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Applications Engineering Manual - Chilled

1. eeeee 23 Heating coil electric hot water Or steam n s 23 Gastired BUMMER desncs cadet asteced eE E aA 24 Recovered heat oo cccecccccccccccececececeeeeeeeseseseeeseseeaeaeaeeeeeeeees 26 Fans Supply return relief 0 0 ccccceccecceeceeeeeceeeeeeeeeeeeeeeeeeseeaees 26 Supply fan only Are 26 Supply fan and relief fan ooo ccc ccc cece ccccccecececceecessssaueaeeeeeeees 29 Supply fan and return fan oo cccccecccccccccecececceecseestssesaeeeeeees 30 Should the system use a relief fan or a return fan 30 FAN LY POS iiinn ce annaa aana ia aa aaa aaia 32 Blow thru versus draw thru sssssssssisiiisisssesssrrrrriirnriereeree 35 Supply fan capacity modulation sssssssssssiiirisiesneeererrrrrena 38 Air cleaning kana a AO 40 Particulate filters lt 2 cccincivsnadallaudvasaasacndstachemaasanadceandieaen a Ei 40 Gaseous air cleaners ooo ccc cccccessecececeeeeeeceecetestseeaeeeeeess 46 BIOLOGICALS masene e e E 48 Water management ceceeccecceeceeeeeeceeeceeeeneeeeeeeeseeeeseseeeeteaees 50 Casing performance leakage and thermal 0 cccccscceeeeeeee 51 VAV Terminal UNITS oo ccccccccccceeeesesese sess esse eee eeeeeeeeseeeeseseeeeeees 54 Types of VAV terminal UNITS wsciscdcscsveisnccdiadiceacs mencasdoveseeasivedasentates 55 Cooling only VAV terminal UNItS ssssessssiiisiisneerrerrrrrrrrrnn 55 VAV reheat terminal UNITS oo cece cece sees ses eseseeeee eae eeeeeeees 56 F
2. cece ce eeeeeteceeeeeeeeeeeeeeeeees 199 Fan pressure optimization cee ceeeeeeeecceceeeeeeeeeeeteseeeeeeeeees 200 Supply airtemperature reset o oo ceececcccccccccceeecneeseessaeeaes 202 Ventilation optimization cece eeeeeeeceeceeeeeeceeceesteeeeeeeeeeees 205 Pump pressure OPTIMIZATION ooo ceccccccccececeeeseseeeseeeee ones 208 Chilled water temperature reset 0 0 ceececcecceeceeeeeeeneees 208 Hot water temperature reset oo cccccccce cesses eeeseneeeeeees 209 Condenser water temperature chillertower optimization 210 Coordination with other building SySteMS ccceecceeeeeeeeeeee 211 MOSSY AO 213 References oii ccccccccccccecescececesescsteteesetesessststensitereasisereates 225 ao re 230 vi Chilled Water VAV Systems SYS APM008 EN Overview of a Chilled Water VAV System A typical chilled water variable air volume VAV system consists of a VAV air handling unit that serves several individually controlled zones Each zone has a VAV terminal unit that varies the quantity of air delivered to maintain the desired temperature in that zone The primary components of a typical chilled water VAV system Figure 1 include e VAV air handling unit that contains a mixing box filters a chilled water cooling coil possibly a heating coil gas fired burner or electric heater a variable volume supply fan possibly a return or relief fan and controls e VAV terminal unit with a temperature sensor f
3. Chilled Water VAV Systems SYS APM008 EN CO2 sensors increase both the installed cost of the system and the risk These sensors need to be maintained and calibrated or periodically replaced in order to maintain accuracy If the sensor goes out of calibration and signals that the CO2 concentration in the zone is lower than it actually is the system will reduce ventilation to under ventilate that zone degrading indoor air quality On the other hand if the sensor signals that the CO2 concentration is higher than it actually is the system will increase ventilation to over ventilate that zone wasting energy Therefore CO2 sensors should not be used indiscriminately Rather they should be installed only in those zones where they provide the best return on investment and are worth the risk SYS APM008 EN System Controls eT Figure 147 Ventilation optimization DCV at the zone level H communicating BAS lounge rest storage office 0107 0100 vestibule corridor TOD a TOD 2 g 01070 007 o reception area office conference room computerroom With this approach CO2 sensors are installed only in those zones that are densely occupied and experience widely varying patterns of occupancy such as conference rooms auditoriums and gymnasiums These zones are the best candidates for CO2 sensors and provide the biggest bang for the buck These sensors are used to reset the ventila
4. chillers cll bypass line YP cooling towers condenser pump s waterside heat economizer exchanger pump cooling coils with two way control valves AAAS AANANAY ASSN Chilled Water VAV Systems 91 TRANE For more information on adding thermal storage to a chilled water system refer to the Trane Air Conditioning Clinic titled Ice Storage Systems TRG TRC019 EN and the Trane application manuals titled Ice Storage Systems SYS AM 10 and Control of Ice Storage Systems ICS AM 4 For more information on the various types of boilers refer to Chapter 31 Boilers of the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org The Boiler Book from Cleaver Brooks www boiler spec com or the Gas Boilers design guide from the New Buildings Institute www newbuildings org 92 Primary System Components E a aes Thermal storage Adding thermal storage to the chilled water system can reduce utility costs by shifting the operation of the chiller from periods when the cost of electricity is high e g daytime to periods when the cost of electricity is lower e g nighttime This reduces the electricity required to operate the chiller during the periods of high cost electricity and shifts operation of the chiller to the period of low cost electricity The chiller is used during the period of low c
5. Simultaneous heating and cooling limitation For a comfort cooling application Section 6 5 2 1 of ASHRAE 90 1 2007 places limits on the energy used to reheat air that has been previously cooled either by refrigeration equipment or an airside economizer cycle This impacts VAV systems that use heating coils in the VAV terminal units to prevent overcooling zones under low cooling loads Before this heating coil can be activated the primary airflow must first be reduced to the largest of the following Chilled Water VAV Systems SYS APM008 EN FRAME An addendum h to ASHRAE Standard 90 1 2007 removes exceptions a2 0 4 cfm ft2 2 L s m2 and a4 300 cfm 140 L s from Section 6 5 2 1 and adds a new exception that allows the dual maximum control strategy as long as the maximum heating primary airflow lt 50 of maximum cooling primary airflow see Figure 50 p 57 SYS APM008 EN System Design Issues and Challenges Pi eT a1 The volume of outdoor air required to meet the ventilation requirements of Section 6 2 of ASHRAE Standard 62 1 for the zone Voz a2 0 4 cfm ft2 2 L s m2 of the zone conditioned floor area a3 30 percent of zone design supply airflow a4 300 cfm 140 L s this exception is for zones whose design supply airflows total no more than 10 percent of the total system airflow a5 Any higher rate that can be demonstrated to the satisfaction of the authority having jurisdiction to reduc
6. floor by floor VAV air handling units SA to zones When a series desiccant wheel is used in a dedicated OA unit it may be necessary to preheat the entering outdoor air OA when the relative humidity is high on a mild rainy day for example Using the preheat coil to raise the dry bulb temperature slightly 5 F to 20 F 3 C to 11 C lowers the relative humidity Lowering the relative humidity of the air entering the regeneration side of the wheel allows the desiccant to reject water vapor to the air thus enabling it to adsorb water vapor from the air downstream of the cooling coil Typically the amount of heat added by the preheat coil is small and it may be required for only a small number of hours throughout the year Therefore it may be practical to recover the needed heat from the condenser of a water chiller see Condenser heat recovery p 88 A small inexpensive electric heater is another option Alternatively a total energy wheel can be added to the system Figure 96 When high RH conditions occur the total energy wheel transfers moisture from the entering outdoor air OA to the exhaust air EA thus lowering the relative humidity of the air before it enters the regeneration side of the series desiccant wheel OA In such cases adding a total energy wheel reduces and often eliminates the need to add regenerative preheat Of course this requires exhaust air to be ducted back to the dedicated
7. Of course this colder air leaving the coil in a draw thru configuration can also be of benefit At equivalent supply air temperatures the draw thru configuration delivers the supply air at a lower dew point Figure 28 which improves dehumidification performance In a blow thru configuration the air handling unit typically needs to be longer to avoid uneven velocities as the air passes through the cooling coil Alternatively a diffuser set of baffles can be used to provide even airflow across components downstream of the fan and minimize additional length In the example shown in Figure 29 the diffuser section increases the overall length of the air handling unit by 1 5 ft 0 4 m or 8 percent Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components Figure 29 Effect of draw thru versus blow thru with diffuser on AHU length draw thru configuration q al We p 16 0 ft 4 9 m i 17 5 ft 5 3m diffuser q D blow thru configuration Source Images from Trane TOPSS program Note When using a blow thru configuration consider using a plenum fan The required distance between the fan and cooling coil is much shorter and a diffuser is not needed In addition this avoids the negative impact of the abrupt discharge on the per
8. Step 4 Sum the results to determine the acoustical performance of the installation Once the contributions of the individual paths for a particular receiver location are calculated they must be added together to determine the total sound at the receiver A unique sum is required for each critical receiver location Step 5 Compare the summations with the acoustical goals and in the context of the project budget The sum of the sound paths for a particular receiver location is a prediction of the sound level at that location If the sum is lower than the sound target for that receiver location the design does not need to be changed although it may be reviewed for potential cost reductions If the estimate from the analysis exceeds the sound target the paths are reviewed to determine which paths are dominant Alterations to the source and or the path elements are then made to reduce the sound at the receiver location This is typically an iterative process comparing the acoustical effect of various alterations Once a design meets the acoustical goals for the project everyone on the team must understand the work and costs required to implement the design It may also be prudent to review the cost of meeting the acoustical goals and reconsider system layout alternatives or equipment options that were initially rejected due to cost Specific acoustical recommendations It is challenging to put together a list of specific acoust
9. e Add antifreeze to the chilled water system Adding antifreeze such as glycol to the chilled water system lowers the temperature at which the solution will freeze Given a sufficient concentration of glycol no damage to the system will occur For a VAV system since the cooling coil is typically not used during sub freezing SYS APM008 EN Chilled Water VAV Systems 19 FRAME Figure 13 Air mixing baffles 20 Primary System Components a aT weather a concentration that provides burst protection is usually sufficient for the chilled water system Table 13 p 87 A concentration that provides freeze protection is only needed in those cases where no ice crystals can be permitted to form such as a coil loop that operates during very cold weather or where there is inadequate expansion volume available Make sure to also use an inhibitor package to help resist corrosion The advantage of this approach is that it is predictable and relatively easy to maintain However antifreeze degrades the heat transfer performance of cooling coils and chillers often increasing the size and cost of these components In addition it increases the fluid pressure drop through the coils and chillers impacting pumping energy use Preheat the outdoor air before it mixes with the recirculated air Using an electric heater steam coil or hot water heating coil to preheat the sub freezing outdoor air before it enters the mixing box dec
10. might be any of the following e A single room separated by physical boundaries walls windows doors floor and ceiling The individual offices in an office building or individual classrooms in a school could each be a separate zone In this case each office or classroom would be served by a dedicated VAV terminal unit and zone sensor e A group of several rooms Several of the offices or classrooms along the west facing perimeter of the building could be grouped together as one zone In this case one VAV terminal unit would be used to serve the entire group of rooms and a zone sensor would typically be installed in only one of the rooms e A subsection of a large open area An office building might include a large open area that is divided into cubicles The interior portion of this open area might be separated into several zones in order to provide better temperature control If the area is bounded by a perimeter wall the outer 15 ft 4 6 m of this area might be its own zone due to the impact of heat gain and loss through the building envelope In all cases rooms that are grouped together as a single thermal zone should have similar heating and cooling requirements Whenever possible a zone should have definite physical boundaries walls windows doors floor and ceiling Loss of temperature and humidity control can result if air can be supplied to the zone by a VAV terminal unit other than the one connected to the zone sens
11. pre humidify the entering outdoor air reduces the required capacity of the humidification equipment SYS APMO008 EN Chilled Water VAV Systems 161 System Design Variations a a Drawbacks of outdoor air preconditioning However there are some drawbacks that need to be justified e Increases airside pressure drop which increases fan energy and may require larger fan motors Adding an air to air energy recovery device increases the static pressure drop in both the outdoor and exhaust air paths The magnitude of this pressure drop and the configuration of the fans determine how much additional energy is consumed The operating cost savings provided by recovered energy must exceed the increased cost of operating the fans in order to justify the cost of the energy recovery device e May require additional exhaust ductwork Routing most of the exhaust air back to the energy recovery device so that the path is adjacent to that of the entering outdoor air may require more ductwork than a system without energy recovery An advantage of a coil loop is that it can be used to transfer heat between air streams that are physically separated by some distance making them well suited for retrofit situations Also a coil loop can be used to recover heat from multiple separate exhaust air streams using multiple exhaust side coils Best practices for preconditioning outdoor air using air to air energy recovery When using air to air ene
12. The sixth step is to determine system ventilation efficiency Ev Table 20 is an excerpt from ASHRAE 62 1 It lists the default value for system ventilation efficiency based on the largest zone outdoor air fraction Zp Interpolating in this table is allowed Table 20 Ey defaults from Table 6 3 of ASHRAE 62 1 2007 Maximum Zp Ev lt 0 25 0 90 lt 0 35 0 80 lt 0 45 0 70 lt 0 55 0 60 gt 0 55 Use Appendix A For the zones served by the system in this example the largest Zp is 0 50 Table 19 Interpolating in Table 20 the system ventilation efficiency Ev is determined to be 0 65 Notice that if the largest Zp is greater than 0 55 the standard requires the use of the calculated Ey method that is outlined in Appendix A of the standard The final step is to calculate the required outdoor air intake for the system Vot This is determined by dividing the uncorrected outdoor air intake Vou by system ventilation efficiency Ev Vot Vou Ev Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Issues and Challenges Pi a eT For this example the uncorrected outdoor air intake is 2800 cfm 1 3 m3 s with a system ventilation efficiency of 0 65 resulting in a required outdoor air intake Vot flow of 4310 cfm 2 0 m s Appendix A calculated Ev method As mentioned earlier ASHRAE 62 1 provides a second method for determining system ventilation efficiency
13. a D a a surge static pressure a Q Q a Q Q Q Q sensor in supply duct Q sensor setpoint potential energy savings fan pressure optimization airflow By making sure that every VAV terminal unit has just enough pressure to deliver the required airflow this strategy ensures that no zones will be starved for air due to too little pressure in the upstream supply duct Finally fan pressure optimization allows the static pressure sensor to be located anywhere in the supply duct system This presents the opportunity to have it factory installed and tested at the fan outlet inside the central air handling unit In this location it can also serve as the duct high pressure sensor protecting the ductwork from damage in the event of a fire damper closing If the VAV terminal units include pressure independent communicating controls the system level communications are already in place making fan pressure optimization the lowest cost highest energy saving strategy for fan capacity control Supply air temperature reset In a VAV system it is tempting to raise the supply air temperature SAT at part load conditions in an attempt to save cooling or reheat energy Increasing the supply air temperature reduces cooling energy because 1 it allows the control valve on the chilled water to modulate further closed and 2 it improves the ability of the airside economizer
14. method Beginning with the default method which uses Table 6 3 in the standard determining system ventilation efficiency and the required outdoor air intake flow involves several steps 1 Calculate breathing zone outdoor airflow Vpbz Determine the zone air distribution effectiveness Ez 2 3 Calculate zone outdoor airflow Voz 4 Calculate the zone primary outdoor air fraction Zp 5 Determine the uncorrected outdoor air intake Vou Chilled Water VAV Systems 105 System Design Issues and Challenges Pi ae 6 Determine system ventilation efficiency Ev 7 Calculate the system outdoor air intake Vot Figure 85 shows an example eight zone VAV system serving an office building Supply air is delivered to the zones through ceiling mounted diffusers and return air leaves the zones through ceiling mounted return air grilles All zones have VAV reheat terminals The first three steps zone level calculations have already been completed Table 18 see Zone level ventilation requirements p 101 Figure 85 Example office building with a VAV system warehouse north north offices conference room north interior offices west east offices offices south interior offices south conference south room offices Table 18 Zone level ventilation calculations for example office building cooling design Rp cfm p x Pz qty Vbz p cfm Ra cfm ft2 x Az ft Vbz a cf
15. rather than 55 F 13 C using the example in Figure 145 This allows SAT Chilled Water VAV Systems SYS APM008 EN For more information on combining zone level demand controlled ventilation with system level ventilation reset refer to the Trane Engineers Newsletter titled CO2 Based Demand Controlled Ventilation with ASHRAE Standard 62 1 2004 ADM APNO17 EN and the Trane Engineers Newsletter Live broadcast DVD titled CO2 Based Demand Controlled Ventilation APP CMC024 EN An energy saving enhancement to the ventilation reset control strategy is for the BAS to enforce a maximum OA fraction Figure 146 and increase the primary airflow to the critical zone zone with the highest OA fraction at low load conditions While this will activate reheat to avoid overcooling the zone when it is hot or cold outside the energy saved by avoiding the need to increase system level intake airflow will likely outweigh the small amount of reheat needed for this one zone or few zones SYS APM008 EN System Controls aT reset to be used during cooler weather while still offsetting the cooling loads in the interior zones e Design the air distribution system for low pressure losses and use the fan pressure optimization strategy p 200 to minimize the penalty of increased fan energy when the SAT is increased Ventilation optimization The ventilation control function of the zone level VAV terminal units was
16. 116 117 zone primary outdoor air fraction 63 105 zone sensor 100 zone temperature control 183 SYS APM008 EN Chilled Water VAV Systems 235 Index 236 Chilled Water VAV Systems SYS APMO008 EN S TRANE Trane www trane com For more information contact your local Trane office or e mail us at comfort trane com Literature Order Number SYS APM008 EN Date September 2009 Supersedes New Trane has a policy of continuous product and product data improvement and reserves the right to change design and specifications without notice
17. C and the minimum required outdoor airflow for that zone can be reduced typically to the building related or base ventilation rate Ra required by ASHRAE Standard 62 1 see Minimum ventilation rate required in breathing zone Vpbz p 102 The purpose of each of these actions is to save energy In addition the minimum primary airflow setting of the VAV terminal serving that zone can often be lowered to avoid or reduce the need for reheat The minimum primary airflow setting is typically selected to ensure proper ventilation or to increase air circulation for occupant comfort see Minimum primary airflow settings p 62 However with no occupants and a reduced ventilation requirement the minimum primary airflow setting can be lowered significantly minimizing or avoiding reheat energy Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Controls ET Table 31 Example of occupied standby modet occupied mode occupied standby mode Lights on off Zone cooling setpoint 75 F 77 F 24 C 25 C Outdoor airflow required2 310 cfm 60 cfm 153 L s 28 L s Minimum primary airflow 450 cfm 225 cfm setting 212 L s 106 L s 1 Based on a 1000 ft2 93 m2 conference room with a design zone population Pz of 50 people 2 According to Table 6 1 of ANSI ASHRAE Standard 62 1 2007 the required outdoor airflow rates for a conference room are Rp 5 cfm p 2 5 L s p
18. Figure 24 and Figure 26 However the air handling unit may be slightly longer since the fan motor is mounted at end of the shaft e Single versus multiple fans Most fans in VAV systems use a single fan wheel However using multiple fans can shorten the length of the AHU This is often referred to as a fan array Figure 27 For a given airflow a unit with multiple fans uses several smaller diameter fan wheels rather than a single larger diameter fan wheel The distance length required both upstream and downstream of the fan is typically a function of the fan wheel diameter Therefore using multiple smaller diameter fan wheels can shorten the required upstream and downstream spacing required and can shorten the overall length of the air handling unit While the overall length can be reduced significantly by changing from one to two fans the potential length reduction diminishes as the number of fans increases When more than four to six fan wheels are used the upstream and downstream spacing requirements begin to be dictated by the need for access rather than by fan wheel diameter so there is generally little further length reduction benefit Another benefit of using multiple fans is redundancy If one fan fails another fan is available to compensate In most cases three or four fans are able to provide the required level of redundancy However using multiple fans is typically less efficient and increases the cost of th
19. Heating capacity MBh kW 22 8 6 7 22 8 6 7 Entering fluid temperature F C 150 65 6 3 180 82 2 2 Returning fluid temperature F C 116 46 7 146 63 3 Coil flow rate gpm L s 1 34 0 085 1 33 0 084 Fluid pressure drop ft H20 kPa 0 12 0 35 1 10 3 3 Airside pressure drop at heating airflow 0 07 17 6 0 04 10 1 in H20 Pa 1 Assumes airside pressure drop changes with the square of the airflow reduction design cooling airflow 2000 cfm 0 94 m3 s reheat heating airflow 600 cfm 0 28 m3 s 2 Assumes the hot water supply temperature has been reset from 180 F 82 2 C to 150 F 65 6 C during the months when reheat rather than heating is needed see Hot water temperature reset p 209 3 The two row coil is able to provide the required heating capacity using a lower hot water temperature which presents the opportunity to use a condensing boiler see Non condensing versus condensing boilers p 92 Some engineers express concern about the increase in air pressure drop and this certainly should be considered it requires higher static pressure and fan horsepower at design conditions However because the coil air pressure difference drops quickly as airflow is reduced the actual impact on annual fan energy use is small In many cases the energy related benefits of using waterside heat recovery and condensing boilers will outweigh the increase in annual fan energy Pumping ener
20. However as mentioned earlier in cold climates some VAV systems may use baseboard radiant heat located along the perimeter walls within the occupied space This flexibility also makes chilled water VAV systems a popular choice for taller buildings which are not well suited for roof mounted DX equipment and large areas for vertical air shafts Finally using water chillers for cooling centralizes the refrigerant inside a few pieces of equipment This minimizes the risks associated with refrigerant leaks compared to having refrigerant containing equipment spread throughout the facility Flexibility of air handling equipment In general air handling units offer greater flexibility than packaged DX equipment Airhandling units can typically be applied to a wider range of operating conditions making them better suited for systems requiring variable airflow lower cfm ton L s kW such as those with high percentages of outdoor air or colder supply air temperatures and tighter space control requirements In addition air handling units are typically available with a broader range of options such as energy recovery devices dehumidification enhancements fan choices air cleaning equipment sound attenuation choices and casing performance thermal and leakage options Able to adapt to changes in building use Most chilled water VAV systems use an open return air plenum to allow air from all the zones to return back to the VAV air handl
21. Sulfur trioxide Toluene Source NAFA Guide to Air Filtration 4th Edition Figure 11 5 National Air Filtration Association www nafahq org Chilled Water VAV Systems 47 For more information on the various methods for dealing with biological contaminants in the air stream refer to the NAFA Guide to Air Filtration www nafahq org For more information on the benefits and concerns related to using UV C lights in HVAC systems including recommendations and precautions for when they are used refer to the Trane engineering bulletin titled Using Ultraviolet Light to Control Microbial Growth in Buildings CLCH PRB0O14 EN or Chapter 16 Ultraviolet Lamp Systems in the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org 48 Primary System Components a a a Ta Photocatalytic oxidation PCO can also be used to remove VOCs from the air stream VOCs are adsorbed onto the surface of the catalyst commonly a bed of titanium dioxide TiOz2 and ultraviolet light is used to drive a reaction with oxygen and water vapor Ideally this process oxidizes all of the organic compounds into carbon dioxide CO2 and water H20 See p 49 for further discussion of PCO Biologicals Biological or microbial contaminants describe a subset of airborne particles that originate from living or once living organisms e g bacteria fungi viruses Inhaling these agents may cause occupants to experi
22. The Boiler Book http www boilerspec com Dow Chemical Company 2008 HVAC Application Guide Heat Transfer Fluids for HVAC and Refrigeration Systems www dow com heattrans Institute of Electrical and Electronics Engineers IEEE 2006 Wireless Medium Access Control and Physical Layer Specifications for Low Rate Wireless Personal Area Networks IEEE Standard 802 15 4 2006 New York NY IEEE Institute of Environmental Sciences and Technology IEST 2006 HEPA and ULPA Filters IEST RP CC001 4 Mt Prospect IL IEST National Air Filtration Association NAFA 2007 NAFA Guide to Air Filtration 4th Edition Virginia Beach VA NAFA New Buildings Institute NBI 1998 Gas Boilers Advanced Design Guideline Fair Oaks CA NBI http Awww newbuildings org Sheet Metal and Air Conditioning Contractors National Association SMACNA 2006 HVAC Systems Duct Design Chantilly VA SMACNA Stanke D 2004 Addendum 62n Single zone amp Dedicated OA Systems ASHRAE Journal October pp 12 20 Stanke D 2005 Addendum 62n Single Path Multiple Zone System Design ASHRAE Journal January pp 28 35 Stanke D 2005 Standard 62 1 2004 Designing Dual Path Multiple Zone Systems ASHRAE Journal May pp 20 30 Stanke D 2006 Standard 62 1 2004 System Operation Dynamic Reset Options ASHRAE Journal December pp 18 32 Trane 2008 Using Ultraviolet Light to Control Microbial Growth in Buildings CL
23. This eliminates the maintenance chiller condenser tube cleaning freeze protection and water treatment associated with a cooling tower In addition it eliminates the concern for the availability and quality of makeup water This often favors air cooled chillers in areas that have an inadequate or costly water supply or where the use of water for the purpose of air conditioning is restricted On the other hand water cooled chillers typically last longer than air cooled chillers since an air cooled chiller is typically installed outdoors whereas a water cooled chiller is installed indoors In addition water cooled chillers are typically more energy efficient In an air cooled chiller the condensing temperature of the refrigerant is dependent on the dry bu b temperature of the ambient air For example if the ambient dry bulb temperature is 95 F 35 C the refrigerant condensing temperature might be 125 F 52 C Chilled Water VAV Systems SYS APM008 EN For more information on low flow chilled water systems and condenser water systems refer to the Trane application manual titled Chiller System Design and Control SYS APMO01 EN SYS APM008 EN Primary System Components a ase In a water cooled chiller however the refrigerant condensing temperature is dependent on the condenser water temperature which is dependent on the wet bulb temperature of the ambient air For the same 95 F 35 C ambient dry bulb temperatu
24. airflow to space cool primary air 0 design space load design heating load cooling load Figure 121 Control of a dual duct VAV terminal Constant volume to the zone single fan system design supply 100 Tm airflow warm primary air 8 oO a g A 3 cool primary air K se 0 design space load design heating load cooling load Best practices for dual duct VAV systems When using a dual duct VAV system consider the following general For more information on dual duct VAV recommendations systems refer to the following ASHRAE Journal articles e Use a dual fan duct duct configuration to maximize energy efficiency 1 Warden D Dual Fan Dual Duct As described earlier the dual fan arrangement Figure 116 results i less Systems Better Performance at a Lower overall energy use because it avoids wasteful reheat of the mixed air Cost ASHRAE Journal January 1996 when the airside economizer is activated to save cooling energy pp 36 41 Available at www ashrae org In addition the dual fan arrangement allows the heating air handling unit 2 Warden D Dual Fan Dual Duct Goes y ne 3 to School ASHRAE Journal May 2004 to deliver neutral air unheated recirculated air down the hot duct pp 18 25 Available at www ashrae org during the cooling season During very low cooling loads the dual duct VAV terminal mixes this recirculated air with cooling primary air tempering the suppl
25. and Relief Dampers for Air Side Economizer Systems for VAV systems With this approach the relief damper is modulated to directly control building pressure at the desired setpoint and the OA and RA dampers are controlled to ensure that the proper amount of outdoor air is brought in for ventilation e Flow tracking uses airflow measurement devices to measure both supply and return airflows The capacity of the return fan tracks supply airflow to maintain a fixed airflow differential between the two accounting for local exhaust fans and building pressurization Flow tracking works best in applications that have a constant amount of local exhaust and are only minimally affected by wind and stack effect Successful implementation requires well calibrated flow sensors because the difference between supply and return airflows can be a very small fraction of the overall sensed airflow e Signal tracking monitors the pressure in the supply duct and uses that signal to modulate the speeds of both the supply and return fans Building pressure is controlled indirectly and ineffectively because of the disparity between the performance curves of the two fans Therefore this approach is not recommended Safeties The unit level controller for the VAV air handling unit typically includes several safeties that protect the equipment from harm Common examples include e A freeze protection sensor that turns off the fan and or closes the OA dam
26. condensing pressure control 188 constant flow pumping 86 97 208 209 control valve selection 83 controls 98 156 airside economizer control 117 126 130 151 152 156 163 167 174 202 203 building pressure control 178 condensing pressure control 188 differential comparative enthalpy control 177 discharge air temperature control 171 fixed dry bulb control 175 return water temperature control 190 return fan capacity control 181 safeties 182 space pressure control 187 unit level 171 ventilation control 171 184 zone temperature control 183 cooling coil See chilled water cooling coil cooling only VAV terminal units 55 cross leakage 165 Cv See valve flow coefficient D damper 2 3 11 27 54 113 115 125 150 164 172 181 200 212 deadband 3 5 184 dedicated outdoor air unit 115 dehumidification 118 after hours 125 demand controlled ventilation 132 159 differential enthalpy control 177 differential pressure sensor 44 179 direct drive plenum fan 33 34 35 direct fired burner 24 discharge air temperature control 171 downdraft 64 66 75 drain pan 16 37 51 draw thru configuration 35 51 141 dual duct VAV system 165 best practices 169 dual duct VAV terminal units 62 duct breakout 137 SYS APMO008 EN Chilled Water VAV Systems 231 232 Index Ductulator 72 dumping 74 149 158 dust spot efficiency 41 dynamic reset of intake airflow 117 E economizer airside 117 126 130 151 152 156 163 1
27. plenum However consider the impact of only reducing the airhandling unit by one size The right hand column in Table 28 depicts the same cold air system design but the air handling unit is only downsized one size to a size 25 This still results in a significant reduction in footprint 14 percent see Figure 105 and weight 13 percent but the larger coil face area results in a lower airside Chilled Water VAV Systems SYS APM008 EN System Design Variations a pressure drop and even greater fan energy savings a 32 percent reduction in fan input power compared to the conventional 55 F 13 C supply air system This decision often results in the best balance of installed cost savings and energy savings Figure 105 Impact of cold air distribution on AHU footprint plan view Size 21 Size 25 Challenges of cold air distribution Concerns that design engineers have about cold air distribution typically focus on the following three issues e Effects of delivering cold air into the zone on occupant comfort e Impact on overall system energy consumption e Avoiding condensation on components of the air distribution system Effects of delivering cold air into the zone on occupant comfort Design engineers typically use one of two approaches to avoid dumping cold air into the occupied space 1 supply air diffusers with a high aspiration ratio or 2 fan powered VAV terminal units used as air blenders e High aspi
28. savings depends on proper economizer operation as well as on proper integration of the energy recovery device into the control of the system SYS APMO008 EN Chilled Water VAV Systems 163 TRANE System Design Variations To accommodate economizer operation when the energy recovery device is idle add bypass dampers Figure 114 to allow full economizer airflow without significantly increasing the airside pressure drop and corresponding fan energy consumption Alternatively provide two separate paths for outdoor air one for minimum ventilation airflow and the other for economizer airflow Figure 114 Bypass dampers allow for airside economizing and capacity control E Bypass dampers divert airflow around the energy wheel during economizer siaduep ssedAq Z N o N 3 operation f MAKA S E Ge z N 8 3 2 S sA 3 D A 2 a so a y 8 A 25 lt 8 jS e Provide a means to control the capacity of the device at part load During cool weather most VAV systems will require a means to modulate For more information on methods used p the capacity of the energy recovery device to avoid overheating the air for capacity control and frost prevention with various air to air energy recovery Unnecessarily operating the device at full capacity may require rec
29. see Design temperatures and flow rates p 81 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a as Freeze prevention for the chilled water distribution system Adding antifreeze such as glycol to the chilled water system lowers the temperature at which the solution will freeze Given a sufficient concentration of glycol no damage to the system will occur As the temperature drops below the glycol solution s freeze point ice crystals begin to form Because the water freezes first the remaining glycol solution is further concentrated and remains a fluid The combination of ice crystals and fluid makes up a flowable slush The fluid volume increases as this slush forms and flows into available expansion volume Freeze protection indicates the concentration of antifreeze required to prevent ice crystals from forming at the given temperature Burst protection indicates the concentration required to prevent damage to equipment e g coil tubes bursting Burst protection requires a lower concentration of glycol which results in less degradation of heat transfer Table 13 Table 13 Concentration required for freeze protection vs burst protection Ethylene Glycol Propylene Glycol Concentration volume Concentration volume Temperature freeze burst freeze burst F C protection protection protection protection 20 7 16 11
30. 0 0013 A power kW lt L ssupply x 0 0021 A Excerpt from Table 6 5 3 1 1A of ASHRAE Standard 90 1 2007 American Society of Heating Refrigerating and Air Conditioning Engineers Inc www ashrae org Option 1 is based on nameplate motor power for the fan It is easier to apply than Option 2 but not as flexible To comply using Option 1 the sum of the nameplate motor powers for all fans that operate at peak design conditions must be no greater than the value listed in Table 24 For example if the design supply airflow is 30 000 cfm 14 2 m3 s or 14 200 L s the total allowable nameplate motor power for the VAV system is 45 hp 34 kW Allowable Nameplate Motor Power 30 000 cfm x 0 0015 45 hp Allowable Nameplate Motor Power 14 200 L s x 0 0024 34 kW As mentioned previously this limit applies to the sum of all fans that operate at peak design cooling conditions Option 2 is based on input power to the fan shaft brake horsepower To comply using Option 2 the sum of the fan input brake powers for all fans that operate at peak design conditions must be no greater than the value listed in Table 24 This fan power limitation includes the following adjustment factor to account for special filters and other devices A PD x CFMdevice 4131 A x PD x L sdevice 650 100 where PD pressure drop adjustment for each applicable device Table 25 in H20 Pa CFMaevice L Sdevice design airflow th
31. 18 12 10 12 25 17 29 20 0 18 33 22 36 24 10 23 39 26 42 28 20 29 44 30 46 30 30 34 48 30 50 33 40 40 52 30 54 35 50 46 56 30 57 35 60 51 60 30 60 35 Source Dow Chemical Company 2008 HVAC Application Guide Heat Transfer Fluids for HVAC and Refrigeration Systems www dow com heattrans For a chilled water VAV system since the cooling coil is typically shut off during sub freezing weather burst protection is usually sufficient Freeze protection is mandatory in those cases where no ice crystals can be permitted to form such as a coil loop that operates during very cold weather or where there is inadequate expansion volume available When an air cooled chiller is used an alternative approach is to use a packaged condensing unit condenser and compressor located outdoors with a remote evaporator barrel located in an indoor equipment room The two components are connected with field installed refrigerant piping This configuration locates the part of the system that is susceptible to freezing evaporator indoors and still uses an outdoor air cooled condenser Chilled Water VAV Systems 87 For more information on employing heat recovery in a water cooled chilled water system refer to the Trane application manual titled Waterside Heat Recovery in HVAC Systems SYS APMOO5 EN 88 Primary System Components a a as Freeze prevention for the condenser water distribution syste
32. Crosse WI Inland Printing Company 1987 Ice Storage Systems SYS AM 10 La Crosse WI Inland Printing Company 1988 Control of Ice Storage Systems ICS AM 4 La Crosse WI Inland Printing Company Trane 2004 Air Conditioning Fans TRG TRC013 EN Air Conditioning Clinic series La Crosse WI Inland Printing Company 2000 Cooling and Heating Load Estimation TRG TRC002 EN 2001 Chilled Water Systems TRG TRC016 EN 2001 Fundamentals of HVAC Acoustics TRG TRC007 EN 2001 VAV Systems TRG TRC014 EN 2006 Ice Storage Systems TRG TRC019 EN Chilled Water VAV Systems 227 228 References E Trane Eppelheimer D and Bradley B 2000 Cold Air Makes Good ense Trane Engineers Newsletter 29 2 Guckelberger D and Bradley B 2004 Brushless DC Motors Setting a New Standard for Efficiency Trane Engineers Newsletter 33 4 Hsieh C and Bradley B 2003 Green Growing Here to Stay Energy and Environmental Initiatives Trane Engineers Newsletter 32 3 Murphy J and Bradley B 2002 Using CO2 for Demand Controlled Ventilation Trane Engineers Newsletter 31 3 Murphy J and Bradley B 2005 CO2 Based Demand Controlled Ventilation with ASHRAE Standard 62 1 2004 Trane Engineers Newsletter 34 5 Murphy J and Bradley B 2005 Advances in Desiccant Based Dehumidification Trane Engineers Newsletter 34 4 Guckelberger D and Bradley
33. Efficiency by Particle Size HEPA ULPA classifications are defined by IEST RP CC001 4 HEPA and ULPA Filters Source 2008 ASHRAE Handbook HVAC Systems and Equipment Chapter 28 Table 3 American Society of Heating Refrigerating and Air Conditioning Engineers Inc www ashrae org 42 Chilled Water VAV Systems SYS APM008 EN Primary System Components a a aT Key factors to consider when selecting particulate filter types for a specific application include e Target particle size and degree of cleanliness required collection efficiency The collection efficiency of a particulate filter is a function of how well it removes particles of various sizes Filters with higher efficiencies remove a higher percentage of particles and smaller particles than filters with lower efficiencies Since particulate contaminants vary in size Figure 34 it is important to define the contaminants of concern for a given facility when selecting the type of filter to be used e Allowable airside pressure drop A direct correlation usually exists between collection efficiency and airside pressure drop Generally a filter with a higher efficiency will cause a higher pressure drop in the passing air stream increasing fan energy use The number of pleats in a media filter determines the surface area of the media In general the more surface area the lower the airside pressure drop Pressure drop is also related to air velocity higher v
34. OA unit Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Issues and Challenges Pa a eT Another example dedicated OA unit configuration includes a total energy wheel to precondition the entering OA and a fixed plate heat exchanger to reheat the dehumidified OA when desired Figure 97 Figure 97 Example dedicated OA unit with total energy recovery and a fixed plate heat exchanger SS SS SS p DH fixed plate heat exchanger oO gt i v 2 ie 9 supply fan exhaust fan uo f g D D a Q 109 Buljooo The fixed plate heat exchanger is equipped with integral face and bypass dampers an empty core through the middle of the heat exchanger allows air to bypass to modulate the amount of heat recovered and avoid overheating the conditioned air CA This allows the dedicated OA unit to deliver the dehumidified air cold rather than reheated to neutral when it is hot outside but then to reheat the dehumidified air to avoid overcooling spaces during mild weather This configuration may be more typical for applications that do not require dehumidification to as low a dew point as can be achieved with the series desiccant wheel Figure 96 After hours dehumidification Controlling humidity is not only a priority when the
35. Smaller VAV terminals which ease tight installations are less expensive and may be quieter e Smaller ductwork which requires less sheet metal simplifies installation and leaves more space above the ceiling for other services e Shorter floor to floor height attributable to smaller ductwork may reduce the cost of glass and steel in a multi story building Chilled Water VAV Systems 147 148 System Design Variations a ae e Smaller supply and return or relief if equipped fans reduce the cost of the electrical distribution system and lowers operating costs it may also reduce fan generated noise e Potential for lower space humidity levels due to the delivery of colder and therefore drier air The lower relative humidity in a cold air system often allows the zone dry bulb temperature to be slightly warmer than in a conventional system while still achieving an equivalent sensation of comfort Table 28 demonstrates the benefit of smaller air handling units The left hand column in the table depicts a conventional 55 F 13 C supply air system which has a design airflow of 13 000 cfm 6 1 m3 s The VAV air handling unit is a size 30 resulting in a coil face velocity of 435 fpm 2 2 m s Note The unit size typically represents the nominal face area of the cooling coil in terms of ft In this example the face area of the size 30 air handling unit is 29 90 ft 2 78 m2 The middle column in the table de
36. System Components ST the air and avoid temperature stratification in the occupied portion of the zone Increased airflow may also be needed to meet the minimum requirement for safe and proper operation of an electric heating coil Figure 50 shows an alternate method to control a VAV reheat terminal unit When the zone requires cooling the control sequence is unchanged primary airflow is varied between maximum and minimum cooling airflow as required to maintain the desired temperature in the zone Figure 50 Alternate control of a VAV reheat terminal unit 100 F heating coil activated gt maximum primary a x airflow maximum discharge air temperature maximum primary heating airflow minimum primary cooling airflow airflow to space 0 design space load design heating load cooling load When primary airflow reaches the minimum cooling airflow setting and the cooling load continues to decrease the heating coil is activated to warm temper the air to avoid overcooling the zone As more heat is needed the controller resets the discharge air temperature setpoint upward to maintain zone temperature at setpoint dark dashed line in Figure 50 until it reaches a defined maximum limit This control sequence requires a discharge air temperature sensor installed for each VAV terminal The discharge temperature is limited to minimize temperature stratification when delivering warm air through overhead diffuse
37. Use air mixing baffles This configuration of baffles Figure 13 located immediately downstream of the mixing box adds rotational energy and increases the velocity of the air stream which improves mixing blending to prevent or minimize temperature stratification The advantage of this approach is that it works consistently and requires no maintenance However it does increase the cost and length of the air handling unit since distance is needed downstream for the air to finish mixing and slow down before reaching the filters Also the baffles add Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN Primary System Components a a a aT pressure drop typically 0 2 in H20 50 Pal which increases fan energy use and mixing effectiveness decreases as airflow is reduced e Introduce cold outdoor air downstream of the cooling coil This configuration sometimes called a winterizer uses a combination of two different sized air handling units Figure 14 configured to allow the outdoor air to be introduced downstream of the cooling coil whenever the outdoor air is colder than 32 F 0 C The smaller airhandling unit sized for the minimum required ventilation airflow contains filters and possibly a small preheat coil Figure 14 Winterizer configuration preheat coil winter summer cooling coil Source Image adapted from Trane TOPS
38. VAV Systems 17 TRANE 18 Primary System Components E Figure 11 Impact of coil face velocity on AHU footprint see Table 4 Size 25 Size 35 Size 30 Note that in order to deliver equivalent capacity the cooling coil in the smaller sized AHU requires more fins than the coil in the larger sized AHU This slightly increases the cost of the coil and along with the smaller coil face area increases the airside pressure drop 0 69 in H20 173 Pa for size 25 unit versus 0 50 in H20 125 Pa for the size 30 unit This does impact fan energy use However in a VAV air handling unit the airside pressure drop decreases quickly as supply airflow is reduced at part load so the actual impact on annual fan energy use is lessened Finally the smaller coil reduces the fluid pressure drop 12 1 ft H20 36 3 kPa for size 25 unit versus 13 6 ft H20 40 6 kPa for the size 30 unit which may decrease pumping energy If the goal for a project is to minimize footprint and or equipment cost consider specifying that the face velocity at design airflow should not exceed 90 percent for example of the manufacturer s tested and published limit to prevent moisture carryover rather than specifying an arbitrary maximum coil face velocity of 500 fpm 2 5 m s For example if the manufacturer s published limit is 600 fpm 3 0 m s specify that the coil face velocity not exceed 540 fpm 2 7 m s at design airflow However if the goal for
39. Vbz p cfm Ra cfm ft2 x Az ft Vbz a cfm Vbz cfm Ez Voz cfm South offices 5 18 90 0 06 2000 120 210 0 8 263 West offices 5 20 100 0 06 2000 120 220 0 8 275 South conf room 5 30 150 0 06 3000 180 330 0 8 413 East offices 5 20 100 0 06 2000 120 220 0 8 275 South interior offices 5 50 250 0 06 10 000 600 850 1 0 850 North interior offices 5 50 250 0 06 10 000 600 850 1 0 850 North offices 5 16 80 0 06 2000 120 200 1 0 200 North conf room 5 20 100 0 06 2000 120 220 1 0 220 System totals 224 1120 1980 Pz RpxPz zZ RaxAz 112 Table 23 System level ventilation calculations based on calculated Ey method heating design design Vpz Vpzm Voz Zd Evz cfm cfm cfm Voz Vpzm 1 Xs Za South offices 1900 475 263 0 55 0 87 West offices 2000 500 275 0 55 0 87 South conf room 3300 825 413 0 50 0 92 East offices 2000 500 275 0 55 0 87 South interior offices 7000 1750 850 0 49 0 94 North interior offices 7000 1750 850 0 49 0 94 North offices 1600 400 200 0 50 0 92 North conf room 1800 450 220 0 49 0 93 System totals Vps 6650 Vpzm Because zone primary airflows are lower at the heating design condition the overall system primary airflow Vps is also lower Table 23 This results in a higher average outdoor air fraction Xs Xs Vou Vps 2800 cfm 6650 cfm 0 42 For this example higher required zone outdoor airflows Voz increase the
40. Water Control RLC PRB017 EN SYS APM008 EN System Controls eT variable speed drive on one or more of the condenser fans This is typically an option available from the chiller manufacturer For a water cooled chiller the condenser water system may need to be designed to vary either the temperature or flow rate of the water through the condenser For example consider an office building that has been unoccupied during a cool autumn weekend during which time the temperature of the water in the cooling tower sump has dropped to 40 F 4 C Monday is sunny and warm and the building cooling load requires a chiller to be started Pumping this cold water through the chiller condenser results in a very low refrigerant pressure inside the condenser Because the chiller is operating at part load and the tower sump is relatively large the minimum pressure difference may not be reached before the chiller is turned off on a safety The solution is to either increase the temperature of the water entering the condenser or decrease the flow rate of water through the condenser Both of these strategies will increase the temperature and pressure of the refrigerant inside the condenser After the minimum pressure difference is reached system operation may return to normal Common approaches for increasing the evaporator to condenser pressure difference include e Cycling or varying the speed of cooling tower fans to increase the temperature
41. a aT and upstream of the fan is at a negative pressure relative to the air surrounding the unit Only the fan discharge is at a positive pressure This will result in air leaking into the air handling unit Any air that leaks into the unit between the cooling coil and supply fan will raise the temperature of the supply air Since the VAV air handling unit is controlled to maintain the supply air temperature at a setpoint this leakage results in the cooling coil needing to overcool the portion of air that passes though the coil in order to offset the mixing of warmer air downstream of the coil But any air that leaks into the unit upstream of the cooling coil does not impact the supply air temperature because it will still pass through the cooling coil This is simply return air that enters the unit via leakage rather than through the return air damper Of course if the return air is ducted directly to the air handling unit the air that leaks in is not conditioned return air and will likely increase cooling energy Figure 42 Example impact of air leakage draw thru indoor AHU ee z return air supply air outdoor air i i L PNp gz air leakage E E ae air leakage By contrast Figure 43 depicts the same indoor air handling unit configured as a blow thru unit The static pressure inside the unit and downstream of the fan is at a positive pressure relative to the air surrounding the unit This will result in air
42. a plenum fan typically has lower discharge sound levels than a housed fan Figure 24 The reduced sound levels occur because air velocity dissipates more quickly as the air pressurizes the plenum surrounding the fan and because the plenum provides an opportunity for some of the sound to be absorbed before the air discharges from the air handling unit Figure 24 Plenum fan can reduce discharge sound levels r 110 housed FC 22 375 in 568 mm housed AF 25 in 635 mm 2 100 sa Delt drive plenum AF 35 56 in 903 mm fo nm direct drive plenum AF 30 in 762 mm 5 ToN N as hy g 90 Nei a _ eenttn oe gop snnm Min reen is D fay z senuana niany a renn po 70 t teter c trann a anay amp 60 D housed fan plenum fan p a Sn LG 5 50 63 125 250 500 1000 2000 4000 8000 octave band frequency Hz 1 Based on a typical VAV air handling unit configuration with a single front top discharge opening operating at 13 000 cfm 6 1 m3 s and 3 in H2O 750 Pa of external static pressure drop SYS APMO008 EN Chilled Water VAV Systems 33 TRANE Primary System Components To allow a housed fan to achieve similar discharge sound levels a discharge plenum can be added to the air handling unit While this plenum helps reduce discharge sound levels it increases fan input power and increases the overall length of the unit Figure 25 and Figure 26 In this example when a discharge plenum is
43. added to the housed fan the input power for the housed AF fan is higher than for the direct drive plenum fan Figure 26 Figure 25 Plenum fan can reduce overall AHU length EJ a discharge plenum 18 3 ft 5 6m 15 0 ft 4 6m W _ plenum Db fan Source Images from Trane TOPSS program Figure 26 Discharge sound levels with multiple discharge connections 2 housed AF 25 in 635 mm 110 unit length 15 0 ft 4 6 m input power 14 7 bhp 10 9 kW 2 100 housed AF 25 in 635 mm discharge 9 plenum 5 unit length 18 3 ft 5 6 m a input power 16 1 bhp 12 0 kW a 90 A belt driven plenum AF 35 56 in 903 mm 3 unit length 15 0 ft 4 6 m 80 input power 17 4 bhp 13 0 kW g 2 von direct drive plenum AF 30 in 762 mm a 70 unit length 15 9 ft 4 8 m S input power 14 6 bhp 10 9 kW 9 ty SS ty Ia v 60 tn x o ann be ees a 5 50 63 125 250 500 1000 2000 4000 8000 octave band frequency Hz 1 Based on a typical VAV air handling unit configuration with multiple discharge openings Figure 25 operating at 13 000 cfm 6 1 m3 s and 3 in H20 750 Pa of external static pressure drop 2 Discharge sound power for a housed AF fan with a single front top discharge opening Figure 24 included here to demonstrate the sound reduction provided by the discharge plenum 34
44. air stream Also known as a rotary heat exchanger passive desiccant wheel heat wheel or enthalpy wheel Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary a TR Cold spot thermal resistance ratio A measure of thermal resistance of the air handling unit casing transmission loss A term used to measure the effect of a barrier on reducing the amount of transmitted sound It is the ratio of sound power on the receiver side of a barrier to the sound power on the source side Traq damper Trane s flow measuring outdoorair damper underfloor air distribution A method of air distribution in which conditioned air is delivered to the zones under a raised floor and floor grilles unoccupied economizing An energy saving control strategy that attempts to precool the building using cool outdoor air prior to the morning cool down period unoccupied mode The typical nighttime operating mode of a system The building does not require ventilation because it is not occupied and the zone temperatures are allowed to drift to unoccupied setpoints UV C Ultraviolet radiation in the C wavelength band variable air volume VAV system A type of air conditioning system that varies the volume of constant temperature air supplied to meet the changing load conditions of the zone variable frequency drive VFD See variable speed drive variable speed drive VSD A device used to vary the capacity of a fan pump
45. air with the cool primary air using a small terminal fan fan pressure optimization An optimized method of VAV system static pressure control which uses the benefits of DDC control to continuously reset the static pressure setpoint of the system so the VAV terminal requiring the highest inlet pressure is nearly fully open Chilled Water VAV Systems 217 218 Glossary E fan speed control A method of supply fan modulation that affects a fan s capacity by varying its speed of rotation commonly accomplished using a variable speed drive on the fan motor flow tracking Method of return fan capacity control in which the return fan is controlled based on a fixed airflow differential from supply fan airflow flue gases Exhaust gases from a boiler or gas fired burner glycol A liquid that is mixed with water to lower the freezing point of the solution grille A device used to direct air out of the conditioned space into the ceiling plenum or return duct system HEPA High efficiency particulate air filter humidity pull down mode An operating mode for transition from the unoccupied mode to the occupied mode in which the HVAC system operates to lower the humidity inside the building to reach the desired occupied humidity setpoint by the time people enter the building IEEE Institute of Electrical and Electronics Engineers www ieee org indirect fired burner A fuel burning device in which the products of combustion d
46. airflow has less impact on fan energy Finally when the outdoor temperature is colder than 60 F 16 C no further reset occurs and the SAT setpoint remains at 60 F 16 C Limiting the amount of reset to 60 F 16 C allows the system to satisfy the cooling loads in interior zones without needing to substantially oversize the VAV terminal units and ductwork serving those zones Alternatively some systems reset the SAT setpoint based on the temperature in the zone that is most nearly at risk of overcooling which would require activating local reheat A building automation system monitors the temperature in all zones finding the zone that is closest to heating setpoint temperature Then the SAT setpoint is reset to prevent this coldest zone from needing to activate reheat When considering using SAT reset in a VAV system e Analyze the system to determine if the savings in cooling and reheat energy will outweigh the increase in supply fan energy e If higher zone humidity levels are a concern consider either 1 providing an outdoor dew point sensor to disable reset when it is humid outside or 2 providing one or more zone humidity sensors to override the reset function if humidity in the zone exceeds a maximum limit e For interior zones that will likely have nearly constant cooling loads during occupied periods calculate the design supply airflows for those zones based on a warmer than design supply air temperature 60 F 16 C
47. airflow settings if it results in a reduction of overall system energy use This exception recognizes that increasing the minimum airflow setting in the critical zone increases system ventilation efficiency and results in lower system intake airflow However this has to be demonstrated to and approved by the authority having jurisdiction AHJ so it may not be possible in all cases Some engineers use a spreadsheet analysis to demonstrate this while others use whole building energy simulation software One approach is to use a control strategy such as ventilation reset see Ventilation optimization p 205 to minimize overall system energy use In addition exception c under Section 6 5 2 1 allows any amount of air to be reheated if at least 75 percent of the energy required for reheat is recovered on site In chilled water VAV systems that use hot water coils in the VAV terminals heat recovery from the chilled water system may provide enough of the reheat energy to meet this exception see Condenser heat recovery Chilled Water VAV Systems 131 For more information on demand controlled ventilation see Ventilation control p 184 and Ventilation optimization p 205 For more information on energy and high performance building initiatives refer to the Trane Engineers Newsletter titled Energy and Environmental Initiatives ADM APNOO8 EN 132 System Design Issues and Challenges Pi eT p
48. airside economizer or modulating the heating control valve The supply fan modulates to maintain a constant pressure in the supply ductwork and the outdoor air damper brings in at least the minimum required amount of outdoor air Zone is unoccupied When the zone is scheduled to be unoccupied most buildings relax the zone setpoints allowing the temperature in the zone to either increase or decrease In fact this practice is required in many buildings by local codes or Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Overview of a Chilled Water VAV System a aT energy standards These new setpoints are often called setback temperatures and the result is a much wider deadband Figure 4 Figure 4 Unoccupied zone setback temperatures occupied cooling setpoint occupied heating setpoint zone dry bulb temperature ol oO e gt a Eg o el unoccupied heating setback temperature During unoccupied periods as long as the temperature in the zone is within this wider deadband the VAV terminal unit closes to prevent any air from being supplied to the zone Also any local heat heating coil in the VAV terminal or baseboard heat within the zone is off If all zones served by the air handling unit are unoccupied and the zone temperatures are within the deadband the supply fan typically shuts off Because the building is unoccupied no ventilation is required and the outdoor air damper is closed Som
49. also help ensure that the zone receives the required amount of outdoor air for ventilation As mentioned in Dynamic reset of intake airflow p 117 ASHRAE Standard 62 1 permits dynamic reset of intake outdoor airflow Vot as operating conditions change as long as the system provides at least the required breathing zone outdoor airflow Vbz whenever a zone is occupied As the number of people occupying a zone varies the quantity of outdoor air required to properly ventilate that zone also varies Demanc controlled ventilation DCV is a strategy that attempts to dynamically reset the outdoor airflow delivered to a zone based on changing population within that zone Commonly used methods for assessing zone population include e Time of day schedules A time of day schedule can be created in the building automation system BAS to indicate when each zone is occupied versus unoccupied For any hour that a zone is scheduled to be unoccupied even though other zones served by the system are scheduled to be occupied minimum outdoor airflow for that zone is reset to zero or to the building related ventilation rate Ra see Zone level ventilation requirements p 101 Alternatively a time of day schedule can be used to predict the actual number of people in a zone for any given hour This variation in population is then communicated to the unit level controller for the VAV terminal unit and used to reset the minimum outdoor airflow
50. and application concepts assumes no responsibility for the performance or desirability of any resulting system design Design of the HVAC system is the prerogative and responsibility of the engineering professional Trane and the Trane logo are registered trademarks and TRACE System Analyzer and TAP are trademarks of Trane a business of Ingersoll Rand Chilled Water VAV Systems SYS APM008 EN Contents PrefaCe oii cccccccccccccecessescesseetstsesscstvasscsteasscstsasstetersicetensatstenees i Overview of a Chilled Water VAV System 0000000 00000 1 Basic System Operation ooo ccceccccccccesssececsseseeececseeseeeecsessaeeseestseaees 3 Benefits of Chilled Water VAV Systems ou cccccceeesccceesseseeeeessesaeees 6 Drawbacks Challenges of Chilled Water VAV Systems 9 Common Building Types That Use Chilled Water VAV Systems 10 Primary System Components 00000000000 00 0cccccccccecceceeeeees ii VAV AirHandling Unit cece cece ccccceccceeccnesntstssseeeeeeeeeeeeeennenaes 11 Indoor versus outdoor airhandling units c ceeeeeeeeeeeeeeeeeee 12 Chilled water cooling COI cccccccccccceteecceecstteeecesestteeeerscseeeeeeeees 15 Maximum face velocity to prevent moisture carryover 17 Freze PlOVENTIOMN sivessngeatsaasdunandeachenidiancnsiniinacebitanstesbabeateaass 18 EVapPOrative COOMNG lt icsssssiaeicnesnesdecishevediauetoiveckeceaeeaaereaieds 21 Heat source inside the VAV airhandling unit
51. and cleaning e Use sloped drain pans and clean them regularly A flat drain pan retains water and stagnant water can provide a habitat for microbial growth A sloped pan improves drainage considerably and eliminates standing water Be sure that the drain connection is located at the lowest point in the pan and install the air handling unit within the manufacturer s tolerance for levelness e Properly install condensate traps and periodically clean them out If the cooling coil and its associated drain pan are located upstream of the supply fan draw thru configuration the pressure inside the air handling unit casing at that section is less than the pressure outside Without a drain seal air can be drawn in through the condensate drain line from outside Improper condensate trapping results in the wetting of the interior of the unit and may even allow water to leak into the building If the cooling coil and drain pan are located downstream of the supply fan blow thru configuration the pressure inside the casing is greater than the pressure outside Without a drain seal air and condensate are pushed out through the condensate drain line This eliminates concerns for wetting the interior of the unit but results in conditioned air leaking out of the unit wasted energy In either a draw thru or blow thru configuration the condensate drain line must include a properly designed drain seal to allow condensate to flow out of the drain pan
52. and maintain the air seal Although other sealing devices are sometimes used a simple P trap is used in the majority of installations Follow the manufacturer s recommendations for the design and installation of this condensate trap Note that the design of the trap differs depending on whether the cooling coil is a draw thru or blow thru configuration Remember even a well designed trap if plugged causes the drain pan to overflow Inspect traps regularly for blockage Clean and prime the trap if necessary especially just prior to the cooling season Casing performance leakage and thermal The primary role of the air handling unit casing is to direct air through the various components filters coils fans etc To accomplish this in an energy efficient manner the casing must minimize air leaking into or out of the air handling unit and minimize heat loss or gain through the casing Air leakage In addition to the casing of the air handling unit being designed to minimize air leakage the impact of air leakage on the overall energy use of the HVAC system depends on the configuration of the unit and its location within the system For example Figure 42 depicts an indoor draw thru air handling unit that is installed in a mechanical room in which the return air passes through the mechanical room before entering the unit The static pressure inside the unit Chilled Water VAV Systems 51 52 Primary System Components a
53. auditoriums and places of worship Chilled Water VAV Systems 157 158 System Design Variations a ae Note Effective January 1 2010 ASHRAE Standard 90 1 addendum n to ASHRAE 90 1 2007 will require single zone VAV control for any air handling unit that contains a chilled water cooling coil and a supply fan that has a motor greater than or equal to 5 hp 4 kW Compared to a constant volume system that is often used for large zones such as these a single zone VAV system results in fan energy savings and less fan generated noise at part load conditions It also significantly improves dehumidification because unlike a constant volume system a VAV system continues to deliver cool dry air at part load conditions see Full load versus part load dehumidification performance p 118 However because supply airflow varies the air distribution system must be designed carefully to prevent dumping under low load conditions Best practices in a single zone VAV application Consider the following when designing a single zone VAV system Uniformity of loads throughout the zone Like any other single zone system a single zone VAV system does not have the capability to satisfy simultaneous heating and cooling requirements When used to provide comfort for a large zone it may result in undesirable temperature variations in the areas of the zone that are further away from the zone sensor Therefore the loads should be fairly
54. building is occupied When indoor humidity rises too high during unoccupied periods after hours one option may be to turn on the central airhandling unit and dehumidify recirculated air OA damper is closed However there is typically very little sensible load in the spaces at this time so the reheat coils in the VAV terminals may need to be activated to prevent overcooling If a hot water heating system is used this also requires turning on the boiler and circulation pumps An energy saving alternative is to equip the VAV air handling unit with a series desiccant wheel When after hours dehumidification is needed the AHU conditions only recirculated air The series desiccant wheel allows the airhandling unit to dehumidify the air to a lower dew point but deliver it at a warmer dry bulb temperature than if using a cooling coil alone Figure 98 This saves energy by minimizing the need for using new energy to reheat remotely at the VAV terminals maybe even allowing the hot water boiler and pumps to remain off Chilled Water VAV Systems 125 For more information on the various types of humidification equipment including sizing and application refer to the ASHRAE Humidity Control Design Guide for Commercial and Institutional Buildings and Chapter 21 Humidifiers in the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org 126 System Design Issues and Challenges Figure 98 Example performance of
55. calculate the outdoor air intake flow required under the current operating conditions e Demand Controlled Ventilation Resetting intake airflow in response to variations in zone population As the number of people occupying a zone varies the quantity of outdoor air required to properly ventilate that zone varies Demand controlled ventilation DCV is a zone level control strategy that dynamically resets the outdoor airflow requirement for the zone based on changing population in that zone As mentioned earlier in this chapter see Ventilation control p 184 time of day TOD schedules occupancy sensors OCC and carbon dioxide CO2 sensors are commonly used to implement demand controlled ventilation A time of day schedule can be created in the BAS to indicate to the VAV terminal unit when each zone is occupied versus unoccupied or to predict the actual number of people in a zone for any given hour An occupancy sensor such as a motion detector can be used to detect the presence of people in a zone A CO2 sensor is used to monitor the concentration of carbon dioxide in the zone which can be used as an indicator of the current per person ventilation rate cfm person m3 s person In most VAV systems the best approach often combines all three DCV approaches TOD schedules occupancy sensors and CO2 sensors at the zone level using each where it best fits with ventilation reset at the system level Figure 147 and Figure 148
56. coil face area would result in a coil face velocity of 521 fpm 2 6 m s at design airflow While this is higher than the industry rule of thumb it is well below the manufacturer s tested limit for preventing moisture carryover Table 4 Impact of cooling coil face velocity on AHU footprint and weight Size 25 AHU Size 30 AHU Size 35 AHU Coil face area ft2 m2 24 97 2 32 29 90 2 78 34 14 3 17 Face velocity fpm m s 521 2 6 435 2 2 381 1 90 Coil rows 6 rows 6 rows 4 rows Fin spacing fins ft fins m 103 338 83 272 137 449 Total cooling capacity MBh 525 154 525 154 525 154 kW Airside pressure drop in 0 69 173 0 50 125 0 36 90 H20 Pa Fluid pressure drop ft H20 12 1 36 3 13 6 40 6 9 1 27 1 kPa AHU footprint ft m 12 1 x 6 7 12 1 x 7 8 13 5 x 8 0 3 7 x 2 0 3 7 x 2 4 4 1 x 2 4 AHU height ft m 9 1 2 8 9 1 2 8 9 3 2 8 AHU weight Ibs kg 3350 1520 3570 1620 4700 2130 1 Based on a typical VAV air handling unit layout consisting of an OA RA mixing box high efficiency filters hot water heating coil chilled water cooling coil airfoil centrifugal supply fan and a top mounted discharge plenum Selecting the smaller size 25 air handling unit results in lower cost of the equipment reduces the weight of the unit by 6 percent and reduces the footprint floor space required by 14 percent Figure 11 Chilled Water
57. configured inside the terminal unit so its airflow is in series with the primary airflow path The terminal unit fan operates continuously whenever the zone is occupied and draws air from either the primary air stream or the ceiling plenum based on the cooling or heating requirement of the zone The result is a constant volume of air delivered to the zone at all times Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a eT In the cooling mode the primary airflow is reduced as the cooling load in the zone decreases The total airflow to the zone remains constant a combination of cool primary air and warm plenum air Figure 54 When primary airflow reaches the minimum setting for the unit and the cooling load continues to decrease a heating coil can be used to warm the air to maintain zone temperature Figure 54 Control of a series fan powered VAV terminal unit heating coil activated N maximum primary airflow 100 minimum primary airflow airflow to space primary air 0 1 design space load design heating load cooling load Parallel versus series fan powered VAV terminals Series fan powered units are sometimes selected because the zones receive constant airflow Constant airflow simplifies selection of the supply air diffusers and compared to parallel fan powered units increases air motion at part load which can improve occupant comfort However because t
58. currently required for the zone for that hour If the VAV system includes a BAS it probably includes a time of day scheduling function so the only additional cost is time to set up the schedules This approach may be well suited for a classroom where occupancy is predictable and the number of occupants does not vary greatly e Occupancy sensors A sensor such as a motion detector can be used to detect the presence of Chilled Water VAV Systems SYS APM008 EN FRAME For more information on CO2 based demand controlled ventilation refer to the Trane Engineers Newsletter titled CO2 Based Demand Controlled Ventilation with ASHRAE Standard 62 1 2004 ADM APNO17 EN and to the Trane Engineers Newsletter Live broadcast DVD titled CO2 Based Demand Controlled Ventilation APP CMC024 EN SYS APM008 EN System Controls rT people in a zone and send a binary signal to the unit level controller for the VAV terminal unit indicating whether that zone should be considered occupied or unoccupied When the sensor indicates the zone is occupied it requires the design outdoor airflow When the sensor indicates the zone is unoccupied the required outdoor airflow rate is reset to zero If an occupancy sensor is used in combination with a time of day schedule the building may be scheduled as occupied while the sensor indicates the zone is unoccupied Table 29 In this mode called occupied standby
59. curve Room Criteria RC A single number used to describe sound in an occupied space It uses a series of curves and reference lines for plotting sound pressure by octave band and determining the RC value and a descriptor of the sound quality i e hiss rumble sensible energy recovery The transfer of sensible heat between two or more air streams or between two locations within the same air stream sensible heat Heat that causes a change in the dry bulb temperature of the air with no change in moisture content sensor The component of a control loop that measures the condition of the controlled variable and sends an input signal to the controller series fan powered terminal unit A fan powered VAV terminal unit consisting of a primary airflow modulation device and a small integral constant volume fan packaged so that the airflow paths are in series Provides a constant volume of supply air to the zone when operating setback The practice of changing the temperature setpoint of the zone during unoccupied hours in an effort to save energy setpoint The desired condition of the controlled variable in a control loop silencer A device installed in an air distribution system to reduce noise SMACNA Sheet Metal and Air Conditioning Contractors National Association www smacna org stack effect When indoor air is warmer than outdoor air the less dense column of air inside the building results in a negative pressure in the lowe
60. each see Fans p 26 Chilled Water VAV Systems SYS APM008 EN System Controls E If the air handling unit includes a supply fan and a relief fan building pressure is directly controlled by varying the capacity of the central relief fan Figure 131 e A differential pressure sensor monitors the indoor to outdoor pressure difference Its signal is used to adjust relief airflow directly limiting building pressure Relief capacity control can be accomplished by either a modulating the relief damper and allowing the relief fan to ride the fan curve or b equipping the relief fan with a variable speed drive e The relief fan only operates when necessary to relieve excess building pressure When intake airflow is at the minimum required for ventilation as is often the case when not economizing operation of the central relief fan may be unnecessary In some buildings local exhaust fans and exfiltration due to the pressurized building are sufficient to relieve the intake airflow Figure 131 VAV system with central relief fan relief fan and relief exhaust damper ceiling plenum return local exhaust fan DEA return air plenum differential __v7 n pressure CS sensor Q occupied space infiltration gt exfiltration recirculating variable speed damper supply fan VAV box gt INTAKE MA e l gt flow measuring mixed air G Oe ees intake OA damper plenum filte
61. efficient at delivering outdoor air from the intake to the breathing zones then 15 percent of the system primary airflow would need to be outdoor air 110 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Issues and Challenges i Se Table 21 System level ventilation calculations based on calculated Ey method cooling design design Vpz Vpzm Voz Za Evz cfm cfm cfm Voz Vpzm 1 Xs Za South offices 1900 475 210 0 44 0 71 West offices 2000 500 220 0 44 0 71 South conf room 3300 825 330 0 40 0 75 East offices 2000 500 220 0 44 0 71 South interior offices 7000 1750 850 0 49 0 66 North interior offices 7000 1750 850 0 49 0 66 North offices 1600 400 200 0 50 0 65 North conf room 1800 450 220 0 49 0 66 System totals Vpz 26 600 Vps 18 600 block airflow Step 6b is to determine the ventilation efficiency for each zone Evz This describes how efficiently the system delivers outdoor air from the system level intake to the individual breathing zone For single path systems Appendix A provides the following equation for zone ventilation efficiency Evz 1 Xs Za Xs was calculated in step 6a and Zg for each zone was calculated in step 4 For this example zone ventilation efficiencies vary from 0 65 to 0 75 Table 21 For Step 6c system ventilation efficiency Ev is equal to the worst case or lowest zone ventilation ef
62. energy installed cost among VAV than fan powered VAV units systems particularly when electric heat is used in the VAV terminal units VAV terminals with hot water coils require piping and valves for each unit to be installed in the ceiling e Often preferred in retrofit plenum projects because cooling and heating are both provided by a single VAV terminal unit and VAV reheat terminals are smaller than fan powered VAV Potential for downdraft problems in terminals perimeter zones with very high heat loss Figure 59 p 64 Units with electric heat typically require larger power wiring to each unit e Economical method of providing heat to perimeter e High minimum airflow settings may zones in a building with require boiler operation during the relatively small heating loads cooling season e No floor space is required in High minimum heating airflow the zone for the heating settings may be required to minimize system drafts and provide adequate mixing in the zone e Does not require a boiler and water distribution system if electric heating coils are used in the VAV terminal units SYS APM008 EN Chilled Water VAV Systems 67 Primary System Components a a as Interior zones Cooling only VAV Perimeter zones Parallel fan powered VAV Most of the interior zones are served by cooling only VAV terminal units The perimeter zones and certain densely occupied interior zones that experience widely varying occupancy
63. fully ducted return air path 1 In many applications when the system is bringing in minimum ventilation airflow not in airside economizer mode local exhaust fans in restrooms and copy centers for example and exfiltration due to positive building pressurization are often sufficient to relieve all the air brought into the building for ventilation In this case no central relief is needed and the central relief fan can be turned off only needing to operate during economizer mode SYS APM008 EN Chilled Water VAV Systems 31 Primary System Components a a as Fan types VAV air handling units are typically available with several choices for fan types and sizes This affords the opportunity to select a fan that optimizes the balance of energy efficiency acoustics and cost The most common type of fan used in VAV systems is a centrifugal fan in which air enters the center of the fan wheel axially and follows a radial path through it A centrifugal fan may be characterized by the shape of the fan blades whether it contains a fan scroll housed or not plenum whether it is belt driven or direct drive and whether one or multiple fan wheels are used e Shape of fan blades For more information on fan types refer A forward curved FC fan has blades that are curved in the direction of aa A a wheel rotation These fans are operated at relatively low speeds and are used to deliver large volumes of air against relatively low
64. in which some zones require cooling at the same time that other zones require heating They are characterized by two separate duct systems a cold duct and a hot duct Each zone is served by a dual duct VAV terminal unit which consists of two airflow modulation devices Figure 115 One modulation device varies the amount of cool primary air and the other varies the amount of warm primary air These two air streams mix inside the dual duct unit before being distributed downstream to the zone Figure 115 Dual duct VAV terminal unit cool primary air warm primary air SS This system can provide excellent control of both temperature and humidity and it can be very energy efficient when the mixing of cooled and heated air is avoided and two supply fans are used one for heating the other for cooling Figure 116 In addition all heating is done in the central air handling unit There are no heating coils or fans in the dual duct VAV terminals and no associated water distribution system Chilled Water VAV Systems 165 166 System Design Variations i However this system is relatively uncommon because of the need to install two separate duct systems In addition dual duct VAV terminal units typically cost more than VAV reheat terminal units Dual versus single fan system Dual duct VAV systems may use either one or two central air handling units In the dual fan configuration Figure 116 the cooling air handling unit m
65. include a heating coil hot water steam or electric or gas fired burner inside the airhandling unit This centralized source of heat is primarily used to 1 warm up the building in the morning prior to occupancy and 2 maintain the desired supply air temperature during extremely cold weather preventing air that is too cold from being delivered to the zones that may still require cooling such as interior zones Heating coil electric hot water or steam An electric heater installed inside the air handling unit at the factory simplifies jobsite installation and avoids the need to install a boiler and hot water or steam distribution system in the building or to provide gas service to the unit A hot water or steam heating coil can also be mounted inside the AHU in the factory but requires a boiler and hot water or steam distribution system to be installed in the building This approach centralizes the heating equipment and can incorporate various methods of heat recovery to reduce energy use see Condenser heat recovery p 88 A hot water coil that contains pure water no antifreeze should not be used if the coil will be exposed to air that is colder than 32 F 0 C During normal operation with both the circulation pump and boiler operating the water inside the coil may not freeze until it is exposed to air that is much colder than 32 F 0 C But if the pump fails the stagnant water inside the tubes of the coil wil
66. indoor temperature during these unoccupied hours it is possible to use the outdoor air for free precooling This strategy often called night or unoccupied Chilled Water VAV Systems 199 System Controls aT economizing can save energy by precooling the building prior to the morning cool down period Figure 142 Figure 142 Unoccupied economizing unoccupied 9 d economizing scheduled occupied hours unoccupied cooling setpoint zone temperature Cools zone below occupied occupied setpoint cooling setpoint outdoor temperature 4am 5 a m 6am 7am Unoccupied economizing uses the BAS to determine if the condition of the outdoor air is suitable for free cooling and determines which zones could benefit from precooling Unoccupied economizing is allowed whenever the outdoor dry bulb temperature is cooler than the current zone temperature by a defined margin 15 F 8 3 C for example In addition a humidity sensor can be added to prevent unoccupied economizing when the outdoor air is cool but humid Fan pressure optimization As described earlier in this chapter the unit level controller on the VAV air For more information on using fan handling unit varies the capacity of the supply fan to maintain the static pressure optimization in VAV systems cier tie Tane mames Narda pressure at some location in the supply duct at a desired setpoint titled VAV System Optimiz
67. materials such as electrical wiring insulation gaskets caulk and sealants and insulation in close proximity to the lamps should be shielded from direct expose to the UV energy or substituted with other materials Failure to properly protect these materials can result in serious safety and equipment reliability issues Clean and replace lamps as specified by the manufacturer Proper maintenance including periodic cleaning of the lamp surface typically every three months is important to keep the UV lamps operating effectively The UV intensity degrades over time so lamps need to be replaced at intervals specified by the manufacturer typically every 9000 operating hours or annually Since UV lamps contain mercury which is a regulated hazardous waste disposal of lamps is subject to state and federal regulations Photocatalytic oxidation PCO uses ultraviolet light shining on the surface of a catalyst commonly a bed of titanium dioxide TiO2 to create highly reactive hydroxyl radicals The cells of microbiological organisms that contact these hydroxyl radicals are destroyed through a process called lysis destruction of the cell by rupturing its membrane or cell wall Biological organisms also create low levels of VOCs which typically cause odors As mentioned earlier p 48 PCO technology is able to break down VOCs and remove these biological related odors Chilled Water VAV Systems 49 Preventing blowing snow from enterin
68. moisture leaves the air and condenses on surfaces diffuser A device connected to the end of the supply duct system used to distribute the supply air into the conditioned space direct digital control DDC A method of terminal unit control using an electric motor to operate the air modulation damper actuator It uses a microprocessor that enables digital communication between the unit controller and a central building automation system direct drive plenum fan A type of plenum fan in which the motor is mounted directly to the end of the fan wheel shaft eliminating the need for sheaves or belts direct expansion DX system A system that uses the refrigerant directly as the cooling media The refrigerant inside the finned tube evaporator absorbs heat directly from the air used for space conditioning direct fired burner A fuel burning device in which the heat from combustion and the products of combustion are transferred directly to the air stream being heated displacement ventilation See thermal displacement ventilation draft Undesired local cooling of a body caused by low temperature and air movement drain pan A device positioned under a cooling coil to collect condensate and direct it to a drainage system draw thru A configuration where the fan is located downstream and draws air through the cooling coil dual duct terminal unit A type of VAV terminal unit consisting of two airflow modulation devices one for co
69. nearly identical to the design system resistance curve While this results in energy savings as previously mentioned it also keeps the operating point of Chilled Water VAV Systems 201 The fan pressure optimization strategy is not only for the supply fan It can also be used to control the fan in a dedicated outdoor air system see Figure 96 p 124 or to control the central relief fan when floor by floor building pressure control is used see Figure 133 p 181 Impact of SAT reset on airside economizing When the outdoor air is cooler than the supply air temperature setpoint the water chillers are shut off and the outdoor and return air dampers modulate to deliver the desired supply air temperature see Airside economizer control p 174 Raising this supply air temperature setpoint allows the chillers to be shut off sooner and increases the number of hours of modulated economizer operation when the economizer alone provides all the needed cooling However increasing the supply air temperature does increase fan energy So the energy benefit of increasing the number of hours of modulated economizer operation must be weighed against the increased fan energy 202 System Controls the supply fan further away from the surge region than the other fan capacity control methods Figure 144 Figure 144 Energy savings and reduced risk of fan surge P 33 KA a a a a D g
70. not on the top floor of the building e Are there adjacent zones that have approximately the same density of occupants lighting and equipment and are the time of use schedules similar e If so will the occupants accept the temperature varying slightly If adjacent zones meet these criteria they likely can be grouped together into a single zone without much sacrifice in occupant comfort Locating the zone sensor The zone temperature sensor should be installed in a representative location within the zone If the zone consists of more than one room place the sensor in the room where tighter temperature control is most important The temperature in the other rooms may vary more than in the room with the temperature sensor Follow these general guidelines when locating the zone sensor e Do not place the zone sensor where it will be affected by air discharged from a supply air diffuser e Make certain that only the VAV terminal unit that is connected to the zone sensor can influence the temperature being sensed by that sensor This also means that a zone should only be served by diffusers connected to the VAV terminal that is controlled by the temperature sensor located within that zone Do not serve a zone with diffusers connected to two separately controlled VAV terminals e Do not place the zone sensor directly on a wall with a large amount of heat gain or loss or where solar radiation will create a false reading generally th
71. occupied setpoint by the end of the scheduled occupied period outdoor air Air brought into the building from outside either by a ventilation system or through openings provided for natural ventilation parallel fan powered terminal unit A fan powered VAV terminal unit consisting of a primary airflow modulation device and a small integral constant volume fan packaged to provide parallel airflow paths PCO Photocatalytic oxidation An air cleaning technology that uses ultraviolet light shining on the surface of a catalyst to adsorb gaseous contaminants such as VOCs and inactivate biological contaminants perimeter zone A conditioned space with walls and windows that are exposed to the outdoors In most climates these spaces would require seasonal cooling and heating plenum The space between the ceiling and the roof or the floor above plenum fan A fan assembly consisting of a single inlet impeller mounted perpendicular to airflow which pressurizes a plenum chamber in the air distribution system population diversity The ratio of the actual system population to the sum of the peak zone populations pressure dependent VAV control method that uses the zone temperature sensor to directly control the position of the airmodulation damper The actual airflow delivered to the zone is a by product of this position and depends on the static pressure inside the duct at the inlet to the terminal unit pressure independent VAV contr
72. often acceptable in order to improve performance at reduced airflows e When ceiling mounted diffusers will deliver warm air to the zone try to limit the difference between the supply air temperature and the zone temperature to 15 F 8 3 C Limiting the supply air temperature during heating avoids excessive temperature stratification when supplying warm air from overhead This may also increase the zone air distribution effectiveness used to determine required ventilation airflow see Impact of zone air distribution effectiveness p 102 e In perimeter zones with high heat loss through the building envelope discharge diffusers to blanket the perimeter wall or window area Figure 60 p 65 This helps prevent downdraft problems that can occur when large volumes of heated air are distributed through ceiling mounted diffusers Chilled Water VAV Systems 75 Primary System Components a a In addition when using linear slot diffusers e Maintain recommended separation between diffusers Figure 67 The maximum recommended end to end distance between linear slot diffusers is three diffuser lengths The maximum recommended distance between the end of a linear slot diffuser and a perimeter wall when airflow is parallel to the wall is two diffuser lengths Figure 67 Maximum separation distances and collision velocities interior wall 75 FPM max 0 38 M s 150 FPM max perimeter 0 76 s interior wall wal
73. operation prevails Frost forms on a total energy recovery device at a much colder outdoor temperature than it does on a sensible energy recovery device This allows total energy recovery to recover more heat during cold weather and lessens and may even eliminate the need for frost prevention Total energy recovery devices generally have a higher effectiveness than many sensible energy recovery devices so they save more heating energy and may permit further downsizing of the heating equipment Water vapor transferred by a total energy recovery device humidifies the entering outdoor air during the heating season which helps keep the space from becoming too dry Free humidification also reduces the energy used by the mechanical humidification system if installed and permits this equipment to be downsized Most heating climates also include a cooling season Applying a total energy recovery device enables a larger reduction in cooling capacity which can reduce the first cost premium for energy recovery e Integrate control with airside economizer operation In many climates an airside economizer can provide the benefits of free cooling for much of the year While the economizer operates air to air energy recovery offers no additional benefit In fact unless it is turned off the energy recovery device actually increases the cooling load by transferring heat to the outdoor air stream Achieving maximum energy
74. pressure independent VAV controller directly controls the actual volume of primary air that flows to the zone This requires accurate measurement of primary airflow which is typically accomplished with a multipoint airflow sensor mounted on the inlet The position of the modulating damper is not directly controlled and is a by product of regulating the airflow through the unit Because the airflow delivered to the zone is directly controlled it is independent of inlet static pressure Pressure independent control increases the stability of airflow control and allows minimum and maximum airflow settings to become actual airflows rather than physical positions of the modulation device 54 Primary System Components a a as Thermal performance The air handling unit casing also provides an insulating function to minimize heat loss or gain and to minimize condensation from forming on the interior or exterior surfaces of the unit If the air surrounding the air handling unit is unconditioned or significantly warmer or colder than the conditioned air inside the unit heat gain loss and condensation may be a design issue To prevent condensation from forming on the surfaces of the air handling unit the casing thermal resistance must maintain the surface temperature of the casing exterior above the dew point of the surrounding air The following equation defines the cold spot thermal resistance ratio TR required for a specific applicati
75. pump is often controlled to maintain a set pressure differential at some location in the piping system In some systems a differential pressure transducer is located between the inlet and the discharge of the pump while in others it is located at the most distant point in the piping system Similar to the fan pressure optimization strategy discussed previously p 200 it is possible to optimize this pressure control function to minimize pump energy consumption if each air handling unit or VAV terminal unit uses a communicating controller that knows the current position of its modulating valve The BAS continually polls the individual controllers looking for the valve that is the furthest open The pump pressure setpoint is then periodically reset so that at least one valve is nearly wide open The result is that the pump generates only enough pressure to push the required quantity of water through this critical furthest open water valve This concept is often called pump pressure optimization or critical valve reset This optimization strategy results in less pump energy use by allowing the pump to operate at a lower pressure at part load conditions Chilled water temperature reset Raising the temperature of the water leaving the chiller decreases the energy used by the chiller In a constant flow pumping system this will typically Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Controls eT decrease over
76. runout ducts downstream of the VAV terminal units This allows adjustment to deliver the desired airflow to each diffuser served by the VAV terminal Many types of diffusers are available with an integral balancing damper to simplify installation Balancing dampers may also be necessary upstream of some VAV terminals especially those located closest to the supply fan Pressure independent control and using the static regain method may reduce the need for upstream balancing dampers Supply air diffusers Each VAV terminal unit is connected via sheet metal or flexible ductwork to remotely located diffusers Supply air diffusers are used to introduce supply air into the conditioned space Proper air diffusion is an important comfort consideration especially in VAV systems Because most VAV systems require the diffuser to deliver air into the space over a wide range of airflows diffusers that are specifically intended for use in VAV applications should be used to increase circulation and prevent cold air dumping at lower airflow rates Linear slot diffusers Figure 65 are designed to effectively distribute air over a wide range of airflows making them the preferred diffuser for VAV systems They use a principle known as the Coanda effect to distribute air into the conditioned space Figure 66 When air is discharged at a relatively high velocity along the surface of the ceiling it creates an area of low pressure that causes the suppl
77. secondary configuration Variable primary flow configuration variable volume m e oA pump chillers chillers constant volume pump variable volume pump two way control NAAANAN bypass decoupler variable volume pump minimum flow bypass valve two way control valve MASSON cooling coil cooling coil While much less common the condenser water distribution system can also be designed for variable flow Reducing the condenser water flow rate does reduce pumping energy but it increases chiller energy use and can sometimes increase cooling tower energy use As the flow rate through the chiller condenser decreases the temperature of the water leaving the condenser and therefore the refrigerant condensing pressure increases This increases compressor lift and energy use In addition the condenser water flow rate must be maintained above the minimum required for proper operation of the chiller cooling tower and pumps motor cooling For these reasons controlling a variable flow condenser water system to minimize overall energy use is complicated Instead many projects choose to design a low flow condenser water system to minimize pump energy and simplify system level control
78. some of which emit several VOCs and at various generation rates A common way to control the concentration of gaseous contaminants indoors is to dilute them with outdoor air This approach is appealing because many VOCs defy individual treatment However it is only practical if the quality of the outdoor air is suitable and if the resulting supply airflow is consistent and appropriate and if it mixes effectively with the air in the occupied space Removal of gaseous contaminants typically relies on the principles of adsorption oxidation or catalysis Of these adsorption is the most common The media used for adsorption depends on the contaminants to be removed Table 9 Of these granular activated carbon is the most widely used This typically involves installation of a pleated panel or packed bed media in the air stream Figure 39 Figure 39 Activated carbon DX q P X pleated panel impregnated with activated carbon yale ng packed bed media filled with granular activated carbon shown with prefilters Images used by permission from CLARCOR Air Filtration Products www clcair com Chilled Water VAV Systems SYS APM008 EN TRANE All types of gaseous air cleaners require medium to high efficiency particle filtration typically MERV 13 or higher upstream to keep the media or catalytic surfaces clean SYS APM008 EN Primary System Components a S Carbon filtration can reliably remove a variety
79. static pressures Due to the inherently light construction of the fan wheel FC fans are typically used in smaller systems that require static pressures of 4 in H20 1000 Pa or less FC fans are typically less costly but usually less efficient than the other types Systems that require greater than 3 in H20 750 Pa of static pressure are usually best served by the more efficient backward curved BC backward inclined Bl or airfoil shaped AF fan blades In larger systems the higher fan efficiencies can result in significant energy savings A BI fan has flat blades that are slanted away from the direction of wheel rotation while a BC fan has shaped blades that are curved away from the direction of wheel rotation Their rugged construction allows them to operate at higher speeds than FC fans and makes them suitable for moving large volumes of air in higher static pressure applications Also BC and BI fans are typically more efficient than FC fans A refinement of the BI fan changes the shape of the blade from a flat plate to that of an airfoil similar to an airplane wing The smooth airflow across the blade surface reduces turbulence and noise within the wheel The result is that an AF fan typically requires less input power than other fan types Table 6 Table 6 Impact of fan type on input power brake horsepower Fan type and size Input power Rotational speed bhp kW rpm Housed FC 22 375 in 568 mm
80. system design condition Evaporative humidifier cooler in series with another cooling coil Pressure drop through device at fan system design condition Sound attenuation section 0 15 in H20 38 Pa Excerpt from Table 6 5 3 1 1B of ASHRAE Standard 90 1 2007 American Society of Heating Refrigerating and Air Conditioning Engineers Inc www ashrae org For example consider a 30 000 cfm 14 200 L s VAV system that includes a MERV 13 filter and a total energy wheel The filter is installed in the mixed air stream outdoor and recirculated air so the design airflow through it is 30 000 cfm 14 200 L s At design cooling conditions the outdoor airflow through the supply side of the wheel is 10 000 cfm 4 720 L s with a pressure drop of 0 8 in H20 200 Pa The airflow through the exhaust side of the wheel is 8 000 cfm 3 780 L s with a pressure drop of 0 7 in H20 175 Pa For this example the pressure drop adjustment A is 9 8 bhp 7 3 kW A 0 9 in H20 x 30 000 cfm 4131 0 8 in H20 x 10 000 cfm 4131 0 7 in H20 x 8 000 cfm 4131 9 8 bhp A 225 Pa x 14 200 L s 650 100 200 Pa x 4 720 L s 650 100 175 Pa x 3 780 L s 650 100 7 3 kW and the total allowable fan input brake power for the VAV system is adjusted to 48 8 bhp 37 1 kW Allowable Fan Input Power 30 000 cfm x 0 0013 9 8 48 8 bhp Allowable Fan Input Power 14 200 L s x 0 0021 73 37 1 kW
81. temperature of 55 F 13 C This economizer strategy saves cooling energy by avoiding the need to operate the cooling equipment see Airside economizer control p 174 However the heating coil must use more energy to warm the air from 55 F 13 C to the 105 F 41 C primary air temperature If the economizer was not activated the mixed air temperature would be about 65 F 18 C requiring less heating energy but increasing cooling energy For this reason a single fan dual duct system is not as energy efficient as a two fan dual duct system In contrast the dual fan dual duct arrangement avoids this energy penalty The cooling air handling unit uses the airside economizer to save cooling energy and to use the cool outdoor air for free cooling Because the heating air handling unit only conditions recirculated air it is not penalized by the airside economizer and need only warm the air from 75 F 24 C to the 105 F 41 C primary air temperature Figure 116 This eliminates the wasteful reheating of mixed air and allows an airside economizer to provide free cooling whenever possible Variable versus constant volume to the zone A dual duct VAV terminal unit can be controlled to provide either a variable volume or a constant volume of supply air to the zone When designed to deliver a variable volume of supply air to the zone Figure 118 and Figure 119 the cool primary airflow is reduced as the cooling load in
82. the zone single fan system maximum cooling airflow maximum sehaceeneeeeaeeeeseneeneaeseneenaneeeseneennanseneeneaneeeseneennanseneeananeaesennennanseneeneaenanses heating airflow minimum me uusssoncessouuossonasse 0000 ES primary primary air airflow airflow to space cool primary air 0 design space load design heating load cooling load When designed to deliver a constant volume of supply air to the zone Figure 120 and Figure 121 as the cooling load in the space decreases the amount of cool primary air is reduced and the amount of warm primary air is increased The zone receives a constant total supply airflow at a variable temperature Because of this constant airflow to the zone little or no fan energy savings is realized at part load conditions For the same reasons described previously in a dual fan system Figure 116 the VAV terminal needs to maintain a minimum cool primary airflow through the cooling damper to maintain proper ventilation of the zone Figure 120 In a single fan system Figure 117 however the cooling damper can close completely as long as the VAV terminal maintains a constant airflow to the space Figure 121 Chilled Water VAV Systems SYS APM008 EN System Design Variations a Figure 120 Control of a dual duct VAV terminal Constant volume to the zone dual fan system design supply 100 F airflow warm primary air minimum primary airflow
83. the zone decreases When the cool primary airflow reaches the minimum primary airflow setting for the unit and the zone cooling load continues to decrease the heating damper begins to open This allows the warm primary air to mix with the cool primary air and provide warmer supply air to the zone In a dual fan dual duct VAV system Figure 116 all of the outdoor air enters through the cooling air handling unit To ensure proper ventilation of the zone therefore the VAV terminal needs to maintain a minimum cool primary airflow through the cooling damper Figure 118 In a single fan dual duct VAV system Figure 117 outdoor air enters through the common air handling unit so it can be delivered by either the cold or hot duct system To ensure proper ventilation therefore the VAV terminal needs to maintain a minimum combined primary airflow but the cooling damper can close Figure 119 SYS APMO008 EN Chilled Water VAV Systems 167 168 System Design Variations a Figure 118 Control of a dual duct VAV terminal Variable volume to the zone dual fan system maximum cooling 100 F airflow MARRAREN SOCCER EEE CCRC ECE Oe EEC CCE EE PCE ER ECU E RCE ECE E EE CEEE TC A CEC EER EEE CECE ER TE EEE CTE maximum heating airflow 8 g warm 2 primary air 3 minimum primary x airflow cool primary air 0 design space load design heating load cooling load Figure 119 Control of a dual duct VAV terminal Variable volume to
84. to the supply airborne path and 2 by radiating sound from the terminal unit itself Sound data for VAV terminal units should be measured in accordance with ARI Standard 880 Air Terminals Using terminal units with certified sound data guarantees that they will perform as described in the manufacturer s literature e Install VAV terminal units over less sound sensitive areas Good engineering practice is to install VAV terminals above corridors and other less sound sensitive areas such as restrooms or copy rooms This helps reduce sound levels in the occupied space and minimizes disruptions during filter changes or periodic maintenance Installing VAV terminal units directly over sound sensitive spaces can be problematic The ceiling tile is the only barrier between the sound source and the receiver Sufficiently increasing the mass of the ceiling to attenuate the sound can be expensive and may reduce service access to the equipment When a VAV terminal is located directly over an occupied space the length of supply duct downstream of the terminal unit is also typically very short This short section of duct provides minimal opportunity to attenuate the supply airborne sound before it passes through the diffuser which also adds sound and enters the space SYS APM008 EN Chilled Water VAV Systems 145 TRANE System Design Issues and Challenges E e Use lined flexible ductwork to connect VAV terminal units to supply air diffusers Li
85. uniform throughout all areas of the zone Single zone VAV systems should be used to condition relatively large open areas rather than numerous small zones so that any changes in air distribution that occur as the supply fan modulates will have minimal impact on occupant comfort As a general rule a single zone VAV system can be successfully applied in any zone where a single zone constant volume system may traditionally have been applied Design the air distribution system for variable supply airflow In a single zone VAV system it is important to ensure proper air distribution throughout the zone as supply airflow is modulated In general follow these guidelines e Keep the supply duct system as short and as symmetrical as possible As the supply fan modulates systems with long or asymmetric duct runs are more susceptible to unequal air distribution between diffusers that are located at various distances from the fan e Size the ducts for low to medium air velocities Because this system serves a single zone the supply fan is typically located near the zone Using lower air velocities helps to minimize noise generated in the ductwork e Use diffusers that will provide proper air distribution at low airflows For some applications supply air diffusers that are appropriate for Chilled Water VAV Systems SYS APM008 EN FRAME For information on calculating the required ventilation airflow for a single zone VAV system s
86. up or cool down mode As mentioned previously the temperature inside a building is typically allowed to drift when unoccupied usually for the purposes of saving energy This generally requires the HVAC system to start prior to occupancy and operate long enough for the temperature inside the building to reach the desired occupied setpoint by the time people enter the building Figure 139 When the building must be heated prior to occupancy this is called morning warm up When the building must be cooled it is called morning cool down Chilled Water VAV Systems SYS APM008 EN System Controls aT Figure 139 Morning warm up system on system off occupied heating setpoint L zone temperature occupied hours unoccupied heating setpoint 6 a m Noon 6 p m The morning warm up cool down mode typically occurs as a transition from the unoccupied mode to the occupied mode The system attempts to return the temperature inside the building to the occupied setpoint as rapidly as possible In this mode the building does not typically require ventilation because it is not yet occupied Table 33 describes the typical functions of the different system components during the morning warm up mode Table 34 describes the typical functions of the different system components during the morning cool down mode Table 33 Coordination of equipment during morning warm up mode Central air handling unit e Modula
87. used within the facility rather than rejecting it to the outdoors Locating a water cooled chiller in the sidestream configuration Figure 76 allows heat rejected from the condenser to be used within the facility while chiller capacity is controlled to provide only the amount of condenser heat required at a given time Alternatively some water cooled chillers can be equipped with a separate heat recovery condenser bundle to allow for simultaneous heat recovery and heat rejection Some air cooled chillers are available with an integral refrigerant to water heat exchanger Figure 77 which recovers heat from the hot refrigerant vapor for use within the facility Chilled Water VAV Systems SYS APM008 EN Figure 77 Air cooled chiller with heat recovery heat exchanger refrigerant to water heat exchanger for heat recovery SYS APM008 EN Primary System Components Figure 76 Water cooled chiller in sidestream configuration for heat recovery ahah eal 5 I m AAAY C cooling coils a 4 sidestream water cooled chiller In a chilled water VAV system this recovered heat is typically used for preheating domestic or service hot water or for reheat or tempering at VAV terminals
88. wheel in a VAV air handling unit A P channels i in various W hoat transfer media Sensible energy recovery devices transfer only sensible heat Common examples include coil loops fixed plate heat exchangers heat pipes and sensible energy rotary heat exchangers sensible energy wheels Total energy recovery systems not only transfer sensible heat but also water vapor or latent heat Common examples include total energy rotary heat exchangers also known as total energy wheels or enthalpy wheels Figure 112 and fixed membrane heat exchangers In a chilled water VAV system the most common application of an air to air energy recovery device is to precondition outdoor air as it enters the building for ventilation Figure 113 An air to air energy recovery device is arranged to precondition the outdoor air OA by exchanging energy with the exhaust air EA stream While any sensible or total energy recovery technology can be used for this purpose total energy wheels are the most common Chilled Water VAV Systems SYS APM008 EN TRANE System Design Variations Figure 113 Total energy wheel used to precondition outdoor air pi bypass damper m N N RA N relief damper totaltenergy RRA VAV conditioned space wheel pes return air pox i LLL damper N oA p d g SA RA outdoor air eae 7 pi ae damper ap col cooling N eo coil VAV conditioned Imoen box space local EA Benefits of outdoor air preconditio
89. with a heating coil are typically used for zones that require seasonal cooling and heating such as the perimeter zones of a building 68 Chilled Water VAV Systems SYS APM008 EN Interior zones Series fan powered VAV Primary System Components a a as Perimeter zones Series fan powered VAV All of the zones both interior and perimeter are served by series fan powered VAV terminal units Benefits SYS APM008 EN Reduces terminal heating energy by using warm air from the ceiling plenum which has been warmed from heat generated by lights Series fan powered terminals deliver constant airflow to the zone at all times Series fan powered terminals can allow for a smaller central supply fan because the terminal fans overcome the pressure loss between the terminal and the zone Often suitable for retrofit projects because both cooling and heating can be provided by a single VAV terminal No floor space is required in the zone for the heating system Does not require a boiler and water distribution system if electric heating coils are used in the VAV terminal units Fan powered terminals equipped with a heating coil can provide unoccupied heating to the zone without the need to turn on the central supply fan Chilled Water VAV Systems Drawbacks Challenges Series fan powered VAV terminal units typically cost more than either VAV reheat or parallel fan powered VAV terminal units A system that uses serie
90. zone decreases While the supply airflow decreases at part load the supply air temperature remains the same or increases slightly For air blender applications series fan powered VAV terminals are often preferred for large conference rooms or other zones where constant airflow is desirable Parallel fan powered VAV terminals with constant fan operation during occupied periods are well suited for zones where less air motion during off peak conditions is acceptable The advantage of using a parallel fan powered terminal as an air blender is that the terminal fan is smaller than in a series fan powered terminal Therefore the VAV terminals are less expensive to install and operate 150 Chilled Water VAV Systems SYS APM008 EN System Design Variations a Impact on overall system energy consumption Cold air VAV systems require less transport energy energy used to move the air than conventional designs since primary airflow is reduced But not every component of the system contributes to the energy savings e The colder temperature leaving the cooling coil may require the water chiller to operate at a colder refrigerant suction temperature to produce a colder chilled water supply temperature This increases the energy required to operate the chiller e Innon arid climates delivering the air colder also results in delivering the air drier at a lower dew point which reduces relative humidity in the building and increases t
91. 034 EN November 11 DVD La Crosse WI AVS Group U S Green Building Council USGBC 2009 Leadership in Energy and Environmental Design LEED Green Building Rating System Washington D C USGBC http Awww usgbc com Warden D 1996 Dual fan dual duct systems Better performance at a lower cost ASHRAE Journal January pp 36 41 Warden D 2004 Dual fan dual duct goes to school ASHRAE Journal May pp 18 25 SYS APMO008 EN Chilled Water VAV Systems 229 230 Index A absorption distances 127 acoustical analysis 137 acoustics 135 analysis 137 ARI Standard 260 135 141 breakout 136 143 path 136 receiver 136 source 135 specific recommendations 138 AHU See air handling unit air chase wall 143 air cleaners 47 electronic 45 gaseous 46 air cleaning 40 Air Diffusion Performance Index ADPI 75 air distribution 70 air handler See air handling unit air leakage 31 37 51 air cooled chillers 8 80 87 88 89 139 air handling unit 11 44 50 acoustics 141 fewer large AHUs or several smaller units 11 heat source 23 indoor vs outdoor 12 airside economizer control 117 126 130 151 152 156 163 167 174 202 203 air to air energy recovery 20 23 160 allowable nameplate motor power 129 ARI Standard 260 135 141 ARI Standard 880 135 145 ASHRAE Standard 52 2 41 ASHRAE Standard 62 1 44 101 104 107 117 118 170 206 ASHRAE Standard 90 1 57 63 120 127 129 130 aspiration ratio 149 1
92. 100 114 114 93 85 84 89 104 Avoiding condensation on components of the air distribution system Condensation can occur on the outside surface of air handling units For more information on avoiding _ ductwork or supply air diffusers if the surface temperature is at or below the condensate nelualng Meee design dew point temperature of the air that comes in contact with it Two diffuser strategies for indoor equipment rooms refer to the Trane application manual surfaces may be prone to condensation the surface exposed to the occupied titled Managing Building Moisture zone and the surface exposed to the ceiling plenum SYS AM 15 The following practices help minimize the risk of condensation on the components of the air distribution system e Slowly ramp down supply air temperature during morning cool down In humid climates indoor humidity levels may increase if the system is shut off for a significant period of time overnight or over the weekend When the system starts up again the elevated dew point temperature in the zone may result in moisture condensing on the cold zone side surface of the supply air diffusers SYS APMO008 EN Chilled Water VAV Systems 153 TRANE System Design Variations To avoid condensation problems during start up the supply air dry bulb temperature should be ramped down slowly to lower surface temperatures and to dehumidify the building and lower the indoor dew point temperature see
93. 100 kPa and lt 250 F 120 C hot water boilers Non condensing versus condensing boilers Hot water boilers are classified by whether they are condensing or non condensing A conventional non condensing boiler is designed to operate without condensing the flue gases inside the boiler Only the sensible heat value of the fuel is used to heat the hot water All of the latent heat value of the fuel is lost up the exhaust stack This avoids corrosion of cast iron or steel parts Hot water systems with non condensing boilers are often operated to Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a aT ensure that the return water temperature is no lower than 140 F 60 C to prevent condensing A condensing boiler on the other hand uses a high efficiency heat exchanger that is designed to capture nearly all of the available sensible heat from the fuel as well as some of the latent heat of vaporization The result is a significant improvement in boiler efficiency Condensing gas fired boilers have combustion efficiencies that range from 88 to over 95 percent while non condensing boilers have combustion efficiencies that range from 80 to 86 percent Condensing of the flue gases also allows for a lower return water temperature much lower than the 140 F 60 C limit that is common with non condensing boilers In fact the efficiency of a condensing boiler increases as the return water temperatur
94. 16 07 11 98 979 Housed AF 25 in 635 mm 14 65 10 92 1433 Belt drive plenum AF 35 56 in 903 mm 16 15 12 04 1108 Direct drive plenum AF 30 in 762 mm 14 85 11 07 1388 1 Based on a typical VAV air handling unit configuration OA RA mixing box high efficiency filter hot water heating coil chilled water cooling coil and draw thru supply fan with a single discharge opening off the fan section operating at 13 000 cfm 6 1 m3 s and 3 in H2O 750 Pa of external static pressure drop 32 Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components e Housed versus plenum Figure 23 Direct drive plenum fan A variation of the centrifugal fan is a plenum fan This type of fan consists of an unhoused centrifugal fan wheel with an inlet cone and typically airfoil fan blades Figure 23 The fan wheel pressurizes the plenum surrounding the fan allowing the air to discharge in multiple directions Depending on the configuration of the fan inside the air handling unit a plenum fan may be more or less efficient than a housed fan A housed fan is specifically designed to discharge into a straight section of ductwork which minimizes losses as velocity pressure is converted to static pressure In the configuration used in this example a single front discharge opening off the fan section see diagrams in Figure 24 the input power for the housed AF fan is lower than for either of the plenum fans Table 6 However
95. 18 l interior zone 63 65 66 68 69 70 204 L LEED 44 132 134 208 linear slot diffusers 74 load diversity impact on supply fan sizing 7 low flow strategy 82 M MERV 41 42 44 130 minimum efficiency requirements 127 minimum expected primary airflow 107 minimum primary airflow settings 62 mist eliminator 50 modulation curve 39 morning cool down mode 153 157 194 196 199 morning warm up mode 194 195 198 multiple recirculation paths 109 113 170 N night economizing See unoccupied economizing non condensing boiler 92 o occupancy sensors 117 184 187 207 occupied mode 191 194 occupied standby mode 192 operating modes 191 optimal start 198 optimal stop 199 outdoor air preconditioning benefits of 161 best practices 162 drawbacks of 162 overcooling 57 65 89 95 118 119 120 125 130 159 203 P parallel fan powered VAV terminal unit 58 particle sizes 40 particulate media filters 41 peak system population 108 people related ventilation rate 102 108 perimeter zones 63 66 68 69 70 75 100 permanent split capacitor 59 photocatalytic oxidation PCO 48 49 plenum air 58 150 183 plenum fans 33 population diversity 105 107 116 preoccupancy purge 195 196 pressure dependent control 54 pressure independent control 54 primary air 54 62 89 107 112 117 131 150 156 165 167 168 170 183 187 205 pull down of indoor humidity 154 pump pressure optimization 208 R radian
96. 202 e Activate the source of heat in the air handling unit In some applications the supply fan is ramped up to 100 percent airflow when the air handling unit operates in the heating mode This prevents the unit from tripping due to the high temperature rise across electric heaters or conventional non modulating gas fired burners In many climates and for many space types that use single zone VAV systems the air handling unit may only need to provide heat for morning warm up and for a relatively small number of hours during the year When a modulating gas fired burner is used it may allow for varying airflow during the heating mode In this case the supply fan can be modulated based on zone temperature and heating capacity can be modulated to maintain the discharge air temperature at a setpoint Employ demand controlled ventilation Because a single zone VAV system is often used for densely occupied zones with a highly variable population it is typically a good application for CO2 based demanda controlled ventilation DCV See Ventilation control p 184 Carbon dioxide CO2 is produced continuously by the occupants and diluted by the outdoor air so the difference between indoor and outdoor CO2 levels can be used as an indicator of the per person outdoor airflow rate cfm person m s person being delivered to the zone A sensor is used to Chilled Water VAV Systems 159 TRANE For more information on implementing
97. 3 C supply air because they know it has worked in the past The supply airflows that result from this choice directly impact the size and cost of fans air handling units VAV terminal units diffusers and ductwork The size of fan motors is also affected which extends the cost impact to the electrical distribution system Cold air VAV systems typically deliver supply air at a temperature of 45 F to 52 F 7 C to 11 C The appeal of cold air distribution lies in the reduction in the airflow required to offset the sensible cooling loads in the zones As the example in Table 27 suggests lowering the supply air temperature from 55 F 13 C to 48 F 9 C can reduce the supply air volume by 29 percent Table 27 Conventional versus cold air VAV systems conventional VAV system cold air VAV system 48 F 9 C 76 F 24 5 C 28 F 15 5 C 395 cfm ton 0 053 m3 s kW Supply air temperature 55 F 13 C 75 F 24 C 20 F 11 C 553 cfm ton 0 074 m3 s kW Zone setpoint AT Tzone Tsa Supply airflow per ton kW of zone sensible cooling load Benefits of cold air distribution Reducing supply airflow can trigger a series of related benefits e Smaller supply fan and return or relief fan if equipped e Smaller air handling units which can increase usable or rentable floor space see example below e Smaller vertical air shafts which can increase usable or rentable floor space e
98. 3 m s the heating coil in the VAV terminal warms the 55 7 F 13 1 C primary air PA to 57 9 F 14 4 C before delivering it to the zone This avoids Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Issues and Challenges Pi as overcooling the classroom and on this example mild and rainy day results in a relative humidity of 60 percent Figure 93 Reheat tempering at the VAV terminals 70 F DB 69 F WB mild rainy OA part load MA 71 4 F DB RA 74 F DB PA SA 60 RH S57mDB 57 9 F DB epuoj4 ajauosyoef Options include heating coils mounted on the VAV terminals fan powered VAV terminals dual duct air distribution and even radiant heat located within the zone Considerations for reheating tempering in a VAV system e Certain zones in a VAV system such as perimeter zones and interior conference rooms may require reheat even when high sensible cooling loads exist elsewhere To curb operating costs consider using parallel fan powered VAV terminals in these zones e When using electric heating coils comply with the manufacturer s guidelines for minimum airflow limits across the heating elements to ensure safe operation e If using a boiler as the heat source for reheat make sure that it is available to operate during the cooling season when reheat tempering may be needed e Consider the use of on site recovered heat to reduce operating costs For example the heat r
99. 56 B balancing dampers 72 74 BAS See building automation system baseboard radiant heat 2 4 8 23 55 66 92 103 183 biological contaminants 48 block airflow 7 110 blow thru configuration 35 45 51 141 144 boilers 23 92 condensing 92 non condensing 92 breakout See acoustics breathing zone 102 107 111 184 192 building automation system 117 171 181 183 192 198 205 building pressure control 178 burner See gas fired burner Cc calibration 186 carbon dioxide CO2 sensors 185 casing performance 51 CDQ See series desiccant wheel ceiling plenum 2 8 30 58 77 113 137 142 150 156 chilled water cooling coil 2 15 16 19 79 81 chilled water distribution system 81 Chiller System Design and Control SYS APM001 EN Index chilled water system 79 air cooled versus water cooled 80 control valve selection 83 freeze prevention 87 chilled water temperature reset 208 chilled water VAV system benefits of 6 common building types 10 drawbacks and challenges of 9 overview 1 typical combinations 66 chillers types of 79 chiller tower optimization 210 Coanda effect 74 cold air distribution benefits of 147 best practices 155 challenges of 149 comparative enthalpy control 177 condensate drain pipes 155 condensate trap 51 condensation 54 153 condenser heat recovery 88 condenser water distribution system freeze prevention 88 condenser water temperature chiller tower optimization 210 condensing boiler 92
100. 67 174 202 203 waterside 23 91 electric heater 20 23 124 electronic air cleaners 45 electronically commutated motors ECM 60 energy efficiency 127 132 equal friction method 72 evaporative cooling 21 exhaust fan See relief fan F fan control 132 return fan 26 30 179 192 194 supply fan 3 7 26 30 38 158 172 179 192 194 200 fan powered VAV terminal units 58 fan pressure optimization 200 fans 26 blow thru vs draw thru 35 51 141 direct drive plenum 33 34 35 fan array 35 housed 33 plenum 33 supply fan capacity modulation 38 types 32 filters final 45 HEPA cartridge 40 41 42 44 45 particulate 40 types of 41 final filters 45 fixed dry bulb control 175 fixed enthalpy control 176 flexible ductwork 8 70 71 73 146 flow sensor 54 55 63 73 182 flow tracking 182 flue gas 92 fluid temperature drop 95 freeze prevention 18 87 88 frost prevention 163 164 G gaseous air cleaners 46 gas fired burner 2 23 24 26 159 161 grilles See return air grilles H harmonic currents 61 heat recovery air to air 20 23 160 condenser 88 heating coil 23 HEPA filter 40 41 42 44 45 high aspiration diffusers 149 high limit shutoff setting 175 hot water boilers 190 types of 92 hot water distribution 94 hot water system 92 hot water temperature reset 209 housed fans 33 humidification 126 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Index humidifiers 126 humidity control 1
101. 8 ASHRAE Handbook HVAC Systems and Equipment www ashrae org or the NAFA Guide to Air Filtration www nafahq org 40 Primary System Components LT The modulation range of the supply fan is limited by how far the variable speed drive can be turned down typically 30 to 40 percent of design airflow Air cleaning Another requirement of the HVAC system is to ensure that the air delivered to the conditioned space is relatively clean This improves system performance by keeping the coils cleaner for example and keeps the air distribution system relatively clean Some of the contaminants that affect indoor air quality can be classified as particulates gases or biologicals The methods and technologies for effectively controlling these contaminants differ so it is important to define the contaminants of concern for a given facility Particulate filters Particulate matter particulates describes a broad class of airborne chemical and physical contaminants that exist as discrete grains or particles Common particulates include pollen tobacco smoke skin flakes and fine dust Airborne particulates vary in size ranging from submicron to 100 microns um and larger Figure 34 Many types of particle filters are available Figure 35 Some are designed to remove only large particles while others high efficiency particulate air HEPA filters for example also remove particles with diameters less than one micron Figu
102. 88 In this case the minimum primary airflow setting can be higher than the limits listed in exception a VAV fan control If a variable volume supply fan motor is larger than 10 hp 7 3 kW ASHRAE 90 1 requires the motor to be equipped with a variable speed drive or some other device that provides a comparable reduction in fan energy at part load In addition the standard requires that Static pressure sensors used to control variable air volume fans shall be placed in a position such that the controller setpoint is no greater than one third the total design fan static pressure except for systems with zone reset control fan pressure optimization If this results in the sensor being located downstream of major duct splits multiple sensors shall be installed in each major branch to ensure that static pressure can be maintained in each This prohibits locating the duct static pressure sensor at the discharge of the supply fan unless the fan pressure optimization control strategy is used Finally for VAV systems that use direct digital controls DDC on the VAV terminal units fan pressure optimization is required See Supply fan capacity control p 172 and Fan pressure optimization p 200 Demand controlled ventilation For VAV systems that use direct digital controls DDC if the design system level outdoor airflow is greater than 3000 cfm 1 4 m3 s or if the air handling unit is equipped with a modulating ou
103. 95 shows the performance of this VAV system with a series desiccant wheel Air leaves the cooling coil CA at a dry bulb temperature of 51 3 F 10 7 C and a dew point of 50 F 10 C The series desiccant wheel adsorbs water vapor drying the supply air SA to a dew point of 42 9 F 6 1 C Sensible heat added by the adsorption process raises the dry bulb temperature of the supply air to 55 7 F 13 1 C The wheel rotates into the mixed air stream MA where water vapor released from the wheel passes into the mixed air MA and then condenses on the cold coil surface Using the same example classroom from the previous section delivering the supply air at a dew point of 42 9 F 6 1 C results in 40 percent relative humidity in the zone 122 Chilled Water VAV Systems SYS APM008 EN FRAME System Design Issues and Challenges Pi i Figure 95 Example performance of a series desiccant wheel in a VAV system Design condition Peak dry bulb Mild rainy OA 96 0 F DB 76 0 F WB 70 0 F DB 69 0 F WB RA 74 0 F DB 40 RH 74 0 F DB 43 RH MA 80 6 F DB 54 9 F DP 72 0 F DB 60 7 F DP PH 78 7 F DB 60 7 F DP MA 76 2 F DB 59 8 F DP 70 9 F DB 63 6 FDP OA on a CA 51 3 F DB 50 0 F DP 50 1 F DB 49 1 F DP e rai SA 55 7 F DB 42 9 F DP 57 9 F DB 44 3 F DP A PH A amg y MA N a y ae MA Se SA RA Basically adding the series desiccant wheel changes the dehumidification performance of the tra
104. AV Systems SYS APM008 EN TRANE System Design Issues and Challenges Figure 102 T shaped section to attenuate return airborne sound Z no grille duct dia x 4 el A minimum plenum sound return duct ceiling tile source duct dia x 4 d minimum zen source occupied space occupied space plan view elevation view 3 An air chase wall routes the return air from the ceiling plenum down a chase that is constructed along the wall of the mechanical equipment room Figure 103 The chase should be large enough so that the velocity of the return air does not exceed 1000 fpm 5 1 m s The extra turns and absorptive material inside the chase wall help to attenuate the sound that travels along the return air path Figure 103 Example air chase wall section view return air plenum A ceiling tile A 2 in 3 lb density fiberglass p i 14 in chase wall _ i ia 3 sheets 5 8 in Re 2 sheets 5 8 in 12 5 ft sheetrock sheetrock _F return air 9 5 ft 9 ft 3 1 2 in steel a 3 in 3 lb stud density gt fiberglass 2 in 3 lb density gt 14 in fiberglass AA Y Y Systems with a fully ducted return air path have several acoustical advantages see Open ceiling plenum versus fully ducted return p 77 First the casing radiated sound does not enter the return airborne path so less sound is transmitted with the return air S
105. Air Cooling Equipment in the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org 22 Primary System Components a a Ta Figure 15 Direct versus indirect evaporative cooling 65 F DB 58 F WB OA 52 F DP rg en SA supplemental Poras mechanical Coo Ng 93 F DB cooling y OA o 58 F WB SA OA 99 F DP 55ER DE indirect evaporative cooling 22 F DP The leaving air temperature depends on how much the dry bulb temperature of the entering air exceeds its wet bulb temperature For example if the condition of the entering outdoor air OA is 93 F dry bulb and 58 F wet bulb 34 C DB 14 C WB and the direct evaporative process is 80 percent effective the condition of the leaving air OA will be 65 F DB and 58 F WB 18 C DB 14 C WB DBTleaving DBTentering effectiveness x DBTentering z WBTentering DBTleaving 93 F 0 80 x 93 F 58 F 65 F DBTieaving 34 C 0 80 x 34 C 14 C 18 C In a VAV system that is designed to supply air at 55 F 13 C dry bulb a conventional cooling coil is usually required to supplement the evaporative cooling process and further cool the supply air to the desired setpoint Figure 15 The system could be designed for warmer supply air temperature and or use an aggressive supply air temperature reset strategy to minimize the need for supplemental mechanical cooling but these approaches also increase supply airflow and fan ener
106. Air Diffusion Atlanta GA ASHRAE 1992 Gravimetric and Dust Spot Procedures for Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter ASHRAE Standard 52 1 1992 Atlanta GA ASHRAE 2001 Humidity Control Design Guide for Commercial and Institutional Buildings Atlanta GA ASHRAE 2007 Method of Testing General Ventilation AirCleaning Devices for Removal Efficiency by Particle Size ASHRAE Standard 52 2 2007 Atlanta GA ASHRAE 2003 Selecting Outdoor Return and Relief Dampers for Air Side Economizer Systems ASHRAE Guideline 16 2003 Atlanta GA ASHRAE 2007 Standard 62 1 2007 User s Manual Atlanta GA ASHRAE 2007 Ventilation for Acceptable Indoor Air Quality ASHRAE Standard 62 1 2007 Atlanta GA ASHRAE Chilled Water VAV Systems 225 226 References E ASHRAE and Illuminating Engineering Society of North America IESNA 2007 Energy Standard for Buildings Except Low Rise Residential Buildings BSR ASHRAE IESNA Standard 90 1 2007 Atlanta GA ASHRAE 2008 Standard 90 1 2007 User s Manual Atlanta GA ASHRAE California Energy Commission 2003 Advanced Variable Air Volume System Design Guide 500 03 082 A 11 October http www energy ca gov reports 2003 11 17_500 03 082_A 11 PDF Callan D R Bolin and L Molinini 2004 Optimization of air handling systems for federal buildings Engineered Systems April pp 39 50 Cleaver Brooks 2009
107. B 2000 Sound Ratings and ARI Standard 260 Trane Engineers Newsletter 29 1 Stanke D and Bradley B 1991 VAV System Optimization Critical Zone Reset Trane Engineers Newsletter 20 2 Stanke D and Bradley B 2002 Managing the Ins and Outs of Building Pressurization Trane Engineers Newsletter 31 2 Stanke D and Bradley B 2006 Keeping Cool with Outdoor Air Airside Economizers Trane Engineers Newsletter 35 2 ______ Stanke D and Harshaw J 2008 Potential ASHRAE Standard Conflicts Indoor Air Quality and Energy Standards Trane Engineers Newsletter 37 4 Trane 2005 ASHRAE Standard 62 1 2004 Ventilation Requirements Engineers Newsletter Live satellite broadcast APP CMC023 EN September 21 DVD La Crosse WI AVS Group 2005 CO2 Based Demand Controlled Ventilation Engineers Newsletter Live satellite broadcast APP CMC024 EN November 16 DVD La Crosse WI AVS Group 2002 Commercial Building Pressurization Engineers Newsletter Live satellite broadcast APP APV013 EN April 17 videocassette La Crosse WI AVS Group 2006 HVAC Systems and Airside Economizers Engineers Newsletter Live satellite broadcast APP CMC026 EN May 3 DVD La Crosse WI AVS Group Chilled Water VAV Systems SYS APM008 EN References E 2008 ASHRAE Standards 62 1 and 90 1 and VAV Systems Engineers Newsletter Live satellite broadcast APP CMC
108. CH PRBO14 EN La Crosse WI Inland Printing Company 2009 Trane Catalytic Air Cleaning System CLCH PRB0O23 EN La Crosse WI Inland Printing Company Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN References E 2009 Direct Drive Plenum Fans For Trane Climate Changer Air Handlers CLCH PRB021 EN La Crosse WI Inland Printing Company 2004 Trane CDO Desiccant Dehumidification CLCH PRB020 EN La Crosse WI Inland Printing Company Bradway B Hallstrom A Stanke D and Bailey N 1998 Managing Building Moisture SYS AM 15 La Crosse WI Inland Printing Company Guckelberger D and Bradley B 2006 Acoustics in Air Conditioning ISS APM001 EN La Crosse WI Inland Printing Company Murphy J and Bradley B 2002 Air to Air Energy Recovery in HVAC Systems SYS APM003 EN La Crosse WI Inland Printing Company Murphy J and Bradley B 2002 Dehumidification in HVAC Systems SYS APM004 EN La Crosse WI Inland Printing Company 1981 Variable Air Volume Duct Design AM SYS 6 La Crosse WI Inland Printing Company Schwedler M and Brunsvold D 1999 Absorption Chiller System Design SYS AM 13 La Crosse WI Inland Printing Company Hanson S Schwedler M and Bakkum B 2009 Chiller System Design and Control SYS APM001 EN La Crosse WI Inland Printing Company Schwedler M and Brunsvold D 2003 Waterside Heat Recovery in HVAC Systems SYS APM005 EN La
109. CM refer to the Trane Engineers Newsletter titled Setting a New Standard for Motor Efficiency Brushless DC Motors ADM APNO13 EN 60 Primary System Components a a Ta powered VAV also with PSC motors consumed 20 percent less HVAC energy in Atlanta 28 percent less in Los Angeles and 12 percent less in Minneapolis Even when the series fan powered VAV terminals are equipped with ECMs they still used more energy than parallel fan powered VAV Figure 55 Energy consumption of parallel versus series fan powered VAV 100 80 60 40 20 HVAC energy consumption of base Atlanta Los Angeles Minneapolis series FPVAV PSC H series FPVAV ECM parallel FPVAV PSC Another difference between the two types of fan powered terminals is sound Series fan powered units typically produce higher sound levels in the occupied space but some people may prefer the constant sound level of a series unit to the on off sound generated by the cycling fan in a parallel fan powered unit Adding an ECM to a parallel fan powered unit allows the terminal fan to ramp up slowly when activated which minimizes the distraction of the fan cycling on and off Finally a series fan powered unit typically costs more than an equivalent parallel fan powered unit because it requires a larger terminal fan sized to deliver design airflow to the zone And a system that uses series fan powered VAV terminals typically uses them in all zones This appr
110. CO2 based DCV in a single zone system refer to Standard 62 1 2004 System Operation Dynamic Reset Options published in the December 2006 issue of the ASHRAE Journal www ashrae org For more information on air to air energy recovery including its application and control in a VAV system refer to the Trane application manual titled Air to Air Energy Recovery in HVAC Systems SYS APM003 EN 160 System Design Variations a monitor the concentration of CO2 in the zone and this concentration is then communicated to the air handling unit controller and used to reset the intake airflow required for the zone see Figure 111 p 157 Because mixing box pressure varies as supply fan airflow changes see Figure 86 p 114 it is important to make some provision for maintaining the minimum required intake airflow In a VAV air handling unit this typically involves the use of an airflow measuring station or flow measuring OA damper to directly measure intake airflow and prevent it from dropping below the minimum ventilation requirement And because DCV reduces OA intake flow during periods of partial occupancy some method of directly controlling building pressure should be included see Figure 111 See Building pressure control p 178 Air to Air Energy Recovery Air to air energy recovery refers to the transfer of sensible heat or sensible heat and moisture latent heat between air streams Figure 112 Total energy
111. Chilled Water VAV Systems SYS APM008 EN FRAME For more information on direct drive fans and using multiple versus single fans refer to the Trane engineering bulletin titled Direct Drive Plenum Fans for Trane Climate Changer Air Handlers CLCH PRB0O21 EN Figure 27 Fan array with direct drive plenum fans a oy y i Ay j DE upstream inlet side downstream outlet side SYS APM008 EN Primary System Components E aT When space is a prime consideration and multiple supply duct connections are desired a housed fan requires a discharge plenum to allow for the multiple connections If a plenum fan is used however multiple duct connections can be made to the fan module itself eliminating the need for a discharge plenum and resulting in a shorter air handling unit Figure 25 e Belt driven versus direct drive Historically most large fans used in VAV systems were belt driven However with the increased use of VFDs direct drive fans have become popular primarily with plenum fans With a direct drive plenum fan the fan wheel is mounted directly on the motor shaft rather than using a belt and sheaves Figure 23 Because there are no belts or sheaves and fewer bearings direct drive fans are more reliable and require less maintenance In addition there are no belt related drive losses so direct drive fans are typically more efficient quieter and experience less vibration Table 6
112. D DDC VAV terminals e Required outdoor airflow V e Actual primary airflow Va e OA fraction Z communicating BAS e New OA setpoint V e Max OA fraction i The controller on each VAV terminal continuously monitors primary airflow Vpz being delivered to the zone The DDC controller also knows the outdoor airflow required by the zone Voz The BAS periodically gathers this data from all the VAV terminal units and dynamically solves Chilled Water VAV Systems 205 FRAME 206 System Controls eT the equations prescribed by ASHRAE Standard 62 1 to calculate the quantity of outdoor air that must be brought in through the system level outdoor air intake Vot in order to satisfy the ventilation requirements of the individual zones see Calculating system intake airflow Vot p 105 The BAS then sends this minimum outdoor airflow setpoint to the air handling unit controller which modulates the flow measuring outdoor air damper to maintain this new setpoint Ventilation reset ensures that all zones are properly ventilated at all operating conditions while minimizing energy wasted by overventilating In a DDC VAV system this strategy is fairly easy to implement because all of the necessary real time information is already available digitally so no new sensors are required All of the equations are defined in an industry wide standard ASHRAE 62 1 and can be solved dynamically to
113. Figure 20 VAV system with supply fan only 25 outdoor air VAV terminal unit zone return air grille AHU main supply supply duct zone ceiling plenum ductwork runouts return ductwork diffusers static pressure relative to outdoors return air damper However the pressure inside the mixing box D must still be lower than the ambient pressure in order for air to be drawn in through the outdoor air damper So the return air damper must be closed far enough to create the necessary pressure drop from C to D This requires the supply fan to generate a larger pressure differential inlet to outlet from B to A in order to deliver the desired supply airflow and bring in the required amount of outdoor air It also results in a higher pressure in the zone which could cause doors to stand open without latching For proper control of building pressure this supply fan only configuration should usually be avoided and either a relief fan or return fan should be used Chilled Water VAV Systems SYS APM008 EN Primary System Components a a as Supply fan and relief fan As an alternative a relief fan can be added to the system In this configuration Figure 21 the supply fan must still create a high enough pressure at its outlet A to overcome the pressure losses associated with the supply air path and create a low enough pressure at its inlet B to overcome the pressure losses associated with the return air path and t
114. Hot water plant Control of the boilers and pumps must be coordinated to provide hot water to the air handling units VAV terminals and or baseboard radiant heat when needed The primary issues to address thorough hot water plant control include e When should the hot water plant be enabled or disabled Chilled Water VAV Systems 197 System Controls eT e Ina plant with multiple boilers when a boiler must be turned on or off which boiler should it be e fan attempt to turn on a boiler or pump fails or if there is a malfunction what should be done next e How can the energy cost of operating the overall hot water system be minimized System optimization When a building automation system BAS provides system level coordination of the various pieces of HVAC equipment the next logical step is to optimize the control of that system For this discussion optimization is defined as minimizing the cost to operate the entire HVAC system while still maintaining acceptable comfort In other words this means maximizing the efficiency of the entire system not just an individual component Optimal start The morning warm up or cool down mode was discussed previously in this chapter In some buildings a simple time clock or a time of day schedule is used to start and stop the HVAC system In this case the time at which the morning warm up or cool down mode begins is typically set to ensure that the indoor temperature reaches t
115. Many times reheat loads can be met with water temperatures of 105 F 40 6 C or lower because the heat required for tempering air is minimal This is an ideal application for heat recovery from any chiller AAN heating coils The example in Table 14 depicts a VAV terminal selected to deliver 2000 cfm 0 94 m3 s at cooling design conditions with a minimum airflow setting of 600 cfm 0 28 m3 s During reheat tempering mode the heating coil needs to reheat the 600 cfm 0 28 m3 s of cool primary air just enough to avoid overcooling the space When the space cooling load is zero when the reheat load is highest it requires heating the air from 55 F 12 8 C to 75 F 23 9 C in this example In order to use 105 F 40 6 C water for reheat the VAV terminal unit must be equipped with a two row heating coil Table 14 compares the performance of a two row coil operating in the reheat mode to a one row coil operating at the same condition using 150 F 65 6 C water While the two row coil is able to provide the required reheat capacity with the lower water temperature both the airside pressure drop and fluid flow rate are higher which increase fan and pumping energy Table 14 also compares the performance of the same coils operating at design heating conditions delivering 90 F 32 2 C air to the space The two Chilled Water VAV Systems 89 90 Primary System Components a a aT row coil is a
116. Maximum allowable fan system power oo ee ecceeeeececee eee 128 Simultaneous heating and cooling limitation o s 130 WAV fan COMMON si cetutceetacscediaieduts shpoceemesaneseuelangeintennseeacwand 132 Demand controlled ventilation ccceceeceeeeeeeeeeeeeeeeeeee 132 Opportunities to further reduce system energy use 132 ACOUSTICS seriea oa S aa E EE KAE ORES EE 135 Defining an acoustical model oo ccc cece eeeetteeeeeeeeeteeeeeees 135 Specific acoustical recommendations ccc ceeceeeeesteeeeeeeeeeeeeee 138 Aircooled chillers cccceecccececceeeeecceeececeeeeeeeeeeeeeeeeeeneennees 139 Watercooled Chillers ccciesssisaiicsoctigcsantvetlesdammsdeescesbeieeacuusiaads 140 VAV airhandling UNITS ee cee eeeeeeeceeceeeeeeteeeceeeeeeeeeeeeeeeeeeees 141 WAV terminal UNITS cdiedi die anwadacddisansinadacedtdancadebecigonatca steamed 145 System Design Variations 2 0 0 ccceeee 147 Cold Air VAV Systems oo cccceeeccccccceescsececsssseeeeeesseseeeescstsseeessesssaeees 147 Benefits of cold air distribution cccceceeeeeeeeeeeteeteeeeeeeeees 147 Challenges of Cold air diStriDUtiON ccc cceeceeeceeeeesseeeeeeenaees 149 Effects of delivering cold air into the Zone n se 149 Impact on overall system energy consumption 0 151 Avoiding condensation on components eeeeeeeeees 153 Best practices when using cold air distribution cccceeeeeeee 155 SINGIE Z
117. Morning warm up or cool down mode p 194 The temperature of the zone side surface will be somewhere between the supply air dry bulb temperature and the zone dry bulb temperature The ASHRAE Cold Air Distribution System Design Guide suggests that for metal diffusers the zone side surface temperature is generally 3 F 1 7 C warmer than the supply air dry bulb temperature During morning cool down if the supply air temperature is controlled so that it is no more than 3 F 1 7 C below the dew point of the zone the risk of condensation on the zone side surface of the diffuser should be minimal Use of a draw thru supply fan which raises the dry bulb temperature of the supply air a few degrees above the dew point of the supply air and heat gain through the supply ductwork allow for a wider margin and quicker pull down of indoor humidity and provide a greater safety factor to avoid condensation In addition monitoring the indoor dew point during unoccupied periods and turning on the system as necessary to prevent indoor humidity from rising too high also helps minimize the risk of condensation problems during morning cool down See After hours dehumidification p 125 e Properly insulate supply ductwork and supply air diffusers Insulate the supply ductwork and supply air diffusers to prevent condensation on the surfaces exposed to the ceiling plenum Figure 109 However insulation alone cannot prevent condensation A pro
118. ONE VAV o eeecceccccccccssnceccecssteeeeeccsneeeeeesnseeeeesssaeeeersesaeeeeens 157 Best practices in a single zone VAV application 005 158 Airto Air Energy Recovery c cee ceccccccecceeeceeceeseeeeeeeeeteeeeeesentsntaees 160 Benefits of outdoorair precOnditiOning cccccceeeeseeeeeeeeeeee 161 Drawbacks of outdoorair preconditioning ccceeeeeeeees 162 Best practices for preconditioning outdoor alf cceeceeeeeeeeees 162 Dual Duct VAV SYSTEMS ooo eccccccccceccssececeecseeeeeeseaeeeeeeessseeseeseaaees 165 Dual versus Single fan SYStOM cce cee eeeeeeeteeeeeeeeeteeeetneeteees 166 Variable versus constant volume to the Zone ceee 167 Best practices for dual duct VAV SySteEMS cccceeeeeeeeeeeees 169 System Controls oo ccccccccccccc cece cececeneteeetetteeeeenenenes 171 Unit Level CONTOS ss2isiestasdeee miaeahenieieiahocs edecassabdiads iE 171 VAV alrhandling unit o ccc cee ceecceseeceeeeeeeceeeeeeceeeaeeaeeeeeeeeeeeees 171 Discharge air temperature control c ce eeeeeeeeeeeeeeeeeeeeees 171 Ventilation control o oo ce eeeeeeeeee cece cee eeeeceeeeeeeeeeeeeeeeeeeee es 171 Supply fan capacity control ccceccccecceeesssseeeeeeesteeeeeeeaee 172 Airside economizer control ccecceeeeeceeeeeeeeeeeeeeeseeteeeeees 174 Building pressure control oo ccc cee ceeeeeeteteeeeeeteeeeeeee ee 178 Return fan capacity control oo cee ceeeeeectec
119. Ra 0 06 cfm ft2 0 3 L s m2 During occupied mode Vbz Rp x Pz Ra x Az 5 cfm p x 50 people 0 06 cfm ft2 x 1000 ft 310 cfm 2 5 L s p x 50 people 0 3 L s m2 x 93 m2 153 L s During occupied standby mode Vbz 5 cfm p x 0 people 0 06 cfm ft2 x 1000 ft 60 cfm 2 5 L s p x 0 people 0 3 L s m2 x 93 m2 28 L s When the occupancy sensor indicates that the zone is again occupied the zone is switched back to occupied mode Unoccupied mode When the building is unoccupied the BAS can allow the temperature in the zones to drift away cooler or warmer from the occupied setpoints see Zone is unoccupied p 4 But the system must still prevent the zones from getting too cold perhaps 55 F 13 C or too hot perhaps 90 F 32 C In addition when unoccupied the building does not typically require outdoor air for ventilation or to replace exhaust air so the outdoor air dampers can be closed Allowing the indoor temperature to drift during the unoccupied mode often called night setback typically saves energy by avoiding the need to operate heating cooling and ventilation equipment In some cases it may be important to control humidity in addition to temperature when the building is unoccupied The BAS can monitor indoor humidity levels and take action if the humidity rises above a maximum limit see After hours dehumidification p 125 or drops below a minimum limit Chilled Wa
120. S FRANE Applications Engineering Manual Chilled Water VAV Systems September 2009 SYS APM008 EN Chilled Water VAV Systems John Murphy applications engineer Beth Bakkum information designer All rights reserved Preface As a leading HVAC manufacturer we deem it our responsibility to serve the building industry by regularly disseminating information that promotes the effective application of building comfort systems For that reason we regularly publish educational materials such as this one to share information gathered from laboratory research testing programs and practical experience This publication focuses on chilled water variable air volume VAV systems These systems are used to provide comfort in a wide range of building types and climates To encourage proper design and application of a chilled water VAV system this guide discusses the advantages and drawbacks of the system reviews the various components that make up the system proposes solutions to common design challenges explores several system variations and discusses system level control We encourage engineering professionals who design building comfort systems to become familiar with the contents of this manual and to use it as a reference Architects building owners equipment operators and technicians may also find this publication of interest because it addresses system layout and control Trane in proposing these system design
121. S program This winterizer configuration removes the length and pressure drop associated with a preheat coil from the main air path resulting in a shorter air handling unit than if a conventional preheat coil air mixing baffles or an energy recovery device is used And since it adds no static pressure drop to the design of the supply fan it has less impact on fan energy than these other approaches However the cost of the second smaller air handling unit is typically higher than an air mixing baffles and it requires a second smaller set of filters that need to be replaced periodically Evaporative cooling Using an evaporative process to cool the air can reduce the energy used by mechanical cooling equipment However it requires careful attention to water treatment periodic cleaning and routine maintenance to ensure safe and efficient operation Finally it consumes water which may be in limited supply in the arid climates where evaporative cooling provides the greatest energy saving benefit Direct evaporative cooling introduces water directly into the air stream usually with a spray or wetted media The water evaporates as it absorbs heat from the passing air stream which lowers the dry bulb temperature of the air Evaporation of the water however also raises the dew point of the air Figure 15 Chilled Water VAV Systems 21 FRAME For more information on evaporative cooling refer to Chapter 40 Evaporative
122. Static pressure sensor located at outlet of supply fan pman _ i static pressure sensor ay P E a fi VAV terminal units supply fan It is not as energy efficient as other methods however because the setpoint must be equal to the pressure at the fan discharge when the supply fan is delivering full design airflow For this reason ASHRAE 90 1 prohibits the Chilled Water VAV Systems SYS APM008 EN For more information on the impact of the static pressure sensor location on supply fan energy use in VAV systems refer to the Trane Engineers Newsletter titled VAV System Optimization Critical Zone Reset ENEWS 20 2 SYS APM008 EN System Controls aT sensor from being installed in this location unless the fan pressure optimization control strategy is used see VAV fan control p 132 A more common practice is to locate the static pressure sensor approximately two thirds of the distance between the supply fan outlet and the inlet to the critical VAV terminal unit Figure 124 The critical VAV terminal unit is at the end of the supply duct path that represents the largest overall static pressure drop Figure 124 Static pressure sensor located 2 3 of distance down supply duct static pressure sensor VAV supply fan terminal units In this configuration the pressure sensor s must be field installed In larger systems with many VAV terminals determining the best location for
123. This calculated Ey method may be used for any system but it must be used if the maximum primary outdoor air fraction Zp is greater than 0 55 Calculating Ey rather than using the default value from Table 6 3 of the standard usually results in a lower outdoorair intake requirement Vot The steps involved in this method are very similar to those in the default method 1 Calculate breathing zone outdoor airflow Vpbz Determine the zone air distribution effectiveness Ez Calculate zone outdoor airflow Voz Calculate the zone discharge outdoor air fraction Za Determine the uncorrected outdoorair intake Vou Determine system ventilation efficiency Ev vn O O FP W N Calculate the system outdoorair intake Vot Using the same eight zone VAV system example the first three steps zone level calculations have already been completed Table 18 The fourth step is to calculate the fraction of outdoor air that is required in the discharge air for each zone The discharge outdoor air fraction Za is the ratio of zone outdoor airflow to the discharge airflow being delivered to the zone Zd Voz Vdz where Voz zone outdoor airflow cfm m s Vaz discharge airflow cfm m3 s Note that Appendix A uses the term discharge outdoor air fraction Za rather than primary outdoor air fraction Zp In this way the equations can be applied to systems with multiple recirculation paths such as fan powered VAV or du
124. a project is to reduce energy use consider selecting a slightly larger air handling unit The example in Table 4 also shows a selection for a size 35 unit With more surface area the cooling coil can deliver equivalent capacity with only four rows of tubes rather than six Fewer rows along with the larger coil face area decreases the airside pressure drop to 0 36 in H20 90 Pa and decreases the fluid pressure drop to 9 1 ft H20 27 1 kPa The result is reduced fan and pumping energy However this does increase the cost footprint Figure 11 and weight of the air handling unit Freeze prevention As discussed in Ventilation p 101 at part load conditions a properly controlled VAV air handling unit typically brings in a high percentage of outdoor air During cold weather it is difficult to mix the outdoor air and recirculated return air when the two air streams are at widely differing Chilled Water VAV Systems SYS APM008 EN Primary System Components a Te temperatures Incomplete mixing results in distinct temperature layers stratification in the resulting mixed air stream Figure 12 If a layer of sub freezing air moves through an unprotected chilled water or hot water coil the water can freeze and damage the coil Figure 12 Temperature stratification during cold weather 80 F return air warm air 10 F 12 C s outdoor air cold air cooling coil Typically a low limit the
125. a series desiccant wheel used for after hours dehumidification RA ja 3 e 5 2 a oO 8 8 8 D o m SA O fo Pa RA 72 5 F DB 58 9 F DP CA e e 57 5 F DB O RA PH 55 1 F DP 3 SA 74 0 F DB 80 0 F DB 65 0 F DB 50 RH 49 7 F DP Humidification In some buildings or specific areas within a building minimum humidity levels must be maintained for comfort or process requirements Types of humidifiers For buildings that commonly use VAV systems steam humidifiers are the most common type of humidifier used Application considerations When including humidification equipment in a VAV system consider the following e Add moisture to the supply air stream not the outdoor air stream When it is cold outside the outdoor air being brought in for ventilation does not have much capacity to hold additional moisture After the outdoor air has mixed with the warmer return air and or has been warmed by a heating coil it has a much greater capacity to absorb moisture e Avoid oversizing the humidification equipment An oversized humidifier typically results in unstable control with large swings in humidity levels In applications where humidification is provided for comfort avoid the use of overly conservative assumptions or safety factors During cold weather adding too much moisture also increases the likelihood of moisture
126. airflow settings that comply with both ASHRAE Standards 62 1 and 90 1 Finally for certain types of spaces building codes or process requirements dictate a certain minimum air change rate air changes per hour or ACH for the zone This is most common in certain health care facilities laboratories and manufacturing plants In this case the minimum primary airflow setting for the VAV terminal unit is set equal to this minimum air change rate minimum airflow setting cfm minimum required air change rate x volume of zone ft 60 min hr minimum airflow setting m3 s minimum required air change rate x volume of zone m3 3600 s hr Perimeter versus interior zones For simplicity a typical building can be described as having two types of thermal zones perimeter and interior As mentioned earlier each zone is typically served by an individual VAV terminal unit allowing independent control of cooling and heating Perimeter zones In many climates perimeter zones with walls and windows exposed to the outdoors require seasonal cooling or heating Such zones require cooling in the summer it is warm outside the sun is shining through the windows people are occupying the zone and the lights are turned on In the winter these zones can require heating to offset the heat loss through the exterior walls and windows even though some heat is generated in the zone by people lights and equipment Before a VAV terminal unit can b
127. al duct systems However in a single path system VAV reheat like the one used in this example outdoor air only enters the zone from the central air handling unit Therefore discharge airflow and primary airflow are the same and Zq and Zp are also the same In a VAV system the discharge airflow may change with the space load so Za increases when the zone discharge airflow decreases For this reason for design purposes ASHRAE 62 1 recommends using the minimum expected discharge airflow Vdzm for VAV systems Determining this minimum Chilled Water VAV Systems 109 System Design Issues and Challenges Pi a eT expected value requires judgment by the design engineer It may be the minimum airflow setting for the VAV terminal or it might be a higher value see the sidebar What is the minimum expected primary airflow p 107 For this example the discharge outdoor air fractions are the same as the primary outdoor air fractions calculated in the previous section ranging from 0 40 to 0 50 Table 19 The fifth step is to determine the uncorrected outdoor air intake Vou Again this step is identical to the default Ev method discussed earlier p 107 Taking credit for occupant diversity the people related component is reduced and the uncorrected outdoor air intake for this example is 2800 cfm 1 3 m s The sixth step is to calculate system ventilation efficiency Ey The equations for this step are incl
128. al unit varies the quantity of air delivered to the zone cycles the terminal fan on and off and or modulates or stages a heating coil see Types of VAV terminal units p 55 When a heating coil is part of the VAV terminal unit the coordination of cooling and heating is done by the unit level controller However when a remote heat source is used such as baseboard radiant heat installed under windows careful coordination is required to prevent the two systems from fighting each other providing cooling and heating simultaneously and wasting energy The most straightforward approach to prevent simultaneous cooling and heating is to allow the unit level controller on the VAV terminal unit to also control the remote heat source When the zone requires heating the unit level controller responds just as if it was controlling a heating coil inside the VAV terminal unit e For acooling only VAV terminal or series fan powered VAV terminal with no heating coil primary airflow is at the minimum setting and the unit level controller sends signals to modulate or stage the remote heat source e Fora parallel fan powered VAV terminal with no heating coil primary airflow is at the minimum setting The first stage of heating is provided by activating the small terminal fan to mix warm plenum air with the cool primary air If further heating is required the unit level controller sends signals to modulate or stage the remote heat sour
129. al zoning 99 three way valve 84 96 throw 75 149 time of day schedule 184 191 192 197 206 211 TRACE 700 133 152 153 trap See condensate trap two way valve 84 96 U ultraviolet light UV C 48 underfloor air distribution 103 104 unit level controls See controls unoccupied economizing 199 200 unoccupied mode 193 V valve flow coefficient 85 97 valves 84 three way valve 84 96 two way valve 84 96 vapor retarder 154 155 variable flow pumping 208 209 variable speed drive 31 39 114 132 172 179 181 188 203 VAV air handling units 141 Chilled Water VAV Systems SYS APM008 EN Index VAV control pressure dependent vs pressure independent 54 VAV fan control 132 VAV system modulation curve 39 VAV terminal units 54 145 183 cooling only 55 dual duct 62 electronically commutated motors 60 fan powered 58 minimum primary airflow settings 62 parallel vs series fan powered 59 reheat 56 types of 55 ventilation 3 14 62 101 102 107 113 207 ventilation control 171 184 ventilation optimization 205 ventilation rate 102 building related 102 people related 102 108 Ventilation Rate Procedure 101 115 ventilation requirements 104 ventilation requirements 101 ventilation reset 117 131 205 Ww water chiller 188 water cooled chillers 80 140 waterside economizer 23 91 wireless sensor 101 wireless technology 101 Z zone discharge outdoor air fraction 109 zone outdoor airflow 104 109 112
130. all system energy use as long as control of indoor humidity is not compromised As chilled water temperature is increased the temperature of the air leaving the cooling coil may increase above the desired setpoint and indoor humidity levels can increase see Resetting supply air temperature p 120 In a variable flow pumping system however raising the chilled water temperature will often increase overall system energy use Because the water is warmer those VAV air handling units that require cooling will need more water to offset the cooling load This increases pumping energy Hot water temperature reset Lowering the temperature of the water leaving the boiler decreases the energy used by the boiler In a constant flow pumping system this will typically decrease overall system energy use In a variable flow pumping system however lowering the hot water temperature will often increase overall system energy use Because the water is not as hot the heating coils will need more water to offset the heating load This increases pumping energy Hot water heating coils in VAV terminal units may be used for two purposes 1 to provide heated supply air to offset heating loads in the zone and 2 to reheat the cool supply air to avoid overcooling the zone at low cooling loads When the zone requires heating the hot water coil heats the supply air to a temperature that is significantly warmer than the zone temperature When reheating ho
131. aller VAV terminals air handling units and ductwork and can save supply fan energy Increased reheat energy and fewer hours of airside economizer operation partially offset the supply fan energy savings Therefore intelligent system control is crucial to fully realize the potential savings See Cold Air VAV Systems p 147 e Add a desiccant dehumidification wheel in series with the cooling coil Figure 94 depicts a VAV air handling unit with a desiccant For more information on using a series dehumidification wheel configured in series with the cooling coil The desiccant wheel in a VAV system refer 3 desiccant wheel adsorbs water vapor from the process air downstream of to the Trane Engineers Newsletter titled g Advances in Desiccant Based the cooling coil and then releases the collected moisture upstream of that Dehumidification ADM APN016 EN coil enabling the AHU to deliver drier supply air at a lower dew point ang Ee eee ee Bulletin titled without lowering the coil temperature In addition the moisture transfer Trane CDQ Desiccant ine Dehumidification CLCH PRB020 EN occurs within a single air stream a separate regeneration air stream is not needed Figure 94 VAV air handling unit with a series desiccant wheel m lt MA OA 41 b IN N NNN O O pe is oO oO S a RA cooling coil O D gt N ll sesiccant whe fl Figure
132. am of the heating and cooling coils Figure 37 helps keep the coils cleaner for a longer period of time and allows the system to operate more efficiently SYS APM008 EN Chilled Water VAV Systems 43 FRAME An addendum c to ASHRAE Standard 62 1 2007 requires a MERV 11 filter if the building is located in an area of the country that exceeds the U S EPA limit for airborne particles with a diameter of 2 5 microns or less PM2 5 One of the requirements for earning the Indoor Chemical and Pollutant Source Control credit Indoor Environmental Quality section of LEED 2009 is to install a MERV 13 or higher filter to clean both return and outside air that is delivered as supply air 44 Primary System Components Figure 37 Particulate filters in a VAV air handling unit angled particulate filters final HEPA filter Source Image adapted from Trane TOPSS program Some high efficiency filtration systems incorporate a lower efficiency pre filter upstream to capture larger particles and thus extend the useful life of the higher efficiency filter downstream The benefit of this longer life however must be carefully weighed against the additional cost and pressure drop of the upstream pre filters as well as the labor required to periodically replace them ASHRAE Standard 62 1 2007 Section 5 9 requires that a filter with a MERV rating of at least 6 be installed upstream of all wet sur
133. amper actuator or variable speed drive and the pressure sensor is difficult to situate because this plenum is usually small and turbulent See Return fan capacity control p 181 Requires more fan power at part load The pressure in the return air plenum inside the air handling unit must always be high positive enough to force air out through the relief damper The return air damper must therefore create a significant pressure drop between the positive return air plenum and the negative mixed air plenum This added pressure drop requires more combined fan power than a system with a relief fan Limited layout flexibility The return fan must be situated between the air handling unit and the closest leg of the return air path usually near the air handling unit because it must draw the entire return path negative relative to the occupied spaces It must also discharge into the return air plenum during modulated economizer operation Relief fan Advantages Disadvantages Lower operating cost In some applications the relief fan can remain off during non economizer hours and operate at low airflow during many economizer hours Also the return air damper can be sized for a lower pressure drop Simpler control scheme One less sensor and one less actuator simplifies installation and air balancing Applications with a relatively low pressure drop through the return air path which is common in a system that u
134. an powered VAV terminal UNITS oo cee eseesereneeeees 58 Dual duct VAV terminal units oo cece cc cce cece se sess eeeeeeeeeees 62 Minimum primary airflow settings ccceeeeeeeeeeeeeeeeeeeeeeeeeeees 62 Perimeter versus interior ZONES oo cece cece ccc ecec eee ee esses eee eeeeeeeeeeeseees 63 Typical combinations used in chilled water VAV systems 66 SYS APMO008 EN Chilled Water VAV Systems iii Air DIStiDUTON uisiccgasecancutenceseievcantuet E A 70 SUDDIY duct SyS EMI svccarsnadenesesassasssaacenatndansssfarncteadnaatacnsuanageneseashda 70 Supply air GITFUSEIS ccc c s cccceaeacncscedesnenssavadnnscdesansansdtdansanenesansdnnnss 74 Return air path eee cccccce cece cee cneeceeeeeeeeeeeeeteeceeceesaeeeeeeeeeeeeeteneeea 77 Chilled Water SVStemi ccacsextagtscewnstniedetasidagiels elagiimseatanguesserdanicueians 79 Types of water chillers ccccccceccecceceeeceeceeeceeeeeeeeeeeeteeeeeeeenees 79 Chilled water and condenserwater distribution 0 00 cceeeeee eee 81 Design temperatures and flow rates ou eecceeeeeeeeeeeeeeeees 81 Control valve SEISCTION Aescdsicsusveidivsossvnsacdsieeasannce aaa 83 Variable versus constant flow PUMPING ce eee 86 Freeze prevention oo eee cee cece ceeeeeeeeceeeeeeeeeeeceeeeeeeeeeeeeeeseeeeenees 87 Condenser heat recovery cccccceccccecsssseeececseeeeeeeesenseeeesestseeess 88 Waterside CCOMOMIZEL ccccceececeeceeceeeeeeceeeeeeeeeeeeeeeeee
135. anes or a variable speed drive in combination with inlet vanes The controller also monitors chiller operation and protects it from damage by preventing it from operating outside acceptable limits Some water chiller controllers are capable of adapting to unusual operating conditions keeping the chiller operating by modulating its components and sending a warning message rather than doing nothing more than shutting it down when a safety setting is violated Specific details about the water chiller controller should be obtained from the manufacturer Condensing pressure control Every vapor compression chiller requires a minimum refrigerant pressure difference between the evaporator and the condenser in order to ensure that refrigerant and oil circulate properly inside the chiller This pressure difference varies based on the chiller design and operating conditions For a packaged air cooled chiller this function is performed by the unit level controller However if the chiller is expected to operate when it is very cold outside it may need to be equipped with either a discharge damper or Chilled Water VAV Systems SYS APM008 EN For more information on condensing pressure control for water cooled chillers refer to the Trane engineering bulletins titled Condenser Water Temperature Control For CenTraVac Centrifugal Chiller Systems CTV PRBOO6 EN and Water Cooled Series R Chiller Models RTHB amp RTHD Condenser
136. ants that are generated inside the building by people equipment processes or furnishings Of course the introduction of outdoor air requires the removal of an equal quantity of air from the building The Ventilation Rate Procedure Section 6 2 in ASHRAE Standard 62 1 Ventilation for Acceptable Indoor Air Quality prescribes the quantity of outdoor air that must be delivered to each zone based on the expected use of that zone and then prescribes how to calculate the outdoor airflow needed at the system level intake In addition Section 5 of this standard includes several requirements related to the design of the ventilation equipment and distribution system The requirements related to ventilation system controls particulate filtration and humidity control are each discussed in other sections of this manual Note Because ASHRAE 62 1 is under continuous maintenance it can change frequently This manual is based on the 2007 published version of the standard Refer to the most current version for specific requirements Zone level ventilation requirements ASHRAE 62 1 requires the following three step procedure to determine the outdoor airflow required for each ventilation zone 1 Calculate the outdoor airflow that must be delivered to the breathing zone Vbz using the prescribed rates in Table 6 1 of the standard Chilled Water VAV Systems 101 Caution Occupant load or exit population is often determined for use i
137. arallel fan powered VAV terminals Since the fans inside parallel fan powered VAV terminals do not operate in the cooling mode when the supply fan is operating at peak design conditions they do not need to be included e Series fan powered VAV terminals Since the fans inside series fan powered VAV terminals operate continuously during the occupied mode they are operating when the supply fan is operating at peak design conditions so they must be included e Central relief fan In some systems the central relief fan operates only during the economizer mode but not at the peak design cooling condition In this case it does not need to be included On the other hand if the central relief fan does operate at the peak design cooling condition it must be included e Central return fan A central return fan operates at the peak design cooling condition so it must be included e Small local exhaust fans Individual exhaust fans with nameplate motor power of 1 hp 0 75 kW or less are exempt and do not need to be included The 2007 version of ASHRAE 90 1 includes two options for compliance Table 24 128 Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN System Design Issues and Challenges SS Table 24 Fan power limitation for VAV systems Option 1 Allowable nameplate motor hp lt CFMsupply x 0 0015 power kW lt L Ssupply x 0 0024 Option 2 Allowable fan input brake bhp lt CFMsupply x
138. ard until the system balances at an operating point B that brings the system static pressure back down to the setpoint This response over the range of system supply airflows causes the supply fan to modulate along the VAV system modulation curve The most common method used to modulate supply fan capacity in a VAV system is to vary the speed at which the fan wheel rotates This is commonly accomplished using a variable speed drive or variable frequency drive VFD on the fan motor Figure 32 Figure 32 Fan speed control supply fan variable speed drive When the system static pressure controller sends a signal to reduce fan capacity the variable speed drive reduces the speed at which the fan wheel rotates Reducing fan speed rpm shifts the performance curve of the fan downward until the system balances at an operating point B along the VAV system modulation curve bringing the system static pressure back down to the setpoint Figure 33 Figure 33 Performance of fan speed control in a VAV system part load system resistance curve 5 full load system resistance curve 800 rpm o 2 3 N wn g VAV system I T modulation curve s l sensor I setpoint l l 900 rom 1 000 rpm airflow SYS APM008 EN Chilled Water VAV Systems 39 For more information on the various types of particulate filters refer to Chapter 28 Air Cleaners for Particulate Contaminants in the 200
139. ation Critical Zone Reset ENEWS 20 2 When communicating controllers are used on the VAV terminal units it is possible to optimize this static pressure control function to minimize fan energy consumption Each VAV unit controller knows the current position of its airmodulation damper The BAS can continually poll the individual VAV terminal unit controllers looking for the one with the most open damper The static pressure setpoint is then reset so that at least one damper is nearly wide open The result is that the supply fan generates only enough static pressure to push the required quantity of air through this critical furthest open VAV terminal unit This concept is often called fan pressure optimization Figure 143 200 Chilled Water VAV Systems SYS APM008 EN For VAV systems that use direct digital controls DDC on the VAV terminal units fan pressure optimization is required by ASHRAE Standard 90 1 see VAV fan control p 132 SYS APM008 EN System Controls Figure 143 Fan pressure optimization supply fan i static pressure sensor This optimization strategy has several benefits First it results in less supply fan energy use at part load conditions A comparison of the three common fan control methods demonstrates the energy savings potential Table 35 At full load design airflow all three control strategies are the same However at part load conditions fan pressure optimization allows
140. ative to outdoors In addition the supply fan must create low enough pressure at its inlet B to overcome the pressure losses associated with drawing the return air out of the zones and through the return air grilles through the open ceiling plenum and or return ductwork and then through the return air damper filter and coils inside the air handling unit Figure 19 depicts a typical supply fan only system operating with 100 percent recirculated air as it might operate during unoccupied periods or morning warm up no outdoor air is being brought into the building Due to the pressure drop through the return air path the pressure at the inlet to the air handling unit C is lower than the ambient pressure With this negative pressure differential no air will be forced out of the relief damper With this supply fan only configuration the only way for any air to leave the building which is required if outdoor air is to be brought into the building is for the pressure in the zone and return air path to increase SYS APMO008 EN Chilled Water VAV Systems 27 28 Primary System Components a aS Figure 20 depicts this same system operating with 25 percent outdoor air The pressure in the zone and return air path has increased to the point that the pressure at the inlet to the air handling unit C is now higher than the ambient pressure This positive pressure differential will force air out through the relief damper
141. aximize economizer savings and avoid bringing in outdoor air that has higher enthalpy than the return air e During startup after the system has been shut off slowly ramp down the supply air temperature to pull down the humidity in the building and avoid condensation problems See Morning warm up or cool down mode p 194 e Monitor indoor humidity during unoccupied periods and turn on the system as necessary to prevent indoor humidity from rising too high and causing condensation problems during morning cool down Single Zone VAV Recall that in a conventional multiple zone VAV system a damper in the VAV terminal unit modulates to maintain the zone temperature by varying the volume of supply air delivered to that zone The central supply fan modulates to maintain the static pressure setpoint in the supply ductwork A single zone VAV system on the other hand serves only one zone so no VAV terminal units are required Instead a temperature sensor in the zone is used to directly vary the volume of supply air delivered by the fan Figure 111 Just like in a conventional VAV system cooling capacity is modulated to maintain the discharge air temperature at a setpoint Figure 111 Single zone VAV system flow measuring rT OA damper OA Single zone VAV is an excellent system to use in large densely occupied zones that have variable cooling loads Common examples include gymnasiums lecture halls arenas
142. because it supplies air at a relatively constant low dew point temperature at all load conditions As long as any zone needs cooling the VAV air handling unit provides supply air at a dew point that is usually low enough sufficiently dry to offset the latent loads in the zones Full load versus part load dehumidification performance To demonstrate consider a 10 000 ft 283 m3 classroom in Jacksonville Fla that accommodates 30 people During cooling mode the zone sensor setpoint is 74 F 23 3 C dry bulb At the traditional design condition peak outdoor dry bulb temperature the system delivers 1500 cfm 0 7 m s of supply air of which 450 cfm 0 2 m3 s is outdoor air required for ventilation at 55 7 F 13 1 C to offset the sensible cooling load in the zone and maintain the zone temperature at setpoint Plotting this system on a psychrometric chart the resulting relative humidity in the zone is 52 percent at this design condition Figure 90 At part load conditions the VAV system responds to the lower sensible cooling load in the zone by reducing the quantity of air supplied to the zone while maintaining a relatively constant supply air temperature 55 7 F 13 1 C in this example At this example part load peak outdoor dew point condition the supply airflow is reduced to 900 cfm 0 42 m s to avoid overcooling the zone Because the supply air is still cool and dry low dew point however the relative humidity in t
143. between the occupied space and the ceiling plenum A space to plenum pressure difference of no more than 0 02 to 0 03 in H20 5 0 to 7 5 Pa is acceptable under most conditions When a suspended T bar ceiling is used a high pressure difference between the occupied space and the ceiling plenum typically causes some of the return air to be forced around the edges of the ceiling tiles This causes soiling of the tiles which increases the frequency of cleaning or replacement e Avoid undersizing return air openings within the ceiling plenum When the return air path must pass through an interior partition wall that extends from floor to floor make sure the opening through the wall is large enough to avoid an excessive pressure drop In addition the opening into the return air ductwork must be large enough to avoid an excessive pressure drop e Use an open ceiling plenum rather than a ducted return whenever possible Table 10 Using an open ceiling plenum for the return air path reduces installed cost and lowers airside pressure drop which results in less fan energy used However open plenum returns should not be used when prohibited by local codes or when space to space pressure differentials must be controlled 78 Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components Table 10 Open plenum versus fully ducted return air path Open plenum return Advantages Disadvantages e Lower installed cost l
144. ble to provide the required heating capacity using a lower hot water temperature 150 F 65 6 C compared to 180 F 82 2 C for the one row coil When combined with the use of a condensing boiler this lower water temperature can significantly increase the efficiency of the heating system see Non condensing versus condensing boilers p 92 In addition the fluid pressure drop due to the coil is larger for the single row coil increasing the size and energy use of the pumps Table 14 Using condenser water heat recovery and 105 F 40 6 C water for reheat tempering 2 row coil 1 row coil Airside pressure drop at design cooling 0 79 196 0 45 112 airflow in H20 Pa Operating in reheat tempering mode 2 row coil 1 row coil Entering air temperature F C 55 12 8 55 12 8 Leaving air temperature F C 75 23 9 75 23 9 Heating capacity MBh kW 13 0 3 8 13 0 3 8 Entering fluid temperature F C 105 40 6 150 65 6 2 Returning fluid temperature F C 91 32 8 107 41 9 Coil flow rate gpm L s 1 88 0 12 0 61 0 04 Fluid pressure drop ft H20 kPa 0 23 0 68 0 29 0 85 Airside pressure drop at reheat airflow 0 07 17 6 0 04 10 1 in H20 Pa Operating at design heating conditions 2 row coil 1 row coil Entering air temperature F C 55 12 8 55 12 8 Leaving air temperature F C 90 32 2 90 32 2
145. boiler Note Common unit level and system level control functions for a chilled water VAV system are discussed in detail in the chapter System Controls p 171 Specific details should be obtained from the equipment or controls manufacturer Chilled Water VAV Systems SYS APM008 EN Single or multiple air handling units A building may use a few large air handling units or several smaller units depending on size load characteristics and function See VAV Air Handling Unit p 11 Definition of a zone A space or group of spaces within a building with heating and cooling requirements that are sufficiently similar so that desired conditions e g temperature can be maintained throughout using a single sensor e g temperature sensor SYS APM008 EN System Design Issues and Challenges This chapter proposes solutions to several common challenges of designing a chilled water VAV system This is not an exhaustive list of all challenges or all solutions but is meant to cover the most common The chapter System Design Variations p 147 addresses several variations on the typical chilled water VAV system Thermal Zoning In a VAV system each thermal zone has a VAV terminal unit that is controlled to maintain the temperature in the zone it serves Defining the zones in a VAV system is often more of an art than a science and requires judgment by the system design engineer An individual zone
146. by ASHRAE Standard 52 2 that depicts how efficiently a filter removes particles of various sizes mixed air A mixture of outdoor air and recirculated return air modulated economizer mode An operating mode of an airside economizer when the outdoor air is cool enough to handle the entire cooling load and the compressors are off The controller modulates the positions of the outdoor air and return air dampers so that the mixture of outdoor and return air provides supply air at the desired setpoint moisture carryover Retention and transport of water droplets in an air stream morning cool down mode A typical operating mode for transition from the unoccupied mode to the occupied mode during the cooling season It establishes the zone occupied comfort conditions because they were allowed to drift from the occupied setpoint during the unoccupied mode usually to save energy morning warm up mode A typical operating mode for transition from the unoccupied mode to the occupied mode during the heating season It establishes the zone occupied comfort conditions because they were allowed to drift from the occupied setpoint during the unoccupied mode usually to save energy night setback See setback nighttime economizing See unoccupied economizing Noise Criteria NC A single number used to describe sound in a occupied space It uses a series of curves for plotting sound pressure by octave band and determining the NC value non co
147. bypassing the coil remains relatively providing the opportunity to significantly constant at all load conditions which reduce pumping energy at part load results in relatively constant pumping Temperature of the water returning to the energy boiler remains relatively constant as the e Temperature of the water returning to the heating load decreases boiler increases as the heating load e A variable flow system is less sensitive to decreases water balance than most constant flow e Water flow balance is critical to ensure systems proper operation because flow is constant 96 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components The most common approach to select the control valve is the valve flow coefficient Cv or Kv Cv 2L iid Pe o where Cv valve flow coefficient Kv OQ fluid flow rate gpm L s AP pressure drop across the valve psi kPa SG specific gravity of the fluid 1 0 for water p density of fluid kg m3 1 000 kg m3 for water In general the pressure drop AP across the valve should be equal to or slightly greater than the pressure drop through the hot water coil The coil pressure drop can be obtained from the manufacturer of the VAV terminal unit For example consider a hot water coil that has a design flow rate of 1 5 gom 0 095 L s with a pressure drop of 0 61 psi 4 2 kPa Ideally the pressure drop across the valve should be equal t
148. ccesscccceeesteeeeenes 105 Appendix A calculated Ey method 0 0 0 cece 109 Heating Versus cooling design ceccceeceeeeettetteceeeeeeeeeeees 111 Systems with multiple recirculation paths ccccceeee 113 Fixed outdoorair damper position eeeeeeeeeeeeeeeeeeeeees 113 Proportional outdoorair damper control n se 114 Direct measurement and control of outdoor airflow 115 Dedicated Outdoorair SYStEMS ccc cee eeeeeeeceeeeeeeeeeeeeeeeeeeees 115 Dynamic reset of Intake airflow 0 0 2 2 cccccceeceeeeeeeeceeeeeeeeeeeeeeenes 117 HMIGity COMEO sasesana in aa ARE iiaa 118 DENUMICITICATION isitgan aaa O EEEE 118 Full load versus part load dehumidification performance 118 Impact of minimum airflow settings for VAV terminal units 119 Resetting Supply air temperature oo cccc cece ceases eee ee ees 120 Reheating supply air at the VAV terminal units 120 iv Chilled Water VAV Systems SYS APM008 EN Methods for improving dehumidification performance 121 Afterhours Cehumidification ccccccccceecccccecsteeeeesestteeeeeees 125 FLUPIGIIGA TOM vseeaiceetcvsesteaiintunsiguteaddedaateadsbaatcdauasadedimacuanevaitaddeeaids 126 Energy Effici ncy ccccececcceeceeeceeceenceceeeeeeeeeeeeeesenseeneesaeeeeeeeeeeenes 127 Minimum efficiency requirements ccceeeeeeeeeeeeeeeeeeeteeeeeees 127 Minimum equipment efficiencies ceeeeeeceeeeeeeeeeeeeeeeees 127
149. ce Some systems use a separate outdoor air temperature sensor to control the baseboard heating system As the temperature outside gets colder the capacity of the baseboard heat is increased to provide more heat This approach is not recommended in VAV systems because it is difficult to coordinate and often results in simultaneous heating baseboard and cooling VAV terminal Another approach uses two separate zone temperature sensors one to control the cooling provided by the VAV terminal unit and the other to control the baseboard heating system As long as there is a suitable deadband and the two temperature sensors are well calibrated and located next to each other this approach can be effective at preventing the cooling and heating systems from fighting each other and wasting energy Chilled Water VAV Systems 183 For VAV systems that use direct digital controls DDC demand controlled ventilation may be required by ASHRAE Standard 90 1 for densely occupied zones see Demand controlled ventilation p 132 An addendum j to ASHRAE Standard 62 1 2007 helps clarify the intent of the standard by adding the following to Section 8 3 Systems shall be operated such that spaces are ventilated when they are expected to be occupied While the outdoor airflow delivered to the breathing zone Vbz can be reset as zone population Pz varies per Section 6 2 7 the system must deliver at least the building related
150. ch can increase usable or rentable floor space Cold air VAV systems typically deliver supply air at a temperature of 45 F to 52 F 7 C to 11 C For more information see Cold Air VAV Systems p 147 Chilled water cooling coil Figure 9 Actual cooling coll Cooling in a chilled water VAV system is accomplished using a chilled water coil in the VAV air handling unit Cooling coils are finned tube heat exchangers consisting of rows of tubes that pass through sheets of formed fins Figure 9 As air passes through the coil and contacts the cold tube and fin surfaces heat transfers from the air to the water flowing through the tubes Figure 10 SYS APM008 EN Chilled Water VAV Systems 15 FRAME 16 Primary System Components Figure 10 Chilled water cooling coil airflow in airflow out water in water out In most applications the cooling coil also dehumidifies as water vapor in the air condenses on the cold fin surfaces of the coil This water then drains down the coil surfaces drops into the drain pan located beneath the cooling coil and is piped away by the condensate drain line The coil tubes are usually constructed of copper and the coil fins of aluminum For some applications coils may use copper fins or a manufacturer may cover the coil surfaces with a coating to minimize corrosion Selection of the cooling coil impacts the cost of installing operating and maintaining both the VAV a
151. complaints from air cooled chillers come from neighboring properties or buildings Most areas of the country have sound ordinances that define the maximum amount and sometimes quality of sound permitted at the property boundary or lot line In most cases the lot line maximums are given in terms of dBA with different maximum levels for daytime and nighttime periods However sometimes an ordinance includes limits by octave band and limits for special types of sound such as tones or impulse sound In any case the ordinance defines the acoustical target for the project If the acoustical analysis indicates that the air cooled chiller will exceed the ordinance consider moving the chiller adding sound attenuation options to the chiller or building a barrier wall e Move the chiller Sound diminishes with distance so consider moving the chiller away from the sound sensitive areas and using existing barriers such as hills or buildings to block the sound e Investigate sound attenuation options available from the chiller manufacturer Air cooled chillers are considered to contain three sound sources compressors condenser fans and structure Compressors and portions of the structure can be attenuated using sound wraps Attenuating the fans is more difficult because airflow through the condenser coils and fans cannot be blocked by any type of enclosure Purchasing the chiller with special low noise fans is the simplest way to attenuate fan
152. cooling loads allowing them to be satisfied with a warmer supply air temperature Systems in which part load dehumidification is a concern Hot climate with few hours when the outdoor dry bulb temperature is below 60 F 16 C Inefficient design of the air distribution system high pressure loss increases penalty for the higher airflows that result from raising SAT Systems serving some zones that have nearly constant cooling loads consider using a separate system for such zones such as computer centers so they do not force the system to operate at a colder supply air temperature Efficient part load fan modulation such as a variable speed drive This table is an excerpt from the California Energy Commission s Advanced VAV System Design Guide SYS APM008 EN The SAT reset strategy should be designed to minimize overall system energy use considering the trade off between cooling reheat and supply fan energy It should also minimize the negative impact on zone humidity levels These competing issues are often best balanced by first reducing fan airflow taking advantage of the significant energy savings from unloading the fan Once fan airflow has been reduced somewhat raise the supply air temperature to minimize reheat energy and enhance the benefit of the airside economizer To accomplish this the SAT setpoint is typically reset based on either 1 the changing outdoor temperature or 2 by monitoring the cooling needs
153. d near the pump the added pressure drop will probably not impact the size of the pump On the other hand if this valve is part of the critical path highest pressure drop path the added pressure drop through the valve may necessitate the selection of a larger pump If a valve with a larger Cv Kv is selected the pressure drop through the valve will be less than the pressure drop through the coil If this difference in pressure drops is too large it could result in poor controllability low valve authority SYS APM008 EN Chilled Water VAV Systems 85 TRANE For more information on variable flow chilled water or condenser water systems refer to the Trane application manual titled Chiller System Design and Control SYS APM001 EN 86 Primary System Components ST Variable versus constant flow pumping As previously mentioned using two way control valves results in variable water flow through the chilled water system which provides the opportunity to significantly reduce pumping energy at part load A variable flow chilled water distribution system Figure 75 however requires the chillers be equipped to handle variable water flow in a variable primary flow configuration or the system be designed to provide constant water flow through the chillers such as a primary secondary or decoupled configuration Figure 75 Variable flow chilled water systems constant volume ory Primary
154. d return if possible This results in a conditioned ceiling plenum which can reduce the risk of condensation on components of the air distribution system When specifying VAV terminal units request the manufacturer provide actual thermal performance of the cabinet to determine the risk of condensation HVAC system controls Consider raising the zone cooling setpoint 1 F or 2 F 0 5 C to 1 1 C to further reduce the supply airflow in cold air systems The lower relative humidity in a cold air system often allows the zone dry bulb temperature to be slightly warmer than in a conventional system while still achieving an equivalent sensation of comfort Maintain positive building pressure during the cooling season to minimize infiltration of humid outdoor air Use supply air temperature reset see Supply airtemperature reset p 202 to minimize the use of reheat and maximize the benefit of the airside economizer Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Variations a eT e Implement fan pressure optimization see Fan pressure optimization p 200 to minimize supply fan energy and improve zone control e Consider using a differential enthalpy economizer see Airside economizer control p 174 In a cold air VAV zone the relative humidity and therefore the enthalpy of the return air is typically lower than a conventional system A differential enthalpy economizer will m
155. discharged from the unit Figure 2 Figure 2 Typical air handling unit used in a VAV system discharge plenum p filter return air damper supply fan chilled water cooling coil outdoor air hot water damper heating coil The supply air is distributed through ductwork that is typically located in the ceiling plenum above each floor Figure 1 The supply ductwork delivers air to each of the VAV terminal units then this air is introduced into the zones through supply air diffusers Each independently controlled zone has a VAV terminal unit that varies the quantity of air delivered to maintain the desired temperature in that zone Air typically returns from the zones through ceiling mounted return air grilles and travels through the open ceiling plenum to a central return duct that directs this return air back to the air handling unit The chilled water for cooling is provided by a chilled water system which includes one or more water chillers with associated water distribution pumps and heat rejection equipment cooling towers for water cooled chillers condenser fans for air cooled chillers Heating can be accomplished in several ways One approach uses a heating coil hot water steam or electric or gas fired burner inside the air handling unit In this configuration the airhandling unit can warm the supply air during cold weather or during a morning warm up period A second approach uses individual heating c
156. discussed earlier in this chapter see Ventilation control p 184 Ina multiple zone VAV system zone level ventilation control must be coordinated to determine how to control the outdoor air damper in the system level air handling unit As mentioned in Dynamic reset of intake airflow p 117 ASHRAE Standard 62 1 permits dynamic reset of intake outdoor airflow as operating conditions change as long as the system provides at least the required breathing zone outdoor airflow whenever a zone is occupied e Ventilation Reset Resetting intake airflow in response to variations in ventilation efficiency The system ventilation efficiency Ey of a multiple zone VAV system changes as both zone airflows and system level primary airflow change due to variations in building load This ventilation reset strategy dynamically resets the system outdoorair intake Vot based on this changing efficiency Implementing this strategy requires a communicating controller on each VAV terminal unit a building automation system that can gather data from all the VAV controllers and a method to measure and control outdoor airflow at the central air handling unit Figure 146 This is typically accomplished using a flow measuring device in the outdoor air stream Figure 88 p 115 Figure 146 Ventilation reset in a chilled water VAV system VAV air handling unit with a flow measuring OA OA damper e Reset intake airflow V
157. ditional cooling coil trading sensible capacity for more latent capacity This system is able to deliver drier air without requiring a significantly colder leaving coil temperature without the desiccant wheel the cooling coil would need to cool the air to about 43 F 6 1 C dry bulb in order to achieve a supply air dew point of 42 9 F 6 1 C When the relative humidity of the air entering the upstream side of the wheel is high on a mild rainy day for example it may be necessary to preheat this air to lower its relative humidity RH and allow the desiccant to regenerate Figure 95 also shows the performance of the series desiccant wheel on a mild rainy day In this example the mixed air MA is preheated from 72 0 F 22 2 C to 78 7 F 25 9 C in order to lower the RH of the air entering the upstream side of the wheel PH This allows the wheel to supply air at the desired 57 9 F 14 4 C dry bulb but at a dew point of 44 3 F 6 8 C The resulting relative humidity in the zone is 43 percent e Condition the outdoor air with a separate dedicated unit Another way to improve dehumidification performance is to use a dual path air handling unit see Dual path configuration p 13 or a dedicated Genumisinestionpenommance Ea O outdoor air unit to separately dehumidify all of the outdoor air to a dew the Trane application manual titled point that is drier than the zones This conditioned outdoor air is then Dehumidificatio
158. dling unit on the overall energy use of the building Figure 44 Impact of AHU leakage on building energy use blow thru indoor AHU with ducted return Houston Los Angeles Philadelphia St Louis 105 104 wn oO Q 103 a a 102 A 5 101 gt D D 100 G o D 99 s E 98 pe 97 S 96 95 0 1 5 10 0 1 5 10 0 1 5 10 0 1 5 10 base base base base air handling unit leakage of design airflow Regardless of unit or system configuration all penetrations through the casing and base of the air handling unit should be sealed including any electrical and piping connections Chilled Water VAV Systems 53 For more information on preventing condensation of air handling units including design strategies for the mechanical equipment room refer to the Trane application manual titled Managing Building Moisture SYS AM 15 Pressure dependent versus pressure independent control A pressure dependent VAV controller uses the zone temperature sensor to directly control the position of the modulating damper The actual airflow delivered to the zone is a by product of this position and depends on the static pressure at the inlet of the terminal unit Although the zone temperature sensor will continually correct the position of the damper the response can be sluggish and may cause unacceptable temperature variations within the zone particularly if reheat is not used In contrast a
159. drop ft H20 kPa 13 6 40 6 6 2 18 4 By lowering the entering fluid temperature this coil can deliver the same cooling capacity with 36 percent less flow at less than half of the fluid pressure drop with no impact on the airside system For water cooled systems this low flow strategy can also be used for the condenser water distribution system The benefits of applying this coil in a low flow chilled water system include reduced installed cost lower flow rates can allow the pipes fittings pumps valves and cooling tower to be smaller and or reduced pumping energy since the system has to pump less water And although the chiller will consume more energy to make the colder water in most applications the total system chillers pumps and cooling tower for water cooled systems energy consumption will be reduced Figure 73 82 Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components Figure 73 Chilled water system energy use standard flow versus low flow 350 300 o z standard flow rates a 250 a low flow rates 2 200 m o o a 150 4 _ g z an 100 2 5 50 0 jo r 25 load 50 load 75 load full load Standard flow conditions are 2 4 gpm ton 0 043 L s kW chilled water and 3 0 gpm ton 0 054 L s kW condenser water Low flow conditions are 1 5 gpm ton 0 027 L s kW chilled water and 2 0 gpm ton 0 036 L s kW condenser water If further e
160. e The controller also monitors boiler operation and protects it from damage by preventing it from operating outside acceptable limits Specific details about the boiler controller should be obtained from the manufacturer Return water temperature control Non condensing boilers require that the return water temperature be no lower than 140 F 60 C to prevent condensing In some boilers this function is performed using a temperature actuated valve internal to the boiler to divert hot water from the supply and mix it with cooler return water Alternatively the hot water system may need to be designed to ensure a suitably warm return water temperature Common approaches include a pressure actuated bypass valve or a primary secondary pumping configuration System Level Control System level control refers to the intelligent coordination of the individual pieces of equipment so they operate together as a reliable efficient system Typically the central air handling unit VAV terminal units water chillers and boilers are each equipped with separate unit level controllers and these unit level controllers are connected to a centralized system level controller Figure 137 With this configuration each unit level controller is capable of performing its functions even if communication with the system level controller is lost Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN System Controls Figure 137 Syste
161. e air handling unit Blow thru versus draw thru In a blow thru configuration the fan blows air through a cooling coil located downstream of the fan Table 7 p 37 The heat generated by the fan and motor is added to the air upstream of the cooling coil Chilled Water VAV Systems 35 Primary System Components a ee In a draw thru configuration the fan draws air through a cooling coil located upstream The heat generated by the fan and motor is added to the air downstream of the cooling coil Assuming equivalent supply airflows in a draw thru configuration the air must leave the cooling coil at a colder temperature in order to achieve the same supply air temperature delivered down the duct Because the fan heat gain is equal the sensible cooling capacity required is the same for both configurations Figure 28 However because the cooling coil in the draw thru configuration must make the air colder since the fan heat is added downstream of the coil it also makes the air drier in non arid climates This increases the latent cooling capacity Therefore achieving the same 55 F 13 C supply air temperature with the draw thru configuration requires slightly more total cooling capacity but offers the benefit of slightly drier air delivered to the zones Figure 28 Effect of fan heat gain on cooling capacity equal supply airflows blow thru sensible coil 1 draw thru capacity i MA O a ee ee ee coil capacity
162. e airside economizer is activated the air handling unit brings in more than minimum outdoor air for the purposes of free cooling The supply air therefore contains a higher percentage of outdoor air than is required for ventilation This may allow the minimum airflow settings on some VAV terminal units to be lowered possibly saving reheat energy or supply fan energy if any of the VAV terminals are closed to their minimum primary airflow settings For more information on these ventilation control strategies see Ventilation control p 184 and Ventilation optimization p 205 SYS APMO008 EN Chilled Water VAV Systems 117 Section 5 10 of ASHRAE 62 1 2007 requires that systems be designed to limit the relative humidity in occupied spaces to 65 percent or less at the peak outdoor dew point design condition and at the peak indoor design latent load For more information on the dehumidification performance of a VAV system including the impact of minimum airflow settings and SAT reset refer to the Trane application manual titled Dehumidification in HVAC Systems SYS APM004 EN 118 System Design Issues and Challenges i Set Humidity Control While humidity control is apt to imply special applications such as museums or printing plants managing humidity should be a key consideration in any HVAC application Dehumidification A VAV system typically dehumidifies effectively over a wide range of indoor loads
163. e both the dry bulb temperature and humidity of both the outdoor air and return air The controller calculates the enthalpy of both air streams and uses the lower enthalpy air to satisfy the cooling load The economizer is disabled whenever the outdoor air enthalpy is higher than the return air enthalpy Figure 130 SYS APMO008 EN Chilled Water VAV Systems 177 FRAME For more information on the issues related to improper building pressure control and the various methods of controlling building pressure in VAV systems refer to the Trane Engineers Newsletter titled Commercial Building Pressurization ADM APNOO3 EN and the Trane Engineers Newsletter Live broadcast DVD titled Commercial Building Pressurization APP APV013 EN 178 System Controls aT Figure 130 Differential enthalpy control of the airside economizer high limit 5 shutoff aia Q Ta minimum Pag OA intake eae a PRL A z 9804 5 H es i os i i z N m i gt E economizer ON The installed cost of differential enthalpy control is higher than for the other control methods because it requires humidity sensing for both outdoor and return air But it typically results in the most cooling energy saved compared to the other control types which may make up for the first cost increase Climate building use and utility costs impact the operating cost differences of these different methods of economizer control Building pres
164. e decreases Figure 79 Therefore to maximize the efficiency of a condensing boiler it is important that the rest of the heating system be designed to operate at these lower return water temperatures Figure 79 Impact of return water temperature on boiler efficiency 100 NS natural gas 1050 Btu ft3 39 MJ kg 98 NI 96 DN stoichiometric air 17 24 Ib lb of fuel N 1724 kg kg of fuel 94 92 PARC clanecinrimnr 3 a NONCONUECNSINY mode O t E 2 90 D dew point 2 88 JT 4 2 al condensing mode 10 oio gt Xcess ar 84 82 80 60 100 140 180 220 16 38 60 82 104 inlet water temperature F C Source 2008 ASHRAE Handbook HVAC Systems and Equipment Chapter 31 Figure 6 American Society of Heating Refrigerating and Air Conditioning Engineers Inc www ashrae org Because of the potential for corrosion a condensing boiler must be constructed of special materials that resist the corrosive effects of the condensing flue gases This typically results in a higher first cost Finally condensing boilers must be vented with a corrosion resistant stack However since most of the heat has been removed from the combustion gases the stack for a condensing boiler is usually smaller than for a non condensing boiler In addition it can often be constructed out of PVC pipe Chilled Water VAV Systems 93 TRANE For more information on the design of hot water distribution systems refe
165. e design supply airflow for the space is 1000 cfm 0 47 m3 s and the air balancing contractor determines that the return airflow must be 800 cfm 0 38 m s in order for the space to be at the desired positive pressure relative to the adjacent spaces this offset is a negative 200 cfm 0 09 m3 s During operation the VAV terminal unit in the return duct is controlled to an airflow setpoint that is 200 cfm 0 09 m3 s lower than the measured airflow currently passing through the VAV terminal unit in the supply duct Safeties The unit level controller for a VAV terminal unit typically includes several safeties that protect the equipment from harm A common example is a low airflow limit to avoid operating electric heating coils at airflows below the manufacturer s recommendation for safe operation Specific details on safeties should be obtained from the equipment manufacturer Water chiller Each water chiller is equipped with a dedicated unit level controller that varies the cooling capacity of the chiller to supply chilled water at the desired temperature For a chiller using the vapor compression refrigeration cycle this is accomplished by varying the capacity of the compressor Scroll compressors generally cycle on and off Reciprocating compressors either cycle on and off or use cylinder unloaders Helical rotary or screw compressors typically use a slide valve or a similar unloading device Centrifugal compressors typically use inlet v
166. e fan pressure optimization control strategy is used see VAV fan control p 132 A third approach uses the communicating controllers mounted on the VAV terminal units to optimize the static pressure setpoint based on the position of the damper in the critical terminal unit that is the VAV terminal that requires the highest pressure at the inlet This strategy called fan pressure optimization is discussed further on p 200 Fan pressure optimization results in the greatest amount of fan energy savings at part load and allows the static pressure sensor to be located anywhere in the supply duct system This presents the opportunity to have it factory installed and tested at the fan outlet inside the air handling unit Airside economizer control An airside economizer is a common energy saving control strategy that uses outdoor air as a source of free cooling whenever possible When the outdoor air is cool enough the airhandling unit uses it to offset as much of the cooling load as possible When the dry bulb temperature of the outdoor air is colder than the current supply air temperature SAT setpoint 55 F 13 C for the example shown in Figure 126 the controller modulates the positions of the outdoor air and return air dampers so that the mixture of outdoor and return air provides supply air at the desired setpoint In this modulated economizer mode the outdoor air is cool enough to handle the entire l
167. e overall system annual energy usage by offsetting reheat recool energy losses through a reduction in outdoor air intake for the system Exception a1 For multiple zone VAV systems the minimum primary airflow setting typically must be greater than the zone ventilation requirement Voz to avoid excessively high zone outdoor air fractions Za which could significantly increase the required system intake airflow see Ventilation p 101 Therefore exception a1 does not usually apply to multiple zone VAV systems but it does apply to single zone VAV systems see Single Zone VAV p 157 and dedicated outdoor air systems see Dedicated outdoorair systems p 115 Exceptions a2 and a3 In many VAV systems the minimum airflow setting for each VAV terminal can be set equal to either 0 4 cfm ft2 2 L s m2 or 30 percent of the design supply airflow whichever is the largest In this case ASHRAE 90 1 permits the use of new energy for reheat Exception a4 For some smaller zones the 300 cfm 140 L s limit may allow for a minimum airflow setting that is higher than 0 4 cfm ft 2 L s m2 or 30 percent of the design supply airflow However this exception can only be used for a limited number of zones since the sum of the design airflows for the zones using this exception can represent no more than 10 percent of design system fan airflow Exception a5 Alternatively ASHRAE 90 1 allows the use of reheat at higher minimum
168. e pressure inside the mixing box where outdoor air mixes with recirculated return air becomes less negative and outdoor airflow will decrease Figure 86 p 114 The ventilation control loop ensures that the required quantity of outdoor air is brought into the system at all operating conditions Chilled Water VAV Systems 171 The modulation range of the supply fan is limited by how far the variable speed drive can be turned down typically 30 to 40 percent of design airflow Measuring fan airflow In a VAV system measurement of actual fan airflow can be very useful for troubleshooting and ensuring proper system operation A piezometer ring Figure 122 is a device that can be mounted within the inlet cone for many types of fans It measures the pressure drop from the inlet of the cone to the throat which is used to estimate airflow without obstructing airflow through the inlet of the fan Figure 122 Piezometer ring inlet tap mounted on face of inlet cone gt i piezometer ring mounted at throat of inlet cone 172 System Controls S Two common methods of controlling the outdoor air OA damper are 1 Varying the position of the OA damper in proportion to the change in supply airflow which is determined by the signal being sent to the variable speed drive on the supply fan 2 Using an airflow measuring device to directly measure and control the outdoor airflow Both of these methods were discussed
169. e quickly as the air pressurizes the plenum surrounding the fan and because the plenum provides an opportunity for some of the sound to be absorbed before the air discharges from the air handling unit Figure 24 p 33 includes an example comparison of discharge sound power for plenum fans versus housed fans 3 Adding a discharge plenum to a housed fan allows some of the sound to be absorbed before the air discharges from the air handling unit and it allows for the multiple duct connections which splits the sound before it leaves the unit Figure 104 If a plenum fan is used however multiple duct connections can be made to the fan module itself eliminating the need for a discharge plenum Figure 26 p 34 includes an example of the impact of multiple plenum connections on discharge sound power 4 Using a perforated interior casing surface in the discharge plenum or in the fan section for a plenum fan allows some of the sound to be absorbed before the air discharges from the air handling unit 5 Locating the supply fan upstream of the cooling coil blow thru configuration often lowers the discharge sound levels slightly although it may increase inlet sound levels see Table 7 p 37 If further attenuation is necessary consider the following changes to the supply airborne path 1 Locate main ducts and when possible branch ducts and VAV terminal units above corridors and other less sound sensitive areas such as restrooms or co
170. e return duct system return air path is easier to clean than an open ceiling return e More fan energy use due to higher pressure drop through the e Allows the system to be designed for space to space pressure return air path control which may be required in certain areas of healthcare e Often dictates the need to use a return fan rather than barometric facilities and in some laboratories relief dampers or a central relief fan e Often requires a taller ceiling plenum to allow space for ducting Chilled Water System The chilled water cooling coil located inside each VAV air handling unit is For more information on chilled water connected to one or more water chillers by the chilled water distribution systems refer to the Trane application manuals titled Chiller System Design and system Control SYS APM001 EN and Absorption Chiller System Design SYS AM 13 Types of water chillers Water chillers are used to cool water that is subsequently transported to the chilled water cooling coils by pumps and pipes The refrigeration cycle is a key differentiating characteristic between chiller types Vaporcompression water chillers use a compressor reciprocating scroll helical rotary or screw centrifugal to move refrigerant around the chiller The most common energy source to drive the compressor is an electric motor Absorption water chillers do not have a mechanical compressor but instead use heat to drive the refrigeration cycle St
171. e selected to serve a perimeter zone the design engineer must determine the heating load for that zone This determines whether the heating load can be satisfied by supplying warm air through overhead diffusers or if the load must be offset by a separate perimeter heating system e g baseboard radiant heat The following guidelines for heating a perimeter zone are based on the heat loss per unit length of perimeter wall Chilled Water VAV Systems 63 Primary System Components a ae If the design heat loss in a perimeter zone exceeds 450 Btu hr per linear foot 430 W m of perimeter wall an under the window or baseboard heating system is typically used Figure 59 With this much heat loss supplying a high quantity of warm air from overhead diffusers can cause downdrafts leading to occupant discomfort Figure 59 Perimeter heating in a zone with a high amount of heat loss heat loss gt 450 Btu hr ft 430 W m i diffuser cooling only VAV terminal unit baseboard heating system The cooling requirements of these perimeter zones are satisfied by a cooling only VAV terminal unit The supply air diffusers are located in the center of the room to distribute cool air throughout the zone During the heating mode the terminal unit provides a minimum airflow to the zone to meet ventilation requirements The baseboard heating system is separate but can be controlled by the same VAV unit controller Having only one controlle
172. e systems incorporate a timed override button on the zone temperature sensor which allows the occupant to temporarily switch the system into the occupied mode even though it is scheduled to be unoccupied After a fixed period of time three hours for example the system automatically returns the zone to the unoccupied mode In addition an occupancy sensor can be used to indicate that a zone is actually unoccupied even though it is scheduled to be occupied This unoccupied signal can be used to switch the zone to an occupied standby mode in which all or some of the lights can be shut off the temperature setpoints can be raised or lowered slightly the ventilation delivered to that zone can be reduced and the minimum airflow setting of the VAV terminal can be lowered When the occupancy sensor indicates that the zone is again occupied the zone is switched back to the normal occupied mode Chilled Water VAV Systems 5 For more information on the impact of load diversity in multiple zone systems refer to the Trane Air Conditioning Clinic titled Cooling and Heating Load Estimation TRG TRCO02 EN Overview of a Chilled Water VAV System a a eT Benefits of Chilled Water VAV Systems A comprehensive list of system benefits depends on which type of system is the basis of comparison The following section discusses some of the primary benefits of using a chilled water VAV system Provides multiple zones of comf
173. eam hot water or the burning of oil or natural gas are the most common energy sources for absorption chillers SYS APMO008 EN Chilled Water VAV Systems 79 80 Primary System Components a ae Air cooled versus water cooled Besides the refrigeration cycle and type of compressor the most distinctive difference is the type of condensing used air cooled versus water cooled Figure 71 Figure 71 Air cooled versus water cooled chiller air cooled chiller water cooled chiller A packaged air cooled chiller contains all the refrigeration components compressor air cooled condenser expansion device evaporator and interconnecting refrigerant piping wiring and controls The components are usually assembled and tested in the factory which can result in faster installation and improved system reliability Air cooled chillers are typically available in packaged chillers ranging from 7 5 to 500 tons 25 to 1 760 kW A packaged water cooled chiller also contains all the refrigeration components assembled and tested in the factory But a water cooled system requires the design and installation of a condenser water system piping pumps cooling tower and associated controls Packaged water cooled chillers are typically available from 10 to 3 800 tons 35 to 13 000 kW Air cooled chillers are popular because of their simplicity and convenience due to the elimination of the cooling tower and condenser water distribution system
174. econd the additional ductwork provides attenuation especially if duct liner is used Sound can also break out though the walls of the return ductwork so be sure to check the impact of the return duct breakout path for spaces nearest the equipment room If necessary the return duct breakout path can be attenuated by routing the duct over a less sound sensitive area using multiple smaller return ducts that enter on different sides of the equipment room or switching to round ductwork which has a much higher transmission loss than rectangular ductwork SYS APM008 EN Chilled Water VAV Systems 143 TRANE System Design Issues and Challenges e Supply airborne sound The acoustical analysis for the supply airborne sound path begins with the discharge sound data for the air handling unit Since the supply airflow is typically ducted the sound paths are duct breakout for spaces nearest the equipment room and airborne sound that follows the airflow path to the nearby spaces If necessary the supply airborne path can be attenuated by changing the configuration of the air handling unit to lower the discharge sound levels Potential strategies include 1 Selecting a fan with a different blade shape such as airfoil or AF ora fan wheel with a larger diameter may reduce discharge sound levels See Fan types p 32 2 Using a plenum fan can result in lower discharge sound levels than a housed fan because air velocity dissipates mor
175. ed for the supply and return sound paths while outdoor sound data is used for the wall roof transmission sound path Chilled Water VAV Systems 135 136 System Design Issues and Challenges Pi ae Outdoor sound data is also used to calculate the sound level for outdoor receiver locations such as at the lot line Receiver The receiver is simply the location where the sound will be heard and judged against some defined criteria This could be a private office a conference room an open office area a classroom a theater the neighboring lot line and so on Typically a building has several indoor receiver locations and possibly even a few outdoor locations if there is a concern about sound at neighboring properties Path The path is the route the sound travels from the source to the receiver Sound from a single source may follow more than one path to the receiver location Figure 100 For example sound from the supply fan follows the supply ductwork and enters the occupied space through the supply air diffuser Supply fan sound also travels through the walls of the supply duct breakout and then through the ceiling into the space Sound from the intake of the air handling unit makes its way to the space through the return ductwork and grilles or through the wall that separates the mechanical room from the occupied space Figure 100 Sound paths in a VAV air handling unit me return supply breakout return airb
176. eded at the system level intake Vot in order to make sure that the required quantity of outdoor air is delivered to each zone Voz Challenge of ventilating a multiple zone VAV system In a multiple zone recirculating system one supply fan delivers a mixture of outdoor air and recirculated return air to more than one zone This supply air contains a certain fraction percentage of outdoor air and all zones receive the same fraction That is if the supply air leaving the air handling unit contains 40 percent outdoor air some first pass outdoor air that enters through the intake and some unused outdoor air that returns from the zones the air delivered to each zone served by that supply fan will contain 40 percent outdoor air regardless of the overall airflow actually being delivered to the zone In the simple example shown in Figure 84 the outdoor airflow required in Zone 1 is 300 cfm 0 14 m3 s while the total supply airflow is 1000 cfm 0 47 m3 s So 30 percent of the air being supplied to Zone 1 must be outdoor air The critical zone that is the zone that requires the highest fraction of outdoor air Zp is Zone 3 It requires 40 percent of the air being supplied to be outdoor air Figure 84 Challenge of ventilating a multiple zone VAV system sjehidincn i ee lt ca vav Dx lt VAV oo some excess unused OA leaves building zone 1 zone 2 zone 3 We 1000 Ve 1200 Vz 1000 Va 300 Vz 400 V
177. ee i supply fan X VAV box A sa Q gt INTAKE gt MA flow measuring mixed air intake OA plenum filters coils damper Regardless of fan configuration direct control of building pressure requires a differential pressure sensor to monitor the indoor to outdoor pressure difference A common approach is to use an electronic transducer to convert the pressure difference into an electrical signal that is sent to the controller on the air handling unit Two sensing tubes one measuring indoor pressure and the other measuring outdoor pressure are attached to the transducer Proper location of these pressure sensing tubes is important e The indoor sensor is typically located on the ground floor because the effects of over or under pressurization are most noticeable at the external doors Many design engineers locate the sensor in a large open space near the door while others isolate the indoor sensor from the door in a central hallway for example to dampen the effect of rapid pressure changes caused by door operation In either location the indoor pressure sensor should include sufficient signal filtering to minimize the effects of high speed pressure changes It is also important to avoid perimeter locations that can be influenced by wind induced pressure fluctuations e Many design engineers place the outdoor sensor on the roof of the building Others use multiple sensors one at each corner of the building at least 15 f
178. ee p 200 can also be used to control the central relief fan In this case the BAS continually polls the individual relief dampers looking for the one with the most open damper The static pressure setpoint is then reset so that at least one of the relief dampers is nearly wide open The result is that the relief fan generates only enough negative pressure in the relief shaft to maintain building pressure on the critica floor furthest open relief damper Return fan capacity control In a system that includes a return fan this fan operates whenever the supply fan does and pulls return air from the occupied zones back to the return air plenum inside the airhandling unit Figure 132 The air in this plenum should be at a positive pressure relative to outdoors so that it either passes through the recirculating damper into the mixed air plenum or exits the building through the relief damper Historically system design engineers have used several approaches for controlling the capacity of a return fan in a VAV system including e Return air plenum pressure control uses a pressure sensor to measure the static pressure inside the return air plenum in the air handling unit and modulates the capacity of the return fan to maintain Chilled Water VAV Systems 181 FRAME 182 System Controls aT this pressure at a desired setpoint Figure 132 This approach is recommended by ASHRAE Guideline 16 Selecting Outdoor Return
179. ee the sidebar on p 104 SYS APM008 EN System Design Variations a ae conventional VAV systems such as linear slot diffusers Figure 65 p 74 are used For spaces with tall ceilings it may be desirable to distribute the air from the side walls and allow for temperature stratification within the zone Consider the need for heating A single zone VAV system has no VAV terminal units to be equipped with heating coils Therefore in most cases the air handling unit must be capable of providing heat because the zones that these systems typically serve gymnasiums arenas auditoriums etc often do not include baseboard radiant heat The modulation range of the supply fan is limited by how far the variable speed drive can be turned down typically 30 to 40 percent of design airflow In some applications the sensible cooling load in the zone may decrease to the point where it is less than the cooling capacity of the unit at the minimum airflow of the supply fan Under this condition the zone will be at risk of being overcooled To prevent overcooling at low cooling loads consider one of the following strategies e Reset the supply air temperature Reset the supply air temperature upward at low load conditions after the supply fan reaches its minimum airflow This avoids overcooling the zone but the cooling coil also removes less moisture so the humidity level in the zone increases See Supply air temperature reset p
180. eeeeeeeeeeeeees 181 Salelesi ee ae Ea EA Ea EE 182 SYS APM008 EN Chilled Water VAV Systems vV VAV terminal UMTS seieren iinne EARNE EE E 183 Zone temperature Control oo cecccccceeeeeseeececeeeeeceeeeeeeeeeeeeeees 183 Ventilation COntroOl oo cececcccccccccccceeceeeceeeceeeeeesseaeaeauanaaanenes 184 Space pressure CONTIOL ooo cece cceecestececeeceeeeeeceeennestseaes 187 SEGS na E E Ea 188 Water CHIET csaccciazensimentnaasigatenseabuhiniaasenstmaraveRheatume nidiir inanis 188 Condensing pressure Control ooo cccccceeeenssesteeeeeeeeees 188 Hotwater DOST sevcsnecncvedativasisseadsdsandeconmacniine lehbendades erinin is 190 Return water temperature control ssssssssssisisriiririrerrrnnee 190 System Level Control oo ccccccceessssesaeeececeeecceeestesaeeeeeeeeeees 190 Coordination during different operating modes s 191 OCCUPIER MOUS srera a rE EAEEREN REEE 191 Occupied standby mode ou ecceccececcceeessteeeeeeessteeeeeeseaaees 192 Unoccupied MOAS siseses E E 193 Morning warm up or cool down mode ss ssssisisisirieceeen 194 SCHE CUMIN Genese E A EA 196 Chilled water plant cccccccccccseseccccsssseeeeecsseeeesessseeesesensaeess 197 H twater plant 2 ss 4 cacsnnceedessonaaattinsant died eA E ANTER EA 197 System OPTIMIZATION s ssissesessiniiiiirrisreenserriiriiirrnreenrerrrrrrn 198 Optimal Stani sesadean aE Ea E 198 Op mal Siop siseeissnanias iieiaei aiei anaa 199 Unoccupied economizing
181. eeeeeeeneed 91 TGTIMaStONa GC nean E N 92 Hot Water SyStemM ccccccceccccccceesseececeeeseeeeeeesseeeeeessseeeeessseeesenesaas 92 Types of hot water boilers ccccccccceeceeeeeeceeeeceeceeeeeeeeeeeteeeees 92 Hot water distribution 2 2 2 2 cece eect cece eee eeeeeceeeeeceeeeeeeeeeeeeeeeeeeed 94 Design temperatures and flow rates ou cececceeeeeseceeeeeeeees 94 Control valve selection oo ccccccccceccsteeeeeeceseeeeeeesseeeeeeeeeees 95 Variable versus constant flow PUMPING cee eects 97 CONTOS onnee A A EA A A O AAE 98 System Design Issues and Challenges 0 0 0 000000 99 Thermal ZONING oo cecccccccecececceeceeceeeeeeeeeeeeeeceeceeaeeeeeeeeeeeeeeesteeseeeaees 99 Optimizing the number Of ZONES cceceeeceetceceeessteeeeeetsaeeeeeeenaes 99 Locating the ZONE Sensor ccceeeeeeeeeeeeceeeeeeeeesteeeeeeeeeeeeteeeenes 100 Using wireless technology c cccccccccceeeceeceeceeeeeeeeeeteeeetecteneeea 101 NENTIATION seers tsnscestmtsnsnucenenhednesinascabodumstnddgeantasasrtbiniansstiteneneernianet tas 101 Zone level ventilation require MENtS ccccceceeeeeeeeeeeeeeeeeeeeeees 101 Minimum ventilation rate required in breathing zone Vbz 102 Impact of zone airdistribution effectiveness 102 System level ventilation requirement ccccccceeeeeeeeeetteeeeeeeees 104 Challenge of ventilating a multiple zone VAV system 104 Calculating system intake airflow Vot ccccccc
182. ejected by the condenser of a water cooled chiller can be distributed to VAV terminals throughout the building Condenser heat recovery is particularly well suited for supply air tempering applications because it provides the relatively small amount of heat needed for tempering and allows the primary heating equipment boilers for example to be turned off during the summer Any water cooled chiller can be used to provide sensible heat for supply air tempering see Condenser heat recovery p 88 Methods for improving dehumidification performance As mentioned previously VAV systems can provide effective dehumidification over a wide range of load conditions for most applications Avoiding supply air temperature reset during humid weather and providing heat for tempering at low load conditions are generally sufficient for most comfort cooling applications Chilled Water VAV Systems 121 System Design Issues and Challenges Pi eT If necessary the basic design of the VAV system can be altered to enhance dehumidification performance e Investigate the practicality of delivering colder supply air Lowering the dry bulb temperature of the air leaving the cooling coil causes more moisture to condense out of the supply air In a VAV system this colder drier supply air results in a drier zone lower humidity at all load conditions Another benefit of a cold air VAV system is reduced supply airflow which can allow for the use of sm
183. eliminate the need for a boiler and hot water or steam distribution system in the building In addition gas fired burners do not require freeze protection and may cost less to operate than an electric heater Direct fired burners locate the flame directly in the air stream while indirect fired burners separate the combustion process from the air stream through the use of a heat exchanger Figure 16 Direct fired burners are simpler because no heat exchanger is needed and more efficient because there are no heat transfer losses associated with the heat exchanger than indirect fired burners However direct fired burners introduce the products of combustion into the air stream This requires careful control of the combustion process but is generally considered safe in most commercial and industrial applications However many building codes and industry standards prevent the use of direct fired burners for any parts of a building that contain sleeping quarters e Location within the air handling unit Indirect fired burners should be located downstream of the supply fan Figure 17 In this location the pressure inside the casing of the air handling unit is greater than the pressure outside This positive pressure difference reduces the likelihood that combustion gases will be drawn into the supply air stream and therefore into the occupied spaces Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN Primary System C
184. eliver 5 990 cfm 2 82 m3 s This is often called the block airflow This zone by zone load variation throughout the day called load diversity is the reason that VAV systems can deliver less air 18 percent less in this example and use smaller supply fans than multiple zone constant volume systems Similarly the peak cooling coil loads do not necessarily occur at the same time for all VAV air handling units served by a central chilled water plant This system by system load variation block load is the reason that the chilled water plant serving VAV air handling units can be sized for less total capacity than if the building is served by multiple rooftop VAV systems Table 1 Sum of peaks versus block airflow 8 00 a m 4 00 p m space sensible supply airflow space sensible supply airflow cooling load cfm m3 s cooling load cfm m3 s Btu hr W Btu hr W Room 101 44 300 2 040 74 600 3 440 faces West 13 000 0 96 21 900 1 62 Room 102 62 400 2 880 55 300 2 550 faces East 18 300 1 36 16 200 1 20 Block airflow 4 920 5 990 2 32 2 82 Sum of peaks 3 440 2 880 6 320 cfm airflow 1 62 1 36 2 98 m3 s Opportunity to save energy The part load energy savings inherent with a VAV system is twofold First reducing the amount of air delivered at part load creates an opportunity to reduce the fan energy required to move this air The magn
185. elocity through a media filter results in a higher static pressure drop One common method of reducing airside pressure drop is to angle the Figure 36 Angled filters increase filters within the air handling unit Figure 36 This decreases air velocity surface area and lower airside h h th di di h f fth di hich pressure drop through the media and increases the surface area of the media whic increases its dirt holding capacity e Dirt holding capacity Dirt holding capacity is an indication of how much dirt the filter will hold at the dirty or final pressure drop This indicates how often the filter will need to be replaced In general a filter with more media surface area will hold more dirt and will need to be replaced less frequently This varies with the brand of filter e Available space In general filters with higher efficiencies lower airside pressure drop and or greater dirt holding capacity require more space than filters that perform more poorly in these categories This can impact the physical size of the air handling unit e Available budget Filters with more media surface area generally cost more than filters with less surface area This impacts both the installed cost and maintenance replacement cost of the filter system In a typical VAV system the mixture of outdoor air and recirculated return air passes through a particulate filter to remove airborne particulate contaminants Locating these filters upstre
186. emaining at its minimum airflow setting The temperature range between the cooling and heating setpoints is called the deadband Figure 3 Chilled Water VAV Systems 3 Overview of a Chilled Water VAV System Figure 3 Occupied zone temperature setpoints occupied cooling setpoint occupied heating setpoint zone dry bulb temperature a oO o a o o a The air handling unit is controlled to maintain a constant supply air temperature by either modulating the chilled water control valve using the airside economizer or modulating the heating control valve The supply fan modulates to maintain a constant pressure in the supply ductwork and the outdoor air damper brings in at least the minimum required amount of outdoor air for ventilation Zone is occupied and requires heating In many chilled water VAV systems zones that require heating include a heating coil hot water or electric in the VAV terminal unit Alternatively some systems use baseboard radiant heat located along the perimeter walls within the zone When the temperature in the zone drops below the heating setpoint the controller on the VAV terminal unit activates the heating coil warming the air supplied to the zone If baseboard radiant heat is used instead it is activated to add heat directly to the zone The air handling unit is controlled to maintain a constant supply air temperature by either modulating the chilled water control valve using the
187. ence hypersensitivity and allergic reactions respiratory disorders or infectious diseases Many of the larger biological particles such as fungal and bacterial spores are 3 micron to larger than 10 micron in size which makes them easily captured with medium efficiency particle filters see Figure 34 p 40 Other biological particles such as viruses are sub micron in size and cannot be practically removed by particle filtration Given sufficient dose time and intensity C band ultraviolet light UV C can inactivate kill microorganisms In applications where sufficient residence times exist such as on stationary surfaces it is possible and often practical to deliver a killing dose of UV energy to the organism In this application the UV C lamps are sized and positioned inside the air handling unit to irradiate interior surfaces and components typically the cooling coil and condensate drain pan to limit fungal and bacterial growth on those stationary surfaces In addition to controlling microbial growth on surfaces UV C lights are sometimes applied to inactivate microbes in moving air streams referred to as airborne or on the fly kill The high air velocity through most air handling units or air distribution systems requires the use of much higher intensity lamps to deliver the required dose of UV energy to the moving microbes Figure 40 This approach has been effectively applied in some health care applications
188. endent control a controller and possibly a heating coil small terminal fan or filter Modulating airflow to the zone is typically accomplished by using a rotating blade damper that changes airflow resistance by rotating the damper and adjusting the size of the air passage to the zone Chilled Water VAV Systems SYS APM008 EN Figure 46 Cooling only VAV terminal unit primary air SYS APM008 EN Primary System Components a a Types of VAV terminal units This section describes the various types of VAV terminal units used in VAV systems Figure 45 See Typical combinations used in chilled water VAV systems p 66 for further discussion on the typical applications for these different terminal units Figure 45 Types of VAV terminal units parallel fan powered VAV reheat series fan powered Cooling only VAV terminal units The cooling only VAV terminal unit Figure 46 consists of an airflow modulation device with a flow sensor and controls packaged inside a sheet metal enclosure Primary airflow to the zone is reduced as the cooling load decreases Responding to the zone sensor primary airflow is modulated between maximum and minimum settings Figure 47 The maximum setting is determined by the design cooling load of the zone The minimum setting is normally determined to ensure that the zone is properly ventilated or to meet requirements for proper operation of the terminal unit or supply air diffus
189. ents Paths are defined by the end points the source location and the receiver location There may be many receiver locations depending on the installation but the number can be reduced by determining the critical receiver locations In general sound diminishes with distance so the spaces closest to the sound sources are typically the loudest If spaces distant from the unit have sound targets that are considerably below the level required in the space Chilled Water VAV Systems 137 For more information on the Trane Acoustics Program TAP acoustical analysis software visit www trane com 138 System Design Issues and Challenges i eT closest to the source these spaces should also be analyzed Common examples include conference rooms executive offices and classrooms After the critical receiver locations are defined the sound paths from the source to each receiver can be identified Step 3 Perform a path by path analysis Once each path has been identified individual elements are analyzed for their contribution For example the supply airborne path includes various duct elements elbows straight duct junctions diffusers and so on and a room correction factor Algorithms available from ASHRAE can calculate the acoustical effect of each duct element The effect of changing an element such as removing the lining from a section of ductwork can be calculated Software tools make these algorithms easier to use
190. eo 400 Chilled Water VAV Systems SYS APM008 EN For more information on ASHRAE Standard 62 1 and its procedures for calculating zone level and system level outdoor airflow requirements in a VAV system refer to the Trane Engineers Newsletter Live broadcast DVD titled ASHRAE Standard 62 1 Ventilation Requirements APP CMC023 EN or the following ASHRAE Journal articles 1 Stanke D Addendum 62n Single Path Multiple Zone System Design ASHRAE Journal 47 January 2005 28 35 Available at www ashrae org or www trane com 2 Stanke D Standard 62 1 2004 Designing Dual Path Multiple Zone Systems ASHRAE Journal 47 May 2005 20 30 Available at www ashrae org or www trane com SYS APM008 EN System Design Issues and Challenges Pi eT As a result in order to properly ventilate the critical zone Zone 3 in this example the system must over ventilate all other zones Therefore the air that returns from the non critical zones Zones 1 and 2 contains some unused or excess outdoor air Most of this unused outdoor air recirculates back through the system which allows the air handling unit to bring in less than 40 percent outdoor air through the intake But some of the unused outdoor air leaves the building in the exhaust air stream In other words some outdoor air enters the system passes through non critical zones and then leaves the building without being fully used to dilute indoo
191. epends on the device Typically one of the following two approaches is used 1 reduce the heat transfer capacity of the energy recovery device which results in a warmer exhaust side surface temperature or 2 preheat either the outdoor or exhaust air before it enters the device which also raises the surface temperature of the device to prevent frosting 164 Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN System Design Variations i For most applications and climates reducing capacity by modulating an OA bypass damper for example is sufficient for frost prevention And if the VAV air handling unit is equipped with an airside economizer this bypass damper is probably already included see Figure 114 However for applications with extremely cold outdoor air and higher indoor humidity levels during cold weather preheat may be desirable e Decide what amount of cross leakage is acceptable Many types of air to air energy recovery devices permit some degree of cross leakage Through fan configuration and properly adjusted seals the amount of leakage is usually less than 5 percent even for wheels in most applications Cross leakage between the exhaust and supply air streams is seldom problematic in VAV systems since most of the air that returns RA from the spaces is recirculated RRA to the spaces as supply air anyway Figure 113 p 161 Dual Duct VAV Systems Dual duct VAV systems are intended for buildings
192. er of these two dedicated OA system configurations At heating design conditions Table 23 p 112 the required intake airflow for the dedicated OA system would be 3350 cfm 1 6 m3 s compared to 3220 cfm 1 5 m s for the multiple zone VAV system This difference occurs because a multiple zone recirculating system recirculates most of the unused outdoor air that bypasses the breathing zone when Ez lt 1 0 In the third configuration the dedicated OA unit delivers outdoor air to one or more multiple zone recirculating systems In this case intake at the dedicated OA unit is the sum of the system level intake airflows Vot required for each VAV air handling unit so it does not result in an overall reduction in outdoor airflow Chilled Water VAV Systems SYS APM008 EN System Design Issues and Challenges Pi a eT For more discussion of using a dedicated OA system with VAV see Methods for improving dehumidification performance p 121 Dynamic reset of intake airflow Section 6 2 7 of ASHRAE 62 1 explicitly permits dynamic reset of intake outdoor airflow Vot as operating conditions change as long as the system provides at least the required breathing zone outdoor airflow Vbz whenever a zone is occupied The standard specifically mentions the following reset control strategies e Resetting intake airflow in response to variations in zone population As the number of people occupying a zone varies the quantity
193. er to the Trane Engineers bring in cool but humid outdoor air which may actually increase mechanical Newsletter Live broadcast DVD titled cooling energy Figure 127 HVAC Systems and Airside Economizers APP CMC026 EN and the Trane Engineers Newsletter titled Fixed enthalpy control uses sensors to measure both the dry bulb Airside Economizers ADM APNO20 temperature and humidity of the outdoor air The controller then calculates EN the enthalpy of the outdoor air and compares it to a predetermined high limit shutoff setting The economizer is disabled whenever the outdoor air enthalpy is above this limit Figure 128 Figure 128 Fixed enthalpy control of the airside economizer return air temperature z 3 Ay a 2 4 lt 7 ry o bd faa a Ei high limit i a shutoff x 3 aan an 1 5 minimum a a 5 gt OA intake tala s we S 3 ae 2 e lt 5 aa w possible increased mechanical cooling Sa energy dry bulb temperature F Fixed enthalpy control requires an added outdoor humidity sensor so it costs more than fixed dry bulb control But for a VAV system in most climates 176 Chilled Water VAV Systems SYS APM008 EN System Controls aT fixed enthalpy control saves more cooling energy so it might be worth the added first cost However in hot and dry climates bringing in 100 percent outdoor air can actually increase mechanical cooling energy even if the outdo
194. ering the outdoor air to the breathing zone Vou Dx x Rp x P2 Ra x Az Most of these values were determined for the zone level ventilation calculations see Table 18 As part of this step the designer can take credit for population diversity within the system D since buildings are seldom occupied with peak population in all zones simultaneously For example if all Chilled Water VAV Systems 107 108 System Design Issues and Challenges Pi eT of the work spaces in an office building are fully occupied the conference rooms will be sparsely occupied Mathematically occupant diversity D is the ratio of the expected peak system population Ps to the sum of the peak populations in the individual zones P2 D Ps Pz In this same example the sum of the peak zone populations Pz is 224 Table 18 Assuming the maximum expected system population Ps is only 164 people the occupant diversity factor D is 0 73 164 224 The architect or owner usually prepares a program indicating the building s expected occupancy and use This program may be a good source for information related to expected system population Note that this occupant diversity value is multiplied by only the people related component of the ventilation requirement Rp x Pz not the building related component Ra x Az For this example the uncorrected outdoor air intake is 2800 cfm 1 3 m3 s Vou 0 73 x 1120 cfm 1980 cfm 2800 cfm
195. ers Most cooling only units are used for zones that require year round cooling like the interior zones of a building When cooling only units are applied to zones that do have a need for heat the heat is typically provided by a remote source such as baseboard radiant heat located along the perimeter wall in the zone When the zone requires heating primary airflow is at the minimum setting and the remote heat source is activated Figure 47 Many VAV unit controllers provide an output signal to control this remote source of heat Chilled Water VAV Systems 55 TRANE Primary System Components Figure 47 Control of a cooling only VAV terminal unit maximum 100 remote source of heat activated T primary 4 gt airflow 8 oO P 2 3 i minimum speseeaeeneenneneesnennsnneneenneneenmeneenmesmenmesmenmenmenmennes i sansa adnensensensensensensensensensensensenes primary x airflow 0 i design space load design heating load cooling load VAV reheat terminal units Figure 48 VAV reheat terminal unit The VAV reheat terminal unit Figure 48 also contains an airflow modulation device flow sensor and controls but it also contains an electric or hot water primary air heating coil In the cooling mode the unit is controlled in the same manner as the cooling only unit Primary airflow is reduced as the cooling load in the space decreases Figure 49 When primary airflow reaches the minimum setting for the unit and the c
196. ers or operators define only one or a few time of day schedule to operate the entire building However if areas of the building have significantly different usage patterns this approach wastes energy since the entire building may be operating to maintain occupied temperature setpoints even though only part of the building is in use A more energy efficient approach is to create separate time of day operating schedules for areas of the building with significantly different usage patterns If the facility already has a BAS it probably includes a time of day scheduling function so the only additional cost is the operator s time to set up the schedules To reduce the number of schedules to be created and maintained group zones with similar usage patterns together and create one schedule for each group Chilled water plant Control of the chillers pumps and cooling towers in a water cooled system must be coordinated to provide chilled water to the VAV air handling units when needed The primary issues to address thorough chilled water plant control include e When should the chilled water plant be enabled or disabled e Ina plant with multiple chillers when a chiller must be turned on or off which chiller should it be e If an attempt to turn ona chiller pump or cooling tower fails or if there is a malfunction what should be done next e How can the energy cost of operating the overall chilled water system be minimized
197. es benefit from constant airflow many zones can accommodate variable airflow Controlling the dual duct VAV terminal to deliver variable airflow to the zone Figure 118 and Figure 119 results in less fan energy used However as discharge airflow Vaz to the zone decreases the zone outdoorair fraction Za increases which can result in a high system intake flow Vot In some cases this benefit of additional fan energy savings may be accompanied by an increase in the energy required to condition this additional outdoor air brought in at the air handling unit Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Controls This chapter discusses the control of a chilled water VAV system Unit level control refers to the functions required to control and protect each individual piece of equipment System level control refers to the intelligent coordination of the individual pieces of equipment so they operate together as a reliable efficient system Unit Level Controls Unit level control for a piece of HVAC equipment typically involves the use of several control loops to employ specific functions plus various safeties to protect the equipment In addition alarms and diagnostic messages assist the building operator or service personnel in responding to or preventing problems with the equipment While this section identifies many of the unit level control functions for the primary components of a chilled water VAV sys
198. essive scheduling and equip zone temperature sensors with timed override buttons If a person wants to use a space during a time when it has been scheduled as unoccupied they simply press the timed override button see Figure 138 p 194 and the BAS switches that zone into the occupied mode This returns the temperature to the occupied setpoint and delivers ventilation air to that zone Typically the BAS automatically returns this Chilled Water VAV Systems SYS APM008 EN FRAME For more information on the control of the chilled water plant refer to the Trane application manual titled Chiller System Design and Control SYS APM001 EN For more information on the control of the hot water plant refer to the 2008 ASHRAE Handbook HVAC Systems and Equipment Chapter 12 and The Boiler Book by Cleaver Brooks www boilerspec com SYS APM008 EN System Controls aT zone to the unoccupied mode after a defined fixed period of time two or three hours for example Using the timed override feature provides the opportunity to be more aggressive with time of day operating schedules Avoid wasting energy by starting and stopping the HVAC system based on typical usage not worst case or once a year use Once occupants are educated about using the timed override feature energy savings and minimal complaints can coexist e Use separate time of day schedules for areas with differing usage patterns For simplicity many building manag
199. faces including cooling coils Note ASHRAE 62 1 Section 6 2 1 also requires a MERV 6 filter if the building is located in an area of the country that exceeds the U S EPA limit for airborne particles with a diameter of 10 microns or less PM10 In general this requirement can be met with most standard throwaway or pleated filters see Table 8 However when a higher level of filtration is desired bag or cartridge filters are sometimes used Figure 35 p 41 Bag or cartridge filters typically cost more than pleated filters which may make them a good candidate for using a pre filter to extend their useful life In order to maintain the desired level of cleanliness and minimize system energy use never operate the air handling unit without the filters in place especially during construction of the building Filters used during construction should be replaced prior to building occupancy It is important to maintain and replace filters as recommended by the manufacturer Typically filters are replaced based on the length of time in operation or on the measured air pressure drop across the filter A differential pressure sensor can be used to indicate when the filters are dirty the pressure drop exceeds a pre determined limit and can send an alarm to the building automation system to indicate that they need to be replaced Filter manufacturers typically publish a dirty or final pressure drop However this is sometimes overr
200. ficiency Evz That is 0 65 for the north offices in this example The final step is to calculate the required outdoor air intake at the air handling unit Vot This is determined by dividing the uncorrected outdoor air intake Vou by system ventilation efficiency Ev Vot Vou Ev For this example the uncorrected outdoor air intake is 2800 cfm 1 3 m3 s with a system ventilation efficiency of 0 65 resulting in a required outdoor air intake flow of 4310 cfm 2 0 m s In this example using Appendix A resulted in an intake airflow that is the same as if determined using Table 6 3 The default values in that table are based on an assumed average outdoor air fraction Xs of 0 15 For systems with a higher average fraction Appendix A results in a higher system ventilation efficiency and a lower intake airflow Heating versus cooling design As mentioned earlier supplying warm air to the zone may result in a zone air distribution effectiveness Ez that is less than one How does this impact Chilled Water VAV Systems 111 FRAME System Design Issues and Challenges Pi eT system intake airflow Table 22 and Table 23 include the calculations for this same example system at the heating design condition with several zones having a zone air distribution effectiveness of 0 8 Table 22 Zone level ventilation calculations for example office building heating design Rp cfm p x Pz qty
201. flow at the air handling unit An addendum ak to ASHRAE Standard 90 1 2007 requires pump pressure optimization if differential pressure control is used to control a variable speed pump and DDC controls are used 208 System Controls Figure 148 Ventilation optimization Ventilation reset at the system level a VAV air handling unit with a flow measuring OA OA damper e Reset intake airflow V OCC DDC VAV terminals e Required outdoor airflow V e Actual primary airflow V e OA fraction Z communicating BAS e New OA setpoint V e Max OA fraction Besides being a cost effective reliable and energy efficient approach to ventilating a VAV system combining zone level DCV with system level ventilation reset can ensure that each zone is properly ventilated without requiring a CO2 sensor in every zone CO2 sensors are used only in those zones where they will bring the most benefit This minimizes installed cost avoids the periodic calibration and cleaning required to ensure proper sensor operation and minimizes risk see sidebar For the other zones occupancy sensors and or time of day schedules are used to reduce ventilation Pump pressure optimization When variable flow pumping is used in either the chilled water or hot water distribution system some method is needed to control the pump capacity Similar to the supply fan in a VAV system the
202. flow delivered through the diffusers Voz Vbz 0 8 during the heating mode to compensate for the zone air distribution effectiveness Ez 0 8 of using ceiling mounted return air grilles e Locate the return air grilles in the floor or at the base of a side wall Ez 1 0 during both cooling and heating modes e Design the system so that the supply air temperature Tsa during the heating mode is less than 15 F 8 C above the zone temperature Tzone and select the supply air diffusers to achieve a velocity of 150 fpm 0 8 m s within 4 5 ft 1 4 m of the floor With this design a zone air distribution effectiveness Ez of 1 0 can be achieved even with overhead supply of warm air and overhead return See VAV reheat terminal units p 56 e Use baseboard radiant heat as the source of heat within the zone Since the supply air is not used for heating the supply air temperature Tsa SYS APMO008 EN Chilled Water VAV Systems 103 Calculating system level intake Vot for a single zone VAV system For a single zone VAV system see p 157 in which one air handling unit delivers a mixture of outdoor air and recirculated air to only one zone ASHRAE 62 1 prescribes that the system level intake Vot needs to equal the calculated zone outdoor airflow Voz Vot Voz There is no need to account for system ventilation efficiency Ev when designing a single zone system Note For more information on calculating
203. formance of a housed fan Finally systems that use the blow thru configuration have often experienced problems with final filters getting wet Experience indicates that this problem can often be minimized by using a draw thru supply fan or by adding a few degrees of heat to the air before it passes through the final filters Table 7 Blow thru versus draw thru configuration Blow thru Advantages Disadvantages e Heat generated by the fan and motor is added to the air upstream Often results in a longer AHU or the use of a diffuser section to of the cooling coil allowing for a warmer leaving coil temperature develop an acceptable velocity profile for air passing through the to achieve a desired supply air temperature cooling coil Figure 29 e Locating the fan upstream of the cooling coil often lowers the e If final filters are used this configuration often results in problems discharge sound levels slightly but also raises inlet sound levels with final filters getting wet Figure 38 p 45 e Greater concern with air leaking out of the AHU since more of the casing is pressurized See Air leakage p 51 Draw thru Advantages Disadvantages e Typically results in a shorter AHU since less distance is needed e Heat generated by fan and motor is added to the air downstream of between the upstream cooling coil and the fan Figure 29 the cooling coil requiring a colder leaving coil temperature to e Colde
204. g through the intake of an outdoor air handling unit is difficult so the unit should be designed to minimize the impact of the snow on downstream components e g wetting the filters Consider including a drain pan in the intake module or mixing box to allow melted snow to drain away If a preheat coil is used to help melt snow configure the unit with the filters located downstream of the preheat coil to keep them from getting wet Installing a separate throwaway filter upstream of the preheat coil helps keep the coil clean and allows that filter to be removed during winter to avoid it getting wet from snow 50 Primary System Components E PCO technology can be applied directly to the air stream since it has a very low airside pressure drop and the reaction takes place at ambient temperatures Figure 41 The primary drawback is high installed cost and the operating costs associated with cleaning replacing and disposing of the UV lamps Figure 41 Photocatalytic oxidation PCO Water management Preventing moisture problems in buildings is a shared responsibility among all parties involved in the design construction maintenance and use of the building As for preventing water related problems within the air handling unit itself follow these basic practices e Prevent rain from entering through outdoor air intakes or from leaking through the equipment casing or roof curb An outdoorair intake sized to avoid too hig
205. g proposed building There is a real potential to save energy in chilled water VAV systems through optimized system design and control strategies This energy savings reduces Chilled Water VAV Systems SYS APM008 EN TRANE For more information on acoustical analysis and the topic of HVAC acoustics in general refer to the Trane application manual titled Acoustics in Air Conditioning ISS APM001 EN SYS APM008 EN System Design Issues and Challenges Pi eT operating costs for the building owner and can help in achieving points toward LEED certification Acoustics HVAC equipment creates sound and in a well designed application that sound provides a positive effect on occupant comfort That is it provides an appropriate level of background sound for speech isolation or permits clear communication in a classroom However it is also possible for the sound from HVAC equipment to be considered noise because it disrupts the intended function of the building Sound from HVAC equipment impacts the sound levels in the building but a larger role is played by how the equipment is applied One common approach to addressing HVAC acoustics is to use a fixed set of design practices on every job With sufficient experience this may be all a design engineer needs to create an installation that is free of sound problems However this may also unnecessarily inflate the installed cost of some projects and may not provide sufficient attenuati
206. ge of air through the drain line condenser The component of the refrigeration system where refrigerant vapor is converted to liquid as it rejects heat to water or air Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary E condensing boiler A type of boiler that uses a high efficiency heat exchanger designed to capture nearly all of the available sensible heat from the fuel as well as some of the latent heat of vaporization The result is a significant improvement in boiler efficiency condensing pressure Pressure of the refrigerant vapor when it condenses into a liquid constant volume system A type of air conditioning system that varies the temperature of a constant volume of air supplied to meet the changing load conditions of the zone controller The component of a control loop that compares the measured condition of the controlled variable to the desired condition setpoint and transmits a corrective output signal to the controlled device cool down mode See morning cool down mode cooling only terminal unit The simplest type of single duct VAV terminal unit It has the capability of varying the airflow but has no method to provide heating for the zone COP A dimensionless ratio of the rate of heat removal to the rate of energy input in consistent units for a complete refrigerating system or some specific portion of that system under designated operating conditions A higher COP designates a hig
207. guration the RA lower cooling coil need only be sized for the 9 500 cfm 4 5 m s of recirculated air For this path a size 21 AHU casing results in a coil face velocity of 456 fpm 2 3 m s The OA upper coil which is sized for the 3 500 cfm 1 6 m s of outdoor air requires a size 8 AHU casing which results in a coil face velocity of 438 fpm 2 2 m s The overall footprint of the dual path unit size 8 casing stacked on top of a size 21 casing is smaller than that of a dual path unit size 30 casing although the dual path unit is taller Figure 8 and Table 3 SYS APM008 EN TRANE Primary System Components Figure 8 Example footprint reduction from using a dual path air handling unit see Table 3 A Dual path AHU nee Size 21 on bottom Size 8 on top 7 8 ft 2 4 m lt lt 11 4 ft 3 5 m p Single path AHU Size 30 q l 11 7 ft 3 6 m p y Table 3 Impact of dual path configuration on AHU footprint and weight Single path AHU Dual path AHU Size 30 bottom Size 21 top Size 8 AHU footprint ft m 11 7 x 7 8 11 4 x 6 7 3 6 x 2 4 3 5 x 2 0 AHU height ft m 5 1 1 6 7 5 2 3 m AHU weight Ibs kg 2800 1270 2800 1270 e Cold air distribution By reducing the supply air temperature less supply airflow is required to offset the sensible cooling loads in the zones Reducing supply airflow can allow for the selection of smaller air handling units whi
208. gy may increase during reheat mode but will likely decrease Chilled Water VAV Systems SYS APM008 EN TRANE For more information on incorporating a waterside economizer into a chilled water system refer to the Trane application manual titled Chiller System Design and Control SYS APM001 EN SYS APM008 EN Primary System Components E a aT during heating mode The overall impact on annual pumping energy use depends on climate and building operating characteristics Waterside economizer In a chilled water system there are several ways to accomplish free cooling through the use of a waterside economizer The three most common approaches are to 1 use a plate and frame heat exchanger in the condenser water system 2 allow for refrigerant migration inside a centrifugal chiller or 3 use cool water from a well river or lake In the case of the plate and frame heat exchanger it is used to keep the chilled water distribution system separate from the condenser water system When the outdoor wet bulb temperature is low enough the cooling tower cools the condenser water to a temperature that is colder than the water returning to the chiller s In the sidestream configuration shown in Figure 78 this plate and frame heat exchanger is able to pre cool the warm water returning from the cooling coils reducing chiller energy use Figure 78 Example waterside economizer using a plate and frame heat exchanger
209. gy use Any cooling energy saved is offset somewhat by the increased fan energy use as the evaporative media increases the airside pressure drop that the supply fan must overcome Indirect evaporative cooling typically uses an evaporative cooling tower to cool water and then pumps this water through a conventional cooling coil to cool the air This approach does not involve the evaporation of water into the air stream so it does not increase the dew point of the air Figure 15 The evaporation process occurs outside the building in the cooling tower In some applications indirect evaporative cooling is implemented using a stand alone cooling tower or similar device and a separate coil located upstream of the conventional cooling coil However in a chilled water VAV system because a water distribution system is already part of the system a more common approach is to add a plate and frame heat exchanger to the chilled water system allowing cool condenser water from the cooling tower Chilled Water VAV Systems SYS APM008 EN Primary System Components a aT to cool the chilled water see Figure 78 p 91 This configuration is often called a waterside economizer Heat source inside the VAV air handling unit Heating in a VAV system can be accomplished in several ways While many systems include heating coils hot water or electric in the VAV terminal units or baseboard radiant heat installed within the zone some systems also
210. h of air velocity and a hood mounted over the intake both help to prevent rain from being drawn into the ductwork or air handling unit A mist eliminator just inside the intake opening can be used to remove water droplets that may be carried along with the intake air For an outdoor air handling unit the casing should prevent water from leaking in by including water tight seams gasketed doors and a sloped roof to prevent water from puddling Puddling standing water can lead to rust and eventually water leaking into the unit e Use nonporous cleanable interior surfaces Smooth double wall interior surfaces allow for easier inspection and cleaning and also isolate the casing insulation from the air stream For acoustically sensitive installations consider a perforated interior surface with matte faced coated or wrapped insulation The perforated surface is still cleanable but the casing will absorb some of the generated sound Ensure that the air handling unit includes easily removable panels and or access doors to allow for regular inspection and cleaning Poor location of Chilled Water VAV Systems SYS APM008 EN For more information on water management in buildings including proper condensate trap design refer to the Trane application manual titled Managing Building Moisture SYS AM 15 SYS APM008 EN Primary System Components a a aT the air handling unit or limited service clearance can also discourage inspection
211. he amount of latent cooling by the coil Offsetting this increase in latent coil load however is the substantial and continuous reduction of sensible heat generated by the supply and return fans e The economizer in a cold air VAV system is not able to shut off the water chiller as soon as it can in a conventional VAV system For the example in Figure 107 when the outdoor temperature is between 48 F and 55 F 9 C and 13 C the conventional system will be in the modulated economizer mode with the chiller off see Airside economizer control p 174 The cold air system however will be in the integrated economizer mode outdoor air damper is wide open but the chiller is still required to operate to deliver the air at 48 F 9 C Figure 107 Impact of supply air temperature on airside economizer operation s3 180 80 160 zZ 140 lt 1 2 Ro 120 amp 70 a amp 100 we 65 a a 2 i 60 fliin 80 3 Mee ec S tan O loy 55 a high Miz 60 9 50 cele lim er a 15 ie Its 2 35 Oi 48 F jiita tofp 40 g d p TS l EA SAT setpoint integrated iea 30 economizer SAT reset economizer a i 0 30 40 50 60 70 80 90 100 110 dry bulb temperature F To minimize this loss of modulated economizer hours in a cold air VAV system consider using supply air temperature SAT reset see Supply air temperature reset p 202 At part load when the supply air temperature is reset upwards the line separa
212. he classroom only rises to 57 percent Figure 90 Chilled Water VAV Systems SYS APM008 EN System Design Issues and Challenges SS Figure 90 Full versus part load dehumidification performance of a VAV system Design Full Load Part Load condition Peak dry bulb Peak dew point OA 96 0 F DB 76 0 WB 76 0 F DP 84 0 F DB RA 74 0 F DB 52 RH 74 0 F DB 57 RH MA 80 6 F DB 79 0 F DB SA 55 7 F DB 55 7 F DB peak dew point part load f OA F peak dry bulb full load OA 2pO 4 ajauosyoer By continuing to deliver cool dry air at part load VAV systems typically provide effective dehumidification over a wide range of load conditions Impact of minimum airflow settings for VAV terminal units Eventually the sensible cooling load in the zone becomes small enough that the required supply airflow is less than the minimum airflow setting of the VAV terminal unit see Minimum primary airflow settings p 62 For this same example classroom assume that the minimum airflow setting is 700 cfm 0 33 m s Now consider a cool rainy day 70 F dry bulb 69 F wet bulb 21 1 C DB 20 6 C WB At this condition if 700 cfm 0 33 m3 s is supplied at 55 7 F 13 1 C the space will be overcooled to 71 8 F 22 1 C As the dry bulb temperature in the space decreases the relative humidity increases to 66 percent in this example and the classroom will feel cool and damp Figure 91 Figure 91 Pote
213. he components inside the air handling unit Figure 21 VAV system with supply and relief fans 25 outdoor air VAV terminal unit zone return air grille AHU supply supply duct zone ceiling plenum ductwork runouts return ductwork diffusers static pressure relative to outdoors relief damper However the relief fan is used to raise the pressure of the air to be exhausted from C to E so that it is high enough to overcome the pressure loss associated with the relief damper and force the excess air out of the building Adding the relief fan allows the system to exhaust the air that is to be replaced by fresh outdoor air and does so without increasing the pressure in the zone or requiring the supply fan to generate a larger pressure differential In smaller VAV systems that do not use an airside economizer cycle local exhaust fans serving restrooms or copy centers and local barometric relief dampers may exhaust enough air without the need for a central relief fan SYS APMO008 EN Chilled Water VAV Systems 29 30 Primary System Components a a Supply fan and return fan The final configuration uses a fan in the return air path rather than in the exhaust air path When the system includes a return fan the supply fan must still create the same pressure at its outlet A to overcome the pressure losses associated with the supply air path Figure 22 Figure 22 VAV system with supply and return fans 25 outdoor a
214. he desired occupied setpoint prior to occupancy on the coldest or warmest morning of the year In other words the system is programmed to start early enough so that the building will warm up or cool down fast enough on the worst case morning As a result for most days the system starts earlier than it needs to This increases the number of operating hours and increases energy use An alternative approach is a strategy called optimal start The BAS is used to determine the length of time required to bring each zone from its current temperature to the occupied setpoint temperature Then the BAS waits as long as possible before starting the system so that the temperature in all zones reaches the occupied setpoint just in time for occupancy Figure 140 Figure 140 Optimal start optimal start occupied heating setpoint t zone occupied hours temperature unoccupied heating setpoint 6 a m Noon 6 p m 198 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Controls aT The optimal starting time is determined using the difference between the actual zone temperature and the occupied setpoint temperature heating or cooling It compares this difference with the historical performance of how quickly the zone has been able to warm up or cool down Some systems also compensate for the current ambient outside temperature This strategy reduces the number of system operating hours and saves energy compared to mainta
215. he heating coils in the VAV terminal units or with baseboard radiant heat within the zone e If the system includes baseboard heat within the zone it could be used to warm up the zone prior to occupancy and the central air handling unit could remain off during this mode e If the system includes VAV reheat terminals only the heating coil for those zones requiring morning warm up would need to be activated e If the system includes fan powered VAV terminals equipped with heating coils only the terminal fan and heating coil for those zones requiring morning warm up would need to be activated The central air handling unit could remain off 3 If flow tracking is used the central return fan modulates to maintain a fixed airflow differential from the supply fan airflow 4 The air modulation damper in the VAV terminal unit may be fully open allowing for a fast warm up or it may modulate to achieve a more controlled warm up and limit supply and return if equipped fan airflow to avoid potential motor overload SYS APMO008 EN Chilled Water VAV Systems 195 FRAME System Controls E Table 34 Coordination of equipment during morning cool down mode Central air handling unit Modulates the supply fan to maintain static pressure in supply duct at the desired setpoint and shuts off when all zones have reached their occupied cooling setpoints Modulates the cooling valve to discharge air at the desired setpoint Outdoor air damper
216. he terminal fan runs continuously whenever the zone is occupied and because it must be sized for the design airflow to the zone a series fan powered unit consumes more energy than a parallel fan powered unit The use of series fan powered terminals can decrease the energy used by the central supply fan because the series fan overcomes the pressure loss between the terminal and zone However the actual impact on overall system energy use depends on the efficiency differences between the supply and terminal fans and motors In general the larger centralized supply fan has a significant efficiency advantage both fan and motor efficiency over the terminal fan even with the use of ECM technology see Electronically commutated motors on fan powered VAV terminal units p 60 In addition the heat generated by the terminal fan motor in a series unit is added to the air stream during both cooling and heating operation while the fan heat in a parallel unit is only added to the air stream when it provides a benefit that is only when heating is required Figure 55 shows the impact of using parallel versus series fan powered VAV terminals in an example office building The base case is a chilled water VAV system that uses series fan powered units equipped with conventional permanent split capacitor PSC motors For this example parallel fan Chilled Water VAV Systems 59 For more information on electronically commutated motors E
217. heating effect of using a three way versus a two way valve The hot water distribution system however sees a great difference Table 16 Chilled Water VAV Systems 95 TRANE Primary System Components Figure 80 Three way versus two way control valves airflow three way modulating valve two way modulating valve With a three way valve the terminal water flow rate water flowing through the coil plus the water bypassing the coil is relatively constant at all loads Therefore pumping energy will remain relatively constant regardless of the heating load In addition because hot water is bypassing the coil the temperature of the water returning to the boiler increases as the zone heating load decreases With a two way valve however the terminal water flow varies proportionately with the load This provides the opportunity to significantly reduce pumping energy at part load Because there is no mixing of coil and bypassed water the temperature of the water leaving the terminal remains relatively constant at all load conditions Table 16 Three way versus two way control valves Characteristics of system that uses Characteristics of system that uses three way modulating control valves two way modulating control valves e Water flow rate through each load terminal e Water flow rate through each load terminal water flowing through the coil plus water varies proportionately to the load
218. her efficiency critical valve reset See pump pressure optimization critical zone reset See fan pressure optimization Cv Flow coefficient Term used for the selection of fluid control valves in I P units damper A device used to vary the volume of air passing through a confined cross section by varying the cross sectional area deadband The temperature range between the cooling and heating setpoints dedicated outdoor air system DOAS A system that uses a dedicated air handling unit to cool heat dehumidify or humidify all of the outdoor air brought into the building for ventilation This system then delivers this conditioned outdoor air directly to the conditioned spaces or to HVAC equipment dedicated outdoor air unit An airhandling unit used to cool heat dehumidify or humidify all of the outdoor air brought into the building for ventilation This conditioned outdoor air may be delivered directly to the zone s or to other air handlers or terminal equipment Also called a makeup air unit or 100 percent outdoor air unit demand controlled ventilation DCV A control strategy that attempts to dynamically reset the system outdoorair intake based on changing population in the zone Chilled Water VAV Systems 215 216 Glossary E desiccant Adsorbent or absorbent liquid or solid that removes water or water vapor from an air stream or another material dew point temperature DPT The temperature at which
219. hilled water distribution system 14 F 7 8 C AT e High efficiency water cooled centrifugal chillers 5 96 COP or 7 33 COP for Los Angeles e Chiller tower optimization control strategy e High efficiency direct drive plenum fans in the VAV air handling units e Total energy wheel to precondition the entering outdoor air except for Los Angeles e Cold air distribution 48 F 9 C supply air temperature with same size ductwork e Improved supply air temperature reset control strategy e Ventilation optimization control strategy e Parallel fan powered VAV terminal units serving the perimeter zones e Airside economizer with fixed enthalpy control fixed dry bulb control in Los Angeles For a centrifugal chiller operating at these water temperatures and flow rates which differ from the ARI standard rating conditions the minimum efficiency required by ASHRAE 90 1 2007 is 5 11 COP 5 90 COP for Los Angeles For this example even with no improvements to the building envelope lighting or other systems the proposed chilled water VAV system reduced the overall building energy cost by 26 percent for the building in Houston by 10 percent in Los Angeles by 24 percent in Philadelphia and by 21 percent in St Louis Figure 99 Example energy savings versus a LEED 2009 baseline building 100 80 60 40 20 0 St Louis overall building energy cost of base Houston Los Angeles Philadelphia m baseline buildin
220. ical practices that should be used on every project Nearly everything on the list increases the installed cost cost that may or may not be justified by the acoustical requirements For this reason an acoustical analysis is preferred to meet the acoustical goals at the lowest cost Chilled Water VAV Systems SYS APM008 EN System Design Issues and Challenges Pi eT The following sections may be used to identify potential problems for each component of the chilled water VAV system Consider both source attenuation and path attenuation when determining the most cost effective way to achieve the acoustical goals Air cooled chillers The acoustical advantage of an air cooled chiller is that the compressor sound is generated outside of the building allowing this sound source to be placed away from the building If the air cooled chiller is located near the building it must be far away from operable windows doors and sound sensitive areas When possible locate the chiller near a space with a high tolerance for sound such as a storage or equipment room Walls near the chiller must have sufficient sound transmission loss to meet the acoustical goals of the adjacent space Special consideration is required for air cooled chillers that are located on the roof Typical built up roofs have low transmission loss which makes unit placement critical Vibration isolation must also be tailored to the specific roof structure The majority of noise
221. idden by the air handling equipment manufacturer based on the ability of the fan to overcome this pressure drop The replacement filters should have similar performance characteristics as the filters originally specified by the design engineer Critical characteristics include efficiency MERV rating for example airside pressure drop at the desired operating airflow and physical size In addition air bypass can reduce the effectiveness of the filtration system During replacement the filter assembly should be carefully inspected to identify any areas that can allow air to bypass around the filters These areas should be sealed with Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a as tape or gasketing for example to minimize airflow through the space between adjacent filters Some applications such as certain areas of hospitals clinics laboratories or manufacturing facilities require final filters located downstream of all other components in the airhandling unit Figure 37 When HEPA filters are used locate them in the furthest downstream section of the air handling unit If these high pressure drop filters are located upstream of the supply fan the pressure inside the unit casing between the filters and fan inlet will likely be at a significantly negative pressure with respect to the air surrounding the casing Any air that leaks into the air handling unit at this location will in
222. in Ventilation p 101 In addition system level ventilation optimization is discussed in Ventilation optimization p 205 Supply fan capacity control As cooling load changes the VAV terminal units modulate to vary the airflow supplied to the zones This causes the pressure inside the supply ductwork to change A sensor measures the static pressure at a particular location in the supply duct The controller compares this measured pressure to the desired setpoint and adjusts the capacity of the supply fan to deliver enough air to maintain the desired static pressure in the supply duct The location of this duct static pressure sensor impacts the energy use of the supply fan as well as the ability of the system to provide acceptable comfort to the building occupants In some VAV systems this static pressure sensor is mounted near the outlet of the supply fan Figure 123 The appeal of this approach is that the sensor can be factory installed and tested resulting in greater reliability and no field installation cost If fire dampers are included in the supply duct system the sensor will be on the fan side of the fire dampers In this location this sensor can also be used as the high pressure cutout to shut off the fan and protect the ductwork from damage in the event that the fire dampers close Also depending on the layout of the duct system this method may eliminate the need for multiple duct mounted sensors Figure 123
223. in method is recommended for sizing the main supply ducts upstream of the VAV terminal units The higher air velocity increases the benefit of regaining static pressure Because of the lower pressures associated with the ductwork downstream of the VAV terminal units these benefits are not as significant So the equal friction method is often used for this portion of the system Best practices for the design and layout of the supply duct system Other publications contain more complete details related to duct design but following are a few general recommendations e Keep the duct layout as simple and symmetrical as possible Use low pressure drop duct fittings and follow the best practices published by the Sheet Metal and Air Conditioning Contractors National Association SMACNA for designing and installing duct systems Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components e Use at least three diameters of straight duct downstream of the discharge from the air handling unit before the first elbow junction etc Satisfactory fan performance and distribution of air throughout the system requires unrestricted and relatively uniform airflow from the discharge of the air handling unit This first section of supply ductwork should be straight for at least three duct diameters to allow turbulence to decrease and for a uniform velocity profile to develop before the air encounters an elbow or junction However when
224. ing see Air to Air Energy Recovery p 160 The advantage of this approach is that it also reduces cooling and heating energy use and can allow for downsizing of cooling and heating equipment However such a device does increase the cost of the air handling unit and adds a pressure drop to both the outdoor and exhaust air streams which increases fan energy use Steam heating coils can also freeze if the condensate is allowed to remain inside the tubes of the coil If a steam coil is likely to be exposed to air that is colder than 32 F 0 C e Use a distributing type steam coil This type of coil has steam distributing tubes inside the larger condensing tubes Orifices located in the bottom of the distributing tubes are directed toward the condensate return header improving condensate drainage e If possible pitch the steam coil toward the condensate connection to assist with drainage Also make sure the air handling unit is installed within the manufacturer s tolerance for levelness e Properly size install and maintain the steam trap e Because steam coils are very sensitive to piping practices it is extremely important to follow the manufacturer s instructions regarding installation of the condensate piping steam trap and vacuum breakers Gas fired burner Alternatively gas fired burners can be installed inside the airhandling unit at the factory This simplifies installation at the jobsite and may
225. ing unit This combined with the use of flexible ductwork to connect the VAV terminal units to supply Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Overview of a Chilled Water VAV System a eT air diffusers provides flexibility for the system to adapt to potential changes in building use Also using chilled water as the cooling medium offers more flexibility to adapt to future changes in the building load It is easier to design a chilled water air handling unit with reserve capacity than it is for a direct expansion DX refrigeration system In addition centralized chilled water and hot water plants can also be designed to have reserve capacity which can be used by any of the air handling units they serve Drawbacks Challenges of Chilled Water VAV Systems Similar to the discussion of benefits a list of drawbacks is dependent on which type of system is the basis of comparison The following section however discusses some of the primary challenges related to chilled water VAV systems along with some potential ways to address those challenges More sophisticated system to design control and operate The flexibility of a chilled water VAV system achieved through the use of more sophisticated equipment and controls also means that it can be more challenging to design control and operate properly than a system that uses more packaged components Single zone packaged DX equipment is simple
226. ining the indoor temperatures at occupied setpoint even though the building is still unoccupied Optimal stop A related strategy is optimal stop As mentioned previously at the end of the occupied period most systems are shut off and the indoor temperature is allowed to drift away from occupied setpoint However the building occupants may not mind if the indoor temperature drifts just a few degrees before they leave for the day Optimal stop uses the BAS to determine how early heating and cooling can be shut off for each zone so that the indoor temperature drifts only a few degrees from occupied setpoint Figure 141 In this case only cooling and heating are shut off the supply fan continues to operate and the outdoorair damper remains open to continue ventilating the building Figure 141 Optimal stop a time clock cee ee drift below occupied optimal start optimal stop setpoint occupied bg heating setpoint ie Sone occupied hours temperature unoccupied heating setpoint 6 a m Noon 6 p m The optimal stop strategy also reduces the number of system operating hours saving energy by allowing indoor temperatures to drift early Unoccupied economizing During the unoccupied mode the indoor temperature can sometimes drift up warmer than the occupied cooling setpoint This requires the system to enter the morning cool down mode prior to occupancy However if the outdoor dry bulb temperature is cooler than the
227. ion p 205 Space pressure control For certain types of spaces the air pressure within the space is required to be controlled to be either higher positive pressure or lower negative pressure than the adjacent spaces This is most common in certain health care facilities and laboratories One common approach for controlling space pressure is to install a second VAV terminal unit in the return duct leaving the space Figure 135 The VAV terminal unit in the supply duct modulates to maintain space temperature and to ensure the minimum air change rate required The VAV terminal unit in the return duct modulates to maintain either a positive or negative pressure in the space This can be done either through direct pressure measurement or by controlling the return airflow to a pre defined offset from the current measured supply airflow airflow tracking Figure 135 Space pressure control via airflow tracking supply VAV terminal unit communication link primary air from main AHU return return VAV terminal unit SYS APMO008 EN Chilled Water VAV Systems 187 188 System Controls eT When the airflow tracking method is used this offset which is determined by the air balancing contractor when the system is commissioned is the amount of excess or deficit return air needed to maintain the space at the desired negative or positive pressure with respect to the adjacent space s For example if th
228. ion Process by which fluid molecules are concentrated on a surface by chemical or physical forces AHRI Air Conditioning Heating and Refrigeration Institute www ahrinet org air cooled condenser A type of condenser in which refrigerant flows through the tubes and rejects heat to outdoor air that is drawn across the tubes air diffusion Distribution of air within a conditioned space by an outlet discharging supply air in various directions and planes air handling unit AHU A piece of equipment used to move clean and condition heat cool humidify dehumidify air air mixing baffles A device located immediately downstream of the mixing box that adds rotational energy and increases the velocity of the air stream improving the mixing blending and minimizing temperature stratification air separator A component of a closed piping system that removes air that is entrained in the water distribution system airside economizer A method of free cooling that involves using cooler outdoor air for cooling instead of recirculating warmer indoor air air to air energy recovery The transfer of sensible heat or sensible plus water vapor latent heat between two or more air streams or between two locations within the same air stream ANSI American National Standards Institute www ansi org ARI Former Air Conditioning amp Refrigeration Institute See AHRI ASHRAE American Society of Heating Refrigerating and Air Cond
229. ion efficiency and in some cases the dirt holding capacity of the filter Chilled Water VAV Systems 45 For more information on the various types of gaseous air cleaners refer to Chapter 29 Industrial Gas Cleaning and Air Pollution Control in the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org or the NAFA Guide to Air Filtration www nafahq org 46 Primary System Components a a aT system Some types of electrostatic precipitators are cleanable which significantly reduces the cost of system maintenance The benefit of this technology is higher collection efficiency and increased dust holding capacity at a lower airside pressure drop The primary drawback is an increase in first cost and it typically increases the cost and size of the air handling unit Many actively charged electrostatic precipitators are not MERV rated but are rated by collection efficiency at a specific particle size 99 8 percent efficient at capturing 0 5 micron particles for example Gaseous air cleaners The presence of undesirable gases and vapors particularly carbon monoxide radon oxidants and volatile organic compounds or VOCs such as formaldehyde indoors can be detrimental to building occupants materials and contents Controlling VOC concentrations is particularly challenging hundreds of them are present often at very low concentrations few are unique to any one source and there are many potential sources
230. ion is to vary the position of the OA damper in proportion to the change in supply airflow First with the system operating at design full load supply airflow the OA damper position is set to bring in the minimum required quantity of outdoor airflow Vot Then with the system operating at the minimum expected system airflow the OA damper is adjusted to a more open position so it brings in the same quantity cfm m s of outdoor air as it did at design airflow During system operation the position of the OA damper is varied in proportion to the change in supply airflow which is determined by the signal being sent to the variable speed drive on the supply fan Figure 87 114 Chilled Water VAV Systems SYS APM008 EN Figure 88 Flow measuring outdoor air damper Trane Traq damper SYS APM008 EN System Design Issues and Challenges i Figure 87 Proportional control of the outdoor air damper OA damper position at minimum supply airflow OA damper position at design supply airflow 10 20 30 40 50 60 70 80 90 100 of VFD signal This method is somewhat inaccurate over the entire range of supply airflows resulting in some over ventilation and is unable to respond to pressure fluctuations caused by wind or stack effect But it results in significantly less over ventilation than a fixed OA damper position thus saving energy and it is relatively inexpensive Direct measurement and control of outdoor airflo
231. ior zones Interior zones are typically surrounded by other zones at the same temperature so they do not experience the same heat gain and heat loss fluctuations as a perimeter zone Therefore many interior zones require year round cooling due to the relatively constant amount of heat generated by people lights and equipment and the absence of heat losses through the building envelope Interior zones on the top floor of a building might need to be treated as a perimeter zone if they experience a significant heat loss through the roof Most interior zones are served by cooling only VAV terminal units that modulate supply airflow in response to the changing cooling load Some interior zones such as conference rooms may require some amount of tempering reheat to avoid overcooling the zone at partial occupancy Chilled Water VAV Systems 65 66 Primary System Components a aT Typically either VAV reheat or fan powered VAV terminal units are used to provide the tempering needed at lower cooling loads They also have a minimum airflow setting to serve ventilation requirements Typical combinations used in chilled water VAV systems Most buildings use a combination of VAV terminal unit types Following is a discussion of the most common combinations Interior zones Cooling only VAV Perimeter zones Cooling only VAV with baseboard radiant heat All zones interior and perimeter are served by cooling only VAV terminal units Howe
232. iping at the desired setpoint Turns on boilers and varies boiler capacity to heat water to the desired setpoint 1 If flow tracking is used the central return fan modulates to maintain a fixed airflow differential from the supply fan airflow An addendum j to ASHRAE Standard 62 1 2007 helps clarify the intent of the standard by adding the following to Section 8 3 Systems shall be operated such that spaces are ventilated when they are expected to be occupied While the outdoor airflow delivered to the breathing zone Vbz can be reset as zone population Pz varies per Section 6 2 7 the system must deliver at least the building related or base ventilation rate Ra whenever the zone is expected to be occupied see Minimum ventilation rate required in breathing zone Vbz p 102 192 Occupied standby mode As mentioned a time of day schedule in the BAS is typically used to define when a zone is to operate in the occupied versus unoccupied mode In addition when an occupancy sensor is used in combination with a time of day schedule this sensor can be used to indicate if the zone is actually unoccupied even though the BAS has scheduled it as occupied This combination is used to switch the zone to an occupied standby mode see example in Table 31 In this mode all or some of the lights in that zone can be shut off the temperature setpoints can be raised or lowered by 1 F to 2 F 0 5 C to 1
233. ir VAV terminal unit zone return air grille AHU main supply supply duct zone ceiling plenum ductwork runouts return ductwork diffusers Ae supply static pressure relative to outdoors return air damper However the pressure at the inlet of the supply fan B only needs to be low enough to overcome the pressure losses associated with drawing the return air through the return air damper filter and coils or the pressure associated with drawing the outdoor air through the outdoor air damper whichever is higher The return fan is used to overcome the pressure losses associated with drawing the return air out of the zones and through the return air grilles open ceiling plenum and or return ductwork It must also generate enough pressure F to push any relief air out through the relief damper Should the system use a relief fan or a return fan The supply fan and relief fan configuration usually works best in VAV systems that use an open ceiling plenum for part of the return air path Table 5 Systems with relief fans are easier to control typically lower the cost of the airhandling unit and are often less costly to operate than systems with return fans See Building pressure control p 178 When the pressure drop through the return air path is very high which may be the case in a larger system with a fully ducted return air path evaluate both the relief and the return fan configurations If the sup
234. ir AC counterparts Potential for disruptive harmonic currents Harmonic currents are created when AC power is converted to DC power In some cases these currents can overheat conductors and connectors Chilled Water VAV Systems 61 FRAME 62 Primary System Components a a Ta interfere with the operation of sensitive equipment and in severe cases burn out transformers and motors Determining whether harmonic currents will cause a problem in a particular building requires review of the electrical system before it s installed so that appropriate steps can be taken When necessary it is possible to alter the design of the system by oversizing the neutral wire for example and or reduce motor generated harmonics by adding a harmonic filter for example Dual duct VAV terminal units A dual duct VAV terminal unit consists of two airflow modulation devices packaged inside a sheet metal enclosure Figure 58 One modulation device varies the amount of cold primary air and the other varies the amount of warm primary air These two air streams mix inside the dual duct unit before being distributed downstream to the zone Figure 58 Dual duct VAV terminal unit cool primary air warm primary air Dual duct VAV systems are intended for buildings that require seasonal cooling and heating The energy use of this system is generally low and it can provide excellent control of both temperature and humidity However this s
235. ir handling unit and the chilled water system For example the amount of material used to construct the coil overall size number of tubes number of fins determines the initial cost more material increases the cost But the size of the cooling coil also dictates the weight and footprint of the air handling unit the larger the coil the larger the AHU must be to house it A larger AHU may require a larger mechanical room reducing usable or rentable floor space limit access for service impact the amount of structural support needed or challenge the arrangement of ductwork and piping Because the cooling coil is also part of the air distribution system its geometry size number of rows fin spacing and fin profile contributes to the airside pressure drop and affects the energy used and sound generated by the fans A larger AHU will typically result in a lower airside pressure drop through its components which can reduce fan energy see example in Table 4 Finally because a chilled water cooling coil is also part of the chilled water system its geometry contributes to the waterside pressure drop and affects the energy used by the pumps And the extent to which coils raise the chilled water temperature dramatically affects both the installed cost of chilled water piping and pumping energy Coil performance can even influence the efficiency of the water chiller For further discussion see Chilled Water System p 79 Chi
236. is closed unless the condition of the outdoor air is suitable to provide free cooling economizer Modulates the central return fan if equipped to maintain static pressure in the return air plenum of the air handling unit at the desired setpoint3 VAV terminal Opens the air modulation damper until zone temperature reaches occupied cooling setpoint then the damper closes2 Chilled water plant Turns on chilled water pumps if a variable flow system varies the speed of the pumps to maintain pressure in the chilled water piping at the desired setpoint Turns on chillers and varies chiller capacity to cool water to the desired setpoint If water cooled turns on condenser water pumps when chillers need to operate and modulates cooling tower fans to cool condenser water to the desired setpoint Hot water plant if included Hot water pumps are off Boilers are off 1 In some buildings outdoor air is brought into the building during the morning cool down mode to dilute contaminants that have accumulated inside the building during the unoccupied mode This is often called a preoccupancy purge In this case the central relief damper or relief fan should modulate to maintain indoor to outdoor static pressure difference at the desired setpoint 2 The air modulation damper in the VAV terminal unit may be fully open allowing for a fast cool down or it may modulate to achieve a more controlled cool down and limit supp
237. is means placing the sensor on an interior wall Chilled Water VAV Systems SYS APM008 EN FRAME Figure 82 Wireless zone sensor SYS APM008 EN System Design Issues and Challenges Pe Using wireless technology In the HVAC industry the use of wireless technology can often result in an overall lower installed cost when compared to traditional wired sensors especially in historical or difficult to wire buildings renovations or in locations with high labor rates By eliminating the wires between a zone temperature sensor and a VAV terminal the sensor Figure 82 can be easily placed in the best location to accurately measure the zone temperature This might be on a cubicle wall a concrete or brick wall or some other difficult to wire location A wireless zone sensor is easy and inexpensive to move when the layout or use of the zone changes or if the initial placement of the sensor turns out to be a poor location To ensure reliable operation make sure the wireless technology adheres to the Institute of Electrical and Electronics Engineers IEEE Standard 802 15 4 This standard was created to minimize the risks of interference with other wireless devices In addition ensure that the wireless sensor has a long battery life at least five years and a visible low battery indicator to minimize ongoing maintenance Ventilation For this manual ventilation refers to the introduction of outdoor air to dilute contamin
238. itioning Engineers www ashrae org aspiration ratio Total room air circulation divided by the air discharged from the outlet Also called entrainment ratio attenuation The reduction in the sound level as it travels along the path from a source to the receiver block airflow Calculated by finding the single instance in time when the sum of the zone airflows is the greatest This method is used for sizing a VAV supply fan that delivers a varying amount of air to the system Chilled Water VAV Systems 213 214 Glossary eT blow thru A configuration where the fan is located upstream and blows air through the cooling coil boiler A pressure vessel that typically consists of a water tank or tubes with water flowing through them a heat exchanger fuel burners exhaust vents and controls Its purpose is to transfer the heat generated by burning fuel to either water or steam breathing zone The region within an occupied space between planes 3 in and 72 in 75 mm and 1800 mm above the floor and more than 2 ft 600 mm from the walls or fixed air conditioning equipment building automation system BAS A centralized control and monitoring system for a building CDQ Trane s Cool Dry Quiet technology See series desiccant wheel chiller tower optimization A control strategy that uses the BAS to dynamically determine the optimal condenser water temperature that minimizes the combined energy use of the water chille
239. ittle or no return ductwork smaller fan motor Potential exists to lower the pressure in the ceiling plenum below due to lower pressure drop through the return air path outdoor pressure if return air grilles are not sized properly and e Often results in lower supply airflow because some of the heat building pressure is not properly controlled generated by recessed lights and some of the heat conducted e Local code may require surfaces and materials used in the plenum through perimeter walls or the roof is picked up by the return air to meet a certain fire rating stream rather than being transferred through the ceiling into the e Space to space pressure control is not possible zone e Less fan energy use due to lower pressure drop through the return air path and possibly less supply airflow e Allows use of barometric relief dampers or a central relief fan rather than a return fan More difficult to clean than a fully ducted return air path Often allows the use of a shorter ceiling plenum because space is not required for ducting Fully ducted return Advantages Disadvantages e May avoid the need for surfaces and materials used in the plenum e Higher installed cost due to more return ductwork a larger fan to meet a certain fire rating required by local code motor to offset the increased pressure drop through the return air e While still difficult to clean due to limited access a fully ducted path and possibly VAV terminals in th
240. itude of this energy savings depends on the method used to modulate the capacity of the fan Second the reduced airflow across the cooling coil allows the chilled water control valve to reduce water flow in order to deliver a constant supply air temperature This results in a reduction in cooling energy compared to a constant volume reheat system Chilled Water VAV Systems 7 Overview of a Chilled Water VAV System a a rT In addition using chilled water as the cooling medium presents further opportunity for energy savings through the use of centralized higher efficiency cooling equipment and a water distribution system Flexibility of equipment location Chilled water VAV systems offer significant flexibility when locating the various components of the system The design team can maximize the amount of usable floor space in the building by using an air cooled chiller outdoor air handling units and VAV terminal units installed in the ceiling plenum Or equipment can be located indoors water cooled chillers and indoor air handling units to improve access for maintenance and prolong equipment life Centralizing the cooling and heating equipment minimizes disruption of the occupants when maintenance or repair is required Similarly the VAV terminals can be installed above corridors to minimize disruption of the occupants Typically the only equipment located within the occupied space is the temperature sensor mounted on the wall
241. ivate office could also be used to slightly raise or lower the zone temperature setpoints and to reduce the ventilation requirement for that zone when it is unoccupied e Acard access security system could be used to turn on lights start the HVAC system and increase ventilation delivered to a secure work area when the occupants card in for the day e A point of sale ticket system at a theater could be used to vary the ventilation delivered to an individual theater based on the number of people that purchased tickets for the show This integration may also involve changing the operation of one or more components of the HVAC system to assist another system For example Chilled Water VAV Systems 211 System Controls eT providing effective smoke control in larger buildings may involve enlisting the help of the fans in the air handling unit In this case activation of a fire alarm causes the components of the HVAC system to perform smoke control functions While other smoke control functions can be conceived four common functions include e Off In this mode both the supply fan and exhaust or return fan are turned off In addition both the outdoor and exhaust air dampers are closed and cooling and heating are disabled e Pressurize In this mode the outdoor air damper is wide open the supply fan is turned on and operates at 100 percent airflow the VAV terminal units are fully open and the exhaust or return fan is tu
242. ixes all of the outdoor air with a portion of the recirculated return air This mixture is then cooled and delivered through the cold duct system to the cooling airflow modulation device in each dual duct VAV terminal unit The heating air handling unit typically conditions only recirculated return air no outdoor air This air is heated and delivered through the hot duct system to the heating airflow modulation device usually a damper in each terminal unit Figure 116 Dual fan dual duct VAV system cooling airhandling unit gt 55 F 13 C dual duct VAV terminal units mw 105 F 41 C In the single fan configuration Figure 117 all the recirculated return air is mixed with the outdoor air inside a single air handling unit This mixture is then diverted through either the cooling coil or the heating coil and delivered down the respective duct system to the individual dual duct VAV terminal units Figure 117 Single fan dual duct VAV system of 55 F 13 C c l dual duct Z VAV terminal gt 4 unit Z A 105 F 41 C central ai handling unit Chilled Water VAV Systems SYS APM008 EN System Design Variations i While the single fan dual duct configuration requires only one air handling unit it is complicated to control efficiently At the example operating conditions depicted in Figure 117 the airside economizer is modulating the outdoor and return air dampers to deliver the supply air at the desired
243. jobsite conditions dictate that an elbow or junction be installed near the air handling unit consider using a plenum fan or adding a discharge plenum downstream of a housed fan In either case the plenum can often eliminate the need for the elbow or junction by allowing for straight duct takeoffs in multiple directions Figure 64 This reduces pressure losses and the associated impact on fan energy and sound Figure 64 Discharge plenum with multiple straight connections discharge plenum supply fan e Place main duct runs and when possible branch runs and VAV terminal units above hallways and other unoccupied areas This typically eases installation and maintenance and helps minimize sound radiated to the occupied spaces e Limit the use of flexible ductwork upstream of VAV terminal units While flexible ductwork has many benefits improper use can cause numerous problems in a VAV system Flexible ductwork causes turbulent airflow and relatively large static pressure loss In addition using flexible ductwork at the inlet of a VAV terminal unit may impact the accuracy and consistency of the flow sensor due to the turbulence it causes Therefore the use of flexible ductwork upstream of VAV terminal units should be kept to an absolute minimum Ideally flexible ductwork should only be used downstream of the VAV terminal units All runs of flexible ductwork should be kept as short as possible even though the ease of instal
244. k type of material to be used and all applicable codes For long or high pressure drop stacks a flue booster fan may be required And a Chilled Water VAV Systems 25 FRAME 26 Primary System Components a aT barometric damper may be required if a tall chimney produces excessive draft e Controls Because airflow across the gas fired burner varies the heating capacity of the burner must be modulated to prevent the temperature rise through the burner from exceeding the maximum allowable limit For most VAV applications this likely requires a burner with a 10 1 turndown ratio This means the burner can operate at a capacity as low as 10 percent of its rated capacity In addition air temperature sensors on the entering and leaving sides of the burner are needed to protect it from damage Finally allow the supply fan to continue to operate for a period of time after the gas fired burner has been shut off allowing the heat exchanger to dissipate any residual heat Consult the manufacturer for the length of this cool down period In VAV applications the air velocity across a direct fired burner must remain within a specific range for safe operation In this case manufacturers typically provide an adjustable opening that automatically varies the opening size as airflow changes keeping the air velocity relatively constant Recovered heat In some applications heat may be recovered from another part of the HVAC system C
245. l lt 2 diffusers Scuse a lengths max lengths max erimeter wall 150 FPM max P SRNA 0 76 M s e Avoid high collision velocities Figure 67 The collision velocity is the speed at which moving air meets a wall or another air stream Excessively high collision velocities often result in uncomfortable drafts for the occupants For perimeter walls the collision velocity should not exceed 150 fpm 0 76 m s For interior walls the collision velocity should not exceed 75 fpm 0 38 m s When two air streams collide the collision velocity determined by adding the velocities of both air streams at the point of collision should not exceed 150 fpm 0 76 m s e Locate linear slot diffusers to maintain parallel flows and avoid air streams colliding at a right angle Figure 68 This improves air circulation and avoids high collision velocities Figure 68 Avoid locating linear slot diffusers that cause air streams to collide at right angles supply air Z he diffusers TN NEVER do this perpendicular air flows 76 Chilled Water VAV Systems SYS APM008 EN Figure 69 Static pressure in plenum versus occupied space return air grille 0 02 in H20 5 0 Pa e _ SS 0 05 in H20 12 5 Pa SYS APM008 EN Primary System Components a a as Return air path Because a VAV system uses a central supply fan to serve many zones a path is needed to allow air to re
246. l be at risk of freezing and the consequences of a frozen coil burst tubes and water leaks are too severe If a hot water coil is likely to be exposed to air that is colder than 32 F 0 C consider one of the following methods of freeze protection e Add antifreeze to the hot water system Adding antifreeze such as glycol to the hot water system lowers the temperature at which the solution will freeze Given a sufficient concentration of glycol no damage to the system will occur For a VAV system since the hot water coil operates during sub freezing weather a concentration that provides freeze protection to prevent the solution from forming crystals at the coldest expected outdoor temperature is required Table 13 p 87 Make sure to also use an inhibitor package to help resist corrosion At the warmer fluid temperatures used in the hot water system the impact of glycol on pressure drop is much lower than in cooling coils e Use air to air energy recovery to preheat the outdoor air An air to air energy recovery device such as a coil loop heat pipe fixed plate heat exchanger or wheel can typically preheat the entering outdoor SYS APMO008 EN Chilled Water VAV Systems 23 FRAME Figure 16 Direct fired versus indirect fired gas burners Indirect fired gas burner 24 Primary System Components a aT air to a temperature warmer than 32 F 0 C minimizing or possibly avoiding the risk of coil freez
247. lation is an enticement to push the limits of acceptable practice SYS APM008 EN Chilled Water VAV Systems 73 FRAME Figure 65 Linear slot diffuser a 74 Primary System Components a a aT e f needed reducers should be located several duct diameters upstream of VAV terminal units A reducer a transition that reduces the size of the duct causes a significant drop in static pressure velocity pressure increases as air velocity increases then static pressure decreases as velocity pressure increases and it creates turbulence The turbulence is greatest just downstream of the reducer If installed too close to the inlet of the VAV terminal unit this turbulence can impact the accuracy and consistency of the flow sensor possibly increasing fan energy use After the VAV terminals have been selected and approved for purchase the design engineer should coordinate with the installing contractor Installing an upstream duct that is the same diameter as the inlet to the VAV terminal will eliminate the need for a reducer If a reducer is needed locate it at least three duct diameters upstream of the inlet to the VAV terminal unit This allows most of the turbulence to dissipate before air enters the VAV terminal unit improving airflow measurement accuracy and control stability Be sure to consider the pressure loss of reducers in the sizing of the supply ductwork and selection of the supply fan e Add balancing dampers to the
248. ld be measured in accordance with ARI Standard 260 Sound Rating of Ducted Air Moving and Conditioning Equipment to properly account for the acoustical effect of every unit configuration In addition work with the manufacturer to determine which configurations result in the desired sound level for the application An acoustical analysis helps determine which of the sound paths are critical and which source of unit sound is most important to attenuate e Casing radiated sound Casing radiated sound from outdoor air handling units is rarely a problem unless the unit includes an exhaust fan and is close to a sound sensitive area If the unit cannot be selected with sufficiently low casing radiated sound to meet the outdoor acoustical goals the solutions are similar to those used with air cooled chillers Attempt to locate the air handling unit away from the sound sensitive area or increase the transmission loss of the adjacent roof or walls Barrier walls can be used to redirect sound away from sensitive areas but the barrier must allow outdoor air to reach the air handling unit and avoid recirculating exhaust air back through the intake Note If the unit is relocated and the supply and return ducts are routed above the roof some of the airborne sound will break out to the outdoors thereby reducing the sound level in the supply and return airborne paths Casing radiated sound from indoor air handling units must not be overlooked especia
249. leaking out of the air handling unit This will increase fan airflow and fan energy use because the fan is moving more air than is being delivered to the spaces In addition the air that leaks out of the unit has already passed through the cooling coil so the unit is using more cooling energy to condition air that is not being sent to the spaces However since the return air from the spaces passes through the mechanical room where the air handling unit is located any air that leaks out of the unit mixes with and pre cools this return air so not all of the excess cooling energy is lost Of course if the return air is ducted directly to the air handling unit the energy used to cool the air that leaks out is lost Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a aa Figure 43 Example impact of air leakage blow thru indoor AHU return air supply air outdoor air a eo The actual impact of air leakage on the overall energy use of the HVAC system depends on whether the air handling unit is located indoors or outdoors the location of the fan within the overall unit draw thru or blow thru whether the return air is ducted directly to the air handling unit or returns through the mechanical room in which the unit is located and many other factors As an example Figure 44 illustrates the impact of air leakage from a blow thru indoor air han
250. ling and into the occupied space e Structure borne This path differs from the others in that energy is transmitted through the framework of the building This energy may come directly from the vibration of the sound source or may be airborne sound that is transferred to the structure An acoustical analysis consists of five basic steps Step 1 Set acoustical goals for the finished space It is critical to establish realistic acoustical goals for the occupied space at the outset of any HVAC project There are always implicit often subjective expectations and it is much easier if you understand these expectations before designing the system Acoustical goals vary depending on how the space is used Once the goals are understood they can be stated using an appropriate descriptor such as Noise Criteria NC or Room Criteria RC for indoor environments or dBA for classrooms or outdoor environments Remember the following when defining the desired sound levels e Asa general rule lower sound levels cost more to achieve e The entire building does not have the same acoustical requirements Bathrooms and hallways do not need to be as quiet as executive offices and conference rooms A low cost quiet installation takes advantage of this point e Successful acoustics requires a team effort including the owner design engineer architect equipment manufacturer and installing contractor Step 2 Identify each sound path and its elem
251. lled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a as Maximum face velocity to prevent moisture carryover If a cooling coil also dehumidifies it must be selected to prevent moisture carryover at design air velocities While a long time industry rule of thumb has been to select cooling coils for a face velocity no greater than 500 fpm 2 5 m s at design airflow many of today s heat exchanger surfaces have been engineered to prevent moisture carryover at much higher velocities The footprint of the AHU cabinet is typically dictated by the size of the cooling coil and the size of the cooling coil is often dictated by the allowable face velocity An overly restrictive limit on coil face velocity results in the selection of a larger air handling unit than may be necessary This increases the cost of the equipment results in heavier equipment that requires more structural support and requires more floor space Table 4 shows an example selection of a 13 000 cfm 6 1 m3 s VAV air handling unit selected to provide 525 MBh 154 kW of total cooling capacity Arbitrarily limiting coil face velocity to 500 fpm 2 5 m s results in the need to select a size 30 unit Note The unit size typically represents the nominal face area of the cooling coil in terms of ft2 In the example depicted in Table 4 the face area of the size 30 air handling unit is 29 90 ft 2 78 m2 A size 25 unit with its reduced
252. lly when the return air is not ducted directly to the unit Sound radiated from the casing can be transmitted through the walls SYS APMO008 EN Chilled Water VAV Systems 141 TRANE System Design Issues and Challenges of the equipment room into adjacent spaces When the return air is not ducted directly to the unit sound radiates into the equipment room from both the casing and the unit inlet This increases the equipment room sound level so more sound is available to transfer through the equipment room walls and or follow the return air path opposite the direction of airflow to the occupied space Finally use sufficient vibration isolation to prevent structure borne sound Fan housings should have flexible materials where they connect to the air handling unit and the base structure should be isolated e Return airborne sound If the return air is not ducted directly to the air handling unit the acoustical analysis for this sound path should begin with the inlet plus casing sound data for the air handling unit In this configuration the return air typically enters the equipment room from the ceiling plenum through one or more holes in the equipment room wall A portion of the sound inside the equipment room is transmitted through these holes into the ceiling plenum and is then transmitted through the acoustical ceiling tile into the occupied space As such this path has very little attenuation If necessary an unducted return
253. ly and return if equipped fan airflow to avoid potential motor overload 3 If flow tracking is used the central return fan modulates to maintain a fixed airflow differential from the supply fan airflow 196 In some climates it may necessary to reduce the indoor humidity level prior to occupancy This is often called humidity pull down Similar to morning cool down this requires the HVAC system to start prior to occupancy and operate long enough for the humidity inside the building to reach the desired occupied humidity setpoint by the time people enter the building During the humidity pull down mode the supply air dry bulb temperature should be ramped down slowly to lower the indoor dew point temperature and avoid condensation of surfaces of the air distribution system Scheduling Determining the times at which to start and stop the HVAC system is typically based on assumptions regarding building usage Most building managers or operators want to avoid complaints from the occupants and the time needed to respond to those complaints For this reason they usually take a very conservative approach starting the system very early and stopping it very late This can be costly from an energy perspective since the entire building may be operating to maintain occupied temperature setpoints even though only a few spaces are occupied Following are a few simple solutions to minimize comfort complaints and avoid wasting e Use aggr
254. m In a water cooled system the cooling tower and sections of the condenser water piping located outdoors are often drained during cold weather to avoid freezing Alternatively some systems use special control sequences sump basin heaters heat tape on outdoor piping or even locate the sump indoors to prevent freezing In another configuration the cooling tower can be located indoors with outdoor air ducted to and from the tower This typically requires the use of a centrifugal fan to overcome the static pressure losses due to the ductwork which increases tower fan energy are compared to a conventional outdoor cooling tower with propeller fans For systems with year round cooling requirements that cannot be met with an airside economizer air cooled chillers are often used Air cooled condensers have the ability to operate in below freezing weather and can do so without the problems associated with operating the cooling tower in these conditions Alternatively a dry cooler can use the cold ambient air to cool the water directly without needing to operate a chiller or cooling tower Condenser heat recovery A water cooled chiller rejects heat to the condenser water then the cooling tower rejects this heat to the outdoors An air cooled chiller rejects heat from the hot refrigerant vapor leaving the compressor to the outside air flowing across the air cooled condenser coils In some systems all or part of this heat can be recovered and
255. m Vbz cfm Ez Voz cfm South offices 5 18 90 0 06 2000 120 210 1 0 210 West offices 5 20 100 0 06 2000 120 220 1 0 220 South conf room 5 30 150 0 06 3000 180 330 1 0 330 East offices 5 20 100 0 06 2000 120 220 1 0 220 South interior offices 5 50 250 0 06 10 000 600 850 1 0 850 North interior offices 5 50 250 0 06 10 000 600 850 1 0 850 North offices 5 16 80 0 06 2000 120 200 1 0 200 North conf room 5 20 100 0 06 2000 120 220 1 0 220 System totals 224 1120 1980 Pz RpxPz RaxAz 106 Chilled Water VAV Systems SYS APM008 EN What is the minimum expected primary airflow Conservative designers use the minimum airflow setting for the VAV terminal see Minimum primary airflow settings p 62 as the minimum expected primary airflow in this calculation While straightforward this approach results in a lower system ventilation efficiency Ev and a higher outdoor air intake flow Vot at design Less conservative designers might use a value for minimum expected primary airflow that exceeds the minimum airflow setting for the VAV terminal For most VAV systems the highest outdoor air intake flow Vot occurs when the total system primary airflow is at design block airflow When system airflow is at design it is unlikely that any VAV terminal will be closed down to its minimum primary airflow setting Using a higher more realistic value for minimum expected primary airflow will
256. m level control of a chilled water VAV system cooling tower water cooled chiller VAV terminal unit exhaust fan system level controller BAS boiler Coordination during different operating modes One of the primary system level control functions is to coordinate the central airhandling unit VAV terminal units and other pieces of equipment during the various modes of operation The primary system level operating modes in a VAV system are e Occupied mode e Unoccupied mode e Morning warm up or cool down mode Typically a time of day schedule in the building automation system is used to define when the system is to operate in these various modes Occupied mode When the building is occupied the VAV system must maintain the temperature in each occupied zone at the desired setpoint cooling or heating and provide the required minimum amount of outdoor air for ventilation Table 30 describes the typical functions of the different system components during the occupied mode In many buildings the occupied mode occurs during daytime hours and the unoccupied mode occurs at night Depending on building usage however the occupied mode could extend into the evening Chilled Water VAV Systems 191 System Controls E Table 30 Coordination of equipment during occupied mode Central air handling unit VAV terminal Modulates supply fan to maintain static pressure in the supply duct at the de
257. m2 Then determine the peak number of people expected to occupy the zone during typical usage Pz and occupiable floor area Az Finally solve the following equation to find the minimum outdoor airflow required for the breathing zone Vpbz Vbz Rp x Pz Ra x Az Impact of zone air distribution effectiveness In addition to defining the breathing zone outdoor airflow Vbz ASHRAE 62 1 also prescribes a zone air distribution effectiveness Ez that accounts for how well the ventilation air which is delivered to the zone by supply air diffusers actually gets into the breathing zone Figure 83 The breathing zone outdoor airflow Vpz is divided by this effectiveness Ez to determine the outdoor airflow that must be delivered through the supply air diffusers Voz Table 17 is an excerpt from ASHRAE 62 1 and provides default values for Ez for air distribution configurations commonly used in VAV systems It is based on the placement of supply air diffusers and return air grilles and the temperature of the air being supplied Chilled Water VAV Systems SYS APM008 EN TRANE System Design Issues and Challenges Table 17 Zone air distribution effectiveness Ez Location of supply air Location of return air Supply air temperature Ez diffusers grilles Tsa ceiling ceiling cooler than zone 1 0 ceiling floor cooler than zone 1 0 ceiling ceiling warmer than zone 0 8 2Tzone 15 F 8 C ceiling ceiling
258. mal performance of the AHU casing is more critical since extreme outdoor temperatures result in more heat loss gain Installing the air handling units indoors requires floor space that could have otherwise been used by the building owner or leased to a tenant Following are several strategies used to minimize the floor space required by an indoor air handling unit e Stacked configurations If the frame of the air handling equipment is sturdy enough components can be stacked on top of one another to minimize the footprint or floor space required The top unit in Figure 6 depicts an example VAV air handling unit with both a supply fan and relief fan configured on one level The overall length is 23 2 ft 71 m and the width is 78 ft 2 4 m 12 Chilled Water VAV Systems SYS APM008 EN FRAME For more information on direct drive fans and using multiple versus single fans refer to the Trane engineering bulletin titled Direct Drive Plenum Fans for Trane Climate Changer Air Handlers CLCH PRB0O21 EN SYS APM008 EN Primary System Components Figure 6 Example of a stacked configuration to reduce unit length Source Images from Trane TOPSS program The bottom unit in Figure 6 depicts the same components configured in a stacked arrangement where the diverting box and relief fan are stacked on top of the o
259. mance of the air handling unit casing See Casing performance leakage and thermal p 51 Because intake airflow is a larger percentage of supply airflow freeze protection for the coils is more important see Freeze prevention p 18 SAT reset can also help reduce the risk of freezing coils because it increases supply airflow which lowers the percentage of outdoor air Supply ductwork VAV terminal units and supply air diffusers Insulate and vapor seal all supply ductwork and plenum side surfaces of supply air diffusers and follow the manufacturer s recommendations for insulating and sealing VAV terminal units Select linear slot diffusers with a high aspiration ratio to provide sufficient air movement Use parallel fan powered VAV terminals with the terminal fan cycling on only as the first stage of heat in perimeter zones and in interior zones that experience wide variations in cooling load to recover heat from the ceiling plenum and minimize reheat energy If using conventional diffusers use a fan powered VAV terminal as an air blender to blend warm air from the ceiling plenum with the cold primary air before delivering it to the zone Figure 106 Use parallel fan powered terminals with the terminal fan operating continuously during occupied periods whenever possible to minimize the installed cost and energy consumption of these terminal fans Use an open ceiling plenum return rather than a fully ducte
260. marily on installed first cost often ignore such factors as energy use maintenance requirements or expected life of the equipment Life cycle cost includes the total cost of owning and operating the HVAC system over a given period of time This includes installed cost energy cost maintenance cost replacement cost and any other known and expected costs As mentioned in other parts of this manual chilled water VAV systems are in many ways inherently energy efficient Reducing the quantity of air delivered at part load allows this system to reduce the fan energy required to move this air And the reduced airflow across the cooling coil allows the chilled water plant to unload reducing cooling energy at part load In addition various control strategies and design options provide the opportunity to further reduce the energy use of this type of system Minimum efficiency requirements Many state and local building codes include requirements for minimum levels of energy efficiency Some of these requirements relate to the efficiency of specific equipment such as water chillers or boilers while others relate to the design and control of the overall HVAC system ANSI ASHRAE IESNA Standard 90 1 Energy Standard for Buildings Except Low Rise Residential Buildings is the basis for many of these codes Its purpose is to provide minimum requirements for the energy efficient design of buildings and as such it addresses the entire buildi
261. mponents a a aaa Collection Dust spot Typical controlled Typical applications efficiency1 efficiency contaminant and limitations Typical air filter cleaner type IEST Type F n a lt 0 30 um particles e Cleanrooms HEPA ULPA filters 2 99 999 on 0 1 to e Virus unattached e Radioactive materials 0 2 um particles e Carbon dust e Pharmaceutical IEST Type D n a e Sea salt manufacturing 2 99 999 on 0 3 um e All combustion smoke e Carcinogenic materials particles e Radon progeny e Orthopedic surgery IEST Type C n a 2 99 99 on 0 3 um particles IEST Type A n a 2 99 97 on 0 3 um particles MERV 16 n a 0 3 to 1 0 um particles e Hospital inpatient care Bag filters Nonsupported e All bacteria e General surgery flexible microfine fiberglass or MERV 15 gt 95 e Most tobacco smoke e Smoking lounges synthetic media 12 to 36 in Donici ij E T deep 6 to 12 pockets e e R rops nae el sne eze AO a Box filters Rigid style cartridge MERV 14 90 to 95 e Cooking oil g filters 6 to 12 in deep may use e Most smoke lofted air laid or paper wet MERV 13 80 to 90 Insecticide dust laid media e Copier toner e Most face powder e Most paint pigments MERV 12 70 to 75 1 0 to 3 0 um particles e Superior residential Bag filters Nonsupported e Legionella buildings flexible microfine fiberglass or MERV 11 60 to 65 e Humidifier dust e Better commercial buildings ae moia AA a6 In e Lead dust e Hospital laboratories CEP te pocke
262. mum COz2 limit is set equal to the assumed outdoor CO2 concentration see note above the occupancy sensor can help prevent over ventilation and wasted energy when the zone is unoccupied while other zones served by the same system are still occupied Chilled Water VAV Systems SYS APM008 EN System Controls eT Alternatively if the occupancy pattern of a zone is predictable a time of day schedule in the BAS can be used in combination with a CO2 sensor to reduce over ventilation and wasted energy when the zone is unoccupied while other zones served by the same system are still occupied In most cases the best value for a VAV system is to combine all three DCV approaches using each where it best fits Those zones that are densely occupied and experience widely varying population such as conference rooms auditoriums and gymnasiums are good candidates for CO2 sensors However zones that are less densely occupied or have a population that varies only minimally such as private offices many open plan office spaces and many classrooms are probably better suited for occupancy sensors and or time of day schedules Zones with predictable occupancy patterns such as cafeterias and some classrooms are good candidates for time of day schedules In a VAV system that serves multiple zones zone level ventilation control must be coordinated to determine how to control the system level outdoor air intake see Ventilation optimizat
263. n designing egress paths that comply with the fire code However this population is typically much larger than the expected peak zone population Pz used for designing the ventilation system and for calculating cooling loads Using occupant load rather than zone population to calculate ventilation requirements will often result in oversized HVAC equipment and excessive energy use Figure 83 Impact of zone air distribution effectiveness Ez a9 9 breathing zone 102 System Design Issues and Challenges Pa eT 2 Determine the zone airdistribution effectiveness Ez which depends on the location of supply air diffusers and return air grilles using the default values in Table 6 2 of the standard 3 Calculate the outdoor airflow required for the zone typically at the supply air diffusers by dividing the breathing zone outdoor airflow by the zone airdistribution effectiveness Voz Vbz Ez Minimum ventilation rate required in breathing zone Vbz Table 6 1 of ASHRAE 62 1 prescribes two ventilation rates for each occupancy category one for people related sources of contaminants and another for building related sources For step 1 determine the occupancy category for the zone and look up the corresponding minimum outdoor air rates in Table 6 1 The people related ventilation rate Rp is quantified in terms of cfm person L s person and the building related ventilation rate Ra is quantified in terms of cfm ft L s
264. n in HVAC Systems either SYS APM004 EN For more information on using a dedicated outdoor air system to improve 1 Ducted directly to each zone While this manual is focused on chilled water VAV systems dedicated outdoor 2 Ducted directly to individual dual duct VAV terminals that serve each air systems such as the example shown in Figure 96 are also commonly used zone see Figure 89 p 116 with fan coils water source heat pumps 3 PTACs small packaged rooftop units DX split systems chilled beams or chilled ceilings and variable refrigerant flow VRF systems Ducted to the outdoor air intake of one or more VAV air handling units Figure 96 The example dedicated OA unit depicted in Figure 96 includes a total energy wheel to precondition the entering OA see Air to Air Energy Recovery p 160 and a series desiccant wheel to enable the unit to deliver drier air without requiring a significantly colder leaving coil temperature In this configuration the dedicated OA unit delivers the conditioned outdoor air CA to floor by floor VAV air handling units SYS APMO008 EN Chilled Water VAV Systems 123 124 System Design Issues and Challenges Figure 96 Example dedicated OA system delivering conditioned OA to floor by floor VAV air handling units series desiccant total energy wheel wheel 3 preheat K gt coil dedicated OA unit cooling coil sa to zones
265. nal e No heating coils in the dual units duct VAV terminals and no associated water distribution system Potential for downdraft problems in perimeter zones with very high heat loss Figure 59 p 64 e No fans located in the dual duct VAV terminals e No floor space is required in the zone for the heating system Air Distribution For an indoor installation a VAV air handling unit can be configured to discharge the supply air horizontally or vertically depending on the floor space available and the layout of the mechanical room with respect to the area of the building to be conditioned Additionally the return air can enter the airhandling unit from one of many directions For an outdoor installation both the supply air and return air paths are typically routed downward through the roof curb and building roof into the building However architectural or acoustic limitations may require the supply air to be discharged horizontally from the air handling unit The return air may also need to be connected horizontally rather than from below Contact the equipment manufacturer for specific information on available configurations Supply duct system The supply duct system transports air from the air handling unit to each of the VAV terminal units and then on to the supply air diffusers Figure 62 From its connection to the air handling unit the supply duct may be routed through a central vertical shaft and connected to duct
266. nal units p 58 The magnitude of this trade off varies with building usage and climate For example consider a well constructed building in a mild climate During the day the heat generated by people and lights may be more than enough to Chilled Water VAV Systems SYS APM008 EN Primary System Components a a aT overcome the heat loss through the building envelope This can result in a net cooling load on most days even in perimeter zones Therefore during occupied hours the system may not need to activate the heating coils in the VAV terminal units During the night on the other hand when people are generally gone and most of the lights are turned off the heat loss through the building envelope causes the temperature in the zone to drop In the morning prior to occupancy the supply fan is turned on the outdoor air damper is closed and the heating coils in the VAV reheat terminal units are used to warm the zone In this example the VAV reheat terminals either operate in the cooling mode or in the morning warm up mode and the heating coils are never used to reheat previously cooled air The result is that for a well constructed building in a mild climate overall energy use is likely to be very close to that of a fan powered VAV system and the installed cost is typically lower Of course this changes with climate and building type Benefits Drawbacks Challenges e Normally has the lowest e Uses more terminal heating
267. ndensing boiler A conventional boiler designed to operate without condensing the flue gases inside the boiler Only the sensible heat value of the fuel is used to heat the hot water All of the latent heat value of the fuel is lost up the exhaust stack occupied mode The typical daytime operating mode of a system The building must be ventilated and the comfort cooling or heating temperature setpoints must be maintained in all occupied zones occupied standby mode A daytime operating mode of a system when a zone is expected to be occupied but an occupancy sensor indicates that it is not presently occupied All or some of the lights can be shut off the temperature setpoints can be raised or lowered slightly the outdoor airflow required can be reduced typically to the building related ventilation rate Ra and the minimum primary airflow setting of the VAV terminal can be lowered Chilled Water VAV Systems 219 220 Glossary a optimal start An optimized morning warm up routine that determines the length of time required to bring the zone from its current temperature to the occupied setpoint temperature and then waits as long as possible before staring the system so the zone reaches the occupied setpoint just in time for scheduled occupancy optimal stop An optimized system shutdown routine that determines how early heating and cooling can be shut off for each zone so that the indoor temperature drifts only a few degrees from
268. ned flexible ductwork is very effective at attenuating high frequency noise However it also causes turbulent airflow and relatively large static pressure drops It is best to limit the use of flexible duct to no longer than 6 ft 2 m If the overall length of duct between the VAV terminal unit and diffuser is greater than this sheet metal should be used for the initial section of ductwork while limiting the use of flexible duct to no more than the last 6 ft 2 m needed to connect to the supply air diffusers 146 Chilled Water VAV Systems SYS APM008 EN For more information on the benefits challenges and proper application of cold air VAV systems refer to the Trane Engineers Newsletter titled Cold Air Makes Good ense ENEWS 29 2 and the Cold Air Distribution System Design Guide published by ASHRAE www ashrae org For further energy savings consider keeping the same size ductwork and air handling units not downsizing for installed cost savings This also improves the ability of the system to respond to possible future increases in load since the system will be capable of handling an increased airflow rate if needed SYS APM008 EN System Design Variations This chapter explores several variations to the typical chilled water VAV system design Cold Air VAV Systems Many choices in the design of an HVAC system are predetermined by experience System design engineers repeatedly choose to supply 55 F 1
269. nergy savings is desired consider keeping the same sized pipes not downsizing them for installed cost savings This also improves the ability of the chilled water system to respond to possible future increases in load since the pipes will be capable of handling an increased flow rate if needed Control valve selection The purpose of the control valve on a chilled water coil in a VAV air handling unit is to vary modulate the flow of chilled water through the coil to maintain the desired discharge air temperature Either a three way or two way control valve can be used Figure 74 As a three way control valve modulates to allow less water to flow through the coil thus decreasing its capacity the excess water bypasses the coil and mixes downstream with the water that flows through the coil A two way modulating valve does not bypass any unused water It simply throttles the amount of water passing through the coil The coil experiences no difference in the cooling effect of using a three way versus a two way valve The chilled water distribution system however sees a great difference Table 12 p 84 SYS APMO008 EN Chilled Water VAV Systems 83 FRAME Primary System Components Figure 74 Three way versus two way control valves airflow three way modulating valve With a three way valve the termina two way modulating valve water flow rate water flowing through the coil plus the water bypassi
270. ng The HVAC section of ASHRAE 90 1 includes a large number of requirements related to system design control and construction However this section of the manual focuses on only a few of the HVAC related requirements that are of specific interest to designers of typical chilled water VAV systems Note Because ASHRAE 90 1 is under continuous maintenance it can change frequently This manual is based on the 2007 published version of the standard Refer to the most current version for specific requirements Minimum equipment efficiencies ASHRAE 90 1 contains minimum full and part load efficiency requirements for various types of HVAC equipment water chillers packaged rooftop units Chilled Water VAV Systems 127 System Design Issues and Challenges Pi eT boilers gas fired burners cooling towers and so on Meeting both the full and part load efficiencies is mandatory whether the prescriptive or Energy Cost Budget ECB method of compliance is used Maximum allowable fan system power Because fan energy use depends heavily on the design of the air distribution system it is difficult to prescribe a minimum efficiency requirement for a fan Therefore ASHRAE 90 1 prescribes a limit to the allowable fan system power This limit applies to all fans that operate at peak design cooling conditions For VAV systems this typically includes supply return relief and exhaust fans as well as series fan powered VAV terminals e P
271. ng setpoint by 1 F 0 5 C and resetting the supply air temperature up to 55 F 13 C at part load resulted in lower overall HVAC energy use e Run 5 versus Run 4 Resetting the supply air temperature even further at part load up to 60 F 15 6 C in this example was beneficial in all climates but it had the greatest impact in the economizing dominated climates Los Angeles and Portland Ore Chilled Water VAV Systems SYS APM008 EN TRANE System Design Variations E e Run 7 versus Run 8 When fan powered VAV terminals were used as air blenders parallel fan powered terminals with the terminal fan operating continuously during occupied hours used less energy than series fan powered terminals In this example both the parallel and series fan powered terminals were equipped with ECMs Figure 108 TRACE analysis of a cold air chilled water VAV system 110 90 80 70 60 Run 1 Run 2 R HVAC energy consumption of base case Run 4 Run 5 Run 6 Run 7 un 3 Run 8 base case cold air raise zone SAT reset SAT reset parallel parallel series 55 F SA 48 F SA setpoint 1 F to 55 F to 60 F FPVAV FPVAV w ECM FPVAV w ECM cycled constant m Atlanta 100 104 102 91 87 86 90 104 m Minneapolis 100 104 103 97 95 95 98 106 E Los Angeles 100 101 96 85 75 75 81 97 m Denver 100 103 102 91 87 86 90 106 o Philadelphia 100 105 104 95 92 91 95 106 o Portland Ore
272. ng the coil is relatively constant at all loads Therefore pumping energy will remain relatively constant regardless of the cooling load In addition because cold water is bypassing the coil the temperature of the water returning to the chiller decreases as the zone cooling load decreases With a two way valve however the terminal water flow varies proportionately with the load This provides the opportunity to significantly reduce pumping energy at part load Because there is no mixing of coil and bypassed water the temperature of the water leaving the terminal remains relatively constant at all load conditions Table 12 Three way versus two way control valves Characteristics of system that uses three way modulating control valves Characteristics of system that uses two way modulating control valves Water flow rate through each load terminal water flowing through the coil plus water bypassing the coil remains relatively constant at all load conditions which results in relatively constant pumping energy Temperature of the water returning to the chiller decreases as the cooling load decreases Water flow balance is critical to ensure proper operation because flow is constant e Water flow rate through each load terminal varies proportionately to the load providing the opportunity to significantly reduce pumping energy at part load Temperature of the water returning to the chiller remains relatively co
273. ngs This simplifies control lowers the installed cost and may actually increase accuracy because it avoids the potential inaccuracy of an outdoor sensor The measured concentration of CO2 in the zone is then communicated to the unit level controller for the VAV terminal unit and used to reset the outdoor airflow currently required for the zone Figure 134 If the CO2 concentration in the zone is less than or equal to the minimum CO2 limit the zone ventilation setpoint is at a less than design base outdoor airflow On the other hand if the CO2 concentration is greater than or equal to the maximum CO2 limit the zone ventilation setpoint is equal to the design outdoor airflow requirement If the CO2 concentration is between the minimum and maximum CO2 limits the ventilation setpoint Chilled Water VAV Systems 185 FRAME 186 System Controls eT is adjusted proportionally between the base and design outdoor airflows Figure 134 Varying zone level outdoor airflow requirement based on measured CO2 concentration nanunnannnnunnnnunannnnnn snnnnnn design Perrier anne max CO2 min CO2 a zone outdoor airflow aseene base CO2 concentration in zone Note The simplest approach is to use the assumed outdoor CO2 concentration as the minimum COz2 limit In a multiple zone recirculating system however if a zone is unoccupied while other zones are occupied the concentration of CO2 in the unoccupied zone cannot dro
274. ning Preconditioning the outdoor air with air to air energy recovery offers the following benefits e Reduces cooling dehumidification heating and humidification energy During the cooling season in a climate where it is hot and humid outside a total energy recovery device precools and pre dries dehumidifies the outdoor air by transferring both sensible heat and water vapor to the exhaust air stream During the heating season when outdoor conditions are cold and dry the same total energy recovery device preheats and prehumidifies the outdoor air by removing both sensible heat and water vapor from the exhaust air stream and releasing it into the supply air stream A sensible energy recovery device does not transfer water vapor so it precools the entering outdoor air during warm weather and preheats it during cold weather e Allows downsizing of cooling dehumidification heating and humidification equipment At the design cooling condition air to air energy recovery reduces the cooling load due to the outdoor air allowing the cooling equipment to be downsized At the design heating condition it reduces the heating load due to the outdoor air allowing the heating equipment to be downsized Sometimes a source of heat in the VAV air handling unit can be eliminated if morning warm up can be accomplished at the VAV terminals Also if the system includes mechanical humidification the ability of a total energy recovery device to
275. ns to cool condenser water to the desired setpoint Hot water plant if included e Turns on hot water pumps when hot water is needed if a variable flow system varies the speed of the pumps to maintain pressure in the hot water piping at the desired setpoint Turns on boilers and varies boiler capacity to heat water to the desired setpoint 1 If the system includes baseboard radiant heat it could be used to maintain the unoccupied heating setpoint and the main supply fan could remain off In addition if fan powered VAV terminals are equipped with heating coils only the terminal fan and heating coil for the zone requiring heat would need to be activated and the main supply fan could remain off 2 If flow tracking is used the central return fan modulates to maintain a fixed airflow differential from the supply fan airflow Figure 138 Zone temperature sensor with a timed override button 194 Some systems incorporate a timed override feature which allows the occupant to switch the system into the occupied mode during hours when it is scheduled to be unoccupied The most common means for enabling this function is a timed override button located on the zone sensor Figure 138 Typically pressing this button directs the system to operate in the occupied mode for only a fixed period of time three hours for example After this time period expires the BAS automatically returns the zone to unoccupied mode Morning warm
276. nstant as the cooling load decreases A variable flow system is less sensitive to water balance than most constant flow systems 84 Chilled Water VAV Systems SYS APM008 EN Primary System Components The most common approach to select the control valve is the valve flow coefficient Cv or Kv Cv _2 ie AP AP o where Cv valve flow coefficient Kv OQ fluid flow rate gpm L s AP pressure drop across the valve psi kPa SG specific gravity of the fluid 1 0 for water p density of fluid kg m3 1 000 kg m3 for water In general the pressure drop AP across the valve should be equal to or slightly greater than the pressure drop through the chilled water coil obtained from the manufacturer For example consider a chilled water coil that has a design flow rate of 68 gpm 4 3 L s with a pressure drop of 2 5 psi 17 2 kPa Ideally the pressure drop across the valve should be equal to the pressure drop through the coil Therefore if the fluid is pure water the desired valve flow coefficient is 43 0 1180 68gpm Kv 36x4 3L s 1180 Cv EPM _ 43 0 17 2kPa a r j 1000kg m If a valve with a smaller Cv Kv is selected the pressure drop through that valve will be larger than the pressure drop through the coil at design flow This is generally considered good for valve controllability but make sure that the pump can handle this increase in pressure drop If this valve is locate
277. ntial for overcooling due to minimum airflow settings on VAV terminal units mild rainy 70 F DB part load 69 F WB OA MA 70 6 F DB w E Q RA a at 700 cfm z 55 maa 71 8 F DB D 66 RH o w SYS APM008 EN Chilled Water VAV Systems 119 Doesn t ASHRAE Standard 90 1 prohibit the use of new energy reheat in VAV terminals Not necessarily Section 6 3 2 3 of the 2007 edition defines several exceptions for which new energy reheat is permitted Exception A in the standard permits the use of new energy for reheat after the supply airflow is reduced to a defined limit The minimum airflow setting for most zones in a VAV system is less than the limits defined by this section of Standard 90 1 For further discussion see Simultaneous heating and cooling limitation p 130 120 System Design Issues and Challenges Pi as Resetting supply air temperature One way to prevent this type of overcooling is to reset the supply air temperature upward at low sensible load conditions If the minimum airflow setting is 700 cfm 0 33 m s raising the supply air temperature to 57 9 F 14 4 C for example avoids overcooling the classroom on this example mild rainy day but the cooling coil also removes less moisture from the supply air As a result the relative humidity in the space remains high at 65 percent Figure 92 Figure 92 Impact of supply air temperature reset mild rainy 70 F DB part l
278. ntilation optimization control strategy p 204 which reduces outdoor air intake flow during partial occupancy With less intake airflow the pressure drop through the OA path decreases 14 Chilled Water VAV Systems Primary System Components Figure 7 Dual path VAV air handling unit Source Image adapted from Trane TOPSS program The face area of a cooling coil is dictated by the design airflow through that coil and the size of the coil typically dictates the footprint of the air handling unit the larger the coil the larger the AHU must be to house it In a dual path unit because the RA cooling coil only conditions the recirculated air rather than the mixture of outdoor and recirculated air it can be smaller than it would be for a single path unit Consider an example VAV air handling unit that is sized for 13 000 cfm 6 1 m3 s of supply air of which 3 500 cfm 1 6 m s is outdoor air and 9 500 cfm 4 5 m s is recirculated return air In a single path configuration the single cooling coil must be sized for the total 13 000 cfm 6 1 m s For this unit a size 30 AHU casing results in a coil face velocity of 435 fpm 2 2 m s Note The unit size typically represents the nominal face area of the cooling coil in terms of ft2 In this example the face area of the size 30 air handling unit is 29 90 ft 2 78 m2 In a dual path confi
279. ntral air handling unit and the other path is the recirculation of unused outdoor air that the fan powered VAV terminal draws in from the ceiling plenum Dual fan dual duct VAV systems Figure 116 p 166 also have two ventilation paths One path is the primary air stream from the central cooling air handling unit which brings in outdoor air and mixes it with recirculated return air The other path is from the central heating air handling unit that recirculates return air from all zones Fixed outdoor air damper position One common practice which rarely works as intended for ventilating a VAV system is to set a fixed minimum position for the outdoor air damper With the system delivering design supply airflow this position is set by the test adjust and balance TAB contractor when the system is first started and the damper remains at this same position throughout the full range of system airflows The outdoor air damper is allowed to open further for economizer cooling See Airside economizer control p 174 As the system supply airflow drops during part load conditions the pressure losses due to return air grilles ceiling plenum and return ductwork are lessened Figure 86 This causes the pressure inside the mixing plenum where the outdoor air mixes with the recirculated return air to increase That is it is not as negative with respect to the pressure outside as it was at design supply airflow With the outdo
280. o not come into contact with the air stream being heated but are separated from the air stream through the use of a heat exchanger integrated economizer mode An operating mode of an airside economizer when the outdoor air is warmer than the current supply air temperature setpoint The outdoor air dampers remain wide open return air dampers are closed but the unit controller activates compressors to provide the balance of the cooling capacity needed to provide supply air at the desired setpoint interior zone A conditioned space that is surrounded by other conditioned spaces with no perimeter walls windows Typically requires some degree of cooling all year long to overcome the heat generated by people lighting or equipment Kv Flow factor Term used for the selection of fluid control valves in SI units latent heat Heat that causes a change in the moisture content of the air with no change in dry bulb temperature LEED Leadership in Energy and Environmental Design A building rating system created by the U S Green Building Council a building industry coalition www usgbc org linear slot diffuser A type of supply air diffuser in which jets are formed by slots or rectangular openings with a large aspect ratio See Coanda effect makeup air unit See dedicated outdoorair unit Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary a MERV Minimum Efficiency Reporting Value A rating value defined
281. o the pressure drop through the coil Therefore if the fluid is pure water the desired valve flow coefficient is 1 92 52 8 52 8 36x0 095L s Cy L M ig ae 4 2kP z 2kPa pene i l A o00kg Im If a valve with a smaller Cv Kv is selected the pressure drop through that valve will be larger than the pressure drop through the coil at design flow This is generally considered good for valve controllability but make sure that the pump can handle this increase in pressure drop If this valve is located near the pump the added pressure drop will probably not impact the size of the pump On the other hand if this valve is part of the critical path highest pressure drop path the added pressure drop through the valve may necessitate the selection of a larger pump If a valve with a larger Cv Kv is selected the pressure drop through the valve will be less than the pressure drop through the coil If this difference in pressure drops is too large it could result in poor controllability low valve authority Variable versus constant flow pumping As previously mentioned using two way control valves results in variable water flow through the system which provides the opportunity to significantly reduce pumping energy at part load Chilled Water VAV Systems 97 98 Primary System Components a a aT A variable flow hot water distribution system however may require some means to provide constant wate
282. oach costs more than a system that uses parallel fan powered VAV terminals in the perimeter zones and cooling only VAV terminals in the interior zones Electronically commutated motors on fan powered VAV terminal units An electronically commutated motor ECM is a brushless DC motor that combines a permanent magnet rotor wound field stator and an electronic commutation assembly to eliminate the brushes Figure 56 They are more efficient than the single speed fractional horsepower motor technologies that have traditionally been used in fan powered VAV terminals To offset some of the increased energy use of series fan powered VAV systems the use of ECMs has become increasingly popular Some of the benefits include e Energy savings Figure 57 compares the performance of a standard AC motor with that of an ECM at various airflows for a series fan powered VAV terminal unit Although the efficiency advantage is less at the upper end of the Chilled Water VAV Systems SYS APM008 EN FRAME Figure 56 Electronically commutated motor GE Industrial Systems Primary System Components a aT application range for the motor the efficiency benefit of the ECM is significant at the lower end of the airflow range This efficiency difference often allows ECMs to offer substantial energy savings compared to conventional motor technologies Figure 57 Comparison of motor power in series fan powered VAV boxes size 05 0 5 1 05
283. oad 69 F WB OA MA 74 F DB 65 RH 71 4 F DB RA SA 579 F DB epuoj4 al iAuosyoer It may be tempting to raise the supply air temperature SAT at part load conditions to save cooling or reheat energy But in non arid climates warmer supply air means less dehumidification at the coil and higher humidity levels in the zones And because the supply air is warmer those zones that require cooling will need more air to offset the cooling load which increases supply fan energy consumption See Supply air temperature reset p 202 If dehumidification is a concern avoid using SAT reset when it is humid outside unless a system analysis indicates that humidity in the zones will not rise to unacceptable levels For applications that include SAT reset consider either 1 providing an outdoor dew point sensor to disable reset when it is humid outside or 2 providing one or more zone humidity sensors to override the SAT reset function if humidity in the zone or return air exceeds a maximum limit Reheating tempering supply air at the VAV terminal units Both overcooling and increased humidity levels can be avoided by reheating tempering the supply air after it reaches the minimum airflow setting for the VAV terminal unit Figure 93 illustrates the effect of adding a heating coil to the VAV terminal unit that serves this same example classroom When the primary airflow is reduced to the minimum airflow setting of 700 cfm 0 3
284. oad and the control valve on the chilled water cooling coil is closed The outdoor air damper is allowed to modulate between the minimum position required for proper ventilation and wide open Chilled Water VAV Systems SYS APM008 EN TRANE SystemC Figure 126 Modulated versus integrated economizer modes 85 180 80 160 T c 140 a 120 lt 70 D 100 amp Q so S ia Scop a Raa hig OM 6 5 i loh pp Zer 3 teu Mig Shug z 40 a a 40 3 35 SAT setpoint ta 30 modulata integrated iia 20 economizer economizer sas l i 0 30 40 50 60 70 80 90 100 110 dry bulb temperature F When the dry bulb temperature of the outdoor air rises above the current supply air temperature setpoint the system needs more cooling capacity than the outdoor air alone can provide The outdoor air dampers remain wide open return air dampers are closed but the unit controller modulates the valve on the cooling coil to provide the balance of the cooling capacity needed to achieve the desired supply air temperature This is called integrated economizer mode Figure 126 because both the economizer and cooling coil are used to satisfy the cooling load As outdoor air temperature continues to rise it eventually takes more mechanical energy to cool all outdoor air than it would take to cool a mixture of outdoor air and recirculated return air At this point the outdoorair damper closes to the minimum position required fo
285. of air to each VAV terminal unit without excessive noise e Minimizes the static pressure and associated power requirements of the supply fan e Minimizes the installed cost without great sacrifices in system efficiency e Accommodates space limitations without excessive pressure drop To achieve all of the above system designers commonly use one of two methods to design the supply duct system the equal friction method or the static regain method Chilled Water VAV Systems 71 FRAME Figure 63 Trane Ductulator For more information on the static regain method of duct design refer to the Trane application manual titled Variable Air Volume Duct Design AM SYS 6 For more information on the VariTrane Duct Designer software which can use either the equal friction or static regain method visit www trane com For more information on best practices for the design and layout of duct systems refer to the Sheet Metal and Air Conditioning Contractors National Association SMACNA manual titled HVAC Systems Duct Design 72 Primary System Components a a as Equal friction method Duct systems designed using the equal friction method are sized to have a nearly equal static pressure drop per foot meter of duct length The result is that the static pressure is very high near the discharge of the fan and steadily decreases until it is very low at the inlet to the farthest VAV terminal unit The equal friction method is
286. of gases and odors from the air stream Application considerations include e The activated carbon media adds a pressure drop to the airside system which increases fan energy use e Lower air velocity is required through the bed typically 250 to 300 fpm 1 3 to 1 5 m s for effective operation often increasing the size of the air handling unit e Filtration efficiency is reduced by high humidity levels e Activated carbon does not remove some gases such as carbon monoxide or carbon dioxide e The technology requires frequent maintenance to ensure expected performance and is messy No single media works for all gaseous contaminants Table 9 so it is important to define the contaminants of concern for a given facility In addition the lack of standard performance ratings makes it difficult to effectively evaluate and apply gaseous air cleaners Table 9 Recommended removal media for gaseous contaminants Contaminant Activated Carbon Potassium Permanganate Impregnated Impregnated Media Caustic Carbon Phosphoric Acid Impregnated Carbon Acetic acid Acetone Acrolein Amines Ammonia Benzene Chlorine Ethyl alcohol Formaldehyde Gluteraldehyde Hydrogen cyanide Hydrogen sulfide Methyl alcohol Mercaptans Methylene chloride Methyl ethyl ketone Nitric oxide Nitrogen dioxide Ozone Sulfur dioxide
287. of outdoor air required to properly ventilate that zone varies This strategy commonly referred to as demand controlled ventilation attempts to dynamically reset the system outdoor air intake Vot based on changing population in the zone Some of the commonly used methods of assessing zone population include 1 Time of day occupancy schedules in the building automation system BAS that are used to either indicate when a zone is normally occupied versus unoccupied or to vary the zone ventilation requirement based on anticipated population 2 Occupancy sensors such as motion detectors that indicate when a zone is occupied or unoccupied When unoccupied the zone ventilation requirement is reduced 3 Carbon dioxide CO2 sensors that monitor the concentration of CO2 in the zone as an indicator of the per person ventilation rate cfm person m3 s person actually being delivered to the zone e Resetting intake airflow in response to variations in ventilation efficiency The ventilation efficiency Ey of a multiple zone VAV system changes as zone level and system level primary airflows change with variations in building load This strategy which some have referred to as ventilation reset dynamically resets the system outdoorair intake Vot based on this changing efficiency e Resetting VAV minimums in response to variations in intake airflow In most VAV applications each terminal unit has a minimum airflow setting When th
288. of the zones served by the air handling unit Figure 145 shows an example of a SAT reset strategy based on the changing outdoor dry bulb temperature When the outdoor temperature is warmer than 70 F 21 C no reset takes place and the SAT setpoint remains at the design value of 55 F 13 C When it is this warm outside the outdoor air provides little or no cooling benefit for economizing and the cooling load in most zones is likely high enough that reheat is not required to prevent overcooling In this region the fan energy savings likely outweighs the impact on cooling energy In addition the colder and drier supply air allows the system to provide sufficiently dry air to the zones improving part load dehumidification Chilled Water VAV Systems 203 FRAME 204 System Controls aT Figure 145 Example SAT reset based on outdoor temperature 62 61 supply air temperature setpoint F 40 50 60 70 80 90 outdoor dry bulb temperature F When the outdoor temperature is between 60 F and 70 F 16 C and 21 C the supply air temperature setpoint is reset at a 2 to 1 ratio Figure 145 That is for every 2 F 1 1 C change in outdoor temperature the setpoint is reset 1 F 0 6 C In this range SAT reset likely enhances the benefit provided by the economizer and it is possible that some zone level reheat can be avoided Plus the supply fan has likely been unloaded significantly already so the increased
289. of water leaving the cooling tower e Modulating a throttling valve to reduce the flow rate of water through the condenser e Using a chiller condenser bypass pipe to reduce the flow rate of water through the condenser Figure 136 e Using a cooling tower bypass pipe to mix warm water leaving the condenser with the cold tower water thus increasing the temperature of water entering the condenser e Using a variable speed drive on the condenser water pump to reduce the water flow rate through the condenser Each of these strategies has its advantages and disadvantages Selecting the appropriate condensing pressure control scheme will depend on the specific requirements of the application Chilled Water VAV Systems 189 FRAME For more information on the design of hot water distribution systems refer to Chapters 12 Hydronic Heating and Cooling System Design and 46 Valves of the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org or The Boiler Book by Cleaver Brooks www boilerspec com 190 System Controls Figure 136 Chiller bypass for condensing pressure control cooling tower bypass pipe condenser aN evaporator DG condenser water diverting pump valive water cooled chiller Hot water boiler Each hot water boiler is equipped with a dedicated unit level controller that varies the heating capacity of the boiler to supply hot water at the desired temperatur
290. often used because the calculations are simple and are easily performed using a hand held duct sizing calculator like the Trane Ductulator Figure 63 Drawbacks include the possibility of a higher total pressure drop and higher operating costs Static pressures throughout the duct system are balanced at design airflow through the use of balancing dampers but will not remain balanced at part load airflows For this reason the equal friction method is better suited for constant volume systems than for VAV systems If the equal friction method is used for the main supply ductwork in a VAV system the VAV terminal units should have pressure independent control capability and possibly some balancing dampers to avoid excessive flow rates when upstream duct pressures are high Static regain method Duct systems that are designed using the static regain method strive to maintain a fairly constant static pressure in each section of the entire duct system With this method static pressure is regained as the duct size decreases by converting velocity pressure to static pressure Advantages of using the static regain method include the possibility of reduced overall static pressure drop and lower fan operating costs and more equally balanced pressures throughout the system The drawback of this method is the time consuming iterative calculation procedure For large systems this often requires the use of a computer program The static rega
291. oils hot water or electric installed in the VAV terminal units Each coil is controlled to warm the supply air when necessary A third approach uses perimeter baseboard radiant heat within those zones that require heat The baseboard heaters can be controlled separately or by the controller on the VAV terminal unit Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Overview of a Chilled Water VAV System a a rT When hot water is used for heating it is provided by a hot water system which includes one or more boilers with associated water distribution pumps Each VAV terminal unit is equipped with a unit controller that regulates the flow of primary supply air to the zone to provide cooling heating and ventilation for the zone it serves The VAV air handling unit is also equipped with its own controller A system level controller ties the individual VAV terminal unit controllers to the controller on the air handling unit providing intelligent coordinated control so that the individual pieces of equipment operate together as a system Basic System Operation Unlike a constant volume system which delivers a constant quantity of air at varying temperatures a VAV system delivers a varying quantity of constant temperature air The following section describes in a very simple manner how a typical chilled water VAV system operates For a more detailed discussion see System Controls p 171 Zone is occupied and requi
292. ol method that directly controls the actual volume of primary air that flows to the zone The position of the air modulation damper is not directly controlled and is simply a by product of regulating the airflow through the unit Since the airflow delivered to the zone is directly controlled it is independent of inlet duct static pressure primary air Conditioned air delivered by a central supply fan to a terminal unit psychrometric chart A tool used to graphically display the properties of moist air Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary eT pump pressure optimization An optimized method of variable flow pump control which uses the benefits of DDC control to continuously reset the pressure setpoint of the system so the hot water control valve requiring the highest inlet pressure is nearly fully open recirculated return air Air removed from the conditioned space and reused as supply air usually after passing through an air cleaning and conditioning system for delivery to the conditioned space reducer A transition that reduces the size of the air duct return air Air that is removed from the conditioned space s and either recirculated or exhausted return air grille See grille riding the fan curve A method of fan capacity modulation that involves no direct form of control but simply allows the fan to react to the change in system static pressure and ride up and down it s performance
293. ol primary air and one for warm primary air These units can be controlled to provide either a constant volume or variable volume of supply air to the zone dual fan dual duct VAV system A VAV system that consists of two air handlers one that delivers cool primary air and one that delivers warm primary air These two duct systems provide air to a dual duct VAV terminal unit for each zone dust spot efficiency A rating value defined by ASHRAE Standard 52 1 that depicts the amount of atmospheric dust a filter captures ECM Electronically commutated motor A brushless DC motor that combines a permanent magnet rotor wound field stator and an electronic commutation assembly to eliminate the brushes Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary E electronic air cleaner Particulate filter that uses electrostatic attraction either passively charged electret or actively charged electrostatic precipitators to enhance collection efficiency Energy Star A program administered by the U S Environmental Protection Agency and Department of Energy that helps reduce energy costs and protect the environment through energy efficient products and practices www energystar gov enthalpy Describes the total amount of heat energy both sensible and latent in one pound of air at a given condition enthalpy wheel See total energy wheel equal friction duct design method A method of designing an air duct s
294. oling loads When the zone requires heating the hot water coil heats the supply air to a temperature that is significantly warmer than the zone temperature When the air must be reheated to avoid overcooling however the coil warms the supply air to a temperature that is no higher than the zone temperature The water temperature required for reheating can often be much lower than the temperature required for heating Therefore consider using heat recovery see Condenser heat recovery p 88 or temperature reset control strategies see Hot water temperature reset p 209 to reduce boiler energy use Control valve selection The purpose of the control valve is to vary the flow of water through the coil to maintain space comfort Hot water coils in VAV terminal units are typically either staged on off or modulated With staged control the control valve is either fully opened or fully closed With modulated control the position of the control valve is varied to maintain zone temperature Either a three way or two way control valve can be used Figure 80 As a three way control valve modulates to allow less water to flow through the coil thus decreasing its capacity the excess water bypasses the coil and mixes downstream with the water that flows through the coil A two way modulating valve does not bypass any unused water It simply throttles the amount of water passing through the coil The coil experiences no difference in the
295. ommon sources of recoverable heat include e Warm condenser water leaving a water cooled chiller or hot refrigerant vapor leaving the compressor in an air cooled chiller see Condenser heat recovery p 88 e Another air stream or another location in the same air stream using an air to air heat exchanger see Air to Air Energy Recovery p 160 Fans supply return relief Fans are used to move air throughout the various components of a VAV system Depending on the application the system may include 1 a supply fan only 2 a supply fan and a relief or exhaust fan or 3 a supply fan and a return fan Supply fan only In this configuration Figure 19 the supply fan must create high enough pressure at its outlet A to overcome the pressure losses associated with pushing the air through the main supply ductwork VAV terminal units supply duct runouts and supply air diffusers Note If the system uses series fan powered VAV terminals the small terminal fan is used to overcome the pressure losses between the terminal unit and the zone For further discussion see Fan powered VAV terminal units p 58 Chilled Water VAV Systems SYS APM008 EN TRANE Primary System Components Figure 19 VAV system with supply fan only 100 recirculated air VAV terminal unit zone return air grille AHU main supply supply duct zone ceiling plenum ductwork runouts return ductwork diffusers static pressure rel
296. omponents Figure 17 Indirect fired burner located downstream of supply fan th Direct fired burners should be located upstream of the supply fan Figure 18 This helps avoid unstable burner operation and nuisance trips of airflow safety switches which are often caused by localized high velocities when the burner is located in close proximity to the fan discharge indirect fired gas burner supply fan Source Image adapted from Trane TOPSS program Figure 18 Direct fired burner located upstream of supply fan direct fired supply fan gas burner igi Source Image adapted from Trane TOPSS program Gas supply The supply of natural gas connected to the gas train must be within the range of allowable pressures A higher inlet gas pressure may require a pressure regulator while a lower inlet pressure may require an oversized gas train Consult the manufacturer for specific gas pressure and volume requirements Combustion gas flue stack indirect fired burner only For air handling units located outdoors the manufacturer often provides a combustion gas flue stack to be installed at the jobsite Because the burner is located outside of the building concerns about combustion gases are lessened For units located indoors the engineer must design the combustion gas flue stack based on heat output horizontal and vertical lengths of the stac
297. on TRmin DPTout DBTin DBTout DBTin where TRmin minimum cold spot thermal resistance ratio required DPTout dew point of the air outside the casing F C DBTin dry bulb temperature of the air inside the casing F C DBTout dry bulb temperature of the air outside the casing F C For example consider an air handling unit that will be installed in an unconditioned equipment room The worst case conditions for the equipment room are 96 F 36 C dry bulb and 60 percent relative humidity which equates to a 80 F 27 C dew point If the temperature of the air leaving the cooling coil is 55 F 13 C the thermal resistance ratio of the air handling unit casing must be at least 0 61 TRmin 80 F 55 F 96 F 55 F 0 61 TRmin 27 C 13 C 36 C 13 C 0 61 To minimize the risk of condensation either the air handling unit casing must be above this calculated minimum TR value or the mechanical equipment room can be conditioned to lower the dew point of the air surrounding the unit see Figure 110 p 155 and Figure 96 p 124 VAV Terminal Units The supply ductwork delivers air to each of the VAV terminal units Each zone has a VAV terminal unit that varies the quantity of air delivered to maintain the desired temperature in that zone A VAV terminal unit is a sheet metal assembly consisting of an airflow modulation device a flow sensor on units with pressure indep
298. on on others On projects where acoustics is critical or prior experience is lacking the proper approach is to conduct an acoustical analysis early in the design process Even a simple acoustical analysis can help achieve occupant satisfaction while minimizing installed cost Defining an acoustical model A simple acoustical model consists of a source receiver and path Source The source is where the sound originates Chilled water VAV systems contain several sound sources and each should be reviewed separately Each source has a unique sound quality and level but all of them play a role in determining the sound the receiver hears The foundation of an acoustical analysis is the equipment sound data An accurate analysis depends on accurate sound data for the equipment Indoor sound data for air moving equipment should be measured in accordance with ARI Standard 260 Sound Rating of Ducted Air Moving and Conditioning Equipment and ARI Standard 880 Performance Rating of Air Terminals This ensures that the sound data accurately reflects the contributions of all the sound sources and accounts for the effects of the cabinetry Outdoor sound data should be measured in accordance with ARI Standard 370 Sound Rating for Large Outdoor Refrigerating and Air Conditioning Equipment Indoor water chillers should be rated in accordance with ARI Standard 575 Method of Measuring Machinery Sound within an Equipment Space The indoor sound data is us
299. onfigured inside the terminal unit so that its airflow is in parallel with the primary airflow path In the cooling mode the unit is controlled in the same manner as the cooling only unit Primary airflow is reduced as the cooling load in the zone decreases However when primary airflow reaches the minimum setting for the unit and the cooling load continues to decrease the small fan activates to mix warm plenum air with the cool primary air Figure 52 This increases the total airflow to the zone which improves mixing decreases the risk of temperature stratification and allows the diffusers to perform better It also results in a warmer supply air temperature As additional heating is required the terminal fan remains on and a heating coil is used to further warm the supply air Figure 52 Control of a parallel fan powered VAV terminal unit maximum 100 T primary heating coil activated airflow C oO oO ise a N g 3 minimum BU a annem pono Re ERED EE CTC primary x airflow primary air 0 7 g H design space load design heating load cooling load The terminal unit fan cycles on only when the zone requires heating Operating the fan is a form of energy recovery It tempers the supply air with warm return air which has gained heat from the building and lights This delays the need to use new energy by activating the heating coil A series fan powered VAV terminal unit Figure 53 has a relatively large fan
300. onstructed 162 Chilled Water VAV Systems SYS APM008 EN TRANE System Design Variations buildings little exfiltration due to positive building pressurization with a minimal amount of local exhaust In addition if demand controlled ventilation DCV is being used the amount of outdoor air being brought into the building is reduced for many hours during the year The energy recovery device provides less benefit because there is less outdoor air to precondition and with less air entering the building less air must be exhausted Air exhausted by local exhaust fans and exfiltration due to building pressurization are relatively constant so when DCV reduces intake airflow less central exhaust air is available for energy recovery e Sensible or total energy recovery In most climates a total energy recovery device improves the opportunity for downsizing the cooling and heating equipment and usually provides the best payback because it recovers both sensible heat and water vapor latent heat The most notable exceptions are in dry climates where it is often unnecessary to mechanically dehumidify the outdoor air In this case coil loops and fixed plate heat exchangers typically provide the best value There is a common misperception that only hot humid climates justify the need for total energy recovery When compared with sensible energy recovery however total energy recovery devices can provide advantages in climates where heating
301. ontamination between zones allow for greater optimization of setpoints for energy savings and often avoid the need for return fans A survey of five newly constructed buildings revealed that using smaller air handling units lt 50 000 cfm 24 m3 s resulted in 7 to 8 percent energy savings compared to using larger air handling units SYS APM008 EN Primary System Components This chapter discusses the primary components of a typical chilled water VAV system in greater detail For details on specific pieces of equipment consult the manufacturer VAV Air Handling Unit Return air from inside the building is drawn into the VAV air handling unit AHU Figure 5 through the return air damper and is mixed with outdoor air that enters through the outdoor air damper This mixed air passes through a filter a heating coil a chilled water cooling coil the supply fan and possibly a final filter before it is discharged into the supply ductwork Figure 5 Typical air handling unit used for VAV applications discharge plenum return air filter damper supply fan chilled water cooling coil outdoor air hot water damper heating coil VAV air handling units are typically available with a broad range of options such as energy recovery devices dehumidification enhancements fan choices air cleaning equipment sound attenuation choices and casing performance thermal and leakage options Because of this flexibili
302. ooled chillers Water cooled chillers are typically installed inside the building in a mechanical equipment room The critical sound path for this equipment is radiated sound from the chiller that passes through the walls or other parts of the structure and into the occupied spaces Vibration isolation should be considered for the chiller as well as any connections to the chiller such as piping and electrical conduit Attempting to attenuate the source is not acommon approach for water cooled chillers but occasionally portions of the chiller may be encased in a sound blanket Beside selecting equipment with lower sound levels the most common approach to avoid sound problems with water cooled chillers is to locate the equipment room away from any sound sensitive areas and or construct the surrounding walls with sound transmission loss sufficient to achieve the acoustical goals in the adjacent spaces Pay particular attention to doorways and any penetrations through the walls Keep gaps to a minimum and seal them with an acoustical mastic compound The most common sound problems with chillers installed indoors are due to replacing an older chiller with a new chiller that has higher sound levels or a Chilled Water VAV Systems SYS APM008 EN System Design Issues and Challenges Pi eT different sound spectrum In this case it is generally not possible to relocate the chiller and it can be very difficult to increase the sound transmission l
303. ooling devices refer to the Trane application and wastes energy manual titled Air to Air Energy Recovery fin RAAG VSO BUS EN The method used for capacity control depends on the device Coil loops either vary the speed of the circulation pump or use a three way mixing valve to bypass some of the fluid around the exhaust side coil Fixed plate heat exchangers often use a modulating damper to bypass some of the exhaust air Heat pipes may use a tilt controller bypass dampers or a series of solenoid valves to shut off refrigerant flow for individual heat pipes Wheels use a modulating damper to bypass air around the exhaust side of the wheel Figure 114 or vary the rotational speed of the wheel For wheels exhaust air bypass is recommended because it provides more linear control and a wider range of capacity control than a VFD and in a VAV air handling unit the bypass dampers are likely already provided for economizer operation or to bypass the wheel when it shuts off e Provide a method for frost prevention in cold climates Any air to air energy recovery device that preconditions outdoor air is subject to frost buildup during very cold weather If the surface temperature of the device falls below the dew point of the exhaust air water vapor will condense on the exhaust side of the device If the exhaust side surface temperature falls below 32 F 0 C this water freezes eventually blocking airflow The method used for frost prevention d
304. ooling load continues to decrease the heating coil warms tempers the air to avoid overcooling the zone Figure 49 Control of a VAV reheat terminal unit heating coil maximum 100 7 heating coil activated primary a N airflow minimum primary oO H 3 Pe AAE EENEN Ln nnn e A heating airflow g be F minimum primary ee ee eee eee cooling airflow x 0 H design space load design heating load cooling load When the zone heating load requires the air to be delivered at a temperature warmer than the zone the primary airflow may be increased to a higher minimum setting than is used during the cooling mode Figure 49 This heating minimum airflow setting is needed because when warm buoyant air is supplied from the ceiling a higher velocity is required to effectively mix 56 Chilled Water VAV Systems SYS APM008 EN For more information on this alternate control strategy referred to by some as the dual maximum strategy refer to the California Energy Commission s Advanced Variable Air Volume System Design Guide 500 03 082 A 11 October 2003 An addendum h to ASHRAE Standard 90 1 2007 allows this alternate control strategy with a maximum heating primary airflow lt 50 of maximum cooling primary airflow as an exception to comply with the limitation on simultaneous heating and cooling see Simultaneous heating and cooling limitation p 130 SYS APM008 EN Primary
305. or Optimizing the number of zones If a VAV system is designed with too few thermal zones it may result in undesirable temperature variations for many occupants within the zone A smaller zone is typically better able to closely control temperature which contributes to better occupant comfort However increasing the number of independently controlled zones also raises the installed cost of the system Chilled Water VAV Systems 99 100 System Design Issues and Challenges Pi eT Therefore the optimum number of zones best balances occupant comfort requirements with the budgetary limits of the project The first step is to determine the maximum number of potential thermal zones ignoring cost Each room separated by physical boundaries should be a separate zone Larger open areas should be divided up into several smaller zones The next step is to determine how many of these zones can be easily combined using the following criteria For perimeter zones or interior zones on the top floor of the building e Are there adjacent zones in which the perimeter wall and or roof have the same exposure east facing west facing and so on e Ifso do these zones have the same percentage and type of glass e If so do these zones have approximately the same density of occupants lighting and equipment and are the time of use schedules similar e If so will the occupants accept the temperature varying slightly For interior zones
306. or base ventilation rate Ra whenever the zone is expected to be occupied see Minimum ventilation rate required in breathing zone Vpbz p 102 In a VAV system that serves multiple zones zone level DCV must be coordinated to determine how to control the system level outdoor air intake see Ventilation optimization p 205 184 System Controls eT Oftentimes in an attempt to reduce installation costs the heating control zones are larger than the cooling control zones In this case a single temperature sensor might be used to control the baseboard heat for an area of the building that contains several temperature sensors to control the VAV terminal units In this case it is possible for the areas within the heating zone that do not contain the heating sensor to fight with the cooling zones due to variations in loads within the larger heating zone To avoid this problem increase the number of heating control zones or increase the deadband between the heating and cooling setpoints While increasing the deadband may limit the energy wasted when the systems fight each other it may also result in more occupant complaints regarding temperature control Again the best control approach is to allow the unit level controller on the VAV terminal unit to also control the remote heat source Ventilation control In addition to controlling the temperature in the zone the unit level controller on a VAV terminal unit can
307. or air damper set at a fixed position the quantity of outdoor air entering through the OA damper will decrease That is outdoor airflow will not be held constant but instead will decrease as supply airflow decreases Chilled Water VAV Systems 113 System Design Issues and Challenges Pi a eT Figure 86 Impact of variable airflow on mixing plenum pressure VAV terminal unit zone zone return air grille AHU main supply supply ceiling plenum ductwork ductwork return ductwork diffusers ee full design load part load supply fan static pressure relative to outdoors Lan plenum pressure varies as supply airflow changes ASHRAE Standard 62 1 2007 addresses this practice in Section 5 4 Mechanical ventilation systems shall be designed to maintain the minimum outdoor airflow as required by Section 6 under any load condition Note VAV systems with fixed outdoor air damper positions must comply with this requirement at minimum supply airflow To comply with ASHRAE 62 1 using a fixed OA damper position the OA damper must be set at a fairly wide open position to bring in the required outdoor airflow when supply airflow is at minimum This increases the energy required to heat cool and dehumidify the excess OA that is brought into the system whenever supply airflow is higher than the minimum Proportional outdoor air damper control A relatively inexpensive enhancement to using a fixed OA damper posit
308. or air enthalpy is low Figure 128 Note Because of this Table 6 5 1 1 3A of ASHRAE Standard 90 1 2007 prohibits the use of a fixed enthalpy high limit shutoff strategy in many of the dry and marine climate zones Figure 129 shows one of these hot dry days The enthalpy of the outdoor air OA is below the high limit shutoff setting so the fixed enthalpy economizer opens the outdoor air damper to 100 percent The controller then modulates the valve on the cooling coil to cool this air to the desired supply air SA temperature setpoint However if the economizer was disabled recirculated return air RA would mix with the minimum required outdoor air OA and then this mixed air MA would pass through the cooling coil Figure 129 Fixed enthalpy control in hot dry climates BS 180 wet bulb temperature F P 80 160 140 75 economizer enabled 120 A economizer Seten disabled aie Asp jo qj sures6 ones ypruny 0 30 40 50 60 70 80 90 100 110 dry bulb temperature F If the economizer is enabled the enthalpy difference across the cooling coil is greater than if the economizer is disabled Because of the dry outdoor conditions the cooling coil is not dehumidifying it is only removing sensible heat So if the outdoor air OA is warmer than the return air RA it might take more cooling energy to achieve the same supply air temperature Differential or comparative enthalpy control uses sensors to measur
309. or compressor by varying the speed of the motor that rotates the drive shaft VAV box See VAV terminal unit VAV reheat terminal unit A type of single duct VAV terminal unit that can provide heating using a small heating coil VAV system modulation curve A curve that illustrates the VAV system fan static pressure requirement over the range of airflows VAV terminal unit A sheet metal assembly used to vary the quantity of supply air delivered to the conditioned space ventilation The intentional introduction of outdoor air into a zone through the use of the HVAC system in the building ventilation optimization An optimized control strategy that combines various demand controlled ventilation approaches time of day schedules occupancy sensors and CO2 sensors at the zone level with ventilation reset at the system level ventilation reset A control strategy that attempts to dynamically reset the system outdoor air intake based on changing system ventilation efficiency VOC Volatile organic compound Chilled Water VAV Systems 223 224 Glossary eT warm up mode See morning warm up mode water chiller A refrigerating machine used to transfer heat between fluids waterside economizer A method of free cooling that diverts cool condenser water through a separate heat exchanger to precool the water returning to the water chillers Winterizer An air handling unit configuration that uses a combination of two differen
310. or each independently controlled zone e Supply ductwork and supply air diffusers e Return air grilles ceiling plenum and return ductwork e Water chiller s with associated water distribution pumps and heat rejection equipment cooling towers for water cooled chillers condenser fans for air cooled chillers e Hot water boiler s with associated water distribution pumps or electric heat e System level controls Figure 1 Primary components of a chilled water VAV system VAV air handling unit supply ductwork supply air diffuser VAV terminal unit i a d 1 fk chilled water distribution pumps water chillers SYS APM008 EN Chilled Water VAV Systems 1 Overview of a Chilled Water VAV System a a aT The VAV air handling unit can be located either outdoors typically on the roof of the building or indoors typically in a penthouse or mechanical equipment room in the basement or on one of the occupied floors of the building A building may use a single air handling unit or several units depending on its size load characteristics and function Return air from inside the building is drawn back to the air handling unit Some of this air is exhausted while the rest enters the air handling unit through a return air damper to be mixed with outdoor air that enters through a separate damper This mixed air typically passes through a filter a heating coil a chilled water cooling coil and a supply fan before it is
311. orne airborne supply breakout a wall transmission receiver There are several different types of equipment that make up chilled water VAV systems and each requires acoustical analysis For each sound source it is necessary to determine the path that the sound travels from the equipment to the receiver location These sound paths are dependent on the type and location of the equipment but generally fall into the following categories e Airborne Sound follows the airflow path Supply airborne sound travels in the same direction as the supply air In VAV systems the supply airborne path is also influenced by the discharge sound from the VAV terminal unit Return airborne sound travels against the direction of airflow back through the return air path Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Design Issues and Challenges Pi eT e Duct breakout Sound passes through walls of the ductwork supply or return into the ceiling plenum and then through the ceiling into the occupied space e Radiated Sound radiated from the casing of the equipment VAV terminal air handling unit water chiller etc and travels through whatever is between the equipment and the receiver location For a piece of equipment that is located outdoors this radiated sound travels through a wall or roof For equipment located indoors the radiated sound travels either through walls or in the case of a VAV terminal unit through the cei
312. ort control Chilled water VAV systems are popular because they are capable of controlling the temperature in many zones with dissimilar cooling and heating requirements while using a central air handling unit This is accomplished by providing a VAV terminal unit and temperature sensor for each independently controlled zone When the sun is shining against the west side of the building in the late afternoon a VAV system can provide an increased amount of cool supply air to keep the perimeter zones along the west exposure comfortable while throttling back the airflow to the zones along the east exposure so as not to overcool them Load diversity results in less supply airflow and a smaller supply fan When an air handling unit is used to deliver air to multiple zones the method used to size that supply fan depends on whether the system is designed to deliver a constant or variable quantity of air to each zone If the system is designed to deliver a constant quantity of air to each zone a constant volume system the supply fan must be sized by summing the peak design airflow requirements for each of the zones it serves regardless of when those peak requirements occur However if the system varies the quantity of air delivered to each zone as is the case in a VAV system the supply fan can be sized based on the one time worst case overall block airflow requirement of all the zones it serves since all zones do not require peak de
313. oss of the structure The best approach may be to build an enclosure around the chiller In general these enclosures must be large enough to allow an operator to enter and log the performance of the chiller In addition they should be designed to be disassembled and moved out of the way when the chiller requires major service VAV air handling units Air handling units have the greatest diversity in sound paths of all the For more information refer to the Trane equipment in a chilled water VAV system Sound from these units enters the Engineers Newsletter titled Sound A f PREA i PA Ratings and ARI Standard 260 ENEWS occupied space via all of the sound paths described earlier in this section 29 1 sound follows the supply and return airflow paths and is radiated from the casing and from the inlet and outlet openings However air handling units also offer the greatest flexibility of attenuation options It may be possible to configure the unit in a blow thru or draw thru arrangement individual modules can be stacked on top of each other they offer a variety of fan types and locations and they can be equipped with acoustical lining silencers or discharge plenums All of these options impact the sound from the unit In addition the air handling unit can be located inside or outside of the building Consult with the equipment manufacturer to ascertain how the sound data is determined Indoor sound data for air handling units shou
314. ost electricity to cool or freeze water inside storage tanks During the nighttime hours the outdoor dry bulb and wet bulb temperatures are typically lower than during the day This allows the chiller to operate at a lower condensing pressure and regain some of the capacity and efficiency lost by producing the colder fluid temperatures needed to recharge the storage tanks Another potential benefit of thermal storage is to reduce the required capacity of the chiller When thermal storage is used to satisfy all or part of the cooling load the chiller may be able to be downsized as long as there is still enough time to recharge the storage tanks Hot Water System As mentioned earlier heating in a VAV system can be accomplished using either e Baseboard radiant heat installed in the zone e Heating coils hot water or electric in the individual VAV terminal units e A heating coil hot water steam or electric or a gas fired burner that is located inside the VAV air handling unit If any of these options uses hot water a boiler and hot water distribution system is needed Types of hot water boilers A hot water boiler is a pressure vessel that typically consists of a water tank or tubes with water flowing through them a heat exchanger fuel burners exhaust vents and controls It transfers the heat generated by burning fuel to either water or steam The majority of boilers used in VAV systems are low pressure lt 160 psig 1
315. outdoor air fractions Za but the average outdoor air fraction Xs also increases The overall impact is a higher system ventilation efficiency Ey 0 87 and a lower intake airflow Vot Vou Ey 2800 cfm 0 87 3220 cfm Using the calculated Ey method from Appendix A of the standard the system intake airflow is calculated to be 3220 cfm 1 5 m s at heating design compared to 4310 cfm 2 0 m3 s at cooling design Note that if the default Ey method had been used for heating design calculations in this Chilled Water VAV Systems SYS APM008 EN For a detailed discussion of calculations for systems with multiple recirculation paths refer to the May 2005 ASHRAE Journal article titled Standard 62 1 Designing Dual Path Multiple Zone Systems SYS APM008 EN System Design Issues and Challenges Pa eT example the default value for Ey would be 0 60 maximum Zp 0 55 and the required system intake would have been much higher 4670 cfm 2 2 m3 s Note Unlike multiple zone recirculating systems single zone and most 100 percent outdoor air systems for which Vot depends strictly on Voz usually require more intake airflow during heating operation than during cooling operation Systems with multiple recirculation paths fan powered VAV and dual duct Fan powered VAV systems Figure 51 and Figure 53 p 58 have two paths for outdoor air to get into the zone One path is the primary air stream from the ce
316. outdoor air to a low dew point and then duct the conditioned outdoor air CA directly to floor by floor mechanical rooms Figure 96 p 124 The dry outdoor air maintains a low dew point in the mechanical room Best practices when using cold air distribution Proper engineering and construction practices are critical ingredients in any successful HVAC system Consider the following when designing a cold air VAV system Building construction e Use vapor retarder on the warm side of the insulation in perimeter walls to minimize vapor pressure diffusion e Seal all perimeter wall penetrations e g electrical and plumbing services to minimize infiltration VAV air handling unit e Seal all penetrations including connections for coil piping electrical service and controls e Gasket access panels door openings and inspection windows paying special attention to positive pressure sections e Insulate and vapor seal condensate drain pipes if they run though the building SYS APMO008 EN Chilled Water VAV Systems 155 156 System Design Variations a ae Avoid high dew point temperatures in the mechanical room by pressurizing the mechanical room with dehumidified supply air Figure 110 or using a dedicated outdoor air unit to supply dehumidified outdoor air to floor by floor mechanical rooms Figure 96 p 124 Consult the manufacturer to determine the allowable dew point in the mechanical room based on the thermal perfor
317. ower than outdoors resulting in humid air being drawn in from outdoors However this problem can be minimized or avoided by controlling building pressure and properly sizing return air grilles When properly sized the pressure drop through a return air grille is 0 02 to 0 03 in H20 5 0 to 7 5 Pa This means that the pressure in the ceiling plenum will be 0 02 to 0 03 in H2O 5 0 to 7 5 Pa less than the pressure in the occupied space Consider this example Figure 69 If building pressure is controlled to 0 05 in H20 12 5 Pa above the outdoor pressure then the pressure in the ceiling plenum will be 0 02 in H20 5 0 Pa higher than outdoors Best practices for the return air path When designing the return air path for a VAV system consider the following general recommendations e When using return air linear slots to match the appearance of linear slot supply air diffusers place them perpendicular to the supply air slot diffusers Figure 70 This helps avoid supply air from bypassing the occupied portion of the space and allows for proper air circulation Chilled Water VAV Systems 77 TRANE Primary System Components Figure 70 Place return air slots perpendicular to supply air slot diffusers supply diffuser return diffuser slot perpendicular e Avoid undersizing return air grilles If the return air openings are too small they create too much pressure drop and result in a significant pressure difference
318. p 202 Ventilation optimization demand controlled ventilation ventilation reset p 205 Pump pressure optimization p 208 Chilled water temperature reset p 208 Chiller tower optimization p 210 The impact of any energy saving strategy on the operating cost of a specific For more information on the Trane s system depends on climate building usage and utility costs Building EEE ea ep a analysis software tools can be used to analyze these strategies and convert building analysis software programs y b y g i i visit www trane com energy savings to operating cost dollars that can be used to make financial decisions Figure 99 shows the potential energy savings of using various HVAC strategies in an example office building that has a water cooled chilled water SYS APM008 EN Chilled Water VAV Systems 133 Note that the energy savings in this example is due only to changes in the design and control of the HVAC system There were no changes made to the building envelope no lighting or plug load reductions and no cross cutting strategies like shading or daylighting For any actual project it is strongly recommended that these other types of energy saving strategies also be considered 134 System Design Issues and Challenges Pi eT VAV system The system in the proposed building incorporates the following energy saving strategies e Low flow condenser water distribution system 15 F 8 3 C AT e Low flow c
319. p down to the outdoor concentration Recirculated air from the occupied zones causes the CO2 concentration of the supply air to be higher than the outdoor CO2 concentration The result will be over ventilation at very low occupancy levels A more aggressive approach would be to set this minimum CO2 limit higher than the outdoor CO2 concentration but determining this value is difficult and requires making many assumptions CO2 based DCV requires a CO2 sensor in each zone where it is used and requires periodic calibration and cleaning to ensure proper operation Zones that are densely occupied and experience widely varying population such as conference rooms auditoriums and gymnasiums are good candidates for CO2 sensors Other non critical zones which are likely to be over ventilated at all times never approach their maximum COz levels so CO2 sensors in these zones are unnecessary It takes some time for the indoor concentration of CO2 to decrease when people leave a room An occupancy sensor can be used in combination with a CO2 sensor to reduce zone ventilation more quickly thus saving energy When all the people have left the room the occupancy sensor will indicate that the zone is unoccupied and this signal can be used to reduce the zone ventilation setpoint to the base outdoor airflow even though the measured CO2 concentration is still decreasing and has not yet reached the minimum COQz limit In addition when the mini
320. path can be attenuated by adding one of the following elements 1 A silencer can be installed in the return air opening s to the equipment room Silencers can be effective but they add static pressure loss which makes the fan work harder and generate more sound Also their performance both pressure drop and attenuation is affected by inlet and outlet conditions 2 A section of return ductwork with 2 in 5 cm thick exposed duct lining can provide effective attenuation The length of lined duct depends on how much attenuation is needed When possible use either a T or H shaped section of duct to take advantage of duct end reflection loss Figure 102 The return duct should terminate in the ceiling plenum without an obstruction diffuser elbow or grille on the end and should include at least four equivalent duct diameters of unobstructed straight duct prior to the opening Duct end reflection loss also increases as the duct size decreases so splitting the return duct into multiple smaller ducts increases this attenuation Branching into smaller ducts also provides attenuation at the T junction and splits the one large sound path into several smaller paths that can be moved far apart from each other before the return ducts terminate above the ceiling When sizing the return ducts remember that elbows and junctions generate sound if air velocity is too high or if the inside corners are not sufficiently rounded 142 Chilled Water V
321. per when a coil containing water is exposed to air that is cold enough to freeze see Freeze prevention p 18 e A pressure switch that turns off the fan s to prevent damaging the ductwork when it measures an excessively high static pressure in the duct e One or more temperature sensors that avoid operating a gas fired burner or electric heater at temperatures above the manufacturer s recommendation for safe operation e Acondensate overflow float switch that turns off the fan closes the chilled water control valve and closes the OA damper to prevent overflowing the drain pan in the event that the condensate drain line is plugged e Electrical interlocks that turn off ultraviolet lights if installed when a door is opened to avoid inadvertent exposure of service personnel to UV C light These are just examples Specific details on safeties should be obtained from the equipment manufacturer Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN System Controls aT VAV terminal units Similar to the airhandling unit each VAV terminal unit is typically equipped with a dedicated unit level controller that communicates with the building automation system This controller typically performs the following functions Zone temperature control A zone sensor measures the temperature in the zone The unit level controller compares this measured temperature to the desired setpoint and depending on the type of VAV termin
322. perly sealed vapor retarder must be included on the warm side of the insulation to prevent condensation within the insulation itself If the ductwork or diffusers use internal insulation the sheet metal typically acts as the vapor retarder If external insulation is used it must be covered with a well sealed vapor retarder Figure 109 Insulation on supply air diffusers insulation flexible ductwork plenum om occupied zone e Avoid high dew point temperature in the mechanical room Condensation can occur on the outer surface of air handling units if the surface temperature is below the dew point temperature of the air in the 154 Chilled Water VAV Systems SYS APM008 EN TRANE System Design Variations mechanical room The dew point temperature can be reduced by ducting the outdoor air directly to the intake of the air handling unit and then pressurizing the mechanical room with a small amount of dehumidified supply air Figure 110 The dry supply air limits infiltration into the equipment room and maintains a low dew point by filling the room with low dew point air Figure 110 Pressurize the mechanical room with dry supply air Equipment room conditions 80 F DB 53 F DP 27 C DB 12 C DP return air E J hot humid supply air outdoor air proper trapping Exfiltration Alternatively a dedicated outdoor air unit can be used to separately dehumidify all of the
323. permanent split capacitor AC motor with SCR speed control 0 4 0 84 0 3 0 63 0 2 0 42 energy input W cfm W m2 s 0 1 nan electrically commutated motor ECM 0 0 0 500 0 24 1000 0 48 1500 0 72 2000 0 96 2500 1 20 airflow cfm m3 s The added cost of an ECM can be offset more quickly in applications that require a relatively high number of hours of operation For this reason they are more commonly used with series fan powered terminals in which the terminal fan operates whenever the zone is occupied see Parallel versus series fan powered VAV terminals p 59 However even if a zone may not be a good candidate for this type of motor based solely on energy savings the decision to use an ECM may be based on the other benefits listed below Self balancing An ECM is capable of maintaining a relatively constant airflow regardless of future changes to downstream ductwork Gradually changing sound levels The soft start nature of the ECM allows the fan to ramp up slowly when activated This minimizes the distraction of the fan cycling on and offina parallel fan powered VAV terminal Potential drawbacks include SYS APM008 EN Higher installed cost ECMs require power transistors to drive the stator windings at a specified motor current and voltage level This addition coupled with electronic commutation controls currently make them more expensive to purchase than the
324. picts the same system but designed for a 48 F 9 C supply air temperature and a 1 F 0 5 C warmer zone temperature setpoint This reduces the design airflow to 9300 cfm 4 4 m s The air handling unit can be downsized two sizes to a size 21 and still maintain a similar coil face velocity This reduces the footprint of the unit by 25 percent Figure 105 and reduces the installed weight by 22 percent This decreases the cost of the equipment and requires less floor space and less structural support Table 28 Impact of cold air distribution on AHU footprint and weight Size 30 AHU Size 21 AHU Size 25 AHU Supply air dry bulb 55 13 48 9 48 9 temperature F C Zone temperature setpoint 75 24 76 24 5 76 24 5 OF C Design airflow cfm m3 s 13000 6 1 9300 4 4 9300 4 4 Coil face area ft2 m2 29 90 2 78 20 81 1 93 24 97 2 32 Face velocity fpm m s 435 2 2 447 2 3 372 1 9 Fan input power bhp kW 12 8 9 5 12 2 9 1 8 7 6 5 AHU footprint ft m 12 1 x 7 8 10 6 x 6 7 12 1 x 6 7 3 7 x 2 4 3 2 x 2 0 3 7 x 2 0 AHU height ft m 9 1 2 8 8 4 2 5 9 1 2 8 AHU weight installed Ibs kg 3570 1620 2770 1260 3110 1410 1 Based on a typical VAV air handling unit layout consisting of an OA RA mixing box high efficiency filters hot water heating coil chilled water cooling coil airfoil centrifugal supply fan and a top mounted discharge
325. ply fan is capable of handling the pressure drop of both the supply and return air paths then the relief fan configuration is preferred for the reasons mentioned above Use a return fan only if the return air path adds more pressure drop than the supply fan can handle Chilled Water VAV Systems SYS APM008 EN FRAME Primary System Components aE aS a1 Table 5 Return fan versus relief fan in a VAV system Return Fan Advantages Disadvantages Lower differential pressure across the supply fan if the pressure drop of the return air path is greater than the pressure drop of the outdoor air path which is likely in most VAV systems Potentially lower installed cost for a system with a fully ducted return air path A smaller differential pressure across the supply fan can result in a smaller fan motor and variable speed drive e Higher operating costs especially in applications with extended hours of economizer cooling the return fan must run whenever the supply fan operates Potential for air to leak out through the relief damper because the return air plenum inside the air handling unit operates at a positive pressure F in Figure 22 Note Using low leak relief dampers can minimize air leakage to the outdoors More complex expensive fan speed control Controlling the pressure in the return air plenum inside the air handling unit requires an additional pressure sensor and modulating device either a d
326. primary air 48 F TE 9 C primary air 48 F 19 C E supply air 55 F 13 C parallel fan powered air blender supply air 55 F series plenum air 13 C fan powered aor air blender 27 C Typically the fan in a series fan powered VAV terminal operates continuously during occupied hours When used as an air blender the terminal fan is sized to mix cold primary air 48 F 9 C in this example with warm air from the ceiling plenum 80 F 27 C in this example to deliver 55 F 13 C air to the zone at design cooling load when the primary air damper is wide open As the cooling load in the zone decreases and the primary air damper begins to close the series terminal fan continues to deliver a constant quantity of air to the zone However the resulting temperature of the supply air increases at part load Typically the fan in a parallel fan powered VAV terminal operates only when heat is needed When used as an air blender however the terminal fan is controlled to operate continuously during occupied hours It is sized to mix cold primary air with warm plenum air and in this example deliver 55 F 13 C air to the zone at design cooling load when the primary air damper is wide open As the cooling load in the zone decreases and the primary air damper begins to close the parallel terminal fan continues to operate but the quantity of air delivered to the
327. py rooms 2 Add 2 in 5 cm thick acoustical lining to the duct The length of lined duct depends on how much attenuation is needed 3 Split the supply duct into multiple smaller ducts that exit from different sides of the equipment room This is most easily accommodated by adding a discharge plenum to the air handling unit that extends to the height of the ceiling plenum Figure 104 This allows for straight duct takeoffs in multiple directions minimizing pressure losses and the associated impact on fan energy and noise 144 Chilled Water VAV Systems SYS APM008 EN TRANE System Design Issues and Challenges Figure 104 Discharge plenum with multiple straight connections discharge plenum supply fan 4 Switch to round instead of rectangular ductwork Round duct has significantly higher transmission loss than rectangular duct so it will reduce the sound that breaks out of the ductwork However with less breakout this means that the sound continues to travel down the duct airborne 5 Add a silencer to the supply airborne path to absorb sound energy Realize however that the added airside pressure drop of the silencer makes the fan work harder and generate more noise Therefore adding a silencer should be considered only after other air handling unit and ductwork configurations have been explored VAV terminal units VAV terminal units can impact the acoustics of the occupied space in two ways 1 by adding sound
328. r contaminants For this reason multiple zone recirculating systems have a system ventilation efficiency Ev that is less than 100 percent Some of the outdoor air brought into the system is wasted because of over ventilation in non critical zones However as demonstrated below multiple zone recirculating systems can account for population diversity which helps offset some of this inherent inefficiency Calculating system intake airflow Vot ASHRAE Standard 62 1 defines a procedure to account for system ventilation efficiency and to determine the system level intake airflow Vot the quantity brought through the outdoor air damper in the air handling unit required to deliver the proper quantity of outdoor air to each of the individual zones The standard provides two methods for determining system ventilation efficiency Ev e Table 6 3 default Ey method e Appendix A calculated Ey method The default approach is simpler and takes less time than the calculated approach However the calculated approach is more accurate and usually results in higher efficiency and lower intake flow and may be worth the added effort Note The examples below are not intended to reflect a real life system but rather to demonstrate the calculation methods so the numbers used are simple rather than realistic For more realistic examples refer to the ASHRAE Journal articles listed in the sidebar Table 6 3 default Ey
329. r floors and a positive pressure in the upper floors This pressure difference induces outdoor air to enter the lower floors and indoor air to leave the upper floors while air flows upward within shafts and stairwells Chilled Water VAV Systems 221 222 Glossary a static regain duct design method Method of designing an air duct system that strives to maintain a fairly consistent static pressure throughout the entire duct Recommended for sizing the supply ducts upstream of the terminal units in a VAV system The design of a static regain duct system often requires the use of a computer program supply air Air that is delivered to the zone by mechanical means for ventilation heating cooling humidification or dehumidification supply air diffuser See diffuser supply air temperature reset A control strategy that raises the temperature of the primary air leaving the rooftop unit at part load in an effort to reduce overall system energy use supply duct static pressure control Method of VAV system static pressure control that mounts the static pressure sensor somewhere in the supply duct system allowing the supply fan to back down and lower the static pressure in the system under part load conditions supply duct system A system that is typically constructed of ductwork fittings and diffusers This system transports the supply air from the air conditioning equipment to the conditioned space surge A condition of uns
330. r flow through the boilers or the boilers must be equipped to handle variable water flow Common approaches include a pressure actuated bypass valve primary secondary pumping system or a temperature actuated valve internal to the boiler to divert hot water from the supply and mix it with cooler return water Controls The control of a chilled water VAV system is often grouped into unit level and system level control functions Unit level control refers to the functions required to control and protect each individual piece of equipment System level control refers to the intelligent coordination of the individual pieces of equipment so they operate together as an efficient system In a typical direct digital control DDC system the VAV air handling unit VAV terminal units water chiller and boiler if included are each equipped with a separate unit level controller These unit level controllers are connected to a centralized system level controller Figure 81 With this configuration each unit level controller is capable of performing its functions even if communication with the system level controller is lost Acommon analogy is to view the individual unit level controllers as members of an orchestra and the system level controller as the conductor Figure 81 Control of a chilled water VAV system VAV terminal unit cooling tower water cooled chiller exhaust fan i system level controller BAS
331. r for the zone ensures proper sequencing of the cooling and heating systems If the design heat loss in a perimeter zone is less than 450 Btu hr per linear foot 430 W m of perimeter wall downdrafts are typically less problematic and heated air can be supplied through ceiling mounted diffusers When overhead heating is acceptable VAV reheat or fan powered VAV terminal units can be used to provide both cooling and heating for perimeter zones The rate of heat loss is then used to determine proper diffuser location If the design heat loss is between 250 and 450 Btu hr per linear foot 240 to 430 W m of perimeter wall the diffuser airflow pattern should blanket the perimeter walls with warm air Figure 60 64 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components Figure 60 Perimeter heating in a zone with a moderate amount of heat loss heat loss 250 450 Btu hr ft 240 430 W m m or fan powered terminal unit N diffusers blanket perimeter wall e If the design heat loss is less than 250 Btu hr per linear foot 240 W m of perimeter wall diffusers can typically be located in the center of the room and still provide adequate blanketing to handle the perimeter heat loss Figure 61 Figure 61 Perimeter heating in a zone with a minimal amount of heat loss T p heat loss lt 250 Btu hr ft 240 W m VAV reheat or fan powered terminal unit Inter
332. r leaving coil temperature results in increased achieve a desired supply air temperature dehumidification capacity e Proper condensate trapping is critical to avoid wetting the interior of e Heat added by the supply fan located between the cooling coil and the AHU because the drain pan is under a negative pressure final filters typically prevents final filters from getting wet e Less concern with air leaking out of the AHU since less of the casing is pressurized See Air leakage p 51 SYS APMO008 EN Chilled Water VAV Systems 37 TRANE For more information on fan performance curves system resistance curves and VAV fan modulation refer to the Trane Air Conditioning Clinic titled Air Conditioning Fans TRG TRC013 EN 38 Primary System Components E a aa Supply fan capacity modulation To accommodate variable airflow in a VAV system the supply fan must be selected and controlled so that it is capable of modulating over the required airflow range The pressure drop through ducts fittings coils filters and so forth change as airflow varies The capacity of the supply fan must be modulated to generate sufficient static pressure to offset these pressure losses and provide the minimum pressure required for proper operation of the VAV terminal units and supply air diffusers at all airflows To achieve this balance a simple control loop is used Figure 30 A pressure sensor measures the static pressu
333. r outdoors or indoors An outdoor unit is typically installed on the roof of the building An indoor unit is typically installed in a penthouse the basement or a mechanical equipment room located on one of the occupied floors of the building Table 2 describes the advantages and drawbacks of each location Table 2 Indoor versus outdoor location of air handling units Indoor AHU Advantages Disadvantages e Preventive maintenance on the AHU is performed indoors away e Requires indoor floor space that could have otherwise been used by from harsh weather the building owner or leased to a tenant e Thermal performance of the AHU casing is less critical since indoor e May require the equipment room to be conditioned to prevent temperatures are milder resulting in less heat loss gain condensation sweating on the exterior surfaces of the AHU Outdoor AHU Advantages Disadvantages e Frees indoor floor space that can be used by the building owner or e Preventive maintenance on the AHU may need to be performed leased to a tenant during harsh weather or require the construction of an outdoor e Less concern about condensation sweating on the exterior service corridor surfaces of the AHU since any condensation drains onto the roof e May require more space for vertical air shafts reducing the amount surface of usable floor space Roof structure may need to be strengthened to support the added weight of the AHUs Ther
334. r plus cooling tower fans Coanda effect Concept behind the operation of a linear slot diffuser Air is discharged at a relatively high velocity along the surface of the ceiling creating an area of low pressure that causes the supply air to hug the ceiling As it travels along the ceiling air from the occupied space is drawn into and mixed with the supply air stream When the air settles to the occupied levels of the space it has assumed an average temperature collection efficiency Describes how well a particulate filter removes particles of various sizes from the air stream collision velocity The speed at which moving air meets a wall or another air stream When two air streams collide the collision velocity is determined by adding the velocities of the two air streams at the point of collision combustion efficiency A measure of boiler efficiency that is calculated by dividing the fuel input to the boiler minus stack flue gas outlet loss by the fuel input to the boiler This value generally ranges from 75 to 86 percent for most non condensing boilers and from 88 to 95 percent for condensing boilers compressor A mechanical device used in the vapor compression refrigeration cycle to increase the pressure and temperature of the refrigerant vapor condensate trap Device for collecting liquid formed by the condensation of water vapor on a cooling coil as it travels out of the drain pan for the purpose of preventing the passa
335. r proper ventilation and the valve on the cooling coil modulates as necessary to achieve the desired supply air temperature This decision to disable economizer operation is typically made automatically by comparing the condition of the outdoor air to a setpoint called the high limit shutoff setting Figure 126 While there are numerous high limit shutoff strategies the most common strategies used to control the economizer in a VAV system are fixed dry bulb control fixed enthalpy control and differential or comparative enthalpy control Fixed dry bulb control uses a sensor to measure the dry bulb temperature of the outdoor air The controller compares this temperature to a predetermined high limit shutoff setting and disables the economizer whenever the outdoor dry bulb temperature is above this limit Figure 127 SYS APMO008 EN Chilled Water VAV Systems 175 System Controls aT Figure 127 Fixed dry bulb control of the airside economizer high limit shutoff supply air temperature possible increased a mechanical cooling minimum energy OA intake sag CTC ns While this method is simple and relatively inexpensive fixed dry bulb control For more information on the various can result in suboptimal performance For instance in many non arid methods of controlling airside s oe A z n economizer ark their impad in VAY climates if the high limit shutoff setting is too high this control strategy can systems ref
336. r to Chapters 12 Hydronic Heating and Cooling System Design and 46 Valves of the 2008 ASHRAE Handbook HVAC Systems and Equipment www ashrae org or The Boiler Book by Cleaver Brooks www boilerspec com 94 Primary System Components E a aT although stainless steel may be required in some cases and can often be directly vented through an exterior wall of the building Hot water distribution The hot water distribution system consists of piping pumps valves an air separator and expansion tank and other accessories Design temperatures and flow rates It is important to remember that hot water temperatures and flow rates are variables They should be selected to design an efficient and flexible hot water distribution system Many hot water heating systems are currently designed for 180 F 82 2 C fluid temperature Designing the system for a lower temperature may require more coil surface area more rows more fins to deliver the same heating capacity Table 15 shows an example of the same VAV terminal unit with heating coils selected to deliver 22 8 MBh 6 7 kW of capacity using either 180 F 82 2 C or 150 F 65 6 C hot water Note that in order to deliver equivalent capacity the coil selected for 150 F 65 6 C hot water requires two rows of tubes rather than one row This increases the cost of the VAV terminal unit Table 15 Comparison of 180 F 82 2 C and 150 F 65 6 C supply water tempera
337. r to design install and operate But the packaged nature of this equipment limits flexibility minimizes the potential to reduce energy use and can increase maintenance since the equipment is distributed throughout the facility The challenge for the design team is to take advantage of the benefit of flexibility while keeping the system easy to operate and maintain Pre engineered controls and easy to use building automation systems are two technologies that can help achieve this balance Ceiling space and vertical shafts are required to deliver conditioned air Because the conditioned air is delivered by a central supply fan ceiling space is required to duct the air from the airhandling unit to the VAV terminals and eventually to the occupied spaces In addition for a multi story building one or more vertical air shafts may be needed to duct the outdoor air to floor by floor air handling units These vertical shafts take up some usable floor space in the building To minimize impact on the floor plan these shafts are often located in the core of the building next to elevator shafts and restrooms Chilled Water VAV Systems 9 10 Overview of a Chilled Water VAV System a a eT More sophisticated equipment to maintain and repair Water chillers and hot water boilers are more complicated pieces of equipment than packaged DX units Some facilities prefer to use smaller simpler equipment that can be maintained by the facility s
338. ration diffusers A diffuser with a high aspiration ratio induces air from within the occupied space toward the supply air diffuser It enhances comfort in cold air VAV systems by increasing both air motion and diffuser throw A common example of a diffuser with a high aspiration ratio is a linear slot diffuser see Supply air diffusers p 74 It recirculates approximately 1 cfm m s of air from within the zone for each 1 cfm m s of supply air that it delivers through the diffuser Note Non aspirating diffusers such as perforated plates or concentric grilles may not perform as well in cold air applications because cold dense supply air tends to drop quickly to the floor dumping on the occupants If this type of diffuser is used consider using either series or SYS APMO008 EN Chilled Water VAV Systems 149 TRANE System Design Variations parallel fan powered VAV terminals to blend locally recirculated air with the cold primary air before delivering it to the zone e Fan powered VAV terminals as air blenders Operating the terminal fan continuously during occupied hours blends warm air from the ceiling plenum with the cold primary air before it is delivered to the zone Either a series or a parallel fan powered VAV terminal can be used as an air blender in this manner Figure 106 Figure 106 Fan powered VAV terminals used as air blenders in cold air VAV systems plenum air 80 F 27 C IN
339. re if the wet bulb temperature is 78 F 26 C the cooling tower likely delivers 85 F 29 C water to the water cooled condenser and the refrigerant condensing temperature might be 105 F 40 C This lower condensing temperature pressure reduces the amount of work required by the compressors Figure 72 which reduces compressor energy use This efficiency advantage however may lessen at part load conditions because the dry bulb temperature tends to drop off faster than the wet bulb temperature Figure 72 Comparison of condensing pressures air cooled condenser water cooled condenser expansion pressure rN O_o e m compressor evaporator enthalpy Finally when comparing the efficiency of a water cooled versus air cooled chilled water system be sure to consider the impact of cooling tower fan and condenser water pump energy in the water cooled system and condenser fan energy in the air cooled system Performing a comprehensive energy analysis is the best method of estimating the operating cost difference Chilled water and condenser water distribution The chilled water distribution system consists of piping pumps valves an air separator and expansion tank and other accessories When a water cooled chiller is used the condenser water distribution system consists of piping pumps valves and other accessories Design temperatures and flow rates It is important to remember that wa
340. re 34 Typical particle sizes Ultrafine PM Fine PM Coarse PM 001 005 01 05 1 5 1 5 10 50 100 1000 Human Atmospheric Dust Hair g i 25 c SE Be 8 Oil Smoke Paint Pigments je N See er _Panel Filters oo ae See Se J o i l E a aa 1 Paes Rane 1 001 005 01 05 1 5 1 5 10 50 100 1000 particle diameter um Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components Figure 35 Types of particulate media filters bag filter HEPA filter cartridge filter Images used by permission from CLARCOR Air Filtration Products www clcair com Particulate filter efficiency is typically expressed in terms of dust spot efficiency or minimum efficiency reporting value MERV Dust spot efficiency is defined by ASHRAE Standard 52 1 and relates to the amount of atmospheric dust that a filter captures ASHRAE 52 1 is now obsolete and has been replaced by ASHRAE Standard 52 2 The minimum efficiency reporting value MERV is defined by ASHRAE 52 2 and relates to how efficiently a filter removes particles of various sizes from 0 3 to 1 micron Table 8 identifies common types of particulate filters and their typical applications It also approximates equivalent dust spot efficiencies for the various MERV levels Chilled Water VAV Systems 41 FRAME Table 8 Applications guidelines for various filter types Primary System Co
341. re at a particular location in the duct system A controller compares this static pressure reading to a setpoint and the supply fan capacity is modulated to generate enough static pressure to maintain the desired pressure setpoint at the location of the sensor Figure 30 Supply fan capacity control Tr statielpressure sensor controller Figure 31 depicts an exaggerated example to illustrate this control loop As the cooling loads in the zones decrease the dampers in all or most of the VAV terminal units modulate toward a closed position This added restriction increases the pressure drop through the system reducing supply airflow and causing the part load system resistance curve to shift upwards Figure 31 VAV system modulation curve part load system resistance curve full load system v4 resistance curve VAV system modulation curve static pressure 1 000 rom sensor setpoint airflow Chilled Water VAV Systems SYS APM008 EN Primary System Components a a a ae In response the fan begins to ride up the constant speed rpm performance curve from the design operating point A trying to balance with this new system resistance curve As a result the fan delivers less airflow at a higher static pressure The static pressure controller senses this higher pressure and sends a signal to reduce the capacity of the supply fan Modulating the fan capacity shifts the performance curve of the fan downw
342. reases the temperature difference between the two air streams which improves mixing effectiveness and reduces stratification The advantage of this approach is that it is predictable and effective In cold climates a source of heat may already be needed in the centralized VAV air handling unit However because the heat source needs to be located in the outdoor air stream this approach may limit flexibility or increase the cost of the air handling unit And if a hot water or steam preheat coil is used they also require some method of freeze prevention see Heating coil p 23 One common approach is to use a preheat coil with integral face and bypass dampers These dampers modulate to vary the amount of heat transferred to the air while allowing full water or steam to flow through the coil tubes Use air to air energy recovery to preheat the outdoor air As an alternative to the prior method an air to air energy recovery device such as a coil loop fixed plate heat exchanger heat pipe or wheel can be used to preheat the entering outdoor air during cold weather see Air to Air Energy Recovery p 160 The advantage of this approach is that it also reduces cooling and heating energy use and can allow for downsizing of cooling and heating equipment However such a device does increase the cost of the air handling unit and adds a pressure drop to both the outdoor and exhaust air streams which increases fan energy use
343. related problems in the building envelope where surface temperatures below the indoor dew point are likely Sizing the humidifier can be particularly challenging if the VAV system includes an airside economizer Typically when the outdoor air is driest at the winter design condition for example the OA damper is closed to its minimum position But at other times during the year the economizer may open the OA damper when it is relatively cold and dry outdoors While the outdoor air may not be as dry as it is at the winter design Chilled Water VAV Systems SYS APM008 EN For more information refer to ANSI ASHRAE IESNE Standard 90 1 Energy Standard for Buildings Except Low Rise Residential Buildings and the Standard 90 1 User s Manual both available for purchase at www ashrae org SYS APM008 EN System Design Issues and Challenges Pi eT condition the system is introducing a larger quantity of outdoor air The design engineer should estimate the humidification load at both conditions e Follow the manufacturer s recommendations for downstream absorption distance and maximum relative humidity If the steam is not fully absorbed by the air stream it can cause downstream surfaces to get wet The humidifier should be far enough upstream of elbows junction sensors or dampers to allow for sufficient absorption Absorption distances are shorter with lower air velocities Energy Efficiency Decisions based solely or pri
344. res cooling A sensor in each zone compares the dry bulb temperature in the zone to a setpoint and the VAV terminal responds by modulating the volume of supply air to match the changing cooling load in the zone As the cooling load decreases the VAV terminal responds by reducing the quantity of cold air delivered to the zone The VAV air handling unit is controlled to maintain a constant supply air temperature Depending on the condition of the outdoor air this may involve modulating a control valve on the chilled water cooling coil using outdoor air for free cooling airside economizing or modulating a control valve on the hot water heating coil or gas fired burner The central supply fan modulates to maintain the pressure in the supply ductwork at a setpoint this pressure ensures that all zones receive their required quantities of cold air The outdoor air damper allows the required amount of fresh outdoor air to be brought into the system for ventilation Zone is occupied but requires no cooling or heating As the cooling load in the zone decreases the damper in the VAV terminal closes until it reaches the minimum airflow setting As the load continues to decrease further the constant quantity of cool air causes the dry bulb temperature in the zone to drop below the cooling setpoint If the temperature in the zone falls below the cooling setpoint but remains above the heating setpoint the VAV terminal takes no control action r
345. result in a higher system ventilation efficiency Ev and a lower outdoor air intake flow Vot at design One of the keys at this step is to be realistic ASHRAE 62 1 does not require the system to be designed to handle ANY combination of conditions that might conceivably occur just those that can reasonably be expected to occur under normal operation The 62MZCALC spreadsheet provided with the Standard 62 1 User s Manual is a useful tool that allows design engineers to analyze the impact of various operating scenarios For more information on selecting minimum airflow settings that comply with both ASHRAE Standard 62 1 and 90 1 refer to the Trane Engineers Newsletter titled Potential ASHRAE Standard Conflicts Indoor Air Quality and Energy Standards ADM APNO30 EN and to the Trane Engineers Newsletter Live broadcast DVD titled ASHRAE Standards 62 1 and 90 1 and VAV Systems APP CMC034 EN SYS APM008 EN System Design Issues and Challenges a The fourth step is to calculate the fraction of outdoor air that is required in the primary air for each zone The primary outdoor air fraction Zp is the ratio of zone outdoor airflow to the primary airflow being delivered to the zone The larger the ratio the richer the concentration of outdoor air required to meet the zone ventilation demand Zp Voz V pz where Voz zone outdoor airflow cfm m s Vpz primary airflow cfm m3 s In a VAV system the primar
346. rgy recovery to precondition the entering outdoor air consider the following general recommendations e Properly size the energy recovery device In a mixed air VAV system size the air to air energy recovery device to precondition only the minimum outdoor airflow required for ventilation not the maximum airflow expected during economizer operation This minimizes the first cost of the device Bypass dampers or a separate OA path should be used to allow for the increased outdoor airflow needed during airside economizing see Figure 114 e Strive for balanced airflows Duct as much of the exhaust airflow to the energy recovery device as possible The less disparity between the outdoor and exhaust airflows the more energy can be recovered In applications with a moderate amount of outdoor air for ventilation 20 to 30 percent for example when the system is bringing in minimum ventilation airflow not in airside economizer mode local exhaust fans in restrooms and copy centers for example and exfiltration due to positive building pressurization may be sufficient to relieve all the air brought into the building for ventilation so the central relief exhaust fan is turned off In this case there is no air passing through the exhaust side of the energy recovery device and therefore no energy is saved Therefore air to air energy recovery may only make economic sense for systems with higher amounts of outdoor air or for tightly c
347. rmostat or freezestat is installed on the upstream face of the water coil This sensor measures the lowest temperature in any 12 in 30 cm section of the coil face If the temperature of the air entering any section of the coil approaches 32 F 0 C the unit controller responds by stopping the supply fan closing the outdoor air damper or both This adversely affects occupant comfort and indoor air quality If a chilled water coil is likely to be exposed to air that is colder than 32 F 0 C the system must include some method to protect the coil from freezing Several common freeze prevention methods are listed below Choose the method that best suits the application e Drain the chilled water cooling coils during cold weather This requires vent and drain connections on every coil as well as shutoff valves to isolate the coils from the rest of the chilled water distribution system After draining each coil use compressed air to remove as much water as possible add a small amount of antifreeze to prevent any remaining water from freezing and disconnect the freezestat to avoid nuisance trips The advantage of this approach is that it has minimal impact on the cost of the air handling unit and has no impact on energy use However it does increase maintenance cost especially in locations where the temperature can fluctuate widely during seasonal transitions requiring the coils to be drained and filled several times each season
348. rned off With only the supply fan operating exhaust or return fan is off the zones served by the central air handling unit will become pressurized This mode might be used to prevent smoke from migrating in from other areas of the facility during a fire e Depressurize exhaust In this mode the outdoor air damper is closed the supply fan is turned off cooling and heating are disabled and the exhaust or return fan is turned on and operates at 100 percent airflow With only the exhaust or return fan operating supply fan is off the zones served by the central air handling unit will become depressurized This mode might be used to clear the area of smoke from a recently extinguished fire or possibly to prevent smoke from migrating into other areas of the facility during a fire e Purge In this mode the outdoor air damper is wide open the supply fan is turned on and operates at 100 percent airflow the VAV terminal units are fully open and the exhaust or return fan is turned on and operates at 100 percent airflow This mode might be used to purge the smoke or stale air out of a building after a fire has been extinguished or to prevent smoke from migrating into other areas of the building 212 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Glossary ACH Air changes per hour ADPI Air Diffusion Performance Index A measure of a supply air diffuser s performance when delivering cool air to the zone adsorpt
349. rns about the increase in airside pressure drop 0 45 in H20 112 Pa for the one row coil versus 0 79 in H20 196 Pa for the two row coil and its impact on increasing fan power However because the airside pressure drop decreases quickly as airflow is reduced at part load the actual impact on annual fan energy use is typically small In addition to the supply water temperature the design temperature difference AT also impacts the cost and energy consumption of a hot water system Many hot water heating systems are currently designed for either a 20 F 11 C or 30 F 17 C fluid temperature drop AT Designing the system for a larger AT allows a lower fluid flow rate to deliver the same heating capacity Lower flow rates can allow the pipes pumps and valves to be smaller reducing system installed cost In addition lower flow rates can reduce pumping energy at both full load and part load If further energy savings is desired consider keeping the same sized pipes not downsizing them for installed cost savings This also improves the ability of the hot water system to respond to possible future increases in load since the pipes will be capable of handling an increased flow rate if needed Finally recognize that hot water heating coils in VAV terminal units may be used for two purposes 1 to provide heated supply air to offset heating loads in the zone and 2 to reheat the cool supply air to avoid overcooling the zone at low co
350. rough each applicable device Table 25 cfm L s Chilled Water VAV Systems 129 FRAME For more information on selecting minimum airflow settings that comply with both ASHRAE Standard 62 1 and 90 1 refer to the Trane Engineers Newsletter titled Potential ASHRAE Standard Conflicts Indoor Air Quality and Energy Standards ADM APNO30 EN and to the Trane Engineers Newsletter Live broadcast DVD titled ASHRAE Standards 62 1 and 90 1 and VAV Systems APP CMC034 EN 130 System Design Issues and Challenges a ai aS Table 25 Fan power limitation pressure drop adjustments Pressure drop credit Adjustment PD Fully ducted return and or exhaust air systems 0 5 in H20 125 Pa Airflow control devices in the return and or exhaust air path 0 5 in H20 125 Pa Exhaust filters scrubbers or other exhaust air treatment Pressure drop through device at fan system design condition MERV 9 through 12 particulate filtration 0 5 in H20 125 Pa MERV 13 through 15 particulate filtration 0 9 in H20 225 Pa MERV 16 and higher particulate filtration or electronically enhanced filters 2x clean filter pressure drop at fan system design condition Carbon and other gas phase air cleaners Clean filter pressure drop at fan system design condition Heat recovery device e g wheel coil loop heat pipe fixed plate heat exchanger Pressure drop through device at fan
351. rs see Best practices for locating supply air diffusers p 75 When the discharge air temperature reaches the maximum limit and the zone requires more heating primary airflow is increased while the discharge air temperature setpoint remains at the maximum limit Figure 50 The result is that the airflow modulation damper and hot water valve will modulate open simultaneously By actively controlling the discharge air temperature it can be limited so that temperature stratification and short circuiting of supply to return are minimized when the zone requires heating This improves occupant comfort and results in improved zone air distribution effectiveness see Impact of zone air distribution effectiveness p 102 Chilled Water VAV Systems 57 TRANE Figure 51 Parallel fan powered VAV terminal unit air from ceiling plenum Figure 53 Series fan powered VAV terminal unit primary air 58 Primary System Components a E Fan powered VAV terminal units Fan powered VAV terminal units include an airflow modulation device a flow sensor and a small fan packaged inside an insulated cabinet The terminal fan mixes warm air from the ceiling plenum with cool primary air from the central air handling unit to offset heating loads in the zone A heating coil can be added to the terminal unit and activated when additional heat is required A parallel fan powered VAV terminal unit Figure 51 has a small fan c
352. rs coils If the air handling unit includes a supply fan and a return fan building pressure is directly controlled by varying the position of the relief damper Figure 132 e Adifferential pressure sensor monitors the indoor to outdoor pressure difference Its signal modulates the position of the relief damper directly limiting building pressure The outdoor and recirculating air dampers may share the same actuator but the relief damper must be controlled separately to accommodate varying building pressure setpoints e The return fan which operates whenever the supply fan does pulls return air from the occupied zones and pressurizes the return air plenum in the air handling unit Air from this plenum either passes through the recirculating damper into the mixed air plenum or exits the building through the relief damper Various methods for controlling the capacity of the return fan are discussed in Return fan capacity control p 181 SYS APMO008 EN Chilled Water VAV Systems 179 TRANE SystemC Figure 132 VAV system with central return fan relief return air variable speed damper plenum return fan RELIEF d H ducted return RA aues i local i i exhaust fan differential i iii ick pressure Bee eee cee aca tncnseceseeasasneecenesesocnansueececeeasncneuceeecd Q 23 sensor EJ occupied gt infiltration Shoes exfiltration A recirculating bn l Ly amper variable sp
353. s and Challenges Pi a outdoorair unit while the primary air damper regulates recirculated supply air from the air handling unit to control zone temperature 3 Ducted directly to the outdoor air intake of one or more VAV air handling units Figure 96 p 124 Figure 89 Dedicated outdoor air unit delivering OA to dual duct VAV terminals VAV air handling unit dedicated recirculating outdoor air unit dual duct VAV terminal In either of the first two configurations since the outdoor air is delivered directly to the zones or to zone level VAV terminals this is not considered a multiple zone recirculating ventilation system The system level intake airflow Vot delivered by the dedicated OA unit must be the sum of the calculated zone outdoor airflows Voz Vot Voz This may be less than what ASHRAE 62 1 would require of a conventional VAV system multiple zone recirculating Using the same example see Table 18 p 106 if the dedicated OA system delivers 100 percent outdoor air no mixing with recirculated air to the zones the required outdoor air intake flow Vot ZVoz at cooling design conditions would be 3100 cfm 1 5 m3 s compared to 4310 cfm 2 0 m s for the multiple zone recirculating VAV system However since multiple zone recirculating systems are allowed to account for population diversity see p 107 system level intake airflow for a multiple zone VAV system can be close to or even less than eith
354. s fan powered VAV terminals in all zones costs more than a system that uses parallel fan powered VAV terminals in perimeter zones and cooling only VAV terminals in the interior zones Terminals with hot water heating coils require piping and valves for each unit to be installed in the ceiling plenum Typically require larger power wiring to each unit because the terminal fan in a series fan powered unit is larger than the fan in a parallel fan powered unit Potential for downdraft problems in perimeter zones with very high heat loss Figure 59 p 64 Additional source of noise terminal fan is located above or very near the occupied space Continuous operation of the terminal fan operates whenever the zone is occupied consumes more energy than a parallel fan powered unit where the terminal fan only operates when heating is required 69 70 Primary System Components a a as Interior zones Cooling only VAV Perimeter zones Dual duct VAV Most of the interior zones are served by cooling only VAV terminal units The perimeter zones and certain densely occupied interior zones that experience widely varying occupancy such as conference rooms are served by dual duct VAV terminal units Benefits Drawbacks Challenges e Eliminates terminal heating e Two duct systems are required energy by using warm i e Dual duct VAV terminal units typicall centrally recirculated air YP y cost more than VAV reheat termi
355. s in VAV terminal units can likely provide sufficient reheat with a lower water temperature so hot water temperature reset can be used to save boiler energy Finally when the outdoor temperature is warmer than 50 F 10 C no further reset occurs and the hot water temperature setpoint remains at 150 F 66 C Condenser water temperature chiller tower optimization Depending on the system cooling load and outdoor conditions cooling towers may have the ability to supply condenser water at a colder temperature than at design conditions Lowering the temperature of the water leaving the cooling tower decreases the energy used by the chiller but increases the energy used by the cooling tower fans The key to maximizing energy savings is knowing the relationship between cooling tower energy use and chiller energy use At design conditions a chiller typically uses five to ten times more energy than a cooling tower This would suggest that it might be beneficial to use more cooling tower energy to save chiller energy However there is a point of diminishing return where the chiller energy savings is less than the additional energy used by the cooling tower Figure 150 shows the combined annual energy consumption of a chiller and cooling tower in a system that is controlled to various condenser water temperature fixed setpoints At near design conditions the cooling tower may not be able to supply the temperature requested but it will suppl
356. ses an open ceiling plenum for part of the return air path can use lower cost fans as well as fewer less costly controls Greater layout flexibility The relief fan can be positioned anywhere in the return air path because the supply fan draws the return path negative relative to the occupied spaces during modulated economizer operation An air handling unit installed in the basement with a central relief fan installed on the roof can take advantage of winter stack effect to lower operating cost e Potential for negative building pressure at low loads This condition can occur when a variable speed drive controls the relief fan supply airflow is very low and required relief airflow is less than is delivered with the relief fan operating at lowest speed Note Using a constant speed relief fan with a modulating relief damper avoids this problem e Potential for air to leak in through the relief damper because the return air plenum inside the air handling unit operates at negative pressure whenever the relief fan is turned off C in Figure 21 Note Using low leak relief dampers can minimize air leakage from outdoors Higher differential pressure across the supply fan than in a system with a return fan The supply fan must overcome the pressure drop of the return path as well as the supply path For this reason an air handling unit with a relief fan may not be capable of delivering the pressure differential required in a system with a
357. sign airflow simultaneously Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN Overview of a Chilled Water VAV System a ae a Sie aT Impact of load diversity on sizing the supply fan As seen in the example shown in Table 1 the peak space cooling loads do not necessarily occur at the same time for all spaces served by the system Room 101 has several west facing windows so the peak highest space sensible cooling load occurs in late afternoon 4 00 p m when the sun is shining directly through the windows Room 102 has several east facing windows so the peak space sensible cooling load occurs in the morning 8 00 a m when the rising sun shines directly through its windows If a single constant volume system is used to serve these two zones the system must deliver 3 440 cfm 1 62 m3 s to Room 101 and 2 880 cfm 1 36 m3 s to Room 102 at all times So a constant volume supply fan needs to be sized to deliver 6 320 cfm 2 98 m3 s This is often called the sum of peaks airflow Although Rooms 101 and 102 peak at different times during the day there will be a single instance in time when the sum of these two space loads is highest If these two rooms are served by a single VAV system the supply fan need only be sized for the time when the sum of the space sensible cooling loads is the highest which occurs at 4 00 p m in this example So a variable volume supply fan need only be sized to d
358. sired setpoint Modulates the cooling valve or heating valve to discharge air at the desired setpoint Modulates the position of the outdoor air damper to bring in at least the minimum required amount of outdoor air for ventilation May open the outdoor air damper further if the condition of the outdoor air is suitable to provide free cooling economizer Modulates the central relief damper or relief fan to maintain indoor to outdoor static pressure difference at the desired setpoint Modulates the central return fan if equipped to maintain static pressure in the return air plenum of the air handling unit at the desired setpoint Varies primary airflow cycles terminal fan if included and or modulates or stages a local or remote heat source to maintain zone temperature at the occupied setpoint cooling or heating Chilled water plant Turns on chilled water pumps when cold water is needed if a variable flow system varies the speed of the pumps to maintain pressure in the chilled water piping at the desired setpoint Turns on chillers and varies chiller capacity to cool water to the desired setpoint If water cooled turns on condenser water pumps when chillers need to operate and modulates cooling tower fans to cool condenser water to the desired setpoint Hot water plant if included Turns on hot water pumps when hot water is needed if a variable flow system varies the speed of the pumps to maintain pressure in the hot water p
359. sound e Build a barrier wall Sound from the entire chiller can be directed away from the lot line or other sensitive areas by using a barrier wall Barrier walls generally are not designed to absorb sound although sound absorptive materials are sometimes added to the walls Rather barriers are most effective when used to redirect the sound toward a less sound sensitive area For this reason barriers should be constructed around three sides of the chiller SYS APMO008 EN Chilled Water VAV Systems 139 TRANE 140 System Design Issues and Challenges E a eT with the open side facing away from the sound sensitive area Figure 101 Figure 101 Barrier wall for an air cooled chiller elevation view Barrier wall required on three sides for maximum sound reduction benefit Y a Airflow and py Ja Airflow and p gt service service clearance clearance Seal all wall penetrations Barrier walls should be constructed with sufficient mass to lower transmitted sound by 10 dB in all octave bands and must be free of any openings They should be at least 2 ft 0 6 m taller than the chiller but may need to be even taller to achieve the desired sound reduction Follow the manufacturer s recommendations for airflow clearance between the barrier and the chiller In cases where an extreme sound reduction is required it is possible to build a barrier with built in sound reduction baffles Water c
360. such as conference rooms are served by parallel fan powered VAV terminal units Benefits Drawbacks Challenges e Minimizes terminal heating e Fan powered VAV terminal units energy by using warm air from typically cost more than VAV reheat the ceiling plenum which has terminal units been warmed from heat generated by lights Terminals with hot water heating coils require piping and valves for e Often suitable for retrofit each unit to be installed in the ceiling projects because both cooling plenum and heating can be provided by a single VAV terminal Typically require larger power wiring to each unit because of the terminal e No floor space is required in fan the zone for the heating e Potential for downdraft problems in system perimeter zones with very high heat e Does not require a boiler and loss Figure 59 p 64 water distribution system if electric heating coils are used in the VAV terminal units Additional source of noise terminal fan is located above or very near the occupied space e Fan powered terminals equipped with a heating coil can provide unoccupied heating to the zone without the need to turn on the central supply fan e Operation of the terminal fan consumes energy Fan powered terminal units without a heating coil are typically used for those zones that require year round cooling and have relatively high minimum airflow settings such as interior conference rooms Fan powered terminal units
361. sure control The use of an airside economizer dynamic reset of ventilation air e g demand controlled ventilation the presence of wind stack effect and intermittent operation of local exhaust fans can all cause undesirable changes in building pressure During humid weather maintaining the pressure inside the building so that it is slightly higher than the pressure outside positive pressure increases comfort and helps reduce the leakage of humid outdoor air into the building envelope During cold weather the pressure inside the building should be equal to or even slightly less than the pressure outside This reduces the likelihood of forcing moist indoor air into the building envelope and helps minimize uncomfortable cold drafts due to infiltration In either case excessive building pressure whether negative or positive should be avoided This indoor to outdoor pressure difference can be controlled by adjusting either the quantity of air brought into or exhausted from the building In most VAV systems the minimum quantity of outdoor air brought into the building is based on the ventilation requirements of a local building code so controlling building pressure typically involves varying the quantity of air exhausted from the building The method used to control building pressure depends on the configuration of the fans in the system For a discussion of the different fan configurations including the benefits and drawbacks of
362. system level intake Vot for a dedicated OA system see p 115 For more information on ASHRAE Standard 62 1 and its procedures for calculating zone level and system level outdoor airflow requirements in single zone or 100 percent outdoor air systems refer to the Trane Engineers Newsletter Live broadcast DVD titled ASHRAE Standard 62 1 Ventilation Requirements APP CMC023 EN or to the October 2004 ASHRAE Journal article titled Addendum 62n Single zone amp Dedicated OA Systems available at www ashrae org or www trane com 104 System Design Issues and Challenges Pa a eT does not need to be warmer than the zone Tzone during the heating mode and a zone air distribution effectiveness Ez of 1 0 can be achieved Note The only configuration that has a zone air distribution effectiveness greater than 1 is when cool air is delivered to the zone using low velocity thermal displacement ventilation TDV Typical underfloor air distribution UFAD systems which result in partially mixed zones do not use thermal displacement ventilation and Ez 1 0 during cooling However if either of these two systems is used to deliver warm air through their floormounted diffusers and return air through ceiling mounted grilles zone air distribution effectiveness heating is only 0 7 System level ventilation requirement As mentioned earlier ASHRAE Standard 62 1 also prescribes a process to calculate the outdoor airflow ne
363. t 4 6 m above the roof and average their signals In any case select sensors that will minimize wind effect and keep water out of the sensing tube and locate them to minimize the effects of wind Finally due to wind stack effect and different operating patterns of floor by floor air handling units building pressure control can be particularly difficult in taller buildings Figure 133 demonstrates one approach to control building pressure at each floor 180 Chilled Water VAV Systems SYS APM008 EN FRAME SYS APM008 EN System Controls Figure 133 Floor by floor building pressure control dedicated OA unit with a series desiccant wheel and total energy recovery central relief vee fan with VSD to zones floor by i i floor VAV R P air handling floor by floo units pressure sensor and relief damper to zones pressure sensor in central relief shaft A differential pressure sensor monitors the indoor to outdoor pressure difference on each floor Its signal modulates the position of the relief damper for that floor directly controlling building pressure The floor by floor relief dampers are all connected to a relief exhaust shaft with a central relief fan The signal from a pressure sensor in this relief shaft is used to modulate the capacity of the relief fan through the use of a variable speed drive VSD The fan pressure optimization s
364. t heat See baseboard radiant heat recovered heat 26 reducer 74 reheat 56 66 89 90 95 106 110 120 121 125 152 156 192 202 209 relief fan 12 28 29 30 128 Chilled Water VAV Systems 233 234 Index return fan 26 30 77 179 192 194 return water temperature control 190 return air grilles 2 27 30 77 78 103 106 return air path 77 return air plenum 8 31 179 181 pressure control 181 return fan capacity control 181 S safeties 182 188 scheduling 196 series desiccant wheel 122 124 181 series fan powered VAV terminal unit 58 signal tracking 182 silencers 141 simultaneous heating and cooling 57 130 158 single zone VAV system 157 best practices 158 smoke control 212 sound paths 136 space pressure control 187 static regain method 72 supply duct system 70 supply ductwork 1 2 11 26 54 70 71 136 154 156 supply fan 3 7 26 30 158 172 179 192 194 200 capacity modulation 38 impact of load diversity 7 supply air diffusers 74 supply air temperature reset 22 120 152 156 202 supply duct breakout noise See acoustics System Analyzer 133 system intake airflow 63 105 112 131 system optimization 198 system primary airflow 107 110 system resistance curve 38 201 system ventilation efficiency 104 105 107 108 111 131 170 205 system level control 190 T tempering 65 89 90 120 121 thermal displacement ventilation 103 104 thermal performance 54 thermal storage 92 therm
365. t sized units configured to allow the outdoor air to be introduced downstream of the cooling coil whenever the outdoor air is colder than 32 F 0 C protecting the chilled water cooling coil from freezing zone One occupied space or several occupied spaces with similar characteristics thermal humidity occupancy ventilation building pressure zone air distribution effectiveness Ez A measure of how effectively the air delivered to the zone by the supply air diffusers reaches the breathing zone Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN References Air Conditioning and Refrigeration Institute ARI 2008 Air Terminals ARI Standard 880 2008 Arlington VA ARI 2008 Method of Measuring Machinery Sound Within an Equipment Space ARI Standard 575 2008 Arlington VA ARI 2001 Sound Rating of Ducted Air Moving and Conditioning Equipment ARI Standard 260 2001 Arlington VA ARI 2001 Sound Rating of Large Outdoor Refrigerating and Air Conditioning Equipment ARI Standard 370 2001 Arlington VA ARI American Society of Heating Refrigeration and Air Conditioning Engineers Inc ASHRAE 2007 ASHRAE Handbook Applications Atlanta GA ASHRAE 2005 ASHRAE Handbook Fundamentals Atlanta GA ASHRAE 2008 ASHRAE Handbook HVAC Systems and Equipment Atlanta GA ASHRAE 1996 Cold Air Distribution System Design Guide Atlanta GA ASHRAE 2002 Designer s Guide to Ceiling Based
366. table fan operation where the air alternately flows backward and forward through the fan wheel generating noise and vibration system resistance curve A plot of the static pressure drop that the system including the supply ductwork duct fittings terminal units diffusers and grilles coils filters dampers etc creates over a range of airflows system ventilation efficiency Ev A measure of how efficiently the system distributes air from the outdoor air intake to the breathing zone TAB Test adjust and balance tempering The process of adding sensible heat to the supply air at the VAV terminal unit to avoid overcooling the zone thermal displacement ventilation A method of air distribution in which cool air is supplied at low velocity directly to the lower part of the occupied space Heat is carried by convective flows created by heat sources into the upper part of the zone where is extracted throw Horizontal or vertical axial distance an air stream travels after leaving an air outlet before the maximum stream velocity is reduced to a specified terminal velocity defined by ASHRAE Standard 70 total energy recovery The transfer of sensible and latent moisture heat between two or more air streams or between two locations within the same air stream total energy wheel A rotating heat recovery device that recovers sensible temperature and latent humidity heat from one air stream and releases it to another adjacent
367. taff Many buildings that use chilled water VAV systems use facility staff to perform regular maintenance such as changing filters inspecting and tightening fan belts and cleaning drain pans and then hire an outside service provider for repair work and regular maintenance of the larger pieces of equipment The right answer for each project depends on the expertise and availability of the facilities maintenance staff Common Building Types That Use Chilled Water VAV Systems Chilled water VAV systems are used in almost all building types but the most common uses include e Commercial office buildings e Schools both K 12 and higher education e Hospitals clinics and medical office buildings e Large hotels and conference centers e Large retail centers shopping malls e Airports e Laboratories e Industrial facilities and manufacturing processes Chilled Water VAV Systems SYS APM008 EN A few large air handling units or several smaller units A building may use a few large air handling units or several smaller units depending on size load characteristics and function A study commissioned by the U S General Services Administration GSA concluded that using multiple small air handling units is more desirable than using fewer large air handling units Callan Bolin and Molinini 2004 Smaller air handling units allow more diversity for after hours operation provide more flexibility result in less cross c
368. tdoor air damper or airside economizer ASHRAE 90 1 requires some method of demand controlled ventilation for any zone larger than 500 ft2 50 m2 that has a design occupancy of more than 40 people 1000 ft2 of floor area 40 people 100 m2 Small systems in which the design system level outdoor airflow is less than 1200 cfm 0 6 m s or systems for which ASHRAE 90 1 requires exhaust air energy recovery are exempt from this requirement In addition any zone in which the design supply airflow is less than 1200 cfm 0 6 m s is exempt Opportunities to further reduce system energy use While local building codes might include requirements for minimum levels of energy efficiency many building owners desire even higher efficiency levels for their buildings In addition programs like ENERGY STAR administered by the U S Environmental Protection Agency and Department of Energy DOE Rebuild America administered by the DOE and LEED Leadership in Energy and Environmental Design created by the U S Green Building Council a building industry coalition have encouraged higher levels of energy efficiency in buildings Table 26 contains a list of several system design options and control strategies that can help further reduce the energy use of a chilled water VAV Chilled Water VAV Systems SYS APM008 EN System Design Issues and Challenges i eT system This list is not intended to be all encompassing but focuses on those energy sa
369. tem specific details should be obtained from the manufacturer of the equipment Extended discussions in this section will be limited to those unit level control issues that require decisions to be made by the HVAC system designer or system operator VAV air handling unit Typically the central air handling unit is equipped with a dedicated unit level controller that communicates with the building automation system BAS In a VAV application this controller typically performs the following functions Discharge air temperature control A sensor measures the temperature of the air leaving the air handling unit The controller compares this measured temperature to the desired setpoint and modulates a control valve on the chilled water cooling coil varies the position of the outdoorair damper for economizer operation or modulates a control valve on the hot water heating coil or gas fired burner When on off or step control of cooling or heating capacity air is used such as electric heaters or two position valves the actual temperature of the discharge air may swing above and below the desired setpoint However this typically has little impact on comfort within the zone Ventilation control In a VAV application as supply airflow drops at part load conditions so does return airflow With less return airflow the pressure drop from the zone to the air handling unit is reduced If the outdoor air damper remains at a fixed position th
370. ter VAV Systems 193 FRAME System Controls eT Table 32 describes the typical functions of the different system components during the unoccupied mode Table 32 Coordination of equipment during unoccupied mode Central air handling unit Supply fan is off unless the zones require cooling or heating If the supply fan is on modulates the cooling valve or heating valve to discharge air at the desired setpoint Outdoor air damper is closed unless the zones require cooling and the condition of the outdoor air is suitable to provide free cooling economizer or if exhaust fans operate during the unoccupied mode not recommended Central return fan if equipped is off unless supply fan is on and then it is modulated to maintain static pressure in the return air plenum of the air handling unit at the desired setpoint VAV terminal Varies primary airflow cycles terminal fan if included and or modulates or stages a local or remote heat source to maintain zone temperature at the unoccupied setpoint cooling or heating Chilled water plant Turns on chilled water pumps when cold water is needed if a variable flow system varies the speed of the pumps to maintain pressure in the chilled water piping at the desired setpoint Turns on chillers and varies chiller capacity to cool water to the desired setpoint If water cooled turns on condenser water pumps when chillers need to operate and modulates cooling tower fa
371. ter temperatures and flow rates are variables They should be selected to design an efficient and flexible water distribution system Table 11 shows an example of selecting a chilled water cooling coil in a 13 000 cfm 6 1 m3 s VAV air handling unit The left hand column shows the performance of this coil when it is selected with a 44 F 6 7 C entering fluid Chilled Water VAV Systems 81 Primary System Components a a aT temperature and a 10 F 5 5 C fluid temperature rise AT To provide the required 525 MBh 154 kW of cooling capacity the coil requires 105 gpm 6 6 L s of water The right hand column shows the performance of same coil but in this case it is selected with 40 F 4 4 C entering fluid and a 15 5 F 8 7 C AT To provide the equivalent capacity the coil requires only 67 5 gpm 4 3 L s of water Table 11 Impact of entering fluid temperature and flow rate on cooling coil Conventional Low flow system design system design Coil face area ft2 m2 29 90 2 78 29 90 2 78 Face velocity fpm m s 435 2 2 435 2 2 Coil rows 6 rows 6 rows Fin spacing fins ft fins m 83 272 83 272 Total cooling capacity MBh kW 525 154 525 154 Entering fluid temperature F C 44 6 7 40 4 4 Leaving fluid temperature F C 54 12 2 55 5 13 1 Fluid AT F C 10 5 5 15 5 8 7 Fluid flow rate gpm L s 105 6 6 67 5 4 3 Fluid pressure
372. tes the supply fan to maintain static pressure in supply duct at the desired setpoint and shuts off when all zones have reached their occupied heating setpoints Outdoor air damper is closed Modulates the heating valve to discharge air at the desired setpoint Modulates the central return fan if equipped to maintain static pressure in the return air plenum of the air handling unit at the desired setpoint VAV terminal Opens the air modulation damper until zone temperature reaches the occupied heating setpoint then the damper closes Chilled water plant Chilled water pumps are off Chillers are off If water cooled condenser water pumps and cooling tower fans are off Hot water plant if included Turns on hot water pumps if a variable flow system varies the speed of the pumps to maintain pressure in the hot water piping at the desired setpoint Turns on boilers and varies boiler capacity to heat water to the desired setpoint 1 In some buildings outdoor air may be brought into the building during the morning warm up mode to dilute contaminants that have accumulated inside the building during the unoccupied mode This is often called a preoccupancy purge In this case the central relief damper or relief fan should modulate to maintain indoor to outdoor static pressure difference at the desired setpoint 2 If there is no source of heat in the central air handling unit morning warm up can be accomplished with t
373. the supply fan to operate at a lower static pressure and therefore use less energy Figure 144 For this example the supply fan controlled using fan pressure optimization uses only 43 percent of full load power compared to 55 percent with a non optimized strategy that maintains a constant static pressure at a location two thirds of the way down the supply duct Table 35 Comparison of fan capacity control methods control method airflow fan static fan input power full load pressure power full load 24 000 cfm 2 7 in H2O 22 hp 16 4 kW 100 11 3 m3 s 672 Pa part load fan outlet 18 000 cfm 2 1 in H20 13 hp 9 7 kW 60 8 5 m3 s 523 Pa supply duct 18 000 cfm 1 9 in H20 12 hp 8 9 kW 55 8 5 m3 s 473 Pa optimized 18 000 cfm 1 5in H20 9 5 hp 7 1 kW 43 8 5 m3 s 374 Pa A second benefit of fan pressure optimization is lower noise levels Because the supply fan does not need to generate as much static pressure it generates less noise In addition with lower pressures in the supply ductwork the dampers in the VAV terminals will be more open resulting in less noise generated by the VAV terminals Another benefit is reduced risk of fan surge This optimized control method allows the system to operate as if there is a static pressure sensor installed directly upstream of each VAV terminal unit By keeping the worst case damper nearly wide open at all times the VAV system modulation curve is
374. the zone requires a less than design base outdoor airflow typically the building related ventilation rate Ra Table 29 Combining an occupancy sensor with a time of day schedule Time of day Occupancy sensor Operating mode of Outdoor airflow schedule reads indicates VAV terminal unit setpoint occupied occupied occupied design outdoor airflow occupied unoccupied occupied standby base outdoor airflow less than design unoccupied n a unoccupied no outdoor airflow Occupancy sensors are relatively inexpensive do not need to be calibrated and are already used in many zones to control the lights Zones that are less densely occupied or have a population that varies only minimally such as private offices many open plan office spaces and many classrooms are good candidates for occupancy sensing Carbon dioxide CO2 sensors A sensor is used to monitor the concentration of CO2 in the zone which is being continuously produced by the occupants The difference between the CO2 concentration in the zone and the outdoor CO2 concentration can be used as an indicator of the perperson ventilation rate cfm person m3 s person Note In most locations the concentration of CO2 outdoors remains relatively constant Because of this and in lieu of installing an outdoor CO2 sensor most designers use either a one time reading of the outdoor CO2 concentration at the building site or a conservative value from historical readi
375. ther modules This reduces the overall length to 13 8 ft 4 2 m The width of the unit remains the same but of course the unit is taller 10 2 ft 3 2 m compared to 5 1 ft 1 6 m for the non stacked unit In many buildings with slab to slab heights of 12 ft 3 7 m or more the added height of a stacked air handling unit is not an issue And by using a stacked configuration the footprint of this example unit is reduced by 40 percent Of course the larger the airflow capacity the taller the airhandling unit will be So for very large units stacking may not be possible due to the floor to floor height limitation of the building Multiple fans dual fans or fan arrays Using multiple fans rather than a single supply fan can also result in a shorter air handling unit Figure 27 p 35 For a given airflow a unit with multiple fans uses several smaller diameter fan wheels rather than a single larger diameter fan wheel Upstream and downstream spacing length requirements are typically a function of the fan wheel diameter except for very small sizes where access requirements dictate the necessary spacing Therefore using multiple smaller diameter fan wheels can shorten the upstream and downstream spacing required and can shorten the length of the overall air handling unit see Fan types p 32 However using multiple fans is typically less efficient and increases the cost of the air handling unit Dual path configura
376. this sensor at all load conditions can be difficult often determined by trial and error or requiring the installation of multiple sensors Field installation and adjustment of one or possibly several duct pressure sensors increases installation cost However using this method typically allows for more fan energy savings than compared to the fan outlet method because the setpoint is just high enough to maintain the pressure corresponding to that location in the duct system at design supply airflow Figure 125 In addition the sensor in the supply duct allows fan airflow to be reduced before the fan enters the surge region Chilled Water VAV Systems 173 For a chilled water VAV system in which providing an airside economizer is difficult or costly because the outdoor air ductwork must be sized for full economizer airflow or where a significant amount of humidification is needed the system can be designed with a waterside economizer p 91 174 System Controls eT Figure 125 Fan energy saved using static pressure sensor in supply duct rather than at fan outlet fan operating point at design airflow static pressure sensor at fan outlet additional energy savings sensor setpoint sensor in supply duct airflow ASHRAE 90 1 requires the static pressure sensor be located such that the controller setpoint is no greater than one third the total design fan static pressure unless th
377. ting these two modes will slide to the right on the psychrometric chart Figure 107 This strategy can help recover some of the lost modulated economizer hours but likely increases fan energy SYS APMO008 EN Chilled Water VAV Systems 151 FRAME 152 System Design Variations a eT e While a colder supply air temperature reduces the quantity of primary air sent to each zone the outdoorair requirement for the zone remains unchanged The result is that the minimum airflow settings at the VAV terminal units may need to be a higher percentage of design airflow in order to ensure each zone delivers the required outdoor airflow under low load conditions This may cause the reheat coils in the VAV terminal units to activate sooner in a cold air system than in a traditional VAV system Using supply air temperature reset at part load and optimizing the control of ventilation see Ventilation optimization p 205 can help reduce the operating cost impact of reheat e When fan powered VAV terminals are used as air blenders the power needed for continuous operation of the terminal fans during occupied hours adds up Consider using ECMs on terminal fans to minimize energy use see Electronically commutated motors on fan powered VAV terminal units p 60 Also parallel terminals with constant fan operation during occupied hours consume less energy than series terminals The varying impact on overall system energy use reflec
378. tion Another way to reduce AHU footprint is to use a dual path configuration to separately condition the recirculated return air RA and outdoor air OA Each air path includes a dedicated cooling coil but the same fan serves both paths Figure 7 Chilled Water VAV Systems 13 FRAME Avoiding fan surge in a dual path AHU When a dual path configuration is used for a VAV air handling unit preventing the supply fan from operating in the surge region can be more challenging As supply airflow is reduced at part load outdoor airflow through the OA path may remain nearly constant to ensure proper ventilation while the recirculated airflow through the RA path is reduced Therefore the pressure drop through the OA path remains high while the pressure drop through the RA path decreases This high pressure drop through the OA path can cause the supply fan to surge at reduced supply airflow To help prevent the fan from operating in the surge region e Size the components in the OA path for a low airside pressure drop This may involve increasing the casing size for the top section OA path of the air handling unit e Carefully select the supply fan to reduce the potential for surge A direct drive plenum fan p 35 often provides the greatest flexibility for selection e Implement the fan pressure optimization control strategy p 199 which allows the fan to generate less pressure at part load e Implement the ve
379. tion requirement for their respective zones Voz based on measured CO2 Zones that are less densely occupied such as open plan office spaces or do not experience significant variations in occupancy such as private offices and many classrooms are probably better suited for occupancy sensors When the sensor indicates that the zone is unoccupied the controller lowers the ventilation requirement for the zone Finally zones that are sparsely populated or have predictable occupancy patterns may be best controlled using a time of day schedule This schedule can either indicate when the zone will normally be occupied versus unoccupied or can be used to vary the zone ventilation requirement based on anticipated population These various zone level DCV strategies can be used to reset the ventilation requirement Voz for their respective zones for current conditions The BAS periodically gathers this data from all the VAV terminal units and uses the ventilation reset equations to determine how much outdoor air must be brought in at the central air handling unit to satisfy all of the zones served Figure 148 Chilled Water VAV Systems 207 FRAME The ventilation optimization strategy can be used to earn the Outdoor Air Delivery Monitoring credit Indoor Environmental Quality section of LEED 2009 because it involves installing a CO2 sensor in each densely occupied space and an outdoor airflow measuring device to measure intake air
380. to satisfy all or part of the cooling load see sidebar reducing the cooling load on the chiller plant Finally for those zones with very low cooling loads raising the supply air temperature increases air circulation and can avoid or minimize the use of reheat energy However because the supply air is warmer those zones that require cooling will need more air to offset the cooling load This increases supply fan energy And warmer supply air means less dehumidification at the coil in non arid climates and higher humidity levels in the zones see Resetting supply air temperature p 120 Chilled Water VAV Systems SYS APM008 EN FRAME System Controls eT Table 36 lists conditions that favor the use of SAT reset and conditions that reduce the potential to save energy using SAT reset Table 36 Conditions that favor or don t favor the use of SAT reset Conditions that favor using SAT reset Conditions that reduce potential to save energy with SAT reset Mild climate with many hours when the outdoor dry bulb temperature is below 70 F 21 C VAV terminal units that have minimum airflow settings higher than 30 percent of design airflow increases likelihood that reheat will be needed to avoid overcooling the zones Efficient design of the air distribution system low pressure loss causes less of a penalty for the higher airflows that result from raising SAT Systems with interior zones that have varying
381. troduce particles downstream of the high efficiency filters negating some of the benefit of using final filters Finally if possible avoid combining final filters with a blow thru supply fan that is where the cooling coil is located between the supply fan and final filters Figure 38 Systems with this configuration often experience problems with final filters getting wet Experience indicates that this problem can often be minimized by using a draw thru supply fan where the supply fan is located between the cooling coil and the final filters or by adding a few degrees of heat to the air before it passes through the final filters Figure 38 Avoid combining a blow thru supply fan with final filters final filters cooling coil Some particulate filters use electrostatic attraction to enhance efficiency These electronic air cleaners are either passively charged electret or actively charged electrostatic precipitators Passively charged filters are standard panel or pleated media filters that receive an electrostatic charge to the media during the manufacturing process This charge increases the efficiency rating by one to two MERV levels and typically dissipates over time Actively charged air cleaners electrostatic precipitators include an active electrical field that charges the media and in some cases the particles as they enter the filter system This can greatly improve the fine particle collect
382. ts a A i Box filters Rigid style cartridge MERV 10 50 to 55 e Milled flour filters 6 to 12 in deep may use e Coal dust lofted air laid or paper wet MERV 9 40 to 45 e Auto emissions laid media e Nebulizer drops e Welding fumes MERV 8 30 to 35 3 0 to 10 0 um particles e Commercial buildings Pleated filters Disposable e Mold e Better residential buildings xtended surface 1 to 5 in thick MERV 7 25 to 30 e Spores e Industrial workplaces with cotton polyester blend ae Paine bost nieta media cardboard frame e e P all ee SNES OEGE AI Cartridge filters Graded density MERV 6 lt 20 e Fabric protector viscous coated cube or pocket e Dusting aids filters synthetic media MERV 5 lt 20 e Cement dust Throwaway Disposable e Pudding mix synthetic media panel filters e Snuff e Powdered milk MERV 4 lt 20 gt 10 0 um particles e Minimum filtration Throwaway Disposable e Pollen e Residential buildings fiberglass or synthetic panel MERV 3 lt 20 e Spanish moss e Window air conditioners filters i Dust mites Washable Aluminum mesh f latex coated animal hair or foam MERV 2 lt 20 e Sanding dust rubber panel filters e Spray paint dust Passive electrostatic MERV 1 lt 20 e Textile fibers electret Self charging Carpet fibers passive woven polycarbonate panel filter 1 Minimum Efficiency Reporting Value MERV is defined by ANSI ASHRAE Standard 52 2 1999 Method of Testing General Ventilation Air Cleaning Devices for Removal
383. ts the complex relationship between building usage climate and the design and control of the HVAC system Increased reheat energy and fewer hours of airside economizer operation partially offset the fan energy savings Intelligent system control is crucial to fully realize the potential energy savings of cold air VAV systems In addition it can be a challenge to achieve both energy savings and first cost savings with a cold air VAV system Downsizing the ductwork saves first cost but results in less fan energy savings than keeping the same sized ductwork Building analysis software like Trane s TRACE 700 can be used to determine the most desirable balance between energy savings and first cost for any particular project For further energy savings consider keeping the same size ductwork and air handling units not downsizing for installed cost savings This also improves the ability of the system to respond to possible future increases in load since the system will be capable of handling an increased airflow rate if needed Figure 108 includes the results of a TRACE analysis for an office building in six different climates The building uses a chilled water VAV system e Run 2 versus Run 1 Simply lowering the supply air temperature to 48 F 9 C without changing zone setpoints or using supply air temperature reset may use more energy than the conventional 55 F 13 C design e Runs 3 and 4 versus Run 1 Raising the zone cooli
384. ture 180 F 82 2 C 150 F 65 6 C hot water hot water Coil rows 1 row 2 rows Heating capacity MBh kW 22 8 6 7 22 8 6 7 Coil flow rate gpm L s 1 33 0 084 1 34 0 085 Fluid AT F C 34 18 9 34 18 9 Returning fluid temperature F C 146 63 3 116 46 7 Fluid pressure drop ft H20 kPa 1 10 3 3 0 12 0 35 Airside pressure drop at design cooling 0 45 112 0 79 196 airflow in H20 Pa Airside pressure drop at heating airflow 0 04 10 1 0 07 17 6 in H20 Pa 1 Assumes airside pressure drop changes with the square of the airflow reduction design cooling airflow 2000 cfm 0 94 m3 s reheat heating airflow 600 cfm 0 28 m3 s However using a lower entering water temperature and or a larger fluid AT results in a lower return water temperature In this example with 150 F 65 6 C supply water temperature the temperature of the water returning to the boiler decreases to 116 F 46 7 C While this may be too low for a conventional non condensing boiler it can significantly increase the efficiency of a condensing boiler Figure 79 Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a aT In addition the larger two row coil reduces the fluid pressure drop 1 10 ft H20 3 3 kPa for the one row coil versus 0 12 ft H20 0 35 kPa for the two row coil which may decrease pumping power Some engineers raise conce
385. turn from the zones back to the air handling unit Air typically leaves the zones through ceiling mounted return air grilles and travels through the open ceiling plenum to either the mechanical room when floor by floor indoor air handling units are used or a central vertical air shaft when outdoor air handling units are used A return air duct may be used to direct the air returning from the open ceiling plenum into the mechanical room or up the vertical shaft Figure 62 p 71 Open ceiling plenum versus fully ducted return Most VAV systems use the open plenum space above the ceiling to return air from the zones This minimizes installed cost and lowers the pressure loss through the return air path Alternatively some applications use sheet metal ductwork from the return air grilles all the way back to the return air opening of the air handling unit This increases installed cost adds more pressure loss that the fans need to overcome typically requiring a return fan and possibly VAV terminals in the return duct system and makes the system more difficult to balance and control So why do it Sometimes it is required to meet a local building code Sometimes it is done to allow easier cleaning of the return air path Some in the industry recommend using a ducted return to prevent moisture related problems in the ceiling plenum They claim that using an open plenum for the return air path causes the air pressure inside the plenum to be l
386. ty there is generally no standard configuration for a VAV system When multiple units are used a common approach is to dedicate one air handling unit and therefore one VAV air distribution system to serve each floor of the building An advantage of this approach is that it minimizes the number and or size of vertical shafts used to route ductwork inside the building Using fewer and or smaller vertical air shafts will likely increase the amount of usable floor space in the building Another advantage is that if the floors are leased to different organizations it offers a simple way to bill tenants individually for their HVAC energy use An alternative approach is to use one air handling unit to serve each exposure of the building For example all the west facing zones are served by one unit all the east facing zones are served by another unit and so on The advantage of this approach is that all the zones served by an air handling Chilled Water VAV Systems 11 Primary System Components a aT unit are thermally similar potentially allowing for reduced energy use A drawback is that depending on the shape of the building it may necessitate the use of multiple vertical air shafts which can result in less usable floor space This approach may be combined with the use of floor by floor air handling units to serve the interior zones Indoor versus outdoor air handling units The VAV air handling unit can be located eithe
387. uded in Appendix A of ASHRAE 62 1 For systems with a single recirculation path cooling only VAV terminals or VAV reheat terminals like the system used in this example this calculation is a fairly simple process For systems with multiple recirculation paths fan powered VAV terminals or dual duct VAV systems the process is more complex See Systems with multiple recirculation paths p 113 Calculating system ventilation efficiency for a system with a single recirculation path requires three steps Step 6a is to calculate the average fraction of outdoor air in the system Xs Xs Vou Vps where Vou uncorrected outdoor air intake cfm m s Vps system primary airflow cfm m s Appendix A of the standard defines system primary airflow Vps as the sum of the zone primary airflows In a VAV system for cooling design calculations the maximum airflow delivered by the supply fan or block airflow is less than the sum of the zone peak airflows because of load diversity Table 21 This block airflow can be obtained from a load calculation program or by summing the peak airflows for all the zones and multiplying this sum by an assumed load diversity factor For this example system the system block airflow system primary airflow at cooling design conditions is 18 600 cfm 8 8 m3 s So the average outdoor air fraction for the system Xs is 0 15 2800 18 600 1 3 8 8 This means that if the system was 100 percent
388. ver the perimeter zones also include baseboard radiant heat along the perimeter walls Benefits Drawbacks Challenges e Simple dependable heating Requires coordinated control to system prevent the baseboard heating system from fighting with the e Radiant heat below the cooling only VAV system windows helps minimize downdrafts e Hot water baseboard radiant heat normally has a relatively high e Simple control of morning installed cost warm up operation e Baseboard heating systems require floor space within the perimeter zones e No hot water pipes to serve VAV terminals need to be installed in the ceiling plenum e Hot water or steam radiant heat requires additional space in the building for a boiler and water or steam distribution system e No VAV terminal fan energy consumption Interior zones Cooling only VAV Perimeter zones VAV reheat Most of the interior zones are served by cooling only VAV terminal units The perimeter zones and certain densely occupied interior zones that experience widely varying occupancy such as conference rooms are served by VAV reheat terminal units VAV reheat typically has the lowest installed cost among VAV systems particularly when electric heat is used in the VAV terminals But this combination is typically less efficient than systems that use parallel fan powered VAV terminals fan powered VAV terminals recover heat from the ceiling plenum See Fan powered VAV termi
389. vings strategies that are of specific interest to designers of typical chilled water VAV systems Table 26 Potential energy savings strategies for chilled water VAV systems VAV air handling unit Upsize AHU casing p 18 High efficiency fans p 32 Lower pressure drop filters p 43 Cold air distribution p 147 Precondition outdoor air with air to air energy recovery p 160 Series desiccant wheel for low dew point applications p 122 Dedicated OA system p 123 Dual fan dual duct VAV system p 165 Fixed or differential enthalpy control of the airside economizer p 174 Evaporative cooling p 21 Chilled water and hot water systems High efficiency water chillers p 79 Water cooled chiller plant p 79 Lower flow rates on the chilled water and condenser water distribution systems p 81 Variable flow pumping including variable primary flow systems p 86 Waterside economizer p 91 Chillers in series SYS APMO001 EN Condenser heat recovery for reheat p 88 Thermal storage p 92 Condensing boilers and lower water temperatures p 92 VAV terminal units Parallel fan powered VAV terminals for those zones that require heat p 59 Electronically commutated motors ECM on fan powered VAV terminals p 60 System level controls Optimal start stop p 198 Unoccupied nighttime economizing p 199 Fan pressure optimization p 200 Supply air temperature reset
390. w One of the best methods for controlling system level intake airflow Vot ina multiple zone VAV system is to actually measure the outdoor airflow and control it directly This is typically accomplished by using a flow measuring device in the outdoor air stream such as a flow measuring OA damper Figure 88 or a field installed airflow measurement station This method is accurate over a wide range of airflows and can respond to pressure fluctuations caused by wind or stack effect It reduces the energy use associated with over ventilation and has the added benefit of providing a means to document the outdoor airflow brought into the system over time However outdoor airflow measurement does increase the cost of the system Dedicated outdoor air systems As described earlier in a conventional VAV system the Ventilation Rate Procedure in ASHRAE 62 1 often requires the air handling unit to bring in more outdoor air than simply the sum of zone ventilation requirements However some VAV system designs use a dedicated outdoor air unit to condition all the outdoor air for the system This conditioned outdoor air CA is then either 1 Ducted directly to each zone 2 Ducted directly to individual dual duct VAV terminals that serve each zone Figure 89 the ventilation damper in the dual duct VAV terminal maintains the required quantity of outdoor air from the dedicated Chilled Water VAV Systems 115 116 System Design Issue
391. warmer than zone 1 0 lt Tzone 15 F 8 C ceiling floor warmer than zone 1 0 floor thermal ceiling cooler than zone 1 2 displacement ventilation floor underfloor air ceiling cooler than zone 1 0 distribution floor ceiling warmer than zone 0 7 Excerpt from Table 6 2 of ANSI ASHRAE Standard 62 1 2007 In most VAV systems the supply air diffusers are located in or near the ceiling When cool air Tsa lt Tzone is delivered to the zone through these ceiling mounted diffusers the system is 100 percent effective at getting the outdoor air into the actual breathing zone that is Ez 1 0 This is the case whether the return air grilles are located in or near the ceiling or in or near the floor However when hot air Tsa gt Tzone 15 F 8 C is delivered to the zone through the same ceiling mounted diffusers and then leaves the zone through ceiling mounted return air grilles the zone air distribution effectiveness is only 0 8 When supplied and returned overhead the buoyancy of this hot air will tend to cause some of the air to bypass from the supply air diffusers to the return air grilles without reaching the actual breathing zone Therefore this configuration is less than 100 percent effective at delivering outdoor air from the diffusers into the breathing zone For zones that require heating employ one of the following strategies e If Tsa gt Tzone 15 F 8 C increase the outdoor air
392. wever the coil warms the supply air to a temperature that is no higher than the zone temperature The water temperature required for reheating can often be much lower than the temperature required for heating and can be a good application for a hot water temperature reset control strategy to save boiler energy For an example see Condenser heat recovery p 88 Figure 149 shows an example of a hot water temperature reset strategy based on the changing outdoor dry bulb temperature When the outdoor temperature is colder than 20 F 7 C no reset takes place and the hot water temperature setpoint remains at the design value of 180 F 82 C When it is this cold outside it is likely that many of the zones require the hotter water temperature for space heating Chilled Water VAV Systems 209 FRAME 210 System Controls eT Figure 149 Example hot water temperature reset based on outdoor temperature hot water temperature setpoint F 150 0 10 20 30 40 50 outdoor dry bulb temperature F When the outdoor temperature is between 20 F and 50 F 7 C and 10 C the hot water temperature setpoint is reset at a 1 to 1 ratio Figure 149 That is for every 1 F 0 6 C change in outdoor temperature the setpoint is reset 1 F 0 6 C In this range few zones are likely to require space heating but are more likely to require reheat to avoid overcooling at minimum primary airflow In this case the heating coil
393. with specially designed low velocity air systems However it is typically impractical for most conventional HVAC systems The required dose of UV C light varies widely depending on the type and form of the microorganism Viruses require a much lower dose of UV energy than spore forms of fungi such as mold and bacteria Figure 40 Chilled Water VAV Systems SYS APM008 EN FRAME For more information on photocatalytic oxidation PCO refer to the Trane engineering bulletin Trane Catalytic Air Cleaning System CLCH PRB0O23 EN SYS APM008 EN Primary System Components a a aaa Figure 40 Effectiveness of UV C light 100 x o S g E bacteria ta o 2 P O oO amp spores o fungal bacterial low high dose If using UV C lamps in air handling units be aware of the following safety and material considerations Include interlock switches and warning signs at each entry location UV C energy poses a significant health hazard to unprotected human skin and eyes To safeguard against inadvertent exposure by operators or maintenance personnel electrical interlock switches should be provided at all entry locations to the air handling unit to disconnect power to the lamps when a door is opened Industry recognized warning signs should also be placed at all entry locations Substitute or shield any susceptible materials UV energy can also prematurely age many polymeric plastic materials Susceptible
394. work that is located in the ceiling plenum above each floor of the building Between the air handling unit and the VAV terminal units most of the supply ductwork is constructed of sheet metal However a short section of flexible Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a aT duct is commonly used to connect the sheet metal duct to the inlet of each VAV terminal unit This allows for some flexibility when the sheet metal ductwork and VAV terminals are not installed at the same time Figure 62 Air distribution components of a chilled water VAV system VAV air handling unit ot supply air diffuser gt i P Sa VAV terminal unit F return ductwork supply ductwork system level controller The supply ductwork between the VAV terminal units and the supply air diffusers is constructed of either sheet metal or flexible duct It is best however to limit the use of flexible duct to no longer than 6 ft 2 m sections to reduce the turbulence and high pressure drop associated with flexible duct If the overall distance between the VAV terminal and diffuser is greater than 6 ft 2 m sheet metal should be used for the initial sections of ductwork while limiting the use of flexible duct to no more than the last 6 ft 2 m needed to connect to the supply air diffusers A successful design of the supply duct system achieves the following e Supplies the required quantity
395. y air to avoid overcooling the zone Similar to a SYS APM008 EN Chilled Water VAV Systems 169 TRANE For a detailed discussion of the ventilation calculations for a dual fan dual duct VAV system refer to the May 2005 ASHRAE Journal article titled Standard 62 1 Designing Dual Path Multiple Zone Systems available at www ashrae org 170 System Design Variations a fan powered VAV terminal mixing this recirculated air with cool primary air avoids the need to use new energy for heating Elimination of reheat is the greatest energy benefit of the dual fan dual duct VAV system Use Appendix A calculated Ey method from ASHRAE Standard 62 1 to calculate the system level ventilation requirement Dual fan dual duct VAV systems have two paths for delivery of outdoor air to the zone one path is the primary air stream from the cooling air handling unit which brings in outdoor air and mixes it with recirculated return air and the other path is the heating air handling unit which recirculates unused ventilation air with return air from all zones Appendix A in ASHRAE 62 1 provides a calculation method for determining system ventilation efficiency calculated Ey method which gives credit to systems with multiple recirculation paths and typically results in a lower outdoor air intake requirement Vot Implement variable rather than constant airflow to the zone wherever possible While some zon
396. y air to hug the ceiling and air from the space to be drawn into and mixed with the supply air stream This increases air circulation and allows the air to reach an average temperature before it settles to the occupied levels of the space Chilled Water VAV Systems SYS APM008 EN FRAME For more information on space air diffusion refer to e 2005 ASHRAE Handbook Fundamentals Chapter 33 www ashrae org e ASHRAE s Designer s Guide to Ceiling Based Air Diffusion www ashrae org SYS APM008 EN Primary System Components Figure 66 Coanda effect of linear slot diffusers supply air linear slot diffuser supply duc 55 F see air from space 5 ft 1 5 m Best practices for locating supply air diffusers Proper selection and placement of supply air diffusers generates air movement throughout the conditioned space eliminating areas of stagnant and stratified air Other publications contain more complete details related to sizing and locating supply air diffusers but the following are a few general recommendations e Select and lay out supply air diffusers to achieve at least an 80 percent ADPI at cooling design airflow Air Diffusion Performance Index ADPI is a measure of the diffuser s performance when delivering cool air to the zone e Keep air throw as long as possible to maximize the effectiveness of air diffusion For a VAV system some amount of overthrow at design airflow is
397. y airflow changes with the cooling load so Zp increases when the zone primary airflow decreases For this reason for design purposes ASHRAE 62 1 requires using the minimum expected primary airflow Vpz Vpzm for VAV systems Determining this minimum expected value requires judgment by the design engineer It may be the minimum airflow setting for the VAV terminal or it might be a higher value see sidebar Note Do not set the minimum primary airflow Vpzm equal to the zone outdoor airflow Voz This approach results in an intake airflow equal to the system primary airflow that is 100 percent outdoor air Using the minimum primary airflow Vpzm the outdoor air fractions Zp vary from 0 40 to 0 50 for this example system Table 19 So 0 50 represents the critical highest demand of the system Table 19 System level ventilation calculations based on default Ey method design Vpz Vpzm Voz Zp cfm cfm cfm Voz Vpzm South offices 1900 475 210 0 44 West offices 2000 500 220 0 44 South conf room 3300 825 330 0 40 East offices 2000 500 220 0 44 South interior offices 7000 1750 850 0 49 North interior offices 7000 1750 850 0 49 North offices 1600 400 200 0 50 North conf room 1800 450 220 0 49 The fifth step is to determine the uncorrected outdoor air intake Vou This is the intake airflow that would be required if the system was 100 percent efficient at deliv
398. y the water at the coldest temperature possible Chilled Water VAV Systems SYS APM008 EN TRANE SYS APM008 EN System Controls Figure 150 Optimized control of condenser water temperature 300 000 i cooling tower ales chiller at 5 Ooo E i a 200 000 O O i i oO c v 100 000 85 F 70 F 55 F optimized 29 C 21 C 13 C control condenser water temperature setpoint The fourth column shows a system that uses the system level controller to dynamically determine the optimal condenser water temperature that minimizes the combined energy use of the chiller plus cooling tower fans Coordination with other building systems System level control provides the opportunity to coordinate the operation of the HVAC system with other building systems such as lighting security and fire protection Following are some examples e A time of day schedule that is used to turn on and off the HVAC system could also be used to turn on and off lights inside or outside the building In addition an occupancy sensor could be used to indicate that a zone is actually unoccupied even though the BAS has scheduled it as occupied see Occupied standby mode p 192 and turn off all or some of the lights and or plugged in equipment When the occupancy sensor indicates that the zone is again occupied the lights are turned back on e An occupancy sensor that is used to turn on and off lights in a pr
399. ystem is relatively uncommon because of the need to install two separate duct systems For more discussion see Dual Duct VAV Systems p 165 Minimum primary airflow settings In most applications each VAV terminal unit has a minimum primary airflow setting to ensure proper ventilation or to ensure proper operation of the terminal unit or supply air diffusers Providing less than this required minimum airflow may e Underventilate the zone and degrade indoor air quality e Result in poor distribution mixing of supply air into the conditioned space some types of supply air diffusers require a minimum discharge velocity Chilled Water VAV Systems SYS APM008 EN SYS APM008 EN Primary System Components a a aT e Cause erroneous airflow readings that interfere with proper control of the VAV terminal unit the accuracy of the flow sensor in the VAV terminal is based upon a specific airflow range For multiple zone VAV systems this minimum primary airflow setting should be greater than the zone ventilation requirement Voz This avoids excessively high zone primary outdoor air fractions Zp which could significantly increase the required system intake airflow see Ventilation p 101 However ASHRAE Standard 90 1 places limits on using new energy to reheat air that has been previously cooled See Simultaneous heating and cooling limitation p 130 for more information on how to select minimum primary
400. ystem that results in an equal static pressure drop per foot meter of duct Equal friction duct systems can be easily designed by hand evaporative cooling Sensible cooling obtained by latent heat exchange from water sprays or jets of water evaporator The component of the refrigeration system where cool liquid refrigerant absorbs heat from air causing the refrigerant to boil exhaust air Air that is removed from the conditioned space s and then discharged to the outdoors expansion device The component of the refrigeration system used to reduce the pressure and temperature of the refrigerant expansion tank A component of a closed piping system that accommodates the expansion and contraction of the water as temperature and therefore density changes face velocity Velocity of the air as it passes through a device airflow rate divided by the face area of the device fan array A configuration that uses multiple fans stacked to allow for parallel air paths fan outlet static pressure control A method of VAV system static pressure control that mounts the static pressure sensor near the outlet of the main supply fan and maintains a constant static pressure at the sensor fan performance curve A plot of a specific fan s airflow capacity at a given speed rpm versus the static pressure it generates fan powered terminal unit A type of single duct VAV terminal unit that can provide heating to a zone by mixing warm plenum

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