Trane trg-trc001-en User's Manual
Contents
1. Psychrometry period seven Review Let s review some of the main concepts from this clinic on psychrometry Properties of Air Humidity Ratio grains Ib of dry air 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F The lines of the psychrometric chart represent five physical properties of air dry bulb wet bulb dew point humidity ratio and relative humidity If any two of these properties are known the remaining properties can be determined from the chart 52 TRG TRC001 EN TRG TRC001 EN period seven Review Determining Mixed Air Conditions 95 F x0 25 23 75 F 80 F x0 75 60 00 F 80 mixture 83 75 F oney Aupruny In Period Two a method was discussed to determine the resulting properties of an air mixture By plotting the conditions of the outdoor air and recirculated air and using the percentage of outdoor air the resulting condition of the air mixture was calculated and plotted on a straight line from A to B connecting the two air conditions Determining Sensible Heat Ratio 7 Sensible Heat Gain Sensible Heat Gain Latent Heat Gain B SHR 60 r SE 70 30 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F In Period Three the ratio of sensible heat gain to total heat gain was discussed With the aid of the sensible heat ratio scale on the chart an SHR line was dra2. so a Caoling Heating ae n aa 4 FE pi 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F ah 14 Put all of these changes together on one chart and they show the direction the air condition will move when the dry bulb temperature or moisture content is altered Removing Sensible Heat and Moisture ogey Aypruny 40 Cooling tH And a e een 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F In actual practice however both the dry bulb temperature and moisture content of the air generally change simultaneously When this happens the resulting air conditions move from A at some angle The exact angle and direction depend upon the proportions of sensible and latent heat added or removed Sensible heat causes a change in the air s dry bulb temperature with no change in moisture content Latent heat causes a change in the air s moisture content with no change in dry bulb temperature To provide summer comfort air is cooled and dehumidified moving the air condition downward and to the left resulting in a lower dry bulb temperature and a lower moisture content 17 S TRANE period two Air Mixtures Psychrometry period two Air Mixtures Before an air conditioning problem can be analyzed on the psychrometric chart the conditions of the air to be cooled or heated must be known Determining Ente
3. latent load and the horizontal leg represents the amount of change in dry bulb temperature through the coil i e sensible load 40 TRG TRC001 EN TRG TRC001 EN S TRANE period five Tons of Refrigeration Sensible and Latent Coil Loads 4 5 x 3 430 cfm x 29 6 23 5 94 150 Btu hr 7 8 tons of refrigeration sensible 4 5 x 3 430 cfm x 32 7 29 6 47 850 Btu hr 4 0 tons of refrigeration latent By determining the enthalpy values for these three points the same equation can be used to calculate both the sensible and the latent portions of the coil s refrigeration load Sensible Refrigeration Load 4 5 x 3 430 cfm x 29 6 23 5 94 150 Btu hr 94 150 Btu hr 12 000 Btu hr ton 7 8 tons of refrigeration sensible Latent Refrigeration Load 4 5 x 3 430 cfm x 32 7 29 6 47 850 Btu hr 47 850 Btu hr 72 000 Btu hr ton 4 0 tons of refrigeration latent 41 TRANE period six Psychrometric Analyses Psychrometry period six Psychrometric Analyses Now we will look at a few ways that the psychrometric chart can help us analyze air conditioning systems For simplicity we will limit our examples to systems serving a single zone SHR at Full Load Conditions 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F In the previous example the sensible heat ratio was based on full load or design load conditions It must be understood th
4. Air Conditioning Clinic Psychrometry One of the Fundamental Series TRG TRC001 EN Comment Card We want to assure that our educational materials meet your ever changing resource development needs Please take a moment to comment on the effectiveness of this Air Conditioning Clinic Psych rometry Level of detail circle one Too basic Just right Too difficult One of the Fundamental Series Rate this clinic from 1 Needs Improvement to 10 Excellent TRG TRC001 EN Content 1 2 3 4 5 6 y 8 9 10 Booklet usefulness 1 2 3 4 5 6 7 8 9 10 Slides illustrations 1 2 3 4 5 6 7 8 9 10 Presenter s ability 1 2 3 4 5 6 7 8 9 10 Training environment 1 2 3 4 5 6 7 8 9 10 Other comments About me Type of business Job function Optional name phone address Give the completed card to the presenter or drop it in the mail Thank you TRANE The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company Response Card We offer a variety of HVAC related educational materials and technical references as well as software tools that simplify system design analysis and equipment selection To receive information about any of these items just complete this postage paid card and drop it in the mail Education materials Air Conditioning Clinic series About me
5. Determine the enthalpy difference between the entering and leaving air conditions g Calculate the refrigeration load in tons TRG TRC001 EN TRG TRC001 EN Answers 0 a A O N 93 grains lb 64 8 F DP 49 5 RH 70 5 F WB 85 F DB 110 grains Ib 41 RH 64 F WB 56 F DP 32 RH a 4 000 cfm _ 20 000 cfm ae 95 F x 0 20 19 F 80 F x 0 80 64 F Mixed Air Temperature 19 F 64 F 83 F See Figure 90 oney Aupnuny Dry Bulb Temperature F 69 4 F WB 6 000 cfm 40 000 cfm 7 7 95 F x 0 15 13 5 F 78 F x 0 85 66 3 F Mixed Air Temperature 13 5 F 66 3 F 79 8 F See Figure 91 67 4 F WB See Figure 91 59 TRANE Answers w F 70 8 50 n Sea f sn FER F 4 10 30 35 40 45 50 55 60 65 70 75 Dry Bulb Temperature F 105 110 8 a The SHR line crosses the saturation curve at 56 F WB See Figure 92 60 F DB 58 F WB See Figure 92 60 70 80 MIE TROY BES ji 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F 9a _ 42 000 Btu hr SHR 0 75 56 000 Btu hr b See Figure 93 TRG TRC001 EN TRG TRC001 EN Answers NEY aH aysus 60 70 80 90 00 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Tempe
6. Engineered Systems Clinic series Name Trane Air Conditioning Manual Title Trane Systems Manual Business type Software tools Equipment selection Phone fax System design amp analysis E mail address Periodicals Engineers Newsletter Company Other Address Thank you for your interest TRANE The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse WI 54601 7599 www trane com An American Standard Company BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE THE TRANE COMPANY Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES Crop to width of 7 75 Psychrometry One of the Fundamental Series A publication of The Trane Company Worldwide Applied Systems Group S TRANE Preface Psychrometry A Trane Air Conditioning Clinic The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research testing programs and field experience The Trane Air Conditioning
7. Psychrometric Analyses SHR at Part Load Conditions 47 000 Btu hr 1 085 x 78 F Supply DB 3 430 cfm s T Supply DB 65 4 F SSS See eee Dry Bulb Temperature F In response to the reduction in room sensible heat gain the coil capacity is throttled raising the supply air temperature from D to D to balance the new room sensible heat gain This new supply air temperature is dictated by the equation Sensible Heat Gain Supply Airflow cim ag x Room DE Supply DB Since the supply airflow and the desired room dry bulb temperature are constant the only variable that responsed to this change in sensible heat gain is supply air temperature aE ie 1 085 x 78 F Supply DB 3 430 cfm Supply DB 65 4 F This new supply air temperature D is delivered in sufficient quantity to absorb the room s sensible heat gain but it does not fall on the part load SHR line and is not dry enough to completely absorb the latent heat gain When the conditioned air enters the room it mixes with room air along the 0 70 sensible heat ratio line from D to A The resulting room condition A where the SHR line intersects the room dry bulb temperature line 78 F shows that the relative humidity increased to 6 44 TRG TRC001 EN TRG TRC001 EN S TRANE period six Psychrometric Analyses Constant Volume System a modulating cooling coil constant quantity of variable temperature
8. period one The Psychrometric Chart Dry Bulb Thermometer Dry bulb temperatures are read from an ordinary thermometer that has a dry bulb Wet Bulb Thermometer Wet bulb temperatures are read from a thermometer whose bulb is covered by a wet wick The difference between the wet bulb temperature and the dry bulb temperature is caused by the cooling effect produced by the evaporation of moisture from the wick This evaporation effect reduces the temperature of the bulb and therefore the thermometer reading Consequently the difference between dry bulb and wet bulb temperature readings is a measure of the dryness of air The drier the air the greater the difference between the dry bulb and wet bulb readings TRG TRC001 EN 3 TRANE period one The Psychrometric Chart Condens ation Occurs at Dew Point The third property dew point temperature is the temperature at which moisture leaves the air and condenses on objects just as dew forms on grass and plant leaves Fog Occurs When Air Is Saturated When the dry bulb wet bulb and dew point temperatures are the same the air is saturated It can hold no more moisture When air is at a saturated condition moisture entering the air displaces moisture within the air The displaced moisture leaves the air in the form of fine droplets When this condition occurs in nature it is called fog TRG TRC001 EN S TRANE period one The Psychrometric Chart Relat
9. the specific heat of air and the conversion factor of 60 minutes per hour These properties of air at standard conditions 69 5 F DB dry air at sea level result in the value 1 085 Air at other conditions and elevations will cause this factor to change Density 0 075 lb ft Specific Heat 0 24 Btu lb F 0 075 x 0 24 x 60 min hr 1 085 TRG TRC001 EN period four Air Quantity Determining Supply Airflow STEP 4 Solve the supply airflow equation 80 000 Btu hr 3 430 cfm 1 085 x 78 F 56 5 F For this example the supply airflow is calculated as follows 80 000 Btu hr 7 085 x 75 F 565 7 2 430 cfm Supply Airflow cfm Determining Supply Airflow a 3 430 cfm 56 5 F DB 55 2 F WB The cooling coil must cool and dehumidify 3 430 cfm of air from the entering condition C to the supply air condition D to maintain the desired room conditions TRG TRC001 EN 35 mane 36 period four Air Quantity Arbitrarily Using 55 F Supply Air 47 000 Btu hr sensible heat gain 20 000 Btu hr latent heat gain 47 000 Btu hr SHR 0 70 67 000 Btu hr Some designers prefer to set the supply air temperature at 55 F or use a 20 F temperature differential Room DB Supply DB without regard for the actual sensible heat ratio of the room Using our same example let s examine how this has the potential for creating a problem Assume that the building design changes
10. Btu hr 4 5 x Supply Airflow x A h3 38 TRG TRC001 EN TRG TRC001 EN S TRANE period five Tons of Refrigeration Realize that 4 5 is not a constant It is the product of density of air and the conversion factor of 60 minutes per hour The density of air at standard conditions 69 5 F DB dry air at sea level results in the value 4 5 Air at other conditions and elevations will cause this factor to change Density 0 075 Ib ft 0 075 x 60 min hr 4 5 Determining Tons of Refrigeration STEP 2 Solve the total refrigeration load equation 4 5 x 3 430 cfm x 32 7 23 5 142 000 Btu hr 142 000 Btu hr 11 8 tons of refrigeration 12 000 Btu hr ton Using the supply airflow calculated during Period 4 and the enthalpy values read from the psychrometric chart Refrigeration Load Btu hr 4 5 x 3 430 cfm x 32 7 23 5 142 000 Btu hr Converting to the more common units of tons 142 000 Btu hr 72 000 Btu hr ton 11 8 tons of refrigeration 39 TRANE period five Tons of Refrigeration Sensible and Latent Coil Loads sensible load 15 9 65 70 75 80 Ory Bulb Temperature F 15 20 The psychrometric chart can also be used to determine the sensible and latent components of the coil s refrigeration load First draw a right triangle though the coil entering and leaving air conditions The vertical leg represents the amount of moisture removed by the coil i e
11. Clinic series is one means of knowledge sharing It is intended to acquaint a nontechnical audience with various fundamental aspects of heating ventilating and air conditioning We ve taken special care to make the clinic as uncommercial and straightforward as possible Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text This particular clinic introduces the concept of psychrometry the science concerned with the physical laws that govern air water mixtures 1999 American Standard Inc All rights reserved TRG TRC001 EN Contents period one The Psychrometric Chart 1 Properties Of Air 2 Constructing a Simple Psychrometric Chart 8 Effect of Sensible Heat and Moisture Changes 15 period two Air Mixtures 18 period three Sensible Heat Ratio 22 period four Air Quantity ceteteee 30 period five Tons of Refrigeration 37 period six Psychrometric Analyses 42 period seven Review sissies 52 QU Z enguanar eaan ds 57 Answers 25 said aan nren 59 Glossary aieiaa 64 TRG TRC001 EN At S TRANE TRG TRC001 EN S TRANE period one The Psychrometric Chart Psychrometry period one The Psychrometric Chart Psychro
12. and Air Conditioning Engineers coil curves These represent the changes in dry bulb and wet bulb temperatures as air passes through a typical cooling coil 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 space dew point temperature The temperature at which moisture leaves the air and condenses on surfaces dry bulb temperature A measure of the amount of sensible heat in the air enthalpy A quantity that describes the total amount of heat energy both sensible and latent in one pound of air at a given condition humidity ratio A quantity that describes the actual weight of the water in an air water vapor mixture interior space A conditioned space that is surrounded by other conditioned spaces with no perimeter walls or windows It typically requires some degree of cooling all year long to overcome the heat generated by people lighting etc latent heat Heat that causes a change in the air s moisture content with no change in dry bulb temperature mixed air bypass A method of part load control that uses face and bypass dampers located in front of the cooling coil to vary the portion of the supply air that passes through the coil This varies the supply air temperature as the two airstreams mix downstream of the coil outdoor air Air brought in to the building either by a ventilation system or throug
13. room thermostat assumes control of the face and bypass dampers which reduces the amount of air passing through the cooling coil Since the coil is now running wild the reduced airflow through the coil 1 870 cfm is cooled and dehumidified more than at full load D When the conditioned air mixes with the bypass air 1 560 cfm the required supply air condition 3 430 cfm at E results TRG TRC001 EN TRG TRC001 EN S TRANE period six Psychrometric Analyses Effect of Mixed Air Bypass EY JEH ASUS en os 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F This supply air E mixes with room air along the part load 0 70 SHR line arriving at the resulting new room conditions A While the quantity and temperature of supply air are suitable to absorb the room s sensible heat gain they are unable to completely absorb the latent heat gain The result is a shift in room conditions from the design point A to 78 F DB 58 RH A Variable Air Volume VAV System v 7 E variable speed drive variable quantity of constant temperature air The final method of part load control we will analyze is to vary the supply airflow to the room Let s look at the same example again this time using a simple variable air volume VAV system This system responds to part load conditions by supplying a variable quantity of constant temperature air At full load this syst
14. 00 105 110 Dry Bulb Temperature F Supply air with any combination of dry bulb and wet bulb temperatures that falls on this line will be able to absorb the room s sensible and latent heat in the correct proportions needed to maintain the desired room conditions A 78 F DB 65 F WB Each of these combinations however requires a different quantity of air to do the task Recall the analogy with the container of water If the supply air is warm a higher quantity of air is required than if the supply air is cold TRG TRC001 EN TRG TRC001 EN period three Sensible Heat Ratio Drawing an SHR Line index point po a 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F e TRANE Wy BA sIqQIsuaS 70 90 1 00 Sensible heat ratio lines for other conditions are drawn in the following manner Assume that room design conditions are 80 F DB and 60 RH and that the SHR is calculated as 0 60 First line up the index point with the 0 60 marking on the sensible heat ratio scale and draw a line Next draw a second line parallel to the first through the point B that represents the design room conditions This is the 0 60 SHR line for a room at 80 F DB and 60 RH Supply air at C 60 F DB and 58 F WB will maintain the desired room conditions as will supply air at D 70 F DB and 64 F WB To do so each of these combinations will require a different q
15. 5 110 Dry Bulb Temperature F To complete this basic chart the wet bulb temperature lines must be added Once again at a saturated condition the wet bulb dry bulb and dew point temperatures are equal Therefore the wet bulb temperature lines start at the saturation curve To observe what happens to wet bulb temperatures when air is heated start with saturated air at 50 F dry bulb At this condition the air has a moisture content of approximately 54 grains per pound as shown by A If the temperature of this air is increased to 75 F dry bulb without changing its moisture content the air condition will move along the constant humidity ratio line 54 grains Ib to B The wet bulb temperature of this warmed air is approximately 60 1 F A line drawn from B to a point on the saturation curve that represents 60 1 F saturation temperature B gives an indication of the direction the wet bulb temperature lines will run By taking numerous wet bulb readings under different conditions the wet bulb temperature lines can be added to the chart 13 TRANE 14 period one The Psychrometric Chart Properties of Air 200 180 160 140 120 100 80 60 Humidity Ratio grains Ib of dry air 40 aan fe ee Re dry bull a relative humidity 25 30 35 40 45 50 55 60 65 70 75 8 85 90 95 100 105 110 Dry Bulb Temperature F The psychrometric chart now defines these five properties of air dry bulb temperature vertic
16. 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F In fact this curve forms the 100 relative humidity curve or saturation curve All points on this curve represent the moisture content that constitutes complete saturation of air at the various dry bulb temperatures TRG TRC001 EN 9 S TRANE period one The Psychrometric Chart Dry Bulb Wet Bulb and Dew Point oon A B c D dry bulb 60 F 62 F 75 F 90 F 180 wet bulb 60 F 60 8 F 65 2 F 70 F 160 dew point 60 F 60 F 60 F 60 F rel humidity 100 92 60 140 120 100 60 F 80 wet bulb 4 60 F dew point 60 Humidity Ratio grains Ib of dry air 40 20 25 30 3 40 4 50 55 60 65 70 75 80 8 90 Dry Bulb Temperature F 95 100 105 110 Another fact about saturated air should be discussed before we proceed Assume a volume of moist air has the initial conditions indicated in column D of the table The air has a 90 F dry bulb temperature and a 60 F dew point A wet bulb thermometer shows the wet bulb temperature to be 70 F From a moisture content table the relative humidity of the air is approximately 37 With no change in the moisture content of this volume of air the table shows the progressive change that occurs as the air cools Point C As the dry bulb temperature drops from 90 F to 75 F the wet bulb temperature drops from 70 F to 65 2 F yet the dew point remains the same at 60 F The relative humidity rises from 3
17. 7 to 60 Point B When the dry bulb temperature reaches 62 F the wet bulb temperature is about 60 8 F the dew point remains constant at 60 F and the relative humidity is 92 Point A Finally when the dry bulb temperature reaches the 60 F dew point temperature the wet bulb cannot be reduced any lower because evaporation can no longer occur the air is saturated and contains all the moisture it can hold The relative humidity is now 100 At any point on a 100 relative humidity curve the three air temperatures dry bulb wet bulb and dew point are equal 10 TRG TRC001 EN TRG TRC001 EN TRANE period one The Psychrometric Chart Adding Sensible Heat 200 i we 100 saturation 160 E w w E 60 F 3 60 F a wet bulb a ee ees cew goin k i E 60 H T 25 30 3 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Additionally the dew point temperature does not change as the dry bulb temperature changes provided that the moisture content of the air remains the same Merely heating the air does not change its moisture content Therefore as the air is heated its condition will move horizontally along a constant humidity ratio line In this example heating 60 F saturated air moves the air condition along a horizontal humidity ratio line that corresponds to a constant 77 56 grains of moisture per pound of dry air Horizontal lines can be drawn from each saturation point
18. For example if sensible heat is added to air the air condition moves horizontally to the right Effect of Removing Sensible Heat H oa a A i L tJ 10 0 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F Conversely if sensible heat is removed from air the air condition moves horizontally to the left As long as the moisture content of the air remains unchanged the humidity ratio remains the same Therefore this movement follows the horizontal humidity ratio lines 15 S TRANE period one The Psychrometric Chart Effect of Adding Moisture Dry Bulb Temperature F On the other hand if moisture is added to air without changing the dry bulb temperature the air condition moves upward along a dry bulb temperature line Effect of Removing Moisture lt p sn i eee L 0 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Finally if moisture is removed from the air without changing its dry bulb temperature the air condition moves downward along a dry bulb temperature line 16 TRG TRC001 EN TRG TRC001 EN S TRANE period one The Psychrometric Chart Removing Sensible Heat and Moisture h TTT amp Hu l lfjirlgi TANEYE ra EEH HE m f a 1100 Patt FHH sg 14 A og n
19. across to the right side of the chart A horizontal line can be provided for each humidity ratio value 11 TRANE period one The Psychrometric Chart Relative Humidity Curves 220 relative humidity 180 KJ gt 160 a 140 2 oy Ei 120 N R a Ss od g 100 EO g amp gt 0 eS 3 els 5 60 rk ae al 20 a 40 25 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F Additional curves can be added to the chart to represent relative humidity conditions that are less than 100 The curves shown are at 10 intervals and represent humidity conditions ranging from completely saturated air to completely dry air When air is completely dry its relative humidity cannot change with temperature The 0 condition is therefore represented by the horizontal axis of the chart Properties of Air 200 180 160 140 120 Cj i humidity ratio 60 Humidity Ratio grains Ib of dry air 40 3 A dry bulb 2 relative humidity 25 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F This basic chart now shows three air conditions dry bulb temperature vertical lines humidity ratio horizontal lines and relative humidity diagonally curved lines 12 TRG TRC001 EN TRG TRC001 EN e TRANE period one The Psychrometric Chart Determining Wet Bulb Lines Humidity Ratio grains Ib of dry air 25 30 3 40 45 50 55 60 65 70 75 80 85 90 95 100 10
20. air This is the manner in which a constant volume variable temperature system with a modulating coil performs It provides a constant quantity of air to the room and responds to part load conditions by varying the supply air temperature This is performed by modulating the flow of the cooling fluid through the coil typically using a two way or three way control valve controlled by a thermostat that senses the room dry bulb temperature Such a system can provide good dry bulb temperature control As the sensible heat ratio changes from full load however it may lose control of the room relative humidity Constant Volume with Reheat thermostat adding reheat improves f humidity control One method of improving the constant volume system s ability to control room humidity is to reheat the supply air In this example reheat is provided by a heating coil located downstream of the air handler This reheat coil is controlled by a thermostat sensing the room dry bulb temperature while the cooling coil is controlled to provide a constant leaving air temperature 45 TRANE period six Psychrometric Analyses Effect of Adding Reheat 47 000 Btu hr 1 085 x 78 F Supply DB 3 430 cfm Supply DB 65 4 F 60 80 30 1 00 30 35 40 4 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Using the part load conditions from the previous example the room s sensible
21. al lines humidity ratio and dew point temperature horizontal lines relative humidity curved lines and wet bulb temperature diagonal lines Remember if any two of these five air conditions are known the other three can be found on the psychrometric chart by locating the point of intersection of the two known conditions Specific Volume 200 180 100 specific volume lines cubic feet pound of dry air 120 100 80 Humidity Ratio grains lb of dry air 40 20 2 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F There is one more property of air that is displayed on the psychrometric chart specific volume Specific volume is defined as the volume of one pound of dry air at a specific temperature and pressure As one pound of air is heated it occupies more space the specific volume increases TRG TRC001 EN TRG TRC001 EN e TRANE period one The Psychrometric Chart Effect of Adding Sensible Heat eof tT er 44 Hk4S aPrenZaak EHO o 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Effect of Sensible Heat and Moisture Changes When either the sensible heat content or the moisture content of air changes the point on the psychrometric chart that represents the original air condition moves to a position that represents the new condition of temperature and or humidity
22. at the sensible portion of total heat gain is particularly subject to change throughout the day causing the ratio of sensible to total heat gain to change 42 TRG TRC001 EN TRG TRC001 EN S TRANE period six Psychrometric Analyses SHR Changes with Room Load Full Load SHR 80 000 Btu hr 0 80 80 000 Btu hr 20 000 Btu hr Part Load SHR 47 000 Btu hr 0 70 47 000 Btu hr 20 000 Btu hr For example assume that at full load the room is subject to an 80 000 Btu hr sensible heat gain and a 20 000 Btu hr latent heat gain The full load sensible heat ratio is 0 80 At other times during the day clouds block the sun and reduce the solar heat gain and some of the lights are turned off This reduces the room s sensible heat gain from 80 000 Btu hr to 47 000 Btu hr The room s latent heat gain originates primarily from people Assuming that the occupancy of the room remains constant the latent heat gain is still 20 000 Btu hr and the part load sensible heat ratio becomes 0 70 SHR at Part Load Conditions Dry Bulb Temperature F To maintain the design room conditions A for this part load sensible heat ratio a different supply air condition one that falls on the 0 70 SHR line and a different airflow are required But suppose the system in this example was designed to deliver a constant quantity of air and vary its supply temperature to meet the changing loads 43 S TRANE period six
23. ative humidity When any two of these five properties of air are known the other three can be quickly determined from the psychrometric chart 6 TRG TRC001 EN TRANE period one The Psychrometric Chart Summer Design Conditions a 95 F DB dry bulb A 78 F WB wet bulb For example let s assume that the summer design conditions are 95 F dry bulb and 78 F wet bulb What is the relative humidity humidity ratio and dew point Point of Intersection 72 F Only one point on the psychrometric chart represents air with both of these conditions This point is located where the vertical 95 F dry bulb DB and diagonal 78 F wet bulb WB temperature lines intersect From this intersection the remaining three air properties can be read from the chart Both the dew point and humidity ratio lines are horizontal and the values are shown on the right side of the chart In this example the humidity ratio is about 118 grains of moisture per pound of dry air and the dew point temperature is approximately 72 F TRG TRC001 EN 7 S TRANE period one The Psychrometric Chart Notice that the point of intersection falls between two relative humidity curves 40 and 50 By interpolation the relative humidity at this condition is approximately 47 Constructing a Simple Psychrometric Chart To better understand the psychrometric chart and show why the lines intersect as they do we will construct a simple chart Pr
24. e A 78 F DB 65 F WB 32 TRG TRC001 EN TRG TRC001 EN period four Air Quantity Determining Supply Airflow STEP 3 Identify supply air conditions See 30 35 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F The third step is to determine the required supply air conditions This is where the coil curves are used Using the curvature of the nearest coil line as a guide draw a curve from the mixed air condition C until it intersects the SHR line Point D at which this curve crosses the SHR line represents the supply air condition that will absorb the room s sensible and latent heat in the correct proportions needed to maintain the desired room conditions Here this supply air condition is found to be 56 5 F DB and 55 2 F WB 33 S TRANE 34 period four Air Quantity Determining Supply Airflow STEP 4 Solve the supply airflow equation le E Airflow 4 085 x Room DB Supply DB With the supply air conditions known the next step is to calculate the specific quantity of air cfm or cubic feet per minute needed to satisfy the room heat gains The required supply airflow is determined using the following formula where the sensible heat gain is expressed in Btu hr and the two temperatures are in F Sensible Heat Gain Supply Airflow cfm 7585x Room DB Supply DB Realize that 1 085 is not a constant It is the product of density
25. e new room condition A to calculate the mixed air entering conditions Then use the coil curves to find the condition of the 56 5 F dry bulb supply air Finally draw the SHR line through this new supply air condition to find the resulting room conditions Repeating this process a few times allows the room condition to converge and be equal to the condition used in the previous iteration 50 TRG TRC001 EN TRG TRC001 EN S TRANE period six Psychrometric Analyses Software Tools The psychrometric chart is a visual tool that helps designers find solutions to many common HVAC problems by plotting conditions on the chart Today many of these same problems can be quickly solved by computers which can often eliminate the need for a graphical solution altogether Still a basic understanding of psychrometric principles is required to use these tools and is fundamental to the science of air conditioning Instead of relying solely on the typical coil curves printed on the psychrometric chart many manufacturers provide computerized coil selection programs to determine the actual performance of specific coils Software tools are also available to assist you in performing psychrometric calculations such as determining the properties of air at a given set of conditions and finding the conditions that result when two air streams are mixed 51 TRANE period seven Review
26. em looks the same on the psychrometric chart as the constant volume system it supplies 3 430 cfm of 56 5 F air to the room 49 S TRANE period six Psychrometric Analyses Calculating Part Load Airflow 47 000 Btu hr 2 015 cfm a RRR 1 085 x 78 F 56 5 F HH tt Sess Sea Sees Dry Bulb Temperature F At part load when the SHR of the room is reduced from 0 80 to 0 70 the VAV system responds by reducing the quantity of 56 5 F air supplied to the room to match the reduced sensible heat gain The part load sensible heat gain of 47 000 Btu hr and the constant supply air temperature 56 5 F DB are used to determine the required part load air quantity 47 000 Btu hr 1085 x 78 F 565 R 2015 cfm Supply Airflow cfm When the conditioned supply air D enters the room it mixes with room air along the part load SHR line from D to A This quantity and temperature of supply air are suitable to absorb the room s sensible heat gain but are unable to completely absorb the latent heat gain The result is a shift in the room conditions from the design point A to 78 F DB and 59 RH A While the simple VAV system does a better job of controlling room humidity than the simple constant volume system it is still unable to maintain the desired condition of 50 RH To more accurately determine the final room conditions you would cycle through the psychrometric chart again First use th
27. ensure that both sensible and latent heat are removed in the proper proportions There are several combinations of dry bulb and wet bulb temperatures that will produce the desired room conditions Each of these combinations requires a different quantity of air 25 TRANE 26 period three Sensible Heat Ratio Heat Gain This relationship between the conditions and quantity of the supply air can be described using the analogy of maintaining a constant temperature within a container of water In this illustration the container of water is capable of absorbing heat The amount of heat it absorbs is called heat gain To maintain the water temperature at a constant 75 F any heat gain must be offset by mixing cool water with the water already in the container The rate at which this cool water is added to the container is determined by its temperature For a given water temperature there is a certain flow rate measured in gallons per minute gom that will offset the heat gain and maintain the desired temperature in the container If the water is warm a higher flow rate is required than if the water is very cold TRG TRC001 EN S TRANE period three Sensible Heat Ratio Sensible Heat Ratio SHR Sensible Heat Gain SHR MMMM Sensible Heat Gain Latent Heat Gain The sensible heat ratio abbreviated as SHR refers to the comparison of sensible heat gain to total heat gain sensible heat plus latent heat Once
28. h openings provided for natural ventilation from outside the building psychrometric chart A tool used to graphically display the properties of air psychrometry The science dealing with the physical laws of air water mixtures 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 relative humidity A comparison of the amount of moisture that a given amount of air is holding to the amount of moisture that the same amount of air can hold at the same dry bulb temperature return air Air that is removed from the conditioned space s and either recirculated or exhausted TRG TRC001 EN TRG TRC001 EN TRANE Glossary saturation curve This represents the moisture content that constitutes complete saturation of air at the various dry bulb temperatures saturation point The maximum amount of water vapor that one pound of dry air can hold at a given dry bulb temperature sensible heat Heat that causes a change in the air s dry bulb temperature with no change in moisture content sensible heat ratio SHR The ratio of sensible heat gain to total sensible latent heat gain specific volume The volume of one pound of dry air at a specific temperature and pressure supply air Air that is delivered to the conditioned space by mechanical means for ventilation heating cooling hu
29. halpy is usually designated as h TRG TRC001 EN 37 S manwe period five Tons of Refrigeration Determining Tons of Refrigeration STEP 1 a Find enthalpies 45 TR S entering and CECE FTI 40 leaving coil y aA 11 e s7 Ari ER X o a Warr so Sa B 25 LETETT PLAI LA 45 20 15 55 60 65 70 15 80 85 90 95 Dry Bulb Temperature F 15 20 105 110 Using the previous example for calculating supply airflow the first step is to determine the enthalpies of the air entering and leaving the cooling coil This is accomplished by lining up three points on the chart including the entering air condition and identical points on the two enthalpy scales one each on the left and right edges of the psychrometric chart Using this method the enthalpy of the mixed air entering the coil C is found to be 32 7 Btu lb Similarly the enthalpy of the supply air leaving the coil D is found to be 23 5 Btu Ib Determining Tons of Refrigeration STEP 2 Solve the total refrigeration load equation Refrigeration _ _ Load Btu hr 4 5 x Supply Airflow x h h3 hy h enthalpy of air entering coil Btu lb enthalpy of air leaving coil Btu lb The total refrigeration load in terms of Btu per hour is then calculated using the following formula where the supply airflow is expressed in cfm h is the entering air enthalpy in Btu lb and h is the leaving air enthalpy in Btu lb Refrigeration Load
30. heat gain is reduced from 80 000 Btu hr to 47 000 Btu hr while the latent heat gain remains the same Sensing the reduction in dry bulb temperature due to the lower sensible heat gain the room thermostat assumes control of the reheat coil The cooling coil is controlled to provide a constant supply air temperature D 56 5 F DB while the reheat coil is controlled to add just enough heat to the supply air to offset the reduction in room sensible heat gain Since the supply airflow is constant and the desired room dry bulb temperature and sensible heat gain are known we can calculate the required re heated supply air temperature 47 000 Btu hr OO 4 f 1 085 x 78 F Supply DB ee Supply DB 65 4 F Since the supply air leaving the coil is sensibly heated i e no moisture is added or removed it moves horizontally along a constant humidity ratio line from D to E The resulting supply air conditions are 65 4 F DB 58 9 F WB 46 TRG TRC001 EN TRG TRC001 EN S TRANE period six Psychrometric Analyses Effect of Adding Reheat WEH IR BPqrsuac E a mee 65 Ct it t 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F This supply air mixes with room air along the part load 0 70 SHR line from E to A arriving at the desired room conditions A Now if the room s latent heat gain were also changed the resulting room conditions would
31. hrometric chart plot represents the changes that a volume of air undergoes as it travels through a typical air conditioning system In this illustration recirculated air A is mixed with outdoor air B producing a mixed air condition C This air mixture passes through the cooling and dehumidifying coil with the changes in dry bulb temperature and humidity ratio represented by the coil curve from C to D This supply air D enters the room and mixes with the room air along the SHR line from D to A absorbing the room s sensible and latent heat gains to maintain the room at desired conditions A Again for this specific supply air condition a specific airflow is required to maintain the desired room conditions 55 S TRANE 56 period seven Review S TRANE An American Standard Company For more information refer to the following references Trane Air Conditioning Manual ASHRAE Handbook Fundamentals Fundamentals of Thermodynamics and Psychrometrics ASHRAE self directed learning course Psychrometrics Theory and Practice ASHRAE Visit the ASHRAE Bookstore at www ashrae org For information on additional educational materials available from Trane contact your local Trane sales office request a copy of the Educational Materials price sheet Trane order number EM ADV1 or visit our online bookstore at www trane com bookstore TRG TRC001 EN TRANE Quiz Questions for Period 1 1 Given air co
32. in is 20 000 Btu hr First divide the sensible heat gain by the total heat gain The resulting sensible heat ratio SHR is 0 80 Determining Supply Airflow room 78 F DB 50 RH outdoor air OA 95 F DB 78 F WB ventilation 25 OA The second step is to determine the entering air conditions Design room air is 78 F DB 50 RH design outdoor air is 95 F DB 78 F WB Twenty five percent 25 outdoor air is required for ventilation purposes TRG TRC001 EN 31 S mane period four Air Quantity Determining Supply Airflow STEP 2 Plot room outdoor and entering conditions 95 F x 0 25 23 75 F 78 F x 0 75 58 50 F mixture 82 25 F 40 50 60 2 70 5 0 90 1 00 30 35 40 45 50 55 60 65 70 75 80 85 Dry Bulb Temperature F 95 100 105 110 Plot the outdoor air B 95 F DB 78 F WB and indoor air A 78 F DB 65 F WB conditions on the psychrometric chart Then calculate the mixed air conditions using the method learned in Period 2 95 F x 0 25 23 75 F 78 F x 0 75 58 50 F Mixed Air Temperature 23 75 F 58 50 F 82 25 F Locate the mixed air conditions C on the psychrometric chart 82 25 F DB 68 6 F WB Next establish the SHR line by aligning the 0 80 mark on the scale with the index point and drawing a line through both points to the saturation curve In this case the room design conditions and the index point are the sam
33. io of sensible heat gain to total heat gain is one of the most important factors affecting air conditioning system requirements Effect of Removing Sensible Heat fr o O S E ER jj 1 1140 amp ACHE oney upnunp Dry Bulb Temperature F If only sensible heat is removed from the air the line representing this change moves from the original condition horizontally to the left This results in a lower dry bulb temperature while the moisture content the humidity ratio remains constant 22 TRG TRC001 EN S TRANE period three Sensible Heat Ratio Effect of Removing Latent Heat Lt L 0 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Conversely if only latent heat is removed the line moves vertically downward along a constant dry bulb temperature line This results in a lower moisture content or humidity ratio Removing Sensible and Latent Heat 30 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F If both sensible and latent heat are removed from the air the resulting air condition will be to the left and below the initial condition The proportions of sensible and latent heat removed will determine the exact direction the resulting air condition follows TRG TRC001 EN 23 TRANE 24 period three Sensible Heat Ra
34. ive Humidity Amount of moisture that a given Relative amount of air is holding Humidity Amount of moisture that a given amount of air can hold The fourth property relative humidity is a comparison of the amount of moisture that a given amount of air is holding to the amount of moisture that the same amount of air can hold at the same dry bulb temperature Relative Humidity 50 100 saturated Relative humidity is expressed as a percentage For example if the relative humidity of the air is 50 it contains one half the amount of moisture possible at the existing dry bulb temperature TRG TRC001 EN 5 S TRANE period one The Psychrometric Chart Humidity Ratio Finally humidity ratio describes the actual weight of water in an air water vapor mixture In other words if one pound of air were wrung completely dry comparing the weight of the water to the weight of the dry air would yield its humidity ratio Humidity ratio can be expressed as pounds of moisture per pound of dry air or as grains of moisture per pound of dry air There are 7000 grains of water in a pound To appreciate the magnitude of these units of measurement at sea level one pound of 70 F air occupies approximately 13 5 cubic feet and one grain of water in that air weighs about two thousandths 0 002 of an ounce Properties of Air a Dry bulb temperature 4 Wet bulb temperature 4 Dew point temperature 4 Humidity ratio a Rel
35. metry is the science dealing with the physical laws of air water mixtures When designing an air conditioning system the temperature and moisture content of the air to be conditioned and the same properties of the air needed to produce the desired air conditioning effect must be known Once these properties are known the air conditioning task can be determined This analysis can be performed using the psychrometric chart The psychrometric chart graphically displays several physical properties of air over a broad range of conditions Knowing the relationship of these air properties aids the task of air conditioning system design and analysis TRG TRC001 EN 1 TRANE period one The Psychrometric Chart Psychrometric Chart s e ragga ss Va 3 7 a Properties of Air At first glance the psychrometric chart appears to be an imposing network of lines When properly used however it provides valuable information about the properties of air During this session the psychrometric chart and its use in solving many air conditioning problems will be explained Properties of Air a Dry bulb temperature a Wet bulb temperature 4 Dew point temperature 4 Relative humidity 4 Humidity ratio 98 The psychrometric chart contains five physical properties to describe the characteristics of air Dry bulb temperature Wet bulb temperature Dew point temperature Relative humidity Humidity ratio TRG TRC001 EN TRANE
36. midification or dehumidification supply duct system A system that transports the primary air from the central air handler to the VAV terminal units and then on to the space diffusers ton of refrigeration A quantity that is equal to 12 000 Btu hr 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 space wet bulb temperature A measure of the dryness of the air obtained by using a thermometer with a bulb that is covered by a wet wick 65 S 3 RANE Literature Order Number TRG TRC001 EN File Number E AV FND TRG TRC001 1099 EN The Trane Company i X 199 Worldwide Applied Systems Group Supersedes ED FND TRG TRC001 199 EN 3600 Pammel Creek Road La Crosse WI 54601 7599 Stocking Location Inland La Crosse www trane com An American Standard Company Since The Trane Company has a policy of continuous product improvement it reserves the right to change design and specifications without notice
37. nditions of 80 F DB and 60 RH find the humidity ratio and dew point temperature 2 Given air conditions of 85 F DB and a humidity ratio of 90 grains lb find the relative humidity and wet bulb temperature 3 Given air conditions of 74 F WB and 60 RH find the dry bulb temperature and humidity ratio 4 Given air conditions of 80 F DB and a humidity ratio of 64 grains lb find the relative humidity and wet bulb temperature 5 Given air conditions of 90 F DB and 68 F WB find the dew point temperature and relative humidity Questions for Period 2 6 Given outdoor air conditions of 95 F DB 78 F WB indoor design conditions of 80 F DB 67 F WB total airflow of 20 000 cfm and outdoor airflow of 4 000 cfm a Find the dry bulb temperature of the mixture b Plot all three conditions on the psychrometric chart c Find the wet bulb temperature of the mixture 7 Given outdoor air conditions of 90 F DB 80 F WB indoor design conditions of 78 F DB 65 F WB total airflow of 40 000 cfm and outdoor airflow of 6 000 cfm a Find the dry bulb temperature of the mixture b Plot all three conditions on the psychrometric chart c Find the wet bulb temperature of the mixture Questions for Period 3 8 Given a sensible heat ratio of 0 80 and indoor design conditions of 78 F DB and 66 F WB a Draw the SHR line on the psychrometric chart b Find the dry bulb and wet bulb temperatures of the air where this SHR line crosses
38. not fall exactly on A but on the appropriate SHR line that runs through E Adding reheat permits better room humidity control at various part load conditions while maintaining room dry bulb temperature control Realize however that this system uses more energy than the previous constant volume system with a modulating cooling coil it constantly cools the supply air to 56 5 F then reheats the air as necessary when the building sensible load drops Mixed Air Bypass OA face and bypass W dampers mixed air bypass improves humidity control Another method of improving the constant volume system s ability to control room humidity is to bypass mixed air around the cooling coil In this example face and bypass dampers are placed in front of the cooling coil and used to vary the portion of the supply air that actually passes through the coil thus 47 S TRANE 48 period six Psychrometric Analyses varying the supply air temperature as the two airstreams mix downstream of the air handler The face and bypass dampers are controlled by the room dry bulb thermostat The cooling coil is allowed to run wild causing the air that does pass through it to be cooled more at partial airflows Effect of Mixed Air Bypass 82 25 F x 0 455 37 4 F REETA 51 40 F x 0 545 28 0 F M TT mixture 65 4 F nry wap aysus Dry Bulb Temperature F At our example part load conditions the
39. operties of Saturated Air dry bulb humidity dry bulb humidity temp ratio temp ratio 25 F 19 14 60 F 77 61 30 F 24 19 65 F 92 89 35 F 29 94 70 F 110 82 40 F 36 51 75 F 131 83 45 F 44 34 80 F 156 38 50 F 53 63 85 F 185 03 55 F 64 63 90 F 218 42 The amount of moisture contained in saturated air depends on dry bulb temperature This table shows the maximum amount of water vapor that one pound of dry air can hold at various dry bulb temperatures For example at 25 F one pound of dry air can absorb and hold 19 14 grains of water at 30 F it can absorb 24 19 grains at 35 F it can absorb 29 94 grains and so on Each of these conditions is a saturation point TRG TRC001 EN S TRANE period one The Psychrometric Chart Plotting Saturation Points 18 42 200 180 195 03 T 160 o 156 38 o a 140 E 5 131 83 Ss 1 i 2 110 82 g 100 4 i o 92 89 o 5 i 77 61 6 5 64 63 5 53 63 40 o 3 ns 200 4 19 79 94 19 14 0 25 30 3 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F These saturation points can be plotted on a chart with dry bulb temperature along the horizontal axis and humidity ratio along the vertical axis When several saturation points are plotted the curve created resembles the relative humidity curves of the psychrometric chart Saturation Curve 100 relative humidity curve Humidity Ratio grains lb of dry air 25 30 3 40 45 50
40. r mixture Determining Entering Air Conditions 95 F x 0 25 23 75 F 80 F x 0 75 60 00 F mixture 83 75 F This is done by multiplying the dry bulb temperature of each air condition by its percentage and summing the results For example if the outdoor dry bulb temperature is 95 F and it represents 25 of the air mixture it contributes 23 75 F or 0 25 x 95 to the dry bulb temperature of the air mixture Similarly if the dry bulb temperature of the recirculated air is 80 F it contributes 60 F or 0 75 x 80 to the dry bulb temperature of the air mixture The sum of 23 75 F and 60 F equals 83 75 F the resulting dry bulb temperature of this air mixture TRG TRC001 EN period two Air Mixtures Determining Entering Air Conditions ogey Anpruny Returning to the psychrometric chart point C at which the 83 75 F dry bulb temperature falls on the line from A to B represents the conditions of the air mixture 83 75 F DB and approximately 70 F WB Because the recirculated air quantity constitutes a larger percentage 75 of the mixture the mixed air condition C is much nearer to the indoor design condition A than to the outdoor design condition B TRG TRC001 EN 21 TRANE period three Sensible Heat Ratio Psychrometry period three Sensible Heat Ratio This period is devoted to understanding the term sensible heat ratio and how it is represented on the psychrometric chart The rat
41. rature F 10 80 000 Btu hr Shit 80 000 Btu hr 20 000 Btu hr 0 80 58 3 F WB See Figure 94 Dry Bulb Temperature F 1 000 Btu hr 7 085 x 80 F 57 F 7 40 fM 1 Supply Airflow cfm 61 TRANE Answers 12 See Figure 95 Refrigeration Load 4 5 x 7 000 cfm x 33 2 24 2 283 500 Btu hr 283 500 Btu hr 72 000 Btu hr ton 23 6 tons of refrigeration 45 85 15 Dry Bulb Temperature F 3 225 15 20 13 a See Figure 96 b 95 F x 0 25 23 75 F 78 F x 0 75 58 50 F Mixed Air Temperature 23 75 F 58 50 F 82 25 F Mixed Air Conditions 82 25 F DB 67 7 F WB See Figure 96 c 156 000 Btu hr SHR 756 000 Btu hr 39 000 Btu h 80 S Figure 96 d Leaving Coil Conditions 57 F DB 54 4 F WB See Figure 96 62 TRG TRC001 EN Answers lt 1 fj aai a M as O OS E na t 0 SeT Lit s lg c e TEELT FR ER 1 H Hf Ht 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F e 156 000 Btu hr Airflow cfm 1 085 x 78 F 57 F 6 847 cfm f n h 32 1 23 7 8 4 Btu lb g Refrigeration Load 4 5 x 6 847 cfm x 8 4 Btu lb 258 817 Btu hr 258 817 Btu hr 12 000 Btu hr ton 7 218 tons TRG TRC001 EN 63 S TRANE 64 Glossary ASHRAE American Society of Heating Refrigerating
42. ring Air Conditions outdoor supply air OA fan recirculated air RA The air entering the cooling coil may be 100 recirculated A 100 outdoor B or a mixture of the two C 18 TRG TRC001 EN TRG TRC001 EN period two Air Mixtures Determining Entering Air Conditions outdoor air oney Aupruny amp If outdoor air B is mixed with recirculated air A the conditions of the resulting mixture are found somewhere on a straight line connecting the two points If the mixture is half and half this condition falls on the midpoint of the line between A and B If more than half of the mixture is recirculated air A the condition of the mixture will fall closer to A than to B Determining Entering Air Conditions 4 000 cfm mixed air 1 000 cfm OA 4 000 cfm _ 4000cim 9 2 3 000 cfm OA 25 RA RA 75 mixture 100 In this example 1 000 cfm of outdoor air OA is mixed with 3 000 cfm of recirculated air RA for a total supply airflow of 4 000 cfm First the percentage of outdoor air within the mixture is determined This is done by dividing the outdoor air quantity by the total air quantity 1 000 cfm _ 4 000 cfm 29 19 S TRANE 20 period two Air Mixtures The outdoor air quantity in this example constitutes 25 of the mixture while the recirculated air makes up the remaining 75 The next step is to determine the dry bulb temperature of the ai
43. the 90 RH curve TRG TRC001 EN 57 S TRANE 58 Quiz Questions for Period 4 9 Given a room with a 42 000 Btu hr sensible heat gain and a 56 000 Btu hr total heat gain excluding ventilation heat gain and indoor design conditions of 80 F DB and 50 RH a Determine the sensible heat ratio b Draw the SHR line 10 Given a room with an 80 000 Btu hr sensible heat gain a 20 000 Btu hr latent heat gain excluding ventilation heat gain and indoor design conditions of 80 F DB and 67 F WB find the wet bulb temperature of the supply air if it leaves the cooling coil at 60 F DB Questions for Period 5 11 Given indoor design conditions of 80 F DB and 67 F WB if the air leaves the cooling coil at 57 F DB find the airflow in cfm required to satisfy a 1 000 Btu hr sensible heat gain 12 Given that air enters the cooling coil at 85 F DB 69 F WB and leaves at 58 F DB 56 4 F WB if the supply airflow is 7 000 cfm find the total refrigeration load in tons 13 Given indoor design conditions of 78 F DB 65 F WB outdoor conditions of 95 F DB 75 F WB a sensible heat gain of 156 000 Btu hr a latent heat gain of 39 000 Btu hr and 25 outdoor air a Plot the indoor and outdoor design conditions on the psychrometric chart Find the mixed air conditions entering the cooling coil Draw the SHR line Draw the coil curve and determine the coil leaving air conditions Calculate the supply airflow h 0 a 0 F
44. this ratio is known an SHR line can be drawn on the psychrometric chart Sensible Heat Gain sensible Feat Ratio SMR Sensible Heat Gain Latent Heat Gain Sensible Heat Ratio Scale 40 60 70 80 90 1 00 ro Eele T t CI Suss PE 40 45 50 55 60 65 70 75 80 8 90 95 100 105 110 Dry Bulb Temperature F A scale around the right and top edges of the chart gives the SHR values Also there is an index point in the middle of the chart at the 78 F DB and 65 F WB condition Using a straight edge a sensible heat ratio line can be drawn by aligning the appropriate SHR value on the scale with the index point TRG TRC001 EN 27 TRANE 28 period three Sensible Heat Ratio Drawing an SHR Line MEY E 1 00 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Assume that room design conditions A are 78 F DB and 65 F WB and that the sensible heat ratio is calculated as 0 80 That is sensible heat gain represents 80 of the total sensible plus latent heat gain The SHR line is found by aligning the index point with the 0 80 marking on the sensible heat ratio scale and drawing a line from the index point to the saturation curve SHR Dictates Supply Air Condition p 80 60 70 6 80 a 60 A 90 55 1 00 s 2 sensible heat ratio ine 0 30 HH Ty T 30 35 40 45 50 55 60 65 0 15 80 85 90 5 1
45. tio Heat and Moisture Transfer supply a AMD return air sensible heat latent heat Imagine conditioned supply air as a sponge As it enters a room it absorbs heat and moisture The amount of heat and moisture absorbed depends on the temperature and humidity of the supply air This sponge the supply air must be cool enough to pick up the room s excess sensible heat gain and dry enough to pick up the room s excess latent heat i e moisture Therefore the excess sensible and latent heat in the room determine the required dry bulb and wet bulb temperatures of that supply air TRG TRC001 EN TRG TRC001 EN e TRANE period three Sensible Heat Ratio Removing Sensible and Latent Heat 80 75 F Pa 9 gooler 8 damper LJ 3 s cr Fa 60 Ei we 70 50 45 _ 40 re so 38 I warmer drier When the required amount of sensible and latent heat are not properly removed from the room the desired room conditions cannot be maintained For example if too much sensible heat and not enough latent heat are removed the room feels cold and damp On the psychrometric chart room conditions move up and to the left On the other hand if too much latent heat but not enough sensible heat is removed the room feels warm and dry On the psychrometric chart room conditions move down and to the right Therefore the conditions of the supply air must be controlled accurately to
46. to use a much higher quality glass that will reduce the sensible portion of the design load from 80 000 Btu hr to 47 000 Btu hr This reduces the SHR to 0 70 Arbitrarily Using 55 F Supply Air y VA gs 30 35 40 45 50 55 60 6 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F Plotting this new SHR line on the psychrometric chart we find that the SHR line crosses the coil curve at approximately 49 F DB D If the system is arbitrarily designed with a 55 F supply air temperature D the resulting room conditions will fall on the 0 70 SHR line drawn through D The resulting room conditions A will be 78 F DB 57 RH This arbitrary design practice results in a higher room relative humidity than desired TRG TRC001 EN S TRANE period five Tons of Refrigeration Psychrometry period five Tons of Refrigeration The psychrometric chart can also be used to determine the total load on the refrigeration equipment expressed in Btu per hour or tons of refrigeration One ton equals 12 000 Btu hr What is Enthalpy The total heat energy in one pound of air Btu lb at its present condition Enthalpy h Sensible Heat Latent Heat Another property of air enthalpy must now be defined Enthalpy describes the total amount of heat energy both sensible and latent in one pound of air at its present condition It is expressed in Btu per pound of dry air Btu lb When displayed in formulas ent
47. uantity of air 29 S TRANE period four Air Quantity Psychrometry period four Air Quantity Next we will determine the flow rate of supply air necessary to maintain a given set of design room conditions Coil Curves o E 80 coil curves oney Anpruny 30 35 40 45 50 55 60 6t 7 75 80 85 90 95 100 105 110 Before proceeding one more set of curves on the psychrometric chart must be identified These curved lines represent the changes in dry bulb and wet bulb temperatures as air passes through a typical cooling coil These are commonly referred to as coil curves they depict approximate coil performance Exact coil performance depends on the actual coil geometry and can be precisely determined by coil selection software These curves were established from hundreds of laboratory tests of various coil geometries at different air and coolant temperatures They let you determine leaving coil conditions and postpone coil selection until the final design The use of these coil curves will be discussed later 30 TRG TRC001 EN period four Air Quantity Determining Supply Airflow STEP 1 Calculate the sensible heat ratio SHR 80 000 Btu hr sensible heat gain 20 000 Btu hr latent heat gain 80 000 Btu hr SHR 0 80 100 000 Btu hr To demonstrate how the required supply airflow is determined assume that a room s sensible heat gain is 80 000 Btu hr and its latent heat ga
48. wn It was also shown that any combination of air conditions that fall on this line will maintain the desired room conditions A Each set of conditions requires a different supply airflow 53 TRANE period seven Review Determining Coil Leaving Conditions Supply _ Sensible Heat Gain Airflow 4 085 x Room DB Supply DB H Iva aqsuas oe ee 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 Dry Bulb Temperature F After determining the entering air conditions for the coil and the slope of the SHR line the coil curves were used to find the required supply air conditions This point D was established by the intersection of the coil curve and the SHR line By knowing the design room conditions A and the required supply air conditions D the corresponding supply airflow could be calculated Determining Tons of Refrigeration Refrigeration Load 4 5 x Supply Airflow x h h gt as 15 30 35 5 55 60 65 70 75 80 5 90 95 Dry Bulb Temperature F 15 20 After the entering C and leaving D coil conditions were established the enthalpies for each were read from the psychrometric chart These enthalpy values and the previously calculated supply airflow were used to determine the refrigeration load in Btu hr or tons 54 TRG TRC001 EN TRG TRC001 EN period seven Review S Psychrometric s Analysis 3 s The resulting psyc
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