good practice guide
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1. Solenoid valve am Condensate gt Once the level of condensate in the collection container has risen high enough a sensor causes a low tower deus voltage solenoid valve on the probe sensor outlet to open This removes the differential pressure holding the main condensate drain valve closed and so the main valve opens and the condensate drains away When the condensate level falls below a lower set level the solenoid valve closes Upper level probe sensor Dip tube Weir so closing the main valve and hence ensuring that no air is released while Fig 11 Cutaway diagram of an electronic condensate the condensate is ejected drain trap 6 16 United Glass Alloa This major glass works operates two compressed air systems the air quality of which is essential to the production process The high level of air loss from cracked open condensate drains on the 45 psi system was a major energy concern for the company However the problem was how to justify the capital expenditure on an air system that was adequately meeting production needs During an energy reduction review of the compressed air strategy the total loss from a bank of 11 receivers at the compressor house was measured At 1 150 cfm the loss was equivalent to 27 000 per annum or 5 of the system air costs In addition many more drains around the site were found to be open The 11 receivers were fitted with level s2. Fig 6 A deliquescent dryer There is little or no energy loss However due to the poor dewpoint high maintenance corrosion problems and Health and Safety considerations this method is now uncommon 6 8 3 Membrane Dryers 4 C to 40 C Dewpoint These dryers use hollow fibre membranes which diffuse the moisture from the compressed air to atmosphere see Fig 7 They are available to dry the air to dewpoints from 4 C to 40 C They are designed for localised low dewpoint requirements and are currently available only up to 50 cfm capacity Certain types of membrane can reduce the oxygen content of the compressed air and therefore should not be used in breathing air applications 16 Clean H20 compressed air H O Fig 7 Operation of a membrane dryer Their constant purging can prove very costly particularly if they are lightly loaded Typically 28 purging may be required to guarantee a pressure dewpoint of 20 C 6 8 4 Desiccant Dryers 20 C to 70 C Dewpoint When better dewpoints are needed than those which can be achieved from refrigerated dryers desiccant dryers are used In this class of machine the desiccant bed is regenerable Therefore all designs have an alternating duty section through which the compressed air being dried is fed and a non duty section which is being regenerated as shown in Fig 8 These units can remove vapour phase moisture only Liquid water will pass through or d
3. In general terms the energy requirements for similar air quality classifications are much the same for these units as the heatless or pressure swing unit Improved heater designs particularly in bed integral designs with lower energy consumptions are becoming available with optimised desiccant beds for the duty Such features lead to reductions in energy costs to around 10 of the generation cost for Class 1 2 1 air quality However research and development in this field is leading to changes which do reduce the energy required to regenerate the desiccant It is possible to obtain a unit with an external blower with heater elements This obviates the need for any purge of compressed air The heating of the regenerating air stream can be by steam which is often available on site at little or no cost Such a system is found to produce Class 1 2 1 quality air for 8 additional generation costs Duty times and consequently regeneration times for the towers in large TSA units are much longer than PSA units This feature enables off peak electrical power to be used with excellent savings being achieved Dewpoint sensing control of heated dryers which have long cycle times is achieved by extending the duty phase when regeneration is not yet required as shown in Fig 10 Energy savings Energy savings Energy savings Fig 10 Dewpoint sensing control of heated desiccant dryers 19 Regeneration by Waste Heat of Compression
4. WALES Business and Environment Branch National Assembly for Wales Cathays Park Cardiff CF10 3NQ Tel 029 2082 5172 Section 4 1 4 2 6 8 6 9 6 10 6 11 6 12 6 13 6 14 6 15 6 16 6 17 6 6 1 6 6 2 6 7 1 6 7 2 6 7 3 6 7 4 6 7 5 6 8 1 6 8 2 6 8 3 6 8 4 6 15 1 CONTENTS INTRODUCTION COMPRESSED AIR FILTRATION AND DRYING HOW HIGH ARE YOUR TREATMENT COSTS CLASSIFICATION CLASSES How to Apply Classifications Typical Applications SPECIFYING AIR TREATMENT EQUIPMENT TREATMENT SYSTEMS After cooling Air receivers Filtration Aughinish Alumina Ltd Askeaton Compressor Pre filtration Pre filters General Purpose Filters High Efficiency Oil Removal Filters After filters Dust Removal Filters Activated Carbon Filters Breathing Air Sterile Filters Point of Use Filters Drying Refrigerated Air Dryers 3 C Dewpoint Deliquescent Dryers 10 C Dewpoint Membrane Dryers 4 C to 40 C Dewpoint Desiccant Dryers 20 C to 70 C Dewpoint Barbican Centre London Zeneca Macclesfield Micron Mills Ltd Orpington Sony Manufacturing Company Bridgend Rosyth Royal Dockyard Rosyth Vauxhall Motors Ellesmere Port Condensate Collection Electronic Condensate Drain Traps ECDTs United Glass Alloa Lubricators APPENDIX Useful addresses Page No Section Page No FIGURES Fig 1 Water removal each week from 500 l sec of 7 bar g air Fig 2 Treatment needed for a selection of co
5. Govt Office for the East Midlands The Belgrave Centre Stanley Place Talbot Street Nottingham NGI 5GG Tel 0115 971 2476 North East Sustainability and Environment Team Govt Office for the North East Wellbar House Gallowgate Newcastle upon Tyne NEI 4TD Tel 0191 202 3614 NORTHERN IRELAND IRTU Scientific Services 17 Antrim Road Lisburn Co Antrim BT28 3AL Tel 028 9262 3000 North West Environment Team Govt Office for the North West Cunard Building Pier Head Water Street Liverpool L3 IQB Tel 0151 224 6401 South East Sustainable Development Team Govt Office for the South East Bridge House 1 Walnut Tree Close Guildford Surrey GUI 4GA Tel 01483 882532 East Sustainable Development Awareness Team Govt Office for the East of England Heron House 49 53 Goldington Road Bedford MK40 3LL Tel 01234 796194 SCOTLAND Energy Efficiency Office Enterprise and Lifelong Learning Dept 2nd Floor Meridian Court 5 Cadogan Street Glasgow G2 6AT Tel 0141 242 5835 South West Environment and Energy Management Team Govt Office for the South West The Pithay Bristol Avon BS1 2PB Tel 0117 900 1700 West Midlands Regional Sustainability Team 77 Paradise Circus Queensway Birmingham B1 2DT Tel 0121 212 5300 Yorkshire and the Humber Sustainable Development Unit Govt Office for Yorks and the Humber PO Box 213 City House New Station Street Leeds LS1 4US Tel 0113 283 6376
6. designed to minimise network pressure drops with a peak flowing velocity of 6 to 10 metres per second It is a false economy often encountered to fit a filter with small connections to larger pipework because pressure loss will be incurred which will cost energy to overcome Maintenance of filters is most important A careful check on the differential pressure will alert the user of the need for a replacement element Consider fitting duplex filters in critical systems that cannot be shut down to change elements This avoids having to tolerate unacceptable pressure drops 11 6 4 Aughinish Alumina Ltd Askeaton This major bauxite processing plant uses compressed air for plant and instrument applications The instrument air is treated by a desiccant dryer fitted with pre and after filters Maintaining the pressure of the instrument air system is the priority of the site compressor control system During an efficiency review of the compressed air system the instrument air treatment system was found to be causing restrictions due to an increased demand for air A total pressure drop of 1 3 bar was attributed to overloading and subsequent deterioration of the two filters A further 0 6 bar pressure drop was measured across the dryer In order to overcome this restriction an older dryer filter unit was re commissioned and operated in parallel to the other The flows through the two dryers were carefully balanced to avoid overloading either drye
7. filter air particularly in ageing large systems at the point of use to remove such particles and thereby avoid problems Point of use filters add pressure losses to the system particularly as they are often overlooked and hence waste energy 6 8 Drying Relying on filtration only the water classification will remain high at Class 6 or worse This is because it is necessary to depress the dewpoint see Fig 4 below ambient for condensation to occur Therefore following pre filtration when better water content quality is required some form of drying by depression of the dewpoint is necessary Dewpoint is the temperature below which water vapour will condense to liquid water at given conditions Fig 4 shows how atmospheric air at 10 C will have a dewpoint of 46 C at 7 bar g If the temperature of the compressed air system drops below 46 C water vapour will condense within the pipework and receivers etc System pressure barg 502515 7 3 1 0 Dewpoint C at pressure indicated 70 60 50 40 30 20 10 0 10 20 30 40 50 60 Dewpoint C at atmospheric pressure Fig 4 Dewpoint and the water holding content of air If the same air is dried by refrigeration to a pressure dewpoint of 3 C equivalent to a dewpoint of 21 C at atmospheric pressure then no water vapour will condense until the system temperature reaches 3 C If a desiccant dryer
8. purging air losses These figures are averages Some individual types of dryer are better than others as discussed later Additional energy will also be consumed to overcome the pressure drop when point of use filters are required for removal of odours particulates and or microbial contamination 4 CLASSIFICATION CLASSES In order to bring some logic to the design and application of treatment systems the International Standards Organization ISO in conjunction with PNEUROP have brought air contamination classes into use for the major contaminants These are shown in Table 3 Table 3 Air contamination classifications ISO 8573 1 Dirt Dirt Water Oil Particle size in Concentration Pressure dewpoint including microns mg m C ppm vol at vapour 7 bar g mg m 0 1 0 1 70 0 3 0 01 1 1 40 16 20 128 15 8 3 940 40 10 7 1 240 10 1 500 N A NB This standard is currently under review 4 1 How to Apply Classifications Prior to the purchase of new treatment systems or when reviewing energy saving opportunities with existing systems the user should assess carefully the actual needs The classification class required should be specified to all vendors in order that correct analysis of bids can be made in terms of price and energy consumption 4 2 Typical Applications Table 4 shows typical compressed air applications and the classification classes neede
9. so more water and oil will condense A receiver also creates a quiet zone where the turbulence is considerably reduced Some moisture can pass through the after cooler condensate removal separator at high velocity and the receiver can help to trap this 6 3 Filtration This is a far ranging topic Fig 2 attempts to show the many points where filters are fitted and the different types that are fitted Compressors are often found generating at a pressure well above that required for the process in order to cater for undersized filters the wrong filters for the duty too many filters or poorly maintained filters Fig 3 shows the additional cost of generating the compressed air which is needed to overcome pressure drops such as those resulting from the use of filters On average it is found that for every 1 bar additional generation pressure there is a loss of 7 in specific energy When designing treatment systems the number of filters to be employed to meet the classification class should be considered along with the peak design flow and a maximum service pressure differential should be specified to the vendors Class _ _ _ Large pneumatic tools Some foundry machines Class 2 4 4 Welding machines C AC WS R GP RD Pneumatic tools 1 1 Machine tools Sas Hand tools Workshop air Air motors 4 Fluidic
10. 3066 British Compressed Air Society 33 34 Devonshire Street London WIN IRF Tel 0171 935 2464 Fax 0171 935 3077 applications of new ntures into new d information on energy managers
11. GOOD PRACTICE GUIDE 21 6 Energy saving in the filtration and drying of compressed air PRAG o m 5 B amp D ENERGY EFFICIENCY BEST PRACTICE PROGRAMME 9L AGIND ADILOVed GOOD ENERGY SAVING IN THE FILTRATION AND DRYING OF COMPRESSED AIR This Guide is No 216 in the Good Practice Guide series It provides advice on practical ways of improving energy efficiency in the filtration and drying of compressed air Case histories are also included providing practical examples of how savings have been made Prepared for the Department of the Environment Transport and the Regions by ETSU Harwell Didcot Oxfordshire OX11 ORA and Air Technology Ltd 6 amp 7 Falcon Street Loughborough Leicestershire LE11 1EH ETSU also acknowledges the help of the following Domnick Hunter Denco Miller plc Ultrafilter and Atlas Copo Crown copyright 1998 First published March 1998 LIST OF RELEVANT GOOD PRACTICE GUIDES 2 ENERGY SAVINGS WITH ELECTRIC MOTORS AND DRIVES 3 INTRODUCTION TO SMALL SCALE COMBINED HEAT AND POWER 13 HEAT RECOVERY FROM HIGH TEMPERATURE WASTE GAS STREAMS 18 REDUCING ENERGY CONSUMPTION COSTS BY STEAM METERING 30 ENERGY EFFICIENT OPERATION OF INDUSTRIAL BOILER PLANT 31 COMPUTER AIDED MONITORING AND TARGETING FOR INDUSTRY 36 COMMERCIAL REFRIGERATION PLANT ENERGY EFFICIENT OPERATION AND MAINTENANCE 37 COMMERCIAL REFRIGERAT
12. ION PLANT ENERGY EFFICIENT DESIGN 38 COMMERCIAL REFRIGERATION PLANT ENERGY EFFICIENT INSTALLATION 42 INDUSTRIAL REFRIGERATION PLANT ENERGY EFFICIENT OPERATION AND MAINTENANCE 44 INDUSTRIAL REFRIGERATION PLANT ENERGY EFFICIENT DESIGN 48 REDUCING ELECTRICITY USE IN INJECTION MOULDING 59 ENERGY EFFICIENT DESIGN AND OPERATION OF REFRIGERATION COMPRESSORS 69 INVESTMENT APPRAISAL FOR INDUSTRIAL ENERGY EFFICIENCY 84 MANAGING AND MOTIVATING STAFF TO SAVE ENERGY 85 ENERGY MANAGEMENT TRAINING 91 MONITORING AND TARGETING IN LARGE MANUFACTURING COMPANIES 126 COMPRESSING AIR COSTS 168 CUTTING YOUR ENERGY COSTS A GUIDE FOR THE TEXTILE DYEING AND FINISHING INDUSTRY 169 TOTAL QUALITY MANAGEMENT 213 SUCCESSFUL PROJECT MANAGEMENT FOR ENERGY EFFICIENCY 214 MAKING USE OF BUSINESS STANDARDS 215 REDUCING ENERGY COSTS IN INDUSTRY WITH ADVANCED COMPUTING AND CONTROL 217 CUTTING ENERGY LOSSES THROUGH EFFECTIVE MAINTENANCE TOTALLY PRODUCTIVE OPERATIONS Copies of these Guides may be obtained from Energy Efficiency Enquiries Bureau ETSU Harwell Didcot Oxfordshire OX11 ORA Tel 01235 436747 Fax 01235 433066 E mail etsueng aeat co uk Overseas customers please remit 3 per copy minimum of 6 with order to cover cost of packaging and posting Please make cheques drafts or money orders payable to ETSU FOREWORD This Guide is part of a series produced by the Government under the Energy Efficiency Best Practice P
13. Some special dryers are available for dedication to oil free compressors These take all or a proportion of the hot compressed air before after cooling to regenerate the non duty section of the dryer The load on the compressor and the efficiency of its cooling system will determine the delivered dewpoint which is normally in the range 20 C to 30 C depending on load and cooling The cost of providing 1 5 1 quality air can be as low as an additional 3 on generation costs 6 9 Barbican Centre London The Barbican Arts Centre requires instrument air for control purposes and to power actuators The electrical cost for the compressed air at this site is approximately 5 500 per annum The conditions in the vast underground Power House are far from ideal for the compressed air system Consequently the air temperature onto the dryer is high requiring a larger dryer to be installed than would otherwise be necessary Due to the critical requirement of this air a standby dryer is also installed A previous dryer failure has resulted in both dryers being run on duty to ensure the integrity of the system When the dryers were due for replacement it was thought that energy saving controllers on the new dryers would reduce electrical costs considerably The new dryers were purchased with moisture sensor controls which ensure that the dryer towers purge only when required This enabled both dryers to be operated with optimum efficiency Th
14. d to suit the duty Care should be taken when using this information as it is for guidance only since individual uses can vary If in doubt consult the equipment supplier Table 4 Typical application requirements Application classes Typical quality classes Water e Air agitation 3 Air bearings Air gauging Air motors Brick and glass machines Cleaning of machine parts Construction Conveying granular products Conveying powder products Fluidics power circuits Fluidics sensors Foundry machines Food and beverages UA N FIL Ns Nn gt tA NM WY Hand operated air tools n 1 ds Machine tools Mining Micro electronics manufacture Packaging and textile machines Photographic film processing Pneumatic cylinders Pneumatic tools Process control instruments Paint spraying amp po vw m sm a Ol a wl es Jm es Sand blasting Welding machines A General workshop air RICO rw Bi t90 WTR tA W MW My BG GB NM RB KD 5 SPECIFYING AIR TREATMENT EQUIPMENT Whether designing new systems or reviewing existing systems the first step should be to define exactly what are the compressed air purity requirements There is a very wide range of requirements for air quality all o
15. e controllers proved successful saving 30 of the total compressed air usage and providing a payback of less than 1 5 years The reduced demand on the compressors meant that temperatures were lowered and the periods between maintenance extended 6 10 Zeneca Macclesfield This major pharmaceutical site uses compressed air for a variety of instrumentation purposes The air is compressed at a central compressor station and distributed site wide Each department treats the air to the required quality for the duty Traditionally as an ICI site the emphasis on equipment specification has been to ensure that the capacity of the equipment more than meets the required duty In the case of desiccant compressed air dryers this means that purge air costs are high This problem has been recognised and moisture sensing controls are usually specified in order to save energy and related costs One such instance is the recent purchase of two 152 cfm heatless desiccant dryers for the SPP Plant Both dryers are more than twice the capacity for their required duty However they are interlinked so that either can act as a backup when the other requires maintenance work Both were purchased with energy saving controls The dryer feeding SPP Plant sees the heaviest duty Its energy controller has been shown to save 65 of the dryer s rated purge air This has provided a payback period for the controller of less than 1 5 years 20 6 11 Micron Mills Ltd Orp
16. e pressure dewpoints achievable are in the range of 40 C to 70 C Drying air to classification Class 1 2 1 with this machine is found to add some 15 to the generation costs when the purge air loss and filter pressure drops are taken into account This type of dryer is the only configuration capable of drying air to 70 C classification Class 1 1 1 However this is found to add some 21 to the generation costs when the purge air loss and filter pressure drops are taken into account Due to the energy costs and problems associated with the additional compressor capacity required to provide the purge air this method is normally limited to fairly small capacities unless Class 1 air is required Under elevated inlet temperature conditions in excess of 40 C heatless dryers can be more energy efficient than the heated type which need to be considerably de rated on flow when operating at these conditions 18 Energy saving a NN ON _ t Energy saving L_ Duty L Regenerating Fig 9 Dewpoint sensing control of heatless desiccant dryers Dewpoint sensing control of heatless dryers provides savings by skipping unnecessary purge cycles as shown in Fig 9 Heated Desiccant Dryers This configuration also known as the temperature swing absorber TSA typically uses a combination of embedded electrical heaters in the desiccant and a smaller amount of purge air to drive out the moisture laden air from the non duty tower
17. ensing automatic drains at a cost of under 4 000 The consequent payback for these was within the first few months A further 19 drains were also installed around the site and a similar payback was predicted 23 6 17 Lubricators Many air tools and other devices using compressed air need lubrication to give good service life Compressor oil which is carried forward in poor air quality systems cannot be relied upon to provide such lubrication since it is often broken down or of incorrect grade and may be mixed with solid dirt particulates It is normal to employ lubricators which will inject an oil aerosol at the usage point These are often used as part of a final filter and pressure regulator set These devices work on the basis of a pressure differential causing the oil to atomise Hence they can waste energy If a large number of usage points connected to a common line need to be lubricated it is recommended that a carefully sized oil film creation device is fitted at the feeding point Such a device generates bubbles of oil as the compressed air flows through it which can travel long distances through the pipework and provide excellent lubrication without causing excessive pressure drops and energy loss when compared to traditional lubricators 24 7 APPENDIX USEFUL ADDRESSES Energy Efficiency Best Practice Programme Enquiries Bureau ETSU Harwell Didcot Oxfordshire OXII ORA Tel 01235 436747 Fax 01235 43
18. ercome additional pressure drops or the cost of purging air This Good Practice Guide GPG starts by describing the contaminants that may be present in air and then gives guidance on selecting the level of treatment appropriate to the application It describes the energy costs associated with the different equipment available for treating air and how these can be minimised through best practice Case studies are included which demonstrate the savings that can be made Energy Consumption Guide 42 Compressing air costs Treatment which is a companion to this GPG shows how energy savings of 35 were made at a sample of ten sites using the techniques described in this Guide Most industrial applications for compressed air are at a pressure of 7 bar 100 psig Therefore unless otherwise stated this Guide concentrates on equipment rated at or around this working pressure 2 COMPRESSED AIR FILTRATION AND DRYING Air compressors inhale many different contaminants prior to the compression cycle These include water vapour dirt atmospheric pollution During compression the volume of the air reduces in proportion to the absolute compression ratio The contamination is therefore increased per unit of volume Table 1 below shows typical levels of atmospheric contamination per m of free air and the change in concentration when the air has been compressed to 7 bar g Table 1 Typical contamination levels for atmospheric and com
19. es such as food pharmaceuticals and micro electronics there can be direct contact between the air and the product In some of these cases sterile air is required which can be achieved by additional filtration or special filters that can be steam sterilised 3 HOW HIGH ARE YOUR TREATMENT COSTS The results from many practical tests of the energy consumption versus the achieved air quality in dewpoint terms of all the types of treatment systems known are shown in Table 2 The additional spend on energy is expressed as a percentage of the basic cost of compressed air Table 2 Typical additional costs for drying compressed air Pressure Dryer type Typical levels of Added dewpoint filtration installed energy cost 10 C Deliquescent Nil 1 one Refrigeration General purpose 5 20 C Membrane High efficiency 28 20 C Waste heat Depends on 3 5 regenerative compressor configuration 40 C Desiccant heatless High efficiency before 10 15 and dust removal after 40 C Desiccant heated or High efficiency before 8 12 external blower and dust removal after 70 C Desiccant heatless High efficiency before 21 and dust removal after The additional energy costs are made up of the extra generation pressure needed to overcome the pressure drop across the dryer and accompanying pre and after filters actual direct energy usually electricity required to run the equipment
20. estroy the desiccant These machines can be high energy users particularly if misapplied or poorly maintained For this reason the manufacturers are bringing improved units to the market place which offer reduced energy consumption Tower A Duty Purge Duty Purge Tower B Purge Duty Purge Duty Heat 8 hours regenerated 5 mins Heatless Fig 8 Operation of desiccant dryers 17 There are two basic types the heated and heatless dryer Both types traditionally use a control with a fixed purge loss and purge cycle timer This can prove expensive when the demand is less than the dryer rated flow Over sizing desiccant dryers with fixed control can result in a considerable waste of compressed air energy Most recently manufacturers are introducing dewpoint dependent switching or in bed sensing devices These sense the compressed air load on the dryer by measuring the dewpoint at the dryer exit or the moisture content of the desiccant bed If the load is low then the amount of regeneration needed is less and the regeneration cycle is automatically altered to take this into account Excellent energy savings are being achieved by the use of this technique Unless an energy management system is employed it is important not to oversize desiccant dryers particularly the heatless models A dryer will have a fixed purge loss and purge cycle t
21. f applications requiring a high quality dewpoint Heat regenerated desiccant air dryers were to be purchased to treat the air at the two compressor houses Realising that there is a high energy cost for desiccant dryers it was considered that improved control could reduce this expense The operating cost for the standard model of the 16 dryers would be 56 700 per annum 19 of the site annual electric cost for compressed air This figure includes the air purge and heater electric consumption during the dryer cycle It is a high figure since the dryers needed to be sized for the installed compressor capacity not the mean demand for air Consequently the dryer towers would often be regenerating before the moisture level had reached design limits The dryers were purchased with optional discharge air moisture sensors and controls to pause the changeover cycle until the drying tower reached a saturation level to need regeneration A timer was fitted on this control for one of the dryers so that the site engineers could easily monitor the energy savings obtained On this dryer the moisture sensing control system reduced the regeneration cost by 37 1 Assuming a similar saving for the other dryers this provided a total annual electric saving of 21 000 The project payback for the controls was much less than two years without any loss of compressed air quality A further benefit is that each dryer is now fitted with a dewpoint sensor which can supp
22. f which can be met with the right equipment However unnecessary levels of treatment can significantly increase the associated energy costs The points to be considered at this stage are the air quality needed for each duty within the facility the estimated air demand for each duty the working pressure needed at the usage point the atmospheric contamination conditions the position of the compressor intake the temperature conditions of the compressed and cooling air at the treatment system location e the type of compressor the quality of compressed air being delivered by the site compressors the condition and configuration of existing pipework Health and Safety Executive and other legislative requirements Many plants need only part of the air treated to a very high quality In these cases excellent savings are achievable by treating all the generated air to the minimum acceptable level and improving the quality to the desired level close to the usage point If the majority of the air is needed at a high quality it can make sense to treat all the compressed air to the level required by the highest quality user This is not usually the most economic method in terms of capital expenditure or energy However this method can save on pipework and the users can be confident that wherever the compressed air is taken from the quality will be good Having made the basic decisions each of the methods available s
23. hould be reviewed to decide on which configuration is required and determine the total associated additional energy expenditure When comparing equipment specifications look not just at the energy consumption at maximum or rated flow but also at the efficiency at lower flows more typical of actual use 6 TREATMENT SYSTEMS Each part of the treatment system listed below is now discussed in some detail and includes typical energy costs and associated energy saving opportunities e After cooling Air receivers Filtration Pre filters After filters Drying Refrigeration dryers A Deliquescent dryers Membrane dryers A Desiccant dryers e Condensate collection 6 1 After cooling The first step in removing water and some oil vapour through condensing is in the after cooler which is a standard fitting on most modern compressors Some 6846 of the water is removed in the after cooler when the air temperature is reduced to 35 C The after cooler power requirement is normally included in the total package electrical consumption Some 2 of the package power is needed by the after cooler 6 2 Air Receivers Following the after cooler the air is normally fed to an air receiver the volume of which is normally some 1046 of the compressor rated output in volume per minute terms The receiver should wherever possible be placed outside in a cool location which will further reduce the temperature of the compressed air and
24. ime unless dewpoint switching or other methods of control are employed Therefore if the demand is considerably less than the dryer design flow much energy will be wasted Desiccant dryers are very reliable well proven machines but problems beyond the control of the manufacturer are often found which impact on their efficiency and hence the energy consumption of the compressed air system Quite often desiccant dryers are found to be operating at very poor dewpoints while taking the full energy requirement This can be due to high inlet temperature operating at the wrong pressure operating over capacity or poor maintenance It is recommended that a regularly calibrated dewpoint meter is fitted at the discharge of all such dryers to monitor effectiveness Other areas which should be checked if the dewpoint is poor are poor cooling poor pre filtration causing liquid phase water and oil carryover to contaminate or destroy the desiccant bed high peak loads causing desiccant bed fluidisation faulty change over valves causing continuous purge faulty controls causing poor or no regeneration of individual towers e desiccant contamination by oil Heatless Desiccant Dryers 40 C to 70 C Dewpoint In this machine also known as the pressure swing absorber PSA the desiccant in the non duty tower is regenerated by purging with compressed air from the system This can take from 10 20 of the dryer design flow in purge air Th
25. ington Micron Mills Ltd uses compressed air as the grinding gas for the fine milling process of pharmaceutical products As such the supply of high quality air is vital to production The compressed air system needs to be sized and available to meet peak production but by the nature of the product full capacity is rarely required Consequently the old heatless desiccant dryers were proving costly with their fixed purge cycle When the company was planning a major upgrade of the compressed air system it was decided to replace the dryers and the option of a dewpoint led control system was considered In addition to cost savings the dewpoint meters associated with the controls would provide a valuable alarm should loss of dewpoint occur and therefore avoid the possibility of product wastage The two new 480 cfm dryers proved very successful with the delivered dewpoint being better than before The dewpoint control system supplied with the dryers was fitted with timers which give an indication of the energy savings For this application 85 of the purge costs have been saved On average the air is required for 50 hours per week so savings of around 1 800 per annum are achieved 6 12 Sony Manufacturing Company Bridgend Sony Manufacturing a division of Sony United Kingdom Ltd established in South Wales since 1973 has two highly efficient manufacturing plants Staff there are committed to their Environmental Policy in which energy savi
26. itional 28 of the initial water content following the after cooler When used in conjunction with a high efficiency oil removal filter air of the classification Class 1 4 1 is delivered Condenser d Air in nd Air out Air air exchanger Air refrigerant exchanger Water separator Liquid separator Condensate Expansion Compressor valve Receiver Fig 5 A refrigerant air dryer 15 Some 5 is added to the cost of generating the compressed air by this method when the power needed for the refrigeration circuit and the filter pressure drop are taken into account These are well proven machines which give few problems in service if properly installed and maintained However problems have occurred which can affect the performance and hence energy consumption These include poor dewpoint due to internal contamination pre filtration is recommended especially with oil injected compressors high compressor delivery temperatures faulty condensate drain traps allowing the passage of liquids downstream of the dryer loss of refrigerant 6 8 2 Deliquescent Dryers 10 C Dewpoint This is a simple form of chemical dryer in which the compressed air is passed through a bed of salt tablets see Fig 6 This non regenerable medium produces a pressure dewpoint of around 10 C to 11 C below the compressed air inlet temperature Air outlet I Deliquescent bed Condensate outlet
27. itional filters These are discussed below 6 7 1 Dust Removal Filters With a desiccant dryer the desiccant is packed into two chambers The action of the air flow through the duty bed takes desiccant fines downstream of the dryer With these dryers a dust removal filter is always installed to remove particles down to 1 um 6 7 2 Activated Carbon Filters All the previous stages of filtration will remove water liquid oil and oil aerosols and dirt However the smell of oil will remain in the system If the compressed air is to be used for breathing mixing with food or pharmaceutical products or other such duties an activated carbon filter is required 6 7 3 Breathing Air In some work areas the atmospheric air is too contaminated for operators to breathe without risk to health In these cases it is normal to provide a clean air supply which can be breathed safely BS4275 1997 applies to breathing air quality standards in the workplace Care should be taken with this duty If the demand for breathable air is high some plants have a dedicated generation system with treatment to the standard required at the point of generation or point of use Very small demands for breathable air can be met by the use of portable equipment as used by the Fire Service and scuba divers The general air system is often used to provide breathable air In this case special considerations are necessary to ensure adequate air supplies of acceptable quality Poi
28. ly an alarm should a loss of dewpoint occur 6 14 Vauxhall Motors Ellesmere Port The ageing compressor station at Vauxhall Motors was being reviewed to provide a more modern reliable and efficient system The air is generated at a central compressor station prior to distribution throughout the site Previously the air had been treated by a refrigerant dryer but the new engine and paint shops required a much higher quality of air The old dryer had reached the end of its life and was due for replacement One option was to replace the dryer with a new desiccant version providing the required air quality to the whole site However the two areas needing desiccant dried air accounted for only 38 of the total site usage Following a simple cost analysis project it was decided to replace the dryer in the compressor station with another refrigerant version Desiccant dryers were fitted at the air feeds to the two critical areas The project justification was that 55 000 per annum 4 5 of the annual compressed air electricity costs would be saved Compressed air supplied to most of the site continued to be dried to an acceptable level while high quality air is now available where needed The capital required for the extra dryers was recovered within two years 6 15 Condensate Collection Some of the greatest wastages often up to 10 of the mean demand are found at the condensate collection points at the inter cooler after cooler receive
29. mmon applications Fig 3 Increased cost of generating compressed air to overcome pressure drops on a nominal 7 0 bar system Fig 4 Dewpoint and the water holding content of air 13 Fig 5 A refrigerant air dryer 14 Fig 6 A deliquescent dryer 15 Fig 7 Operation of a membrane dryer 16 Fig 8 Operation of desiccant dryers 16 Fig 9 Dewpoint sensing control of heatless desiccant dryers 18 Fig 10 Dewpoint sensing control of heated desiccant dryers 18 Fig 11 Cutaway diagram of an electronic condensate drain trap 22 TABLES Table 1 Typical contamination levels for atmospheric and compressed air Table 2 Typical additional costs for drying compressed air Table 3 Air contamination classifications ISO 8573 1 nn A N Table 4 Typical application requirements ENERGY SAVING IN THE FILTRATION AND DRYING OF COMPRESSED AIR 1 INTRODUCTION The wide range of uses for compressed air means that there is a great variation in air quality requirements The concentration of airborne contaminants during compression means that the compressed air direct from the compressor package can rarely be used without some form of treatment Fortunately there is a wide range of equipment available which can satisfy the most demanding of applications The careful selection installation and maintenance of treatment equipment can help reduce the associated energy costs of treating air namely due to direct energy costs for running equipment the extra generation cost needed to ov
30. n that eight or more stages of filtration can be employed all in the pursuit of good practice In fact several more stages are often found because engineers have sought to overcome poor air quality Over 3 bar pressure differential has been measured at some sites just to overcome the filtration This should never be more than 0 5 bar when the filters are new and up to a maximum of 1 bar in service on full air demands When designing new or reviewing older systems it is recommended that the following points are considered the peak air demand the piping size the piping material the contamination levels within the pipe the effect of any treatment changes on the piping contamination the air classification class needed at each of the usage points the air demand for each class the configuration of compressor employed the compressed air and ambient temperature at the filtration points the air dryer configuration isolation valves in the system This information will enable the designer to arrive at the minimum number of filters required for the duty Having arrived at the filtration required the filter size should be considered Most suppliers rate filters in flowing capacity the designer should work on the peak potential flows The screwed or flanged connections to the housing are often found to be much smaller than the pipework diameter upstream and downstream of the filter The pipework should have been
31. ng is a key criterion The Display Device plant requires air with a dewpoint better than 40 C for the manufacture of television tubes The compressed air was treated by a bank of eight heat regenerated desiccant dryers installed in parallel The system was struggling to achieve the desired quality and was using excessive energy to maintain this result The company worked with a dryer manufacturer to develop a specially sized dryer which met the needs of better efficiency greater reliability and reduced maintenance energy usage and noise One twin tower desiccant dryer with external heat regeneration replaced the previous equipment The new dryer used heated atmospheric air for the regeneration so saving on expensive purge losses The regeneration cycle was timed such that the heating used off peak electricity tariffs The system was considerably simpler than previously used so reducing the cost of maintenance After commissioning the quality of the air was measured as better than 60 C which assisted the product quality The new dryer installation has reduced the electrical power consumption cost of the compressed air system by 10 and reduced maintenance costs by 77 This has provided a Suitable payback for the project The benefits also include better air quality reduced noise levels and the release of a compressor for standby duties 21 6 13 Rosyth Royal Dockyard Rosyth The Rosyth dockyard uses compressed air for a wide variety o
32. nt of use purification is best since only the quantity needed will be treated Specially designed breathing air packages can be purchased to connect to the air distribution network at the point of use which will contain all the components needed to clean up the air to the standard required It is important that maintenance of all breathing air equipment is conducted at the correct intervals to ensure that the air quality is within specification In addition the air purity must be tested periodically to prove compliance with the Control of Substances Harmful to Health COSHH Regulations 1994 6 7 4 Sterile Filters Micro organisms in compressed air can be a serious problem in some processes Some can be as small as 0 04 um they are living organisms and are able to multiply freely under the right conditions The passage of even a few viable organisms into a clean area process or system causes contamination This can result in reduced product quality complete rejection or serious infection 13 To protect against such events sterile filters are employed These are designed such that they can be sterilised in place with steam 6 7 5 Point of Use Filters With the exception of breathing air and sterile filters all the other types discussed previously are normally found at or close to the point of generation However as has already been mentioned piping systems can add contamination due to rust and pipescale It is often necessary to
33. oler This is normally found upstream of the air dryer and will remove particles down to 1 um including coalesced liquid water and oil giving an air quality of 2 6 4 6 6 2 High Efficiency Oil Removal Filters When improved levels of oil removal are required it is normal to fit a high efficiency oil removal filter This will remove 99 999 of the particles down to a size of 0 01 um including water and oil aerosols giving an air quality of 1 6 1 The oil quality at this point is as good as can be delivered by an oil free machine which would still need this filter to remove any inhaled atmospheric contamination These filters are normally found upstream of the air dryer 12 It should be noted that if the duty is for truly oil free air Class 1 oil content 0 01 um oil free compressors or lubricated machines with the correct filtration can be specified While it is possible to provide air of Class 1 quality with lubricated compressors and filters the filters must be installed under the correct operational conditions typically 21 C and be properly maintained Where the oil free application is critical a duplex arrangement will be required to ensure that the treatment is not bypassed for maintenance requirements In addition oil free rotary compressors are more efficient and have a longer life between major overhaul than their lubricated counterparts 6 7 After filters Following the dryer it is normal to find add
34. pressed air Matter Contamination levels Atmospheric air Compressed air Dust 140 x 106 m 1 120 x 10 m Water 50 80 in UK 100 condensate Industrial pollution Acids hydrocarbons aerosols etc Multiplied by 8 Natural entities Pollen bacteria viruses etc Multiplied by 8 The compressor can add additional contaminants depending on its construction These can include oil oil vapour wear particles e carbonaceous products from compressor oil With normal 7 bar g systems at the final discharge pressure the temperature prior to the after cooler will range from 100 C to 190 C depending on the compressor configuration the water and oil content will remain mainly as vapours Following compression the compressed air is cooled and water and oil vapours condense into liquids which collect in the distribution system if left untreated The delivered air from a compressor package will typically have a pressure dewpoint of 35 C for an explanation of dewpoint see page 13 and an oil content of 4 ppm The situation as regards water at the specified conditions is shown in Fig 1 4 gt 4 41 wk remaining 1 080 I wk v Compressor 1 2 3 6896 2396 596 735 I wk 250 I wk into 54 I wk removed by pipework system cooled aftercooling to 35 C cooled to 15 C to 2 C Fig 1 Water removal each week from 500 l sec of 7 bar g air Since ambient air con
35. r The filter elements were replaced The result of this work enabled the compressor generating capacity to be reduced from 8 8 to 7 8 bar g The low cost of the actions provided an estimated payback period of within one month 6 5 Compressor Pre filtration To protect the compressor from incoming dirt a pre filter is always provided In the case of piston vane and screw machines these filters have a dirt retention capacity of around 5 um Centrifugal machines are more sensitive to incoming dirt and inlet filtration is normally in two stages the final filtration level being 0 2 um Some additional power is required to overcome the pressure drop of the inlet filter This is taken into account with compressor package performance figures However as these items become contaminated on duty the pressure drop and hence the power required will increase Increases in specific power consumption of 3 are not uncommon due to this problem Consult the compressor service manual and change the filters when the pressure differential has reached the maximum recommended 6 6 Pre filters Before any drying takes place it is usual to fit pre filters to reduce the oil content and the water burden on the drying stage 6 6 1 General Purpose Filters Following the after cooler and air receiver the next part of the treatment process before drying is filtration It is normal with oil injected compressors to fit a general purpose filter following the after co
36. r and distribution system If the compressor is lubricated the oil vapour condenses with the water and forms an emulsion Sometimes wear particles and pipescale add to the problem by becoming trapped in valves and diaphragms causing drains to leak permanently Often drain traps are unsuited to the duty or incorrectly installed and after a short while they fail causing the contamination to pass on to the 22 usage points To avoid this problem many traps are found by passed or valves left cracked open causing the wastage The problem with oil free machines which leads to similar wastages is that many traps do not work well without some lubrication and the internal parts rust causing them to fail Special oil water separation systems are needed to comply with the Environmental Protection Act 1990 This act and its implications for the disposal of condensate from compressed air systems are described in the booklet Condensate Disposal available from the British Compressed Air Society BCAS 6 15 1 Electronic Condensate Drain Traps ECDTS ECDTs are now a popular alternative to conventional automatic drain traps or manual drains having the advantages of no air loss during condensate discharge low maintenance and high reliability They are available in a range of sizes from very small units attached to the bottom of filters to much larger units fitted to air receivers see Fig 11 Condensate out Diaphragm valve
37. rogramme The aim of the programme is to advance and spread good practice in energy efficiency by providing independent authoritative advice and information on good energy efficiency practices Best Practice is a collaborative programme targeted towards energy users and decision makers in industry the commercial and public sectors and building sectors including housing It comprises four inter related elements identified by colour coded strips for easy reference Energy Consumption Guides blue energy consumption data to enable users to establish their relative energy efficiency performance Good Practice Guides red and Case Studies mustard independent information on proven energy saving measures and techniques and what they are achieving New Practice projects light green independent monitoring of new energy efficiency measures which do not yet enjoy a wide market Future Practice R amp D support purple help to develop tomorrow s energy efficiency good practice measures If you would like any further information on this document or on the Energy Efficiency Best Practice Programme please contact the Environment and Energy Helpline on 0800 585794 Alternatively you may contact your local service deliverer see contact details below ENGLAND London Govt Office for London 6th Floor Riverwalk House 157 161 Millbank London SWIP 4RR Tel 020 7217 3435 East Midlands The Sustainable Development Team
38. s power circuits m Class 1 4 2 Pneumatic cylinders Drain Class 2 2 2 Powder conveying Sensors Instruments Paint spraying Class 1 1 1 Drain 70 C Micro electronics Photographic HE CT Some food and Terminal filtration beverages depends on level of upstream filtration Instruments i Tools Breathing air GP LUB BA HE Key C Compressor HE High efficiency filter AC After cooler DD Desiccant dryer WS Water separator DR Dust removal filter R Receiver CT Carbon tower GP General purpose filter LUB Lubricator RD Refrigeration dryer BA Breathing air set Fig 2 Treatment needed for a selection of common applications 12 A o energy costs 96 A o Increase in compressor 0 0 2 0 4 06 08 10 12 1 4 Pressure drop bar Fig 3 Increased cost of generating compressed air to overcome pressure drops on a nominal 7 0 bar system 10 All filters in a system should be fitted with pressure differential gauges which show when filter replacement is needed The inlet air temperature to each filter should not be greater than the manufacturer s recommendations or else the capability of the filter to retain efficiently the contaminants will be greatly reduced In the following Sections air contamination classes met are given against each treatment system They are always given in the order dirt water oil In can be seen from this Sectio
39. tains 12 5 g of water vapour for each m of free saturated air at 15 C a 500 1 5 1 000 cfm compressor will inhale 1 080 litres of water vapour per 48 hour week This quantity is still present in the air at terminal pressure and discharge temperature The vapour will begin to condense as the air temperature is cooled to or below that of the pipework downstream of the compressor If there are atmospheric contaminants or oil is added to the air for compressor lubrication a proportion of these contaminants will condense with the water as the air is cooled Other contaminants are added to the compressed air by the piping network including e pipescale rust sludge These contaminants tend to dry out at the extremes of the network where they are seen as particulate matter or dirt In critical applications the pipework is made from materials such as polished stainless steel copper and PVC to avoid corrosion at times point of use filters are still used to prevent contamination of the product or process In less critical applications where normal pipework is employed it is usual practice to employ point of use filters to avoid contamination When air is used for breathing BS4275 1997 and for certain processes the smell of oil vapours and toxic gases are problems for which additional treatment is required Uses for compressed air are becoming more and more exacting In many processes such as brewing and paint spraying and industri
40. were used to dry the air to a pressure dewpoint of 40 C the atmospheric dewpoint will become 57 C and the air will be almost completely dry 14 All configurations of dryers can suffer from some common problems which impact on their efficiency and energy costs elevated inlet temperatures above design which cause poor dewpoints bad installation and poor ventilation A common problem evidenced by poor dewpoint is when dryers are arranged in parallel taking compressed air from a common wet manifold and delivering into a common dry manifold Often different makes of dryer or dryers of different capacities are connected in parallel Great care should be taken in these cases to avoid preferential i e unbalanced flow between dryers since this adversely affects the dewpoint Air dryers are available in many configurations depending on the air quality required Each configuration with the associated additional energy requirement is given below 6 8 1 Refrigerated Air Dryers 3 C Dewpoint The most common method of dewpoint depression is the refrigerated air dryer see Fig 5 The refrigerant dryer depresses the dewpoint by cooling the incoming air so condensing moisture out of the air The dried air is then re heated by the incoming air in the air to air exchanger This machine comes in many forms such as direct expansion direct contact and thermal mass Typically pressure dewpoints of 3 C are reached which will remove an add
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