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Large refrigeration systems oftencontain multiple compressorsoperating in parallel. Thesecompressors can be controlled tomaximize the efficiency of theentire system. This is particularlyimportant with screwcompressors. Based on Universityof Wisconsin research, one largerefrigerated warehouse inWisconsin could save 5% to 15%annually by improvingcompressor control.

PART-LOAD PERFORMANCEScrew compressor s Most screw compressors canmodulate their capacity from10% to 100% of full load. This ismost often accomplished byusing a slide valve to shorten orlengthen the suction volume. A

typical screw compressorunloading curve is shown inFigure 1. Below about 50% of full-load capacity, theefficiency of a screw compressor decreasessigni ficantly. At ful l-load capacity, screwcompressors generally perform better thanreciprocating compressors, though theirperformance decreases at part-loads. This meansthat proper sizing and control is crit ical.

Recipr ocating compressors Reciprocating compressors unload more linearly

than screw compressors. A typical unloading curveis shown in Figure 1. At full-load, reciprocatingcompressors do not perform as well as screwcompressors, especially as the refrigerationtemperature is lowered. However, reciprocatingcompressors are better suited for systems that havehigh load variability.

OPTIM AL LOAD SHARINGIf there are two compressors in your refrigerationsystem, how you split the load between them canaffect both energy use and operating costs. The

following rules assume that the two compressorshave a similar design.

Screw comp ressor s Equal-sized compressors. For system loads lessthan 6570% of full capacity, the load should besplit equally between the two compressors. Once thesystem load is greater than 6570% of full capacity,one compressor should be fully loaded and the othershould meet the balance of the load.

Unequal-sized compressors . Because screwcompressor performance deteriorates at less than50% full-load, try to avoid running any individualcompressor at less than 70% load. For intermediateloads, the smaller compressor should be fullyloaded. For large loads, the larger compressorshould be fully loaded. Further research is neededto develop guidelines if the side-inlet economizerports are active. However, the general rule stillholds that these compressors should be as fullyloaded as possible.

(Figure 1) Part-load characteristi cs for screw and r eciprocating compressors.

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ENERGY CENTEROF WISCONSIN

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Opt im izing Load Sharing w ith M ult ipleCompressorsTechniques to cut energy w aste in large refrigeratio n system s

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Recipr ocating compressors

Equal-sized compressors . Reciprocatingcompressors have li ttle performancedegradation when unloaded, so the relativeload distribution between two equal-sizedcompressors is not very important. Splittingthe load to equalize pressure losses in the drysuction line yields the optimal performancebecause this will minimize suction line losses.

Unequal-sized compressors . When unequal-sized reciprocating compressors are running inparallel, the load should be splitproportionally to minimize the pressure dropin the dry suction lines.

Screw and recipro cati ng compressors used together When both reciprocating and screwcompressors operate together, baseload thescrew compressor and use the reciprocatingcompressor to meet the varying load.

LEARN M ORE

Find out more about performanceoptimization of industrial refrigerationsystems:

Cutt ing Energy Waste in Large Refr igerati on Systems. 1999. Energy Center of Wisconsinfact sheet. Available at www.ecw.org.

Evaporator Condenser Control in Industr ial Refr igerati on Systems. 2000.By Kyle A. Manske, Douglas T.Reindl, and Sanford A. Klein.Submitted for publication in theInternational Journal of Refrigeration.

Load Shari ng Str ategies in Mult ipl e Compressor Refr igerati on Systems.1999. By Kyle A. Manske, Douglas T.Reindl, and Sanford A. Klein.Presented at the 8th International

Refrigeration Conference at Purdue University,July 2000.

Performance Optimizati on of Industr ial Refr igerati on Systems. 1999. Masters thesis byKyle A. Manske. University of Wisconsin,Madison. Energy Center of Wisconsinpublication 196-1.

Refr igerat ion System Malfuncti ons (Monitor ing to Prevent Problems). 1999. By Kyle A.Brownell, Sanford A. Klein, and Douglas T.Reindl. ASHRAE Journal, February 1999.Available on-line at www.ashrae.org .

ABOUT US

The Energy Center o f W isconsin

is a private nonprofit

organizat ion dedicated t o

improving energy efficiency in

Wisconsin. With suppor t f rom

Wisconsins energy u tilities, the

Center provides energy eff iciency

prog rams, research, and

education to residents,

businesses, and government.

ENERGY CENTEROF WISCONSIN

Copyr ight 2001Energy Center o f W isconsin

Learn other ways to save compressor energy in thefact sheet Cutting Energy Waste in Large Refrigeration

Systems .

Hiddeninefficienciesinyour refriger-ationsystemcouldberobbingyou ofthousandsof dollarsinenergycostsandshorteningthelife of your equip-ment.At anicearenainMadison,Wisconsin,for example,a mechanicalengineeringgraduatestudent from theUniversityof Wisconsintook acloselook at a newrefrigerationsystem.What hefoundaddedup toproblems

costingmorethan$19,000ayear.

Your refrigerationsystemscouldbesufferingfromhiddenequipmentfaultsandinefficient controls.Youmight besurprisedat howmuchmoneyyoucansaveandhoweasilyyoucanprevent costlyproblemssimplyby installingproper monitoringequipmentandadoptinganefficient control strategy.

SOMECOSTLY CULPRITSA CASESTUDYA p lugged fi l ter: $2300Suctionlinefi lterskeeptherefrigerant cleanandprotect the compressor from ingestingdamagingdebris.Onenormallydoesnot expect thesefiltersto get dirtybecausetherefrigerantsystemissealed,but attheicearenaoneof thesuctionline filterswasfoundto beplugged.Therestrictedflowcauseda pressuredropinthe systemthat,accordingtocomputer models,wascostingthe facil-ity$2300 ayear,or about fivepercent of their annu-al operatingcost.

A low refrigeran t charge: $7800Thesysteminspectionat thei cearenaalsorevealedalow refrigerant chargedueto ahiddenrefrigerantleak.Theleak wasrepairedandthesystemrecharged,bringingabout adramaticperformanceboost.Accordingtooperational datatakenbefore

andafter therepair,thelow chargecauseda33 per-cent performancedropandwasadding$7800t oannual operatingcosts.

A standard con t ro l st rategy : $9600Sometimesacommon, acceptedpracticeis not thebest practice.In manyrefrigeration systems,com-

pressor discharge (head)pressureis kept at afixed,high level toassuresafe,reliableoperationover arangeof outdoortemperatures.Fixedhighheadpressuremaintainsadequaterefrigerant flow,freezeprotection for theevaporative condenser,andan adequatepressure

differenceacrossthe expansion valve.But fixedheadpressureisnt theonly wayto providetheseassurances.

Itsfar moreefficient toallowheadpressuretofloat withambient wet-bulbtemperature,down toaminimum safelevel for agivensystem.With float-ingheadpressure,thesystemworksonly ashardasit needstounder all weather conditions,yet safetyandreliability aremaintained.

Cutting Energy Waste inLarge Refrigeration SystemsStandard practicesoften arent enoug h to keep costly inefficiency at bay

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ENERGYCENTEROFWISCONSIN

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Weareafourth generationbutter manufacturer.Wevebeenworkingwith arefrigerationcontractor since1989tocut theenergycost forour operation.Wevebeenableto savetensofthousandsof dollarsbyapplyingideassuchasfloatingheadpressure.

Stan Zander Zander s Creamery, Inc., Cross Plains,Wisconsin

Engineering graduatestudent KyleBrownell demonstratesthe effect of lettingrefrigeration head pressurefloata technique that should savean icearena$9600 a year in energy costs.

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