Refrigeration and Air Conditioning

ElectricalEnergyEquipment:RefrigerationandAirConditioningEnergyEfficiencyGuideforIndustryinAsia–©UNEP1REFRIGERATION&AIRCONDITIONINGSYSTEM1.INTRODUCTION..........................................................................................................12.TYPESOFREFRIGERATIONANDAIRCONDITIONING.....................33.ASSESSMENTOFREFRIGERATIONANDAIRCONDITIONING.....94.ENERGYEFFICIENCYOPPORTUNITIES...................................................125.OPTIONCHECKLIST..............................................................................................176.WORKSHEETS...........................................................................................................197.REFERENCES..............................................................................................................211.INTRODUCTIONThissectionbrieflydescribesthemainfeaturesoftherefrigerationandairconditioningsystem.1.1WhatisRefrigerationandAirConditioningRefrigerationandairconditioningisusedtocoolproductsorabuildingenvironment.Therefrigerationorairconditioningsystem(R)transfersheatfromacoolerlow-energyreservoirtoawarmerhigh-energyreservoir(seefigure1).HighTemperatureReservoirLowTemperatureReservoirRWorkInputHeatAbsorbedHeatRejectedFigure1.SchematicrepresentationofrefrigerationsystemElectricalEnergyEquipment:RefrigerationandAirConditioningEnergyEfficiencyGuideforIndustryinAsia–©UNEP2ThereareseveralheattransferloopsinarefrigerationsystemasshowninFigure2.Thermalenergymovesfromlefttorightasitisextractedfromthespaceandexpelledintotheoutdoorsthroughfiveloopsofheattransfer:§Indoorairloop.Intheleftloop,indoorairisdrivenbythesupplyairfanthroughacoolingcoil,whereittransfersitsheattochilledwater.Thecoolairthencoolsthebuildingspace.§Chilledwaterloop.Drivenbythechilledwaterpump,waterreturnsfromthecoolingcoiltothechiller’sevaporatortobere-cooled.§Refrigerantloop.Usingaphase-changerefrigerant,thechiller’scompressorpumpsheatfromthechilledwatertothecondenserwater.§Condenserwaterloop.Waterabsorbsheatfromthechiller’scondenser,andthecondenserwaterpumpsendsittothecoolingtower.§Coolingtowerloop.Thecoolingtower’sfandrivesairacrossanopenflowofthehotcondenserwater,transferringtheheattotheoutdoors.1.2Air-ConditioningSystemsDependingonapplications,thereareseveraloptions/combinationsofairconditioning,whichareavailableforuse:§Airconditioning(forspaceormachines)§Splitairconditioners§Fancoilunitsinalargersystem§Airhandlingunitsinalargersystem1.3RefrigerationSystems(forprocesses)Thefollowingrefrigerationsystemsexistsforindustrialprocesses(e.g.chillingplants)anddomesticpurposes(modularunits,i.e.refrigerators):§Smallcapacitymodularunitsofthedirectexpansiontypesimilartodomesticrefrigerators.§Centralizedchilledwaterplantswithchilledwaterasasecondarycoolantforatemperaturerangeovertypically5oC.Theycanalsobeusedforicebankformation.Figure2.AtypicalHeatTransferLoopinRefrigerationSystem(BureauofEnergyEfficiency,2004)ElectricalEnergyEquipment:RefrigerationandAirConditioningEnergyEfficiencyGuideforIndustryinAsia–©UNEP3§Brineplants,whichusebrinesasalowertemperature,secondarycoolantfortypicallysub-zerotemperatureapplications,whichcomeasmodularunitcapacitiesaswellaslargecentralizedplantcapacities.§Theplantcapacitiesupto50TR(tonsofrefrigeration)areusuallyconsideredassmallcapacity,50–250TRasmediumcapacityandover250TRaslargecapacityunits.Alargecompanymayhaveabankofunits,oftenwithcommonchilledwaterpumps,condenserwaterpumps,coolingtowers,asanoffsiteutility.Thesamecompanymayalsohavetwoorthreelevelsofrefrigerationandairconditioningsuchasacombinationof:§Comfortairconditioning(20–25oC)§Chilledwatersystem(80–100C)§Brinesystem(sub-zeroapplications)2.TYPESOFREFRIGERATIONANDAIRCONDITIONINGThissectiondescribesthetwoprincipletypesofrefrigerationplantsfoundinindustry:VapourCompressionRefrigeration(VCR)andVapourAbsorptionRefrigeration(VAR).VCRusesmechanicalenergyasthedrivingforceforrefrigeration,whileVARusesthermalenergyasthedrivingforceforrefrigeration.2.1VapourCompressionRefrigerationSystem2.1.1DescriptionCompressionrefrigerationcyclestakeadvantageofthefactthathighlycompressedfluidsatacertaintemperaturetendtogetcolderwhentheyareallowedtoexpand.Ifthepressurechangeishighenough,thenthecompressedgaswillbehotterthanoursourceofcooling(outsideair,forinstance)andtheexpandedgaswillbecoolerthanourdesiredcoldtemperature.Inthiscase,fluidisusedtocoolalowtemperatureenvironmentandrejecttheheattoahightemperatureenvironment.Vapourcompressionrefrigerationcycleshavetwoadvantages.First,alargeamountofthermalenergyisrequiredtochangealiquidtoavapor,andthereforealotofheatcanberemovedfromtheair-conditionedspace.Second,theisothermalnatureofthevaporizationallowsextractionofheatwithoutraisingthetemperatureoftheworkingfluidtothetemperatureofwhateverisbeingcooled.Thismeansthattheheattransferrateremainshigh,becausetheclosertheworkingfluidtemperatureapproachesthatofthesurroundings,thelowertherateofheattransfer.Therefrigerationcycleis