跨临界二氧化碳制冷技术世界第一台多功能两阶段二氧化碳概要文档格式.docx
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跨临界二氧化碳制冷技术世界第一台多功能两阶段二氧化碳概要文档格式.docx
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ThetwostagetranscriticalCO2refrigerationplantreplaces22independentcoolingandheatingsystemscomprisingaircooledsinglestageHCFCandHFCcondensingunitsforoneblastfreezer,onecoldstore,twochillers,oneprocesschilledwaterchillerandfourreversecycleACunits,sixR134aairtowaterheatpumps,threegasfiredwaterheatersandtwosetsofelectricunderflooranddoorheatersfortheexistingblastfreezerandcoldstore.
Operatingconditionsare+5°
CSSTforofficeAC,–5°
CSSTforhighstageandchillingdutiesand–35to–40°
CSSTfortheboosterdutiesservingthenewcoldstoreandblastfreezer.TheACcompressorsalsoserveaseconomisercompressorsforthemainchillingandfreezingplant,whichthereforerunsathighCOP’swithavirtualgascoolerCO2exittemperatureof+5°
C.
Duringcommissioningseveraldesignfaultswerediscovered,i.e.firstly,greatdifficultytocontrolthepressureinthe+10and+5°
Cvesselswithconventionalmechanicalspringloadedconstantpressureregulators;
secondly,thehighmiscibilityofCO2refrigerantinPOEoilsresultedina−5°
Coilstillnotperformingatall;
thirdly,thelackofsuctionsuperheatonthehighstageandACcompressorsresultedinlowcompressordischargetemperaturesandhencesomedifficultyinheatingwatertoadequatelyhightemperatures.Thiswascausedbythedesigner’sdesiretoachievethehighestpossibleCOPswithlowCO2gascoolerexittemperatures.Fourthlycoldstoreandchillerevaporatordesignsgotmixedduringmanufacture.Intheenditwasnecessarytochangethecircuitingoftheseunitsinthefield,adifficultjobrequiringgreatskill.Finallythereisstillsomeuncertaintyinthedesigner’smindaboutthebestwaytohandleCO2liquidflowfromthegascoolerduringsubcriticaloperations.Aseparatesubcriticalliquidreceivermayberequired.
Therewerealsoseveralsupplierissuesliketheinabilitytosupplysomehighpressurevalves,thedeletionofessentialcomponentsfromtheoilseparatorsandtheoilreturnarrangementsfromtheoilseparatorstothecompressorcrankcases,whichcausedoilcarryrates200timeshigherthanthosespecifiedatmaximumvaluesof5ppmofCO2mass.
Fortunatelyalldifficultieswereresolvedandnowtheplantisperformingsatisfactorily,althoughnotyetattheefficienciespredictedbythedesigner,whoestimatedthatthenewCO2plantwouldultimatelyreducethespecificelectricalenergyconsumptionby75%,gasconsumptionby60%,globalwarmingemissions–includingHFCandHCFCfugitivegases–by40%andcoolingwaterconsumptionby62,000litres/annum,i.e.40%whentheplantachievesthefulldesigncapacityof7,500kg/day.
1.INTRODUCTION
ItwasnoneotherthanhislatedearfriendProf.Dr.GustavLorentzenwhorevivedthewriter’sinterestinCO2refrigerationinthemid1980’swhentheOzoneDepletionpotentialofCFCsandHCFCsbecameevident.ThisresultedintheuniqueInternationalAgreementcalledtheMontrealProtocolin1987tophaseouttheuseofCFCsandHCFCsandtoprohibittheproductionanduseaftercertaindates.
TheeminentrefrigerationscientistDrS.ForbesPearsondesignedthefirstapplicationofCO2inthemodernerain1992.ThesystemcomprisedtwofloodedCO2evaporators.TheCO2vapouriscondensedinanammoniacooledplateheatexchanger.Asmalldemonstrationunitwasinstalledinasmall-23oCcoldstoreatMarksandSpencerp.l.c.,Kilmarnock,Scotland.CO2hotgasfordefrostwasgeneratedinaCO2boilerheatedbyammoniafromthedischargeofanammoniacompressor|1|.GustavLorentzencalledRef.1“Aremarkablepaper.”
10thIIRGustavLorentzenConference:
NaturalWorkingFluids,Delft,25-27June2012.
2.THEREVIVALOFCO2
GustavLorentzencalledpubliclyfortherevivaloftheuseofCO2in1993.AsProfessorRistoCiconkovshowssoeloquentlyinFigure1|2|,CO2andammoniawerecommonlyusedinallmannerofcoolingandfreezingapplicationsfromthe1870stothe1940s,includingcoolingforhumancomfort.ThecoolinginsomecinemasintheGeorgeStreetcomplexinSydneywaseffectedbyaCO2refrigerationsystemuntilabout1966!
ButaftertheadventofCFCs(R12etc.)inthe1930s,theuseofCO2rapidlydeclined.Luckilyammonia(NH3)survivedasaNaturalRefrigerantforindustrialapplications.
3.EVALUATIONOFCOP’s
3.1Subcriticaloperation
HCs
Kyoto1997HFC(CFC)HCFCNH3
CFC
NH3CO2HCs
1987
Figure1:
Abriefhistoryofrefrigerants
ReferringtoFig.2theCOPsforsubcriticalCO2operationdependonsuctionanddischargepressures,andthedegreeofliquidsubcooling,likeallsubcritical,i.e.conventionalrefrigerationsystems.3.2Transcriticaloperation
AsshowninFig.3theCOPoftranscriticalCO2compressorsisdependentonthesuctionanddischargepressureand–toamuchlargerextent–ontheCO2exittemperaturefromthegascooler.HencetheendeavourtohavethehighstagecompressorsrunwithaliquidCO2temperatureof+5°
C,whichisthevirtualgascoolerexittemperature.ThisisachievedbyusingtheACcompressorsaseconomisercompressorsaswellastheyeffectadegreeofparallelcompression.3.3SeasonallyweightedCOPs
Figure3.TranscriticalCO2compressorCOP
variationwithSat.SuctionTemp.atvariousgascoolerexittemperatures
TheMelbourneclimatehasameantemperatureof+15°
C.Giventheneedforwaterheating,thefactthatforashortperiodoftheyeartheplantwouldneedtorunintranscriticalconditionsandthefactthatthehighestrefrigerationloadsoccurduringthefivedaysthatproductiontakesplace,itisassumedthattheplantwillrunintranscriticalmode20%oftheyear,i.e.1752hours.Thisisequalto250daysproductionat7hours/day.
Therestofthetimethemainplantrunspurelytocompletetheblastfreezingcycleandmaintainoperatingconditions.TheACsystemrunspartiallyasaneconomisingcompressor.InFigure4,theambientdryandwetbulbtemperatureoccurrenceshavebeenplottedfrommorethan30,000datareadingstakenattheMelbourneCityweatherstationfrommidnighttomidnightfor10years|3|.WehaveselectedtheMelbourneCitydesignconditionsof+35°
Cdrybulb,+21°
Cwetbulb,whichwereexceededonly0.4%and0.3%ofthetimerespectivelyduringtheperiod1997–2006.
InTable1,wehaveevaluatedtheSeasonallyWeightedCOP(SWCOP)basedontheMelbourneCityclimate.AnadvantageofCO2isitshighpressure,whichallowsclosetemperatureapproachesbetweencondensingtemperatureandcoolingairorwater.Thelowertheambienttemperature,thehighertheCOPandthehighertheheatingloadwillbeinadditiontohotwaterrequirements,butthelowerthecoolingloadwillbe.It
hasbeenassumedthatonaverage20%oftheplantFigure4.AmbientDry&
WetBulbTemperaturecapacitywouldrunintranscriticalmodetoprovideheatProfile–MelbourneCity,10yearperiodforallpurposesandwhentheambientcoolingair1997–2006,24hrs/day|3|.temperatureistoohightoreducethedischargepressurebelow7.38MPa,thecriticalpressure.
Table1.EvaluationofWeightedCOPwithambienttemperatureconditionsfor12months
runningofCO2coolingintheCityofMelbournein:
a.DXCO2with+5°
CevaporatingtemperatureforfactorycoolingandofficeAC
b.DX
CO2with–5°
Cevaporatingtemperatureforhighstage,chiller,chilledwaterandpackingroom
4.SYSTEMFUNCTIONSANDCAPACITIES,ANDOPERATINGMETHOD
4.1Background
InSeptember2009ExquisinePtyLtddecidedtoinstallatwostagetranscriticalCO2refrigerationplanttoreplace22independentsystemsprovidingheatingandcoolingattheirThornbury,Victoria,foodprocessingfacilitywheretheymanufacturehighendfrozendairydesserts.Theprojectwassupportedbya50%grantfromAusIndustryundertheRe-ToolingforClimateChangeprogram.ACO2/ammoniacascadeplantwasbrieflyconsideredbutwithresidentialpropertiesborderingthesite,itwasjudgedbestnottouseammonia.Plantoperatingnoisewasalsoapotentialproblem.
4.2Definitionofplantfunctions|4|
AcompleterangeofhighenddessertsaremanufacturedattheExquisinefactorywherethetwostagetranscriticalCO2planthasbeeninstalled.
.1Refrigerationdutiesandcapacities
.1Cartonblastfreezingof1,500kgofproductperdaygrowingto7,500kgperdayatanaverageair
temperatureof–33°
C.Q=70kW@To=–40°
C
.2Coldstoragewiththesameplantofabout50tonsoffrozenproductsmostlyinshippercartons,but
someunpacked.Q=20kW@To=–40°
.3Chillstorageforingredientsat0°
C.Q=10kW@To=–5°
.4Packingareacoolingtoatemperatureof+10°
Cmaximum.Q=25kW@To=–5°
.5Generate5,000litresof4°
Cprocesschilledwaterperdayfromamainswatertemperatureof+20°
10kW@To=0.5°
C,backpressurecontrolled.
.6FactorycoolingandAC.Q=25kW@To=+5°
.7Officecoolingandheating.Q=10kW@To=+5°
.2Heatingduties
.1Underfloor,anddoor
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