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

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