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英文文献

英文原文

Theoreticalanalysisoflow-temperaturehotsourcedriventwo-stageLiBr/H20absorptionrefrigerationsystem

W.B.Ma

GuangzhouInstituteofEnergyConversion,ChineseAcademyofSciences,

81CentralMartyrs'Road,Guangzhou,China

S.M.Deng

DepartmentofBuildingServicesEngineering,TheHongKongPolytechnicUniversity,HungHom,Kowloon,HongKong

Received19December1994;revised6June1995

Abstract:

Adetailedtheoreticalanalysisispresentedforatwo-stageLiBr/H20absorptionrefrigerationsystem,whichconsistsofanevaporator,alow-pressureabsorber,alow-pressuregenerator,ahigh-pressureabsorber,ahigh-pressuregenerator,acondenser,alow-pressureheatexchangerandahigh-pressureheatexchanger,drivenbyalow-temperaturehotsource.Acomparisonofresultsfromthetheoreticalanalysisandpreliminaryexperimentindicatesthatthetheoreticalanalysisdevelopedcanrepresentarealsystemwithareasonableaccuracy,andisusefulforfuturedevelopment.

Keywords:

absorption；water-lithium；bromide；two-stage

Literatureonabsorptionrefrigerationsystemsdrivenbyalow-gradeenergysourcesuchassolarenergyorwasteheatinindustrieshasbeenpresented1-5.Thesesystemscaningeneralbeclassifiedintotwotypesaccordingtothedifferentworkingfluids:

absorptionrefrigerationsystems,usingwater-lithiumbromide（LiBr/H20）,water-ammonia（H20/NH3）,water-lithiumchloride（LiCI/H20）;andadsorptionsystems,usingammoniacalciumchloride（CaCI2/NH3）,water-silica-gel,waterzealot,activatedcharcoal-methanol（CH3OH）.However,onlyabsorptionrefrigerationsystemsusingwaterlithiumbromideasworkingfluidhavebeenoperatedsuccessfullyandhavefoundcommercialapplications.

Asingle-stageLiBr/H20absorptionrefrigerationsystemgenerallyconsistsofanevaporator,absorber,generator,condenserandsolutionheatexchanger.Thissystemoperateswithwaterasrefrigerantandlithiumbromideasabsorbent,andtheheatsourcerequiredtorunsuchasystemshouldhaveatleastatemperatureofover86°CinordertoachieveareasonableCOP6.However,thereexistsalargeamountoflow-temperatureheatsourceoflessthan86°C,suchaswasteheatinindustries,solarenergyandgeologicalheat.Iftheselowtemperatureheatsourcescanbeusedorreused,itwillnotonlyimprovetheoverallsystemenergyefficiency,butdecreasetheheatpollutiontotheenvironmentaswell.

Atwo-stageLiBr/H20absorptionrefrigerationsystem,withwaterasrefrigerantandlithiumbromideasabsorbent,canhoweverbeoperatedwithalowertemperatureheatsourcefrom75to86°C,andacoefficientofperformance（COP）ofabove0.38forsuchasystemcanbeachievedwhenthecondensercoolingwatertemperatureis32°Candthechilledwatersupplytemperatureis9°C,notablywhentheoutlettemperatureoftheheatsourceislowerthan64°C7.Thereforeatwo-stageLiBr/H20absorptionrefrigerationsystemisusefulandsignificantinrecoveringlowtemperaturewasteheatinindustries,andinapplyingsolarenergyandgeologicalheat.Researchworkforsimulatingsingle-stageabsorptionrefrigerationsystems,double-effectabsorptionrefrigerationsystemsandabsorptionheatpumpshasbeenreportedbyanumberofresearchers,includingBogart8,Vilestetal9,andGrossmanetal1°-12,butlittleresearchfortwo-stageLiBr/H20absorptionrefrigerationsystemshasbeenpresented.Thispaperdescribesthetheoreticalanalysisfortheperformanceofatwo-stageabsorptionrefrigerationsystemunderdifferentoperatingconditions,andacomparisonbetweenthetheoreticalanalysisandpreliminaryexperimentalresultsispresented

.

Assumptions

Thefollowingassumptionsweremadeinthecourseofanalysis.

1.ThetemperatureandconcentrationofLiBraqueousareinequilibriumatthesaturatedpressureofLiBraqueous.

2.Heatlosstooutsidethesystemisneglected.

3.TheLiBrfractionpressureisneglected:

i.e.thepressureinthevaporphaseisequaltothesaturatedpressureofwater.

Systemdescription

Thetwo-stageLiBr/H20absorptionrefrigerationsystemshownschematicallyinFigure1consistsofanevaporator,low-pressure（LP）absorber,LPgenerator,LPsolutionheatexchanger,high-pressure（HP）absorber,HPgenerator,HPsolutionheatexchanger,condenserandtwosolutionpumps.ThepressureintheevaporatorisequaltothatintheLPabsorber,Pe.ThepressuresintheLPgeneratorandtheHPabsorberareequal（Pm）asarethepressuresintheHPgeneratorandthecondenser（Pc）.TheorderofthesepressuresisPe

Therefrigerantwateriscirculatedthroughtheevaporator,LPabsorber,LPgenerator,HPabsorber,HPgeneratorandcondenser.Afterwatervaporhascondensedinthecondenser,itreturnstotheevaporatorthroughanexpansionvalve.However,theabsorbentLiBraqueoussolutioniscirculatedwithintwoseparatecycles:

alow-pressurecyclebetweentheLPabsorberandtheLPgenerator,andahigh-pressurecyclebetweentheHPabsorberandtheHPgenerator.

Figure2showsthetwo-stageLiBr/H20absorptionrefrigerationcycleonalogP-Tdiagram.Thecycleindicatedby21-71--51--41--81--91--21isthelow-pressurestage,andthatby2h--7h--5h--4h--8h--9h--2histhehighpressurestage,correspondingtopointsinFigure1.Ascomparedwithasingle-stageabsorptionrefrigerationsystem,therearetwoadditionalcomponents,istheHPabsorberandLPgenerator,inatwo-stagesystem.TheseareusedtoconcentratetheLiBraqueoussolutionintheLPstagecyclesothatitispossibletoproduce7-9°Cchilledwaterintheevaporator.

Theoreticalanalysisoftheabsorptioncycle

Low-pressurecycle

WatervaporizedintheevaporatorisabsorbedbytheconcentratedsolutionofLiBrfromtheLPgeneratorthroughtheLPsolutionheatexchanger;thentheLiBraqueoussolutionchangestotheweaksolution.Theoutlettemperatureandconcentration（T12andXl,）oftheLiBrsolutionaredeterminedbasedonthepressurePeandtemperatureTwiofthecoolingwater：

TheweaksolutionisthenpumpedtotheLPgeneratorthroughtheLPsolutionheatexchanger,inwhichitexchangesheatwiththeconcentratedsolutionfromtheLPgenerator.IntheLPgenerator,weaksolutionisheatedtoT~4andconcentratedtostrongsolutionXlr.T14andX'lrareinequilibriumatpressurePm：

ThenstrongsolutionreturnstotheLPabsorbertoformthelow-pressurecycle.

High-pressurecycle

ThestrongsolutionXhrintheHPabsorberfromtheHPgeneratorthroughtheHPsolutionheatexchangerabsorbsvaporgeneratedintheLPgenerator.Theweaksolution）（haisthenpumpedtotheHPgeneratorthroughtheHPsolutionheatexchanger.Similarly,Xh,andTh2aredeterminedbythetemperatureTwiofthecoolingwaterandPro：

TheweaksolutionintheHPgeneratorisheatedtoTh4bytheheatsourceandconcentratedtostrongsolutionXhr.Th4andXhrareinequilibriumatpressurePc：

ThestrongsolutionconcentratedintheHPgeneratorthenreturnstotheHPabsorber,formingthehighpressurecycle.

Massandenergybalance

Evaporator：

LPabsorber：

LPgenerator：

LPheatexchanger：

（19）

HPabsorber：

HPgenerator：

HPsolutionexchanger：

（26）

Condenser：

Totalinputheat：

Totaloutputheat：

Totalenergybalance：

Coefficientofperformance（COP）：

Statevariables,temperatures,concentrationsandenthalpiesofLiBraqueoussolutionatpoints1-9werecalculatedbyliterature13-19.

Thetemperaturedifferences（ΔT）betweentwofluidsattheexitsoftheheatexchangersinthesystemweredeterminedbasedontheoperatingparametersandheatexchangerstructureindifferentheatexchangers.Forexample,intheevaporator,thetemperaturedifferencesbetweenthechilledwaterandrefrigerantattheentranceandexitare7and2°Crespectively.

Results

Analysisresultswereobtainedusinglow-temperaturehotwaterasheatsourceandarediscussedbelow：

Effectofhotwatertemperature

Figure3showstheeffectofthetemperatureofthehotwateronthecoefficientofperformance（COP）.Itisclearthatthetwo-stageLiBrabsorptionrefrigerationcannotbeoperatediftheoutlettemperatureoftheLiBraqueoussolution,Th4orTl4,intheHPgeneratorandLPgeneratorisbelow58°C,undertheconditionwhenthetemperatureofchilledwateris7°candthatofthecondensercoolingwateris32°c.IfthetemperaturedifferencebetweentheinlettemperatureofthehotwaterandtheoutlettemperatureoftheLiBraqueoussolution,Th4orT14,intheHPgeneratorandLPgeneratorisassumedtobe15°C,thenatwo-stageLiBr/H20absorptionrefrigerationsystemcannotbeoperatedwhenthetemperatureofthehotwaterThi,isunder73°C.TheCOPofatwo-stageLiBr/H20absorptionrefrigerationsystemincreaseswiththetemperatureofthehotwater,butwhenthetemperatureThi,isabove87°C,orTh4andT14areabove72°C,theincreaseofCOPisverysmall.Becausetheheatlossincreaseswhenthetemperatureoftheheatsource,Thi,isincreased,thetemperatureoftheheatsourceThiabove87°Cisnotadvantageousforatwo-stageLiBr/H20absorptionrefrigerationsystem.Figure3showsthatthebesttemperaturerangeoftheheatsourceisbetween75°Cand87°Cforatwo-stageLiBr/H20absorptionrefrigerationsystem.

Effectofchilledwatertemperature,Tcho

TheinfluenceofdifferentchilledwatertemperatureTchoontheCOPofatwo-stageLiBr/H20absorptionrefrigerationsystemisshowninFigure4.Atwo-stageLiBr/H20absorptionrefrigerationsystemismoresuitableforsupplyingchilledwaterofover9°Cforairconditioningorotherapplications.Ifthetemperatureofthechilledwaterisunder7°C,theCOPisverysmallwhenthetemperat