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1、2018年可锐考研英语阅读分析详解（三）Theuseofheatpumpshasbeenheldbacklargelybyskepticismaboutadvertisers’claimsthatheatpumpscanprovideasmanyastwounitsofthermalenergyforeachunitofelectricalenergyused,thusapparentlycontradictingtheprincipleofenergyconservation. Heatpumpscirculateafluidrefrigeran
2、tthatcyclesalternativelyfromitsliquidphasetoitsvaporphaseinaclosedloop.Therefrigerant,startingasalow-temperature,low-pressurevapor,entersacompressordrivenbyanelectricmotor.Therefrigerantleavesthecompressorasahot,densevaporandflowsthroughaheatexchangercalledthecondenser,whichtra
3、nsfersheatfromtherefrigeranttoabodyofair.Nowtherefrigerant,asahigh-pressure,cooledliquid,confrontsaflowrestrictionwhichcausesthepressuretodrop.Asthepressurefalls,therefrigerantexpandsandpartiallyvaporizes,becomingchilled.Itthenpassesthroughasecondheatexchanger,theevaporator,whi
4、chtransfersheatfromtheairtotherefrigerant,reducingthetemperatureofthissecondbodyofair.Ofthetwoheatexchangers,oneislocatedinside,andtheotheroneoutsidethehouse,soeachisincontactwithadifferentbodyofair:roomairandoutsideair,respectively. Theflowdirectionofrefrigerantthroughaheatpu
5、mpiscontrolledbyvalves.Whentherefrigerantflowisreversed,theheatexchangersswitchfunction.Thisflow-reversalcapabilityallowsheatpumpseithertoheatorcoolroomair. Now,ifundercertainconditionsaheatpumpputsoutmorethermalenergythanitconsumesinelectricalenergy,hasthelawofenergyconservat
6、ionbeenchallenged?No,notevenremotely:theadditionalinputofthermalenergyintothecirculatingrefrigerantviatheevaporatoraccountsforthedifferenceintheenergyequation. Unfortunatelythereisonerealproblem.Theheatingcapacityofaheatpumpdecreasesastheoutdoortemperaturefalls.Thedropincapaci
7、tyiscausedbythelesseningamountofrefrigerantmassmovedthroughthecompressoratonetime.Theheatingcapacityisproportionaltothismassflowrate:thelessthemassofrefrigerantbeingcompressed,thelessthethermalloaditcantransferthroughtheheat-pumpcycle.Thevolumeflowrateofrefrigerantvaporthrought
8、hesingle-speedrotarycompressorusedinheatpumpsisapproxi
