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考研英语阅读真题及答案2017年考研英语阅读真题及答案推荐：Theuseofheatpumpshasbeenheldbacklargelybyskepticismaboutadvertisers’claimsthatheatpumpscanprovideasmanyastwounitsofthermalenergyforeachunitofelectricalenergyused,thusapparentlycontradictingtheprincipleofenergyconservation.Heatpumpscirculateafluidrefrigerantthatcyclesalternativelyfromitsliquidphasetoitsvaporphaseinaclosedloop.Therefrigerant,startingasalow-temperature,low-pressurevapor,entersacompressordrivenbyanelectricmotor.Therefrigerantleavesthecompressorasahot,densevaporandflowsthroughaheatexchangercalledthecondenser,whichtransfersheatfromtherefrigeranttoabodyofair.Nowtherefrigerant,asahigh-pressure,cooledliquid,confrontsaflowrestrictionwhichcausesthepressuretodrop.Asthepressurefalls,therefrigerantexpandsandpartiallyvaporizes,becomingchilled.Itthenpassesthroughasecondheatexchanger,theevaporator,whichtransfersheatfromtheairtotherefrigerant,reducingthetemperatureofthissecondbodyofair.Ofthetwoheatexchangers,oneislocatedinside,andtheotheroneoutsidethehouse,soeachisincontactwithadifferentbodyofair:roomairandoutsideair,respectively.Theflowdirectionofrefrigerantthroughaheatpumpiscontrolledbyvalves.Whentherefrigerantflowisreversed,theheatexchangersswitchfunction.Thisflow-reversalcapabilityallowsheatpumpseithertoheatorcoolroomair.Now,ifundercertainconditionsaheatpumpputsoutmorethermalenergythanitconsumesinelectricalenergy,hasthelawofenergyconservationbeenchallenged?No,notevenremotely:theadditionalinputofthermalenergyintothecirculatingrefrigerantviatheevaporatoraccountsforthedifferenceintheenergyequation.Unfortunatelythereisonerealproblem.Theheatingcapacityofaheatpumpdecreasesastheoutdoortemperaturefalls.Thedropincapacityiscausedbythelesseningamountofrefrigerantmassmovedthroughthecompressoratonetime.Theheatingcapacityisproportionaltothismassflowrate:thelessthemassofrefrigerantbeingcompressed,thelessthethermalloaditcantransferthroughtheheat-pumpcycle.Thevolumeflowrateofrefrigerantvaporthroughthesingle-speedrotarycompressorusedinheatpumpsisapp
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