SVW-HVAC introduction

1Airconditioner2Airconditioningsystemshavelongceasedtoberegardedasluxuryequipment.Airconditionershavebecomeafactorinactivesafety,andtodaycanalmostbeconsideredasanintegralpartofavehicle'ssafetyspecification.10yearsago,onlyabout10percentofallnewlyregisteredvehicleswerefittedwithanairconditioningsystem.By1996,airconditionerswerebeinginstalledasstandardinmorethanoneinfournewlyregisteredvehicles.Customerdemandforairconditioningisrisingcontinually.3Whyairconditioning(1)Peoplefeelcomfortableatacertainambienttemperatureandatmospherichumidity.Asacomponentpartofactivesafety,thedriver'swell-beingisakeyfactorindrivingability.The“in-carclimate“hasadirectbearingonthedriver,fatigue-freedrivinganddrivingsafety.4ComfortcurvesAcomfortableinteriortemperatureisdependentupontheprevailingambienttemperatureanduponsufficientairflow:Lowambienttemperature,e.g.–20oCÆHigherinteriortemperature28oCÆHighairflowrate:8kgpermin.Highambienttemperature,e.g.40oCÆLowinteriortemperature23oCÆHighairflowrate:10kgpermin.Moderateambienttemperature,e.g.10oCÆLowinteriortemperature21.5oCÆLowairflowrate:4kgpermin.5Whyairconditioning(2)ScientificstudiesconductedbytheWHO(WorldHealthOrganization)haveshownthatone'sabilitytoconcentrateandreactionsareimpairedwhenunderstress.Heatputsastrainonthebody.Thebesttemperatureforthedriverisbetween20and22oC.ThisisequivalenttoclimaticloadA,thecomfortrange.Strongsunlightcanincreasetheinteriortemperaturebymorethan15oCabovetheambienttemperature–particularlyintheheadarea.Thisiswheretheeffectsofheataremostdangerous.Thebodytemperaturerisesandtheheartrateincreases.Heavierperspirationwilltypicallyoccur,too.Thebrainisnotreceivingenoughoxygen.AlsorefertoclimaticloadrangeB.ClimaticloadsinrangeCputanexcessivestrainonthebody.Physiciansspecialisingintraffic-relatedillnessesrefertothisconditionas“climaticstress“.Studieshaveshownthatanincreaseintemperaturefrom25to35oCreducesone'ssensoryperceptionandpowersofreasoningby20%.Ithasbeenestimatedthatthisfigureisequivalenttoaloodalcoholconcentrationof0.5millilitresalcohollevel.6PhysicsofthecoolingsystemManysubstancesareknowntoexistinthreeaggregatestates.Takewaterforexample:solid–liquid–vapour.Theprincipleofcoolingfollowsthislaw.Eveninancienttimestherewasaneedforcooling.Oneofthefirstmethodsusedtocoolfoodstuffswastostoretheminan“icebox“.Theice(waterinasolidaggregatestate)absorbstheheatofthefoodstuffs,therebycoolingthemdown.Theicemeltsasaresult,assuminganotheraggregatestate,namelythatofaliquid(water).Ifthewaterisheatedfurther,itwillboilandevaporate.Thewaterisnowinthegaseousstate.Thegaseoussubstancecanbeconvertedbacktoaliquidbycoolingitandwillbecomeasolidagainifcooledfurther.7PressureandboilingpointIfthepressureischangedusingaliquid,theboilingpointchanges.Allliquidsbehavesimilarly.BoilingpointH2O/water=100oCMachineoil=380-400oCTheevaporationprocessisalsousedinvehicleairconditioners.Asubstancewithalowboilingpointisusedforthispurpose.Thissubstanceisknownasarefrigerant.BoilingpointRefrigerantR12–29.8oCRefrigerantR134a–26.5oCIncurrentautomotiveairconditioners,onlyrefrigerantR134aisused.8StateofrefrigerantR134ainthecycleinanairconditioner9Toocooldownanobject,heatmustbegivenoff.Acompressionrefrigerationsystemisusedinmotorvehiclesforthispurpose.Arefrigerantcirculatesintheclosedcircuit,continuallyalternatingchangingfromaliquidtoagasandviceversa.Therefrigerantis:–compressedinthegaseousstate,–condensedthroughheatdissipation–andevaporatedthroughpressurereductionandheatabsorption.Coolairisnotproduced,heatisextractedfromtheairflowinthevehicle.10Coolingprocess11CoolingsystemThecomponentsAtoHexistineverycircuit.Additionalconnectionscanbeprovidedforservicework,temperaturesensors,pressureswitchesinthehigh-andlow-pressurecircuitandoildrainscrewsdependingonthecircuitdesignandrequirements.Thelayoutofcomponentswithinthecircuitalsodiffersfromonevehicletypetoanother.Somesystemshaveadamperbeforethecompressorinordertodampenrefrigerantvibrations.12ThecompressorThecompressordrawsincold,gaseousrefrigerantthroughtheevaporatorunderlowpressure.Itisvitalforthecompressorthattherefrigerantbeinagaseousstate,becauseliquidrefrigerantcannotbecompressedandwoulddestroythecompressor(inmuchthesamewayasawatershockcandamageanengine).Thecompressorcompressestherefrigerantandforcesittowardsthecondenserasahotgasonthehigh-pressuresideofrefrigerantcircuit.Thecompressorthereforerepresentstheinterfacebetweenthelow-pressureandhighpressuresidesoftherefrigerantcircuit.13ThecondenserHot,gaseousrefrigerantcomingfromthecompressoratatemperatureofapprox.50-70oCisinjectedintothecompressor.Thetubesandfinsofthecondenserabsorbheat.Coolambientairisductedoverthecondenser,absorbingheatintheprocessandtherebycoolingdowntherefrigerant.Whentherefrigerantcoolsdown,itcondensesataspecifictemperatureandpressureandbecomesaliquid.Atthebottomofthecompressor,therefrigerantemergesfromthecondenserasaliquid.14ThefluidcontaineranddrierTheliquidrefrigerantcomingfromthecondenserentersthecontainerattheside.Therefrigerantiscollectedinthecontainer,thenitflowsthroughthedrierandalongthe