heat exchangers.pdf

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1、cen58933_ch13.qxd9/9/20029:57AMPage667CHAPTERHEATEXCHANGERS13eatexchangersaredevicesthatfacilitatetheexchangeofheatbetweentwofluidsthatareatdifferenttemperatureswhilekeepingthemfromCONTENTSHmixingwitheachother.Heatexchangersarecommonlyusedinprac-13–1TypesofHeatExchangers668ticeinawiderangeofapp

2、lications,fromheatingandair-conditioningsystemsinahousehold,tochemicalprocessingandpowerproductioninlargeplants.13–2TheOverallHeatTransferCoefficient671Heatexchangersdifferfrommixingchambersinthattheydonotallowthe13–3AnalysisofHeattwofluidsinvolvedtomix.Inacarradiator,forexample,heatistransferr

3、edExchangers678fromthehotwaterflowingthroughtheradiatortubestotheairflowing13–4TheLogMeanTemperaturethroughthecloselyspacedthinplatesoutsideattachedtothetubes.DifferenceMethod680Heattransferinaheatexchangerusuallyinvolvesconvectionineachfluid13–5TheEffectiveness–NTUandconductionthroughthewallse

4、paratingthetwofluids.IntheanalysisofMethod690heatexchangers,itisconvenienttoworkwithanoverallheattransfercoeffi-13–6SelectionofHeatcientUthataccountsforthecontributionofalltheseeffectsonheattransfer.Exchangers700Therateofheattransferbetweenthetwofluidsatalocationinaheatex-changerdependsonthemag

5、nitudeofthetemperaturedifferenceatthatlocation,whichvariesalongtheheatexchanger.Intheanalysisofheatex-changers,itisusuallyconvenienttoworkwiththelogarithmicmeantemper-aturedifferenceLMTD,whichisanequivalentmeantemperaturedifferencebetweenthetwofluidsfortheentireheatexchanger.Heatexchangersarema

6、nufacturedinavarietyoftypes,andthuswestartthischapterwiththeclassificationofheatexchangers.Wethendiscussthede-terminationoftheoverallheattransfercoefficientinheatexchangers,andtheLMTDforsomeconfigurations.WethenintroducethecorrectionfactorFtoaccountforthedeviationofthemeantemperaturedifferencef

7、romtheLMTDincomplexconfigurations.Nextwediscusstheeffectiveness–NTUmethod,whichenablesustoanalyzeheatexchangerswhentheoutlettemperaturesofthefluidsarenotknown.Finally,wediscusstheselectionofheatexchangers.667cen58933_ch13.qxd9/9/2