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1、CHAPTER11HeatExchangersIntroductionHeatexchangerThedeviceusedtoimplementheatexchangebetweentwofluidsthatareatdifferenttemperaturesandseparatedbyasolidwall.ApplicationsSpaceheatingandair-conditioning,powerproduction,wasteheatrecovery,andchemicalprocessing.Objectives(1)Tointroduceperformance
2、parametersforassessingtheefficacyofaheatexchanger(2)Todevelopmethodologiesfordesigningaheatexchangerorforpredictingtheperformanceofanexistingexchangeroperatingunderprescribedconditions.11.1HeatExchangerTypesHeatexchangersaretypicallyclassifiedaccordingtoflowarrangementandtypeofconstruction
3、.concentrictubeParallel-flowThehotandcoldfluidsenteratthesameend,flowinthesamedirection,andleaveatthesameend.CounterflowThefluidsenteratoppositeends,flowinoppositedirections,andleaveatoppositeends.Alternatively,thefluidsmaymoveincrossflow(perpendiculartoeachother),asshownbythefinnedandunfi
4、nnedtubularheatexchangersoffigure11.2.11.1HeatExchangerTypesAnothercommonconfigurationistheshell-and-tubeheatexchanger.Specificformsdifferaccordingtothenumberofshell-and-tubepasses,andthesimplestform,whichinvolvessingletubeandshellpasses,isshowninfigure11.3.11.1HeatExchangerTypesBafflesare
5、usuallyinstalledtoincreasetheconvectioncoefficientoftheshell-sidefluidbyinducingturbulenceandacross-flowvelocitycomponent.11.1HeatExchangerTypesAspecialandimportantclassofheatexchangersisusedtoachieveaverylarge(≥700m2/m3)heattransfersurfaceareaperunitvolume.Termedcompactheatexchangers,thes
6、edeviceshavedensearraysoffinnedtubesorplatesandaretypicallyusedwhenatleastoneofthefluidsisagas,andishencecharacterizedbyasmallconvectioncoefficient.11.1HeatExchangerTypes11.2TheoverallHeatTransferCoefficientForcylindricalwallsSurfacefoulingThesubsequentdepositionofafilmorscaleonthesurfacec
7、angreatlyincreasetheresistancetoheattransferbetweenthefluids.Thiseffectcanbetreatedbyintroducinganadditionalthermalresistance,termedthefoulingfactor,Rf.Itsvaluedependsontheoperatingtemperature,fluidvelocity,andlengthofserviceoftheheatexchanger.FineffectFinsare