传热学教学课件：CHAPTER 11 Heat Exchangers

HEATTRANSFERCHAPTER11HeatExchangers#1HeatExchangers,LMTDMethodWherewe’vebeen……Sofarhavefocusedondetailedheattransferanalysisofspecificconditions,suchasexternalheattransfercoefficientWherewe’regoing:Investigatemethodsforlargersystemlevelanalysisthatcombineallthesemodesofheattransferinheatexchangers#2HeatExchangers,LMTDMethodKEYPOINTSTHISLECTURETypesofheatexchangers,advantagesanddisadvantagesOverallheattransfercoefficient,conceptoffoulingfactorLogmeantemperaturedifferenceApplicationofLMTDtoheatexchangeranalysisTextbooksections:§11.1–11.3#3HeatExchangerTypesExample:#4HeatExchangerTypes(Cont’d)ShellandTube:ShellandTube:ShellandTube:ShellandTube:

(commoninchemicalprocessindustry)#5HeatExchangerTypes(Cont’d)ShellandTube:#6HeatExchangerTypes(Cont’d)PlateandFrameSeriesofplateswithflowchannelsembossedinthem.ThetwofluidsareguidedthroughalternatingrowsoftheplatesAdvantages:__________________________Applicationpictured:Electrocoatpaintinautomotiveassemblyplant#7HeatExchangerTypes(Cont’d)PlateandFinDensearrayofplatesthatguidealternatingchannelsoffluids(typicallyair)SeriesoffinsconnecttheplatesandgreatlyincreasetheheattransferareaAdvantage:verylargeheattransfersurfaceareaperunitvolume.Onecommonapplication:Aircraftenvironmentalcontrolsystems#8OverallheattransfercoefficientforHXRecallfromearliertheoverallthermalresistanceconcept:Typesofresistancesinvolvedwithheatexchangers(coveredinprevioussessions)ColdsideinternalconvectionColdsidefoulingfactorConductionthroughwallHotsideexternalconvection(smoothwallormayinvolvefins)HotsidefoulingfactorReviewhow

thesewere

calculated#9AnalysisofheattransferTotalheattransferrateisfoundthroughenergybalance,regardlessoftheHXtypeorflowpathForhotfluid:

Energybalance:DefineForthecoldfluid:(Note:nominussign“-”inthisequation,

sinceheatflowin)q#10Analysisofheattransfer(Cont’d)Energybalancegives:FortheentireflowlengthAconvenientwaytocomputetheheattransferisfromthemeantemperaturedifferencebetweenthehotandcoldfluidsNext:

EvaluationofTmdifferentforparallelandcounterflow#11AnalysisofparallelflowheattransferParallelflowheatexchangerAtanylocationalongtheheatexchangerWhere:So:IntegratingfromtheinlettotheoutletdqEq.11.13#12Analysisofparallelflowheattransfer(Cont’d)Fromtheoverallenergybalance,totalheattransfer:thus:CombiningForparallelflow:Eq.11.14#13Analysisofparallelflowheattransfer(Cont’d)Temperatureprofileforparallelflow:InOutFig11.7#14AnalysisofcounterflowheattransferForcounterflow:Temperatureprofileforcounterflow:InOutFig11.8#15TypicaluseoftheLMTDmethod:Given:NeedtocoolacertainmassflowrateoffluidA

fromTA,itoTA,ousingthefluidBatTB,i

Find:Design/sizetheheatexchangerSolutionMethod:Usetheoverallenergybalancetofind

Selecttheheatexchangertype(basedontheotherprojectneeds,availableresources,sizeandweightconsiderations,etc.,etc.)Selecttubediametersandtypesofheattransfersurfaces(fins,nofins,etc.)Use

todeterminetheneededheatexchangerheattransferarea(length)#16SpecialcasesForacondensingvaporForanevaporatingliquidWhatifCh

=Cc

inacounter-flowHX?xTInOutxTInOutxTInOut#17Multipassandcross-flowheatexchangersTheequationsarethesame.Counter-flowconditionsTofindF,pleaserefertothefigures11.10-13.#18TypicalExample

E11.1(textbook,pp619)

#19HeatExchangers,LMTDMethodKEYPOINTSTHISLECTUREVarioustypesofheatexchangersthatarecommonlyusedinindustryandproductdesigns.