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1、HeatexchangersHeatexchangers:types1Plateheatexchangers3Shell-and-tubeheatexchangers4Boilers6Characteristics7Heatexchangeranalysis8Overallheat-transfercoefficient9Fouling10Heatexchangermodelling10Non-dimensionalmethodofheat-exchangermodelling:theNTUapproach13Pr
2、eliminarysizing15HEATEXCHANGERS:TYPESHeatexchangersareoff-the-shelfequipmenttargetedtotheefficienttransferofheatfromahotfluidflowtoacoldfluidflow,inmostcasesthroughanintermediatemetallicwallandwithoutmovingparts.Weherefocusonthethermalanalysisofheatexchangers,
3、butproperdesignanduserequiresadditionalfluid-dynamicanalysis(foreachfluidflow),mechanicalanalysis(forclosureandresistance),materialscompatibility,andsoon.Heatlossesorgainsofawholeheatexchangerwiththeenvironmentcanbeneglectedincomparisonwiththeheatflowbetweenbo
4、thfluidflows;i.e.aheatexchangercanbeassumedgloballyadiabatic.Thermalinertiaofaheatexchangerisoftennegligibletoo(exceptinspecialcaseswhenamassiveporoussolidisusedasintermediatemedium),andsteadystatecanbeassumed,reducingthegenericenergybalanceto:(1)wherethetotal
5、enthalpyhthasbeenapproximatedbyenthalpy(i.e.negligiblemechanicalenergyagainstthermalenergy),andDmeansoutputminusinput.Althoughheatflowsfromhotfluidtocoldfluidbythermalconductionthroughtheseparatingwall(exceptindirect-contacttypes),heatexchangersarebasicallyhea
6、tconvectionequipment,sinceitistheconvectivetransferwhatgovernsitsperformance.Convectionwithinaheatexchangerisalwaysforced,andmaybewithorwithoutphasechangeofoneorbothfluids.Whenonejustreliesinnaturalconvectiontotheenvironment,likeinthespace-heatinghot-waterhome
7、radiator,orthedomesticfridgeback-radiator,theyaretermed'radiators'(inspiteofconvectionbeingdominant),andnotheatexchangers.Whenafanisusedtoforcetheflowofambientair(orwhennaturalorartificialwindapplies,likeforcarradiator)thenameheatexchangerisoftenreservedforthe
8、casewheretheambientfluidisducted.Othernamesareusedforspecialcases,like‘condenser’forthecasewhenonefluidflowchangesfromvapourtoliquid,‘vaporiser’(orevaporator,orboiler)whenafluidcha
