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Convection—themeansofheattransferinfluidsresultingfromcurrents.
Counterflow—referstothemovementoftwoflowstreamsinoppositedirections;
alsocalledcountercurrentflow.
Crossflow—referstothemovementoftwoflowstreamsperpendiculartoeachother.
Differentialpressure—thedifferencebetweeninletandoutletpressures;
representedasΔP,ordeltap.
Differentialtemperature—thedifferencebetweeninletandoutlettemperature;
representedasΔT,ordeltat.
Fixedhead—atermappliedtoashell-and-tubeheatexchangerthathasthetubesheetfirmlyattachedtotheshell.
Floatinghead—atermappliedtoatubesheetonaheatexchangerthatisnotfirmlyattachedtotheshellonthereturnheadandisdesignedtoexpand(float)insidetheshellastemperaturerises.
Fouling—buildupontheinternalsurfacesofdevicessuchascoolingtowersandheatexchangers,resultinginreducedheattransferandplugging.
Kettlereboiler—ashell-and-tubeheatexchangerwithavapordisengagingcavity,usedtosupplyheatforseparationoflighterandheaviercomponentsinadistillationsystemandtomaintainheatbalance.
Laminarflow—streamlineflowthatismoreorlessunbroken;
layersofliquidflowinginaparallelpath.
Multipassheatexchanger—atypeofshell-and-tubeheatexchangerthatchannelsthetubesideflowacrossthetubebundle(heatingsource)morethanonce.
Parallelflow—referstothemovementoftwoflowstreamsinthesamedirection;
forexample,tube-sideflowandshell-sideflowinaheatexchanger;
alsocalledconcurrent.
Radiantheattransfer—conveyanceofheatbyelectromagneticwavesfromasourcetoreceivers.
Reboiler—aheatexchangerusedtoaddheattoaliquidthatwasonceboilinguntiltheliquidboilsagain.
Sensibleheat—heatthatcanbemeasuredorsensedbyachangeintemperature.
Shell-and-tubeheatexchanger—aheatexchangerthathasacylindricalshellsurroundingatubebundle.
Shellside—referstoflowaroundtheoutsideofthetubesofashell-and-tubeheatexchanger.SeealsoTubeside.
Thermosyphonreboiler—atypeofheatexchangerthatgeneratesnaturalcirculationasastaticliquidisheatedtoitsboilingpoint.
Tubesheet—aflatplatetowhichtheendsofthetubesinaheatexchangerarefixedbyrolling,welding,orboth.
Tubeside—referstoflowthroughthetubesofashell-and-tubeheatexchanger;
seeShellside.
Turbulentflow—randommovementormixinginswirlsandeddiesofafluid.
TypesofHeatExchangers
换热器的类型
Heattransferisanimportantfunctionofmanyindustrialprocesses.Heatexchangersarewidelyusedtotransferheatfromoneprocesstoanother.Aheatexchangerallowsahotfluidtotransferheatenergytoacoolerfluidthroughconductionandconvection.Aheatexchangerprovidesheatingorcoolingtoaprocess.Awidearrayofheatexchangershasbeendesignedandmanufacturedforuseinthechemicalprocessingindustry.
Inpipecoilexchangers,pipecoilsaresubmergedinwaterorsprayedwithwatertotransferheat.Thistypeofoperationhasalowheattransfercoefficientandrequiresalotofspace.Itisbestsuitedforcondensingvaporswithlowheatloads.
Thedouble-pipeheatexchangerincorporatesatube-within-a-tubedesign.Itcanbefoundwithplainorexternallyfinnedtubes.Double-pipeheatexchangersaretypicallyusedinseries-flowoperationsinhigh-pressureapplicationsupto500psigshellsideand5,000psigtubeside.
Ashell-and-tubeheatexchangerhasacylindricalshellthatsurroundsatubebundle.Fluidflowthroughtheexchangerisreferredtoastubesidefloworshell-sideflow.Aseriesofbafflessupportthetubes,directfluidflow,increasevelocity,decreasetubevibration,protecttubing,andcreatepressuredrops.Shell-and-tubeheatexchangerscanbeclassifiedasfixedhead,singlepass;
fixedhead,multipass;
floatinghead,multipass;
orU-tube.Onafixedheadheatexchanger(Figure7.1),tubesheetsareattachedtotheshell.Fixedheadheatexchangersaredesignedtohandletemperaturedifferentialsupto200°
F(93.33°
C).Thermalexpansionpreventsafixedheadheatexchangerfromexceedingthisdifferentialtemperature.Itisbestsuitedforcondenserorheateroperations.Floatingheadheatexchangersaredesignedforhightemperaturedifferentiaisabove200°
C).Duringoperation,onetubesheetisfixedandtheother“floats”insidetheshell.Thefloatingendisnotattachedtotheshellandisfreetoexpand.
Figure7.1FixedHeadHeatExchanger
Reboilersareheatexchangersthatareusedtoaddheattoaliquidthatwasonceboilinguntiltheliquidboilsagain.Typescommonlyusedinindustryarekettlereboilersandthermosyphonreboilers.
Plate-and-frameheatexchangersarecomposedofthin,alternatingmetalplatesthataredesignedforhotandcoldservice.Eachplatehasanoutergasketthatsealseachcompartment.Plate-and-frameheatexchangershaveacoldandhotfluidinletandoutlet.Coldandhotfluidheadersareformedinsidetheplatepack,allowingaccessfromeveryotherplateonthehotandcoldsides.Thisdeviceisbestsuitedforviscousorcorrosivefluidslurries.Itprovidesexcellenthighheattransfer.Plate-and-frameheatexchangersarecompactandeasytoclean.Operatinglimitsof350to500°
F(176.66°
Cto260°
C)aredesignedtoprotecttheinternalgasket.Becauseofthedesignspecification,plate-and-frameheatexchangersarenotsuitedforboilingandcondensing.Mostindustrialprocessesusethisdesigninliquid-liquidservice.
Air-cooledheatexchangersdonotrequiretheuseofashellinoperation.Processtubesareconnectedtoaninletandareturnheaderbox.Thetubescanbefinnedorplain.Afanisusedtopushorpulloutsideairovertheexposedtubes.Air-cooledheatexchangersareprimarilyusedincondensingoperationswhereahighlevelofheattransferisrequired.
Spiralheatexchangersarecharacterizedbyacompactconcentricdesignthatgenerateshighfluidturbulenceintheprocessmedium.Asdootherexchangers,thespiralheatexchangerhascold-mediuminletandoutletandahot-mediuminletandoutlet.Internalsurfaceareaprovidestheconductivetransferelement.Spiralheatexchangershavetwointernalchambers.
TheTubularExchangerManufacturersAssociation(TEMA)classifiesheatexchangersbyavarietyofdesignspecificationsincludingAmericanSocietyofMechanicalEngineers(ASME)constructioncode,tolerances,andmechanicaldesign:
●ClassB,Designedforgeneral-purposeoperation(economyandcompactdesign)
●ClassC.Designedformoderateserviceandgeneral-purposeoperation(economyandcompactdesign)
●ClassR.Designedforsevereconditions(safetyanddurability)
HeatTransferandFluidFlow
Themethodsofheattransferareconduction,convection,andradiantheattransfer(Figure7.2).Inthepetrochemical,refinery,andlaboratoryenvironments,thesemethodsneedtobeunderstoodwell.Acombinationofconductionandconvectionheattransferprocessescanbefoundinallheatexchangers.Thebestconditionsforheattransferarelargetemperaturedifferencesbetweentheproductsbeingheatedandcooled(thehigherthetemperaturedifference,thegreatertheheattransfer),highheatingorcoolantflowrates,andalargecross-sectionalareaoftheexchanger.
Conduction
Heatenergyistransferredthroughsolidobjectssuchastubes,heads,baffles,plates,fins,andshell,byconduction.Thisprocessoccurswhenthemoleculesthatmakeupthesolidmatrixbegintoabsorbheatenergyfromahottersource.Sincethemoleculesareinafixedmatrixandcannotmove,theybegintovibrateand,insodoing,transfertheenergyfromthehotsidetothecoolerside.
Convection
Convectionoccursinfluidswhenwarmermoleculesmovetowardcoolermolecules.Themovementofthemoleculessetsupcurrentsinthefluidthatredistributeheatenergy.Thisprocesswillcontinueuntiltheenergyisdistributedequally.Inaheatexchanger,thisprocessoccursinthemovingfluidmediaastheypassbyeachotherintheexchanger.Bafflearrangementsandflowdirectionwilldeterminehowthisconvectiveprocesswilloccurinthevarioussectionsoftheexchanger.
RadiantHeatTransfer
Thebestexampleofradiantheatisthesun’swarmingoftheearth.Thesun’sheatisconveyedbyelectromagneticwaves.Radiantheattransferisaline-of-sightprocess,sothepositionofthesourceandthatofthereceiverareimportant.Radiantheattransferisnotusedinaheatexchanger.
LaminarandTurbulentFlow
Twomajorclassificationsoffluidflowarelaminarandturbulent(Figure7.3).Laminar—orstreamline—flowmovesthroughasysteminthincylindricallayersofliquidflowinginparallelfashion.Thistypeofflowwillhavelittleifanyturbulence(swirlingoreddying)init.Laminarflowusuallyexistsatlowflowrates.Asflowratesincrease,thelaminarflowpatternchangesintoaturbulentflowpattern.Turbulentflowistherandommovementormixingoffluids.Oncetheturbulentflowisinitiated,molecularactivityspeedsupuntilthefluidisuniformlyturbulent.
Turbulentflowallowsmoleculesoffluidtomixandabsorbheatmorereadily