换热器外文翻译 2.docx

换热器外文翻译 2.docx

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换热器外文翻译2

HeatExchangers

KeyTermsBaffles—evenlyspacedpartitionsinashellandtubeheatexchangerthatsupportthetubes,preventvibration,controlfluidvelocityanddirection,increaseturbulentflow,andreducehotspots.

Channelhead—adevicemountedontheinletsideofashell-and-tubeheatexchangerthatisusedtochanneltube-sideflowinamultipassheatexchanger.

Condenser—ashell-and-tubeheatexchangerusedtocoolandcondensehotvapors.

Conduction—themeansofheattransferthroughasolid,nonporousmaterialresultingfrommolecularvibration.Conductioncanalsooccurbetweencloselypackedmolecules.

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°F（93.33°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