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75TWISTEDTUBEHEATEXCHANGERTECHNOLOGYR.DonaldMorganBrownFintubeCompany12602FM529RoadHouston,Texas77240-0082USAPhone:1(713)849-8197Fax:[phone]e-mail:[e-mail]85%ofallnewheatexchangerapplicationsinoilrefining,chemical,petro-chemical,andpowergenerationareaccommodatedthroughtheuseofconventionalshellandtubetypeheatexchangers.Thefundamentalbasisforthisstatisticisshellandtubetechnologyisacosteffective,provensolutionforawidevarietyofheattransferrequirements.However,therearelimitationsassociatedwiththetechnologywhichincludeinefficientusageofshellsidepressuredrop,deadorlowflowzonesaroundthebaffleswherefoulingandcorrosioncanoccur,andflowinducedtubevibration,whichcanultimatelyresultinequipmentfailure.Thispaperpresentsarecentinnovationanddevelopmentofanewtechnology,knownasTwistedTubetechnology,whichhasbeenabletoovercomethelimitationsoftheconventionaltechnology,andinaddition,providesuperioroverallheattransfercoefficientsthroughtubesideenhancement.Thispapercomparestheconstruction,performance,andeconomicsofTwistedTubeexchangersagainstconventionaldesignsforvariousmaterialsofconstructionincludingreactivemetals.KEYWORDSheatexchanger,twistedtubetechnology,heattransfer,corrosionresistanceCONVENTIONALSHELLANDTUBEDESIGNConventionalTEMA(TubularExchangerManufacturersAssociation)typeshellandtubetypeheatexchangersconsistofanumberofroundtubesattachedtoatubesheetinsideacylindricalvessel,withtubesizes,tubelengths,andshelldiametersvaryingdependingontherequirementsoftheapplication.Heattransfersurfaceareascanvaryfromafewsquarefeettoover25,000squarefeet.Thetubebundlenormallycontainsanumberofbafflestoaccomplishthedualobjectivesofprovidingasupportstructureforthetubes,andtodirecttheshell-sideflowacrossthetubesratherthanalongthetubes(Fig1).Theresultingbackandforthshell-sideflowwillyieldahigherthanexpectedpressuredropperunitofheattransferbecauseenergyisusedtoreversetheflowratherthantoenhanceheattransfer.Also,theenergyconsumedinreversingtheflowwilltendtoforcetheshell-sidefluidthroughbaffle-to-tubeandbaffle-to-shellclearancesyieldinglowercrossflowandlowerheattransfercoefficients.Finally,fluidflowaroundthebafflesisnon-uniformresultinginareasoflowflowanddeadspots,whicharepronetofoulingaccumulation,corrosion,andpoorheattransfer.Thethermaleffectiveness(
),ofashellandtubeexchangerisnormallycalculatedassumingperfectradialandnoaxialmixingoftheshellsidestream.Inpracticehowever,thereisconsiderableaxialmixingwithinabafflecompartment,andfurther,thestreamisincross-flowforpartofthetimeratherthanaxialflow.Theseeffectsarefurthercomplicatedbyleakageofflowthatoccursatthebaffle-to-tube76andbaffle-to-shelljointsthatdoesnottakefullpartintheheattransferinthebundle.Theoveralleffectoftheselimitationsistheactualthermaleffectiveness(
)willbelowerthanthetheoreticalvalue,anditwillbelowerthanthevaluesobtainedforothertypesofheatexchangersthatdonotsufferfromtheselimitations.Typically,thermaleffectivenessofaconventionalshellandtubetypeexchangerwillbeintherangeof60%to80%TheTwistedTubeHeatExchangerTheTwistedTubeheatexchangeroriginatedinEasternEuropeandbecamecommerciallyavailableinScandinaviainthemid1980’s.ItwasdevelopedprimarilytoFigure1.ConventionalHeatExchangerovercomethelimitationsinherentwithconventionalshellandtubetechnology.ApplicationsofTwistedTubetechnologywereprimarilyinsinglephaseandcondensingdutiesinpulpandpaperanddistrictheatingwithlimitedexposureintheprocessindustries.In1991,Kochlicensedthetechnologyandin1995subsequentlyacquiredthetechnologyoutright.ConstructionTheTwistedTubeexchangerconsistsofabundleofuniquelyformedtubesassembledinabundlewithouttheuseofbaffles(Fig2).ThetubeshavebeensubjectedtoauniqueformingprocesswhichresultsinanovalcrosssectionwithasuperimposedhelixFigure2.TwistedTubeHeatExchangerBundle77providingahelicaltube-sideflowpath(Fig3).Theformingprocessensuresthattubewallthicknessremainsconstantandthematerialyieldpointisnotexceededtherebyretainingmechanicalintegrity.Thetubeendsareroundtoallowconventionaltubetotubesheetjoints.Figure3.Tube-sideFlowPathAwiderangeoftubematerialscanbeusedincludingcarbonandstainlesssteels,Cr-Moalloys,duplexandsuperduplexalloysaswellastitanium,zirconiumandtantalum.Tubesizesmayvaryfrom½inchto1inch.Tubesareassembledintoabundleonatriangularpitchonerowatatimewitheachtubebeingturnedtoalignthetwistsateveryplanealongthebundlelength.Thisalignmentresultsintubescontactingadjacenttubesatmanypointsalongthelengthofthetubeinthebundle(Fig4).Thecompletedbundleisthentightlystrappedcircumferentiallytoensurenotubemovementandarobustbundleistheendresult.Bundlescanbeconstructedwithmorethan5000tubesandupto6feetindiameterwithtubelengthsupto80feet(Fig5).Figure4.TubeAlignmentandSupport78Figure5.CompletedTwistedTubeBundleTheshell-sideflowpathiscomplexandpredominantlyaxialinnature(Fig6).Typically,theshellsideflowareaisapproximatelyequaltothetubesideflowarea.Thebundleisoftenshroudedtoensureshellsideflowremainsinthebundleandminimizesbypassing.Pathsareavailabletoallowthefluidtoflowintoandoutofthebundleateachend.Whenhighinletandoutl