CONTROLLING SHELL AND TUBE EXCHANGERS.docx
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CONTROLLING SHELL AND TUBE EXCHANGERS.docx
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CONTROLLINGSHELLANDTUBEEXCHANGERS
CONTROLLINGSHELLANDTUBEEXCHANGERS
©WalterDriedger,P.Eng.,2000May20.walter(at)driedger(dot)ca
FirstpublishedinHydrocarbonProcessing,March1998.
This Adobe®fileisavailablefordownload.
INTRODUCTION.Shellandtubeheatexchangersareamongthemoreconfusingpiecesofequipmentfortheprocesscontrolengineer.Theprincipleofoperationissimpleenough:
Twofluidsofdifferenttemperaturesarebroughtintoclosecontactbutarepreventedfrommixingbyaphysicalbarrier.Thetemperatureofthetwofluidswilltendtoequalize.Byarrangingcounter-currentflowitispossibleforthetemperatureattheoutletofeachfluidtoapproachthetemperatureattheinletoftheother.Theheatcontentsaresimplyexchangedfromonefluidtotheotherandviceversa.Noenergyisaddedorremoved.
Sincetheheatdemandsoftheprocessarenotconstant,andtheheatcontentofthetwofluidsisnotconstanteither,theheatexchangermustbedesignedfortheworstcaseandmustbecontrolledtomakeitoperateattheparticularraterequiredbytheprocessateverymomentintime.Theheatexchangeritselfisnotconstant.Itscharacteristicchangeswithtime.Themostcommonchangeisareductionintheheattransferrateduetofoulingofthesurfaces.Exchangersareinitiallyoversizedtoallowforthefoulingwhichgraduallybuildsupduringuseuntiltheexchangerisnolongercapableofperformingitsduty.Onceithasbeencleaneditisagainoversized.
WHEREDOWEMEASURE?
Atthefundamentallevel,thereisonlyonevariablethatcanbecontrolled--theamountofheatbeingexchanged.Inpracticalsituationsitisnotpossibletomeasureheatflux.Itisalwaysthetemperatureofonefluidortheotherwhichisbeingmeasuredandcontrolled.Itisnotpossibletocontrolbothsincetheheataddedfromoneistakenfromtheother.Thereforethefirstconsiderationistospecifytheplaceatwhichthetemperatureistobekeptconstant.Thisisusuallywithinapieceofequipmentsomewheredownstreamoftheoutletofoneofthefluids.Assumingthereisnotmuchtemperaturechangealongthepiping,themeasurementmaybeanywherebetweentheoutletitselfandthepointofinterest,perhapsatthebaseofadistillationtower.Incaseswherethemeasurementisbeingmadedownstreamofabypassvalve,thefurtherdownstream,thebetterthemixingwillbe,andthemorerepresentativethemeasurement.Ontheotherhand,toofardown-streammayresultinprocessdeadtimethatcanmakecontroldifficult.Incaseswherethe"other"fluidistheonebeingmanipulated,itisoftenquitesufficienttomakethemeasurementdirectlydownstreamoftheoutletnozzleoftheexchanger.
WHICHSTREAMDOWEMANIPULATE?
Thesecondconsiderationiswhichstreamtomanipulate.Thecomplicationsarisefromthefactthatexchangershavefourportsandinvolvetwodifferentfluids,eitherofwhichmaychangephase.Theformerfeaturealoneallowseightdifferentvalvearrangements.Figure3-1allowsthereadertofigurethemallout.Thediagramassumesthatitisthefluidontheshellsidewhosetemperatureisbeingcontrolled.Aslikelyasnot,itistheoneonthetubeside.Itdoesn'treallymakeanydifferencetothecontrolstrategy.Therealissueiswhichfluidistobemanipulatedbythevalves.Forthesakeofdiscussionwewilltermthetwostreamsthe"process"sideandthe"heatexchangemedium"side.Acompletetabulationofallthepossibilitiesis:
a-Processside,outletthrottling.
b-Processside,inletthrottling.
c-Processside,bypasswithoutletrestriction.
d-Processside,bypasswithinletrestriction.
e-Mediumside,outletthrottling.
f-Mediumside,inletthrottling.
g-Mediumside,bypasswithoutletrestriction.
h-Mediumside,bypasswithinletrestriction.
Amongthisprofusionofalternatives,somemustbebetterthanothers.Thepreferredchoicedepends,asalways,ontheparticularsituation.
Thereareanumberofvarietiesofthebasicshellandtubeexchangerthatcanbecontrolledalongsimilarlines.Plateexchangersconsistofthinsheetsofcorrugatedmetal.Thecorrugationsareformedtoproducepassagessothatthetwofluidspassinoppositedirectionsonoppositesidesofeachsheet.The"shell"sideandthe"tube"sideareessentiallyinterchangeable.
Aerialcoolers,sometimescalledfinfancoolers,aresimilartoshellandtubeexchangersexceptthattheyarealltube.Theairblowingpastthetubescanbeconsideredtobeinanextremelylargeshell.
THROTTLINGTHEPROCESSFLUID.Itisquitemeaninglesstoattempttocontroltheprocesstemperaturebythrottlingeithertheinletortheoutletoftheprocessfluid.Thedesiredprocessflowrateissetbyotherrequirementsandthesewouldbeinterferedwithbymanipulatingtheprocessflow.Temperaturewillchangesomewhatsinceflowreductionincreasestheresidencetimeofthefluidandtheoutlettemperaturewillmorecloselyapproachtheinlettemperatureofthemedium.
Ontheotherhand,variationsinprocessflow,causedbysomeexternalinfluence,isoneofthemajorcausesoftemperaturevariation.Itisoftenthereasonwhywemustmanipulatesomeotherparametertomaintainconstanttemperature.
BYPASSINGTHEPROCESSFLUID.Processtemperaturecanbecontrolledbymanipulatingprocessflowifabypassisinstalled.Astheoutlettemperaturerises(assumethisisaheater),morefluidisbypassedaroundtheex-changerwithoutbeingheated.Asthetwostreamsareblendedtogetheragain,thecorrecttemperatureisachieved.
SPLITRANGE.Bypassmanipulationsoundssimplebutthereareafewtrickstoit.Firstly,therearetwowaysofarrangingthevalvecontrols:
Wecanattempttominimizepressuredropatalltimes,orwecanattempttokeepthepressuredropconstant.Inneithercasedowewanttointerruptthetotalflow.Ifwewishtominimizepressuredrop,abutterflyvalveisthelikeliestchoice.However,evenawideopenbutterflyhassomepressuredrop.Itmaybegreaterthanthatoftheheatexchangeritself.Thismeansthatevenwhenthevalveiswideopenonlyhalftheflow,orless,willbypasstheexchanger.Toaccomplishagreaterdegreeofbypass,arestrictionmustbeplacedontheflowthroughtheexchanger.Therestrictionshouldbeadjustablesinceconditionschangeandwedonotwantmorerestrictionthannecessary.Theeasiestwaytodothisiswithahandvalve.Sincethesevalvesareofteninrelativelyinaccessibleplaces,remoteactuatorsmaybeadded.Oncethatisdoneitbecomesanobviousmattertoarrangeautomaticcontrolssothatoncethebypassisfullyopen,therestrictionvalvestartstoclose,andviceversa.
Thisis,ofcourse,asplitrange.Thevalvepositioners,orI/Ps,arecalibratedsothat0→50%signalopenstheoutletand50→100%signalclosesthebypassvalve.Withthisarrangement,atleastoneofthevalvesisfullyopenatalltimesandtheeffectiveCvrangesfrom100%to200%ofthatofasinglevalve.Itshouldbenotedthatbyarrangingforminimumpressuredropwemustacceptthatthepressuredrop,andconsequentlytheflow,willvaryasthevalvepositionschange.NotealsothattheremustbeeitherasingleI/Ptubedtobothvalves,ortwoI/Ps(orpositioners)mustbewiredinseries.Eitherway,specialcaremustbetakenduringconstructionandmaintenance.
TheexampleshowninFigure3-2showstheFailOpenvalvetobetheoneinthebypass.Letusassumethattheprocessstreamisbeingheated.Thefailuremodesofthetwovalvesissuchthatasignalfailuretoeither,orboth,valveswillresultinlessheatbeingdeliveredtotheprocessstream.Failurealsomeansthatthemediumwillnotbecooled.Theoppositefailureresponseiseasilyarranged.Itisamatterofchoice.Oncethechoicehasbeenmade,thecontrolactionofthecontrollerbecomesamatterofdeduction:
a)Assumethattheprocessstreamoutletistoohot.Thatis,itisabovethesetpoint.
b)Thenthedeviationofthecontrollerispositive.
c)Assumethecontrolleractionispositive.Thisproducesarisingvalveoutputsignalthatwilltendtoopentheoutletandclosethebypass.
d)Thatwouldraisethetemperatureoftheprocesssteam.Wrong!
Thesituationisgettingworse.
e)Thiscontrollermustbeconfiguredtobereverseacting.
f)Nowarisingoutlettemperaturewillcauseafallingvalvesignal.
g)Thatwillopenthebypassandclosetheoutlet.
h)Thiswilllowerthetemperatureoftheprocessstreamthusbringingthemeasurementbacktothesetpoint.
Thesimplestwaytocarryoutsuchacontrolactionanalysisistotracearoundtheloopfromthemeasurementtothecontroller,tothevalve,andbacktothemeasurement.Assumethecontrolactionispositive.Ifthemeasurementisbroughtbacktothesetpoint,everythingisOK.Ifthingsgetworse,reversethecontrolleraction.
Moderndistributedcontrolsystems(DCS)haveabuilt-inoptionforreversingthesignaltothevalve.Ifthisisused,itisnotnecessarytotakethefailuremodeofthevalveintoaccountwhendoingacontrolactionanalysis.Firstthinkpositivethenselectareverseoutput(notcontrolaction)ifthevalveisFailOpen.
Anastuteobserverwillrealizethatthereisapossibilityforbothvalvestobeclosed.Iftheoutletvalvefailsclosed,thecontrollerwillsensealowtemperature.Itsoutputwillthenriseandclosethebypass.Itisveryunlikelyforthevalveitselftofailwhenthesignalisstillgood.Nevertheless,ifthisremotepossibilityisunacceptable,bothvalvesmustbemadeFailOpen.Howcanthisbedone?
Itcan't!
Aslongasbothvalvesaredrivenbythesamesignal,theirfailuremodesmustbeoppositebecausetheireffectonthepro
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