HeatRecoverySystems

APIRECOMMENDEDPRACTICE669FIRSTEDITION,JUNE2022

American

PetroleumInstitute

i

SpecialNotes

APlpublicationsnecessarilyaddressproblemsofageneralnature.Withrespecttoparticularcircumstances,local,state,andfederallawsandregulationsshouldbereviewed.

NeitherAPInoranyofAPI'semployees,subcontractors,consultants,committees,orotherassigneesmakesanywarrantyorrepresentation,eitherexpressorimplied,withrespecttotheaccuracy,completeness,orusefulnessoftheinformationcontainedherein,orassumeanyliabilityorresponsibityforanyuse,ortheresultsofsuchuse,ofanyinformationorprocessdisclosedinthispublication.NeitherAPInoranyofAPI'semployees,subcontractors,consultants,orotherassigneesrepresentthatuseofthispublicationwouldnotinfringeuponprivatelyownedrights.

APlpublicationsmaybeusedbyanyonedesiringtodoso.EveryefforthasbeenmadebytheInstitutetoassuretheaccuracyandreliabilityofthedatacontainedinthem;however,theInstitutemakesnorepresentation,warranty,orguaranteeinconnectionwiththispublicationandherebyexpresslydisclaimsanyliabilityorresponsibilityforlossordamageresultingfromitsuseorfortheviolationofanyauthoritieshavingjurisdictionwithwhichthispublicationmayconflict.

APlpublicationsarepublishedtofacilitatethebroadavailabilityofproven,soundengineeringandoperatingpractices.Thesepublicationsarenotintendedtoobviatetheneedforapplyingsoundengineeringjudgmentregardingwhenandwherethesepublicationsshouldbeused.TheformulationandpublicationofAPlpublicationsisnotintendedinanywaytoinhibitanyonefromusinganyotherpractices.

AnymanufacturermarkingequipmentormaterialsinconformancewiththemarkingrequirementsofanAPIstandardissolelyresponsibleforcomplyingwithalltheapplicablerequirementsofthatstandard.APIdoesnotrepresent,warrant,orguaranteethatsuchproductsdoinfactconformtotheapplicableAPIstandard.

Usersofthisrecommendedpracticeshouldnotrelyexclusivelyontheinformationcontainedinthisdocument.Soundbusiness,scientific,engineering,andsafetyjudgmentshouldbeusedinemployingtheinformationcontainedherein.

Allrightsreserved.Nopartofthisworkmaybereproduced,translated,storedinaretrievalsystem,ortransmittedbyanymeans,electronic,mechanical,photocopying,recording,orotherwise,withoutpriorwrittenpermissionfromthepublisher.ContactthePublisher,APIPublishingServices,200MassachusettsAvenue,NW,Suite1100,Washington,DC20001-5571.

Copyright◎2022AmericanPetroleumInstitute

iii

Foreword

NothingcontainedinanyAPlpublicationistobeconstruedasgrantinganyright,byimplicationorotherwise,forthemanufacture,sale,oruseofanymethod,apparatus,orproductcoveredbyletterspatent.Neithershouldanythingcontainedinthepublicationbeconstruedasinsuringanyoneagainstliabilityforinfringementofletterspatent.

Theverbalformsusedtoexpresstheprovisionsinthisdocumentareasfollows.

Shall:Asusedinastandard,"shall"denotesaminimumrequirementtoconformtothestandard.

Should:Asusedinastandard,"should"denotesarecommendationorthatwhichisadvisedbutnotrequiredtoconformtothestandard.

ThisdocumentwasproducedunderAPlstandardizationproceduresthatensureappropriatenotificationandparticipationinthedevelopmentalprocessandisdesignatedasanAPIstandard.QuestionsconcerningtheinterpretationofthecontentofthispublicationorcommentsandquestionsconcerningtheproceduresunderwhichthispublicationwasdevelopedshouldbedirectedinwritingtotheDirectorofStandards,AmericanPetroleumInstitute,200MassachusettsAvenue,NW,Suite1100,Washington,DC20001.Requestsforpermissiontoreproduceortranslatealloranypartofthematerialpublishedhereinshouldalsobeaddressedtothedirector.

Generally,APIstandardsarereviewedandrevised,reaffirmed,orwithdrawnatleasteveryfiveyears.Aone-timeextensionofuptotwoyearsmaybeaddedtothisreviewcycle.StatusofthepublicationcanbeascertainedfromtheAPIStandardsDepartment,telephone(202)682-8000.AcatalogofAPlpublicationsandmaterialsispublishedannuallybyAPI,200MassachusettsAvenue,NW,Suite1100,Washington,DC20001.

SuggestedrevisionsareinvitedandshouldbesubmittedtotheStandardsDepartment,API,200MassachusettsAvenue,NW,Suite1100,Washington,DC20001,standards@.

V

Contents

Page

1Scope 1

2NormativeReferences 1

3Terms,Definitions,Acronyms,andAbbreviations 1

3.1Definitions 1

3.2AcronymsandAbbreviations 3

4GeneralInformation 3

4.1UtilitySystems 3

4.2MechanicalDescriptions 3

4.3SpecificHeatRecoveryApplications 4

AnnexA(informative)SteamSystems 5

AnnexB(informative)HotOilandOtherHeatTransferFluids 12

AnnexC(informative)HotWater/GlycolSystems 17

AnnexD(informative)WatertubeWHRUConfigurations 20

AnnexE(informative)FiretubeWHRUConfigurations 23

AnnexF(informative)FCCU/RCCUProcessUnitsHeatRecoverySystems 48

AnnexG(informative)ProcessHeaterConvectionSections 50

AnnexH(informative)GasTurbineExhaustHeatRecoverySystem 51

AnnexI(informative)ThermalOxidizersHeatRecoverySystems 52

AnnexJ(informative)Process/FlueGasHeatRecoverySystems 56

AnnexK(informative)ProcessLiquidHeatRecoverySystems 60

AnnexL(informative)SulfurRecoveryUnitHeatRecoverySystems 61

AnnexM(informative)HeatFluxandCirculationRatio 68

Bibliography 73

Figures

A.1TypicalSteamSystemComponentsasPartofaWatertubeSteamGenerator 7

B.1TypicalMoltenSaltSystem 13

B.2TypicalHeatTransferFluidSystemwithWasteHeatRecoveryUnit 14

D.1TypicalNaturalCirculationWatertube 21

D.2TypicalForcedCirculationSystem 2

Contents

vi

Page

E.1TypicalHorizontalFiretubewithExternalDrumHRSG 26

E.2TypicalVerticalFiretubewithExternalDrumHRSG 26

E.3TypicalFiretubeKettleTypeHRSG 27

E.4TypicalInsulatedMetalFerrule 28

E.5TypicalStraightCeramicFerrule 29

E.6TypicalHeadedCeramicFerrule 30

E.7TubeArrangementswithStraight,Hex-head,andSquare-headFerrules 31

E.8TypicalDamagewhenFerruleFitisTooTight 32

E.9ConventionalStrengthWelds 33

E.10FulDepthStrengthWeld 33

E.11BackFace(ShellSide)Weld 34

E.12TypicalChanneltoTubesheet-shellInterconnections 36

E.13TypicalKnucklesforFullDiameterKettles 37

E.14TypicalDual-compartmentFiretubeHRSG 39

E.15TypicalTwo-pass,SeparateShells,FiretubeHRSG 40

E.16TypicalTwoTubePassFiretubeHRSG 42

E.17TypicalInternalBypassSystemwithValveandDamper 43

M.1ExampleofCirculationRatioVariabilitywithSteamRate 71

Tables

C.1FreezingPointsofEthyleneGlycolBasedWaterSolutions 19

C.2FreezingPointsofPropyleneGlycolBasedWaterSolutions 19

M.1HRSGFiretubeandWatertubeLocalHeatFlux 69

vii

Introduction

Usersofthisrecommendedpractice(RP)needtobeawarethatfurtherordiferingrequirementscanbeneededforindividualapplications.ThisRPisnotintendedtoinhibitavendorfromoffering,orthepurchaserfromaccepting,alternativeequipmentorengineeringsolutionsfortheindividualapplication.Thiscanbeparticularlyapplicablewherethereisinnovativeordevelopingtechnology.Whereanalternativeisoffered,thevendorshouldidentifyanyvariationsfromthisRPandprovidedetails.

ThisRPrequiresthepurchasertospecifycertaindetailsandfeatures.

InthisRP,SystemInternational(SI)unitsareusedandwherepractical,U.S.customary(USC)unitsareincludedinparenthesesforinformation.

1

HeatRecoverySystems

1Scope

1.1Thisrecommendedpractice(RP)providesguidelinesforheattransferequipmentusedinwasteheatrecoverysystemsinthepetroleum,petrochemical,andnaturalgasindustries.Detailsofrelatedequipmentdesignsareincludedonlywherethesearenecessarytoensureproperdesignandoperationandsafeinteractionwiththeheatrecoverysystem.Itindicatesareasthatneedattentionandoffersinformationanddescriptionsofvarioustypesofheattransferequipmentavailabletoaidintheselectionoftheappropriateheatrecoverysystem.

1.2ThisRPdoesnotincludedetailedrequirementsfor:—processtoprocessheatrecoverysystems,

—gasturbineexhaustheatrecovery

—COboilers,or

—convectionsectionsusedwithinfiredheaters.

1.3ThewasteheatrecoverysystemsincludedinthisRParetypicalofthosecurrentlyinuseandshouldnotlimittheuseofalternatesystems.

1.4Instrumentationandcontrolsforwasteheatrecoverysystemsaresometimesmentionedbutisnotmeanttocompletelydefineallrequirements.Otherstandardsanddocuments,suchasAPIRecommendedPractice556,Instrumentation,Control,andProtectiveSystemsforFiredHeaters,shouldbeusedtofullydefinetheneededinstrumentsandcontrols.

2NormativeReferences

Therearenonormativereferencesinthisdocument.

3Terms,Definitions,Acronyms,andAbbreviations

3.1Definitions

Forthepurposesofthisdocument,thefollowingtermsanddefinitionsapply.

3.1.1

approachtemperature

Thedifferencebetweenthesaturationtemperatureofthesteamattheselectedpressureandthetemperatureofthewaterleavingtheeconomizer.

3.1.2

desuperheater

attemperator

AdevicelocatedinternalorexternaltotheHRSGthatcontrolstheexittemperatureofthesteamfromthesuperheater.Thedevicetypicallyinjectswaterwithverylowsolidscontentintothesteamtocontrolthesteamtemperaturebyreducingit.Alsocalledanattemperator.

3.1.3

downcomer

Aheatedorunheatedpipecarryingwaterfromthesteamdrumtoanevaporator/generatorsectionofanHRSG.

APLRECOMMENDEDPRAGTICE669

2

3.1.4

evaporator

generator

TheportionoftheHRSGinwhichwaterisboilingtoformsteam.Typically,amixtureofwaterandsteamexistsattheexitofthisportion.Insomeconfigurations,thesteamandwaterareseparatedpriortoleavingthisportionoftheHRSG.Alsoreferredtoasasteamgeneratorsection.

3.1.5

economizer

TheportionoftheHRSGwhereincomingfeedwatertemperatureisraisedtolessthansaturationtemperaturebyrecoveryoftheheatfromtheheatingmedium.

3.1.6

ferrule

Ahightemperatureresistantmetallicorceramicshapedtubeinsertprovidedatthetubeandtubesheetareatoinsulatecomponentsfromhightemperaturesandtolimitheattransfer.

3.1.7

firetubeHRSG

Ashell-and-tubeheatexchangerinwhichsteamisgeneratedontheshellsidebyheattransferredfromhotfluidflowingthroughthetubes.

3.1.8

heatrecoverysteamgenerator(HRSG)

Asysteminwhichsteamisgeneratedandmaybesuperheatedorwaterheatedbythetransferfromheatmediumusuallygaseousproductsofcombustionorotherhotprocessfluids.

3.1.9

heatrecoverysystem

Asystemthattransfersenergyfromonemediumtoasecondmediumforreuse.

3.1.10

pinchtemperature

Thediferencebetweentheheatingmediumtemperatureleavingthesteamgeneratorsectionandthesteam'ssaturationtemperatureattheselectedpressure.

3.1.11

processfluid

Theheatingmediumusedtosupplytheheattotheutilityfluid.

3.1.12

riser

Aheatedorunheatedpipecarryingwaterandsteamfromanevaporator/generatorsectionofanHRSGtothesteamdrum.

3.1.13

shell-and-tubewatertubeHRSG

Ashell-and-tubeheatexchangerinwhichsteamisgeneratedinthetubesbyheattransferredfromahotfluidontheshellside.

3.1.14

steamdrum

Apressurevesselwhoseprimarypurposeistoseparatewaterandsteam.

3.1.15

superheater

TheportionoftheHRSGinwhichsaturatedsteamisheatedtohighertemperatures.

HEATREcOVERYSySTEMS3

3.1.16

wasteheatrecoveryunit(WHRU)

Anon-firedheatexchangerinwhichwasteheatistransferredtoacoldfluidwhichisnotwaterbeinggeneratedtosteam.

3.1.17

watertube

Amultipletubecircuitheatexchangerinwhichtheutilityfluidflowsinsidethetubesandtheheatingmediumflowsoverthetubes.Mostcommonlythetubecontainswaterinawater/steamsystem.

3.1.18

watertubelow-pressurecasingHRSG

Amultipletubecircuitheatexchangerwithinagas-containingcasinginwhichsteamisgeneratedinsidethetubesbyheattransferredfromahotgasflowingoverthetubes.

3.2AcronymsandAbbreviations

Forthepurposesofthisdocument,thefollowingacronymsandabbreviationsapply.

BFWboilerfeedwater

CRcirculationratio

FACflowacceleratedcorrosion

FCCUfluidcatalyticcrackingunit

HRSGheatrecoverysteamgenerator

HTFheattransferfluid

MAWPmaximumallowableworkingpressure

NPSHnetpositivesuctionhead

RCCUresidualcatalyticcrackingunit

SRUsulfurrecoveryunit

TOthermaloxidizer

TRSGthermalreactorsteamgenerator

WHBwasteheatboiler

WHRUwasteheatrecoveryunit

4GeneralInformation

4.1UtilitySystems

4.1.1SeeAnnexAforinformationon"SteamSystems".

4.1.2SeeAnnexBforinformationon"HotOilandOtherHeatTransferFluidSystems".

4.1.3SeeAnnexCforinformationon"HotWater/GlycolSystems".

4.2MechanicalDescriptions

4.2.1SeeAnnexDforinformationon"WatertubeWHRUConfigurations".

4.2.2SeeAnnexEforinformationon"FiretubeWHRUConfigurations".

4APLRECOMMENDEDPRAGTICE669

4.3SpecificHeatRecoveryApplications

4.3.1SeeAnnexFforinformationon"FCCU/RCCUProcessUnitHeatRecoverySystems".

4.3.2SeeAnnexGforinformationon"ProcessHeaterConvectionSections".

4.3.3SeeAnnexHforinformationon"GasTurbineExhaustHeatRecoverySystems".

4.3.4SeeAnnexIforinformationon"ThermalOxidizersHeatRecoverySystems".

4.3.5SeeAnnexJforinformationon"Process/FlueGasHeatRecoverySystems".

4.3.6SeeAnnexKforinformationon"ProcessLiquidHeatRecoverySystems"

4.3.7SeeAnnexLforinformationon"SulfurRecoveryUnitHeatRecoverySystems".

4.3.8SeeAnnexMforinformationon"HeatFluxandCirculationRatio".

5

AnnexA

(informative)

SteamSystems

A.1General

A.1.1Steamsystemsareusedforrecoveringwasteenergyfromaprocessorfluegasstreamandreturningthatenergybyvaporizingwatertosteamforlow,medium,orhigh-pressuresteamsystemswithintheplant.TherearealsoopportunitiesforpreheatingBFWaswellassuperheatingsteamiftheconfigurationbenefits.

A.1.2Steamsystemsmaybeclosed,inwhichthecondensateleavingthesteamconsumersisrecoveredandreturnedtoboiler/steamgenerator,ortheymaybeoncethroughsystems,inwhichthesteamproducedisoftenusedindirectcontactwiththeprocessmediumandisnon-recoverable.

A.1.3Therearemanyapplicationsforsteamgeneratingsystemsincludingsulfurrecoveryunits(sulfurcondensersandthermalreactorsteamgenerators),gasturbineexhaustsystems,incineratorwasteheatexchangers,processsteamgenerators.

A.2BoilerFeedWater/Condensate

A.2.1Waterconservationandqualitycontrolareimportantforclosedsystemsandtheboilerfeedwater(BFWycondensateisideallycleanwaterwithminimalcontaminant.Typically,de-aerationandchemicaldosingareappliedtothemakeupwater.Blowdownfromtheboilerdrum(s)isusedtomaintainthequalityoftheboilerfeedwaterandpreventthebuildupofimpurities.Therelativelyhighcostassociatedwiththesupplyandtreatmentofmakeupwatermeansthatitisimportanttominimizelossesfromthesystemandrecirculatecondensatewhereverpossible.

A.2.2ThesteamexitingtheHRSGshouldbeofspecifiedpurity(contaminants)andquality(watercontent).Failuretocontrolthesteampurityislikelytoacceleratedamagetodownstreamequipment.Forsteamthatisusedindirectcontactapplicationsandisnotrecovered,thewaterqualityanddrynessofthesteammaybelessstringentbutmuststillbeadequatetopreventscalinganddamagetotheboiler/steamgeneratoranddownstreamequipment.Contaminantscanbecontrolledaswithanysteamgeneratingorboilersystem.RefertoAPIRecommendedPractice538foradditionaldetailonfeedwaterpreparation,chemicaltreatment,andsteampurity.

A.2.3Inmostclosedsystemapplications,itispreferableforthequalityofthesteam(thedryness)tobehigh.Waterdropletsentrainedwithinthesteamcanhaveadetrimentalimpactonthedownstreamequipment,possiblycausingfoulingorscaling.Waterdropletscarriedalongathighspeedinsteampipingsystemscancauseerosionofpipes,fitingsandvalvesandcancauseconsiderabledamageifallowedtoimpactsteamturbineblades.Ifthereisalargeamountofwatercarriedoverintothesteammains/headersandinsufficientmeansofremovingit,poolingcouldoccurwithintheselineswhichmayresultinwaterhammerorslugging.

A.2.4Toachieveahighsteamqualityatitssource,demisterpadsorothertypesofsteam/waterseparationdevicesmaybeinstalledattheoutletofthesteamgenerator,orthesemaybeusedinternalorexternaltoasteamdrum.Asthesteamflowsthroughthedistributionmains/headers,somecoolingandcondensingwilloccur(unlesssuperheated)andthequalitywillbereduced.Steamseparatorsandsteamtrapsmaybeinstalledwithinthemains/headerstoensurethatthesteamqualityreachingtheconsumersremainshigh.RefertoAPIRecommendedPractice538foradditionaldetailsonsteampurity.

A.2.5Superheatingcanbeusedtoensurethedrynessoftheexportedsteam.Ahighsteamqualityintothesuperheatersectionisdesirabletoavoidfoulingofthesuperheaterheattransfersurface.Reducingthesteampressurecanbeanothermeansofincreasingthedrynessofthesteamdependinguponwherethesteamsystem

6APIRECOMMENDEDPRACTICE669

operatingconditionsareontheMollierdiagram;e.g.reducingthepressureofhigh-pressuresaturatedsteamtoaround4000kPa(a)(580psia)orhigherwillreducethesteamquality.Inoncethroughapplicationslowqualitysteammaybeacceptableandthemeasuresdescribedabovemaybeunnecessarybutthepotentialissuesoferosionandcondensatebuildupinthedistributionlinesmuststillbeaddressed.

A.3SteamSystemComponents

A.3.1TypicalSteamSystemComponents

FigureA.1showsasketchoftypicalsteamsystemcomponents.Notallcomponentsareusedineverydesignnorarealltheneededcomponentsshown.

A.3.2SteamDrum

A.3.2.1Thesteamdrumprovidesseveralfunctionswithinthesteamsystem.Primarilyitprovidesphaseseparationforthesteam/boilerwaterstreamreturningfromtheevaporatorbutalsoservestoprovideadequateboilerwaterholduptoensureconstantflowtotheevaporator.Adrumwilltypicallybeprovidedwithlevelcontrolandsafeguardingtoensurethatthemakeupboilerwaterflowisbalancedwiththesteamexportrate,blowdownrate,andtheliquidlevelwithinthedrumremainsbetweensafelimitsforoperation.Maintainingthevaporspaceabovetheliquidlevelisalsoimportantforachievingtherequiredsteamqualityleavingthesteamdrum.

A.3.2.2Theboilerwaterqualitywithinthedrumshouldberoutinelymonitoredbelowtheliquidlevelthroughsampling.Blowdownlines(continuousand/orintermittent)areemployedtocontrolthebuildupofdissolvedsolids.Therateofcontinuousblowdownorfrequencyofintermittentblowdownwilldependonthequalityofthemakeupboilerwater.

A.3.2.3Inanaturalcirculationsystem,thedrummustalwaysbemountedabovetheevaporator.Inaforcedcirculationsystem,thedrumistypicallyabovetheevaporatorbutcanbebelowtheevaporatorwhenthesystemisproperlydesigned.Inallcasesitmustbeelevatedabovetheboilerwatercirculationpumpstopreventcavitationatthepumpinletbyensuringsufficientnetpositivesuctionhead(NPSH).Designingtoensurethatallheattransfersurfacesarefiled/coveredwithwaterevenifthepumpsstopmakesthesystemintrinsicallysafe.Thismaymeanstoppingtheheatingmediumflowintothesystem.

A.3.3Liquid-vaporSeparationDevices

Thesizingofthevaporspaceabovetheliquidlevelofthesteamdrumwilldeterminetheamountofwaterentrainmentwithintheexportsteam-someentrainmentisinevitable.Wheretheprocessdemandisforverydrysteamitwillbenecessarytoinstallseparationdevicestominimizeliquidentrainment.Thetypeandsizeoftheseparationdevicecanbetailoredtoachieveaspecificsteamqualityforthegivenexportflow.Separationequipmentwillcreatepressuredropandthiscouldbesignificantwhenselectingtheseparationdevice.

HEATRECOVERYSYSTEMS7

Key

1.Boilerfeedwatersupply

2.Steamdrum

3.Superheater

4.Evaporator/steamgenerator5.Economizer

6.Ductwork

7.Water-steamseparator

8.Desuperheater

9.Safetyvalve

10.Sootblowersystem

11Boilerfeedwaterpumps

12.Stack

13.Riser

14.Downcomer

15Boilerwatercirculationpumps16Desuperheatingwater

17.Steamtoconsumers

18.Heatsource

19.Blowdown

FigureA.1-TypicalSteamSystemComponentsasPartofaWatertubeSteamGenerator

A.3.4Downcomers

A.3.4.1Inanaturalcirculationsystem,thedowncomerscarryboilerwateratthesaturationtemperaturefromthesteamdrumtotheevaporator.Theelevationofthesteamdrumprovidesthestaticheadthatdrivestheboilerwaterthroughthesteamgenerationcircuit.Thenumberandsizeofdowncomersmustbedeterminedaccordingtothethermalandhydraulicrequirementsofthesystem.

A.3.4.2Inaforcedcirculationsystem,thedowncomerwillormallybeasinglelinetotheboilerwatercirculationpump(s).ThestaticheadgainedbyelevatingthesteamdrummustexceedtherequiredNPSHofthepumpcombinedwiththefrictionallossesinthedowncomertoensurevaporgenerationdoesnotoccuratthepumpinlet.

A.3.5Risers

Theriseristheportionofthecircuitwhichcarriesthesteam/boilerwaterfromtheevaporatortothesteamdrum.Arisercanbeheated,orunheated,dependinguponitscontactwiththeheatingmedium.

A.3.6Evaporator

A.3.6.1Theheatappliedtotheevaporator/steamgeneratorcausesthesaturatedboilerwatertovaporize.Inanaturalcirculationsystemwaterflowthroughtheevaporatorwillnormallybevertical.Slopedsystemscanalsobeused.Theflowgeneratedbythethermosiphoneffectisafunctionoftheamountoftheheatrecovered/

8APLRECOMMENDEDPRAGTICE669

steamgeneratedintheevaporatorandthefluidhydraulicswithinthecircuit.Thetotalflowratethroughthecircuitrelativetotheflowrateofsteamproducedintheevaporatorisknownasthecirculationratio(CR).ThedesignermustselecttheCRandheatfluxsothatphaseseparationdoesnotoccurwithintheevaporatorwhichcanleadtooverheatingandfailureofthetubes.Inabankofevaporatortubeswithalargereductioninheatingmediumtemperatureacrossthebank,considerationmustbegiventothevariationinheatfluxesandsteamgenerationrateswithinthevariouspartsoftheevaporator.

A.3.6.2Inforcedcirculationsystemstheevaporatortubesaremorecommonlyhorizontal.Thesecircuitsoperatewithhighervelocitiestostabilizetheboilingandcharacteristicallyhavefewerparallelheatingpathsandtypicallyhavemultiplepassesofeachpaththroughtheheatedzone.Wherethereisachangeinelevationbetweeninletandoutletoftheheatingcoilsthenthedirectionofflowshouldbeupwardsthroughthecoilbundle.Sincehydraulicsystemlossesareovercomebytheboilerwatercirculationpumpthedesignerhasgreaterflexibilityintermsofcirculationratiosandmassvelocities.Thedesignershouldselectamassvelocityhighenoughtoensureevenflowdistributiontoeachparallelpass.

A.3.7EconomizerandSuperheater

A.3.7.1EconomizersareusedtomaximizeheatrecoverybypreheatingtheBFWbeforeitentersthesteamdrum.Economizerswillbelocatedatthebackendoftheheatrecoverysystemusuallyatthepointwherethereisnolongersuficientheatintheheatingmediumtogeneratesteam.Whenusingexhaustgasfromacombustionsourceastheheatingmediumconsiderationmustbegiventothedewpointofexhaustgases,particularly