锅炉水循环和水处理外文翻译.docx

锅炉水循环和水处理外文翻译.docx

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锅炉水循环和水处理外文翻译

专业外文资料翻译

一.英文资料

BoilerCirculation＆WaterTreatment

Anadequateflowofwaterandwater-steammixtureisnecessaryforsteamgenerationandcontroloftubemetaltemperaturesinallthecircuitsofasteam-generatingunit.Atsupercriticalpressures,thisflowisproducedmechanicallybymeansofpumps.Atsubcriticalpressures,circulationisproducedeithernaturallybytheactionoftheforceofgravity,bypumps,orbyacombinationofthetwo.

Fig-1Simplenatural-circulationcircuit（diagrammatic）

includingprimarysteamseparatorindrum

Theforceofgravityavailabletoproduceflowinnaturalcirculationcomesfromthedifferencebetweenthedensities（1b/cuft）ofthefluidsinthedowncomer（downflow）andriser（upflow）portionsofthecircuit（Fig-1）.Maximumpumpingeffectoccursifthefluidinthedowncomersiswateratorslightlybelowsaturationtemperatureandfreeofsteambubbles.Heat-absorbingrisesatsaturationtemperatureconveytotheboilerdrumawater-steammixtureoflessdensitythanthatofthewaterinthedowncomers.Thisdifferenceindensityestablishestheforceavailableforcirculation.

Theflowinthevariouscircuitsofboilerunitsdesignedforforcedcirculationatsubcriticalpressures,isproducedbymechanicalpumps.Therearetwogeneraltypesofforced-circulationsystems,a“once-through”systemanda“recirculating”system.

The“once-through”force-circulationtypereceiveswaterfromthefeedsupply,pumpingittotheinletoftheheat-absorbingcircuits.Fluidheatingandsteamgenerationtakeplacealongthelengthofthecircuituntilevaporationiscomplete.Furtherprogressthroughtheheatedcircuitsresultsinsuperheatingthevapor.Conventionallythistypeofforcecirculationrequiresnosteam-and-waterdrum.Amodificationofthe“once-through”typeevaporatestopartialdryness（90%quality）removeingtheexcesswaterinaseparator.

The“recirculation”forced-circulation-typeunithaswatersuppliedtotheheat-absorbingcircuitsthroughaseparatecirculatingpump.Thewaterpumpedisconsiderablyinexcessofthesteamproducedand,likeanatural-circulationboiler,asteam-and-waterdrumisrequiredforsteamseparation.Theseparatedwatertogetherwithfeedwaterfromthefeedpumpisreturnedthroughdowncomercircuitstothecirculatingpumpforanother“roundtrip.”

Intherecirculatingtypeofforcedcirculationthereisanetthermallossfortheboilunitbecauseoftheseparatecirculatingpump.Whilepracticallyalltheenergyrequiredtodrivethepumpsreappearsinthewaterasaddedenthalpy,thisenergyoriginallycamefromthefuelataconversion-to-useful-energyfactoroflessthan1.0.Ifanelectricmotordriveisused,thenetenergylost,referredtothefuelinputinaplantwith33%thermalefficiency,isabouttwicetheenergysuppliedtothepumpmotor.

Naturalcirculation

Inanatural-circulationsystem,circulationincreaseswithincreasedheatinput（andincreasedsteamoutput）untilapointofmaximumfluidflowisreached.Beyondthispoint,anyfurtherincreaseinheatabsorptionresultsinaflowdecrease.Thefromofthecurve,showninFig-2,isproducedbytwoopposingforces,Anincreaseindowncomersandrisersastheheatabsorptionincreases.Atthesametime,thefrictionandotherflowlossesinbothdowncomersandrisersalsoincrease.Whentherateofincreaseintheselosses（causedprimarilybytheincreaseinspecificvolumeintherisercircuits）becomesgreaterthanthegainfromincreasingdensitydifference,istodesignallthecircuitstooperateintheregionoftherisingpartofthecurve,totheleftofthepeakinFig-2.

Whendesignconditionsarelimitedtotherisingportionofthecirculationcurve,anatural-circulationboilertendstobeself-compensatingforthenumerousvariationsinheat-absorptionconditionsencounteredinanoperatingunit.Theseincludesuddenoverloads,changeinheat-absorbing-surfacecleanliness,nonuniformfuelbedorburnerconditions,andeventheinabilitytoforecastpreciselyactualconditionsovertheoperatinglifetime.

Fig-2typicalrelationshipbetweencirculationinaboilercircuit（atagivenpressure）andamountofsteamsteamproduced（scalearbitraiy）

Nosimilarcompensatingeffectisinherentinaforced-circulationunitoperatingatsubcriticalpressures,sincealargepartofthetotalresistanceoftherisercircuits,muchgreaterthanthenaturalcirculationeffect,iscausedbyflow-distributiondevicesrequiredatthecircuitinlets.Undertheseconditions,becauseofthedisproportionatelylargeresistanceofthedistributors,anincreaseinheatabsorptiontoanindividualcircuitorgroupofcircuitscausesonlyaslightchangeintheflowrate.

Themethodofproducingflowinboilercircuits,whethernaturalormechanical,hasvirtuallynobearingontheeffectivenessofheat-absorbingsurfacesaslongastheinsidesurfaceiswerredatalltimesbythewaterinawater-steammixtureofsuitablequalitytomaintainnucleateboiling.Providedthisfundamentalrequirementismet,thewater-filmresistancetoheatflowisnegligiblysmall,andtheoverallheatconductancedependsongas-sideandtube-wallresistances.Withinthenucleateboilingregime,boilerheat-absorbingsurfaceinthefurnaceorconvectionportionoftheunitabsorbsabsorbssubstantiallythesameamountofheatpersqftregardlessofwhetherthecirculatingflowisproducedbynaturalmethodsorbypumps.

Witheithertypeofcirculation,anydeparturefromthenucleateboilingregimerequiresspecialconsiderationoftheforced-convectionstearn-filmheattransfercoefficientanditsrelationtopermissiblemetaltemperatures.

Forcedornaturalcirculation

Undercertainconditionsforcedcirculationcanbeusefullyappliedtosteamgeneration.Mechanicalmeanstomovethefluidwithinthecircuitsareemployedforboilersdesignedtooperateaboveornearthecriticalpressure（3208.2psia.）Thereareinstances,also,intheprocessandwaste-heatfieldswheretemperaturecontrolorconsolidatingheatpickupfromwidelyseparatedpointscanbeeconomicallyeffectedbytheuseofpumps.Theconditionwhereforcedcirculationcanbeappliedtoadvantagearequitespecific.

Naturalcirculationismosteffectivelyemplovedwhenlargechangesindensityusuallyrestrictedtosubcriticalpressuredesignswherethereisaconsiderabledifferenceindensitybetweensteamandwater.Atpressuresabove2900psianatural-circulationsystembecomesincreasinglylargeandcostly,andapumpmaybemoreeconomicaltoassurepositiveflow.

Theforced-circulationprinciple,however,isequallyoperableinboththesupercriticalandsubcriticalpressureranges.Theselectionoftheidentifyingname“Universal-Pressure”boilerreflectsthebroadapplicabilityoftheonce-throughforced-circulationprinciple.Itschoice,asopposedtotheretentionofnaturalcirculationinthesubcriticalrange,isessentiallydeterminedbytheeconomicsoftheinstallation.

Fig-3Densitiesofsteamandwateratsaturatedsteamtemperature

forpressuresfromatmospherictocritical

Thedifferentialindensitiesofsteamandwaterfortherangeof14.7to3208.2psiaisnotedinFig-3.Asubstantialdifferentialpersistswelluptowardthecriticalpressure.Aslongasthemaximumeffectivenessoffthisdifferentialismaintainedbytheefficientseparationofthesteamfromthewaterinthecircnlatingsystem,aswiththeuseofcyclonesteamseparator,mechanicalaidtocirculationisnotessential.

InternalTreatmentofWater

Therearenumerouschemicals,somecalledboilercompounds,onthemarketandrecommendedfor“internal”watersofteningandothertreatment.Theirdutyistostabilizehardnessagents,preventscalingorresidualmakedepositseasilyremovable.Suchchemicalsarealsousedforneutralizationofresidualhardnessinsystemsafterexternaltreatment.Chemicalsofthisnatureareintroducedatamoreorlessconstentrateindissolvedstateintofeedtanksorsystems.Themostfrequentlyusedchemicalswithcalciumandmagnesiumsaltsresultsinconsolablecalciumandmagnesiumphosphates.Otherinternaltreatmentagentsarecalciumhydroxide,sodiumcarbonate,sodiumhyates,tanninsandamines.

Themosteffectivemethodofintroducinginternaltreatmentcompoundstoensureareasonabledegreeofquantitycontrolisbymeansofdosageapparatusconsistingofoneormorecontainershavingmanualormotordrivenagitatorsandadosagepump.

Asageneralruleinternaltreatmentalone,forwatersofmuchmorethan5°（UK）（７０p.p.m）hardnessisnotrecommendedbecausesystemandmake-upquantitiesandwatercompositionmaybecomecriticalandprecipitatedsludgeandsaltsareliabletoincreasedensityofwaterandsolidificationofsludgewithconsequentpropensitytofoamingandpriminginsteamplantandcirculationcomplications.

Internaltreatmentisgenerallyusedwithinlimitsforsmallishsystems,sayupto23MWorasresidualtreatmentandtoraisepHvaluetoarequisiteamount.

ExternalTreatmentofWater

Externalsofteningofsystemandmake-upwateristhemoreeffectiveandcomprehensivemethodofwatertreatmentforanysizeofplantandnetworkanditcansuitmostwatercharacteristics.Nowadaysexternaltreatmentinvolvestheprincipleofion-exchange,whichensureswaterofvirtuallyzerohardness.

Theion-exchangeprocesscanbedescribedasfollows.Moleculesofdissolvedsaltsaredissociatedinfromoffreeelectricallychargedionsinliquidsolution.Intheforchfieldtheytendtobeattractedtooppositecharges:

positivecationstothenegativecathodesandnegativeanionstothepositiveanode.Ion-exchangematerialsareinsolubleartificialresinstowhichactivatedchemicalgroupswithtieddissociabl