风机选型资料

风机根底讲义OutlineAirfundamentals,volume,pressure&powerFanandAirSystemsFantesting&ratingsFanperformancecurvesFanlaws&densityeffectsAirsystems&systemresistancecurveInteractionofsystemcurveandfancurveFantypes&arrangementsDuctsystemdesignconsiderationsFundamentalsofAirAirhasweightandexertsapressureAtmosphere-theenvelopeofairsurroundingtheearthAtmosphericpressure-thepressureexertedbytheatmosphere,oftenexpressedinforceperunitareaStandardatmosphericpressureSinceatmosphericpressurechangeswithelevation,weatherconditions,etc.,standardatmosphericpressureservesasabasisforcomparisonMeasuredatsealevel,or29.92〞ofmercurypersq.in.Alsocalled“StandardAirDensity〞StandardAirDensityStandardairdensity=0.075lb./ft3Airtemperatureof70ºF(21ºC)Altitudeat0ft.elevation(sealevel)Barometricpressureof29.92〞Hg(101kPA)Specificvolumeof13.33ft3/lb.Non-StandardAirDensitiesNon-standardairdensitiesarecausedby:Temperaturesotherthan70ºFElevationsaboveorbelowsealevelBarometricreadingsotherthan29.92HgPartialvacuuminairflowcausedbyfansuctionRelativehumiditychangesGasesotherthan“air〞

FanAirVolumeThepurposeofafanistoaddenergytotheair,sothatairismovedfromoneareatoanotherThevolumeofairmovedbythefanisusuallyexpressedincubicfeetperminute(CFM)ACFM=volumeofairmovedbythefanatanyairdensitySCFM=volumeofairmovedbymovedbythefanatstandardairdensity(.075lb./ft.3)

FanTotalPressureFanTotalPressure(TP)TotalmechanicalenergyaddedtotheairbythefanTP=TPatoutlet-TPatinletor...TP=staticpressure(SP)+velocitypressure(VP)

FanStaticPressureFanStaticPressure(SP)ThefantotalpressurelessthefanvelocitypressureSP=SPatoutlet-TPatinletor...SP=totalpressure(TP)-velocitypressure(VP)

FanVelocityPressureFanVelocityPressure(VP)ThepressurecorrespondingtothefanoutletvelocityThekineticenergyperunitvolumeofflowingairVP=totalpressure(TP)-staticpressure(SP)VP=(V/1096)2=Densityinlb./ft3

V=VelocityinFPMifstandardair:VP=(V/4005)2FanPowerAirHorsepower(AHP)Assuming100%efficiency,thehorsepowerrequiredtomoveagivenvolumeofairagainstgivenpressureStaticAHP=(CFMxSP)/6356TotalAHP=(CFMxTP)/6356BrakeHorsepower(BHP)TheactualhorsepowerafanrequiresBHP>AHP,becauseafanisnot100%efficientInadditiontofanpowerinput,BHPmayincludepowerabsorbedbyV-beltdrives,accessories,etc.FanEfficiencyStaticEfficiency(SE)RatiooffanpoweroutputtothefanpowerinputUsesSP,whichdoesnotincludekineticenergySE=(CFMxSP)/(6356xBHP)MechanicalEfficiency(ME)orTotalEfficiencyRatiooftotalfanpoweroutputtothefanpowerinputUsesTP,whichincludeskineticenergyME=(CFMxTP)/(6356xBHP)FanTestingandRatingFansaretestedinsetupsthatsimulateinstallationsFourstandardinstallationtypes:FanTestingandRatingFansaretestedagivenspeed(RPM)Volumetricflowrate(CFM)ismeasuredat:Noresistance(wideopenvolumeorfreedelivery),Completeresistance(shutoff),andEverythinginbetweenCompleteResistanceNoResistanceFanPerformanceCurveThetestdataisplotted,producinga“fancurve〞BrakeHorsepowerStaticPressureAirVolume(CFM)FanPerformanceCurve-ExampleFanTestingandRatingNotallfansizesaretestedforratingByuseofequationscalled“fanlaws〞testdataisusedtocalculateperformanceofalargerfanthatisgeometricallysimilar(notsmaller)NotpracticaltotestafanateveryspeedByuseof“fanlaws〞itispossibletopredictfanperformanceatvariousspeedsFansaretested&ratedatstandardairdensityFanperformancecanbepredictedatvaryingairdensitiesbyusing“fanlaws〞FanLaws-EffectofSpeedChangeCFMvariesdirectlyasRPMCFM1RPM1CFM2RPM2SPvariesasthesquareoftheRPMSP1RPM1SP2RPM2HorsepowervariesthecubeoftheRPMBHP1RPM1BHP2RPM2===23Multi-SpeedFanCurve-ExampleEffectofChangeinSpeed-ExampleCurrent:600RPM,2〞SP,6.5BHP,19,000CFMWhatwillthefanperformancebeat678RPM?CFMvariesdirectlyasRPMRPM1CFM1 600 19,00021,500CFM2RPM2CFM2 678 xSPvariesasthesquareoftheRPMRPM1SP1 600 2.02.55SP2RPM2SP2 678 xHorsepowervariesthecubeoftheRPMRPM1BHP1 600 6.59.37BHP2RPM2BHP2 678 x=======23==23DensityAffectsonFanPerformanceFanperformancechangeswiththedensityofthegasbeinghandledForcomparativepurposes,fancatalogratingsareusuallyshownatstandardairdensityTouseanymanufacturer’sratings,youmustconvertyourperformancerequirementstostandardairdensityFanSPandBHPvaryindirectproportiontotheairdensityatthefaninletSP1BHP1Density1 SP2BHP2Density2Theheaviertheair,themorepressurewillbeproducedandmorehorsepowerwillberequiredAirvolume(CFM)isnotaffectedbydensityCFM1=CFM2AfanisaconstantvolumemachineandwillproducethesameCFMatanyairdensity==FanLaws-DensityEffectsTemperature&AltitudeEffectsTemperaturesotherthan70ºFaffectdensityAttemperaturesabove70ºF,airdensityisless(lighterair),thusSPandBHParelessAttemperaturesbelow70ºF,airdensityisgreater(heavierair),thusSPandBHParegreaterAltitudesotherthansealevelaffectdensityAtaltitudesabovesealevel,airdensityisless(lighterair),thusSPandBHParelessAtaltitudesbelowsealevel,airdensityisgreater(heavierair),thusSPandBHParemoreTemperatureandAltitudeCorrectionsHowtoconvertfanperformanceatactualconditionstostandardconditions:Step1.IfACFM,donotconvert(useACFMtoselectfan) IfSCFM,converttoACFMbymultiplyingbyfactorStep2.DivideoperatingSPbyfactortogetSPatstandardStep3.Selectfanfromcatalogratingtablesusingvalues fromStep1&2(obtainRPMandBHPfromtable)Step4.DetermineoperatingBHPbymultiplyingBHPat standardconditionsbyfactorStep5.Determine“cold〞orstartingBHPbymultiplyingBHP atstandardconditionsbyaltitudefactorat70ºFTemperatureandAltitudeCorrectionFactorsTemperatureandAltitudeCorrectionFactorsUsingcorrectionfactorsfromtableTodetermineACFM,SPandBHPusefactorfoundatintersectionofoperatingtemperatureandaltitudeTodetermine“cold〞orstartingBHP,usefactorfoundatintersectionofoperatingaltitudeand70ºFWhenoperatingdensityisknown,determinethecorrectionfactorbydividingtheoperatingdensitybythestandarddensityCorrectionFactorOperatingDensity x StandardDensity0.075==DensityCorrectionExampleA365BC,SWSIistohandle17,000CFMat2.5〞SP,operatingat300ºFand3000ft.altitudeStep1.DonotconvertACFM,use17,000CFMtoselectfanStep2.OperatingSP/factor=2.5/.624=4〞SPatStd.Step3.Using17,000CFMand4〞SP,selectfanfromthe catalogratingtables(RPMis918,BHPis14.36)Step4.Std.BHPxfactor=14.36x.624=8.96OperatingBHPStep5.Std.BHPxaltitudefactorat70ºF=14.36x.896= 12.87ColdorStartingBHPColdorStartingBrakeHorsepowerCautionshouldbeusedonthebrakehorsepowerconversionandmotorselectionAsthefanmayberequiredtostartat“cold〞temperature(beforetheairwarmsup)Therefore,youshouldselectthemotorforthemaximumBHPatjobconditionsbymultiplyingbythealtitudefactorat70ºFInletSuctionEffectSuctionisacommoninfluenceondensity,especiallyonexhaustsystemsSuctioncreatedbysystemresistanceonafaninletcreatesapartialvacuumattheinletThispartialvacuum(negativepressure)lowerstheairdensityatthefaninletSincelowpressurecorrectionsaresmall,suctionpressurelessthan10〞SPisusuallyignoredStep1.DeterminesuctionpressurefactorSuctionpressurefactor=AP+(-InletSP) APStep2.DeterminecombinedfactorCombinedfactor=suctionpressurefactorxfactorfromtableStep3.DetermineSPatstandardSPatstandard=operatingSP/combinedfactorWhere:AP=atmosphericpressure=407〞wgatsealevel(TodetermineAPatotherthansealevel,divide407byaltitudefactorat70ºF)InletSPisnormallyanegativenumber

CorrectingforInletSuctionEffectAfanistodeliver10,750ACFMat22〞SP,at200ºF,where20〞ofSPisatthefaninletStep1.Suctionpressurefactor=407+(-20)=.951 407Step2.Combinedfactor=.951x.803=.764Step3.SPatstandard=22〞SPoperating/.764=28.8〞Results:Thefanwouldnowbeselectedfor10,750CFMat28.8〞SPResultingspeedis2073RPMandBHPis79.64atstandardTheBHPatconditionsis60.84(79.64x.764=60.84)At200F,thefancouldoperatewitha75HPmotor,howeveriffanisstarted“cold〞withoutashutoffdamper,thena100HPmotorisrequired(79.64BHPstd.x.951=75.73coldorstartingBHP)InletSuctionCorrectionExampleHumidAirandSpecialGasesInadditiontotemperature,altitude&inletsuction,densitiesvaryfromstandarddueto:Moisture(humidair)GasesormixturesofgasesotherthanairForhumidair,densityeffectcanbecalculatedusingpsychometricchartsHumidairislessdensethandryairForgasesotherthanair,youwillneedtoknowthecompositionandrelative%ofeachgastocalculatedensityeffectFanSafeSpeedandTemperatureTemperaturesotherthan70ºFmaycauseanalloytobecometoopliableorbrittleSpeedadjustmentscanexceedthelimitsofthewheel,shaftandbearingsGasesotherthanairmaybecorrosivetovitalstructuralcomponentsFanStallAerodynamiceffectFanunstableoperationLessseverethansurgeFanSurgeInteractionwithsystemFanoperationisveryunstablePulsatingflowatinletanddischargeFanUsefulApplicationRangeHowAirSystemsWorkTomoveairthroughasystem,pressureisrequiredtoovercomesystemresistanceFormostsystemsductairflowisturbulent,characterizedbyrapidrandomfluctuationsofvelocityandpressureThepressurerequiredchangesasthesquareofthechangeinflowrateSP1Flow1 SP2Flow2Plottedasa“systemresistancecurve〞2=SystemResistanceCurve-ExampleCFMin10,000’sStaticPressurePoint1:40,000CFM4in.wg.SPPoint2:60,000CFM9in.wg.SPFan&System-PointofOperationCFMFanPerformanceCurveSystemResistanceCurvePointofOperationBHPRequirementBHPCurveStaticPressureAfanwilloperateatthepointwherethesystemresistancecurveintersectsthefancurveFan&SystemCurve-ExampleFan&SystemPressureRelationshipsDuctlocatedonsuctionsideoffanFan&SystemPressureRelationshipsDuctlocatedondischargesideoffanFansinSeriesCFMremainsthesame,SP&BHPdoubleCFM2=CFM1

SP2=2SP1

BHP2=2BHP1FansinParallelSPremainsthesame,CFM&BHPdoubleSP2=SP1

CFM2=2CFM1

BHP2=2BHP1DuctSystemExampleDuctSegmentCalculationsVelocitypressuremethodFrictionalanddynamic(fitting)lossesinductsandhoodsarefunctionofthevelocitypressureLossescanbecalculatedbymultiplyingthevelocitypressurebyalosscoefficientLosscoefficientsexistfor:hoods,ducts,elbows,branchentries,contractions,expansions&frictionDuctSegmentCalculationsVelocitypressuremethodDistributionofAirflow-BalancingBalancingobjective:toobtainthedesiredflowrateateachhoodinthesystem,whilemaintainingthedesiredvelocityineachbranchandmainTodothis,allflowpaths(ducts)enteringajunctionmusthaveequalcalculatedstaticpressurerequirementsTwobalancingmethods:BlastgatemethodBalancebydesignmethodBlastGateBalancingMethodBlastgatesareadjustedafterinstallationtoachievedesiredflowrateateachhoodAdjustinggatesettingswillchangeflowratesinallotherbranchesTheresultingSPincreasecanresultinincreasedfanBHPCanleadtoplugginginductduetosettledparticulateBalancebyDesignMethodOftencalled“staticpressurebalancemethod〞Calculationbeginsathoodfarthestfromfanandproceeds,segmentbysegment,tothefanAteachjunction,theSPrequiredtoachievedesiredflowinonestreammustequalSPinjoiningairstreamThestaticpressuresarebalancedbysuitablechoicesofductsizes,elbowradii,etc...Thismethodisrequiredfortoxicmaterials,explosives,radioactivedustsandbiologicalsEliminatestheriskofexposureduetoblastgatetamperingNopossibilityofaccumulationduetoblastgateobstructionDuctSystemDesignConsiderationsHooddesignHoodtypes,location,capturevelocity,flowrateDuctworkdesignTransportvelocity,ductmaterials,ducttransitions,branchentryloss,elbowdesign&lossStackcapdesignStacktypes,exhauststackoutlets,effectivestackheightOthersystemcomponentsBlastgates,dampers,filtersHoodTypesEnclosethehoodasmuchaspossibleThemorecompletelyenclosed,thelessairrequiredforcontrolHoodConfigurationLocatehoodsocontaminantisremovedawayfromthebreathingzoneoftheoperatorThecloserthehoodtothesource,thelessairrequiredforcontrolHoodLocationCaptureVelocityTheminimumhoodinducedairvelocitynecessarytocaptureandconveythecontaminantintothehoodAirflowratemustincreaseasthesquareofdistanceofthesourcefromthehoodHoodAirFlowRateV=Q/A=CFM/AreaAresultofhoodconfiguration&airflowrateHoodCaptureVelocityMinimumdesignvelocityisrequiredtopreventmaterialfrom“settling〞andpluggingtheductExcessivelyhighvelocitiesarewastefulofpowerandmaycauserapidabrasionofductsDuctTransportVelocityMinimumDuctDesignVelocitiesDuctMaterialConsiderationsDuctsareoftenmadeofgalvanizedsheetsteelorblackironCorrosivegases,erosiveparticulate&hightemperaturesmayrequirespecialmaterialsFlexibleconnectorsmayberequiredtopreventtransmittingvibrationtoductworkDuct&hangersshouldwithstand:Weightofduct1/2fullofparticulateWeightofductinsulationWeightofworkerwalkingonductworkDuctmaterialshouldbesmoothtominimizefrictionStraightductfrictionlossequation:DuctMaterialConsiderationsGalvanizedsheetmetalductDuctMaterialFrictionLossFactorsBlackIron,aluminum,stainlesssteel&PVCductsDuctMaterialFrictionLossFactorsDuctBranchEntriesBranchentryangleshouldbe30ºorless(45ºmax)Expansionshouldbe15ºmaximumBranchesshouldenteratgradualexpansionsandbestaggeredDuctBranchEntriesSizetheducttomaintaintheselectedorhighertransportvelocityAdifferenceof.10〞wgormorebetweenthemainVPandVPofthetwobranchesshouldbecorrectedDuctSymmetricalWyesDuctwyesshouldbesymmetricalandgradualAngleto30ºto60ºispreferredDuctBranchEntryLossesBranchentrylossassumedtooccurinbranchDonotincludeanenlargementregaincalculationforbranchentryenlargementsDuctContractionsDuctsarereducedtofitintotightplaces,fitequipment,ortoprovidehigherdischargevelocityDuctExpansionsUsedtofitequipmentorreduceenergyconsumedinthesystembyreducingvelocityandfrictionUndesirableintransportsystems,asductvelocitymaybereducedbelowminimumtransportvelocitycausingmaterialtosettleinductStaticPressureRegainandLossDuctlocatedonsuctionsideoffanStaticPressureRegainandLossDuctlocatedondischargesideoffanSPRegainforExpansionsSPLossesforContractionsDuctElbows-ElbowRadiusElbowsshouldbe2to2.5xdia