外文翻译--离心风机.doc

CentrifugalfanAcentrifugalfan(alsosquirrel-cagefan,asitlookslikeahamsterwheel)isamechanicaldeviceformovingairorgases.Ithasafanwheelcomposedofanumberoffanblades,orribs,mountedaroundahub.AsshowninFigure1,thehubturnsonadriveshaftthatpassesthroughthefanhousing.Thegasentersfromthesideofthefanwheel,turns90degreesandacceleratesduetocentrifugalforceasitflowsoverthefanbladesandexitsthefanhousing.1Figure1Centrifugalfanscangeneratepressurerisesinthegasstream.Accordingly,theyarewell-suitedforindustrialprocessesandairpollutioncontrolsystems.Theyarealsocommonincentralheating/coolingsystems.1.FancomponentsThemajorcomponentsofatypicalcentrifugalfanincludethefanwheel,fanhousing,drivemechanism,andinletand/oroutletdampers.2.TypesofdrivemechanismsThefandrivedeterminesthespeedofthefanwheel(impeller)andtheextenttowhichthisspeedcanbevaried.Therearethreebasictypesoffandrives.12.1DirectdriveThefanwheelcanbelinkeddirectlytotheshaftofanelectricmotor.Thismeansthatthefanwheelspeedisidenticaltothemotorsrotationalspeed.Withthistypeoffandrivemechanism,thefanspeedcannotbevariedunlessthemotorspeedisadjustable.2.2.1BeltdriveFigure2:CentrifugalfanwithabeltdriveBeltdrivenfansusemultiplebeltsthatrotateinasetofsheavesmountedonthemotorshaftandthefanwheelshaft.Thistypeofdrivemechanismisdepictedinfigure2.Thebeltstransmitthemechanicalenergyfromthemotortothefan.Thefanwheelspeeddependsupontheratioofthediameterofthemotorsheavetothediameterofthefanwheelsheaveandcanbeobtainedfromthisequation:1where:rpmfan=fanwheelspeed,revolutionsperminuterpmmotor=motornameplatespeed,revolutionsperminuteDmotor=diameterofthemotorsheaveDfan=diameterofthefanwheelsheaveFanwheelspeedsinbelt-drivenfansarefixedunlessthebeltsslip.Beltslippagecanreducethefanwheelspeedseveralhundredrevolutionsperminute(rpm).2.2VariabledriveVariabledrivefansusehydraulicormagneticcouplings(betweenthefanwheelshaftandthemotorshaft)thatallowcontrolofthefanwheelspeedindependentofthemotorspeed.Thefanspeedcontrolsareoftenintegratedintoautomatedsystemstomaintainthedesiredfanwheelspeed.1Analternatemethodofvaryingthefanspeedisbyuseofanelectronicvariable-speeddrivewhichcontrolsthespeedofthemotordrivingthefan.Thisoffersbetteroverallenergyefficiencyatreducedspeedsthanmechanicalcouplings.2.3FandampersFandampersareusedtocontrolgasflowintoandoutofthecentrifugalfan.Theymaybeinstalledontheinletsideorontheoutletsideofthefan,orboth.Dampersontheoutletsideimposeaflowresistancethatisusedtocontrolgasflow.Dampersontheinletsidearedesignedtocontrolgasflowandtochangehowthegasentersthefanwheel.Inletdampersreducefanenergyusageduetotheirabilitytoaffecttheairflowpatternintothefan.13.Backward-curvedbladesBackward-curvedblades,asinFigure3(b),usebladesthatcurveagainstthedirectionofthefanwheelsrotation.Thebackwardcurvaturemimicsthatofanairfoilcrosssectionandprovidesgoodoperatingefficiencywithrelativelyeconomicalconstructiontechniques.Thesetypesoffanwheelsareusedinfansdesignedtohandlegasstreamswithlowtomoderateparticulateloadings.Theycanbeeasilyfittedwithwearprotectionbutcertainbladecurvaturescanbepronetosolidsbuild-up.Backwardcurvedfanscanhaveahighrangeofspecificspeedsbutaremostoftenusedformediumspecificspeedapplications-highpressure,mediumflowapplications.Backward-curvedfansaremuchmoreenergyefficientthanradialbladefansandso,forhighhorsepowerapplicationsmaybeasuitablealternativetothelowercostradialbladedfan.4.StraightradialbladesRadialfanblades,asinFigure3(c),extendstraightoutfromthehub.Aradialbladefanwheelisoftenusedonparticulate-ladengasstreamsbecauseitistheleastsensitivetosolidsbuild-upontheblades,butitisoftencharacterizedbygreaternoiseoutput.Highspeeds,lowvolumes,andhighpressuresarecommonwithradialfans,andareoftenusedinvacuumcleaners,pneumaticmaterialconveyingsystems,andsimilarprocesses.Figure35.CentrifugalfanratingsRatingsfoundincentrifugalfanperformancetablesandcurvesarebasedonstandardairSCFM.Fanmanufacturersdefinestandardairasclean,dryairwithadensityof0.075poundsmasspercubicfoot(1.2kg/m),withthebarometricpressureatsealevelof29.92inchesofmercury(101.325kPa)andatemperatureof70F(21C).Selectingacentrifugalfantooperateatconditionsotherthanstandardairrequiresadjustmenttobothstaticpressureandbrakehorsepower.Thevolumeofairwillnotbeaffectedinagivensystembecauseafanwillmovethesameamountofairregardlessoftheairdensity.Ifacentrifugalfanistooperateatanon-standarddensity,thencorrectionsmustbemadetostaticpressureandbrakehorsepower.Athigherthanstandardelevation(sealevel)andhigherthanstandardtemperature,airdensityislowerthanstandarddensity.Centrifugalfansthatarespecifiedforcontinuousoperationathighertemperaturesneedtobeselectedtakingintoaccountairdensitycorrections.Again,acentrifugalfanisaconstantvolumedevicethatwillmovethesameamountofairattwodifferenttemperatures.If,forexample,acentrifugalfanmoves1,000ft/min(28m/min)at70F(21C)itwillalsomove1,000ft/min(28m/min)at200F(93C).Centrifugalfanairvolumedeliveredbythecentrifugalfanisnotaffectedbydensity.However,sincethe200F(93C)airweighsmuchlessthanthe70F(21C)air,thecentrifugalfanwillcreatelessstaticpressureandwillrequirelessbrakehorsepower.Selectingacentrifugalfantooperateatconditionsotherthanstandardairrequiresadjustmenttobothstaticpressureandpower.WhenacentrifugalfanisspecifiedforagivenCFMandstaticpressureatconditionsotherthanstandard,anairdensitycorrectionfactormustbeappliedtoselectthepropersizefantomeetthenewcondition.Since200F(93C)airweighsonly80%of70F(21C)air,thecentrifugalfanwillcreatelesspressure.Togettheactualpressu