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CentrifugalfanAcentrifugalfanalsosquirrelcagefan,asitlookslikeahamsterwheelisamechanicaldeviceformovingairorgases.Ithasafanwheelcomposedofanumberoffanblades,orribs,mountedaroundahub.AsshowninFigure1,thehubturnsonadriveshaftthatpassesthroughthefanhousing.Thegasentersfromthesideofthefanwheel,turns90degreesandacceleratesduetocentrifugalforceasitflowsoverthefanbladesandexitsthefanhousing.1Figure1Centrifugalfanscangeneratepressurerisesinthegasstream.Accordingly,theyarewellsuitedforindustrialprocessesandairpollutioncontrolsystems.Theyarealsocommonincentralheating/coolingsystems.1.FancomponentsThemajorcomponentsofatypicalcentrifugalfanincludethefanwheel,fanhousing,drivemechanism,andinletand/oroutletdampers.2.TypesofdrivemechanismsThefandrivedeterminesthespeedofthefanwheelimpellerandtheextenttowhichthisspeedcanbevaried.Therearethreebasictypesoffandrives.12.1DirectdriveThefanwheelcanbelinkeddirectlytotheshaftofanelectricmotor.Thismeansthatthefanwheelspeedisidenticaltothemotorsrotationalspeed.Withthistypeoffandrivemechanism,thefanspeedcannotbevariedunlessthemotorspeedisadjustable.2.2.1BeltdriveFigure2CentrifugalfanwithabeltdriveBeltdrivenfansusemultiplebeltsthatrotateinasetofsheavesmountedonthemotorshaftandthefanwheelshaft.Thistypeofdrivemechanismisdepictedinfigure2.Thebeltstransmitthemechanicalenergyfromthemotortothefan.Thefanwheelspeeddependsupontheratioofthediameterofthemotorsheavetothediameterofthefanwheelsheaveandcanbeobtainedfromthisequation1whererpmfanfanwheelspeed,revolutionsperminuterpmmotormotornameplatespeed,revolutionsperminuteDmotordiameterofthemotorsheaveDfandiameterofthefanwheelsheaveFanwheelspeedsinbeltdrivenfansarefixedunlessthebeltsslip.Beltslippagecanreducethefanwheelspeedseveralhundredrevolutionsperminuterpm.2.2VariabledriveVariabledrivefansusehydraulicormagneticcouplingsbetweenthefanwheelshaftandthemotorshaftthatallowcontrolofthefanwheelspeedindependentofthemotorspeed.Thefanspeedcontrolsareoftenintegratedintoautomatedsystemstomaintainthedesiredfanwheelspeed.1Analternatemethodofvaryingthefanspeedisbyuseofanelectronicvariablespeeddrivewhichcontrolsthespeedofthemotordrivingthefan.Thisoffersbetteroverallenergyefficiencyatreducedspeedsthanmechanicalcouplings.2.3FandampersFandampersareusedtocontrolgasflowintoandoutofthecentrifugalfan.Theymaybeinstalledontheinletsideorontheoutletsideofthefan,orboth.Dampersontheoutletsideimposeaflowresistancethatisusedtocontrolgasflow.Dampersontheinletsidearedesignedtocontrolgasflowandtochangehowthegasentersthefanwheel.Inletdampersreducefanenergyusageduetotheirabilitytoaffecttheairflowpatternintothefan.13.BackwardcurvedbladesBackwardcurvedblades,asinFigure3b,usebladesthatcurveagainstthedirectionofthefanwheelsrotation.Thebackwardcurvaturemimicsthatofanairfoilcrosssectionandprovidesgoodoperatingefficiencywithrelativelyeconomicalconstructiontechniques.Thesetypesoffanwheelsareusedinfansdesignedtohandlegasstreamswithlowtomoderateparticulateloadings.Theycanbeeasilyfittedwithwearprotectionbutcertainbladecurvaturescanbepronetosolidsbuildup.Backwardcurvedfanscanhaveahighrangeofspecificspeedsbutaremostoftenusedformediumspecificspeedapplicationshighpressure,mediumflowapplications.Backwardcurvedfansaremuchmoreenergyefficientthanradialbladefansandso,forhighhorsepowerapplicationsmaybeasuitablealternativetothelowercostradialbladedfan.4.StraightradialbladesRadialfanblades,asinFigure3c,extendstraightoutfromthehub.Aradialbladefanwheelisoftenusedonparticulateladengasstreamsbecauseitistheleastsensitivetosolidsbuildupontheblades,butitisoftencharacterizedbygreaternoiseoutput.Highspeeds,lowvolumes,andhighpressuresarecommonwithradialfans,andareoftenusedinvacuumcleaners,pneumaticmaterialconveyingsystems,andsimilarprocesses.Figure35.CentrifugalfanratingsRatingsfoundincentrifugalfanperformancetablesandcurvesarebasedonstandardairSCFM.Fanmanufacturersdefinestandardairasclean,dryairwithadensityof0.075poundsmasspercubicfoot1.2kg/m³,withthebarometricpressureatsealevelof29.92inchesofmercury101.325kPaandatemperatureof70°F21°C.Selectingacentrifugalfantooperateatconditionsotherthanstandardairrequiresadjustmenttobothstaticpressureandbrakehorsepower.Thevolumeofairwillnotbeaffectedinagivensystembecauseafanwillmovethesameamountofairregardlessoftheairdensity.Ifacentrifugalfanistooperateatanonstandarddensity,thencorrectionsmustbemadetostaticpressureandbrakehorsepower.Athigherthanstandardelevationsealevelandhigherthanstandardtemperature,airdensityislowerthanstandarddensity.Centrifugalfansthatarespecifiedforcontinuousoperationathighertemperaturesneedtobeselectedtakingintoaccountairdensitycorrections.Again,acentrifugalfanisaconstantvolumedevicethatwillmovethesameamountofairattwodifferenttemperatures.If,forexample,acentrifugalfanmoves1,000ft³/min28m³/minat70°F21°Citwillalsomove1,000ft³/min28m³/minat200°F93°C.Centrifugalfanairvolumedeliveredbythecentrifugalfanisnotaffectedbydensity.However,sincethe200°F93°Cairweighsmuchlessthanthe70°F21°Cair,thecentrifugalfanwillcreatelessstaticpressureandwillrequirelessbrakehorsepower.Selectingacentrifugalfantooperateatconditionsotherthanstandardairrequiresadjustmenttobothstaticpressureandpower.WhenacentrifugalfanisspecifiedforagivenCFMandstaticpressureatconditionsotherthanstandard,anairdensitycorrectionfactormustbeappliedtoselectthepropersizefantomeetthenewcondition.Since200°F93°Cairweighsonly80of70°F21°Cair,thecentrifugalfanwillcreatelesspressure.Togettheactualpressurerequiredat200°F93°C,thedesignerwouldhavetomultiplythepressureatstandardconditionsbyanairdensitycorrectionfactorof1.25i.e.,1.0/0.8togetthesystemtooperatecorrectly.Togettheactualpowerat200°F93°C,thedesignerwouldhavetodividethepoweratstandardconditionsbytheairdensitycorrectionfactor.
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