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1、TechnicalBulletinTBN003.0/1998FANLAWSItisnotpracticabletotestthenotproportioned,slightlybetterperformanceofeverysizeoffaninaperformanceisgenerallyobtainedwhenmanufacturer’srangeatallspeedsatprojectingfromagivenfansizetoawhichitmaybeapplied.Noritislargerone.po
2、ssibletosimulateeveryinletdensitywhichmaybeencountered.Fanlawsequations:-Fortunately,byuseoftheFanLaws,itispossibletopredictwithgoodaccuracy3⎛N⎞⎛D⎞⎜2⎟⎜2⎟theperformanceofafanatotherspeedsQ2=Q1x⎜⎟x⎜⎟NDanddensitiesthanthoseoftheoriginal⎝1⎠⎝1⎠ratingtest.22⎛N⎞⎛D⎞⎛
3、d⎞⎜2⎟⎜2⎟⎜2⎟Itisimportanttonote,however,thatP2=P1x⎜⎟x⎜⎟x⎜⎟NDdtheseLawsapplytoagivenpointof⎝1⎠⎝1⎠⎝1⎠operationonthefancharacteristic.Theycannotbeusedtopredictotherpoints35⎛N⎞⎛D⎞⎛d⎞⎜2⎟⎜2⎟⎜2⎟onthischaracteristiccurve.W2=W1x⎜⎟x⎜⎟x⎜⎟NDd⎝1⎠⎝1⎠⎝1⎠TheseLawsaremostoften
4、usedtocalculatechangeinflowrate,pressureandpowerofafanwhenthesize,whereQ=flowrate,CMHspeedorgasdensityischanged.P=pressure(TP,SPorVP),Pa3d=gasdensity,kg/mThefanLawswillbeaccurateforN=fanspeed,rpmgeometricallyproportionedfans;D=impellerdiameter,mmhowever,becau
5、setolerancesareW=impellerpower,kWusually1-8TechnicalBulletin-TBN003.0/1998ChangeinFanSpeedFirstconsideredarethefanlawsapplyingtoachangeonlyinspeed(constantsystem)withagivenfanandagivensystemhandlingairatagivendensity.(Fig.1)⎛N⎞Q=Qx⎜2⎟21⎜⎟N⎝1⎠2⎛N⎞⎜2⎟P=Px21⎜N⎟⎝
6、1⎠3⎛N⎞W=Wx⎜2⎟21⎜⎟NFig.1ChangeinRPM⎝1⎠Efficiencywillnotchange.ChangeinFanSizeFanLaws2accountforchangesinperformanceduetoproportionedchangesinfansize,basedonconstanttipspeed,withconstantspeed,airdensity,fanproportionsandfixedoperatingpoint.(Fig.2)⎛W⎞Q=Qx⎜2⎟21⎜⎟
7、⎝W1⎠2⎛D⎞=Qx⎜2⎟1⎜⎟D⎝1⎠P=P21⎛D⎞Fig.2ChangeinwheeldiameterN=Nx⎜1⎟21⎜D⎟(constanttipspeed)⎝2⎠Itisusedmostlybyfandesignersandrarelyhasapplicationinthefield.2-8TechnicalBulletin-TBN003.0/1998FanLaws3alsoaccountforchangesinperformanceduetoproportionedchangesinfansize
8、butitbasedonconstantfanspeed,withairdensity,fanproportionsandfixedoperatingpoint.(Fig.3)3⎛D⎞Q=Qx⎜2⎟21⎜⎟D⎝1⎠2⎛D⎞⎜2⎟P=Px21⎜⎟D⎝1⎠⎛D⎞TS=TSx⎜2⎟21⎜⎟D⎝1⎠5⎛D⎞W=Wx⎜2⎟21⎜⎟Fig.3Changeinwheeldiameter
