涡轮分子泵基本理论

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1、TurbomolecularpumpBasictheoryTurbomolecularpumpoperatingprincipleThepumpingeffectofanarrangementconsistingofrotorandstatorbladesisbaseduponthetransferofimpulsesfromtherapidlyrotatingbladestothegasmoleculesbeingpumped.Moleculesthatcollidewiththebladesareadsorbedthereandleavethebladesagainafte

2、racertainperiodoftime.Inthisprocess,bladespeedisaddedtothethermalmolecularspeed.Toensurethatthespeedcomponentthatistransferredbythebladesisnotlostduetocollisionswithothermolecules,molecularflowmustprevailinthepump,i.e.themeanfreepathmustbegreaterthanthebladespacingInthecaseofkineticpumps,aco

3、unterpressureoccurswhenpumpinggas;thiscausesabackflow.ThepumpingspeedisdenotedbyS0.Thevolumeflowratedecreasesaspressureincreasesandreachesavalueof0atthemaximumcompressionratioK0.Figure4.22:OperatingprincipleoftheturbomolecularpumpCompressionratioThecompressionratio,whichisdenotedK0,canbeesti

4、matedaccordingtoGaede[25].Thefollowingappliesforavisuallydensebladestructure(Figure4.22):??0=??.????.?̅Formula4-8:Turbopumpcompressionratio-1?̅:Meanmoleculevelocity[m.s]-1v:Circumferentialspeed[m.s]ThegeometricratiosaretakenfromFigure4.22.Thefactorgisbetween1and3[26].Fromtheequation,itisevid

5、entthatK0increasesexponentiallywithbladevelocityvaswellaswith√Mbecause8.?.??̅=�(Formula1-10)?.?Consequently,thecompressionratiofornitrogen,forexample,issignificantlyhigherthanforhydrogen.Volumeflowrate(pumpingspeed)PumpingspeedS0isproportionaltotheinletareaAandthemeancircumferentialvelocityo

6、fthebladesv,i.e.rotationalspeed.Takingthebladeangleαintoaccountvields:11?=.?.?.????.????=.?.?.???2?024Formula4-9:TurbopumppumpingspeedTakingintoaccountboththeentryconductanceoftheflange?̅???=.?(Formula1-24)4aswellastheoptimalbladeangleof45°,producestheapproximateeffectivepumpingspeedSeffofat

7、urbopumpforheavygases(molecularweight>20)inaccordancewiththefollowingformula:?0.????.?????==??0+???4.(+1)?̅Formula4-10:TurbopumpeffectivepumpingspeedDividingtheeffectivepumpingspeedbythebladedentrysurfaceoftheuppermostdiskandtakingtheareablockedbyt