A Conductance Effect on the Pumping Speed Characteristics of Screw Vacuum Pump

In a volume displacement type vacuum pump, the pumping speed S is expressed in terms of a displacement volume V and the repetition rate f as S=Vf In a screw vacuum pump, a kind of volume displacement type vacuum pump, the displacement volume is tublar in shape and acts as a resistance in the pumping action. In the report, we analysed the effect of flow conductance of the displacement volume on the pumping speed. The result of the analysis shows that the resistance effect on the pumping speed is negligible in high pressure region, but not negligible in a low pressure of free molecular pressure region. In some typical examples, we have obtained correction factor K of less than 50% under an operating condition of free molecular pressure region. It was certifyed that the gradational screw pump, which is a novel type of screw pump, has a better value of K than that of the old one by about 10% or more.