AVS 49th International Symposium
    Vacuum Technology Thursday Sessions
       Session VT-ThM

Paper VT-ThM8
Experimental Analysis of Tapered Gaede Pumps

Thursday, November 7, 2002, 10:40 am, Room C-104

Session: Gas Dynamics and Flow
Presenter: S. Giors, Varian S.p.A., Italy
Authors: S. Giors, Varian S.p.A., Italy
R. Gotta, Varian S.p.A., Italy
J.C. Helmer, Varian S.p.A., Italy
Correspondent: Click to Email
In 1993 Varian S.p.A. revived the Gaede design and developed it for commercial hybrid turbomolecular pumps. The Gaede stages exhaust at pressures above 10 Torr, operating in laminar viscous flow. Classical Gaede stages are characterized by uniform cross section of the channel, from inlet to exhaust. Both modelling and experimental analysis agrees in showing that the maximum compression in the viscous regime is inversely proportional to the square of the height h between disk and channel surface, while channel pumping speed is proportional to the inlet cross section of the channel and hence to h. From this reasoning comes the simple idea that tapering the height h of the channel from inlet to exhaust, keeping the same entrance section, can greatly increase compression, without compromising the pumping speed of the channel. An experiment was designed to test these concepts. A uniform channel and one with a 3:1 taper, with the same inlet channel section, are compared in viscous conditions. Results show that the channels must be compared not just in terms of maximum compression and maximum speed, but in terms of the complete speed vs. compression characteristics. Actually the tapered channel shows a slight decrease in maximum pumping speed, but that is highly compensated by increased compression and improved shape of the speed vs. compression characteristics. Another promising advantage of the tapered channel with respect to the uniform one, is reduction in power dissipation with the same exhaust pressure and flow. Comparison of experimental results with model calculations, indicates some ways in which the Couette-Poiseuille model needs to be improved.