Giampaolo Levi

Transition gas flow in drag pumps and capillary leaks

Modern turbomolecular pumps include a drag stage in the exhaust, operating roughly in the pressure range of 10 mTorr–10 Torr. Flow conditions range from molecular flow at the drag inlet, to viscous flow at the outlet, known as ‘‘transition’’ flow. In general, models of transition flow in drag pumps have not been developed. Moreover, the model of a Gaede pump given in journals and textbooks up to the present, gives values of compression ratio that are orders of magnitude too high. In 1913, Gaede proposed a differential equation for transition flow in a drag pump. He did not solve the general equation, and the model was incomplete. We have developed a new model that takes transition flow in a differential element and integrates it over the length of the pump. This model is modified by a ‘‘pumping leak’’ expression for the gas stripper, which separates the inlet from the outlet. The result is compared with experimental measurements, and good agreement is obtained over the entire pressure range from molecular...