The statistical theory of turbomolecular pumps

Abstract

The working principle of a modern turbomolecular pump is analyzed from the point of view of statistical mechanics. The pumping action is mainly due to the collision probabilities of the gas molecules with the upper and lower blade surfaces caused by the high-speed relative motion between the rotating blades and the gas molecules. In turbomolecular pumps the molecular drag action only serves as an approximate mathematical description when the rotating speed of the blades is not very high, and both the pumping speed and the compression ratio will saturate as the blade speed approaches the average thermal velocity of the gas molecules, i.e., they cannot increase limitlessly with increasing blade speed as suggested by the molecular drag principle. Finally, the compression ratio of a multistage pump with blades of finite lengths under different rotating speeds are calculated and compared with the experimental results.