Engineering aspects of turbomolecular pump design

Abstract

The development of modern (thin-bladed) turbomolecular high-vacuum pumps began in 1957 with the demonstration of the possibility of obtaining high compression ratios with axial flow compressors in the molecular flow regime. Thirty years later, such pumps had become the major method for high vacuum pumping. It had been apparent from the beginning that pneumatic compressors can be useful at any pressure provided a proper number of suitable impellers were used. However, theoretical studies, initially by Prof. A. Shapiro's group at MIT (Massachusetts Institute of Technology) dealt primarily with the pumping mechanism in molecular flow rather than with an optimum practical pump design. Some observations in such studies were misunderstood and the first pump designs were not optimized. Later, compound or hybrid pumps were introduced, which incorporated molecular drag pumping stages. In more recent years, pumps have been made which can exhaust directly to the atmosphere by means of added centrifugal-regenerative impellers. The use of different impeller types provides freedom to the designer to create pumps that match any reasonable desired performance. This paper will attempt to explore some of the engineering aspects of design, especially relationships of volume and mass flows, permissible pressure ratios in various density domains, and their relevance to power consumption.