We have made, and are making, an incessant effort to develop a series of turbomolecular pumps that ensure a high pumping throughput in the medium vacuum region (0.1～10 Pa), as well as in the high vacuum region, which is specifically required in the production of semiconductors and liquid crystal displays. The accurate evaluation of the evacuation efficiency of such pumps is indispensable to the designing of the turbine blades and thread grooves of turbomolecular pumps that ensure the optimum performances in those vacuum regions. We have developed a prediction program which enables the analysis of performance of turbomolecular pumps in the free molecule flow region and the transition flow region, and applied it to the designing of such pumps. To estimate the maximum compression ratio and the maximum pumping speed of turbomolecular pumps in the transition flow region, we employed the integral equation method for the 2-dimensional blade rows, and then corrected the results using the Stokes flow approximation method. The results thus calculated agree quite well with the experimental results, both in the free molecule flow region and the transition flow region.
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