ABSTRACT
Turbomolecular pumps are essentially axial-flow compressors designed for pumping rarefied gases. Original designers adapted more or less traditional axial-flow compressor stage arrangements using mathematical modeling that was based on studies of molecular trajectories inside alternating rotating and stationary blade rows. The maximum possible pumping speed is obviously limited by the cross sectional area of the pumping channel and its conductance, and by the velocity of the moving surface or the rotor. Modern turbomolecular pumps of an open, thin-bladed, axial-flow type appeared in the early sixties following the publication of the article on the performance of an axial flow compressor under rarefied gas conditions. Turbomolecular pumps are employed in the same pressure region as that of vapor jet pumps. Throughout most of a turbomolecular pump, molecular flow conditions prevail, but at the point of discharge the flow is usually in the transition region and may even approach the low end of the viscous flow region.
