The turbomolecular pump, its design, operation and theory; calculation of the pumping speed for various gases and their dependence on the forepump

Abstract

The turbomolecular pump is a mechanical ultrahigh-vacuum pump. Its design is explained and compared with the molecular pump of Gaede. The equations of Gaede for the molecular pump are used in a modified form for the turbomolecular pump. taking into consideration also the internal losses due to back-diffusion and leakage. By this means the pressure ratio of the pump can be calculated for different gases and different speeds of revolution. The calculated values are then compared with the values found by experiment. The logarithm of the pressure ratio is proportional to the square root of the molecular weight. The pressure ratio for mass 120 is 1016. For heavier molecules, eg oil vapour, the value of the pressure ratio is so high that it cannot be measured even with the most modern instruments. Furthermore, it is shown that the pressure ratio can be ascertained in relation to the pumping speed for the gas admitted. A procedure is then indicated, for calculating from this curve the pumping speed for a molecular pump in combination with a backing pump of known speed. If the backing pump is of adequate size the pumping speed for the light gases is somewhat higher than that for heavy gases. On the other hand, if the backing pump is to small the pumping speed for hydrogen decreases more than that for the heavier gases. The curves given are valid only in the molecular flow region; the pumping speed for the turbomolecular pump decreases in the transition range and in that of laminar flow. Finally a short summary of the applications of turbomolecular pumps is presented.