Prevention of overload in high-vacuum systems

Abstract

High-vacuum pumps, like other compressors, have basic limitations in regard to the maximum pressure difference (and pressure ratio) and maximum mass (and volume) flow rates that they can produce. Because high-vacuum pumps are usually made to discharge into another pump, their tolerable discharge pressure must be associated with the characteristics of the backing pump. However, it is also necessary to coordinate the performance of the high-vacuum pump with the performance of the pumping system used for pre-evacuation of the vacuum chamber. The traditional concept of a single absolute value for the crossover pressure used for the initiation of high-vacuum pumping is fundamentally incorrect because it is not based on a clear mass flow limitation. To prevent overloading high-vacuum pumps during and immediately after switching from pre-evacuation to high-vacuum pumping, a simple rule must be observed: the crossover must be performed when the gas mass flow from the vacuum chamber is less than the maximum throughput capacity of the high-vacuum pump. Typically, at the end of the pre-evacuation period, there are two somewhat distinct gas quantities associated with the vacuum chamber, the gas in the space of the chamber and the quasisteady outgassing rate. There are distinct pressure decays associated with those two gas quantities. The overloading of the high vacuum pump due to the space gas can be prevented by opening the high-vacuum valve slowly or by using a parallel low-conductance bypass. However, the overload due to the outgassing rate can only be prevented by following the golden rule of mass flow limitation. An immediate corollary of matching mass flows is that the larger the roughing pump, the lower the crossover pressure must be. In capture pumps, the maximum throughput value for the crossover condition must be correlated with the period of regeneration (for cryopumps) or cathode replacement (for ion pumps).