The gas ballast facility as used in the vacuum pumps permits not only pumping of permanent gases but also even larger quantities of condensable gases. The gas ballast facility prevents condensation of vapors in the pump chamber of the pump. When pumping vapors these may only be compressed up to their saturation vapor pressure at the temperature of the pump. If pumping water vapor, for example, at a pump temperature of 70 °C, the vapor may only be compressed to 312 mbar (saturation vapor pressure of water at 70 °C). When compressing further, the water vapor condenses without increasing the pressure. No overpressure is created in the pump and the exhaust valve is not opened. Instead the water vapor remains as water in the pump and emulsifies with the pump’s oil. This very rapidly impairs the lubricating properties of the oil and the pump may even seize when it has taken up too much water. The gas ballast facility developed in 1935 by Wolfgang Gaede inhibits the occurrence of condensation of the vapor in the pump as follows. Before the actual compression process begins , a precisely defined quantity of air (“the gas ballast”) is admitted into the pumping chamber of the pump. The quantity is such that the compression ratio of the pump is reduced to 10:1 max. Now vapors which have been taken in by the pump may be compressed together with the gas ballast, before reaching their condensation point and ejected from the pump. The partial pressure of the vapors which are taken in may however not exceed a certain value. It must be so low that in the case of a compression by a
factor of 10, the vapors can not condense at the operating temperature of the pump. When pumping water vapor this critical value is termed the “water vapor tolerance”.. Two requirements must be met when pumping vapors:
- The pump must be at operating temperature.
- The gas ballast valve must be open.
(With the gas ballast valve open the temperature of the pump increases by
about 10 °C. Before pumping vapors the pump should be operated for half
an hour with the gas ballast valve open).
Fig. 2.13 Working process within a rotary vane pump with gas ballast,
Fig. 2.14 Diagram of pumping process in a rotary vane pump without (left) and with (right) gas
ballast device when pumping condensable substances.