A displacement vacuum pump is generally a vacuum pump in which the gas which is to be pumped is sucked in with the aid of pistons, rotors, vanes and valves or similar, possibly compressed and then discharged. The pumping process is effected by the rotary motion of the piston inside the pump. Differentiation should be made between oiled and dry compressing
displacement pumps. By the use of sealing oil it is possible to attain in a single-stage high compression ratios of up to about 10^5. Without oil, “inner leakiness” is considerably greater and the attainable compression ratio is correspondingly less, about 10.
As shown in the classification Table 2.1, the oil sealed displacement pumps include rotary vane and rotary plunger pumps of single and two-stage design as well as single-stage trochoid pumps which today are only of historic interest. Such pumps are all equipped with a gas ballast facility which was described in detail for the first time by Gaede (1935). Within specified engineering limits, the gas ballast facility permits pumping of vapors (water vapor in particular) without condensation of the vapors in the pump.
Table 2.1 Classification of vacuum pumps