Rotary vane oil-sealed mechanical pump

A volume of gas is enclosed in the space bounded by the rotor, the stator and the two vanes (see Fig. 1.6). The pump removes the gas by compressing it to a pressure slightly higher than the atmospheric pressure. This overpressure opens the spring-loaded outlet valve, and the gas escapes to the atmosphere. A thin film of oil makes the final seal therefore the ultimate pressure depends also on the oil vapour pressure. With one stage, the lowest attainable pressure is about 10 2torr and with two stages in series 10-3 torr. 

Due to the friction of the sliding vanes, the biggest pump size available (water cooled) is about 4001/s. Smaller pumps (0.5-51/s) are air cooled. 

Condensation of vapours must be avoided since it causes a deterioration of the oil water vapour condense at 234 torr at 70°C, which is the typical working temperature of the pump. To avoid condensation, most pumps have a ballast valve in the high-pressure stage. After the gas has been closed off from the inlet, some gas is bled in through the ballast valve. This causes the discharge valve to open sooner, with a reduction of the compression ratio. Rotary piston pumps have similar characteristics. 

Most mechanical pumps exhibit vibration that may represent a crucial drawback for very low-temperature refrigerators. 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

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Fig. 1.6. Scheme of a rotary vane oil-sealed mechanical pump. 

Mechanical pumps are used directly to produce a low and medium vacuum, as well as extensively to back Roots vacuum, turbomolecular and diffusion pumps. These pumps are also called oil-sealed rotary vane pumps as they rely on the use of vanes or blades to compress gases. 

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