Vacuum pump

Vacuum pump

A device that reduces the pressure of a gas (usually air) in a container. When gas in a closed container is lowered from atmospheric pressure, the operation constitutes an increase in vacuum in this container.

Vacuum pumps are evaluated for the degree of vacuum they can attain and for how much gas they can pump in a unit of time. In practice, where high vacuum is required, two or more different types of pumps are used in series.

In the rotary oil-seal pump (see illustration), gas is sucked into chamber A through the opening intake port by the rotor. A sliding vane partitions chamber A from chamber B. The compressed gas that has been moved from position A to position B is pushed out of the exit port through the valve, which prevents the gas from flowing back. The valve and the rotor contact point are oil-sealed. Since each revolution sweeps out a fixed volume, it is called a constant-displacement pump. Other mechanical pumps are the rotary blower pump, which operates by the propelling action of one or more rapidly rotating lobelike vanes, and the molecular drag pump, which operates at very high speeds, as much as 16,000 rpm. Pumping is accomplished by imparting a high momentum to the gas molecules by the impingement of the rapidly rotating body.

The water aspirator is an ejector pump. When water is forced under pressure through the jet nozzle, it will force the gas in the inlet chamber to go through the diffuser, thus lowering the pressure in the inlet chamber. When high-pressure steam is used instead of water, it is called a steam ejector.

In addition, a number of pumps have been developed which meet special pumping requirements. The ion pump operates electronically. Electrons that are generated by a high voltage applied to an anode and a cathode are spiraled into a long orbit by a high-intensity magnetic field. These electrons colliding with gas molecules ionize the molecules, imparting a positive charge to them. These are attracted to, and are collected on, the cathode. Thus a pumping action takes place. Sorption pumping is the removal of gases by adsorbing and absorbing them on a granular sorbent material such as a molecular sieve held in a metal container. Cryogenic pumping is accomplished by condensing gases on surfaces that are at extremely low temperatures.