In petroleum and chemical production, certain production processes are often carried out at sub-atmospheric pressure. Any pressure lower than atmospheric pressure is called vacuum, and vacuum degree is commonly used to indicate the degree of lower than atmospheric pressure. The machine used to suck gas from equipment below atmospheric pressure is called vacuum pump. In the petroleum industry, vacuum pumps are used for vacuum distillation, lubricating oil denitrification, furfural recovery, etc. In the chemical industry, they are used for vacuum filtration, evaporative concentration, drying, and crystallization of solutions. Vacuum pumps can also be used as large centrifugal pumps for water diversion and irrigation.

Mechanical vacuum pumps can be divided into three categories: reciprocating type, rotary type (slide type) and water ring type according to their structure.

Any vacuum pump generally uses the following two main parameters to express its working performance
(1) Pumping rate refers to the volume of gas sucked by the vacuum pump from the intake pipe under the residual pressure per unit time, that is, the production capacity of the vacuum pump (or V), expressed in m³ or L/s.

(2) Residual pressure or ultimate vacuum refers to the lowest pressure (absolute) that the pump can achieve.

A certain vacuum pump is used to pump the gas in a closed container, no matter how long the pumping time is, the pressure in the container cannot be infinitely reduced to zero (ie, absolute vacuum). This is because when the intake pressure is lower than a certain value, either because the liquid in the pump is vaporized, or because the air plate leaking back from the high-pressure side is the same as the suction of the vacuum pump. Or because the compression ratio of the vacuum pump is too high , the volume coefficient is reduced to zero. It will make the pump unable to continue to inhale fresh gas. In this case, the pressure in the container will never drop again. The absolute pressure value at this time is called residual pressure or ultimate vacuum degree.