Any pressure below atmospheric pressure is called a vacuum. The degree of vacuum is the degree of lower than atmospheric pressure, generally expressed as a percentage or a millimeter of water column (or mercury column). If the vacuum degree of a certain equipment is 80%, it means that the pressure in the equipment is 20% of the atmospheric pressure, that is, when the atmospheric pressure is 1 kg/cm², the pressure in the equipment is 0.2 kg/cm². Therefore, when the degree of vacuum is expressed as a percentage, it expresses the degree of gas extraction. When expressed in mm water column, it means its residual pressure, if the vacuum degree in the equipment is 3x10-2 mm water column, it means that the pressure in the equipment (that is, the residual pressure) is 3x10-2 mm water column.

In chemical plants, some production processes are often carried out under sub-atmospheric pressure, such as vacuum evaporation, vacuum distillation and vacuum suction filtration.

In order to achieve a vacuum, a device for extracting gas is required - a vacuum pump. Vacuum pumps can be roughly divided into reciprocating, rotary, and jet pumps.
1. Reciprocating vacuum pump
The working principle of the reciprocating vacuum pump is exactly the same as that of the reciprocating compressor, except that the air distribution mechanism, that is, the intake and exhaust valves, is different. Spring valves are generally used in reciprocating compressors, while so-called spool valve air distribution mechanisms are mostly used in reciprocating vacuum pumps.

2. Water ring vacuum pump
Water ring vacuum pump is a kind of rotary vacuum pump, which can be used for compressing gas in addition to suction gas.