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According to the working principle of vacuum pump, vacuum pump can be divided into two types: gas transfer pump and gas trap pump. With the application of vacuum technology in the field of production and scientific research, its application pressure range is more and more wide. Most of them need several kinds of vacuum pumps to form a vacuum pumping system to meet the requirements of production and scientific research process. Therefore, the vacuum pumping units composed of different types of vacuum pumps are often used for air extraction. For the sake of convenience, it is necessary to classify these pumps according to their working principle or structural characteristics. They are as follows:

1、 Gas transfer pump

Gas transfer pump is a kind of vacuum pump which can make the gas continuously inhaled and discharged, so as to achieve the purpose of pumping

1) Variable volume vacuum pump

Variable volume vacuum pump is a kind of vacuum pump which uses the periodic change of pump chamber volume to complete the process of suction and exhaust. The gas is compressed before it is discharged. This kind of pump is divided into reciprocating and rotary type

(1) reciprocating vacuum pump: it uses the piston in the pump chamber to do reciprocating motion to suck in, compress and discharge the gas. Therefore, it is also called piston vacuum pump.

(2) rotary vacuum pump: it uses the piston in the pump chamber to do rotary movement to suck in, compress and discharge the gas. There are several types of rotary vacuum pump as follows:

① Oil sealed vacuum pump: it is a rotary variable volume vacuum pump which uses oil to seal the clearance between moving parts and reduce harmful space. This kind of pump is usually equipped with gas ballast device, so it is also called air ballast vacuum pump. According to its structural characteristics, it can be divided into the following five types.

a) Rotary vane vacuum pump: the rotor is installed in the pump shell with a certain eccentricity and close to the fixed surface of the inner surface of the pump shell. Two (or more) rotating blades are installed in the rotor groove. When the rotor rotates, the vane can slide back and forth along its radial groove and always contact with the inner wall of the pump shell. This vane rotates with the rotor, and the pump cavity can be divided into several variable volumes.

b) Slide valve vacuum pump: a slide valve is installed outside the eccentric rotor. The rotation of the rotor drives the slide valve to slide and roll along the inner wall of the pump housing. The slide valve rod on the upper part of the slide valve can slide in the swing slide valve guide rail, and the pump chamber is divided into two variable volumes.

c) Fixed vane vacuum pump: an eccentric rotor close to the inner surface of the pump is installed in the pump shell, and a radial sliding vane which always contacts with the rotor surface is installed on the pump shell. When the rotor rotates, the sliding vane can slide up and down, dividing the pump cavity into two variable volumes.

d) Cycloidal vacuum pump: a cycloidal rotor is eccentrically installed in the pump cavity, which rotates along the inner wall of the pump chamber and divides the pump cavity into two variable volumes.

e) Multi chamber rotary vane vacuum pump: a rotary vane vacuum pump with multiple independent working rooms driven by the same motor is installed in a pump shell in parallel.

② Dry vacuum pump: it is a variable volume vacuum pump without oil (or liquid) sealing.

③ Liquid ring vacuum pump: the rotor with multiple blades is eccentrically installed in the pump shell. When it rotates, the liquid (usually water or oil) is thrown to the pump shell to form a concentric liquid ring of the pump shell. The liquid ring and the rotor blade form several small volumes with periodic volume change, so it is also called rotary variable volume vacuum pump.

④ Roots vacuum pump: the pump is equipped with two opposite directions of synchronous rotation of the double blade or multi blade rotor, between the rotor and the inner wall of the pump shell to maintain a certain gap. It belongs to rotary variable vacuum pump. The supercharged pump is this type of vacuum pump.

2) Momentum transfer pump

This kind of pump relies on high-speed rotating blade or high-speed jet to transfer momentum to gas or gas molecules, so that the gas continuously transfers from the inlet to the outlet of the pump. It can be divided into the following types.

(1) molecular vacuum pump: it is a kind of vacuum pump which uses the high-speed rotating rotor to transfer energy to the gas molecules for compression and exhaust. It has the following types:

① Traction molecular pump: gas molecules collide with a high-speed rotor to obtain momentum, which is sent to the outlet. Therefore, it is a momentum transfer pump.

② Turbomolecular pump: the pump is equipped with a grooved disk or a rotor with blades, which rotates between the stator disks (or stator plates). The linear speed of the rotor circumference is very high. This kind of pump usually works in the state of molecular flow.

③ Compound molecular pump: it is a compound molecular vacuum pump which is composed of turbine type and traction type molecular pumps in series.

(2) jet vacuum pump: it is a momentum transfer pump that uses the high-speed jet generated by the pressure drop of Venturi effect to transport gas to the outlet, which is suitable for working in viscous flow and transition flow. This kind of pump can be divided into the following types in detail:

① Liquid jet vacuum pump: a jet vacuum pump with liquid (usually water) as working medium.

② Gas jet vacuum pump: a jet vacuum pump with non condensable gas as working medium.

③ Steam jet vacuum pump: a jet vacuum pump with steam (steam such as water, oil or mercury) as working medium.

(3) diffusion pump: jet vacuum pump with low pressure and high speed steam flow (oil or mercury vapor) as working medium. The gas molecules diffuse into the steam jet and are sent to the outlet. The density of gas molecules in the jet is always very low. This kind of pump is suitable for working in the state of molecular flow. It can be divided into:

① Self purification diffusion pump: the volatile impurities in the pump fluid are transported to the outlet by special machinery and do not return to an oil diffusion pump in the boiler.

② Fractionation diffusion pump: this kind of pump has fractionation device, which can make the working liquid vapor with low vapor pressure