Some processes in chemical production often need to be carried out under the condition of sub-atmospheric pressure. A vacuum pump is a mechanical device that obtains an absolute pressure lower than atmospheric pressure. There are many structural types of vacuum pumps. Several types of vacuum pumps commonly used in chemical plants are introduced as follows.

1. Reciprocating vacuum pump
The working principle of the reciprocating vacuum pump is basically the same as that of the reciprocating compressor, and the structural difference is not big, but the valve used must be lighter and easier to open and close. When the required vacuum degree is high, the compression ratio is large, and the influence of the residual gas in the clearance is great, so the clearance of the vacuum system must be small. In order to reduce the influence of the backlash, a balance air passage is set between the left and right ends of the vacuum pump cylinder. When the piston is exhausted, leave the balance air passages open for a short period of time, allowing the gas remaining in the clearance to flow from one side of the piston to the other, thereby reducing its pressure.

2. Water ring vacuum pump
There is an eccentrically mounted impeller in the circular shell, and the pump contains a certain amount of water. When the rotor rotates, due to the centrifugal force, the water is thrown to the shell wall to form a water ring. This water ring acts as a seal and closes the gap between the blades into many small cells of different sizes. When the rotor rotates to the right half, the volume of the sealed chamber gradually expands, and the gas is inhaled from the suction port on the right; when the rotor rotates to the left half, the volume of the chamber gradually shrinks, and the gas is discharged from the exhaust port on the left.

The air volume formed by the water ring vacuum pump is limited by the water temperature, generally reaching about 83.4x103Pa. The pump can also be used as a blower, and the generated pressure (gauge pressure) does not exceed 98.07x103Pa.

The water ring vacuum system has a simple and compact structure, is easy to manufacture and maintains, and has a long service life because the rotating part has no mechanical friction.

Water ring vacuum pumps are suitable for conveying corrosive or explosive gases. The disadvantage is that the efficiency is low, generally 30~50%.

3. Water jet vacuum pump
If the working fluid of the jet pump is water, it is called a water jet pump; if it is water vapor, it is called a steam jet pump.

The pressure of the single-stage steam jet pump can reach 0.01MPa. To achieve a higher degree of vacuum, a multi-stage jet system can be used.

The advantages of jet vacuum pump are wide working pressure range, large pumping capacity and simple structure. The disadvantage is that the efficiency is very low.