A vacuum pump is a device that uses various methods to create, improve and maintain a vacuum in a closed space. A vacuum pump can be defined as a device or equipment that uses mechanical, physical, chemical or physical-chemical methods to pump a container to obtain a vacuum. With the development of vacuum applications, the types of vacuum pumps have developed many kinds, and their pumping speeds have risen from a few tenths of a second to hundreds of thousands or millions of liters per second. The ultimate pressure (ultimate vacuum) ranges from coarse vacuum to higher vacuum above 10-12 Pa.
Due to the wide range of working pressures involved in the vacuum application department, it is impossible for any type of vacuum pump to be fully applicable to all working pressure ranges. Different types of vacuum pumps can only be used according to different working pressure ranges and different working requirements. In order to facilitate the use and the needs of various vacuum processes, sometimes various vacuum pumps are combined according to their performance requirements and applied in unit type.
Where the use of mechanical movement (rotating or sliding) to obtain a vacuum pump, called a mechanical vacuum pump. Mechanical vacuum pump according to its working principle and structural characteristics are as follows:
1. Variable volume vacuum pump
It is a vacuum pump that uses the periodic change of the volume of the pump chamber to complete the suction and exhaust to achieve the purpose of pumping. The gas is compressed before exiting the pump chamber. This pump is divided into reciprocating and rotary two.
(1) Reciprocating vacuum pump
The reciprocating movement of the piston in the pump chamber is used to inhale, compress and discharge the gas. Also known as piston vacuum pump.
(2) Rotary vacuum pump
The rotary motion of the rotor member in the pump chamber is used to suck, compress and discharge the gas. It roughly has the following classification:
a. Oil-sealed vacuum pump
It is the use of vacuum pump oil to seal the gap between the moving parts in the pump to reduce the harmful space in the pump, a rotary variable capacity vacuum pump. This pump is usually equipped with a gas ballast device. It mainly includes rotary vane vacuum pump, fixed vane vacuum pump, slide valve vacuum pump, residual cycloid vacuum pump, etc.
B, liquid ring vacuum pump
Install the rotor with multiple blades eccentrically in the pump casing. When it rotates, the working liquid is thrown to the pump casing to form a liquid ring concentric with the pump casing. The liquid ring and the rotor blades form several small rotating variable volume suction and exhaust cavities with periodic changes in volume. The working liquid is usually water or oil, so it is also called water ring vacuum pump or oil ring vacuum pump.
C, dry vacuum pump
It is a kind of pump without oil (or liquid) sealed variable capacity vacuum pump. Since the dry vacuum pump does not require working liquid in the pump chamber, it is suitable for the semiconductor industry, chemical industry, pharmaceutical industry and food industry that require oil-free clean vacuum environment.
D, Roots vacuum pump
The pump is equipped with two opposite directions of synchronous rotation of the double-leaf or multi-leaf rotor. A certain gap is maintained between the rotor and the inner wall of the pump casing.
2. Momentum transfer pump
It relies on high-speed rotating blades or high-speed jets to transfer momentum to the gas or gas molecules, so that the gas is continuously transmitted from the inlet to the outlet of the pump. This type of pump can be divided into the following forms:
(1) Molecular vacuum pump
It is the use of high-speed rotating rotor to transfer momentum to the gas molecules, so that the directional speed, which is compressed, driven to the exhaust port for the front stage of a vacuum pump. This pump can be specifically divided:
a. Traction molecular pump
The gas molecules collide with the high-speed rotor to gain momentum and are driven to the outlet of the pump.
B, turbo molecular pump
By high-speed rotating moving blades and stationary fixed blades cooperate with each other to achieve pumping. Such pumps typically operate in a molecular flow regime.
C, compound molecular pump
It is a kind of compound molecular vacuum pump which is composed of two kinds of molecular pumps of turbine type and traction type in series.
Mechanical vacuum pump is a common type of pump used in vacuum applications, and it is an important part of vacuum equipment. Its detailed classification is shown in Figure 1.

Selection of vacuum pump
The function of the vacuum pump is to remove gas molecules from the vacuum chamber, reduce the gas pressure in the vacuum chamber, and make it reach a certain degree of vacuum. Generally speaking, there is a large range from atmosphere to extremely high vacuum, and so far no vacuum pump can cover this range. Therefore, in order to achieve the process index of different products, work efficiency and equipment working life requirements, different vacuum sections need to choose different vacuum pump configuration. In order to achieve the ideal configuration, the following points should be considered when selecting the vacuum system:
Determining Operating Vacuum Range
The vacuum level required for each process must first be checked. Because each process has its own vacuum range.
Determining the ultimate vacuum degree
On the basis of determining the vacuum degree required by the process, the ultimate vacuum degree of the vacuum pump system is checked, because the ultimate vacuum degree of the system determines the ideal working vacuum degree of the system. Generally speaking, the ultimate vacuum degree of the system is 20% lower than the working vacuum degree of the system and 50% lower than the ultimate vacuum degree of the front pump.
The type of gas to be pumped and the amount of gas pumped
Check and determine the type and amount of extraction required by the process. Because if the gas species to be pumped reacts with the liquid in the pump, the pump system will be contaminated. At the same time, it must be considered to determine the appropriate exhaust time and the amount of gas generated during the extraction process.
Vacuum volume
Check the time required to reach the required vacuum, the flow resistance and leakage of the vacuum pipe.
Consider the pumping rate required to maintain vacuum under certain process requirements after reaching the required vacuum.
Selection calculation of main vacuum pump s = tlog(p1/p2)
Where: s is the pumping rate of the vacuum pump (l/s)
v is the volume of the vacuum chamber (l)
t is the time required to reach the required vacuum (s)
p1 is the initial vacuum (torr)
p2 is the required degree of vacuum (torr).

Vacuum pump selection need to pay attention to the problem
1. Does the vibration generated by the vacuum pump affect the process and the environment. If the process is not allowed, should choose no vibration pump or take anti vibration measures.
2. Understand the composition of the pumped gas, whether the gas contains condensable vapor, whether there is particle dust, whether there is corrosion, etc. When selecting a vacuum pump, you need to know the gas composition and select the corresponding pump for the pumped gas. If the gas contains steam, particles, and corrosive gases, it should be considered to install auxiliary equipment on the inlet pipeline of the pump, such as condenser, dust collector, etc.
3. The vacuum pump should be able to discharge all the gas generated during the process of vacuum equipment under its working pressure.
4, the correct combination of vacuum pump. Because the vacuum pump has selective pumping, sometimes a pump can not meet the pumping requirements, and several pumps need to be combined to complement each other to meet the pumping requirements. Such as titanium submersion pump for hydrogen has a high pumping speed, but can not pump helium, and three-pole type sputtering ion pump, (or two-pole type asymmetric cathode sputtering ion pump) for argon has a certain pumping speed, the combination of the two, will make the vacuum device to get a better degree of vacuum. In addition, some vacuum pumps cannot work at atmospheric pressure and require pre-vacuum; some vacuum pumps have outlet pressures lower than atmospheric pressure and require pre-pumps, so they all need to be combined.
5. Requirements of vacuum equipment for oil pollution. If the equipment is strictly required to be oil-free, various oil-free pumps should be selected, such as water ring pump, molecular sieve adsorption pump, sputtering ion pump, cryogenic pump, etc. If the requirements are not strict, you can choose an oil pump, plus some anti-oil pollution measures, such as adding cold traps, baffles, oil traps, etc., can also meet the requirements of clean vacuum.
Correctly select the working point of the vacuum pump. Each pump has a certain working pressure range, such as: 2BV series water ring vacuum pump working pressure range 760mmHg ~ 25mmHg (absolute pressure), in such a wide pressure range, the pumping speed of the pump changes with the pressure (refer to the pump performance curve for detailed changes), and its stable working pressure range is 760~60mmHg. Therefore, the working point of the pump should be selected within this range, and it should not be allowed to work for a long time under 25~30mmHg.
7. How does the oil vapor discharged from the vacuum pump affect the environment. If the environment does not allow pollution, you can choose an oil-free vacuum pump, or the oil vapor discharged to the outdoor.
The working pressure of the vacuum pump should meet the requirements of the ultimate vacuum and working pressure of the vacuum equipment. For example, a vacuum drying process requires a working vacuum degree of 10mmHg, and the ultimate vacuum degree of the selected vacuum pump should be at least 2mmHg, ideally reaching 1mmHg. Usually select the ultimate vacuum of the pump to be higher than the vacuum equipment working vacuum half to an order of magnitude.
9. The price, operation and maintenance cost of the vacuum pump.

1. Vacuum
The state of a gas below the ambient atmospheric pressure in a designated space.
2. Vacuum degree
Indicates the degree of rarefaction of the gas in a vacuum state, usually expressed by a pressure value.
3. Division of vacuum area
Low vacuum 105Pa-102Pa
Medium vacuum 102Pa-10-1Pa
High vacuum 10-1Pa-10-5Pa
High vacuum 10-5Pa-10-9Pa
Extremely high vacuum <10-9Pa
4. Full pressure
The combination of the pressures of all the components in the gas mixture.
5. Sub-pressure
The pressure of a component of a gas mixture.
6. Standard environmental conditions
The temperature is 20 ℃, the relative humidity is 65%, and the atmospheric pressure is 1 standard atmospheric pressure 1atm = 101325Pa.
7. Standard state of gas
The temperature is 0 ℃, the pressure is 1 standard atmospheric pressure 1atm = 101325Pa.
8. Average free course
The distance traveled by a molecule in every two consecutive collisions with other gas molecules is called the free path. The average of quite a number of different free paths, called
Mean free path.

9. Viscous flow
The mean free path of the gas molecules is much smaller than the flow regime of the smallest cross-sectional dimension of the conduit. Therefore, the flow depends on the viscosity of the gas, viscous flow can be
It is laminar or stagnant.
10. Molecular Flow
The mean free path of the gas molecules is much larger than the flow regime for the largest dimension of the conduit cross-section.
11. Intermediate Flow
The flow of gas through a conduit in a state between viscous flow and molecular flow.
12. Flow
The amount of gas passing through a section in a given time interval divided by that time.
13. Adsorption
The trapping of a gas or vapor by a solid or liquid.
14. Desorption
The release of gases or vapors adsorbed by a material.
15. Gone.
The desorption of gas from the material is considered.
16. Bleed
Natural desorption of gas from the material.
17. Penetration
The phenomenon of gas passing through a solid barrier.