Vacuum Acquisition Technology
It is common to refer to the equipment that can expel gas from a closed container or make the number of gas molecules in the container decrease continuously as vacuum obtaining equipment or vacuum pump. At present in vacuum technology, using different methods, it has been possible to obtain and measure a pressure range from 105 Pa to 10-13 Pa at atmospheric pressure, up to 18 orders of magnitude wide. Obviously, it is very difficult to obtain such a wide range of gas states in low pressure space using only one vacuum pump. In the vacuum acquisition technology, there are two technical methods used to obtain vacuum, one is to discharge the gas directly from the closed container by the movement of a certain mechanism; the other is to adsorb or condense the gas molecules on the cryogenic surface by physical and chemical methods. There are many kinds of vacuum pumps made by these two methods and different classification methods, however, the most common method is to classify them according to the working principle of the pump or its structural characteristics.
Classification of vacuum pumps
According to the working principle of vacuum pumps, vacuum pumps can be basically divided into two types, namely gas transfer pumps and gas trapping pumps. As the application of vacuum technology in the field of production and scientific research requires wider and wider pressure range, most of them need to be pumped by several vacuum pumps to meet the requirements of production and scientific research process, so there are more cases of using vacuum pumping units composed of different types of vacuum pumps for pumping. For the sake of convenience, it is necessary to classify these pumps according to their working principle or their structural characteristics. They are classified as follows.
1. Gas transfer pump
Gas transfer pump is a kind of vacuum pump which can make the gas continuously inhaled and discharged to achieve the purpose of pumping, there are basically two types of such pumps.
1) Variable volume vacuum pump
Variable volume vacuum pump is a kind of vacuum pump that uses the periodic change of pump chamber volume to complete the process of suction and discharge. The gas is compressed before discharge. This kind of pump is divided into two types: reciprocating and rotary type.
(1) Reciprocating vacuum pump: It uses the reciprocating motion of the piston in the pump chamber 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 make rotating movement to suck in, compress and discharge the gas. There are several types of rotary vacuum pumps as follows.
(1) Oil-sealed vacuum pump: It is a kind of rotary variable capacity vacuum pump that uses oil to seal the gap between the moving parts to reduce the harmful space. This kind of pump usually has gas ballast device, so it is also called gas ballast vacuum pump. According to its structural characteristics, it is divided into the following five types.
a) Rotary vane vacuum pump: the rotor is installed in the pump casing with a certain eccentric distance and close to the fixed surface of the inner surface of the pump casing, two (or more) rotary vane are installed in the rotor groove, when the rotor rotates, the rotary vane can slide along its radial groove and always contact with the inner wall of the pump casing, this rotary vane rotates with the rotor and can divide the pump cavity into several variable volumes.
b) Slide-valve vacuum pump: a slide valve is installed outside the eccentric rotor, the rotor rotates and drives the slide valve to slide and roll along the inner wall of the pump casing, the slide valve rod on the upper part of the slide valve can slide in the swingable slide valve guide, and the pump cavity is divided into two variable volumes.
c) Constant vane vacuum pump: There is an eccentric rotor inside the pump casing which is close to the inner surface of the pump, and there is a radial slide on the pump casing which is always in contact with the surface of the rotor, when the rotor rotates, the slide can slide up and down to divide the pump cavity into two variable volumes.
d) Remaining cycloid vacuum pump: A rotor with the shape of remaining cycloid is eccentrically installed in the pump chamber, which rotates along the inner wall of the pump chamber and divides the pump chamber into two variable volumes.
e) Multi-chamber rotary vane vacuum pump: it is a rotary vane vacuum pump with several independent workshops driven by the same motor in parallel in one pump casing.
②Dry vacuum pump: it is a kind of variable volume vacuum pump without oil (or liquid) seal.
③Liquid ring vacuum pump: the rotor with multiple vanes is eccentrically installed in the pump casing, when it rotates, the liquid (usually water or oil) is thrown to the pump casing to form the concentric liquid ring, the liquid ring forms several small volumes with the rotor vanes, so it is also called rotary variable capacity vacuum pump.
④Roots vacuum pump: the pump is equipped with two double or multi-vane rotors rotating synchronously in opposite directions, and a certain clearance is maintained between the rotors and the inner wall of the pump casing. It is a rotary variable vacuum pump. Mechanical booster pump is this type of vacuum pump.
2)Momentum transfer pump
This kind of pump relies on high speed rotating vane or high speed jet to transfer momentum to the gas or gas molecules, so that the gas is continuously transferred from the inlet to the outlet of the pump. Specifically, it can be divided into the following types.
(1) Molecular vacuum pump: It is a kind of vacuum pump that uses a high-speed rotor to transfer energy to gas molecules to compress and exhaust. It has the following types.
①Traction molecular pump: the gas molecules collide with the rotor in high speed and get momentum, which is sent to the outlet, so it is a kind of momentum transfer pump.
②Turbine molecular pump: the pump is equipped with a slotted disc or a rotor with blades, which rotates between the stator discs (or stator blades). The linear velocity of the rotor circumference is high. This pump usually works in the molecular flow state.
③Compound molecular pump: It is a compound molecular vacuum pump combined with two kinds of molecular pumps in series, turbine type and traction type.
(2) Jet vacuum pump: It is a kind of momentum transfer pump which uses the high speed jet generated by the pressure drop of Venturi effect to transport the gas to the outlet, suitable for working in the viscous flow and transition flow state. This pump can be further divided into the following categories in detail.
① Liquid jet vacuum pump: jet vacuum pump with liquid (usually water) as working medium.
②Gas jet vacuum pump: Jet vacuum pump with non-condensable gas as working medium.
③Vapor jet vacuum pump: jet vacuum pump with vapor (water, oil or mercury vapor, etc.) as working medium.
(3) Diffusion pump: jet vacuum pump with low pressure and high speed vapor stream (vapor such as oil or mercury) as working medium. The gas molecules are diffused into the vapor jet and sent to the outlet. The density of gas molecules in the jet is always very low, and this kind of pump is suitable for working in the molecular flow state. It can be divided into.
① Self-purifying diffusion pump: an oil diffusion pump in which volatile impurities in the pump fluid are transported to the outlet by special machinery and not returned to the boiler.
② Fractionated diffusion pump: this pump has a fractionation device, so that the working fluid vapor with lower vapor pressure enters the nozzle of high vacuum work, while the working fluid vapor with higher vapor pressure enters the nozzle of low vacuum work, it is a multi-stage oil diffusion pump.
(4) Diffusion jet pump: It is a kind of momentum transfer pump composed of a single-stage or multi-stage nozzle with diffusion pump characteristics and a single-stage or multi-stage nozzle with jet vacuum pump characteristics in series. Oil booster pump that belongs to this type.
(5) Ion transfer pump: It is a kind of momentum transfer pump that delivers the ionized gas to the outlet under the action of electromagnetic field or electric field.
2. Gas trapping pump
This kind of pump is a kind of vacuum pump which makes gas molecules be adsorbed or condensed on the inner surface of the pump, thus reducing the number of gas molecules in the container and achieving the purpose of pumping, there are several types as follows.
1) Adsorption pump
It mainly relies on the physical adsorption of adsorbent with large surface (such as porous material) to pump the gas, a kind of trapping vacuum pump.
It is a kind of vacuum pump that uses suction agent to capture gas by chemical combination. The suction agent is usually a metal or alloy in the form of block or deposited fresh film. The sublimation pump is of this type.
3) Absorbent ion pump
It is to make the ionized gas adsorbed on the surface of the absorbent material by the action of electromagnetic field or electric field to achieve the purpose of pumping. It has several types as follows.
(1) Evaporation ion pump: the pump is ionized gas adsorbed on the suction material which is sublimated (or evaporated) in an intermittent or continuous manner and covered on the inner wall of the pump in order to achieve pumping of a vacuum pump.
(2) Sputtering ion pump: The ionized gas in the pump is adsorbed on the suction material dispersed by continuous sputtering from the cathode to achieve the purpose of pumping a vacuum pump.
4)Low temperature pump
A vacuum pump that uses low temperature surface to trap gas
3.Performance parameters and scope of use of vacuum pump
Performance parameters of vacuum pump
1)Ultimate pressure of vacuum pump
The ultimate pressure of the pump is Pa, which is the lowest pressure that tends to stabilize when the pump is equipped with a standard test hood at the entrance and works according to the specified conditions without introducing gas to work normally.
2)Pumping rate of vacuum pump
Pump pumping rate in m3/s or l/s is the ratio of the gas flow from the test hood to the equilibrium pressure measured at the specified position of the test hood when the pump is equipped with a standard test hood and works under the specified conditions. It is abbreviated as pumping speed of the pump.
3) Pumping volume of vacuum pump
The pumping capacity of a vacuum pump is Pa m3/s or Pa l/s, which is the gas flow rate at the pump inlet.
4)Starting pressure of vacuum pump
The starting pressure of vacuum pump is in Pa, which is the pressure when the pump starts without damage and has pumping effect.
5)Pre-stage pressure of the pump
The pre-stage pressure of vacuum pump is in Pa, it is the outlet pressure of vacuum pump whose exhaust pressure is lower than one atmosphere.
6)The maximum pre-stage pressure of vacuum pump
The maximum pre-stage pressure of vacuum pump is Pa, it means the pre-stage pressure which can make the pump damaged.
7)The maximum working pressure of vacuum pump
The maximum working pressure of vacuum pump is Pa, which is the inlet pressure corresponding to the maximum pumping volume. Under this pressure, the pump can work continuously without deterioration or damage.
8) Compression ratio
The compression ratio is the ratio of the outlet pressure of the pump to the inlet pressure for a given gas.
The ratio of the actual pumping speed on the pump pumping channel area to the theoretical pumping speed calculated by molecular diarrhea at that place.
10) Pumping speed coefficient
The ratio of the actual pumping speed and the theoretical pumping speed calculated by molecular diarrhea at the pump inlet.
11) Return rate
The pump reflux rate in g/cm2.s. It refers to the pump flow mass flow rate through the pump inlet unit area when the pump works according to the specified conditions.
12)Allowable amount of water vapor
The allowable amount of water vapor unit is kg/h. It refers to the mass flow rate of water vapor that can be pumped out by the gas town pump in continuous operation under normal environmental conditions.
13) Maximum allowable water vapor inlet pressure
The maximum allowable water vapor inlet pressure unit is Pa. It refers to the highest inlet pressure of water vapor that can be pumped out by the gas ballast pump in continuous operation under normal environmental conditions.
The role played by the pump in different working fields is summarized as follows
1) Being the main pump in the system
The so-called main pump is the vacuum pump that directly pumps the pumped vessel of the vacuum system to obtain the vacuum level required to meet the process requirements.
Rough pumping pump is a vacuum pump that starts from atmospheric pressure to reduce the pressure of vacuum system to reach another pumping system can start working.
Pre-stage pump is a vacuum pump used to maintain the pre-stage pressure of another pump below its maximum permitted pre-stage pressure.
Maintenance pump is a pump that can't effectively use the main pre-stage pump when the vacuum system is pumping very little air. For this reason, another auxiliary pre-stage pump with smaller pumping speed is used in the vacuum system to maintain the normal work of the main pump or to maintain the low pressure required for the evacuated container.
5)Coarse vacuum pump or low vacuum pump
Coarse or low vacuum pump is a vacuum pump that starts from the atmosphere and works in the range of low or coarse vacuum pressure after reducing the pressure of the pumped container.
6)High vacuum pump
High vacuum pump refers to the vacuum pump working in high vacuum range.
7)Ultra-high vacuum pump
Ultra-high vacuum pump refers to the vacuum pump working in ultra-high vacuum range.
Booster pump usually refers to the vacuum pump working between low vacuum pump and high vacuum pump to improve the pumping volume of pumping system in the middle pressure range or reduce the pumping rate requirement of the former pump.
Model and specification of vacuum pump
According to the standard JB/T7673-95 of machinery industry of the People's Republic of China, the domestic vacuum pumps are composed of two parts: basic model and auxiliary model, with a horizontal line in the middle. The expression type is 123-456. The number 123 in the grid indicates the basic model and 456 indicates the auxiliary model.
The models of domestic vacuum pumps are usually expressed by the hanyu pinyin letters in Table 3-2. If there is "2" in front of the pinyin letter, it means the pump is double-stage pump in structure. The pumping rate of some vacuum pump series is graded by geometric steps. The unit is "L/S". There are 18 grades, namely 0.2, 0.5, 1, 2, 4, 8, 15, 30, 70, 150, 300, 600, 1200, 2500, 5000, 10000, 20000, 40000. ".