General Knowledge Of Vacuum Pump

- Mar 13, 2019-

General knowledge of vacuum pump

 微信图片_20190321134200

1. The method of obtaining vacuum

 

The equipment which can discharge gas from the closed container or reduce the number of gas molecules in the container is called vacuum acquisition equipment or vacuum pump. At present, various methods have been used in vacuum technology to obtain and measure pressures ranging from 105Pa to 10-13pa, up to 18 orders of magnitude. Obviously, it is very difficult to obtain the gas state in such a wide low-pressure space with only one kind of vacuum pump.

 

In the technology of vacuum acquisition, there are two ways to obtain the vacuum at present. One is to discharge the gas directly from the closed container through the movement of a certain mechanism. The other is to adsorb or condense the gas molecules on the low-temperature surface by physical and chemical methods. There are many kinds of vacuum pumps manufactured by these two methods, and the classification methods are different. However, the most commonly used method is to classify the vacuum pumps according to their working principles or structural characteristics. This will be covered in the next section.

 vacuum c1

2. Classification of vacuum pumps

 

According to the working principle of vacuum pump, vacuum pump can be basically divided into two types, that is, gas transfer pump and gas capture pump. As vacuum application technology in the field of production and scientific research to the requirements of the application scope of the pressure is more and more wide, mostly need consists of several common after extraction of vacuum pump vacuum extraction system can meet the requirements of production and scientific research process, so choose different types of vacuum pump in the vacuum pump suction conditions more. For the sake of convenience, it is necessary to classify these pumps in detail according to their working principles or their structural characteristics. It is described 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. There are basically two types of this pump:

1) variable volume vacuum pump

Variable capacitance 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 being expelled. This pump is divided into reciprocating and rotary two types:

(1) reciprocating vacuum pump: is the use of the pump cavity piston reciprocating movement, the gas inhalation, compression and out. Therefore, also known as piston vacuum pump.

The rotary vacuum pump: is the use of the pump chamber piston to do rotation movement, the gas inhalation, compression and out. Rotary vacuum pump has the following types:

 

 oil seal vacuum pump: it is the use of oil seal between the moving parts of the gap, reduce the harmful space of a rotary variable capacity vacuum pump. This pump is usually equipped with air ballast device, it is also known as air ballast vacuum pump. According to its structural characteristics are divided into the following five types.

 

A) rotary vane vacuum pump: rotor with certain eccentricity installed in the pump shell and the pump shell on the surface of the fixed surface near the rotor slot is equipped with two (or more) rotary vane, can when the rotor rotating rotary-vane along its radial slot reciprocating sliding and contact with the pump casing wall always, the rotary vane with rotor rotation, the pump cavity can be divided into several variable displacement.

B) slide valve vacuum pump: a slide valve is installed outside the eccentric rotor. The rotor rotates to drive the slide valve to slide and roll along the inner wall of the pump shell. The slide valve stem on the upper part of the slide valve can slide in the swinging slide valve guide, and the pump chamber is divided into two variable volumes.

C) stator vane vacuum pump: an eccentric rotor which is close to the inner surface of the pump is installed in the pump shell, and a radial sliding vane which is always in contact with the surface of the rotor is installed on the pump shell. When the rotor rotates, the sliding vane can move up and down to divide the pump chamber into two variable volumes.

D) trochoidal vacuum pump: eccentrically equipped with a trochoidal rotor in the pump chamber, it 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: multiple independent studios rotary vane vacuum pumps driven by the same motor are arranged in parallel in a pump shell.

 

(2)Dry vacuum pump: it is a non - oil (or liquid) sealed variable volume vacuum pump.

(3) liquid ring vacuum pump: with many blades of the rotor eccentric installed in the pump shell, when it rotating, the liquid (usually water or oil) into the pump shell concentric liquid ring, liquid ring and rotor blade formed a few small volume volume period change, it is also called rotary variable capacity vacuum pump.

(4) roots vacuum pump: pump equipped with two opposite direction of synchronous rotation of the double blade shape or multi-blade shaped rotor, between the rotor, the rotor and the pump shell wall are maintained between a certain gap. It belongs to rotary vacuum pump. Mechanical booster pump is this type of vacuum pump.

 

2) momentum transfer pump

The pump relies on high-speed rotating blades or high-speed jets to transmit momentum to the gas or gas molecules, so that the gas continuously from the pump inlet to the outlet. Specific can be divided into the following types.

 

(1)  molecular vacuum pump: it is the use of high-speed rotating rotor energy transmission to the gas molecules, so that the compression, exhaust a vacuum pump. It comes in the following types:

Traction molecular pump: gas molecules and high-speed motion of the rotor collision and gain momentum, was sent to the export, therefore, is a kind of momentum transfer pump.

(2) turbine molecular pump: pump with grooved disc or rotor with blade, it in the stator disc (or stator) rotation. The linear velocity of the circumference of the rotor is very high. The pump normally operates in a molecular flow state.

(3) compound molecular pump: it is by turbine and traction type two molecular pump series combination of a compound molecular vacuum pump.

 

The jet vacuum pump: it is a kind of momentum transfer pump which takes advantage of Venturi effect pressure drop to produce high-speed jet to transport gas to the outlet, suitable for working in viscous flow and transition flow state. This pump can be divided into the following in detail:

Liquid injection vacuum pump: to liquid (usually water) as the working medium of the injection vacuum pump.

(2) gas injection vacuum pump: non-condensable gas as the working medium of the injection vacuum pump.

(3) steam injection vacuum pump: to steam (water, oil or mercury and other steam) as the working medium of the injection vacuum pump.

 

Diffusion pump is low pressure high speed steam flow (oil or mercury vapor) as the working medium of the jet vacuum pump. The molecules of the gas diffuse into the steam stream and are sent to the outlet. The molecular density of the gas is always very low in the jet flow. Can be divided into:

(1) self-purification diffusion pump: pump fluid volatile impurities by special mechanical transmission to the export and not back to the boiler in an oil diffusion pump.

(2) fractionation type diffusion pump: this pump has a fractionation device, so that the steam pressure is low working liquid vapor into the high vacuum working nozzle, and the steam pressure is high working liquid vapor into the low vacuum working nozzle, it is a multi-stage oil diffusion pump.

 

Diffusion jet pump: it is a 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 is of this type.

 

Ion transfer pump: it is a momentum transfer pump that transfers ionized gas to the outlet under the action of electromagnetic field or electric field.

 

2. Gas capture pump

 

This pump is a gas molecules adsorption or condensation on the inner surface of the pump, thereby reducing the number of gas molecules in the container to achieve the purpose of pumping vacuum pump, there are the following types.

 

1) adsorption pump

It mainly relies on the large surface of the adsorbent (such as porous material) of the physical adsorption to pump a type of collector vacuum pump.

2) getter pump

It is a vacuum pump that USES getter to capture gas by chemical combination. A metal or alloy usually in the form of a lump or deposited fresh film. Sublimation pump is of this type.

3) getter ion pump

It is to make the ionized gas through the action of electromagnetic field or electric field adsorption on the surface of the material has inhaled, in order to achieve the purpose of pumping gas. It comes in several forms as follows.

 

Evaporative ion pump: a vacuum pump in which ionized gas is adsorbed on a suction material that is sublimated (or evaporated) in a discontinuous or continuous manner and is covered by the inner wall of the pump to achieve suction.

 

The sputtering ion pump: the pump is ionized gas adsorption by the cathode continuous sputtering out of the getter material, in order to achieve the purpose of a vacuum pump.

 

Low temperature pump

A vacuum pump that USES a low temperature surface to trap gas

 

3. Vacuum pump performance parameters and range of use

 

Performance parameters of vacuum pump

1) ultimate pressure of vacuum pump

The ultimate pressure unit of the pump is Pa, which refers to the lowest stable pressure when the pump is equipped with a standard test cover at the entrance and operates under specified conditions without introducing gas for normal operation.

2) the extraction rate of the vacuum pump

The pumping rate unit of the pump is m3/s or l/s, which refers to the ratio of the gas flow through the test cover and the equilibrium pressure measured at the designated position of the test cover when the pump is equipped with a standard test cover and operates under specified conditions. Speed of pump.

3) the extraction capacity of the vacuum pump

The unit of extraction capacity of the vacuum pump is Pa m3/s or Pa l/s. Refers to the pump inlet gas flow.

 

4) starting pressure of vacuum pump

The unit of starting pressure of the vacuum pump is Pa, which refers to the pressure when the pump is started without damage and has the effect of pumping.

5) pre-stage pressure of the pump

The vacuum pump pressure unit is Pa, it refers to the exhaust pressure less than one atmosphere of vacuum pump outlet pressure.

6) maximum pre-stage pressure of the vacuum pump

The maximum pre-pressure unit of the vacuum pump port is Pa, which refers to the pressure in excess of the pre-pressure that can damage the pump.

7) maximum working pressure of vacuum pump

The maximum working pressure unit of a vacuum pump is Pa, which refers to the inlet pressure corresponding to the maximum amount of air pumped. At this pressure, the pump can continue to work without deterioration or damage.

8) compression ratio

The compression ratio is the ratio of the pump's outlet pressure to the inlet pressure for a given gas.

 

9) ho's coefficient

The ratio of the actual pumping speed on the area of the pumping channel to the theoretical pumping speed calculated on the basis of molecular effusion.

10) pumping speed coefficient

The ratio of the actual pumping speed of the pump to the theoretical pumping speed calculated by molecular effusion at the pump inlet.

11) reflux rate

The unit of reflux rate of the pump is g/cm2.s. It is to point to when the pump works according to formulary condition, the mass flow rate that passes pump inlet unit area.

12) allowable amount of water vapor

The allowable unit of water vapor is kg/h, which refers to the mass flow of water vapor that can be removed by the air ballast pump during continuous operation under normal conditions.

13) maximum allowable vapor inlet pressure

The maximum allowable unit of vapor inlet pressure is Pa. It refers to the maximum inlet pressure of water vapor that can be removed by the ballast pump during continuous operation under normal environmental conditions.

 

4. Scope of use of vacuum pump

 

Because all kinds of vacuum pumps have different working pressure range and starting pressure, so the selection of vacuum pump must meet these requirements. Table 3-1 shows the working pressure range and starting pressure of various common vacuum pumps for reference.

Table 3-1 working pressure range and starting pressure of common vacuum pumps

 

Vacuum pump type

Operating pressure range (Pa)

Starting pressure (Pa)

Piston vacuum pump

Rotary vane vacuum   pump

Water ring vacuum pump

Roots vacuum pump

Turbomolecular pump

Steam jet pump

Oil diffusion pump

Oil vapor jet pump

Molecular sieve adsorption pump

Sputtering ion pump

Titanium sublimation pump

Zirconium aluminum getter pump

Low temperature pump

1×1051.3×102

1×1056.7×10-1

1×1052.7×103

1.3×1031.3

1.31.3×10-5

1×1051.3×10-1

1.3×10-21.3×10-7

1.3×101.3×10-2

1×1051.3×10-1

1.3×10-31.3×10-9

1.3×10-21.3×10-9

1.3×101.3×10-11

1.31.3×10-11

 

1×105

1×105

1×105

1.3×103

1.3

1×105

1.3×10

<1.3×105

1×105

6.7×10-1

1.3×10-2

1.3×10

1.31.3×10-1

 

 

5. The task of vacuum pump in vacuum system

Because the performance of all kinds of vacuum pumps can meet the common point of vacuuming the container, there are still differences. Therefore, it is very important to make clear the work tasks of the pump in the vacuum system. The functions of the pump in various work fields can be summarized as follows

1) main pump in the system

The so-called main pump is the vacuum pump which directly vacuums the vacuum system being pumped by the vessel in order to obtain the vacuum degree needed to meet the technological requirements.

2) rough pumping pump

Crude pump refers to the vacuum system from atmospheric pressure to reduce the pressure of another pumping system can start working vacuum pump.

3) front stage pump

Front stage pump is a vacuum pump used to maintain the pressure of the front stage of another pump below its maximum permissible pressure

4) maintenance pump

Maintenance pump refers to when the vacuum system pumping is very small, can not effectively use the main front pump. To this end, in the vacuum system with a smaller pumping speed of the auxiliary front pump to maintain the normal operation of the main pump or maintain the vacuum pump has been evacuated the container required low pressure.

5) coarse vacuum pump or low vacuum pump

Coarse and low vacuum pump refers to the vacuum pump working in the low vacuum or rough vacuum pressure range after starting from the atmosphere and reducing the pressure of the pumped container.

6) high vacuum pump

High vacuum pump refers to the vacuum pump operating in the high vacuum range.

7) ultra high vacuum pump

Ultra high vacuum pump refers to the vacuum pump operating in the ultra high vacuum range.

8) booster pump

Booster pump usually refers to the work between the low vacuum pump and the high vacuum pump, in order to improve the pumping system in the middle pressure range or reduce the pumping rate of the front stage pump vacuum pump requirements.

 

6. Vacuum pump models and specifications of the method of representation

According to the mechanical industry standard JB/ t7673-95 of the People's Republic of China, various domestic vacuum pumps are made up of two parts: basic type and auxiliary type, with a horizontal line between them. The expression pattern is 123 -- 456. The number 123 indicates the basic model and 456 indicates the auxiliary model.

Domestic vacuum pump models are usually represented by the Chinese phonetic alphabet in table 3-2. If the pinyin letter in front of the word "2", it means that the pump in the structure of the two-stage pump.

Some series of vacuum pumps have a geometric grade for their extraction rate. The units are L/S. There are 18 grades, 0.2, 0.5, 1, 2, 4, 8, 15, 30, 70, 150, 300, 600, 1200, 2500, 5000, 10000, 2000, 40000. Vacuum pump series are sometimes expressed in terms of pump inlet dimensions in "mm". As more types of pumps, the selection should refer to the different manufacturers of product specifications, is very important.

 

Table 3-2 Chinese pinyin codes and names of commonly used vacuum pumps

Code

Name

Code

Name

W

reciprocating pump

Z

Oil diffusion jet pump (oil booster pump)

D

rotary piston vacuum pump

S

sublimation pump

X

sliding vane rotary vacuum pump

LF

Compound ion pump

H

·           Rotary   plunger vacuum pump

GL

Zirconium aluminum getter pump

ZJ

Roots vacuum pump (mechanical booster pump)

DZ

·           refrigerator   cryopump

 

YZ

trochoid vacuum pump

DG

Infusion cryogenic pump

L

sputter ion pump

IF

·           molecular   sieve adsorption pump

XD

Single stage multirotary vane vacuum pump

SZ

water ring pump

F

 molecular   pump

PS

water jet pump

K

Oil diffusion vacuum pump

P

steam ejector pump

 

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