ConceptsTechnology

What is Vacuum Pump?

Device used to obtain and maintain a vacuum

It occupies an important position in vacuum technology and is the core part of the vacuum system.

Vacuum pump classification type a lot, by the suction mechanism can be divided into three categories: use a pump chamber working volume change to pumping displacement pumps, reciprocating pumps, vane vacuum pumps rotary, rotary piston vacuum pumps, trochoid pump, Roots vacuum pumps and liquid ring vacuum pumps, etc.

; Momentary drive vacuum pumps that use high-speed jets of steam (steam) or high-speed rotating rotors to pump air, such as oil diffusion pumps, oil diffusion jet pumps, water vapor jet vacuum pumps and turbomolecular pumps, etc.

; Usage Trapping vacuum pumps, such as molecular sieve adsorption pumps, titanium sublimation pumps, ion getter pumps and cryogenic pumps, are used for adsorption and condensation to store gas molecules permanently or storm at the pump

Vacuum pumps can also be classified by other methods

For example, according to the working medium, it can be divided into oil vapor vacuum pump, water vapor vacuum pump and water ring pump; According to the role played in the vacuum system, it can be divided into main pump, booster pump, front pump, rough pump and maintenance pump.

; According to the working pressure range, it can be divided into low (rough) vacuum pump, medium vacuum pump, high vacuum pump and ultra-high vacuum pump.

Various vacuum pumps such as variable volume vacuum pumps and turbomolecular pumps such as drive transmission vacuum pumps are also called mechanical vacuum pumps due to mechanical movement; Among them, rotary vane vacuum pumps, slide valve vacuum pumps and trochoid vacuum pumps are among the surfaces. There is oil to seal each working quarter, so it is also called sealed mechanical vacuum pump. with oil

Install an inflation valve (called gas ballast valve) in the compression chamber of the oil-sealed mechanical vacuum pump, and fill the compression chamber with an appropriate amount of non-condensable gas to reduce the degree of condensation of the condensable gas. at the pump

The pump is called gas ballast mechanical vacuum pump

PerformanceThe pumping performance of the vacuum pump varies with the pumping mechanism

Some pumps can start work directly from atmospheric pressure, while some pumps cannot start work from atmospheric pressure, there must be a pre-vacuum at the start of work.

The discharge pressure of some pumps is higher than atmospheric pressure, which can directly discharge the gas into the atmosphere.

The exhaust pressure of other pumps is very low, and the gas cannot be directly discharged into the atmosphere.

It is necessary to select a vacuum pump that can discharge the gas to atmosphere as your backup pump.

First-line pumps typically use oil-sealed mechanical vacuum pumps, liquid ring vacuum pumps, and molecular sieve adsorption pumps.

Generally speaking, the exhaust pressure of a vacuum pump that can start from atmospheric pressure is usually higher than atmospheric pressure

Some vacuum vessels require a very low working vacuum and only use a suitable vacuum pump for pumping, while others are very demanding.

It is necessary to combine two or more vacuum pumps with different pumping mechanisms to form a vacuum system for pumping.

Therefore, when choosing a vacuum pump, you must also consider the nature of the gas to be pumped: is it clean, dry air, or is it water vapour, dusty gas, or explosive and flammable gas? vacuum are the pumping speed, the maximum pressure, the working pressure range and the coefficient of Ho

Pumping speed S : called the pumping speed

Given any pressure p at the pump suction port, the volume of gas flowing into the pump per unit time is: S = V / t , where V is the volume of gas flowing into the pump within t

The unit of pumping speed is usually expressed in liters per second (1 / s) or cubic meters per second ( m 3 / s)

Due to the pump suction or discharge q V = Sp , another expression for the pumping speed is S = q V / p

End Pressure: The lowest pressure gradually reached by the pump over a long period of time when air leakage and wall deflation are negligible

Working pressure range: the pressure range when the pump has a large pumping capacity

Figure 1 shows the working pressure range of vacuum pumps commonly used in industrial production and scientific experiments (the solid line represents the normal working pressure range of the pump)

Ho coefficient: the ratio of the actual pumping rate in the area between the pump nozzles to the maximum theoretical pumping rate calculated by the molecular effusion there

The Ho coefficient was first proposed by the Chinese physicist He Zenglu in his work on diffusion pumps in the early 1930s.

When designing steam jet pumps, oil diffusion pumps, and oil diffusion jet pumps or when comparing product performance, the coefficient of Ho is often used

Vacuum system an air extraction system consisting of a vacuum vessel, a vacuum pump, a vacuum valve, a vacuum measuring instrument and connecting pipes (Figure 2)

In the vacuum system, the main pump is used to obtain the working vacuum, and its final pressure should generally be half an order of magnitude less than the working pressure.

The working pressure of the vacuum vessel must be within the pressure range of the maximum pumping speed of the main pump

Figure 2a shows the (approximate) low vacuum system, which only needs to use a vacuum pump that can discharge the gas directly into the atmosphere.

These pumps typically use oil-sealed mechanical vacuum pumps, water ring vacuum pumps, or steam jet vacuum pumps.

If the pumped gas contains dust or water vapour, it is recommended to use a water ring vacuum pump (see liquid ring vacuum pump) or a water vapor jet vacuum pump

Figure 2b shows the medium vacuum system. The main pump is a diffusion jet pump or Roots vacuum pump, and the backup pump is a sliding vane vacuum pump.

Figure 2c shows the high vacuum system. To reduce oil contamination of the vacuum vessel, the lead pump is an oil diffusion pump with a water-cooled baffle and the backup pump is a slide valve vacuum pump.

To increase the pumping capacity of the system in the intermediate pressure range and reduce the pumping rate capacity of the foreline pump, this system connects a Roots vacuum pump in series between the oil diffusion pump and the foreline pump ( also called mechanical booster pump at this time), can also be connected in series with oil diffusion jet vacuum pump (also known as oil booster pump at this time)

To shorten the pumping time, during the start-up phase, the vacuum canister is directly connected to the foreline pump through a bypass instead of the normal working system (the dashed box in Figure 2b and Figure 2c )

At this time, the backup pump is called the raw pump

Figure 2d shows the ultra-high vacuum system, which is composed of sputtering ion pump, titanium sublimation pump, molecular sieve adsorption pump, and corresponding valves.

When using, first make the molecular sieve adsorption pump in an effective working state, and then open valve 1 and valve 2 to pump the system to a certain vacuum, and then start the titanium sublimation pump and sputtering ion pump

The pumping unit assembled on the same frame only by the vacuum pump, the vacuum valve and the connecting pipe is called a vacuum unit.

Can be moved arbitrarily with the place of use

To meet the needs of different situations, there are also diffusion pump units, roots vacuum pump units, and turbomolecular pump units.

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