Roots vacuum pump system

Roots vacuum pump can not be used alone, it must be equipped with the former pump to form a unit. The use of the former vacuum pump is to reduce the discharge pressure of Roots vacuum pump. Because the compression ratio of Roots vacuum pump is limited, it is important to select the pumping speed of the former vacuum pump. The maximum compression ratio of Roots vacuum pump varies with the discharge pressure. At high discharge pressure, the maximum compression ratio is up to 3:1, and the maximum compression ratio is between 1 and 10 mbar (1 mbar = 0.75 Torr), which can reach 50:1, or even higher.

Therefore, the selection of the pumping speed of the front stage pump should be based on the long-term working pressure range of Roots vacuum pump.

For example, there is a system with Roots vacuum pump unit for pumping, which requires 600 litres/sec pumping speed at 1 mbar inlet pressure, and long-term operation. How much pumping speed should be selected for the front stage pump?

It is assumed that the maximum discharge pressure is 30 mbar when the compression ratio K=600:30=20 is used in the 30 liter/second front stage pump.

It is concluded that it is feasible to use a 30 liter/second front-stage pump when the inlet pressure is below 1.5 mbar, but a larger front-stage pump is selected when the inlet pressure is above 1.5 mbar. In this case, the working vacuum of 1 mbar is less than 1.5 mbar, so the 30 liter/second front pump can meet the requirements of the above vacuum system.

In addition, we should consider the requirement of pre-pumping time, if the normal working time is much longer than pre-pumping time, we can choose a smaller pre-pumping pump; if the vacuum chamber is large and requires pre-pumping quickly, we need to use a larger pre-pumping pump. Sometimes in order to better play the efficiency of equipment, the unit is equipped with a larger pre-pumping pump. Stop it after normal operation, and maintain it with a smaller front pump, in order to save power.

The requirement of the limit pressure of Roots vacuum pump unit must be considered when choosing the front pump. The high requirement of the limit vacuum degree (up to 1 X10-3 PA-1 X10-2 Pa pressure) generally chooses two-stage rotary vane oil seal mechanical pump or two-stage slide valve mechanical pump as the front pump. The limit vacuum degree requires that a single stage oil seal mechanical pump be selected as the front pump between (1X10-2Pa~1X10-1Pa), and a reciprocating vacuum pump or a liquid ring vacuum pump be selected as the front pump between 133 and 1333 Pa.

When the reciprocating or liquid ring rough vacuum pump is selected as the front pump of Roots vacuum pump, the speed of the front pump is generally 1/2-1/4 of that of Roots vacuum pump, which can not be too large, otherwise the compression ratio will be too large, resulting in high exhaust temperature of Roots vacuum pump and unable to work normally.

When choosing the front-stage pump, it is also necessary to consider whether there is corrosive medium in the pumped gas and whether there is condensable gas, which is large or small. If there are corrosive gases, oil-seal mechanical pumps can not be selected. Oil-seal mechanical pumps are not suitable for extracting gases containing a large amount of condensable steam. Otherwise, the sealing fluid will be quickly emulsified. When extracting gases containing trace condensable steam, oil-seal mechanical pumps with gas-holder devices must also be used as front-stage pumps.

In order to obtain high pumping rate at low inlet pressure (1-100 pa), a Roots vacuum pump can be used as the first stage pump of another Roots vacuum pump to form a three-stage or four-stage unit. At this time, the pumping speed ratio between Roots vacuum pumps is generally between 2-5.

Usually, the principle of selecting the front stage pump is as follows:

1. Require the pre-vacuum condition for the main pump to work.

2. Under the maximum allowable exhaust pressure of the main pump, the front pump must discharge the maximum amount of gas discharged from the main pump in time, and the pump speed of the front pump must be in accordance with the actual pumping speed of the pump at the actual inlet pressure.

3. The pre-pumping pump which is used as pre-pumping pump should meet the requirements of pre-pumping time and pre-pumping vacuum.