How to choose the front pump of Roots vacuum pump?

Everyone knows that Roots vacuum pump can not be used alone, it must be equipped with a front-stage pump to form a pump unit. The use of the front-stage vacuum pump is to reduce the discharge pressure of Roots vacuum pump. Because the compression ratio of Roots vacuum pump is limited, it is very important to choose the speed of the front-stage vacuum pump. The maximum compression ratio of Roots vacuum pump varies with the discharge pressure. Under high discharge pressure, the maximum compression ratio is 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 pumping speed of the front stage pump should be considered according to 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?

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 smaller pre-pumps; if the vacuum chamber is large and requires pre-pumping quickly, we need to use larger pre-pumps. Sometimes in order to better play the efficiency of equipment, the unit has larger pre-pumps, which are stopped after normal work, and maintained by smaller pre-pumps. This is done to save power.

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

When the reciprocating or water-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, which will cause the exhaust temperature of Roots vacuum pump to be high and can not work properly.

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. German imported water-ring vacuum pump can also be used as the front pump of Roots vacuum pump. The imported water-ring vacuum pump can be used as the front pump with high efficiency. The combination of water-ring vacuum pump and Roots vacuum pump can achieve the best efficiency.

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