Roots vacuum pump can not be used alone. It must be equipped with a front stage pump to form a unit. The purpose of using the front stage vacuum pump is to reduce the discharge pressure of Roots vacuum pump. Since the compression ratio of Roots vacuum pump is limited, it is important to select the pumping speed of the front stage vacuum pump. As shown in the right figure, the maximum compression ratio of Roots vacuum pump varies with the discharge pressure, The maximum compression ratio is 3:1 at most, and the maximum compression ratio can reach 50:1 or even higher when the discharge pressure is between 1 and 10mbar (1 mbar = 0.75torr).

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, in one system, roots vacuum pump unit is used for air extraction, and the pumping speed is required to be 600 L / s at 1 mbar inlet pressure, and the pump operates for a long time. What pumping speed should be selected for the front stage pump.

It is assumed that the compression ratio k = 600:30 = 20 with a 30 L / s front stage pump, and the maximum discharge pressure is 30 mbar when k = 20.

It is concluded that when the 30 L / s front stage pump is selected, it is feasible to work under the inlet pressure lower than 1.5 mbar, but when the inlet pressure is higher than 1.5 mbar, a larger front stage pump is selected. In this case, the working vacuum of 1 mbar is less than 1.5 mbar, so the 30 L / s front stage pump can meet the requirements of the vacuum system mentioned above.

In addition, the requirement of pre pumping time should be considered. If the normal working time is much longer than the pre pumping time, the smaller front stage pump can be selected; If the vacuum chamber is large and requires rapid pre pumping, a larger front stage pump should be used. Sometimes, in order to give better play to the efficiency of the equipment, the unit is equipped with a larger pre pumping pump, which is stopped after normal operation and maintained with a smaller front stage pump. This is to save power.

The requirement of limit pressure of Roots vacuum pump unit must be considered when selecting the front stage pump. The requirement for extreme vacuum degree is high (up to 1x10-3Pa ~ 1x10-2pa). Generally, double-stage rotary vane oil seal mechanical pump or double-stage slide valve mechanical pump is selected as the front stage pump. If the limit vacuum degree is between (1x10-2 PA ~ 1x10-1pa), single-stage oil sealed mechanical pump can be selected as the front stage pump, and the reciprocating vacuum pump or liquid ring vacuum pump can be selected as the front stage pump if the limit vacuum degree is between 133 ~ 1333pa.

When the reciprocating or liquid ring type crude vacuum pump is selected as the front stage pump of Roots vacuum pump, the pumping speed of the former stage 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 the high exhaust temperature of Roots vacuum pump and can not work normally.

When selecting the front stage pump, it is necessary to consider whether there is corrosive medium in the extracted gas, whether it contains condensable gas, whether it is large or small. If there is corrosive gas, oil seal mechanical pump can not be selected. Oil seal mechanical pump is not suitable to remove the gas containing a large amount of condensable steam, otherwise the sealing liquid will be emulsified quickly. When extracting the gas containing a small amount of condensable steam, the oil seal mechanical pump with gas ballast device must be used as the front stage pump.

In order to obtain high pumping rate at low inlet pressure (1 ~ 100Pa), one roots vacuum pump can be used as the third or fourth stage unit composed of another roots vacuum pump. At this time, the pumping speed ratio between roots vacuum pumps is generally between 2 ~ 5.