Roots vacuum pump cannot be used alone. It must be equipped with a foreline pump to form a unit. The foreline vacuum pump is used to reduce the discharge pressure of the roots vacuum pump. Since the compression ratio of the roots vacuum pump is limited, the pumping speed of the foreline vacuum pump is selected. Importantly, as shown in the figure on the right, the maximum compression ratio of a Roots vacuum pump varies with the discharge pressure. Under high discharge pressure, the maximum compression ratio is up to 3:1, and the maximum compression ratio is at the discharge pressure of 1 to 10 mbar. Between (1 mbar=0.75Torr) can reach 50:1, or even higher.

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

For example, there is a system that uses a Roots vacuum pump unit to pump air. It requires a pumping speed of 600 liters/sec at an inlet pressure of 1 mbar, and it works for a long time. What is the pumping speed of the backing pump should be selected.

First assume that when using a 30 liter/sec backing pump, the compression ratio K=600:30=20. From the above figure, the maximum discharge pressure when K=20 is 30 mbar.

It can be concluded that when the 30 liters/sec foreline pump is selected, it is feasible to work at an inlet pressure below 1.5 mbar, but a larger foreline pump is used when the inlet pressure is higher than 1.5 mbar. In this example, the working vacuum of 1 mbar is lower than 1.5 mbar, so the 30 liters/sec foreline pump can meet the pumping requirements of the above vacuum system.

In addition, it should be considered that the pre-pumping time is required. If the normal working time is much longer than the pre-pumping time, a smaller backing pump can be selected; if the vacuum chamber is larger and the pre-pumping is required quickly, use Larger backing pump, sometimes in order to better utilize the efficiency of the equipment, the unit is equipped with a larger pre-suction pump, it is stopped after normal operation, and a smaller backing pump is used to maintain it. This is to save power .

The requirements for the ultimate pressure of the roots vacuum pump unit must also be considered when selecting the backing pump. The ultimate vacuum requirement is relatively high (up to 1X10-3Pa～1X10-2Pa pressure), generally two-stage rotary vane oil-sealed mechanical pump or The two-stage spool valve mechanical pump is used as the fore-stage pump. For ultimate vacuum requirements (1X10－2Pa～1X10－1Pa), a single-stage oil-sealed mechanical pump can be used as the backing pump, and ultimate vacuum requirements between 133～1333Pa can be selected as a reciprocating vacuum pump or a liquid ring vacuum pump. Stage pump.

When a reciprocating or liquid ring rough vacuum pump is used as the fore pump of a Roots vacuum pump, the pumping speed of the fore pump is generally 1/2-1/4 of that of the Roots vacuum pump, which should not be too large, otherwise the compression ratio will be too high. The exhaust temperature of the Roots vacuum pump is high and cannot work normally.

When selecting the foreline pump, you must also consider whether there is corrosive medium in the pumped gas, whether it contains condensable gas, whether it is a large amount or a trace amount. If it contains corrosive gases, oil-sealed mechanical pumps cannot be used. Oil-sealed mechanical pumps are not suitable for pumping gas containing a large amount of condensable vapor, otherwise the sealing liquid will be quickly emulsified and the gas containing trace condensable vapor will be removed. At the same time, the oil-sealed mechanical pump with gas ballast must also be used as the backing pump.

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

Generally, the principle of selecting the backing pump is as follows:
1. The foreline pump is required to create the pre-vacuum conditions required for the main pump to work.
2. Under the maximum discharge pressure allowed by the main pump, the foreline pump must discharge the maximum amount of gas discharged by the main pump in time, and the pumping speed of the foreline pump must be in accordance with the actual pumping speed of the pump at the actual inlet pressure.
3. The backing pump that doubles as a pre-pump must meet the requirements of pre-pumping time and pre-pumping degree.