Why do roots vacuum pumps need backing pumps to work properly?

In the vacuum pump family, the Roots pump is a bit special, its operation requires the cooperation of the backing pump, why is this? To understand the reason, let's first look at the structural characteristics and performance characteristics of the roots pump.
The vacuum ratio of the roots pump is too low to be used alone, so a backing pump is required

The Roots pump is a vacuum pump without internal compression, usually the compression ratio is very low, so it can not be used alone, medium and high vacuum pumps need to use backing pumps to match. Roots pump generally can not be started under normal pressure, if directly started, the motor power is very large, will hurt the vacuum pump, and with the backing pump (such as rotary vane pump), you can help the main pump Roots pump smooth start, continuous operation.
When used in conjunction with a backing pump, the performance characteristics of the Roots pump can be fully utilized

Secondly, the vacuum unit composed of the vacuum pump with a high vacuum degree as the main pump and the vacuum unit composed of the backing pump can effectively play the performance characteristics of the Roots pump; When used in combination, compared with a single vacuum pump, the vacuum degree and pumping rate can be significantly improved, and the range of gas types that can be extracted by the unit can be broadened. For example, with the water ring type as the backing pump and the roots pump as the main pump, you can extract a large amount of condensable steam and gases with a certain corrosiveness.

According to the requirements of different working conditions, roots pumps can be equipped with the following vacuum pumps as backing pumps: rotary vane vacuum pumps, water ring vacuum pumps, spool valve vacuum pumps.