Roots pumps are used in combination with backing pumps to achieve lower pressures, higher suction capacities and larger capacities. While conventional Roots pumps are protected by a pressure switch, Roots pumps that are operated with a frequency converter and those with so-called bypass lines can be operated continuously.
In combination with backing pumps that compress against the atmosphere, these pumps can improve the achievable ultimate pressure of a pump system by a factor of 10. With two Roots vacuum pump stages and a corresponding backing pump, pressures in the range of up to 10-5mbar (0.75 x 10-5 Torr) can be achieved at high suction capacities.
Roots pumps are feed pumps without internal compression. This means that they cannot usually compress directly against the atmosphere, but always work in combination with a suitable backing pump. When selecting a combination of backing pump and Roots pump, the ratio of the suction capacities of the two pumps is decisive. This is referred to as the gradation. Typical ratios are between 1:2 and 1:8, i.e. Roots pumps generally have a suction capacity that is around 2 to 8 times higher than the backing pump below them. The gradation determines the pressure difference between the inlet and outlet of the Roots pump at the respective operating point. The required motor power is directly dependent on