How to select a proper primary vacuum pump for Roots pump?

vacuum system primary pump

This article is to introduce the key points to select of a primary vacuum pump for Roots pumps.

Among all vacuum pumps, the Roots pump, also known as booster vacuum pump, is quite special. It has a very limited compression ratio and therefore, the mostly common used single stage Roots pump cannot work alone. 

1. Primary vacuum pump for the Roots pump

The Roots vacuum pump has to operate together with a primary pump (or fore pump, or backing pump), which is usually an oil-sealed rotary vane pump. The reason is that the single stage Roots pump cannot start under atmospheric pressure. 

With the help of a backing pump (such as a rotary vane pump), the Roots pump can operate and improve (or boost) the ultimate pressure and pumping speed properly. (Exception: a multi-stage Roots pump can operate from atmospheric pressure without any primary pump due to different internal structure.)

2. How to select the primary vacuum pump

You have to consider the following key points to select the fore vacuum pump for the Roots pump to build up a workable vacuum system:

  • Pumping speed ratio between the Roots pump and the primary pump

Generally speaking, the ratio shall be selected from the range of 2 – 10. For example, if the Roots pump has a flow rate of 1200 m3/h, the pumping speed range of the fore pump shall be 120 – 600 m3h.

  • Pre-pumping time
If the operation allows long time pre-pumping, a small fore pump can be used without problem. In case of big vacuum chamber and requirement of quick equipment operation, it’s necessary to choose a bigger pump with higher pumping speed.
  • Required ultimate pressure of the pump system
For high vacuum requirement (1×10-3 Pa to 1×10-2 Pa), it’s common to have a dual-stage rotary vane pump as fore pump. For less requirement (1×10-2 Pa to1x10-1 Pa), a single stage rotary vane pump is enough. A liquid ring vacuum pump can be selected for even less requirement (133 – 1333 Pa).
  • Pumping media
The oil-sealed mechanical pump cannot be considered for corrosive gases. For gases with small amount of water vapor, such pump can be used with gas ballast open.

Click Roots Pumps and Rotary Vane Pumps to find out how to build up a proper vacuum system.

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