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
- Required ultimate pressure of the pump system
- Pumping media