Reliable pumping for large volumes
Leybold RUVAC roots blowers are used in combination with backing pumps to reach lower pressures and higher pumping speeds, and where greater capacities are required. Conventional manually controlled blowers are fitted with a pressure switch to protect against thermal overload, while roots blowers controlled by variable frequency drives or with internal bypass lines can run continuously.
Combined with backing pumps, which compress against atmosphere, these pumps can improve the attainable ultimate pressure of the pumping system by a factor of 10. With two roots vacuum pump stages and a suitable backing pump, pressures down to the 10⁻⁵ mbar range (0.75 × 10⁻⁵ Torr) are achievable.
Roots blowers are positive displacement pumps without internal compression. This means they cannot compress directly against atmosphere and need a suitable backing pump. When selecting the combination of backing and roots pump, the ratio of the two pumps’ pumping speeds is decisive. This is called the staging ratio. Typical staging ratios are between 1:2 and 1:8, meaning the roots blower usually has a pumping speed 2 to 8 times that of the attached backing pump.
The staging ratio determines the pressure difference between the roots pump’s inlet and outlet at the relevant operating point. In principle the required motor power depends directly on and is proportional to this pressure difference. To avoid overloading the motor and overheating the pump, the pressure difference across the roots pump must not exceed a certain value.
Staging ratio
- Typical staging ratios 1:2 to 1:8
- Roots pump 2–8× faster than the backing pump
- A backing (fore) pump is mandatory
- Pressure switch protection against overload
- Continuous operation with a variable frequency drive
