The roots vacuum pump is a kind of rotary volumetric vacuum pump. Its structure is evolved from the roots blower. It first appeared in Germany in 1944 and it was used as a mechanical booster pump to accommodate a vacuum pumping system with a large pumping speed in the pressure range of 10 to 1000 Pa.
According to the different working pressure range, the roots vacuum pump is divided into three types:
1. Low-vacuum roots pump: Dry-type single-stage roots pump (10000Pa) for roughing pumping and wet-type roots pump (2000Pa).
2. Medium-vacuum roots pump: Mechanical booster pumps (1×10-1 Pa).
3. High-vacuum roots pump: Multi-stage dry-type roots pump (high vacuum multi-level roots pump.
The most used roots pump in domestic is medium-vacuum roots pumps (hereinafter referred to as roots pumps). Compared with other oil-sealed mechanical pumps，roots pump has the following features:
1. Larger pumping speeds within a wider pressure range.
2. The rotor has a good geometrical symmetry, so is has small vibration and stable operation. There is a gap between rotors and housing, so there is no need of lubrication. Also the small friction loss can greatly reduce the driving power, which can achieve higher revolving speed.
3. The oil seal and lubrication are no needed in pump chamber, which can reduce the oil vapor pollution to the vacuum system.
4. There is no compression or exhaust valve in the pump chamber. So the structure is simple and compact. It is also insensitive to dust and water vapor in the exhausted gas.
5. The compression is relatively low, and the effect of hydrogen pumping is relatively poor.
6. The shape of rotor surface is a complex curved cylinder, so the processing and inspection is a little difficult.
Application of Roots Pump in Vacuum Engineering
1. It is usually used as a mechanical booster pump in medium vacuum range. Generally it is used in series with the backing pump (rotary vane pump, slide valve pump and water ring pump, etc.) to constitute a vacuum unit.
2. Two-stage or multi-stage roots pump unit can obtain high vacuum
3. Air-cooled roots pump unit is usually used for dry, clean and oil-free pumping system.
4. Wet-type roots pump is usually used for pumping system that contains water vapor.