A roots pump does not discharge to atmosphere on its own. It sits between the chamber and a backing pump such as a rotary vane pump, and its job is to raise pumping speed over a particular pressure band. Whether it earns its place depends on two things: how much gas there is, and at which pressure the extra speed is needed.
Where it fits
- A large chamber that must be pumped down quickly, where a backing pump alone would take too long;
- A process that needs substantial flow at a lower working pressure, where the backing pump’s speed curve has already fallen away;
- A small amount of condensable vapour or dust that needs to be handled together with a compression ratio and pre-capture.
Where it does not, or needs care
- Duty that demands a genuinely low pressure: a booster remains a medium-vacuum machine, and the high-vacuum stage needs a turbomolecular or diffusion pump;
- Small chambers and small gas loads, where the gain is limited and the extra machine only adds maintenance and footprint;
- Gas carrying heavy dust or liquid, which will accelerate rotor and clearance wear unless filtration and capture are properly arranged ahead of it.
Configuration points: ratio the booster to the backing pump, and where the ratio is high use an overflow valve or bypass to limit the differential so the backing pump is not overloaded. Start the backing pump first to establish a base vacuum, then bring the booster in. Size against the actual pressure-versus-speed curve; comparing only peak pumping speeds is an easy way to select wrongly.
This page describes a selection method, not a specific project record.