How do I choose the ratio between Roots pump and backing pump?

A speed ratio of roughly 4:1 to 6:1 — Roots to backing pump — is the common industrial range. Going wider raises the differential pressure across the Roots rotors and makes it overheat; going narrower wastes the Roots pump's capacity and your money.

The constraint behind that ratio is differential pressure, not speed. A Roots pump has no internal compression: its rotors run at a small clearance and do not seal against each other, so it cannot compress against atmosphere at all. All it does is move a large volume from a lower pressure to a slightly higher one. The backing pump has to take that gas away fast enough that the pressure difference across the Roots stays within its permissible limit — a per-model figure on the data sheet, typically a few thousand Pa for direct-drive designs.

What happens when you exceed it: the Roots pump has no way to shed the compression heat, the rotors grow thermally, clearances close, and you get contact. That is why the ratio is a thermal limit dressed up as a speed number, and why an oversized Roots on an undersized backing pump fails in a way that looks sudden.

Three mechanisms let you run wider ratios safely, and they are worth knowing before you buy: a bypass (overflow) valve that opens at a set differential during rough-down and closes for normal operation; a hydrokinetic or frequency-controlled drive that limits the Roots speed until the pressure has fallen; and simply switching the Roots on only below a crossover pressure rather than from atmosphere.

Match the pair to the pressure range you actually work in, not to the pump-down. A Roots pump earns its cost between roughly 10 mbar and 10⁻² mbar, where the backing pump's own speed has rolled off badly. Above that range it contributes little; below it, the combination's ultimate pressure is set by the backing pump's ultimate divided by the Roots pump's compression ratio.

Tell us the chamber volume, the working pressure, the gas load and whether the process is continuous or batch, and we will propose the pair with the differential-pressure and crossover arrangement written into the quotation — not just two model numbers.