How to choose a Roots vacuum booster
A Roots pump (mechanical booster) has two figure-of-eight lobes rotating in opposite directions with tight clearances and no contact. It cannot exhaust to atmosphere on its own — it always sits on top of a backing pump and multiplies that system's speed, typically by a factor of four to ten, in the pressure range where the backing pump alone has gone slow.
Two numbers govern the pairing: the booster's displacement relative to the backing pump's speed, and the maximum allowable differential pressure across the lobes. Too large a ratio and the booster overheats or trips; too small and you gain nothing. Look for the zero-flow compression ratio, the differential-pressure limit, the bypass or overflow-valve arrangement, and whether the drive is a fixed motor or a frequency-controlled one that can start earlier in the cycle.
Typical duties: vacuum furnaces and heat treatment, freeze drying, degassing, metallurgy, and any process that needs high throughput in the 10⁻¹–10⁻³ mbar range. Roots-plus-rotary-vane, Roots-plus-screw and multi-stage Roots sets are the standard building blocks; the right combination depends on whether the process is dirty, wet, or has to stay oil-free.
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FAQ
Why does a Roots pump need a backing pump?
A Roots pump has no internal compression against atmosphere — the rotors do not seal, so it can only compress against a already-reduced pressure. The backing pump establishes that pressure and carries the exhaust. Running a Roots pump without one overheats it and the motor overloads.
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. A higher ratio gets you a better ultimate pressure but needs a bypass or a variable-speed drive to survive the rough-down phase without tripping on differential pressure.
What is the maximum permissible differential pressure?
It is a per-model figure printed on the data sheet, typically a few thousand Pa for the ZJ/ZJP families, and it is a hard limit — exceeding it overheats the rotors and can seize the pump. This is why the pump is started only after the backing pump has pulled the system down, or is protected by a bypass valve.
ZJ, ZJP or ZJQ — what is the difference?
They are different constructions within the Roots family: they differ in permissible differential pressure, cooling arrangement and sealing, which is what decides the pressure range each one is comfortable in. Note the model number is the pumping speed in L/s, not m3/h — a ZJP-300 is 300 L/s, roughly 1080 m3/h.
Can a Roots pump handle dust or condensable vapour?
Not directly. The rotors run at a small clearance, so particulates cause contact and condensate washes the bearing lubricant out. Fit an inlet filter or knock-out pot for dust, and for vapour either heat the pump body or use a condenser upstream.