Roots Pumps

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.

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.

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.

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.

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.