How a Rotary Vane Vacuum Pump Works
The rotary vane pump is the workhorse of rough and medium vacuum. An off-centre rotor with sliding vanes sweeps gas from the inlet around the housing and squeezes it out through an exhaust valve. Every revolution repeats the same four steps.
Step by step
- Suction. As a vane sweeps past the inlet, the space behind it grows. Gas from the chamber expands into this growing pocket.
- Transport. The following vane closes the pocket off from the inlet. The trapped gas is carried around the bottom of the housing.
- Compression. On the exhaust side the gap between rotor and housing narrows, so the pocket shrinks and the gas is compressed to just above atmospheric pressure.
- Exhaust. When the pressure overcomes the valve spring and the atmosphere outside, the exhaust valve opens and the gas leaves, in oil-sealed pumps through an oil mist separator.
- Sealing. Oil seals the vane tips and side clearances, lubricates and cools. At the top, rotor and housing almost touch, which separates the exhaust side from the inlet.
Gas ballast: water vapour drawn in with the gas would condense during compression and spoil the oil. Opening the gas ballast valve admits a little air into the compression stage, so the vapour leaves with the exhaust before it can condense.
Pressure range and performance
- Ultimate pressure
- ≈ 10⁻¹ mbar single-stage, ≈ 10⁻³ mbar two-stage
- Pumping speed
- 1 – 1,600 m³/h
- Working range
- Atmosphere to ≈ 10⁻³ mbar
- Dry?
- No — oil-sealed (dry-running versions exist for rough vacuum)
Advantages
- Low cost per m³/h
- Simple and robust
- Good ultimate pressure in two-stage form
- Starts and runs from atmospheric pressure
Limitations
- Regular oil changes; risk of oil backstreaming
- Condensable vapours contaminate the oil without gas ballast
- Pumping speed falls near the ultimate pressure
Typical applications
- Evacuating HVAC/R systems
- Vacuum packaging
- Freeze drying
- Backing pump for turbomolecular and diffusion pumps
- Laboratory vacuum
Useful calculators
References & further reading
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
- CERN Accelerator School. CAS Vacuum in Accelerators, Platja d’Aro, Spain, 2006 — proceedings (CERN-2007-003). CERN (2007), free to read. Lectures on gas dynamics, conductance, pumps and outgassing.
Frequently asked questions
Why does a rotary vane pump need oil?
The oil seals the tiny gaps between vanes, rotor and housing, lubricates the sliding vanes and carries away heat. Without it, gas would leak back from the exhaust side and the pump could not reach a low pressure.
What is the difference between single-stage and two-stage rotary vane pumps?
A two-stage pump has two pumping stages in series; the first discharges into the second instead of to atmosphere. Less gas leaks back to the inlet, so it reaches about 10⁻³ mbar instead of about 10⁻¹ mbar.
When should I use the gas ballast?
Whenever the pump handles water vapour or other condensable vapours, for example during the first hours of drying or HVAC evacuation. It slightly raises the ultimate pressure, so close it once the vapour load is gone.
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