How a Liquid Ring Vacuum Pump Works
A liquid ring pump uses a rotating ring of liquid, usually water, as its piston. It is simple and tough, and it happily swallows vapour, droplets and dirt that would ruin most other vacuum pumps.
Step by step
- The ring forms. The casing is partly filled with sealing liquid. When the impeller turns, the liquid is thrown outward into a ring that follows the casing wall.
- Off-centre impeller. The impeller sits off-centre. On one side its blades dip deep into the ring; on the other they almost leave it. The space between two blades and the ring forms a cell.
- Suction. Where the blades move out of the liquid, the cells grow and draw gas in through the suction port in the side plate.
- Compression and discharge. Further round, the blades dip back into the ring, the cells shrink, and the compressed gas leaves through the discharge port together with some liquid.
- Cooling. The liquid absorbs the heat of compression, so the gas stays cool. Part of the liquid is continuously replaced to carry that heat away.
The vacuum is limited by the vapour pressure of the sealing liquid: with water at 25 °C the pump cannot go much below about 32 mbar, because the water itself starts to boil. Colder water, a low-vapour-pressure liquid or an air ejector in front of the pump extends the range.
Pressure range and performance
- Ultimate pressure
- ≈ 25 – 33 mbar with water; lower with colder liquid or a gas ejector
- Pumping speed
- 25 – 30,000 m³/h
- Working range
- Atmosphere to ≈ 30 mbar
- Dry?
- No — water or process liquid ring
Advantages
- Tolerates liquid, vapour and dirt
- Cool, nearly isothermal compression, common for wet and flammable gas streams
- Few wearing parts
- Available in very large sizes
Limitations
- High power and liquid consumption
- Vacuum limited by the liquid’s vapour pressure
- Discharged liquid must be treated or recycled
Typical applications
- Paper machines
- Venting power plant condensers
- Chemical and pharmaceutical processing
- Sterilisers
- Vacuum filtration and mining
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.
- IAPWS. SR1-86(1992): Revised Supplementary Release on Saturation Properties of Ordinary Water Substance. International Association for the Properties of Water and Steam. Saturation vapour pressure equation used for liquid water.
Frequently asked questions
What limits the vacuum of a liquid ring pump?
The vapour pressure of the sealing liquid. Once the pressure reaches it, the liquid boils (cavitates) and the pump stops gaining vacuum. With water at 15 °C the limit is about 17 mbar, at 25 °C about 32 mbar.
Can a liquid ring pump handle liquid carry-over?
Yes. Liquid entering with the gas simply joins the ring and is discharged with the gas, which is why these pumps are used on wet, dirty processes.
Which sealing liquids are used?
Water in most cases. For solvent recovery or to avoid contaminating the process, the ring can run on oil or a process solvent, usually in a closed loop with a heat exchanger.
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