How a Vacuum Ejector Works
A vacuum ejector, or jet pump, makes vacuum with no moving parts at all. A fast jet of steam, compressed air or water drags the surrounding gas along with it and carries it out through a widening diffuser.
Step by step
- Motive fluid. Steam, compressed air or water enters at pressure. This is the energy source of the pump.
- Nozzle. The nozzle turns pressure into velocity. For steam and air it is a converging–diverging nozzle that reaches supersonic speed, and the pressure in the jet drops far below the suction pressure.
- Entrainment. Around the jet, in the suction chamber, gas from the process is drawn in. Friction between the fast jet and the slow gas drags the gas along.
- Mixing. In the mixing section the motive fluid and the suction gas exchange momentum and become one stream.
- Diffuser. The diffuser widens, slowing the stream and turning its velocity back into pressure, so the mixture leaves at the discharge pressure.
Three common types: steam ejectors, often several stages with condensers in between, serve refineries and evaporators down to about 0.1 mbar. Compressed-air vacuum generators drive suction cups in pick-and-place handling. The water jet pump (aspirator) in the lab reaches about the vapour pressure of the water, roughly 20–30 mbar.
Pressure range and performance
- Ultimate pressure
- Single stage ≈ 50–100 mbar; water jet ≈ 20–30 mbar; multi-stage steam ≈ 0.1 mbar
- Capacity
- From laboratory aspirators to very large refinery systems
- Moving parts
- None
- Dry?
- Depends on the motive fluid
Advantages
- No moving parts, minimal maintenance
- Handles corrosive, dirty and wet gases
- Low purchase cost and compact
- Scales to very large capacities
Limitations
- High consumption of steam, air or water
- Low energy efficiency
- Steam ejectors produce condensate that may need treatment
- Can be noisy
Typical applications
- Refinery vacuum distillation
- Evaporators and crystallisers
- Deodorising edible oils
- Suction cups in pick-and-place automation
- Laboratory water aspirators
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
How does a water jet pump (aspirator) work?
Tap water flows through a narrow nozzle into a wider tube. The fast stream entrains air from a side port, creating suction for filtration or small distillations. It cannot go below the vapour pressure of the water, about 23 mbar at 20 °C.
What vacuum can a steam ejector reach?
A single stage reaches roughly 50–100 mbar. Several stages in series, with intercondensers removing the steam between them, reach about 0.1 mbar.
Why are compressed-air vacuum generators used in automation?
They are small, light and respond within milliseconds, can be mounted right at the suction cup, and need only compressed air, which most factories already have.
Other pumps
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How a rotary vane vacuum pump works, animated: suction, transport, compression and exhaust, why it needs oil, gas ballast and pressure range.
How a Turbomolecular Pump Works
How a turbomolecular pump works, animated: molecular flow, rotor and stator stages, compression, why it needs a backing pump, and pressure range.
How a Liquid Ring Vacuum Pump Works
How a liquid ring vacuum pump works, animated: eccentric impeller, suction and discharge ports, why water vapour limits the vacuum, and uses.
All vacuum pump types compared
13 pump types with ultimate pressure, speed and applications.