How does a steam-jet or air ejector actually work?
An ejector has three working zones. In the motive nozzle, high-pressure steam or air expands to supersonic velocity, so its pressure falls well below the suction pressure. In the suction chamber and mixing section, that fast jet drags the slower process gas along with it by momentum exchange. In the diffuser, the combined stream slows down again and its velocity converts back into pressure, so the mixture leaves at a pressure higher than the suction.
Because the only energy input is the motive fluid, an ejector is simple and rugged. There is no shaft seal, no lubricant in contact with the process and nothing to seize if the process carries liquid slugs, solids or polymerising vapour. The price of that simplicity is efficiency: much of the motive energy is lost in mixing and shock, and the motive fluid then has to be condensed or discharged along with the process load.
A single ejector stage can only achieve a limited compression ratio economically. Deeper vacuum is reached by putting stages in series, often with a condenser between them so that the steam from the previous stage is removed before the next stage has to compress it.
Where are ejectors used, and where are mechanical pumps better?
Steam ejectors remain common in refineries, petrochemical and chemical plants, edible-oil deodorising and refining, sugar and evaporation duties, vacuum distillation, steam stripping, deaeration, and drying or degassing where the process stream is aggressive or fouling. Compressed-air ejectors (often called venturi vacuum generators) are widely used in packaging, handling and pick-and-place, where small, local vacuum is needed close to the point of use.
Mechanical vacuum pumps such as liquid-ring, rotary-vane, screw, claw and Roots boosters usually use far less energy per unit of gas handled at steady load. The decision is therefore not “which is better” but which fits the process, the utilities available and the cost of running them over the life of the plant.
| Factor | Steam-jet ejector | Compressed-air ejector | Mechanical vacuum pump |
|---|---|---|---|
| Moving parts | None | None | Rotors, vanes, bearings, seals |
| Motive utility | Steam at stable pressure and dryness | Clean, dry compressed air | Electrical power, sometimes seal liquid |
| Tolerance of wet, dirty or corrosive gas | Very high, with suitable materials | Moderate; small passages can foul | Depends strongly on pump type |
| Running energy | High, continuous while operating | High per unit of vacuum flow | Usually lowest at steady load |
| Turndown | Limited for fixed-geometry stages | Simple on/off at point of use | Good with speed control or staging |
| Typical role | Process vacuum in chemical, refinery and oil plants | Local vacuum for grippers and handling | Plant and process vacuum where gas is compatible |
What decides the size of an ejector system?
Ejector performance is fixed by the conditions at its boundaries, so the design starts with an honest load definition rather than a catalogue curve. The specialist designing the stages needs the following, for normal running, start-up and credible upsets:
- Suction pressure required at the process vessel, and the pressure drop of the line and knock-out between vessel and first stage.
- Mass flow and composition of the suction load: condensable vapours, noncondensable gases, and air leakage into the vacuum system.
- Suction temperature, because hot vapour increases the volume each stage must handle.
- Motive steam pressure, temperature and dryness at the nozzle, not at the boiler header. Wet or low-pressure steam is one of the most common reasons an ejector underperforms.
- Cooling-water temperature and flow available for the condensers, including the warmest summer condition.
- Discharge pressure the last stage must reach, such as atmosphere, a vent header or a scrubber with its own resistance.
- Required turndown and whether the process runs continuously or in batches.
Air leakage deserves particular attention. It is a noncondensable load that the condensers cannot remove, so every stage after the first condenser must compress it. A system that met its vacuum on commissioning can lose it years later simply because flange, gland and instrument leaks have grown.
How does a compressed-air ejector fit into this picture?
Compressed-air ejectors use the same physics on a small scale. Air at typically 4–6 bar (58–87 psig) passes through a nozzle and produces vacuum for suction cups, small hoppers or local evacuation. They are compact, instant-acting and ideal for distributed handling, but every ejector is a continuous compressed-air user while it is switched on.
From the compressor room, that means ejectors should be treated as a measurable demand. Multi-stage air ejectors, air-saving controls that switch the air off once vacuum is reached, and correct supply pressure at the ejector (not just at the receiver) all reduce the load on the compressor. For large or continuous vacuum duties, a central mechanical vacuum system is often a better use of energy than many air ejectors running together.
Why is my ejector not pulling the design vacuum?
Ejector faults usually trace back to a boundary condition rather than the ejector itself. A structured check works through them in order:
- Measure motive steam or air pressure and, for steam, check dryness at the nozzle. Check strainers and traps on the motive line.
- Confirm cooling-water inlet and outlet temperatures and flow at each condenser, and compare with the design case.
- Check the discharge pressure. A partly blocked vent, scrubber or silencer raises back-pressure and can stall the last stage.
- Isolate the process and test the ejector train on its own to separate system air leakage from ejector performance.
- Check condensate drains, barometric seals and hot-well levels. A flooded condenser behaves like a blocked one.
- Inspect nozzles and diffusers for erosion, scale or deposits, and confirm the nozzle is the original size.
- Only then compare the measured operating point with the manufacturer’s performance curve for that specific stage.
| Symptom | Likely cause | What to check |
|---|---|---|
| Vacuum slowly worsening over months | Growing air leakage or nozzle wear | Isolation test, leak survey, nozzle bore |
| Vacuum unstable or surging | Low motive pressure, wet steam, high back-pressure | Motive pressure trend, trap condition, vent path |
| Vacuum lost in hot weather | Condenser running too warm | Cooling-water temperature and flow, fouling |
| Sudden loss after start-up | Condensate flooding or seal loss | Barometric leg, hot-well level, drain pump |
| Erratic air-ejector vacuum | Low air pressure at the ejector or dirty air | Pressure at point of use, filters, dryer |
What should be measured at commissioning and in service?
Ejector systems are only as reliable as the evidence kept about them. At commissioning, record suction pressure at the vessel and at the first stage, motive pressure and temperature at each nozzle, cooling-water temperatures and flows, discharge pressure and a measured air-leakage test with the process isolated. These become the baseline for every later fault investigation.
In service, a simple log of the same points, taken at the same plant state, will show slow drift long before vacuum is lost. Note the season as well, because cooling-water temperature changes the baseline. Where the process is safety-critical, vacuum loss and motive pressure alarms should be part of the plant’s own control and safety design.
When should a hybrid ejector and mechanical system be considered?
Many modern plants combine stages. One common arrangement uses steam ejectors for the deep-vacuum first stages, where the gas volume is large, and a liquid-ring vacuum pump in place of the final ejector stages, where steam use would otherwise be highest. Another uses a Roots booster in front of a mechanical backing pump. These combinations can reduce steam consumption, but they change condenser duty, discharge conditions, turndown and maintenance, and they must be designed as one system by a specialist using the actual equipment data.
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand compressed-air, vacuum and blower specialist based at Tarnaka, Secunderabad, serving industry since 2002. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India. Ejector stage design is the work of the ejector manufacturer or a process specialist, and we say so plainly; our part is the utilities and equipment around it.
- Supplying, servicing and maintaining the compressed-air plant that feeds air ejectors, including dryers and filtration for clean, dry motive air.
- Measuring compressed-air demand from ejector-based handling systems and advising on air-saving controls or a central vacuum alternative.
- Supplying and servicing mechanical vacuum pumps such as liquid-ring, rotary-vane and screw types, including as backing or replacement stages in hybrid systems, across ELGi, Atlas Copco, Ingersoll Rand, KAESER and other makes.
- Selling, renting and servicing Roots and other blowers, and supplying their spares.
- Collecting the boundary data a specialist needs, and helping plant teams carry out isolation and baseline tests.
Call +91 99590 60718 or +91 92468 88812, or write to reddycompressors@gmail.com. Our office is at Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017.
What to send us
- Process description, required suction pressure and normal and upset gas loads.
- Motive utility details: steam pressure and quality, or compressed-air pressure and quantity available.
- Existing ejector or vacuum equipment make, model and nameplate photos.
- Cooling-water temperature and flow, including summer values.
- Recent operating log readings and a description of the fault or the improvement wanted.
Frequently asked questions
What is the difference between a steam ejector and a vacuum pump?
A steam ejector uses a high-velocity steam jet to entrain and compress process gas, with no moving parts, while a vacuum pump uses rotating elements driven by a motor. Ejectors tolerate wet, dirty and corrosive vapours very well but use steam continuously. Mechanical pumps usually use less energy at steady load but must be compatible with the process gas, so the choice depends on the process and the utilities available.
Why does my steam jet ejector lose vacuum in summer?
Most often the condensers are running warmer because cooling-tower water temperature has risen. A warmer condenser leaves more vapour in the gas passed to the next stage, which then cannot compress it to the required pressure, and the train can break vacuum suddenly. Check cooling-water temperature, flow and condenser fouling, and confirm the design allowed for the hottest ambient conditions at your site.
How much steam does a vacuum ejector use?
Steam consumption depends on the suction pressure, the gas load, the number of stages, the motive steam conditions and the condenser water temperature, so there is no single figure. The ejector manufacturer calculates it from the full load definition. On an existing plant, it can be measured from motive line flow or estimated from condenser heat load, and compared with the original data sheet to show deterioration.
Can compressed air be used instead of steam in an ejector?
Yes, compressed-air ejectors are widely used for local vacuum such as suction cups and small hoppers. They need clean, dry air at a stable pressure at the ejector, typically in the 4–6 bar range. For large or continuous process vacuum they become expensive to run, because the compressor supplies the motive air continuously, so a mechanical vacuum pump is often more economical.
What is a barometric leg on a vacuum condenser?
A barometric leg is a vertical drain pipe from a vacuum condenser into a sealed hot well. The column of liquid in the pipe balances atmospheric pressure, letting condensate flow out without air leaking back in. For water this needs a height of a little over 10 m. If the seal is lost or the hot well floods, the condenser and ejector stage can lose vacuum.
How do I check whether my vacuum problem is the ejector or an air leak?
Isolate the process vessel and run the ejector train on its own, recording the suction pressure it achieves with motive and cooling conditions at design values. Then reconnect the process and repeat. If the ejector alone performs correctly, the shortfall is in the process side, usually air leakage or extra vapour load. A rate-of-rise test on the isolated vessel helps quantify the leakage.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.