An eductor moves fluid using nothing but a pressure drop across a nozzle. There is no rotating assembly, no seal to leak and no bearing to fail, which is why the principle keeps earning its place in plants that have long since automated everything around it.
How it works
A motive fluid is accelerated through a converging nozzle. The velocity rise produces a local pressure drop, which draws a second fluid in through the suction branch. The two streams mix in the throat and recover pressure in the diverging section before leaving through the discharge.
The three connections
- Motive inlet – the driving fluid, usually steam, water or compressed air.
- Suction inlet – the fluid being entrained.
- Discharge – the combined stream at an intermediate pressure.
Where an eductor earns its keep
The economics rarely turn on efficiency in isolation. An eductor is usually less efficient than a pump on paper, and still the better answer once the whole installation is counted.
- Duties where a motive fluid is already available at pressure and would otherwise be throttled away.
- Intermittent service, where a pump would spend most of its life idle but would still need maintaining.
- Hazardous or abrasive media, where eliminating the seal removes the dominant failure mode.
- Locations with no power supply, or where installing one is the largest line in the budget.
The question is not whether an eductor is efficient. It is whether the energy it uses was going to be recovered by anything else.
Sizing considerations
Performance depends on the ratio between motive, suction and discharge pressures rather than on any single figure. A unit that performs well at one discharge pressure can stall at a slightly higher one, so the sizing case should be the worst combination the system can present, not the design point.
Common mistakes
The two that recur most often are specifying against an optimistic motive pressure, and forgetting that the suction line itself contributes losses. Both show up as a unit that entrains far less than the curve promised.