Steam Eductor Design Basics
Steam eductors use high-velocity steam as the motive fluid to entrain and compress a suction gas or liquid. Understanding the key design parameters — motive pressure, suction pressure, and compression ratio — is essential for correct sizing.
How a Steam Eductor Works
A steam eductor (also called a steam jet ejector) consists of three sections: a converging-diverging steam nozzle, a mixing chamber, and a diffuser. High-pressure steam enters the nozzle and expands to supersonic velocity, creating a low-pressure zone at the nozzle exit. This low pressure draws in the suction fluid (gas, vapor, or liquid). The two streams mix in the mixing chamber, and the diffuser converts velocity back to pressure.
The Three Pressures
Compression Ratio — The Key Design Parameter
The compression ratio (CR) is the ratio of discharge pressure to suction pressure: CR = P₃ / P₂. This single number largely determines eductor performance and steam consumption.
| Compression Ratio | Typical Application | Steam Consumption |
|---|---|---|
| 1.5–3:1 | Tank heating, low-pressure boost | Low |
| 3–8:1 | Vacuum generation (rough vacuum) | Moderate |
| 8–20:1 | Medium vacuum (25–100 mmHg abs) | High |
| 20–100:1 | High vacuum (5–25 mmHg abs) | Very high — multi-stage required |
For compression ratios above 8:1, multi-stage eductor systems (with inter-condensers between stages) are typically more economical than a single large eductor.
Sizing Inputs Required
Motive steam pressure
PSIG at eductor inlet
Motive steam quality
Saturated or superheated
Suction fluid
Gas, vapor, or liquid
Suction pressure
PSIA or mmHg absolute
Suction flow rate
lb/hr or SCFM
Discharge pressure
PSIA required at outlet
Common Applications
Tank Heating
Steam motive fluid heats tank contents directly — 95% thermal efficiency, no heat exchanger required.
Vacuum Generation
Draw vacuum on vessels, evaporators, and distillation columns — no mechanical vacuum pump.
Priming Centrifugal Pumps
Remove air from pump suction before startup — eliminates cavitation on startup.
Condensate Pumping
Lift condensate from below-grade condensate pots to return headers.
Desuperheating
Inject water into superheated steam to reduce temperature to saturation.
Chemical Injection
Use steam to entrain and inject liquid chemicals into a process stream.
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