How to use the steam ejector sizing calculator
An ejector compresses gas using nothing but the velocity energy of a motive steam jet, with no moving parts at all. They are used for process vacuum, for pulling non-condensables out of surface condensers and, as injectors, for feeding boilers. Performance comes down to three numbers: compression ratio, expansion ratio and the entrainment ratio of suction load to motive steam.
Compression ratio is discharge absolute pressure divided by suction absolute pressure, and expansion ratio is motive absolute pressure divided by suction absolute pressure. Motive nozzle throat area is back-calculated from the choked steam flow relation W = 51.5 × A × P × Cd. The nozzle only chokes when motive pressure is roughly 1.8 times discharge pressure or more, so the tool stops and warns you when that condition is not met.
The suggested stage count reflects common practice for steam ejectors discharging to atmosphere. Above a compression ratio of about 10 the job is normally split into two stages, and above 80 into three, with intercondensers between them. Actual entrainment ratio depends heavily on nozzle and diffuser geometry, so always confirm it against the manufacturer performance curve.
Frequently asked questions
Enter the combined air, non-condensable gas and water vapour that has to be pulled out, in lb/hr. For a surface condenser, size it from the HEI air leakage tables rather than guessing.
Less steam passes the nozzle, so suction capacity falls away sharply and the ejector can become unstable and hunt. Running below the design motive pressure is not recommended.