An ejector has two inlets: one to admit the motive fluid, usually steam (inlet 1), and the other to admit the gas/vapor mixture to be evacuated or pumped (inlet 2).
Motive steam, at high pressure and low velocity, enters the inlet 1 and exits the steam nozzle at design suction pressure and supersonic velocity, entraining the vapor to be evacuated into the suction chamber through inlet 2. The nozzle throat diameter controls the amount of steam to pass through the nozzle at a given pressure and temperature.
The entrained gas/vapor flow and the motive fluid (steam) flow mix while they move through the converging section of the diffuser, increasing pressure and reducing velocity. The velocity of this mixture is supersonic and the decreasing cross sectional area creates an overall increase in pressure and a decrease in velocity. The motive fluid slows down and the inlet gas stream picks up speed and, at some point in the throat of the diffuser, their combined flow reaches the exact speed of sound. A stationary, sonic-speed shock wave forms there and produces a sharp rise in absolute pressure. The shock wave in the diffuser throat changes the velocity from supersonic to sub-sonic.
Then, in the diverging section of the diffuser, the velocity of the mixture is sub-sonic and the increasing cross sectional area increases the pressure but further decreases the velocity.
The net result of these energy transformations is an increase of the absolute pressure of the mixture on discharge to several times the pressure at which it entered the ejector inlet.
The choice of the ejector system will primarily depend on the final vacuum to be achieved. The presence and, indeed, the amount of condensables in the gas stream will influence the choice of system, as will the operating costs and capital available.
A Nash vacuum pump may, of course, be used as the final stage of any package, an option that is often employed to reduce operation costs.
Pressurized motive fluids other than steam, such as ethylene glycol, air or nitrogen, follow the same operating principles as steam ejectors.
This also applies to ejectors using air at atmospheric pressure as motive fluid, but these systems are limited in compression ratio. The energy in motive air is developed with the help of a vacuum pump, operating at lower pressures, pulling atmospheric air thru a diverging nozzle, increasing its velocity and dropping its pressure at the nozzle exit. This process, as in a steam ejector system, entrains process gases thru the suction connection.
See more ejector information:
All ejector systems
Ejector/pump hybrid systems
Ejector systems for refineries (pdf)
Ejector systems for the chemical industry (pdf)
Ejectors for steel degassing (pdf)