An ejector has two inlets: one to admit the motive fluid, usually steam, and the other to admit the gas/vapor mixture to be evacuated or pumped.
Motive steam, at high pressure and low velocity, enters the inlet and exits the steam nozzle at design suction pressure and supersonic velocity, entraining the vapor to be evacuated into the suction chamber. 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.