A vacuum system typically consists of one or more stages of ejectors with intermediate condensers which produces the condensation of the vapors coming from the previous stage.
The suction pressure varies in a range from 0.100 to 200 Torr. For higher absolute pressures, a single steam ejector is enough, that is, only one stage.
When vacuum systems with steam ejectors consist of several stages, air, gas and steam drawn out from the facility where vacuum is being created, undergo a compression in each stage. The number of stages is equal to the number of ejectors connected in series.
Intermediate condensers are used for condensing motive steam from ejectors, additionally to the steam aspirated from the facility where vacuum is being created, thereby suction load is reduced in each stage. These condensers can be either mixing or direct contact as well as surface.
The required amount of stages depends on the level of vacuum required in accordance with the following figure: