Air ejectors are essentially jet pumps or eductors, as illustrated in Figure 10. In operation, the
jet pump has two types of fluids. They are the high pressure fluid that flows through the nozzle,
and the fluid being pumped which flows around the nozzle into the throat of the diffuser. The
high velocity fluid enters the diffuser where its molecules strike other molecules. These
molecules are in turn carried along with the high velocity fluid out of the diffuser creating a low
pressure area around the mouth of the nozzle. This process is called entrainment. The low
pressure area will draw more fluid from around the nozzle into the throat of the diffuser. As the
fluid moves down the diffuser, the increasing area converts the velocity back to pressure. Use
of steam at a pressure between 200 psi and 300 psi as the high pressure fluid enables a singlestage air ejector to draw a vacuum of about 26 inches Hg.
Normally, air ejectors consist of two suction stages. The first stage suction is located on top of
the condenser, while the second stage suction comes from the diffuser of the first stage. The
exhaust steam from the second stage must be condensed. This is normally accomplished by an air ejector condenser that is cooled by condensate. The air ejector condenser also preheats the condensate returning to the boiler. Two-stage air ejectors are capable of drawing vacuums to 29 inches Hg.
A vacuum pump may be any type of motor-driven air compressor. Its suction is attached to the condenser, and it discharges to the atmosphere. A common type uses rotating vanes in an elliptical housing. Single-stage, rotary-vane units are used for vacuums to 28 inches Hg. Two stage units can draw vacuums to 29.7 inches Hg. The vacuum pump has an advantage over the air ejector in that it requires no source of steam for its operation. They are normally used as the initial source of vacuum for condenser start-up.