Jet Pump:

Air ejectors are fundamentally jet pumps or eductors, as described in Figure. In operation, the jet pump has two categories of fluids. They are the high pressure fluid which flows by the nozzle, and the fluid being pumped that flows around the nozzle within the throat of the diffuser. A high velocity fluid enters the diffuser whereas its molecules strike other molecules. Those 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 procedure is known as entrainment. The low pressure area will draw more fluid from around the nozzle within the throat of the diffuser. Since the fluid moves down the diffuser then the rising area converts the velocity back to pressure. By Use of steam at a pressure among 200 psi and 300 psi as the high pressure fluid enables a singlestage air ejector to draw a vacuum of about 26 inches Hg.

Figure: Jet Pump

Generally, air ejectors consist of two suction levels. The first level suction is situated on top of the condenser, when the second stage suction comes from the diffuser of the first level. The exhaust steam from the second stage must be condensed. This is generally accomplished through an air ejector condenser which is cooled through condensate. An air ejector condenser also preheats the condensate returning to the boiler. Two-level air ejectors are capable of drawing vacuums to 29 inches Hg.

A vacuum pump might be any categories of motor-driven air compressor. Their suction is attached to the condenser, and it discharges to the atmosphere. A general category uses rotating vanes in an elliptical housing. Single-level, rotary-vane units are used for vacuums to 28 inches Hg. Two levels units could draw vacuums to 29.7 inches Hg. The vacuum pump has a benefit over the air ejector in which it needs no source of steam for its operation. They are generally used as the initial source of vacuum for condenser start-up.