The pumps discussed so far in this chapter have had a variety of moving parts. One type of pump you find in the engine room is the jet pump, usually called an eductor. Figure at left shows an eductor, which has no moving parts. These pumps are used for pumping large quantities of water overboard in such applications as pumping bilges and dewatering compartments. As an engineer, you will think of eductors as part of the main and secondary drainage system; you will also become familiar with them as part of the ship's damage control equipment.
Eductors use a high-velocity jet of seawater to lower the pressure in the chamber around the converging nozzle. Seawater is supplied to the converging nozzle at a relatively low velocity and exits the nozzle at a high velocity. As the seawater leaves the nozzle and passes through the chamber, air becomes entrained in the jet stream and is pumped out of the chamber. Pressure in the chamber decreases, allowing atmospheric pressure to push the surrounding water into the chamber and mix with the jet stream. The diverging nozzle (diffuser) allows the velocity of the fluid to decrease and the pressure to increase; the discharge pressure is then established.
