Jet Pump

The general term, jet pump, includes all machines whose operation is based on the transfer of energy by impact from a fluid jetting at high velocity into a slowly moving or stagnant fluid, giving the mixture of fluids a moderately high velocity which is then reduced carefully so as to give a final pressure greater then the initial pressure of the low-velocity fluid. An injector is a jet pump using a condensable gas to entrain a liquid and discharging at pressure higher than the initial pressure of the motive fluid or the entrained fluid. It is now practically restricted to boiler feed-water injection. An ejector is a jet pump more general in character, using either gas or liquid for either the motive or the entrained fluid and discharging at a pressure intermediate between the motive pressure and the suction pressure. An exhauster, blower, or compressor is an ejector with gases both the motive and entrained fluids; a siphon is a ejector with gas as the motive fluid and liquid as the entrained fluid; an eductor is an ejector with liquids both as motive and entrained fluid; and a fume absorber is an ejector with liquid as the motive fluid and gas as the entrained liquid.

Figure B. Typical steam-jet ejector.

Figure C. Booster ejector with multiple steam nozzles.

An ejector is a simplified type of vacuum pump or compressor which has no pistons, valves, rotors, or other moving parts. Figure B illustrates a steam-jet ejector. It consists essentially of a nozzle which discharges a high-velocity jet across a suction chamber that is connected to the equipment to be evacuated. The gas is entrained by the steam and carried into a venturi-shaped diffuser which converts the velocity energy into pressure energy. Figure C shows a largesized ejector, sometimes called a booster ejector, with multiple nozzles. Nozzles are devices in subsonic flow that have a decreasing area and accelerate the flow. They convert pressure energy to velocity energy. A minimum area is reached when velocity reaches sonic flow. In supersonic flow, the nozzle is an increasing area device. A diffuserin subsonic flow has an increasing area and converts velocity energy into pressure energy. A diffuser in supersonic flow has a decreasing area.
Two or more ejectors may be connected in series or stages. Also, a number of ejectors may be connected in parallel to handle larger quantities of gas or vapor.
Liquid- or air-cooled condensers are usually used between stages. Liquid-cooled condensers may be of either the direct-contact (barometric) or the surface type. By condensing vapor the load on the following stage is reduced, thus minimizing its size and reducing consumption of motive gas. Likewise, a precondenser installed ahead of an ejector reduces its size and consumption if the suction gas contains vapors that are condensable at the temperature condition available. An aftercondenser is frequently used to condense vapors from the final stage, although this does not affect ejector performance
Uses of Ejectors For the operating range of steam-jet ejectors in vacuum applications, see the subsection “Vacuum Systems.”
The choice of the most suitable type of ejector for a given application depends upon the following factors:

1. Steam pressure. Ejector selection should be based upon the minimum pressure in the

supply line selected to serve the unit.
2. Water temperature. Selection is based on the maximum water temperature.
3. Suction pressure and temperature. Overall process requirements should be

considered. Selection is usually governed by the minimum suction pressure required

(the highest vacuum).

4. Capacity required. Again overall process requirements should be considered, but

selection is usually governed by the capacity required at the minimum process

pressure.

Ejectors are easy to operate and require little maintenance. Installation costs are low. Since they have no moving parts, they have long life, sustained efficiency, and low maintenance cost. Ejectors are suitable for handling practically any type of gas or vapor. They are also suitable for handling wet or dry mixtures or gases containing sticky or solid matter such as chaff or dust.
Ejectors are available in many materials of construction to suit process requirements. If the gases or vapors are not corrosive, the diffuser is usually constructed of cast iron and the steam nozzle of stainless steel. For more corrosive gases and vapors, many combinations of materials such as bronze, various stainless-steel alloys, and other corrosion-resistant metals, carbon, and glass can be used.