An injector, ejector, steam ejector, steam injector, eductor-jet pump or thermocompressor is a type of pump. There are two varieties of injector, non-lifting and lifting. The non-lifting injector cold water input is fed by gravity. It uses the principle of induced current (Impulse (physics)) to push water up to the boiler check valve. It avoids the premature boiling of feed water at very low absolute pressure, by avoiding the Venturi effect. The steam cone minimum orifice diameter is keep larger than the combining cone minimum diameter. The non-lifting Nathan 4000 injector used on the Southern Pacific 4294 could push 12,000 gallons per hour at 250 psi. The lifting injector uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid. After passing through the throat of the injector, the mixed fluid expands and the velocity is reduced which results in recompressing the mixed fluids by converting velocity energy back into pressure energy. The motive fluid may be a liquid, steam or any other gas. The entrained suction fluid may be a gas, a liquid, a slurry, or a dust-laden gas stream. The adjacent diagram depicts a typical modern injector. It consists of a motive fluid inlet nozzle and a converging-diverging outlet nozzle. Water, air, steam, or any other fluid at high pressure provides the motive force at the inlet. The Venturi effect is a particular case of Bernoulli’s principle. Fluid under high pressure is converted into a high-velocity jet at the throat of the convergent-divergent nozzle which creates a low pressure at that point. The low pressure draws the suction fluid into the convergent-divergent nozzle where it mixes with the motive fluid. In essence, the pressure energy of the inlet motive fluid is converted to kinetic energy in the form of velocity head at the throat of the convergent-divergent nozzle. As the mixed fluid then expands in the divergent diffuser, the kinetic energy is converted back to pressure energy at the diffuser outlet in accordance with Bernoulli’s principle. Steam locomotives use injectors to pump water into the steam-producing boiler and some of the steam is used as the injector’s motive fluid. Such steam injectors take advantage of condensation of the motive steam resulting from the mixing with cold feed water. Depending on the specific application, an injector can take the form of an eductor-jet pump, a water eductor, a vacuum ejector, a steam-jet ejector, or an aspirator.

Leave a Comment