2.0.01 Ejector

February 23, 2009

My first laboratory experiment of this semester:

An ejector, or injector, or steam ejector or steam injector is a device used to transport fluid (the suction fluid) using the motion of another fluid (motive fluid).

So an ejector works like this: A high-pressure motive fluid is flowed through a nozzle (preferably a convergent nozzle), and the nozzle converts the pressure energy of a motive fluid into velocity energy, and a low pressure zone is created in front of the nozzle, since the motive fluid is expanded (V>>> therefore P<<<). This low pressure zone then draws in the suction fluid.

After passing through the mixing chamber of the ejector, the mixed fluid expands and the velocity is reduced, resulting in the recompression of the mixed fluid (by converting velocity energy back into pressure energy).

The motive fluid may be a liquid, steam or any other gas. The suction fluid may be a gas, a liquid, a slurry, or a dust-laden gas stream.

The diagram below depicts a typical modern ejector. It consists of a motive fluid inlet nozzle and a converging-diverging outlet nozzle.

ejector1

Water, air, steam, or any other fluid at high pressure provides the motive force at the inlet. The Venturi effect, a particular case of Bernoulli’s principle, applies to the operation of this device. 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.

A bit confused? So am I… >.<

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2 Responses to “2.0.01 Ejector”

  1. john-leon Says:

    would it be wrong of me to say that the motive fluid fluid also force the fluid through the diffuser throat where the low pressure occurs thus cause suction.

  2. Vie Says:

    No, that’s true. The low pressure in the diffuser throat also causes suction, and the velocity of the motive fluid prevents atmospheric air from entering the diffuser (my lecturer says it’s called back pressure).


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