Injector

  An injector, ejector or steam ejector is a pump-like device that 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 and then recompresses 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.^[1]

The adjacent diagram depicts a typical modern ejector or 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, 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.

Depending on the specific application, an injector is commonly also called an eductor, a steam-jet ejector, a jet pump or an aspirator.

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Key design parameters

The compression ratio of the injector, P₂ / P₁, is defined as ratio of the injectors's outlet pressure, P₂, to the inlet pressure of the suction fluid, P1.

The entrainment ratio of the injector, W_s / W_v, is defined as the amount of motive fluid, W_s (in kg/hr), required to entrain and compress a given amount, W_v (in kg/hr), of suction fluid..

The compression ratio and the entrainment ratio are key parameters in designing an injector or ejector.

History

The injector was invented by a Frenchman, Henri Giffard in 1858 and patented in the United Kingdom by Messrs Sharp Stewart & Co. of Glasgow. Motive force was provided at the inlet by a suitable high-pressure fluid.

The injector was originally used in the boilers of steam-driven railroad locomotives for injecting or pumping the boiler feedwater to and from the boiler.

Steam locomotives dominated rail transport from the mid 19th century until the mid 20th century, after which they were superseded by diesel and electric locomotives.

Uses

The use of injectors (or ejectors) in various industrial applications has become quite common due to their relative simplicity and adaptability. For example:

• To inject chemicals into the boiler drums of small, stationary, low pressure boilers. In large, high-pressure modern boilers, usage of injectors for chemical dosing is not possible due to their limited outlet pressures.

• In thermal power stations, they are used for the removal of the boiler bottom ash, the removal of fly ash from the hoppers of the electrostatic precipitators used to remove that ash from the boiler flue gas, and for creating a vacuum pressure in steam turbine exhaust condensers.

• For use in producing a vacuum pressure in steam jet cooling systems.

• For the bulk handling of grains or other granular or powdered materials.

• The construction industry uses them for pumping turbid water and slurries.

Similar devices called aspirators based on the same operating principle are used in laboratories to create a partial vacuum and for medical use in suction of mucus or bodily fluids.

Multi-stage steam ejectors

In practice, for suction pressure below 100 mbar absolute, more than one ejector will be used, usually with condensors between the ejector stages. Condensing of motive steam greatly improves ejector set efficiency. Both barometric and shell-and-tube surface condensers are used for this purpose.

Construction materials

Injectors or ejectors are fabricated in carbon steell, Stainless steel, titanium, PTFE, Carbon:carbon and other materials.

See also

• Aspirator
• De Laval nozzle
• Diffusion pump
• Nozzle
• Steam injector
• Surface condenser
• Venturi effect

References