Injector
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An injector, ejector, steam ejector or steam injector 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][2]
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-jet pump, a water eductor, a vacuum ejector, a steam-jet ejector, or an aspirator.
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[edit] 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, Ws / Wv, is defined as the amount of motive fluid Ws (in kg/hr) required to entrain and compress a given amount Wv (in kg/hr) of suction fluid..
The compression ratio and the entrainment ratio are key parameters in designing an injector or ejector.
[edit] History
The injector was invented by a Frenchman, Henri Giffard in 1858[3] 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 locomotives for injecting or pumping the boiler feedwater to and from the boiler. The injector consisted of a body containing a series of three or more nozzles, "cones" or "tubes". The motive steam passed through a nozzle that reduced its pressure below atmospheric and increased the steam velocity. Fresh water was entrained by the steam jet, and both steam and water entered a convergent "combining cone" which mixed them thoroughly so that the water condensed the steam. The condensate mixture then entered a divergent "delivery cone" which slowed down the jet, and thus built up the pressure to above that of the boiler. An overflow was required for excess steam or water to discharge, especially during starting. There was at least one check valve between the exit of the injector and the boiler to prevent back flow, and usually a valve to prevent air being sucked in at the overflow.
After some initial skepticism resulting from the unfamiliar and superficially paradoxical mode of operation, the injector was widely adopted as an alternative to mechanical pumps in steam-driven locomotives. The injectors were simple and reliable, and they were thermally efficient.
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.
[edit] 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.
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.
[edit] 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.
[edit] Construction materials
Injectors or ejectors are fabricated in carbon steel, Stainless steel, titanium, PTFE, carbon and other materials.
[edit] See also
[edit] References
- ^ Perry, R.H. and Green, D.W. (Editors) (2007). Perry's Chemical Engineers' Handbook, 8th Edition, McGraw Hill. ISBN 0-07-142294-3.
- ^ Power, Robert B. (1993). Steam Jet Ejectors For The Process Industries, First Edition, McGraw-Hill. ISBN 0-07-050618-3.
- ^ Strickland L. Kneass (1894). Practice and Theory of the Injector. John Wiley & Sons (reprinted Lindsay Publications Inc.). ISBN 1-55918-306-3.
[edit] Additional reading
- J.B. Snell (1973). Mechanical Engineering: Railways. Arrow Books. ISBN 0-09-908170-9.
- J.T. Hodgson and C.S. Lake (1954). Locomotive Management, Tenth Edition, Tothill Press.
