User:Milton Beychok/Sandbox: Difference between revisions

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[[Image:Ejector or Injector.png|right|thumb|325px|{{#ifexist:Template:Ejector or Injector.png/credit|{{Ejector or Injector.png/credit}}<br/>|}}Figure 1: Schematic diagram of a typical injector or ejector. For a steam-jet ejector, the motive fluid is steam.]]
An '''injector''', '''ejector''', '''steam ejector''' or '''steam injector''' is a device that uses the [[Venturi effect]] of a [[de Laval nozzle|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.<ref>{{cite book|author=Perry, R.H. and Green, D.W. (Editors)|title=[[Perry's Chemical Engineers' Handbook]]|edition=8th Edition|publisher=McGraw Hill|year=2007|id=ISBN 0-07-142294-3}}</ref><ref>{{cite book|author=Power, Robert B.|title=Steam Jet Ejectors For The Process Industries|edition=First Edition|publisher=McGraw-Hill|year=1993|id=ISBN 0-07-050618-3}}</ref>


Figure 1 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''.
==Key design parameters==
The compression ratio of the injector, <math>P_2/P_1</math>, is defined as ratio of the injector's outlet pressure <math>P_2</math> to the inlet pressure of the suction fluid <math>P_1</math>.
The entrainment ratio of the injector, <math>W_s/W_v</math>, is defined as the amount of motive fluid <math>W_s</math> (in kg/hr)  required to entrain and compress a given amount <math>W_v</math> (in kg/hr) of suction fluid..
The compression ratio and the entrainment ratio are key parameters in designing an injector or ejector.
==History==
[[Image:Giffard's Injector drawing.jpg|right|thumb|300px|Drawing of Henri Giffard's Injector<br>
A- Steam from boiler, B- Needle valve, C- Needle valve handle, D- Steam and water combine, E- Water feed, F- Combining cone, G- Delivery nozzle and cone, H- delivery chamber and pipe, K- Check valve]]
The injector was invented by a [[France|Frenchman]], [[Henri Giffard]], in 1858<ref>{{cite book|author=Strickland L. Kneass|title=Practice and Theory of the Injector|edition=|publisher=John Wiley & Sons (Reprinted by Kessinger Publications, 2007)|year=1894|id=ISBN 0-548-47587-3}}</ref> and patented in the [[United Kingdom]] by Messrs. Sharp Stewart & Co. of [[Glasgow]].  Motive force was provided at the inlet by high-pressure steam. 
The injector was originally used in the steam-generating boilers of [[steam locomotive]]s for injecting or pumping the boiler feedwater to and from the boiler. 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 [[Condensation|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, 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 locomotive|diesel]] and [[electric locomotive]]s.
==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 [[fossil fuel power plants]]s, they are used for the removal of the boiler [[bottom ash]], the removal of [[fly ash]] from the hoppers of the [[electrostatic precipitator]]s used to remove that ash from the boiler [[flue gas]], and for creating a vacuum pressure in [[steam turbine]] exhaust [[Surface condenser|condensers]]. 
* For use in producing a vacuum pressure in [[steam jet cooling]] systems.
* For the bulk handling of [[food grains]] and 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]].
==Multi-stage steam-driven ejectors==
For suction pressure below 100 [[Bar (unit)|mbar]] absolute (1.5 [[psia]]), more than one steam-driven ejector will be used in series, usually with steam [[condenser]]s between the ejector stages. Condensing of motive steam greatly improves the efficiency of the set of ejectors. Both [[barometric condenser]]s and shell-and-tube [[surface condenser]]s are used for this purpose.
==References==
{{reflist}}
==External links==
*[http://www.croll.com/_website/pr/vetheory.asp Ejector Pumps and Theory]

Revision as of 09:54, 9 August 2008

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