Editing Injector (section)

==Operation==
The injector consists of a body filled with a secondary fluid, into which a motive fluid is injected. The motive fluid induces the secondary fluid to move. Injectors exist in many variations, and can have several stages, each repeating the same basic operating principle, to increase their overall effect.

It uses the [[Venturi effect]] of a [[De Laval nozzle|converging-diverging nozzle]] on a steam jet to convert the [[pressure]] energy of the steam to [[velocity]] energy, reducing its pressure to below that of the atmosphere, which enables it to entrain a fluid (e.g., water). After passing through the convergent "combining cone", the mixed fluid is fully condensed. The condensate mixture then enters a divergent "delivery cone" which slows the jet, converting kinetic energy back into static pressure energy above the pressure of the boiler enabling its feed through a non-return valve.<ref>“THE STEAM INJECTOR.” BY MR.F.T.BARWELL,  G.W.R. MECHANICS’ INSTITUTE. SWINDON ENGINEERING SOCIETY. TRANSACTIONS, 1929-30. ORDINARY MEETING. — JANUARY 21ST, 1930</ref><ref name=goldfinch>{{cite book|last1=Goldfinch & Semmens|title=How Steam Locomotives Really Work|date=2000|publisher=Oxford University Press|isbn=978-0-19-860782-3|pages=92–97}}</ref>

Most of the heat energy in the condensed steam is returned to the boiler, increasing the [[thermal efficiency]] of the process. Injectors are therefore typically over 98% energy-efficient overall; they are also simple compared to the many moving parts in a feed pump.
[[Image:Injektor Dampfstrahlpumpe.jpg|thumb|350px|Steam injector of a locomotive boiler]]

===Key design parameters===
Fluid feed rate and operating pressure range are the key parameters of an injector, and vacuum pressure and evacuation rate are the key parameters for an ejector.

Compression ratio and the entrainment ratio may also be defined:

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_m</math>, is defined as the amount <math>W_s</math> (in kg/h) of suction fluid that can be entrained and compressed by a given amount <math>W_m</math> (in kg/h) of motive fluid.

===Lifting properties===
Other key properties of an injector include the fluid inlet pressure requirements i.e. whether it is lifting or non-lifting.

In a non-lifting injector, positive inlet fluid pressure is needed e.g. the cold water input is fed by gravity.

The steam-cone minimal orifice diameter is kept larger than the combining cone minimal diameter.<ref>{{cite book|last1=Pullen|first1=William Wade Fitzherbert|title=Injectors: their Theory, Construction and Working|date=1900|publisher=The Technical Publishing Company Limited|location=London|page=51|edition=Second |isbn=0951936751}}</ref> The non-lifting Nathan 4000 injector used on the [[Southern Pacific 4294]] could push 12,000 US gallons (45,000&nbsp;L) per hour at 250 psi (17&nbsp;bar).<ref>{{cite book|last1=Anderson|first1=David N.|last2=O'Day|first2=Russell M. H.|title=Cab-Forward Notes Southern Pacific Railroad's Signature Locomotive|date=17 July 2013|publisher=Gerald Rood|location=Sacramento, California|page=66|edition=Revision 1}}</ref>

The lifting injector can operate with negative inlet fluid pressure i.e. fluid lying below the level of the injector. It differs from the non-lifting type mainly in the relative dimensions of the nozzles.<ref>The Model Injector, Ted Crawford, Tee Publishing</ref>

===Overflow===
An overflow is required for excess steam or water to discharge, especially during starting. If the injector cannot initially overcome boiler pressure, the overflow allows the injector to continue to draw water and steam.

===Check valve===
There is at least one [[check valve]] (called a "clack valve" in locomotives because of the distinctive noise it makes<ref name="goldfinch"/>) 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.

===Exhaust steam injector===
Efficiency was further improved by the development of a multi-stage injector which is powered not by live steam from the boiler but by exhaust steam from the cylinders, thereby making use of the residual energy in the exhaust steam which would otherwise go to waste. However, an exhaust injector also cannot work when the locomotive is stationary; later exhaust injectors could use a supply of live steam if no exhaust steam was available.

===Problems===
Injectors can be troublesome under certain running conditions, such as when vibration causes the combined steam and water jet to "knock off". Originally the injector had to be restarted by careful manipulation of the steam and water controls, and the distraction caused by a malfunctioning injector was largely responsible for the [[1913 Ais Gill rail accident]]. Later injectors were designed to automatically restart on sensing the collapse in vacuum from the steam jet, for example with a spring-loaded delivery cone.

Another common problem occurs when the incoming water is too warm and is less effective at condensing the steam in the combining cone. That can also occur if the metal body of the injector is too hot, e.g. from prolonged use.

The internal parts of an injector are subject to erosive wear, particularly damage at the throat of the delivery cone which may be due to [[cavitation]].<ref>{{cite web | url=https://www.clan-line.org.uk/tech/injectors/ | title=Clan Line : Injectors }}</ref>

===Vacuum ejectors===
{{Main|Vacuum ejector}}
[[Image:Ejector or Injector.svg|thumb|right|396px|Diagram of a typical modern ejector]]
An additional use for the injector technology is in vacuum ejectors in [[railway brake|continuous train braking systems]], which were made compulsory in the UK by the [[Regulation of Railways Act 1889]]. A vacuum ejector uses steam pressure to draw air out of the vacuum pipe and reservoirs of continuous train brake. Steam locomotives, with a ready source of steam, found ejector technology ideal with its rugged simplicity and lack of moving parts. A steam locomotive usually has two ejectors: a large ejector for releasing the brakes when stationary and a small ejector for maintaining the vacuum against leaks. The exhaust from the ejectors is invariably directed to the smokebox, by which means it assists the blower in draughting the fire.  The small ejector is sometimes replaced by a reciprocating pump driven from the [[crosshead]] because this is more economical of steam and is only required to operate when the train is moving.

Vacuum brakes have been superseded by air brakes in modern trains, which allow the use of smaller brake cylinders and/or higher braking force due to the greater difference from atmospheric pressure.

===Earlier application of the principle===
[[File:Smokebox ejection mechanism.png|thumb|right|200px|Sketch of the smokebox of a steam locomotive, rotated 90 degrees. The similarity to the generic injector diagram at the top of this article is apparent.]]
An empirical application of the principle was in widespread use on steam locomotives before its formal development as the injector, in the form of the arrangement of the [[blastpipe]] and chimney in the locomotive smokebox. The sketch on the right shows a cross section through a smokebox, rotated 90 degrees; it can be seen that the same components are present, albeit differently named, as in the generic diagram of an injector at the top of the article. Exhaust steam from the cylinders is directed through a nozzle on the end of the blastpipe, to reduce pressure inside the smokebox by entraining the flue gases from the boiler which are then ejected via the chimney. The effect is to increase the draught on the fire to a degree proportional to the rate of steam consumption, so that as more steam is used, more heat is generated from the fire and steam production is also increased. The effect was first noted by [[Richard Trevithick]] and subsequently developed empirically by the early locomotive engineers; [[Stephenson's Rocket]] made use of it, and this constitutes much of the reason for its notably improved performance in comparison with contemporary machines.