Injector
An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic pump with no moving parts except a valve to control inlet flow.
Depending on the application, an injector can also take the form of an eductor-jet pump, a water eductor or an aspirator. An ejector operates on similar principles to create a vacuum feed connection for braking systems etc.
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.
Steam injector
The steam injector is a common device used for delivering water to steam boilers, especially in steam locomotives. It is a typical application of the injector principle used to deliver cold water to a boiler against its own pressure, using its own live or exhaust steam, replacing any mechanical pump. When first developed, its operation was intriguing because it seemed paradoxical, almost like perpetual motion, but it was later explained using thermodynamics. Other types of injector may use other pressurised motive fluids such as air.History
Giffard
The injector was invented by Henri Giffard in early 1850s and patented in France in 1858, for use on steam locomotives. It was patented in the United Kingdom by Sharp, Stewart and Company of Glasgow.After some initial scepticism resulting from the unfamiliar and superficially paradoxical mode of operation, the injector became widely adopted for steam locomotives as an alternative to mechanical pumps.
Kneass
was a civil engineer, experimenter, and author, with many accomplishments involving railroading. Kneass began publishing a mathematical model of the physics of the injector, which he had verified by experimenting with steam. A steam injector has three primary sections:- Steam nozzle, a diverging duct, which converts high pressure steam to low pressure, high velocity wet steam
- Combining tube, a converging duct, which mixes high velocity steam and cold water
- Delivery tube, a diverging duct, where a high velocity stream of steam and cold water become a slow high pressure stream of water
Nozzle
An ideal gas cools during adiabatic expansion, releasing less energy than the same gas would during isothermal expansion. Expansion of steam follows an intermediate thermodynamic process called the Rankine cycle. Steam does more work than an ideal gas, because steam remains hot during expansion.
The extra heat comes from enthalpy of vaporization, as some of the steam condenses back into droplets of water intermixed with steam.
Combining tube
At the end of the nozzle, the steam has very high velocity, but at less than atmospheric pressure, drawing in cold water which becomes entrained in the stream, where the steam condenses into droplets of water in a converging duct.Delivery tube
The delivery tube is a diverging duct where the force of deceleration increases pressure, allowing the stream of water to enter the boiler.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 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. 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.
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,, is defined as ratio of the injector's outlet pressure to the inlet pressure of the suction fluid.
The entrainment ratio of the injector,, is defined as the amount of suction fluid that can be entrained and compressed by a given amount 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. The non-lifting Nathan 4000 injector used on the Southern Pacific 4294 could push 12,000 US gallons per hour at 250 psi.
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.
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 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.
Vacuum ejectors
An additional use for the injector technology is in vacuum ejectors in 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.