Injector

"Ejector" redirects here. For other uses, see Injector (disambiguation) and Ejector (disambiguation).
Diagram of a typical modern ejector.
Exhaust Steam Injector

An injector, ejector, steam ejector, steam injector, eductor-jet pump or thermocompressor is a type of pump. There are two varieties of injector, non-lifting and lifting.

The non-lifting injector cold water input is fed by gravity. It uses the principle of induced current (Impulse (physics)) to push water up to the boiler check valve. It avoids the premature boiling of feed water at very low absolute pressure, by avoiding the Venturi effect. The steam cone minimum orifice diameter is kept larger than the combining cone minimum diameter.[1] The non-lifting Nathan 4000 injector used on the Southern Pacific 4294 could push 12,000 US gallons (45,000 L) per hour at 250 psi (17 bar).[2]

The lifting injector 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. After passing through the throat of the injector, the mixed fluid expands and the velocity is reduced which results in recompressing 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.[3][4]

The adjacent diagram depicts a typical modern 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 is a particular case of Bernoulli's principle. 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. Steam locomotives use injectors to pump water into the steam-producing boiler and some of the steam is used as the injector's motive fluid. Such steam injectors take advantage of condensation of the motive steam resulting from the mixing with cold feed water.

Depending on the specific application, an injector can take the form of 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, P_2/P_1, is defined as ratio of the injector's outlet pressure P_2 to the inlet pressure of the suction fluid P_1.

The entrainment ratio of the injector, W_s/W_v, is defined as the amount W_s (in kg/h) of suction fluid that can be entrained and compressed by a given amount W_v (in kg/h) of motive fluid.

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

History

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, L- Overflow
A more modern drawing of the injector used in steam locomotives.
Steam injector of a steam locomotive boiler.

The injector was invented by a Frenchman, Henri Giffard in 1858[5] and patented in the United Kingdom by Messrs Sharp Stewart & Co. of Glasgow. Motive force is provided at the inlet by a suitable high-pressure fluid.

Feedwater injectors

The injector was originally used in the boilers of steam locomotives for injecting or pumping the boiler feedwater into the boiler.

Cones

The injector consists of a body containing a series of three or more nozzles, "cones" or "tubes". The motive steam passes through a nozzle that reduces its pressure below atmospheric and increases the steam velocity. Fresh water is entrained by the steam jet, and both steam and water enter a convergent "combining cone" which mixes them thoroughly so that the water condenses the steam, releasing the latent heat of evaporation of the steam. This raises the heat of the feed water but the mixing also imparts extra velocity to the water. The condensate mixture then enters a divergent "delivery cone" which slows down the jet, and because of the additional energy thus imparted, builds up the pressure to above that of the boiler.[6]

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 [6]) 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.

Initial skepticism and advantages over mechanical feed pumps

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 addition of heat to the flow of water lessens the effect of the injected water in cooling the water in the boiler compared to the case of cold water injected via a mechanical feed pump. Most of the heat energy in the condensed steam is therefore returned to the boiler, increasing the thermal efficiency of the process. Injectors are therefore thermally efficient; they are also simple compared to the many moving parts in a feed pump.

Additionally, the amount of water supplied by a mechanical feed pump cannot easily be adjusted; hence a feed pump must be able to supply the maximum demand for water, but then will overfill the boiler at all other times, so an overflow must be installed returning the high-pressure water to the pump's intake. If the feed pump is attached to the motion of the locomotive, it naturally provides water at a rate proportional to the locomotive's speed, which reduces this problem but then means the boiler cannot be refilled when stationary. Traction engines often use feed pumps and can disconnect the motion from the road wheels, and can be seen stationary with their flywheels turning in order to refill their boilers.[6]

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 have gone 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, when vibration caused 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. This can also occur if the metal body of the injector is too hot, e.g. from prolonged use.

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 small ejector is sometimes replaced by a reciprocating pump driven from the crosshead because this is more economical of steam.

Vacuum brakes have been superseded by air brakes in modern trains, which use pumps, as diesel and electric locomotives no longer have a suitable working fluid for vacuum ejectors.

Earlier application of the principle

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 create a negative pressure inside the smokebox and entrain 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.

Modern uses

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

Well pumps

Main article: Water well pump

Jet pumps are commonly used to extract water from water wells. The main pump, often a centrifugal pump, is powered and installed at ground level. Its discharge is split, with the greater part of the flow leaving the system, while a portion of the flow is returned to the jet pump installed below ground in the well. This recirculated part of the pumped fluid is used to power the jet. At the jet pump, the high-energy, low-mass returned flow drives more fluid from the well, becoming a low-energy, high-mass flow which is then piped to the inlet of the main pump.

The S type pump is useful for removing water from a well or container.

Shallow well pumps are those in which the jet assembly is attached directly to the main pump and are limited to a depth of approximately 5-8m to prevent cavitation.

Deep well pumps are those in which the jet is located at the bottom of the well. The maximum depth for deep well pumps is determined by the inside diameter of and the velocity through the jet. The major advantage of jet pumps for deep well installations is the ability to situate all mechanical parts (e.g., electric/petrol motor, rotating impellers) at the ground surface for easy maintenance. The advent of the electrical submersible pump has partly replaced the need for jet type well pumps, except for driven point wells or surface water intakes.

Multi-stage steam vacuum ejectors

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

In operation a two-stage system consists of a primary high-vacuum (HV) ejector and a secondary low-vacuum (LV) ejector. Initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached, the HV ejector is then operated in conjunction with the LV ejector to finally pull vacuum to the required pressure.

In operation a three-stage system consists of a primary booster, a secondary high-vacuum (HV) ejector, and a tertiary low-vacuum (LV) ejector. As per the two-stage system, initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached, the HV ejector is then operated in conjunction with the LV ejector to pull vacuum to the lower intermediate pressure. Finally the booster is operated (in conjunction with the HV & LV ejectors) to pull vacuum to the required pressure.

Construction materials

Injectors or ejectors are made of carbon steel, stainless steel, titanium, PTFE, carbon, and other materials.

See also

References

  1. ↑ Pullen, William Wade Fitzherbert (1900). Injectors: their Theory, Construction and Working (Second ed.). London: The Technical Publishing Company Limited. p. 51.
  2. ↑ Anderson.- O'Day., David N. - Russell M. H. (17 July 2013). Cab-Forward Notes Southern Pacific Railroad's Signature Locomotive (Revision 1 ed.). Sacramento, California: Gerald Rood. p. 66.
  3. ↑ Perry, R.H. and Green, D.W. (Editors) (2007). Perry's Chemical Engineers' Handbook (8th ed.). McGraw Hill. ISBN 0-07-142294-3.
  4. ↑ Power, Robert B. (1993). Steam Jet Ejectors For The Process Industries (First ed.). McGraw-Hill. ISBN 0-07-050618-3.
  5. ↑ Strickland L. Kneass (1894). Practice and Theory of the Injector. John Wiley & Sons (Reprinted by Kessinger Publications, 2007 ). ISBN 0-548-47587-3.
  6. 1 2 3 Goldfinch & Semmens (2000). How Steam Locomotives Really Work. Oxford University Press. pp. 92–97. ISBN 978-0-19-860782-3.
  7. ↑ "Steam-assisted jet pump". General Electric. Retrieved 17 March 2011. United States Patent 4847043 ... recirculation of a coolant in a nuclear reactor

Additional reading

External links

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