Injector

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Diagram of a typical modern ejector.

An injector, ejector, steam ejector, steam injector, eductor-jet pump or thermocompressor is a type of pump 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. 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.[1][2]

The adjacent diagram depicts a typical modern ejector. 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. 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 takes the form of an eductor-jet pump, a water eductor, a vacuum ejector, a steam-jet ejector, or an aspirator.

Key design parameters[edit]

The compression ratio of the injector, P_2/P_1, is defined as ratio of the injectors'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 of motive fluid W_s (in kg/h) required to entrain and compress a given amount W_v (in kg/h) of suction fluid.

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

History[edit]

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[3] 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[edit]

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

Cones[edit]

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 reduced 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. The condensate mixture then enters a divergent "delivery cone" which slows down the jet, and thus builds up the pressure to above that of the boiler.

Overflow[edit]

An overflow is required for excess steam or water to discharge, especially during starting.

Check valve[edit]

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.

Initial skepticism[edit]

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 key to understanding how it works is to appreciate that steam, having a much lower density than water, attains a much higher velocity than water would in flowing from a high pressure to a low pressure through the steam cone. When this jet of steam meets cold water in the combining cone, the principle of conservation of momentum applies. The steam is condensed by mixing with the cold water but the flow of water is accelerated by absorbing the momentum of the high velocity water molecules condensed from the steam. Since the steam, in condensing, gives up its latent heat energy, this causes the temperature of the resultant jet of water to be raised. When this accelerated jet of water passes through the delivery cone, it is capable of developing a much higher pressure than that of the original supply of steam and is thus able to overcome the boiler pressure at the check valve, thereby allowing water to enter the boiler. Furthermore, 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 simple and reliable and also thermally efficient.

Exhaust steam injector[edit]

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.

Problems[edit]

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[edit]

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[edit]

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[edit]

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.
  • For the bulk handling of grains or other granular or powdered materials.
  • The construction industry uses them for pumping turbid water and slurries.
  • Some aircraft (mostly earlier designs) use an ejector attached to the fuselage to provide vacuum for gyroscopic instruments such as an attitude indicator.
  • Aspirators are vacuum pumps based on the same operating principle and are used in laboratories to create a partial vacuum and for medical use in suction of mucus or bodily fluids.
  • Water eductors are water pumps used for dredging silt and panning for gold, they're used because they can handle the highly abrasive mixtures quite well.
  • To create vacuum system in vacuum distillation unit (oil refinery)

Well pumps[edit]

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[edit]

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[edit]

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

See also[edit]

References[edit]

  1. ^ Perry, R.H. and Green, D.W. (Editors) (2007). Perry's Chemical Engineers' Handbook (8th Edition ed.). McGraw Hill. ISBN 0-07-142294-3. 
  2. ^ Power, Robert B. (1993). Steam Jet Ejectors For The Process Industries (First Edition ed.). McGraw-Hill. ISBN 0-07-050618-3. 
  3. ^ Strickland L. Kneass (1894). Practice and Theory of the Injector. John Wiley & Sons (Reprinted by Kessinger Publications, 2007 ). ISBN 0-548-47587-3. 
  4. ^ "Steam-assisted jet pump". General Electric. Retrieved 17 March 2011. "United States Patent 4847043 ... recirculation of a coolant in a nuclear reactor" 

Additional reading[edit]

  • 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 ed.). Tothill Press. 

External links[edit]