Injector
Key design parameters
The compression ratio of the injector, P2 / P1, is defined as ratio of the injectors’s outlet pressure P2 to the inlet pressure of the suction fluid P1.
The entrainment ratio of the injector, Ws / Wv, is defined as the amount of motive fluid Ws (in kg/hr) required to entrain and compress a given amount Wv (in kg/hr) of suction 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
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 and patented in the United Kingdom by Messrs Sharp Stewart & Co. of Glasgow. Motive force was provided at the inlet by a suitable high-pressure fluid.
The injector was originally used in the boilers of steam locomotives for injecting or pumping the boiler feedwater to and from the boiler. The injector consisted of a body containing a series of three or more nozzles, “cones” or “tubes”. The motive steam passed through a nozzle that reduced its pressure below atmospheric and increased the steam velocity. 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 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 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 injectors were simple and reliable, and they were thermally efficient.
Efficiency was further improved by the development of a multi-stage injector which was 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.
Steam locomotives dominated rail transport from the mid 19th century until the mid 20th century, after which they were superseded by diesel and electric locomotives.
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 thermal power stations, they are used for the removal of the boiler bottom ash, the removal of fly ash from the hoppers of the electrostatic precipitators used to remove that ash from the boiler flue gas, and for creating a vacuum pressure in steam turbine exhaust condensers.
For use in producing a vacuum pressure in steam jet cooling systems.
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.
Similar devices called aspirators based on the same operating principle are used in laboratories to create a partial vacuum and for medical use in suction of mucus or bodily fluids.
Multi-stage steam ejectors
In practice, for suction pressure below 100 mbar absolute, more than one ejector will be used, usually with condensors between the ejector stages. Condensing of motive steam greatly improves ejector set efficiency. Both barometric and shell-and-tube surface condensers are used for this purpose.
Construction materials
Injectors or ejectors are fabricated in carbon steel, Stainless steel, titanium, PTFE, carbon and other materials.
See also
Aspirator
De Laval nozzle
Diffusion pump
Nozzle
Surface condenser
Venturi effect
References
^ Perry, R.H. and Green, D.W. (Editors) (2007). Perry’s Chemical Engineers’ Handbook (8th Edition ed.). McGraw Hill. ISBN 0-07-142294-3.
^ Power, Robert B. (1993). Steam Jet Ejectors For The Process Industries (First Edition ed.). McGraw-Hill. ISBN 0-07-050618-3.
^ Strickland L. Kneass (1894). Practice and Theory of the Injector. John Wiley & Sons (Reprinted by Kessinger Publications, 2007 ). ISBN 0-548-47587-3.
Additional reading
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
Wikimedia Commons has media related to: Feedwater injectors
How an Ejector works – Ejector Specialists
Ejector Pumps and Theory
Ejectors
Use of Eductor for Lifting Water
Categories: Chemical engineering | Mechanical engineering | Fluid dynamics | Pumps | Locomotive parts | Steam locomotive technologies
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