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An injector, ejector, steam ejector or steam
injector is a pump-like device 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 and then
recompresses 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.
The adjacent diagram depicts a typical modern
ejector or 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, 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.
Depending on the specific application, an injector
is commonly also called an Eductor-jet pump, a water
eductor, a vacuum ejector, a steam-jet ejector, or
an aspirator.
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 was provided
at the inlet by a suitable high-pressure fluid.
The injector was originally used in the boilers of
steam-driven railroad 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.
Steam locomotives dominated rail transport from the
mid 19th century until the mid 20th century, after
which they were superseded by diesel and electric |
