STEAM INJECTOR & USES - SKengineers
STEAM INJECTOR -
Steam injectors use steam to raise the temperature of water
or other liquids. The injector draws in cold liquid, mixes it with steam inside
the injector and distributes the heated liquid in the tank. In many
applications the circulation induced by the injector is an advantage ensuring
thorough mixing and avoiding temperature stratification.
How Steam Injectors Work -
Before the invention of internal combustion engines the
hand-car was the primary vehicle used for maintaining hundreds of thousands of
miles of railroad tracks. Since it is a human powered vehicle, and considering
the iron construction of the vehicle, it was not a joy to ride as seen in the
movies. Also, considering the additional load on these hand cars the drivers
should be a muscular and lean man to save on the extra weight.
Imagine a moment that you want to pump water from
atmospheric pressure to a boiler which has pressure level 10 to 15 times above
the atmospheric pressure. Before the inventor of steam injector the cold water
pumped to boiler by steam reciprocating pump, pumping water to the boiler. The
first problem with this design is that, steam used to operate the pump
discharged to the atmosphere which reduces overall efficiency of the
locomotive. The second problem is that we are pumping cold water into the
boiler which reduces boiler temperature. The third but not the last problem is
that you have to maintain a pump which has many mowing parts.
The steam injectors solve all these problems in one simple
an ingenious design. The sketch you see here is not a proportional technical
drawing, it is just an animation, which puts emphasize on the injector and
makes the rest of the parts such as boiler disproportionally small.
Here how a stem injector works. The steam is taken from the
boiler is send to the injector with high pressure and with low velocity to the
injector. This flow indicated by red background with white dots showing the
direction of the flow.
The flow is the directed to the converging, diverging steam
cone. In this cones converging section, the steam velocity is increased to
speed of the sound. At the diverging section, the pressure further reduced and
converted to the kinetic energy where steam is moving faster than the speed of
the sound. By the time steam lives the cone, speed of the steam is much faster
than the speed of sound, but its pressure is below the atmospheric pressure.
This flow is shown with light red background.
The low pressure created at the exit of the steam cone
causes the water in the reservoir to be sucked in to this vacuum shown as blue
backgrounded water flow.
The water leaving steam cone with high velocity enters to
the combining cone. It is called combining cone, because the steam is also
sucking large amount of water from reservoir. During this combining process,
steam begin to condense and the water begin to get warmer. This mixing and
condensing flow proses is show with purple background. During combining
process, speed of water was increasing due to hammer effect of the steam jet,
but steam was disappearing from the flow as water. Further narrowing the
combining cone increases the speed of water further. This flow is show as light
blue background.
What lays in between combining cone and the delivery cone is
overflow chamber. Adding this chamber into the steam injector was ingenuity of
steam injector inventor Henri Giffard. This chamber and overflow pipe allowed
the excess water to be send back to reservoir and preventing to steam injector
to choke, especially when it is starting to operate the first time. It allowed
the injector to operate smoothly.
Steam injectors operation will be interrupted by turning the
valve shown here manually or automatically off. This will cut the steam flow to
the injector, in turn, water flow from reservoir to stop. During this time, the
check valve shown in here will prevent the boiler water to escape to the
reservoir.
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 (e.g., water). After passing through the convergent
"combining cone", the mixed fluid is fully condensed, releasing the
latent heat of evaporation of the steam which imparts extra velocity to the
water. 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.
Steam
injector of a locomotive boiler -
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.
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 (45,000 L) per hour
at 250 psi (17 bar).
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 (called a "clack
valve" in locomotives because of the distinctive noise it makes 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.
Vacuum
ejectors -
Diagram
of a typical modern ejector -
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.
Earlier
application of the principle -
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 reduce pressure inside the smokebox by entraining 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 -
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 drawing a vacuum pressure in steam turbine exhaust condensers.
Jet pumps have been used in boiling water nuclear reactors
to circulate the coolant fluid.
For use in producing a vacuum pressure in steam jet cooling
systems.
For expansion work recovery in air conditioning and
refrigeration systems.
For enhanced oil recovery processes in the oil & gas
Industry.
For the bulk handling of grains or other granular or
powdered materials.
The construction industry uses them for pumping turbid water
and slurries.
Educators are used in ships to pump residual ballast water,
or cargo oil which cannot be removed using centrifugal pumps due to loss of
suction head and may damage the centrifugal pump if run dry, which may be
caused due to trim or list of the ship.
Educators are used on-board ships to pump out bilges, since
using centrifugal pump would not be feasible as the suction head may be lost
frequently.
Some aircraft (mostly earlier designs) use an ejector
attached to the fuselage to provide vacuum for gyroscopic instruments such as
an attitude indicator (artificial horizon).
Educators are used in aircraft fuel systems as transfer
pumps; fluid flow from an engine-mounted mechanical pump can be delivered to a
fuel tank-mounted educator to transfer fuel from that tank.
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 educators 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).
Vacuum autoclaves use an ejector to pull a vacuum, generally
powered by the cold water supply to the machine.
Low weight jet pumps can be made out of paper mache.
Construction
materials -
Injectors or ejectors are made of carbon steel, stainless
steel, brass, titanium, PTFE, carbon, and other materials.
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