air ejector system .pdf
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Air ejector is an essential component of any type of feed system on the ship. As the name
suggests, the equipment completely removes the air from any system and also draws out the
vapours released from the condensing steam in the condenser.
Presence of air in the system leads to several detrimental effects on the boiler. Corrosion is one
such effect that in long run results is failure of the boiler from the inside. Also, the accumulation
of air inside the condenser affects the condensing process, which results in the production of
back pressure. The back pressure thus generated leads to increase in exhaust steam pressure and
reduction in thermal efficiency of the plant. Let's take a look at the construction and working of
conventional air ejectors.
For understanding purpose, we would consider a two stage twin element air ejector. This type of
air ejectors are usually steam operated. The air ejector system is divided into two main stages.
The first stage acts as a pump to draw in the air and vapours from the condenser and the
condensing unit of the system forms the second part. Not all the part of the steam entering the
second stage is converted into condensate and for this reason, a return line is provided for the
non-condensed steam to go back to the second stage. The feed water for the cooling purpose is
supplied to both the stages through a U-tube arrangement as shown in the figure.
The air from the condenser or any other part of the system is sucked in by the first stage of the
air ejector, which acts as a pump. The air and vapours then passes to the second stage which is a
condensing unit. A continuous circulation of feed water inside the condensing unit cools the
steam and vapours entering the second stage. The condensed steam through a drain goes to the
main condenser unit of the feed system. All the remaining air and vapour that are not condensed
in the second stage are sent back to the condensing unit through a return line and the process is
repeated again. Any other gases are released to the atmosphere through a vacuum retaining
Air ejectors are always mounted in pairs in the feed system. Though only one is required for
appropriate functioning of the system, the alternate air ejectors act as standby or helper units.
Adequate numbers of valves are fitted on the steam, vapour and feed water line to isolate each
line whenever required.
It works on the principle of convergent /divergent nozzle as it provides the venturi effect at the
point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases and
because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat where
the suction line will be provided.
An air ejector which uses the high pressure motive fluid such as air or steam to flow through the
convergent nozzle the function of the convergent nozzle is to convert the pressure energy of the
motive fluid into the velocity energy.
As the pressure energy before entering the convergent nozzle is greater and the velocity is less
for the fluid. At the point of discharge the pressure energy is converted into the velocity so the
velocity will be greater and the pressure will be less during the discharge.
Divergent nozzle the opposite effect takes place velocity energy is convert into pressure energy,
at the point of diffusion there is a low pressure or vacuum is created which is used to suck the
other fluid for the motion.
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