As the name suggests, air preheater heats up air. It absorbs the heat from the flue gas
emitting from the outlet and then transfers the heat to the incoming cold air.
Its main objective is to enhance the thermal efficiency of the process. It is
actually one of the most important boiler accessories used in industrial units
where heating is an integral part.
The air preheater is nothing but a waste heat
recovery device used to reduce the heat lost from hot air. In the boiler, steam
is produced on being heated with coal. It emits hot flue gas with extremely
high temperature. The air preheater absorbs the heat from this flue gas and
transfers this heat to the air going to coal or boiler. The flue gas coming
through the chimney is of no use but its heat can definitely be used. Air
preheater captures this heat and passes this heat to incoming cold air into the
heating system. So in this way, the preheater acts as an energy saving device
because without its absence the coal in the system would have received cold air
and hence it would have required more heat energy to heat up the water in the
boiler.
There are generally two categories of air preheaters available – tubular type
and regenerative. Now-a-days regenerative ones are most commonly used though
tubular ones also exist. The tubular air preheater consists of some vertical
and horizontal tubes for the movement of air.
The hot flue gases come through the tubes in the top and move downward
through the tubes. On the other hand, ambient air enters through the tubes at
the bottom and moves upward. There is a fan installed in the middle to help the
movement. The cold ambient air gets heated up by the downward movement of the
hot flue gases. So inside the preheater, the cold air gets heated prior to the
process and thus increases the thermal efficiency of the boiler. In case of
regenerative preheater, the process of absorbing heat from the hot flue gas is
done with the help of an absorbing material unlike the tubular preheater where
the process is purely mechanical. The two regenerative types – rotating plate
and stationary consist of devices which absorb heat and confine the heat in a
smaller area to heat up the cold ambient air.
The most common type of air preheater that is
being used is the tri sector based rotating plate regenerative air preheater.
Here the major portion of the device is connected to the hot air; whereas a
smaller portion is linked with ambient air by a fan. As the hot flue gasses
heat up the central element by the rotor, the cold atmospheric air gets heated
up.
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