S team G eneration in M odern
B oilers
There are water and steam tubes
contained in two large chambers known
as the combustion and flue gas chamber
respectively, and are both lined with
refractory material. The hot gas generated
rise upward and pass into the flue gas
chamber from which they go to the stack
and are expelled. A force draught fan blows
air into the chamber to keep a pressure
inside higher than atmospheric.
When preheated water is fed (say at 270
oC) it enters through the economizer
where it is further heated to 350 oC. The
economizer is a coil of tubes placed at the
lower side of the flue gas flow path and
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PECM Issue 31
exchanges heat with the flue gas. The feed
water advances to the drum and down the
downcommers to the bottom header and
up to the risers in the combustion chamber
where it partly vaporizes. It returns to the
steam disengaging drum where steam and
water separates, where the steam proceeds
to the primary superheater while the
water returns though the downcommer.
There are cyclones in the steam drum
to prevent carry over of water droplets
and other particles. The steam leaves the
primary super heater at a temperature
of about 4300C and pass through two
special cylinders called de-superheaters
where is mixes with water and immediately
vaporizes, thereby lowering the
temperature of the steam. After steam
proceeds to the secondary super heater
where it is heated up to 530 oC.
The heat in the boiler is obtained from
combustion of fuel oil and gas. The air
required for the combustion is sucked in
from outside and sent to the combustion
chamber by draught air fans. The air is
preheated to 60 oC by a network of coil
carrying steam. There is the special air
heaters (sometimes the Ljungstorms)
in which the air further get heated
considerably. The air heater is placed in the
flue gas pass way to the stack. The flue gas
passes through the air heater to the stack
at control temperature.
Note: This article was first featured in the
May 2009 issue (Volume 3 and Issue 5) of
Steam & Boiler Review magazine.