Boilers can also be further classified on the basis of the construction of the reversal chamber in fire tube design
Reversal Chamber
This is the posterior portion of
the combustion chamber through which the flue gases travel from the first pass
(furnace) to the second-pass tubes.
Wet Back Boilers
In wet back boilers as the name suggests the reversal chamber is surrounded
by water. Refer the fig. As we can see in the fig, the combustion reversal
chamber is surrounded by water and therefore the heat in the flue gases is
optimally utilized. Radiation losses are reduced as none of the parts of the
combustion chamber are open to the atmosphere instead, they are surrounded by
water.
Dry Back Boilers
The
reversal chamber in dry back boilers is not completely surrounded by water. The
posterior part is exposed to the atmosphere. This leads to increased
radiation losses, as the radiant heat is lost to the atmosphere instead of
going to the water as in wet back boilers. Earlier generation boilers used to
be dry back.
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