3 PASS WET BACK BOILER (VS) DRY BACK BOILERS

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.

3 PASS Wet Back Boilers

In 3 pass 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.