Classification of boilers

 According to the substance flowing through the tube


Fire tube boiler
In fire tube boiler, hot gases pass through the tubes and water surrounds them. Heat from the gases (produced by combustion) is transferred to water, which is then converted to steam.
Examples: Cochran, Lancashire, Locomotive boilers
Water tube boilers
In water tube boiler, water flows inside the tubes and the hot gases flow outside the tubes.
Examples: Babcock and Wilcox boiler, Stirling boiler.

According to Method of firing

Internally fired: Furnace region is provided inside the boiler shell and is completely surrounded by water-cooled surfaces. E.g., Lancashire, Locomotive
Externally fired: Furnace region is provided outside. E.g., Babcock and Wilcox boiler

According to Pressure of steam

Low pressure boiler: Steam pressure below 80 bars.
Examples: Cochran, Cornish, Lancashire, locomotive boilers
High pressure boiler: Pressure above 80 bars.
Examples: Babcock and Wilcox, Lamont


According to circulation of water

Natural circulation: Circulation of water in the boiler takes place by natural convection current produced by the application of heat.
Examples: Lancashire, Locomotive, Babcock and Wilcox boilers
Forced circulation:

Lancashire boiler
- Two large fire tubes
Cornish boiler
- Single large fire tube
Cochran boiler
Locomotive boiler