Boiler is defined as a closed metallic vessel in which the water is heated beyond the boiling state by the application of heat liberated by the combustion of fuels to convert water into steam.

Function of a boiler is to supply the steam at the required constant pressure with its quality either dry, or as nearly as dry, or super-heated. The steam can be supplied from the boiler at a constant pressure by maintaining the steam generation rate and the steam flow rate equal.