A boiler is also known as steam generator made of steel which is usually a closed vessel. The main function of boiler is to transfer the heat which is produced by the combustion of fuel to water and ultimately to generate steam.
The steam produced may be supplied to:
1.     Power Generation.
2.     Heating.
a.      Space heating.
b.     Hot water supply.
3.     Industrial Processes.
a.      Textile mills.
b.     Sugar mills.
c.     Chemical industries.
Classification of Boilers: Generally boiler is two types. These are,
1.     Fire tube boiler.
2.     Water tube boiler.
Fire tube boiler: Fire tube boiler is surrounded by water which passes hot gasses through the tubes. Horizontal return tubular, Lancashire, Comish, Scotch marine, Vertical tubular, Cochran etc.
Water tube boiler: Water tube boiler contains water and the hot gasses flow outside. Babock and Wilcox, La-mont boiler, Stirling boiler, Benson boiler and Loeffler boiler.