A boiler, as the name suggests, is a device that boils (water to produce steam). More scientifi­cally, a boiler converts the chemical energy in fuel into the heat energy in steam, or the heat energy of hot gases into the heat energy of steam when no firing is involved. The earliest form of boiler is the kettle.

Boilers have been used for over a century and a half in many countries of Europe and America and also in Japan, South Africa, Russia, and Australia. In the recent years, with rapid industrialization of the developing world, vast power-generating capacities have been added, particularly in countries such as China and India. The intent of this chapter is to, first of all, define the boiler terminology and then study the preliminary aspects such as boiler classification and specification. Thereafter, certain common topics, namely boiler efficiency, performance testing, boiler evaluation, boiler design for optimum performance, boiler codes, and design data preservation, are examined.