The working medium of the steam boiler is water, and the output working medium is steam. Steam can be divided into saturated steam and superheated steam. The working medium of the hot water boiler is water, and the output working medium is unsaturated hot water. Boiler with special working medium Other chemical fluids other than water, such as mercury vapor boiler.

The boiler is an energy conversion equipment. The energy input to the boiler includes chemical energy and electric energy in the fuel, and the boiler outputs steam, high-temperature water or organic heat carrier with certain heat energy.