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 original meaning of the boiler refers to the water container heated on the fire, and the furnace refers to the place where fuel is burned. The boiler includes the boiler and the furnace. The hot water or steam generated in the boiler can directly provide the required heat energy for industrial production and people's life, and can also be converted into mechanical energy through the steam power device, or converted into electrical energy through the generator. The boiler that provides hot water is called hot water boiler, which is mainly used for life, and also has a small number of applications in industrial production. The boiler that generates steam is called steam boiler, often referred to as boiler for short, and is mainly used in thermal power plants, ships, locomotives and industrial and mining enterprises.