The combustion and endothermic evaporation of shell type boiler are completed in a cylinder, which can be divided into horizontal type and vertical type, such as the early Lanzhou Development and firecracker furnace. Water tube boiler is a boiler whose main heating surface is tubes, which is a major improvement of early boilers, and its safety and reliability are greatly improved.

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.