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 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.