Drum type boilers and boilers whose drums are not heated outside the fire side are of double drum, single drum and multi drum type, and the drums are of horizontal and vertical type.

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.