Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A Water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. FUel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. IN smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.THe heated water then rises into the steam drum. HEre, saturated steam is drawn off the top of the drum. IN some services, the steam will reenter the furnace through a superheater to become superheated. SUperheated steam is defined as steam that is heated above the boiling point at a given pressure. SUperheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.