Steam Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid (steam) exits the boiler for use in various processes or heating applications.

A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

Figure 1: Locomotive fire tube boiler schematic

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.

Figure 2: Water tube boiler schematic