Fire-tube boiler

boiler where hot gases from a combustion process pass through tubes surrounded by water, heating the water to create steam

A fire-tube boiler is a type of boiler in which hot gases pass from a fire through many tubes. These run through a sealed container of water. The heat of the gases passes through the walls of the tubes, and this heats the water and creates steam.

Diagram of a fire-tube boiler

Most steam locomotives have fire-tube boilers.