A fire tube boiler consists of a drum with a tube sheet on each end in which the fire tubes are fastened. Water is contained within the drum surrounding the fire tubes. Fuel is burned in a combustion chamber associated with the boiler and arranged in such a manner that the heat and products of combustion (flue gases) pass through the inside of the fire tubes to heat the water surrounding them. The combustion chamber may be a refractory-lined box located against one end of the drum or a steel chamber located within the drum and surrounded on all but one side by the water in the drum. In the first instance, the boiler may be described as externally fired; in the second, as internally fired.
Horizontal-return-tube boilers were popular in the early refineries. The Scotch marine boiler is of a fire tube design commonly employed in refinery package-type sulfur plants.