Fire-tube boiler

      A fire-tube boiler is a type of boiler in which hot gasses from a fire pass through one or more tubes running through a sealed container of water. The heat of gasses is transferred through the walls of the tubes by thermal conduction , heating the water and ultimately creating steam.
      The fire-tube boiler developed as the third of the four major historical types of boilers : low-pressure tank or "haystack" boilers , flued boilers with one or two large flues , fire-tube boilers with many small tubes and high-pressure water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water perforated by tubes that carry the hot flue gasses from the fire. The tank is usually cylindrical for the most part - being the strongest practical shape for a pressurized container and this cylindrical tank may be either horizontal or vertical.

Sectioned fire-tube boiler from a DRB Class 50 locomotive. Hot flue gases created in the firebox (on the left) pass through the tubes in the centre cylindrical section, which is filled with water, to the smokebox and out of the chimney (far right).

       This type of boiler was used on virtually all steam locomotives in the horizontal "locomotive" form. This has a cylindrical barrel containing the fire tubes , but also has an extension at one end to house the "firebox". This firebox has an open base to provide a large great area and often extends beyond the cylindrical barrel to form a rectangular or tapered enclosure. The horizontal fore-tube boiler is also typical of marine applications , using the scotch boiler. Vertical boilers have also been built of the multiple fire-tube type , although these are comparatively rare : most vertical boilers were either flued or with cross water-tubes.