Cornish boilers (Fire-tube boilers)

      The earliest form of fire-tube boiler was Richard Trevithick's "high-pressure" Cornish boiler. This is a long horizontal cylinder with a single large flue containing the fire. The fire itself was on an iron grating placed across this flue , with a shallow ashpan beneath to collect the non-combustible residue. Although considered as low-pressure (perhaps 25psi) today , the use of a cylindrical boiler shell permitted a higher pressure than the earlier "haystack" boilers of Newcomen's day. As the furnace relied on natural draught (air flow) , a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire.
      For efficiency , the boiler was commonly encased beneath by a brick-built chamber.Flue gases were routed through this , outside the iron boiler shell , after passing through the fire-tube and so to a chimney that was now placed at the front face of the boiler.

Operation

Schematic diagram of a "locomotive" type fire-tube boiler

      In the locomotive-type boiler , fuel is burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water connected to the long , cylindrical boiler shell. The hot gases are directed along a series of fire tubes or flues , that penetrate the boiler and heat the water thereby generating saturated ("wet") steam. The steam rises to the highest point of the boiler , the steam dome , where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.
      In the locomotive boiler , the saturated steam is very often passed into a superheater , back through the larger flues at the top of the boiler , to dry the steam and heat it to superheated steam. The superheated steam is directed to the steam engine's cylinders or very rarely to a turbine to produce mechanical work. Exhaust gases are fed out through a chimney , and may be used to pre-heat the feed water to increase the efficiency of the boiler.
      Draught for firetube boilers , particularly in marine applications , is usually provided by a tall smokestack. In all steam locomotives  , since Stephenson's Rocket , additional draught is supplied by directing exhaust steam from the cylinders into the smokestack through a blastpipe , to provide a partial vacuum. Modern industrial boilers use fans to provide forced or induced draughting of the boiler.
      Another major advance in the Rocket was large numbers of  small-diameter fire tubes (a multi-tubular boiler) instead of a single large flue. This greatly increased the surface area for heat transfer , allowing steam to be produced at a much higher rate. Without this , steam locomotives could never have developed effectively as powerful prime movers.

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.