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.