Fire-tube Boiler - Operation

Operation

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 firetubes (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.