(as related to a fire-tube boiler
) is the flow of combustion products going from the firebox, through the boiler and up the stack
. Draft may be natural, produced by the rising of the combustion gases in the stack, or by mechanical means, for example a blower
. Since the stack of a locomotive
is too short to provide natural draft, during normal running forced draft is achieved by directing the exhaust steam from the cylinders through a cone (“blast pipe
”) upwards and into a skirt at the bottom the stack. When the locomotive is stationary or in a restricted space “live” steam from the boiler is directed through an annular ring surrounding the blast pipe to produce the same effect.