A
smokebox is one of the major basic parts of a
steam locomotive exhaust system.
Smoke and hot gases pass from the
firebox through tubes where they
pass heat to the surrounding water in the
boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the
chimney (or
funnel). Early locomotives had no smokebox and relied on a long chimney to provide natural draught for the fire but smokeboxes were soon included in the design for two main reasons. Firstly and most importantly, the blast of exhaust steam from the cylinders, when directed upwards through an airtight smokebox with an appropriate design of exhaust nozzle, effectively draws hot gases through the boiler tubes and flues and, consequently, fresh combustion air into the firebox. Secondly, the smokebox provides a convenient collection point for ash and cinders ("char") drawn through the boiler tubes, which can be easily cleaned out at the end of a working day. Without a smokebox, all char must pass up the chimney or will collect in the tubes and flues themselves, gradually blocking them.
