Smokebox

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.

The dimensions of the blastpipe and chimney are critical to the steam-generating capacity of the locomotive and its fuel economy, since there is a natural trade-off between a high-velocity steam jet giving a strong draw on the fire, and back-pressure on the exhaust.

More complex exhaust designs such as Kylchap, Lempor and Giesl, can achieve better results than the simple blastpipe and chimney arrangement.

The smokebox incorporates the main steam pipes from the regulator (or superheater header), one leading to each valve chest, a part of the cylinder casting.

British Railways standard classes use this design, where a robust mesh grille is incorporated into the smokebox, forming a filter between the front tubeplate and the exhaust.

Any large pieces of char passing through the boiler tubes tend to be broken up on impact with the mesh, creating finer particles that are swept up into the chimney instead of accumulating in the bottom of the smokebox.

Some designs reversed the layout to avoid problems (asphyxiation and poor visibility) caused by having the exhaust blowing back onto the crew; these were called cab forward locomotives.

Spark arresters are to prevent excessively large fragments of hot ash from being exhausted into the environment where they may pose a fire risk.

The presence of a spark arrester may have a thermodynamic effect, distorting the draw of air over the fire and thereby reducing total power output.

The advantage of superheating is that the steam has greater expansive properties when entering the cylinders, so more power can be gained from a smaller amount of water and fuel.