Smokebox

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.

The smokebox appears to be a forward extension of the boiler although it contains no water and is a separate component. Smokeboxes are usually made from riveted or welded steel plate and the floor is lined with concrete to protect the steel from hot char and acid or rainwater attack.

To assist the passage of the smoke and hot gases, a blower is often used. This is a pipe ending in a ring containing pin-sized holes, which creates a "ring" of steam jets. The steam forces out the smoke and draws further gases through the tubes. This in turn causes air to be drawn through the grate and firehole, making the fire burn hotter.

When the locomotive is in motion, exhaust steam passes through the blastpipe, which is located within the smokebox. The steam is ejected through the chimney, again drawing the fire. The blastpipe is what produces the characteristic "chuff" sound.

Ashes and soot which may be present in the smoke are often deposited in the smokebox. The front of the smokebox has a door which is opened to remove these deposits at the end of each locomotive's working day. The handle(s) must be tightened fully to prevent air leaks, which would reduce the draw on the fire and can also allow any unburnt char at the bottom of the smokebox to catch fire there. Some smokebox doors have a single handle in the form of a wheel; many British-built locomotives have a pair of smokebox door handles resembling the hands of a clock; other designs also exist. On many steamrollers an extension to the body of the smokebox also houses the bearing which supports the front roller. Due to limitations of space, these rollers usually have a drop-down flap instead of a circular smokebox door.

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