Firebox (steam engine)

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Section of typical British boiler and firebox. The firebox is the 'spotty' area on the right of the image, although the spots actually represent stays. The brick arch, firetubes and grate can also be seen.

In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name.

In the standard steam locomotive firetube type boiler, the firebox is surrounded by water space on five sides. The underside is not so surrounded. If the engine burns solid fuel, there is a grate covering most of the bottom of the firebox to hold the fuel. An ashpan collects the solid combustion waste below. Combustion air generally enters at the base, and the airflow is usually controlled by damper doors. There is a large brick arch (made from fire brick) at the front of the box which directs heat and flames back towards the firedoor at the rear. Without the arch, flames would be sucked straight into the firetubes and only the front of the box would receive heat. Both the brick arch and the bars of the grate require periodic replacement due to the extreme heat to which they are subjected.

Firetubes are attached to one wall of the firebox (the front wall for a longitudinal boiler, the top for a vertical boiler) and carry the hot gaseous products of combustion through the boiler water, heating it, before they escape to the atmosphere.

The metal walls of the firebox are normally called sheets, which are separated by stays. Since any corrosion is hidden, the stays may have longitudinal holes, called tell-tales, drilled in them which leak before they become unsafe. The crown sheet is the top of the firebox.

Normally the top of the firebox is semicircular to match the contour of the boiler, however the Belpaire firebox has more of a square shape and is usually made as large as possible within the loading gauge, to offer the greatest heating surface where the fire is hottest.

The Wootten firebox was very tall and wide to allow combustion of anthracite coal waste. Its size necessitated unusual placement of the crew, examples being camelback locomotives.

Some fireboxes were equipped with a so-called combustion chamber which placed additional space between the fire and the boiler. This allowed more complete combustion and thus greater heat.

The fireman's chief duty on a steam locomotive is to maintain the boiler water level so that it covers the firebox crown sheet at all times. Otherwise, the latter will desiccate, overheat and weaken, and a boiler explosion may result.

Normal round-top firebox
A 4-8-2 locomotive with a normal firebox. The round top of the firebox make attaching the boiler easier.
Belpaire firebox
A PRR N1s. The flat sides and square corners show the shape of the Belpaire firebox. This offers a greater heating surface, increasing the efficiency of the engine.
Wootten firebox
An Erie L-1. The Wootten firebox can be seen as the large construction just in front of the tender. Note the unusual position of the drivers cab. The fireman was left exposed between firebox and tender.