Firebox (steam engine)

For other uses, see Firebox (disambiguation).

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.

Steam Locomotive Fire Tube Firebox

The firedoor into the firebox of a steam locomotive

Diagramatic section through an earlier steam locomotive boiler and firebox to the right. Note the boiler is not fitted with a superheater.

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.

Schematic of a later steam locomotive firebox boiler, with firebox to the left and indicatively showing two superheater elements to the right. These elements receive wet steam in parallel from the boiler and discharge collected dry steam to the engine. The colours blue and red represent cooler and hotter water respectively. The colours dark and light green represent wet and dry steam respectively. The colours orange and yellow represent hot and cooler exhaust gas respectively. In the diagram three small diameter orange\yellow empty firetubes are indicated to the bottom with two larger diameter orange\yellow firetubes to the top that contain the superheater elements.

Brick arch

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. The brick arch and the bars of the grate require periodic replacement due to the extreme heat they endure.

Firetubes

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.

Sheets and stays

Cutaway of locomotive firebox and boiler. Note the stays to support the "sheets" (plates) against pressure, the fusible plugs and the "mudhole" to allow access for scraping away scale

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.

Belpaire 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.

Wootten firebox

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.

Combustion chamber

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.

Fireman's duties

The fireman's role on a steam locomotive is to ensure the driver has an adequate supply of steam at his disposal at all times. This is achieved by maintaining a supply of fuel to the fire, and by maintaining the boiler water level so that it covers the firebox crown sheet at all times – otherwise, the latter will overheat and weaken, and a boiler explosion may result. In addition, the fireman also serves as a backup for the driver, keeping a lookout ahead.

Gallery

• 

  Locomotive with a normal firebox. The round top of the firebox makes attaching the boiler easier

• 

  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

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  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

Road locomotive firebox

Road locomotives, such as traction engines, usually had fireboxes similar to those on railway locomotives but there were exceptions, e.g. the Sentinel steam waggon which had a vertical water tube boiler.

Stationary boiler firebox

There were, and are, many different designs of firebox for stationary boilers. In flue-type boilers (e.g. the Lancashire boiler) the flues themselves form the firebox. In water-tube boilers, the firebox is usually a firebrick-lined compartment below the water tubes.

Marine boiler firebox

In marine boilers there are also various types of firebox. The main distinction is, again, between fire-tube types (e.g. the Scotch boiler, with internal firebox) and water-tube types (e.g. the Yarrow boiler, with external firebox).

See also

• Firetube boiler
• Flue
• Fusible plug