Firebox (locomotive)

Steam engine firebox

The firedoor into the firebox of a steam locomotive. The firebox peak temperature is approximately 2,500 degrees F (~1371 ℃)

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. The hot gases generated in the firebox are pulled through a rack of tubes running through the firebox boiler.

In the standard steam locomotive firetube type boiler, the firebox is surrounded by water space on five sides. The bottom of the firebox is open to atmospheric pressure, but covered by fire grates (solid fuel) or a firing pan (liquid fuel). If the engine burns solid fuel, like wood or coal, there is a grate covering most of the bottom of the firebox to hold the fire. An ashpan, mounted underneath the firebox and below the grates, catches and collects hot embers, ashes, and other solid combustion waste as it falls through the grates. In a coal-burning locomotive, the grates may be shaken to clean dead ash from the bottom of the fire. They are shaken either manually or (in larger locomotives) by a powered grate shaker. Wood-burning locomotives have fixed grates that can't be shaken. Wood ash is generally powder which will fall through the grates with no more agitation required than the vibrations of the locomotive rolling down the track. The fire grates must be replaced periodically due to the extreme heat they must endure. Combustion air enters through the bottom of the firebox and airflow is usually controlled by damper doors above the ash collection pocket of the ash pan. A locomotive that burns liquid fuel - usually "Bunker C" fuel oil or similar heavy oil - does not have grates. Instead, they have a heavy metal gauge firing pan bolted tight against the bottom of the firebox. The firing pan is covered with firebrick and the firebox has a firebrick lining, usually up to the level of the firebox door, all the way around the firebox. The oil burner is a nozzle containing a slot for the oil to flow out onto a steam jet which atomizes the oil into a fine mist which ignites in the firebox. The oil burner nozzle is usually mounted in the front of the firebox, protected by a hood of firebrick, and aimed at the firebrick wall below the firebox door. Dampers control air flow to the oil fire.

There is a large brick arch (made from fire brick) the front third to half of the firebox. It is supported on arch tubes, thermic syphons, or circulators. The brick arch directs heat, flames, and smoke back over the fire towards the rear of the firebox. Visible smoke contains unburned combustible carbon particles and combustible gasses. The purpose of this redirection is to cause more complete combustion of these particles and gasses which make the locomotive more efficient and causes less visible smoke to be emitted from the stack. Without the arch, flames and visible smoke would be sucked straight into the firetubes without having been fully burned, causing visible smoke to be emitted at the stack. The brick arch and its supports (arch tubes, thermic syphons, and circulators) require periodic replacement due to the extreme heat they endure.