Firebox (steam engine)
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Steam Locomotive Fire Tube Firebox
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.
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. Firetubes serve the additional purpose of staying the flat tube (flue) sheets (front and rear) so that only the top of the front flue sheet and the bottom of the rear flue sheet must be separately braced.
Sheets and stays 
The sheets on the left and right are called "side sheets." The sheet in the front of the firebox is the "rear flue sheet." The "front flue sheet" is in the front of the boiler and at the rear of the smokebox. The "rear sheet" is at the back of the firebox and has the door opening in it. The crown sheet is the top of the firebox. The crown sheet must be covered by water at all times. If the water level drops below the crown sheet, it will become overheated and start to melt and deform, usually sagging between the crown stays. If the condition continues, the crown sheet will eventually be forced off the crown stays by the pressure in the boiler, resulting in a boiler explosion. This condition, usually caused by human error or inattention, is the single greatest cause of a locomotive boiler explosion. A low water condition is always attended by great danger and if discovered, the crew must immediately take steps to 1) save human life, and 2) try to save the locomotive. The train should be stopped, preferably in an area of low population (if possible) and the train crew (conductor, brakemen, flagmen, etc) should begin evacuations of the immediate area after securing the train with hand brakes. The locomotive crew (engineer and fireman) must determine the level of the water in the boiler. If the water level can't be determined with any certainty, it must be assumed it is below the crown sheet and the fire must immediately be extinguished. Oil fires are extinguished by closing the oil firing valve, then closing the oil tank valve in the tender. Coal fires are extinguished by shaking the grates with full strokes of the shaker bar and dumping the fire into the ash pan, which must then be raked out to completely remove the heat source. Since the grates are immobile in a wood burner, wood fires must be extinguished by dousing the fire with water or covering it with earth or similar methods. If the water level is below the crown sheet, water should not be added to the boiler in an attempt to raise it. If water comes into contact with an overheated crown sheet, it may flash to steam causing a sudden overpressure in the boiler which could initiate an explosion (i.e. similar to a blacksmith dousing a red-hot horse shoe in a bucket of cold water). The firebox door may be opened and the crown sheet examined, but this may only reveal the extent of the damage. Once the fire is out and the locomotive secured, the locomotive crew should abandon the locomotive and leave the area. It should also be noted that the train's air brakes should be set in the emergency position before the locomotive crew leaves the locomotive. To avoid a runaway train, the train crew must set sufficient hand brakes on the train to hold it in the event the air brakes leak off.
Normally the top of the boiler (wrapper sheet) over the firebox is radial to match the contour of the boiler; however, due to the problem of placing stays at right angles to both the wrapper sheet and the crown sheet (see above) the Belpaire firebox was developed. In the Belpaire design, the wrapper sheet is roughly parallel with the firebox sheets to allow better placement of the stays. This arrangement gives the firebox end of the boiler a more 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 most notable user of the Belpaire firebox in the United States was the Pennsylvania Railroad. Other railroads, such as the Great Northern and Illinois Central, had locomotives with Belpaire fireboxes. Illinois Central 4-6-0 #382, Casey Jones' engine, had a Belpaire firebox.
Some fireboxes were equipped with a so-called combustion chamber which placed additional space between the fire and the rear flue sheet. This allowed more complete combustion and increased firebox surface area for greater heat transfer.
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. Both the engineer and fireman must read the train orders, the fireman being a double-check on the understanding of the orders and the compliance of the engineer to the orders. The fireman keeps a lookout on his side of the locomotive and train for signal indications, turnout settings, and general train conditions and confirms the engineer's observations of signal aspects and turnout settings. Unless the locomotive is equipped with an air-powered bell ringer for the engineer's use, the fireman will ring the bell at all places required (before moving, at road crossings, etc). The fireman also serves as a backup for the engineer in case the engineer becomes incapacitated. If the train is stopped on the road (mainline) under circumstances which it may be overtaken by another train and the locomotive end of the train needs to be protected, the fireman may be called upon to act as flagman (assuming no other crewman, like a head brakeman, is available) and walk ahead of the train a sufficient distance to stop an oncoming train. At the terminal before the trip, the fireman makes sure the engineer's oil cans and grease guns are filled, the tender wheel bearings (journals) are properly oiled, and the locomotive is fully supplied (tools, water, fuel, sand, etc) to start out the day's work. At the terminal after the day's work is done, the fireman will fill the boiler with water and either bank or dump the fire (ie extinguish the fire), according to company policy, and chock the driver wheels to prevent the locomotive from moving while it is unattended.
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
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).
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