Keep the Fire Burning, Keep the Burning Hot (Ctrl+ Click)
The typical steam locomotive employs a steel fire-tube boiler that contains water and steam under pressure. A firebox is normally located in the rear of the boiler (chimney or stack in front). The firebox has a water filled steel chamber surrounding the top and sides of the flame in the firebox. If wood or coal is used to make the fire in the firebox it is built on a set of grates where ashes may be separated from the burning fuel. These ashes must periodically be removed from the engine. If wood or coal are the fuel used in the firebox there is a door at the rear of the firebox that is opened to add more fuel. If oil is used there nearly always is a door for adjusting the air flow, maintenance or for cleaning the oil jets.
To extract even more heat, the smoke and hot gases produced by combustion in the firebox travel horizontally through a bundle of parallel tubes submerged in the water in the boiler from the front of the firebox to the front of the boiler. The heat extracted in the firebox and tubes in the boiler converts the water in the boiler to high-pressure steam. To minimize heat loss from the boiler it is normally surrounded with layers of insulation or lagging.
The water and steam in the boiler are kept pressurized to raise the boiling temperature of the water and generate high-pressure steam. The amount of pressure in the boiler is monitored by the engineer or fireman by a gauge mounted in the cab. Excess steam pressure can be released manually or may blow off through a safety valve. Excessive pressure may cause the boiler to violently burst, resulting in injuries and fatalities to nearby individuals, as well as extensive damage to the locomotive itself and nearby structures.
At the front of the boiler is the smokebox, where exhaust steam is ejected into the chimney or stack, drawing the smoke and combustion gases through the fire tubes in the boiler and out the top of the chimney. The combustion in a typical steam engine is not very complete leading to the production of prodigious amounts of smoke, as well as sparks. This characteristic made these engines very dirty to live around, as well as being an acute hazard while passing through a forest, tunnel or snow shed.
The steam generated in the boiler is used to drive the locomotive and also for other purposes (whistles, brakes, pumps, passenger car heating, etc.). The constant use of steam requires the boiler to have water continually pumped into it (usually by automatic means). The source of this water is an unpressurized tank that is usually part of the locomotive's tender. Periodic stops are required to refill the tender.
During operation, the boiler's water level is constantly monitored, normally via a transparent tube referred to as a sight glass, or with a gauge. Maintaining a proper water level is crucial to the efficient and safe operation of the boiler. If the water level is too high, steam production is decreased, efficiency is lost and in extreme cases, water will be carried out with the steam into the cylinders, possibly causing mechanical damage. More seriously, if the water level gets too low, the crown (top) and/or side sheets of the firebox may become exposed. Without sufficient water to absorb the heat of combustion, the firebox sheets will soften and melt, resulting in high-pressure steam being ejected with tremendous force through the firebox and into the locomotive's cab.
Scale may build up in boiler to prevent proper heat transfer, and corrosion will eventually degrade the boiler's materials to the point where it needs to be rebuilt or replaced. Start-up on a large engine may take hours of preliminary heating of the boiler water before sufficient steam is available. The boiler is typically placed horizontally. For locomotives designed to work on steep slopes, it may be placed vertically or mounted instead at an angle.