Although other types of boiler have been tried both historically and laterally with steam locomotives, their use did not become widespread, and the Firebox Fire-tube boiler has been the dominant source of power in the age of steam locomotion from the Rocket in 1829 to the Mallard in 1938 and beyond.
The steam locomotive, when fired up, typically employs a steel firebox fire-tube boiler that contains a heat source to the rear, which generates and maintains a head of steam within the pressurised partially water filled area of the boiler to the front.
The heat source, contained within the firebox, is the energy released by the combustion, typically of a solid or liquid fuel, with the by-product of hot combustion gases. If wood, coal or coke is used as the combustion material it is introduced through a door, typically by a fireman, onto a set of grates where ashes fall away from the burning fuel. If oil is used a door provides for adjusting the air flow, maintenance or for cleaning the oil jets.
The fire-tube boiler is characterised by internal tubes connected to the firebox that guide the smoke and hot combustion gases through the pressurised wet area of the boiler. These tubes greatly increase the contact area between the hot and the wet areas of the boiler and this increases the efficiency of the thermal conduction and thermal radiation processes of heat transfer between the two. The combustion gases emerge from the ends of the fire-tubes at the front of the boiler and are discharged via the smokebox to the chimney (or stack US). Surrounding the boiler layers of insulation or lagging minimise heat loss to the surroundings.
The amount of pressure in the boiler can be monitored by a gauge mounted in the cab and excessive steam pressure can be released manually by the driver or fireman. Alternatively in conditions of high boiler pressure, a safety valve may be triggered to reduce pressure and prevent the boiler violently bursting, which had previously resulted 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 used exhaust steam is injected, with the effect of increasing the volume (or draw) of smoke and combustion gases pulled through the fire tubes in the boiler and out through the chimney. Thermal efficiency considerations of a typical fire-tube boiler led Engineers such as Nigel Gresley to consider innovations such as the Water-tube boiler which he trialled on the LNER Class W1, however these designs required a lengthy teething process and there was insufficient will at the time.
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 unpressurised tank that is usually part of the locomotive's tender or is wrapped around the boiler in the case of a tank locomotive. Periodic stops are required to refill the water.
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 central 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 may soften and melt, with the possible result of high-pressure steam being ejected with tremendous force through the firebox and into the locomotive's cab. The development of the Fusible plug to release pressure in conditions of excessively high temperature and low water levels was designed to protect against this occurrence.
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.
Although the boiler is typically placed horizontally, for locomotives designed to work in locations with steep slopes, it may be more appropriate to consider vertically or mounted such that the boiler remains horizontal but the wheels are inclined to suit the slope of the rails.
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