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A flued boiler, sometimes known as a shell boiler, is a type of device used to contain water, heat that water, and turn it to steam that can be used to power machinery. Such components were used in the 19th century primarily as a means of driving steam engines for trains or other equipment. The flued boiler features a large, cylindrical shell in which water can be stored; within that shell, a flue or duct runs across the length, sometimes returning to the original side of entry. This flue is heated by fire, which in turn heats the water and creates steam.
The use of steam as a power source was common in the 19th century, both for vehicles and factory components. Older boiler designs had a tendency to explode because of flaws in the manner in which the water was boiled; the flued boiler helped to reduce the amount of boiler explosions while creating steam at higher pressures by ensuring the heating surface was round rather than flat. Round surfaces tend to be stronger than flat ones, especially when heated, so the flues in a flued boiler were better equipped to handle the heat stresses required for efficiency.
Very often the end of the flued boiler chamber will be domed. This provides more strength when it comes to resisting the pressures created by the steam production. This is especially common on shorter boiler cylinders, since the pressure can end up being much higher. The high pressures are increased even more when a return flued boiler design is used; this means the flue enters the cylinder, then curves in a U-shape and returns to the original point of entry on one side of the chamber. This design was created to ensure sufficient heat could be produced in a smaller space, since the flue has to be quite long in order to heat the water effectively.
Often a large chimney is required to ensure plenty of oxygen flow to the fire. On many flued boiler models, the fire itself was stocked by this air flow, and without it, the fire would die out and the flue would not produce enough heat to heat the water and produce steam. The chimney is usually positioned at one end of the cylinder to ensure quick access to the flue and quick transfer of oxygen directly to the stoking fire that heats the flue.