A shell or flued boiler is an early and firedoor menu simple form of boiler used to make steam, usually for the purpose of driving a steam engine. Flued boilers were developed in an attempt to raise steam pressures and improve engine efficiency. Early haystack designs of Watt’s day were mechanically weak and often presented an unsupported flat surface to the fire.
Boiler explosions, usually beginning with failure of this firebox plate, were common. The simplest boiler for locomotives had a single straight flue. It was widely used by many of the early locomotive makers, including Blenkinsop’s locomotives for the Middleton railway and Stephenson’s Locomotion. A simple flue must be long if it is to offer adequate heating area. In a short boiler shell, such as required for a steam locomotive, this may be done by using a U-shaped return flue that bends back on itself.
Richard Trevithick had already used a return flue with his first 1802 Coalbrookdale locomotive design and 1804 Pen-y-darren engine. William Hedley used this pattern of boiler for his 1813 locomotives Puffing Billy and Wylam Dilly. Timothy Hackworth’s 0-6-0 Royal George of 1827 also used a return-flued boiler, although it is best known for its pioneering use of a deliberate blastpipe to encourage draught on the fire. Marion, Ohio for their “New Huber” traction engines, from 1885 to 1903. These were not, however, return-flue boilers in the sense used here, but rather return-tube boilers.
