Return-flue boiler
 "New Huber" traction engine
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Huber traction engine of 1897
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Huber advertisement, and boiler sectional diagram
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A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and the later multi-tube fire-tube boilers. A flued boiler is characterized by a large cylindrical boiler shell forming a tank of water, traversed by one or more large flues containing the furnace. These boilers appeared around the start of the 19th century and some forms remain in service today. Although mostly used for static steam plants, some were used in early steam vehicles, railway locomotives and ships.
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. It was known that an arched structure was stronger than a flat plate and so a large circular flue tube was placed inside the boiler shell. The fire itself was on an iron grating placed across this flue, with a shallow ashpan beneath to collect the non-combustible residue. This had the additional advantage of wrapping the heating surface closely around the furnace, but that was a secondary benefit.
Although considered as low-pressure (perhaps 25 psi (1.7 atm)) today, this was regarded as high pressure compared to its predecessors. This increase in pressure was a major factor in making locomotives (i.e. small self-moving vehicles) such as Trevithick's into a practical proposition.
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.
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Wikipedia