Early haystack designs of Watt's day were mechanically weak and often presented an unsupported flat surface to the fire.

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.

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.

This type of boiler is simple to manufacture and strong enough to support "high pressure" (for the period) steam with expansive working in the cylinders.

There is also good gas flow through the large flue, so that the fire receives sufficient draught from the action of a tall chimney alone.

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.

His 1804/5 "Newcastle" locomotive (actually built in Gateshead) began to show one characteristic feature of the return-flued boiler, a prominent dome shape to resist steam pressure in the solid end opposite both furnace and chimney.

[2] 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.

They had a single large cylindrical furnace tube, a combustion chamber external to the boiler's pressure shell, then multiple, narrow fire-tubes returning to a horseshoe-shaped smokebox above and around the firedoor.

Compared to this, the advantage of the Huber boiler was that the firetubes could be replaced more easily, without needing to work from within an enclosed firebox.

As the furnace relied on natural draught, a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire.

Wagon or haystack boilers were heated from beneath and any scale or impurities that formed a sediment settled upon this plate, insulating it from the water.

[7] It is basically a Cornish boiler with the lower half of the shell around the furnace removed, so as to permit a large fire to be lit.

It is generally considered to be the invention of William Fairbairn and John Hetherington in 1844, although their patent was for the method of firing the furnaces alternately, so as to reduce smoke, rather than the boiler itself.

[9] Stephenson's early 0-4-0 locomotive "Lancashire Witch" had already demonstrated the use of twin furnace tubes within a boiler 15 years earlier.

[1] Fairbairn had made a theoretical study of the thermodynamics of more efficient boilers, and it was this that had led him to increase the furnace grate area relative to the volume of water.

William Fairbairn's work on the Lancashire boiler had demonstrated the efficiency virtues of multiple furnaces relative to a reduced water volume.

Fairbairn's research on the strength of cylinders[13] led him to design another improved boiler, based around far-smaller tube diameters, which would thus be able to operate at higher pressures, typically 150 psi (1,000 kPa).