Return-flue boilers

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 Pen-y-darren engine and 1803 Coalbrookdale locomotive design. These boilers were heavily built of cast iron, short and flat-ended. His 1805 "Newcastle" locomotive 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. In this case, the boilermaking, now of wrought iron plates, must have been complicated by Trevithick's single long-travel horizontal cylinder (9 inch diameter × 36 inch stroke) which emerged through this domed end. This did make work easier for the fireman though, as he was no longer trying to reach a firedoor beneath the long crosshead of the piston.

William Hedley used this pattern of boiler for his 1813 locomotives Puffing Billy and Wylam Dilly. Through the Wylam colliery and its owner Christopher Blackett, Hedley would have been familiar with Trevithick's engine.