Cornish boiler

The simplest form of flued boiler was Richard Trevithick's "high-pressure" Cornish boiler, first installed at Dolcoath mine in 1812. This is a long horizontal cylinder with a single large flue containing the fire. 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.

For efficiency, Trevithick's innovation was to encase beneath the boiler with a brick-built chamber. Exhaust gases passed through the central flue and then routed outside and around the iron boiler shell. To keep the chimney clear of the firing space, the brick flue passed first underneath the centre of the boiler to the front face, then back again along the sides and to the chimney.

Cornish boilers had several advantages over the preceding wagon boilers: they were composed of mostly curved surfaces, better to resist the pressure. Their flat ends were smaller than the flat sides of the wagon boiler and were stayed by the central furnace flue, and sometimes by additional long rod stays. A less obvious advantage was that of boiler scale. 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. This reduced heating efficiency and could in extremis lead to local overheating and failure of the boiler plates. In the flued boiler, any sediment fell past the furnace flue and settled out at the bottom of the boiler shell, where it had less effect.

In model engineering, the Cornish boiler, particularly when fitted with Galloway tube is an excellent choice for gas-fired boilers and model steam boats. It is simple to build and as efficient as any small-scale boiler.