Haycock and wagon top boilers

Haycock and wagon top boilers

Newcomen steam engine.
– Steam is shown pink and water is blue.
– Valves move from open (green) to closed (red)

For the first Newcomen engine of 1712, the boiler was little more than large brewer’s kettle installed beneath the power cylinder.

Kettle from a Korean tea house.

Because the engine’s power was derived from the vacuum produced by condensation of the steam, the requirement was for large volumes of steam at very low pressure hardly more than 1 psi (6.9 kPa) The whole boiler was set into brickwork which retained some heat.

Pump to demonstrate vacuum

A large vacuum chaber

A wall built in Flemish bond

A voluminous coal fire was lit on a grate beneath the slightly dished pan which gave a very small heating surface; there was therefore a great deal of heat wasted up the chimney. In later models, notably by John Smeaton, heating surface was considerably increased by making the gases heat the boiler sides, passing through a flue. Smeaton further lengthened the path of the gases by means of a spiral labyrinth flue beneath the boiler. These under-fired boilers were used in various forms throughout the 18th Century. Some were of round section (haycock). A longer version on a rectangular plan was developed around 1775 by Boulton and Watt (wagon top boiler). This is what is today known as a three-pass boiler, the fire heating the underside, the gases then passing through a central square-section tubular flue and finally around the boiler sides.