Stirling boiler

The Stirling boiler is an early form of water-tube boiler, used to generate steam in large land-based stationary plants. Although widely used around 1900, it has now fallen from favour and is rarely seen.

Stirling boilers are one of the larger arrangements for a water-tube boiler: acceptable for stationary use, but impractical for mobile use, except for large ships with modest power requirements. They consist of a large brick-built chamber with a sinuous gas path through it, passing over near-vertical water-tubes that zig-zag between multiple steam drums and water drums.

They are amongst the older, "large-tube" designs of water-tube boilers, having water-tubes that are around 3¼ inches (83 mm) in diameter. The tubes are arranged in near-vertical banks between a number of cylindrical, horizontal steam drums (above) and water drums (below). The number of drums varies, and the Stirling designs are categorized into 3-, 4- and 5-drum boilers. The number of tube banks is one less than this, i.e. 2, 3 or 4 banks.

Gas flow from the furnace passes through each bank in turn. Partial baffles of firebrick tiles are laid on each bank, so as to force the gases to flow first up, and then down through each bank. Unusually, much of the gas flow is along the tubes' axis, rather than across them.

All circulation, both up and down, is through the heating tubes and there are no separate external downcomers. The steam drums and, (in a 5-drum boiler) the water drums, are however linked by short horizontal pipes and these form part of the circulation circuit.

The tubes themselves are seamless-drawn steel and mostly straight, with gently curved ends. The setting of the boiler is a large brick-built enclosure, but the steam drums are suspended from a separate girder framework inside this, so as to allow for expansion with heat. The tubes, and the water drums in turn, are hung from the steam drums, again to allow free expansion without straining the tube ends. Owing to their curved ends the water-tubes may enter the drums radially, allowing easy sealing, but this was also a feature considered, according to the fashion of the time, to be important on account of expansion.

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