A "Scotch" marine boiler (or simply Scotch boiler) is a design of steam boiler best known for its use on ships.
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The general layout is that of a squat horizontal cylinder. One or more large cylindrical furnaces are in the lower part of the boiler shell. Above this are many small-diameter fire-tubes. Gases and smoke from the furnace pass to the back of the boiler, then return through the small tubes and up and out of the chimney. The ends of these multiple tubes are capped by a smokebox, outside the boiler shell.[1]
The Scotch boiler is a fire-tube boiler, in that hot flue gases pass through tubes set within a tank of water. As such, it is a descendant of the earlier Lancashire boiler, and like the Lancashire it uses multiple separate furnaces to give greater heating area for a given furnace capacity. It differs from the Lancashire in two respects: many small-diameter tubes (typically 3 or 4 inches [75 or 100 mm] diameter each) are used to increase the ratio of heating area to cross-section. Secondly, the overall length of the boiler is halved by folding the gas path back on itself.