Fairbairn's Five-tube Boiler
William Fairbairn's work on the Lancashire boiler had demonstrated the efficiency virtues of multiple furnaces relative to a reduced water volume. It was also widely understood that higher steam pressures improved the efficiency of engines. Fairbairn's research on the strength of cylinders led him to design another improved boiler, based around far-smaller tube diameters, which would thus be able to operate at higher pressures, typically 150 psi (10 atm). This was the "five tube" boiler, whose five tubes were arranged in two nested pairs as water drum and furnace, with the remaining tube mounted above them as a separate steam drum. The water volume was extremely low compared to previous boiler designs, as the furnace tubes almost filled each of the water drums.
The boiler was successful according to its goals and provided two large furnaces in a small water capacity. The separate steam drum also aided the production of "dry" steam, without the carryover of water and risk of priming. However it was also complex to manufacture, and did not offer a great deal of heating area for the work involved. It was soon superseded by multi-tube boilers such as the locomotive and the Scotch boilers.
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