Thermic siphons are heat-exchanging elements in the firebox or combustion chamber of some steam locomotive designs. They were invented and patented [1] by the American locomotive engineer John L. Nicholson [2] and are utilised to increase the evaporative capacity of a locomotive boiler by increasing the heating surface. A thermic siphon is an integral part of the boiler and is quite different from a feedwater heater, which is an external accessory.
Design principles
Thermic siphons, viewed from the cab through the open fire doors
Located within a steam locomotive firebox, the thermic siphon is a funnel-shaped steel fabrication that connects the bottom of the throat sheet (the front wall of the firebox) with the crown sheet (the roof of the firebox). Feed water flows upward through the siphons from the boiler barrel. By exposing the water to the heat of the firebox, thermic siphons increase thermal efficiency and help to improve water circulation in the boiler through ensuring more uniform temperatures within it. Fresh cold water introduced from the injectors is pre-heated by passage through the siphons before arriving at the crown sheet, the hottest part of the boiler. Thermic siphons are located in the body of the firebox or, if applied, in the combustion chamber. The technique has been utilised on several designs. In Great Britain, the principal advocate of this technology was Oliver Bulleid, with the concept used in his Merchant Navy, Leader Class, West Country and Battle of Britain Classes.
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