Combustion chamber

A Scotch marine boiler

A " Scotch" marine boiler is a design of steam boiler used for many years, mostly on ships. 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 is a large number of small-diameter firetubes. 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. 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 aspects: the notion of smaller diameter tubes to increase the ratio of heating area to cross-section is taken even further to use a great many small return tubes (typically 3 or 4 inches diameter each). Secondly the overall length of the boiler is halved by folding the gas path back on itself.

Combustion chamber

Inside the combustion chamber, looking up at the tubeplate

The far end of the furnace is an enclosed box called the combustion chamber which extends upwards to link up with the firetubes. The front wall of the combustion chamber is supported against steam pressure by the tubes themselves. The rear face is stayed by rod stays through the rear shell of the boiler. Above the combustion chamber and tubes is an open steam collecting space. Larger long rod stays run the length of the boiler through this space, supporting the ends of the boiler shell. With multiple furnaces, there is a separate combustion chamber for each furnace. A few small boilers did connect them into one chamber, but this design is weaker. A more serious problem is the risk of reversing the draught, where exhaust from one furnace could blow back and out of the adjacent one, injuring the stokers working in front of it.