Scotch Marine Firetube Boilers
The Scotch Marine Boiler is a fire-tube boiler, in that hot flue gases pass through tubes set within a tank of water. 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 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.
There are two basic designs for Scotch Marine Boilers. The typical design is the “wetback”, where the rear face of the combustion chamber is water-jacketed as a heating surface. The “dryback” variation has the rear of the combustion chamber as an open box, backed or surrounded only by a sheetmetal jacket. This simplifies construction, but also loses much efficiency.