Scotch marine boiler

      The Scotch marine boiler differs dramatically from its predecessors in using a large number of small-diameter tubes. This gives a far greater heating surface area for the volume and weight. The furnace remains a single large-diameter tube with the many small tubes arranged above it. They are connected together through a combustion chamber - an enclosed volume contained entirely within the boiler shell - so that the flow of flue gas through the fire-tubes is from back to front. An enclosed smokebox covering the front of  these tubes leads upwards to the chimney or funnel. Typical Scotch boilers had a pair of  furnaces , larger ones had three. Above this size , such as for large steam ships , it was more usual to install multiple boilers.

Side-section of a Scotch marine boiler: the arrows show direction of flue gas flow; the combustion chamber is on the right, the smokebox on the left