What is a FIRETUBE BOILER?  A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat energy from the gases passes through the sides of the tubes by thermal conduction, heating the water and ultimately creating steam.

What is a SCOTCH MARINE FIRETUBE 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 firetubes 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.

What is a PACKAGED FIRETUBE BOILER?  The term “package” boiler evolved in the early- to mid-20th century from the practice of delivering boiler units to site already fitted with insulation, electrical panels, valves and gauges. This was in contrast to earlier practice where little more than the pressure vessel was delivered and the ancillary components were fitted on-site.

What is the difference between WET BACK AND DRY BACK FIRETUBE BOILERS?  The typical design is the “wet back”, where the rear face of the combustion chamber is water-jacketed as a heating surface.

The “dry back” 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. It is only used for small boilers where capital cost outweighs fuel costs. Although the Scotch boiler is nowadays rarely the primary steam generator on a ship, small dry-back designs such as the Minipac are still encountered, for supporting secondary demands whilst alongside in port with the main boilers cold.  One interesting variant of the dry-back design has been a patent for burning ash-prone fuels. The rear of the combustion chamber is used as an access point for an ash separator, removing the ash before the small-diameter tubes.