The firetube boilers is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes and in these cases water tube boilers can improve uneconomic. Operation is simple and feedwater of medium quality may be employed. The name ‘tank boiler’ is sometimes used for firetube boilers because of their large water capacity. The terms ‘smoke tube’ and ‘donkey boiler’ are also in use.
Of the variety proprietary designs of firetube or tank type boiler, many are composite, including sections for diesel exhaust gas heat recovery as well as for direct firing. In the Cochran boiler (above), the products of combustion and exhaust gases pass through separate sets of tubes immersed in the boiler water. These tubes are expanded into tube plates which form part of the boiler pressure shells. With the Aalborg AQ5 the gas streams pass horizontally over the outside of vertical tubes expanded into tube plates forming part of the boiler pressure vessel, in such a way that boiler water flows upwards throughout the tubes. Large downcomer tubes complete the circulation system.
A G Weser produced a boiler unit where the products of combustion pass through tubes surrounded by boiler water whilst diesel exhaust passes over tubes through which boiler water passes . A design similar in principle came from Howaldtswerke.
The Sunrod oil fired boiler combines a firetube and a watertube by arranging the latter inside the former. The watertube surface is extended by having steel pins electric resistance welded on its outer surface. The furnace is arranged either as a water cooled shell with a refractory floor or as a completely water cooled shell. In the largest sizes the furnace walls are of watertube construction. In each case a number of firetubes of large diameter extend upwards from the furnace top to a tube plate forming the top pressure shell. Inside each of these is arranged a watertube with extended surface. The top and bottom of each is connected through the wall of its firetube into the water space of the boiler.
