In fire-tube boilers, products of combustion, or hot gases, flow through ducts (mostly tubes), which are wholly contained within a water vessel (shell). Combustion may also take place within a large tube (referred to as flame tube) also enclosed in that vessel. The fire-tube boiler is the most prevailing boiler used for heating purposes, as well as commercial and industrial applications. Boiler configurations are influenced by heat-transfer requirements, so that as much as possible of the heat released by a fuel may be extracted and transferred to the water.
Fire-tube boilers are classified into horizontal-return-tubular, economic or firebox-type, locomotive firebox-type, scotch-marine-type, vertical tubular, and vertical tubeless boilers. The scotch marine boiler design is the dominant fire-tube type for both heating and industrial process uses of up to about 20,000-kg/hr capacity. Above this capacity, water-tube boilers are generally used.
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