What are the shell boilers?

Shell boilers may be defined as those boilers in which the heat transfer surfaces are all contained within a steel shell. Shell boilers may also be referred to as "fire tube" or "smoke tube" boilers because the products of combustion pass through the boiler tubes, which in turn transfer heat to the surrounding boiler water.

Several different combinations of tube layout are used in shell boilers, involving the number of passes the heat from the boiler furnace will usefully make before being discharged.