F22B7/00—Steam boilers of furnace-tube type, i.e. the combustion of fuel being performed inside one or more furnace tubes built-in in the boiler body
F22B7/12—Steam boilers of furnace-tube type, i.e. the combustion of fuel being performed inside one or more furnace tubes built-in in the boiler body with auxiliary fire tubes; Arrangement of header boxes providing for return diversion of flue gas flow