Variations - Package 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.

Variations - Reverse flame

      In homage to the Lancashire design , modern shell boilers can come with a twin furnace design. A more recent development has been the reverse flame design where the burner fires into a blind furnace and the combustion gasses double back on themselves. This results in a more compact design and less pipework.

Variations - Water tubes

      Fire-tube boilers sometimes have water-tubes as well , to increase the heating surface. A Cornish boiler may have several water-tubes across the diameter of the flue (this is common in steam launches). A locomotive boiler  with a wide firebox may have arch tubes or thermic syphons. These increase the heating surface and give additional support to the brick arch.
      Not all shell boilers raise steam ; some are designed specifically for heating pressurized water.