Configurations

Configurations

      Boilers can be classified into the following configurations :

* "Port boilers" or "Haycock boilers" : a primitive "kettle" where a fire heats a partially-filled water container
   from below. 18th century Haycock boilers generally produced and stored large volumes of very
   low-pressure
   steam , often hardly above that of the atmosphere. These could burn wood of most often , coal. Efficiency
   was very low.
* Fire-tube boilers. Here , water partially fills a boiler barrel with a small volume left above to accommodate
   the steam (steam space). This is the type of   boilers used in nearly all steam locomotives. The heat source is
   inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the
   temperature of the heating surface just below boiling point. The furnace can be situated at one end of
   fire-tube which lengthens the path of the hot gases , thus  augmenting the heating surface which can be
   further increased by making the gases reverse direction through a second parallel tube or a bundle of
   multiple tubes (two-pass or return flue boiler) ; alternatively the gases may be taken along the sides and then
   beneath the boiler through flues (3-pass boiler). In the case of a locomotive-type boiler , a boiler barrel
   extends from the firebox and the hot gases  pass through a bundle of fire tubes inside the barrel which
   greatly increase the heating surface compared to a single tube and further improve heat transfer. Fire-tube
   boilers usually have a comparatively low rate of steam production , but high steam storage capacity.
   Fire-tube boilers mostly burn solid fuels, but are readily adaptable to those of the liquid or gas variety.
* Water-tube boiler. In this type , the water tubes are arranged inside a furnace in a number of possible
   configurations : often the water tubes connect large drums , the lower ones containing water and the upper
   ones , steam and water ; in other cases , such as a monotube boiler , water is circulated by a pump through
   a succession of coils. This type generally gives high steam production rates , but less storage capacity than
   the above. Water tube boilers can be designed to exploit any heat source and are generally preferred in high
   pressure applications since the high pressure water/steam is contained within small diameter pipes which can
   withstand the pressure with a thinner wall.
* Flash boiler. A specialized type of water-tube boiler.

1950s design steam locomotive boiler , from a victorian Railways J class
      * Fire-tube boiler with water-tube firebox. Sometimes the two above types have been combined in the
         following manner : the firebox contains an assembly of water tubes , called thermic syphons. The gases
         then pass through a conventional firetube boilers. Water-tube firebox were installed in many Hungarian
         locomotives , but have met with little success in other countries.
      * Sectional boiler. In a cast iron sectional boiler , sometimes called a "pork chop boiler" the water is
         contained inside cast iron sections. These sections are assembled on site to create the finished boiler.