A boiler is a closed vessel in which water or other fluid is heated.
The fluid does not necessarily boil.(In North America the term "furnace" is normally used if the purpose is not actually to boil the fluid.) The heated or vaporized fluid exits the boiler for use in various processes or heating applications,12 including water heating, central heating, boiler-based power generation, cooking, and sanitation. Źródło: https://en.wikipedia.org/wiki/Boiler
Boilers can be classified into the following configurations: Pot boiler or Haycock boiler/Haystack boiler: a primitive "kettle" where a fire heats a partially filled water container from below.
This is the type of boiler 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 below the boiling point.
The furnace can be situated at one end of a 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 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 increases the heating surface compared to a single tube and further improves 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, tubes filled with water 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 mono-tube 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 flash boiler is a specialized type of water-tube boiler in which tubes are close together and water is pumped through them.
A flash boiler differs from the type of mono-tube steam generator in which the tube is permanently filled with water.
Flash boilers had some use in automobiles in the 19th century and this use continued into the early 20th century.. 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 siphons.
The gases then pass through a conventional firetube boiler.Water-tube fireboxes were installed in many Hungarian locomotives,citation needed but have met with little success in other countries. Źródło: https://en.wikipedia.org/wiki/Boiler#Configurations.