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 heatin applications! #$ %$ includin water heatin! central heatin! boiler&based power eneration! coo'in! and sanitation. Contents hide$ • # aterials • % nery • * +onfiurations • , -afety • -uperheated steam boilers o .# -upercritical steam enerator • / Accessories o /.# 0oiler fittins and accessories o /.% -team accessories o /.* +ombustion accessories o /., 1ther essential items o /. 2as safe chec' • 3 4rauht • 5 -ee also • 6 7eferences • #8 9urther readin Materialsedit$ The pressure vessel of a boiler is usually made of steel (or alloy steel)! or historically of wrouht iron. -tainless steel! especially of the austenitic types! is not used in wetted parts of boilers due to corrosion and stress corrosion crac'in. *$ :owever! ferritic stainless steel is often used in superheater sections that will not be exposed to boilin water ! and electrically&heated stainless steel shell boilers are allowed under the uropean ";ressure <uipment 4irective" for production of steam for sterilizers and disinfectors.,$ In live steam models! copper or brass is often used because it is more easily fabricated in smaller size boilers. :istorically! copper was often used for fireboxes (particularly for steam locomotives)! because of its better formability and hiher thermal conductivity= however! in more recent times! the hih price of copper often ma'es this an uneconomic choice and cheaper substitutes (such as steel) are used instead. 9or much of the >ictorian "ae of steam"! the only material used for boilerma'in was the hihest rade of wrouht iron! with assembly by rivettin. This iron was often obtained from specialist ironwor's! such as at +leator oor (?@)! noted for the hih <uality of their rolled plate and its suitability for hih&reliability use in critical applications! such as hih&pressure boilers. In the %8th century! desin practice instead moved towards the use of steel! which is stroner and cheaper! with welded construction! which is <uic'er and re<uires less labour. It should be noted! however! that wrouht iron boilers corrode far slower than their modern&day steel counterparts! and are less susceptible to localized pittin and stress&corrosion. This ma'es the lonevity of older wrouht&iron boilers far superior to those of welded steel boilers. +ast iron may be used for the heatin vessel of domestic water heaters. $ Althouh such heaters are usually termed "boilers" in some countries! their purpose is usually to produce hot water! not steam! and so they run at low pressure and try to avoid actual boilin. The brittleness of cast iron ma'es it impractical for hih&pressure steam boilers. 4iaram of a fire&tube boiler 4iaram of a water&tube boiler. Energy edit$ The source of heat for a boiler is combustion of any of several fuels! such as wood! coal! oil! or natural as. lectric steam boilers use resistance& or immersion&type heatin elements. Nuclear fission is also used as a heat source for eneratin steam! either directly (07) or! in most cases! in specialised heat exchaners called "steam enerators" (;7). :eat recovery steam enerators (:7-2s) use the heat reBected from other processes such as as turbine. Configurationsedit$ 0oilers can be classified into the followin confiurationsC • "Pot boiler" or "Haycock boiler" C a primitive "'ettle" where a fire heats a partially filled water container from below. #5th century :aycoc' boilers enerally produced and stored lare volumes of very low&pressure steam! often hardly above that of the atmosphere. These could burn wood or most often! coal. fficiency was very low. • 9ire&tube boiler C :ere! water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). 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 'ept permanently surrounded by the water in order to maintain the temperature of the heating surface below the boilin point. The furnace can be situated at one end of a fire&tube which lenthens the path of the hot ases! thus aumentin the heatin surface which can be further increased by ma'in the ases reverse direction throuh a second parallel tube or a bundle of multiple tubes (two& pass or return flue boiler)= alternatively the ases may be ta'en alon the sides and then beneath the boiler throuh flues (*&pass boiler). In case of a locomotive&type boiler! a boiler barrel extends from the firebox and the hot ases pass throuh a bundle of fire tubes inside the barrel which reatly increases the heatin surface compared to a sinle tube and further improves heat transfer. 9ire&tube boilers usually have a comparatively low rate of steam production! but hih steam storae capacity. 9ire&tube boilers mostly burn solid fuels! but are readily adaptable to those of the li<uid or as variety. • ater&tube boiler C In this type! tubes filled with water are arraned inside a furnace in a number of possible confiurations! often the water tubes connect lare drums! the lower ones containin water and the upper ones! steam and water= in other cases! such as a mono&tube boiler! water is circulated by a pump throuh a succession of coils. This type enerally ives hih steam production rates! but less storae capacity than the above. ater tube boilers can be desined to exploit any heat source and are enerally preferred in hih&pressure applications since the hih&pressure waterDsteam is contained within small diameter pipes which can withstand the pressure with a thinner wall. • 9lash boiler C A flash boiler is a specialized type of water&tube boiler in which tubes are too close toether and water is pumped throuh them. A flash boiler differs from the type of mono& tube steam enerator in which the tube is permanently filled with water. In a flash boiler! the tube is 'ept so hot that the water feed is <uic'ly flashed into steam and superheated. 9lash boilers had some use in automobiles in the #6th century and this use continued into the early %8th century. #68s desin steam locomotive boiler ! from a >ictorian 7ailways E class • 9ire&tube boiler with ater&tube firebox. -ometimes the two above types have been combined in the followin mannerC the firebox contains an assembly of water tubes! called thermic siphons. The ases then pass throuh a conventional firetube boiler. ater&tube fireboxes were installed in many :unarian locomotives! but have met with little success in other countries. • -ectional boiler. In a cast iron sectional boiler! sometimes called a "por' chop boiler" the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler..