A boiler is a closed vessel where water or other liquid is heated. The fluid does not necessarily boil. (In North America, the word "furnace" is generally used if the purpose is never to boil the liquid.) The heated or vaporized fluid exits the boiler for use in a variety of heating or processes applications,[1][2] including drinking water heating, central heating system, boiler-based power era, cooking, and sanitation.

Materials
The pressure vessel of the boiler is usually made of steel (or alloy steel), or of wrought iron historically. Stainless steel, of the austenitic types especially, is not found in wetted elements of boilers due to corrosion and stress corrosion breaking.[3] However, ferritic stainless is often found in superheater sections that will not be exposed to boiling water, and electrically heated stainless steel shell boilers are allowed under the Western "Pressure Equipment Directive" for creation of steam for sterilizers and disinfectors.[4]
https://en.wikipedia.org/wiki/Boiler
In live steam models, copper or brass is often used because it is more easily fabricated in smaller size boilers. Historically, copper was often used for fireboxes (particularly for vapor locomotives), because of its better formability and higher thermal conductivity; however, in newer times, the high price of copper often makes this an uneconomic choice and cheaper substitutes (such as steel) are used instead.

For a lot of the Victorian "age group of vapor", the only materials used for boilermaking was the best quality of wrought iron, with set up by rivetting. This iron was extracted from specialist ironworks, such as at Cleator Moor (UK), observed for the high quality of their rolled plate and its own suitability for high-reliability use in critical applications, such as high-pressure boilers. In the 20th century, design practice relocated towards the utilization of steel instead, which is stronger and cheaper, with welded construction, which is quicker and requires less labour. It ought to be mentioned, however, that wrought iron boilers corrode considerably slower than their modern-day steel counterparts, and are less vunerable to localized stress-corrosion and pitting. This makes the durability of older wrought-iron boilers considerably more advanced than those of welded metal boilers.

Cast iron may be used for the heating vessel of local water heaters. Although such heaters are usually termed "boilers" in a few countries, their purpose will be to produce warm water, not steam, and so they run at low pressure and try to avoid boiling. The brittleness of cast iron makes it impractical for high-pressure steam boilers.
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Energy
The foundation of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or gas. Electric vapor boilers use resistance- or immersion-type heating system elements. Nuclear fission is used as a heat source for producing steam also, either straight (BWR) or, in most cases, in specialised high temperature exchangers called "steam generators" (PWR). High temperature recovery vapor generators (HRSGs) use heat rejected from other procedures such as gas turbine.

Boiler efficiency
there are two methods to measure the boiler efficiency 1) direct method 2) indirect method

Immediate method -immediate approach to boiler efficiency test is more useful or more common

boiler efficiency =Q*((Hg-Hf)/q)*(GCV *100 ) Q =Total vapor movement Hg= Enthalpy of saturated vapor in k cal/kg Hf =Enthalpy of feed drinking water in kcal/kg q= level of gasoline use in kg/hr GCV =gross calorific value in kcal/kg like pet coke (8200 kcal/KG)

indirect method -to measure the boiler efficiency in indirect method, we are in need of a following parameter like

Ultimate analysis of fuel (H2,S2,S,C moisture constraint, ash constraint)
percentage of O2 or CO2 at flue gas
flue gas temperature at outlet
ambient temperature in deg c and humidity of air in kg/kg
GCV of energy in kcal/kg
ash percentage in combustible fuel
GCV of ash in kcal/kg
Configurations
Boilers can be classified into the following configurations:

Container boiler or Haycock boiler/Haystack boiler: a primitive "kettle" where a open fire heats a partially filled drinking water box from below. 18th century Haycock boilers produced and stored large quantities of very low-pressure vapor generally, often barely above that of the atmosphere. These could burn off wood or frequently, coal. Efficiency was suprisingly low.
Flued boiler with a couple of large flues-an early type or forerunner of fire-tube boiler.

Diagram of the fire-tube boiler
Fire-tube boiler: Here, drinking water partially fills a boiler barrel with a small volume remaining above to accommodate the steam (vapor space). This is the kind of boiler used in almost all steam locomotives. The heat source is in the furnace or firebox that has to be kept completely surrounded by water in order to keep the temperature of the heating system surface below the boiling point. The furnace can be situated at one end of a fire-tube which lengthens the road of the hot gases, thus augmenting the heating surface which may be further increased by making the gases invert direction through another parallel tube or a lot of money of multiple tubes (two-pass or come back flue boiler); on the other hand the gases may be taken along the edges and then beneath the boiler through flues (3-pass boiler). In case of a locomotive-type boiler, a boiler barrel stretches from the firebox and the hot gases go through a lot of money of fire tubes inside the barrel which greatly escalates the heating system surface compared to a single tube and further improves heat transfer. Fire-tube boilers have a comparatively low rate of vapor production usually, but high steam storage capacity. Fire-tube boilers mostly burn solid fuels, but are easily adaptable to those of the liquid or gas variety.

Diagram of the water-tube boiler.
Water-tube boiler: In this kind, tubes filled with water are arranged inside a furnace in a number of possible configurations. Often the drinking water pipes connect large drums, the low ones containing water and the top ones water and steam; in other instances, such as a mono-tube boiler, drinking water is circulated with a pump through a succession of coils. This kind gives high vapor production rates generally, but less storage space capacity than the above mentioned. Water pipe boilers can be made to exploit any heat source and tend to be preferred in high-pressure applications because the high-pressure drinking water/steam is included within small diameter pipes which can withstand the pressure with a thinner wall structure.
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 pipe is permanently filled up with water. In a flash boiler, the pipe is kept so hot that the water feed is quickly flashed into steam and superheated. Flash boilers experienced some use in cars in the 19th century which use continued into the early 20th century. .

1950s design steam locomotive boiler, from a Victorian Railways J class
Fire-tube boiler with Water-tube firebox. Sometimes both above types have been combined in the next manner: the firebox contains an set up of water tubes, called thermic siphons. The gases pass through a conventional firetube boiler then. Water-tube fireboxes were installed in many Hungarian locomotives,[citation needed] but have fulfilled with little success far away.
Sectional boiler. Inside a solid iron sectional boiler, sometimes called a "pork chop boiler" water is included inside solid iron sections.[citation needed] These sections are assembled on site to generate the finished boiler.
Safety
See also: Boiler explosion
To define and secure boilers safely, some professional specialized organizations such as the American Society of Mechanical Designers (ASME) develop specifications and regulation rules. For example, the ASME Boiler and Pressure Vessel Code is a standard providing an array of rules and directives to ensure compliance of the boilers and other pressure vessels with security, design and security standards.[5]

Historically, boilers were a source of many serious injuries and property destruction due to poorly understood engineering principles. Thin and brittle metallic shells can rupture, while welded or riveted seams could start poorly, resulting in a violent eruption of the pressurized steam. When drinking water is converted to steam it expands to over 1,000 times its original quantity and moves down vapor pipes at over 100 kilometres per hour. Because of this, steam is a great way of moving energy and warmth around a site from a central boiler house to where it is needed, but without the right boiler give food to water treatment, a steam-raising plant will suffer from scale formation and corrosion. At best, this boosts energy costs and can lead to poor quality vapor, reduced efficiency, shorter vegetation and unreliable operation. At worst, it can result in catastrophic loss and failing of life. Collapsed or dislodged boiler tubes can also aerosol scalding-hot steam and smoke from the air intake and firing chute, injuring the firemen who fill the coal in to the fire chamber. Extremely large boilers providing a huge selection of horsepower to operate factories could demolish entire structures.[6]

A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous. If supply water is then sent into the vacant boiler, the small cascade of incoming water instantly boils on contact with the superheated metallic shell and leads to a violent explosion that can't be controlled even by security vapor valves. Draining of the boiler can also happen if a leak occurs in the vapor supply lines that is bigger than the make-up water supply could replace. The Hartford Loop was developed in 1919 by the Hartford Steam Boiler and INSURANCE PROVIDER as a strategy to assist in preventing this problem from taking place, and therefore reduce their insurance promises.[7][8]

Superheated steam boiler

A superheated boiler on a steam locomotive.
Main article: Superheater
Most boilers produce steam to be utilized at saturation heat range; that is, saturated steam. Superheated vapor boilers vaporize water and then further warmth the steam in a superheater. This provides vapor at much higher temperature, but can reduce the overall thermal efficiency of the vapor generating plant because the bigger vapor temperature takes a higher flue gas exhaust temperatures.[citation needed] There are many ways to circumvent this problem, typically by providing an economizer that heats the feed water, a combustion air heating unit in the hot flue gas exhaust route, or both. There are benefits to superheated vapor that may, and will often, increase overall efficiency of both steam generation and its own utilization: benefits in input temperature to a turbine should outweigh any cost in additional boiler problem and expense. There may also be useful restrictions in using damp steam, as entrained condensation droplets will harm turbine blades.

Superheated steam presents unique safety concerns because, if any operational system component fails and allows steam to flee, the ruthless and temperature can cause serious, instantaneous injury to anyone in its path. Since the escaping steam will at first be completely superheated vapor, detection can be difficult, although the extreme heat and sound from such a leak obviously indicates its presence.

Superheater procedure is similar to that of the coils on an fresh air conditioning unit, although for a different purpose. The steam piping is directed through the flue gas route in the boiler furnace. The temp in this area is between 1 typically,300 and 1,600 °C (2,372 and 2,912 °F). Some superheaters are radiant type; that is, they absorb warmth by radiation. Others are convection type, absorbing temperature from a fluid. Some are a mixture of both types. Through either method, the extreme high temperature in the flue gas route will also heat the superheater vapor piping and the steam within. While the heat of the steam in the superheater rises, the pressure of the steam will not and the pressure remains exactly like that of the boiler.[9] Almost all steam superheater system designs remove droplets entrained in the steam to prevent damage to the turbine blading and associated piping.

Supercritical steam generator

Boiler for a power plant.
Main article: Supercritical steam generator
Supercritical steam generators are used for the production of electric power frequently. They operate at supercritical pressure. As opposed to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi or 22 MPa) that the physical turbulence that characterizes boiling ceases that occurs; the fluid is neither water nor gas but a super-critical liquid. There is absolutely no era of steam bubbles within the water, because the pressure is above the critical pressure point at which vapor bubbles can develop. As the fluid expands through the turbine levels, its thermodynamic state drops below the critical point as it does work turning the turbine which turns the power generator that power is eventually extracted. The fluid at that point may be considered a mix of steam and liquid droplets as it goes by into the condenser. This leads to slightly less gas use and therefore less greenhouse gas creation. The word "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" occurs in this product.
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Accessories
Boiler accessories and fittings
Pressuretrols to control the vapor pressure in the boiler. Boilers generally have 2 or 3 3 pressuretrols: a manual-reset pressuretrol, which functions as a basic safety by setting the top limit of steam pressure, the operating pressuretrol, which controls when the boiler fires to keep pressure, and for boilers equipped with a modulating burner, a modulating pressuretrol which handles the amount of fire.
Security valve: It is utilized to relieve pressure and stop possible explosion of the boiler.
Water level signals: They show the operator the amount of fluid in the boiler, also known as a sight cup, water measure or water column.
Bottom blowdown valves: They offer a means for removing solid particulates that condense and lie on underneath of the boiler. As the name indicates, this valve is usually located directly on underneath of the boiler, and is occasionally opened to use the pressure in the boiler to drive these particulates out.
Constant blowdown valve: This enables a small level of water to flee continuously. Its purpose is to avoid the water in the boiler becoming saturated with dissolved salts. Saturation would lead to foaming and cause water droplets to be carried over with the steam - a disorder known as priming. Blowdown is also often used to monitor the chemistry of the boiler water.
Trycock: a kind of valve that is often use to manually check a liquid level in a tank. Most found on a drinking water boiler commonly.
Flash container: High-pressure blowdown enters this vessel where the vapor can 'flash' safely and become used in a low-pressure system or be vented to atmosphere as the ambient pressure blowdown flows to drain.
Automatic blowdown/continuous heat recovery system: This system allows the boiler to blowdown only when make-up water is moving to the boiler, thereby transferring the utmost amount of heat possible from the blowdown to the make-up water. No flash container is generally needed as the blowdown discharged is near to the temperatures of the make-up water.
Hand openings: These are steel plates installed in openings in "header" to permit for inspections & installing pipes and inspection of inner surfaces.
Vapor drum internals, some display, scrubber & cans (cyclone separators).
Low-water cutoff: It is a mechanical means (usually a float change) that is used to turn off the burner or shut off energy to the boiler to avoid it from working once the drinking water runs below a certain point. If a boiler is "dry-fired" (burnt without drinking water in it) it can cause rupture or catastrophic failing.
Surface blowdown range: It offers a means for removing foam or other lightweight non-condensible chemicals that have a tendency to float together with the water inside the boiler.
Circulating pump: It really is made to circulate water back to the boiler after they have expelled a few of its heat.
Feedwater check valve or clack valve: A non-return stop valve in the feedwater collection. This may be installed to the medial side of the boiler, below water level just, or to the top of the boiler.[10]
Top give food to: Within this design for feedwater injection, water is fed to the top of the boiler. This can reduce boiler fatigue triggered by thermal stress. By spraying the feedwater over some trays water is quickly heated and this can reduce limescale.
Desuperheater pipes or bundles: A series of pipes or bundles of tubes in the water drum or the vapor drum designed to cool superheated vapor, in order to supply auxiliary equipment that does not need, or may be damaged by, dry steam.
Chemical injection line: A connection to add chemicals for controlling feedwater pH.
Steam accessories
Main steam stop valve:
Steam traps:
Main vapor stop/check valve: It is utilized on multiple boiler installations.
Combustion accessories
Fuel oil system:energy oil heaters
Gas system:
Coal system:
Soot blower
Other essential items
Pressure gauges:
Feed pumps:
Fusible plug:
Inspectors test pressure gauge attachment:
Name dish:
Registration plate: