A boiler is a closed vessel where water or other liquid is heated. The liquid does not always boil. (In THE UNITED STATES, the word "furnace" is generally used if the purpose is not to boil the fluid.) The warmed or vaporized liquid exits the boiler for use in various heating system or procedures applications,[1][2] including water heating, central heating, boiler-based power generation, 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, especially of the austenitic types, is not found in wetted parts of boilers credited to corrosion and stress corrosion breaking.[3] However, ferritic stainless is often used in superheater sections that will not come in contact with boiling water, and electrically heated stainless steel shell boilers are allowed under the Western "Pressure Equipment Directive" for production 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 (especially for steam locomotives), due to 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 much of the Victorian "age of vapor", the only materials used for boilermaking was the highest grade of wrought iron, with set up by rivetting. This iron was obtained 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 transferred towards the utilization of steel instead, which is more powerful and cheaper, with welded construction, which is quicker and requires less labour. It should be noted, however, that wrought iron boilers corrode considerably slower than their modern-day steel counterparts, and are less susceptible to localized pitting and stress-corrosion. This makes the longevity of older wrought-iron boilers significantly superior to those of welded metal boilers.

Cast iron might be utilized for the heating vessel of home 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 stay away from boiling. The brittleness of cast iron helps it be impractical for high-pressure vapor boilers.
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Energy
The source of heat for a boiler is combustion of some of several fuels, such as wood, coal, oil, or natural gas. Electric vapor boilers use resistance- or immersion-type heating system elements. Nuclear fission is also used as a heat source for producing steam, either directly (BWR) or, generally, in specialised high temperature exchangers called "vapor generators" (PWR). High temperature recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbine.

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

Immediate method -immediate method of boiler efficiency test is more usable or even more common

boiler efficiency =Q*((Hg-Hf)/q)*(GCV *100 ) Q =Total steam circulation Hg= Enthalpy of saturated vapor in k cal/kg Hf =Enthalpy of give food to drinking water in kcal/kg q= level of gas use in kg/hr GCV =gross calorific value in kcal/kg like family pet coke (8200 kcal/KG)

indirect method -to gauge the boiler efficiency in indirect method, we need a subsequent parameter like

Ultimate analysis of energy (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 fireplace heats a partially filled water box 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 or most often, coal. Efficiency was suprisingly low.
Flued boiler with one or two large flues-an early forerunner or type of fire-tube boiler.

Diagram of the fire-tube boiler
Fire-tube boiler: Here, water partially fills a boiler barrel with a small volume still left above to accommodate the vapor (steam space). This is the type of boiler used in nearly all steam locomotives. The heat source is in the furnace or firebox that needs to be held completely surrounded by water in order to keep the temp 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 system surface which can be further increased by making the gases reverse direction through another parallel pipe or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the gases may be taken along the edges and then under the boiler through flues (3-move boiler). In case of a locomotive-type boiler, a boiler barrel expands from the firebox and the hot gases go through a lot of money of fire tubes inside the barrel which greatly increases the heating system surface in comparison to a single pipe and further boosts heat transfer. Fire-tube boilers will often have a comparatively low rate of vapor creation, but high vapor storage capacity. Fire-tube boilers mainly burn solid fuels, but are readily adaptable to those of the gas or liquid variety.

Diagram of the water-tube boiler.
Water-tube boiler: In this kind, pipes filled up with drinking water are arranged inside a furnace in several possible configurations. Often the water pipes connect large drums, the low ones including water and the top ones steam and water; in other cases, such as a mono-tube boiler, drinking water is circulated by a pump through a succession of coils. This kind generally gives high steam creation rates, but less storage capacity than the above mentioned. Water tube boilers can be designed to exploit any warmth source and are generally preferred in high-pressure applications because the high-pressure 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 where tubes are close collectively and water is pumped through them. A flash boiler differs from the kind of mono-tube vapor generator in which the pipe is permanently filled with water. Super fast boiler, the tube is kept so hot that the water give food to is quickly flashed into vapor and superheated. Flash boilers acquired some use in automobiles 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 the two above types have been combined in the following manner: the firebox consists of an assembly of water tubes, called thermic siphons. The gases then go through a typical firetube boiler. Water-tube fireboxes were installed in many Hungarian locomotives,[citation needed] but have fulfilled with little success far away.
Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside solid iron areas.[citation needed] These sections are assembled on site to create the finished boiler.
Safety
See also: Boiler explosion
To define and secure boilers safely, some professional specialized organizations such as the American Culture of Mechanical Designers (ASME) develop specifications and regulation rules. For example, the ASME Boiler and Pressure Vessel Code is a typical providing an array of rules and directives to ensure compliance of the boilers and other pressure vessels with security, security and design standards.[5]

Historically, boilers were a source of many serious injuries and property destruction as a consequence to badly understood engineering principles. Thin and brittle metallic shells can rupture, while welded or riveted seams could start badly, leading to a violent eruption of the pressurized vapor. When water is changed into vapor it expands to over 1,000 times its original travels and volume down steam pipes at over 100 kilometres per hour. Because of this, steam is a great way of moving energy and temperature around a niche site from a central boiler house to where it is necessary, but with no right boiler feed water treatment, a steam-raising flower are affected from size formation and corrosion. At best, this increases energy costs and can result in poor quality steam, reduced efficiency, shorter vegetation and unreliable operation. At worst, it can lead to catastrophic failure and lack of life. Collapsed or dislodged boiler tubes can also squirt scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who weight the coal in to the open fire chamber. Extremely large boilers providing a huge selection of horsepower to use factories could demolish entire structures.[6]

A boiler which has a loss of feed water and is permitted to boil dry out can be hugely dangerous. If nourish drinking water is sent in to the unfilled boiler then, the tiny cascade of inbound drinking water instantly boils on contact with the superheated metal shell and leads to a violent explosion that cannot be controlled even by safety steam valves. Draining of the boiler can also happen if a leak occurs in the steam source lines that is bigger than the make-up drinking water source could replace. The Hartford Loop was invented in 1919 by the Hartford Vapor Boiler and INSURANCE PROVIDER as a method to assist in preventing this condition from taking place, and thereby reduce their insurance promises.[7][8]

Superheated steam boiler

A superheated boiler on the steam locomotive.
Main article: Superheater
Most boilers produce vapor to be utilized at saturation heat; that is, saturated vapor. Superheated steam boilers vaporize the water and then further high temperature the vapor in a superheater. This provides vapor at much higher heat, but can reduce the overall thermal efficiency of the steam generating plant because the higher steam heat takes a higher flue gas exhaust heat range.[citation needed] There are several ways to circumvent this issue, by providing an economizer that heats the give food to water typically, a combustion air heating unit in the hot flue gas exhaust path, or both. A couple of advantages to superheated steam 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 limitations in using damp vapor, as entrained condensation droplets will harm turbine blades.

Superheated steam presents unique safety concerns because, if any system component fails and allows steam to escape, the high temperature and pressure can cause serious, instantaneous harm to anyone in its path. Since the escaping steam will be completely superheated vapor, detection can be difficult, although the intense heat and sound from such a leak obviously indicates its presence.

Superheater operation 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 path in the boiler furnace. The heat in this field is between 1 typically,300 and 1,600 °C (2,372 and 2,912 °F). Some superheaters are glowing type; that is, they absorb warmth by radiation. Others are convection type, absorbing temperature from a fluid. Some are a combination of the two types. Through either method, the extreme warmth in the flue gas path will also temperature the superheater steam piping and the vapor within. While the heat of the steam in the superheater increases, 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 avoid harm to the turbine blading and associated piping.

Supercritical steam generator

Boiler for a power vegetable.
Main article: Supercritical steam generator
Supercritical steam generators are used for the production of energy frequently. They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical vapor generator operates at such a higher 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 generation of steam bubbles within water, because the pressure is above the critical pressure point of which vapor bubbles can form. As the liquid expands through the turbine levels, its thermodynamic state drops below the critical point as it can work turning the turbine which changes the power generator from which power is eventually extracted. The liquid at that time may be a mix of steam and liquid droplets as it passes in to the condenser. This leads to slightly less energy use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" occurs in this device.
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Accessories
Boiler fittings and accessories
Pressuretrols to control the vapor pressure in the boiler. Boilers generally have two or three 3 pressuretrols: a manual-reset pressuretrol, which functions as a basic safety by setting top of the limit of steam pressure, the working pressuretrol, which controls when the boiler fires to keep up pressure, and for boilers outfitted with a modulating burner, a modulating pressuretrol which handles the quantity of fire.
Protection valve: It is used to relieve pressure and stop possible explosion of a boiler.
Water level indications: They show the operator the level of liquid in the boiler, also known as a view glass, water gauge or drinking water column.
Bottom level blowdown valves: They provide a means for removing solid particulates that condense and lay on the bottom of a boiler. As the name suggests, this valve is usually located directly on underneath of the boiler, and is occasionally opened up to use the pressure in the boiler to press these particulates out.
Constant blowdown valve: This enables a small quantity of water to escape continuously. Its purpose is to avoid the water in the boiler becoming saturated with dissolved salts. Saturation would business lead to foaming and cause drinking water droplets to be carried over with the steam - a condition known as priming. Blowdown is often used to monitor the chemistry of the boiler water also.
Trycock: a kind of valve that is often use to manually check a water level in a container. Most commonly entirely on a drinking water boiler.
Flash tank: 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 technique allows the boiler to blowdown only when makeup water is flowing to the boiler, thereby transferring the utmost amount of heat possible from the blowdown to the makeup water. No flash container is normally needed as the blowdown discharged is close to the temperatures of the makeup water.
Hand openings: They are steel plates installed in openings in "header" to permit for inspections & installing pipes and inspection of inner surfaces.
Steam drum internals, some display screen, scrubber & cans (cyclone separators).
Low-water cutoff: It is a mechanical means (usually a float switch) that can be used to turn from the burner or shut down fuel to the boiler to avoid it from running once the water runs below a certain point. If a boiler is "dry-fired" (burnt without drinking water in it) it can cause rupture or catastrophic failure.
Surface blowdown range: It offers a way for removing foam or other light-weight non-condensible chemicals that have a tendency to float together with the water inside the boiler.
Circulating pump: It is designed to circulate water back to the boiler after they have expelled some of its heat.
Feedwater check valve or clack valve: A non-return stop valve in the feedwater line. This may be installed to the side of the boiler, just below water level, or to the top of the boiler.[10]
Top give food to: In 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 a series of trays the water is quickly heated and this can reduce limescale.
Desuperheater tubes or bundles: Some tubes or bundles of tubes in water drum or the vapor drum made to cool superheated vapor, in order to provide auxiliary equipment that does not need, or may be damaged by, dry steam.
Chemical substance 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:fuel 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 dish: