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A boiler is a closed vessel in which
fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
(generally water) is heated. The fluid does not necessarily
boil A boil, also called a furuncle, is a deep folliculitis, infection of the hair follicle. It is most commonly caused by infection by the bacterium ''Staphylococcus aureus'', resulting in a painful swollen area on the human skin, skin caused by an a ...
. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including
water heating Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...
,
central heating A central heating system provides warmth to the number of spaces within a building and optionally also able to heat domestic hot water from one main source of heat unlike heating, ventilation, and air conditioning File:Control circuit in hou ...
, boiler-based power generation,
cooking Cooking, cookery, or culinary arts is the art, science, and craft of using heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy ...

cooking
, and
sanitation Sanitation refers to public health Public health has been defined as "the science and art of preventing disease", prolonging life and improving quality of life Quality of life (QOL) is defined by the World Health Organization The ...

sanitation
.


Heat sources

In a
fossil fuel power plant A fossil fuel power station is a thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to produce high-pressure ...
using a steam cycle for power generation, the primary heat source will be combustion of
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...
,
oil An oil is any nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound ...

oil
, or
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
. In some cases byproduct fuel such as the carbon monoxide rich offgasses of a coke battery can be burned to heat a boiler; biofuels such as
bagasse Bagasse ( ) is the dry pulpy fibrous material that remains after crushing sugarcane Sugarcane or sugar cane is a species of (often hybrid) tall, perennial A perennial plant or simply perennial is a plant Plants are predominan ...
, where economically available, can also be used. In a nuclear power plant, boilers called steam generators are heated by the heat produced by nuclear fission. Where a large volume of hot gas is available from some process, a
heat recovery steam generator A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, ra ...
or
recovery boilerRecovery boiler is the part of Kraft process: Kraft paper mill The wiktionary:kraft, kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibers, ...
can use the heat to produce steam, with little or no extra fuel consumed; such a configuration is common in a
combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbi ...
power plant where a gas turbine and a steam boiler are used. In all cases the combustion product waste gases are separate from the working fluid of the steam cycle, making these systems examples of
external combustion engine An external combustion engine (EC engine) is a reciprocating heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and a ...
s.


Materials

The
pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamen ...
of a boiler is usually made of
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appe ...
(or
alloy steel Alloy steel is steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, s ...
), or historically of
wrought iron Wrought iron is an iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . ...
.
Stainless steel Stainless steel is a group of ferrous alloys that contain a minimum of approximately 11% chromium Chromium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistr ...
, especially of the
austenitic 250px, Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron I ...
types, is not used in wetted parts of boilers due to corrosion and
stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a environment. It can lead to unexpected and sudden failure of normally metal s subjected to a , especially at elevated temperature. SCC is highly chemically specific in tha ...
. However, ferritic stainless steel is often used in superheater sections that will not be exposed to boiling water, and electrically-heated stainless steel shell boilers are allowed under the European "Pressure Equipment Directive" for production of steam for sterilizers and disinfectors. In
live steam Live steam is steam Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for ...
models,
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
or
brass Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

brass
is often used because it is more easily fabricated in smaller size boilers. Historically, copper was often used for fireboxes (particularly for
steam locomotive A steam locomotive is a railway locomotive, rail vehicle that provides the Tractive force#Rail vehicles, force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, ...

steam locomotive
s), because of its better formability and higher thermal conductivity; however, in more recent 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 steam", the only material used for boilermaking was the highest grade of
wrought iron Wrought iron is an iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . ...
, with assembly by
rivet A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite to the head is called the ''tail''. On installation, the rivet is placed in a punched ...

rivet
ing. This iron was often obtained from specialist
ironworks An ironworks or iron works is an industrial plant where iron is smelted and where heavy iron and steel products are made. The term is both singular and plural, i.e. the singular of ''ironworks'' is ''ironworks''. Ironworks succeed Bloomery, bl ...
, such as those in the
Cleator Moor Cleator Moor is a town, civil parish and two Wards and electoral divisions of the United Kingdom, electoral wards (north and south) in the English county of Cumbria and within the boundaries of the Historic counties of England, historic count ...
(UK) area, noted for the high quality of their rolled plate, which was especially suitable for use in critical applications such as high-pressure boilers. In the 20th century, design practice moved towards the use of steel, with
welded Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

welded
construction, which is stronger and cheaper, and can be fabricated more quickly and with less labour. Wrought iron boilers corrode far more slowly than their modern-day steel counterparts, and are less susceptible to localized pitting and stress-corrosion. That makes the longevity of older wrought-iron boilers far superior to that of welded steel boilers.
Cast iron Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impuritie ...
may be used for the heating vessel of domestic water heaters. Although 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 boiling. The brittleness of cast iron makes it impractical for high-pressure steam boilers.


Energy

The source of heat for a boiler is
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
of any of several
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical con ...

fuel
s, such as
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

wood
,
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
,
oil An oil is any nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound ...

oil
, or
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
. Electric steam boilers use resistance- or
immersion-
immersion-
type heating elements.
Nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

Nuclear fission
is also used as a heat source for generating
steam Steam is water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fl ...

steam
, either directly (BWR) or, in most cases, in specialised
heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (m ...

heat exchanger
s called "steam generators" (PWR).
Heat recovery steam generator A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, ra ...
s (HRSGs) use the heat rejected from other processes such as
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
.


Boiler efficiency

There are two methods to measure the boiler efficiency according ASME performance test code (PTC) for boilers ASME PTC 4 and for HRSG ASME PTC 4.4 and EN 12952-15for water tube boilers: # Input Output Method (Direct method) # Heat Loss Method (Indirect method)


Input Output Method (Direct method)

Direct method of boiler efficiency test is more usable or more common. Boiler efficiency = power out / power in = (Q * (Hg - Hf)) / (q * GCV) * 100% Q = rate of steam flow in kg/h
Hg =
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...

enthalpy
of saturated steam in kcal/kg
Hf = enthalpy of feed water in kcal/kg
q = rate of fuel use in kg/h
GCV = gross calorific value in kcal/kg (e.g. pet coke 8200 kcal/kg)


Heat Loss Method (Indirect method)

To measure the boiler efficiency in indirect method, parameter like these are needed: * Ultimate analysis of fuel (H2, S2, S, C, moisture constraint, ash constraint) * Percentage of O2 or CO2 at flue gas *
Flue gas Flue gas is the gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior th ...
temperature at outlet * Ambient temperature in °C and humidity of air in kg/kg * GCV of fuel in kcal/kg * Ash percentage in combustible fuel * GCV of ash in kcal/kg


Configurations

Boilers can be classified into the following configurations: *''Pot boiler'' or ''Haycock boiler''/
Haystack boiler There have been a vast number of designs of steam boiler, particularly towards the end of the 19th century when the technology was evolving rapidly. A great many of these took the names of their originators or primary manufacturers, rather than a mo ...

Haystack boiler
: 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 or most often, coal. Efficiency was very low. *
Flued boiler A shell or flued boiler is an early and relatively simple form of boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a country located in Central Europe. It is divided into ...
with one or two large flues—an early type or forerunner of
fire-tube boiler A fire-tube boiler is a type of boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a country located in Central Europe. It is divided into 16 Voivodeships of Poland, adminis ...
. *
Fire-tube boiler A fire-tube boiler is a type of boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a country located in Central Europe. It is divided into 16 Voivodeships of Poland, adminis ...
: Here, 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 locomotive A steam locomotive is a railway locomotive, rail vehicle that provides the Tractive force#Rail vehicles, force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, ...

steam locomotive
s. 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 boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating the alcohol, the vapors fill in the space, inc ...
. 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. Fire-tube boilers may also be referred to as "scotch-marine" or "marine" type boilers. *
Water-tube boiler A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a country located in Central Europ ...
: 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. These boilers are commonly constructed in place, roughly square in shape, and can be multiple stories tall. **
Flash boiler A flash boiler is a type of water-tube boiler A high pressure watertube boiler (also spelled water-tube and water tube) is a type of Boiler (steam generator), boiler in which water circulates in tubes heated externally by the fire. Fuel is burned i ...
: 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. In a flash boiler, the tube is kept so hot that the water feed is quickly flashed into steam and . 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 siphon Thermic siphons (alt. thermic syphons) are Heat-exchanger, heat-exchanging elements in the Firebox (steam engine), firebox or Combustion chamber#Steam engine, combustion chamber of some steam boiler and steam locomotive designs. As they are direct ...

thermic siphon
s. The gases then pass through a conventional firetube boiler. Water-tube fireboxes were installed in many
HungarianHungarian may refer to: * Hungary, a country in Central Europe * Kingdom of Hungary, state of Hungary, existing between 1000 and 1946 * Hungarians, ethnic groups in Hungary * Hungarian algorithm, a polynomial time algorithm for solving the assignmen ...

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.


Safety

To define and secure boilers safely, some professional specialized organizations such as the
American Society of Mechanical Engineers#REDIRECT American Society of Mechanical Engineers {{rcat shell, {{R from other capitalisation ...
(ASME) develop standards and regulation codes. For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a wide range of rules and directives to ensure compliance of the boilers and other
pressure vessels A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size of ...
with safety, security and design standards. Historically, boilers were a source of many serious injuries and property destruction due to poorly understood engineering principles. Thin and brittle metal shells can rupture, while poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized steam. When water is converted to steam it expands to over 1,000 times its original volume and travels down steam pipes at over . Because of this, steam is an efficient method of moving energy and heat around a site from a central boiler house to where it is needed, but without the right boiler feedwater treatment, a steam-raising plant will suffer from scale formation and corrosion. At best, this increases energy costs and can lead to poor quality steam, reduced efficiency, shorter plant life and unreliable operation. At worst, it can lead to catastrophic failure and loss of life. Collapsed or dislodged boiler tubes can also spray scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who load the coal into the fire chamber. Extremely large boilers providing hundreds of horsepower to operate factories can potentially demolish entire buildings. A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous. If feed water is then sent into the empty boiler, the small cascade of incoming 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 supply lines that is larger than the make-up water supply could replace. The ''Hartford Loop'' was invented in 1919 by the Hartford Steam Boiler Inspection and Insurance Company as a method to help prevent this condition from occurring, and thereby reduce their insurance claims.


Superheated steam boiler

When water is boiled the result is
saturated steam Steam is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere an ...
, also referred to as "wet steam." Saturated steam, while mostly consisting of water vapor, carries some unevaporated water in the form of droplets. Saturated steam is useful for many purposes, such as
cooking Cooking, cookery, or culinary arts is the art, science, and craft of using heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy ...

cooking
,
heating File:Pelletkessel in Wohnhaus.JPG, upHot water central heating unit, using wood as fuel A central heating system provides warmth to the number of spaces within a building and optionally also able to heat water heating, domestic hot water from one ...

heating
and
sanitation Sanitation refers to public health Public health has been defined as "the science and art of preventing disease", prolonging life and improving quality of life Quality of life (QOL) is defined by the World Health Organization The ...
, but is not desirable when steam is expected to convey energy to machinery, such as a ship's
propulsion system Propulsion is the action or process of pushing or pulling to drive an object forward. The term is derived from two Latin words: '' pro'', meaning'' before'' or ''forward''; and ''wikt:pellere, pellere'', meaning ''to drive''. A propulsion system ...
or the "motion" of a
steam locomotive A steam locomotive is a railway locomotive, rail vehicle that provides the Tractive force#Rail vehicles, force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, ...

steam locomotive
. This is because unavoidable temperature and/or pressure loss that occurs as steam travels from the boiler to the machinery will cause some condensation, resulting in liquid water being carried into the machinery. The water entrained in the steam may damage turbine blades or in the case of a
reciprocating steam engine , internal combustion piston engine. A reciprocating engine, also often known as a piston engine, is typically a heat engine (although there are also pneumatic motor, pneumatic and hydraulic motor, hydraulic reciprocating engines) that uses one or ...

reciprocating steam engine
, may cause serious mechanical damage due to hydrostatic lock.
Superheated steam Superheated steam is steam Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vita ...
boilers evaporate the water and then further heat the steam in a ''superheater'', causing the discharged steam temperature to be substantially above the boiling temperature at the boiler's operating pressure. As the resulting " dry steam" is much hotter than needed to stay in the vaporous state it will not contain any significant unevaporated water. Also, higher steam pressure will be possible than with saturated steam, enabling the steam to carry more energy. Although superheating adds more energy to the steam in the form of heat there is no effect on pressure, which is determined by the rate at which steam is drawn from the boiler and the pressure settings of the
safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds p ...
s. The fuel consumption required to generate superheated steam is greater than that required to generate an equivalent volume of saturated steam. However, the overall energy efficiency of the steam plant (the combination of boiler, superheater, piping and machinery) generally will be improved enough to more than offset the increased fuel consumption. Superheater operation is similar to that of the coils on an
air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...
unit, although for a different purpose. The steam piping is directed through the flue gas path in the boiler furnace, an area in which the temperature is typically between . Some superheaters are radiant type, which as the name suggests, they absorb heat by radiation. Others are convection type, absorbing heat from a fluid. Some are a combination of the two types. Through either method, the extreme heat in the flue gas path will also heat the superheater steam piping and the steam within. The design of any superheated steam plant presents several engineering challenges due to the high working temperatures and pressures. One consideration is the introduction of
feedwater Boiler feedwater is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the ma ...
to the boiler. The
pump A pump is a device that moves fluids (liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the ...
used to charge the boiler must be able to overcome the boiler's operating pressure, else water will not flow. As a superheated boiler is usually operated at high pressure, the corresponding feedwater pressure must be even higher, demanding a more robust pump design. Another consideration is safety. High pressure, superheated steam can be extremely dangerous if it unintentionally escapes. To give the reader some perspective, the steam plants used in many
U.S. Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors = Blue and gold  , colors_label = Colors , march = " Anchors Aweigh" ...
destroyers In naval A navy, naval force, or maritime force is the branch of a Nation's armed forces A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically ...
built during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
operated at pressure and superheat. In the event of a major rupture of the system, an ever-present hazard in a
warship A warship or combatant ship is a naval ship that is built and primarily intended for naval warfare. Usually they belong to the armed forces of a state. As well as being armed, warships are designed to withstand damage and are usually faster and ...
during
combat Combat (French language, French for ''fight'') is a purposeful violence, violent conflict meant to physically harm or kill the opposition. Combat may be armed (using weapons) or unarmed (Hand-to-hand combat, not using weapons). Combat is sometim ...
, the enormous energy release of escaping superheated steam, expanding to more than 1600 times its confined volume, would be equivalent to a cataclysmic explosion, whose effects would be exacerbated by the steam release occurring in a confined space, such as a ship's
engine room On a ship A ship is a large watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functi ...
. Also, small leaks that are not visible at the point of leakage could be lethal if an individual were to step into the escaping steam's path. Hence designers endeavor to give the steam-handling components of the system as much strength as possible to maintain integrity. Special methods of coupling steam pipes together are used to prevent leaks, with very high pressure systems employing
welded Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

welded
joints to avoided leakage problems with threaded or
gasket Some seals and gaskets A gasket is a mechanical seal 236px, Compression seal example A mechanical seal is a device that helps join systems or mechanisms together by preventing leakage (e.g. in a pumping system), containing pressure Pres ...

gasket
ed connections.


Supercritical steam generator

Supercritical steam generators are frequently used for the production of
electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been conve ...
. They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over ) that the physical turbulence that characterizes boiling ceases to occur; the fluid is neither liquid nor gas but a super-critical fluid. There is no generation of steam bubbles within the water, because the pressure is above the
critical pressure In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is govern ...
point at which steam bubbles can form. As the fluid expands through the turbine stages, its thermodynamic state drops below the critical point as it does work turning the turbine which turns the electrical generator from which power is ultimately extracted. The fluid at that point may be a mix of steam and liquid droplets as it passes into the
condenser__NOTOC__ Condenser may refer to: Heat transfer * Condenser (heat transfer), a device or unit used to condense vapor into liquid. Specific types include: ** Heat exchanger#HVAC air coils, HVAC air coils ** Condenser (laboratory), a range of laborat ...
. This results in slightly less fuel use and therefore less
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhou ...
production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" occurs in this device.


Accessories


Boiler fittings and accessories

* Pressuretrols to control the steam pressure in the boiler. Boilers generally have 2 or 3 pressuretrols: a manual-reset pressuretrol, which functions as a safety by setting the upper limit of steam pressure, the operating pressuretrol, which controls when the boiler fires to maintain pressure, and for boilers equipped with a modulating burner, a modulating pressuretrol which controls the amount of fire. *
Safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds p ...
: It is used to relieve pressure and prevent possible explosion of a boiler. * Water level indicators: They show the operator the level of fluid in the boiler, also known as a
sight glass A sight glass or water gauge is a type of level sensor, a transparent tube through which the operator of a tank or boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a count ...

sight glass
, water gauge or water column. * Bottom blowdown valves: They provide a means for removing solid particulates that
condense Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
and lie on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out. * Continuous blowdown valve: This allows a small quantity of water to escape continuously. Its purpose is to prevent 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 condition known as
priming Priming may refer to: * Priming (agriculture), a form of seed planting preparation, in which seeds are soaked before planting * Priming (immunology), a process occurring when a specific antigen is presented to naive lymphocytes causing them to dif ...
. Blowdown is also often used to monitor the chemistry of the boiler water. * Trycock: a type of
valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries A slurry composed of glass beads in silicone oil flowing down an inclined plane A slurry is a mix ...

valve
that is often use to manually check a liquid level in a tank. Most commonly found on a water boiler. * Flash tank: High-pressure blowdown enters this vessel where the steam can 'flash' safely and be used in a low-pressure system or be vented to atmosphere while the ambient pressure blowdown flows to drain. * Automatic blowdown/continuous heat recovery system: This system allows the boiler to blowdown only when makeup water is flowing to the boiler, thereby transferring the maximum amount of heat possible from the blowdown to the makeup water. No flash tank is generally needed as the blowdown discharged is close to the temperature of the makeup water. * Hand holes: They are steel plates installed in openings in "header" to allow for inspections & installation of tubes and inspection of internal surfaces. *Steam drum internals, a series of screen, scrubber & cans (cyclone separators). * Low-water cutoff: It is a mechanical means (usually a float switch) or an electrode with a safety switch that is used to turn off the burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastrophic failure. *Surface blowdown line: It provides a means for removing foam or other lightweight non-condensible substances that tend to float on top of the water inside the boiler. * Circulating
pump A pump is a device that moves fluids (liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the ...

pump
: It is designed to circulate water back to the boiler after it has expelled some of its heat. * Feedwater
check valve A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or ...
or clack valve: A non-return stop valve in the
feedwater Boiler feedwater is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the ma ...
line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler. *Top feed: In this design for feedwater injection, the water is fed to the top of the boiler. This can reduce boiler fatigue caused 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: A series of tubes or bundles of tubes in the water drum or the steam drum designed to cool superheated steam, 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 steam stop/check valve: It is used on multiple boiler installations.


Combustion accessories

* Fuel oil system:fuel oil heaters * Gas system: * Coal system:


Other essential items

* Pressure measurement, Pressure gauges: * Boiler feedwater pump, Feed pumps: * Fusible plug: * Insulation and lagging; * Inspectors test pressure gauge attachment: * Name plate: * Registration plate:


Draught

A
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fuel
-heated boiler must provide air to oxidize its fuel. Early boilers provided this stream of air, or ''draught'', through the natural action of convection in a Flue-gas stack, chimney connected to the exhaust of the combustion chamber. Since the heated flue gas is less dense than the ambient air surrounding the boiler, the flue gas rises in the chimney, pulling denser, fresh air into the combustion chamber. Most modern boilers depend on mechanical draught rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draught hard to attain and therefore make mechanical draught equipment much more reliable and economical. Types of draught can also be divided into ''induced draught'', where exhaust gases are pulled out of the boiler; ''forced draught'', where fresh air is pushed into the boiler; and ''balanced draught'', where both effects are employed. Natural draught through the use of a chimney is a type of induced draught; mechanical draught can be induced, forced or balanced. There are two types of mechanical induced draught. The first is through use of a steam jet. The steam jet oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The second method is by simply using an induced draught fan (ID fan) which removes flue gases from the furnace and forces the exhaust gas up the stack. Almost all induced draught furnaces operate with a slightly negative pressure. Mechanical forced draught is provided by means of a fan forcing air into the combustion chamber. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure. Balanced draught is obtained through use of both induced and forced draught. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draught fan works in conjunction with the forced draught fan allowing the furnace pressure to be maintained slightly below atmospheric.


See also

* Babcock & Wilcox,
boiler manufacturer * Combustion Engineering,
boiler manufacturer * Deaerator, Boiler feed water deaerator * Dealkalization of water * Electric water boiler
(for drinking water) * Heat-only boiler station * Hot water reset * Internally rifled boiler tubes
(also known as Serve tubes) * Lancashire boiler * List of boiler types, by manufacturer, List of boiler types * Natural circulation boiler * Outdoor wood-fired boiler * Tube tool


References

*


Further reading

*
American Society of Mechanical Engineers#REDIRECT American Society of Mechanical Engineers {{rcat shell, {{R from other capitalisation ...
: ''ASME Boiler and Pressure Vessel Code, Section I''. Updated every 3 years. * Association of Water Technologies: Association of Water Technologies (AWT). * {{Authority control Boilers, Boilers Chemical equipment Plumbing Heating, ventilation, and air conditioning