A moveable (mobile) boiler
(preserved, Historic Silver Mine in Tarnowskie Góry Poland).
A stationary boiler
(United States).


Heat sources

In a fossil fuel power plant using a steam cycle for power generation, the primary heat source will be combustion of coal, oil, or 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, 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 or recovery boiler can use the heat to produce steam, with little or no extra fuel consumed; such a configuration is common in a combined cycle 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 engines.




For much of the Victorian "age of steam", the only material used for boilermaking was the highest grade of wrought iron, with assembly by riveting. This iron was often obtained from specialist ironworks, such as those in the Cleator Moor (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 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.[citation needed]




Boiler efficiency

There are two methods to measure the boiler efficiency:

  1. Direct method
  2. Indirect 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 of saturated steam in kcal/kg
Hf = enthalpy of feed water in kcal/kg
q = rate of fuel use in kg/h

Indirect method: To measure the boiler efficiency in indirect method, we need 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 fuel in kcal/kg
  • Ash percentage in combustible fuel
  • GCV of ash in kcal/kg


Boilers can be classified into the following configurations:

  • Flued boiler with one or two large flues—an early type or forerunner of fire-tube boiler.
  • Diagram of a fire-tube boiler
  • Diagram of a water-tube boiler.
  • 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.
  • Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections.[citation needed] These sections are assembled on site to create the finished boiler.





Superheated steam boiler

A superheated boiler on a steam locomotive.

When water is boiled the result is saturated steam, 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, heating and sanitation, but is not desirable when steam is expected to convey energy to machinery, such as a ship's propulsion system or the "motion" of a 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, may cause serious mechanical damage due to hydrostatic lock.

Superheated steam 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 valves.[10] 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.



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 destroyers built during World War II operated at 600 psi (4,100 kPa; 41 bar) pressure and 850 degrees Fahrenheit (454 degrees Celsius) superheat. In the event of a major rupture of the system, an ever-present hazard in a warship during combat, 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. 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 joints to avoided leakage problems with threaded or gasketed connections.

Supercritical steam generator

Boiler for a power plant.



Boiler fittings and accessories

  • 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. Blowdown is also often used to monitor the chemistry of the boiler water.
  • Trycock: a type of valve that is often use to manually check a liquid level in a tank. Most commonly found on a water boiler.
  • 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.

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:
  • Soot blower

Other essential items








See also


  5. ^ a b "Steam Generation in Canneries". U.S. Food & Drug Administration. Retrieved 25 March 2018.
  6. ^ Boiler and Pressure Vessel Inspection According to ASME
  8. ^ Dan Holohan."What you should know about Hartford Loops".
  9. ^ "The Hartford Loop on Steam Boilers".
  11. ^ Bell (1952: 1 35)

Further reading

  • American Society of Mechanical Engineers: ASME Boiler and Pressure Vessel Code, Section I. Updated every 3 years.
  • Association of Water Technologies: Association of Water Technologies (AWT).
  • The Babcock & Wilcox Co. (1902): "Steam, its generation and use", New York-London, republished by Nabu Press, ISBN 978-1147-61244-8 (2010)