A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet (a few metres to some hundred metres).


  • A radiant superheater is placed directly in radiant zone of the combustion chamber near the water wall so as to absorb heat by radiation.
  • A convection superheater is located in the convective zone of the furnace usually ahead of economizer ( in the path of the hot flue gases). These are also called primary superheaters.
  • A separately fired superheater is a superheater that is placed outside the main boiler, which has its own separate combustion system. This superheater design incorporates additional burners in the area of superheater pipes. This type of superheater is not popularly used, and even tends to be extinct due to efficiency of combustion ratio with steam quality that is not better than other superheater types.

Steam turbines

A simplified diagram of a coal-fired thermal power station. The superheater is the element 19.

Steam engines

In a steam engine, the superheater re-heats the steam generated by the boiler, increasing its thermal energy and decreasing the likelihood that it will condense inside the engine.[1][2] Superheaters increase the thermal efficiency of the steam engine, and have been widely adopted. Steam which has been superheated is logically known as superheated steam; non-superheated steam is called saturated steam or wet steam. Superheaters were applied to steam locomotives in quantity from the early 20th century, to most steam vehicles, and to stationary steam engines. This equipment is still used in conjunction with steam turbines in electrical power generating stations throughout the world.


General arrangement of a superheater installation in a steam locomotive.
Superheater viewed from the smokebox. Top centre is the superheater header, with pipes leading to cylinders. Tubes below feed steam into and out of the superheater elements within the flues. The stack and the damper have been removed for clarity.

In steam locomotive use, by far the most common form of superheater is the fire-tube type. This takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet in the smokebox. The steam is then passed through a number of superheater elements—long pipes which are placed inside large diameter fire tubes, called flues. Hot combustion gases from the locomotive's fire pass through these flues just like they do the firetubes, and as well as heating the water they also heat the steam inside the superheater elements they flow over. The superheater element doubles back on itself so that the heated steam can return; most do this twice at the fire end and once at the smokebox end, so that the steam travels a distance of four times the header's length while being heated. The superheated steam, at the end of its journey through the elements, passes into a separate compartment of the superheater header and then to the cylinders as normal.

Damper and snifting valve

The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes this cooling effect is absent, and thus a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives (particularly on the London and North Eastern Railway) were fitted with snifting valves which admitted air to the superheater when the locomotive was coasting (drifting). This kept the superheater elements cool and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives.

Front-end throttle

A superheater increases the distance between the throttle and the cylinders in the steam circuit and thus reduces the immediacy of throttle action. To counteract this, some later steam locomotives were fitted with a front-end throttle—placed in the smokebox after the superheater. Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox. This arrangement also allows superheated steam to be used for auxiliary appliances, such as the dynamo and air pumps. Another benefit of the front end throttle is that superheated steam is immediately available. With the dome throttle, it took quite some time before the super heater actually provided benefits in efficiency. One can think of it in this way: if one opens saturated steam from the boiler to the superheater it goes straight through the superheater units and to the cylinders which doesn't leave much time for the steam to be superheated. With the front-end throttle, steam is in the superheater units while the engine is sitting at the station and that steam is being superheated. Then when the throttle is opened, superheated steam goes to the cylinders immediately.

Cylinder valves

Locomotives with superheaters are usually fitted with piston valves or poppet valves. This is because it is difficult to keep a slide valve properly lubricated at high temperature.


Early color photograph from Russia taken by Sergey Prokudin-Gorsky in 1910 of steam locomotive with a superheater

The first practical superheater was developed in Germany by Wilhelm Schmidt during the 1880s and 1890s. The first superheated locomotive Prussian S 4 series, with an early form of superheater, was built in 1898, and produced in series from 1902.[3] The benefits of the invention were demonstrated in the U.K. by the Great Western Railway (GWR) in 1906. The GWR Chief Mechanical Engineer, G. J. Churchward believed, however, that the Schmidt type could be bettered, and design and testing of an indigenous Swindon type was undertaken, culminating in the Swindon No. 3 superheater in 1909.[4] Douglas Earle Marsh carried out a series of comparative tests between members of his I3 class using saturated steam and those fitted with the Schmidt superheater between October 1907 and March 1910, proving the advantages of the latter in terms of performance and efficiency.[5]

Other improved superheaters were introduced by John G. Robinson of the Great Central Railway at Gorton locomotive works, by Robert Urie of the London and South Western Railway (LSWR) at Eastleigh railway works, and Richard Maunsell of the Southern Railway (Great Britain), also at Eastleigh.

Robert Urie's design of superheater for the LSWR was the product of experience with his H15 class 4-6-0 locomotives. In anticipation of performance trials, eight examples were fitted with Schmidt and Robinson superheaters, and two others remained saturated.[6] However, the First World War intervened before the trials could take place, although an LSWR Locomotive Committee report from late 1915 noted that the Robinson version gave the best fuel efficiency. It gave an average of 48.35 lb (21.9 kg) coal consumed per mile over an average distance of 39,824 mi (64,090.5 km), compared to 48.42 lb (22.0 kg) and 59.05 lb (26.8 kg) coal for the Schmidt and saturated examples respectively.[6]

However, the report stated that both superheater types had serious drawbacks, with the Schmidt system featuring a damper control on the superheater header that caused hot gases to condense into sulphuric acid, which caused pitting and subsequent weakening of the superheater elements.[6] Leakage of gases was also commonplace between the elements and the header, and maintenance was difficult without removal of the horizontally-arranged assembly. The Robinson version suffered from temperature variations caused by saturated and superheated steam chambers being adjacent, causing material stress, and had similar access problems as the Schmidt type.[6]

The report's recommendations enabled Urie to design a new type of superheater with separate saturated steam headers above and below the superheater header.[7] These were connected by elements beginning at the saturated header, running through the flue tubes and back to the superheater header, and the whole assembly was vertically arranged for ease of maintenance.[7] The device was highly successful in service, but was heavy and expensive to construct.[7]

Advantages and disadvantages

The main advantages of using a superheater are reduced fuel and water consumption but there is a price to pay in increased maintenance costs. In most cases the benefits outweighed the costs and superheaters were widely used. An exception was shunting locomotives (switchers). British shunting locomotives were rarely fitted with superheaters. In locomotives used for mineral traffic the advantages seem to have been marginal. For example, the North Eastern Railway fitted superheaters to some of its NER Class P mineral locomotives but later began to remove them.

Without careful maintenance superheaters are prone to a particular type of hazardous failure in the tube bursting at the U-shaped turns in the superheater tube. This is difficult to both manufacture, and test when installed, and a rupture will cause the superheated high-pressure steam to escape immediately into the large flues, then back to the fire and into the cab, to the extreme danger of the locomotive crew.


  1. ^ "Superheater".
  2. ^ "Archived copy". Archived from the original on 2008-12-21. Retrieved 2008-12-28.CS1 maint: Archived copy as title (link)
  3. ^ Herbert Rauter, Günther Scheingraber, 1991: Preußen-Report. Band 2: Die Schnellzuglokomotiven der Gattung S 1 – S 11. Hermann Merker Verlag , ISBN 3-922404-16-2 (in German), pp. 85-88.
  4. ^ Allcock, N.J.; Davies, F.K.; le Fleming, H.M.; Maskelyne, J.N.; Reed, P.J.T.; Tabor, F.J. (June 1951). White, D.E. (ed.). The Locomotives of the Great Western Railway, part one: Preliminary Survey. Kenilworth: RCTS. p. 56. ISBN 0-901115-17-7. OCLC 650412984.
  5. ^ Bradley (1974)
  6. ^ a b c d Bradley (1987), p. 15
  7. ^ a b c Bradley (1987), p. 16


  • Bradley, D. L. (1974). Locomotives of the London Brighton & South Coast Railway, 3. London: London, Railway Correspondence and Travel Society, 1974. pp. 88–93.
  • Bradley, D. L. (1987). LSWR Locomotives: The Urie classes. Didcot Oxon: Wild Swan Publications. ISBN 0-906867-55-X.
Boiler (power generation)

A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure (7–2,000 kPa or 1–290 psi) but, at pressures above this, it is more usual to speak of a steam generator.

A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam locomotives, portable engines and steam-powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stationary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately-generated steam is transferred to a receiver (tank) on the locomotive.

Boiling Nuclear Superheater (BONUS) Reactor Facility

The Boiling Nuclear Superheater (BONUS) Reactor Facility, also known to the locals as "Domes", or formally as Museo Tecnologico BONUS Dr. Modesto Iriarte, is a decommissioned nuclear plant in Rincón, Puerto Rico. It was listed on the U.S. National Register of Historic Places in 2007.

Combustion Engineering

Combustion Engineering (C-E) was a multi-national American-based engineering firm and a leader in the development of both fossil and nuclear steam supply power systems in the United States with approximately 42,000[10] employees worldwide. Originally headquartered in New York City, C-E moved its corporate offices to Stamford, Connecticut in 1973. C-E owned over three dozen other companies including Lummus Company, National Tank Company and the Morgan Door Company. Former workers have gone on to hold leadership positions in major engineering firms and governments around the world. The company was acquired by Asea Brown Boveri in early 1990. The boiler and fossil fuel businesses were purchased by Alstom in 2000, and the nuclear business was purchased by Westinghouse Electric Company also in 2000.

KkStB Class 429

The steam locomotive class kkStB 429 was a class of passenger locomotive operated by the Imperial Austrian State Railways (Kaiserlich-königliche österreichische Staatsbahnen), kkStB.

As Wilhelm Schmidt's superheater went into series production, Karl Gölsdorf modified the Class 329 into the superheated variant 429.

The smokebox was lengthened, the boiler barrel reduced accordingly, high-pressure cylinders were given piston valves, the low-pressure cylinders slide valves.

The Lokomotivfabrik Floridsdorf, the Wiener Neustädter Lokomotivfabrik and the Lokomotivfabrik der StEG delivered 57 units (429.01–57) to the kkStB.

In spite of the small superheater area, problems arose with the slide valves on the low-pressure side. As a result, the following 126 engines were supplied with piston valves on both sides (429.100–225).

At the same time a two-cylinder variant with piston valves was tried, of which in the end 197 units were procured by the kkStB (429.900–999 and 429.1900–1996).

The Austrian Southern Railway procured six compound locomotives (with piston valves), that were numbered 429.01–06.

The 429s were employed for all duties and with good coal generated up to 1,200 PS (883 kW; 1,184 hp).

They were to be found in almost all parts of the Danube Monarchy.

After the First World War the former 429s became Class 354.7 with the ČSD, Class Ol12 in the PKP, Class 106 in the JDŽ, Class 688 in the FS and were also used by the Romanian State Railways retaining their original numbers.

A total of 87 units (46 compound, 41 two-cylinder locomotives) remained in the BBÖ.

In 1939 the Deutsche Reichsbahn (DRB) reclassified the two-cylinder engines as 35 201–241 and the compounds as 35 301–346.

During the course of the war, several locomotives from the ČSD and JDŽ ended up in the DRB.

After the Second World War 46 two-cylinder machines were left in the ÖBB and became their Class 35, as well as 39 compound engines which became Class 135.

The serial numbers were not changed from those allocated by the DRB.

MGWR Class H

The Midland Great Western Railway (MGWR) Class H were an 0-6-0 locomotive bought in 1880 from Avonside Engine Company. After 1925 they became Great Southern Railways (GSR) class 619 / Inchicore class J6.

The MGWR acquired these engines at a favourable price from the Avonside Engine Company when the original customer, the Waterford, Dungarvan and Lismore Railway, refused the locomotives due to late delivery and other potential buyers had rejected them. They were notable for being fitted with more comfortable and spacious cabs compared to contemporary MGWR designed locomotives. Due to lack of vacuum train brakes they were confined to North Wall freight yard workings until their first and very extensive rebuild in 1906-1908.Locomotive no. 99 was used in the trials of the patent Cusack-Morton superheater from 1915 to 1916. All the class were then rebuilt with either a Belpaire(Robinson) superheater boiler with piston values between 1918 and 1922.Noted as powerful free steaming engines capable of hauling 55 wagons on the main line they proved for goods work they were also suitable for slower speed passenger services.

NZR UB class

The NZR UB class were a series of Ten Wheelers built by American manufacturers around the start of the twentieth century. Two batches were built by Baldwin in 1898 and 1901 (ten each). The earlier engines had slide valves and Stephenson motion, the later had piston valves and Walschaerts valve gear, as well as a higher boiler pressure. Two additional locomotives were obtained in 1901 from ALCO, one each from Brooks and Richmond. The Brooks locomotive (#17) was heavier (30.1 LT adhesive) with attendant increase in tractive effort (18340 lbf), and had a larger grate (17 sq ft). This locomotive was very popular with crews. The Richmond locomotive had less evaporative heating surface but included a superheater. Boiler pressure was lower (180 psi) and tractive effort was marginally lower. The locomotives were initially assigned to Dunedin to Christchurch expresses and were reassigned as newer power replaced them. The last assignment for the class was on the West Coast Region.

Prussian P 6

The Prussian Class P 6s were passenger locomotives operated by the Prussian state railways with a leading axle and three coupled axles.

The P 6 was conceived as a so-called universal locomotive. The first vehicle was manufactured in 1902 at Düsseldorf by the firm of Hohenzollern. This engine has a number of features that are characteristic of its designer, Robert Garbe: a narrow chimney located well forward and the unusual position of the boiler. As a result, and in spite of the relatively small, 1,600 mm (62.99 in) diameter, driving wheels (on the prototype they were only 1,500 mm or 59.06 in), the locomotives were authorised to travel at up to 90 km/h (56 mph), a speed which could not be attained in practice due to its poor riding qualities.

The smokebox superheater installed on the first machines was soon replaced by a smoke tube superheater. In all, 275 engines of this class were built up to 1910. 110 examples had to be handed over after the First World War as reparations. 163 locomotives were taken over by the Deutsche Reichsbahn as DRG Class 37.0-1, where they were allocated the running numbers 37 001–163. The locomotives with numbers 37 201–206 were, by contrast, G 6 and P 6 class engines respectively of Lübeck-Büchen Railway (LBE), that had a different design from the Prussian locomotives.

The Prussian P 6s were retired by about 1950. The few engines left after the Second World War were no longer employed by the Deutsche Bundesbahn and the Reichsbahn.

The locomotives taken over by the Polish State Railways (PKP) were given the designation Oi1.

One of them has been preserved and can be viewed in the Warsaw Railway Museum.

The engines were equipped with Prussian tenders of class pr 2'2' T 16.

Queensland C18 class locomotive

The Queensland Railways C18 class locomotive was a class of 4-8-0 steam locomotives operated by the Queensland Railways.

Russian locomotive class А

The Russian locomotive class A was a series of Russian steam locomotives from the early 20th Century, among the most powerful produced in the country at that time, with a top speed of 115 kilometers per hour.One example, shown here, Ab type, with a Schmidt superheater, with the number between the couplers indicates Ab 132, produced at the Briansk locomotive factory in 1909.


A smokebox is one of the major basic parts of a steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the chimney (or funnel). Early locomotives had no smokebox and relied on a long chimney to provide natural draught for the fire but smokeboxes were soon included in the design for two main reasons. Firstly and most importantly, the blast of exhaust steam from the cylinders, when directed upwards through an airtight smokebox with an appropriate design of exhaust nozzle, effectively draws hot gases through the boiler tubes and flues and, consequently, fresh combustion air into the firebox. Secondly, the smokebox provides a convenient collection point for ash and cinders ("char") drawn through the boiler tubes, which can be easily cleaned out at the end of a working day. Without a smokebox, all char must pass up the chimney or will collect in the tubes and flues themselves, gradually blocking them.

The smokebox appears to be a forward extension of the boiler although it contains no water and is a separate component. Smokeboxes are usually made from riveted or welded steel plate and the floor is lined with concrete to protect the steel from hot char and acid or rainwater attack.

Snifting valve

A snifting valve (sometimes snifter valve) is an automatic anti-vacuum valve used in a steam locomotive when coasting. The word Snift imitates the sound made by the valve.

South African Class 10B 4-6-2

The South African Railways Class 10B 4-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.

In March 1910, the Central South African Railways placed ten Class 10-2 steam locomotives with a 4-6-2 Pacific wheel arrangement in service, of which five were built with and five without superheaters. In 1912, when the five superheated locomotives were assimilated into the South African Railways, they were renumbered and designated Class 10B. During 1912, the South African Railways placed five more Class 10B locomotives in service.

South African Class 10D 4-6-2

The South African Railways Class 10D 4-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.

In 1910, the Central South African Railways placed one American-built Class 10 4-6-2 Pacific type steam locomotive in service. When the South African Railways classification and renumbering took place in 1912, this locomotive was designated the sole member of Class 10D.

South African Class 6L 4-6-0

The South African Railways Class 6L 4-6-0 of 1904 was a steam locomotive from the pre-Union era in the Cape of Good Hope.

In 1904, the Cape Government Railways placed its last two 6th Class 4-6-0 bar-framed steam locomotives in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and designated Class 6L.

Superheated steam

Superheated steam is a steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.

The steam can therefore cool (lose internal energy) by some amount, resulting in a lowering of its temperature without changing state (i.e., condensing) from a gas, to a mixture of saturated vapor and liquid. If unsaturated steam (a mixture which contain both water vapor and liquid water droplets) is heated at constant pressure, its temperature will also remain constant as the vapor quality (think dryness, or percent saturated vapor) increases towards 100%, and becomes dry (i.e., no saturated liquid) saturated steam. Continued heat input will then "super" heat the dry saturated steam. This will occur if saturated steam contacts a surface with a higher temperature.

Superheated steam and liquid water cannot coexist under thermodynamic equilibrium, as any additional heat simply evaporates more water and the steam will become saturated steam. However this restriction may be violated temporarily in dynamic (non-equilibrium) situations. To produce superheated steam in a power plant or for processes (such as drying paper) the saturated steam drawn from a boiler is passed through a separate heating device (a superheater) which transfers additional heat to the steam by contact or by radiation.

Superheated steam is not suitable for sterilization. This is because the superheated steam is dry. Dry steam must reach much higher temperatures and the materials exposed for a longer time period to have the same effectiveness; or equal F0 kill value. Superheated steam is also not useful for heating. Saturated steam has a much higher wall heat transfer coefficient.Slightly superheated steam may be used for antimicrobial disinfection of biofilms on hard surfaces.Superheated steam’s greatest value lies in its tremendous internal energy that can be used for kinetic reaction through mechanical expansion against turbine blades and reciprocating pistons, that produces rotary motion of a shaft. The value of superheated steam in these applications is its ability to release tremendous quantities of internal energy yet remain above the condensation temperature of water vapor; at the pressures at which reaction turbines and reciprocating piston engines operate.

Of prime importance in these applications is the fact that water vapor containing entrained liquid droplets is generally incompressible at those pressures. If steam doing work in a reciprocating engine or turbine, cools to a temperature at which liquid droplets form; the water droplets entrained in the fluid flow will strike the mechanical parts of engines or turbines, with enough force to bend, crack or fracture them. Superheating and pressure reduction through expansion ensures that the steam flow remains as a compressible gas throughout its passage through a turbine or an engine, preventing damage of the internal moving parts.


This article is about the phenomenon where a liquid can exist in a metastable state above its boiling point. See superheated water for pressurized water above 100 °C. See superheater for the device used in steam engines.In physics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. This is a so-called metastable state or metastate, where boiling might occur at any time, induced by external or internal effects. Superheating is achieved by heating a homogeneous substance in a clean container, free of nucleation sites, while taking care not to disturb the liquid.

WAGR F class

The WAGR F class was a class of 4-8-0 heavy goods steam locomotives operated by the Western Australian Government Railways (WAGR) between 1902 and 1970.

Water-tube boiler

A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

High Pressure Water Tube Boiler:

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers in thermal power stations are also called steam generating units.

The older fire-tube boiler design, in which the water surrounds the heat source and gases from combustion pass through tubes within the water space, is a much weaker structure and is rarely used for pressures above 2.4 MPa (350 psi). A significant advantage of the watertube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

A water tube boiler was patented by Blakey of England in 1766 and was made by Dallery of France in 1780.

Yarrow boiler

Yarrow boilers are an important class of high-pressure water-tube boilers. They were developed by

Yarrow & Co. (London), Shipbuilders and Engineers and were widely used on ships, particularly warships.

The Yarrow boiler design is characteristic of the three-drum boiler: two banks of straight water-tubes are arranged in a triangular row with a single furnace between them. A single steam drum is mounted at the top between them, with smaller water drums at the base of each bank. Circulation, both upwards and downwards, occurs within this same tube bank. The Yarrow's distinctive features were the use of straight tubes and also circulation in both directions taking place entirely within the tube bank, rather than using external downcomers.

Simple boilers
Fire-tube boilers
Water-tube boilers
Electric boilers
Boiler components
Boiler peripherals

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