Flued boiler

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Trevithick's engine of 1806 is built around an early example of a flued boiler (specifically, a return-flue type) Trevithick high pressure engine of 1806.jpg
Trevithick's engine of 1806 is built around an early example of a flued boiler (specifically, a return-flue type)

A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and the later multi-tube fire-tube boilers. A flued boiler is characterized by a large cylindrical boiler shell forming a tank of water, traversed by one or more large flues containing the furnace. These boilers appeared around the start of the 19th century and some forms remain in service today. Although mostly used for static steam plants, some were used in early steam vehicles, railway locomotives and ships.

Boiler closed vessel in which water or other fluid is heated

A boiler is a closed vessel in which fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.

Steam water in the gas phase, which is formed when water boils. Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses

Steam is water in the gas phase, which is formed when water boils or evaporates. Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses. At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 °C (212 °F) at standard pressure. If heated further it becomes superheated steam.

Steam engine Heat engine that performs mechanical work using steam as its working fluid

A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force is transformed, by a connecting rod and flywheel, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine.


Flued boilers were developed in an attempt to raise steam pressures and improve engine efficiency. Early haystack designs of Watt's day were mechanically weak and often presented an unsupported flat surface to the fire. Boiler explosions, usually beginning with failure of this firebox plate, were common. It was known that an arched structure was stronger than a flat plate and so a large circular flue tube was placed inside the boiler shell. The fire itself was on an iron grating placed across this flue, with a shallow ashpan beneath to collect the non-combustible residue. This had the additional advantage of wrapping the heating surface closely around the furnace, but that was a secondary benefit.

James Watt British engineer

James Watt was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.

Boiler explosion catastrophic failure of a boiler

A boiler explosion is a catastrophic failure of a boiler. As seen today, boiler explosions are of two kinds. One kind is a failure of the pressure parts of the steam and water sides. There can be many different causes, such as failure of the safety valve, corrosion of critical parts of the boiler, or low water level. Corrosion along the edges of lap joints was a common cause of early boiler explosions.

Although considered as low-pressure (perhaps 25  psi (1.7  atm )) today, this was regarded as high pressure compared to its predecessors. This increase in pressure was a major factor in making locomotives (i.e. small self-moving vehicles) such as Trevithick's into a practical proposition.

Pounds per square inch unit of pressure or stress

The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.

The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference or standard pressure. It is approximately equal to the atmospheric pressure at sea level.

Richard Trevithick Cornish inventor, engineer and steam locomotive builder

Richard Trevithick was a British inventor and mining engineer from Cornwall, England, UK. The son of a mining captain, and born in the mining heartland of Cornwall, Trevithick was immersed in mining and engineering from an early age. He performed poorly in school, but went on to be an early pioneer of steam-powered road and rail transport. His most significant contribution was the development of the first high-pressure steam engine. He also built the first full-scale working railway steam locomotive. The world's first locomotive-hauled railway journey took place on 21 February 1804, when Trevithick's unnamed steam locomotive hauled a train along the tramway of the Penydarren Ironworks, in Merthyr Tydfil, Wales.

Centre-flue boilers

Middleton locomotive Blenkinsop's rack locomotive, 1812 (British Railway Locomotives 1803-1853).jpg
Middleton locomotive

The simplest boiler for locomotives had a single straight flue. It was widely used by many of the early locomotive makers, including Blenkinsop's locomotives for the Middleton railway and Stephenson's Locomotion.

John Blenkinsop British engineer

John Blenkinsop was an English mining engineer and an inventor of steam locomotives, who designed the first practical railway locomotive.

Middleton Railway

The Middleton Railway is the world's oldest continuously working public railway, situated in the English city of Leeds. It was founded in 1758 and is now a heritage railway, run by volunteers from The Middleton Railway Trust Ltd. since 1960.

This type of boiler is simple to manufacture and strong enough to support "high pressure" (for the period) steam with expansive working in the cylinders. There is also good gas flow through the large flue, so that the fire receives sufficient draught from the action of a tall chimney alone. However it also has little heating area, so is inefficient and burns a large amount of coal.

Chimney structure that provides ventilation for exhausting the hot or toxic flue gases, aerosols and smokes produced by a boiler, stove, furnace or fireplace inside a building to the outside atmosphere

A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator or fireplace from human living areas. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney effect. The space inside a chimney is called the flue. Chimneys are adjacent to large industrial refineries, fossil fuel combustion facilities or part of buildings, steam locomotives and ships.

Return-flue boilers

Puffing Billy of 1813, showing the domed end of its return-flue boiler (centre of picture) Puffing Billy steam engine.JPG
Puffing Billy of 1813, showing the domed end of its return-flue boiler (centre of picture)

A simple flue must be long if it is to offer adequate heating area. In a short boiler shell, such as required for a steam locomotive, this may be done by using a U-shaped return flue that bends back on itself.

Steam locomotive Railway locomotive that produces its pulling power through a steam engine

A steam locomotive is a type of railway locomotive that produces its pulling power through a steam engine. These locomotives are fueled by burning combustible material – usually coal, wood, or oil – to produce steam in a boiler. The steam moves reciprocating pistons which are mechanically connected to the locomotive's main wheels (drivers). Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in wagons (tenders) pulled behind.

Richard Trevithick had already used a return flue with his first 1802 Pen-y-darren engine and 1803 Coalbrookdale locomotive design. [1] These boilers were heavily built of cast iron, short and flat-ended. His 1805 "Newcastle" locomotive (actually built in Gateshead) began to show one characteristic feature of the return-flued boiler, a prominent dome shape to resist steam pressure in the solid end opposite both furnace and chimney. In this case, the boilermaking, now of wrought iron plates, must have been complicated by Trevithick's single long-travel horizontal cylinder (9 in × 36 in (230 mm × 910 mm) diameter×stroke) which emerged through this domed end. [1] This did make work easier for the fireman though, as he was no longer trying to reach a firedoor beneath the long crosshead of the piston.

Return flue boiler.jpg

William Hedley used this pattern of boiler for his 1813 locomotives Puffing Billy and Wylam Dilly. Through the Wylam colliery and its owner Christopher Blackett, Hedley would have been familiar with Trevithick's engine. [2]

Timothy Hackworth's 0-6-0 Royal George of 1827 also used a return-flued boiler, although it is best known for its pioneering use of a deliberate blastpipe to encourage draught on the fire. [1] His lighter weight 0-4-0 version for the Rainhill Trials, Sans Pareil was very similar. [2] Even though they appeared antiquated as soon as the Trials were over, the Canadian Samson of this pattern was built in 1838 and still in service in 1883. [2]

Huber boilers

Huber boiler Farm engines and how to run them; the young engineer's guide with special attention to traction and gasoline farm engines, and a chapter on the science of successful threshing (1910) (14578624980).jpg
Huber boiler
External images
Searchtool.svg "New Huber" traction engine
Searchtool.svg Huber traction engine of 1897
Searchtool.svg Huber advertisement, and boiler sectional diagram

The last return-flue boilers constructed (other than some stationary boilers) are often considered to be those built by the Huber Co. of Marion, Ohio for their "New Huber" traction engines, from 1885 to 1903. [3]

These were not, however, return-flue boilers in the sense used here, but rather return-tube boilers. They had a single large cylindrical furnace tube, a combustion chamber external to the boiler's pressure shell, then multiple, narrow fire-tubes returning to a horseshoe-shaped smokebox above and around the firedoor. The proximity of this smokebox to the fireman led to their nickname of "belly burners". Their design thus has more in common with the horizontal launch-type boilers (as used by Sir Arthur Heywood) or the Scotch marine boiler than they do with the simple single-flue boiler.

By this time, the locomotive boiler had become ubiquitous for traction engines. Compared to this, the advantage of the Huber boiler was that the firetubes could be replaced more easily, without needing to work from within an enclosed firebox.

Cornish boiler

Cornish boiler Moonta historic copper mines 1.JPG
Cornish boiler

The simplest form of flued boiler was Richard Trevithick's "high-pressure" Cornish boiler, first installed at Dolcoath mine in 1812. [4] This is a long horizontal cylinder with a single large flue containing the fire. As the furnace relied on natural draught, a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire.

For efficiency, Trevithick's innovation was to encase beneath the boiler with a brick-built chamber. Exhaust gases passed through the central flue and then routed outside and around the iron boiler shell. To keep the chimney clear of the firing space, the brick flue passed first underneath the centre of the boiler to the front face, then back again along the sides and to the chimney.

Cornish boilers had several advantages over the preceding wagon boilers: they were composed of mostly curved surfaces, better to resist the pressure. Their flat ends were smaller than the flat sides of the wagon boiler and were stayed by the central furnace flue, and sometimes by additional long rod stays. A less obvious advantage was that of boiler scale. Wagon or haystack boilers were heated from beneath and any scale or impurities that formed a sediment settled upon this plate, insulating it from the water. This reduced heating efficiency and could in extremis lead to local overheating and failure of the boiler plates. In the flued boiler, any sediment fell past the furnace flue and settled out at the bottom of the boiler shell, where it had less effect. [5]

In model engineering, the Cornish boiler, particularly when fitted with Galloway tubes (see Lancashire Boiler, below), is an excellent choice for gas-fired boilers and model steam boats. It is simple to build and as efficient as any small-scale boiler. [6]

Butterley boiler

Butterley boiler, from Fairbairn's lecture of 1851 Butterley boiler - Fairbairn - On Boiler Explosions.jpg
Butterley boiler, from Fairbairn's lecture of 1851

The Butterley or "whistle mouth" boiler is a little-known design derived from the Cornish pattern, produced by the noted Butterley boilerworks of Derbyshire. [7] It is basically a Cornish boiler with the lower half of the shell around the furnace removed, so as to permit a large fire to be lit. This made it popular in the textile mills of the Pennines, where the hard Northern coal was of less calorific value than the Welsh coal used in the South West and required a larger fire. [8] Alternatively it may be considered as a shortened Cornish boiler with a wagon boiler placed in front of it with a larger fire beneath that. It suffers the same drawback as the wagon boiler: the concave firebox plate is mechanically weak and this either limits the working pressure or requires extra mechanical staying.

Lancashire boiler

Lancashire boiler, from Fairbairn's lecture Lancashire boiler - Fairbairn - On Boiler Explosions.jpg
Lancashire boiler, from Fairbairn's lecture
Lancashire boiler at Pinchbeck Pumping Station Lancashire boiler, Pinchbeck Pumping station - geograph.org.uk - 1157301.jpg
Lancashire boiler at Pinchbeck Pumping Station

The Lancashire boiler is similar to the Cornish, but has two large flues containing the fires instead of one. It is generally considered to be the invention of William Fairbairn in 1844, although his patent was for the method of firing the furnaces alternately, so as to reduce smoke, rather than the boiler itself. [9] Stephenson's early 0-4-0 locomotive "Lancashire Witch" had already demonstrated the use of twin furnace tubes within a boiler 15 years earlier. [1]

Fairbairn had made a theoretical study of the thermodynamics of more efficient boilers, and it was this that had led him to increase the furnace grate area relative to the volume of water. A particular reason for this was the so-far poor adoption of the Cornish boiler in the cotton mills of Lancashire, where the harder local coal couldn't be burned satisfactorily in the smaller furnace, in favour of the older low-pressure wagon boiler and its large grate. [8]

The difficulties of the Cornish boiler were that a boiler of any particular power would require a known area of furnace tube as the heating area. Longer tubes required a longer and more expensive boiler shell. They also reduced the ratio of grate area relative to the heating area, making it difficult to maintain an adequate fire. Increasing the tube diameter reduced the depth of water covering the furnace tube and so increased the need for accurate control of water level by the fireman, or else the risk of boiler explosion. Fairbairn's studies of hoop stress in cylinders also showed that smaller tubes were stronger than larger tubes. His solution was simple: to replace one large furnace tube with two smaller ones.

The patent [9] showed another advantage of twin furnaces. By firing them alternately and closing the firebox door between firings, it was also possible to arrange a supply of air past the furnace (in the case of a Lancashire boiler, through the ashpan beneath the grate) which would encourage the flue gases produced by the fire to burn more completely and cleanly, thus reducing smoke and pollution. [10] A key factor in this was the distinctive shuttered rotating air damper in the door, which became a feature from the 1840s.

The use of two flues also has a strengthening effect, acting as two long rod stays that support the end plates. [7]

Later developments added Galloway tubes (after their inventor, patented in either 1848 [11] or 1851 [12] ) crosswise water tubes across the flue, thus increasing the heated surface area. As these are short tubes of large diameter and the boiler continues to use a relatively low pressure, this is still not considered to be a water-tube boiler. The tubes are tapered, simply to make their installation through the flue easier. [6]

Lancashire boilers often show corrugated flues, which absorb thermal expansion without straining the riveted seams. Another development was the "kidney flue" or Galloway boiler, where the two furnaces join together into a single flue, kidney-shaped in cross-section. This widened and flat-topped flue was stayed by the use of Galloway tubes.

Although the Lancashire boiler is considered to be an antiquated design, provided that the flue is long enough it can be reasonably efficient. This does lead to a bulky boiler though, particularly for its length, and this has always limited its use to stationary installations. It was the standard boiler in Greater Manchester and Lancashire cotton mills.

Fairbairn's five-tube boiler

Fairbairn five-tube boiler,
end section Fairbairn five-tube boiler, end section (Molesworths, 1883).jpg
Fairbairn five-tube boiler,
end section

William Fairbairn's work on the Lancashire boiler had demonstrated the efficiency virtues of multiple furnaces relative to a reduced water volume. It was also widely understood that higher steam pressures improved the efficiency of engines. Fairbairn's research on the strength of cylinders [13] led him to design another improved boiler, based around far-smaller tube diameters, which would thus be able to operate at higher pressures, typically 150  psi (1,000  kPa ). This was the "five tube" boiler, whose five tubes were arranged in two nested pairs as water drum and furnace, with the remaining tube mounted above them as a separate steam drum. [14] The water volume was extremely low compared to previous boiler designs, as the furnace tubes almost filled each of the water drums.

The boiler was successful according to its goals and provided two large furnaces in a small water capacity. The separate steam drum also aided the production of "dry" steam, without the carryover of water and risk of priming. However it was also complex to manufacture, and did not offer a great deal of heating area for the work involved. It was soon superseded by multi-tube boilers such as the Fairbairn-Beeley and the Scotch boilers.

See also

Further reading

Related Research Articles

Steam locomotive components

This is a glossary of the components found on typical steam locomotives.

Fire-tube boiler

A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or (many) more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

Firebox (steam engine) part of a steam engine

In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name. The hot gases generated in the firebox are pulled through a rack of tubes running through the boiler.

Marc Seguin French engineer

Marc Seguin was a French engineer, inventor of the wire-cable suspension bridge and the multi-tubular steam-engine boiler.


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. 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 blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.

Boiler (power generation) device used to create steam by applying heat energy to water

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 but, at pressures above this, it is more usual to speak of a steam generator.

Scotch marine boiler design of steam boiler best known for its use on ships

A "Scotch" marine boiler is a design of steam boiler best known for its use on ships.

Vertical boiler with horizontal fire-tubes

A vertical boiler with horizontal fire-tubes is a type of small vertical boiler, used to generate steam for small machinery. It is characterised by having many narrow fire-tubes, running horizontally.

Boilers for generating steam or hot water have been designed in countless shapes, sizes and configurations. An extensive terminology has evolved to describe their common features. This glossary provides definitions for these terms.

Sentinel boiler

The Sentinel boiler was a design of vertical boiler, fitted to the numerous steam waggons built by the Sentinel Waggon Works.

Three-drum boiler class of water-tube boiler used to generate steam, typically to power ships

Three-drum boilers are a class of water-tube boiler used to generate steam, typically to power ships. They are compact and of high evaporative power, factors that encourage this use. Other boiler designs may be more efficient, although bulkier, and so the three-drum pattern was rare as a land-based stationary boiler.

Transverse boiler

A transverse boiler is a boiler used to generate steam to power a vehicle. Unlike other boilers, its external drum is mounted transversely across the vehicle.

Vertical cross-tube boiler

A cross-tube boiler was the most common form of small vertical boiler. They were widely used, in the age of steam, as a small donkey boiler, for the independent power of winches, steam cranes etc.

Vertical fire-tube boiler

A vertical fire-tube boiler or vertical multitubular boiler is a vertical boiler where the heating surface is composed of multiple small fire-tubes, arranged vertically.

Launch-type boiler

A launch-type, gunboat or horizontal multitubular boiler is a form of small steam boiler. It consists of a cylindrical horizontal shell with a cylindrical furnace and fire-tubes within this.

Steam locomotive exhaust system

The Steam locomotive exhaust system consists of those parts of a steam locomotive which together discharge exhaust steam from the cylinders in order to increase the draught through the fire. It usually consists of the blastpipe, smokebox, and chimney, although later designs also include second and third stage nozzles.

Fairbairn-Beeley boiler

The Fairbairn-Beeley boiler was a design of fire-tube stationary boiler developed in the late 19th century. It takes its name from its two developers, Sir William Fairbairn and Thomas Beeley


  1. 1 2 3 4 The British Railway Locomotive, 1803–1853. Science Museum. 1958. ISBN   0-11-290152-2.
  2. 1 2 3 Snell, J.B. (1964). Early Railways. London: Weidenfeld & Nicolson.
  3. David Burgess Wise (1973). Steam on the Road. Hamlyn. p. 26. ISBN   0-600-38018-1.
  4. Hills, Power from Steam, p. 103.
  5. Hills, Power from Steam, p. 129.
  6. 1 2 K. N. Harris (1974). Model Boilers and Boilermaking. MAP. ISBN   0-85242-377-2.
  7. 1 2 Fairbairn, On Boiler Explosions.
  8. 1 2 Hills, Power from Steam, p. 133.
  9. 1 2 GB 10166 , Fairbairn, 1844
  10. Hills, Power from Steam, p. 138.
  11. "Lancashire Boiler" (pdf). Museum of Science & Industry, Manchester.
  12. Hills, Power from Steam, p. 134.
  13. Fairbairn, Useful Information for Engineers.
  14. Molesworth, Guildford L. (1882). Molesworth's Pocket Book of Engineering Formula (21st ed.). London: Spon. p. 467.