Water-tube boiler

A high pressure watertube boiler[1] (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.[2]

Water tube boiler schematic
Schematic diagram of a marine-type watertube boiler


“The ability of watertube boilers to generate superheated steam makes these boilers particularly attractive in applications that require dry, high-pressure, high-energy steam, including steam turbine power generation”.[3]

Owing to their superb working properties, the use of watertube boilers is highly preferred in the following major areas:

  • Variety of process applications in industries
  • Chemical processing divisions
  • Pulp and Paper manufacturing plants
  • Refining units

Besides, they are frequently employed in power generation plants where large quantities of steam (ranging up to 500 kg/s) having high pressures i.e. approximately 16 megapascals (160 bar) and high temperatures reaching up to 550 °C are generally required. For example, the Ivanpah solar-power station uses two Rentech Type-D watertube boilers.[4]


Modern boilers for power generation are almost entirely water-tube designs, owing to their ability to operate at higher pressures. Where process steam is required for heating or as a chemical component, then there is still a small niche for fire-tube boilers.


Their ability to work at higher pressures has led to marine boilers being almost entirely water-tube. This change began around 1900, and traced the adoption of turbines for propulsion rather than reciprocating (i.e. piston) engines – although watertube boilers were also used with reciprocating engines.


There has been no significant adoption of water-tube boilers for railway locomotives. A handful of experimental designs were produced, but none of these were successful or led to their widespread use.[5] Most water-tube railway locomotives, especially in Europe, used the Schmidt system. Most were compounds, and a few uniflows. The Norfolk and Western Railway's Jawn Henry was an exception, as it used a steam turbine combined with an electric transmission.[6]

Rebuilt completely after a fatal accident
Using a Yarrow boiler, rather than Schmidt. Never successful and re-boilered with a conventional boiler.[7]


A slightly more successful adoption was the use of hybrid water-tube / fire-tube systems. As the hottest part of a locomotive boiler is the firebox, it was an effective design to use a water-tube design here and a conventional fire-tube boiler as an economiser (i.e. pre-heater) in the usual position.

One famous example of this was the USA Baldwin 4-10-2 No. 60000, built in 1926. Operating as a compound at a boiler pressure of 2,400 kilopascals (350 psi) it covered over 160,000 kilometres (100,000 mi) successfully. After a year though, it became clear that any economies were overwhelmed by the extra costs and it was retired to become a stationary plant.[8] A series of twelve experimental locomotives were constructed at the Baltimore and Ohio Railroad's Mt. Clare shops under the supervision of George H. Emerson, but none of them was replicated in any numbers.[9]

Brotans boiler
Brotan boiler

The only railway use of water-tube boilers in any numbers was the Brotan boiler, invented in Austria in 1902 by Johann Brotan and found in rare examples throughout Europe. Hungary, though, was a keen user and had around 1,000 of them. Like the Baldwin, this combined a water-tube firebox with a fire-tube barrel. The original characteristic of the Brotan was a long steam drum running above the main barrel, making it resemble a Flaman boiler in appearance.[10][11]


While the traction engine was usually built using its locomotive boiler as its frame, other types of steam road vehicles such as lorries and cars have used a wide range of different boiler types. Road transport pioneers Goldsworthy Gurney and Walter Hancock both used water-tube boilers in their steam carriages around 1830.

Most undertype wagons used water-tube boilers. Many manufacturers used variants of the vertical cross-tube boiler, including Atkinson, Clayton, Garrett and Sentinel. Other types include the Clarkson 'thimble tube' and the Foden O-type wagon's pistol-shaped boiler.[12]

Steam fire-engine makers such as Merryweather usually used water-tube boilers for their rapid steam-raising capacity.

Many steam cars used water-tube boilers, and the Bolsover Express company even made a water-tube replacement for the Stanley Steamer fire-tube boiler.[13]

Design variations

D-type Boiler

The 'D-type' is the most common type of small- to medium-sized boilers, similar to the one shown in the schematic diagram. It is used in both stationary and marine applications. It consists of a large steam drum vertically connected to a smaller water drum (a.k.a. "mud drum") via multiple steam-generating tubes. These are surrounded by walls made up of larger water-filled tubes, which make up the furnace.

M-Type Boilers

The M-Type boilers were used in many US World War II warships including hundreds of FLETCHER class destroyers. Three sets of tubes form the shape of an M, and create a separately fired superheater that allows better superheat temperature control. In addition to the mud drum shown on a D-type boiler, an M-Type has a water-screen header and a waterwall header at the bottom of the two additional rows of vertical tubes and downcomers.

Low Water Content

The 'Low Water Content' boiler has a lower and upper header connected by watertubes that are directly impinged upon from the burner. This is a "furnace-less" boiler that can generate steam and react quickly to changes in load.

Babcock & Wilcox boiler

Babcock and Wilcox boiler (Heat Engines, 1913)
Babcock & Wilcox boiler

Designed by the American firm of Babcock & Wilcox, this type has a single drum, with feedwater drawn from the bottom of the drum into a header that supplies inclined water-tubes. The watertubes supply steam back into the top of the drum. Furnaces are located below the tubes and drum.

This type of boiler was used by the Royal Navy's Leander-class frigates.

Stirling boiler

The Stirling boiler has near-vertical, almost-straight watertubes that zig-zag between a number of steam and water drums. Usually there are three banks of tubes in a "four drum" layout, but certain applications use variations designed with a different number of drums and banks.

They are mainly used as stationary boilers, owing to their large size, although the large grate area does also encourage their ability to burn a wide range of fuels. Originally coal-fired in power stations, they also became widespread in industries that produced combustible waste and required process steam. Paper pulp mills could burn waste bark, sugar refineries their bagasse waste. It is a horizontal type of boiler.


Yarrow boiler end section (Stokers Manual 1912)
End-view of a Yarrow boiler

Named after its designers, the then Poplar-based Yarrow Shipbuilders, this type of three-drum boiler has three drums in a delta formation connected by watertubes. The drums are linked by straight watertubes, allowing easy tube-cleaning. This does, however, mean that the tubes enter the drums at varying angles, a more difficult joint to caulk. Outside the firebox, a pair of 'cold-leg' pipes between each drum act as 'downcomers'.[14]

Due to its three drums, the Yarrow boiler has a greater water capacity. Hence, this type is usually used in older marine boiler applications. Its compact size made it attractive for use in transportable power generation units during World War II. In order to make it transportable, the boiler and its auxiliary equipment (fuel oil heating, pumping units, fans etc.), turbines, and condensers were mounted on wagons to be transported by rail.


The White-Forster type is similar to the Yarrow, but with tubes that are gradually curved. This makes their entry into the drums perpendicular, thus simpler to make a reliable seal.[14]


Thornycroft boiler end section (Stokers Manual 1912)
End-view of a Thornycroft boiler

Designed by the shipbuilder John I. Thornycroft & Company, the Thornycroft type features a single steam drum with two sets of watertubes either side of the furnace. These tubes, especially the central set, have sharp curves. Apart from obvious difficulties in cleaning them, this may also give rise to bending forces as the tubes warm up, tending to pull them loose from the tubeplate and creating a leak. There are two furnaces, venting into a common exhaust, giving the boiler a wide base tapering profile.[14]

Forced circulation boiler

In a forced circulation boiler, a pump is added to speed up the flow of water through the tubes.[15]

Other types

See also


  1. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2016-10-11. Retrieved 2013-11-21.CS1 maint: Archived copy as title (link)
  2. ^ Marshall, Chapman Frederick (16 December 2014). "A History of Railway Locomotives down to the End of the Year 1831". BoD – Books on Demand – via Google Books.
  3. ^ "Boiler Efficiency: Water Tube Boilers".
  4. ^ "eCRMS" (PDF). Docketpublic.energy.ca.gov. Retrieved 2018-09-24.
  5. ^ "High Pressure Steam Locomotive Technology". Loco Locomotive gallery.
  6. ^ "The Jawn Henry". Trains Magazine. Retrieved 2008-10-28.
  7. ^ "LNER 10000 High Pressure Locomotive". Loco Locomotive gallery D&H 1402 James Archibald
    : The Delaware and Hudson E7 class of water tube boiler locomotives consisted of three examples 1400-1402.
  8. ^ "Baldwin 60000". Loco Locomotive gallery.
  9. ^ See chapter on "Experimentals" in Sagle, Lawrence W. (1964). B&O Power. Staufer. p. 288 ff.
  10. ^ "Brotan". Loco Locomotive gallery.
  11. ^ "Brotan".
  12. ^ Kelly, Maurice A. (1975). The Undertype Steam Road Waggon. Cambridge: Goose and Son. ISBN 0-900404-16-7.
  13. ^ Harris, K. N. (1967). Model Boilers and Boilermaking. Hemel Hempstead: Model Aeronautical Press.
  14. ^ a b c Stokers' Manual ((1912 edition) ed.). Admiralty, via HMSO, via Eyre & Spottiswoode. 1901.
  15. ^ Newest on Process Equipments (2012-11-25). "Boilers circulation systems: natural circulation and forced circulation". Enggcyclopedia. Retrieved 2013-09-30.

External links

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.

Corner tube boiler

A cornertube boiler is a type of natural circulation water-tube boiler which differentiates itself from other water tube boilers by its characteristic water-steam cycle and a pre-separation of heated steam from the steam-water mixture occurs outside the drum and the unheated downcomers.

Flash boiler

A flash boiler is a type of water-tube boiler. The tubes are close together and water is pumped through them. A flash boiler differs from the type of monotube 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 superheated. Flash boilers had some use in automobiles in the 19th century and this use continued into the early 20th century.

HMCS Fredericton (K245)

HMCS Fredericton was a Flower-class corvette of the Royal Canadian Navy. She was ordered from Marine Industries Ltd. in Sorel, Quebec and laid down on 22 March 1941. She was launched on 2 September 1941 and commissioned on 8 December 1941. She was named after the community of Fredericton, New Brunswick.

Fredericton's design was slightly revised from the earlier Flower-class ships. Corvettes built before 1941 behaved poorly in heavy seas, so her length and weight were increased. She was also outfitted with a water-tube boiler which was more powerful and stable than earlier models. Her armament was limited to a 4-inch (100 mm) gun forward and a 2-pound (0.9 kg) pom-pom gun aft gun as well as depth charge throwers which suited her escort duties and anti-submarine capabilities. Later on she was outfitted with the improved Hedgehog anti-submarine device. She was manned by a crew of 85 which included six officers. Her unofficial emblem was a badge emblazoned with a flying tiger dropping a depth charge on a U-boat.

Fredericton served during the Battle of the Atlantic from 1941 to 1945. During 1942 she escorted oil tankers from the Caribbean to New York City. From 1943 to 1945 she escorted convoys in the Western Atlantic and then later on across the Atlantic to Northern Ireland. She was decommissioned on 14 July 1945. Some question as to her final disposition lies with a possible error in Lloyd's Register. Either she was sold for scrap in 1946 or ended up as a Panamanian-flagged Japanese whaler which was used until 1979.

High-pressure steam locomotive

A high-pressure steam locomotive is a steam locomotive with a boiler that operates at pressures well above what would be considered normal. In the later years of steam, boiler pressures were typically 200 to 250 psi (1.38 to 1.72 MPa). High-pressure locomotives can be considered to start at 350 psi (2.41 MPa), when special construction techniques become necessary, but some had boilers that operated at over 1,500 psi (10.34 MPa).

LNER Class Y1

The LNER Class Y1 was a class of 0-4-0 geared steam locomotives built by Sentinel Waggon Works for the London and North Eastern Railway and introduced in 1925. They passed into British Railways ownership in 1948 and were numbered 68130-68153 but 68134 was withdrawn almost immediately and may not have carried its BR number.

LNER Class Y10

The LNER Class Y10 was a class of two 0-4-0T geared steam locomotives built by Sentinel Waggon Works for the London and North Eastern Railway and introduced in 1930. The LNER numbered them 8403 and 8404 but they were later re-numbered 8186 and 8187. This was the second use of the classification Y10 by the LNER. The first was for an ex-North British Railway 0-4-0 steam tender locomotive, withdrawn 1925.Both the Sentinels passed into British Railways ownership in 1948 but 8187 was withdrawn almost immediately. Number 8186 was allocated the BR number 68186, but never actually carried it, and was withdrawn in 1952.

LNER Class Y3

The LNER Class Y3 was a class of 0-4-0 geared steam locomotives built by Sentinel Waggon Works for the London and North Eastern Railway and introduced in 1927. They passed into British Railways ownership in 1948 and were numbered 68154-68185. At least one was based at Immingham in 1950.

LaMont boiler

A LaMont boiler is a type of forced circulation water-tube boiler in which the boiler water is circulated through an external pump through long closely spaced tubes of small diameter. The mechanical pump is employed in order to have an adequate and positive circulation in steam and hot water boilers.

List of boiler types, by manufacturer

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 more descriptive name. Some large manufacturers also made boilers of several types. Accordingly, it is difficult to identify their technical aspects from merely their name. This list presents these known, notable names and a brief description of their main characteristics.

O-type boiler

An O-type boiler is a form of water-tube boiler. It is named, like the D-type and A-type boilers, from the approximate shape of its tubes.

They are characterised by single steam and water drums vertically above each other, with curved vertical water tubes to the sides forming an overall cylindrical volume. There is no grate at the base of this furnace space, so they are fired by liquid burners, oil or gas, rather than a solid fuel furnace producing ash.O-type boilers first appeared as marine boilers, such as the Johnson boiler, once the switch from coal firing to oil firing began. The large radiant heating area available allows a combustion rate, for a given furnace volume, of around twice that for a contemporary boiler, such as the Yarrow.

Small examples of the O-type are used as some package boilers. Most package boilers are fire-tube boilers, often used for heating or process steam, and generally work at lower pressures than propulsion boilers. If high pressure steam is needed, such as for steam turbines, then a water-tube boiler may be preferred and these are mostly O-type. O-type package boilers appeared post-World War II, with the general shift away from coal and to more automated boilers needing fewer human operators.O-type boilers are available with or without end water-walls. They may also use tube and refractory cement infill, or else a steel membrane wall between the tubes.

Reed water tube boiler

The Reed water tube boiler was a type of water tube boiler developed by J. W. Reed, manager of the engine works at Palmers Shipbuilding and Iron Company of Jarrow, England, where it was manufactured from 1893 to 1905. At this time, Palmers was a vertically integrated business: in its shipyard at Jarrow, using iron ore from its own mine in North Yorkshire, it produced the iron and steel needed for its ships, and engines and boilers of its own design.

Intended for use in the steam propulsion of ships, the Reed water tube boiler was similar to other boilers such as the Normand and Yarrow, themselves developments of the du Temple boiler. These differed from locomotive boilers, also known as "fire tube boilers", in that, whereas the fire tube boiler consisted of a cylinder filled with water, which was heated by tubes passing through it carrying exhaust gases from a furnace, in the water tube boiler the situation was reversed, with water passing through steam-generating tubes mounted directly above the furnace. Advantages of the water tube boiler included comparative lightness and the ability to run at higher pressures. About 170 of Reed's water tube boilers were installed in ships of the Royal Navy, in two of which they were installed to replace boilers rejected by the Admiralty.

SS Clovelly (ferry)

SS Clovelly was a steam ferry that operated on Okanagan Lake in British Columbia, Canada. She was commissioned by Captain L. A. Hayman and built by DeFoe in Vancouver, British Columbia in the fall of 1907. She was named after Clovelly, a small village on the Bristol Channel in England. She was launched by Captain J. B. Weeks and began a service of two trips a week hauling lumber, feed, and fruit between the communities of Westbank, Bear Creek, and Kelowna. She was the fourth ferry on the lake. Shortly after her launch, it was discovered that her vertical boiler leaked, so a water tube boiler was built by A. Brunette of the Leckie Hardware Company of Kelowna. She was inspected and passed by J. H. Thompson, Dominion Government Steamboat Inspector for the Province. In 1911, Clovelly was sold to E. Hankinson. Complaints about poor service reached the government and Hankinson lost the charter. Clovelly went to J. Y. Campbell, who operated her from 1912 to 1916 and also built MV Aricia in 1912.

Steam drum

A steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam at the top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture. The difference in densities between hot and cold water helps in the accumulation of the "hotter"-water/and saturated-steam into the steam-drum.

Stephen Wilcox

Stephen Wilcox, Jr. (February 12, 1830 – November 27, 1893) was an American inventor, best known as the co-inventor (with George Herman Babcock) of the water-tube boiler. They went on to found the Babcock & Wilcox Company. He was born in Westerly, Rhode Island.

Stirling boiler

The Stirling boiler is an early form of water-tube boiler, used to generate steam in large land-based stationary plants. Although widely used around 1900, it has now fallen from favour and is rarely seen.

Thimble tube boiler

A thimble tube boiler is a form of steam boiler, usually provided as an auxiliary boiler or heat-recovery boiler. They are vertical in orientation and would be considered a form of water-tube boiler.

Vertical boiler

A vertical boiler is a type of fire-tube or water-tube boiler where the boiler barrel is oriented vertically instead of the more common horizontal orientation. Vertical boilers were used for a variety of steam-powered vehicles and other mobile machines, including early steam locomotives.

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.

Operating cycle
See also

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