Yarrow boiler
Encyclopedia
Yarrow boilers are an important class of high-pressure water-tube boiler
Water-tube boiler
A water tube boiler 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...

s. They were developed by
Yarrows
Yarrow Shipbuilders
Yarrow Limited , often styled as simply Yarrows, was a major shipbuilding firm based in the Scotstoun district of Glasgow on the River Clyde...

 and were widely used on ships, particularly warship
Warship
A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

s.

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.

Early watertube boilers

Early use of the water-tube boiler
Water-tube boiler
A water tube boiler 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...

 within the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 was controversial at times, giving rise to the 'Battle of the Boilers' around 1900. These first boilers, such as the Belleville and Niclausse, were large-tube designs, with simple straight tubes of around 4" diameter, at a shallow angle to the horizontal. These tubes were jointed into cast iron headers and gave much trouble with leakage at these joints. At the time, an assumption was that thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 in these straight tubes was straining the joints. These boilers were also large, and although fitted to many pre-dreadnought battleships, could not be fitted to the small torpedo boat
Torpedo boat
A torpedo boat is a relatively small and fast naval vessel designed to carry torpedoes into battle. The first designs rammed enemy ships with explosive spar torpedoes, and later designs launched self-propelled Whitehead torpedoes. They were created to counter battleships and other large, slow and...

s and the early destroyer
Destroyer
In naval terminology, a destroyer is a fast and maneuverable yet long-endurance warship intended to escort larger vessels in a fleet, convoy or battle group and defend them against smaller, powerful, short-range attackers. Destroyers, originally called torpedo-boat destroyers in 1892, evolved from...

s then under very active development.

To provide a lighter boiler for smaller vessels, the 'Express'  types were developed. These used smaller water-tubes of around 2" diameter, giving a greater ratio of heating area to volume (and weight). Most of these were of the three-drum pattern
Three-drum boilers
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...

, particularly of the Du Temple and Normand designs. This gave a more vertical arrangement of the water-tubes, thus encouraging thermosyphon circulation in these narrow tubes. The previous problems of tube expansion were still a theoretical concern and so the tubes were either curved, or even convoluted into hairpins and S shapes, so as to increase heating area. In practice these shapes gave rise to two more practical problems: difficulty in cleaning the tubes and also difficulty in forming a reliable joint into the water drums, particularly where tubes entered the drum at a variety of angles.

Yarrow's water-tube boiler

Alfred Yarrow
Alfred Yarrow
Sir Alfred Fernandez Yarrow, 1st Baronet, of Homestead was a British shipbuilder who started a shipbuilding dynasty, Yarrow Shipbuilders.-Life and career:...

 developed his boiler as a response to others that had already developed water-tube boiler
Water-tube boiler
A water tube boiler 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...

s. This was a long process based on theoretical experiment rather than evolution of practical boilers. Work began in 1877 and the first commercial boiler was not supplied until 10 years later, a torpedo boat
Torpedo boat
A torpedo boat is a relatively small and fast naval vessel designed to carry torpedoes into battle. The first designs rammed enemy ships with explosive spar torpedoes, and later designs launched self-propelled Whitehead torpedoes. They were created to counter battleships and other large, slow and...

 of 1887.

Despite this long gestation, the boiler's origins appear to have been most direct. Yarrow's initial conversation with William Crush, head of the boiler department, is recorded to have included a rather direct approach and Yarrow's statements, "We must wake-up about water-tube boilers", "Why not a boiler like this?" (placing his fingers together as if praying), and "Straight tubes?" already expressed two of the boiler's three basic design principles.

Straight tubes

Early water-tube designers had been concerned with the expansion of the boiler's tubes when heated. Efforts were made to permit them to expand freely, particularly so that those closest to the furnace might expand relatively more than those further away. Typically this was done by arranging the tubes in large looping curves, as for the Thornycroft boiler. These had difficulties in manufacturing and required support in use.

Yarrow recognised that the temperature of a water-filled tube was held relatively low and was consistent amongst them, provided that they remained full of water and boiling was not allowed to occur within the tubes themselves. High temperatures and variations only arose when tubes became steam filled, which also disrupted circulation.

His conclusion was thus that straight water-tubes were acceptable, and had obvious advantages for manufacture and cleaning in service.

Obtaining tubes capable of withstanding the increasing boiler pressures was difficult and most makers had already experienced problems with the welds in the tubes. A less obvious benefit of straight tubes was that they could make use of the newly developed seamless-drawn tubes now being produced for bicycle manufacture.

Yarrow's circulation experiments

It was already recognised that a water-tube boiler relied on a continuous flow through the water-tubes, and that this must be by a thermosyphon effect rather than impractically requiring a pump.

The heated water-tubes were a large number of small diameter tubes mounted between large drums: the water drums below and steam drums above. Fairbairn
William Fairbairn
Sir William Fairbairn, 1st Baronet was a Scottish civil engineer, structural engineer and shipbuilder.-Early career:...

's studies had already showed the importance of tube diameter and how small diameter tubes could easily withstand far higher pressures than large diameters. The drums could withstand the pressure by virtue of their robust construction. Manholes fitted to them allowed regular internal inspection.

The assumption was that flow through the water-tubes would be upwards, owing to their heating by the furnace, and that the counterbalancing downward flow would require external unheated downcomers. In most water-tube designs these were a few large-diameter external pipes from the steam drum to the water drum. These large-diameter pipes were thus a problem for reliability owing to their rigidity and the forces upon them.

Alfred Yarrow
Alfred Yarrow
Sir Alfred Fernandez Yarrow, 1st Baronet, of Homestead was a British shipbuilder who started a shipbuilding dynasty, Yarrow Shipbuilders.-Life and career:...

 conducted a famous experiment where he disproved this assumption. Sources are unclear as to whether he discovered this during the experiment, or conducted the experiment merely to demonstrate a theory that he already held.

A vertical U-shaped tube was arranged so that it could be heated by a series of Bunsen burner
Bunsen burner
A Bunsen burner, named after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion.- Operation:...

s on each side. A simple flow meter indicated the direction and approximate strength of any flow through the tank at the top linking the two arms of the U.

When only one side of the U was heated, there was the expected upward flow of heated water in that arm of the tube.

When heat was also applied to the unheated arm, conventional theory predicted that the circulatory flow would slow or stop completely. In practice, the flow actually increased. Provided that there was some asymmetry to the heating, Yarrow's experiment showed that circulation could continue and heating of the cooler downcomer could even increase this flow.

Yarrow then repeated the experiment, first with the U-tube at a shallow angle to the horizontal, finally with the entire system under pressure. The results were the same and circulation was maintained.

The Yarrow boiler could thus dispense with separate external downcomers. Flow was entirely within the heated watertubes, upwards within those closest to the furnace and downwards through those in the outer rows of the bank.

Description

Yarrow's production boiler had a simple and distinctive design that remained broadly unchanged afterwards. Three drums were arranged in a triangular formation: a single large steam drum at the top and two smaller water drums below. They were linked by straight watertubes in a multi-row bank to each water drum.

The furnace was placed in the space between the tube banks. Early boilers were manually coal fired, later oil fired. The boiler was enclosed in a sealed casing of steel, lined with firebricks. Brick-lined end walls to this casing housed the firedoors or oil burner quarls, but had no heating surface. The uptake flue from the boiler was in the centre top of the casing, the exhaust gases passing around the steam drum. To reduce corrosion from flue gases over the drum, it was sometimes wrapped in a simple deflector shroud. Usually the lower part of the water drums was exposed outside the casing, but only the ends of the steam drum emerged. The water level was at around one-third of the steam drum diameter, enough to cover the ends of the submerged water-tubes.

The weight of the boiler rested on the water drums, and thus on supports from the firing flat's deck. The steam drum was only supported by the watertubes and was allowed to move freely, with thermal expansion. If superheated, the superheater elements were hung from this drum. Compared to the earlier Scotch
Scotch marine boiler
A "Scotch" marine boiler is a design of steam boiler best known for its used on ships.The general layout is that of a squat horizontal cylinder. One or more large cylindrical furnaces are in the lower part of the boiler shell. Above this is a large number of small-diameter fire-tubes...

 and locomotive boilers, water-tube boilers with their reduced water volumes were considered lightweight and didn't require extensive supports.

Water drums

The first Yarrow water drums or "troughs" were D-shaped with a flat tubeplate, so as to provide an easy mounting for the tubes. The tubeplate was bolted to the trough and could be dismantled for maintenance and tube cleaning.

This D shape is not ideal for a pressure drum though, as pressure will tend to distort it into a more circular section. Experience of boiler explosion
Boiler explosion
A boiler explosion is a catastrophic failure of a boiler. As seen today, boiler explosions are of two kinds. One kind is over-pressure in the pressure parts of the steam and water sides. The second kind is explosion in the furnace. Boiler explosions of pressure parts are particularly associated...

s had shown that sharp internal corners inside boilers were also prone to erosion by grooving.

Later boilers used a more rounded section, despite the difficulty of inserting and sealing the tube ends when they were no longer perpendicular. These later drums had a manhole in the ends for access.

Downcomers

The circulation in a Yarrow boiler depended on a temperature difference between the inner and outer tube rows of a bank, and particularly upon the rates of boiling. Whilst this is easy to maintain at low powers, a higher pressure Yarrow boiler will tend to have less temperature difference and thus will have less effective circulation. This effect can be counteracted by providing external downcomers, outside of the heated flue area.

Although most Yarrow boilers did not require downcomers, some were fitted with them.

Double-ended boilers

The first double-ended boiler was built in 1905 for the Spanish government. The design was already well-suited to being fired from both ends and it was discovered that double-ended boilers were slightly more efficient in use.

Yarrow's shipyard was always restricted in the size of ships that it could build. Many of their boilers were intended for larger warships and Yarrow supplied these as components to the building yards with larger slipways.

Superheaters

Early Yarrow boilers were not superheated, but with the introduction of steam turbine
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

s, there was a demand for increasingly higher steam temperatures.

Asymmetric boilers

The Yarrow superheater consisted of hairpin tubes, parallel to the existing steam generator tubes. One bank of the generator tubes was separated in two, with individual lower water drums for them. The superheater was placed in the gap formed between these, with both ends of its tubes connected to a single superheater header drum, and an internal baffle to separate wet and dry steam.

A secondary effect of the superheater was to increase the temperature differential between inner and outer tubes of the bank, thus encouraging circulation. The two water drums were often linked by unheated downcomers, to allow this flow between the drums. This effect was later encouraged in the Admiralty boiler, where the tubes of a bank were curved apart to leave space for a superheater, whilst retaining the single water drum.

Controlled flow

Only a single superheater was ever installed, on just one side of the boiler. The simplest, and smallest, boilers moved their exhaust flue to this side, passing all of the exhaust through the bank with the superheater. The now-asymetric boiler could pass all of its exhaust gas through the superheated side as the 'single flow'type. The other bank remained in use for purely radiative heating, often with fewer rows of tubes.

Alternatively the 'double flow' boiler retained full gas flow through both sides, although only one of these contained a superheater. A controllable baffle on the non-superheated side could be closed to increase flow through the superheater. These boilers usually incoproated additional feedwater heater
Feedwater heater
A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system...

s in the updraught above these baffles.

Admiralty three-drum boiler

A later development from the Yarrow was the Admiralty three-drum boiler, developed for the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 between the wars.

This was broadly similar to later, high-pressure and oil-fired, versions of the Yarrow. The waterdrums were cylindrical and downcomers were sometimes, but not always, used. The only major difference was in the tube banks. Rather than straight tubes, each tube was mostly straight, but cranked towards their ends. These were installed in two groups within the bank, so that they formed a gap between them within the bank. Superheaters were placed inside this gap. The advantage of placing the superheaters here was that they increased the temperature differential between the inner and outer tubes of the bank, thus encouraging circulation.

Marine use

, a Havock class destroyer
Havock class destroyer
The Havock class was a class of torpedo boat destroyer of the British Royal Navy. The two ships, and , built in London in 1893 by Yarrow & Company, were the first TBDs to be completed for the Royal Navy, although the equivalent pair from J.I...

. , the lead ship of the class, was built with the then current form of locomotive boiler, Hornet with a Yarrow boiler for comparison.

The first Yarrow boilers were intended for small destroyers and filled the entire width of the hull. In the early classes, three boilers were used arranged in tandem, each with a separate funnel
Funnel
A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....

. The later sets supplied for capital ship
Capital ship
The capital ships of a navy are its most important warships; they generally possess the heaviest firepower and armor and are traditionally much larger than other naval vessels...

s used multiple boilers and these were often grouped into sets of three, sharing an uptake.

Land-based boilers

In 1922, Harold Yarrow decided to exploit the increasing boom for electricity generation as a market for Yarrows to build land-based boilers.
The first boilers, at Dunston Power Station
Dunston
Dunston may refer to one of the following places in England:*Dunston, Derbyshire*Dunston, Lincolnshire**Dunston Pillar, a nearby landmark**Nocton and Dunston railway station*Dunston, Norfolk*Dunston, Staffordshire*Dunston, Tyne and Wear...

 and Brighton
Brighton
Brighton is the major part of the city of Brighton and Hove in East Sussex, England on the south coast of Great Britain...

, were of the same marine pattern. As for their naval success, they were recognised for having a large radiant heating area and being quick to raise steam.

Large land-based turbines required high efficiency and increased superheat
Superheat
Superheat is a live album by Dutch alternative rock band The Gathering, released on 25 January 2000 by Century Media. The album was recorded at Paradiso, Amsterdam, Netherlands on 16 April 1999, with the exception of "Rescue Me" & "Strange Machines", which were recorded at 013, Tilburg, Netherlands...

, so the marine pattern was revised to the distinctive land-based Yarrow boiler. This became asymmetrical. One wing was enlarged and received most of the gas flow. The inner tube banks remained and received radiant heat from the furnace, but the gases then flowed through one of them, over a superheater bank, then through an additional third bank to increase the heat extracted.

Working pressures also increased. From a working pressure of 575 psi in 1927, by 1929 an experimental boiler was operated at 1,200 psi.

Engine 10000

Only one "Yarrow" boiler was used in a railway locomotive, Nigel Gresley
Nigel Gresley
Sir Herbert Nigel Gresley was one of Britain's most famous steam locomotive engineers, who rose to become Chief Mechanical Engineer of the London and North Eastern Railway . He was the designer of some of the most famous steam locomotives in Britain, including the LNER Class A1 and LNER Class A4...

's experimental Engine 10000 of 1924 for the LNER
London and North Eastern Railway
The London and North Eastern Railway was the second-largest of the "Big Four" railway companies created by the Railways Act 1921 in Britain...

 company. Having observed the benefits of higher pressures and compound engines in marine practice
Marine steam engine
A marine steam engine is a reciprocating steam engine that is used to power a ship or boat. Steam turbines and diesel engines largely replaced reciprocating steam engines in marine applications during the 20th century, so this article describes the more common types of marine steam engine in use...

, Gresley was keen to experiment with this approach in a railway locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

. As with the land-based boilers, Harold Yarrow was keen to expand the market for Yarrow's boiler.

The boiler was not the usual Yarrow design, and although designed by Yarrow was built by the Sheffield-based arm of the John Brown
John Brown & Company
John Brown and Company of Clydebank was a pre-eminent Scottish marine engineering and shipbuilding firm, responsible for building many notable and world-famous ships, such as the , the , the , the , the , and the...

 shipyard. In operation, particularly its circulation paths, the boiler had more in common with other three-drum designs such as the Woolnough.
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