Yarrow boiler

Yarrow boilers are an important class of high-pressure water-tube boilers. They were developed by
Yarrow & Co., 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 within the tube bank, rather than using external downcomers.

Early watertube boilers

Early use of the water-tube boiler within the Royal Navy 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 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 boats and the early destroyers 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. Most of these were of the three-drum pattern, 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

developed his boiler as a response to others that had already developed water-tube boilers. 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 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?", 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'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 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 burners 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 were 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 and locomotive boilers, water-tube boilers with their reduced water volumes were considered lightweight and didn't require extensive supports.

Later evolution in design

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 explosions 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 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.