Three-drum boiler
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.
The fundamental characteristic of the "three-drum" design is the arrangement of a steam drum above two water drums, in a triangular layout. Water tubes fill in the two sides of this triangle between the drums, and the furnace is in the centre. The whole assembly is then enclosed in a casing, leading to the exhaust flue.
Firing can be by either coal or oil. Many coal-fired boilers used multiple firedoors and teams of stokers, often from both ends.
Development
Development of the three-drum boiler began in the late 19th century, with the demand from naval ships that required high power and a compact boiler. The move to water-tube boilers had already begun, with designs such as the Babcock & Wilcox or the Belleville. The three-drum arrangement was lighter and more compact for the same power.The new generation of "small-tube" water-tube boilers used water-tubes of around diameter, compared to older designs of 3 or 4 inches. This gave a greater ratio of tube surface heating area to the tube volume, thus more rapid steaming. These small-tube boilers also became known as "express" boilers. Although not all of these were three-drum designs, most were some variation of this. As the tubes of the three-drum are close to vertical, this encourages strong circulation by the thermosyphon effect, further encouraging steaming.
The development of the three-drum pattern was generally one of simplification, rather than increasing complexity or sophistication. Even the first boilers packed a large heating area into a compact volume, their difficulty was in manufacturing and particularly for their maintenance on-board ship.
Tubes
The convoluted tubes of early designs such as the [|du Temple] and Normand were the first to go. A multi-row bank of tubes could provide adequate heating area, without this complexity. Tubes also became straighter, mostly to ease their cleaning. [|Yarrow] had demonstrated that straight tubes did not cause any problems with expansion, but circular drums and perpendicular tube entry were both valuable features for a long service life. Where tubes entered drums at an angle, heating and cooling tended to bend the tube back and forth, leading to leaks. A perpendicular entry was easier to expand the tubes for a reliable seal and to avoid these sideways stresses. It was worth the compromise of the Admiralty boiler's bent tube ends to keep these two features, and these tubes were still simple enough in shape to clean easily.Some of the first boiler tubes, particularly the du Temple with its sharp corners, could not be cleaned of scale internally. Tubes were later cleaned internally by attempting to pass a hinged rod through, with a brush at the end. For the curved tube designs, often only part of the tube could be reached. Another method was to pass a chain down the tube from above, pulling a brush behind it, although this was unworkable for boilers like the Thornycroft where the tubes first travelled horizontally or upwards. The eventual method was to use 'bullet' brushes that were fired from one drum into the other by use of compressed air. Sets of brushes were used, one for each tube, and they were carefully numbered and counted afterwards to ensure that none had been left behind, blocking a tube.
Downcomers
Separate downcomers were used by most designs, even after Yarrow's experiments had demonstrated that circulation could still take place amongst the heated tubes alone. Again, the Admiralty boiler was the culmination of this approach, placing the superheater within the tube bank, so as to encourage the necessary temperature difference.Furnaces
The Admiralty boiler is usually considered to be a direct evolution of the Yarrow, although the White–Forster also had an influence, probably as a result of the large number in service with the Royal Navy. The circular water drums, and their raising above the furnace floor, are White–Forster features. The first reduces the risk of grooving, the latter is appropriate for oil firing.Types
du Temple boiler
The du Temple was an early naval water-tube boiler, patented in 1876. It was invented by Félix du Temple in France and was tested in a Royal Navy torpedo gunboat. Water tubes were convoluted, arranged in four rows to a bank, and S-shaped with sharp right angle bends. This packed a large tube heating area into a small volume, but made tube cleaning impractical. The drums were cylindrical, with perpendicular tube entry and external downcomers between them.White–Forster boiler
The White–Forster was of simple construction, with tubes that had only a gentle curvature to them. This was sufficient to allow them to be replaced in-situ, working through the manhole at the end of the large steam drum. Each tube was sufficiently curved to allow it to be extracted through the steam drum, but sufficiently straight that a single tube could be replaced from a tube bank, without requiring other tubes to be removed so as to permit access. This was one of many features of the White–Forster intended to make it reliable in naval service and easy to maintain. These tubes were of particularly small diameter, only and especially numerous, a total of 3,744 being used in some boilers. The tubes were arranged in 24 rows to a bank, each requiring a different length of tube, and 78 rows per drum. All tubes were curved to the same radius, facilitating repair and replacement on board, but requiring the tube holes in the drums to be reamed to precise angles on a jig during manufacture. This small tube diameter gave a high heating surface, but probably too much: the ratio of surface to volume became excessive and gas flow through the tube banks was affected, giving the boiler furnaces something of a reputation as poor burners.Downcomers were used, either the usual two large pipes, or an unusual but characteristic arrangement of four small tubes to each drum. This was a feature intended to improve survivability after damage, when used on-board warships. The boiler could remain in service with a damaged downcomer tube plugged.
The mud drums were raised above the floor of the furnace on steel girder stools, increasing the furnace volume available for combustion. This feature was intended to encourage the use of oil burning, an innovation on warships around this time. The general appearance of the White–Forster is similar to that of the later Admiralty pattern. Features such as the raised mud drums and the shape of the tubes were an influence.
White–Forster boilers were introduced into the Royal Navy from 1906, for light cruisers and torpedo boat destroyers.
Normand boiler
The Normand boiler was developed by the French Normand shipyard of Le Havre. It was used by the navies of several nations, notably those of France, Russia, Britain and United States. In 1896, the Royal Navy had them installed in twenty-six boats, more than any other water-tube design.Initial design of the Normand boiler was as a development of the Du Temple, with the sharp corners of the tubes replaced by a smooth radiused bend, but still retaining the S shape.
The design of the Normand gave a particularly large heating area in relation to the grate area. The cost of this was a dense nest of tubes, where each of the numerous rows of tubes was bent into a different and complex shape. Tube ends entered the cylindrical drums perpendicularly, for good sealing. The space needed for all these tubes filled the entire lower half of the steam drum, requiring both a large drum and a separate steam dome from which to collect dry steam. The external boiler casing entered the flue uptake at one end, usually enclosing this dome. The ends of the drums extended outside the casing as hemispherical domes. Cold downcomers outside the casing linked these drums, providing a path for the return circulation of cold water.
A further development was the Normand-Sigaudy, where two Normand boilers were coupled back-to-back, for use in large ships. This effectively gave a double-ended Normand that could be fired from both ends.
Reed boiler
The Reed boiler was used by Palmers of Jarrow. It was similar to the Normand, with downcomers and curved tubes that entered cylindrical drums perpendicularly.Thornycroft boiler
The Thornycroft boiler is a variant that splits the usual central furnace into two. There are four drums: two main drums vertically in the centre – a steam and a water drum – also two wing drums at the outside edges of the furnace. The design was notable for its early use of the water-wall furnace. The outer bank of tubes was shallow, consisting of only two rows of tubes. These rows were spaced closely, so that the tubes formed a solid wall, without gasflow between them. The inner bank of tubes was similar: the two rows of tubes closest to the furnace formed a similar water wall. These tubes were splayed apart at their base, so as to provide space for gasflow between them. Within the tube bank, gas flow is mostly parallel to the tubes, similar to some early designs, but contrary to the crossflow design of later three-drum boilers. The exhaust gas emerged into the heart-shaped space below the upper central drum, exiting to the funnel through the rear wall.The steam drum is circular, with perpendicular tube entry. The tube ends span a considerable circumference of the drum, so that the upper tubes enter above the water level. They are thus 'non-drowned' tubes.
The upper and lower central drums are linked by downcomers. Unusually these are internal to the boiler and are heated, although not strongly, by the exhaust gases. They are formed as several vertical tubes on the centreline of the boiler. They are formed into a shallow S-shape to give a little flexibility against thermal expansion. The small wing drums are connected to the lower central drum alone, by large external pipes outside the rear casing of the boiler.
Owing to its early use in the Thornycroft-built destroyer HMS Daring of 1893, this design became known as the
A small single-sided version of this boiler was also produced for launches. The first small version of this also dispensed with the wing drum, the water-wall tubes bending at right angles and passing back to the central water drum, the tubes also forming the grate to support the fire.