The Reed water tube boiler was developed and
patented in 1893 by J. W. Reed, manager of the engine works at
Palmers Shipbuilding and Iron Company, which manufactured it. By the last quarter of the 19th century Palmers had become one of Britain's largest shipbuilders and, during its operation between 1851 and 1933, it produced "more than 900" ships. However, it was a
vertically integrated business: from about 1857 it possessed its own source of
iron ore, mined near the
North Yorkshire coast in the vicinity of
Whitby and
Saltburn, and, according to the local historians Jim Cuthbert and Ken Smith, "it was said that [Palmers' shipyard] brought in iron ore at one end ... and sent it away again at the other end in the form of finished ships." Thus the Reed water tube boiler was a natural addition to the company's output, which had previously included other designs of boiler, such as the
Belleville boiler, besides steam engines. It was similar to its antecedent the
du Temple boiler, and other developments from it such as the
Normand and
Yarrow boilers, in that each featured three cylindrical water chambers arranged to form a triangle or, viewed from one end, an inverted "V" shape: the entire boiler was filled with water but for the upper part of the top chamber, which allowed for the collection of steam, and was connected by two banks of steam-generating tubes to the two lower chambers, between which was a
furnace. Water tube boilers could operate at higher pressures and were much lighter than locomotive boilers, also known as "
fire tube boilers" or, when used in ships, as "marine boilers". In these, water was contained in a single drum through which tubes carried exhaust gases from a furnace: a locomotive boiler had to be constructed from heavier gauge materials, since the greater size of the single drum required a thicker shell, and, while the tubes in a water tube boiler were subject only to tension from the steam and pressurised water within, a locomotive boiler's tubes were subject to compression from without, again requiring thicker materials. of 1895: the black lines adjacent to and above the steam-generating tubes in the cross section are baffles designed to optimise the passage of hot gases around the tubes. Both diagrams illustrate the designed water level in the top chamber, below which the steam-generating tubes were connected. In the Normand boiler, the tubes were comparatively straight and a portion of those in the inner and outer rows of each bank were formed into "tube walls" to direct hot gases generated by the furnace through the boiler. In the Reed boiler, the tubes were bent into pronounced curves of varying radii to maximise surface area and therefore steam production, and baffles were used to direct hot gases. The lowest section of the lowest tubes of Reed boilers was originally bent into tight, "wavy" curves, also to maximise surface area, but this was discontinued by 1901 as it inhibited the flow of water and hence also steam. Further, the external diameter of the tubes tapered at their lower ends from inches (27 mm) to inch (22 mm) to improve the passage of hot gases between them. They were connected perpendicularly to the chambers at each end, as were the tubes in the Normand boiler, to reduce stress. However, in the Reed boiler these connections were made by hemispherical faces, which allowed "a certain angular play". The tubes were secured by
nuts inside the chambers at each end.
Handholes gave access to the bottom chambers, and a
manhole gave access to the top chamber, allowing rapid replacement of defective tubes. In both types of boiler, the steam-generating tubes joined the top chamber below the designed water line to stop them overheating: in another type of water tube boiler, the
Thornycroft, the steam-generating tubes joined the top chamber above the water line, and their tops were "observed to get red-hot when the water was low." Overheated tubes were liable to fail. Large, external "down-comer" tubes transferred water from the top chamber to the two bottom ones. The down-comers thus promoted
convection within the boiler, which needed to be rapid because of the small diameter of the tubes, and formed "a substantial part of [its] framework." Steam was collected inside a
dome on top of the top chamber, from which it passed out of the boiler for use via engine room controls, and in the Reed boiler all but the dome and the ends of the three water chambers was enclosed in a double-layered casing with an air gap and
asbestos lining that reduced the temperature of the outer layer. The casing rose at the top to form an outlet for hot gases into a
funnel. The furnace was fed with coal by
stokers through firebox doors at one end, and, whereas the Normand boiler required a fire of about 18 inches (460 mm) depth, the Reed boiler required a shallower one of between 8 and 12 inches (200–300 mm). Air was admitted to the furnace through the air gap in the boiler casing, thus providing a supply of heated air that was directed to the rear of the
ashpan. This air entered the ashpan through three doors that, along with the firebox doors, closed automatically if a tube failed, the intention being to prevent flames, steam and debris escaping into the
boiler room. A constant supply of
pure water was essential for this type of boiler, as a shortage of water would rapidly result in an empty boiler liable to severe damage from the furnace, and the deposition of any contaminant, such as
limescale, would result in a significant loss of efficiency and could block tubes. To surmount this problem,
boiler feedwater circulated in a
closed system from the boiler as steam to the engines and then to
condensers, from which it returned as water to the boiler, thus completing a cycle. However, some incidental loss of water from the system was unavoidable, and the French naval engineer
Louis-Émile Bertin regarded a 5% loss of water per cycle as the maximum that could be sustained in a water tube boiler installation. Therefore, additional feedwater was required, and it was supplied by apparatus such as an
evaporator, as was fitted in , built by Palmers and launched in 1899. While each of eight Reed boilers in
Pegasus had a grate area of about and a heating area of about , together they produced up to 7,127 IHP . ==Production and use==