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Modern Marine Boiler Types

Considerable improvement in the power and efficiency of marine boilers followed upon the adoption of the Scotch boiler about 1870 and has continued with the subsequent development of the water tube boilers


SCOTCH BOILERS were introduced in 1862, and until water tube boilers were perfected Scotch boilers were generally used in ships. Even to-day they are found in most merchant steamers. Above are models of the Scotch boilers fitted in H.M.S. Trafalgar, built at Portsmouth in 1890. Six boilers of this type were arranged in two stokeholds.

FROM 1870 until the introduction of the water tube boiler the “Scotch” type predominated in the steamers and warships of the world. In merchant vessels the Scotch boiler is still the type most frequently employed and is able to work at pressures up to 300 lb.

When first generally introduced the working pressure of the Scotch boiler was about 60 lb. This was steadily advanced until it had reached 135 lb. by 1889-90. A model of a Scotch boiler of this period is preserved in the Science Museum at South Kensington and represents one of a set of six boilers fitted in H.M.S. Trafalgar at Portsmouth in 1890.

The shell of the boiler was made in the form of two rings joined together by trebly-riveted straps or bands inside and outside. The end plates were in three pieces, flanged for attachment to the furnace flues and the shell, and were stayed by long bars passing through the boiler above the tubes and combustion chambers and below the furnaces. There were four furnace flues 7 ft. 4 in. long of the Fox corrugated type.

On either side of the boiler, adjacent pairs of furnace flues were led into a common combustion chamber. These chambers were approximately quadrant-shaped in section and separated by a water space 5 in. wide. The ends of the furnace flues in the combustion chambers were provided with firebrick deflecting arches, and adjacent arches were separated by a firebrick wall.

The flat sides of the combustion chambers were stayed together by screwed rods, and their roofs were strengthened by girder stays. Each chamber was provided with 245 return tubes, 2½ in. in external diameter, leading to a smoke-box and uptake built on to the outside of the boiler. The ends of the tubes were protected from the effects of the hot gases by special ferrules.

Supported in cradles, the boilers were secured to the inner plating of the hull. The boilers, three on either side, were placed with their backs against the plating of a central tunnel, 10 ft. 6 in. wide, which divided the engine and boiler rooms at the middle of the ship and was used for the transfer of ammunition from one end of the vessel to the other.

There were thus two separate stokeholds and either could be rendered airtight by screens and air-locks when working under forced draught. Air was supplied to either stokehold by three fans.

The Belleville boilerThe boilers in H.M.S. Trafalgar were each 16 ft. 1 in. in diameter and 10 ft. 3 in. long. In working order, with fittings and including its proportion of uptake and funnel, each boiler weighed 84 tons. The heating surface was 3,050 sq. feet and the steam output under natural draught was equivalent to 1,400 indicated horse-power when used in triple-expansion engines. Forced draught yielded 2,100 indicated horsepower.

Scotch boilers have been improved considerably in detail since their introduction, but their general working principles are still incorporated in modern designs for this type of marine steam generator.

BOILERS OF FRENCH ORIGIN were the first of the water tube type to be extensively used. The Belleville boiler, illustrated here, was introduced about 1850, but not until 1893 was an improved type used in a British warship. In 1895 H.M. cruisers Powerful and Terrible were equipped with Belleville boilers.

The water tube boiler has now superseded the Scotch boiler for Naval use and is being used more extensively in the Merchant Service, especially for large passenger liners. Its story is even older than that of some other types of boilers. In 1766 William Blakey patented a boiler that was fitted with inclined water tubes. A water tube boiler was used by James Rumsey in his jet-propulsion steamer described in the chapter “Marine Engines and Their Story”.

Colonel John Stevens's twin-screw launch of 1804, referred to in the chapter “Marine Engines and Their Story”, was also fitted with a boiler of the water tube type. This boiler comprised two sets of inclined tubes closed at their lower ends and with the upper ends opening into a central drum surmounted by a steam dome. The firegrate was situated below the lower ends of one set of tubes, around which passed the heated gases, thence around the central drum to the other set of tubes and so to the funnel. The famous John Penn also used water tube boilers in his Thames paddle steamers as early as 1842. The use of high pressures was associated with water tube boilers at quite an early stage, following their adoption for marine purposes. In 1878 the steam yacht Anthracite was fitted by Loftus Perkins, a pioneer of the marine boiler, with a generator consisting of layers of horizontal tubes contained in a lined sheet-iron case. The tubes were closed at either end, but all were connected together at intervals in their length by short vertical tubes or “thimbles”. This boiler worked at a pressure of 500 lb.

The first water tube boiler to be used extensively in marine practice was the Belleville, named after its inventor, a distinguished French engineer. First introduced about 1850, the Belleville boiler was modified in later years, and by 1880 it comprised a series of slightly inclined, large diameter tubes arranged close together in separate groups.

The improved Belleville boiler was successfully used in French warships and passenger steamers. In the Royal Navy, Belleville boilers were fitted in the torpedo-gunboat Sharpshooter in 1893 and in two large cruisers, H.M.S. Powerful and H.M.S. Terrible, built in 1895-97. These two cruisers had engines of 25,000 indicated horse-power. Each boiler comprised a set of “elements” that consisted of continuous inclined tubes passing ten times to and fro over the fire and opening at their upper ends into a steam drum. The lower ends of the elements were connected together by a “collector”, into which was pumped the feed water.

Four boilers were built together into one unit and each boiler contained eight elements. The mild steel tubes had an external diameter of 4½ in. and a length of 7 feet. They were screwed into malleable cast iron “bends” or junction boxes placed one on top of another. Steam generated in the element tubes carried with it much water to the steam drum. Such water was, however, separated by a system of baffles and returned by vertical pipes to mud drums at either end of the feed collector. Lime-water, was introduced into the mud drums with the feed water to precipitate solid impurities. The supply of feed water was automatically controlled by a vertical cylindrical float.

Eighteen Niclausse boilers, as shown here, were fitted in H.M.S. New Zealand. Each boiler had a heating surface of 1,992 square feet and worked at a pressure of 220 lb. per square inch.

In the front of the boiler a square tube was fitted, and this was supplied with compressed air discharged through nozzles into the furnace to assist in the combustion of the fuel. Above the water tubes were placed deflector plates to help in the circulation of the furnace gases.

Each boiler had a heating surface of 1,500 sq. feet and worked at a pressure of 250 lb., giving a steam supply equivalent to over 500 indicated horse-power. About 1898 an improved form of this boiler was introduced in which a set of smaller elements was placed above the main water tubes to serve as a feed water heater or economizer. The first British warship to be fitted with this improved boiler was H.M.S. Diadem. Further use of the Belleville boiler in the Royal Navy was not made after 1901.

Another interesting boiler of French origin was the Niclausse. In this type of boiler inclined tubes, closed at one end, had within them circulating tubes that led into separate water compartments or headers. Eighteen of these boilers were fitted in H.M.S. New Zealand in 1904. Each boiler contained sixteen elements, each comprising a malleable cast iron header with twenty steel tubes. The headers were divided into two compartments, but all were connected at the top to a common steam drum. The back compartments of the headers communicated with the larger or outer inclined tubes. The smaller tubes inside were connected to the front header compartments. These double tubes ensured circulation of the water in the boiler which had a heating surface of 1,992 sq. feet and worked at a pressure of 220 lb. A type of inclined tube generator that was originally introduced into the United States Navy in 1889 was the Babcock and Wilcox boiler. This has been extensively used in the Royal Navy also since 1898. The inclined steel tubes of this boiler are straight and are expanded into wrought steel headers arranged vertically. The tubes slope downwards towards the front of the boiler, and the upper ends of the front headers are connected with an overhead steam drum. This drum is also connected with the tops of the back headers by horizontal tubes. The lower ends of the front headers are connected to a steel sediment box of square section.

First Naval Water Tube Boiler

The first water tube boiler to be used in the Royal Navy was patented by Sir John I. Thornycroft in 1885. This type of boiler was fitted in H.M.S. Speedy, a torpedo-gunboat of 810 tons built at Chiswick in 1892-93. The vessel had eight of these boilers and they were known as the “Speedy” type. Their total heating surface was 14,720 sq. feet, and the working pressure was 200 lb.

In the “Speedy” Thornycroft boiler a central steam drum, arranged at the apex of an inverted V, was joined by a large number of small-diameter steel tubes to a pair of water drums flanking the furnaces. An improved Thornycroft boiler known as the “Daring” type was fitted in H.M. torpedo boat destroyer Daring in 1893. This boiler had increased heating surface and grate area, and three water drums instead of two.

A special feature of the Thornycroft boiler is the provision of large-diameter tubes, in addition to the furnace tubes, connecting the steam and water drums. These large tubes, or “downcomers”, maintained the water circulation in the boiler.

A modified form of the above type was the Thornycroft-Schulz boiler, with an arrangement of tubes patented by Herr R. Schulz. Boilers of this type were fitted in H.M. torpedo boat destroyers Kennet and Jed in 1903-04.

This type of boiler had an upper steam drum connected to a central lower water drum by eight curved downcomers, and to each of two outer water drums by straight downcomers outside the boiler casing. The steam and water drums were also connected by water tubes forming the sides and crowns of the furnaces. There were 1,764 water tubes, with a total heating surface, including internal downcomers, of 4,040 sq. feet. The furnaces were lined with firebrick and the whole boiler was enclosed in a thin sheet-steel casing lined with asbestos. The working pressure was 220 lb.

the Thornycroft-Schulz water tube boiler

A MODIFIED FORM of the Thornycroft boiler patented in 1885 was the Thornycroft-Schulz water tube boiler. In this type was incorporated an arrangement of tubes patented by Herr R. Schulz. In 1903, boilers of this type were fitted in H.M. torpedo boat destroyers Jed and Kennet. In each boiler were 1,764 water tubes, which gave a total heating surface of 4,040 square feet.

In 1893 a type of boiler used in the Royal Navy for torpedo boat destroyers was patented by J. W. Reed. Boilers of this design were installed in the “River” class of destroyers in 1904. The Reed boiler had a steam drum connected at either end by downcomers with two horizontal water drums. The steam and water drums were also connected by a large number of curved water tubes within a double steel casing lined with asbestos.

Another type of boiler extensively used in the Royal Navy was that originally patented by Sir Alfred F. Yarrow in 1889. For use in torpedo-boat destroyers these boilers were provided with small water tubes, but for cruisers and battleships large-diameter tubes were used to help continuous steaming.

In 1903 a number of large-tube Yarrow boilers were fitted in H.M. battleships Swiftsure and Triumph. These generators resembled in general construction the inverted V arrangement of the Thornycroft boilers. There were two water drums of D-section. The chord of the D was, however, curved to the same radius as the upper steam drum to which both water drums were connected by 1,008 straight steel tubes. There were no downcomers in these boilers. The flames and hot gases from the two furnaces went round the inclined tubes and thence along the inside of the boiler casing to the uptakes and funnel. The boilers, each with a heating surface of 3,127 sq. feet, supplied steam equivalent to 1,000 indicated horse-power at a pressure of 280 lb.

An interesting contrast is the type of Yarrow boiler built about 1924 and arranged for oil-firing. Among the improvements incorporated in the design of this boiler is a superheater of the type first introduced in 1911. This fitting comprises a number of inclined U-tubes arranged on one side of the boiler, and the tube ends enter a special drum divided longitudinally into two compartments by a diaphragm. The usual steam drum is fitted, but the two water drums are of circular instead of the D-section of earlier types.

The water drum on the superheater side of the boiler is smaller than that on the other side and has fewer water tubes. This construction is adopted to balance the resistance of both sides of the boiler to the flow of furnace gases.

The water tubes connecting steam and water drums are arranged in unequal “nests” and the tubes of the two rows of each nest nearest the fire are thicker and larger than the remainder of the tubes.

Another important fitting in this boiler is a funnel uptake damper used for deflecting hot gases from the superheater when the engines are stopped or slowed down. The output of this type of boiler is equivalent to 12,000 indicated horse-power.

The Yarrow boiler is the form most extensively used for marine purposes and is fitted in all types of British warships. In general principles the design of this generator conforms to the original ideas of Sir Alfred F. Yarrow, but, with the introduction of oil-firing, improvements have been made in details. The oil burners are arranged along the side of the boiler beneath the lowest and smallest of the water drums. Above this drum is that of the superheater, and above the superheater drum is the intermediate-sized water drum. The third and largest water drum is on the opposite side of the boiler.

A modern Yarrow boiler has 12,444 sq. feet of heating surface, 6,577 sq. feet of superheating surface and another 14,629 sq. feet for heating the air that is forced in for the combustion of the fuel oil. There are seven oil burners, and the working pressure is 425 lb., with steam superheated to 725° F.

Another type of boiler which has given satisfactory results is that introduced by Mr. J. J. Johnson about 1929. The Johnson boiler consists of two large drums placed vertically one above the other and connected together by curved tubes. In addition to these tubes there is a water tube wall between the drums. This divides the combustion space into two compartments. Either compartment is provided with three oil burners of the spraying type. Interspaced between the generator tubes, superheater tubes are arranged at the top of the boiler.

The furnace gases, having given up most of their heat to the generator and superheater tubes, pass through a large tubular air heater on their way to the funnel uptakes. The generator tubes of the Johnson boiler give 6,841 sq. feet of heating surface, with an additional 3,840 sq. feet in the superheater. The air heater has an area of 23,943 sq. feet. The boiler pressure is usually about 425 lb., with a superheat to 725° F.

BLARGE-TUBE YARROW BOILERoilers of these types, however, are not restricted to a working pressure of 425 lb. Steam may be generated in them to a pressure of 575 lb., but even this figure does not represent finality. A type of boiler known as the Benson generates steam at 3,200 lb. per sq. in., supplied through a reducing valve to high-pressure turbines working at 880 lb. In 1930 a boiler of this type was tried in the Hamburg-Amerika liner Uckermark, 7,021 tons gross.

LARGE-TUBE YARROW BOILERS were fitted in H.M. battleships Swiftsure and Triumph in 1903. Four years earlier Sir Alfred F. Yarrow had patented his design. Originally small water tubes were used and proved sufficient for smaller craft such as torpedo boat destroyers. The large-tube boilers had two water drums of D-section and these were connected to the upper steam drum by 1,008 straight steel tubes.

The marine steam boiler has certainly seen many and drastic changes since its introduction to man’s service.

Perhaps the most spectacular change of all is the change from coal to oil firing. Gone are the days of the “blacksquad” toiling in stokehold infernos on board liners and warships. The quietly swirling sheets of oil-fed flame may now be watched through glass windows in the furnace walls. In a later chapter will be described the burning of oil in the cylinders of the engines — the development of the diesel, the coming of the mighty motorship.

You can read more on “Marine Engines and Their Story”, “Propelling the Modern Ship” and

“The Queen Mary’s Engines” on this website.

You can read more on the “The Yarrow Water Tube Boiler” in Wonders of World Engineering