The Probert Encyclopaedia of Science & Technology

BOILER

Boiler is the name applied to any vessel for boiling large quantities of liquor in, but most commonly used as the designation of a metallic vessel in which water is converted into steam by the agency of fire, the steam being intended by its expansive force to give motion to a steam-engine, or to be used for a variety of manufacturing purposes. Around 1900 boilers were generally constructed of mild steel, having a tensile strength of about 28 tons per square inch. In old locomotive boilers the fire-boxes are almost always constructed of copper, but in some cases, especially in America, mild steel was used here also. The tubes of locomotive boilers were of brass; those of water-tube boilers mostly of mild steel. The tubes of ordinary marine boilers are usually of wrought iron or mild steel.

Boilers may be grouped in two classes, namely, shell or tank boilers and water-tube boilers. The former consist of a large shell or tank, usually cylindrical, with flat ends. This shell contains the water and steam, so that the whole shell is exposed to the full pressure of the steam. In some cases the furnaces are external, but generally they are contained within the shell. Boilers may also be classified as flue boilers and multitubular boilers, and the latter may be again subdivided into fire-tube and water-tube boilers. Flue boilers are so called because they have large internal flues passing through them and traversed by the products of combustion. In fire-tube boilers the products of combustion pass through a large number of tubes of small diameter, whereas in water-tube boilers the water flows through the tubes, and the products of combustion flow over the outsides of the tubes.

Cornish and Lancashire boilers are examples of shell boilers of the flue type. The Lancashire boiler was 20 or 30 feet long, and 7 or 8 feet in diameter. Within it were two cylindrical flues, at the front ends of which were the two furnaces. Conical 'Galloway tubes' were usually fitted into the internal flues for the purpose of improving the circulation of the water. At the rear end of the fire-grate was a firebrick bridge, which served to prevent the fuel from falling over the end of the grate, but its main object was to cause the gases to mix more rapidly in order to facilitate rapid and complete combustion. The boiler was cased in brickwork, so built as to form external flues at the sides and beneath. The products of combustion, after leaving the internal flues, passed down to the bottom flue, along which they flowed to near the front end of the boiler. They then flowed through the side flues to the rear end, and thence into the flue leading to the economizer or to the chimney.

Marine and locomotive boilers were multitubular fire-tube boilers. Marine boilers, also called Scotch boilers, were single-ended or double-ended ; that is, they had furnaces at
one end only or at both ends. The furnaces were in cylindrical flues communicating with internal combustion chambers, from which there were a large number of return tubes above the flues leading to the uptake. In a locomotive boiler it was usual to have a firebrick arch dividing the fire-box into two compartments, the upper of which served as a combustion-chamber. The gases flowed from the fire-box through the small-diameter tubes to the smoke-box at the front end. As a high chimney could not be used, there had to be an artificial or forced draught, and this was always obtained by means of a steam blast, the exhaust steam from the engine cylinders being used for the purpose.

Water-tube boilers, also known as tubulous boilers, have been arranged in three classes, namely: (1) those with limited circulation, (2) those with free circulation, (3) those with accelerated circulation. The special feature of the first class, of which the Belleville boiler is the type, is the use of tubes rising in successive coils or folds, each receiving water at the lower end and discharging steam and water at the upper. The circulation is limited to what is necessary to replace the water evaporated, and there is no water-reservoir. Boilers of the second class, those of free circulation, have horizontal or slightly inclined generating tubes connecting two flat vertical water-spaces. The principal kinds in use around 1900 were the Niclausse, the Babcock and Wilcox the Durr, and the D'Allest. In boilers of accelerated circulation there are vertical tubes of various shapes connecting horizontal reservoirs, generally of a cylindrical shape. The water is returned from the top steam-drum to the water-drum at the bottom by large down-take tubes. The principal examples of this type of boiler are the Thornycroft, the Yarrow, the Normand, the Normand-Sigaudy, and the Stirling. Water-tube boilers of all three classes were being used to an increasing extent in war-vessels and in the mercantile marine from the start of the 20th century, and some kinds of the first two types were at the same time displacing shell boilers on land, especially in electric light and power stations. They require more careful stoking than a Lancashire boiler, but were less liable to explosion, and it was felt around 1900 that in densely peopled districts no other type should be allowed.

In the 19th century the Belleville boiler was more used in naval vessels than any other water-tube boiler, but a special British admiralty committee condemned it in 1904, and recommended the Yarrow and the Babcock and Wilcox.

A Green's economizer was an arrangement of vertical cast-iron tubes which was generally attached to Lancashire boilers in order to heat the feed-water by means of the products of combustion after they left the boiler. The products of combustion then rounded the outsides of these tubes, and the tubes were kept clean by automatic scrapers, which continuously removed the soot from them. The advantages of superheated steam are now generally recognized, and superheaters for supplying heat to the steam after it leaves the boiler, were often added to boilers. Mechanical stokers were also devised. The principal boiler fittings are as follows; pressure-gauges, to show the pressure of steam; safety-valves, of which there should be two on each boiler, one beyond the attendant's control; stop-valves, controlling the passage of steam from the boiler to the steam-piping; separators, for drying the steam; water-gauges, showing the water-level; feed check-valves, for regulating the supply of water; also automatic feed-water regulators, injectors, and feed-pumps.
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