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BOILERS 283 reason for the adoption of the water-tube system in war heavy in proportion to the evaporative power ; but experience shows vessels. According to the particulars given by Sir A. J. it to be durable, and not very expensive in upkeep. The provision combustion-chambers, which the furnace gases must traverse, and Durston and Mr H. J. Oram, R.N., in a paper read of the change of direction which they must take before entering the before the Institution of Civil Engineers in 1899, ordinary tubes, give an opportunity for the gases to be burned before they cylindrical boilers and boiler-room fittings weigh 130 lb are cooled below the ignition-point; and this form of boiler is thereper indicated h.p. The similar weights of the Belleville fore fairly economical under ordinary conditions. To these qualities water-tube boilers fitted in similar vessels only amount of economy and durability is due its almost universal adoption in merchant steamers, where questions of weight and compactness are g » ^ g: not so important as in war vessels. The boilers are generally worked with chimney or “natural” draught; but forced draught is not uncommon, the Howden system being that usually adopted. In this the ashpits are closed, and the air for combustion is supplied Fig.5- Mor/son Type Fig. 8 Fumes Type to the fire partly through the grate-bars and partly through openings in the furnace front above the fire, having previously passed through a regenerative apparatus where it absorbs heat from the smoke, &c., leaving the ordinary boiler-tubes. In all recent war vessels, both of the British and foreign

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navies, the boilers fitted are exclusively of the water-tube - 9type. These may be divided into two distinct classes :— (1) those in which large tubes ranging from 4J inches to IfFig. 6 Fox Type Fig. 9. Brown's Arched inch in diameter are used ; and (2) the “ Express ” boilers, & Ribbed Type in which the tubes are from 1 inch to 1|- inch diameter. The former are adopted in the larger ships, the latter in the smaller vessels which do not keep the sea for long periods, and in which the conditions of durability are not so important as to outweigh the requirement of extremely high power with a minimum of weight. The large-tube type hitherto generally used is the “Belleville” Fig. 7. Deigfihort Type F/q. 10. Brown's Cambered boiler (which, however, has been adversely criticized, and was Sechon reported against by the Admiralty Boiler Committee , e „ev e „ to 100 lb, while the weights of various types of small in 1901). Two views of this boiler are shown in Fig. ' tube water-tube boilers and their fittings as supplied to 11. It is composed of two parts, the boiler proper and the cruisers amount to 55 lb, and as adopted in torpedo-boat “economizer.” Each of these consists of several sets of elements placed side by side; those of the boiler proper are situated immedestroyers are less than 40 lb per indicated h.p. diately over the lire, and those of the economizer in the uptake The sketches of the ordinary cylindrical marine boiler, both single and double ended, given in the Ency. Brit. Cylindrical. fairly YoL, xxii ‘ P- 499existing still represent practice ; but, mainly owing to the improvements in the manufacture of steel plates, some modifications in detail have become common. Circular steel boilerplates can now be procured up to 11£ feet in diameter, of thicknesses ranging from jj inch to If inches; and rectangular plates of the same thicknesses are made of all widths up to 11 feet and of such length as will keep their weight from exceeding 7 tons. As a result, many single-ended boilers have only one plate in their length, and doubleended boilers only two ; in this way some of the circular seams of rivets in the shell are avoided. Boilers are also made of larger size and for higher steam-pressures than formerly. Pressures up to 260 lb per square inch have been used at sea with ordinary cylindrical boilers in some few instances, and several boilers with a diameter of 17 feet are now successfully working at a pressure of 210 lbs. The furnaces used in the smaller sizes are often of the plain cylindrical type, of thicknesses up to f inch ; hut for large furnaces some of the corrugated or ribbed types are generally adopted. Sketches of the sections of these are given in Figs. 5-10. The first three are made from plates originally rolled of a uniform thickness, the differences being only in regard to the shapes Fig. 11.—Belleville Boiler. of the corrugations ; while in the second three the plates are rolled with ribs or thickened portions at above the boiler, the intervening space being designed to act as a distances of 9 inches. In the more recent boilers steam-chests or combustion-chamber. Each element is constructed of a number domes are not used, sufficient steam space being provided in the of straight tubes connected at their ends by means of screwed cylindrical shell. joints to junction - boxes which are made of malleable cast This type of boiler contains a considerable qnantity of water in iron. These are arranged vertically over one another, and, proportion to the heating surfaces. The thickness of the plates, and except in the case of the upper and lower ones at the front of the numerous stays required to support the flat portions, render it the boiler, each connects the upper end of one tube with the lower