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282

BOILERS

the bottom of this drum, where the heating is not great, and where therefore their presence will not be injurious. Locomotive Boilers.—At the present time locomotive boilers do not differ very much from the type illustrated in the Encyclopcedia Britcinnicci, vol. xxii. p. 498, except that they are considerably larger and arranged for a higher working pressure than formerly. A pressure of 200 lb per square inch is common on the North-Eastern and London and North-Western Railways, while 230 lb per square inch is being used in some instances on the Continent and in America. One of the largest of recent engines, made for the Lancashire and Yorkshire Railway Company (illustrated in the Engineer for 17th March 1899), has a working pressure of 175 lb per square inch, a grate surface of 26 square feet, and a total heating surface of 2052 square feet. There are 239 tubes, 2 inches in outside diameter and 15 feet long. The shell of the boiler is 4 feet 9 inches in diameter. The fire - box is of copper, and the stays of bronze set at the usual pitch of 4" x 4". The rest of the boiler, including the tubes, is of steel. In some of the large boilers, especially on the London and South-Western Railway, watertubes have been fitted. These are usually Fig. 2.—Babcock and Wilcox Water-tube Boiler fitted with Superheaters, about 2 f inches in diameter, and are in500 lb per square inch. Like all water-tube boilers, they require clined across the fire-box to facilitate the circulation of to be frequently cleaned if impure feed-water is used ; but the the water. They are expanded into and beaded over the straightness of their tubes enables their condition to be ascertained side plates of the fire-box, and the outer plates of the at any time when the boiler is out of use, and any accumulation shell are provided with stays passing through the tubes. of scale to be removed. The superheaters which are sometimes fitted consist of two cross-boxes or headers placed transversely under the They considerably increase the heating surface, and are cylindrical drum and connected by numerous C-shaped tubes. regarded as very efficient. Experience show's that they The steam is taken by an internal pipe passing through the bottom stand well. of the drum into the upper cross-box, then through the C tubes into the lower box, and thence to the steam-pipe. When steam is being raised the superheater is flooded with water, which is drained out through a blow-off pipe before communication is opened with the steam-pipe. Another type of water-tube boiler in use for stationary purposes is the “Stirling,” illustrated in Fig. 4. This boiler usually consists of five horizontal drums, of which the three upper 4 St ‘‘,ag ,~ ones form the steam space and the two lower contain ' the water. In some cases only one lower chamber is used. The lower drums are connected to each other at about the middle of their height by horizontal tubes, and to the upper drums by numerous nearly vertical tubes which form the major portion of the heating surfaces. The central upper drum is at a slightly higher level than the others, and communicates with that nearest the back of the boiler by a set of curved tubes entirely above the water-level, and with the front drum by two sets—the upper one being above, and the lower entirely below, the water-level. The whole boiler is enclosed in brickwork, into which the supporting columns and girders are built. Brickwork baffles compel the furnace gases to take specified courses among Fig. 4.—Stirling Boiler. the tubes. It will be seen that the space between the boiler Marine Boilers.—For marine work the varied conditions front and the tubes forms a large Fig. 3.—Handhole Fittings. combustion-chamber, into which to be fulfilled in different classes of vessels have led to the all the furnace gases must pass before they enter the spaces employment of several different types of boilers. In merbetween the tubes ; another chamber is formed between the first chant vessels, except in a fewr passenger steamers making and second sets of tubes. The feed-water enters the back upper short runs, ordinary cylindrical return-tube boilers are drum, and must all pass down the third set of tubes into the lower’ drum before it reaches the other parts of the boiler. Thus always used, while in war vessels and in a few passenger the coldest water is always where the temperature of the furnace vessels of high speed engaged in cross-Channel services, gases is lowest; and as the current through the lower drum is various forms of water-tube boilers are found. The great slight, the solid matters separated from the feed-water while its difference in the weights of these types forms the principal temperature is being raised have an opportunity of settling to

in Fig. 3, the vratertightness of the joints being secured by the outer cover-plates. The holes being oval, the inside fitting can be placed in position from outside, and it is so made as to cover the opening and prevent any great outrush of steam or water should the bolt break. Any desired working pressure can be provided for in these boilers ; in some special cases it rises as high as