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BOILERS 285 by a “lanterne” to two plates, they are fitted by conical joints to of two, three, or four to nearly all of the sinuosities of the header, the inner plate only, the holes in the outer plate being closed by the purpose of this arrangement being to give opportunities small round doors. The tubes are closed at the lower end, and for the furnace gases to become well mixed together, and to ensure circulation is provided for by a diaphragm in the watei’-chamber, their contact with the heating surfaces. Access for securing and inner water-tubes as in the “Niclausse” boiler. the tubes in the headers is provided by a hole formed on the other The “ Lagrafel-D’Allest ” boilers also have large tubes. They side of the header opposite each of the tubes, where they are are necessarily arranged in pairs having one combustion-chamber grouped in threes or fours, and by one larger hole opposite each “ Lavrafel- common 1° both. Each of a pair consists of a nearly group of two tubes. The larger holes are oval, and are closed by D’AlIest ” horizontal cylindrical steam-chest, the fittings similar to those used in the land boiler (Fig. 3). The ends with large flat water - chambers,connected between at which smaller holes are conical, with the larger diameter on the inside, the numerous water-tubes are fitted above the fire-level. The and are closed by special conical fittings ; the conical portion and nests of tubes, except at the upper portion, do not extend the bolt are one forging, and the nut is close-ended. In case of the whole width of each of the pairs of boilers, but a space is left on breakage of the bolt, the fitting would be retained in place by the the inner side of each to form a combustion-chamber. The grate- steam-pressure. The feed-purifier is a small horizontal cylinder bars are placed below the tubes, and a fire-bridge is formed at the placed above and in front of the steam-chest, and connected to it inner side of each fire, separating the grate from the combustion- by a pipe which provides an uninterrupted communication between chamber. On the outer side of the fires the iron casing of the the upper parts of the two chests, so that when in use the upper boiler is protected by fire-brick lining. The spaces between the part of the purifier is always fully charged with steam. The lower tubes of the lower row, and also of the upper horizontal row, are half of the small chest is divided into several compartments by closed by suitably-shaped tiles, and iron plates are fitted to cover means of diaphragm plates. The feed-water is led into the chest the upper portion of the spaces between the outside vertical rows in the upper part at the middle of the length, and falls over a of tubes. The products of combustion are therefore compelled to dished plate _ through the steam in the form of a cascade. Its pass over the fire, under the lower horizontal row of tubes, over temperature is thus raised nearly to the boiling-point, and most of the fire-bridge into the combustion-chamber, thence returning the impurities are separated out in a solid form and deposited in in an opposite direction across all the tubes, and finally leaving the various chambers through which the water flows. The purified the nest of tubes at its lowest part. The course of the gases thus feed-water passes from the end divisions down the vertical pipes bears a marked resemblance to that obtaining in the ordinary into the square-sectioned cross-pipe, whence it is carried into the return-tube type of boiler. general circulation of the boiler. The other boilers to be described are the types, sometimes called “ Express ” boilers, which are largely used in torpedo-boats, torpedo-boat destroyers, and in small cruisers, where the most important requirement is very high power with a very small weight of boiler. They all consist of a horizontal steam-chest in the upper part, connected by numerous small tubes of from 1 inch to 1] inch outside diameter to two or three smaller horizontal chambers placed near the fire-level. The tubes are always made of seamless steel, and are usually galvanized on the outside by an electrical process. In the “Normand” boiler (Fig. 14) both the upper and lower chambers are considerably longer than the fire-grate, and are connected by large pipes at each end of the boiler, out- „ „ side the action of the fire, which serve as downcasts Normand.” and facilitate the circulation of water. The heating-tubes are all bent to the forms shown in the cross-section ; the outer rows usually enter the steam-chest at about the water-line, and the others lower dowm. The two inner rows on each side, for a part of the length of the fire-grate, touch one another along nearly all their length, and are separated only at their ends where they have to enter the horizontal chambers through separate holes. Arranged thus they form “ water-walls.” The two outer rows on each side are similarly arranged along the whole length of the boiler, and protect its outer casing from the action of the fire. These water-walls direct the flow of the products of combustion amongst all the heating-tubes. The ends of the casing, between the tubes and the fire-grate, are protected by fire-brick linings. In the “Yarrow” boiler, which is shown in cross-section in Fig. 15, the tubes are all straight, and there are no water-walls. The products of combustion pass across all the tubes the lt ,, whole length of the boiler. The outer casings are arrow. made of double thickness, with an air-space between them. Outside circulating pipes, similar to those in the “Normand” boiler, Pio. 13. —Babcock and Wilcox Marine Boiler—longitudinal section. are occasionally fitted, but it is often found that there is sufficient The “Babcock and Wilcox” marine boiler is in use in the American circulation without them. In the larger boilers the inner rows and British navies and al-o in several yachts and merchant steamers. of tubes are slightly larger than the others, and are arranged “ Babcock ^ rccent example is shown in Fig. 13. It consists of a with wider spaces between them, and the lower chambers are and horizontal cylindrical steam-chest placed transversely made with riveted plates, access to examine the tube ends being a rou Wilcox.” over £ °f elements beneath is the confire. obtained through manholes in the ends. In smaller boilers the Each elementP consists of a front andwhich back header lower parts of the chambers are portable, being bolted to the tubenected by numerous water-tubes which have a considerable inclina- plates. tion to facilitate the circulation in the boiler. The upper ends of The “ Blechynden ” boiler bears considerable resemblance to the the back headers are situated immediately under the steam-chest, “Yarrow” boiler, but the heating-tubes, instead of being straight, and are connected to it by short nipples ; by a similar means they are all slightly curved. The object is to enable any of (( , are connected at the bottom to a pipe of square section which them to be drawn out or replaced without disturbing ~ extends across the width of the boiler. Additional connexions the remainder, the curvature permitting them to be led are made by nearly vertical tubes between this cross-pipe and the through several small holes in the upper part of the steam-chest bottom of the steam-chest, and also between the pipe and the feed- which are closed by ordinary doors. In the earlier type the outer purifier. The front headers are connected at their upper ends, by rows of tubes form water-walls at their lower part to protect the means of long horizontal tubes, with the steam-chest, the bottom outer casings; but their upper portions, not being in contact, permit ends being closed. The headers are made of wrought steel, and the furnace gases to pass between them. In the more recent form ’ except the outer pairs, which are flat on the outer portions, they there are no water-walls, but a baffle is fitted between the tubes to are sinuous on both sides, the sinuosities fitting into one another. ensure that the products of combustion take a definite course The tubes are If inch outside diameter, and are arranged in groups amongst them. Each lower chamber is connected to the upper