284
BOILERS
end of the next tube of the element. The boxes at the back of peculiar construction. The lower or back ends are reduced in the boiler are all close-ended, but those at the front are provided diameter and screwed and fitted with cap-nuts which entirely close with a small oval hole, opposite to each tube end, closed by an them. The front ends are thickened by being upset, and the internal door with bolt and cross-bar ; the purpose of these openings parts where they are to fit into the header walls and in the diais to permit the inside of the tubes to be examined and cleaned. phragm are carefully turned to gauge. The upper and lower parts The lower front box of each element of the boiler proper is connected of the tubes between these fitting portions are then cut away, to a horizontal cross-tube of square section, called a “feed-collector,” the side portions only being retained, and the end is termed a Avhich extends the whole width of the boiler. When the boiler is “lanterne.” A small water-circulating tube of thin sheet steel, not in use, any element can be readily disconnected and a spare one fitted inside each generating tube, is open at the lower end, and at inserted. The lower part of the steam-chest is connected to the the other is secured to a smaller “lanterne,” which, however, only feed-collector by vertical pipes at each end of the boiler, and pro- extends from the front of the header to the diaphragm. This longations of these pipes below the level of the feed-collector form smaller “lanterne” closes the front end of the generating tube, closed pockets for the collection of sediment. The tubes are usually and the whole arrangement is such that when the tubes. are in made of seamless steel, although lap-welded tubes have sometimes place only the small inner circulating tubes communicate with the been used. They are generally about 4| inches in external diameter ; the two lower rows are f inch thick, the next two rows inch, and the remainder about l inch. The construction of the economizer is similar to that of the boiler proper, but the tubes are shorter and smaller, being generally about 2| inches in diameter. The lower boxes of the economizer elements are connected to a horizontal feed-pipe which is kept supplied with water by a feed-pumping engine, and the upper boxes are connected to another horizontal pipe from which, the heated feed-water is taken into the steamchest. Both the boiler proper and the economizer are enclosed in a casing which is formed of two thicknesses of thin iron separated by non-conducting material and lined with fire-brick at the part between the fire-bar level and the lower rows of tubes. Along the front of the boiler, above the level of the firing-doors, there is a small tube having several nozzles directed across the fire-grate, and supplied with compressed air at a pressure of about 10 lb per square inch. In this way not only is additional air supplied, but the gases issuing from the fire are stirred up and mixed, their combustion being thereby facilitated before they pass into the spaces between the tubes. A similar air-tube is provided for the space between the boiler proper and the economizer, and further combustion of the furnace gases is found to take place in this chamber. Any water suspended in the steam is separated in a special separator fitted in the main steam-pipe, and the steam is further dried by passing through a reducing-valve which ensures a steady pressure on the engine side of the valve, notwithstanding fluctuations in the boiler. The boiler pressure is usually maintained at about 50 lb per square inch in excess of that at which the engines are working, the excess forming a reservoir of energy to provide for irregular firing or feeding. Another type of large-tube boiler Fig. 12.—Niclausse Boiler—transverse section. which has been used in the British and in other navies is the “Mclausse,” shown in Fig. 12. front space between the front of the header and the diaphragm, It is also in use on land in several electric-light while the annular space in the generating tubes around the water“ Ni= " installations. It consists of a horizontal steam-chest circulating tubes communicates only with the space between the clausse. un(jer which is placed a number of elements arranged diaphragm and the back of the header. The steam, therefore, side by side over the fire, the whole being enclosed in an iron casing formed in the tubes escapes from them into this back space, lined with fire-brick where it is exposed to the direct action of the through which it rises into the steam-chest, whilst the space in fire. Each element consists of a header of rectangular cross-section, the front of the header always contains a down-current of water fitted with two rows of inclined close-ended tubes, which slope supplying the inner circulating tubes. The tubes are maintained downwards towards the back of the boiler with an inclination of in position by cross-bars, each secured by one stud-bolt screwed 6° to the horizontal. The headers are usually of malleable cast into the header wall, and each serving to fix two tubes. The iron with diaphragms cast in them, but sometimes steel has been products of combustion ascend directly from the fire amongst the employed, the bottoms being closed by a riveted steel plate, and tubes, and the combustion is rendered more complete by the introthe diaphragms also made of the same material. The headers are duction of jets of high-pressure air immediately over the fire, as in bolted to socket - pieces which are riveted to the bottom of the the “Belleville” boiler. The “Diirr” boiler, in use in several vessels in the German steam-chest, so that any element may be easily removed. The tube-holes are accurately bored, at an angle to suit the inclination navy, and in 1901 fitted for trial in two vessels of the British of the tube, through the front and back qf the headers and through navy, in some respects resembles the “ Niclausse.’ The ' Durr. the diaphragm, those in the header walls being slightly conical. separate headers of the latter, however, are replaced by The tubes themselves, which are formed of seamless steel, are of one large water-chamber, and instead of the tubes being secured
