The horizontal tubular boiler has many excellent points, not the least of which is that it is accessible for examination and cleaning in practically every part. No boiler can be ex pected to work ideally when the feed water is bad, but the horizontal tubular type gives as good service, even under this trying condition, as can be had from any known type. Its weak points are (1) that it is not so well adapted to extremely high pressures as some of the water-tube types, of which one will be presently noticed; and (2) when it ruptures (as must happen occasionally with every type of boiler) the explosion is likely to be considerably more destructive than the explosion of a sectional boiler, because the large quantity of energy that it contains is liberated more suddenly.
Another class of externally fired boilers that is becoming more and more widely used, both in the United States and Europe, is the ((water tube* type, which is characterized by the fact that its tubular elements contain water, instead of serving for the transmission of the furnace gases as in all the other forms that have been considered above. One of the best-known boil ers of this class is the Babcock and Wilcox, which is shown in Fig. 5. This boiler is built up of lap-welded wrought-iron tubes, placed in an inclined position, and attached, both at the front and at the rear, to an upper drum that is made of extra thick steel or iron plates, and double-riveted, or riveted with a butt strapped joint. The tubes are not vertically over one another, but are so that each tube comes directly over a space in the row below it. The boiler is suspended from steel girders, which rests upon iron columns that are entirely independent of the brickwork; and hence the brickwork may be repaired, or may even be removed altogether, without dis turbing the boiler itself. The fire is situated under the front or higher end of the inclined tubes, and the products of combustion are guided by division plates and bridges so that after rising from the fire grate they pass be tween the tubes to the combusion chamber of the drum, then downward among the tubes again, and finally upward and to the chimney.
This devious course as well as the staggering i of the tubes, is intended to bring the hot gases into intimate contact with the tubes at every point. As the water in the boiler becomes heated, it rises toward the higher end of the tubes, becoming meanwhile partially converted into steam. The column of mixed water and steam ascends into the drum, where its con stituents separate, the steam remaining in the drum, while the water flows to the rear, where it passes down through the long, upright tubes, and so completes the circulation.
Water-tube boilers are now used to some extent in marine work, and especially in the naval service. Attention was particularly di rected to this branch of the subject by the recent elaborate investigations of the commis sion appointed by the British Admiralty, for the purpose of recommending a standard type of boiler for use in the British navy. (Consult Engineering News, 4 Sept. 1902, p. 176). The Belleville boiler, which has been somewhat extensively used in the past in the British naval service, consists essentially of a series of water tubes, slightly inclined to the horizontal, and opening at the bottom into malleable iron col lector boxes, and at the top into a drum to which the main steam pipe is attached. The feed water is introduced at the middle of the upper drum, and. is injected under a pressure in excess of that which is carried upon the boiler itself. To prevent the comparatively c cool feed water from enterin any of the tubes in which steam is generate these tubes are caused to project at least eight inches into the drum. The feed passes down through return pipes at the sides of the boiler, and enters the tubes below, after its temperature has been raised by the heat of the furnace sufficiently to prevent injury from contraction strains. The proper regulation of the feed-water sup ply is one of the difficult practical points about the Belleville boiler; and to overcome it as far as possible an ingenious automatic feed device is provided. There is little or no true circula tion in boilers of this type. The tube-groups discharge a mixture of steam and water into the drum, where the steam is supposed to be freed from the water by the aid of a system of baffle plates. An economizer is placed in the stack above the boiler in the most approved modern installations, the construction of the economizer being similar to that of the boiler itself, except that the tubes composing the elements are smaller.
All boilers are supposed to be provided with certain appliances intended to secure safety and uniformity of working. Noteworthy among these are the safety valve and the gauges that indicate the pressure of the steam and the position of the water level. These are described under separate headings.