Babcock & the Babcock Wilcox type, shown in Figure r (pt. 77), the principal portion is a number of ranks of inclined wrought-iron water-tubes connected. at front and back by suitable " head ers," which are in connection with a horizontal cylindrical drain, above, in which the water-line is carried about the centre of height. The whole being incased in brick-work, the gases of combustion pass upward from the grate through and among the ranks of water-tubes, their course being held along these tubes by a baffle-plate, so that from the time they leave the combustion-chamber by the grate to their entrance into the smoke chamber, at the back, they are in contact with the tubes. Nearly the entire front half of the tubes receives heat by direct radiation from the incandescent fuel. There is constant circulation through these tubes, and, as the water at the back and lower portion is more quiet than elsewhere, there is at this part a transverse horizontal mud-drum in which solid impurities may collect. The main cylindrical shell has above it a hori zontal crosswise steam-drum. The tubes do not lie above one another in vertical lines, but are "staggered "—that is, one tube lies above the space between two others. They are expanded into the headers, and each has opposite it a'hand-hole, to permit perfect cleaning.
The Babcock boiler has as its distinguishing characteristics, among others of the same general class, sinuous headers for each vertical row of tubes; a separate and independent connection with the drum, both front and rear, for each such vertical row of tubes; all joints between the parts of the boiler proper are made without bolts or screw-threads; no surfaces are used which require to be stayed; the boiler supported independent of the brick-work, that it may be free to expand and contract as it is heated and cooled; has large drums; and every part is accessible for cleaning and repair.
The Abeadroth & Roof Boiler (fig% 3) consists of a nest of inclined water-tubes connected at their front and rear ends with a series of horizontal overhead drums, each drum with its underlying row of tubes forming a "section" of the boiler. The Heine boiler is of a somewhat similar type. The Kelly generator has the saute inclined tubes, hut there is no lengthwise drum; the tubes discharge into a number of vertical chambers connected with a steam-drum set crosswise above them. The Firmenich boiler has at the bottom two partially cylindrical horizontal wrought-iron shells, one on each side of the grate. Ranks of nearly vertical water-tubes connect these shells with two upper horizontal and nearly cylindrical drums, which have in common a horizontal cylindrical steam-drum.
Figure 5 (p1. 77) shows a sectional boiler not known in America. The water is contained principally in a series of rows of conical cast-iron sec tions connected above and below.
The Harrison Sectional (fig. generator character ized by ample combustion-space—is principally composed of cast-irou spheres made in sections of four. These are so connected as to give con siderable water-space and are in connection with a suitable steam-drum.
The Sterling Boiler (fig. 4) has a horizontal water-drum connected by inclined tubes having bent ends with two upper horizontal water and steam-drums parallel with the lower drum. The products of com
bustion pass beneath an arch, striking the tubes connecting the lower with the front upper drum, the tubes being at an angle of 50° with the hori zontal. The gases then strike a baffle-plate or partition-wall composed of fire-tiles, and are sent along the front ranks of tubes until they rise above the baffle-plate. They cross to the ranks of tubes connecting the rear upper drum with the lower drum, these tubes being at an angle of 6o° with the horizontal and following them downward to the out-take at the rear of the lower drum. The upper drums are connected by two series of pipes arranged above and below the water-line.
should have their contacting sur faces planed smooth before riveting. The outer of the two edges of a joint should be bevelled by a planing-machine, and to make the joint steam tight under great pressure it should be " calked " along this planed edge by repeated blows with a round-nosed tool, to compact the surfaces more perfectly. The strength of riveted joints is only from forty to sixty per cent. of that of the solid plate. Chain-riveting (p1. 76, fig. 8) is twenty-four per cent. stronger than zigzag riveting (jig. 9) and from sixteen to nine teen per cent. stronger than single riveting (jig. 7). The tendency of flat boiler surfaces to bulge outward is resisted by stays or tension mem bers, which are either rods, tubes, or plates. If the latter, they are gene rally called "gussets." At present wrought-iron and steel plates are generally joined by riveting, but hydraulic welding has been attended with some degree of success, and electrical welding has been introduced.
The of Boilers is a term frequently misapplied and mis understood. Strictly speaking, a boiler has no horse-power. The amount of steam that may produce oue horse-power in a common non-condensing slide-valve engine might in a compound condensing engine with improved valves develop four horse-power, but if used in heating only, would develop no power. But there is a conventional rating by which one boiler horse power is assumed to be the evaporation of thirty pounds of water per hour from 70° Fain% into steam at one hundred pounds gauge-pressurt. The effective heating surface of a horizontal tube is approximately that due to only one-half its circumference. One pound of good coal will evaporate, under favorable conditions, from nine to twelve pounds of water from and at 2 2 ° Fahr., the rate of evaporation from 1.5 to r.9 pounds per square foot of heating surface. As vertical surfaces do not absorb heat so well from the gases of combustion as do inclined surfaces, and these not so well as horizontal surfaces, it becomes desirable to give as much inclined and horizontal, and comparatively as little vertical, surface as possible. lu horizontal tubular boilers the shell is about ten times inure efficient, surface for surface, than are the tubes. One square foot of heating sur face at right angles to the current of heating gases, so as to receive them by direct impact, equals four diagonal or eight parallel to their flow.