Among the kinds of internally fired boilers that are more familiar to the engineers of the United States, the vertical tubular boiler and the locomotive boiler deserve special mention. The vertical tubular boiler consists of a cylin drical shell, with flat heads at the top and bottom, and traversed by a large number of small vertical tubes. The Manning boiler, shown in Fig. 2, is a good example of this type. At the lower end, the shell of this boiler is enlarged to provide a greater space for the fire-box than could be had if the shell were of the same diameter all the way. Another object that the designer had in view, in increas ing the diameter of the shell in this way, was to give the boiler a certain degree of elasticity. The tubes are likely to be hotter, in service, than the outer shell; and hence they tend to expand more, and thus throw stresses upon the heads and the tube ends. The reversed flange by which the main shell is secured to the fire box is supposed to yield sufficiently, under the bending stress thus thrown upon it, to relieve the more vulnerable parts of the boiler from the expansion strains to which they would otherwise be subjected. The fire-box of the Manning boiler is surrounded by an annular space (or "water leg") containing water, the inner plates of this space being secured to the outer ones by screw stay bolts that are spaced evenly, at short distances, so that they form the corners of a system of small squares. These bolts are supposed to be screwed into each of the shells of the water leg, and afterward riveted over at both ends. They are also com monly made hollow, or drilled lengthwise with a small central hole, so that if one of them should break or corrode away seriously, the escaping steam or water would attract the at tention of the fireman. Vertical tubular boilers are particularly useful when the avail able floor space in the boiler room is small; but they are often hard to clean out, and hence are not to be recommended when the water supply is known to form considerable deposits of scale matter. Such scale matter, in what ever part of the boiler it is formed, will event ually fall upon the lower tube sheet, or else into the water leg. That which falls into the water leg will do no great harm unless it is allowed to accumulate to an unreasonable ex tent. Handholes are provided along the bot tom of the water leg, on the outer shell, and these should be opened as often as experience with the particular feed water that is used indi cates to be necessary, and the water leg thoroughly freed from scale and mud. Hand holes should also be provided at or near the level of the lower tube sheet, for a like pur pose; but it is not so easy to remove the scale from this sheet as it is to remove it from the water leg. That which lodges around the edges of the tube sheet can be removed without any great trouble, but the deposit that lies toward the middle of the tubes can hardly be got at from the handholes. Yet it is of the highest importance that the tube sheet should be kept free from such deposits, because otherwise the ends of the tubes will become overheated and loosened, and serious mischief, or even disastrous explosions, may follow.
The locomotive boiler is built in a great variety of forms and proportions, but the fun damental principles of design are substantially the same in most of them. Like the vertical tubular boiler, it has a fire-box that is sur rounded by a water leg on all sides, though it is open at the bottom for the discharge of ashes, and for the admission of air for com bustion. The inner and outer walls of the fire-box are connected by stay-bolts, and the upper sheet of the furnace (technically known as the "crown-sheet") is supported in some efficient manner, so that the pressure of the steam shall not force it down out of position. The support thus necessary for the crown-sheet is sometimes afforded by running "sling stays" from it to the neighboring parts of the outer shell, and sometimes by providing parallel, hori zontal girders over the sheet, these being se cured to the crown-sheet, at short intervals, by means of hangers, or long, thimbled rivets. Not infrequently these two methods of sup port are combined in the same boiler, as sug gested in the illustration (Fig. 3). The prod ucts of combustion pass forward from the furnace, through a bank of small tubes that conduct them to a "smoke-box" or "extension)) at the front end, to which the stack is attached. When the locomotive type of boiler is used in stationary practice, the draft required for com bustion is provided by a chimney or tall stack, as in stationary boilers of other types; but when used in railway service it is impossible to obtain the draft in this manner, and a "blast-pipe" is therefore provided, through which the exhaust steam from the cylin ders is discharged up the stack. The gaseous products of combustion are expelled from the "front extension" by the blast of steam, and an equivalent quantity of air is drawn up through the fire. The draft produced in this way is quite powerful. "Baffle plates" are therefore provided in the furnace, in many cases, to deflect the hot gases that come from the fire, and bring them into contact with a considerable portion of the surface of the fire box, before they pass out into the tubes. The weakest points about the locomotive type of boiler are the crown-sheet and the stay-bolting. If sediment lodges upon the crown-sheet, and thereby keeps the water from direct contact with the metal there, overheating is sure to occur, and the sheet may become so softened and burned as to lose its strength, tear away from its fastenings and permit the entire con tents of the boiler to be discharged into the furnace. Many of the explosions of locomotive boilers are due to this action. The stay-bolting at the sides of the fire-box is likewise a source of frequent trouble, because it is found that the stay-bolts sometimes corrode away very rapidly, so that they are in reality badly wasted and weakened, when the engineer in charge believes them to be still sound and strong. As
in the vertical boiler, the stay-bolts are com monly made hollow, so that they may auto matically give notice of breakage by leaking. This artifice is helpful, but unfortunately it does not invariably work as it is intended to, and broken or badly corroded stay-bolts exist, not infrequently, without giving the alarm that they are supposed to give.
Externally Fired Boilers.— The common est type of externally fired boiler, in the United States, is the horizontal tubular. A standard design of this boiler is shown, with its brick setting. Three pairs of lugs are often pro vided, but two pairs are sufficient except when the boiler is very long; and two pairs can be brought to a good bearing upon the side walls more readily than three. The boiler should be °anchored') by the front pair of lugs, and the rear pair should be provided with rollers so that the boiler may expand and contract freely, without producing strains in the setting or in itself. The course of the feed-pipe, through which water is introduced into the boiler, is indicated quite plainly in the engraving. If there are several boilers set together in one battery, the main feed-pipe runs along the fronts, just under the projecting ends of the boilers. From this main feed-pipe a branch pipe is taken off for each boiler. The branch pipe is taken off on the left-hand side of the boiler, and near the main pipe it is provided with a ground union, or with a flanged connec tion. Immediately above the union there is a check valve, and above this is the globe valve which controls the feed. The feed-pipe enters the boiler just above the tubes, and passes down work (or partially torn away, in Fig. 4. It consists of a cylindrical shell, usually composed of three courses or of plates, riveted together. The circular joints in these boilers are almost invariably single-riveted; but the longitudinal joints are double-riveted, triple-riveted or riveted in some even more substantial manner, according to the pressure that the boiler is to .carry. The longitudinal joints, which are not shown in the engraving, should be high enough to be well out of the way of the hot gases from the furnace. A multitude of tubes extend through the boiler from end to end, and the furnace gases pass from the furnace back underneath the shell of the boiler to the °combustion chamber" at the rear, after which they rise and return to the front end through the tubes. They then enter the °smoke box" at the front end, and finally pass upward into the flue that leads to the chimney. The weight of the boiler is sus tained by means of cast-iron (or steel) pro jections, or "lugs," that are not shown in the illustration, but which are riveted to the shell, and rest upon the side walls of the brick the boiler on the inside, nearly to the back head. It then crosses over to the right-hand side, and discharges downward between the tubes and the shell. It is found by experience that when feed-water is introduced in this way it becomes heated almost to the temperature of the water in the boiler before it is discharged, so that the annoying and often dangerous effects that are produced when the shell is chilled by cooler feed-water are entirely avoided. On large boilers the feed-pipe should have a diameter of at least an inch and a half. The blow-off pipe (which is used for drawing off the contents of the boiler) should be located at the rear end, and should enter the boiler at the bottom, and not through the back-head. To strengthen the construction, the shell should be reinforced, where the blow-off enters it, by a ring of boiler plate securely riveted in place, about the point of attachment of the blow-off. The neglect of this simple matter of reinforce ment has led to many serious accidents. through the blow-off pipe pulling out and permitting the contents of the boiler to be discharged through the opening so made. As the blow-off is exposed to the action of the fire, it is also important that it should be encased in some sort of a protecting sleeve. A piece of larger pipe, slipped over the blow-off, is often used for this purpose, but it has the disadvantage of rendering the blow-off itself inaccessible for examination. A piece of asbestos rope coiled about the pipe is equally satisfactory, and per mits of easy inspection of the pipe. The blow off pipe of a boiler that is properly cared for is not likely to burn nor to become otherwise injured. Most of the accidents from the burn ing of such pipes have been primarily due to permitting the pipes to become choked up with mud or scale, so that water could not enter them freely from the boiler, to keep them properly cool. This may be almost certainly avoided by opening the blow-off (say) twice a day for a moment or two, until any sediment that may have fallen into it has been thoroughly swept out. The blow-off pipe is often so arranged that the elbow comes in the combustion cham ber; but this is not good practice, and it is much better to carry the pipe down until it passes below the floor of this chamber. The pipe itself should be about , two inches in diameter. It should be provided with a plug cock or with a gate valve, but a globe valve should never be used upon it, since valves of this type do not have a straight passage through them, and are therefore likely to catch and retain pieces of scale, which often prove to be very troublesome impediments. It should be mentioned that those parts of the heads of a horizontal tubular boiler that lie above the tubes are intrinsically weak, and must there fore be sustained in some manner. The neces sary support is usually secured by running braces from the heads to the side of the boiler shell, though sometimes the braces are run through the entire length of the boiler, from one end to the other.