BOILER 87 BOILER ers are almost invariably horizontal, but small vertical boilers are often used. They are employed in steam cranes and other situations where great length would be an inconvenience, and otherwise very frequently when small powers are required, especially for temporary pur- poses. As the size of a furnace limits the fuel which it can burn, this fre- quently involves having a much larger grate than could be conveniently ar- ranged inside the boiler, and on this and other accounts boilers are usually ex- ternally fired. Under the head of plain boilers come all ordinary cylindrical boil- ers, with or without internal furnaces, horizontal or vertical. They are the cheapest and simplest which can be made, and, if properly proportioned, pos- sess a considerable evaporative efficiency. When it is necessary, however, to econ- omize fuel, or space, or both, multitu- bular boilers are frequently used. These derive their name from the fact that in them the flame and gases of combustion are made to pass through a great num- ber of small tubes (surrounded by the water) on their way to the chimney. The steam generating power of a boiler depends greatly on the extent of surface which it presents to the flame, and it is obvious that a great number of small tubes present a much larger surface than one large tube occupying the space of them all. Thus, with the same heating surface, a multitubular boiler will oc- cupy much less space than a plain one, and, at the same time, the efficiency of its surface is found to be greater. It is, however, necessarily more expensive and more liable to get out of order. Tubu- lous boilers differ from multitubular boil- ers in not only containing tubes, but con- sisting of them, and having no large cyl- inders whatever. Their chief advantages are their great strength, for it is easy to make a metal tube strong enough to withstand pressures far higher than any at present in use; and the peculiarity, that if any accident happens or explo- sion occurs, it will only be to one tube at a time, and not to an immense boiler shell, and its evil consequences will thus be greatly reduced. For this reason tubulous boilers are often called safety boilers. Locomotive boilers ai'e always multi- tubular, for much the same reason as marine boilers. The boiler of a single locomotive often contains 1,500 or 1,800 square feet of heating surface, and oc- casionally as much as 2,000 square feet. The principal test of the efficiency of a boiler is the quantity of water (gener- ally expressed either in pounds or gal- lons), which it will evaporate from and at a temperature of 212° F., with the con- sumption of one pound of coal. Of course, this varies very much with the quality of the fuel, but with good pit coal (not dross), a Cornish boiler often evaporates 6 to 8 pounds of water per pound of coal, and a multitubular boiler about 10 or 11 pounds per pound of coal. Good Cornish or Lancashire boilers, however, often attain as great economy as those of any other type. The best rate of combustion on the grate varies with the construction of the boiler, from 10 to 18 or 20 pounds per square foot of grate surface per hour, and much more with forced draft, as in a locomotive. For all first class work, and work where high pressures are to be used, the common material for boiler construction is mild steel, made by the open hearth process, and having a tenacity of about 28 tons per square inch. Copper is often used in the fire boxes of locomotives, but seldom in any other description of boiler. Brass boiler tubes are sometimes seen, and on account of its better conducting qualities, brass is to be pi'eferred to iron, but its costliness prevents it superseding iron in the great majority of cases. Every boiler has, to render it complete and workable, a number of fittings or mountings, of which the following are the principal: A glass gauge to show the level of the water inside the boiler, and gauge cocks for the same purpose; a gauge to show the pressure of the steam.; a valve for admitting water; a cock at the bottom for emptying or blow- ing off; a valve for the discharge of the steam ; one or two safety valves, weighted so that when the pressure of steam in the boiler reaches a certain height, they open and allow the steam to rush into the air; a door by which a man can get in to clean the boiler, etc. The Cornish boiler has often two in- ternal flues or tubes, which is a much more advantageous construction. In the Galloway boiler, there are two furnaces, but these join together in one chamber just behind the bridges, and the gases , are made to pass through a space con- siderably narrowed by side pockets pro- jecting inward in order that they may be well mixed. From this point to the back of the boiler there is just one flue, made oval in section, and crossed by a considerable number of vertical taper tubes, which form a direct communica- tion between the water beneath and that above the flue. These tubes both pro- mote circulation and strengthen the flue. A later development of the boiler is the water tube boiler, which is extensively used. In this type one or more drums of water legs connected by tubes of com-
