BOILER, the name given to a vessel in which steam is generated. In its sim plest form, it consists of a close vessel made of metal plate, having apertures for the admission of water and egress of steam, fitted with apparatus for show ing the level of the water and the pres sure of the steam, and in connection with a furnace. When water is boiled in an open pan, the temperature of the water and of the steam rising from it remains at 212° F., and the tension or pressure of the steam is no more than sufficient to make its way into the atmosphere, being exactly equal to that exerted in all di rections by the atmosphere itself-14.7 pounds per square inch. In a close ves sel, on the other hand, the temperature and pressure to which we can raise the steam are only limited by the strength of the boiler.
The form of a boiler is determined by two considerations—namely, strength to withstand internal pressure, and effi ciency in producing steam. The globular form is that best adapted for strength, and was the earliest to be used. It pre sents to the fire, however, the minimum area in proportion to its contents, and, therefore, has a minimum efficiency. Af ter spherical boilers, cylindrical ones came into use, at first set on end, and afterward laid on their sides, and, later on, these were furnished with internal cylindrical tubes for furnaces.
Boilers may be classified in several ways—as horizontal and vertical; inter nally and externally fired; and plain, multitubular and tubulous. Large boil 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 are 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.