BOILER (Fr. ehaudiere, Ger. kessel), the name given to a vessel in which steam, usually for a steam-engine, is generated. In its simplest form, it consists of a dose ves sel made of metal plate. having apertures for the admission of water and egress of steam, fitted with apparatus for showing the level of the water and the pressure of the steam, and in connection with a furnace, either internal or external. When water is boiled in an open pan, the temperature of the water and of the steam rising from :t, remains at or very near 212' F , and the tension or pressure of the steam is no more than sufficient to make its way ht.() the atmosphere, being exactly- equal to that exerted in all directions by the atmosphere itself—natnely, 14.7 lbs. per square inch. In a close vessel, on the other hand, the temperature and pressure to which we can raise the steam are only limited by the strength of the vessel or boiler against bursting.
'rhe form of :t boiler is determined by two considerations—namely, strength to with stand internal pressure, and efficiency in producing steam; and the object of the designer is to combine in one apparatus sufficient strength to work safely at the proposed pressure, with such a form and arrangement as shall abstract the maximum of heat from the !vases of combustion, and at the same time be in all respects suitable to the special circumstances of the case. 'rile globular form is that best adapted for strength, and was the earliest to he used. It presents to the fire, however, the minimum area in pro portinn to its contents, and therefore has a minimum efficiency. After spherical boilers, cylindrical ones came into use, at first set on end, and afterwards laid on their sides, and later on, these were furnished with internal cylindrical tubes for furnaces. Watt's wagon boiler" (so called front its shape) was used for many years, but being quite unfit for auy but the lowest pressures, it has-long been discarded; and the " eg.g-end" boiler, or plain cyliuder with hemispherical ends. also much used at one time, has now almost
disappeared ou account of its small evaporative efficiency. At present, it is quite com mon to use a working steam-pressure of 30 Ibs. per square inch in ordinary factory boilers, and in some cases this is already greatly exceeded, while the tenelency to use higher pressures seems to grow yearly. tnder these pressures, the only forms of boiler which can be used without heavy and expensive internal stays to prevent the danger of are the globular and the cylindrical. The former shape is rejected for the reason already given. and the latter form is used almost invariably in the construction of modern boilers, ac will be seen from the examples given below. The ends of the cylinders, when it is necessary to make them fiat, must, of course, be strengthened by stays.
Boilers may be classified in several ways—as (1) horizontal and vertical; (2) inter nally and externally fired; and (3) plain, multitulnilar. and tubulons. Large boilers are almost invariably horizontal. but small vertical boilers are often used. They are employed in steam-cranes (q.v.) and other situations where great length would he an inconvenience, and often in traelion-engines, where steep inclines have to be traversed. and where. if a locomotive boiler were used, one or the other end of its tubes might become uncovered, and so get burned. In Great 2ritaip, when moderately good fuel is used, boilers with an internal furnace are generally preferred; but on the continent the common brown coal is much inferior to our fuel. and a correspondingly larger quantity of it must be used to generate a given volume of steam. As the size of a furnace limits the fuel which it can burn, this frequently invnlves having a much larger grate than could be conveniently arranged inside the boiler, and on this other account:: boilers are, ou the continent, more frequently externally fired than ui this country.