Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/877

 FURNACE 841 FIG. 2. Elevation of Catalan Forge. the fuel some way down. In the work of iron-smelting, the ore is laid in a heap upon the fuel (charcoal) tilling up the hearth, and is gradually brought to the metallic state by the reducing action of the carbonic oxide formed at the tuyere. The metal sinks through the ignited fuel, forming, in the hearth, a spongy mass or ball which is lifted out by the smelters at the end of each operation, and carried to the forge hammer. The earthy matters form a fusible glass or slag melt, and collect at the lowest point of the hearth, whence they are removed by opening a hole pierced through the front wall at the bottom. The active portion of such a furnace is essentially that above the blast pipe, the function of the lower part being merely the collection of the reduced metal; the fire may therefore be regarded as burning in an unconfined space, with the waste of a large amount of its heating power. By continuing the walls of the hearth above the tuyere, into a shaft or stack either of the same or some other section, we obtain a furnace of increased capacity, but with no greater power of consuming fuel, in which the material to be treated can be heated up gradually by load ing it into the stack, alternately with layers of fual, the charge descending regularly to the point of combustion, and absorbing a proportion of the heat of the flame that went to waste in the open fire. This principle is capable of very wide extension, the blast furnace being mainly limited in height by the strength the column of materials or &quot;burden&quot; lias to resist crushing, under the weight due to the head adopted, and the power of the blowing engine to supply blast of sufficient density to overcome the resistance of the closely packed materials to the free passage of the spent gases. The consuming power of the furnace or the rate at which it can burn the fuel supplied, is measured by the number of tuyeres and their section. In the largest modern blast furnaces used for smelting iron ores, they may be as many as six or eight, but as a rule the smaller number of from three to five of larger area is adopted, and with these there is no difficulty experienced in burning from 80 to 100 tons of coke per day in hearths of 5 to 7 feet in diameter. The profile adopted for blast furnaces has been very much varied at different times. The earliest examples were invariably square or rectangular in horizontal section, and the same class of form has been retained in many instances up to the present time; but the general tendency of modern practice is to substitute round sections, their construction being facilitated by the use of specially moulded bricks which have entirely superseded the sandstone blocks formerly used. The vertical section, on the other hand, is subject to considerable variation according to the work to which the furnace is applied. Where the operation is simply one of fusion, as in the iron -founder s so-called cupola, in which there is no very great change in volume in the materials on their descent to the tuyeres, the stack is nearly or quite straight-sided, but when, as is the case with the smelting of iron ores with limestone flux, a large proportion of vola tile matter has to be removed in the process, a wall of varying inclination is used, so that the body of the furnace is formed of two dissimilar truncated cones, joined by their bases as in fig. 3, the lower one passing downwards into a short, nearly cylindrical, position. The upper cone DC is known as the stack proper, the lower one, from the broadest part C to the tuyeres B, as the boshes, and the lower cylin drical part AB as the hearth. The further consideration of this subject belongs, however, more particularly to the FIG. 3. Section of Blast Furnace. article IRON. It may be sufficient to say that all blast fur naces of large size are more or less reducible to this pattern. The use of bellows or of analogous con trivances is not essen tial to the working of stack furnaces; as tho supply of air may, in furnaces of small size, be equally well ob tained by the draught of a chimney, or a steam jet aspirator. The former plan is adopted in the so- called economic fur nace (hornoeconomico) of the Cartagena lead district in Spain, used in re-smelting old slags and waste pro ducts of the Roman mines, and the latter in Woodward s and Ireland s cupolas for iron foundries. In either case numerous holes representing tuyeres are provided around the lower part of the furnace, often in two rows at different heights. These furnaces have not, however, been very generally adopted; and even in Spain the chimney draught has in many instances been replaced by a fan blower. A more primitive form of the same contrivance is still in use in Burmah, the furnace, about 5 or 6 feet high, being placed on the side of a bank facing the prevailing wind, which enters through a series of small round tuyere holes. Similar contrivances were used by the ancient Celtic inhabit ants of the Rhine valley, the ruins of furnaces and slag heaps being found in the Nassau and Eifel hill countries, in high exposed situations far away from streams, where water power for bellows could have been obtained. Reverberatory Furnaces. Blast furnaces are, from the intimate contact between the burden to be smelted and the fuel, the least wasteful of heat ; but their use supposes the possibility of obtaining fuel of good quality and free from sulphur or other substances likely to deteriorate the metal produced. In all cases, therefore, where it is desired to do the work out of contact with the solid fuel, the operation of burning or heat-producing must be performed in a special fire-place or combustion chamber, the body of flame and heated gas being afterwards made to act upon the surface of the material exposed in a broad thin layer in the work ing bed or laboratory of the furnace by reverberation from the low vaulted roof covering the bed. Such furnaces are known by the general name of reverberatory or reverbatory furnaces, also as air or wind furnaces, to distinguish, them from those worked with compressed air or blast. Originally the term cupola was used for the reverberatory furnace, but in the course of time it has changed its mean ing, and is now given to a small blast furnace such as that used by iron-founders, reverberatory smelting furnaces in the same trade being called air furnaces. Figs. 4, 5, and 6 represent a reverberatory furnace such as is used for the fusion of copper ores for regulus, and may be taken as generally representing its class. The fire place A is divided from the working bed B by a low wall C known as the fire bridge, and at the opposite end there is sometimes, though not invariably, a second bridge of less IX. 106