Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/880

 844 FURNACE of Yorkshire, and the analogous substance knowu as Diuas clay, which is really nearly pure silica, containing at most about 2i per cent, of bases. Dinas clay is found at various places iii the Vale of Neath in South Wales, in the form of a loose disintegrated sandstone, which is crushed between rollers, mixed with about 1 per cent, of lime, and moulded into bricks that are fired in kilns at a very high tempera ture. These bricks are specially used for the roof, fire arches, and other parts subjected to intense heat in reverbera- tory steel melting furnaces, and, although infusible under ordinary conditions, are often fairly melted by the heat without fluxing or corrosion after a certain amount of ex posure. Canister, a slightly plastic siliceous sand, is simi larly used for the lining of Bessemer steel converters ; it is found in the neighbourhood of Sheffield. Alumina as a refractory material is chiefly used in the form of an hydrated aluminous iron ore known as bauxete, found in the south of France, in Carniola, and in Antrim; but its applications are somewhat special. It has been found to stand well for the linings of rotatory puddling furnaces, where, under long-continued heating, it changes into a substance as hard and infusible as natural emery. In th-3 Paris Exhibition of 1878 bricks very hard and dense in character, said to be of pure alumina, were exhibited by Muller & Co. of Paris, as well as bricks of magnesia, the latter being specially remarkable for their great weight. They are intended for use at the extreme temperatures obtainable in steel fur naces, or for the melting of platinum before the oxy-hydrogen blowpipe. For the latter purpose, however, limo is generally used ; but as this substance has only small stability, it is usually bedded in a casing of fire-brick. Fig. 9 is a section of a lime furnace as used for platinum melting. The flamo of the gas jet is introduced through the hole at the top, and plays over the surface of the metal in the hollow below. Oxide of chromium and chromic iron ore have been proposed as refractory crucible materials by Andouin of Paris. The former may be used as a bed for melting platinum in the same way as lime or magnesia, without affecting the quality of the metal. Ferric oxide, though not strictly infusible, is largely used as a protecting lining for furnaces in which malleable iron is made, a portion of the ore being reduced and recovered in the process. In an oxidizing atmosphere it is indifferent to silica, and therefore siliceous bricks contain ing a considerable proportion of ferric oxide, when used in flues of boilers, brewers coppers, &c. t and similar situations, are perfectly fire-resisting so long as the heated gas con tains a large proportion of unconsumed air. The red fire bricks known as Windsor bricks, which are practically similar in composition to soft red sandstone, are of this character. Furnace Count ruction. In the construction of furnaces provision has to be made for the unequal expansion of the different parts under the effect of heat. This is especially necessary in the case of reverberatory furnaces, which are es sentially weak structures, and therefore require to be bound together by complicated systems of tie rods and uprights or buck staves. The latter are very commonly made of old flat bottom rails, laid with the flat of the flange against the wall. Puddling furnaces are usually entirely cased with iron plates, and blast furnaces with hoops round each course of the stick, or in those of thinner constructions the fire brick work is entirely enclosed in a wrought iron casing or jacket. Such parts as may be subjected to extreme heat and the fretting action of molten material, as the tuyere aud slag breasta of blast furnaces, and the fire Fio. 9. Sect; on of Lime Furnace. bridges and bed plates of reverberatory furnaces, are often made in cast iron with double walls, a current of water or air being kept circulating through the intermediate space. In this way the metal, owing to its high con ductivity and low specific heat as compared to that of water, is kept at a temperature far below its melting point if the water is renewed quickly enough. It is of course necessary in such cases that the circulation shall be perfectly free, in order to prevent the accumulation of steam under pressure in the interior of the casting. This method has received con siderable extension of late years, notably in furnace-smelting of iron ores containing manganese, where the entire hearth is often completely water-cased, and in some lead furnaces where no fire-brick lining is used, the lower part of the furnace stack being a mere double iron box cooled by water sufficiently to keep a coating of slag adhering to the inner shell which prevents the metal from being acted upon. Furnaces with special Methods of Firing. In the examples hitherto noticed, the use of solid fuel has alone been con sidered, whether in admixture with the charge in blast furnaces or burnt upon a grate in reverberatory furnaces. In either case the useful heating effect realized is consider ably below that indicated as possible by theory, and for the same reason, namely, that the carbon factor of the fuel is to a considerable extent only partially oxidized, producing carbonic oxide CO, instead of carbonic acid COo, as it should do if the combustion were complete. This is attended with great loss of heat, unless steps be taken to ensure the sub sequent combustion of the carbonic oxide, by bringing it into contact with more air at an appropriate temperature. The production of carbonic oxide is a necessary consequence where coal is used in large masses, the carbonic acid in the gases resulting from complete combustion at any spot being reduced more or less completely to carbonic oxide by contact with the ignited carbon immediately adjacent. To obtain the most perfect combustion it is, therefore, essential that the layer of fuel upon the bars in a grate fire should be as thin as is consistent with preventing the passage of an undue amount of air, which is attended with a strong cooling effect. This condition is, however, only possible in such furnaces as require an oxidizing atmosphere, as, for example, boiler fires and the different forms of calciners. Coal-Dust Furnace. A special method of providing more intimate contact between air and fuel has been adopted in a furnace designed by Mr T. R. Crampton, who grinds the whole of the coal to a fine powder in a flour mill, and pro pels a current of coal dust and air, mixed in the right pro portion for burning, into the combustion chamber represent- ting the fire-place of an ordinary furnace, either by a fan blower or by chimney draught. In the special application of welding iron considerable economy has been obtained with this furnace over ordinary coal-firing, but its use has up to the present time been exceedingly limited. Gas Furnaces. A more general remedy has been found in what is known as gas-firing, where the whole of the fuel is of design first imperfectly burned, i.e., converted into car bonic oxide or rather into a mixture of carbonic oxide and nitrogen, by reducing the supply of air through the grate to a minimum and completing the combustion on the fire bridge by a further supply of air introduced through special channels either at ordinary atmospheric temperature or artificially heated. In this case, the fire-place proper is replaced by a gas-producer or gazogene, which may either form one construction with the other parts of the furnace or be separated from them. Fig. 10 represents a gas-producer intended for heating retort furnaces in gas works. The coal is charged into a deep barrel-shaped stack a, terminated below by a small inclined grate b, with flat bars placed edge wise, like the steps of a ladder or the laths of a Venetian blind, allowing sufficient_air to pass between for a smothered