Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/321

IRON the mica along the axis of the nozzle a view of the furnace interior is obtained, whilst by removing the stopper a jet of hot blast rushes out, by means of which the temperature 305 FIG. 22. I. Siemens Electrical Pyrometer. II. Section of Heated End III. Double Voltameter. can be ascertained by holding rods of zinc, &c., in the jet; or the ball of a fSiemens pyrometer can be introduced into the tuyere through the orifice. In the early days of the hot blast it speedily became manifest that unless the tuyere nozzles were artificially cooled they became so rapidly eaten away that practically the hot blast was inapplic able ; to remedy this the " water tuyere " was in vented by Condie. This simply consists of a nozzle with double walls, the outer one forming a "jacket" round the inner one or nozzle proper, water being allowed to circulate through the space between the walls. Another way of effecting the cooling is by bending a coil of wrought iron piping into a conical spiral (fig. 23), placing this in the cavity of a mould furnished with a core, and casting round it a hollow cone of cast iron, so that by connecting the projecting ends of the coil with a water main and escape pipe respectively a continual circulation of water is kept up through the coil, whilst the blast passes through the central cavity. Sometimes bronze and especially "phosphor-bronze" nozzles are employed; these have the advantage that the molten pig iron as it runs down inside the furnace does not adhere to them so readily as it does to iron tuyeres. The tuyeres are generally arranged sym metrically in a horizontal plane round the base of the furnace, this effects an equal distribution, of blast, and facilitates regular working. With small-sized furnace.s such as are used in various parts of the Continent (with charcoal as fuel), two tuyeres only are fre quently used ; with larger ones three, four, or five are usually employed, the diameter of the orifice of the nozzle being greater the greater the furnace; thus whilst some 2 inches diameter suffices with the smaller furnaces, 4, 5, G, and even 8 inch nozzles are used with the larger furnaces, especially in America (e.g., at Pittsburg), and when the number of tuyeres is small. In other cases a larger number of tuyeres, sometimes as many as eight, are substituted for increased dimensions, so as to enable the requisite amount of air to pass into the furnace with out unduly increasing the pressure of the blast, which varies from 2 to 50 per- cent, of an atmosphere (i.e., from about half an inch to 15 inches of mercury, representing from 4 oz. to 7J Ib per square inch), the lightest pressure being employed in small charcoal furnaces, and the heaviest in the English hard coke large furnaces and the American anthracite furnaces, especially the latter, on account of the tendency of the anthracite to disintegrate and so plug up the passages between the lumps of ore, &c. The pressure of the blast in ordinary large English furnaces, such as those of the Cleve land district, usually averages about 4 to 4 5 R> per square inch, equivalent to about one-third of an atmosphere, or some 10 inches of mercury. When the nozzle of a tuyere gets injured or burnt through, the water intended to keep it cool is apt to find its way into the furnace. As long as the quantity of water thus introduced is small, the only eil c ct is a reduction of temperature opposite to the tuyere owing to the heat absorption in the conversion of the water into steam and the reaction of the water vapour on the red hot coke, forming carbon oxide and hydrogen (the presence of extra hydrogen thus formed also modifies to some extent the chemical actions taking place in the upper part of the furnace in a direction rather the reverse of economical as regards consumption of fuel); if, on the other hand, a large volume of water is suddenly introduced, and especially if by a "slip" (or sudden jerky motion downwards of a mass of material that had previously more or less " scaffolded ") it is forced into the mass of molten cinder and pig in the hearth, or, what amounts to the same thing, it the cinder and molten pig are suddenly forced or splashed up by the slip, a sudden explosive generation of steam (and probably decomposition into oxygen and hydrogen, or formation of iron oxide and free hydrogen) takes place, sometimes giving rise to serious accidents. Why contact with vitreous matter (such as cinder, &c.) should cause a more explosive formation of water vapour or gases than contact with metal is unknown, but probably the cause is the same as that in virtue of which a piece of sodium in contact with water only will evolve hydrogen quickly but not explosively, whereas if the sodium touch glass, glazed crockery, &c., and water simultaneously, a violent explosion often occurs. In foundries and during the refining of iron ( 23), if water be thrown on the surface of the molten or semi-fused metal, and a piece of solid cinder or slag bo mechani cally carried under the surface of the hot metal, a more or less loud explosion often occurs, sometimes sufficiently violent to pro duce fatal consequences and do much damage (Menelaus); on the other hand, in cold blast furnaces where water tuyeres are not used, explosions of this class never happen, although in all kinds of fur naces explosions may occur duo to admixtures of air and blast furnace gases being formed when the blast is cut off for tapping, &c., and then being fired on putting on the blast again. This class of accident is usually guarded against as far as possible by means of appropriately constructed valves in the gas main, &c. Sudden violent mechanical squirting out of molten pig or cinder by a slip inside the furnace sometimes occurs with serious consequences; but this is a different thing from (though often combined with) the effect of water being carried suddenly into contact with the cinder, XIII. 39