Messrs. Hopkins, Gilkes & Co., at Tees-side, gave 76 feet height, and 20,000 cubic feet capacity. In 1867, the furnaces at Norton were made 78 feet high, and 26,000 cubic feet. In 1868, Messrs. Bolckow & Vaughan enlarged their two furnaces of 1866, the one to 26,000 cubic feet, the other to 29,000 cubic feet capacity, the original height being retained -viz., 96 feet. In 1870, Mr. Cochrane erected a monster furnace at Ormesby, 92 feet high, and 41,000 cubic feet, and at Ferryhill, westward of Middlesborough, with a greater height they combined a smaller capacity-106 feet high, 33,000 cubic feet; and lastly, in 1871, Mr. Cochrane built a furnace 92 feet high, and 42,500 cubic feet capacity. The internal section of the greater number of these furnaces is given in the plate, copied from the historical account of the gradual development of the blast furnaces in Cleveland, by Mr. J. Gjers. We see by these that the form is very various-lofty furnaces almost cylindrical alongside of barrels very stumpy, enlarged at the belly, and much contracted at the top. These forms, as well as the height, the total capacity, the mode of charging, etc., have, as we all know, a certain influence in the working of the furnaces. The yield and consumption of raw material vary with these elements. Unfortunately, the short notice of M. Gjers does not give any details on this subject, not even an indication of the maximum yield; but this incompleteness I have, in part at least, been able to supplement by data given in the Memoirs of Mr. Bell, and the reports of the discussions which * Journal of the Iron and Steel Institute, Nov. 1871. these Memoirs gave rise to at the meetings of the Iron and Steel Institute. What strikes us immediately is, that by common consent it is allowed that the yield of these big furnaces does not increase in the proportion of their capacity. Thus, at Clarence Works, Mr. Bell's own works, we find, for four types of very different dimensions from each other, yielding forge iron Nos. 3 and 4, the yields as follows:- A Elements of the furnaces. A On the other hand, the numerous furnaces of Messrs. Bolckow & Vaughan and those of Ferryhill, in which the same ores and the same coke are used as at Clarence, the blast being heated to the same temperature of 400° C. to 450° C., and the pig being also Nos. 3 and 4, gave the following results: Lastly, by comparing the three successive types put up by Mr. Samuelson, at Newport, we again found the same figures c. ft. 45 c. ft. An old furnace of . 5000 yields 23 tons in 24 hours 218 p. ton. 342 66 430 Mr. Bell, in the discussions of the Iron and Steel Institute in 1871, affirms, with perfect reason, that he "never found that a furnace of 25,000 cubic feet did twice the work as well as one of half the size." In fact, we know that in the old furnaces of 5000 cubic feet to 7000 cubic feet capacity, the mean capacity is 210 cubic feet per ton of Nos. 3 and 4 pig, whereas in the more modern furnaces of 10,000 cubic feet to 15,000 cubic feet, we find from 280 to 320 cubic feet capacity per ton of yield, and in the most recent furnaces of 25,000 cubic feet, the capacity per ton of yield is 420 cubic feet to 490 cubic feet. In other words, the descent of the charges requires 60 to 70 hours in the large furnaces, and only 20 to 40 hours in the small ones. This extreme slowness in the descent of the charges may, in a certain point of view, have advantages. Variations in the raw material are less sensibly felt in the large furnaces. It may also happen that reduction goes on under better conditions-that the ore should arrive in the zone of fusion better prepared. But is there no limit to this successive development of the blast furnace? May not the juste milieu corresponding to a maximum of economy be overstepped? If the work goes on very slowly, is not the carbonic acid (CO2), arising from the reduction of the ores, exposed to be converted into carbonic oxide (CO) by contact with incandes cent carbon, in proportions increasing as the descent of the charges is slow? In short, is the consumption necessarily so much the less as their dimensions are large, and the descent of the charges slow? The figures above tabulated answer this question to a certain extent. Previously to 1860, the blast furnaces in Cleveland consumed 1.5 to 1·7 tons of coke for one ton of pig -gray forge-yielded, or, at the very least, 1.45 tons = 29 cwt., according to the statements of Mr. Bell. At present, the consumption in the enlarged furnaces is reduced to 1·125 = 22 cwt., the blast being heated to 400 to 500 degrees Centigrade: but it is quite certain that there is no difference in the consumption of the furnaces of 10,000, 16,000, and 28,000 cubic feet capacity, or even beyond these monstrous dimensions. If, therefore, beyond a certain limit, the large dimensions produce neither increased yield nor economy of fuel, it does not look very rational to go on increasing the capital for establishing furnaces with these vast dimensions.* This is what has at last occurred to our neighbors on the other (English) side of the Channel. A reaction has taken place in England, at all events in those districts in which the fuel and the ores are liable to crush and compress under their own weight. Thus at Askam-in-Furness, the height has been reduced from 75 feet to 61; at Consett, the furnaces have been reduced from 70 feet to 55; at Workington and at Barrow, situated like Askam in the district of rich hæmatites of Cumberland, the body of the furnaces has, in like manner, been reduced in height-at Workington, from 70 * See note on estimate of cost, Appendix. feet to 55, and at Barrow, from 75 to 61;* and lastly, at Creusot, a blast furnace which had been raised to 88 feet has likewise been decapitated. These examples are sufficient to show that a certain height is accompanied by proved inconveniences; but if we desire to appreciate at its true value the influence of these exaggerative dimensions, we must, in the first place, endeavor to form exact notions of the chemical and calorific reactions upon which the working of the blast furnace is based. § 3. Principal reactions in blast furnaces.-In every furnace of this type, there are two contrary currents in motion, and reacting the one upon the other—a gaseous current ascending, the temperature of which is at first very high, and decreases gradually till it quits the furnace at the tunnel-head or top; and a solid descending current, composed of the ores, the fluxes, and the fuel, the temperature of which goes on increasing always under the action of the gaseous current in the opposite direction. Of these two currents the one is slow, the other very rapid. The solid materials of the charges descend rarely with a greater speed than 20 inches per hour, whilst the gases pass upwards with a velocity of 20 inches per second. Further, the mass of air blown into the furnace is generally greater than that of the solid charges by the tunnel-head in the same time; and the weight of gases going off from the furnace is often more than double that of the melted substances (pig iron and slag) flowing out by the "tap" and slag discharge. The air, blown into the furnace at the twyres, is almost * See Journal of Iron and Steel Institute, Nov. 1871, p. 409. |