§ 21. Consequences derived from the preceding examples:Before applying this method of analysis of the working of blast furnaces to other examples, let us endeavor to extract from the numbers we have arrived at the lessons they teach. Let us examine in turn the influence of the capacity, and of the height, of the furnaces, and then that of the temperature of the blast. For this purpose we have gathered into a single table the most salient results of the examples we have chosen as illustrations. Let us remark now, at once, on the subject of the five examples compared in this synoptical table, that the furnaces of Consett are not rigorously comparable with the three others. The ores treated are richer-a mixtare of hematites and calcined Cleveland oxides; and, above all, the pig-iron is cooler, so to speak-it is Nos. 4 and 5, and not Nos. 3 and 4, like those of Clarence and Ormesby. On this account we should calculate for Consett furnaces rather less than 330 and 550 calories for the caloric absorbed by the fusion of pig and slags. This circumstance explains the relatively feeble amount, found by difference, for the caloric lost by the walls, etc., of the furnace at Consett. Having mentioned this, let us now compare the two furnaces of Clarence, which differ from each other only in capacity and in height, whilst they are charged with exactly the same ores, fluxes, and coke, and with blast at the same temperature, and yield pig-iron of the same quality. If the consumption be different, it must chiefly arise from this difference in capacity and in height in the two furnaces. It is necessary to remark, however, that the section of the furnace of 1866 is more tapering than that of 1853 (figs. 2 and 6). For very different heights, the waist of the large furnace is 6 inches more than that of the small one. The distribution of the reducing gases, and consequently the reduction itself, will be less uniform in the small furnace, and this circumstance alone may involve a less economical working. Nevertheless, we know by the general experience of blast furnaces that this difference of section cannot have any great influence if the mode of charging be suited to the section, which doubtless is the case in Clarence works. The descent of the charges is slower in the large furnace. The internal capacity is 300 cubic feet per ton of pig yielded, whereas it is only 195 cubic feet in the small. But this circumstance alone cannot explain the remarkable difference in consumption in the two furnaces. In the greater number of old blast furnaces in Britain and on the Continent, the internal capacity does not exceed 175 to 210 cubic feet, and yet the consumption is often less than in these Cleveland furnaces; and besides, as remarked in § 2, the most modern furnaces of Cleveland, which give 350, 420, and even 500 cubic feet capacity per ton of yield, consume no less-the temperature of blast being the same-than the furnaces of smaller capacity, provided the height of these furnaces be suited to the nature of the ores treated, and the temperature of the blast. We have only to consult the numbers referring to Consett furnaces to be convinced that even with heights of 52 feet, and capacities less than 175 cubic feet per ton of yield, we may, with suitable ores, have a very small consumption of fuel; and we therefore already conclude that it would be very rash to assert, in any general terms, that, in all circumstances, blast furnaces of great height and great capacity are necessarily more economical of fuel than those of smaller dimensions. That which first strikes us, in comparing the two blast furnaces of Clarence, is the difference of temperature of the gases, 452° in the small, against 332° in the large furnace. Then comes the difference in the ratio CO2 CO' which is 0.387 in the small, and 0-6865 in the large. This low value of m in the small furnace indicates an unfavorable combustion of carbon; that is, an abundant formation of CO at the expense of CO2 due to the reduction. We see, in fact, that each lb. of carbon burned produces 3854 calories in the large furnace, Difference 609 calories. We have found, above all, a great difference in the carbon burned in the zone of reduction: 0.058 per lb. of iron in the large furnace, and 0.1245 in the small one; a difference which shows that the small furnace is much further from the ideal working than the large one. In reality, the large furnace consumes per lb. of pig yielded, 0.990 is burned, 1lb .020 carbon, of which The small furnace consumes 1lb 318 carbon, of which { { 0.030 is taken up by the iron. 1.288 is burned, 0.030 taken by the iron. Diff. 0 298 in favor of the large furnace. The synoptical table shows, besides per lb. of pig yielded, Total caloric received by the small furnace 4935 cal. And this excess is supplied partly by the hot blast, partly by combustion, properly so called. Notwithstanding the equality of temperature, the blast carries in more caloric into the small than into the large furnace; because, as it consumes more fuel, it requires more blast. The small furnace receives from the hot blast. 755 cal. Difference 602 153 cal. |