Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/332

 316 IKON effected ; so that, instead of requiring an amount of coke to be consumed equal in weight to about 1 09 times that of the pig iron made (nearly 2 If cwts. per ton), which as stated in the previous section is the minimum practical average consumption possible when the amount of heat brought in by the blast about equals that carried out by the waste gases, a smaller amount of fuel will suffice, from O95 to 90 parts of coke (19 and 18 cwts. per ton) only being necessary with blast heated to 700 and upwards by a Whitwell stove, the heat carried out by the waste gases being less than that brought in by the blast by an amount equal to that which would otherwise have been generated by the fuel saved. With ores other than Cleveland iron stone the same kind of result is obtained ; not only in England and Wales, but also in France, Germany, Switzer land, America, and elsewhere, the Siemens-Cowper and Whitwell stoves have been extensively adopted on account first of the saving of fuel effected by them, and secondly of the greater regularity and efficiency in working. On the other hand there is no doubt that increasing the height of a furnace smelting calcined Cleveland ironstone from 48 to 80 feet causes a considerable saving in fuel ; as shown in the previous section, the increased height acts partly by permitting the gases to escape at a lower temperature, and partly by enabling the fuel to be burnt with the formation of a smaller relative proportion of carbon oxide and a larger one of carbon dioxide than is the case with the smaller furnace. When, however, a still greater height is given to the furnace, a further saving in fuel and larger relative production of carbon dioxide do not seem to occur, furnaces of 90 and 100 feet in height not exhibiting any marked advantages over 80-foot furnaces working under the same conditions, so far as con sumption of fuel is concerned. Even if Bell s views as regards the non-apparent advantage of increasing the blast temperature above 500 C. with the furnaces smelting Cleveland ore experimented on by him be admitted to apply in all cases where this ore is used, it does not follow that they are applicable to other furnaces smelting different kinds of ore, nor does it follow that 80 feet in height is the limit beyond which no visible saving is effected in all cases; thus for instance with a furnace smelting (at Consett) a mixture of Cleveland ore and haematite in such proportions that about half the iron made came from each ore, a distinct saving of coke was found to accompany the substitution ot Whitwell stoves giving blast at about 720 for iron stoves giving blast at about 450 ; whereas with the less heated blast the coke consumption was 2275 cwts. per ton of iron, with the hotter blast it was only 18 cwts., the conditions, character of ore and flux used and pig produced, &c., being pretty nearly the same, the furnace being 55 feet in height in each case. Again, on rebuilding such a furnace (for the purpose of using the same mixture of ores) to a height of about 70 feet, it was found that the increased dimensions, so far from pro ducing the beneficial effects which such a change would have given had Cleveland ore only been used, introduced such irregularities in working that the height had to be reduced to the former amount, 55 feet or so. Similar results have also been found with furnaces using Lancashire haematite only ; thus a Barrow furnace built to* the height of 75 feet, and using Cowper stoves, worked so badly that it was reduced to 61 feet, when it worked well. Analogous results were also obtained at Workington, a 70-foot furnace work ing much better when cut down to 55 feet ; in America also it has been found that charcoal furnaces, increased materially above the original height, worked irregularly until the height was reduced again, when the working again became good. With easily reducible Belgian ores furnaces of 50 to 60 feet in height are found to give the best results both as to quantity of iron run and as to economy of fuel. On the other hand, an increase in height from 45 to 60 feet in furnaces smelting Staffordshire ores was found to cause an average saving of upwards of 10 cwts. of coal per ton of iron (Plum, Journal I. and 8. Inst., 1871, ii. 227), whilst two furnaces at Stanhope (New Jersey) using magnetic ore, one 80 feet high and of 16,400 cubic feet capacity, the other 55 feet high and of 9200 cubic feet capacity, differed by several cwts. in the amount of coal re quisite to produce a ton of pig, the difference being in favour of the larger furnace ; similarly at Glendon, U.S., two furnaces, 72 and 50 feet in height and 11,900 and 4800 cubic feet capacity respectively, differed by 3 to 4 cwts. of coal per ton of pig, the taller furnace requiring the least fuel (F. Firmstone, Trans. Am. Journ. Mining Engineers). With charcoal furnaces smelting grey iron, increasing the blast temperature from about 200 to 400 or 500 C. causes a marked saving of fuel ; but the utility of heating the blast above 300 or even a somewhat lower limit for such furnaces when making white iron is regarded by Tiinner as extremely doubtful : thus ho states (Journal I. and 8. Inst., 1873, 442) that charcoal furnaces at Neu- berg which used 23 to 24 cwts. of charcoal per ton of grey Bessemer pig (and only about 15^ for white iron) when the blast was at 200, only required 19 to 20 cwts. per ton of grey pig when the blast was heated to 500, representing a saving of some 4 cwts. per ton of charcoal ; similarly at Heft the charcoal consumption was 20 cwts. and upwards per ton of first class grey Bessemer pig with blast at 200; and only 17 to 18 cwts. with blast at 350 to 400 C., repre senting a saving of at least 2 cwts. of charcoal per ton of pig. Analogous results have also been recorded as obtained with Cailu- thian furnaces, a saving of 25 to 30 per cent, in the charcoal used accompanying the heating of the blast to 500 D or 600 J instead of the much lower temperature formerly employed ; similarly with Swedish charcoal furnaces smelting bog ores, the use of hot blast at 350 instead of cold blast producing a saving in fuel estimated as averaging one-third, or 33 per cent, of the larger amount, and the employment of blast at 200 effecting a saving of one-fifth, or 20 per cent., with mountain ores (Jcru-Kontorcts Annaler, 1859, p. 273). The much smaller consumption of fuel in the Wrbna charcoal furnaces and others using certain Austrian ores as com pared with English coke furnaces appears, from the results of Tiinner, quoted in the last section, to be mainly due to the smaller amount of slag produced, and the consequently diminished furnace requirements, a portion of the diminution being due to the some what larger formation of carbon dioxide relatively to the carbon oxide in the escaping gases ; with other ores producing more cinder, the quantity of charcoal used per ton of iron run does not seem to be materially less than the amount of coke employed with English furnaces, so far as comparisons can be instituted. On the whole, the precise details as regards the dimen sions of furnace, temperature of blast, &amp;lt;fcc., the use of which will enable iron to be smelted from a given kind of ore with a minimum expenditure of fuel, cannot always be distinctly stated, the requisite data not existing : evi dently the data available for one class of ore and fuel and iron produced are not applicable without material correction to other classes. 1 All existing experience, how ever, goes to show that the blast furnace is an instrument in which it is impossible completely to utilize the calorific power of the fuel burnt. Owing to the nature of the com plex reactions regulating the resultant chemical changes taking place in the furnace, a considerable proportion of the fuel inevitably must escape as carbon oxide, and it is not practicable to restore to the furnace the whole of the heat thus not utilized by employing the waste gases to heat up the blast, although a portion of this heat may thus be saved. Thus, in the case of average Cleveland ironstone, the 3850 units of heat per unit of weight of pig requisite for the various items of furnace consumption would be obtained by the combustion of less than 5 part by weight of carbon (representing 10 cwts. of carbon or less per ton of pig, i.e., about 10 5 cwts. of average coke per ton of pig), could all the carbon be burnt to carbon dioxide ; whilst for ores containing less earthy matter, and hence requiring less flux and producing less cinder, considerably smaller amounts would be required, in some cases not mor3 than 7 or 8 cwts. per ton. In practice, even with the most economical arrangements at present known, the consumption of fuel is largely in excess of the minimum quantity thus theoretically requisite, tho coke used varying from 18 or 19 cwts. in the most favourable cases to 25 and even 30 cwts. per ton of pig under less economical conditions. A portion of the excess of fuel thus burnt may be utilized in generating steam by burning the waste gases under the boilers. Bell calculates that somewhat upwards of 1400 heat units per unit weight of pig iron were thus utilized in the works serving as the chief basis of his inquiries, 1 An instructive essay by J. Walters on the best-known means of increasing the production of blast furnaces without at the same time interfering with the quality of their products, discussing the dimensions as regards height, diameter of both size and throat, &o. , and other conditions best suited for certain classes of ores, is to be found in the Revue Universelle, 1877, and in abstract in the Journal I. and S. Inst. , 1877 (Foreign Report}, p. 125. See also papers by T. Whitwell (Iron, 1878) &quot; On the Construction, Dimensions, and Management of Blast Furnaces. 1