Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/333

IRON 317 the total amount of water required for steam purposes being 2 55 times the weight of the pig iron made (in cluding the steam used for blast engine, pumps, &c., and allowing 15 per cent, for waste by priming, cleaning boiler.-;, etc.), this water being raised to 100 in a hot well (by the waste steam) and simply converted into steam at 45 Ib pressure (non-condensing engines used). Adding this to 3850, a total of 5250 is obtained as heat actually accounted for in smelting average Cleveland ore when the ste.im power is obtained solely from waste gases as fuel, representing consequently about f^0- = 656 parts of carbon, say 07 parts of coke, or 14 cwts. per ton. Hence even when the consumption of coke is 18 cwts. per ton of pig (below which even with the most highly heated blast continuous production never seems to have been effected, whilst usually a considerably greater amount is used), a largo waste of heat through imperfect combustion below the boilers, and radiation, &c., therefrom, is occasioned. A fortiori the same argument applies to a blast furnace using raw coal, even when close-topped. When compared with other modes of burning fuel in metallurgical operations, &c., however, the blast furnace does not seem to be so wasteful as many of these appliances ; thus Griiner calculates the following values as approximately the percentages utilized of the total heat capacity of the fuel employed in various kinds of furnaces : Air furnaces ; steel melted in crucibles 1.7 lleverberatory furnaces, ,, 2 Siemens s crucible furnace 3 to 3 "5 ,, glass furnace 5 5 ,,6 Well arranged Siemens and Ponsard s furnaces .... 15 ,, 20 Old cupola melting furnaces 29 ,, 30 Modern ,, ,, 50 and upwards. Large blast furnaces for iron smelting (exclusive } of the heat obtained by combustion of the > 3-1 to 30 waste gases) ) The rate of production in a blast furnace is, up to a certain extent, variable with its dimensions ; but no well marked correlative increase appears to have been effected in the make of furnaces of considerably upwards of 15,000 or 20,000 cubic feet capacity above that of furnaces of these dimensions. The quality of the ore smelted also largely affects the rate, the furnace being of such dimensions as to give the maximum saving of fuel practicable, and the least crushing of the ore by its own weight, together with the minimum tendency to scaffolding, slips, and other practical inconveniences; thus, whilst from furnaces smelting Cum berland and North Lancashire haematite an output of GOO and even 800 tons per week has been accomplished, from 400 to 500 tons per week is the usual result with large furnaces smelting clay ironstone, such as that of the Cleveland district. Somewhat smaller yields than these are obtainable from furnaces of less capacity. Charcoal furnaces usually make more pig for a given amount of cubic capacity than when coke, anthracite, or raw coal is employed as fuel : thus, whilst some Styrian charcoal furnaces have been made to produce for every 1000 cubic feet capacity from 110 to 130 tons weekly (the capacity being only 500 to 1200 cubic feet), and whilst the Swedish and Norwegian and some American charcoal furnaces of 1000 to 3000 cubic feet capacity produce per 1000 cubic feet 50 to 70 tons weekly, the large English coke blast furnaces of 15,000 to 20,000 cubic feet and upwards usually produce only 15 to 30 tons weekly per 1000 cubic feet. Those of the coke, anthracite, and coal burning furnaces of Europe and America of somewhat less capacity than these largest sizes usually produce somewhat more than 20 to 30 tons weekly per 1000 cubic feet ; but in many cases this is done at the expenditure of a greater amount of fuel than that employed in the larger furnaces (i.e., after making allowance for the difference in the amount of flux added, and cinder produced, &c. ). This is not the case with the European charcoal furnaces, for in some of these the consumption of fuel is not greater, and in other cases is notably less, per ton of iron made than in the largest Eng lish coke-employing furnaces, even after making these allowances. lu many American charcoal furnaces, however, notwithstanding that a purer ore is smelted than that used in some of the European charcoal furnaces, the consumption of charcoal appears to be not ably higher, approaching 18 and 19 and even 20 cwts. of charcoal per ton of iron instead of 15 to 17 cwts. ; still, as compared with coke, these charcoal furnaces ordinarily consume a smaller amount of fuel. According to Akermann the charcoal used in America is usually very much more dense than that employed in the Swedish charcoal furnaces, so that a bushel sometimes represents some 30 per cent, more of weight of fuel. With charcoal as fuel it does not appear that an increased rate of driving the furnace (by putting on more blast) necessarily causes an increase in the fuel consumption ; indeed, the opposite result has been observed in certain cases, at least to a certain extent, the cause being the relatively smaller loss of heat by radiation, &c. , from the furnace. For any given furnace and ore, &c. , there is a particular rate of driving which gives the minimum fuel consumption : a more rapid rate requires more fuel because the gases have not time to eil ect their full action on the ores, and less carbon dioxide is formed ; a slower rate causes more loss by_ radiation, &c., relatively to the output. Up to a certain extent it is often advantageous to use a little extra fuel, and increase the rate of production beyond the rate that would correspond to the minimum fuel consumption ; which is probably the reason why in many instances the fuel employed per ton of iron is somewhat larger than that found to be requisite in other analogous cases, where the rate of production is somewhat lower ; the exact point at which the advantages of increased rate of production are counter balanced by extra cost for fuel, and extra wear and tear, &c., necessarily varies in each particular case. Cold Blast as compared with Hot. In reference to the employ ment of cold blast for the production of iron, the saving in fuel occasioned by the use of heated air has been practically proved to be so great that excepting for certain special brands of iron the use of hot blast has almost entirely superseded that of cold ; the evi dence in support of the alleged deterioration in quality thereby caused is, however, not so conclusive as that in behalf of the economy produced. "With a cold blast the mass of fuel in front of the tuyeres is visibly much less brightly incandescent than that in a hot blast furnace, being comparatively black, indicating considerable local refrigeration, and hence probably differences in the amount of silicon, sulphur, phosphorus, &c. , reduced in the hearth; but analyses of hot and cold blast pig irons made from the same ore do not always show such marked differences as might be anticipated ; opinions are in fact somewhat divided even at the present day on this point, but such of these opinions as admit of being checked by figures usually incline to the non-existence of any material diifer- ence between the English pig irons produced from a given ore, flux, and fuel. by cold and hot blast respectively. On the other hand, it was for many years after Neilson s patent was taken out a matter of belief, especially in A 7 ales, that the increased impurity of the pig made with hot blast necessitated so much more labour and expendi ture of fuel in puddling, to give a wrought iron equally good with that made by cold blast, as to render the actual saving doubtful ; whilst with certain Swedish charcoal irons of the highest brands, c. g. , Dannemora iron from magnetite, cold blast is still adopted on the ground that experience has shown a marked deterioration in the character of the iron produced when the blast was heated. With other similar Swedish and Norwegian brands, on the other hand, n heated blast is in use, it being considered that no perceptible deterioration in quality is thereby occasioned ; this remark equally applies to the Styrian and Carinthian furnaces employing Eisenerz and Lolling spathic ores, and to those at Fullonica where the Elba specular ore is smelted ; Tiinner states that the use of hot blast for Eisenerz charcoal iron production in no way necessarily produces any deterioration in quality ; and Bell is of the same opinion so far as English irons made with hot blast up to 500 0. are concerned. In many cases the superiority of cold blast over hot blast iron alleged to exist, as shown by chemical analyses, and more especially by mechanical tests, is really due to the fact that the ores used for the two are not identical, the cold blast metal being made from a purer quality. In fact, the notion that cold blast iron is vastly superior to hot seems to have been originally to a considerable extent the result of a trade manoeuvre ; thus the ironstone of the Scotch coal-fields near Glasgow being of a refractory nature required the consumption of a much larger amount of fuel with cold blast than did the more easily reducible South Wales ores ; but with hot blast a much greater saving in fuel was produced with the Scotch than with the Welsh ore; as early as 1834 Dufrenoy (director-general of mines, France) specially investigated the relative advantages of hot and cold blast with these two ores, and found that, whilst with the Scotch ore the saving produced at the Clyde works by heating the blast to about 320 by an expenditure of 8 cwts. of coal per ton of iron was (after allowing for this 8 cwts. , and taking into account the coal used for the blowing engines) eqiiivalent on the whole to a diminution of coal consumed from 153 to 59 cwts. of coal per ton of iron, 1 or 1 Dr Clark read before the Royal Society of Edinburgh in 1835 a paper " On the Application of the Hot Blast in the Manufacture of Iron," in which he stated that at the Clyde works, prior to the end of 1829, the average consumption of coke (45 parts of which were