474 



NA TURE 



[Sept. 12, 1889 



Down to the beginning of the seventeenth century the only 

 fuel used in the blast furnace, and, indeed, in the manufacture 

 of iron generally, was charcoal. In 1620, Dudley made several 

 attempts to substitute mineral coal for vegetable fuel in his 

 smelting works, which, by the exhaustion of timber, had become 

 very expensive. He failed in this, and in consequence the 

 British iron trade gradually fell until it was not equal to the 

 production of one modern blast furnace. This happened in 

 1740, when Darby, by treatirg pit coal in the same fashion as 

 the charcoal burners had dene with wood, i. e. by charring it, 

 restored vitality to an expiring industry. It is true the restora- 

 tion must have been of a languid character, for in half a century 

 afterwards it is said the weekly produce of a furnace did not 

 exceed fifteen or sixteen tons. 



Various changes we>-2 introduced into the manufacture of iron 

 in the first quarter o*^ the present century, but these were rather 

 of a mechanical than of a chemical nature. They chiefly owed 

 their origin to the lessons taught by the chemist Black to James 

 Watt, who profited by them in the application of steam as a 

 motive power. This brings us to the year 1828, a year which 

 will always be distinguished in the annals of the iron trade by 

 the discovery of Neilson of the value of heated air in smelting 

 the ores of this metal. I never heard it pretended that the 

 inventor had any pretensions to be considered a man of sciencee 

 Had it been otherwise, the knowledge of the virtues of the hot 

 blast might have been indefinitely postponed, and this opinion 

 is founded on the fact that for many a long year no satisfactory 

 explanation was given why heat obtained by burning coal in the 



hot air apparatus was capable of saving three or four times its 

 weight in the fuel consumed in the furnace itself. I propose, 

 with your permission, to consider this subject with more atten- 

 tion than I shall devote to other portions of this address, and I 

 am led to do this, not only because it is one of some scientific 

 interest, but because its study seems to afford a solution to some 

 questions in respect to which great differences of opinion 

 prevailed among those whose daily work led them to pay much 

 attention to their details. These questions have all a reference 

 to the quantity of fuel consumed in smelting the ore, as this may 

 be aff"ected by the temperature of the blast and the dimensions 

 of the furnaces employed for this purpose. 



As is well understood, the heat excited in an iron furnace may 

 be classified under three heads : — 



First, that derived from the combustion of the coke at the 

 point where the blast enters the furnace, the ultimate product 

 being carbonic oxide. 



Second, the conversion of a portion of this carbonic oxide 

 into carbon dioxide. 



Third, the heat carried into the furnace by the blast. 



For the better illustration of the relations which the heat 

 derived from these sources bear to each other, a table (No. I.) 

 has been prepared in which the quantities of each are given in 

 Centigrade calories, and reckoned upon 20 units of iron to corre- 

 spond with English weights. The information upon which the 

 calculations are based is derived from actual observation gathered 

 from furnaces of different sizes and fed with air at different 

 temperatures. 



Table I. 



Height of furnace in feet 



Calories — per unit of coke burnt to CO 



,, from portion of this CO burnt to COj .. 



Total calories from coke " .. 



Calories in blast 



Total heat per unit of coke 



Temperature of blast C 



Cwts. blast per ton of metal 



,, escaping gases per ton of metal 



,, slag per ton of metal 



48 



2078 



560 



2638 



2638 

 0° 

 228 

 285 



34 



2028 



1059 



3087 



508 



3595 



485° 



128 



170 



31 



80 

 2018 

 1636 

 3654 



534 

 4188 



485° 

 103 



138 



29 



80 

 2045 



1463 

 3508 



732 



4240 



695° 



94 



126 



28 



A second table contains statements showing the manner in 

 which the heat so generated is appropriated in the various 

 divisions of the duty the furnaces had to perform, and for facility 

 of comparison, alongside the quantities of heat so required, their 

 equivalents in the coke used have been added. 



In the table No. II. the appropriation of the heat is separated 

 into Constants and Variables. The first consists of items where 

 the quantity of heat in making a particular quality of iron is 

 only liable to alterations of trifling amount. On the other hand, 

 the variables exhibit in A and B diff"erences so considerable that 

 work which in the furnace blown with cold air absorbed 73,388 

 calories per 20 cwts. of pig-iron, was done with 58,645 calories 

 by merely raising the blast to 485°. 



The cause of this great variation in the amount of heat re- 

 quired for a given weight of pig-iron, produced under different 

 circumstances as to temperature of blast and size of furnace, de- 

 pends on changes in the actual amount of work to be performed. 

 How this arises will be best seen in the description of the four 

 examples set forth in the two tables. 



Beginning with A, which is a furnace 48 feet in height, blown 

 \yith cold air and consuming 45 cwts. of coke and 18 cwts. of 

 limestone per ton of metal, the volume of gas produced may be 

 taken at 14,460 cubic yards at ordinary temperatures and pres- 

 sures. At the temperature at which they escape we may assume 

 the volume per ton of iron to be about 36,000 cubic yards, pass- 

 ing out of the furnace at the rate of 357 cubic yards per minute. 



In comparing the consumption of coal formerly burnt in the 

 hot-air stoves with the saving of coke in the furnace, account 

 must be taken of the different conditions of the combustion. In 

 Table I., owing to the small quantity of carbon dioxide formed, 

 the heat evolved is only 2638 calories per unit of coke, whereas 

 each unit of the coal consumed in heating the air aff'orded three 

 times this quantity of heat. Doubtless there was a great loss in 

 the operation of heating the air, for it would not appear that 



much above one-fourth of the theoretical quantity of heat capable 

 of being generated by the coal reached the furnace through the 

 tuylres. 



We have now to consider the nature of the change which is 

 effected in a furnace where, for every 2638 calories generated by 

 the combustion of the coke, 508 calories are carried in by the 

 blast. It will b2 readily understood that with the velocity at 

 which the gases are passing out of the cold-blast furnace they 

 have but little time to impart their heat to the incoming solids, 

 or to have the carbonic oxide they contain converted into carbon 

 dioxide. The withdrawal of so much coke, and its place taken 

 by heat contained in the blast, means that the 14,460 cubic yards 

 of escaping gases are reduced to about 12,120 cubic yards. The 

 effect of this is not only to alter the speed at which the gases 

 are passing through the materials, but to alter the relation in 

 point of quantity which the ironstone present in the furnace 

 bears to the coke, so that, in point of fact, a larger space is 

 occupied by the ore than was before, and a lesser one by the 

 fuel. We have thus the carbonic oxide passing more slowly 

 over the oxide of iron at the same time that there is a greater 

 quantity of the oxide exposed to the influence of the reducing 

 gas. To illustrate how this operates, a table has been prepared, 

 showing how each 1000 cubic feet of furnace space is occupied 

 in the four cases we are considering : — ■ 



A B 



48-feet 48-feet 



cold blast. hot blast. 



736 686 



63 

 201 



Coke cubic feet 

 Limestone ,, 

 Ironstone ,, 



75 

 239 



C 



80-feet 



blast. 



590 



86 



324 



D 



80-feet 



blast. 



590 



77 



333 



1000 



The immediate effect of the introduction of the hot blast is to 

 alter the spaces filled by the three minerals from those given in 



