TRANSACTIONS OF SECTION B. 



525 



The cause of this preat variation in the amount of heat required for a given 

 ■weight of pig iron, produced under different circumstances as to temperature of 

 blast and size of furnace, depends on changes in tlie actual amount of work to bo 

 performed. How this variation 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 with 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 14,4G0 cubic yards at ordinary temperatures and 

 pressures. At the temperature at which it escapes we may assume the volume 

 per ton of iron to be about 36,000 cubic yards, passing out of the furnace at the rate 

 of 675 cubic feet 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 2,638 calories per unit of coke, whereas each unit 

 of the coal consumed in heating the air afforded 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 tuj-eres. 



We have now to consider the nature of the change which is effected in a furnace 

 where, for every 2,638 calories generated by the combustion of the coke, 508 

 calories are carried in by the blast. It will be readily understood that, with the 

 velocity at which the gases are passing out of the cold-blast fiu-nace, 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 ia 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 car- 

 bonic 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 1,000 

 cubic feet of furnace space is occupied in the four cases we are considering: — 



P,The immediate effect of the introduction of the hot blast is to alter the spaces 

 filled by the three minerals from those given in Column A to coke 686. limestone 

 75, and ore 239 cubic feet. This is followed by a twofold advantage. Volume for 

 volume, ore and limestone possess double the heat-intercepting power of coke, and 

 there is 19 per cent, more ore ready to oxidise the carbonic oxide passing over it 

 at a reduced speed of 16 per cent, than there was when using cold air. The in- 

 creased efhciency of the coke, due to a more perfect cooling of the gases and higher 

 oxidation of the carbonic oxide, permits its further suppression until the relative 

 spaces filled by the materials are those shown under Column B. These advantages 

 would not of themselves suffice to save 16 cwts. of coke or thereabouts out of 

 45 cwts., but the reduction in the coke consumed is followed by a diminution in the 

 quantity of air used and in the weight of gases and slag produced. A reference to 



