127 THE MINING MAGAZINE: DEVOTED TO Mines, Mining Operations, Metallurgy, &c., &c. VOL. VII.-SEPTEMBER, 1856.-No. III. ART. I. THE IRON MANUFACTURE OF GREAT BRITAIN-THEORETICALLY AND PRACTICALLY CONSIDERED, BY WM. TRURAN, C. E. No. 8. (Continued from page 58, vol. 7.) SECTION VII.-THE BLAST, OR COMPRESSED ATMOSPHERIC AIR EMPLOYED IN SMELTING. DENSITY OF THE BLAST. THE pressure or density of the blast is a matter of considerable importance in smelting. It is regulated by the height of the furnace; size of the hearth; and qualities of the fuel. In the infancy of the manufacture, owing to the low state of mechanical science, the blowing machines employed were very deficient in the power necessary for compressing the air. The blast obtained was inferior in quantity and of a low pressure; but the furnaces were of small capacity, and the produce of iron was on the same limited scale. With the improvements in the manufacture of machinery, attention was directed to the blowing machinery of iron works; greater volumes of blast, compressed to a higher density, were obtained; and with additional blowing power, the capacity and height of the furnace was increased, and the produce of metal augmented in a similar ratio. The density of the blast is dependent on the height of the furnace, inasmuch that the greater or lesser weight of the column of solid materials affects the density of the stratum of fuel under combustion. But the blast furnaces of the present day are built nearly of one uniform height;-a deviation of more than 5 feet from the average height is rare. The variation, then, in the pressure exerted by the superincumbent materials on the fuel, from the greater or lesser height of the column in different furnaces, is not great, and may safely be omitted. Therefore, in proportioning the density and volume of the blast, we must consider the internal dimensions of the furnace, and the qualities of the fuel, as the governing agents. In the present state of the manufacture, 2 lbs. to the square VOL. VII.-8 inch is a minimum degree of compression. A few years since, the experiment of a lower blast was tried at the Wingerworth furnaces, but after great expense and waste of material, it was finally abandoned, and a blast of 21 to 23 lbs. to the square inch substituted. And more recently we have witnessed numerous instances of the injurious effects produced by a weak scattered blast on the make and quality of the crude iron. The maximum density of blast is dependent, to a certain extent, on the fuel. If the coal is of a hard compact structure, containing a large percentage of carbon, a pressure of 4 to 5 lbs. to the inch may advantageously be adopted. With a weak, friable. coal, containing a low percentage of carbon, 2 to 25 lbs. is very commonly used. The dense cokes of a highly bituminous coal support a blast of 2 to 3 lbs. As a general rule, from which we have seen no exceptions, the density of blast applicable to any given coal-the other conditions being equal-is proportionable to the density of the coke produced from such coal. Hence, the hardest and heaviest coke will carry the strongest blast, and vice versa. Generally speaking, the increase in the density of the blast has not kept pace with the enlargements of the blast furnace, and yet they are directly dependent on each other. The density must be such, that the requisite quantity of atmospheric air penetrates to the opposite side of the hearth, in order to maintain active combustion in the fuel descending farthest from the tuyere. The wider the hearth the more numerous will be the obstructions to its passage across. Any increase, then, in the width of the hearth should be met by a corresponding increase in the density of the blast. The degree of compression is undoubtedly carried farther now than formerly, but it is not sufficient for the large hearths constructed. In the old blast furnaces the width of the hearth seldom exceeded 3 feet; through a mass of materials of this thickness only had the blast to penetrate. For this purpose the density usually was 1 to 2 lbs. If the obstruction offered by the thickness of material required a blast of this density for the perfect combustion of the fuel, it is very evident that with a wider hearth a proportionately higher density is required. If to penetrate 3 feet required a mean pressure of 14 lbs. to penetrate twice the distance requires twice the pressure, or 34 lbs. to the square inch. Considering the pressure of 14 lbs. as suitable for the 3 feet hearth, an approximation may be made to the pressure required for any other width. The 1 lbs. to 3 feet, is equal to six tenths of a pound nearly for each foot. Hearths are now constructed up to 8 feet in breadth: for such, the pressure of blast should, by this rule, be 4 lbs. to the inch. For the more common breadth of 6 feet the pressure will be 3 lbs; but in practice a lower density is used. Partly from not justly considering the relation which the density of the blast should bear to the breadth of the hearth, and partly from the blowing engines having been constructed to deliver blast of a fixed density, an increase, if desired, is not attainable. Furnaces having the widest hearths are blown with engines which maintain a pressure of 21 to 3 lbs. to the inch but the diameter of the discharging pipes are such that, at their termination the pressure is commonly under 2 lbs. Complete combustion of the fuel with this low pressure is not to be expected the portions farthest from the tuyere do not receive the requisite volume of oxygen gas for creating a high temperature; consequently, the carbon of such fuel does not contribute by its combustion a maximum quantity of caloric for the reduction of the ore and flux. The entire calorific powers of the fuel are not developed and brought to bear on the fusion of the ore; hence, the quality of the resulting crude iron is deteriorated. The distinction between volume and density is not sufficiently attended to by practical smelters. With the enlarged capacity of the blast furnace, an increased volume of blast has been employed; but the mere increase of volume is not sufficient to obtain the entire benefits accruing from the employment of large furnaces. Augmenting the volume alone, implies the ability of a blast of given density to penetrate the mass of materials, whatever may be the breadth of the hearth,-a manifest error. From inatten. tion to this circumstance the capacity of the blast furnace has been increased from about 80 to 260 and 280 yards, with an equally great, if not greater increase in the volume of the blast; but the density remains nearly the same. Had the density of the blast been augmented in the same ratio as the increased dimensions of the furnace, the produce of the largest blast furnaces would be more in unison with their capacity. But while we advocate the employment of a blast of adequate volume and density, we freely admit that no form of tuyere yet invented can produce a perfect combustion of the carbon. The blast of low density supplies the nearest coal, but fails to penetrate the materials to the farthest: a high density, while it supplies. ample air to the farthest, affords scarcely any to the nearest coal. A tuyere which shall afford to each the requisite quantity of air for perfect combustion is much wanted, and until it is provided a high economy of fuel will not be attained. We are of opinion that a compound tuyere, consisting of a central discharge of very dense blast, and an exterior annular discharge of a less dense blast, eventually will supersede the present single tuyere. The necessity for compressing the blast to a less or greater density is generally admitted by iron smelters; for under no other circumstances can sufficient air be brought in contact with the fuel: occasionally, however, the patent records of this country contain ingenious substitutes for the common plan of forcing a draught. Recently a furnace was built in the neighborhood of Caerphilly, in which the requisite draught was attempted to be maintained by exhaustion, but without success, and the operations were discontinued. A different result could scarcely be anticipated; for if a brief consideration is given to the rapid rate of combustion necessary to the maintenance of the high temperature of a blast furnace, the impossibility of supplying the atmospheric air by an indraught becomes manifest. MODE OF APPLYING THE BLAST. A diversity of opinion exists as to the best mode of applying the blast. The usual plan is by three tuyeres, one on each side and the third at the back; but departures from this mode are to be seen at several works. It is seldom that a fewer number are employed with modern furnaces, but a greater number is considered by some smelters as being more advantageous, and furnaces may be seen receiving blast through 10 and even 12 pipes. The volume of blast is fixed; therefore, when more than 3 pipes are used, and the original density of the blast is desired to be maintained, their diameter is reduced, so that the total sectional area remains unaltered. The blast, then, being delivered through a larger number, the discharge by each pipe will be reduced in the same ratio as its reduced sectional area. The advantages which are supposed to attend the employ. ment of a large number of small pipes, are, a more perfect combustion of the fuel, and, consequent thereon, a greater economy of blast and smelting materials, and an increased produce. It is maintained by some smelters, that such advantages have been realized whenever the number of tuyeres has been increased. Certain it is, however, that more than one has obtained letters patent for the supposed discovery of such improved (?) mode of bringing the blast in contact with the fuel, and the employment of four or more tuyeres is now common in several districts. We cannot subscribe to the general approval of numerous tuyeres. After a lengthy trial at several furnaces, we were unable to discern any superiority over the fewer number. And if due consideration be paid to the circumstances which determine the volume and density of the blast, any other result is not to be expected. The admission of the blast at several points in the circumference of the hearth, doubtless conduces to a more perfect combustion of so much of the fuel as is within the range of the blast, but it is detrimental to the combustion of the remotest portions. For the profitable consumption of the fuel, each and every piece within the zone of fusion requires to be supplied with the necessary volume of atmospheric air. This can only be accomplished by having a blast proportioned in |