brake HP. hour, about 76 feet of producer gas, and about 32 feet of coke-oven gas. An 800 HP. engine used 5 gallons of oil per twentyfour hours. Mazout of special quality will do, and some of it can be used again after filtration. About 12 to 14 gallons of water per brake HP. hour must be passed through the jackets. About 3 per cent. is lost by evaporative cooling. Blast Furnace.-The modern blast furnace differs from the ancient furnaces in the fact that the product is well under control, and is always cast iron, instead of an uncertain product which partook more of the character of malleable than of cast iron. The blast furnace dates from about the beginning of the sixteenth century, but its true era came with the successful utilisation of coke by Abraham Darby, about 1733. In 1740 the production of British pig was only 17,350 tons, in 1796 the annual make per furnace was only 1,032 tons, or less than 20 tons a week. From 1804 to 1818 the price of pig ranged from £7 to £9 per ton. In 1828 the hot blast was introduced. With the introduction of the fire-brick regenerative hotblast Cowper stoves (1860) the economy was increased. Later, Whitwell's apparatus for heating the blast effected further saving. the height and capacity of furnaces grew. Lowthian Bell gave the following figures: Also Sir within the lining, and having nozzles 8 in. diameter. The output is 11,000 tons of pig per month, using ores containing 62 per cent. of iron. At the Duquesne works, there are furnaces 100 ft. high, boshes 22 ft. diameter, angle of bosh 74°, throat 17 ft., crucible 14 ft., bell 12 ft., ten tuyeres 8 in. diameter, placed 9 ft. 8 in. above the level of the hearth. The Youngstown furnaces of the National Steel Co., blown in in 1900, are 106 ft. high, 23 ft. diameter in the bosh, 15 ft. in the hearth. Blast furnaces vary much in shape and proportions, which cannot be satisfactorily explained on a theoretical basis. There are national and local differences, in the ores, fuels, and fluxes used, in blast pressure, in slow or rapid working. As the ores smelted vary much in percentage of iron, the poorer ones require a larger furnace capacity than the richer ones, and at the same time the furnace yield is less. The best guide for the shape of a furnace working under any given conditions is obtained after one has been blown out. The lining will be found to have worn more in one locality, and have received deposits in others. Thus the original form may be modified with advantage. Although the tendency has been to a regular increase in height and diameter as being conducive to economical results, yet a limit comes bell, the open-topped type having fallen mostly into disuse. Compare with Fig. 218, which illustrates one of the furnaces at the Cockerill works at Seraing. The Throat, or mouth, is closed with a bell, or cup and cone that prevents the escape of the gases, which are led away through pipes, and utilised for heating boilers, or driving gas engines. The throat is several feet less in diameter than the body of the furnace. The Body, which is the preliminary heating area for fuel and ore, and has by far the largest proportion of furnace capacity. Generally the sides of this are straight, and tapered, enclosing a truncated cone, but sometimes they are curved or bellied. The amount of coning depends on the relation between the dimensions of the throat, the boshes, and the height. The Boshes is the name given to the lower or melting area, and here much difference in The intense heat of the blast furnaces burns out the lining of the boshes. Formerly less than 250,000 tons was obtained during the life of the lining, now there are cases of 1,000,000 tons having been exceeded. This is due to the use of water-cooled plates of cast iron, or copper, or bronze built into the walls horizontally. The plates are spaced at increased distances apart from above the tuyere belt to the upper part of the bosh. Water under pressure is brought in at the lowest row of plates whence it ascends in the series. See Fig. 219. The furnace Hearth rests upon a foundation of brick-work built in clay or other material. Masonry built round this receives columns, to carry the body or stack, which is iron or steel plated, and lined with fire-brick. The Charge of a blast furnace consists of ore, fuel, and flux (limestone), and the relative B Fig. 219.-Water Cooled Blocks or Plates. A. Kennedy's two-pass water cooled bronze block. B. Cooling plate fitted with water baffles. C. Gayley plate, used in Edgar Thomson furnaces. practice exists, especially in the angle, which has to be ascertained for the best work. It ranges from 60° in the older Cleveland furnaces to 75° in many modern ones. The angle, the diameters at bottom and top, and depth are mutually related. At the bottom of the boshes the tuyere zone must be of moderate diameter, to permit the blast to penetrate to the centre of the materials, at the top a large area is required to give the solids as they descend to the hearth an exposure of several hours to the heated gases. The Crucible. This is the smallest part of the furnace below the hearth, in which the metal collects, and above which the tuyeres enter. These point to the centre, and are generally arranged horizontally. These are water jacketed. In the Scotch tuyeres the water is circulated round a coil of pipes encasing the passage; in the Lloyd's, now more often used, water is sprayed into a jacket surrounding. proportions of each vary with the quality of the ore. Coal, coke, and a mixture of each is used. Approximately a ton of coke and 2 tons of ore are required for the production of a ton of pig, and about 30 cwt. of slag results. To melt this, a weight of air is required equal to about half the weight of the materials,-ore, limestone, and coke, and the waste gases comprise about two-thirds of the weight of pig and slag. The solid materials take about seventy hours to pass from the top of the furnace to the hearth, during which period a gradual heating up occurs. About 75 per cent. of the calorific power theoretically available is utilised in the blast furnace. Blast pressure ranges from 3 to 20 lb. per square inch. The reactions which go on are the reduction of the metal from its ore in the presence of carbonic oxide. These have been treated very exhaustively by Lowthian Bell, and Dr Percy. Substantially they may be summed up in the words of the latter: "Temperature, carbonic oxide, and incandescent carbon explain all the phenomena of the blast furnace." The successive reactions are not so simple as they appear, because so many elements enter into the composition of ores, and fuels; besides carbon, there are phosphorus, silicon, alumina, and others, besides which there are differences in working. Iron ore, in its descent, meeting the ascendant blast becomes hotter and hotter, passing through zones which are only approximately defined, in which warming up, reduction, carbonisation, and fusion occur, losing first its water, then its carbonic acid, then its oxygen. CO2 is soon reduced to CO, and this at the high temperature reduces the metal by abstracting oxygen from the ore, the gas becoming again CO,. With regard to hard or easy driving; hard working wears the lining badly, and it requires a large hearth, and relatively small diameter of bosh, which is American practice. A large hearth is necessary to burn sufficient fuel, for which a large volume of high-pressure blast is necessary, and when this is combined with a bosh of small diameter, more oxygen is removed from the ores by the gases, and less by solid carbon. Hard driving ensures a more even quality of iron with less silicon than easy working. With regard to the more rapid wear of the lining it is held by those who adopt it, that more frequent relining is better economy than the long service working. The hard driving of the American furnaces may be judged from the following: An 80 feet high Edgar Thomson furnace, with a capacity of 18,200 cubic feet, receives 25,000 cubic feet of air per minute, through seven 6-inch tuyeres, at a pressure of 9 lb. Over 10,000 tons of iron a month have been produced here. The Youngstown furnaces, 106 feet high, with a capacity of 26,500 cubic feet, receive from 50,000 to 60,000 cubic feet of air per minute, through sixteen 6-inch tuyeres, at a pressure of 15 lb. Over 17,000 tons a month have been produced in these. At the Duquesne furnaces 793 tons of pig have been produced from one furnace in a day, or at the rate of over 5,000 tons in a week. Blast Furnace Gas.-The great advantage of utilising blast furnace gas is that it does away with the steam boilers hitherto heated by such gas, and employs it direct for the driving of engines. At the Cockerill works it is estimated that while the boilers which are heated by the furnace gas supply steam for about 2,500 HP. the same if used in gas engines would provide about 12,000 HP. The difficulties which are met with in the utilisation of blast furnace gas are due less now to the presence of dust than to that of tar. The dust can be reduced to of a gramme per cubic métre, but the tar passes on and causes the valves and piston rings to stick, and it be comes deposited in the cylinder and passages. In most cases these engines are used in installations where sudden stoppages would derange the works, as for blowing, for electric lighting, and for mines, hence the necessity for purifying the gas beyond that necessary to prevent the cutting and scoring of working parts. Blast Furnace Gas Engines. - Somewhere about the year 1893, Mr B. H. Thwaite was struck by the great similarity of producer gas and blast furnace gas, and, with a view to the employment of the latter for power purposes, he made use of a demonstration producer gas plant for the purpose of synthesising a gas of the same proportions as certain analyses of blast furnace gas, or B.F. gas as it is now often called. The synthesised gas, even with a high percentage of CO2 was found to work perfectly in the gas engine. Thwaite then prepared plans of a cleaning plant, and ultimately, with the co-operation of James Riley of the Glasgow Iron Co., he established a small plant and gas engine for the lighting by electricity of the works at Wishaw. Thwaite had already published something of his work in the Iron and Coal Trades Review, which drew Continental attention to the great possibilities of B.F. gas. Unfortunately, however, the Continental engineers, who should have been better informed, repudiated Thwaite's insistence upon the necessity of cleaning the gas before using it in engines. It is hardly necessary to state that the attempts to use uncleaned gas failed, with the result of retarding progress. |