centre of each fragment, before it reaches the region of fusion, and as a rule, the furnace is built smaller in proportion to the ease with which the various oxides are reduced. The horizontal section given to the hearth must vary according to the pressure of the blast, and the porosity of the materials employed in the furnace. When hard coke is used, and the ore is in large pieces, a longer time is required for the air to penetrate to the centre of each lump, and a greater pressure is then needed. But this tends to produce a higher temperature, and, consequently, greater reducing energy, which necessarily makes the metal more impure. The height of the furnace should be limited when the fuel is friable, such as anthracite, and the ore in small pieces; for if the charge is too compact the gas can only circulate with difficulty. Moreover, in a mass of different materials, descending gradually, the difference of density becomes greater as the height of the furnace is greater; the heavier pieces of ore tend to descend vertically while the lighter particles of fuel are forced to the sides, which circumstance limits the possible height. The in ternal shape of a blast-furnace, should be that of the general form which it tends to assume after some weeks of working. It has been found in practice that the section is modified where the heat has been greatest, and that the sharp angles of the hearth and boshes of the older forms were invari ably burned away. When a furnace is Fig.16. Fig. 17. working irregularly, which often arises from an accumulation of lime and unreduced ore in the hearth and boshes, it is generally due to scaffolding. Fig. 16 is a section of a furnace showing an excrescence of this kind given by Mr. R. Howson who recommends the shape Fig. 17 as the best form for avoiding scaffolds. A the introduction of twyers which convey the blast of air into the furnace. On the front or working side the hearth is extended outwards for a short distance, forming a rectangular cavity known as the fore-hearth, which is bounded in front by a refractory stone termed the dam-stone. The arch covering this cavity is called the tymp-arch. The tymp is made either of a block of refractory stone, or of a hollow cast-iron box built in the masonry, and through this box a current of water constantly circulates, in order to keep it cool. In fig. 15, A is the charging gallery, B the cup and cone arrangement for charging, c the throat, D the body, EE the boshes, F the blast-main, & the iron ring supporting the body, Hн the pillars, I the hearth, K the twyers, L the dam, 7 the iron dam-plate, m the fore-hearth. The dam is formed of fire-brick, and is carried up to the twyer level, a semi-circular notch in the top edge serving as a passage for the slag. The tap-hole for the molten iron is a narrow slit through the bottom of the dam. (t) is the tymp. N is the opening for collecting the waste gases which are utilised for heating the blast, boilers, etc. G H Fig. 15. E H The charge is tipped into the cup or hopper (x), and allowed to fall into the furnace by lowering the cone B, which acts very advantageously in distributing the charge over the surface of the materials already in the furnace. Form and interior dimensions.-The descent of the materials must, in all cases, be sufficiently slow for the reducing action of the gas and carbon to penetrate to the centre of each fragment, before it reaches the region of fusion, and as a rule, the furnace is built smaller in proportion to the ease with which the various oxides are reduced. The horizontal section given to the hearth must vary according to the pressure of the blast, and the porosity of the materials employed in the furnace. When hard coke is used, and the ore is in large pieces, a longer time is required for the air to penetrate to the centre of each lump, and a greater pressure is then needed. But this tends to produce a higher temperature, and, consequently, greater reducing energy, which necessarily makes the metal more impure. The height of the furnace should be limited when the fuel is friable, such as anthracite, and the ore in small pieces; for if the charge is too compact the gas can only circulate with difficulty. Moreover, in a mass of different materials, descending gradually, the difference of density becomes greater as the height of the furnace is greater; the heavier pieces of ore tend to descend vertically while the lighter particles of fuel are forced to the sides, which circumstance limits the possible height. The internal shape of a blast-furnace, should be that of the general form which it tends to assume after some weeks of working. It has been found in practice that the section is modified where the heat has been greatest, and that the sharp angles of the hearth and boshes of the older forms were invariably burned away. When a furnace is Fig.16. Fig. 17. working irregularly, which often arises from an accumulation of lime and unreduced ore in the hearth and boshes, it is generally due to scaffolding. Fig. 16 is a section of a furnace showing an excrescence of this kind given by Mr. R. Howson who recommends the shape Fig. 17 as the best form for avoiding scaffolds. Greater height may be given to a furnace to increase its capacity, and to intercept the heat more completely. Combustion should only occur in the neighbourhood of the twyers, and the greater the distance of the upper end of the charge from the zone of combustion, the more perfectly will the heat be extracted from the ascending gases, so that furnaces are now built 80 to 90 feet in height. Fig. 18. The section of most blastfurnaces is round, which economises the heat, and causes it to be more uniformly distributed; but there is always a difficulty in forcing the blast to the centre, since the charges sink more there than at the circumference. This circumstance induced Rachette to adopt an elliptical or rectangular section. The large production of 30 tons of grey iron in 24 hours in this small furnace is due to the suitable distribution of the blast, and the non-conducting nature of the walls. Truman states that the charges descend uniformly to the twyers, thus utilising the fuel more completely, and that the smelting is rapid. This furnace Fig. 18 is oblong and rectangular in shape, being 3 feet wide at the twyers, 7 feet at the throat, and about 30 feet high, with a capacity of 2000 cubic feet. The object of this shape is to keep the ascending gases |