A-shaped cast-iron pipes, that connect two parallel horizontal main pipes, embedded in the masonry on either side of the rectangular fire-place that extends throughout the stove. A modification of this second type is the so-called pistol-pipe stove (Fig. 74). In this case the arch is replaced by a single pipe divided longitudinally, the division reaching nearly to the top, which is enlarged in the form of the stock of a pistol. Two types of regenerative hot-blast stoves are employed. These are based on the principle of the intermittent absorption of heat by masses of fire-brick and the transference of the heat to the blast. The first stove of this type was constructed by Cowper in 1860; it is similar in arrangement to a Siemens regenerator. It is enclosed, however, in an iron case so as to withstand the pressure of the blast. The first stove of the second type was constructed by Whitwell in 1865 for the Thornaby works at Stockton. It is essentially a serpentine pipe-stove constructed of fire-brick. The Cowper stove, which is represented in sectional elevation and plan in Figs. 75 and 76, consists of a sheet-iron tower I, of circular horizontal section, closed with a dome-shaped roof B, and lined internally with fire-brick. A circular flame-flue F, extends from the base to the dome, whilst the remainder of the stove is filled with fire-brick chequer-work, and forms the regenerator R. The waste gases from the blast-furnace pass in by the valve G, and are burnt at N, the necessary air for combustion entering by the valve A. The flame descends through the chequer-work and passes out by the chimney-valve V. In this way the brickwork becomes heated. The valves G, A, and V are closed, and cold blast, admitted through the valve C, is passed through in the reverse direction. It absorbs heat from the chequer-work and is delivered as hot-blast by the valve H. The chequer-work is constructed of Cowper's honeycomb bricks. Two stoves are worked in conjunction, one being heated while the blast passes through the other. It is advisable to have a third in reserve. These stoves are 40 to 65 feet high and 26 to 28 feet in diameter. Compared with pipe-stoves the saving of fuel is about 20 per cent. and the increased make is also 20 per cent. An exact average of over 100 stoves shows the saving in fuel to amount to a little over 5 cwt. of coke per ton of iron.* In 1883 there were 360 of these stoves at work in Europe and America. The Whitwell stove is shown in Fig. 77. It is merely a brickwork serpentine pipe formed by vertical walls, and enclosed in a cylindrical case. The waste gases from the furnace enter * E. A. Cowper, Journ Iron and Steel Inst., 1883, p. 576. at A and are burnt, air being admitted through the passages a, a. The flame passes up and down the passages formed by the dividing B H FIG. 75. walls, and escapes to the chimney by the passage C. When the stove is heated, the gas and chimney valves are closed, and cold blast is admitted at D, and passes out heated at B. The walls forming the regenerator consist of 5-inch brickwork. The older forms of Whitwell stove were 25 feet in height. Recently the height has been greatly increased, the largest size adopted being about 70 feet high and 25 feet in diameter. A domed top is now used. The relative merits of fire-brick and iron hot-blast stoves may be seen from the following comparison drawn by Lürmann :*. Fire-brick stoves may be heated to very high temperatures without interfering with their working, if the dust contained in the blast-furnace gases contains no basic constituent which has a detrimental action on the fire-brick. The heating surface of fire-brick stoves must be considerable if the temperature of the blast is not to be altered too rapidly, and if the working is not to be interrupted too often. The cost of erection of these stoves is consequently high. The cost of maintenance, on the other hand, is very low. The older forms of fire-brick stoves are not easily cleaned. By rendering cleaning possible, the construction becomes more complicated, and the cost is consequently increased. Fire-brick stoves require a skilled attendant, as the working is intermittent. Iron stoves can only be heated to the temperature at which the castiron, of which they are made, softens. Iron stoves, with sufficient heating surface for the amount of blast to be heated, necessitate a smaller expenditure of capital than the older forms of fire-brick stoves, and give uniform temperatures. They, however, present the danger of the working being interfered with on account of pipes having to be replaced, and consequently the cost of maintenance is high. On the other hand, the value of the wornout material of the iron stoves is higher than that of the fire-brick ones. Iron stoves are easily cleaned. Iron stoves may be entrusted to the ordinary workmen, and demand no special skill. Fire-brick stoves, as compared with those of iron, possess the advantage of effecting a saving in heat by lowering the temperature at which the products of combustion enter the chimney. The quantity of coke consumed when fire-brick stoves are employed being less than when iron stoves are used, the weight of furnace gases is correspondingly affected. Sir Lowthian Bell† assumes that for 20 units of iron made there are 122 units of gas in the former against 129 units in the latter. Allowing 10 per cent. of free atmospheric air in the burnt gases as they leave the stoves, their weight may be taken at 212 and 240 units respectively per 20 units of iron produced. The two accounts may be stated as follows: *Stahl und Eisen, 1882, p. 361. + Journ. Iron and Steel Inst., 1883, p. 119. |