Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/881

FURNACE 845 combustion, the gases produced passing out by the largo flue c. The charging hole d is covered by a closely fitting valve making a gas-tight joint, the escape of gas being prevented by the great thickness of coal above the FIQ. 10. Gas-producer (transverse section). flue and the strong draught in the latter. From the ex ceedingly poisonous nature of carbonic oxide it is of the utmost importance to prevent the issue of uuburnt gas; and if this cannot be prevented, the escape must be fired when the charging hole is opened. This is regularly done in blast furnaces working with gas-collecting flues, and even the native iron-smelters of India, in starting their small furnaces with stacks only a few feet high, observe the same precaution. Another point of equal importance is to pre vent the access of air to the gas anywhere except at the point of ignition. Any leakage of air into the gas flues must as a rule produce explosion. An ingenious and efficacious method of establishing the draught in gas- producers is adopted by Dr C. W. Siemens. The gas delivery tube rises to a certain height, is then laid horizontal for a short distance, and finally descends verti cally to the original level. The gas in passing along the horizontal tube loses heat by radiation, and on arriv ing at the downward tube is sensibly denser than when it started, so that the second vertical tube acts as siphon and maintains constant exhaust ou the producer. In some cases the gases from the fuel are modified in composition by an injection of steam from a pipe below the grate e (fig. 10), which, impinging upon ignited coal, is decomposed into hydrocarbon and carbonic oxide gases. This transforma tion is necessarily attended with considerable cooling effect, from the large amount of heat expended in the decomposi tion of water, so that the method is only of limited appli cation ; but the " richness " or fuel value of the gases is very considerably increased by its use. It is more particularly of value with dry or anthracitic coals. The gas-producer is used with advantage with very inferior fuels, such as peat, lignite, sawdust, <fcc., containing much ash and water, and if the latter be removed by passing the crude gas through a condenser, according to the method adopted by Lundin in Sweden, the poorest material, such as wet sawdust, may be employed for puddling and steel melting, operations that require the highest attainable temperatures. Mechanical Furnaces. The introduction and withdrawal of the charges in fusion furnaces is effected by gravitation, the solid masses of raw ore, fuel, and flux being thrown in at the top, and flowing out of the furnace at the taphole or slag run at the bottom. Vertical kilns, eucli as tlios-s used for burning limestone and iron ores, are worked in a similar manner, the raw stone going in at the top, and the burnt product falling through holes in the bottom when allowed to do so. With reverberatory calciners, however, where the work is done upon a horizontal bed, a consider able amount of hand labour is expended in raking out the charge when finished, and in drawing slags from fusion furnaces; and more particularly in the puddling process of refining iron the amount of manual exertion required is very much greater. To diminish the item of expenditure on this head, various kinds of mechanical furnaces have been adopted, all of which can be classified under three heads of gravitating furnaces, mechanical stirrers, and revolving furnaces. 1. In gravitating furnaces the bed is laid at a slope just within the angle of repose of the charge, which is introduced at the upper end, and is pushed down the slope by fresh material, when necessary, in the contrary direction to the flame which enters at the lower end. This method is used in Styria for burning the dust of spathic iron ore which cannot be put into the kilns with the large lumps. The fuel used is blast furnace gas, the calciners being directly over the furnace top. Gerstenhofer s pyrites burner, another furnace of this class, has a tall vertical chamber heated from below, and traversed by numerous narrow horizontal cross bars at different heights. The ore in fine powder is fed in at the top, through a hopper, in a regular thin stream, by a pair of rollers, and in falling lodges on the flats of the bars, forming a talus upon each of the height corresponding to the angle of rest of the material, which is, however, at short intervals removed to lower levels by the arrival of fresh ore from above. In this way a very large surface is exposed to the heat, and the ore, if containing sufficient sulphur to main tain the combustion, is perfectly burned when it arrives at the bottom ; if, however, it is imperfectly sized or damp, or if it contains much earthy matter, the result is not very satisfactory. Stetefeld s furnace, used for the conversion of sulphuretted silver ores into chlorides, is of a somewhat similar character. It is now largely used in the silver mines of the western States of America. 2. Mechanical stirrers constitute a second division of mechanical furnaces, in which the labour of rabbling or stirring the charges is performed by combinations of levers and wheel-work taking motion from a rotating shaft, and more or less perfectly imitating the action of hand labour. They are almost entirely confined to puddling furnaces, and have not been very generally adopted for these. 3. Revolving furnaces, the third and most important division of mechanical furnaces, are of two kinds. The first of these resemble an ordinary reverberatory furnace by having a flat bed which, however, has the form of a circular disk mounted on a central shaft, and receives a slow movement of rotation from a water-wheel or other motor, so that every part of the surface is brought succes sively under the action of the fire, the charge being stirred and ultimately removed by passing under a series of fixed scraper arms placed above the surface at various points. Brunton s calciner, used in the " burning " of the pyritic minerals associated with tin ore, is the most familiar example of this type. The revolving hearth is also repre sented in Pernot and Pousard s steel-melting furnaces. In these, however, the hearth rotates on an inclined axis, so that the path of its surface is oblique to that of the flame. In the second class of revolving furnaces the working part is a hollow cylinder, between the fireplace and flue, with its axis horizontal or nearly so, whose inner surface represents the working bed. It is mounted upon friction rollers, and receives motion from a special steam