1902 Encyclopedia > Furnace


FURNACE. Under this name are included all con-trivances for the production and utilization of heat by the combustion of fuel.

The word is common to all the Romance tongues, appearing in more or less modified forms of the Latin fornax. But in all those languages the word has a more extended meaning than in English, as it covers every variety of heating apparatus; while here, in addition to furnaces proper, we distinguish other varieties as ovens, stoves, and kilns. The first of these, in the form Of en, is used in German as a general term like the Freuch four; but in English it has been restricted to those apparatus in which only a moderate temperature, usually below a red heat, is produced in a close chamber. Our bakers' ovens, hot-air ovens or stoves, annealing ovens for glass or metal, &c, would all be called fours in French and Oefen in German, in common with furnaces of all kinds. Stove, an equi-valent of oven, is from the German Stube, i.e.. a heated room, and is commonly so understood; but is also applied to open fire-places, which appears to be somewhat of a departure from the original signification.

Furnaces are constructed according to many different patterns with varying degrees of complexity in arrange-ment ; but all may be considered as combining three essen-tial parts,—namely, the fire-place in which the fuel is con-sumed, the heated chamber, laboratory, hearth, or working bed, as it is variously called, where the heat is applied to the special work for which the furnace is designed, and the ap-paratus for producing rapid combustion by the supply of air under pressure to the fire. In the simplest cases, the functions of two or more of these parts may be combined into one, as in the smith's forge, where the fire-place and heating chamber are united, the iron being placed among the coals, only the air for burning being supplied under pressure from a blowing engine by a second special con-trivance, the tuyere, tuiron, twyer, or blast pipe; but in the more refined modern furnaces, where great economy of fuel is an object, the different functions are distributed over separate and distinct apparatus, the fuel being con-verted into gas in one, dried in another, and heated in a third, before arriving at the point of combustion in the working chamber of the furnace proper.

The most obvious distinction that can be used in the classification of furnaces is founded on the method adopted for supplying air, which may either be blown into the fire, under a pressure above that of the atmosphere sufficient to overcome the resistance presented by the packed columns of fuel and other materials to its free passage, or be drawn through it by a partial vacuum in a chimney formed by the heated gases on their way to the atmosphere. The former are known as blast furnaces, and the latter as chimney draught, air, or wind furnaces.

Stack and Blast Furnaces.-—The blast furnace in its simplest form is among the oldest, if not the oldest, of metallurgical contrivances. In the old copper smelting district of Arabia Petreea, clay blast pipes dating back to the earlier dynasties of the ancient empire of Egypt have been found in great numbers, buried in slag heaps; and in India the native smiths and iron workers continue to the present day the use of furnaces of similar primitive types. These, when reduced to their most simple expression, are mere basin-shaped hollows in the ground, containing ignited charcoal and the substances to be heated, the fire being urged by a blast of air blown in through one or more nozzles from a bellows at or near the top. This class of furnace is usually known as an open fire or hearth, and is represented in a more advanced stage of development by the Catalan, German, and Walloon forges formerly used in the production of malleable iron, and still current to some extent in Sweden, Corsica, and a few places in central Europe. Figs. 1 and 2 represent a Catalan forge in use a few years since at Mont-gaillard, in Allege, then one of the few localities in which the process survived. It is now probably com-pletely abandoned. In all of these the parts are essentially the same: the cavity in the ground is represented by a pit of square or rectangular section lined with brick or stone of a kind not readily acted on by heat, about 1J or 2 feet deep, usually some-what larger above than below, with a tuyere or blast pipe of copper penetrating one of the walls near the top, with a considerable downward inclination, so that the air meets the fuel some way down. In the work of iron-smelting, the ore is laid in a heap upon the fuel (charcoal) filling up the hearth, and is gradually brought to the metallic state by the reducing action of the carbonic oxide formed at the tuyere. The metal sinks through the ignited fuel, forming, in the hearth, a spongy mass or ball which is lifted out by the smelters at the end of each operation, and carried to the forge hammer. The earthy matters form a fusible glass or slag melt, and collect at the lowest point of the hearth, whence they are removed by opening a hole pierced through the front wall at the bottom. The active portion of such a furnace is essentially that above the blast pipe, the function of the lower part being merely the collection of the reduced metal; the fire may therefore be regarded as burning in an unconfined space, with the waste of a large amount of its heating power. By continuing the walls of the hearth above the tuyere, into a shaft or stack either of the same or some other section, we obtain a furnace of increased capacity, but with no greater power of consuming fuel, in which the material to be treated can be heated up gradually by load-ing it into the stack, alternately with layers of fuel, the charge descending regularly to the point of combustion, and absorbing a proportion of the heat of the flame that went to waste in the open fire. This principle is capable of very wide extension, the blast furnace being mainly limited in height by the strength the column of materials or "burden" has to resist crushing, under the weight due to the head adopted, and the power of the blowing engine to supply blast of sufficient density to overcome the resistance of the closely packed materials to the free passage of the spent gases. The consuming power of the furnace or the rate at which it can burn the fuel supplied, is measured by the number of tuyeres and their section. In the largest modern blast furnaces used for smelting iron ores, they may be as many as six or eight, but as a rule the smaller number of from three to five of larger area is adopted, and with these there is no difficulty experienced in burning from 80 to 100 tons of coke per day in hearths of 5 to 7 feet in diameter.

Tie profile adopted for blast furnaces has been very much varied at different times. The earliest examples were invariably square or rectangular in horizontal section, and the same class of form has been retained in many instances up to the present time ; but the general tendency of modern practice is to substitute round sections, their construction being facilitated by the use of specially moulded bricks which have entirely superseded the sandstone blocks formerly used. The vertical section, on the other hand, is subject to considerable variation according to the work to which the furnace is applied. Where the operation is simply one of fusion, as in the iron-founder's so-called cupola, in which there is no very great change in volume in the materials on their descent to the tuyeres, the stack is nearly or quite straight-sided, but when, as is the case with the smelting of iron ores with limestone flux, a large proportion of vola-tile matter has to be removed in the process, a wall of varying inclination is used, so that the body of the furnace is formed of two dissimilar truncated cones, joined by their bases as in fig. 3, the lower one passing downwards into a short, nearly cylindrical, position. The upper cone DC is known as the stack proper, the lower one, from the broadest part C to the tuyeres B, as the boshes, and the lower cylindrical part AB as the hearth. The further consideration oof this subject belongs, however, more particularly to the article IRON. It may be sufficient to say that all blast fur-naces of large size are more or less reducible to this pattern.

The use of bellows or of analogous con-trivances is not essen-tial to the working of stack furnaces; as the supply of air may, in furnaces of small size, be equally well ob-tained by the draught of a chimney, or a steam jet aspirator. The former plan is adopted in the so-called economic fur-nace (homo economico) of the Cartagena lead district in Spain, used in re-smelting old slags and waste pro-ducts of the Roman mines, and the latter in Woodward's and Ireland's cupolas for iron foundries. In either case numerous holes representing tuyeres are provided around the lower part of the furnace, often in two rows at different heights. These furnaces have not, however, been very generally adopted; and even in Spain the chimney draught has in many instances been replaced by a fan blower.

A more primitive form of the same contrivance is still in use in Burmah,—the furnace, about 5 or 6 feet high, being placed on the side of a bank facing the prevailing wind, which enters through a series of small round tuyere holes. Similar contrivances were used by the ancient Celtic inhabit-ants of the Bhine valley,—the ruins of furnaces and slag heaps being found in the Nassau and Eifel hill countries, in high exposed situations far away from streams, where water power for bellows could have been obtained.

Reverberatory Furnaces.—Blast furnaces are, from the intimate contact between the burden to be smelted and the fuel, the least wasteful of heat; but their use supposes the possibility of obtaining fuel of good quality and free from sulphur or other substances likely to deteriorate the metal produced. In all cases, therefore, where it is desired to do the work out of contact with the solid fuel, the operation of burning or heat-producing must be performed in a special fire-place or combustion chamber, the body of flame and heated gas being afterwards made to act upon the surface of the material exposed in a broad thin layer in the work-ing bed or laboratory of the furnace by reverberation from the low vaulted roof covering the bed. Such furnaces are known by the general name of reverberatory or reverbatory furnaces, also as air or wind furnaces, to distinguish them from those worked with compressed air or blast.

Originally the term cupola was used for the reverberatory furnace, but in the course of time it has changed its meaning, and is now given to a small blast furnace such as that used by iron-founders,—reverberatory smelting furnaces in the same trade being called air furnaces.

Figs. 4, 5, and 6 represent a reverberatory furnace such as is used for the fusion of copper ores for regulus, and may be taken as generally representing its class. The fire-place A is divided from the working bed B by a low wall C known as the fire bridge, and at the opposite end there is sometimes, though not invariably, a second bridge of less height called the flue bridge D. A short diagonal flue or up-take E conveys the current of spent flame to the chimney

FIG. 4.—Longitudinal section of Reverberatory Furnace.

E which is of square section diminishing by steps at two or three different heights, and provided at the top with

FIG. 5. —Reverberatory Furnace (horizontal section).

a covering plate or damper G, which may be raised or Lowered by a chain reaching to the ground, and serves for regulating the speed of the exhaust gases, and thereby the draught of air through the fire. Where several furnaces are connected with the same chimney stack, the damper takes the form of a sliding plate in the mouth of the connecting flue, so that the draught in one may be modified without affecting the others. The fire bridge is partially protected against the intense heat of the body of flame issuing through the fire arch by a passage to which the air has free access. The material to be melted is intro-duced into the furnace from the hoppers HH through the charging holes in the roof. When melted the products separate on the bed (which is made of closely packed sand or other infusible substances), according to their density; the lighter earthy matters forming an upper layer of slag are drawn out by the slag hole K at the flue end into an iron waggon or bogie, while the metal subsides to the bot-tom of the bed, and at the termination of the operation is run out by the tap hoie L into moulds or granulated into water. The opposite opening M is the working door, through which the tool for stirring the charge is introduced. It is covered by a plate suspended to a lever, similar to that seen in the end elevation (fig. 6) in front of the slag hole.

According to the purposes to which they are applied, reverberatory furnaces may be classed into two groups, namely, fusion or melting furnaces, and calcining or wasting furnaces, also called calciuers. The former have a very extended application in many branches of industry, being used by both founders and smelters in the fusion of metals; in the concentration of poor metallic compounds by fusion intoregulus; in the reduction of lead and tin ores; for refining copper and silver; for making malleable iron by the puddling processes and welding; and for the manufacture of carbonate from sulphate of sodium in chemical works, &c. Calcining furnaces have a less extended application, being chiefly employed in the conversion of metallic sulphides into oxides by continued exposure to the action of air at a temperature far below that of fusion, or into chlorides bj roasting with chloride of sodium. As some of these substances (for example, sulphide of read and copper pyrites) are readily fusible when first heated, but become more re-fractory as part of the sulphur is dissipated and oxygen takes its place, it is important that the heat should be very carefully regulated at first, otherwise the mass may become clotted or fritted together, and the oxidizing effect of the air soon ceases unless the fritted masses be broken small again. This is generally done by making the bed of the furnace very long in proportion to its breadth and to the fire grate area, which may be the more easily done as a not inconsiderable amount of heat is given out during the oxida-tion of the ore,—such increased length being often obtained by placing two or even three working beds one above the other, and allowing the flame to pass over them in order from below upwards. Such calciners are used especially in roasting zinc blende into oxide of zinc, and in the con-version of sulphides of copper into chlorides in the wet ex-traction process. In some processes of lead smelting, where the minerals treated contain sand, the long calciner is pro-vided with a melting bottom close to the fire-place, so that the desulphurized ore leaves the furnace as a glassy slag or silicate, which is subsequently reduced to the metallic state by fusion with fluxes in blast furnaces.

Muffle Furnaces.—A third class of furnaces are so arranged that the work is done by indirect heating; that is, the material under treatment, whether subjected to calcina-tion, fusion, or any other process, is not brought in contact either with fuel or flame, but is raised to the proper tem-perature by exposure in a chamber heated externally by the products of combustion. These are known as muffle or chamber furnaces; and by supposing the crucibles or retorts to represent similar chambers of only temporary duration, the | ordinary pot melting air furnaces, and those for the reduc-tion of zinc ores or the manufacture of coal gas, may be in eluded in the same category. These are almost invariably air furnaces, though sometimes air under pressure is used ; as for example in the combustion of small anthracitic coal, where a current of air from a fan-blower is sometimes blown under the grate to promote combustion.

Crucible Melting Furnaces.—Figs. 7 and 8 represent a series of one of the simplest furnaces of this class, the ordinary crucible air furnace, or pot melting hole, according to the Sheffield term used in cast-steel works. It is a chamber of brick-work a, either straight in the sides or more generally some-what barrel-shaped, with a grate at the bottom, and of sufficient capacity to hold one or two crucibles b, containing from 50 to 70 lb weight of steel each, enough room being left around them for coke to bring the con-tents up to the melting point in five or six hours. The crucibles are supported upon disks, or "cheeses" of fire-brick, placed upon the grate; the draught is Jg-maintained by a chimney c about 30 feet high, communicating FIG- 7.— Crucible Melting Fur-with the furnace by a short flue nace (longitudinal section), near the top of the latter. The furnace is placed nearly or quite level with the floor of the casting shop /, and covered with a square fire tile or quarry g, set in an iron frame with a projecting handle, the fire grate being accessible from the cellar below.

Assayers' Muffle Furnace.—The construction of a muffle furnace as used by assayers has already been described in the article ASSAYING, vol. ii. p. 726. It is simply a small square air furnace with a D-shaped chamber of fire-clay fixed in the middle, so as to be surrounded with incandescent fuel, a current of air being drawn through it by a series of draught holes or slits in the roof or sides. Larger-sized muffles are used by enamellers and painters, and in the production of enamelled iron goods, as Flo. 8. -Crucible Melting Furnace well as for calcining minerals (transverse section), containing arsenic where it is to be collected for sale, and in the production of metallic colours where the material has to be kept free from the influence of flame and smoke.

Furnace Materials.—The materials used in the construction of furnaces are divisible into two classes, namely, ordinary, and refractory or fire-resisting. The former are used principally as casing, walls, pillars, or other supporting parts of the structure, and include ordinary red or yellow bricks, clay-slate, granite, and most building stones : while the latter are reserved for the parts immediately in contact with the fuel and flame, such as the lining of the fire-place, the arches, roof, and flues, the lower part if not the whole of the chimney lining in reverberatory furnaces, and the whole of the internal walls of blast furnaces. Among such substances are fire-clay and fire-bricks, certain sand-stones, silica in the form of ganister, and Dinas stone and bricks, ferric oxide and alumina, carbon (as coke and graphite), magnesia, lime and oxide of chromium,—their relative importance being indicated by their order, the last two or three indeta. being only of limited use.

The most essential point in good fire-clays, or in the bricks or other objects made from them, is the power of resisting fusion at the highest heat to which they may be exposed. This supposes them to be free from metallic oxides, forming easily fusible compounds with silica, such as lime or iron, the presence of the former even in comparatively small proportion being very detrimental. As clays they must be sufficiently plastic to be readily moulded, but at the same time possess sufficient stiffness not to contract too strongly in drying, whereby the objects produced would be liable to be warped or cracked before firing. In most cases, however, the latter tendency is guarded against, in making up the paste for moulding, by adding to the fresh clay a certain proportion of burnt material of the same kind, such as old bricks or potsherds, ground to a coarse powder. Coke dust or graphite is used for the same pur-pose in crucible making.

The most highly valued fire-clays are derived from the CoalMeasures. Amongthe chief localities are the neighbourhood of Stourbridge in Worcestershire and Stannington near Sheffield, which supply most of the materials for crucibles used in steel and brass melting, and the pots for glass houses; Newcastle-on-Tyne, and Glenboig near Glasgow, where heavy blast furnace and other fire-bricks, gas retorts, &c, are made in large quantities. Coarse-grained but very strong fire-bricks are also made of the waste of the china clay works at Lee Moor, Devon. See FIRE-CLAY.

In Belgium, the clay raised at Andenne is very largely used for making retorts for zinc furnaces, not for local use alone, as it is exported for the same purpose to England and other countries. The principal French fire-clays are derived from the Tertiary strata in the south, and more nearly resemble porcelain clays thau those of the Coal Measures. They give wares of remarkably fine texture and surface, combined with high refractory character. The principal centre of manufacture is in Paris, where small crucibles, tubes, furnaces, and other articles for the use of assaying and chemical laboratories, as well as for gold and silver refining, are produced in large quantities.

In Germany, Ips and Passau on the Danube, and Gross Almerode in Hesse, are the best known localities producing fire-clay goods, the crucibles from the last-mentioned place, known as Hessian crucibles, going all over the world. These, though not showing a great resistance to extreme heat, are very slightly affected by sudden alternations in heating, as they may be plunged cold into a strongly heated furnace without cracking, a treatment to which French and Stourbridge pots cannot be subjected with safety. The Cornish crucibles used in copper assaying, made at Bedruth, are generally similar in quality and behaviour to the Hessian, but are not quite so rapidly perforated by corrosive fluxes.

Plumbago or graphite is largely used in the production of crucibles, not in the pure state but in admixture with fire-clay; the proportion of the former varies with the quality from 25 to nearly 50 per cent. These are the most enduring of all crucibles, the best lasting out 70 or 80 meltings in brass foundries, about 50 with bronze, and 8 to 10 in steel melting. The most important manufactory is that of the Patent Plumbago Crucible Company, Batter-sea, on the Thames, where the best Ceylon graphite is the basis of the composition employed. They are also made in all the principal crucible works of the continent of Europe and in the graphite-producing localities of Canada and the United States.

Silica is used in furnace-building in the forms of sand, ganister, a finely-ground sandstone from the Coal Measures of Yorkshire, and the analogous substance known as Dinas clay, which is really nearly pure silica, containing at most about 2 J per cent, of bases. Dinas clay is found at various places in the Vale of Neath in South Wales, in the form of a loose disintegrated sandstone, which is crushed between rollers, mixed with about 1 per cent, of lime, and moulded into bricks that are fired in kilns at a very high tempera-ture. These bricks are specially used for the roof, fire arches, and other parts subjected to intense heat in reverbera-tory steel melting furnaces, and, although infusible under ordinary conditions, are often fairly melted by the heat without fluxing or corrosion after a certain amount of ex-posure. Ganister, a slightly plastic siliceous sand, is simi-larly used for the lining of Bessemer steel converters; it is found in the neighbourhood of Sheffield.

Alumina as a refractory material is chiefly used in the form of an hydrated aluminous iron ore known as bauxete, found in the south of France, in Carniola, and in Antrim ; but its applications are somewhat special. It has been found to stand well for the linings of rotatory puddling furnaces, where, under long-continued heating, it changes into a substance as hard and infusible as natural emery. In the Paris Exhibition of 1878 bricks very hard and dense in character, said to be of pure alumina, were exhibited by Muller & Co. of Paris, as well as bricks of magnesia, the latter being specially remarkable for their great weight. They are intended for use at the extreme temperatures obtainable in steel fur-naces, or for the melting of platinum before the oxy-hydrogen blowpipe. For the latter purpose, however, lime is generally used ; but as this substance has only small stability, it is usually bedded in a casing of fire-brick. Fig. 9 is a section of a lime furnace as used for platinum melting. The flame of the gas jet is introduced through the hole at the top, and plays over the surface of the metal in the hollow below. Oxide of chromium and chromic iron ore have been proposed as refractory crucible materials by Andouin of Paris. The former may be used as a bed for melting platinum in the same way as lime or magnesia, without affecting the quality of the metal.

Ferric oxide, though not strictly infusible, is largely used as a protecting lining for furnaces in which malleable iron is made, a portion of the ore being reduced and recovered in the process. In an oxidizing atmosphere it is indifferent to silica, and therefore siliceous bricks contain-ing a considerable proportion of ferric oxide, when used in flues of boilers, brewers' coppers, &c, and similar situations, are perfectly fire-resisting so long as the heated gas con-tains a large proportion of unconsumed air. The red fire-bricks known as Windsor bricks, which are practically similar in composition to soft red sandstone, are of this character.

Furnace Construction.—In the construction of furnaces provision has to be made for the unequal expansion of the different parts under the effect of heat. This is especially necessary in the case of reverberatory furnaces, which are essentially weak structures, and therefore require to be bound together by complicated systems of tie rods and uprights or buck staves. The latter are very commonly made of old flat bottom rails, laid with the flat of the flange against the wall. Puddling furnaces are usually entirely cased with iron plates, and blast furnaces with hoops round each course of the stack, or in those of thinner constructions the fire-brick work is entirely enclosed in a wrought iron casing or jacket. Such parts as may be subjected to extreme heat and the fretting action of molten material, as the tuyere and slag breasts of blast furnaces, and the fire bridges and bed plates of reverberatory furnaces, are often made in cast iron with double walls, a current of water or air being kept circulating through the intermediate space. In this way the metal, owing to its high con-ductivity and low specific heat as compared to that of water, is kept at a temperature far below its melting point if the water is renewed quickly enough. It is of course necessary in such cases that the circulation shall be perfectly free, in order to prevent the accumulation of steam under pressure in the interior of the casting. This method has received con-siderable extension of late years, notably in furnace-smelting of iron ores containing manganese, where the entire hearth is often completely water-cased, and in some lead furnaces wdiere no fire-brick lining is used, the lower part of the furnace stack being a mere double iron box cooled by water sufficiently to keep a coating of slag adhering to the inner shell which prevents the metal from being acted upon.

Furnaces with special Methods of Firing.—In the examples hitherto noticed, the use of solid fuel has alone been con-sidered, whether in admixture with the charge in blast furnaces or burnt upon a grate in reverberatory furnaces. In either ease the useful heating effect realized is considerably below that indicated as possible by theory, and for the same reason, namely, that the carbon factor of the fuel is to a considerable extent only partially oxidized, producing carbonic oxide CO, instead of carbonic acid C02, as it should do if the combustion were complete. This is attended with great loss of heat, unless steps be taken to ensure the subsequent combustion of the carbonic oxide, by bringing it into contact with more air at an appropriate temperature. The production of carbonic oxide is a necessary consequence where coal is used in large masses, the carbonic acid in the gases resulting from complete combustion at any spot being reduced more or less completely to carbonic oxide by contact with the ignited carbon immediately adjacent. To obtain the most perfect combustion it is, therefore, essential that the layer of fuel upon the bars in a grate fire should be as thin as is consistent with preventing the passage of an undue amount of air, which is attended with a strong cooling effect. This condition is, however, only possible in such furnaces as require an oxidizing atmosphere, as, for example, boiler fires and the different form3 of calciners.

Coal-Dust Furnace.—A special method of providing more intimate contact between air and fuel has been adopted in a furnace designed by Mr T. R. Crampton, who grinds the whole of the coal to a fine powder in a flour mill, and propels a current of coal dust and air, mixed in the right proportion for burning, into the combustion chamber representing the fire-place of an ordinary furnace, either by a fan blower or by chimney draught. In the special application of welding iron considerable economy has been obtained with this furnace over ordinary coal-firing, but its use has up to the present time been exceedingly limited.

Gas Furnaces.—A more general remedy has been found in what is known as gas-firing, where the whole of the fuel is of design first imperfectly burned, i.e., converted into car-bonic oxide or rather into a mixture of carbonic oxide and nitrogen, by reducing the supply of air through the grate to a minimum and completing the combustion on the fire bridge by a further supply of air introduced through special channels either at ordinary atmospheric temperature or artificially heated. In this case, the fire-place proper is replaced by a gas-producer or gazogene, which may either form one construction with the other parts of the furnace or be separated from them. Fig. 10 represents a gas-producer intended for heating retort furnaces in gas works. The coal is charged into a deep barrel-shaped stack a, terminated below by a small inclined grate b, with flat bars placed edge-wise, like the steps of a ladder or the laths of a Venetian blind, allowing sufficient air to pass between for a smothered combustion, the gases produced passing out by the large 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 flue and the strong draught in the latter. From the exceedingly poisonous nature of carbonic oxide it is of the utmost importance to prevent the issue of unburnt 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 prevent 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 on 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 advautage with very inferior fuels, such as peat, lignite, sawdust, &c., 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, such as those 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 underthree 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 engine by means of a central belt of spur gearing. Furnaces of this kind were first used in alkali works for the conversion of sulphate into carbonate of sodium in the process known as black ash fusion, but have since been applied to puddling in America and elsewhere by Danks and others; but for the latter purpose they are still to some extent in the experi-mental stage. As calciners they are used in tin mines and for the chlorination of silver ores.

Use of Heated Air.—The calorific intensity of fuelis found to be very considerably enhanced, if the combustion be ef-fected with air previously heated to any temperature between that of boiling water and a dull red heat, the same effect being observed both with solid and gaseous fuel. The latter, especially when brought to the burning point at a high temperature, produces a heat that can be resisted by the most refractory substances only, such as silica, alumina, and magnesia. This is attained in the regenerative furnace of Siemens, detailed consideration of which belongs more properly to the subject of iron.

Economy of Waste Heat and Gas.—In every system of artificial heating, the amount of heat usefully applied is but a small proportion of that developed by combustion. Even under the most advantageous application, that of evaporation of water in a steam boiler where the gases of the fire have to travel through a great length of flues bounded by thin iron surfaces of great heat-absorbing capacity, the tempe-rature of the current at the chimney is generally much above that required to maintain an active draught in the fire-place; and other tubes containing water, often in considerable numbers, forming the so-called fuel economizers, may often be interposed between the boiler and the chimney with marked advantage as regards saving of fuel. In re-verberatory and air furnaces used in the different operations of iron manufacture, where an extremely high temperature has to be maintained in spaces of comparatively small extent, such as the beds of puddling, welding, and steel-melting furnaces, the temperature of the exhaust gases is exceedingly high, and if allowed to pass directly into the chimney they appear as a great body of flame at the top, It is now general to save a portion of this heat by passing the flame through flues of steam boilers, air-heating apparatus, or both—so that the steam required for the necessary operations of the forge and heated blast for the furnace itself may be obtained without further expenditure of fuel. The most perfect method of utilizing the waste heat hitherto applied is that of the Siemens regenerator, in which the spent gases are made to travel through chambers, known as regenerators or recuperators of heat, containing a quantity of thin fire-bricks piled into a cellular mass so as to offer a very large heat-absorbing surface, whereby their tem-perature is very considerably reduced, and they arrive at the chimney at a heat not exceeding 300 or 400 degrees. As soon as the bricks have become red hot, the current is diverted to an adjacent chamber or pair of chambers, and the acquired heat is removed by a current of cool gas or air passing towards the furnace, where it arrives at a temperature sufficiently high to ensure the greatest possible heating effect in combustion. This system being alternative, four regene-rators, two for air and two for gas, are required for each furnace ; but in some of the newer French patterns of so-called recuperative furnaces, a system of tubular bricks is adopted in the chambers and only the air is heated, the gas being brought hot from the producer to the furnace instead of cooling it first by atmospheric exposure in a long tube in the manner adopted by Siemens. ' This allows a consider-able simplification in the apparatus; only a single regene-rator is required working continuously, the flame travelling outwards though one set of passages in the bricks and the air inwards through another; and as the former consists only of burnt gases, no explosion can take place if a communi-cation be established between the two currents through a leaking joint or broken brick.

In iron-smelting blast furnaces the waste gases, though not escaping at as high sensible temperatures as those of the fur-naces previously considered, are of considerable fuel value, and may render important services if properly applied. Owing to the conditions of the work, which require the maintenance of a sensibly reducing atmosphere, they contain a very notable proportion of carbonic oxide, and are drawn off by large wrought-iron tubes near the top of the furnace and conveyed by branch pipes to the different boilers and air-heating apparatus, which are now as a rule entirely heated by the combustion of such gases. Formerly they were allowed to burn to waste at the mouth of a short chimney place above the furnace top, forming a huge body of flame, which was one of the most striking features of the Black Country landscape at night, but is now less commonly seen than formerly. Perhaps the greatest number of flaming fur-naces to be seen at present are those of the Scotch founding iron district about Gartsherrie, Coatbridge, &c.

Figs. 11, 12, and 13 represent a modern furnace heated by gas burnt with hot air as applied to heating '

gas retorts. The retorts of fire clay r r r, seven in number, are mounted upon supports at both ends in bricks is adopted in the chambers and only the air is heated, the gas being brought hot from the producer to the furnace instead of cooling it first by atmospheric exposure in a long tube in the manner adopted by Siemens. ' This allows a consider-able simplification in the apparatus; only a single regene-rator is required working continuously, the flame travelling outwards though one set of passages in the bricks and the air inwards through another; and as the former consists only of burnt gases, no explosion can take place if a communi-cation be established between the two currents through a leaking joint or broken brick.

In iron-smelting blast furnaces the waste gases, though not escaping at as high sensible temperatures as those of the fur-naces previously considered, are of considerable fuel value, and may render important services if properly applied. Owing to the conditions of the work, which require the maintenance of a sensibly reducing atmosphere, they contain a very notable proportion of carbonic oxide, and are drawn off by large wrought-iron tubes near the top of the furnace and conveyed by branch pipes to the different boilers and air-heating apparatus, which are now as a rule entirely heated by the combustion of such gases. Formerly they were allowed to burn to waste at the mouth of a short chimney place above the furnace top, forming a huge body of flame, which was one of the most striking features of the Black Country landscape at night, but is now less commonly seen than formerly. Perhaps the greatest number of flaming furnaces to be seen at present are those of the Scotch founding iron district about Gartsherrie, Coatbridge, &c.

Figs. 11, 12, and 13 represent a modern furnace heated by gas burnt with hot air as applied to heating '

gas retorts. The retorts of fire clay r r r, seven in number, are mounted upon supports at both ends in an arched chamber having a long flue running along the centre line at the bottom, covered with bricks pierced at intervals by narrow slits a a a, which form the gas admission passages from the gas-producer flue /, the supply being regu-lated by a valve v. These alternate with similar slits b b of less depth, communicating with a lateral flue, supplying heated air ; the mixture being effected at a great number of points ensures uniformity of combustion along the whole length of the furnace. The flame, after heating the retorts, descends by passages under the ends of the side retorts in the lower series to a number of arched divisions in the substructure containing the air-heating pipes, which are of cast iron in horse-shoe coils. By this means the temperature of the gases is considerably reduced by the time they reach the chimney flue, the heat intercepted being returned by the air to the combustion chamber.

Laboratory and Portable Furnaces.—Small air-furnaces with hot plates or sand bath flues were formerly much employed in chemical laboratories, as well as small blast furnaces for crucibles heated with charcoal or coke. The use of such furnaces has very considerably diminished, owing to the general introduction of coal gas for heat-ing purposes in laboratories, which has been rendered possible by the invention of the Bunsen burner, in which the mixture of air and gas giving the least luminous but most powerfully heating flame is effected automatically by the effluent gas. These burners, or modifications of them, have also been applied to muffle furnaces, which are convenient when only a few assays have to be made —the furnace being a mere clay shell and soon brought to a working temperature ; but the fuel is too expensive to allow of their being used habitually or on a large scale. Petroleum, or rather the heavy oils obtained in tar refineries, having an equal or superior heating power to coal gas, may also be used in laboratories for producing high temperatures. The oil is introduced in a thin stream upon a series of inclined and channelled bars, where it is almost im-mediately volatilized and burnt by air flowing in through parallel orifices. Furnaces of this kind may be used for melting cast-iron or bronze in small quantities, and were employed by M. St Claire Deville in experiments in the metallurgy of the platinum group of metals.

Sefstrom's blast furnace, used in Sweden for the assay of iron ores, is one of the most convenient forms of portable furnaces applied to melting in crucibles. It consists of a sheet-iron cylinder about 8 or 9 inches in diameter, within which is fixed one of smaller size lined with fire-clay, as shown in part section in fig. 14. The space between the two cylinders serves as a heater and distributor for the blast, which is introduced through the nozzle at the bottom, and enters the furnace through a series of several small tuyeres arranged round the inner lining. Charcoal is the fuel used, and the crucibles stand upon the bottom of the clay lining. When a large body of fuel is required, the cylinder can be lengthened by an iron hoop which fits over the top ring. Deville's portable blast furnace is very similar in principle to the above, but the body of the furnace is formed of a single cast-iron cylinder lined with fire-clay, closed below by a cast-iron plate perforated by a ring of small holes—a hemispherical basin below forming the air-heating chamber.

The literature of furnaces is co-extensive with that of metallurgy. Most of the different patterns in use will be found described and fully illustrated in Percy's and Phillip's Metallurgy, Jordan's Album du Cours de Métallurgie, ko. The atlas to Karsten's great work, and the plates in the Encyclopédie Méthodique, are also of much interest, but the types of apparatus represented are chiefly antiquated and out of use. (H. B.)


In the figures fire-brick work is represented by closer, and casing walls in ordinary bricks by more open shading.

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