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FURNACE

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Originally appearing in Volume V11, Page 359 of the 1911 Encyclopedia Britannica.
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FURNACE , a contrivance for the See also:

production and utilization of See also:heat by the See also:combustion of See also:fuel. The word is See also:common to all the See also:Romance See also:tongues, appearing in more or less modified forms of the Latin fornax. But in all those See also:languages the word has a more extended meaning than in See also:English, as it covers every variety of See also:heating apparatus; while here, in addition to furnaces proper, we distinguish other varieties as ovens, stoves and kilns. The first of these, in the See also:form Ofen, is used in See also:German as a See also:general See also:term like the See also:French 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 See also:close chamber. Our bakers' ovens, hot-See also:air ovens or stoves, See also:annealing ovens for See also:glass or See also:metal, &c., would all be called fours in French and Ofen in German, in common with furnaces of all kinds. See also:Stove, an See also:equivalent of See also:oven, is from the German Stube, i.e. a heated See also:room, and is commonly so understood; but is also applied to open See also:fire-places, which appears to be somewhat of a departure from the See also:original signification. Furnaces are constructed according to many different patterns with varying degrees of complexity in arrangement; but all may be considered as combining three essential parts, namely,the fire-See also:place in which the fuel is consumed, the heated chamber, laboratory, See also:hearth or working See also:bed, as it is variously called, where the heat is applied to the See also:special See also:work for which the furnace is designed, and the apparatus for producing rapid combustion by the See also: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 See also:smith's forge, where the fire-place and heating chamber are See also:united, the See also:iron being placed among the coals, only the air for burning being supplied under pressure from a blowing See also:engine by a second special contrivance, the tuyere, tuiron, twyer or blast-See also:pipe; but in the more refined See also:modern furnaces, where See also:great See also:economy of fuel is an See also:object, the different functions are distributed over See also:separate and distinct apparatus, the fuel being converted into See also: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. Furnaces may be classified according as the products of combustion are employed (I) only for heating purposes, or (2) both for heating and bringing about some chemical See also:change. The furnaces employed for See also:steam-raising or for heating buildings are invariably of the first type (see See also:BOILER and HEATING), while those employed in metallurgy are generally of the second. The essential difference in construction is that in the first class the substances heated do not come into contact with either the fuel or the furnace gases, whereas in the second they do. Metallurgical furnaces of the first class are termed crucible, muffle or See also:retort furnaces, and of the second See also:shaft and reverberatory furnaces.

The following is a detailed subdivision: (I) Fuel and substance in contact. (a) Height of furnace greater than See also:

diameter =shaft furnaces. (a) No blast =kilns. (i3) With blast =blast furnaces. (b) Height not much greater than diameter =hearth furnaces. (2) Substance heated by products of combustion=reverberatory furnaces. (a) See also:Charge not melted =roasting or calcining furnaces. (b) Charge melted melting furnaces. (3) Substance is not directly heated by the fuel or by the products of combustion. (a) Heating chamber fixed and forming See also:part of furnace= muffle furnaces. (b) Crucible furnaces. (c) Retort furnaces.

Another See also:

classification may be based upon the nature of the heating See also:agent, according as it is See also:coal (or some similar combustible) oil, gas or See also:electricity. In this See also:article the general principles of metallurgical furnaces will be treated; the subject of gas- and oil-heated furnaces is treated in the article FUEL, and of the electric furnace in the article See also:ELECTROMETALLURGY. For special furnaces reference should he made to the articles on the See also:industry concerned, e.g. GLASS, Gas, § Manufacture, &c. Shaft, Blast and Hearth Furnaces.—The blast furnace in its simplest form is among the See also:oldest, if not the oldest, of metallurgical contrivances. In the old See also:copper-smelting See also:district of See also:Arabia Petraea, See also:clay blast-pipes dating back to the earlier dynasties of See also:ancient See also:Egypt have been found buried in slag heaps; and in See also:India the native smiths and iron-workers continue to use furnaces of similar types. These, when reduced to their most See also:simple expression, are See also:mere See also:basin-shaped hollows in the ground, containing ignited See also: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 See also:bellows at or near the See also:top. They are essentially the same as the smith's forge. This class of furnace is usually known as an open fire or hearth, and is represented in a more advanced See also:stage of development by the Catalan, German and Walloon forges formerly used in the production of malleable iron. Fig. 1 represents a Catalan forge. The cavity in the ground is represented by a See also:pit of square or rectangular See also:section lined with See also:brick or See also:stone of a See also:kind not readily acted on by heat, about 11 or 2 ft. deep, usually somewhat 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 iron-smelting the ore is laid in a heap upon the fuel (charcoal) filling up the hearth, and is gradually brought to the metallic See also:

state by the reducing See also:action of the See also:carbon monoxide formed at the tuyere. The metal sinks through the ignited fuel, forming, in the hearth, a spongy See also:mass or See also:ball, which is lifted out by the smelters at the end of each operation, and carried to the forge See also:hammer. The earthy matters form a fusible glass or slag melt, and collect at the lowest point of the nearrn, wnence they are removed ally representing its class. The fire-place A is divided from the by opening a hole pierced through the front See also:wall at the bottom. working bed B by a See also:low wall C known as the fire See also:bridge, and at the The active portion of such a furnace is essentially that above the opposite end there is sometimes, though not invariably, a second blast-pipe, the See also:function of the See also:lower part being merely the collection bridge of less height called the flue bridge D. A See also:short See also:diagonal. flue of the reduced metal; the fire may therefore be regarded as burning in an unconfined space, with the See also:waste of a large amount of its heating See also: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 loading it into the stack, alternately with layers of fuel, the charge descending regularly to the point of combustion, and absorbing a See also:pro-portion of the heat of the See also:flame that went to waste in the open fire. This principle is capable of very wide See also:extension, the blast furnace being mainly limited in height by the strength the See also:column or up-take E conveys the current of spent flame to the See also:chimney of materials or " See also:burden " has to F, which is of square section, diminishing by steps at two or three resist crushing, under the See also:weight due to the See also:head adopted, and the different heights, and provided at the top with a covering See also:plate or power of the blowing engine to supply blast of sufficient See also:density to overcome the resistance of the closely packed materials to the See also:free passage of the spent gases. The consuming power of the furnace or the See also:rate at which it can See also:burn the fuel supplied is measured by the number of tuyeres and their section. The development of blast furnaces is practically the development of iron-smelting. The See also:profile has been very much varied at different times. The earliest examples were square or rectangular in See also:horizontal section, but the general tendency of modern practice is to substitute See also:round sections, their construction being facilitated by the use of specially moulded bricks which have entirely superseded the See also:sandstone blocks formerly used. The See also:vertical section, on the other See also:hand, is subject to considerable variation according to the work to which the furnace is applied.

Where the operation is simply one of See also:

fusion, as in the iron-founder's See also:cupola, in which there is no very great change in See also:volume in the materials on their descent to the tuyeres, the stack is nearly or quite straight-sided; but when, as is the See also:case with the smelting of iron ores with See also:limestone See also:flux, a large proportion of volatile See also:matter has to be removed in the See also:process, a wall of varying inclination is used, so that the See also:body of the furnace is formed of two dissimilar truncated cones, joined by their bases, the lower one passing downwards into a short, nearly cylindrical, position.

End of Article: FURNACE

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