furnace, enclosed space for the burning of fuel. There are many kinds of furnaces, the type depending upon the fuel and the use to which the heat produced within it is put. Most familiar are the furnaces used in the heating of buildings. In the hot-air furnace, fuel is burned within an inner wall and air, led into a space between the inner and the outer wall, is heated and is led away to the various rooms of the building. Hot-water furnaces, by which water is heated to be led through pipes to radiators, and furnaces that turn water to steam for heating purposes are common. The kiln is a kind of furnace. In metallurgy, the separation of many metals from their ores is accomplished by the use of various kinds of furnaces, e.g., the blast furnace and the reverberatory furnace. The structure of these furnaces makes possible a good control of temperature. In the production of steel, however, the open-hearth furnace and the Bessemer converter are used in the treatment of cast iron. The electric furnace is extensively employed in the production of high-grade steels for use in making steel alloys and for the manufacture of high-speed tools. Heat may be generated in such a furnace by using an electric arc or by sending an electric current through resistive elements in the furnace. If the material to be processed is electrically conductive, heat may also be generated by creating an electric current in the material by induction or by inserting into it electrodes to which a voltage is applied. In the preparation of phosphorus from calcium phosphate, this compound of phosphorus is mixed with sand and coke and treated in an electric furnace. An electric current is sent from one electrode to another through the mass to create the extremely high temperature needed to bring about the chemical action that results in the production of free phosphorus. Graphite is produced from coal or coke in an electric furnace, and the extremely hard substance carborundum is made there by the combination of carbon and silicon (from sand). Nitrogen is obtained from the air (in the Birkeland-Eyde process) by passing a stream of air through an arc. The nitrogen and oxygen of the air combine to form nitric oxide.

electric furnace: see furnace.

blast furnace, structure used chiefly in smelting. The principle involved in this means of extracting metals is that of the reduction of the ores by the action of carbon monoxide, i.e., the removal of oxygen from the metal oxide in order to obtain the metal. Blast furnaces differ in construction. The one used in the production of iron consists of a chimneylike structure (usually 80-100 ft/24-30 m high) made of iron or steel and lined with firebrick. It is narrow at the top, increasing in diameter downward, but narrowing again suddenly almost at the bottom, to form the hearth or crucible. There the fine molten products are caught. The furnace is fed from the top with a charge of definite quantities of ore, coke, and a flux, mostly limestone. Preheated compressed air is introduced at the bottom through pipes (tuyères) entering just above the hearth. The air passes upward through the charge. The coke is oxidized to carbon dioxide, which changes to carbon monoxide at the high temperature. The carbon monoxide then reduces the ores and, taking on oxygen, reverts to carbon dioxide. This gas, together with unused carbon monoxide, nitrogen, and other constituents of the air originally introduced, is led off through a pipe from the top of the furnace and, being still at a high temperature, is employed to heat the stoves into which fresh air for the process is brought. As the operation proceeds, the mass in the furnace becomes molten and descends into the crucible. The iron sinks to the bottom; impurities, called the slag, being lighter, float on top. The slag is drained through a pipe in the upper portion of the crucible. The iron is tapped from below and run into sand molds to harden. The product is known as pig iron or cast iron (see iron). Efforts to increase production rates have led to the addition of pure oxygen and steam and the sizing of ore to obtain better gas-solid contact. Flux and ore are sometimes combined into pellets. Pig iron prepared in the blast furnace is converted into steel by the Bessemer process. Copper ore treated in a blast furnace yields a copper matte, from which only a part of the impurities are removed. It is usually further refined by electrolytic methods (see copper).

Hopewell Furnace National Historic Site: see National Parks and Monuments (table).

Furnace used for smelting, refining, or melting in which the fuel is not in direct contact with the contents but heats it by a flame blown over it from another chamber. Such furnaces are used in copper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum recycling. In steelmaking, this process (now largely obsolete) is called the open-hearth process. The heat passes over the hearth and then radiates back (reverberates) onto the contents. The roof is arched, with the highest point over the firebox. It slopes downward toward a bridge of flues that deflects the flame so that it reverberates.

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Chamber heated with electricity to very high temperatures, for melting and alloying metals and refractories. Modern electric furnaces generally are either arc furnaces or induction furnaces. Arc furnaces produce roughly two-fifths of the steel made in the U.S. In the induction furnace, a coil carrying alternating electric current surrounds the container or chamber of metal; circulating eddy currents induced in the metal produce extremely high temperatures.

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Type of electric furnace in which heat is generated by an arc between carbon electrodes above the surface of the material (commonly a metal) being heated. William Siemens first demonstrated the arc furnace in 1879 at the Paris Exposition by melting iron in crucibles; horizontally placed carbon electrodes produced an electric arc above the container of metal. The first commercial arc furnace in the U.S. (1906) had a capacity of four tons (3.6 metric tons) and was equipped with two electrodes. Modern furnaces range in heat size from a few tons up to 400 tons (360 metric tons), and the arcs strike directly into the metal bath from vertically positioned, graphite electrodes to remelt scrap steel or refine briquettes of direct-reduced iron ore.

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Vertical shaft furnace that produces liquid metals by the reaction of air introduced under pressure into the bottom of the furnace with a mixture of metallic ore, fuel, and flux fed into the top. Blast furnaces are used to produce pig iron from iron ore for subsequent processing into steel; they are also employed in processing lead, copper, and other metals. The current of pressurized air maintains rapid combustion. Blast furnaces were used in China as early as 200 BC, and appeared in Europe in the 13th century, replacing the bloomery process. Modern blast furnaces are 70–120 ft (20–35 m) high, have 20–45-ft (6–14-m) hearth diameters, use coke fuel, and can produce 1,000–10,000 tons (900–9,000 metric tons) of pig iron daily. Seealso metallurgy, smelting.

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