furnace(redirected from electric heater)
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a structure in which heat, evolved as a result of the combustion of fuel or the conversion of electrical energy, is used for the thermal processing of materials and workpieces or for heating. Steam boilers, agglomerators, and certain electrical heating devices and other heating units (for example, a catalytic reaction furnace), which essentially meet the above definition have not traditionally been considered as furnaces. On the other hand, complex units for thermal or thermochemical treatment of metal items, in which the item is not only heated and maintained at a given temperature but also cooled (in a different section of the unit) to a certain temperature at a controlled rate, are called furnaces. The stack annealing furnace is an example of such a complex furnace unit.
In addition to their many industrial uses, furnaces are used in homes for heating and, in the form of stoves, for baking and other kinds of cooking.
The major parts of a furnace are the heat generator, which is a section of the furnace or a device in which heat is evolved; the working chamber, which holds the material or workpieces to be processed; and the cooling chamber (in units for thermal or thermochemical processing), for cooling the items. Processing furnaces must also have devices for fuel or electrical power supply and outlets for the combustion products, as well as feed and transport mechanisms to move items through the furnace and then to remove them. Other major components include systems for automatic control of the furnace operation, structural members (base, supportive framework, servicing platforms), and devices such as recuperators and regenerators for using the heat of the fuel combustion products.
Depending on the type of furnace, the heat generators and the cooling chambers may be structurally separate from the working chamber, as is the case with furnaces that have external fireboxes. However, in most modern furnaces, these sections are integrated with the working chamber. In some furnaces, such as induction furnaces, the heat is generated in the heated item itself. Heating furnaces have no working chamber at all, as the heat evolved in these furnaces is transferred to the heated space.
Furnaces are extremely varied in design and operation, using all types of fuel, electrical power, and solar energy. The operating temperature in furnaces may range from 100° to 5000°C. There is no generally accepted classification of furnaces. Furnaces may be divided into the two broad classes of industrial and lighter-duty furnaces.
Industrial furnaces may be classified according to their function. Furnaces for the removal of moisture are divided into drying furnaces for solid materials (lumber, molds, hides, and ceramic items) and evaporation furnaces, or evaporators, for solutions. Evaporators are used in the food-processing and chemical industries. Heating furnaces are designed for heating materials or workpieces without changing their state of aggregation. Furnaces of this type are used, for example, to heat metal prior to pressure shaping in metallurgy and machine building. Heat-treating furnaces are used for imparting new mechanical properties to materials and workpieces. They are used, for example, for the thermal and thermochemical treatment of rolled products in metallurgy and for processing items in machine building. Roasting furnaces are used in processing mineral raw materials (ores, clay, and limestone) in order to change the structure and chemical composition of these minerals prior to subsequent treatment. Roasting furnaces are also used for roasting items made of these raw materials, including construction and fire-resistant materials, clay and porcelain goods, and enameled metal items.
Smelting furnaces of various types are used for converting materials to the liquid state by heating them above their melting point. Thus, water-jacketed furnaces are used to produce metals from ores; open-hearth and two-bath furnaces are used in the smelting of steel and nonferrous metals in foundry production, and for fusing various materials. Furnaces for the decomposition and sublimation of materials, usually with changes in the state of aggregation, are used in the dry distillation of fuels for producing coke and charcoal. (The furnace that produces coke is called a coke oven.) They are also used in the sublimation of volatile metals, the cracking of petroleum, and many processes in the chemical industry. Included among the classes of lighter-duty furnaces is the large group of heating stoves. Many lighter-duty stoves, and especially the larger varieties of, for example, baking and confectionery ovens, are essentially industrial furnaces, and they may also be found under this classification.
Based on the method of heating, a distinction is made between fired and electrical furnaces. Fired furnaces are heated by some type of fuel. They are divided into direct-heating and indirect-heating. In the former (continuous and shaft furnaces), fuel combustion products come into contact with the heated material or workpiece. In the latter, heat from the combustion process is transferred to the heated material or workpiece through a wall of radiant tubes of muffles or, in several cases, through liquids (tank furnace).
Electric furnaces are divided into resistance furnaces, in which electric current is passed either through the item itself or through electrical heating resistances located in the furnace; induction furnaces; and arc furnaces, in which heat is evolved in an electric arc.
A distinct group is formed by furnaces with special types of heating, such as plasma-arc furnaces, electron-beam furnaces, and optical furnaces (including solar furnaces).
Industrial furnaces are also classified according to their heat exchange processes. A distinction is made between furnaces with radiant heat exchange (for example, open-hearth and continuous furnaces), furnaces is which heat exchange occurs mainly through convection (low-temperature heaters and driers), and furnaces with a layer mode of operation, in which the material being processed is a fixed or movable layer (shaft and fluidized-bed furnaces). In furnaces that process materials in layers, radiation and convection are usually of equal importance.
Furnaces are also classified according to such other features as the method of moving the heated items (conveyor furnace, roller hearth furnace, pusher furnace, drawing furnace, and walking hearth furnace) and design features (rotary furnace, Hoffman rotary-ring furnace, and bell furnace). They are also classified according to modes of heating and temperature control (continuous furnace, chamber furnace, and reverberatory, indirect heating furnace). Some furnaces, such as the Martin open-hearth furnace and the Siemens furnace, are known by the names of their inventors.
REFERENCESGlinkov, M. A. Osnovy obshchei teorii pechei, 2nd ed. Moscow, 1962.
Spravochnik konstruktora pechei prokatnogo proizvodstva. Edited by V. M. Tymchak. Moscow, 1970.
V. M. TYMCHAK