Furnaces, Electric

Abstract

Abstract The term electric furnace applies to all furnaces that use electrical energy as their sole source of heat. Electric furnaces are used for heating solid materials to desired temperatures below their melting points for subsequent processing, or melting materials for subsequent casting into desired shapes. Classification is by the manner in which the electrical energy is converted into heat. Three types of furnaces are widely used in industry: electric resistance furnaces, arc furnaces, and electric induction furnaces. The most widely used and best known resistance furnaces are indirect‐heat resistance furnaces or electric resistor furnaces. They are categorized by a combination of four factors: batch or continuous; protective atmosphere or air atmosphere; method of heat transfer; and operating temperature. The primary method of heat transfer in an electric furnace is usually a function of the operating temperature range. The three methods of heat transfer are radiation, convection, and conduction. Radiation and convection apply to all of the furnaces described. Conductive heat transfer is limited to special types of furnaces. Operating temperature ranges are classified as low, medium, and high; there is no standard or precise definition of these ranges. Arc furnaces used in electric melting, smelting, and electrochemical operations are of two basic designs; indirect and direct arc. The arc of the indirect‐arc furnace is maintained between two electrodes and radiates heat to the charge. The arcs of the direct‐arc furnace are maintained between the charge and the electrodes, making the charge a part of the electrical power circuit. Not only is heat radiated to the charge, but the charge is heated directly by the arc and the current passing through the charge. Direct‐arc furnaces include open‐arc furnaces, d‐c arc furnaces; submerged arc‐furnaces; and arc‐resistance furnaces. All new furnace installations require pollution control equipment. This normally consists of off‐gas afterburning (sometimes with energy recovery), and dust collection equipment, typically a baghouse. Induction furnaces utilize the phenomena of electromagnetic induction to produce an electric current in the load or workpiece. This current is a result of a varying magnetic field created by an alternating current in a coil that typically surrounds the workpiece. Power to heat the load results from the passage of the electric current through the resistance of the load. Physical contact between the electric system and the material to be heated is not essential and is usually avoided. Nonconducting materials cannot be heated directly by induction fields. The efficiency of an induction furnace installation is determined by the ratio of the load useful power to the input power drawn from the utility. Losses that must be considered include those in the power converter transmission lines, coil electrical losses, and thermal loss from the furnace. A unique capability of induction heating is apparent in its ability to heat the surface of a part to a high temperature while the interior remains at room temperature. Electric furnaces are used for annealing, brazing, carburizing, galvanizing, forging, hardening, melting, sintering, enameling, and tempering metals, most notably aluminum, copper, iron and steel, and magnesium alloys.