Electric Furnace

Electric Furnace, electrically heated device used industrially for melting metals or firing ceramics. It is also known as an electrothermic furnace.

The simplest type of electric furnace is the resistance furnace, in which heat is generated by passing a current through a resistance element surrounding the furnace or by utilizing the resistance of the material being heated. The heating element in an externally heated furnace may take the form of a coil of metal wire wound around a tube of refractory material or it may be a tube of metal or other resistive material such as carborundum. Resistance furnaces are particularly useful in applications in which a small furnace, with precisely controlled temperatures, is needed. Small resistance furnaces are widely used in laboratories and in shops for the heat treatment of tools. Larger furnaces are used for firing ceramics and melting brass. The highest temperature at which resistance furnaces are operated, for example, in the manufacture of graphite, is in the neighborhood of 4100° C (7366° F).

The electric-arc furnace is the most widely used type of electric furnace for the production of quality alloy steels and range in capacity from 227 kg (500 lb) to 181 metric tons. In these furnaces the heat is generated by an arc struck between the metal being heated and one or more electrodes suspended above the metal. A typical form of arc furnace has three electrodes, fed by a three-phase power supply, giving three heating arcs. The electrodes are made either of graphite or of carbon.

A more recently developed type of electric furnace is the induction furnace, consisting of a crucible in which a metallic charge is heated by eddy currents induced magnetically. Around the crucible is wound a coil through which high-frequency alternating currents are passed. The magnetic field of this coil sets up eddy currents in the metal in the crucible. Induction furnaces have a number of advantages, chief among them being the speed at which metal can be melted. At comparatively low frequencies the induced eddy currents exert a stirring action on the molten metal. Because the higher frequencies are the most effective for heating, some induction furnaces have two coils, one for high-frequency current and one for low-frequency. The earlier types of induction furnaces operated at frequencies between 60 and 60,000 cycles per second, but some modern furnaces are designed to use frequencies of 1 million cycles or more per second.

A special type of furnace, called an electrolytic furnace, is used in the production of aluminum, magnesium, and sodium. In the electrolytic furnace, a salt is fused by the heat generated by the passage of a large electric current and is at the same time electrolyzed so that the pure metal is deposited at one electrode.

Date: 2015-01-02; view: 1378