Resistance Furnace

(also electric resistance furnace), an electric furnace in which heat is generated by conductors that offer resistance to the passage of a current through them. Resistance furnaces are widely used in heat treatment, for heating prior to pressure shaping, and to dry or melt materials.

Resistance furnaces are used extensively because of their numerous advantages. Any temperature up to 3000°C can be obtained in the furnace chamber. Articles can be uniformly heated either by appropriately locating the heating elements along the walls of the furnace chamber or by means of forced circulation of the furnace atmosphere. Automatic control of the power and, consequently, of the temperature conditions in such a furnace is easily implemented. Resistance furnaces are readily mechanized and automated, thus alleviating the work of personnel and facilitating the inclusion of such furnaces in automatic transfer lines. In addition, they are compact. Such furnaces provide a secure environment for various processes. A resistance furnace may be well sealed, in which case the heating is carried out in a vacuum, or it may contain either a gaseous medium that prevents oxidation or a special atmosphere for chemical case hardening, for example, for carburization or nitriding.

Most resistance furnaces are of the indirect type. In indirect-heat resistance furnaces, electric energy is converted into heat when a current flows through the heating elements. The heat is transmitted to the articles to be heated by radiation, convection, or conduction. Such a furnace consists of a working chamber formed by a lining composed of a layer of firebrick that supports both the articles to be heated and the heating elements and that is insulated from a metal casing by a layer of heat insulation (Figure 1). The parts and mechanisms that operate in the chamber, as well as the heating elements, are made of heat-resistant steels, refractory steels, or other refractory materials.

Figure 1. Diagram of a batch-type indirect-heat resistance chamber furnace: (1) heating elements, (2) refractory lining, (3) heat insulation, (4) refractory hearth plate

Continuous furnaces are used to heat large lots of identical parts. In such furnaces, articles move continuously from one end to the other. As compared with other types of furnaces, the output of continuous furnaces is higher, the heating of articles is more uniform, and the power consumption is lower. As a rule, such furnaces are highly mechanized.

In both batch-type and continuous resistance furnaces with a working temperature of up to 700°C, forced circulation of the furnace gases is widely used. The gases are circulated by fans or blowers that either are installed in the furnace or are located, together with the heating elements, outside the furnace in electric heaters.

Indirect-heat resistance furnaces that are used to melt fusible metals—for example, lead, babbitt metal, aluminum alloys, or manganese alloys—are built either as crucible furnaces with a metal crucible and an external heater or as reverberatory furnaces with a bath and, above the bath, heating elements in the roof.

Laboratory resistance furnaces include small pipe furnaces, muffle furnaces, and chamber furnaces, as well as thermostats (constant-temperature chambers) and drying ovens.

Figure 2. Schematic diagram of a direct-heat resistance furnace: (1) article to be heated, (2) step-down transformer, (3) and (4) contacts; (P) power

In direct-heat resistance furnaces, an article—for example, a rod or a tube—is heated directly by a current that passes through it (Figure 2). Direct heating makes it possible to concentrate a large amount of power in the article and provides for very rapid heating, on a scale of seconds or fractions of a minute.

Almost all industrial and laboratory resistance furnaces have automatic temperature control.