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HIGH FREQUENCY FURNACES
The three main types of electric melting furnaces are resistance, arc, and induction. Resistance furnaces are very simple, but high temperatures cannot be obtained. Arc furnaces contaminate the charge with carbon from the electrodes and volatile elements may be lost from the melt. Induction furnaces may be constructed with or without a magnetic core.
The induction furnaces operate with a current that may be generated by an alternating current (a. c.) generator, which has a relatively low fixed frequency, or by an electronic tube, where a triode tube functions as the source of the alternating current.
Electric tube induction furnaces have many advantages for small charges, especially of expansive metals. The frequency may be as high as several million cycles per second and is easily regulated. An adjustment of the circuit when the charge melts is not so important as it is with furnaces using an a. c. generator. Melting is very rapid. The operation of these furnaces is very flexible and uniform. The crucible is deep to minimize oxidation. Moreover, the melting can be conducted in vacuum or under a controlled atmosphere if desired.
Since the heat is generated in the metal, there is no danger of carburizing the melt. The equipment is easy to maintain as there are no moving parts. The crucibles are cheap and have a long life. However, the equipment is more expensive than an a. c. generator and is justified only for small charges.
One of the most powerful furnaces of this type has the following characteristics. It is a 2-crucible, 250 kw. installation of the electronic tube type. The nominal capacity is 110 lbs., but actually 287 lb. may be melted. The average melting times are 21 min. for 192 lb. of steel and 32 min. for 287 lb. However, some heats are much shorter. Each pound of steel melted requires 0.57 kwh. (kilowatt-hour). With 176 lb. heats of steel, each crucible will last about 100 heats. Particular attention is given to the simplicity of operation and to the safety provisions.