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.

Operating furnaces on dry blast

Dried blast, or dry blast, as it is commonly known, is the practice of controlling, at a predetermined point or level, the water vapour content of air which serves a blast furnace or cupola. It means reducing the water vapour content in summer months, but may include the reverse, that is, raising the water vapour content of the blast, during the winter months, up to that same predetermined level. From another angle, it is the practice of eliminating, from furnace operation, throughout the 12 months, both the fast moving and long-term variables which otherwise are introduced by variations in weather. The main advantage of such a process is control of furnace temperature which is made possible by the fact that part of the water vapour is broken down into hydrogen and oxygen, and that the heat of dissociation is absorbed in the melting zone.

Fundamentally, dry blast is a tool for eliminating one of the variables in blast furnace operation and can be made to serve two different purposes: to drive for maximum production and to secure fuel economy, smoothness of operation, uniformity of product, and low cost.

It gives five major advantages:

1. The operation of other equipment is simplified by its feeding air to the blowers at a constant temperature and a constant humidity. With that type of plant, there is no need of continually, hour by hour, readjusting blower speed to make it supply the blast furnace with a constant weight of oxygen.

2. The cost of maintenance is reduced by feeding clean air to the blowers.

3. With respect to increased production, constant humidity permits driving the furnace safely up to the limit of other variables or factors. Sudden changes in humidity cause sudden changes in the temperature of critical furnace zones. Without dry blast, there must be held a reserve for meeting sudden changes in humidity. But when this danger is eliminated, all this reserve can be safely applied to the production of more tonnage.

4. With respect to fuel economy it is known that, as the humidity of furnace blast increases, more carbon must be burned to produce a ton of iron.

5. With respect to a more uniform product, regularity and smoothness of operation reduce off-grades both as to specification and temperature. In the case of mill iron, the advantages of uniformity extend on through the Bessemer and the open-hearth. In the case of foundry iron, the furnace can be changed quickly and with certainty from one specification to another, it can reduce off-grade product and supply iron of more precise specifications.