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Experimental Electric Furnaces

By JEROME S. MARCUS, B. Sc. (Chem. Eng.)

An electric furnace is an apparatus for the production of high temperatures by electricity. The advantages of such an apparatus are - the direct application of heat to the material, thus eliminating excessive losses by conduction thru the walls of a containing vessel; the production of high temperature, usually above those obtainable from fuel in common uses; simple and accurate regulation, giving absolute control of a process and an economical use of power; and finally, with sources of water-power, a low operating cost.

There are several types of electric furnaces in use. The general division are the Induction type, and the Resistance type. The purpose of this article is to give the experimenter the simple construction details and operating principles of these furnaces.

Before going further, it is well to inform the operator of any of these devices to watch his fuses, as many will be blown without the proper regulation of the rheostat in Fig. 1. The experimenter will find that a transformer is not necessary for a small arc furnace, but in the case of the resistance or induction types a higher voltage than the ordinary lighting current is required for good results.

The author has found the simplest rheostat to be of the water-barrel type. A wooden pail is first filled with strong salt water. A metal plate in the bottom is attached to one lead, which is well insulated; a piece of rubber hose over the wire is excellent. To this other lead is soldered a metal electrode of any sort. The distance between the plate and the electrode regulates the current; the closer they are the less the resistance. The experimenter may fit up a support for his adjustable electrode to suit his convenience. Fig. 2.

The arc furnace is by far the best for the amateur. It is the simplest and cheapest in construction, the easiest of operation and regulation, the most economical and is productive of higher temperature than the other types.

A furnace which will give practical results can be made from two blocks of slaked lime, hollowed out and grooved for two carbon electrodes, as shown. This apparatus can be run on the ordinary lighting current the same as an arc-light. To start the arc in operation the carbons are touched together and then drawn a small distance apart, giving a steady arc. The material to be melted is placed in the hollow beneath the arc. To stop the arc, the carbons are drawn far apart, thus breaking the arc. The use of a rheostat or "ballast" improves the steadiness of the arc. A transformer is not necessary as was stated before.

Since an electric arc between carbon wears down the positive electrode, adjustment is frequently necessary in order to maintain the flow of current. One carbon should therefore be made loose in order to feed it in as required.

The above apparatus is not one that the experimenter cares to have as a permanent part of his laboratory. Below is given a description of a highly efficient furnace with which any experiment can be readily performed.

A clay flower pot is drilled to permit the carbon to pass thru, and is lined with fire clay or lime. The carbons are attached to wooden blocks, as shown. The pot is set on a board base with a circle of asbestos beneath. The adjustment of the carbon is made by means of threaded bolts moving the blocks in grooves. A single pole, single throw-knife switch may be mounted on the base. A clay cover is placed over the pot when in operation. The details are shown in Figs. 4 A, B and C. One inch wood is used for blocks and base.

A three-electrode arc furnace is shown in Fig. 4-D. The positive electrode only needs to be adjusted to keep the arc in the center of the chamber.

The adjustment of the electrodes is eliminated if water cooled metal ones are utilized in place of the carbon. These are more expensive to make, however, and unless carefully made soon come apart. A copper disc is welded or brazed on the end of a copper or brass tube. The cooling water is introduced thru a small metal pipe, see Fig. 5. An electrode can be made of iron pipe with a cap screwed on the end but is less efficient, owing to the high resistance of the iron.

The resistance type of furnace depends on the resistance offered by the material to an electric current, for its source of heat. This form of furnace is the one used in the manufacture of carborundum, the smelting of ores, especially aluminum and in the refining of zinc.

The simplest resistance furnace consists of a flower pot or other clay container, in which the material is placed around two carbon electrodes, as shown in Fig. 6. Very often it is necessary to place a little granulated carbon between the electrodes to start the flow of current.

Another type of resistance furnace utilizes the container as one electrode. A graphite crucible is generally used, altho an iron pot lined with fire clay in which a large amount of carbon has been mixed, may be used for the lower temperatures.

Only one connection is then made to the carbon electrodes, of which there may be one or several, depending on the size of the furnace, the other connection being made to the container itself. Higher voltages than 110 are best for this type of furnace, the amperage varying with the resistance offered by the material. The carborundum furnace at Niagara Falls runs on potentials as high as 22,000 volts.

The induction furnace (Fig. 8) is not a practical one for the experimenter. It requires some of the molten material to start it, and due to the high reactance resulting from the distance between the primary and secondary is uneconomical for a laboratory device. The induction furnace operates as a transformer, the secondary winding in this case being the "charge," which is contained in the circular channel A, and is heated by the secondary current. The amount of energy put into the secondary can be varied by varying the applied primary voltage.

It is hoped that these few notes will prove useful to the electrical experimenter. There is a large number of unsolved problems concerning the behavior of various substances at high temperatures yet to be worked out, and the results of some experimenter's research may be, for all we can tell, of great commercial or scientific value.

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