US2349678A - Electric salt bath furnace - Google Patents

Description

May 23, 1944.

H. A. ROLNICK ELECTRIC SALT BATH FURNACE 2 Sheets-Sheet 1 Filed July 17, 1943 4 INVENTOR. fiQ/PRYA- Ram/wan May 23, 1944.

H. A. ROLNICK ELECTRIC sum BATH rummcn Filed July 1'7, 1943.. 2 Sheets-Sheet 2 uvmvrox A #04 zv/c/r ATTORNEY Patented May 23, 1944 ELECTRIC SAL-T BATH FURNACE Harry A. Rolnick, Philadelphia, Pa., assignor to Rolnick Testing & Manufacturing 00., Philadelphia, Pa.

Application July 17, 1943, Serial No. 495,132

4 Claims.

This invention relates to electrically heated salt bath furnaces which are being increasingly used, particularly for the hardening of metals such as high speed steel and aluminum. The reason for the increasing use of these furnaces for this purpose resides in the fact that the molten salt when heated by electrical current is found to form a very desirable heat transmitting medium. A major problem has arisen, however, in the practical employment of such furnaces, which is the maintenance of uniform temperature throughout the bath. This is a fundamental requisite of a heating furnace which is to be practical and successful, as will be understood when it is stated that at a temperature of 920 F. a variation in excess of ma make the difference between success and failure in the heating of aluminum parts.

Heretofore the maintenance of such uniform temperature has not been achieved by the method of heat introduction generally employed. This method consisted of introducing current into the salt bath by means of a pair of electrodes extending from the top of the furnace downwardly into the bath. An example of the results achieved by this arrangement is as follows: For hardening aluminum alloys I employed a salt bath furnace 8 long, 5 deep, and 3' wide, with standard electrodes arranged in the standard manner, that is, the electrodes extended from the top of the furnace downwardly into the bath along one side of the furnace. I then determined by measurement that the temperature at the top of the bath was 920 F., the temperature 2' below the top was 910 F., the temperature 3 below the top was 870 F., while at the bottom of the bath the tem' perature was 750 F. Such a temperature differential from top to bottom of the salt bath is highly objectionable because the temperature at which the hardening of aluminum is carried out must range between 910 F. and 930 F.

It is therefore the principal object of my invention to provide an electric salt bath furnace wherein substantial uniformity of temperature is obtained throughout the bath. For this purpose I take maximum advantage of the natural convection currents which are caused when molten salts with a lower density rise and are replaced by the colder salt possessing a higher density. To accomplish the foregoing result I provide an arrangement of electrodes whereby substantially all of the heat in the bath is generated closely adjacent the bottom. By this arrangement I obtain in the same furnace hereinbefore described, the remarkable temperature uniformity within 4 F. from top to bottom of the salt bath as against the variation of F. with the stand-. ard arrangement of standard parts.

Further objects and advantages of this invention will become apparent in the following de tailed description thereof:

In the accompanying drawings,

Fig. 1 is an isometric projection, with parts broken away, of an electric furnace embodying one form of my invention.

Fig. 2 is a section taken substantially on the line 22 of Fig. l.

Fig. 3 is a view similar to Fig. l, but showing a modified form of electrode.

Referring to the drawings, there is shown an.

electric salt bath furnace comprising a chamber H), which in Fig. 1 has two of the walls broken away for the purpose of disclosing the interior thereof. The chamber, which may be made entirely of ceramic, diatomaceous, or other earth materials which are heat resisting, is adapted to contain the melt consisting of a mixture of salts which fuses at the desired high temperature to form a molten mass or bath. The heat for melting the salts and maintaining the bath at the desired temperature is provided by electrodes indicated generally at H and I2, and extending downwardly into the bath. One or more pairs of electrodes may be employed. The electrodes are connected at their upper ends to a suitable source of alternating current. Heat is generated mainly along the adjacent edges of the electrodes, and the quantity of heat generated depends upon the distance between these edges, the length of the edges, and the current density.

I-Ieretofore, as described in the introduction hereto, the electrodes were placed with their adjacent edges relatively close together, and they extended downwardly to a predetermined distance above the bottom of the bath. The net result was a temperature gradient which was higher at the top and lower at the bottom, and in the instance cited hereinbefore, amounted to a 170 F. differential. As a result, the lighter salt bath mixture was formed at the top and the denser mixture at the bottom.

My invention operates on a principle exactly opposite to that described above, and as a result, I am enabled to obtain a salt bath mixture where the lighter elements constantly tend to form at the bottom, thus tending to rise and be replaced by the denser mixture from the top. A maximum utilization of convection currents is thus effected, which results in remarkable uniformity of temperature. In actual figures, a bath which does not vary more than :4" F. throughout its entire volume has'been obtained in the same furnace which heretofore was subject to a 170 F. variation under the same operating conditions,

For carrying out the above novel principle of my invention, I cause substantially all of the heat in the bah to be generated closely adjacent, and substantially parallel, to the bottom of the bath. 1 can achieve this result in several ways. As shown in Fig. 1, I may form the electrodes of the usually downwardly extending portions I4 and I5, but instead of terminating the electrodes at the bottom of these portions, I provide the downwardly extending portions I4 and I5 with substantially horizontal extensions I6 and I! adjacent and substantially parallel to the bottom of the bath. The downwardly extending portions I4 and I5 are positioned as far apart as possible so that'relatively little current will passbetween them, while the horizontal extensions I6 and I! are so formed that their adjacent edges I9 and 20 are relatively close to carry substantially all of the current. Under these conditions practically all of the heat will be generated closely adjacent the bottom. The lighter heated mass will rise and be replaced continuously by the cooler, heavier mass from thetop, thus producing maximum convection currents, insuring uniformity of temperature throughout the bath. Such uniformity is the highly desired goal in all salt bath furnaces.

In a modified form of my invention I may secure the. same result by insulating the downwardly extending portions of the electrodes so that current can pass only between the adjacent edges of the horizontal portions of the electrodes. In this formof the invention it would not be necessary to space the downwardly extending portions I4 and I5 as far apart as shown in Fig. 1, if such spacing were not otherwise desired.

In the form of the invention where the downwardlyextending portions are not insulated there will be a tendency for a relatively small amount of current to pass between these portions. To compensate for this tendency, I may form the horizontal portions of the electrodes as shown in Fig. 3, at I6, and IT, with the parts of the horizontal portions farthest away from the downwardly-extending portions being positioned closest together and spreading apart as they approach the downwardly-extending portions. This will give a toeing-in effect so that more current will pass and more heat will be increasingly generated adjacent the bottom the further the distance from the downwardly-extending portions.

v In' accordance with the provisions of the patent statutes, I have herein described the principle and operation of my invention, together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative, and that the invention can be carried out by other equivalent means. Also, while it ments in the combination and relations described, some of these may be altered and others omitted without interfering with the more general results outlined, and the invention extends to such use.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. An electric salt bath ,furnace comprising a plurality of electrodes for passing current through the bath and adapted to be connected to a source of current supply, said electrodes having portions extending downwardly toward the bottom of the bath and having portions connected to said first portions and extending in a single plane substantially parallel to the bottom of the bath, said last named portions being positioned with their adjacent edges relatively close togenerate large currents and produce vigorous thermal circulation.

2. An electric salt bath furnace comprising a plurality of electrodes for passing current through the bath and adapted to be connected to a source of current supply, said electrodes having portions extending downwardly toward the bottom of the bath and having portions connected to said first portions and extending in a single plane substantially parallel to the bottom of the bath and substantially the full width of the bath, said last named portions being positioned with their adjacent edges relatively close to generate large currents and produce vigorous thermal circulation.

3. An electric salt bath furnace comprising a plurality of electrodes for passing current is designed to use the various features and elethrough the bath and adapted to be connected to a source of current supply, said electrodes having portions extending downwardly toward the bottom of the bath and having portions connected to said first portions and extending in a single plane substantially parallel to the bottom of the bath, said last named portions being positioned with their adjacent edges relatively close to generate large currents and produce vigorous thermal circulation, the electrodes being so formed that the adjacent edges of the laterally- I extending portions are closer to each other than the adjacent edges of the downwardly-extending portions.

4. An electric salt bath furnace comprising a plurality of electrodes for passing current through the bath and adapted to be connected to a source of current supply, said electrodes having portions extending downwardly toward the bottom of the bath and having portions connected to said first portions and extending in a single plane substantially parallel to the bottom of the bath and substantially the full width of the bath, said last named portions being positioned with their adjacent edges relatively close to generate large currents and produce vigorous thermal circulation, said adjacent edges being positioned closer together the, further their distance from the downwardly-extending portions.

HARRY A. ROLNICK.

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