US1952143A - Electric resistance furnace - Google Patents

Description

March 27, 1934. F. L. PRESTON ELECTRIC RESISTANCE FURNACE Original Filed Oct. 20. 1930 2 Sheets-Sheet 1 March 27, 1934. F. L. PRESTON ELECTRIC RESISTANCE FURNACE Original Filed Oct. 20. 1930 2 Sheets-Sheet 2 .surrounding the electrodes.

crucible and to arcing -white hot and Patented Mar. 27, 1934 PATENT OFFICE ELECTRIC RESISTANCE FURNACE Frederick L. Preston, Mobile, Ala.

Application October 20, 1930, Serial No. 489,723 Renewed August 28, 1933 11 Claims.

This invention relates to improvements in electric resistance furnaces and generally to improvements in the construction of a furnace whereby very high voltage current may be used to produce high temperatures within the reaction chamber without appreciable injury or deterioration of the electrodes or other parts of the furnace.

Electric furnaces as commonly constructed are limited as to the temperature which may be consistently maintained by the burning of the electrodes or by other improper operation of the furnace which can be prevented by improved in- 'sulation.

It is the purpose of this invention to so construct an electric furnace that relatively long electrodes extending into the furnace chamber are protected against oxidation by tightly packing around the electrodes an insulating mass of finely divided material whereas the reaction .chamber which is supported between the elecby a mass of coarse mateupon the passage of the trodes is surrounded rial, thus resulting,

current, in a high resistance field surrounding the reaction chamber and low resistance fields Preferably I intend to form my electrodes of solid graphite and surround them by finely-divided graphite while the reaction chamber comprises a graphite crucible .30.

surrounded by a packing of coarse coke. Upon the passage of a relatively high current, heat is developed in the reaction chamber due both to the relatively high resistance path around the within the crucible. The high temperature causes the coke to become the heat developed in the high resistance field is absorbed in the low resistance fields so that the supported ends of the electrodes do not attain a temperature that will cause appreciable damage. The furnace chamber is tightly sealed and forms a substantially air tight fich'amber so that the tightly packed mass around the electrodes protects the electrodes against burning.

'It is further a purpose of my invention to so construct the furnace chamber by coating the 1. side of the refractory walls v inner surface of the refractory walls of the chamher with a mixture consisting of powdered graphite and a binder, so that the high heat developed within the reaction chamber will not cause appreciable injury to the furnace walls. The outare preferably covered with asbestos to protect the supporting frame work.

It is another purpose of my invention to so form the reaction chamber of a pair of crucibles one of which is inverted and supported upon the top of the other that the chamber is made substantially air tight and is maintained in tight sealing relation with the furnace walls. Thus I support an upright crucible on a block positioned on the bottom wall and seat an inverted crucible upon the top of the first mentioned crucible, the seating edges of the two crucibles being suitably ground or fitted to form a tight seal. The bottom of the inverted crucible is removed to form a pouring opening which may be closed by a conical cap. It will be apparent that the curvature of the walls of the inverted crucible allows it to be forced into tight sealing relation with the opening in the top furnace wall by suitably adjusting the height of the supporting block for the lower crucible.

Another feature of my invention consists in the mounting of the furnace in such a manner 15 that it may be tilted by manual or power operation in order to discharge the contents from the reaction chamber and to provide means whereby, upon the tilting of the furnace, the electrical connections to the furnace are simultaneously 30 broken.

Additional features and advantages of my improved construction will be more readily apparent as the description proceeds in connection with the attached drawings in which one em-- bodiment of an electric resistance furnace including the improved features is illustrated.

in the drawings:

Figure l is a top plan View of the furnace showing the connections to the transformer box.

Figure 2 is a front elevation of the furnace.

Figure 3 is a vertical end elevation taken from the right hand end of Figure 1.

Figure 4 is a sectional elevation taken longitudinally through the furnace.

Figure 5 is a detail section of the spring con nection for one of the bus bars.

With reference to the drawings, 1 is a base frame supporting a pair of end stands 2 and 3 which include trunnion bearings 4 and 5 formed .100 at the upper ends of the stands and adapted to pivotally support a saddle 6 having end stub shafts Z and 8. The saddle 6 serves as a swinging support for the furnace generally denoted as 9. The furnace 9, as shown, is of a box-like 105 rectangular shape but it will be apparent that other forms may be utilized without departing from the spirit of my invention. The walls of the furnace are secured by framing 10 which encloses the walls of refractory fire brick 11. Ad- 110 jacent the furnace is supported a transformer 12 which has output connections 13 and 14. Extending from the furnace are a pair of copper bus bars 15 and 16 which carry the current to the furnace electrodes. The ends of the bus bars 15 and 16 are adapted to engage the spring connections secured to the transformer output connections when the furnace is in operative position. An upwardly extending handle 19 is shown in Figure 2 which serves as means for tilting the furnace. It will be apparent that the tilting movement of the furnace will serve to break the connections between the bus bars 15 and 16 and the spring connections 17 and 18. The furnace may, if so desired, be tilted by power means.

A pair of electrodes preferably cylindrical in form are supported longitudinally and co-axially within the furnace. The electrodes extend through intermediate end walls 40, 41 into open spaces 42, 43. The bus bars 15 and 16 extend into the spaces 42, 43 and are attached to the electrodes by U-shaped straps 44 which are bolted to the bus bars and extend around the bus bars. By this construction the electrodes may be readily adjusted for the desired spacing between their ends. The electrodes are preferably formed of solid graphite.

Centrally within the furnace and spaced from the ends of the electrodes is supported a crucible 24 also preferably composed of graphite. A solid preferably graphite block support 25 may be utilized resting upon the bottom wall of the furnace to hold the crucible 2a in place. Upon the top of the crucible 2a is mounted an inverted crucible 26 which has the bottom removed to form a pouring opening 27 normally closed by an air tight refractory plug 28. The meeting edges 29 of the crucibles are suitably ground or fitted to provide a tight seal. It should be noted that the narrowing curvature of the upper crucible pro vides a tight fit for the curved opening 31 of the upper Wall of the furnace, thus the upper crucible cannot slip out and, by suitably adjusting the spacing block 25 both crucibles will be tightly held in place.

The entire outer surface of the fire brick 11 is preferably covered by asbestos sheeting 32 which also extends through the opening 31 between the crucible 26 and the fire brick 11. The inner surface of the fire brick 11 is painted with a plurality of coats of a mixture of powdered graphite and a suitable binder which provides a covering protection for the fire brick and sub stantially seals the furnace against the admission of air. This protecting paint or coating is indicated by the heavy line 33 shown in Figure 4. The remainder of the interior of the furnace is filled with suitable filling material which will serve as heat insulation and resistance material. Generally the filling of an electric furnace consists of loose coke which is packed as tightly as possible around the electrodes and reaction chamber, but it is my intention to provide an improved filling which will protect the electrodes and, at the same time, give greater efiiciency by allowing higher temperatures to be attained without injury to the apparatus. Thus entirely surrounding the electrodes at the ends of the furnace is a compact mass of powdered graphite, indicated at 30. This filling material is carried a little past the end face of the electrodes. The central portion of the furnace surrounding the crucibles or reaction chamber is packed with coarse coke 34. Thus the central portion of the furnace provides a high resistance field whereas the electrodes are surrounded with low resistance fields upon the passage of the electric current.

It may be desired to allow the escape of the occluded air which is expanded by the heat developed and a vent plug 35 is shown in Figure 1 which extends into the coke mass and may be removed during the initial heating of the furnace. After all possible air has been forced out, the plug may be replaced to maintain a slight vacuum within the furnace chamber.

It has been found especially desirable to use relatively long electrodes, a greater dissipation of heat is permitted and the supported ends do not reach excessively high temperatures. In the construction of an electric furnace, such as I have described, I have used a current as high as 3600 amperes and 12 volts which results in a temperature within a graphite crucible of substantially 4000 C. It is found that the heat is developed not only by the passage of the current between the electrodes through the high resistance path around the crucible 4, but also that there is an arcing between the walls within the interior of the crucible. Even with this high temperature sustained for several hours, it is still possible to touch the outside of the furnace without burning the hand.

It is my intention to include within the scope of my invention all modifications which may occur to those skilled in the art and which fall within the spirit of my invention as expressed in the appended claims.

I claim:

1. In an electric resistance furnace, a tightly sealed furnace chamber, electrodes extending into said furnace chamber for a substantial distance, a reaction chamber between said electrodes, powdered resistance material tightly packed around said electrodes to form low resistance fields, and coarse resistance material surrounding said reaction chamber to form a high resistance field.

2. In an electric resistance furnace a tightly sealed furnace chamber, graphite electrodes extending into said furnace chamber for a substantial distance, a reaction chamber between said electrodes, powdered graphite tightly packed around said electrodes to form low resistance fields and coarse coke material packed around said reaction chamber to form a central high resistance field.

3. In an electric resistance furnace, a tightly sealed furnace chamber, cylindrical graphite electrodes axially extending into said furnace chamber for a substantial distance, a reaction chamber between said electrodes, powdered graphite tightly packed around said electrodes to form low resistance fields and coarse coke material packed around said reaction chamber to form a central high resistance field.

4. In an electric resistance furnace, a tightly sealed furnace chamber, comprising refractory walls coated on the inner side with a mixture of powdered graphite and a binder, cylindrical graphite electrodes extending within said iur-' nace chamber and facing each other, a reaction chamber supported between the ends of said electrodes, resistance material filling the interior of said furnace chamber surrounding said electrodes and said reaction chamber, said filling material comprising powdered graphite surrounding the electrodes and coarse coke surrounding the reaction chamber. I

5. In an electric resistance furnace, a tightly sealed furnace chamber comprising refractorywalls covered on the outside with asbestos and on the interior by a layer of powdered graphite and a binder, electrodes supported by the end walls of said furnace chamber and extending towards the central portion thereof, a reaction chamber supported between said electrodes comprising an upright crucible and an inverted crucible supported by the upper edge of said upright crucible, said inverted crucible extending through the upper wall of said chamber and being tightly sealed against the outwardly converging open- 6. In an electric resistance furnace, the combination of a tightly sealed chamber, oppositely arranged electrodes within said chamber, a reaction chamber arranged between said electrodes in such a manner that upon the passage of a current of sufficiently high voltage an arc will form within said reaction chamber, resistance fields within said sealed. chamber comprising fields of low resistance surrounding the electrodes, and a field of high resistance surrounding said reaction chamber.

'7. In an electric furnace, a pair of spaced electrodes, a crucible positioned between said electrodes and adapted to receive the charge, an inverted graphite crucible positioned on top of said first mentioned crucible, a removable closure in the bottom of the inverted crucible and means for tilting said furnace whereby the charge may be poured through the opening in the bottom of said inverted crucible.

8. In an electric resistance furnace, a tightly sealed furnace chamber, electrodes extending coaxially into said chamber through the end walls thereof, a crucible positioned within said chamber between the opposed ends of said electrodes, finely powdered resistance material tightly packed around each of said electrodes within said chamber completely surrounding each of said electrodes in planes transverse thereto and covering the end face of said electrodes and a body of coarse resistance material tightly packed around said crucible and between the bodies of resistance material surrounding said electrodes whereby said finely powdered resistance material constitutes fields of low resistance around the electrodes to absorb heat therefrom and said coarse resistance material constitutes a higher resistance field around the crucible.

9. In an electric resistance furnace, a tightly sealed furnace chamber, a pair of electrodes extending through the end walls of said chamber into the interior thereof, each of said electrodes being extended within said chamber a distance greater than the cross sectional dimensions of the electrodes, a reaction chamber positioned within said furnace chamber between the opposed ends of said electrodes, a tightly packed body of finely powdered resistance material surrounding each of said electrodes within each of said chambers and covering the end faces thereof, and a tightly packed body of relatively coarse resistance material around said reaction chamber and between the ends of said electrodes.

10. In an electric resistance furnace, a tightly sealed furnace chamber, a pair of graphite electrodes extending through the end walls of said chamber into the interior thereof, each of said electrodes being extended within said chamber a distance greater than the cross sectional dimensions of the electrode, a graphite crucible positioned within said chamber between the opposed ends of said electrodes, a tightly packed body of finely powdered resistance material surrounding each of said electrodes within said chamber and covering the end faces thereof and a tightly packed body of relatively coarse resistance material around said crucible and between the ends of said electrodes.

11. In an electric resistance furnace, a tightly sealed furnace chamber, electrodes supported by the end walls of said chamber and extending towards the center portion thereof, a reaction chamber supported in said chamber comprising an upright crucible and an inverted crucible supported by the upper edge of said upright crucible, said inverted crucible extending through the upper wall of said chamber and being tightly sealed against the upwardly converging opening.

FREDERICK L. PRESTON.

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