US1841537A - Electric furnace resistor - Google Patents

Description

-lllllllllllllll il m m L Fll Jan. 19, 1932. J. KELLEHER ELECTRIC FURNACE RESISTOR Filed Nov. 27, 1925 I avwmwtoz Jame; hflllakar 3511 W 511 1014404 4 *Bowoc Patented Jan. 19, 1932 UNITED STATES PATENT OFFICE JAMES KELLEHER, OF CHIPPAWA, ONTAEBJO, CANADA, ASSIGNOR TO HARPER ELECTRIC FURNACE CORPORATION, A. CQRYORATION OF NEW YORK ELECTRIC FURNACE RESISTOR Application filed November 27, 1925. Serial No. 71,516.

This invention relates to a resistor adapted for use as a heating element of an electric furnace, and to the method of forming same.

Among the objects of my invention is to produce a resistor of this character which is highly eflicient in operation, strong and durable in use, and which may be operated at a high temperature without deterioration, and Without the necessity of enclosing the same in an air tight compartment to prevent oxidation.

In accordance with my invention, I employ a carbonaceous resistance element around which is moulded refractory material, which is inert with respect to the resistance element, and which has high heat conductivity. The refractory body, preferably, not only encloses the resistance element but also encloses a portion of the conductor terminals which are connected to the resistance element. While the refractory body encloses the resistance element, special provision is made to prevent breaking of the resistance element or the enclosing sheath 5 due to differences in expansion of the resistance element and sheath when both are heated. Enclosing the refractory body is a coating of fused material which forms a seal to prevent access of gas through the refractory body to the carbonaceous resistance element, and in this manner prevents oxidation of the carbonaceous material when the resistor is operated in an oxidizing atmosphere.

In the preferred form of my invention, the

refractory material is formed of silicon carbide, recrystallized in whole or in part, and the outer sealing coating is formed of fused borax.

In making up the resistor, a graphite or other carbonaceous resistance element of the zigzag type or other suitable form is employed. The usual conducting terminals are secured to the resistance element and the element and terminals are then coated with a 46 readily fusable or volatilizable material such as paraffin wax. The assembled coated resistance element and terminals are then placed in a mould which conforms to the outside shape of the finished body. The space between the. assembled resistor and mould is then rammed with the body refractory. Any one of several mixtures may be employed. One such mixture is composed of sand and coke in such proportions that, when the resistor is connected to a source of electrical energy to heat it to a suffioiently high temperature, silicon carbide is formed. Again the so-called carborundum firesand which is silicon carbide in the amorphous state, may be used, and this material, when heated to a sufficiently high temperature, will crystallize forming silicon carbide. I prefer, however, to use a body refractor'y mixture composed of crushed silicon carbide grains because less time is occupied in preparing the resistor body for use once the resistance element and terminals have been moulded into the body. \Vhen crushed silicon carbide grains are employed, I preferably employ a mixture of coarse and fine grains because such mixture moulds better into the shape desired.

T o whatever mixture employed, I add 1% to 15% by weight of borax, and then add enough Water so as to make the mixture of such consistency that it can readily be rammed into the mould. The borax content depends upon the use for which the resistor is designed. The thickness of the body surrounding the resistance element and terminals depends upon the use for which the resistor is designed, and this thickness may vary between a very thin layer such as to 1 or even thicker.

Having finished moulding the body around the resistance element and terminals, the

mould is removed from around the body and the whole is then dried out by passing current through the resistor. As the body dries out, the energy dissipated in the resistor is increased until the whole has attained a white heat on the outside.

Upon cooling, the borax will be found to have melted forming a gas tight film on the outside of the body, and when the mixture, composed of crushed silicon carbide grains has been employed, it will be found on transverse fracture that the silicon carbide next to the resistor is recrystallized forming a solid or monolithic recrystallized silicon carbide resistor sheath. Although recrystallization is not essential, it is preferable. During the heating operation, the wax coating on the resistance element will have melted and the wax seeped through the body of the sheath leaving a space between the resistance element and the silicon carbide sheath. This space will eliminate any danger of cracking the sheath or resistance element, due to difference in the expansion of the resistance element and sheath when the resistor is heated in use. As the silicon carbide sheath is an excellent conductor of heat, it permits the heat to be transmitted from the resistor to the furnace chamber with a minimum loss.

Another method of making one of these resistors complete with its case or protection, is to mould the silicon carbide around the coated resistance element and terminals as described above, but without the addition of borax. Instead of borax, glue or someother temporary bond is mixed with the silicon carbide. The mould is then removed and the resistor is dried out at a temperature just suflicicnt to drive off the moisture used in moulding the body. When sufficiently dried, the whole is then placed in a furnace and surrounded b a mixture of sand, coke and sawdust, simi ar to that used in making silicon carbide. The terminals are then connected to a source of electrical energy and the resistor is run at sufiicient energy consumption and length of time to recrystallize part or the whole of the body. When the furnace is sufficiently cool, so as to permit the handling of the resistor and terminals as a unit, it is removed and any adherent silicon carbide mixture is scraped off and the body cleaned. A solution of borax and water is then poured over the body. The body being porous, absorbs the solution readily. The proper amount of borax being added in this manner the moisture is again driven off by heating carefully and slowly. When dried the resistor is connected as before, to a source of electrical energy until a white heat has been attained, and then allowed to cool off. The resistor is now ready to be placed in any furnace adapted for its use.

. An advantage of this type of resistor is that no special chamber need be built for the resistor so as to exclude oxygen. The resistor may be operated in an atmosphere which is highly oxidizing without deterioration. Furthermore, due to the strength of the heatshows partly in elevation ing unit, comprising the resistorand termi nals, it may be placed in any convenient position in the furnace. Electrical connection to the resistor may be made outside the furnace and in case of failure or fracture, the resistance element, terminals and refractory may be removed as a unit and readily replaced by a spare unit.

By the use of the zigzag type of carbona ceous heating element, it is possible to have a resistor of large energy dissipating qualities, confined to a small space thus permitting high furnace temperatures, which may be obtained in a very short space of time. It will, however, be understood that if desired, other forms of carbonaceous resistance elements ma be used.

Iii the accompanying drawings, Fig. 1

and partly in central longitudinal section, a heating unit constructed in accordance with one form of my invention, and Fig. 2 shows a fragmentary transverse section of this unit taken on the line 2-2 of Fig. 1. In the drawing, 10 representsa zigzag graphite or other carbonaceous resistance element to which is secured in any suitable manner conducting terminals 11. The resistance element and terminals are embedded or incased in the refractory body material 14, which, as shown, envelops the entire outer surface of the resistance element. It does not penetrate the slots in the zigzag element, and is slightly spaced from the outer surface of the resistance element and terminals as is indicated at 13 except at the outer ends of the terminals 11 where the sheath is in direct contact with the surface of the terminals. The outer surface of the refractory material 14 is sealed by a coating or film 15 of fused borax. While the drawing shows the borax as forming a film on the outside of the surface of the refractory material, it will be understood that borax may penetrate to some extent the refractory material serving to fill the pores or interstices in the surface of the. refractory body.

I claim:

1. A resistor comprising a carbonaceous resistance element incased in a monolithic mass of silicon carbide, said mass being slightly spaced from the surface of said resistance element so as to independently expand and contract with changes in temperature.

2. A heater unit comprising a carbonaceous resistance element, a silicon carbide sheath enclosing said element, said sheath and element being positioned with a slight space therebetween so as to be free to expand and contract independently of one another.

8. A heating element consisting of a carbonaceous resistance element, terminals secured to said element and a monolithic silicon carbide sheath enclosing said element and terminals and being separate from said resistance element so as to be free to expand and contract independently of said element and terminals.

4. A resistor comprising a carbonaceous resistance element incased in a monolithic mass of silicon carbide, the silicon carbide next to the surface of the resistance element being recrystallized, said mass of silicon carbide being separate from said resistance element so as to be free to expand and contract independently of said resistance element.

5. A carbon resistance element, a silicon carbide sheath enclosing the same and spaced therefrom sufficiently to permit relative expansion and contraction of said element and sheath and fused borax at the outer surface of said sheath for rendering the sheath impervious to gas.

6. A heating element consisting of a carbon resistance element, conducting terminals secured thereto and a monolithic mass of silicon carbide forming a sheath enclosing said element and terminals, said mass being supported in contact with the terminals near their outer ends and being spaced from other portions of said terminals and from said resistance element sufficiently to permit independent relative expansion and contraction of said mass and said element and terminals.

7. A heating element consisting of a carbon resistance element, conducting terminals secured thereto, a monolithic mass of silicon carbide forming a sheath enclosing said element and terminals, said mass being supported in contact with the terminals near their outer ends and being spaced from other portions of said terminals and from said resistance element sufficiently to permit independent relative expansion and contraction of said mass and said element and terminals and fused borax at the outer surface of said sheath for rendering said sheath impervious to gas.

8. A method of forming a resistor which consists in coating a carbon resistance element with a readily fusible material, moulding around the coated element a mixture of crushed silicon carbide grains and a temporary binder, slowly heating the resistance element by the passage of current therethrou h bide grains are converted into a monolithic mass of recrystallized silicon carbide, which is in contact with the outer 'ortions of the terminals and is spaced sufiiciently from the inner portions of said terminals and from the resistance element so as to permit relative expansion of said terminals and element and said mass of recr stallized silicon carbide, and whereby said orax forms a sealing medium at the outer surface of said mass of recrystallized silicon carbide.

JAMES KELLEHER.

until the entire mass has attained a whlte v heat.

9. A method of forming a heating element which consists in connecting conducting terminals to a zigzag graphite resistance element, coating said resistance element and the inner portions of said terminals with a read- I ily fusible material without filling the slots between sections of the resistance element, moulding around said coated element and terminals a mixture of coarse and fine silicon carbide grains together with a binder com-' prising borax, slowly heating the mass to a white heat by passing current through the resistance element whereby the silicon car-

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