US2268691A - Electrically heated furnace - Google Patents

Description

Patented Jan. 6, 19,42

ELECTRICALLY HEATED FURNACE Frank W. Brooke, Pittsburgh, Pa., assignor to Swindell-Dressler Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application March 20, 1939, Serial No. 262,860

Claims. (Cl. 13-20) The vgeneral object of the present invention is to provide an electrical heated furnace; and particularly a metallurgical furnace, adapted-for use under conditions requiring a relatively high temperature and high furnace heat supply rates, and

making it highly iniportant to maintain a furnace chamber atmosphere which is non-oxidiz- V ing. There is -need for such-a furnace, for example, in theproduction of so-called oxygen-freev copper, by a process in which copper is-melted and'rened in a furnace chamber in which a reducing atmosphere is maintained.

A specific object of the ypresent invention is to provide furnace heating means comprising hollow porous electric resistors of refractory material, such as silicon carbide, Alundum, chromite, and carbon in amorphous. semiamorphous or graphitic forms, and to maintain said resistors Vin operative condition and to control atmospheric conditions within the furnace chamber, by continuously passing into the hollow resistors a reducing gas including a carbon constituent which will crack .at high temperatures, whereby the l ducing gas envelope about each resistor.

A specic object of the present invention is to providefor the use in a furnace of electrical heating elements comprising heating resistors in 3 the form of such cylinders of amorphous or semiamorphous carbon, varying in diameter from six inches to 24' inches or so, and ranging in length up to 72 inches, as are ow commercially available in large quantities, and at a relatively low 4 cost, for use v as the electrodes of electric arc furnaces, employed for various metallurgical pur- Poses.

Amore specific object of the present invention ,is to provide simple and effective means for graphite which have a specific resistance much 55 lower than that of the electrode material. and which are mounted in opposing walls of the furnace, and have their outer ends attached to metallic contact or terminal members, which may be artificially cooled, if necessary, to keep them from attaining injuriously high temperatures; and each heating element is formed with a longitudinal gas passage or space extending through the carbon resistor' body and graphite end members of the heating element, and means are provided for passing a suitable gas into said space vunder a pressure sufficient to create an outflow of gas throughcracks and joints of the heater element structure, and through the pores of the carbon and graphite portions of the element.

Carbon is ordinarily cracked out of the gas entering the cracks and joints, so as to seal the latter, quite rapidly, in consequence of the relatively high temperatures to which the cracks and joint walls are subjected as a result of the relatively high crack and joint resistance to electric current flow. The carbon thus deposited in element cracks and joints, not only closes and seals the cracks and joints, but provides a good mechanical and electrical bond between the crack orjoint walls.

The major sealing action is effected during an initial operating period, ordinarily of a few hours or days duration and while the composition of the gas employed during that period may be the same as that supplied in subsequent regular operation, I consider it generally desirable to eifect the major sealing action by means of a gas such as propane from which carbon may be liberated by a cracking action mbre freely than from a T more stable, less expensive and more easily obtainable reducing gas which may be supplied for flow through the carbon and graphite pores of the heating element in regular operation.

Initially, the pore or void space in the carbon and graphite bodies of the improved heating element, amounts to 20 or 30 per cent of the overall volume of said bodies. While in use, the pores tend to fill up with deposited carbon, cracked out of the gas, the rate at which carbonv may ordinarily .be expected to deposit in the pores, is not high enough to prevent the heating elements from having an effective operating life of commercially acceptable length, and much longer than they would normally have but for the reducing gas flow through their carbon and graphite pores.

A furtherA specific object of the invention is to connect the resistance body end members and metallic contact or terminal members of each lture changes.

heating element by .threaded joints and to associate therewith a torsion device by which the joints can be kept tight, notwithstanding the joint loosening tendency of heating element tempera- Such a torsion device may comprise means for subjecting one end of the heat- -ing element to the torque produced by a weight or a spring suitably connected to the element. v Another speelde-object of the invention is to provide a furnace chamber roof specially shaped to assist 'in the maintenance of.- reducing gas envelopes about the heating elements.

A further specic object of the invention is to connect the heating elements in a multi-phase heating circuit in a novel 'and advantageous manner. v

The variousfeatures of novelty which characterize my invention are pointed out with particulOl " outer end of the end member,

larity in the claims annexed to and forming a part of this specification. For a better understanding of the invention. however, its advantages, and specic objects' attained with its use, reference should be had to th'e accompanying drawing and descriptive matter in which I have illustrated and described apreferred embodiment of the invention.

Of the drawing:

Fig. 1 is a side elevation of amelting furnace partly broken away and in section;

`Fig. 2 is a partial section on the line 2-2 of Fig. 1, with parts broken away; and

Fig. 3/is a diagrammatic representation of a furnace heating element and heating circuit arrangement.

'Ihe furnace A shown in the drawing comprises walls formed of suitable refractory and heat insulating material and covered by a sheet metal casing B to avoid leakage and enclosing a furnace chamber A', with a hearth A2 supportving the metal melted, or other material treated.

Extending across the furnace chamber in the upper portion thereof,v are a suitable plurality of' parallel horizontally disposed electric heating elements, each comprising an elongated resistor left hand side -of'the furnace, as seen in'Fig. 2,

has an externally threaded inner end portion F' of reduced diameter, which is screwed into the corresponding end member` socket D3, so as to normally hold a collar or flange portion F2 of the contact members in snug engagement with the Each contact member F is formed with an axial passage F5i which communicates with the corresponding end member passage D'. Each member F is formed with a cooling fluid space and is formed at vthe outer end of the element with inlet and outlet ports F5 and F* for the passage of' `a cooling uid,

usually water, into and out of the space F4.

The outer surface of the body portion of each contact member is cylindrical and extends away from the furnace wall through a stufiing box comprising a Achambered member G mounted on the furnace wall, and an adjustable gland G' extending into the chamberedmember and adapted to compress packing material G2 within the latter. As shown,'a gasket G3 of asbestos or the like `is interposed between each memberrG and the adjacent portion of the outer side of the furnace wall. To further reduce theheat transfer to each stuffing box, the adjacent. endY of the corresponding end member D is advantageously spaced inwardlyy some distance from the outer 'side of the furnace, so that a portion of the heat insulating material forming the outer portion of body C and end members D connected to thex ends of the body C and mounted in the side walls of the furnace.

-Asshown,- each resistor body C is an elongated cylindrical body of carbon in an yamorphous form having a relatively high velectrical resistance.'

Each body C is formed with an axial passage C. I now contemplate that i'n the ordinary practice of the invention, the heating resistors C employed will be commercially available carbon electrodes which are now manufacturedy for use' in electric arc furnaces.v Each end member D is a cylindrical body of graphite `which is a much better electrical conductor than amorphous carbon, and is formed with an axial passage D'.

In the preferred construction shown, each re sistor body C is connected at each end to the c'orresponding end member D, by an externally` threaded nipple E approximately one half of which is screwed into an internally threaded counterbore or socket C? formed in the body C, while the remainder of the nipple is threaded into an internally threaded counterbore or socket D2 formed in the adjacent end of the end member D. Eachmember D may well be somewhat larger in diameter than the resistor body C. in order" that the heating element end member resistance and heat losses may be correspondingly reduced.l

The nipples E may well be such connector parts, formed of amorphous carbon, as are now in use in connection with furnace electrodes. Aa

the, furnace wall may be interposed between the gasket' G3 and the end member D.

Atthe right hand side of the furnace as seen y in Fig. 2, the outer socket D3 of each end member D receives the reduced and externally threaded -inner c nd portionvH' of a fluid cooled electric contact or terminalmeinber H, comprising parts H', Fig/Ht H5 and H6', corresponding tothe parts F', F4, F5 and F6, respectively, of each terminal part F. Each contact member H, which need not include an axial passageH, is provided at itsv outer end with an externally threaded portion Hl, which is screwed into a socket formed in a corresponding copper -jumper or connector I, which has a second socket receiving the connector end portion H", of another heating element, the. different heating lelements being thus connected in vpairs at one sideof the furnace.

In the preferred form of the present invention,

the different threaded connections by which Athe parts C, D, E, F and H of each heating element are united. to one -another and to the connector I, are all threaded in the same direction, so that a suitable torsion device may subject the contact device F of the element to a turning torque in the proper direction to neutralize any tendency of the different threaded connections to loosen up in use, and to maintain' suiiicient contact pressure' between the engaging thread surfaces,

and between the abutting ends of the, resistor body C and end members D, and between lthe outer ends of the latter and the contact flanges F2 and H, to suitably minimize the corresponding resistance of the joints between the engaging ythe same diameter as the axial surfaces. In the simple form illustrated in the drawing, each torsion device comprises a weight J suspended from an arm J' adjustably secured to the corresponding contact F so that the arm may extend in a generally horizontal direction away from the contact.

, In accordance with the present invention, a suitable gas is continuously supplied to the bores or axial passages C' and D', of the non-metallic portions of each heating element, through the axial passage F3 in the copper contact or terminal member F, by a corresponding gas supply pipe or hose connection K. The gas thus supplied passes slowly through the pores of the carbon and graphite bodies C, D, and E, to the outer surface of the parts C and D of the element, and passes more rapidly through such joints or cracks as may normally be expected to initially 7exist, or which may develop from time to time in the operation of a heating element. vSuch joints or cracks E, will normally be sealed in a comparatively short operating period by carbon cracking out of the gas and depositing in the cracks or joints. The carbon thus deposited not only seals the joints or cracks, but forms a good mechanical and electric conducting bond between the walls of the cracks or joint. While there willr be some cracking of the gas flowing through the graphite and carbon pores, the resultant deposit of carbon in the pores will be very much slower than in the first mentioned joints or cracks, which offer sufiicient resistance to electric current iiow to produce appreciable local increases in heating element temperatures, greatly increasing the cracking action on the gas owing through the cracks or joints. Under normal operating conditions, carbon will not be deposited in the carbon and graphite pores at a rate rapid enough to prevent the heating element from having'an operating life of commercially acceptable length The reducing gas which slowly passesl outwardly from the bore of each heating element to its outer surface, tends to maintain a reducing atmosphere inthe furnace chamber. Inorder that the furnace chamber atmosphere in immediate proximity to the heating elements may be maintained definitely reducing under conditions in which the general chamber atmosphere may be less definitely reducing, I advantageously the carbon and graphite portion of the element.

The continuous introduction into the furnace chamber of a suitable reducing gas supplied to the pores of the heating elements as labove described, not only maintains or contributes signicantly to the maintenanceeof the propel furnace atmosphere required for the production of oxygen-free copper, and required for other metallurgical furnace operations, but contributes directly to a desirably long operating life of the heating elements by protecting the latter against contact with oxidizing gases.

As those skilled in the art will understand, the composition of the gas passed into the heating elements may vary with conditions and particularly with the relative availability of different gases suitable for the purpose. As previously indicated, I advantageously useY a free cracking gas, such for example, as propane, in an initial joint sealing period of operation, and use under subsequent regular operating conditions, a more stable, but definitely reducing, gas. Ordinarily the latter may well be of a commercially known type practically free from smoisture and other objectionable impurities, and of about the following composition.

Per cent Carbon monoxide 6 Carbon dioxide 6 Hydrogen 11 Nitrogen I7 f one another and with one of the three separate `shape the roof of the furnace chamber to provide 'a separate horizontal space or cell L for each heating element in whichtheupper portion of the element is received, each space or cell L being open -at its underside. 'I'he cells L may be formed, as shown, in a furnace of satisfactory construction, by providing a plurality of masonry arches M,l which alternate along the length of the furnace Awith the cell spaces L, and which have their ends in abutting relation with the right and left hand furnacewalls, as seen vsin Fig. 2. Each of\ the arches M is shaped to provide a top portion M extending horizontally from one side of the furnace. to the other and of keystone shape in cross se'ction. The opposite sides M3 of each top portion M' diverging downwardly and away from one another, form the abutments for adjacent masonry arch portions N'of the furnace roof, each o'f the arches N forming the top wall, orcrown portion, of a corresponding cell or space L. The location of a heating element in its space or cell L, obviously tends to retard the movement laway from the heating element, of the reducing gas slowly leaking from its axial passage through the pores of phase sections of the secondary winding 0 of a transformer, which may be of any conventional type and hence need not be further illustrated or described. It is to be noted, however, that the transformer must in practice serve to reduce the supply voltage, which may well be 10,000 volts or higher, to a secondary voltage of 10 volts or so. Such a small voltage supplies energy at a suitable rate, for example, in a furnace of the type shown, having amorphous carbon heating elements C, which are eight inches in diameter, sixty inches long, and are each formed with an axial passage C three inches in diameter.

.The connection of the heating elements to a multiphase supply circuit in the general manner illustrated in Fig. 3, insures desirable balanced load conditions and simple circuit connections. For an ordinary metal melting or rening furnace, six heating elements of the character described will be adequate to supply heat at the required rate. In the case of a furnace in which more than six heating elements are required, use may be made of two or more 4 sets of six heating elements each, with each set connected to a multiphase supply circuit in the general manner illustrated in Fig. 3. Although the present invention is especially advantageous for use in furnaces employed in melting and refining metals which need to be protected against oxidation l by the, maintenance of a'reducing atmosphere in the furnace chamber, the invention may well claim as new and desire to secure by Letters be employed in furnaces used riety of other purposes.

While in accordance with' the provisions of the statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of 'the apparatus disclosed .without departing fromthe spirit of my invention as set forth in the appended claims and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features. Y Having now described my invention, what I for a wide va- Patent is:

l. In a furnace, the combination with the furnace chamber, of electrical heating means therefor comprising a hollow porous resistor of refractory material within said chamber, and means for closing cracks and joints in said hollow resistor comprising means for passing into said hollow resistor a gas including a carbon constituent which cracks at high temperatures and deposits solid carbon thereby formed in said cracks and joints.

2'. An electrical furnace heating element comprising two tubular bodies of graphite adapted to be incorporated in opposed walls of a furnace chamber, and a tubular body portion of amorphous carbon adapted to extend across 1`said chamber and in threaded connection at each end with the adjacent end-portion, said tubu1ar bodies of graphite having a specic resistance much lower than that of said body portion of amorphous carbon.

3. 'Ina furnace, the combination with the opposing walls of a furnace chamber, of electrical heating elements each having end portions of graphite mounted in said walls and a body portion of amorphous carbon extending across'the chamber and each formed with a longitudinal gas channel, and means for supplying a reducing gas to the said channel of each heating element, said end portions of graphite having a specific resistance much lower than that of said body portion of amorphous carbon.

4. The /combination with an electric heating element comprising a plurality of conducting sections arranged end to end with threaded connections between the adjacent ends of adjacent sec tions, all of said connections being threaded in the same direction, means for holding the sec-v tion at one end of the element against rotation and means for continuously subjecting the section at the other end of the element to a torque in the direction to tightenv said threaded connections.

5. An electric heating unit comprising two similar heating elements arranged side by side and each comprising two end portions and an intermediate body portion with a threaded connection between each end' of the latter and the adjacent end portion, .an element connecting member at one end of the unit, a separate threaded connection between said member and each of the two adjacent element end portions, the said threaded connections being all threaded in the v same direction so that all may be maintained tight, by impressing torque on'each of the two element end portions remote from said connecting member, and means for continuously impressing torque on each of the two last mentioned element portions lin the direction to maintain said threaded connections tight. 7

6. An electric heating unit comprising two similar heating elements arranged side by side and each comprising end portions of graphite, a body portion of carbon, a threaded connection between each end of the body portion and the corresponding end portion, an element connecting member at one' end of the unit, a separatethreaded connection between said member and each of the two adjacent element ends, a separate metallic l0. terminal for each element at the end of the latter remote from said connector and a threaded connection between each terminal and the adjacent end portion of the corresponding element, the said threaded connections connecting the end porti-ons of each element to the body portion thereof and to said connectingv member and the corresponding terminal being all threaded in the same direction so that the dilerent threaded connections of each element can be maintained tightA by a torque impressed onA the corresponding terminal, and means for continuously subjecting each of said terminals to a torque to thereby maintain said connectionstight.

7. In a furnace, the combination with the opposing walls of a furnace chamber of electric heating elements extending across said chamber and each comprising graphite end portions mounted in the opposing walls of the furnace chamber and a carbon rod extending across the furnace chamber and in threaded engagement means for passing into the said longitudinal channel of each element a gas including a carbon constituent which cracks at high temperatures and deposits solid carbon thereby formed in said cracks and joints. l

8. In 'a furnace, the combination with the opposing walls of a furnace chamber, of two side by side electrical heating elements each having end portions -of graphite mounted in said walls anda body portion of amorphous carbon extending across the chamber, a threaded connection between each end of the body portion and the` corresponding end portion, aconnecting member at one side of the furnace chamber, a threaded connection between said member and the adjacent end of each of said elements, a separate metallic terminal for each element at the end of the latter remote from said connector and a threaded connection between said terminal and the adjacent end portion of the element, the said threaded connections connecting the end portions of each element to the body portion thereof, and to the corresponding terminal and connecting member, being all threaded in the same direction, and a torsion device connected to each terminal and subjecting the latter to a torque tending to tighten each of the diierent threaded connections of the element.

9. In a furnace, the combination with the furnace chamber and its enclosing wall, of an electrical heating resistance unit therefor comprising a hollow porous resistor of amorphous 'carbon within said chamber, hollow resistor end connections of graphite mounted in the chamber wall and engaging ands'upporting end portions of said resistor, and means for closing cracks and joints` in said hollow resistor comprising means for passing into said hollow resistor a gas including a will crack and passing electrical current through the resistor to heat the walls of joints and cracks therein to a temperature high enough to crack said constituent and thereby cause said joints and cracks to be sealed by carbon cracked'out of the gas. v

FRANK W. BROOKE.

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