US1306289A

US1306289A - Vanxa

Info

Publication number: US1306289A
Authority: US
Publication date: 1919-06-10
Anticipated expiration: 1936-06-10

Description

Patented J une 10, 1919.

Fly.

F. THORNTON. 1R., AND 0. A. COLBY.

ELECTRIC FURNAC.

APvucAnuN mm ocr.24.19|1.

INVENTOR ATTORNEY UNITED STATES PATENT OFFICE.

FRANK THORNTON, JR., F PITTSBURGH. AND ORA A. COLBY, 0F LARIMER, PENNSYL- VARIA, ASSIGNORS T0 WESTNGHUSE ELECTRIC & MANUFACTURING COMPANY, A

CORPORATION QF YENNSYLVANIA.

ELECTRIC FURNACE.

' To all 'whom it may concern.' i

Be it known that we,FnANx THORNTON,

. Jr.z a citizen of the United States, and a consequently, fails to heat. u A tetes a, removal of the consumed resistor maresident of Pittsburgh, in the county of Alleghen and State of Penns lvania, and ORA A. nur, a citizen of the nited States, and a' resident of Larimer, in the county of Westmoreland and State of Pennsylvania, have invented a new and useful Improvenient in Electric Furnaces, of which the following is a specification.

This invention relatesto electric furnaces and, in particular, to high-temperature elcctric furnaces for heating and treating metals and also for other heating applications.

In working with those types of electricresistancerfurnaces in which the resistor conl tains `granular material, we have found that therateof combustion of the granular resistor, material increases during1 the operation ofthe furnace and, in a comparatively short time, becomes so great as to cause a rapid inclease in resistance, and the resistor,

This news-:si-

terial and a renewal of the same which interferes with the operation of the furnace and is also expensive.

We have found also that the limits of temperatures for a furnace using granular carbonaceous material, such as granulated aphite, colto, and charcoal, as a resistor f 1s around .900 C., because, when a temJ perature of about 1000 C. is maintained on the surface of the resistor which coirtitutes the hearth of the heating chamber, the teniperature in the interior of the resistor is around 2000 C., which exceeds the safe limit of the refractory material of the furnace. This condition is due to the fact that carbon has a negative temperature-resistanoe coefficient and thc .current through a m'ass of any form of carbon or ,graphite tends to concentrato in the zone of highest y temperature which, in this case, is in 4=. part of t e resistor away fromthe heating chamber, since the portion of the resistor exposed in the heating chamber will be cooler than other portions of the resistor. Accordingly. an object of this invention is to provide an electr1c-resistanco furnace in which high tcm eratures are obtainable without a rate of eterimaton that will rtm1?? *he fumes@ Specification of Letters Patent` Iatntea .j une 10, 1919.

Application lled October 24, 1917. Serial No. 198,275; i i .i

too expensive to operate with ,commercial success. p l y Another object of our inventionv is to -provide a furnace which has a preheating cham bel' in which the articles to be treatedinay be gradually brought up to a. moderately hightemperature before they are introduced into the main heating chamber ffth'e furnace. By arranging the preheatin chamber in the furnace structure near t e main heating chamber, with the resistor between the two chambers, a path is provided foil the escape of heat from the lower part of the resistor, thereby equalizing the temperature of the resistor and its lining, so that higher temperatures may be safely ymaintained in the heating chamber.

In that type of furnace in which the heat is generated only in the Hoor of the hearth or heating chamber, the side walls and roof obtain their heat only by direct radiation, and pieces of metal that are laid closely over the surface of the hearth will interfere with the radiation of heat directly in proportion to the amount of hearth surface that they cover. lIn a continuous] operated furnace, this condition prevents t c roof .of the furnace from reaching a temperature equal to that of the hearth or the metal, and such a furnace can, therefore, only bo operated at a very low rate of production. In order to avoid this condition, it is an object of this invention to provide an electric-furnace resistor of carborundum forming the top and bottoni walls of the heating chamber. so that heat may be generated at every point in the roof and floor of the heating chamber. A high temperature is thereby maintained ouali radiating surfaces in the chamber, and heat will be fed into the work through all its exposed surtions of another electric-resistance furnace embodying our invention; and Fig. 4 1s a sectional View, taken on the line II-III of Fi 3.

5 Rerringto Figs. 1 and 2, the furnace construction comprises a suitable iron framework and exterior shell 10, that carries the vertical walls 11 and a bottom wall 12 of refractory material, the latter supporting a refractory brick base structure 13. A center wall 14 is spaced from the side walls 11 to form hoppers 15 into which granular 'material, such as graphite or coke, `may be charged -for a purpose to be described. The hoppers 15 extend downwardly into the fur-fV nace structure and in each of them is a solid terminal electrode 16 vof graphite or amorphous carbon for leading current to the resister. The electrodes 16 are located in the bottoms of the hoppers 15 and, at their lower ends, are connected to metal plates 17 which extend downwardly throu h the ioor of the furnace and form `termina s to which the supply mainsare connected.

The main heating chamber 18 of the furnace is open, at one end, and otherwise inclosed by top and

bottom walls

19 and 20 separated by the spacers 21 which form the si e walls of the heating chamber. The

'

walls

19 and 20 consist of carborunduin or silicon carbid bricks or slabs, and the intermediate s acers 21 may also consist of solid carborun um blocks, but any highly refractory material will serve the purpose, since it is not necessary that these spacers ybe of electrical conducting material.

The

carborundum walls

19 and 20 of the chamber `18 constitute the resistor of the furnace, which is spaced from the up er taperingl ends of the terminal electro es 16. These spaces between the resistor and the electrodes are filled with masses 22 of granular graphite or coke, packed to provide a medium between the electrodes and the resister that is an electrical conductor and also a thermal insulator of greater crosssectionthan the resistor to prevent overheatin and consumption of the expensive electroe 16. The masses 22 oxidizc and a-re 5 gradually consumed and may bc replaced by additional material introduced :1nd tamped into ythe upper ends of the hoppcrs 15.

The bottom Wall 20 forms the floor-of the hea-ting chamber 18, and the upper and lower surfaces of thc top and

bottom walls

19 and 20 respectively, arc covered by n lining 23 of a highly refractory.composition consisting of carboru nduin sand and silicate of soda or earborundun sand, cruciblc clay and graphite. This lining serves to retard the leakage of heat from the

resistor blocks

19 and 20, and, since the resistance of carborundum changes slowly at high tempera tures, the top and bottom walls of the chamlber, after a few hours run and heat intcrchange, will have approximately the same resistance and eventually will have approximately the same temperature. Thelresistor inclosin -the main heatingchamber 18 rests on the re-brick base 13, and within the latter is inclosed `a prehealting chamber 24 directly beneath the heating chamber l8r and separated from the latter by thelower portion of the refractor lini 23 and the refractory wa1l25. T e pre eat' chamber 24 is thus located so that it will erive heat from the lower carborundum wall 20, which loses art of its heat throu h the lining 23 and t e wall 25. Cold arties, such as cold ieces of metal, would be damaged if intro- 80 duced directlg' into the,A high-temperature Y heating cham er 18 and they are therefore, first heated to a moderately hi htempe'rature in the preheati 'chamber 24, 'after which they are intro uced into the high/35 temperature chamber 18. 1

The silicon carbid or carborundum is very suitablefor use as avresistor 4since -it resists fusion and oxidation up to an ext'remelydiigh temperature, is ri -d, of good heat conductivity and is availab ein various molded shapes.\ Although carborundum has a larger negative temperature-resistance co-v efficient than carbon, it is superior to carbon as a resist-or for high-temperature `fur-- )laces because of the combustion which takes place on the exposed surface of graphite or carbon blocks at high,tempe`ratures, and the diiculty of repair or'replacement of burned y portions. Moreover, it 1s possible to utilize 100 the negative temperature-resistance coeicient characteristic of carborundum 'as 'a basis for controllin the furnace. The lack of combustion of t e carborundum resistor material results in there being an oxidizing atmosphere within the furnace, but a nonoxidizing atmosphere `may beobtained byA "spreading granulatedcharcoal or Vcarbon over the surface ofythe resistor near the door where it is burned by the airthat enters. 4With a furnace constructed according to .this invention, it has been -pOSSible to obtaincori- 'v tinuous operation at temperatures as'4 high as 1250 C. without injury to any part of? the furnace and"the' temperature dp 11b through the silicon carbid or carborundum is' very much less than that which has been observed through a granular resistance bed, uml the danger of damaging the lining is very much reduced. i

IdFigs. v3 and 4, we have illustrated n furnace construction embed ing features oi the' furnace illustrated in figs- 1 and 2-but in which only the floor 26 of the heating chamber is built up of carborundum or silicon carbid bricks. "lhesc bricks constitute the resistor of the furnace and replace the granular resistance bed illustratedand describediu our copending application` In the particular form illustrated, there are 13o insulating conducting three parallel spaced rows 261 of bricks, iid on edge end to end, and between the p titiller row and the twoouter rows are rows 2C2`=0fbric`ks which break joints with the center and outer ro'ws and are laid on their wider sides. Metal bars placed in the heating chamber 181 will then be supported at intervals" onlyand will be` spaced from the intermediate rrows 262. The fioor 26 is inclosedat the bottom andsides by a highly refractory carbonaeeous lining 2T similaito the lining 23, the lining 27 being supported by the fire-brick wall 28 that forms the roof of the preheating chamber 241. The ends of "the resistor formed by the floor 26 are connected to suitable terminal electrodes (not shown), such as `the electrodes 1G of lf 1, by the granular masses of thermal Same manner as those illustrated in Fig. 1. Temperatures, up to about 1200o C., are obtainedvin this furnace without difficulty and the furnace is adapted to be used with success for hardening tools and dies and for firing porcelain. A non-oxidizing atmosphere may be obtained within the furnace by spreading granular charcoal or carbon over the floor 26 near the door where it is burned by the entering air.

ile We have shown and described our invention in detail, it is to be understood that numerous changes and adaptations may be made without departing from the spirit of the invention as defined in the appended claims.

We claim as our invention:

i 1. In an electric furnace, the combination with a heating chamber, of a resistor adjacent the same comprising blocks of refractory electrical conductin7 material in contact with one another and forming the top and bottom walls of said chamber, terminal electrodes, and a mass of thermalinsulating and electrical-conducting granular material interposed between each of the electrodes and the resistor to prevent overheating of the electrodes.

In an electric furnace, the combination with a heating chamber, of a resistor comprising blocks of refractory electricalconducting material in contact with one another and forming certain walls of said. chamber, hoppers on opposite sides 'of said chamber, terminal electrodes in said hop ers, and a mass of thermal-insulating an electrical-conducting material between each of said electrodes and the resistor and of greater cross-section than the resistor to prevent overheating of the electrodes.

3. In a high-temperature electric furnace, the combination with a heating chamber having walls formed of blocks of refractory electrival-conducting material constituting the resistor of the furnace, of hoppers on opposite sides of said Chamber, solid material 221, in the`` 'faire resistorof the furnace.

tibll carbonaceous terminal electrodes in Said hoppers, and aniass of thermal-insulating and electrical-condoeting material between l euch of said electrodes and the resistor, said masses of material being granular and of 70 greater cross-section than the resistor to prevent overheating and consumption of the l electrodes.

4. An electric furnace comprising a heating chamber having an open end, top and 15 'bottom walls formed of solid refracto electrical-conducting `material which constltutes the resistor of the furnace, and side" walls of refractory material for separating the top and bottom walls.

5. A high-teuriieratl1re electric furnace comprising a heating chamber having walls formed of solid refractory cle'ctrical-conf ducting i'uaterial, said material constituting terminal elec- 86 and a mass of thermalinsulating`a`nd" electrical-conducting material between said electrodes and theopposite outer sides of said chamber.

G. A high-temperature .electric Vfurnace 90 comprising a heating ch'amber having topand bottom walls formed of blocks uflrefracbory electrical-conducting material con`- stituting the resistor of the furnace.

7. A higb-temperature electric heating furnace comprising a resistor consisting of blocks of electrical-conducting refractory material inclosim;r a space which constitutes the heating chamber of the furnace, terminal electrodes, and a mass of thermal-insulating and electrical-conducting material between each of said electrodes and said resistor, said masses of conducting material being granular and of greater cross-Section than the resistor to prevent overheating and consumption of said electrodes. Y

8. An electric furnace comprising a heating chamber, a preheating chamber, and a resistor for supplying heat to both of said chambers.

9. An electric heating furnace comprising a heating chamber, a preheatingchamber and a resistor between said chambers and arranged to supply a higher degree of heat to said heating chamber than' to Said 115 preheating chamber.

10. `An electric furnace comprising a heating chamber` a preheating chamber, and a resistor having a part disposed between said chambers and closer to the heating chamber than to the preheating chamber so as to supply a higher degree of heat to the former than to the latter.

11. An electric furnace comprising a heating chamber, a preheating chamber, and a resistor having apart disposed between said chambers to constitute a wall ,for the heating chamber and separated by a layer of material from the prcheating chamber` ,ll-2. .An electric furnace comprising a heating chamber and a preheating chamber dis-I poscd one above the other, a resistor having a part disposed between said chambers to constitute a wall for thc heating chamber and separated by a layer-.of material from the preheating chamber, terminal electrodes on opposite sides of said chambers, and -a mass of electrical conducting' material between each electrode and said resistor.

13. An electric furnace comprisin a heating chamber, a preheatingchamoer isposed directl `below said heating chamber, a resistor, aving a part constituting 'the floor 1 .of said heating chamber, a layer of material of low thermal and electrical conductivity between said` resistor and said preheating chamber, hoppers at onnosite sides of said resistor, terminal electrodes in said hpprs,

and a mass of electrical-conducting, thermal-insulating granular material between cach electrode and the resistor to prevent overheating of said electrodes.

14. An electric-resistance furnace having a heating chamber provided with top and bottom walls comprising material that has la larger negative temperatureresistance coefficient than carbon, said walls constituting the resistor of the furnace.

15. An electrical-resistance furnace having a heating chamber provided with a top wall of carbid that has a negative temperature-resistance coefficient, said wall constitutin g the resistor of the furnace.

16. An electric-resistance furnace comprising a heatinfr chamber provided with top and bottom walls of silicon carbid that has a large ne ative temperature-resistance coecient, saiv walls constituting the resistor of the furnace.

17. A high-temperature electric-resistance furnace comprising top and bottom walls consisting of silicon carbid that has a large negative temperaturefresistance coeilicient and side walls which inclose a s ace that constitutes the heating chamber o the fur-,

nace. i

18. An electric-resistance furnace com prising a resistor consisting of blocks of silicon carbid s aced apart to form the heatin chamber o the furnace.

n testimony whereof, lwe have hereunto subscribed our names 1917.

FRANK THORNTOMJB; ORA A. coLBY. Y

`this 22nd day 0f 0613-,