US 2417953 A
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March 25, 1947. s. HQSTUPAKOFF HIGH TEMPERATURE ELECTRICALLY-HEATED FURNACE Filed June 2, 1944 rif.-
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Patented Mar.l25, 1947 HIGH TEMPERATURE ELECTRICALLY- HEATED FURNACE Semon H. Stupakoi, Latrobe, Pa., assigner to Stupakof! Ceramic Manufacturing Co., Latrobe, Pa., a corporation of Pennsylvania Application June 2, 1944, Serial No. 538,393
2 Claims. 1 A
This invention pertains to electrically-heated furnaces, and more particularly to electricallyheated furnaces using resistance heaters, and is for a furnace 'capable of operating at high temperatures in an oxidizing atmosphere.
According to the present invention, it is proposed to use as the heating elements for the furnace, a ceramic material which is a conductor of the second class. Material available commercially under the trade name Insulcon is a typically satisfactory material. Such materials are made from a mixture of rare earth oxides and have a negative coefilcient of electrical resistance. They are insulators when cold, but when-heated to about 700 C. or above, they will readily conduct electricity and the passage of current therethrough will of course thereafter sustain them at a conducting temperature.
The melting temperature of the conductor will lvary according to the ingredients used, but may be in the neighborhood of 2700 C. Being cornposed of completely oxidized materials, a conductor of this kind is chemically' stable when heated in an oxidizing atmosphere up to or near its melting temperature.
For many years, I have tried to use Insulcon" conductors as a heating means for furnaces intended to operate with anoxidlzing atmosphere at temperatures at or near, or possibly somewhat in excess of 2000 C., there being an industrial need for a furnace of this character. However, difficulty has been encountered in providing for the conduction of electrical current to the heaters operating at or above this region of temperature. Platinum is the metal most resistant to oxidization at high temperatures, but platinum has a melting point of approximately 1650" C., so that the temperature of a furnace using these oxide heating elements, where platinum is used to carry current to the elements, is confined to a temperature below 1650 C. Metals which melt at a higher temperature than platinum, suchas molybdenum (melting point 2620 C.), or tungsten (melting point 3370 C.), oxidize or burn readily when heated in air or other oxidizing atmospheres, so
that such metals are of no value as leads or terminals for "Insulcon or similar heaters.
According to the present invention, I have found that by using high frequency electrical curent, perhaps 100,000'jf2cycles, more or less, electrical current can beaconducted from a lead for a reasonable distance` through air to the "Insulcon units. This avoids the necessity for Iany metal terminals on the Insulcon units themselves. and hence enables `the furnace to be oper- `a furnace embodying my invention;
Fig. 2`is a transverse section in the plane of line II-II of Fig. 1;
Fig. 3 is a fragmentary view illustrating partly in section and partly in elevation, an arrangement for including the ceramic heaters in a high frequency circuit; and
Fig. 4 is a detail section in the plane of line IV--IV of Fig. 1, showing the relation oi the Insulcon or other heaters to the source of current supply.
In the drawings,4 the structure has been illustrated only schematically for the purpose of indicating one manner of employing my invention. In the drawings, 2 designates generally a. refractory furnace structure which is here illustrated to be a small capacity or laboratory size of unit. It has a refractory bottom 3, refractory side walls I, and a refractory top 5, enclosing a chamber C. In the roof of the furnace are transversely extending hangers or inverted piers 5a, also formed oi' refractory material, and having a plurality of small holes extending transversely therethrough. These holes loosely receive the heating elements or conductors 6. Provision is made for several of these conductors to be supported in parallel relation.
As heretofore indicated, the conductors or elements 6 are conductors of the second class formed of Insulcon or a like compound of rare earth oxides having a negative coeii'lcient of electrical resistivity and which are non-conducting when cold, but which become conductors when heated to a temperature at or above 700 C.
The ends of the conductors 6 terminate in spaced relation to water-cooled electrodes l and 8, one of these electrodes being at one end oi the resistors and one being at the opposite end, the electrodes being spaced from the ends of the resistors in such manner that a reasonably short gap exists between the resistors and the electrodes. The electrodes themselves are illustrated as being formed of metal with a water-circulating space 9 therein. Each electrode has a connection for two water pipes, these being designated i0, so that water may be continuously circulated therethrough, enabling the electrodes to be kept cool enough to prevent damage thereto. In the drawings, the terminals 1 and 8 are shown as being located entirely within the furnace chamber, but it is understood that provision may` be made for locating them otherwise, or surrounding them with heat insulating material to avoid excess loss of furnace heat. The water pipes I may also conveniently comprise conducting leads whereby the electrodes may be coupled into a high frequency circuit includingthe wires I I and a source of high frequency current I2. By high irequency I mean a frequency above the range of audibility and preferably of the order of around 100,000 cycles or more. Any frequency that will f able the required amount of current to trans- 'se the air gap between the electrodes and the :,r the ceramic conductors 6 may be employed. While frequencies considerably in excess 100,000 cycles may be used, there is little adultra-high frequencies. 135:. f z necessary for the rods 9 to be "o a conducting temperature before viii operate electrically, This may use ci' gas burners or any other y means not shown, which can be employ to bring the furnace up to a, temperature of around 000 C. and thereafter removed or the operation thereof discontinued. At this time the high frequency circuit will be energized and our will travel. from the electrodes through the airrgap at each end thereof .into or from the conductors 6, which, according to the amount oi current employed, can be operated in an atmosphare of air or other oxidizing atmosphere at a` temperature of about 2000 C. or greater, the ltempera'ture however v being maintained somewhere at a point below themelting point of the conductors around 270W C.
The heating units. themselves are preferably rod-like in form 'and of relatively small cross section. This section may be, for example, of the order of a quarter of an inch or less. Where the cross section is large, there may be n0n-unlform temperature lconditions within the unit itself due to the tendency oi the current to seek a path through the conductor causing overheating in Some polntsrand perhaps underheating in others. The smaller the diameter of the rods, the more nearly the likelihood of there being uniform conductivity across the entire section, and less likelihood of the current seeking a path of less section than the full diameter of the sectionD It will of course he understood that the drawing is schematic and that the heating units may be arranged at the bottom or sides of the furnace chamberu While I have illustrated and described one embodiment or my invention and the method of operating the same, it will be understood that this merely we? of illustration and that various changes and modifications may be made within the contemplation of my invention and under the scope of the `following claims:
i claim, as my invention: y
l., electric furnace designed to operate at very high temperatures in an oxidizing atmos 0 Number phere, comprising an enclosure, arod-like heating element supported therein comprising rare earth oxides and having a negative coemcient o! electrlcalresistivlty, a pair of terminals o! high electrical conductivity each being in proximity to but spaced from diil'erent ends of said conductor, means for cooling said terminals, and a high frequency circuit connected across said terminals t0 pass a high frequency current throughout substantiallLthe entire cross-sectional area of said heating element.
2. An electric furnace designed to operate at very high temperatures in an oxidizing atmosphere, comprising furnace enclosing walls. a plurality of rod-like conductors of the second class supported from a portion o! said walls, a pair of hollow terminals of high electrical conductivity, each being in proximity to but spaced from corresponding ends of said plurality of conductors of the s econd class, means for circulating a c001- ing fluid through said hollow terminals so as to maintain their temperature substantially lower than that of said second class conductors, and a source o1 high frequency current connected in series with said terminals for passing a high trequency current throughout substantially the entire cross-sectional area ct said rod-like condoctors of the second class for maintaining them at very high temperatures after they have been brought up to such temperatures so as to be# come conducting. v
SEMON H. S'IUPAKOFF.
REFERENCES CITED The following references are oi record in the ille oi this patent:
UNITED STATES PATENTS Name Date Shaw Apr. 12, 1932 Saunders Sept. '7, 1920 Dowiatt Oct. 29, 1929 Murthrup Mar. 3, 1931 .Benner et al July 19. 1932 Fowler Nov. 21, 1882 Colby May 13, 1924 Hull Jan. 6, 1925 Adams June 24, 1930 McArthur Mar. 7, 1933 Meissner Apr. 10, 1934 Rouy Dec. l5, 1942 Millar Nov. 24, 1931 Ruckstahl .1 May 10, 1932 Benner et al Apr. 16, 1935 Henke Dec. 13, 1938 Geller Aug. 22, 1944 FOREIGN PATENTS Country Date Australian Apr. 10, 1929 French June 11, 1934 German Oct. 13, 1939 British Dec. ll, 1912 Number