US2657247A - High-temperature electric furnace and process of operation - Google Patents

Description

Oct. 27, 1953 o. BRETSCHNEIDER 2,657,247

HIGH-TEMPERATURE ELECTRIC FURNACE AND PROCESS OF OPERATION Filed Oct. 5, 1950 2 Shefs-Sheet 1 2 INV V OR Orro Emerscuzvzmsaz Oct. 27, 1953 o. BRETSCHNEIDER 2,657,247 HIGH-TEMPERATURE ELECTRIC F NACE.

PRO I CEISS OF OPERATI AND Filed Oct. 5, 1950 '2 Sheets-Sheet 2 I/Vl/Ell/ TOR Patented Oct. 27, 1953 HIGH-TEMPERATURE ELECTRIC FURNACE AND PROCESS OF OPERATION Otto Bretschneider, Frankfurt am Main, Germany, assignor to Deutsche Goldund Silber- Scheideanstalt vormals Roessler, Frankfurt am Main, Germany, a corporation of the German Republic Application October 5, 1950, Serial No. 188,565 In Germany October 5, 1949 8 Claims.

The present invention relates to improvements in high temperature electric furnaces and more particularly, to high temperature electric furnaces of the resistance type.

Resistance type high temperature furnaces have generally been provided with carbon resistance elements in the form of tubes or rods. Carbon resistance elements have usually been preferred over those of graphite in view of the higher electrical resistance of carbon. The carbon resistance elements, however, have the drawback that they must be replaced. rather frequently, and in view of the diflicult workability of carbon, it was always necessary to have an adequate supply of finished replacement heating elements to avoid undue interruptions in the operation of the furnaces. Furthermore, at temperatures around 2000 C. one has to take into account the shrinkage of carbon and at temperatures over 2000 C. the graphitization thereof. These disadvantages have hindered the development of large high temperature furnaces with carbon resistance heating elements. In the construction of granular carbon furnaces, one is, for all practical purposes, limited to the use of ceramic containers for the support of the granular carbon. As, at temperatures of 2000 C. carbon reacts with all normally usable ceramic materials, such furnaces have not been usable for high temperature work. In special instances, where graphite was used for the resistance heating element, it was necessary to employ relatively small cross-sections or extremel costly high operating current strengths were necessary.

It is an object of the. invention to provide high ber of intermediate resistances are formed at the r joints of the subdivisions. For example, the heating element, preferably of graphite, can be in the form of a composite rod or tube formed from a plurality of individually formed graphite bodies which are placed against each other and 2 form intermediate resistances at the contacting surfaces transverse to the longitudinal axis of the rod or tube.

The effectiveness of such a composite heating element was unexpected as it was to be feared that the contact at the joints between the individual shape bodies from which it is formed, which contact is to some degree dependent upon the pressure holding such bodies together, would not be completely uniform, and that, therefore, small arcs would occur leading to damage or destruction of the surrounding area. It was, however, found that the current flow provided by the resistance elements (formed of graphite or other suitable resistance material) in accordance with the in ntion is entirely uniform and that, consequentl uniform furnace operation with low current consumption is obtained.

In accordance with a preferred modification of the invention, a gas-tight housing is provided for the furnace so that the furnace can be operated under vacuum or that it can be operated under vacuum at least during the preheating period until a emperature of over 2080 C. is reached. It found that, when temperatures over 2000 C. are reached, the vacuum can be lowered or entirely dispersed with as it was found, danger of damage to the joints by arcing substantially does not exist above this temperature. For this reason, the heating elements in accordance with the invention, possess an especially long life. The furnace can also be heated and operated with inert gases as, for instance, argon, hydrogen, nitrogen and the like.

Materials other than graphite can be used for the resistance elements in accordance with the invention. For example, carbides which are stable at the operating temperatures can be used, such as tungsten, titanium, zirconium, tantalum or boron carbides or sintered mixtures thereof. Carbon which has been heated for a short period to temperatures over 2500 C. and graphitizable carbon which gives a material of high resistance after graphitiaation have also been found well suited. The resistance elements can also be formed from heterogeneous materials, for example, carbides such as tungsten. carbide and carbon such as lampblack. It is possible, therefore, by selection of the number of intermediate resistance forming joints and the composition of the resistance material to provide the requirements which are suitable for the desired individual heating process.

In accordance with one modification of the invention, the heating element is composed of a plurality of rings or segments of rings which are assembled into a round or polygonal tube. The term ring, as employed herein, is not limited to circular rings, but is intended to cover rings or annuli of other shapes. The joining surfaces of the rings or ring segments can be fiat or toothed to provide interlocking joints or to provide relatively small contacting supporting areas with reference to the cross-section of the rings or ring segments. The walls of the tube, which are formed from the individual graphite pieces, can be of the thickness usually employed, as the main resistance of the resulting heating element is in the joints between the individual pieces and depends primarily upon the number of such joints. The insulation for the heating element can be a packing of powdered or granular carbon or charcoal pieces in which the element is bedded. Such packing increases the stability of the heating element. Ceramic insulation generally cannot touch the heating element, and where ceramic insulation is used, it must be spaced from the heating element. Composite insulation of ceramic and carbon powder grains or pieces are useable, as long as a chemically indiiferent barrier ring, such as, for example, of silicon carbide is placed between the carbon and the ceramic portion of the insulation. The silicon carbide ring may be built up of shaped pieces, or it can be formed by pouring in silicon carbide loosely. For high vacuum operation, reflectors can be employed for insulation.

The invention may be more fully understood from the following description in connection with the accompanying drawings, in which- Figure 1 is a view in vertical longitudinal sec tion of a furnace illustrating an embodiment of the invention.

Figures 2 through 6 are sectional views illustrating various modifications of heating elements in accordance with the invention.

Figures '7 and 8 are plan views illustrating further modifications of heating elements in accordance with the invention.

In Figure l, the bottom of the furnace housing I carries a copper cone 2, which is securely hard soldered thereto to provide a gas-tight joint. The interior of such cone 2 is strongly cooled with water from sprayer [3 during operation of the furnace. A cylindrical piece of graphite 3, which serves to conduct electricity from the hot zone of the furnace fits tightly over cone 2 and is. therefore, in electric contact with the housing through such cone. The housing is grounded during operation of the furnace.

The composite heating element 4, formed of a plurality of superposed graphite rings, is supported Within the furnace on the graphite cylinder 3. The hollow interior of the heating element serves for the reception of the material to be heated. The wall thickness of the heating element can, for example, be several centimeters. The height of the individual rings depends upon the desired furnace voltage. The lower the height of the individual rings, the greater the number of intermediate resistances between the joints, whereby the operating voltage is increased. The rings or ring segments, in accordance with the invention, can be provided with supporting feet so that the entire surfaces of the rings are not in contact and the resistance, consequently, is increased. Heating elements produced in this manner have many times the resistance of unitary graphite tubes of the same dimensions. The use of ring segments instead of rings for forming the heating elements is of advantage in the production of large technical furnaces.

The upper end of heating element 4 is joined with graphite conductor 5, the upper tapered portion of which fits in graphite conductor plate 6. Heat insulating graphite plate 8 lies over conductor plate 6 and a ceramic cover plate 9 is located thereover. Electric current is supplied to conductor plate 5 through water cooled copper tubes 1 which are insulated from the furnace housing I by gas tight insulating closures Ill. Packing l i is of granular charcoal which prevents radiation losses of heat from the heating element to a large degree. The evacuation of the furnace can be effected through conduit l2. To assist cooling of the furnace, the housing can be sprayed with water or other cooling liquid from sprayer nozzles M.

The provision of a gas tight housing for the furnace renders it possible to work at temperatures between 2000 C. and 3000 C. in vacuum. The life of the heating element is practically without limit when operating in vacuum as long as it is not damaged by the furnace charge.

Heating elements in accordance with the invention were not visibly al ered after hours operation. The time required for preheating the furnace described, preferably under vacuum, to 2080" C. is only about 45 minutes. The construction of such furnace is very simple and unskilled personnel can easily be trained to operate An important advantage of the furnace, in accordance with the invention resides in the fact that it may be constructed and operated with much larger dimensions than were heretofore economically practical. It is, for example, no longer necesary to employ long, single piece rods of graphite and the like as resistance elements, which rods had to be produced to order for the larger furnaces. In accordance with the invention, it is now possible to build up the resistance element from a plurality of separate pieces which provide a plurality of joints and intermediate resistances transverse to the current flow in the element. The production of large furnaces is, therefore, substantially simplified and for the first time, economically practical in many instances.

The heating elements can be of varied construction, as may be seen from the modifications shown in the drawings, particularly Figures 2-8. They can, for example, be of closed construction. as shown in Figures 1 through 4, 6 and 8, or they can be of open construction, as shown in Figures 5 and '7. As shown in Figures 2 through 5, the upper and lower surfaces of the individual rings forming the resistance element can be shaped so that contact between the stacked rings does not extend over their entire cross section. When the heating element is built up of pieces of such size that there are vertical joints as well as th horizontal joints which provide the intermediate resistances, it is preferable to stagger the pieces as shown in Figures '7 and 8, to provide a brick-work like construction. The pieces can also be spaced from each other in the horizontal direction as shown in Figure 7, to provide a lattice-like heating element, whereby a saving in resistance material employed is obtained.

As shown, for example, in Figure l, the interior of the tubular heating element can serve to receive the material to be heated. Heating elements of brick-work like construction shown in Figures 7 and 8, have been found especially suitable for this purpose. It is also possible, of course, to provide a plurality of heating elements within the furnace housing, each of which serves to hold the material to be heated or between which the material to be heated may be placed in a common heating space.

The high temperature furnaces, in accordance with the invention, have many applications. For example, they can be employed for sintering highly refractory oxides such as alumina or similar heating operations in the ocramic field, which require very high temperatures. Previously, gas heating was employed for such purposes, as electric furnaces, which in themselves provide better operating and temperature control, were considered economically impractical in the large sizes required for large scale production.

The furnaces, in accordance with the invention, are also well suited for the productio of synthetic jewels. Furthermore, high melting metals such as tungsten may be sintered therein, and carbides such as tungsten and boron carbides may be produced with extremely high purity therein.

Also, it is possible to employ the furnaces, in accordance with the invention, for gas reactions. It is preferable, however, to preheat the furnaces in vacuum to 2000 C. before carrying out such gas reactions.

Iclaim:

1. In an electric resistance furnace having an interior walled space serving to receive material to be heated, at least one self-sustaining nonmetallic resistance element essentially forming the walls of the space serving to receive the material to be heated said resistance element being formed of a plurality of shaped bodies of a resistance material selected from the group consisting of graphite and metal carbides in contact with each other to provide joints which are transverse to the direction of electric heating current flowing through such resistance element, a gas-tight housing surrounding said walled space and means for evacuating said walled space.

2. In an electric resistance furnace having an interior walled space serving to receive material to be heated, at least one self-sustaining nonmetalllc substantially tubular resistance element having its interior free to receive the material to be heated, said tubular resistance element being formed of a plurality of shaped bodies of a resistance material selected from the group consisting of graphite and metal carbides arranged in contacting layers transverse to the longitudinal axis of the tubular element, means for supplying electric heating current to the ends of said tubular resistance element, a gas-tight housing surrounding said walled space and means for evacuating said walled space.

3. In an electric resistance furnace having an interior walled space serving to receive material to be heated, at least one self-sustaining nonmetallic upright substantially tubular resistance element having its interior free to receive the material to be heated, said tubular resistance element being formed of a plurality of shaped bodies of a resistance material selected from the group consisting of graphite and metal carbides arranged in contacting layers transverse to the longitudinal axis of the tubular element, means for supplying electric heating current to the ends of said tubular resistance element, a gas-tight housing surrounding said walled space and means for evacuating said walled space.

4. An electric furnace, in accordance with claim 3, in which the individual shaped bodies forming the tubular resistance element are composed of graphite.

5. An electric furnace, in accordance with claim 1, in which said resistance element is composed of graphite.

6. An electric furnace, in accordance with claim 1, in which said resistance element is composed of a material comprising a metal carbide.

7. In a process of operating an electric furnace having an interior walled space serving to receive material to be heated essentially formed by at least one self-sustaining non-metallic resistance element formed of a plurality of shaped bodies of the resistance material selected from the group consisting of graphite and metal carbides in contact with each other to provide joints which are transverse to the direction of electric heating current flowing through such resistance element the step which comprises maintaining a vacuum around said heating element at least until a temperature of 2000 C. is reached within said furnace.

8. An electric furnace in accordance with claim 3, in which th contacting surfaces of the shaped bodies of the resistance elements are tongue and grooved to provide a tongue and groove joint between the layers.

OTTO BRETSCHNEIDER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 882,788 Marsh Mar. 24, 1908 896,413 Reid Aug. 18, 1908 1,091,808 Calhane Mar. 31, 1914 ,132,684 Queneau Mar. 23, 1915 1,147,703 Brown July 27, 1915 1,352,086 Rose Sept. 7, 1920 1,432,505 Weidrick Oct. 17, 1922 1,456,891 Little May 29, 1923 1,553,379 Hellmund Sept. 15, 1925 1,557,074 MacFarland Oct. 13, 1925 1,572,881 Brace Feb. 16, 1926 2,430,171 Hatch Nov, 4, 1947 2,476,916 Rose et al July 19, 1949

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