US2831909A - Electrical resistance element and furnace containing the same - Google Patents

Description

April 22, 1958 w, c. sElFERT ELECTRICAL..RES1STANCE ELEMENT AND FURNACE' 'CONTAINING`- THE' SAME Filed Dec. 3. 1956 VACUUM PUMP www

INVENT OR WALL/,4M c. sE/FERT ATTORNEY ELECTRICAL RESISTANCE ELEMENT AND FURNACE CONTAINING THE SAME William C. Seifert, Ardmore, Pa., assignor to Donald W. Kent, Philadelphia, Pa.

y' rApplication December 3, 1956, Serial No. 626,050

4 Claims. (Cl. 13-31) I This invention is a new and useful electrical resistance element of the direct radiation type and furnace containing the same. The invention will be fully understood fromthe following description read in conjunction with the drawing in which: v

Fig. 1 is a diagrammatic showing of one step in the manufacture of the element of my invention;

Fig. 2 is a diagrammatic showing of a second step in the manufacture of said element; y

Fig. 3 is a longitudinal section through a furnace containingv said electrical resistance element; p

Fig. 4 is a transverse section through the showing in Fig. 3 on the plane indicated by the line 4 4; and

Fig. 5 is a top view through part of the construction shown in Fig. 3 on the plane indicated by 5-5. j

In my prior application, Serial No. 577,128, filed April 9, 1956, I described an electrical resistance element of the direct radiation type consisting of an external, tubular sheath of high melting point metal, surrounding a core of high melting point inorganic oxide, and reduced in diameter uponsaid core by longitudinal extension to compress said core to maximum density. Such resistance elements are produced in the form of long, tubular rods, which arey then bent into loops at evenly spaced intermediate points to form aqseries of parallel rods of even length,

connected at their ends by congruent loops. The blanket heater so formed is held in the furnace by hanging the top.` loops on refractory projections set in the refractory inside walls of the furnace. Such heaters are characterized by structural stability at high temperatures; they do not warp or distort even at temperatures in excess of 2000" F. They have uniform longitudinal resistance and circumferentially uniform radiation characteristics. Such heaters are preeminently adapted to use in furnaces operating under high vacuum although they may also be used in furnaces in which special atmospheres are maintained.

When operating under vacuum the heat is of course communicated to the charge in the heating zone exclusively by radiation, and in such case it is important that the heater elements be capable of transmitting the largest possible amount of radiant heat in a given time. The instant invention is an improvement on the construction described and claimed in said prior application, by which the heat input is maximized and the radiation more uniformly and intensely distributed within the furnace. Although preeminently suited to use under vacuum, it is also 'advantageous where the furnace is operated with air or a special atmosphere.

4Referring to Fig. 1, tube 1 is made of a metal which is cold-workable and which has a high residual, tensile strength and an extremely low vapor pressure even at temperatures in excess of 2000 F. The exact composition will be determined by whether it is to be installed in a furnace operated under vacuum or in which a special atmosphere is to be maintained. For furnaces operating under'high vacuum, the following are examples of alloys which are suitable:

Example I Percent Nickel 77-80 Chromium 17-20 Aluminum l- 3 Small amounts of manganese, carbon, etc.

Example 2 Percent Iron 81-85 Chromium 12-15 Nickel 3 4 In general the alloys conventionally known as NichrOme, Chromel and Inconel are suitable. The alloys commonly known as the stainless irons are also suitable. For use at temperatures considerably in excess of 2000 F. platinum may be employed.

For the purpose of fabricating the element 1 of my invention, the lower end of tube 2 of the specified composition is sealed by squeezed portion 3 and the upper end is connected to hopper 4 provided with vibrator 5 and containing finely divided, high-melting point, inorganic oxide 6, suc-h as alumina, zirconia, or magnesia. By operating vibrator 5, tube 2 is completely filled with oxide. For certain critical uses this operation will be performed under vacuum. The other end of tube 2 is then sealed by squeezed portion 7 and engaged by gripper jaws 8 upon which traction is exerted by chain 9 to draw the tube through swaging die 10 secured in holder 11. The drawing reduces the external diameter without substantially affecting the wall thickness, thereby reducing the enclosed volume and compressing the contained oxide. The reduction is carried to the point at which the oxide has reached maximum density, i. e., at which the voids are substantially eliminated. This results in a current carrier characterized by extreme uniformity of thermal radiation combined with exceptional strength and rigidity at extremely high temperatures and substantially complete freedom from warping.

Referring to Figs. 3 and 4, the furna-ce 12 is enclosed in a shell consisting of tubular section 13 terminating in bumpedl ends 14 and 15. Bumpedend 14 is connected through pipe 17 to vacuum pump 18 discharging through pipe 19. Bumped end 15 carries the anged opening 21 which may be sealed, vacuum-tight, by dished cover 22 suspended from the crane arm 23. Internally shell 13 is provided with refractory lining 24, defining side walls Z5 and 26, roof 27 and lioor 28. If a special atmosphere is to be maintained in the furnace, the gas suitable for this purpose may be introduced through the valved pipe 29 (Fig. 3).

By reference to Figs. 4 and 5 it will be noted that each of the side walls carries a series of refractory projections 30 which extend horizontally at a right angle to the wall, and thence at an angle of about 45 to form supports for the resistance element 31.

In preparing the resistance element 31 the tube 2 hereinabove referred to, kswaged upon the refractory filling and elongated by swaging, is bent into loops at evenly spaced intervals to form a series of parallel rods, spaced apart between 3 and 6 diameters on centers, which can be suspended from these projections or supports without touching the floor 28 of the furnace. The tubes as produced by swaging are long enough so that the free ends may be bent at a right angle, as indicated by numerals 32 and 33 in Fig. 4, to be carried through the shell 13 Ce Patented Apr. 22, 1958 both pairs, is bent with reference to the next succeeding pair in .such a way that the median plane of each intersects at an included angle gb of between 75 and 105 and preferably of substantially 90. This construction is evident from the top viewthereof in Fig. 5. Before the bumped end 15 is placed in position and sealed to shell 13, the heater 31 soforrnedis carried into the space defined byk the refractory lining 24. The` upper ends of the loops are hung on the free ends of the projections 30 extending from one of the side walls 25 or 26, with .the free ends of the heater extending through the refractory lining and through stuffing boxes such as 34, to permit of the establishment of the necessary electrical connections to include the heater 31 in an electric heating circuit. The free ends which project through the refrac- :tory lining and the stufling boxes are reinforced with cxternal metal .sleeves to lower the resistance at this point. Temperatures obtaining in the furance are measurable by thermo-couple 36 in combination with any suitable recording instrument (not shown).

Referring to Fig. 4, transformer 41 receives current lthrough primary connections 42 and 43 andsupplies current through secondary posts 44 and 45. The end 32 of heater 31 shown in the figure is electrically connected through lug 46, conductor 47'and lug 48, to secondary post 44. The end 33 of the tubular heater carried by. the wall 26 .shown in the iigure is electrically connected through lug 51, conducto-r 52 and lug 53, lto secondary post 44. The other ends of the tubular heaters are similarly connected with post 45 through similar conductors and lugs.

In operation the tubular heaters may, for example, be maintained at a temperature of about 2150 F. tohold the furnace interior within 50 F. of the resistor. tem. perature. Radiation within the interior of the furnace is extremely intense and of uniform distribution. If the furnace is to be operated under vacuum, the Valve in valved pipe 29 remains closed, and aftercharging and before heating, the desired degree of vacuum is lcreated within the furnace by the operation of vacuum pump 18. If the furnace is t be operated with a special atmosphere following evacuation, the gas necessary for this purpose isintroduced through valved pipe 29.

I claim:

1. An electrical resistance element of the direct radiation type consisting of an external tubular sheath of high melting point metal surrounding a core of' high melting point inorganic oxide and reduced in diameter upon said core by longitudinal swaging .to compress said core to maximum density, said tube bent to form a continuous series of parallel vertical rods connected at their ends by congruent loops, each connected pair of rods being spaced apart between 3 and 6 diameters on centers and the median plane of each vsucceeding pair sharing one rod in common intersecting at an included angle between and 105.

2. An electrical resistance heater according tokclaim 1 in which said median planes intersectat an angle of about 3. A resistance type electric furnace comprisingy a refractory setting defining` a heating zone, and within said heating zone an electrical resistance element of the direct radiation type comprising an external tubular sheath of high melting point metal surrounding a core of high melting point inorganic oxide reducedin diameter by longitudinal swaging to compress said core to maximum density, said tubular sheath beingbent ytoform a continuous series of parallel rods wit-h.connecting loops at their ends, each connected pair of rods being lspaced apart between 3 and 6 diameters on centers, the median-plane of each succeeding pair sharing one rod in common intersecting at an included angle between 75 and 105, said electrical resistance Velement positioned in saidfumacel with said rods vertically oriented and with the loops at the upper ends of said rods carried by projections extending horizontally from a side wall of said furnace.

4. A resistance type electric furnace according to claim 3 including, in addition, a vacuum-tight shell enclosing said heating zone and means for evacuating said shell` and said heating zone.

References Cited in the file of this patent UNITED STATES PATENTS 953,774 Appelberg Apr. 5, 1910 1,359,400 Lightfoot Nov. 6, 1920 1,555,542 Young Sept. 29, 1925 1,849,288l Fahrenwald Mar. 5, 1932 2,717,915 Shapiro Sept. 13, 1955 FOREIGN PATENTS 11,128 Great Britainv of 1908 142,876 Switzerland Dec. 16, 1930

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