US2481229A - Electrical heating element - Google Patents

Description

p 1949- G. LEWlN ELECTRICAL HEATING ELEMENT Filed Dec. 6, 1944 INVENTOR. GERHARD LEWIN BY ff%u A TTOQNEY PatentedSept. 6, 1949 PATENT OFFICE ELECTRICAL HEATING ELEMENT Gerhard Levin, Geneva, 1ll., assignor to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application December 6, 1944, Serial No. 568,851

3Claims. 1

This invention relates-to an electrical heatin element, particularly for use at high operation temperatures, such as above 900 C. and particularly above 1100 C.

More specifically the invention concerns a heating element comprised of an electrical resistor of metal oi high melting temperature, such as tungsten, molybdenum, iridium, platinum, osmium, and alloys of two or more of them, in particular of tungsten and molybdenum, e veloped by highly refractory insulating material, in particular alumina (ALaOs), magnesia (M80), and compounds of these oxides with silica (8102), such as sillimanite and mullite. Other highly refractory and at high temperatures electrically insulating masses or ceramics can also -be used.

This application is a continuation in part of my copending patent application Ser. No. 477,802, filed March 3, 1943, now Patent #2372312 issued March 27, 1945, for Improvements in electrical heating elements."

Considerable difficulties were encountered in enclosing the resistor gas-tightly within the highly refractory insulation, so as to avoid oxida= tion of the resistor material at high operation temperatures and in open air or other oxidizing media. Usually the heating elements were made in the form of rods and provided with electrodes on opposite ends for enclosing them in an electrical circuit. This necessitated a length of the heating element. exceeding the width of the furnace so that the electrodes would be contacted and held in position by counter-electrodes arranged outside or within holes of the furnace wall, through which the ends of the heating element were passed.

It is therefore an object of the invention to provide an electrical heating element the resistor of which is gas-tightly enclosed within the refractory cover in such a manner that the access of oxidizing or other gases capable of reacting det rlmentally with the resistor material at high operation temperatures is effectively prevented.

It is another object of the invention to arrange .the electrodes on one end of the electrical heating element so that it can be inserted into and removed from the furnace through one opening only.

It is still another object of the invention to provide an electrical heating element which projects into the furnace chamber in any desired direction and to any desired extent.

It is a further object of the invention to provide an electrical heating element which is relatively simple in manufacture and translates the in the electrical heating element 12.

2 heat developed essentially by radiation at temperatures above about 900 to 1100' C.

It is still a further object of the invention to provide an electrical heating element which can be operated in open air or in any other atmosphere per se detrimental .to the resistor material at relatively high operation temperatures, from about 900 to about 1500 C., for a relatively long period of time.

It is still a further object of the invention to provide an electrical heating element, which can be arranged within the space or chamber of the furnace in any desired position and particularly immersed into a batch, liquid or solid, contained therein.

It is still another object of the invention to provid an electrical heating element which can freely expand within the space or chamber of the furnace.

It is still another object of the invention to provide an electrical heating element the effective and heat developing portion of which is completely or almost completely within the space or chamber of the furnace while the electrodes are arranged within or outside of the wall of the furnace and not endangered by excessive heat.

These and other objects of the invention will be more clearly understoodwhen the specification proceeds with reference to the drawings in whichFig.1shows a vertical cross-section through a furnace with heating elements according to the invention, Fig. 2 a more schematical cross-section through a heating element exemplifying a preferred feature of the invention, and Fig. 3 in crosssection a portion of a modification thereof.

Referring to Fig. 1, I0 is a wall of refractory material of the type mentioned above, chamotte, etc, e. g. for heating, such as firing or sintering any articles or melting or refining a batch of metal, metal alloy, glass and the like. In proper level above the bottom of the furnace and the goods or batch contained therein, openings l l are provided in the furnace wall for mounting there- In each opening an annular electrode i3 is mounted by means of an electrically insulating cement or any other suitable insulation. The inner surface of contact i3 is screw threaded so that the lead-in electrode it which is screw threaded on its outside can be secure therein. An insulated conductor i5 is connected with contact i3 and at the outside of wall 10 with a suitable terminal IS. The heating element continues beyond electrode M to the outside and is provided at its end with another electrode II, which might be screw threaded on its outside and upon which another contact i8 is screwed which is connected with an insulated conductor l3 and terminal 23. It will be appreciated that instead of a screw connection between the electrodes and the contacts, any other, for instance, frictional connection between .them can be provided. If the atmosphere in the interior 2| of the furnace be air, there is no need for any gas-tight connection between electrode l4 and contact l3. If there is a particular atmosphere to be maintained within thgfurnace, such as a hydrogen atmosphere, or vapors or gasses are developed .therein during operation the escape of which to the outside of the furnace is undesirable, the connection between electrode I4 and contact i3 should be gas-tight; a screw-connection appears suitable for this purpose, though, as stated above, any other connection can be used.

As is to be seen in Fig. 1, the exemplified heating element can be considered as comprising two portions, one forming the heat radiating portion, arranged entirely within the furnace chamber and extending essentially from the end 23 of the heating element to a plane 39 which is laid through the element perpendicularly to its longitudinal axis. The other, leading-in or current feeding portion comprises the electrodes and their connection with the other heating portion Referring to Fig. 2, the heating element is shown there somewhat out of proportion in that, for claritys sake, the diameters and thicknesses of the different parts are shown on a substantially enlarged scale. The heat radiating portion of the heating element comprises a tube 22 closed at one end 23 and open at the other end 24. Within tube 22 and spaced therefrom, another tube 36 is arranged which projects near to the closed end 23 of tube 22 and also into the leading-in portion of the element. In the cylindrical space between tubes 22 and 36, from near the end 23 of tube 22 to plane 39, a resistor element is arranged consisting for instance of a coiled wire or narrow foil 33. The resistor element is connected on one end with a wire 31 threaded through tube 36 and connected at its free end, at 4|, with the leading-in portion of the element. The other end of coil 36 near plane 39 is connected with another Wire 31 arranged outside tube 36 and connected at 40 with another part of the leading-in portion of the element.

The leading-in portion of the element is comprised of various parts connected with one another and tube 22 by means of proper seals 25 and 33, preferably glass seals, the purpose of which is to establish a gas-tight connection between the various parts of the leading-in portion as well as to insulate electrically certain parts from each other, where required.

In particular, the leading-in or current feeding portion of the element consists of a hollow metal electrode 27 provided with a screw thread 28 on the outside and flanges 29, 33 projecting to opposite sides. A tube 26 is fitted upon and brazed or soldered to flange 29 in a gas-tight manner, and at its free end sealed onto the open end 24 of tube 22, for instance by means of a glass seal 25. Another metal tube 3| is fitted upon and brazed or soldered onto flange 30 in a gas-tight manner, and to it, or to electrode 21, or to metal tube 26all of which combined form an electrically conductive unit-the end of wire 31 is welded, in the exemplification of the invention at 46.

Another metal tube 32 is gas-tightly and insuiatingly connected with metal tube II by means of a glass seal 33, and at its free end gas-tightly connected with a second electrode 34. Since metal tubes 3| and 32 are insulated electrically from one another, wire 31 can be connected either to tube 32, at 4|, by welding or the like, or to the electrode 34.

Electrode 34 is for instance screw threaded at its outside.

It will be appreciated that electrode 21 corresponds to electrode l4, and electrode 34 corresponds to electrode I! of the heating element :12 illustrated in Fig. 1.

In order to permit of producing temperatures in operation from 900 C. to 1450" C. and possibly higher, tube 22 has to be highly refractory as well as gas-tight and is therefore made preferably of aluminum oxide (A1201) or a silicate thereof, such as sillimanite, or magnesium oxide or a silicate thereof, or oxides of thorium, beryllium, zirconium, or highly refractory silicates thereof. These as pure as economically possible, oxides or silicates may either be used singly or in any suitable mixture. Oxides or silicates which vitrify upon firing at suitably high temperatures are pre ferred because they result in gas-tight fragments which retain their gas-tightness over many thousand hours of operation. In manufacturing the tube, any well known ceramic process can be used.

The inner tube 36 is arranged for electrically insulating coil 33 and wire 31 from the other leading-in wire 31. Tube 36 also extends over the heating portion of the element and must therefore be both electrically insulating and highly refractory in the sense that it remains solid and retains its shape at highest operation temperature. Therefore, tube 36 is made preferably of the same type of refractory as tube 22. It may be produced for instance by extruding a slip of the selected highly refractory and electrically insulating material through a nozzle havizntgJ a mandrel, drying and firing the extruded It will also be appreciated that the full voltage prevails between the two wires 3'! inside and outside tube 36, and outside plane 39, while this voltage difference gradually decreases toward the end of tube 36 and close to the end 23 of tube 22. Therefore the portion of tube 26 within the heat radiating zone is under gradually decreasing electrical strain while the portion of tube 36 within the end 24 of tube 22 and the leading-in or current feeding portion of the element is subjected to the full voltage difference. If voltages are used for feeding the element which exceed about to 240 volts, it becomes advisable to make the latter portion of tube 36 of a refractory of higher electrically insulating quality than its other part within the actual heating portion. In such case tube 36 may be made of two different parts, the one within the leading-in portion consisting par ticularly of beryllium oxide which has a very high electrical resistivity, while the other portion is made of aluminum oxide or its silicates, or one or some of the other refractory materials mentioned above. To thi end for instance slips of the different materials are prepared, a tube made of each of them by extrusion, the two tubes in their still plastic condition pressed together at contacting ends, and thereafter pre-flred and finally fired at temperatures well above the highest operation temperature of the heating element.

Coil 38 and wires 31 are conveniently a single piece of wire, the material of which is chosen much from that of tube 3|.

mamas according to the highest temperature to be produced. If operation temperatures not exceeding 1100 C. are contemplated, a nickel-chromium alloy or platinum may be used for them. It is preferred, however, to use molybdenum for coil 38 and wires 81, if operation temperatures up to about 1300" to 1500" C. are contemplated, or a molybdenum-tungsten alloy preferably in equal proportions by weight of the components, and pure tungsten, iridium, osmium or alloys thereof for still higher operation temperatures.

Coil 38 is produced in well known processes and by means of well known machines. The pitch of the windings may be the same over the entire length of the coil if essentially equal temperatures are to be produced over the heat radiating length of tube 22. If another law of temperature is to be realized, for instance, gradually decreasing temperatures over a part of tube 22 near plane 39, then the pitch of the windings of coil 38 near plane 39 can be increased and thereby the temperature produced by them reduced near plane 39.

The parts of the leading-in portion of the element are made of materials suitable for the purposes they have to serve. Tubes 26 and M are preferably made of a nickel-iron alloy or a nickel-cobalt-iron alloy, such as known under the trade names Kovar or Fernico. Both of them are brazed or soldered to flanges 29 and 30, respectively, in a gas-tight manner. If soldering is used for establishing the gas-tight connection, a suitable hard solder, such as that known under the trade name Easy-fie or a silver solder is used, having a flow point above about 630 to 960 C. The electrode 21 may consist of any suitable metal or metal composition, such as nickel, bronze, Monel metal and stainless steel.

The glass seal 25 between metal tube 26 and the end 24 of tube 22 consists of any suitable kind of glass, the heat expansion of which is between those of the metal and the ceramic; the kinds of glass known as "Corning 775 or "Nonex 772 proved suitable. Since this seal provides a gas-tight connection only, any other suitable seal can be used instead, such as illustrated in Fig. 3. A metal deposit 48 is burned onto the preterably non-glazed end of tube 22, and the inner end of metal tube 26 is brazed or soldered upon it, for instance by means of a solder 49 of the type mentioned above.

Tube 32 may consist of the same metal or metal composition as tube 3|; this tube is not exposed to any considerable heat, and therefore may be made of any other suitable metal. However, 'its heat expansion should not differ too The glass seal 33 between tubes 3| and 32 serves the double purpose of gas-tightly connecting and electrically insulating the two tubes from one another.

Hence it should be of a suitable kind of glass, the heat expansion of which matches the one oi the two metal tubes, 1. e. equals them as much as possible within the temperature range to which these tubes are heated during operation. Besides all the kinds of glass mentioned above for effecting the glass seal 25, also a glass known as Corning G12 has proved suitable.

Electrode 34 is provided with a bore 35 for a purpose to be explained later on. It is brazed, welded or soldered into the open end of tube 32. Bore 35 is closed at its outer end by means of a solder 42 of any suitable type, such as mentioned above.

In assembling the heating element, tube 28 is first sealed upon tube 22. Tubes 8| and 32 are separately assembled by producing between them glass seal 38. t

Then tube 38 with coil 38 arranged on its outside and wire 31 threaded through it are slipped into tube 22 with tube 26 sealed thereto; then hollow electrode 21 is threaded over the and oil tube 26 and the ends of the two wires 87 and flanges 29 fitted into tube 26; thereupon the end of the outer wire 31 is welded to the inside of tube 3!, at All, and the end of the inner wire 87 is welded to the inside of the tube 32 at Al, and the unit consisting of tube ti, 32 and glass seal 33 is fitted upon electrode 21. Thereafter the ends of tubes 28 and ti fitted upon flanges 2d and 30. respectively, are welded to the flanges, for instance, by lowering an induction coil into the position shown in Fig. 2 and applying induction current, preferably high frequency current, to it. Instead, tube as may be brazed or soldered individually to electrode 27 before the unit 3t, 32, 33 has been fitted upon flange 30, and thereafter tube N is brazed or soldered upon flange 20 and/or electrode 21.

Now electrode 34 is fitted into the open end of tube 32 and welded to it by induced currents, or brazed or soldered thereto.

One outstanding difficulty encountered in using heating elements of this type consists in the deterioration of coil 38 at the high operation temperatures due to oxidation by vaporsof moisture within tube 22 and the space enclosed by the leading-in or current feeding unit, or the action of other gases or vapors contained therein, such as occluded in the wire and various tubes.

It has been found that this difficulty can be successfully overcome by using proper essentially metallic getter materials within the enclosed space of the heating element. Such getter materials absorb or fix such vapors and gases or combine with them whenever they are present or freed in the interior of the heating element during operation, and in particular form oxides with them which are stable at operation temperature.

If the operation temperature does not exceed about 900 to 1100" C., the use of a getter material of the type of phosphorous (preferably yellow phosphorous), barium and sodium has proved suitable. Such material is mixed in finely divided state with a volatile organic liquid, such as alcohol or acetone, or with water to form a pasty slip which is applied to the windings of coil 38 for instance by brushing the slip upon it.

The coil is heated thereafter to a temperature preferably above C. but below 200 C. whereby the binder evaporates and the getter material is left upon and adheres to the windings oi the coil. By roughening the surface of the wire forming the coil, the adherence of the essentially metallic getter material to the wire can be improved. This application of the getter material is preferably eiiected in open air before the coil and tube 26 are slipped into the tube 22.

If higher operation temperatures than those mentioned above are intended, getter materials of the type of metallic zirconium, melting at 1900 0., and metallic thorium, melting at about 1845 C. are preferred. Due to their metallic condition, these getter materials are apt, however, to short circuit adjacent windings of coil 38 and, therefore, according to the invention, admixed with powdery thorium or zirconium oxide or other highly refractory and insulating oxides or 7 other materials, a slip formed of such mixture by the use of an organic volatile binder or of water, the slip applied to coil 38 and the latter heated within the temperature range stated above so that the binder evaporates and the mixture of the getter and insulating materials is deposited upon and between the preferably roughened windings of coil 38. This procedure is suitably effected before coil 38 and tube 36 are slipped into tube 22.

After the heating element has been prepared and assembled in the way described hereinbefore, it is positioned in a vacuum chamber 46 indicated in dotted lines in Fig. 2. A pipe 41 connects the chamber with a vacuum pump (not shown). An induction coil 43 is arranged within that chamber and its leading-in wires 44, 45 insulatingly led through the top of the chamber to the outside where any suitable alternating, particularly high frequency induction current, can be applied to them. A ring shaped solder 42 was applied to the end of electrode 34 before chamber 46 was closed.

Now vacuum is applied to chamber 46 and thereby air, moisture and other gases removed from the interior of the heating element through bore 35 and the opening in ring 42. A vacuum of a few millimeters to a fraction of one millimeter may be attained at the end of the evacuation period. Thereafter induction current is applied to coil 43, and electrode 34 and solder 42 are heated to a temperature where the latter melts and thereby closes the outer end of bore 34. Thereupon the induction current is shut off, air admitted to chamber 46, and finally the completed heating element is removed from the chamber. However, in view of the small content of air in the confined space within tube 22 and the unit supplyin the heating current (the diameters of which are shown at enlarged scale as hereinbefore stated), evacuation of that space can be omitted in certain cases.

During operation the heating portion of the element is at highest temperature. If the latter be between about 900 to 1100 C., and a getter applied consisting of phosphorous, etc., the latter will be evaporated but sublimate or precipitate again within the interior of the leading-in or current feeding portion of the heating element which is far cooler than its heating portion. An equilibrium between the vapor pressure of the vaporized getter material in the heating zone and that of the Sublimated or precipitated getter in the leading-in portion of the element will -soon be established. Whenever getter-vapors in the heating portion are used up by fixing, etc., vapors of moisture and gases freed during operation, they will be replaced by vaporisation of sublimated or precipitated getter material in the interior of the relatively cool leading-in portion of the element.

If, however, the element is used for producing higher temperatures, and metallic zirconium or thorium is used as getter material, the latter and the admixed powdery refractories will be sintered upon the windings of coil 38 at those high operation temperatures, and remain there as a continuously effective getter material. The sintered metallic getter and insulating material will also serve to keep the windings of the coil in place and to separate them from one another as well as from the inside surface of tube 22. However, if the highly heated coil is not spaced from but contacts the inside surface of tube 22, the efiiciency of the element is still improved by direct conveyance of heat to tube 22 in addition to heat conveyance by radiation from the coil. It is well understood that the outside of tube 22 forms the active heating surface, operating mainly by radiation of heat.

Modifications of the leading-in and/or heat radiating portions of the heating element as exemplifled in my above identified copending patent application may be used; they do not form a subject matter of this invention.

A mode of mounting the heating element in an opening of the furnace wall is illustrated in Fig. 1; other modes, such as exemplified in my above identified copending patent application, can of course be used.

From the above it will be appreciated that the invention provides. a heating element with both electrodes on one side or one electrode at each end, which can be easily mounted, supplied with heating current, removed and replaced and used in any position relative to the furnace. Any law of heating can be realized and in particular high est temperatures produced at some distance from the furnace wall so as to keep the leading-in portion of the element at temperatures at which the seals, and particularly glass seal 25, is not softened and endangered.

The upper limits of the temperatures produced over the effective heating portion of the element are given by the softening temperature of the covers l2 or 22 which must be avoided in order to prevent deformation or sagging of the heating element. The melting temperature of the resistor proper must also be considerably higher than the highest operation temperature.

By the use of proper getter materials safe operation of the heating element over several thousands of hours is secured and its manufacture is simplified since evacuation does not have to be driven too far, and can be omitted in some cases, nor is, in general, heating of the element during evacuation necessary in order to drive off occluded or absorbed moisture and other detrimental vapors and gases.

Although the invention is not limited to any particular material of the resistor and cover as well as their dimensions, it may be said that within a range of operation temperatures from about 1300 to 1500.C. the wall thickness of tube 22, Fig. 2, may amount to about its inner diameter to about ,41" to /2", and the overall outer diameter of coil 38 to /32" to /64 less than the interior diameter of cover 22.

While it has been suggested above to place the essentially metallic getter material onto and between the windings of the coil, it may also be sprayed or applied in any other way onto the inside of tube 22. During operation, any traces of gases and vapors developed or freed from the resistor proper, and/or the interior of tube 22 will be absorbed or fixed by the getter material and thus any deterioration of the resistor, such as oxidation, be prevented over a long time of the life of the heating element.

If a getter material, such as powdery metallic zirconium or thorium, admixed with powdery zirconium oxide, thorium oxide, beryllium oxide, etc., is used and intended to be sintered upon the coil 38 and/or the inside of tube 22, the completed element should be preformed by heating it to sintering temperature of themixture before it is shipped or put into operation. In such a mixture of powdery getter and insulating materials the components may be mixed in equal or any other suitable ratio, securing the desired getter and insulating eiiects. If metallic zirconium and aluminum oxide are used, a mixture of 35% by weight of the former and 65% of the latter proved suitable.

It should be understood that the invention is not limited to any particular exemplification hereinbefore described and shown in the drawings but is to be derived in its broadest aspects from the appended claims.

What I claim is:

1. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a heating portion and at least one leading-in portion for the heating current, said heating portion including a tubular cover of highly refractory, electrically insulating and gas-tight material and within said cover an electrical resistor, such as a wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by a chromium-nickel alloy, tungsten, molybdenum, platinum, iridium, osmium and their alloys, said leading-in portion inmetallic getter material of a melting temperature 2. In an electrical-heating element as set forth in claim 1, a granular getter material of a melting temperature considerably above operation temperature, as exemplified by zirconium and thorium, admixed with grannular highly refractory and insulating material.

3. In an electrical heating element as set forth in claim 1, a granular getter material of a melting temperature considerably above operation temperature, as exemplified by zirconium and thorium, admixed with granular highly refractory -and insulating material, said resistor-being coiled and said mixture sintered onto and between th windings of said resistor.

GERHARD LEWIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,937,706 McCullough Dec. 5, 1933 1,952,717 Lederer Mar. 27, 1934 2,116,432 Gustin Ma 3, 1938 2,167,762 Lockwood Aug. 1, 1939 2,215,587 Kerschbaum Sept. 24, 1940 2,231,236 Wentworth Feb. 11, 1941 2,280,977 Reichmann Apr. 28, 1942 2,297,780 Pugh Oct. 6, 1942 2,372,212 Lewin Mar. 27, 1945 OTHER REFERENCES Yarwoods "High Vacuum Technique, pages exceeding the highest operation temperature ar- 38-49.

ranged within said confined space near said resistor.

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