US2372212A

US2372212A - Electrical heating element

Info

Publication number: US2372212A
Application number: US477802A
Authority: US
Inventors: Lewin Gerhard
Original assignee: American Electro Metal Corp
Current assignee: American Electro Metal Corp
Priority date: 1942-03-03
Filing date: 1943-03-03
Publication date: 1945-03-27
Anticipated expiration: 1962-03-27
Also published as: GB595060A

Description

March 27, 1945. G, LEWIN 2,372,212

ELECTRICAL HEATING ELEMENT Filed Mar ch 3, 1943 3 She ats-Sheet 2 INVENTOR 652144190 A EW/N ATTORNEY March 27, 1945. a. LEWlN 2,372,212

I ELECTRICAL HEATING ELEMENT Filed March 3, 1945 3 Sheets-Sheet 3 34 INVENTOR GEQHA PD LEW/N ATTORNEY Patented Mar. 27, 1945 ELECTRICAL HEATING ELEMENT Gerhard Lewln, Geneva, Ill., asslgnor to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application March 3, 1943, Serial No. 477,802

16 Claims.

This invention relates to an electrical heating 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 of high melting temperature, such as tungsten, molybdenum, iridium, platinum, osmium, and alloys of two or more of them, in particular of tungsten and molybdenum, enveloped by highly refractory insulating material, in particular alumina (A1203), magnesia (MgO), and compounds of these oxides with silica (S102) such as sillimanite and mullite. Other highly refractory and at high temperatures electrically insulating masses or ceramics can also be used.

Considerable difficulties were encountered in enclosing the resistor gas-tightly within the highly refractory insulation, so as to avoid oxidation 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. Thi necessitated a length of the heating element exceeding the width of the furnace so that the electrodes could 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 detrimentally 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 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 provide an electrical heating element which can freely expand within the space or chamber of the furnace.

It is another object of the invention to provide an electrical heating element which can easily and quickly be replaced while the furnace is in operation and/or at high operation temperature.

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.

It is still a further object of the invention to rovide an electrical heating element which can be used substantially independent from the thickness of the wall of the furnace, and the electrodes of which need no artificial cooling.

These and other objects of the invention will be more clearly understood when the specification proceeds with reference to the drawings in which Fig. 1 shows 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, Fig. 3 in a similar manner a modification thereof, Fig. 4 a cross section through a heating element according to Fig. 3 with a changed detail, Fig. 5 in cross section a further modification of the upper part of the element shown in Fig. 2, Fig. 6 in cross section a portion of the latter during manufacture, Figs. 7 and 8 partly in cross section and partly in elevation different mountings of the element in the furnace wall, Fig. 9 in schematical cross section a heating element with electrodes on both ends, and Fig. 10 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 II are provided in the furnace wall for mounting therein the electrical heating elements I2. 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 14 which is screw threaded on its outside can be secured therein. An insulated conductor I is connected with contact I3 and at the outside of wall Ill with a suitable terminal IS. The heating element continues beyond electrode l4 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 18 is screwed which is connected with an insulated conductor l9 and terminal 20. It will be appreciated that instead of a screw connection between the electrodes and the contacts, any other 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 I3. If there is a particular atmosphere to be maintained within the furnace, such as a hydrogen atmosphere, or vapors or gases 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 it is to be seen in Fig. 1, the 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 vertically 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 thedifierent 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 of a coiled wire 38. The resistor element is connected on one end with a wire 31 threaded through tube 36 and connected at its free end, at M, with the leading-in portion of the element. The other end of coil 38 near plane 39 is connected with another wire 31 arranged outside tube- 33 and connected at fill 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 glass seals

25 and 33, 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.

In particular, the leading-in or current feeding portion of the element consists of a hollow metal electrode 21 provided with a screw thread 28 on the outside and

flanges

29, 30 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 connected with the open end 24 of tube 22 bymeans of a glass seal 25. Another metal tube 3! is fitted upon and brazed or soldered onto flange 30 in agas-tight manner, and to it, or to electrode 21, or to metal tube 26-all of which combined form an electrically conductive unit.the end of wire 31 is welded, in this exemplification of the invention at 40. r

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

Electrode 34 is screw threaded at its outside.

It will be appreciated that electrode 21 corresponds to electrode l4, and electrode 34 corresponds to electrode ll of the heating element I2 in Fig. 1.

It will be further appreciated that tube 22 has to be highly refractory as well as gas-tight, in order to produce temperatures in operation from 900 C. to 1450 C. and possibly higher. Therefore tube 22 is made of aluminum oxide Alzm 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 preferred 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. Thus the finely powdered material admixed with a volatile binder is .pressed to shape and fired thereafter at temperatures close to but below the melting temperature of the material, preferably at a temperature which is somewhat higher than the highest operation temperature. Experience shows that thereby gas-tightness of the tube will be retained over a long period of life of the element. There may also be produced a slip of the highly refractory material, the slip shaped in a liquid absorbing mold in a well known manner, the shaped fragment pre-fixed at relatively low temperature and thereafter finally fired at a temperature the same as in producing the tube from powdery initial material.

The inner tube 36 is arranged for electrically insulating coil 38 and wire 31 from the other leading-in wire 31. It therefore has to consist of electrically insulating material. Tube 36 also extends over the heating portion of the element and must therefore be highly refractory in the sense that it remains solid and retains its shape at highest operation temperature. Therefore, tube 36 is made of the same type of refractory as tube 22. It may be produced by preparing a slip of the highly refractory and electrically insulating material selected, extruding it through a nozzle having a mandrel, drying the extruded tube and pre-firing and finally firing it at temperatures the same as tube 22.

produced by them reduced near plane 39.

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 diiference gradually decreases toward the end of tube 36 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 i subjected to the full voltage difference. If voltages are used for feeding the element which exceed about 120 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 particularly of beryllium ox'de which has a very high electrical resistivity, while the other portion is made of aluminum oxide or its silicates, or one or ome of the other refractory materials mentioned above. To this end 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 pro-fired and finally fired; thereby a strongly coherent single tube is obtained theportion of higher electrical resistance of which is arranged within the feeding-in portion of the element projecting into tube 22 near plane 39 where high temperatures and large voltage gradients prevail, while the other portion of lower electrical resistivity lies within the heat radiating portion of tube 22. Tube 36 may also consist of short pieces about 1 to 3" long and of various composiitons, if desired. This facilitates the assembly.

Coil 38 and wires 31 are usually a single piece of wire, the material of which is chosen 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 wres 31, up to operation temperatures of about 1300 to 1500 C., 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 temperatures.

Coil 36 i produced in well known processes and by means of well known machines. It will be appreciated that the pitch of the windings be the same over the ant-re length of the coil if essentially equal temperatures are to be produced over the heat radiating length of tube 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 In this manner overheating can be avoided where the elements are supported within the furnace.

The parts of the leading-in port on of the element are made of materials suitable for the purposes they have to serve. Tube 26 is preferably made of a nickel-iron alloy or a nickel-cobaltiron 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 connect'on, a suitable hard solder, such as that known under the trade 775" or Nonex" proved suitable.

Tube 32 may consist of the same metal or metal composition as tube 3|; it will be appreciated that this tube is not exposed to any considerable heat, and therefore maybe made of any other suitable metal. However, its heat expansion should not diifer too much from that of tube 3|. Theglass 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 of the two metal tubes, i. e., equal them as much as possible within the temperature range to which these tube 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 particularly if tubes 3| and 32 are made of a n ckel-iron alloy.

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, tub 26 is first sealed upon tube 22. This i performed by introducing a hollow cylindrical punch 99, F g. 6. inside tube 26, slipping another hollow punch 98 over tube 22 and pressing both punche against the molten or plastic glass 25 between

tube

22 and 26 which, upon solidification, effects the gastight seal between the tubes.

Tubes 3| and 32 are separately assembled by producing between them glass seal 33 in a similar way as glass seal 25. Also separately, tube or tubes 36 are slipped over the inner wire of coil 38 and the end of the outer wire 3? is welded to the inside of tube 3| at 40, while the end of the inner wire 3i is welded to the inside of tube 32 at it. Then the free end of tube is fitted upon flange 29, with tube 22 attached to tube as by means or the glass seal 25. Thereafter coil is slipped into tube 22 and the free end of tube 3? over flange and the ends of tube 28 and 3| are welded, brazed or soldered to the flanges 23, respectively, 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 26 may be brazed or soldered individually to electrode 2! before the unit 3|, 32, 33 has been fitted upon flange 30, and after the latter has been done, tube 3| can be brazed or soldered upon flange 30 and/or electrode 2'! ina similar way.

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 difliculty encountered in using heating elements of this type consists in the deterioration of coil 38 at the high operation temperatures due to oxidation by vapors of water 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 difilculty can be successfully overcome by using proper 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.

If the operation temperature does not exceed about 900 to 1100 C., the use of a getter material of the type of phosphorus, barium and sodium has proved suitable. Red phosphorus 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 100 C. but below 200 C. whereby the binder evaporates and the getter materialis left upon and adheres to the windings f the coil. By roughening the surface of the wire forming the coil, the adherence of the getter material to the wire can be improved. This application of the getter material is preferably effected in open air before the coil and tube 36 are sunk or slipped into the tube 22. Barium is best applied by using getter pills available on the market and which consist, e. g., of a mixture of Ba and Mg powders pressed into a small tablet. Such pills may be deposited inside tube 22 near plain 39 on top of coil 38.

If higher operation temperatures than those mentioned above are intended, getter materials of the type of metallic zirconium, melting at 1900 C., 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, powdery thorium or zirconium is admixed with powdery aluminum oxide, thorium oxide, zirconium oxide or other highly refractory and insulating oxides or 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 th 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, 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 36 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 38 and solder 82 are heated to a temperature where the latter melts plastic glass to form seal 25.

and thereby closes the outer end of bore '34. Thereupon the induction current is shut off, and the solder permitted to solidify. Then the vacuum is cut off, air admitted to chamber 46, and finally the completed heating element is removed from the chamber.

During operation the heating portion of the element is at highest temperature. be between about 900 to 1100 C., and a getter applied consisting of phosphorus, 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. Phosphorus will be deposited as very active yellow phosphorus.

If, however, the element is used for producing higher temperatures, and metallic zirconium or thorium are 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 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 emciency 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.

Referring to Fig. 5, there is shown a modification of the leading-in portion of the heating element. There tubes 26 and 3| are fitted respec tively into a

recess

86, 81 provided on the inside of flanges 88, 89 which project to opposite sides from electrode 21. Tube 26 consists of Kovar and tube 3| is made of an iron nickel alloy. Since Kovar is preferably brazed with copper, tube 26 is first copper-brazed to electrode 21. Then tube 26 is connected to tube 22 by inserting a punch 38, indicated in dotted lines in Fig. 6, into tube 26 and slipping another punch 98 over tube 22 into the position also shown in dotted lines, and by squeezing between them the viscous or The assembly is done in a manner similar to the procedure described before, the only difference being that in this case the brazing of electrode 21 onto tube 26 is done before sealing

tubes

22 and 26.

There is also illustrated in Fig. 5 another manner of connecting electrode 36 with tube 32. In this exempliflcation, electrode 34 is solid and slipped into tube 32 after the entire heating element has been assembled. An annular piece of solder 18, e. g., of the shape of a wire-ring is put inside the slightly projecting edge of tube 32 onto a radially extending annular portion of electrode 36. Thereupon the assembled heating element is put into a glass container 96 the inner diameter of which is slightly larger than the outer diameter of the screw thread 28 and a stopper 9| is pressed into the open end of container 90. A tube 92 of glass or other material passes airtightly through an opening in stopper Ell, is provided with a cock and connected with a vacuum pump (not shown).

The interior of container 90 is thereafter evacuated, and vapors of water and gases sucked from the interior of the assembled heating element through the fine gap between the annular por- If the latter tion of electrode 34 and the inside of tube 32. After evacuation has been effected to a desired extent, an induction coil 93 is slipped over the outside of container 38 in front of electrode 34, proper induction current is applied thereto and thereby electrode 34 (which may consist of copper, bronze or the like) heated to a temperature at which solder 18 melts and seals the fine gap between the electrode and tube 32. Thereafter the induction current is shut off, the electrode and solder permittedto cool and the latter to solidify, then the vacuum pump is shut off, stopper 9| removed and the completed heating element taken out of container 90.

Reverting to Fig. 3, there is exemplified a modification of the heating element. Identical reference numbers indicate similar parts. Again and for claritys sake the diameters of tube 22 and of the other elements are shown in larger proportion than actually used.

Within tube 22 an insulating tube 36 is arranged through which wire 31 is threaded. Wire 31 branches off at 48 outside the lower end of tube 36, and is connected with coil 38 which is wound upwardly and continued by straight wire 54. Another branch 43 of wire 31 continues through another insulating tube 50 at the lower end of which it is connected with a second coil the upper end of which continues in the straight wire 52 threaded through another insulating tube 53. Wire 31 is connected at 41 with tube 32, while

wires

52, 54 are connected with tube 31 which, in this modification of the invention, is fitted within hollow electrode 21 and projects to opposite sides therefrom. Electrode 21 can be omitted and the outer surface of tube 3| used as terminal. Hence, coils 38 and 5| are arranged electrically in parallel and supplied through

electrodes

21 and 34 respectively with electric heating current. In this or any similar manner it is possible to arrange any number of heating coils within the tubular cover 22 and to operate them inparallel. The cross-sections of the wires forming coils 38 and 5t may be the same or different, and sim larly the distances between the individual windings of the coils may be the same or different. Thus one is enabled to obtain the same heating effect over the free and operative length of the heating element formed by tube 22 to plane 39, or to vary almost at will the law of heating or temperatures produced over that length. It is also possible to vary the distances between adjacent windings of each coil tube 22; thereby the heat developed near that plane will be lower than over the remaining portion of the heating element and the seals more effectively protected against undesired overheating. In the same way, a few windings near the closed end 23 of tube 22 may be more distant so as to avoid dangerous temperatures at that end of the cover.

Fig. 4 shows a crossection along plane 39 through the heating element shown in Fig. 3, but

with the following modification. Instead of using two

separate tubes

36, 53, a single insulating element 13 is used of a type available in the market, made of highly refractory material of the kind stated above and provided with two bores 14 parallel to the longitudinal axis of that element. Within each of these holes, the

wires

31 and 52 are arranged; around the outside of the insulating element 13, coil 38 is arranged.

A mode of mounting the heating element in an opening of the furnace wall is illustrated in Fig. I. Another mode is shown in Fig. '7, in which,

however, only the leading-in portion of the heating element is illustrated.

In the furnace wall It which may be vertical, or inclined, or form the top portion of the furnace, an opening II is provided or drilled, which is of larger diameter where it faces the outside of the furnace and of smaller diameter where it faces the heating chamber. The diameters are such that tube 26 fits into the larger one, considering its relatively small heat expansion during operation, while tube 22 fits in and projects from the part of opening ll of smaller diameter.

The arrangement is preferably such that electrode 21 is outside the furnace wall l0, and so are tubes 3|, 32 and electrode 34 which is, e. g.,-screw threaded and provided with a nut 11 for fastening a conductor between it and tube 32.

An annular contact 13 screw threaded on its inside is screwed upon electrode 21 and connected with a conductor 80. A circular disc 16, e. g., of a copper sheet is welded, brazed or soldered upon contact 18.

In this way a very simple mounting is obtained. In order to replace the heating element, it is drawn out of opening II and another one put in.

'The conductors are easily connected with and disconnected from it.

Fig. 8 shows a similar way of mounting a heating element in the furnace wall It. The opening H is cylindrical, and the heating element slipped through it so that tube 26 faces the opening. Between the tube 26 and the opening some resilient and heat resistant material is inserted, such as a ring 15 of asbestos. A disc 16, e. g., of a copper sheet or the like to form a cooling fin is brazed upon contact 21.

While the exemplifications of the invention dealt with above illustrate a heating element the electrical connections of which are arranged on one end, it is sometimes desirable to arrange the electrical connections on opposite ends of a rodlike heating element. Fig. 9 exemplifies such a proposition.

The heating element consists of a highly refractory tube 22 which is sealed on both ends into a tube 26 each of a metal composition the same exemplified hereinbefore, by means of glass seals 25. In the open end of each of the tubes 25 a solid electrode 34 is fitted and, after evacuation in a manner described hereinbefore, with reference to Fig. 5, sealed into it by means of a ring shaped piece of solder 18.

Inside tube 22 a resistor coil 83 of a metal composition stated above is positioned in such a manner that its ends are spaced from the glass seals 25 so that the highly heated portion of the rod is distant from those seals and the latter are not softened during operation. Coil 83 is continued on both ends by wires 8!, 82 the free ends of which are welded to the inside of a coordinated tube 26 before electrode 34 is inserted. A getter material properly selected and applied in the manner hereinbefore described, is also used. The assembly and evacuation is effected in a similar manner as described hereinbefore, it being understood that one electrode 34 is fitted and welded, brazed or' soldered into one of the tubes 26 before evacuation is effected, while the other solid electrode 34 is fitted into the other tube 26 but soldered to it only at the end of an evacuation process performed in the manner described hereinbefore with reference to Fig. 5. Onto each electrode 34 an annular contact 13 is, e. g., sicrelvged, each of which is provided with a cooling It will be appreciated that a rod of this type can be pushed through opening H on one side of the furnace, across the furnace chamber and through opening II on the other side of the furnace, whereupon the contacts 19 with fins 16 are screwed upon the electrodes 34. An asbestos ring may be inserted betweeneach tube 28 and opening H for the purposes stated hereinbefore.

According to a modification of a rod-like heating element, one end of which is shown in Fig. 10, a rod or tube 84 of highly refractory material the same as used for tube 36, can be inserted into coil 83 in order to keep the latter close to tube 22 during operation and to prevent any sagging of the coil.

From the above it will be appreciated that the invention provides a heating element with both electrodes on one side or one electrode at one end each, which can be easily mounted, fed with heating current, removed and replaced and usedin any position relative to the furnace. It does not need any artificial cooling, such as water cooling, of its ends. Its effective heating portion may be above the batch to be heated in the furnace, or be submerged into the latter at'least partially. The heating element may also cross the entire width of the heating chamber if it is given the form, e. g., of a rod (Figs. 9, 10), or the width of the furnace chamber may be covered almost entirely by an arrangement as shown in Fig. 1.

Any law of heating can be realized and in particular highest 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 glass seals, and particularly seal 25, is not softened and endangered.

The upper limits of the temperatures produced over the efiective 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 in order to avoid softening and deformation of the resistor; this can easily be accomplished by proper choice of the resistor materials exemplified hereinbefore.

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 ofi occluded or adsorbed 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 l300 to 1500 C. the wall thickness of tube 22, Fig. 2, may amount to about 1 its inner diameter to about to /2", and the overall outer diameter of coil 38 to 3 2" to $4 less than the interior diameter of cover 22.

While it has been suggested above to place the 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 the terminal or 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 powder, 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 the mixture 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 effects. 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 tobe 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 a said resistor and gettermaterial, two electrode I terminals, one each electrically connected with one end of said resistor, each of said electrodes gas-tightly connected with one end of a tubular metal body, and gas-tight glass seals for insulating each such body from and associating it with said cover.

2. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a coiled electrical resistor, such as wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium and their alloys, a metallic getter material on windings of said coil, a highly refractory, insulating and gas-tight tubular cover enclosing said coil and getter material, two electrode terminals, one each electrically connected with one end of said coil, each of said electrodes gas-tightly connected with one end of a tubular metal body, and gas-tight glass seals for insulating each such body from and associating it with said cover, the gas-tightly sealed space within said cover, glass seals, electrodes and bodies being evacuated.

3. An electrical heating element adapted for use at high operation temperatures above about 900 C. substantially comprising, in combination, a coiled electrical resistor, such as wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium,

osmium and their alloys, a metallic getter mate rial selected from the group consisting of zirconium, aluminum and thorium and admixed in finely divided state with finely divided refractory and insulating material on windings of said coil, a highly refractory, insulating and gas-tight tubular cover enclosingsaid coil and getter material, two electrode terminals, one each electrically connected with one end of said coil. at least two tubular metal bodies extending essentially co-axially with said tubular cover from the latter, each of said electrodes gas-tightly connected with an end of one of said bodies, and gas-tight glass seals for insulating each such body from and associating it with said cover.

4. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a coiled electrical resistor, such as wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum,

iridium, osmium and their alloys, metallic getter material selected from the group consisting of zirconium and thorium and admixed in finely divided state with finely divided refractory and insulating material sintered upon and between windings of said coil, a highly refractory, insulating and gas-tight tubular cover enclosing said coil and getter material, two electrode terminals, one each electrically connected with one end of said coil, at least two tubular metal bodies extending essentially co-axially with said tubular cover from the latter, each of said electrodes gas-tightly connected with an end of one of said bodies, and gas-tight glass seals for insulating each such body from and associating it with said cover.

5. An electrical heating element adapted for use at high operation temperatures above about 900 (2., substantially comprising, in combination, a coiled electrical resistor, such as wire. of a metal or metal alloy of a melting temperature considerably above operation temperature, as

terial within the gas-tightly sealed interior of the heating element.

6. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a coiled electrical resistor, such as wire, of

a metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by a chromium-nickel alloy, platinum, molybdenum, tungsten, iridium, osmium and their alloys, a highly refractory, insulating and gas-tight tubular cover enclosing said coil, two electrode terminals, one each electrically connected with one end of said coil, at least two tubular metal bodies extending essentially coaxially with said tubular cover from the latter, each of said electrodes gas-tightly associated with an end of one of said bodies, gas-tight glass seals for insulating each such body from and associating it with said cover,'-and a metallic getter material within the gas-tightly sealed interior of the heating element and close to windings of said coiled resistor, said gas-tightly sealed interior being evacuated.

7. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a coiled electrical resistor, such as wire, of a metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by a chromiummickel alloy, platinum,

molybdenum, tungsten, iridium, osmium andv their alloys, a highly refractory, insulating and gas-tight tubular cover enclosing said coil, two electrode terminals, one each electrically connected with one end of said coil, each of said electrodes gas-tightly associated with one end of a tubular metalbody, gas-tight glass seals for insulating each such body from and associating it with said cover, and metallic getter material selected from the group consisting of phosphorus, barium, sodium, zirconium and thorium within the gas-tightly sealed interior of the heating element and close to windings of said coiled resistor, said gas-tightly sealed interior being evacuated.

8. An electrical heating element adapted for operation at relatively high temperatures, such as above about 900 C. and particularly above 1100 C., substantially comprising, in combination, a coiled electrical resistor of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iriridium, osmium and alloys thereof, a metallic getter material of low vapor pressure at operation temperature, as exemplified by zirconium and thorium, attached to windings of said coil, a tubular gastight cover of highly refractory and insulating material, as exemplified by oxides of aluminum, magnesium. titanium, mixtures and silicates thereof, said tubular cover closely enveloping said coil so as to allow heat expansion of the latter, two electrode terminals, each of said terminals electrically connected with one end of said coil, two tubular metal bodies, one each gas-tightly connected with one of said electrodes and by means of a glass seal insulated from and associated with said cover, said metal bodies and glass seals of matched composition so as to maintain their gas-tight connection at elevated temperatures below the softening points of said glass and metal, the gas-tightly sealed space within said tubular cover, metal bodies and electrodes being evacuated.

9. An electrical heating element adapted for relatively high operation temperatures, such as above 900 C., and particularly above 1100 (3., substantially comprising, in combination, at least one resistor wire coil of metal 01' metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium, and alloys thereof, a getter material of a melting temperature considerably above operation temperature, as exemplified by zirconium and thorium, on and between windings of said coil, a tubular gas-tight cover closed at one end, said cover of highly refractory and insulating material and closely enveloping said coil and getter material so as to allow heat expansion of said coil at operation temperature, twoelectrode terminals at the open end of said tubular cover, one of said electrodes being hollow and associated with a first type tubular metal body projecting to opposite sides from said hollow electrode, one of said tubular projections insulatingly connected by means of a glass seal with the open end of said tubular cover, the other one of said electrodes gas-tightly associated with a second type tubusaid glass and metal, electrical connections between one each of said electrodes and one end of said coil, said connections arranged within the space enclosed by said tubular cover, tubular bodies, glass seals and electrodes, and a tubular highly refractory insulation arranged within said coil and projecting therefrom beyond the open Y end of said cover, said tubular insulation passed I by one of said electrical connections.

10. An electrical heating element adapted for use at high operation temperatures above about 900 C., substantially comprising, in combination,

- a coiled electrical resistor, such as wire, of metal or metal alloy of a melting temperature considerably above operation temperature. as exemplified by tungsten, molybdenum, platinum, iridium, osmium and their alloys, 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 attached to windings of said coil, an insulating, highly refractory and gas-tight tubular cover open on both ends closely enveloping said coil so as to allow of its heat expansion at operation temperature, two electrode terminals and two tubular metal bodies, one Of said electrodes each gas-tightly connected with one of said tubular bodies, a gas-tight and insulating glass seal between one each of said tubular bodies and one end of said cover, the metal and glass of said bodies and seals of matched compositions so as to maintain their gas-tight connection at elevated temperatures below the softening points of said glass and metal, said coil spacedly arranged from said glass seals within said tubular cover, and each end of said coil connected with the adjacent electrode by conductive means developing considerably less heat than the coil, said means exemplified by a straight portion of the wire of which the coil consists.

11. An electrical heating element adapted for use at high operation temperatures above about 900 0., substantially comprising, in combination, a coiled electrical resistor, such as wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium and their alloys, 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 attached to windings of said coil, an insulating, highly refractory and gas-tight tubular cover open on both ends closely enveloping said coil 50 as to allow of its heat expansion at operation temperature, two electrode terminals and two tubular metal bodies, one of said electrodes each gas-tightly connected with one of said tubular bodies, a gas-tight and insulating lass seal between one each of said tubular bodies and one end of said cover, the metal and glass of said bodies and seals of matched compositions so as to maintain their gas-tight connection at elevated temperatures below the softening points of said glass and metal, said coil spacedly arranged from said glass seals within said tubular cover, and each end of said coil connected with the adjacent electrode by conductive means developing considerably less heat than the coil, said means exemplified by a straight portion of the wire of which the coil consists, the sealed interior of the heating element being evacuated.

12. An electrical heating element adopted for use at high operation temperatures above about 900 C., substantially comprising, in combination, a coiled electrical resistor, such as wire of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplifled by tungsten, molybdenum, platinum, iridium, osmium, and their alloys, a granular getter material of low vapor pressure at operation temperature, as exemplified by zirconium and thorium, attached to windings of said coil, a rod or tube of highly refractory insulating material within said coil and close to its windings when cold, 9. highly refractory and gas-tight insulating tubular cover closely enveloping said coil so as to allow its heat expansion at operation temperatures, two electrode terminals and two tubular metal bodies, each of said electrodes gas-tightly connected with one end of a tubular body each, a

softening point of said glass and metal, said coil within said tubular cover spacedly arranged from its ends, and each end of said coil connected with the adjacentelectrode by electrically conductive means developing considerably less heat than the coil, said means exemplified by a straight portion of the wire of which the coil consists, the sealed interior of the heating element being evacuated.

13. An electrical heating element adapted for relatively high operation temperatures, such as above 900 C. and particularly above 1100 C., substantially comprising, in combination, a resistor formed by at least one wire coil of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium, a tubular gas-tight and highly refractory insulating cover closed on one end, said resistor coil arranged within said cover spaced from its open end, a hollow electrode terminal gas-tightly and conductively connected, such as by brazing, with a metal tube each on opposite front sides, one of said metal tubes gas-tightlyconnected by means of a glass seal with the open sealed interior of the heating element, and a getter material within said interior space and close to the windings of said resistor coil.

14. An electrical heating element adapted for relatively high operation temperatures, such as above about 900 C., substantially comprising'in combination, at least one resistor wire of metal or metal alloy of a melting, temperature considof, said cover finally fired at a temperature at least as high as operation temperature and closed at one end, said coiled part arranged within said cover so as to be close to its inside surface at operation temperature and being spaced from its open end, two electrode terminals adjacent to the open end of said cover, at least one of said electrodes being hollow, a first type tubular metal bodies projecting to opposite sidesfrom the hollow one of said electrodes, a glass seal between one of said projections and the open end of said cover, the other one of said electrodes conductively connected, such as by brazing, with a second type of tubular metal body, the latter body and the other one of said projections associated by means of a glass seal, the composition of the glass of said seals and metal of said bodies being matched so as to maintain their gas-tight connection at elevated temperatures below the softening points of the glass and metal, said resistor wire continuing from the end of its coiled part adjacent to one of said electrodes to the latter and conductively connected with it, said resistor wire led from the other end of its coiled part through and beyond the space within the latter and conductively connected with the other one of said electrodes, 8. highly refractory insulating tubular member enveloping said latter resistor wire at least within said space, and a getter material within the sealed interior of the heating element and close to the windings of said coiled part, said interior being evacuated.

15. An electric heating element adapted for use at high operation temperatures, substantially comprising, in combination, a heating portion and a leading-in portion, said heating portion including an electric resistor, such as a wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium and their alloys, a metallic getter material near said resistor, a highly rei'ractory, electrically insulating and gas-tight tubular cover enclosing said resistor and getter material, said leading-in portion including two electrode terminals and two tubular metal bodies. one electrode each electrically connected with one end of said resistor and gas-tightly connected with an end of one of said tubular metal bodies, said cover extending from said heating portion into said leading-in portion and associated with and insulated from said tubular metal bodies by means of gas-tight glass seals so that the space Within the thus connected portions is gas-tightly sealed, said space being evacuated.

16. An electric heating element adapted for use at high operation temperatures, substantially comprising, in combination, a heating portion and two leading-in portions, said heating portion having opposite ends each associated with one of said leading-in port-ions, said heating portion including an electric resistor, such as a wire, of metal or metal alloy of a melting temperature considerably above operation temperature, as exemplified by tungsten, molybdenum, platinum, iridium, osmium and their alloys, a metallic getter material near said resistor, a highly refractory, electricall insulating and gas-tight tubular cover enclosing said resistor and getter material, said leading-in portions each including one electrode terminal and one tubular metal body, one electrode each electrically connected with an end of said resistor and gas-tightly connected with an end of said tubular metal body, said cover extending from said heating portion into said leading-in portions and associated with and insulated from each said tubular metal bodies by means of a gas-tight glass seal so that the space within the thus connected portions is gastightly sealed, said space being evacuated.

GERHARD LEWIN.