US2157884A - Electric heating element - Google Patents

Description

y 1939- c. B. BACKER 2,157,884

ELECTRIC HEATING ELEMENT Filed Jan. 16, 1936 AT'ToRNEY Patented May 9, 1939 UNITED STATES PATENT OFFICE Application January 16, 1938, Serial No. 59,391

In Canada May 21, 1935 8 Claims. 20164) My invention relates to electric heating elements and more particularly to bular heating elements in which the coiled resis ance wire with its surrounding insulation is enclosed in a metallic tube or sheath.

There are on the market two types of such tubular heating elements, namely, the so-called Calrod element and the Backer tube element. In the Calrod element the resistance coil is surrounded by a powdered insulation material (generally magnesium oxide which has been fused and powdered), which is compacted around the resistance coil by swaging (hammering) the tube so as to reduce its diameter. In the Backer tube element, on the other hand, the insulation consists of chemically pure crystalline magnesium oxide, which is produced in place from magnesium metal by treating the metal with steam or water under very high pressure and temperature, whereby the magnesium metal is converted into magnesium hydroxide, which is further converted into magnesium oxide by heat-, ing to-dull red heat.

A Backer tube element is made as follows:

, Into a metal tube we insert three magnesium metal strips bent into segmental shape so as to form in effect a magnesium lining inside the metal tube. (Instead of the three segmental strips, a magnesium metal tube may, of course, be used,

, although that would be much more expensive than the strips.) The coiled resistance wire is now inserted into the tube and held in position at both ends. The assembled element, consisting of the outer tube or sheath, the magnesium metal a iining and the coiled resistance wire, is now placed vertically in an autoclave nearly full of water, so that the tube is preferably covered with the water. The autoclave is then closed and heated until the steam pressure is at least 15 w atmospheres, and the temperature corresponds to that of the saturated steam pressure used. (A much higher pressure than 15 atmospheres may be used with advantage, as the process goes faster with higher pressure and temperature.) The 5 water now combines with the magnesium metal to form magnesium hydroxide, while hydrogen gas is liberated and escapes through a suitable outlet valve on the autoclave cover.

During the conversion the crystalline mag- 50 nesium hydroxide formed grows or expands, so

that, when the conversion is finished, the hydroxide takes up about twice as large volume as the original magnesium metal, whereby the resistance coil becomes completely embedded in u the hydroxide. The tube is now dried at a temperature of dull red heat in order to convert the magnesium hydroxide into oxide. The center opening in the resistance coil (through which the water circulated during the conversion in the autoclave) may now be filled with any good re- 5 fractory insulation powder, or a magnesium metal wire may be inserted into the center opening and converted in the autoclave into magnesium hydroxide. Thereafter air-tight terminals are attached to the ends of the resistance wire at both 10 ends of the tube, and after a final drying operation the tube element is ready for use.

The Backer tube element as described above has already been patented in most industrial countries. See United States Patent Reissue No. 18 16,340, Canadian Patent No. 309,366, and corresponding British Patent No. 336,949. My present patent application covers an important improvement in the above described tubular element.

As stated above the magnesium hydroxide must 20 be heated to dull red heat to convert it to magnesium oxide free from water. During this drying process, which must be done at a temperature of about 600 degrees C., the magnesium oxide shrinks, as it loses its water. The result is that circumferential cracks develop in the oxide. These cracks or openings are very objectionable since they make it impossible to produce tube elements which can be flash tested with more than 1500 volts, when the tube is cold, or 800 volts, when the tube is red hot. For many purposes approval specifications call for flash test of the elements with as high a voltage as 1200 or 1500 volts, when the element is up to its maximum temperature, that is, dull red heat, and for 550- volt service the elements must be flash tested with 2100 or 2200 volts.

One object of my invention, therefore, is to provide an improved heating element of the metalclad or sheathed type.

Another object of my invention is to provide a novel method of compressing or compacting the insulating material in a well-known sheathed type of heating element to increase the dielectric strength thereof.

A further object of my invention is to effect such compression or compacting of the insulating material, or deformation of the complete element, in a plurality of steps in conjunction with selected temperature treatments, whereby the desired compacting and deformation is effected without undue strains in the insulating material, which would otherwise develop deleterious cracks. Other objects of my invention will become evident from the following description thereof, a

taken in conjunction with the accompanying drawing, wherein:

Figs. 1 to 5,.inclusive, are cross-sectional views of heating elements deformed in different ways in accordance with my present invention, together with elevational views of simple dies for effecting the desired deformation in each case.

I have developed a new method of deforming the tube or heating element, after it has been baked in the autoclave, so that the magnesium oxide becomes very compacted and may be dried at any temperature without developing any cracks. I proceed as follows: The tube element may be such as that designated generally by reference character I in Fig. 1, comprising the outer metallic tube or sheath 8, the coiled resistance wire 9 (initially an ordinary helical coil) and the intervening insulating material it). After the tube element has been treated in the autoclave as de-. scribed above, it is dried in a suitable drier at a temperature of from 330 to 350 degrees C. A drying period of 30 to 60 minutes is satisfactory. By this drying the magnesium hydroxide loses a part of its bound hydroxide water, without being completely converted into oxide. No objectionable cracks or openings develop in the magnesium hydroxide when it is dried at only 350 degrees C. Nevertheless, it loses sufiicient water and becomes considerably softer, so that it may be easily compacted.

After this first drying operation the center opening in the element is filled with insulation powder and the terminals are assembled into both ends of the tube. Then the tube is submitted to the first deforming operation by pressing it in a suitable die, so that the insulation becomes compacted, or reduced, in volume, the volume reduction in this first deforming operation may be from 15% to 25%.

The simplest method of deforming the tube is to press it between two flat dies ii and i2, as simply illustrated in Fig. l. The tube then takes a more or less rectangular cross section, with half circular sides, as shown in Fig. 1. Although this shape of the tube is cheaply attained, because it requires only fiat dies, and although a tube element flattened as shown in Fig. 1 is very much better in every respect than a tube which has not been deformed at all, it is not entirely satisfactory, because, if the tube is to be flattened enough to sufficiently compact the oxide at the rounded sides of the tube, the thickness of the oxide layer at the flattened sides becomes too small.

A much better shape is shown at l3 in Fig. 2, which is more or less elliptical, rather than rectangular. To deform the tube into a shape as shown in Fig. 2 requires a set of dies i4 and IS with grooves of the corresponding form, as indicated. In this shape an evenly compacted oxide is attained without-reducing the thickness of the insulation layer at any point to an objectionable degree. I have found that the shape as shown in Fig. 2 is the most desirable one, and gives a. perfect result.

Another shape, shown in Fig. 3 at I6, also gives very good results. It is more or less square except that the sides of the square are slightly rounded, the cooperating dies i1 and i8 being correspondingly shaped, as indicated. This rounding of the sides is desirable, because in a section with straight fiat sides the fiat parts of the wall have a tendency to bulge out, when the tube is heated, and thereby destroy the good contact between the tube and the insulation.

Another shape--approxiately semi-circularwhich may be desirable for special purposes is shown in Fig. 4 at is. A tube shaped as per Fig. 4 by means of suitable dies 20 and 2| may be conveniently bent into hair-pin form and pressed together so as to form a return bend element of approximately circular cross section having the terminals near together at the same end of the element. The cross section of such an element would be as shown in Fig. 5. There may also be other shapes into which the tube may be formed for special purposes.

After the first deforming operation, in which the insulation may be compacted to the extent of 15% to 25% of its volume, the tube is again put into the drier and dried for several hours at about 600 degrees C. This drying operation will convert all the magnesium hydroxide into oxide, without any cracks developing in the insulation.

After this final drying the tube is further deformed in the same dies as used for the first deforming operation or in similar dies, as will be understood. In the second deforming operation the tubes are given their final shapes as shown in Figs. 1 to 5, as the case may be, and the oxide insulation is now compacted, so that it occupies about two-thirds of the volume of the original magnesium hydroxide. This great compression of the oxide makes it extremely hard and compact, so that its dielectric strength is increased to more than the double of what it was before the deformation.

It was mentioned above that the well-known Calrod tube element is swaged (hammered) in a rotating swaging machine in order to reduce the diameter of the tube and thereby compact the powder insulation. This swaging operation always produces a considerable elongation of the tube (from 10% to 20% elongation), and the circumference of the tube is considerably reduced. The swaging operation is fundamentally different from my new deforming methad, which is the subject of this patent application. In my deforming operation the circumference of the tube is not decreased by design (although the tube may incidentally get its circumference reduced by anywhere from half of one percent to two percent on account of its wall being slightly compressed at both sides), and the length of the tube is not increased at all. The lengthening of the tube inherent in the swaglng method is a great drawback, which makes manufacturing difficult in all cases where the tubes must be made to exact length. This diificulty has been eliminated by my deformation method. The fundamental differences between the said swaging method (as described in U. S. Patent No. 1,367,341, dated February 1, 1921, and earlier patents such as British Patent No. 18,685, A. D. 1914), and my deformation method are that by the former method a reduction in the circumference and a considerable increase in the length of the tube element are unavoidable, while by my deformation method there is no increase in the length of the tube and the reduction in the cir- 'cumference is so small as to be hardly measurable.

The deformation of a tube element may be done in a strong press by a single blow of the press. If no large press is available the deformation may also be done in a small press by stages, that is, a little at a time. If this method is used, then there is a very small lengthening of the tube, depending upon how many blows of the press are required to deform the whole length of the tube. 75

oxide.

There are also other extremely important difierences in the two methods. If a tube with crystalline magnesium oxide insulation (made by converting magnesium metal by steam in an autoclave) is swaged, the crystalline structure of the oxide is broken up by the thousands of hammer blows, and the insulation becomes a hard mass consisting of extremely fine particles of By this breaking up of the crystalline structure of the oxide, its heat conductivity is very materially reduced, which has the effect of increasing considerably the temperature diiference between the resistance wire and the outer sheath. By the deformation method the crystalline structure of the oxide is not destroyed. The crystals are merely packed closer together, and the good heat conductivity is retained.

I have also found that, if a tube element with crystalline magnesium oxide is swaged, the insulation resistance of the oxide is decreased to a small fraction of what it was before the "swag- 1112, while the deformation of the tube, as described in this application, does not decrease the specific insulation resistance of the oxide. When a tube element has been deformed, its insulation resistance is a little lower than before the deformation, but the decrease is only in proportion to the decreased thickness of the oxide layer.

From the above it will be realized that the swaging method is practically speaking useless for tube elements with crystalline magnesium oxide insulation (made from magnesium metal in an autoclave by the Backer insulation process). On the other hand, the swaging method is excellent for tube elements made by the Calrod powder process. My deformation method could not be successfully used for such elements, because the resistance coil would not deform with the tube, but would have a tendency to remain circular and therefore cut through the insulation layer or reduce its thickness too much.

Various modifications may be made in devices embodying my invention, such as changing the shape into which the tube is deformed, without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are imposed by the prior art or are set forth in the appended claims.

I claim as my invention;

1. The method of making a tubular heating element comprising the steps of disposing a resistance member and moisture-containing material comprising a magnesium oxide compound surrounding said member within a metal tube or enclosing sheath, heating said material to dehydrate it to a considerable degree and initially deforming said sheath to compact said material around said resistance member and close any cracks formed therein by such dehydration, the volume reduction of said material being 15-25%, further heating said material to additionally dehydrate it and further deforming said sheath to additionally compact said material.

2. The method of making a tubular heating element comprising the steps of disposing a resistance member and insulating moisture-containing material surrounding said member within a metal tube or enclosing sheath, partially drying said material and initially deforming said sheath to compact said material around said resistance member, the volume reduction of said material being Iii-25%, further drying said material and further deforming said sheath to additionally compact said material, the. second volume reduction being of an order similar to that of the first.

, 3. The method of making a tubular heating element comprising the steps of disposing a resistance member and insulating moisture-containing material surrounding said member within a metal tube or enclosing sheath, drying said material at a temperature of above 300 C. for a predetermined period of time and initially deforming said sheath to partially compact said material around said resistance member, further drying said material at a higher temperature and for a longer period of time than the first-named drying, and further deforming said sheath to additionally compact said material.

4. The method of making a tubular heating element comprising the steps of disposing a resistance member and insulating moisture-containing material surrounding said member within a metal tube or enclosing sheath, drying said material at a temperature of 330 to 350 C. for a period of 30 to 60 minutes and then initially deforming said sheath to partially compact said material around said resistance member, further drying said material at about 600 C. for several hours and further deforming said sheath to additionally compact said material.

5. The method of making a tubular heating element comprising the steps of disposing a resistance member and insulating moisture-containing material surrounding said member within a metal tube or enclosing sheath, drying said material at a temperature of 330 to 350 C. for a period of 30 to 60 minutes and then initially deforming said sheath to reduce the volume of said insulating material by 15% to 25%, further drying said material at about 600 C. for several hours and further deforming said sheath to reduce the volume of said material to about twothirds of its original volume.

6. The method of making a tubular heating element comprising the steps of disposing a resistance member surrounded by hydroxide within a metal tube or enclosing sheath, partially converting' said hydroxide to oxide, initially deforming said sheath to partially compact the hydroxide and oxide, completing the conversion to oxide, and further deforming said sheath to additionally compact the oxide.

7. The method of making a tubular heating element comprising the steps of disposing a resistance member surrounded by magnesium hydroxide within a metal tube or enclosing sheath, partially converting said magnesium hydroxide to magnesium oxide, initially deforming said sheath to partially compact the hydroxide and oxide, completing the conversion to magnesium oxide, and further deforming said sheath to additionally compact the oxide.

8. The method of making electric heating elements comprising the insertion of a magnesium metal lining intoa metal sheath, placing a resistance member inside the magnesium lining, treating the assembled unit with steam or water under high prssure and temperature to convert the magnesium metal into magnesium hydroxide,

partially dehydrating the hydroxide, reducing the CHRISTIAN B. BACKER.

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