US2285611A - Electric heating device - Google Patents

Description

June 9, 1942. 4 G. E. PRICE ELECTRIC HEATING DEVICE Filed Dec. 29, 1939 INVENTOR @9a/y@ TP/76e TT RNEY Patented June 9, 1942 ELECTRIC HEATING DEVICE George E. Price, Mansfield, Ohio, assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 29, 1939, Serial No. 311,618

9 Claims.

My invention relates to electric heating devices and in particular to methods of forming insulating partitions in electrical heating devices in which substantial amounts of refractory insulating materials, such as magnesium oxide, are embodied. However, my invention will be found applicable to the formation of partitions or Wall structures in enclosures operated at elevated temperatures for many other purposes than the specific one of insulating an electrical terminal wire to which I apply it in the apparatus now to be described in detail.

In certain types of electric ranges an electrical heating element of reslstance.material is enclosed ina metallic sheath of somewhat larger diameter and electrically insulated from the latter by a layer or packing of some refractory insulating substance; for example, magnesium oxide. One method of assembling the above-mentioned structure is to position the electrical heating element central to the axis of the surrounding tube and sift powdered magnesium oxide into the annular space thus left within the tube, tamping the magnesiumkoxide in place to ensure the maximum possible solidity of packing until the tubular sheath is completely lled. However, with any known process of forming the magnesium oxide packing,'it still retains a certain amount of porosity capable of absorbing moisture, gases, and the like, from the air unless some form of enclosing partition is formed at the two ends of the tubular sheath to prevent access of such moisture, gases, or the like. Under such conditions subsequent heating of the resistance unit in its normal use in an electric range or other similar device will vaporize this absorbed moisture, or the like, and unless some vent is provided in the enclosing walls of the heater unit, it is liable to produce dangerous pressures and even explosions.

In an attempt to provide against the foregoing diiliculties the practice has been adopted in some cases of providing a vacuum-'tight seal at each end of the tubular sheath. Since the heater conductor must be highly insulated from' the enclosing sheath, any terminal seals must be of good electrical insulation. The above-mentioned practice has been to form these seals of glass fused into a vacuum-tight bond with both the enclosed sheath and the terminal conductor through which current passes to the heater conductor within the sheath.v However, it has been found impossible in actual commercial practice to ensure that all of the water vapor and other gases are uniformly removed from the sheath and its contents, and the result has been that serious explosions have occurred in apparatus employing vacuum-tight terminal seals.

In view of the above-mentioned different types of difficulty, it is one object of my invention to provide a terminal in which a Wall or proportion of porous insulating material is provided which electrically insulates and holds the terminal of the electrical heater central to the end of the enclosing sheath; which sufiiciently obstructs the inflow of moisture and other gases from the atmosphere when the heater element is being cooled down from its operative condition so that no substantialam'ount of moisture or other gases are absorbed in the magnesium oxide filler; and which nevertheless provides a suilicient vent to permit a rapid enough out-now of any gas or vapor pressures developed within the sheath when the unit is heated, so that explosions cannot take place.

Another object of my invention is to provide a mechanically secure support between the heater terminal and the enclosing sheath.

Still another object of my invention is to provide a structure for electrically insulating partitions or Walls within enclosures likely to be heated to elevated temperatures, said partitions being pervious to a predetermined degree and direction to flow of gases and vapors.

Still other objects of my invention will become evident upon reading vthe following specification in which:

Figure 1 is a view, partly in cross-section of a heater element for electric ranges during an early condition in the course of its construction,

Figs. 2 and 3 of the drawing are respectively a plan View and elevational View, partly in section, illustrating an electric range heater embodying the principles of my invention, and

Fig. 4 is a cross-sectional view taken along the line IV-IV of Fig. 2.

, Referring in detail to Fig. 1, an electrical heater element l which may be of nichrome or other suitable electrical resistance material and which is preferably wound to a helical form is positioned in the central axis of a cylindrical enclosing sheath 2. For one size of heater the nichrome resistance may be 0.010 inch in diameter wound on a mandrel about 0.080 inch in diameter to form a helix of substantially 27 turns per inch. The sheath 2 may be of any suitable material, but that which I prefer is an alloy comprising 17 to 19% chromium, 8 to 11% nickel, 2 to 3% silicon, carbon substantially .08%, manganese not over 1.50%, sulphur not over .03% and columbium at least equal to eight to ten times the carbon content. For some purposes the sheath 2 may suitably consist of` an alloy comprising 30 to 80% nickel, 10 to 20% chromium, and the remainder iron except for minor quantities of carbon, manganese, silicon, phosphorus, sulphur and copper. n

The ends of the heater element I are connected to terminal rods 3 and 3a which may, for example, be 0.12 inch in diameter by substantially 1 to 41/2 inches long and be of the same material as the sheath, or may be of steel such as an open hearth steel containing .15 to .25% carbon, .30 to .60% manganese, .05% phosphorus and .05% sulphur. The ends of the element I are connected to the terminal rods 3 and 3a by twisting its turns tightly against and end portion of slightly reduced diameter on the latter in the manner shown in Fig. 1.

The heater element I and its terminals 3 and 3a are positioned in the central axis of the sheath 2 and the space between the tube filled as completely as possible by sifting in some refractory electrical insulating material, such as magnesium oxide powder, of not over 40 mesh through an open end of the sheath 2, vibrating the powder carefully into place. The ends of the sheath may then be temporarily closed in some suitable way such as by a metal disc I at one end, held by a groove rolled in the sheath near such end and by an enlarged portion 4a. of the terminal 3a at the other end of the tube,likewise held by a groove rolled into the sheath near such end, as shown in Fig. 1.

Once filled, the cylindrical unit is swedged, reducing its diameter from, for example, 0.375 inch with a wall thickness of 0.030 inch down to 0.340 inch. A three-inch length at each end may then be further swedged down to 0.333 inch, and the total length of the unit being in one example 291/2 inches after such swedging.

The cylindrical unit is then heated so as to drive out a large portion of any moisture of any occluded gases which may be present in the magnesium oxide, and to sufficiently annealthe unit so that it may be bent into the spiral of the form r,

indicated in Figs. 2 and 3. Preferably the sheath 2 is,`as indicated by Fig. 4,v flattened on the side opposite to that toward which the terminals extend in Fig. 3. The ends of the sheath containing the metal closing discs of Fig. 1 may then be out off.

In order to form the terminal partitions, which form a special feature of my invention, the magnesium oxide is reamed out for a distance, for example, of approximately one-fourth to five-sixteenth inch from each end of the sheath 2. The inner face of the sheath 2 and the surfaces of the terminal rods 3 and 3a are carefully cleaned. The unit is heated so as to expel substantially all moisture and gases which may be therein and a tubular-shaped bushing 5, of a composition about to be described, is slipped over each terminal rod 3 and 3a, the bushing 5 being of such dimensions as to only loosely fit the annular space from which the magnesium oxide has been removed.

The sheath 2 is then heated `to an elevated temperature for a considerable period of time to free it as completely as possible from absorbed or combined vapors and gases, and its two ends are then heated by any suitable means, for example, a hand torch to a temperature of between 1700 and 2000 F. for a considerable distance back from each end. This renders the bushing 5 thoroughly plastic.

A steel plunger is then fitted over the end of the terminal rods 3 and 3a and pressed down against the plastic bushing 5 with a pressure of from 25 to 30 pounds, thus pressing the subtance of the bushing 5 toadhere to and wet" the inner surface of the sheath 2. The aforesaid plunger is then removed. In order to ensure that the material of the bushing 5 thoroughly wets and adheres to the surface of the terminal rods 3 and 3a, the latter is then heated by suitable means, such as a gas torch, to a temperature of the order of 1850 F. The terminal is then permitted to cool slowly to room temperature. A connectorv 6, suitable for attachment of external electrical conductors may then be screwed onto threads provided on the ends of terminal rods 3 and 3a.

The bushing 5 is preferably made from a mixture comprising glass and some refractory metal oxide or'other highly refractory electrically insulating material. I have found preferable with the latter a material known as Zircon. The glass to be used is finely ground and mixed in proper proportions with the ground Zircon or other refractory material. The size of the zircon or otherrefractory particles and their relative proportions of the glass and the refractory determine in large degree the degree of porosity of the partition in the heater terminal. For a higher degree of porosity I believe a coarse refractory size and a relatively small percentage of glass should be employed; and conversely, relatively fine refractory particles present in relatively small amount in the glass will produce a partition of low porisity to gas. To take one example, I have found by experiment that a partition of substantially the dimensions described above in connection with Fig. 3 cornprising 70% of a soft glass, Corning Glass Company code No. 012, and 30% Zircon of approximately to 200 mesh permitted .35 cubic centimeters of atmospheric air to flow through it with a difference of pressure' of '760 millimeters between its opposed flat surfaces. To take another example, a bushing of the same dimensions made from a mixture comprising 34% of Corning Glass Company code No. 012, 33% of Corning Class Company code No. 172, 22% of 80 mesh Zircon and 11% of 200 mesh Zircon passed .002 cubic centimeter of atmospheric air of the same pressure difference between its two faces.

While a variation in the materials and their relative proportions in the bushings 5 is within the purview of my invention, I have so far found the best results to be obtained from bushings comprising 34% of Corning Glass code No. 012, 33% of Corning Glasscode No. 172, 22% of 80 mesh Zircon and 11% of 200 mesh Zircon.

For many purposes it may be found desirable that the glass mixture employed shall have an expansion curve matching that of the sheath `2 and/or `the rods 3 and 3a from room temperature up to that at which the glass shows substantial plasticity.

Experiments have indicated that the structure illustrated in Fig. 3 permits, under operating conditions, a considerably more ready out-flow of internal high pressure gaseous substances than in-flow of external atmospheric pressure substances, The combination with a heated enclosure of a wall portion which readily permits the out-flow of high pressure gases but which greatly restricts the in-ilow of atmospheric pressure gaseous substances is accordingly within the purview of my invention, regardless of whether such partitions are formed from the particular substances described above, or formed otherwise.

While I have described a specific embodiment of my invention, it will be recognized that the principles thereof are of broad application. For example, porous insulating partitions may be produced thereby for uses other than terminals of electrical range heaters; and the use of porous permeable or semi-permeable terminals for range heaters composed of other materials than the specific ones I have here described are within the broader aspects of my invention.

I claim as my invention:

l. In combination with an electrical heater element, a tubular sheath spaced away therefrom and a powdered refractory insulator occupying the space between said element and said sheath, an end terminal for said heater and sheath comprising a wall portion consisting of a fused mixture of glass and zircon forming fused joints with said sheath and the terminal for said heater, said zircon having a neness and being contained in said mixture in a quantity such as to permit substantial out-flow of gases emitted from said insulator when said insulator is heated and to prevent substantial inflow of gases to said insulator when said insulator is cooled.

2. In combination with an electrical heater element, a tubular'sheath spaced away therefrom and a powdered refractory insulator occupying the space between said element and said sheath, an end terminal for said heater and sheath comprising a wall portion consisting of a fused mixture of substantially 34% Corning Glass code No. 012, 33% of Corning Glass codev No. 172, 22% 80-mesh zircon and 11% 20D-mesh zircon forming fused joints with said sheath and the terminal for said heater.

3. A terminal insulator for enclosures containing electrical heaters comprising an annular bushing in fused bond at its periphery with the wall of said enclosure and at its interior face with an inleading conductor for said electrical heater, said bushing comprising a mixture of glass with a substantial quantity of a high melting insulating refractory said refractory having a neness and being contained in said mixture in a quantity such as to permit substantial outow of gases from said enclosure when said .enclosure isv heated and to prevent substantial inflow of gases into said enclosure when said enclosure is cooled.

4. In combination with a heating unit having a resistor. a terminal at the end thereof, a tubular sheath spaced away from such resistor and terminal and an insulating material occupying the space between the sheath and resistor and a portion of said terminal, a sealing partition for such unit consisting of a fused mixture of glass and zircon adheringfto said sheath and terminal, said zircon having a fineness and being contained in said mixture in a quantity such as to permit substantial out-flow of gases emitted from said material when said material is heated and to prevent substantial inflow of gases to said material when said material is cooled.

5. In combination with a heating unit having a resistor, a terminal at the end thereof, a tubular sheath spaced away from such resistor and terminal and an insulating material occupying the space between the sheath and resistor and a portion of said terminal, a sealing partition for such unit consisting of a fused mixture of substantially 34% Corning Glass code No. 012, 33% of Corning Glass code No. 172, 22% 80-mesh zircon and 11% 20D-mesh zircon forming fused joints with said sheath and the terminal.

6. In combination with an electrical heater element, a tubular sheath spaced away therefrom and a powdered refractory insulator ocfused mixture of two-thirds glass and one-third zircon forming fused joints with said sheath and the terminal for said heater.

7. In combination with an electrical heater element, a tubular sheath spaced away therefrom and joints with said said heater.

8. In combination a resistor, a terminal lar sheath spaced away from such resistor and terminal and an insulating material occupying the space between the sheath and resistor and terminal, a sealing partition substantially 67% glass, 22% 80-mesh zircon and .fused Joints with

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