US2285952A - Electric heater insulating material - Google Patents

Description

June 9, 1942.

O. G. VOGEL ETAL ELECTRIC HEATER INSULATING h/IA'NEIRIAL Filed July 29, 1959 inventors: Ohver G. Vogeh Louis Navias yes W 617W Their" Attorney.

Patented June 9, 1942 ELECTRIC HEATER INSULATING MATERIAL Oliver G. Vogel, Oak Park, Ill., and Louis Navias,

Schenectady, N. Y., assignors to Edison Gencral Electric Appliance Company, Inc., Chicago, 11]., a corporation of New York Application July 29, 1939, Serial No. 287,334

7 Claims.

This invention relates to electric heaters, more particularly to electric heaters of the sheathed type, and it has for its object the provision of a heater of this character having improved insulating material.

While not limited thereto, this invention is particularly applicable to sheathed heaters having a resistance conductor enclosed in a metallic sheath, and embedded in and supported in spaced relation with the sheath by a densely compacted layer of heat refractory and conducting, electrically insulating material. Such a heater is described in the United States patent to Charles C. Abbott, dated February 1, 1921. It is to be understood, however, that this invention is not limited to the specific sheathed heater disclosed in this patent, but has more general application to sheathed heaters.

The heat refractory insulating material that has found wide application in sheathed electric heaters is granular magnesium oxide, and heaters provided with this insulating material have, in the main, been quite satisfactory. Pure magnesium oxide has a very high initial resistance and maintains a high resistance value throughthe electrical resistance of the heaters falls rather rapidly with use.

We have found that commercially available magnesium oxides contain more or less calcium oxide (CaO), commonly called lime, and have discovered that the presence of this material is an important factor in causing the early failure of the heating unit, and the reduction in its insulating resistance with use. ence of very-small quantities of lime causes an increase in the reactivity of the insulating material to metals. As a result, the insulation reacts with the sheath, the resistance conductor and its terminals, resulting in injury to these memhere. At times this reaction causes swelling of the heater to such an extent that the heater Even the pressheath is split and cracked. In addition, the 55 presence of even small amounts of lime results in an increase in the reacitvity of the insulation to moisture, and a marked and rapid decrease in the insulation resistance.

We obviate these difficulties by counteracting the effects of the lime by means of silicon dioxide (S102), commonly called silica. We establish a.

silica-lime ratio in the insulation such that the silica neutralizes the effects of the lime by combining with it and the magnesium oxide to form. silicates of lime and magnesia. Silica should be present in the proper amounts before the magnesium oxide is fused so as to prevent the segregation of any free lime in the fused mass as it cools, that is, to prevent the presence of any uncombined lime.

For a more complete understanding of this invention, reference should be had to the accompanying drawing in which the single figure is a fragmentary view in elevation of an electric sheathed heater embodying this invention, portions of the heater being shown in section so as to illustrate certain details of construction.

Referring to the drawing, this invention has been shown in one form as applied to an electric heater of the sheathed type described in the above-mentioned Abbott Patent No. 1,367,241. As shown, the heater comprises a helical resistance conductor Ill mounted within and substantially centrally of a tubular metallic sheath II. The resistance conductor I0 may be formed of any suitable material, such' as a nickelchromium alloy. The sheath ll preferably will be made of a suitable chromium-iron-nickel al loy. The resistance conductor I0 is secured at its ends to suitable terminals l2. These terminals may be formed of any suitable material, but preferably will be formed of steel. It is to be understood, however, that the conductor l0, the sheath H and terminals I2 may be made of many other suitable materials.

The resistance conductor I0 is embedded in and is supported in spaced relation with reference to the sheath by a mass I3 of heat refractory and heat conducting, electrically insulating material. The mass of insulating material is in granular form and is compactedto a hard dense mass in any suitable manner, as by reducing the diameter of the sheath H after the resistance conductor, terminals, and insulating material have been assembled with the sheath. This reducing operation may be accomplished by swaging, drawing or rolling.

The mass of insulating material I3 is formed of granular magnesium oxide which is treated by fusing the material, crushing it and pulverizing it to the desired granular size. As pointed out previously, commercialmagnesium oxides have more or less lime present, and the presence of this lime is very detrimental to the heater in that it reduces materially the insulating resistance of the heater with use, and increases its reactivity to moisture. and also to heated metal. As a result of the latter of these detrimental characteristics, we have found that the resistance conductor [0, the metallic sheath II and the metallic terminals I? are attacked by the insulating mass 13. At times the attack has been so severe, especially at the junctions of the resistance conductor I with the terminals I2, that the heating unit swells to such an extent that it splits and cracks the metallic sheath. This, of course, seriously impairs the life of the heater.

As pointed out previously, a silica-lime ratio is established in the magnesium oxide to counteract the detrimental eifects of lime; and the ratio should be established before the magnesium oxide is fused so that no free lime will precipitate out when the magnesium oxide cools from fusing.

The presence of the silica reduces materially the reactivity of the insulation i3 to metals, and to moisture, and it maintains the electrical resistance through a very long useful life.

Silica, however, has a tendency to reduce the initial electrical resistance of the magnesium oxide at high temperatures and, therefore, the amount used must be controlled, but within fairly wide limits. A little of the initial electrical resistance, however, may be sacrificed because of the other outstanding benefits obtained from the presence of silica, an important one of which is the maintenance of the resistance. In order that the silica content shall not be too great, it is desirable that the magnesium oxide have a limited maximum lime content. It is desirable that this maximum lime content should not be above around per cent by weight, and preferably it should be below around 2.5 per cent by weight; and it is preferable that the maximum silica content be not greater than around 25 per cent by weight.

Generally speaking, the ratio of silica to lime should be at least as great as 1 to 1, although this minimum ratio may be around 2 to 3. Where the lime content is from around 0.5 per cent to around 5 per cent by weight, the ratio of silica to lime may range from 2 to 3 to 2 0 to l; but the silica content should not in any case be below around 1 per cent nor' above around 25 per cent by weight. Where the lime contents are smaller than 0.5 per cent by weight, the silica content should range from around 1 to 2 per cent by weight.

At the present time, the preferred silica-lime balance should be as follows: for any lime content of around 0.5 per cent or less by weight, the silica content should range from about 1 to 2 per cent by weight; and for any lime content ranging from around 0.5 per cent up to about 5 per cent by weight, the ratio of silica to lime should range from 2 to 1, to 3 to 1.

The following are certain specific examples of higher silica to lime ratios that are permissive: for a lime content of around 2.5 percent by weight, the silica content may be as great as 25 per cent by weight which is a ratio of silica to lime of to 1; if the lime content be around 1.5 per cent by weight, a silica content of 25 per cent by weight may be used, which gives a ratio of silica to lime of around 1'7 to 1; for a lime content of .75 per cent by weight, the silica content may be as high as per cent by weight which gives a ratioof silica to lime of to 1; and. ii. the lime content be around .05 per cent by weight, the silica content may be as high as 1 to 2 per cent by weight, or in the ratio of 20 to 1, to- 40 to 1.

The control of the silica-lime ratio as described above effeets marked improvements in the haeter; its insulation is maintained at a fairly permanent value through a greatly increased useful life; and the heater does not break down prematurely because the reaction of the insulation material to metals is greatly reduced; and in addition, the reaction of the insulation to moisture is greatly lessened.

While we have shown a particular embodiment of our invention, it will be understood, of course,

that we do not wish to be limited thereto since ing material for said resistance element comprising fused magnesium oxide having present before being fused a controlled silica-lime content, the percentage of silica ranging from 1 to 2 per cent by weight when the lime content is around 0.5 per cent or less by weight, and in a ratio of. silica to lime of around 2 to 1 to 3 to 1 when the lime content ranges from around 0.5 per cent to 5 per cent by weight.

2. Electrically insulating and heat conducting material for a sheathed electric heater that is provided with a metallic sheath and a resistance conductor within the sheath embedded in and supported in spaced relation with reference to the sheath by the electrically insulating and heat conducting material comprising a mass of fused magnesium oxide which before being fused has present a predetermined lime-silica content. said lime content not exceeding about 5% by weight and the silica content not exceeding about 25% by weight.

3. In an electric heater provided with a metallic casing and a metallic resistance element in said casing, electrically insulating and heat conducting material adapted to be inserted in the casing to embed the resistance element and hold it in spaced relation with reference to the casing comprising, fused magnesium oxide having present before being fused a controlled silica-lime content, the ratio of the silica to lime being not less than around 2 to 3, but when the lime content is less than around 0.5 per cent by weight. then the silica content being not less than around 1 per cent by weight.

4. In an electric heater provided with a metallic casing and a metallic resistance element within said casing, electrically insulating and heat conducting material adapted to be inserted in the casing to embed and support the resistance element and hold it in spaced relation to the casing, said material comprising fused magnesium oxide which has present before being fused'a controlled silica-lime content, the ratio of the silica to lime being not less than around 2 to 3 and not greater than around 20 to 1 where the lime content ranges from about 0.5 to around 5 per cent by weight, and said lime content being not greater than around 5 per cent by weight and said silica content not greater than around 25 per cent by weight.

5. In an electric heater provided with a metallic casing, and a metallic resistance element within said casing, a mass of fused magnesium oxide adapted to be inserted in said casing for embedding said resistance element and holding it in spaced relation with reference to said. casing, said fused magnesium oxide having present before being fused a controlled silica-lime content, the ratio of silica to lime ranging from 2 to 3 to 20 to 1;

'ly insulating and heat conducting material for embedding said resistance element consisting of fused magnesium oxide having present before be-- ing fused a controlled silica-lime ratio, the proportion of silica being sumciently great to prevent the presence of free lime in said magnesium oxide after being fused.

,7. In an electric heater provided with a metallic sheath and a metallic resistance conductor in said sheath, electrically insulating and heat conducting material adapted to be inserted in the sheath to embed the resistance conductor and hold it in spaced relation with reference to the sheath, said material comprising granular magnesium oxide which has been fused, cooled and then crushed into granular form, and said magnesium oxide having a controlled silica to lime ratio, the silica being present before fusing in suflicient quantities to prevent the segregation of free lime in said magnesium oxide when it cools after fusing.

OLIVER G. VOGEL. LOUIS NAVIAS.

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