US2170692A - Electric heater - Google Patents

Description

Aug. 22, 1939. L. NAVIAS 2,170,692

ELECTRIC HEATER Filed July 3l, 1937 l 1 i v 0 2 3 4 .5 o V/ 2 3 4 5 o 2 .a 4 5 a 7 a 9 /o Inventor: Louis Navas,

Hli s Alstom@ ey.

Patented Aug. 22, 1939 PATENT OFFICE ELECTRIC HEATER Louis Navias, Schenectady, N. Y., assigner to General Electric Company, a corporation of New York Application July 31, 1937, Serial N0. 156,740

8 Claims.

This invention relates to electric heaters and, more particularly, to electric heaters of the sheathed type, the new feature being an improved insulating material.

This invention has particular application to electric heaters of the sheathed type such as described and claimed in the United States patent to C. C. Abbott, No. 1,367,341, dated February 1, 1921, and assigned to the same assignee as the 10 present invention. Briefly, this heater comprises a resistance element enclosed in a metallic sheath, and embedded in and supported in spaced relation to the sheath, by a dense, compact mass of heat-refractory, heat-conducting, electrically insulating material. It is to be understood, however, that this invention is not limited to the speciilc heater disclosed in this patent, but has more general application.

'Ihe heat-refractory insulating material that -20 has found wide application in sheathed heating units is magnesium oxide, usually in finely-divided form. Heating units provided with this material for the most part have been quite satisfactory. However, for various reasons, important among which is the change in the composition of the magnesia which results, for example, from absorption of moisture and gases, for instance carbon dioxide, the electrical resistance of the heater does not remain constant but decreases in 3,) use. This reduces the life of the heater and, of course, is not so satisfactory as would be the case if the electrical resistance remained constant, or substantially so. To obvlate this dilculty it has heretofore been proposed to use beryllium oxide '35 in lieu of magnesium oxide as the insulating mediumin sheathed heating units. Beryllium oxide is comparatively expensive, costing at present at least about ten times as much as magnesium ox- 1de and, in addition, certain disadvantages are '40 encountered in its use not heretofore known or appreciated.

It is an object of the present invention to provide an electric heater which has a more constant electric resistance during the life of the heater 45 than heretofore has been possible by the use of conventional insulating mediums.

Another object of the invention is to provide an electric heater containing heat-refractory, heatconducting insulating material of improved elec- 50 trical resistance.

Another object of the invention is to provide a method for improving the electrical resistance of fused magnesia. 'Ihe scope of the invention also includes the product of such method.

55 Other and further objects of the invention will be apparent to those skilled in the art as a reading of this specification proceeds.

In accordance with this invention, the resistance conductor of the heater is embedded in, and supported in spaced relation with the walls of 5 its sheath by a layer of insulating material comprising a fusion product of magnesium oxide and beryllium oxide, advantageously with the latter present in the initial mixture in an amount not substantially exceeding 10 per cent by weight 10 thereof.

For a more complete understanding of the invention attention is directed to the following description and the accompanying .drawing in which Fig. 1 is a fragmentary view in elevation of an 15 electric heater embodying this invention, portions of the heater being shown in section so as to illustrate certain structural details; and Figs. 2 and 3 are graphs illustrating the results of electrical tests on insulating compositions of this in- 20 vention.

Referring to Fig. 1 of the drawing, this invention is there shown in one form as applied to an electric heater of the type described and claimed in the above-mentioned Abbott Patent No. 1,367,341.

As shown, the heater comprises a helical resistance conductor mounted Within, and -substantially centrally of a tubular metallic sheath I l. The resistance conductor preferably is formed of a nickel-chromium alloy. The sheath also preferably is formed of an alloy consisting chiefly of nickel and chromium.

The resistance conductor I0 is surrounded by, and supported in spaced relation with the sheath by a layer i2 of heat-refractory, heat-conducting, electrically insulating material comprising a product made by fusing together magnesium oxide and beryllium oxide in a manner such as more fully described hereinafter. This insulating material may be compacted in the sheath by any suitable means, for example, by reducing the diameter of the sheath after the resistance conductor and insulating medium have been assembled with the sheath. The reducing operation may be accomplished by swaging or rolling.

The resistance conductor I G has its ends secured to terminals I3 which, as shown, have their inner ends embedded in the insulating material IZ so as to be supported in spaced relation with 5@ the sheath.

In preparing the insulating composition of this invention magnesia prepared from metallic magnesium or from certain grades of magnesite of a high degree of purity may be employed. The

magnesia component may be obtained, for example, by the hydration of pure magnesium metal in an autoclave. The hydroxide is converted into the oxide by heating in an electric munie or other suitable furnace to a temperature of,

for instance, about 650 to 850 C. Beryllium oxide which initially has been ilred to a temperature of about 1450" to 1550 C. advantageously maybeempioyed.

The dry magnesium and beryllium oxides in predetermined proportions are thoroughly mixed, the relative proportions thereof being dependent, for example, upon their purity and the particular characteristics desired in the end-product. For example, the proportions may vary from about 0.50 to 10 parts by weight of .beryllium oxide to about 99.50 to 90 parts by weight of magnesium oxide. 'Ihe use of a higher or lower ratio of beryllia to magnesia is not precluded. 'I'he essential point is to use such ratio ol' beryllia to' magnesia as will impart optimum electrically .insulating properties to the end-product. The mixed oxides then are heated to a temperature sumcient to convert the mixture to liquid-state, for example to a temperature of about 3000v to 3500 C., resulting in a completely liquefied product or what also may be described as a product o f complete liquefaction of the magnesia-beryllia; mixture, as distinguished from a sintered product'. Any other suitable means of bringing about an intimate association of beryllium and magnesium oxides in molten state may be employed. f

The fusion (liquefaction) product is cooled. When cold (solidified) it is crushed and screened to a suitable particle size, for example', to a size such that substantially all of it passes through a U, S. Sieve Series No. 40 sieve. The comminuted material is treated to remove iron, for example by magnetic separation, after which it is fired out of contact with metal surfaces to a temperature of, for example, about 1l00 C. to 1400 C. The red product advantageously is sifted through a screen of suitable size, for example a 40-mesh screen, and thenA stored in containers until put into use,

An examination under a. petrographic microscope of compositions prepared as hereinbefore described reveals that they comprise substantially pure isotropic periclase crystals, or fragments thereof; periclase crystals or fragments having anisotropic beryllia-magnesia eutectic attached to a surface thereof; periclase crystals or fragments having facing siu'faces coated with, and connected by anisotropic beryllia-magnesia eutectic; and, sometimes, small particles of anisotropic beryllia-magnesia eutetic itself.

Fig. 2 shows graphically the results of tests of the average resistance, plotted against the logarith of time, of units containing insulating compositions-of the present invention in comparison with fused magnesia and berylliaalone as insulating media. In this iigure graphs A and B represent the results of testing units containing beryllium oxide and fused magnesium oxide, respectively; and' graphs C, D and E represent the results of testing units containing compositions prepared as herein described from mixtures of 2 per cent, 5 per cent and 10 per cent, respectively, of beryllium oxide-and the remainder magnesium oxide prepared from metallic magnesium.

In making the test, which is in eiIect a life test with pure metal electrodes under isothermal conditions, a series of ten units, two for each sample, was prepared with hydrogen-annealed nickel parts. All of the units were arranged inra bundle having a thermocouple assembly at its center, and were heated uniformly throughout their entire length and during the whole period of test (72 hours) at a temperature of 982 C. (1800 F.)

not be used to predict the values obtainable at any other time, it is to be noted that the best maintenance of resistance is shown by the composition produced from a mixture of 5 per cent beryllium oxide and per cent magnesium oxide. vThe poorest maintenance of resistance is shown by the fused beryllium oxide. All the compositions of this invention which were tested maintain their initial resistance better than fused magnesium oxide or fused beryllium oxide. This is important from the standpoint of design and service use of an electric heating unit.

It is also to be noted that although fused beryllia has a higher electrical resistance both initially and afterl hours heating at 982 C., thereafter its resistance decreases rapidly. The highest resistance to the passage of electric current at the end 'of 7, 24, 46 and 72 hours heating was shown by the composition made from a mixture of 2 per cent beryllia and 98 per cent magnesia.

Fig. 3 shows the results of electrical resistance tests on units containing different magnesites ignited or calcined to oxidlc state and compositions prepared from mixtures containing varying percentages of beryllia with said magnesites. In preparing the compositions the results of tests on which are designated as F, G and H, magnesites designated below for convenience as l, 2 and 3respectively, were used. These magnesites upon analysis of ignited samples were found to have the following chemical compositions:

Varying percentages of beryllia were mixed with the individual calcined magnesites, The individual mixture then was fused and further processed essentially in the same manner as hereinbefore described with reference to compo, sitions utilizing magnesia prepared from metallic magnesium.

Graphs F and G were plotted from electrical resistance values of calcined magnesites l and 2, respectively, and of compositions prepared by fusing 2, 3 and 5 per cent beryllia with each of said calcined magnesites. Graph H was plotted from the resistance values of calcined magnesite 3 and of compositions prepared by fusing l, 2, 3, 5 and l0 per cent beryllia with said magnesite. As shown by these graphs, a composition made from a mixture of 2 per cent beryllia and 98 per cent 'magnesia. (from magnesite) has a higher electrical resistance than compositions utilizing other percentage proportions. Such a composition represents a preferred embodiment of this invention, All compositions of the present invention have practical usefulness as electrically insulating materials.

What I claim as new and desire to secure by Letters Patent of the United States is:

ll. An electric heater comprising a metallic sheath, a resistance element encased by said sheath and between said resistance conductor and said sheath a layer of an insulating composition consisting of the comminuted solidified product of liquefaction of a mixture of magnesium oxide and beryllium oxide, the latter not exceeding substantially 10 per cent by weight of the whole.

2. An electric heater comprising a tubular metallic sheath, a resistance conductor within said sheath and insulating material consisting of the comminuted solidied product of liqueiaction of a mixture of about 0.5 per cent to 10 per cent by weight beryllium oxide and the remainder magnesium oxide, said insulating material being compacted within said sheath about said resistance conductor and supporting the latter in spaced relation with said sheath.

3. An electric heater comprising a tubular metallic sheath, a resistance conductor within 'said sheath arranged centrally thereof and powdered, densely .compacted insulating material consisting of the powdered solidified product oi liqueiaction of a mixture of about 0.5 per cent to per cent by weight beryllium oxide and the remainder magnesium oxide, said insulating material surrounding said resistance conductor and supporting the latter in spaced relation with said sheath.

4. An electric heater comprising a metallic sheath, a resistance conductor within said sheath and insulating material consisting of the comminuted solidiiied product of liquefaction of a mixture of approximately 98 parts by weight oi' magnesium oxide and approximately 2 parts by weight of beryllium oxide, said insulating material being compacted within said sheath about said resistance conductor and supporting the latter in spaced relation with said sheath.

5. An electrically insulating composition consisting of the comminuted solidied product of liquefaction of a mixture of magnesium oxide and beryllium oxide, the latter not exceeding substantially l0 per cent by weight of the whole.

6. The method of preparing a material especially adapted for use as insulation in electric heaters of the sheathed type which comprises forming a liquid mixture of magnesium oxide and not exceeding substantially per cent by weight of the whole or beryllium oxide, cooling the resultant. mass, and comminuting the cooled mass.

7. The method which comprises forming a liquid mixture of magnesium oxide and from 0.5 to 10 per cent by weight of the whole of beryllium oxide, cooling the resulting mass, comminuting the cooled mass, and heating the comminuted mass to a temperature of about 1100 to 1400 C.

8. 'Ihe method which; consists in thoroughly mixing, by weight, approximately 2 per cent beryllium oxide with approximately 98 per cent magnesium oxide, heating the mixture to about 3000 to 3500" C. completely to liquefy it, cooling the liqueiled product, comminuting the cooled product, treating the comminuted product to remove iron, and heating the thus treated product out of contact with metal surfaces to a temperature of about 1100 to 1400 C.

LOUIS NAVIAS.

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