US 2669636 A
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Feb. 16, 1954 w. T. RAWLES 2,669,636
SHEIATHED ELECTRIC HEATER INSULATING MATERIAL Filed May 21, 1951 Inventor: Wi I iam T Rawle s,
bps m H i s Attorne g.
Patented Feb. 16, 1954 SHEATHED ELECTRIC HEATER INSULATING MATERIAL William T. Rawles, Pittsileld, Mass assignor to General Electric Company, a corporation of New York Application May 21, 1951, Serial No. 227,384
This invention relates to sheathed electrical heaters and particularly concerned with an improved insulating material for such heaters. It has as its principal object the provision of a heater of this character including an improved heat-refractory insulating material.
The sheathed heaters of the type with which the present invention is concerned are those in which a resistance conductor is enclosed in a tubular metallic sheath, and embedded in and supported in spaced relation with the sheath by a densely compacted layer of heat-refractory and heat-conducting, electrically insulating material.
Such a heater is shown in the accompanying drawing in which the single figure is a fragmentary view in elevation of a sheathed electric heater embodying the present invention, portions of the heater being shown in sections so as to illustrate certain details of construction.
As shown in the drawings, the heater comprises a helical resistance conductor 19 mounted within and substantially centrally of a tubular metallic sheath H. The resistance conductor Ill may be formed of any suitable material, such as a nickel-chromium alloy. The sheath I l preferably is made of a suitable chrome-iron-nlckel alloy. The resistance conductor It is secured at its ends to suitable terminals [3. 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 in, the sheath I! and terminals 13 may be made of many other suitable materials.
The resistance conductor Ill is embedded in and is supported in spaced relationship with reference to the sheath by a mass 12 of heat refractory and heat-conducting, electrically insulating material. The mass of insulating material is in granular form and is compacted to 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 within the sheath. This reducing operation may be accomplished by swaging, drawing, .or rolling.
The heat refractory insulating material that has found wide application in such sheathed electric heaters is granular magnesium oxide, and heaters provided with this insulating material have on the whole been quite satisfactory. Pure magnesium oxide has a very high initial resistance and maintains a high resistance; value throughout a very long life. The granular magnesium oxide is prepared by fusing magnesium oxide and crushing and pulverizing the fused product to the desired granular size.
Due to the fact that pure magnesium oxide is not available commercially in suflicient quantitles or at a reasonable enough cost, commercial magnesium oxides have ordinarily been employed after first being treated or modified to obviate the undesirable effects of the impurities ordinarily present in such commercial magnesium oxides. These treatments have included the incorporation into the fused magnesium oxide of a small amount of silicon oxide for the purpose of counteracting the small quantities of calcium oxide present as impurities in commercially available magnesium oxides or the treatment of the fuse magnesium oxide with beryllium oxide by the process and for the purposes described in Patent 2,170,692, issued to Louis Navias, and assigned to the same assignee as the present invention. However, even these modified commercial magnesium oxides have not been completely satisfactory from the cost standpoint.
Various attempts have heretofore been made to substitute for magnesium oxide less expensive materials such, for example, as zirconium silicate which is obtainable in substantial quantities at a lower cost. However, the extreme hardness of granular zirconium silicate and the crystal structure thereof do not permit this material to densify easily when pressed, rolled or swaged in the manufacture of the heating element. Granular zirconium silicate has been found to flow to such an extent during the reduction of the sheath that the resistance conductor does not remain centered within the sheath during the reduction operation, Also as the zirconium silicate is not sufficiently densified or compacted, shifting of the helical resistance conductor within the sheath was frequently encountered during use of the heater element.
The present invention is based on the discovcry that a low cost refractory insulating material possessing satisfactory electrically insulating and heat-conducting properties and the proper compacting characteristics can be provided in the form of mixtures of zirconium silicate and magnesium oxide of particular particle sizes. The term "magnesium oxide as used hereinafter is intended to include pure magnesium oxide as well as the modified commercial grades of magnesium oxide, all of which have been prepared by fusing and thereafter crushing and pulverizing the fused material tothe propergranular size.
More particularly, it has been found that mixtures of from about to per cent by weight of granular fused magnesium oxide and from to 80 per cent zirconium silicate can be satisfactorily employed in the manufacture of the above described sheathed electric heating elements provided the particle size of the magnesium oxide is such that at least about 22 per cent is retained on a (SO-mesh screen while all of it passes through a 40-mesh screen and the zirconium silicate is of a particle size less than 80 mesh with at least 80 per cent of the zirconium silicate being of a particle size less than 100 mesh. The term mesh as used herein refers to the U. S. Sieve Series numbers as set forth,,fo r example, in the data relating to Sieves of the Standard Screen Scale, page 2453 of the Handbook of Chemistry and Physics, 26th ed., 1942- 1943.
It has been found that while zirconium silicate alone has poorer electrically insulating properties than magnesium oxide, in mixtures of zirconium silicate and magnesium oxide of the above proportions, these poorer electrically insulating properties are offset by the better heat-conducting properties of the zirconium silicate so that heater elements containing such mixtures were completely satisfactory, both from the heat conductivity and electrical insulating standpoints. .In
fact, the presence of a substantial quantity of quantity of silica, specifically fused silica in granular form and having a particle size within the same ranges specified for the zirconium silicate. In general, the silica may be substituted for up to 60 per cent by weight of the zirconium silicate. In other words, the silica content of the total refractory mixture may be up to 40 per cent by weight.
As a specific example of how the present invention may be carried into effect, a powder mixture consisting of 7 parts zirconium silicate and 3 parts crystalline fused magnesium oxide of the above-mentioned particle sizes was loaded into a sheathed unit employing the standard technique described, for example, in Oakley et a1. patent, 2,483,839. The loaded unit was prepared for a reduction in cross-sectional area by rolling in the standard manner after first being sealed by means of washers crimped into the end of the tubular sheath. The units were thereafter rolled to compact the powder and checked for elongation, degree of compaction, or hardness of the compacted material, reproducibility, and position ofrthe helical resistance elements. "It was found that all of the units loaded satisfactorily and while they showed slightly greater elongation than corresponding units filled only with pure magnesium oxide of the same degree of compaction, they were nevertheless satisfactory and reproducible. In addition the helix was well centered within the tubular sheath and the comin the straight magnesium oxide pacted powder wa sufliciently hard to retain the helix in such position during use of the heating unit. These results are to be compared with previous experiences employing pure zirconium silicate where after rolling, the helical resistance element was poorly positioned within the tubular sheath and the powder in the reduced unit was loose, permitting shifting of the element during use of the heating unit.
In the insulating compositions of the present invention the zirconium silicate or mixture of zirconium silicate and silica comprises the principal portion. The magnesium oxide in the proportions employed permits the proper compaction of the granular mixture which cannot be obtained with either pure zirconium silicate or mixtures of zirconium silicate and silica. It is essential that the magnesium oxide have a grain size larger than the particle sizes or average particle sizes of the zirconium silicate and silica. It is believed that the improved compacting characteristics result from the fact that the magnesium oxide is more easily crushed during the rolling or swaging operations performed on the sheathed unit and because of its cubic crystal habit, it breaks down in all directions with equal ease to fill in between the particles of the zirconium silicate or mixtures of zirconium silicate and silica and effect a satisfactory compacting of the entire mixture.
Furthermore, by employing a mixture of different grain sizes there is obtained a material of a high bulk density and a resultant better heat conductivity. Furthermore, the larger grains of magnesium oxide break down more readily than smaller ones during the reducing operation with resultant better compaction of the mixtures.
It is to be noted that the mixtures of the present invention are particularly characterized by the absence of any binder, such as clay or the like, frequently employed for the purpose of bonding together poorly compactible granular insulating materials ordinarily as a result of a partial sintering or fusing of the clay or similar binder. Such binders are undesirable in the sheathed units with which the present invention is concerned due to their poor electrical characteristics.
The advantages obtained by the substitution of some silica for part of the zirconium silicate in.- clude a decreased cost and a decrease in weight per unit volume. However, as the silica will not withstand as high an operating temperature as the zirconium silicate, the proportions thereof should be limited, particularly in those instances where the heating unit will be operated at relatively high temperatures.
The primary function of the magnesium oxide is that of a binding action without fusion. This binding action results solely from the compacting characteristics of the magnesium oxide and its ability to disintegrate during the reducing operation and fill the voids between the zirconium silicate or zirconium silicate and silica particles.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A binder-free granular, refractory, heatconductive, embedding and electrically insulating material for sheathing electrical resistance elements comprising a mixture of (a) from about 20 to 40 per cent, by weight, granular fused magnesium oxide of a particle size such that at least about 22 per cent is retained on a SO-mesh screen and all passes through a 40-mesh screen, and (b) from about 60 to by weight, of at least one material of the class consisting of (1) zirconium silicate and (2) mixtures of zirconium siilcate and silica, said material (b) being of a particle size less than 80 mesh with at least 80 per cent thereof being of a particle size less than 100 mesh.
2. An insulating material of claim 1 in which the material (b) is zirconium silicate.
3. The insulating material of claim 1 consisting of to 40 per cent magnesium oxide, to 70 per cent zirconium silicate and up to per cent silica.
4. The insulating material of claim 1 consisting of 30 per cent fused magnesium oxide, per cent fused zirconium silicate and 20 per cent silica.
5. A sheathed electrical heating element including an embedding material comprising a 0mm pressed, binder-free, granular mixture of 20 to 40 per cent granular fused magnesia, balance at least one granular material of the group consisting of 1) zirconium silicate and (2) mixtures of zirconium silicate and silica of a particle WILLIAM T. RAWLES.
References Gitetl in the file of this patent UNITED STATES PATENTS Number Name Date 1,359,400 Lightfoot Nov. 16, 1920 1,952,119 Comstock Mar. 27, 1934 OTHER REFERENCES Rees et al.: A Preliminary Investigation of the Magnesia-Zircon Series. Transactions of the English Ceramic Society, May 1930, pp. 309-316. Copy in Div. 3 (106-57).