US2703355A - Electric heater - Google Patents

Description

March 1, 1955 H, HAGGLUND 2,703,355

ELECTRIC HEATER Filed Oct. 23, 1950 2 Sheets-Sheet 1 INVENTOR. fR/K M4 h/IME MATTmssom/M e a; UND

March 1, 1955 E. H. M.-HAGGLUND ELECTRIC HEATER 2 Sheets-Sheet 2 Filed Oct. 23, 1950 A TTO/QIVE) United States Patent ELECTRIC HEATER Erik Haijime Mattiasson Hagglund, Fail-field, Conn., as-

signor to The Kanthal Corporation, Stamford, Conn., a corporation of Connecticut Application October 23, 1950, Serial No. 191,693

4 Claims. (Cl. 201-67) This invention relates to a new and improved alloy metallic sheath of an electric heating unit, especially of an enclosed tubular type comprising a metallic resistance heating element embedded in an electrically insulating heat-conducting mass such, for example, as magnesium oxide which in turn is encased by the metallic sheath. More particularly the invention relates to the alloy metal of the sheath.

It is the principal object of the present invention to provide an alloy metal for the sheath, that will minimize deterioration of the insulating mass at elevated temperatures over extended periods of use, While possessing satisfactory physical qualities, and be susceptible of commercial production.

Another object of the invention is to provide additional strength of the sheath.

The alloy metal of the sheath may have an iron base, or it may have a nickel base where ductility and creep strength are important considerations. In the event an iron base is used the alloy metal will contain also aluminum and chromium. In the event a nickel base is used, the alloy metal will contain also aluminum. In either case (iron or nickel base), small amounts of other elements are present as impurities, or commonly are added during Referring to the drawings, and particularly to Fig. 1, the numeral 1 indicates a metallic resistance heating unit. 2 is an electrically insulating heat-conducting mass. 3 is a metallic sheath. 4 is a sheath outer casing.

Fig. 2 shows graphically the loss of electrical insulating efficiency that is effected by reason of diffusion at elevated temperatures, of commonly found oxides, for example, oxides of aluminum and chromium, into an insulating mass, for example, composed of magnesium oxide.

From numerous tests of various sheath materials, I have observed that commonly found oxides which are developed at elevated temperatures upon the inner surfaces of sheaths of various metallic constituents, when diffused into the electrically insulating mass, Will increase the conduction of minute currents of electricity through said mass as the amounts of such diffused oxides increase. Such commonly found oxides when so developed and diffused into the insulating mass have been found to form a. well defined pattern of relative effectiveness of said oxides in forming said electrical conduction passages through the insulating mass.

Reduction of insulating efficiency of the insulating mass by diffusion of oxides could be avoided entirely by use of a sheath of precious metal such as gold or platinum, for example, neither of which oxidizes at elevated temperatures and, therefore, could not cause penetration of foreign oxides into the insulating mass. Were it not for other considerations, therefore, a sheath of gold or platinum, or with an interior plating of gold or platinum sufficiently thick to eliminate diffusion effects of non-precious metals of the sheath through the gold or platinum plating, would be an ideal solution of the problem here considered. Such a solution is impractical commercially, however, because of the prohibitive cost of the precious metals in what would be the required quantities.

The order of relative electrical conductivity (i. e., of objectionableness from a viewpoint of deterioration of insulation) of some commonly found oxides is as follows:

Resistivity in ohms per ec.

Temp. in Degrees C iron oxide ehrornic 0xide nickel oxide Zirconium oxide silicon oxide. aluminum oxidemagnesium oxide the smelting process in order to carry through the usual metallurgical reactions, and to improve heat resistivity and mechanical properties of the alloy metal.

Mechanical strength of the sheath may be increased by means of a sheath outer casing composed of an alloy metal of greater ductility and creep strength than that of the sheath metal indicated. The sheath outer casing alloy metal may have a nickel base and contain chromium and iron.

I have found that a heater unit sheath composed of any of the alloy metals described above combines the primary requisite of minmal tendency to cause deterioration of the insulating mass, with economic and physical feasibility of production of the sheath.

Other objects and features of my nvention will be apparent from the following detailed description and from the accompanying drawings, wherein,

Fig. 1 is an elevational view, partly in section, showing by way of illustration, a heater unit of the type indicated in a spiral form, with a sheath as described and a sheath outer casing, although, of course, the unit may follow any convenient, form without affecting the invention, such, for example, as straight rectangular, circular, serpentine, labyrinthine, etc. l

Fig. 2 is a graph showing the electrical resistivity, and hence the insulating efiiciency over a range of ternperatures, of pure magnesium oxide, and the electrical resistivity of magnesium oxide with which stated proportions of aluminum oxide and chromic oxide have been mixed.

Of the foregoing oxides, those of zirconium, silicon and magnesium are not developed in substantial quantity by P commercial alloys when used at elevated temperatures,

and therefore do not enter seriously into the problem of providing an alloy of desirable qualities'for the use herein contemplated.

The other oxides tabulated above, viz., those of iron, chromium, nickel and aluminum, and especially those of chromium and aluminum, are developed in considerable amounts on the surfaces of various commercial high temperature alloys when heated in air or oxygen-containing atmospheres to temperatures ranging upward from 700 C., and in progressively greater amounts as the temperature is increased.

From the foregoing table it will be observed that at 1000 C., for example, the electrical resistivity of chromic oxide is less than 7.5 X10 expressed in ohms per cc., whereas the electrical resistivity of aluminum oxide is .i 2.8 x10, expressed in ohms per cc. Translated into terms sulator, actually is the most generally used insulator for Patented Mar. 1, 1955 installations of the type here considered. It is obvious, therefore, that an alloy metal that would diffuse only magnesium oxide into the insulating mass (for example magnesium oxide) would cause no diminution of insulatcasing, with proportions of constituent metals expressed in percentages, are as follows:

Group C.High nickel shea'th outer casing alloy metal ing efficiency of the insulating. mass, and so be: a com- 5 mercially desirable product. if otherwise suitable. This or Mn Ca 06 Fe solution is not practical, however, because, as mentioned 0 B above, magnesium oxide is not developed in substantial 6 L altimeq iw by commercial metals- Eiiiiii: 333% 3 31% 3:3 8:2 Siio Sit 3:8 133:

Consideration of the foregoing facts suggests, as a 10 A solution of the problem of producing a commercially superior sheath alloy metal, ascertainment of an alloy, i sheah Calsmg 1S secufed to fi i most otherwise suitable, that will develop on its inner sur- 3 by i fl Ttils method satlsfagtorg ce r cticall onl' oxides of aluminum to th' exer Or.mary cor. Hons a srpnger on ilusio of the moral detrimental oxides iii any a PPretween the Sheath the casmg 1S1 @qmred for exciable quantity and those in an aggregate amount subgi where i heater g is ig L tha-n p p e more usua sp1ra orrn, or were e ea er un1t igig i g gg iiz ig gg gg fgii igig g W111 be sulgectedi t8 eijrfcesswe vlbratlon, a stronger bond i may be 0 taine y orging or rolling the sheath mag ggi ggg ng fi g i g for example terial afild thelcaising mateirial together, or the casing. may

4 v be we ed c'a or bon ed onto the sheath accordin As a result of numerous studies and expenments, alto usual HIetaHUIgiC-(IL pwcmhmeg loy metals for electrical heater unit sheaths have been In h three foregoing groups f alloy metals, indi. found to be satisfactory with respect to insulation de- Gated as Group A, Group and Group carbon, u teflofatlofl when mp of the following lnfllcated con, and manganese are common metallurgical reaction proportions of constituent elements, expressed 1n perelements that are used generally in alloying processes. centages: I have lncluded also mmor quantities, as indicated, of

Group A.-Ir0n base x1 Cr 0 s1 Mn Ga Ce Zr '11 Nb Ta 00 Fe (1) 5.0 13.0 0.0 0.0 0.6 0.0 0.01 0.0 0.0 .0 0.0 1.0 Balance. (2) 9.0 5.0 0.10 0.0 0.5 0.0 0.05 0.1 1.0 .0 0.0 10.0 Do. a) 4.0 22.0 0.20 0.7 0.5 0.0 0.4 0.15- 0.0 .4 3.0 0.1 Do. 4) 2.0 30.0 0.00 1.2 0.0 0.4 .0.0 0.0 3.0 .0 0.1; 0.0 Do.

Group B.-Nickel base 0 s1 Mn on 00 Zr- Fe Ni 0.00 1.5 0.7 0.4 0.15 0.4 0.0 Balance. 0.11 0.0 0.4 0.05 0.4 0.1 0.2 Do. 020 0.8 0.0 0.0 0.0 0.0 0.8 Do.

I have found that the presence of chromium in the foregoing alloys has certain advantages.

It contributes to the primary requirement that the sheath alloy metal shall develop at elevated temperatures aluminum oxides practically exclusively on the sheaths inner surface. While these aluminum oxides are developed, of course, entirely from the aluminum content of the alloy, I have observed that they more readily form. a surface coating, and that this aluminum oxide coating is more persistently adhesive, where a chromium content is present in the alloy metal than where there is no chromium Different theories might be advanced in explanation of this fact. As these theories are involved and controversial, however, discussion of them, which would have to be rather long and involved to be at all adequate, is omitted. Sulfice it to add that I have found that, within the limits of the above illustrative examples, as the chromium content is increased the required proportion of aluminum may be decreased without detriment to the aluminum oxide formation at elevated temperatures, during the life of the heater unit. This is due to the increased durability of the aluminum oxide coating that is developed in such cases where chromium is present in the alloy metal.

Furthermore, chromium adds to the strength and heat resistivity of the alloy metal, although it will cause brittleness and loss of ductility to an undesirable extent it included in larger proportion than I have indicated. In the instance of the sheath alloy metal with iron base (Group A), Where the chromium content is decreased, similar physical qualities of st ength and heat resistivity are supplied by the addition 0 .obalt.

Ductility and creep strength of the sheath may be increased further by means of a sheath outer casing of a tough and ductile alloy metal, even though, because of its factor of oxidation at elevated temperatures such casing material would not be suitable for use as sheath material throughout. Illustrative examples of alloys that. I'have found to be suitable for this purpose as a sheath the elements of calcium, cerium, and zirconium, because I have found that these elements improve heat resistivity of the sheath alloy metal.

In Group A alloy metals, I have included also the elements of titanium, niobium, and tantalum because I have found that these elements increase ductility of the sheath alloy metal.

In each of the three foregoing groups, A, B, and C, the preferred alloy metal of that respective group is shown as the first of the several illustrative examples of alloy metals found satisfactory for the indicated purpose.

According to the invention, ranges within which the proportions of constituent metals of the above alloy metals may be varied are indicated by the above illustrative examples, although variations within these limi lt sh may be made within the scope of this invention.

The sheath alloy metal with iron base (Group A) preferably should contain from 2.0 to 9.0% aluminum, from 5.0 to 3.0% chromium, from 0.0 to 0.2% carbon, 0.0 to 1.2% silicon, 0.0 to 0.6% manganese, 0.0 to 0.4% calcium, 0.0 to 0.4% cerium, 0.0 to 0.15% zirconium, 0.0 to 3.0% titanium, 0.0 to 3.0% niobium, 0.0 to 3.0% tantalum, 0.0 to 10.0% cobalt, the balance principally iron.

The sheath alloy metal with nickel base (Group B) preferably should contain from 5.0 to 11.0% aluminum, from 0.0 to 8.0% chromium, 0.0 to 0.2% carbon, 0.0 to 1.5% silicon, 0.0 to 0.7% manganese, 0.0 to 0.4% calcium, 0.0 .to 0.4% cerium, 0.0 to 0.4% zirconium, 0.0 to 0.8% i'ron,-the balance principally nickel.

The sheath outer casing alloy metal (Group C) preferably should contain from 14.0 to 22.0% chromium, 0.0 to 0.1% carbon, 0.0 to 1.6% silicon, 0.0 to 0.6% manganese, 0.0 to 0.4% calcium, 0.0 to 0.4% cerium, 0.0 to 0.4% zirconium, 0.0 to 25.0% iron, the balance principally nickel.

Analyses of insulating masses. of heater units with sheaths made of alloy metals as suggested in- Groups A and B above, and of any commercial type of design, have shown, after extended operation at temperatures, up to 1000 C., that as to oxides of chromium, nickel or iron, the increase in the content of these three oxides in the insulating mass is less than 0.2% of each of said oxides.

As a result of this more favorable factor of deterioration of the insulating mass that is obtained by heater unit sheaths formed of alloy metals according to the present invention, a heater unit with a sheath of any one of said materials has a safe heat limit approximately 150 C. higher than that of 850 to 900 C. for any heater unit of this type heretofore known, and correspondingly longer life when used at the same temperatures as those at which any heater unit heretofore known is used.

What I claim and wish to secure by Letters Patent is:

1. An electric heating device including a sheath, an electric heating resistor inside of said sheath, and an electrically insulating mass inside of said sheath and directly contacting the inside surface of said sheath and in which said resistor is embedded and which separates and electrically insulates said resistor from said surface, said sheath being made of a metallic alloy containing a metal of the class consisting of chromium and nickel which during the operation of said device normally would form an oxide on said surface of greater electrical con ductivity than said mass and which would diffuse into said mass and thus lower its electrical resistivity, said alloy containing sufticint aluminum in proportion to said metal to retard the formation of said oxide and to thereby materially prolong the useful life of said device, said alloy otherwise having a composition suitable to serve as said sheath, said alloy otherwise than said aluminum and the aforementioned metal of the class of chromium and nickel consisting essentially of iron excepting impurities and alloys having no deleterious effect on said action of the aluminum.

2. An electric heating device including a sheath, an electric heating resistor inside of said sheath, and an electrically insulating mass inside of said sheath and directly contacting the inside surface of said sheath and in which said resistor is embedded and which separates and electrically insulates said resistor from said surface, said sheath being made of a metallic alloy containing a metal which during the operation of said device normally would form an oxide on said surface of greater electrical conductivity than said mass and which would diffuse into said mass and thus lower its electrical resistivity, said alloy containing sutficient aluminum in proportion to said metal to retard the formation of said oxide and to thereby materially prolong the useful life of said device,

said alloy otherwise having a composition suitable to serve as said sheath, said metal comprising a substantial amount of chromium and said alloy otherwise than said aluminum and chromium consisting essentially of iron excepting impurities and alloys having no deleterious effect on said action of the aluminum.

3. An electric heating device including a sheath, an electric heating resistor inside of said sheath, and an electrically insulating mass inside of said sheath and directly contacting the inside surface of said sheath and in which said resistor is embedded and which separates and electrically insulates said resistor from said surface, said sheath being made of a metallic alloy containing a metal which during the operation of said device normally would form an oxide on said surface of greater electrical conductivity than said mass and which would diffuse into said mass and thus lower its electrical resistivity, said alloy containing sufiicient aluminum in proportion to said metal to retard the formation of said oxide and to thereby materially prolong the useful life of said device, said alloy otherwise having a composition suitable to serve as said sheath, said alloy composition consisting of from 2% to 9% of said aluminum, from 5% to 30% chromium comprising said metal, said aluminum being used in amount approximately inversely in proportion to the amount of chromium used, and with the balance iron excepting small amounts of impurities and alloys having no deleterious effect on said action of the aluminum.

4. An electric heating device including a sheath, an electric heating resistor inside of said sheath, and an electrically insulating mass inside of said sheath and directly contacting the inside surface of said sheath and in which said resistor is embedded and which separates and electrically insulates said resistor from said surface, said sheath being made of a metallic alloy containing a metal which during the operation of said device normally would form an oxide on said surface of greater electrical conductivity than said mass and which would dilfuse into said mass and thus lower its electrical resistivity, said alloy containing suflicient aluminum in proportion to said metal to retard the formation of said oxide and to thereby materially prolong the useful life of said device, said alloy otherwise having a composition suitable to serve as said sheath, said alloy composition consisting of from 2% to 9% of said aluminum, from 5% to 30% chromium comprising said metal, said aluminum being used in amount approximately inversely in proportion to the amount of chromium used, and with the balance iron excepting small amounts of impurities and alloys having no deleterious effect on said action of the aluminum, said sheath being tightly enclosed by a casing made of an alloy consisting of from 14% to 22% chromium with the balance nickel excepting impurities and small amounts of alloys having no material deleterious effect on the toughness and ductility of said casing alloy, the latter being substantially free from aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 859,608 Marsh July 4, 1907 1,359,400 Lightfoot NOV. 16, 1920 2,019,686 Lohr Nov. 5, 1935 2,191,790 Franks Feb. 27, 1940 2,387,980 Cooper Oct. 30, 1945 2,403,926 Johnson July 16, 1946 2,460,590 Lohr Feb. 1, 1949 2,533,736 Lohr Dec. 12, 1950 FOREIGN PATENTS 147,151 Austria Oct. 10, 1936 583,807 Great Britain Dec. 31, 1946 OTHER REFERENCES Broniewski: Treatise in Anales de Chimie et de Physique, 8 Serie, Tome 25, 1912, pp. 106411.

Woldman: Material Engineering of Metal Products, 1949.

Reinhold Pub. Co., New York, chap. XIII, pp. 335- 349.

Serial No. 124,858, Hessenbruch (A. P. C.), published June 1, 1943, abandoned.

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