US3369209A - Electric heating element - Google Patents

Electric heating element Download PDF

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Publication number
US3369209A
US3369209A US430642A US43064265A US3369209A US 3369209 A US3369209 A US 3369209A US 430642 A US430642 A US 430642A US 43064265 A US43064265 A US 43064265A US 3369209 A US3369209 A US 3369209A
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Prior art keywords
sheathing
oxygen
electric heating
heating element
powder
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Expired - Lifetime
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US430642A
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English (en)
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Edwin Bjorn
Hegbom Thor
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Individual
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Individual
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B3/00—Ohmic-resistance heating
    • H05B3/40—Heating elements having the shape of rods or tubes
    • H05B3/42—Heating elements having the shape of rods or tubes non-flexible
    • H05B3/48—Heating elements having the shape of rods or tubes non-flexible heating conductor embedded in insulating material

Definitions

  • This invention relates to an electric heating element including an outer sheath, at least one resistance wire housed within said sheath and surrounded by magnesia powder, and said sheath containing, or alternatively, said powder containing ferrous oxide, nickel oxide, cobalt oxide or copper oxide in an amount sufficient to preclude substantial discoloration of the magnesia powder when the elementis sealed in a gas-tight manner and heated to an elevated temperature.
  • the present invention relates to heating elements and relates more particularly to an electric heating element to be used at operation temperatures above 700 C. in air and comprising an outer metallic sheathing which houses one or more resistance elements embedded in a magnesia filling in the absence of materials of the kind that easily give ofl oxygen.
  • such heating elements are manufactured by starting, for example, with a tubular blank and consisting of a heat resistant material.
  • the tube is placed vertically and filled with a central coil of resistance wire around which there is placed as electrical insulation a suitable material such as magnesia powder.
  • a suitable material such as magnesia powder.
  • the tube is vibrated to ensure a uniform distribution of the insulating material.
  • the tube with its contents is compressed from a diameter, for example, 9.5 mm. down to 8 mm. to ensure that the coil is held firmly and that the heat conductivity of the powder is improved.
  • end seals are applied, if desired.
  • Such an element may be used straight with or without a reflector for use in heating rooms, for example, or it may be wound up in a spiral to be used as a cooking plate in a well known manner. Naturally, it is very essential to maintain a proper insulation under all conditions, and it is this problem with which the instant application is concerned.
  • one of the reasons for the discoloring may be that the atmospheric conditions Within the tube are successively changed, as evidenced on the one hand by a pressure reduction, and on the other hand by an alteration of the gas composition such as decrease of the oxygen content by an oxidation of the sheathing and/or of the resistance wire.
  • a tube material of the most commonly utilized type comprising nickel-chromium-iron alloys
  • this factor appears to be of less relevance inasmuch as the total deteriorating effect may be reduced by a suitable choice of sheathing material.
  • the increased significance of the outer sheathing will be recognized on comparing the relatively large area of the same in respect to the resistance wire housed therein.
  • tubes of various compositions have been filled with magnesia powder and sealed in a gas-tight manner by welding the ends thereof, whereafter the tubes have been heated up to 1125 C. for five hours.
  • the results of this study showed that magnesia in tubes of alloyed materials was strongly discolored. Therefore, it would appear that the discoloring process is related to the ability of the sheathing material to combine with oxygen thereby influencing the oxygen partial pressure within the tube.
  • a primary object of the present invention is to prevent as far as possible the reduction of the electric insulation resistance of magnesia powder which in certain cases arises in tubular elements after extended periods of heating to elevated temperatures. Consistent therewith, the in stant inventive concept provides that the sheathing should be made of a material such that, if a tube of the same with a magnesia powder therein is sealed in a gas-tight manner and heated to 1125 C. for five hours, no substantial discoloration of the powder occurs.
  • this invention utilizes two slightly different methods for achieving the intended result, that is, the use of a suitable sheathing material and/ or, the addition of a material which would result in maintaining the oxygen partial pressure above the critical limit at which the magnesia powder is discolored.
  • a sheathing material a combination of two metals or alloys in such a manner that the innermost part of the sheathing is made of a material with the recited property of being able to prevent discoloration, such as carbon steel, copper or nickel, whereas the outermost part of said sheathing is made of a material with the required heat resistance at elevated temperatures in air.
  • Sheathings of this general kind- may be manufactured in a plurality of different ways, such as forming the tubes from strips of compound material. Also, the sheathings may be formed from two tubes one introduced into the other. Another method contemplated by this invention is to apply, such as by flame spraying, iron, an alloy, an oxide or another oxygen-containing composition on to a strip or the inner surface of a tube, out of which the sheathing is then made.
  • magnesia powder one or more oxides or other oxygen-containing materials, the oxygen partial pressure of which at the operating temperature would be above the limit at which deterioration is encountered.
  • oxides or other oxygen-containing materials examples include FeO, CoO, NiO and CuO.
  • uch additives may be introduced separately into the sheathing or, alternatively, they may be formed by oxidation of an inner portion of the sheathing.
  • Swedish Patent No. 152,930 discloses an addition of certain components having the ability to give off oxygen.
  • the oxides which are used readily give off oxygen and thus have a very essential influence on the atmosphere within the tube in such a manner as to render the same mainly oxygen. This ensures the formation of a superficial protective coating on the resistance wire and/ or the inner surface of the tube.
  • tubular heating elements, after filling and compression thereof, are annealed at temperatures above 1000 C., under which condition the said oxygencontaining additives give off their oxygen content.
  • the present invention contemplates oxides of quite another character such as FeO, CoO, NiO, and the like, which may not be considered as giving off oxygen readily but which nevertheless would be able to increase the oxygen partial pressure in an enclosed space above the above-discussed critical :value at which a discoloration of the magnesia powder occurs.
  • oxides of quite another character such as FeO, CoO, NiO, and the like, which may not be considered as giving off oxygen readily but which nevertheless would be able to increase the oxygen partial pressure in an enclosed space above the above-discussed critical :value at which a discoloration of the magnesia powder occurs.
  • the oxygen partial pressure of the materials mentioned in the Swedish patent is considerably higher than 1 atmosphere, whereas the oxides as now contemplated have oxygen partial pressure values of 10- and 10- atmospheres, respectively.
  • FIGURE 1 is a side elevational view of one form of heating element in accordance with the instant inventive concept, parts being in section for illustrative clarity;
  • FIGURE 2 is an enlarged longitudinal cross-sectional view of a portion of an electric heating element in accordance with one embodiment of this invention.
  • FIGURE 3 is an enlarged longitudinal cross-sectional view of a portion of an electric heating element according to another embodiment of this invention.
  • FIGURE 1 an electric heating element in accordance with the instant inventive concept is shown generally in FIGURE 1 and designated by the reference numeral 10.
  • This element comprises basically an outer tubular sheathing 12 with at least one resistance coil 14 housed therein embedded in magnesia powder 16, end plugs 18 sealing the sheathing 12 in a gas-tight manner.
  • the terminals 20 of the resistance coil 14 extend outwardly from the sheathing 12.
  • the sheathing 12 is formed of a material which is resistant to heat at temperatures above 700 C. in air and the resistance coil 14 has an operating temperature of up to about 1050 C.
  • the sheathing is formed of an outer layer 12a and an inner layer 12b, the former being made of a material resistant to heat at 700 C. in air such as a nickel-chromium-iron alloy, and the latter including an oxygen-containing material. While the drawing shows two distinct layers, it is to be understood that the inner layer 121) may merely be a portion of the sheathing which incorporates as an additive the oxygen-containing material. This material may be, for example, iron oxide, nickel oxide, cobalt oxide, copper oxide and the like.
  • the material in its oxide form may be either incorporated as an additive or as a layer in the sheathing, it may take the form of an oxidizable material which during the formation treatment is heated to such a temperature as to oxidize the same thereby forming the oxygen-containing material in situ.
  • an oxygen-containing powder may be added to the magnesia powder as illustrated in FIGURE 3 wherein the magnesia powder is identified as 16a and the oxygen-containing powder is identified as 1612.
  • the oxygen-containing material may be either incorporated per se or formed in situ as described with reference to FIGURE 2.
  • the significant factor in regard to this instant invention is the incorporation of an oxygen-containing material within the element either as a portion of the outer sheathing or as an additive to the mangesia powder which material has the characteristic of functioning to preclude any substantial discoloration of the magnesia powder when the element is sealed off in a gas-tight manner and heated to a relatively high temperature such as 1125 C. for an extended period such as five hours.
  • This oxygen-containing material maintains the oxygen partial pressure above the critical limit described hereinbefore at which the magnesia powder tends to deteriorate.
  • Example A tubular element having a compound sheathing of 0.65 mm. wall thickness was chosen.
  • This compound sheathing had an outer layer of 0.45 mm. thickness composed of Nikrothal 4R (Reg. Trademark), this ma terial being an alloy of the composition 33% Ni, 20% Cr, the balance essentially being iron, and an inner layer of 0.20 mm. thickness composed of carbon steel, containing 0.1% C, 0.2% Si, and 0.4% Mn, the balance essentially iron.
  • the outer diameter of the tube was 9.5 mm.
  • the tube was filled with magnesia powder, and as a resistance coil an 0.4 mm. diameter wire of Kanthal DSD (Reg.
  • an electric heating element including an outer sheathing, at least one resistance wire housed therein, and magnesia powder embedding the resistance wires
  • said sheathing housing therein a material precluding substantial discoloration of the powder when the element is sealed in a gas-tight manner and heated to 1125 C. for five hours, said material being selected from a group consisting of ferrous oxide, nickel oxide, cobalt oxide and copper oxide and being present in an amount sufficient to provide an oxygen partial pressure at 1125 C. in the order of from about 10 to about 10- atmospheres.
  • an electric heating element including an outer sheathing, at least one resistance wire housed therein, and magnesia powder embedding the resistance wires
  • the improvement which comprising the addition to the powder of a material precluding substantial discoloration of the powder when the element is sealed in a gas-tight manner and heated to 1125 C. for five hours, said material being selected from a group consisting of ferrous oxide, nickel oxide, cobalt oxide and copper. oxide and being present in an amount sufficient to provide an oxygen partial pressure at 1125 C. in the order of from about 10 to about 10- atmospheres.
  • An electric heating element in accordance with claim 1 wherein said sheathing is formed of a composite, two-layer material and said material is housed in the inner layer.

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US430642A 1964-02-07 1965-02-05 Electric heating element Expired - Lifetime US3369209A (en)

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SE01538/64A SE327767B (enrdf_load_stackoverflow) 1964-02-07 1964-02-07

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3476842A (en) * 1966-08-10 1969-11-04 Dow Corning Composition for sealing joints and method of making same
US3571477A (en) * 1968-06-21 1971-03-16 Bert Phillips Protection of oxidizable electric furnace elements at high temperatures
US3828296A (en) * 1970-07-21 1974-08-06 Int Nickel Co Sheathed electric heater elements
US3959001A (en) * 1973-12-21 1976-05-25 Dynamit Nobel Aktiengesellschaft Method of preparing an electrically insulating embedding composition
US4234786A (en) * 1979-02-12 1980-11-18 General Electric Company Magnesia insulated heating elements and method of making the same
US4376245A (en) * 1980-02-06 1983-03-08 Bulten-Kanthal Ab Electrical heating element
EP0079385A4 (en) * 1981-05-18 1983-09-20 Matsushita Electric Ind Co Ltd SHIELDED HEATING ELEMENT AND THEIR PRODUCTION PROCESS.
US4626665A (en) * 1985-06-24 1986-12-02 Shell Oil Company Metal oversheathed electrical resistance heater
US4732792A (en) * 1984-10-08 1988-03-22 Canon Kabushiki Kaisha Method for treating surface of construction material for vacuum apparatus, and the material treated thereby and vacuum treatment apparatus having the treated material
EP3745815A3 (en) * 2012-07-05 2021-02-17 nVent Services GmbH Mineral insulated cable having reduced sheath temperature

Citations (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1763117A (en) * 1930-06-10 House electric
US2036788A (en) * 1934-03-06 1936-04-07 Gen Electric Electric heating unit
US2280516A (en) * 1942-04-21 Method op treating magnesia and electrical insulating
US2280515A (en) * 1942-04-21 Electrical insulating material and method of producing the same
US2360267A (en) * 1942-11-23 1944-10-10 Mcgraw Electric Co Encased heating unit
US2703355A (en) * 1950-10-23 1955-03-01 Kanthal Corp Electric heater
US2767288A (en) * 1954-04-26 1956-10-16 Gen Electric Electric heating unit
US2816200A (en) * 1954-12-15 1957-12-10 Int Nickel Co Electrical heating unit
US2975262A (en) * 1952-09-16 1961-03-14 American Mach & Foundry Electrical heating unit
US3201738A (en) * 1962-11-30 1965-08-17 Gen Electric Electrical heating element and insulation therefor

Patent Citations (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1763117A (en) * 1930-06-10 House electric
US2280516A (en) * 1942-04-21 Method op treating magnesia and electrical insulating
US2280515A (en) * 1942-04-21 Electrical insulating material and method of producing the same
US2036788A (en) * 1934-03-06 1936-04-07 Gen Electric Electric heating unit
US2360267A (en) * 1942-11-23 1944-10-10 Mcgraw Electric Co Encased heating unit
US2703355A (en) * 1950-10-23 1955-03-01 Kanthal Corp Electric heater
US2975262A (en) * 1952-09-16 1961-03-14 American Mach & Foundry Electrical heating unit
US2767288A (en) * 1954-04-26 1956-10-16 Gen Electric Electric heating unit
US2816200A (en) * 1954-12-15 1957-12-10 Int Nickel Co Electrical heating unit
US3201738A (en) * 1962-11-30 1965-08-17 Gen Electric Electrical heating element and insulation therefor

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3476842A (en) * 1966-08-10 1969-11-04 Dow Corning Composition for sealing joints and method of making same
US3571477A (en) * 1968-06-21 1971-03-16 Bert Phillips Protection of oxidizable electric furnace elements at high temperatures
US3828296A (en) * 1970-07-21 1974-08-06 Int Nickel Co Sheathed electric heater elements
US3959001A (en) * 1973-12-21 1976-05-25 Dynamit Nobel Aktiengesellschaft Method of preparing an electrically insulating embedding composition
US4234786A (en) * 1979-02-12 1980-11-18 General Electric Company Magnesia insulated heating elements and method of making the same
US4376245A (en) * 1980-02-06 1983-03-08 Bulten-Kanthal Ab Electrical heating element
EP0079385A4 (en) * 1981-05-18 1983-09-20 Matsushita Electric Ind Co Ltd SHIELDED HEATING ELEMENT AND THEIR PRODUCTION PROCESS.
US4586020A (en) * 1981-05-18 1986-04-29 Matsushita Electric Industrial Company, Limited Sheathed resistance heater
US4732792A (en) * 1984-10-08 1988-03-22 Canon Kabushiki Kaisha Method for treating surface of construction material for vacuum apparatus, and the material treated thereby and vacuum treatment apparatus having the treated material
US4626665A (en) * 1985-06-24 1986-12-02 Shell Oil Company Metal oversheathed electrical resistance heater
EP3745815A3 (en) * 2012-07-05 2021-02-17 nVent Services GmbH Mineral insulated cable having reduced sheath temperature
US11224099B2 (en) 2012-07-05 2022-01-11 Nvent Services Gmbh Mineral insulated cable having reduced sheath temperature

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SE327767B (enrdf_load_stackoverflow) 1970-08-31

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