US2010768A - Electric range heating unit - Google Patents

Description

J. D. MORGAN Filed June l, 1931l ELECTRIC RANGE HEATING UNIT INVENTQR Jq//fz l). Mor an BY ,l

ATTORNEY Aug. 6, 1935.

LIIIIII;

Patented Aug. 6, 1935 UNITED STATES ELECTRIC RANGE HEATING UNIT John D. Morgan, South Orange, N. J., assignor to Doherty Research Company, New York, N. Y., a corporation of Delaware Application June 1, 1931, Serial No. 541,227

7 Claims.

This invention relates to electric heating units and more particularly it concerns refractory resistor-supporting members therefor. It is of especial utility in connection with the manufacture of electric range heating units of the closed top type employed for heating cooking vessels and the like.

Electric range heating elements of this general type have long been known. The predominating 10 form thereof consists of a porcelain member provided with electric resistance wires contained in grooves in its surface. Such open type units are generally objectionable in that water, grease and the like spills into the grooves and causes short circuiting and the emitting of noxious gases and vapors.

Attempts have heretofore been made to produce a satisfactory closed top electric heating element suitable for use for domestic cooking purposes. Elements of this character have been produced in which a sheet steel disc is placed over the conventional open type element. Since such sheet steel readily Warps and distorts, thereby causing material loss of eciency, further attempts were made to substitute for the sheet steel certain cast iron members provided with insulating porcelains secured to their underside upon which the resistance wires were strung. Obviously such construction is very expensive. Likewise attempts were made to prepare an electric heater having a top of refractory material placed upon separate heat insulating bricks or the like of refractory material. The top and backing members were manufactured and handled separately, resulting in considerable breakage in shipment and during assembly of the parts.

The present invention provides an electric range heating unit which is adapted to increase the efficiency and utility of electric equipment for heating and cooking purposes.

Among the more important objects of the present invention are: to provide a novel electric heater unit having a work-carrying member made of molded refractory material of high heat conductivity; to provide in a unitary structure a refractory resistor-supporting element of very high heat conductivity having a backing of a refractory material of extremely low heat-conductivity; to provide in a process for producing an electric heating unit, for molding an electric resistor element in situ within a black refractory body of high heat conductivity while preventing substantial deterioration of the resistor element; and to provide in a novel manner for imbedding a metallic resistance element in a refractory material of high heat-conductivity and low electric conductivity.

In its broadest scope, the present invention relates to a unitary electric resistance heater embodied in a unitary structure, and comprising at least two members or plates of refractory material, preferably intimately secured together by suitable bonding means and fired simultaneously, one of the said members supporting or having embedded therein a resistor wire properly positioned somewhat below the top` surface of the work carrying surface of the unit so as to `be capable of transmitting a maximum of its heat to such surface and to a cooking vessel or the like resting thereon.

The top plate which is in contact with the object to be heated is preferably of black or dark color and has a very high thermal conductivity and a very low electrical conductivity; the thermal conductivity being substantially greater for instance than that of silicon carbide. This top member preferably is made of a refractory mixture containing silicon, with or without Zircon or silicon carbide, and bonded by a compound non-volatile at 2600o F.,-the mixture having a heat conductivity at least twice that of fire clay at temperatures above 2000 F., and approximating that of cast iron at such temperatures. Such compositions are fully disclosed in the U. S. patent of J. D. Morgan, W. G. Bjorkstedt and R. E. Lowe, No. 1,811,242 granted June 23,. 1931, for Heat conducting high temperature resistant refractory. An example of such composition follows:

Native Zircon, milled so that the major portion thereof will pass through a 200 mesh screen is mixed with an equal amount of 140 to 200 mesh silicon or a ferro-silicon alloy high in silicon. To the resultant mixture is then added four to six percent by weight of ortho-phosphoric acid (sp. gr. 1,71) or its equivalent. This mixture of materials is then shaped as desired, and it may be fired at temperatures of about 2t00 F. or at the temperatures at which it is to be used. The preferred composition has a high thermal conductivity as above set forth, and has a limiting strength under compression of around 650 pounds per square inch at 2500 F. It is highly resistant to gaseous corrosion and erosion at 2600u F., and is avery efficient absorber of radiant heat due to its dark color.

The silicon has been satisfactorily used in the above refractory mixture in amounts varying from approximately 50 to 85% ofthe mixture.

Silicon carbide may be substituted for a portion or all of either or both the silicon and the Zircon. Particularly is this advantageous where the heating unit is to be employed at relatively low temperatures, in which case silicon carbide is suitable for use and is less expensive than Zircon. A mixture containing parts silicon, 15 parts silicon carbide and 6 parts ortho-phosphoric acid, gives a satisfactory refractory for use at low temperatures.

The bottom plate of the heating unit is made of a porous, white, or light-colored refractory adapted to facilitate the reflection of heat from the top member, and this refractory has a very low heat conductivity. It is so disposed and designed as to reradiate to the upper or black plate a major portion of the heat transferred to it from the resistor element.

The preferred refractory composition for use in the manufacture of this plate is th-at described in applicants Patent No. 1,873,014, granted August 23, 1932, for High temperature insulating products and process of making same. Such refractory product contains a large percentage of voids, while possessing relatively high crushing strength at 2500 F. It is made of a refractory aggregate, a small amount of finely divided metal, and a bonding agent capable of generating a gas when in contact with the said metal. The refractory aggregate may be composed of any materials known to be capable of resisting high tempera"- tures. The finely-divided metal is given a protective coating of a liquid colloidal material such as glue, gelatine, resin or other adhesive substances which will decompose at relatively low temperatures during subsequent ring of the refractory. TheA protective coating on the metalparticles retards or prevents reaction between the latter and the bonding material until the mixture is properly heat-treated for the formation of the refractory plate.

An example Vof such a composition follows:

Native Zircon sand, or a mixture of the same with milled Zircon, is mixed with approximately 1% *of its weight of aluminum powder, 15% of powdered sulfur or rosin, and 6% of P205 the latter preferably being half in the form of H3PO4 and half in the form of ammonium acid phosphate in aqueous solution.

Other metals such as zinc and magnesium or their alloys may be employed in place of aluminum. The sulfur may be omitted or replaced by -any well-known combustible material adapted to generate gases.

The above refractory mixture is shaped or molded, and is red at a temperature of about 500 to 700 F. As the temperature increases, the mass swells and becomes highly cellular, the bond sets, and the walls of the cells become hard so that the iinal product has a high crushing strength, but, owing to its high percentage of voids, has a high heat-insulating capacity. Final firing temperatures from 500 F. to 2500 F. and above may be employed, -although the swelling and cell formation occur below 500 F.; and a final setting occurs at approximately 700 F.

Other light-colored refractory materials such as kaolin or diatomaceous silica or the like may be substituted in part lor in whole for the Zircon,-

particularly where the resultant electric heating unit is intended for use at relatively low temperatures. The product is not only a good insulator against loss of heat but it is highly refractory and cannot be damaged by any temperature which is tolerable to the black heat-conducting member or plate previously described. A specimen of the material has been found to have over 60% voids when tested by the specific gravity method. In one instance, the black refractory material as used in the upper heat-conducting plate had a heat conductivity of 226 B. t. u. per square foot, per inch thickness', per degree Fahrenheit per hour,-whereas the porous refractory heat-insulating material forming the.

lower light-colored plate had a heat conductivity of v2.9 B. t. u. per square foot, per inch thickness, per degree Fahrenheit per hour.

In the attached drawing, Fig. 1 is a view in vertical section of a unitary electric heater embodying the present invention.

Fig. 2 is a view in vertical section showing a second modification of the invention.

Fig. 3 is a view in vertical section showing a third modification, in which the metal resistor is imbedded within the black refractory plate during manufacture; and

Fig. 4 is a plan view showing the spiral arrangement of the resistor-carrying member; and

Fig. 5 is a vertical section through Still another modificati-on of the invention.

In the drawing, l0 designates an upper member or plate of a black refractory material forming a closed top of an electric range heating unit. This plate is provided in its lower surface with a spirallyarranged groove or grooves I2 adapted to house the usual metal electric resistor element The top member l0 is secured upon the top surface of a light-colored porous refractory plate 20 of a composition heretofore described. The terminals of the electric resistor Ill extend downwardly through suitable apertures 22 formed in the bottom plate 20 and are connected in an eleotric circuit 24%.. In the modification shown in Fig. 2, the closed top plate I0 is of solid construction, and the upper surface of the light-colored porous refractory member 20 is provi-ded with a spiral groove 30 adapted to house the resistor member lil.

In the modification shown in Fig. 3, a metal resistor wire or ribbon 32 is imbedded in the plate l0 at a substantial distance above the top of the bottom member 20 and near the upper surface of top plate l0.

In the forms of the invention shown in Figs. 1 and 2, the respective members I0 and 20 may be shaped and fired in the usual manner, and may then be placed in contact with each other after the proper positioning of the resistor wire in the grooves l2 or 30. The surfaces of contact of the members l0 and 20 are previously coated with a suitable bonding material such as a wash of clay slip made from ball clay. In place of clay slip there may be employed a suitable' refractory mixture containing a low melting flux, which mixture will melt at a temperature below that employed for baking the heater unit and will permit permanent bonding together of the upper and lower plates l0 and 20. 'Ihe unit housing the resistor wire is then baked at temperatures of '700 F. or above for a suitable length of time to form a strong unit splendidly adapted for the purpose for which it is intended.

If desired, heating or firing of the plates l0 and 20 may be effected after their assembly in a composite structure with or without the application of pressure. v l

In the preferred practice of the invention for manufacturing the modification shown in Fig. 3, the porous refractory plate 20 is first formed and fired at temperatures of approximately 500n F. The resistor elementhere shown `for illustration only as a metallic ribbon 32,--is arranged in spiral form on a temporary support such as cardboard and the latter is positioned at a proper distance above the upper surface of the porous refractory plate 2U. The finely-divided mixture of the black refractory material heretofore disclosed is then placed around and upon the resistor element 32 thereby supporting the latter in apropos its position. This material may contain some of the bonding agent heretofore described. The

cardboard support is thereupon removed and additional black refractory material is placed upon the resistor element to the desired depth. Pressure is then applied upon the upper surface of the material I6 While supporting the plate, to strongly compress this material. Simultaneously, the plate 20 and refractory material carried thereon are subjected to temperatures adapted to properly form the upper heat transferring plate l0 and to bond the same with the upper surface of the plate 20. l

According to the modification of the invention shown in Fig. 5, each of a pair of members or plates of the black refractory material of high heat conductivity is mounted on one of the opposite sides of a porous member or plate of lightcolored refractory material having a very low heat-conductivity. The resistor elements It may be mounted in spiral grooves in each of the outer plates I 0, as shown, or such elements may be mounted in grooves provided in the opposite surfaces of the porous member 20. A composite heater unit of this construction can advantageously be employed in an installation providing for the simultaneous utilization of the top member lll. for baking and the bottom member l0 for grilling.

It is preferred to coat the resistor element or wire with a low-melting glaze prior to bringing the same in contact with the black refractory mass IB, particularly when the latter is used in the moist state before firing. Such treatment is especially desirable in connection with the modification shown in Fig. 3. This glaze, which may for example be a low melting glass, serves to protect thev resistor from corrosion by the acidic bonding material prior to the high temperature firing of the mass. The glaze melts and is absorbed into the pores of the refractory aggregate during the ring, and thus provides sufficient space for the expansion of the resistor wire during subsequent use of the heating unit. A bakelite varnish or other varnish may be employed as the glaze. The entire outer surface or the work-carrying surface of the heating unit may likewise be glazed with a suitable borosilicate glass or the like, which may readily be cleaned and which will prevent absorption by the refractory plate of materials accidentally spilled thereon.

In practical use it has been demonstrated that electric stove heating units of the type herein described will heat Water to the boiling point in substantially less than three quarters of the time required by the standard open-top heating unit of well known type now in the market, and in approximately half the time required by a standard heater having a closed top of cast iron,- while respectively employing for the purpose approximately 60% of the current measured in watt hours of that required for the purpose by the open-top unit, and 55% of that required by the heater having the cast iron closed top.

A long series of comparative experiments indicate that the refractory-enclosed electric heating unit of the present invention will bring foods to a proper cooking temperature at a faster rate than will many of the heating units now on the market. Foods can now be cooked in contact with the top surface of the unit which cannot properly be so cooked upon an uncovered heating element or upon the usual cast iron top heater.

The invention is susceptible of modification within the scope of the appended claims.

l claim:

l. As a new article of manufacture, a unitary electric heater assembly consisting of a porous member of light colored refractory material having a high percentage of voids and a very low thermal conductivity, a pair of black refractory members mounted upon opposite surfaces of the said porous member, metallic resistors operatively interposed respectively between the porous member and each of the said black refractory niembers, the last named members being composed of a bonded mixture consisting essentially of silicon and zircon and having a thermal conductivity greater than that of silicon carbide and a low electrical conductivity.

2. As a new article of manufacture, an electric heater unit comprising a refractory body, an electric resistor element mounted in electrically uninsulated and good heat-conducting relationship to said refractory body, said body having a low electrical conductivity and comprising substantially equal parts of zircon and silicon bonded by reaction products of phosphoric acid therewith and having a coecient of heat transmission equal to Abonded carborundum at temperatures above 20.00 F. and adapted for commercial use at 2600 F.

3. An electric heater unit embodying a refractory body prepared by mixing ground silicon with finely divided zii-con, treating said mixture with from 4 to 6% of phosphoric acid, shaping the mixture and firing, said unit comprising an electric resistor associated with said body in electrically uninsulated and good heat-conducting relationship.

4. An electric heater unit embodying a refractory body prepared by mixing ground silicon with finely divided Zircon, treating said mixture with a mineral acid reacting with the elements of said mixture to form products none of which are volatile at temperatures up to 2600 F., shaping the mixture and firing, said unit comprising an electric resistor associated with said body in electrically uninsulated and good heat conducting relationship.

5. An electric heater unit embodying a refractory body comprising substantially equal parts by weight of zircon and silicon bonded by a compound non-volatile at 2600 F., said unit comprising'an electric resistor associated with said body in electrically uninsulated and good heat conducting relationship.

6. An electric heater unit embodying a refractory body comprising substantially equal parts by weight of Zircon and silicon, said unit comprising a resistor associated with said body in electrically uninsulated and good heat-conducting relationship.

7. An electric heater unit embodying a refractory body prepared by mixing ground silicon with finely'divided zircon, treating said mixture with from 4 to 6% of phosphoric acid, shaping the mixture and firing, said unit comprising an electric resistor associated with said body in electrically uninsulated and good heat-conducting relationship, the proportions of silicon being by weight 50-85% to zircon by weight 50-l5%.

JOHN D. MORGAN.

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