05/048390

10/12/1971

06/22/1970

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General Motors Corporation (Detroit, MI)

219/461.100

338/285, 219/553, 338/280, 252/62, 219/452.120

H05B3/74; H05B3/68; H05B3/68

219/464,460-461,345,553,443,463 338/280,296,285 252/62

3500444	ELECTRICAL HEATING UNIT WITH AN INSULATING REFRACTORY SUPPORT	March 1970	Hesse et al.
3500464	INSULATING ELECTRICAL HEATER SUPPORT	March 1970	Hesse et al.
3501621	DUAL RIBBONED SURFACE HEATING ELEMENT	March 1970	Pansing et al.
3014872	Fibrous insulation	December 1961	Scott

Mayewsky, Volodymyr Y.

What is claimed is as follows

1. In combination, an uninsulated resistance element formed as a ribbon into a sinelike curve pattern, a low density composite heater block having groove means formed in an upper surface therein and continuously supporting said resistance element therein, means for attaching said resistance element in said groove means, said heater block consisting essentially of inorganic fibrous-ceramic insulating refractory material having a low thermal mass, said heater block having dispersed therein a finely divided opacifier substance in an amount sufficient to diffusely reflect infrared energy emitted from said resistance element toward said heater block and having a wavelength within the range of approximately 2 to 5 microns, wherein the finely divided opacifier substance is selected from a group consisting of titanium oxide, zirconium oxide, tin oxide and potassium titanate, and said finely divided opacifier substance being present in an amount between 5 percent to 10 percent by weight of said heater block.

2. The combination recited in claim 1, wherein said resistance element is an iron-chromium-aluminum alloy and said attaching means includes a plurality of U-shaped tie members, each of said tie members being in the form of an iron-chromium-aluminum alloy staple.

3. The combination recited in claim 1, wherein said heater block is formed having a density within the approximate range of 10 to 15 pounds per cubic foot.

4. The combination recited in claim 1, wherein a minimum of 70 percent of the infrared energy emitted from the resistance element toward said heater block and having a wavelength within the range of approximately 2 to 5 microns is reflected by said heater block.

5. The combination recited in claim 1, wherein said resistance element is an iron-chromium-aluminum alloy and said block contains a minimum of 40 percent by weight of alumina to prevent loss of aluminum from the resistance element to said block.

6. A radiant cooking unit comprising, an upper utensil supporting cover plate of infrared transmissive material, a fibrous-ceramic insulating block disposed below said plate and having a raised peripheral face defining a central recessed area in spaced relation with the undersurface of said plate, means for supporting said block with said peripheral face on said plate, a ribbon-type infrared emissive resistance element continuously supported within spiral groove means formed in said recessed area, said groove means being channel-shaped in cross section having a flat bottom wall continuously supporting said resistance element thereon, said element formed in a deeply undulating curve pattern from an iron-chromium-aluminum alloy material whose operating temperature is in the range of 1,500 to 2,000° F., said block having dispersed therein sufficient finely divided opacifier substance to reflect a minimum of 70 percent of the infrared energy emitted from said element having a wavelength within the range of approximately 2 to 5 microns, said finely divided opacifier substance being selected from a group consisting of titanium oxide, zirconium oxide, tin oxide and potassium titanate, and said opacifier substance being present in an amount between 5 to 10 percent by weight of the fibrous-ceramic block.

7. In the cooking unit of claim 6, said resistance element ribbon being formed into a deeply undulating curve pattern having an amplitude-to-pitch ratio of the order of one.

8. In the cooking unit of claim 7, said resistance element ribbon having a height of approximately 125 mils and a thickness of the order of 7 to 9 mils, said resistance element ribbon having an amplitude of approximately 0.20 inch and a pitch of approximately 0.20 inch.

9. In the cooking unit of claim 6, said cover plate being formed from a recrystallized glass-ceramic material transmissive to infrared energy having a wavelength within the range of 1 to 5 microns.

10. A radiant cooking assembly comprising an upper utensil supporting cover plate of infrared transmissive material, a rigid fibrous-ceramic heater block disposed below said plate and having a raised peripheral face defining a central recessed area spaced from said plate, means for supporting said block with said peripheral face on said upper plate, an infrared emissive uninsulated iron-aluminum-chromium alloy resistance element ribbon located on said central recessed area in vertically spaced relation below the undersurface of said cover plate, said resistance element being supported in continuous spiral groove means formed in said central recessed area, said groove means being channel-shaped in cross section having a planar bottom wall providing substantially continuous support for said resistance element, said resistance element ribbon being formed in a continuous sinelike curve pattern having generally planar tangent sections throughout the length thereof, said resistance element having an overall width substantially equal to the width of said groove bottom wall whereby said element being constrained against radial movement within said groove means upon heating of said element, means for attaching said resistance element to said heater block including a plurality of U-shaped tie members, each of said tie members being in the form of an iron-chromium-aluminum alloy wire staple having a bridge portion engaging the upper edge of selected tangent sections of said resistance element, and said staples including a pair of spaced legs extending through said heater block with the free ends thereof projecting outwardly from the bottom surface of said heater block, said staple free ends being cemented to the underside of said heater block for fixedly retaining said staples in said heater block, said staples grouped on substantially uniformly spaced radial lines whereby each resulting arcuate section of said element is secured at its end points to said heater block permitting limited accordionlike movement within said groove means as said resistance element expands and contracts due to thermal changes.

This invention relates to domestic ranges and more particularly to electric ranges having infrared radiant-type heating units for so-called smooth top or glass top ranges.

Certain prior art glass-top cooking units have employed an incandescent tungsten filament sealed in an evacuated quartz envelope as the infrared heating element. The tungsten filament is positioned in spaced relation below a glasslike cover plate of material transparent to infrared radiation but absorbent to most of the visible light. The U.S. Pat. No. 3,375,346 is representative of this type of cooking unit. A second approach to infrared cooking units involves the use of the less expensive glowing resistance element operated in an unsealed or unevacuated atmosphere and generally referred to as "open coil" type heating unit. In one form of open coil surface-heating unit the resistance element is maintained in direct contact with the utensil-supporting plate to conductively and radiantly transmit heat to the utensil as exemplified by the disclosures in U.S. Pat. Nos. 2,799,765 and 3,086,101, for example.

An infrared radiant cooking unit design using a sealed tungsten filament has the disadvantage of not only being expensive to build but, owing to its operating temperature of the order of 4,500° F., will emit a considerable amount of its energy in the visible light band which will reduce its overall efficiency.

An open coil radiant unit using electrical resistor alloys which operates with an airgap between the resistance element and a glasslike utensil support plate such as disclosed in U.S. Pat. No. 2,913,565 is still another approach to infrared cooking. As open coil radiant units operate in the temperature range of 1,500° to 2,000° F. and thus closer to the infrared optimum heater element temperature, determined by tests to be approximately 2,350° F., they must incorporate more resistance-element material to provide greater surface radiating area required to achieve a watt density comparable to a tungsten or a plate-contacting resistant element. This factor, together with the increased structure necessary to support the open coil resistance element, has generally meant a unit having substantial thermal mass and resultant thermal lag presenting disadvantages such as a slow arrival at operating temperature and slow cooldown time.

Present open coil alloy resistance elements are economically limited to material from the nickel-chromium, nickel-chromium-iron, iron-chromium family or an iron-chromium-aluminum (Fe-Cr-Al) family or the like having desirable strength and electrical characteristics. Because of the relatively low melting point of these alloys compared to tungsten, as noted above, their operating temperatures will of necessity be much lower. This fact creates two problems. First, the decreased temperature necessitates the usage of a filament of greater surface area for the same total energy emission derived from tungsten. While the element from the Fe-Cr-Al family of alloys is preferred over a Ni-Cr, etc., alloy for use with an open coil unit because of their higher melting points and higher electrical resistivity, allowing higher operating temperatures, a support structure for the Fe-Cr-Al alloy family has been a serious problem because of the elements lack of ductility and weakness or poor "hot strength" at operating temperatures. Moreover, the thermal expansion of Fe-Cr-Al alloys as they are heated to their operating temperatures is considerable, thus making it necessary to devise some means to physically restrain the resistance element during operation. Second, the lower filament temperature causes more of the infrared energy to be emitted at longer wavelengths. Thus, if a maximum amount of the infrared energy is to be directly transmitted or radiated through the utensil support cover plate, the cover plate must have good infrared transmission characteristics within a greater wavelength range, preferably of 1 to 5 microns. To the solution of these problems the teachings of this invention is directed.

Accordingly, an object of the present invention is to provide an infrared radiant heating unit of the open coil type having a low thermal or heat-sink mass supporting structure.

Another object of this invention is the provision of an infrared radiant-type heating unit of the open coil type having a deeply undulating curve patterned electrical resistance element formed of an Fe-Cr-Al type alloy which operates in the temperature range of 1,500° to 2,000° F., wherein a high percentage of the radiated energy emission can be transmitted through an infrared transmissive cover plate with a minimum of the emitted energy being in the visible light band.

It is a further object of the present invention to provide an infrared radiant heating unit providing continuous direct support for an open coil-type ribbon-shaped resistance element formed in a deeply undulating curve pattern wherein an air gap is maintained between the element and an infrared transmissive plate.

A still further object of the present invention is to provide a new infrared reflective low thermal mass heater block for an open coil radiant-heating unit that provides ready and inexpensive means for controlling the thermal expansion and contraction of a deeply undulating or sinelike pattern ribbon resistance element within a spiralled or convolute groove formed in the block.

Yet another object of the invention is to provide an infrared radiant-type heating unit of the air-operated type having a resistance element formed from a ferritic alloy material having an operating temperature range of 1,500° to 2,000° F., wherein the utensil support plate is formed from a glass-ceramic material that transmits the majority of the infrared rays between 1 and 5 microns in wavelength.

Still another object of the instant invention is the provision of an infrared radiant heating unit having an open coil-type resistant element spaced from an infrared transmissive cover plate wherein the element is continuous Fe-Cr-Al alloy ribbon element formed in a deeply undulating or sinelike curve pattern providing substantially planar tangent portions while being continuously supported throughout its length in a spiral groove formed in a cast fibrous-ceramic, low density, infrared reflective heater block and retained at spaced locations in the groove by means of a plurality of radially aligned tie members allowing accordionlike thermal expansion and contraction of arcuatelike sections of the resistance element.

A final object of the present invention is the provision in a radiant-heating unit of a low thermal mass heater support block for an infrared emissive resistance element wherein the block is molded from inorganic fibrous-ceramic material rendering it reflective of more than 70 percent of the infrared energy in the wavelength range of approximately 2 to 5 microns by having dispersed therein a defined amount of a finely divided substance selected from a group consisting of titanium oxide, zirconium oxide, tin oxide and potassium titanate.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIG. 1 is a fragmentary perspective view of a domestic range with parts broken away to show the present invention;

FIG. 2 is an enlarged view in vertical section taken along the line 2--2 in FIG. 1;

FIG. 3 is an enlarged view in top elevation of one of the heating assemblies removed from the range and showing a 6-inch heating unit and 8-inch heating unit;

FIG. 4 is an enlarged view in vertical section taken along the line 4--4 of FIG. 1;

FIG. 5 is an enlarged view in top elevation of the 8-inch radiant-heating unit of the heating assembly;

FIG. 6 is an enlarged view in vertical section taken along the line 6--6 in FIG. 5; and

FIG. 7 is an enlarged view of a portion of the heating element of the invention.

Referring now to the drawings in FIG. 1 a domestic range 10 is illustrated having a metal body 11 supporting an upper casing 12 which includes a collar 14 around the top edge thereof forming a top opening 16 located above upper insulation retainer wall 17 of the range body. The peripheral collar 14 merges with the rearwardly located control panel 18 having a plurality of control knobs 20 thereon for selectively energizing infrared radiant-heating units to be described.

A pair of substantially rectangular heating assemblies, indicated generally at 22 and 23 in FIG. 2, are especially adapted to be installed in the outer casing 12 of the range for location above the upper wall 17 of the range. The heating assemblies 22, 23 are oriented in spaced side by side relationship below the opening 16 with their longitudinal axes oriented parallel to the sides of the range. As the heating assemblies 22, 23 are identical in construction, like reference characters are used to designate like or corresponding parts throughout the several views. It should be noted that while the heating assemblies incorporating the invention are described in conjunction with a freestanding range it is to be understood that the present invention is not intended to be so limited and the heating units could be used, for example, with counter or "drop-in" type cooking arrangements without departing from the scope of the invention.

Turning now to FIGS. 2, 3 and 4 the heating assemblies 22, 23 each have a housing or pan member 24 comprising a bottom wall 26 provided with marginal sidewalls 28 and 30 upstanding from the bottom wall 26 and a pair of marginal end walls 32, 33 shown terminating in substantially horizontal end flange walls 34, 35 outstanding from the top of end walls 32, 33, respectively, having downwardly directed lip portions represented at 36 in FIG. 4 for flange 34. Within the range top opening 16 is located an infrared transmissive cover plate member 38 overlying each of the heating assemblies 22, 23 to define the complete surface heating or cooking area of the range. Plate member 38 has a substantially rectangular configuration providing a continuous upper surface that extends throughout the planar extent of the opening 16, and is mounted therein together with the heating assemblies by securing means including a trim ring indicated at 40 and a plurality of cooperating L-shaped clips 41 incorporating individual threaded pressure screws 42 located in tapped holes in the horizontal leg of the clips. As best seen in FIG. 4 the trim ring 40 is substantially T-shaped in cross section and includes a head 43 spanning the gap 44 between the collar 14 and the plate 38, as well as depending stem 45 disposed in the gap. The stem 45 has formed along its lower end a hook portion 46 which provides shoulder means for interlocking engagement with the cooperating hook portion 47 of the clip members 41.

After the clips 41 are properly located at spaced intervals to engage the underside of the range collar 14 by seating the circular bead portions 48 on the vertical legs of the clips in the radiused area formed by a depending flange 49 of the top opening 16, the clip pressure screws 42 are taken up to retain the plate and heating assemblies in the opening 16. Appropriate sealing material, such as RTV Silicone manufactured by Dow Corning, for example, can be used to provide a liquidtight seal between the trim ring head 43, the collar 14 and the plate 38, if desired, to seal the top surface from spillage thereon.

It will be noted that the heating assemblies 22, 23 are secured to the range collar 14 and cover plate 38 only at their end walls 32, 33 by means of the pressure plate members 50 (FIG. 4) located on pressure screws 42 for engaging the underside of the flanges 34 and 35 positioned along the longitudinal edges 51 of the plate 38. As seen in FIG. 2 the transverse edges 52 of the plate 38 are secured just to the range collar 14 by means of the trim ring 40 and clips 41. In order to cushion the seating of the cover plate 38 within the range collar, suitable resilient means, such as asbestos pad indicated at 53 in FIG. 4, are located between the upper surfaces of flanges 34, 35 and the undersurface of the cover plate. The washer-type pressure members 54 (FIG. 2) used on the clips 41 located adjacent transverse cover plate edges 52 have suitable resilient cushioning means on their upper surfaces such as, for example, heat-resistant asbestos.

The cover plate 38 is preferably formed of a recrystallized glass-ceramic material having a relatively low coefficient of thermal expansion, while being highly transparent to infrared energy. Examples of such material suitable for use with the instant invention are Hercuvit manufactured by Pittsburgh Plate Glass Industries and Cer-Vit manufactured by Owens-Illinois Glass Co. Recrystallized glass-ceramic is capable of transmitting the majority of the infrared rays between 1 and 5 microns in wavelength as contrasted with quartz or high-silica glass which is unable to transmit wavelengths longer than 3.5 microns in usable thicknesses. It is to be understood that Cer-Vit and Hercuvit are only representative of suitable glass-ceramic material having the essential property of being transmissive of the aforementioned range of infrared rays emitted from an open coil type resistant element to be described.

The housing member 24 provides a boxlike receptacle defined by the sidewalls 28, 30; end walls 32, 33 and bottom wall 26 for receiving a flexible insulation mat 60 which is preferably formed from inorganic fibrous material such as rock wool, glass wool, slag wool, asbestos or the like having a thickness in the disclosed form of approximately 1 inch. As seen in FIGS. 2 and 3 the insulation mat 60 extends throughout the housing member 24 to conform with the bottom wall 26 which is formed in the shape of an irregular pentagon having oblique wall portion 55 angled inwardly to accommodate different sized heating units to be described.

As seen in the heating assembly 23 of FIG. 3, the flexible insulation mat 60, positioned in the housing member 24, has located thereon a pair of heating units indicated generally at 61, 62. Each of the heating units 61, 62 comprises a heater support block and a resistance element with the heating unit 61 having its heater block shown generally at 63 supporting a resistance element ribbon 64 while the heating unit 62 has a heater block 65 supporting a resistance element ribbon 66. In the preferred form the continuous element 64 is located within a spiral or convolute-shaped groove 67 formed to define a heater area approximately 8 inches in diameter while the continuous element 66 is positioned within a spiral groove 68 to define a heater area approximately 6 inches in diameter.

In the case of the right-hand heating assembly, partially indicated at 22 in FIG. 2, the assembly is oriented with the larger heating unit 61 at the front of the range while the left-hand heating assembly 23, shown in FIG. 3, has been turned end for end to locate the smaller heating unit 62 in a frontal position in the range top. In this way the cover plate 38 will provide alternate small and large surface-heating or cooking areas at both the front and rear of the cover plate 38. It will be observed that the larger heater block 63 has a generally octagon shape when viewed in plan locating three of its sides respectively adjacent to and parallel with the end wall 32 and sidewalls 28 and 30 of the housing member 24. The smaller heater block 65 has an irregular shape in plan to conform to the oblique wall portion 55 of the housing 24, along with the walls 30 and 33.

As seen in FIG. 2 the heater blocks 63, 65 have an overall thickness, which when combined with the thickness of supporting mat 60, locates their upper peripheral faces 69 and 70, respectively, a defined distance above the housing flange walls 34, 35 such that the peripheral faces 69, 70 are brought into flush contact with the undersurface 71 of the cover plate 38. In the disclosed form the distance is of the order of 0.20 inch. The faces 69, 70 of the blocks are held in cushioned pressure contact with the plate undersurface 71 by means of the trim ring 40 and clip 42 arrangement described above. For a more detailed account of the heating assembly and cover plate mounting arrangement, reference should be made to copending application Ser. No. 55,815, filed July 17, 1970, and assigned to the same assignee as the instant application.

The heater blocks 63, 65 are retained in the housing 24 prior to the fabrication of the heating assemblies and cover plate with the range by means of a holddown pin 72 extending through a central bore in the heater blocks, the mat 60 and the housing bottom wall 26. A suitable push nut 73 of known commercial type is located on the free end of the pin 72 for retention of the heating units. The head portion 74 of the pin is retained in a counterbore 75 formed in central upstanding hub portions 76, 77 of the blocks 63, 65. The upper faces of the hubs 76, 77 are located to provide a clearance between the undersurface 71 of the plate and the hub faces which is of the order of 0.03 inch in the instant embodiment. This clearance is provided to insure that the hubs do not contact the plate undersurface 71 before the faces 69, 70 upon tightening the clips 41 during final assembly of the range top.

The heater blocks 63, 65 are cast or molded to provide disk-shaped hollows defining recessed circular areas 80 and 81 in the block upper faces 69 and 70, respectively. In the preferred form the blocks 63, 65 have substantially identical thicknesses, within casting tolerances, of approximately 11/4 inches and for the purpose of this disclosure, differ only in the total length of their spiral grooves and consequently the size of the heating area provided. Thus, except for differences in watt densities the heating unit designs are similar in all other respects and accordingly only the larger heating unit 61 shown in FIG. 5 will be discussed in detail.

To complete the description of the heating assembly of FIG. 3 it will be observed that each of the blocks 63 and 65 are formed with opposed cavities 78, 79, respectively, to receive one end of a central heat-sensing device indicated generally at 82 for operation with a surface temperature indicating light circuit described and claimed in application Ser. No. 55,816, filed July 17, 1970, and assigned to the assignee of the present invention. Also, each heating unit has associated therewith cover plate temperature cutoff control means generally indicated at 83 and 84. The control means 83, 84 form no part of the instant invention and are described in application Ser. No. 74,399, filed Sept. 22, 1970, also assigned to the assignee of the present invention.

Considering now the heating unit 63 of FIGS. 5 and 6 it will be seen that the spiral groove 67 is channel shaped in cross section with its opposite sidewalls 85, 86 sloped outwardly at an angle of approximately 30° from a flat or planar channel wall 87 defining the bottom of the groove resulting in the continuous spiral ridge 88 formed by the spiral groove 67 having a truncated-cone shape when viewed in section. In the form shown the groove 67 has a depth of approximately 0.14 inches and a width at its bottom wall 87 of approximately 0.40 inch.

The spiral groove 67 provides a path for locating the continuous resistance element ribbon 64 such that one end is positioned for electrical connection to an inner terminal member 90 and its opposite end located for connection to an outer terminal member 91. The terminal members 90, 91 are identical so as to be interchangeable with either heating unit and comprise an insulating column 92 formed of electrical porcelain or the like provided with an axial chamber for receiving a vertical terminal blade 93 therein as shown in FIG. 2 for the smaller heater block 65. Each terminal insulator column 92 has a generally rectangular base 94 extending through a conforming aperture in the bottom wall 26 of the housing, a central pedestal 95 located in a circular bore in the mat 60 and an upper reduced post 96 extending through a hole in the heater block terminating substantially flush with recessed area 80. The terminal posts 96 each have a slotted portion 89 on their upper ends which is aligned with either end of the spiral groove 67 for receiving the associated end contacts 98 of the resistance element 64.

The terminal blade 93 is retained in the terminal insulator by means of a screw 97 extending through a transverse bore in the central pedestal 95 and an aligned hole in the blade 93. The inner and outer connector blades of the heating unit 61 are electrically connected across a suitable power source to self-energize the resistance element 64 into its infrared-radiating temperature. The details of the terminal connectors form no part of the instant invention and are described and claimed in the above mentioned mounting arrangement, the application Ser. No. 55,815.

By virtue of the foregoing description the deeply undulating or sinelike patterned resistance element 64 is supported in spiral groove 67 to establish a defined vertical airgap G (FIG. 6) between the cover plate undersurface 71 and the element which in the form shown provides a minimum spacing therebetween of the order of 1/4-inch. The 1/4-inch gap G is the minimum distance permitted by Underwriters Laboratories between an uninsulated electrical conducting member and an adjacent conductor.

To produce the required wattage rate for the heating units it was determined that an element be used that can be self-heated into the range of 1,500° to 2,000° F. and constructed of a high-temperature resistance material from the iron-chromium-aluminum family. An Fe-Cr-Al ferrite alloy is preferred because it has a higher melting point and a higher electrical resistivity compared to the Ni-Cr series mentioned allowing it to be operated at higher temperatures within the above mentioned range. In the disclosed form of the invention the resistance element has a content by weight of chromium 22.5 percent, aluminum 5.5 percent, silicon 0.5 percent, carbon 0.1 percent with the balance iron. Owing to the lack of tensile and compressive strength at high temperatures a ribbon resistance element formed from the Fe-Cr-Al alloy for use as an open coil infrared emitter located in spaced relation to the infrared transmissive cover plate presented combined thermal, infrared reflection and continuous support structure problems not satisfactorily solved by the prior art.

A ribbon element is important to the unit because Fe-Cr-Al electrical resistance alloys, due to their relatively low melting points compared to tungsten filaments, for example, require that they be operated in the temperature range not exceeding 1,500° to 2,000° F. for long life. The lower temperature range requires an element having sufficient surface area and small mass to achieve an energy emission comparable to a radiant tungsten filament unit or plate contacting infrared surface heating unit. In the preferred form the invention uses a relatively thin ribbon-shaped resistance element having a small cross section in combination with a large surface area formed into a sinelike curve or deeply undulating pattern resulting in a minimum heat sink mass with fast on-off response time.

As seen in FIG. 7 the ribbon element 64 is formed into a sinelike curve providing substantially planar tangents 102 and return bent curved peaks of apices 103. In the preferred form shown in enlarged FIG. 7 the resistance element 64 has an amplitude measurement A of approximately 0.20 inch providing a resultant overall width measurement B of approximately 0.40 inch. The element 64 has a full cycle or pitch measurement C of approximately 0.20 inch resulting in an amplitude-to-pitch ratio of the order of one. The ribbon resistance element 64 is formed having a thickness in section of the order of 9 mils (31 gauge) and height H of the order of 125 mils capable of developing a wattage rating of 2,000 watts at 236 volts AC. It will be noted that in the case of the resistance element 66 for the 6-inch heating unit 62 a ribbon of the same height but having a somewhat reduced thickness of the order of 7 mils (33 gauge) is used to provide a wattage rating of 1,200 watts at 236 volts AC.

The present invention solved the support problem for the open coil resistance element described above by means of the channel-shaped spiral groove arrangement 67 whose planar bottom wall 87 provides substantially continuous support for the resistance element throughout its entire length. This arrangement prevents any sagging or warping of the element, a condition leading to fracture thereof during operation and this is accomplished without significant thermal heat sinking of the element. The heater support block structure is to be contrasted with applicant's issued U.S. Pat. No. 3,346,720 which teaches intermittent support of a "corrugated" resistant element for an infrared heater at widely spaced intervals to attain maximum exposure of the element to the vertically spaced underlying reflective pan member.

Applicant's construction, however, enables the heater block to reflect upwardly through the cover plate 38 the majority of the infrared rays emitted from the resistance element toward the block to supplement the radiant heating from the element and thereby have the maximum radiant heat transfer to a utensil being heated on the cover plate. In particular, the surface reflectivity of the block portion exposed to the resistance element should exceed 70 percent in the wavelength range of 2.0 to 5.0 microns. It is to be noted that the heater block reflective upper recessed area 80 utilizes a "diffuse" reflective effect in a manner to be explained due to its inherent uneven or irregular fibrous surface as contrasted with "specular" reflection that results from a smooth or polished metal surface such as a looking glass.

The heating units of the present invention requires not only that the heater blocks have some means to render them infrared reflective but that they also incorporate means to control the thermal expansion and contraction of the sinelike patterned ribboned element 64 within the spiral groove 67. Without providing some means to physically restrain the free movement of the element a resultant bunching or crowding of certain adjacent tangent portions 102 will occur through the frictional resistance developed between the ribbon element and the groove bottom surface 67 thereby giving rise to an uneven heating pattern or "hot spots" in the cover plate heating area. In addition, uncontrolled thermal expansion and contraction could result in certain adjacent tangent portions 102 making electrical contact so as to short out the element.

In accordance with certain principles of the present invention wherein the heating element is in substantially continuous surface contact with the heater block there is extensive conductive heat transfer from the element to the block thus requiring a low density block material having minimal thermal conductivity and low thermal mass. The invention therefore involves the rendering of a fibrous-ceramic block infrared reflective by uniformly dispersing therein a finely divided opacifier substance in an amount sufficient to reflect the majority of the infrared energy having a wavelength within the range of 1 to 5 microns. It was discovered that a substance selected from a group consisting of titanium oxide, zirconium oxide, tin oxide and potassium titanate if present in the amount of the order of 5 percent to 10 percent by weight of a low density fibrous-ceramic insulating block will reflect at least 70 percent of the infrared rays within the range mentioned.

A specific example of such a heater block used in the present invention is as follows: ------------------------------------------------------------ ---------------

COMPOSITION

The material shall not contain more than 45 percent silica by weight. The block must not contain materials which will give off odors when heated. ____________________________________________________________ ______________ ------------------------------------------------------------ --------------- COMPOSITION

The material shall not contain more than 45% silica by weight. The block must not contain material which will give off odors when heated. ____________________________________________________________ ______________ Element Concentration As the Concentration Compound ____________________________________________________________ ______________ Si SiO ₂ 43.0% AL 18.6% AL ₂ O ₃ 39.8% K 7.4% K ₂ X* 11.5% Ti 4.6% TiO ₂ 7.4% Fe 1.0% Fe ₂ O ₃ 1.4% ____________________________________________________________ ______________ *K is present as KCL, K ₂ 0, and K ₂ SO ₄ ____________________________________________________________ ______________ ------------------------------------------------------------ --------------- PHYSICAL CHARACTERISTICS

Density, p.c.f. 13±1 Tensile Strength, p.s.i. 29.2±3.1 Compression Modulus, p.s.i. 476±64 ____________________________________________________________ ______________

IGNITION LOSS

The maximum weight loss upon heating to 2,000° F. shall not exceed: Heating Max. Weight Loss ____________________________________________________________ ______________ 4 hours 5.1% 24 hours 5.2% ____________________________________________________________ ______________

THERMAL CONDUCTIVITY

B.T.U./inch/hr./°F./sq. ft. Temperature,°F. "K" Factor Temperature,°F. "K" Factor ____________________________________________________________ ______________ 400 0.34 1,200 0.78 600 0.45 1,400 0.88 800 0.56 1,600 1.00 1,000 0.67 ____________________________________________________________ ______________

SURFACE REFLECTIVITY

The surface reflectivity of the block must exceed 70 percent in the wavelength range of 2.0 to 5.0 microns. ____________________________________________________________ ______________

BINDER

Any inorganic binder should be burned out of the block. ____________________________________________________________ ______________

Mention should be made of experiments demonstrating that it is desirable to keep the resistance element and reflector means as close to the cover plate 38 as possible, within permitted Underwriter Laboratory standards previously discussed, to minimize energy losses. It will be appreciated that by rendering the heater blocks infrared reflective, the present invention has significantly improved the art of radiant heating units. It is desirable that the amount of silica (SiO ₂ ) in the block should not exceed approximately 45 percent by weight to avoid a chemical reaction between the heater block and the resistance element. Also, the block preferably should contain a minimum of 40 percent by weight of alumina (Al ₂ O ₃ ) to prevent excessive loss or transfer of aluminum from the Fe-Cr-A1 resistance element to the block during use. The heater block in its preferred form has a density within the approximate range of 10 to 15 pounds per cubic foot with the density of the specific example being about 13 pounds per cubic foot.

My invention also contemplates novel arrangement for physically restraining the resistance element ribbon 64 within the spiral grooves to control thermal expansion and contraction of the element. By virtue of using a low density fibrous-ceramic heater block having the desired infrared reflective properties the invention not only provides continuous support for the resistance element but furthermore allows for novel and inexpensive means to fixedly retain the ribbon element at spaced locations within the spiral groove. As seen in FIG. 6 a plurality of U-shaped tie or staple members indicated at 106 are used in a symmetrical pattern wherein they are aligned or spaced radial lines indicated at 108 at spaced circumferential points therealong.

As viewed in FIG. 6 the tie members 106 of the invention are formed of resistance wire stock in a manner similar to common wire staples having a bridge portion 110 and parallel leg portions 112. It is important that interreaction at the points of contact between the tie members and the resistance element be minimized. Such interreaction destroys the protective aluminum oxide coating that forms on the Fe-Cr-Al alloy elements during heating resulting in possible rupture of the element. Applicant has discovered that if the tie members 106 are fabricated from an Fe-Cr-Al alloy similar to the one used for the resistance elements, no damage occurs to the aluminum oxide coating of the element at the tie member locations to reduce the service life of the resistance elements.

In the preferred form the tie members are fabricated from 23 gauge wire stock with the bridge 110 having a dimension of approximately 0.26 inch and the legs 112 having a length of approximately 1.12 inches. The tie members 106, while capable of being self-driven by hand, are preferably forced into the blocks 64 by means of suitably stapling machines or guns and accordingly must have sufficient stiffness to be self-penetrating.

To provide for the permanent retention of the tie members 106 within the heater blocks a suitable high-temperature resistant cement indicated at 114 in FIG. 6 is used to affix the free ends of the legs 112, shown protruding a defined distance beyond the underside surface 116 of the block to the surface 116. An example of a suitable high-temperature cement for this purpose is manufactured under the trademark Sauerseisen Cement.

As stated above the tie members 106 in the instant invention are located in a pattern whereby they are aligned or grouped substantially on radial lines represented at 108 in FIG. 5. In the preferred form the centerlines are spaced at 45° increments with the result that each approximate 45° of arc of the resistance element has a tie member at either end. In this manner each arcuate segment of the element is free to thermally expand and contract in accordionlike fashion without causing upward bowing or warping of the element. The tangent portions 102 are also restrained from contacting each other by limiting the cumulative buildup of thermal movement of the resistance element to defined arcuate segments between pairs of circumferential spaced tie members while affording relative movement between individual tangents 102.

In order to insure the arcuate expansion and contraction of the resistance element it should be observed that the overall width or double amplitude measurement B is substantially equal to the width of the planar bottom wall 87 of the grooves 67 and 68, i.e., of the order of 0.40 inch. By this construction the resistance element will be constrained against radial-type thermal movement in the groove while promoting the accordionlike arcuate expansion and contraction of the element.

In view of the aforesaid description of the preferred embodiment of the invention, it will be appreciated by those skilled in the art that the infrared radiant heating unit of the present invention is characterized by providing maximum support to the resistance element with minimal thermal heat sinking of the element and furthermore constitute a unit providing an infrared reflective support having a low thermal mass and high-temperature thermal insulation. The infrared radiant units of the present invention are constructed and arranged so that the resistance-heating element thereof is able to be operated either continuously or pulsatingly over long periods of time with a high degree of reliability.

While the embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.