Title:
INFRARED SURFACE HEATING UNIT
United States Patent 3646321
Abstract:
An infrared surface heating unit including an upper utensil supporting infrared transmissive cover plate having a convolute-shaped ribbon resistance element contiguous with the undersurface of the cover plate. The element has corrugations formed across its width throughout its length such that a blend of conductive and radiant heating of the utensil is achieved.
US Patent References:
Heaters
Scofield - April 1963 - 3086101
Electrical heating unit
Scofield - May 1958 - 2833908
Electric hot plate
Gaugler et al. - April 1967 - 3316390
Infrared surface heating unit with corrugated ribbon-shaped filament
Siegla - October 1967 - 3346720
ELECTRICAL RADIANT HEATING UNIT
Kelm - October 1969 - 3471680
Heaters
Scofield - April 1963 - 3086101
Electrical heating unit
Scofield - May 1958 - 2833908
Electric hot plate
Gaugler et al. - April 1967 - 3316390
Infrared surface heating unit with corrugated ribbon-shaped filament
Siegla - October 1967 - 3346720
ELECTRICAL RADIANT HEATING UNIT
Kelm - October 1969 - 3471680
Application Number:
05/048389
Publication Date:
02/29/1972
Filing Date:
06/22/1970
View Patent Images:
Export Citation:
Assignee:
General Motors Corporation (Detroit, MI)
Primary Class:
Other Classes:
219/553, 338/280
International Classes:
F24C15/10; H05B3/74; H05B3/68; H05B3/68
Field of Search:
219/462,464,460,540,553,345 338/280,293
US Patent References:
Primary Examiner:
Mayewsky, Volodymyr Y.
Claims:
What is claimed is as follows
1. In an infrared surface heating unit, the combination of, an outer envelope including a resilient metallic lower plate and an upper infrared transmissive glasslike cover plate for supporting a utensil thereon, said lower plate and said cover plate being located in spaced relationship with one another, said cover plate having an electrical isolating coating formed on the undersurface thereof; an infrared emissive resistance element supported on planar base support means in the space between said lower plate and said cover plate in direct thermal conductive pressure contact with said coated undersurface of said cover plate; said planar base support means including a relatively thick insulation mat formed of resilient low-density refractory material having low thermal mass and a relatively thin sheet of substantially rigid dielectric material on the upper surface of said mat, said resistance element being ribbon shaped and having a plurality of transverse corrugations formed continuously along the length thereof, each of said corrugations having substantially planar upper, side and bottom walls providing a series of alternately inverted quadrilateral-shaped cavities, said corrugation upper walls contacting said cover plate undersurface and said corrugation lower walls maintained in spaced relation to said cover plate undersurface whereby combined conductive and radiant heating is effected between utensils supported on said cover plate and said resistance element, and peripheral clamping means exerting compression forces on said cover plate and said lower plate to maintain said upper walls of said corrugated resistance element in pressure contact with said coated undersurface of said cover plate and to maintain said lower walls of said corrugated resistance element in pressure contact with said sheet of dielectric material.
2. The combination of claim 1, said cover plate undersurface coating being formed of a deposited silicon dioxide material for insulating said resistance element from electrical contact with said cover plate.
3. The combination of claim 1, said cover plate being formed of recrystallized glass-ceramic material and said resistance element formed of an iron-chromium-aluminum alloy material.
4. The combination of claim 1, said resistance element upper and bottom walls being substantially equal in extent.
5. In an infrared surface heating unit, the combination of an outer low profile envelope including a flexible lower metallic plate and an upper utensil supporting cover plate of infrared transmissive glass-ceramic material, said lower plate and said cover plate being located in spaced relationship with one another, said cover plate having a continuous transparent electrical isolating coating of deposited silica formed on the undersurface thereof, a sandwich structure located in the space between said lower plate and said cover plate; said sandwich structure comprising a relatively thick insulation mat of fibrous-ceramic resilient insulating material having low thermal mass, an intermediate rigid sheet of relatively thin mica board and a ribbon-shaped resistance element of convolute pattern located between said intermediate rigid sheet and said cover plate; said resistance element being formed from an iron-chromium-aluminum alloy material, said resistance element having a plurality of transverse corrugations formed continuously along the length thereof, each of said corrugations having substantially planar upper, side and bottom walls providing a series of alternately inverted rectangular-shaped cavities; said corrugation upper and bottom walls being substantially parallel and equal in extent whereby the aggregate upper wall surfaces in conductive heat transfer relation with said coated undersurface of said cover plate comprise less than one-half of the total upper surface area of said resistance element, power source means for energizing said resistance element above 1,500° F. whereby combined conductive and infrared radiant heating is transmitted to utensils supported on said cover plate, and clamping ring means engaging the periphery of said cover plate and said lower plate for exerting compression forces on said sandwich structure to maintain said upper walls of said corrugated resistance element in pressure contact with said coated undersurface of said cover plate.
1. In an infrared surface heating unit, the combination of, an outer envelope including a resilient metallic lower plate and an upper infrared transmissive glasslike cover plate for supporting a utensil thereon, said lower plate and said cover plate being located in spaced relationship with one another, said cover plate having an electrical isolating coating formed on the undersurface thereof; an infrared emissive resistance element supported on planar base support means in the space between said lower plate and said cover plate in direct thermal conductive pressure contact with said coated undersurface of said cover plate; said planar base support means including a relatively thick insulation mat formed of resilient low-density refractory material having low thermal mass and a relatively thin sheet of substantially rigid dielectric material on the upper surface of said mat, said resistance element being ribbon shaped and having a plurality of transverse corrugations formed continuously along the length thereof, each of said corrugations having substantially planar upper, side and bottom walls providing a series of alternately inverted quadrilateral-shaped cavities, said corrugation upper walls contacting said cover plate undersurface and said corrugation lower walls maintained in spaced relation to said cover plate undersurface whereby combined conductive and radiant heating is effected between utensils supported on said cover plate and said resistance element, and peripheral clamping means exerting compression forces on said cover plate and said lower plate to maintain said upper walls of said corrugated resistance element in pressure contact with said coated undersurface of said cover plate and to maintain said lower walls of said corrugated resistance element in pressure contact with said sheet of dielectric material.
2. The combination of claim 1, said cover plate undersurface coating being formed of a deposited silicon dioxide material for insulating said resistance element from electrical contact with said cover plate.
3. The combination of claim 1, said cover plate being formed of recrystallized glass-ceramic material and said resistance element formed of an iron-chromium-aluminum alloy material.
4. The combination of claim 1, said resistance element upper and bottom walls being substantially equal in extent.
5. In an infrared surface heating unit, the combination of an outer low profile envelope including a flexible lower metallic plate and an upper utensil supporting cover plate of infrared transmissive glass-ceramic material, said lower plate and said cover plate being located in spaced relationship with one another, said cover plate having a continuous transparent electrical isolating coating of deposited silica formed on the undersurface thereof, a sandwich structure located in the space between said lower plate and said cover plate; said sandwich structure comprising a relatively thick insulation mat of fibrous-ceramic resilient insulating material having low thermal mass, an intermediate rigid sheet of relatively thin mica board and a ribbon-shaped resistance element of convolute pattern located between said intermediate rigid sheet and said cover plate; said resistance element being formed from an iron-chromium-aluminum alloy material, said resistance element having a plurality of transverse corrugations formed continuously along the length thereof, each of said corrugations having substantially planar upper, side and bottom walls providing a series of alternately inverted rectangular-shaped cavities; said corrugation upper and bottom walls being substantially parallel and equal in extent whereby the aggregate upper wall surfaces in conductive heat transfer relation with said coated undersurface of said cover plate comprise less than one-half of the total upper surface area of said resistance element, power source means for energizing said resistance element above 1,500° F. whereby combined conductive and infrared radiant heating is transmitted to utensils supported on said cover plate, and clamping ring means engaging the periphery of said cover plate and said lower plate for exerting compression forces on said sandwich structure to maintain said upper walls of said corrugated resistance element in pressure contact with said coated undersurface of said cover plate.
Description:
This invention relates to infrared surface heating units for domestic ranges and more particularly to surface heating units of the open coil type.
It has been recognized that surface heating units of the type having "open coil" infrared emissive resistance elements for heating utensils supported on a glasslike cover plate operated most efficiently when the resistance element is located as close as possible to the utensil being heated. The term "open coil" refers to a resistance element operated in an unsealed or unevacuated atmosphere as contrasted with a tungsten element for example requiring a vacuum sealed envelope. An illustration of an infrared surface heater having an open coil resistance element in direct contact with the infrared transmissive cover plate is disclosed in the U.S. Pat. No. 3,086,101 issued Apr. 16, 1963. This type of structure necessitates the glass to be heated prior to emission of infrared radiation which causes a thermal lag effect preventing quick response to heat control. Also, the difference between the coefficient of expansion of the cover plate and the contiguous resistance element produces a thermal stress condition which causes warping or bowing of the element resulting in uneven heating or possible contact and electrical shorting between adjacent coils.
One solution to the problem involves a heating unit wherein the resistance element is completely spaced from the underside of the cover plate such that a utensil supported thereon is heated solely by infrared radiation from the element thus reducing the time lag required to conductively heat the cover plate. An example of this type of heating unit is disclosed in applicant's U.S. Pat. No. 3,345,598 issued Oct. 3, 1967.
Open coil infrared emitters for the aforementioned types of heating units which do not require a sealed atmosphere and are economically useful are relatively limited to the nickel-chromium, nickel-chromium-iron and iron-chromium group of alloys and a group of iron-chromium-aluminum alloys. Both of these alloy groups create a physical support problem because of their inherent low ductility and poor hot strength at high temperatures. Moreover, thermal expansion of these electrical resistance alloys at high temperatures is considerable which makes it necessary to physically restrain the resistance elements during operation. To the solution of an open coil radiant heating unit employing such resistance elements, the present invention is directed.
It is, therefore, an object of the present invention to provide an improved infrared surface heating unit that improves the overall service life efficiency and reliability of the unit by means of a combined conductive and radiant heating open coil resistance element arrangement.
A further object of the present invention is to provide an improved infrared surface heating unit of the type including a utensil supporting cover plate of infrared transmissive material and a low thermal mass insulating base mat located in spaced relationship with the cover plate and having a ribbon-shaped resistance element sandwiched in the space therebetween contiguous with the cover plate, the resistance element formed with corrugations across its width wherein a blend of conductive and radiant heating of the utensil is achieved.
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 view in perspective of a domestic range;
FIG. 2 is a top plan view of a portion of a top surface heating unit with the cover plate thereof partially broken away;
FIG. 3 is a view in vertical section taken along the line 3--3 in FIG. 1; and
FIG. 4 is an enlarged, fragmentary, sectional view taken on line 4--4 in FIG. 3.
Referring now to the drawings, in FIG. 1, the domestic range 10 is illustrated including top 12 having a plurality of openings 14 (FIG. 3) therein in which are located surface heating units 16 constructed in accordance with the principles of the present invention. Each of units 16 is associated operably with suitable power supply means such as a controller 18 located on a rear control panel 20 of the range for varying energization of the heating units 16.
Referring now more particularly to FIGS. 2 through 4, the improved surface heating unit 16 is illustrated as including a supporting ring 22 formed in the sheet metal upper surface 12 of the range 10. Supporting ring 22 includes an upwardly curved portion 24 having a planar top face 25 surrounding the unit 16 and a depending flange 26 for forming a continuous horizontal rim 27. In the illustrated arrangement a sensor support pan 28 is located beneath the opening 14 and supportingly received by the top 12 at an upper outwardly directed lip 30 thereon and secured to the rim 27 by suitable means such as metal screws (not shown).
Each unit 16 has an outer envelope formed by a lower plate 32 spaced beneath an upper infrared transmissive utensil supporting cover plate 34. The cover plate 34 includes a peripheral edge 36 thereon joined to a peripheral brim 38 of the lower plate 32 by a clamping ring 40 directed continuously around the outer periphery of the unit 16 to be supportingly received by the planar face 25 of the ring 22.
Joined to the upper surface of the brim 38 of the lower plate 32 is a rigid continuous annular L-shaped sealing member 44 having a radially upwardly directed flange 46 defining a seat for receiving an annular sealing gasket 48 preferably constructed of a heat resistant resilient material such as asbestos. The gasket 48 is held in sealing engagement with the cover plate 34 and sealing member 44 by the clamping ring 40 to thereby seal against leakage of the utensil spillage or the like interiorly of the space 50. It will be noted that clamping ring 40 comprises upper 52 and lower 53 annular double L-shaped members each of which has a radial flange indicated at 54, 55 which are suitably secured such as by welding to provide a unitary element which is supported on the upper planar portion 25 of the ring 22.
As seen in FIG. 3 on top of the peripheral edge 36 of the glass plate is located an annular seal member 60 preferably constructed of heat resistant material such as fiber glass which separates the upper member 52 of the clamping ring 40 from the cover plate 34. In this manner the cover plate 34 and the lower plate member 32 are joined together by the continuously formed clamp ring 40 having its upper member 52 overlying the peripheral edge 36 of the cover plate 34 and the lower member 53 inserted in underlying relation with the peripheral brim 38 of the lower base plate 32, whereby the low-profile envelope 16 is sealed against entry interiorly of the base plate 32 and the cover plate 34 by the seal rings 60 and gasket 48.
As shown in the preferred form there is physically secured to the underside of the plate at the center thereof a temperature sensing or conducting device 66 such as a thermistor the resistance of which varies in response to changes in temperature sensed by the thermistor. These resistance changes are transmitted electrically by means of the leads 68 shown to a suitable control circuit adjusting the heat input accordingly. Those skilled in the art are familiar with several electrical circuits of which thermistors are now used for the control of surface cooking operations and the type of actual control is not important to the present invention.
In the illustrated arrangement the heating unit envelope 16 is a sandwich structure which consists of subpanel base member 70 preferably made of a resilient refractory mat of fibrous-ceramic material, composed mainly of alumina and silica approximately three-quarter inch in thickness, having minimal thermal conductivity and a relatively low thermal mass. Located at the upper surface 72 of the base member 70 is a relatively thin sheet 74 of dense dielectric material for supporting a corrugated ribbon-shaped resistance element 76 thereon. In the form shown the sheet 74 is composed of mica formed by compression into a rigid board having a thickness of about one-sixteenth inch.
The resistance element 76 has one end thereof connected to the terminal 78 and the opposite end thereof electrically connected to a terminal 80. The terminals 78 and 80 are directed through the base member 70 and plate 32 and are provided with lead wires 82, 84 adapted to be electrically connected across the power source whereby the resistance element 76 is energizable into a maximum temperature range of approximately 1,500° to 2,000° F. The radiation emitted from the portions of the continuously corrugated resistance element not in contact with plate 34 is directed upwardly through the cover plate 34 to heat utensils supported thereon. The ribbon-shaped element 76 is preferably constructed of high temperature resistance alloy material from the nickel-chromium group or the iron-chromium-aluminum group referred to above having a thickness of the order of 4 mils and a width of approximately one-eighth inch providing desirable strength and electrical characteristics.
The cover plate 34 in the disclosed form is constructed of a recrystallized glass-ceramic such as Cer-Vit manufactured by Owens-Illinois or Hercuvit manufactured by Pittsburgh Plate Glass Company. However, other suitable glasslike infrared transmissive material having high strength such as quartz or a high silica glass such as Vycor made by Corning Glass may also be used in the unit without departing from the scope of the invention.
As the electrical conductivity of plate 34 may increase at high temperature, suitable means must be provided to electrically insulate the resistance element 76 from the cover plate 34. Accordingly, in the illustrated arrangement, an electrically isolating transparent coating or film 86 of infrared transmissive material such as vapor deposited quartz or silica is formed on the undersurface of the cover plate.
As seen in the enlarged sectional view of FIG. 4, the resistance element 76 is corrugated across its width throughout its length by a plurality of corrugations, each corrugation including substantially straight upper 90, side 92 and bottom 94 walls to form a series of alternately inverted quadrilateral-shaped cavities 95. While in FIG. 4 the cavities are shown as being rectangular it is to be noted that other geometrical configurations could be used, the prime requirement being that the corrugation pattern can accommodate the thermal growth of the ribbon resistance element 76 by compressing the corrugations. While the cover plate contacting upper walls 90 of the resistance element are shown substantially equal to the noncontacting bottom walls 94 it is to be understood that various contact to noncontact ratios could be employed, depending upon the wattage output and specific size of ribbon used, with the limitation that the aggregate cover plate contacting surfaces of the resistance element upper walls 90 comprise less than one-half the total surface area of the element. The total surface area of the element would, of course, include the surfaces of the sidewalls 92 in addition to the top walls 90 and bottom walls 94.
In this manner less than one-half of the resistance element 76, represented by upper walls 90, will be in conductive heat transfer relation with the cover plate 34 while the bottom walls 94 will radiate infrared energy through the cover plate to a utensil thereon. This design provides a combination of conductive and radiant heating for the cooking utensil supported on the cover plate. In addition, the cover plate functions as a heat-sink for the corrugated resistance element 76 to prevent overheating thereof thus increasing the service life of the element. In the preferred form the resistance element 76 is formed with its sidewalls 92 having a height of the order of 0.20 inches while the pitch dimensions for the corrugations, indicated at P in FIG. 4, is approximately 0.50 inches.
To insure efficient heat conduction between the upper walls 90 of the resistance element 76 and the cover plate 34 it is necessary to provide pressure contact therebetween. In the invention the resilient refractory base member 70 is compressed in the heating unit envelope 16 by means of the resiliency of the refractory base member 70 and the flexibility of the lower metallic plate 32 such that when the clamping ring 40 is positioned a compressive force is exerted on the corrugated resistance element 76 by virtue of being sandwiched between the mica board sheet 74 and the cover plate 34.
The undersurface 35 of the cover plate, having the aforementioned coating 86 applied thereto, presents a smooth planar area to provide maximum contact between a planar uncorrugated ribbon-shaped convoluted resistance element and the cover plate which should insure uniform distribution of heat to the cover plate. Tests showed, however, that after the element has been subjected to repeated heating and cooling cycles, a warping and bowing of a planar ribbon element developes resulting in substantial noncontact areas between the element and the cover plate. Applicant discovered that by forming the resistance element in relatively short, planar segments or upper walls 90, the resistance element is able to move about the transverse fold lines of relatively easy flexure created by the corrugations allowing the upper walls 90 to conform to the cover plate coated undersurface 86 under extended service life conditions.
The corrugated arrangement of the resistance element 76 further allows the element to accommodate for its own thermal expansion and contraction by means of the transverse lines of flexure formed at the junctures of the sidewalls 92 with the upper 90 and bottom 94 walls. By reason of this feature the thermal expansion of the resistance element is controlled to prevent warping which, if uncontrolled, could result in adjacent convolute sections contacting and shorting the element 34. It should be noted that applicant's heating unit also can be in a wide range of ratings and watt densities as well as providing units with zones of different watt densities.
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.
It has been recognized that surface heating units of the type having "open coil" infrared emissive resistance elements for heating utensils supported on a glasslike cover plate operated most efficiently when the resistance element is located as close as possible to the utensil being heated. The term "open coil" refers to a resistance element operated in an unsealed or unevacuated atmosphere as contrasted with a tungsten element for example requiring a vacuum sealed envelope. An illustration of an infrared surface heater having an open coil resistance element in direct contact with the infrared transmissive cover plate is disclosed in the U.S. Pat. No. 3,086,101 issued Apr. 16, 1963. This type of structure necessitates the glass to be heated prior to emission of infrared radiation which causes a thermal lag effect preventing quick response to heat control. Also, the difference between the coefficient of expansion of the cover plate and the contiguous resistance element produces a thermal stress condition which causes warping or bowing of the element resulting in uneven heating or possible contact and electrical shorting between adjacent coils.
One solution to the problem involves a heating unit wherein the resistance element is completely spaced from the underside of the cover plate such that a utensil supported thereon is heated solely by infrared radiation from the element thus reducing the time lag required to conductively heat the cover plate. An example of this type of heating unit is disclosed in applicant's U.S. Pat. No. 3,345,598 issued Oct. 3, 1967.
Open coil infrared emitters for the aforementioned types of heating units which do not require a sealed atmosphere and are economically useful are relatively limited to the nickel-chromium, nickel-chromium-iron and iron-chromium group of alloys and a group of iron-chromium-aluminum alloys. Both of these alloy groups create a physical support problem because of their inherent low ductility and poor hot strength at high temperatures. Moreover, thermal expansion of these electrical resistance alloys at high temperatures is considerable which makes it necessary to physically restrain the resistance elements during operation. To the solution of an open coil radiant heating unit employing such resistance elements, the present invention is directed.
It is, therefore, an object of the present invention to provide an improved infrared surface heating unit that improves the overall service life efficiency and reliability of the unit by means of a combined conductive and radiant heating open coil resistance element arrangement.
A further object of the present invention is to provide an improved infrared surface heating unit of the type including a utensil supporting cover plate of infrared transmissive material and a low thermal mass insulating base mat located in spaced relationship with the cover plate and having a ribbon-shaped resistance element sandwiched in the space therebetween contiguous with the cover plate, the resistance element formed with corrugations across its width wherein a blend of conductive and radiant heating of the utensil is achieved.
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 view in perspective of a domestic range;
FIG. 2 is a top plan view of a portion of a top surface heating unit with the cover plate thereof partially broken away;
FIG. 3 is a view in vertical section taken along the line 3--3 in FIG. 1; and
FIG. 4 is an enlarged, fragmentary, sectional view taken on line 4--4 in FIG. 3.
Referring now to the drawings, in FIG. 1, the domestic range 10 is illustrated including top 12 having a plurality of openings 14 (FIG. 3) therein in which are located surface heating units 16 constructed in accordance with the principles of the present invention. Each of units 16 is associated operably with suitable power supply means such as a controller 18 located on a rear control panel 20 of the range for varying energization of the heating units 16.
Referring now more particularly to FIGS. 2 through 4, the improved surface heating unit 16 is illustrated as including a supporting ring 22 formed in the sheet metal upper surface 12 of the range 10. Supporting ring 22 includes an upwardly curved portion 24 having a planar top face 25 surrounding the unit 16 and a depending flange 26 for forming a continuous horizontal rim 27. In the illustrated arrangement a sensor support pan 28 is located beneath the opening 14 and supportingly received by the top 12 at an upper outwardly directed lip 30 thereon and secured to the rim 27 by suitable means such as metal screws (not shown).
Each unit 16 has an outer envelope formed by a lower plate 32 spaced beneath an upper infrared transmissive utensil supporting cover plate 34. The cover plate 34 includes a peripheral edge 36 thereon joined to a peripheral brim 38 of the lower plate 32 by a clamping ring 40 directed continuously around the outer periphery of the unit 16 to be supportingly received by the planar face 25 of the ring 22.
Joined to the upper surface of the brim 38 of the lower plate 32 is a rigid continuous annular L-shaped sealing member 44 having a radially upwardly directed flange 46 defining a seat for receiving an annular sealing gasket 48 preferably constructed of a heat resistant resilient material such as asbestos. The gasket 48 is held in sealing engagement with the cover plate 34 and sealing member 44 by the clamping ring 40 to thereby seal against leakage of the utensil spillage or the like interiorly of the space 50. It will be noted that clamping ring 40 comprises upper 52 and lower 53 annular double L-shaped members each of which has a radial flange indicated at 54, 55 which are suitably secured such as by welding to provide a unitary element which is supported on the upper planar portion 25 of the ring 22.
As seen in FIG. 3 on top of the peripheral edge 36 of the glass plate is located an annular seal member 60 preferably constructed of heat resistant material such as fiber glass which separates the upper member 52 of the clamping ring 40 from the cover plate 34. In this manner the cover plate 34 and the lower plate member 32 are joined together by the continuously formed clamp ring 40 having its upper member 52 overlying the peripheral edge 36 of the cover plate 34 and the lower member 53 inserted in underlying relation with the peripheral brim 38 of the lower base plate 32, whereby the low-profile envelope 16 is sealed against entry interiorly of the base plate 32 and the cover plate 34 by the seal rings 60 and gasket 48.
As shown in the preferred form there is physically secured to the underside of the plate at the center thereof a temperature sensing or conducting device 66 such as a thermistor the resistance of which varies in response to changes in temperature sensed by the thermistor. These resistance changes are transmitted electrically by means of the leads 68 shown to a suitable control circuit adjusting the heat input accordingly. Those skilled in the art are familiar with several electrical circuits of which thermistors are now used for the control of surface cooking operations and the type of actual control is not important to the present invention.
In the illustrated arrangement the heating unit envelope 16 is a sandwich structure which consists of subpanel base member 70 preferably made of a resilient refractory mat of fibrous-ceramic material, composed mainly of alumina and silica approximately three-quarter inch in thickness, having minimal thermal conductivity and a relatively low thermal mass. Located at the upper surface 72 of the base member 70 is a relatively thin sheet 74 of dense dielectric material for supporting a corrugated ribbon-shaped resistance element 76 thereon. In the form shown the sheet 74 is composed of mica formed by compression into a rigid board having a thickness of about one-sixteenth inch.
The resistance element 76 has one end thereof connected to the terminal 78 and the opposite end thereof electrically connected to a terminal 80. The terminals 78 and 80 are directed through the base member 70 and plate 32 and are provided with lead wires 82, 84 adapted to be electrically connected across the power source whereby the resistance element 76 is energizable into a maximum temperature range of approximately 1,500° to 2,000° F. The radiation emitted from the portions of the continuously corrugated resistance element not in contact with plate 34 is directed upwardly through the cover plate 34 to heat utensils supported thereon. The ribbon-shaped element 76 is preferably constructed of high temperature resistance alloy material from the nickel-chromium group or the iron-chromium-aluminum group referred to above having a thickness of the order of 4 mils and a width of approximately one-eighth inch providing desirable strength and electrical characteristics.
The cover plate 34 in the disclosed form is constructed of a recrystallized glass-ceramic such as Cer-Vit manufactured by Owens-Illinois or Hercuvit manufactured by Pittsburgh Plate Glass Company. However, other suitable glasslike infrared transmissive material having high strength such as quartz or a high silica glass such as Vycor made by Corning Glass may also be used in the unit without departing from the scope of the invention.
As the electrical conductivity of plate 34 may increase at high temperature, suitable means must be provided to electrically insulate the resistance element 76 from the cover plate 34. Accordingly, in the illustrated arrangement, an electrically isolating transparent coating or film 86 of infrared transmissive material such as vapor deposited quartz or silica is formed on the undersurface of the cover plate.
As seen in the enlarged sectional view of FIG. 4, the resistance element 76 is corrugated across its width throughout its length by a plurality of corrugations, each corrugation including substantially straight upper 90, side 92 and bottom 94 walls to form a series of alternately inverted quadrilateral-shaped cavities 95. While in FIG. 4 the cavities are shown as being rectangular it is to be noted that other geometrical configurations could be used, the prime requirement being that the corrugation pattern can accommodate the thermal growth of the ribbon resistance element 76 by compressing the corrugations. While the cover plate contacting upper walls 90 of the resistance element are shown substantially equal to the noncontacting bottom walls 94 it is to be understood that various contact to noncontact ratios could be employed, depending upon the wattage output and specific size of ribbon used, with the limitation that the aggregate cover plate contacting surfaces of the resistance element upper walls 90 comprise less than one-half the total surface area of the element. The total surface area of the element would, of course, include the surfaces of the sidewalls 92 in addition to the top walls 90 and bottom walls 94.
In this manner less than one-half of the resistance element 76, represented by upper walls 90, will be in conductive heat transfer relation with the cover plate 34 while the bottom walls 94 will radiate infrared energy through the cover plate to a utensil thereon. This design provides a combination of conductive and radiant heating for the cooking utensil supported on the cover plate. In addition, the cover plate functions as a heat-sink for the corrugated resistance element 76 to prevent overheating thereof thus increasing the service life of the element. In the preferred form the resistance element 76 is formed with its sidewalls 92 having a height of the order of 0.20 inches while the pitch dimensions for the corrugations, indicated at P in FIG. 4, is approximately 0.50 inches.
To insure efficient heat conduction between the upper walls 90 of the resistance element 76 and the cover plate 34 it is necessary to provide pressure contact therebetween. In the invention the resilient refractory base member 70 is compressed in the heating unit envelope 16 by means of the resiliency of the refractory base member 70 and the flexibility of the lower metallic plate 32 such that when the clamping ring 40 is positioned a compressive force is exerted on the corrugated resistance element 76 by virtue of being sandwiched between the mica board sheet 74 and the cover plate 34.
The undersurface 35 of the cover plate, having the aforementioned coating 86 applied thereto, presents a smooth planar area to provide maximum contact between a planar uncorrugated ribbon-shaped convoluted resistance element and the cover plate which should insure uniform distribution of heat to the cover plate. Tests showed, however, that after the element has been subjected to repeated heating and cooling cycles, a warping and bowing of a planar ribbon element developes resulting in substantial noncontact areas between the element and the cover plate. Applicant discovered that by forming the resistance element in relatively short, planar segments or upper walls 90, the resistance element is able to move about the transverse fold lines of relatively easy flexure created by the corrugations allowing the upper walls 90 to conform to the cover plate coated undersurface 86 under extended service life conditions.
The corrugated arrangement of the resistance element 76 further allows the element to accommodate for its own thermal expansion and contraction by means of the transverse lines of flexure formed at the junctures of the sidewalls 92 with the upper 90 and bottom 94 walls. By reason of this feature the thermal expansion of the resistance element is controlled to prevent warping which, if uncontrolled, could result in adjacent convolute sections contacting and shorting the element 34. It should be noted that applicant's heating unit also can be in a wide range of ratings and watt densities as well as providing units with zones of different watt densities.
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.