CA1048581A

CA1048581A - Low thermal mass solid plate surface unit

Info

Publication number: CA1048581A
Application number: CA75234737A
Authority: CA
Inventors: Bohdan Hurko
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1974-09-30
Filing date: 1975-09-03
Publication date: 1979-02-13
Also published as: IT1042903B; JPS5240851A; GB1508226A; FR2286576A1; SE7510320L; US3895216A; AU8372375A; DE2542512A1

Abstract

ABSTRACT OF THE DISCLOSURE
A lightweight solid plate surface heating unit of low thermal mass with a top plate of thin composite material with a center core of high thermal conductivity and outer reinforcing layers. A coating of high electrical resistivity covers the bottom layer of the composite plate.
A film heater of bifilar spiral pattern having its terminals near the center of the plate is bonded to the electrical resistivity coating. A terminal block is mounted to the underside of the plate for making electrical connection with the terminal sections of the film heater.

Description

9D-RG-11807 ::

The invention relates to solid plate surfa~e heating units for use in the surface cooking of foods, and part-icularly a unit heated by a film heater.
In the United States the principal means of surface cooking of foods is by means of metal sheathed electrical resistance heating elements, where the heating unit has a helical resistance wire that is surrounded by a magnesium oxide insulation that is in turn protected by a metal sheath to form an elongated tube which is usually wound in a spiral configuration in a flat plane so that the cooking utensil may be supported directly on the heating unit. Under ideal ` -conditions, when the heating unit is perfectly flat and the cooking utensil has a flat bottom for good area contact with -the heating unit, the thermal efficiency is somewhat between 70% to 80%. However, part of this heat transfer is stored in the cooking utensil which does not contribute anything to the cooking process The actual useful heat which is stored in the food under ideal testing conditions is between 58% and 66%. In the average home, the surface cooking efficiency is actually much lower. Probably on the order of 50% of the heating energy is wasted. This low thermal efficiency of present day surface cooking is caused mainly by a poor thermal coupling between the utensil and the sur-face unit and large amounts of stored heat in the heating unit A sheath type surface unit has the highest thermal efficiency because of its relatively low thermal mass. It has one disadvantage, however, and that i8 that sheath type surface units operate at relatively high temperatures because they have a relatively small contact area with the bottom of the utensil.
Early prior art patents include United States Patent No 3,067,315 dated December4, 1962, which shows a solid ~~ 9D-RG-11807 1~4l~S~l plate surface heating unit with a high temperature glass having multi-layer film heater in strip form bonded to the underside thereof. Another related patent is my U.S.
Patent No. 3,569,672 dated March 9, 1971, which shows a solid plate surface heating unit formed with a top plate of composite sheet material having a metal sheathed electrical resistance heating unit bonded to the underside thereof. .
The principal object of the present invention is to provide a low thermal mass solid plate surface heating unit having a top plate of high thermal conductivity that is heated by a film heater in strip form bonded to the under-side thereof, where the film heater will operate at relatively low temperatures and the top plate will prevent the form-ation of localized hot spots in the film such that the surface heating unit will have higher thermal efficiency due to low stored heat, low thermal mass and low operating tem-peratures.
A further object of the present invention is to provide ; a low thermal mass solid plate surface heating unit of the class described where the electrical terminations are made adjacent the center of the film heater pattern to create a generally uniform circular heating pattern.
A further object of the present invention is to provide a low thermal mass solid plate surface heating unit of the class described where the film heater is provided with a doubled-over spiral pattern that terminates adjacent to the center of the top plate for making electrical connec-tions with a terminal block that is supported from the top plate.
A further object of the present invention is to provide a low thermal mass solid plate surface heating unit where the film heater is prevented from reaching an operating 1~48S81 temperature greater than a maximum between about 425 F and 5750F.
A still further object of the present invention is to provide a low thermal mass solid plate surface heating unit where the terminal block is provided with resiliently mounted contact pads for making electrical connection with the film heater.
The present invention, in accordance with one form thereof, relates to a low thermal mass solid plate surface heating unit that is formed with the thin plate of high thermal conductivity having a coating on its bottom side of high electrical resistivity on which is bonded a film heater of spiral pattern in combination with a terminal block for making electrical connection with the terminal sections of the film heater.
This invention will be better understood from the following description taken in conjunction with the accompany-ing drawings and its scope will be pointed out in the appended claims.
Figure 1 is a fragmentary cross-sectional elevational view through a portion of the center of a low thermal mass solid plate surface heating unit embodying the present in-vention that is shown supported in an opening in a cooktop.
Figure 2 is a bottom plan view of the solid plate surface heating unit shown in Fig. 1 with the reflector pan removed to show the double spiral pattern of the film heater bonded to the underside of the top plate, as w.sll as the location of the terminal block adjacent the center of the plate.
Figure 3 is an enlarged fragmentary view of the center portion of the heating unit of Fig 1 showing the layered construction of the top plate of composite sheet material , . .

, . . . .
, -~ 9D-RG-11807 1~48581 and the resilient nature of an electrical contact pad for making connection with the terminal sections of the film heater.
Turning now to a consideration of the drawings, and in particular to Fig. 1, there is shown a low thermal mass solid ~ ;
plate surface heating unit 10 embodying the present invention.
This heating unit has four main elements namely a top plate 12 of high thermal conductivity, a film heater 14 located on the bottom side of the top plate, an electrical terminal block 16 for making electrical connection between the film heater and the leads of a power circuit, and finally a bottom reflector pan 18 forming the underside of the heating unit and cooperating with the top plate to form a unitary construction.
The top plate 12 is illustrated in the partial cross-sectional elevation view of Fig. 1 as being a solid plate when actually it is of composite sheet material having many layers of different materials. This is necessary because of the scale of this drawing of Fig. 1. If an attempt were made to illustrate these different layers of material it would be difficult with the naked eye to separate them. Hence, for a more accurate understanding of the nature of the top com-posite plate 12 attention is directed to the enlarged fra-gmentary view of Fig. 3. This circular plate 12 is a thin, lightweight circular plate formed of composite sheet material having an inner core 26 of high thermal conductivity such as copper, silver or aluminum for distributing the heat rapidly over the entire plate so as to obtain a generally uniform temperature distribution. This copper core 26 is of small thickness, on the order of 0.040", and it needs to be reinforced. This core 26 is provided with a top and bottom outer skin 28 which serves to reinforce the center , ~.- r 1~4~S8~ 9D_RG_11807 core and retard oxidation and corrosion. m ese top and bottom outer skins 28 may be selected ~rom a group com-prising stainless steel, nickel, chromium and a low carbon -steel such as Tinamel, since this low thermal mass surface heating unit operates at relatively low temperatures on the order of a maximum of abou~ 500 or in a maximum temperature range somewhere between 4250F and 575F. The composite sheet -material of this top plate 12 is very similar to the composite plate described in my earlier U.S. Patent No. 3~569,672 which used a metal sheet electrical resistance heating element ; bonded to the underside of the top plate as the heating source.
; Another advantage derived from the fact that this heating unitoperates at such a low temperature is that the edge of the composite plate need not be sealed by forming the top and bottom skins or layers 28 over the edge of the center core , 26 so that the copper, silver or aluminum core will not oxidize or corrode excessively.
It may be well to provide some additional means for reinforcing this composite plate 12 so that when it is heated it will not tend to buckle or warp. This need is sat-isfied by forming the composite plate 12 with a slight crown on the order of .030". This crown is advantageous for another reason in that it tends to cooperate with the flexible bot-tom wall of a low thermal mass cooking utensil to obtain a maximum thermal coupling action between the heating unit and the cooking utensil.
As is best seen in Fig. 2, the source of heat for the heating unit 10 of the present invention is a narrow con-tinuous heater strip of film 14 using multiple film layers of noble metals such as gold, platinum, silver and aluminum as is taught in my earlier U S. Patent No. 3,067,315 dated December 4, 1962 entitled, "Multi_Layer Film Heaters In Strip _ 5 --~ 9D-RG-11807 ~ 48S81 Form", Of course, it is necessary to ele~tri~ally insulate the film heater 14 from the composite metal plate 12, and this is accomplished by coating the bottom layer 28 of the plate 12 with an electrically non-conductive material such as an enamel coating 30 of high electrical resistivity.
~ a~a~,Dearar>c e.
For purposes of apparcnoc as well as economy, it is also well to coat the top layer 28 of a composite plate 12 with a decorative protective coating. Hence, the preferred em-bodiment of this solid plate surface heating unit 10 has the same enamel coating 30 on both the top and bottom surfaces of the composite plate 12.
It is deemed wise to limit the operating temperature of the composite plate 12 to a maximum temperature of about 500~, although this may vary within a range between about 425F and 575F. miS can begt be done by introducing a temperature-limiting means to the surface unit 10 such that the power to the film heater 14 is cut off if the tempera-ture of the composite plate 12 rises to a predetermined maximum temperature of about 500F. m is temperature-limiting means may comprise a temperature sensor 34 in theform of an elongated bulb which is positioned outside the outermost turn of the film heater 14 and held firmly against the underside of the composite plate 12 by means that is not shown. This sensor 34 may be filled with a high tem-perature thermostatic fluid, such as sodium potassium (NaK) or the like. me sensor 34 would communicate with a temper-ature responder tnot shown) by means of a capillary tube (not shown), as for example as illustrated in my earlier Patent No. 3,622,754 dated November 23, 1971 entitled, "Glass Plate Surface Heating Unit With Even Temperature Distribution".
It is appreciated that as the technology of film heaters and metals improves in the future that this relative-9D-~G-11807 1~48581 ly low maximum operating temperature of the composite substrate might be raised consiaerably. At the present state of this art it would appear that the maximum temperature range between about 425F
and 575F is the most practical.
Looking at the spiral pattern of the film heater 14 in Fig. 2, ~
it should be noted that it appears as a double spiral that originates -adjacent one side of the outer periphery as at 38 as a doubled-over end that forms a pair of parallel paths as the spiral is wound in ever decreasing coils or turns toward the center of the plate 12. A rather ; lO large opening or clearing 40 is left in the center of the plate minus the film heater so as to form a relatively cool area. Each coil or turn of the spiral film heater is provided with an elongated terminal section 42 which is curved inwardly of the two innermost coils 43 and 45 and arranged in a closely spaced parallel relationship with the other elongated terminal section 42.
An electrical termination is provided this film heater 14 by means of the terminal block 16 of insulating material that is adapted to be mounted directly to the composite plate 12. This mounting means for the terminal block 16 utilizes a stud 44 that is welded directly to the metal bottom layer 30 of the composite plate. This stud 44 is provided with~an elongated head 46 as is best seen in the bottom plan view of Fig. 2.
This head 46 has a pair of spaced vertical walls 48 that extend into the terminal block and prevent relative rotation therebetween. A very efficient electrical ground for the top plate 12 is formed through the -welded stud 44 and the reflector pan 18 so as to eliminate any current leakage problems that might otherwise arise.
The terminal block 16 is of course formed of insulating material and it has a center opening 54 for receiving the stud therethrough. The free end of the stud is threaded at 55 for receiving a fastening nut 56 .
This terminal block 16 also includes a pair of terminal .

, ' ' ~ ' ~ ' ' ! ~

iO48S~1 9D-RG_11807 post 60 for making an electrical connection for the el-ongated terminal sections 42 of the film heater 14 Each terminal post 60 extends through an opening in the terminal ~lock and is fastened therein by virtue of the fact that each post has a head 62 at one end that is seated within the terminal block and is threaded at the other end for receiving the terminal nut 64. A spiral compression spring 66 cooperates with each terminal post 60. One end of this spring 66 is fastened to the head 62 of the post as by silver soldering, while the other end of the spring is fitted with a contact pad 68 that is also fastened to the spring, as by soldering. This provides a resilient spring means for engaging the cold terminal sections 42 of the film heater 14. Notice that a terminal blade 70 of a lead wire 72 is fitted over the terminal post 60 and fastened in place by the terminal nut 64 for making an electrical connection of the heating unit 10 in a power circuit It is well to provide a heat reflector pan 18 beneath the top plate 12 of the heating unit, as it serves to re-flect some of the heat from the film heater 14 upwardly tothe top plate 12 for restricting the heat loss beneath the heating unit. Moreover, this reflector pan serves as a support means for the peripheral edge of the top plate 12 by virtue of the fact that the pan has a vertical peripheral wall 75 with an outer ledge 77 at its top portion for re-ceiving the peripheral edge of the top plate 12 thereon.
Finally, the reflector pan has an outwardly turned flange 79 that serves as a support lip and a trim strip for closing the gap between the heating unit 10 and a cooktop 81 or other supporting surface having an opening in which the heating unit 10 is generally flush mounted. This reflector pan 18 is fitted in place by having a central opening for .
.

~ 9D-RG-11807 1~4~3S81 receiving the welded stud ~4 therethrough. Hence, the fastening nut 56 not only holds the terminal block 16 in place, but it also serves as a mounting means for the reflector pan 18 to the top plate 12. It is necessary to insulate the terminal post 60 from the reflector pan 18 and this is done by forming enlarged holes in the reflector pan for receiving the terminal post 60, and surrounding the terminal post with a bushing-like extension 85 of the terminal block 16 that extends through the enlarged hole in the reflector pan. In order to fasten the terminal block 16 to the reflector pan 18 so the block doesn't shift position, a push nut 86 is driven onto each extension 85. The heating unit 10 is fastened in the opening in the cooktop 81 by a series of spring clips 83 that are fastened to the reflector pan 18 and are adapted to engage under the edge of theopening in the cooktop, as is best seen in Fig. 1.
Having described above my invention of a low thermal mass solid plate surface heating unit, it is well to appreci-ate that sheath type surface heating units have a thermal efficiency between about 70% and 70% under ideal laboratory testing conditions. Probably the actual useful héat that is stored in the food is between 58 and 68%. If warped pans are used, the surface cooking efficiency is probahly about 50% which means that about 50% of the heating energy is wasted. The cause of this low efficiency is poor thermal coupling between the surface unit and the cooking utensil, and the large amount of stored heat in the heating element.
When this low thermal mass solid plate surface heating unitis ~ used in conjunction with a low thermal mass cooking utensil a - 30 thermal efficiency of as high as about 90% may be achieved. The stored heat in a low thermal mass cooking utensil is about 2.5% as compared with about 13% in an aluminum test _ g _ ' - - ' , : . ' .,: . . , . ~ , ' :. ~ : ' .. " . ' . . ' 1~48S81 9D_RG-11807 ; utensil. Therefore, about 87% of the energy is used for actual food cooking, which is almost twiee as much as in conventional surface cooking using sheathed heating elements The performance of surface units depends on their thermal mass and operational temperature. The product of these two paramaeters equals the amount of stored heat, and represents about 75% of all heat losses during the effici-ency tests The sheath type surface units have the highest efficiency because of their relatively low thermal mass For example, a typical 6" diameter heater coil weights about 8.3 ounces and its thermal mass ~specific heat x weight) is as follows:
Helix 0014 BTU/F
MgO .0606 BTU/F
Sheath 0300 BTU~F
Total .0920 BTU~F
However~ sheath type surface units operate at relatively high temperatures between about 800F and 1600F because they have a small contact area with the utensil bottom, and this reduces to some degree their performance.
The heating units of glass-ceramic cooktops also have poor performance; mainly, because of their high thermal mass which is usually two to three times as high as the thermal mass of a sheath type heater. If the present invention of Fig 1 were used in a 6" diameter plate unit, the thermal mass w~uld be slightly lower than the thermal mass of a standard 6" diameter sheath unit. Such a plate weights about 9.5 ounces which is slightly more than a sheath unit, but - the thermal mass is lower because the MgO is not used.
Copper .036 BTU/F
Steel .026 BTU/F
Enamel 007 BTU/F
Total .065 BTU/F

1¢)~8S81 In the preferred embodiment of this invention, the composite plate 12 has a copper core which is about 60%
by weight, and steel top and bottom layers which is about 40% by weight. The film heater 14 is so thin that its thermal mass is negligible. It is felt that the composite plate 12 is an ideal substrate for a film heater. Normally film heaters are applied on a coramic base with a low thermal diffusivity. If a hot ~pot occurs on the film, the ceramic substrate would not conduct heat away and the hot spot would be localized Such a hot spot has self-destructive characteristics and is the main cause of film failures.
The composite plate substrate 12 of the pre~ent in-vention will e~iminate ~uch film failures because of the good thermal conductivity of copper which would make it impossible to generate hot spots on the film. Also due to the same characteristic, the film heater operates at lower average temperatures for the same watts density, and has more even temperature distribution. For instance, on a glass-ceramiQ substrate the film at a power rating of 1250 watts operates at a temperature between 1200F and 1400F
; m e composite metal plate units with similar wattage would operate between 1060F and 1110F at no-load conditions.
Our past experience teaches that there are practically no film failures at such low temperatures However, during performance under load the top plate operates at much lower temperatures. For example~ when water is boiled at 212F
the average plate temperature is about 350 F. This explains the higher efficiency of the unit. Both the thermal mas~
and the operating temperature are low, and it is these par-ameters which contribute to stored heat.
Another advantage which contributes to the high efficiency of the unit of this invention is the large heated .

lQ4~81 9D-RG-11807 surface area. Also there is a very fast response to tem-perature setting giving fast heating up and fast cool down characteristics No problem would exist with cloth ignition hazard because of the low operational temperatures, and there is a good even heat distribution due to the thermal con-ductivity of the composite plate.
Modifications o~ this invention will occur to those skilled in this art, therefore, it is to be understood that this invention is not limited to the particular embodi_ ments disclosed ~ut that it is intended to cover all modifi-cations which are within the true spirit and scope of this invention as claimed . .
.:
. .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A low thermal mass solid plate surface heating unit comprising a thin plate of composite sheet material with a core of high thermal conductivity selected from a group comprising copper, silver and aluminum, and top and bottom layers selected from a group comprising stainless steel, nickel and chromium, and low carbon steel, said top and bottom layers serving to reinforce the core and restrict its oxidation and corrosion, an insulating layer of high electrical resistivity substantially covering the bottom layer of the composite plate, and a film heater of double spiral form bonded to the insulating layer and serving as an electrical resistance heating element, and electrical terminal means cooperating with the composite plate and electrically connected to the film heater and adapted to connect the surface heating unit in a power circuit to provide a low energy unit of highest thermal efficiency, said electrical terminal means being located adjacent the center of the double spiral film heater pattern to create a generally uniform circular heating pattern beneath the composite plate.

2. A low thermal mass solid plate surface heating unit as recited in Claim 1, a heat reflector pan receiving the composite plate across the top thereof, fastening means for joining the reflector pan to the composite plate and cap-turing the said terminal means therebetween.

3. A low thermal mass solid plate surface heating unit as recited in Claim 2 wherein the said electrical terminal means includes resiliently biased contact pads in electrical engagement with the film heater.

4. A low thermal mass solid plate surface heating unit as recited in Claim 1 wherein the double spiral film heater is a doubled back on itself adjacent the periphery of the composite plate, the film heater strip terminating adjacent the center of the composite plate in a separated pair of elongated terminal sections of relatively low re-sistance for unit area compared to the main heated portion of the film heater pattern so that the electrical terminations are made adjacent the center of the composite plate remote from the heated portion of the film heater.

5. A low thermal mass solid plate surface heating unit as recited in Claim 4 wherein the said fastening means comprises a stud means fastened to the bottom layer of the composite plate and engaging the reflector pan for holding these parts together, an insulating terminal block in-cluding at least two fastening bolt means where each fasten-ing bolt is supplied at its inner end with a resiliently biased contact pad in engagement with at least one of the elongated terminal sections of the film heater, while the outer end of each fastening bolt extends outwardly of the reflector pan and has a termination for receiving an el-ectrical power supply lead.

6. A low thermal mass solid plate surface heating unit as recited in Claim 5 wherein the said electrical ter-minal means comprises an insulating terminal block and moun-ting means joined to the composite plate for holding the terminal block in place, terminal post means carried by the terminal block and adapted to make electrical connection between the film heater and electrical power supply means.

7. A low thermal mass solid plate surface heating unit as recited in Claim 6 wherein the composite plate is generally circular in plan view and is provided with a slight crown.

8. A low thermal mass solid plate surface heating unit as recited in Claim 7 wherein each terminal post means includes a resiliently biased contact pad in electrical contact with a terminal section of the film heater strip.

9. A low thermal mass solid plate surface heating unit as recited in Claim 1 wherein there is a temperature limiting means for the composite plate that is set for a maximum temperature between about 425°F and 575°F.

10. A low thermal mass solid plate surface heating unit as recited in Claim 9 wherein there is a temperature limiting means for the composite plate that is set for a maximum temperature between about 425°F and 575°F.