Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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
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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-
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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
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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
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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
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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
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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
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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
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