U.S. patent number 3,622,754 [Application Number 05/058,110] was granted by the patent office on 1971-11-23 for glass plate surface heating unit with even temperature distribution.
This patent grant is currently assigned to General Electric Company. Invention is credited to Bohdan Hurko.
United States Patent |
3,622,754 |
Hurko |
November 23, 1971 |
GLASS PLATE SURFACE HEATING UNIT WITH EVEN TEMPERATURE
DISTRIBUTION
Abstract
A glass-ceramic plate surface heating unit having a heat
spreader plate of high thermal conductivity bearing against the
underside of the glass to create an even temperature distribution.
Means are provided to hold the spreader plate firmly against the
glass plate. A metal-sheathed heating element of high watts density
is attached to the underside of the heat spreader plate.
Reinforcing means are present beneath the spreader plate to prevent
warpage. A temperature-limiting means is combined with the heat
spreader plate to limit the maximum temperature to which the glass
ceramic plate is exposed. The heat spreader plate is grounded to
provide a safety precaution since the glass ceramic plate becomes
more electrically conductive at higher temperatures.
Inventors: |
Hurko; Bohdan (Louisville,
KY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
22014745 |
Appl.
No.: |
05/058,110 |
Filed: |
July 24, 1970 |
Current U.S.
Class: |
219/448.17;
219/465.1; 126/400; 165/185; 219/530 |
Current CPC
Class: |
F24C
15/105 (20130101); H05B 3/748 (20130101); F24C
15/102 (20130101); H05B 3/746 (20130101); H05B
2213/04 (20130101); H05B 2213/07 (20130101) |
Current International
Class: |
F24C
15/10 (20060101); H05B 3/68 (20060101); H05B
3/74 (20060101); H05b 003/68 () |
Field of
Search: |
;219/449-450,461-462,463-464,530,540,430,437-438 ;165/185 ;99/447
;126/400 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mayewsky; Volodymyr Y.
Claims
What is claimed as new and is desired to be secured by Letters
Patent of the United States is:
1. A surface-heating unit comprising a plate of glass ceramic
material supported on a plate of composite metal sheet material
with a center core selected from a group of high thermal
conductivity metals and alloys such as copper, silver and aluminum,
and an outer skin selected from the group of oxidation- and
corrosion-resisting metals and alloys such as stainless steel,
nickel and chromium, an insulated electrical resistance heating
element attached to the underside of the composite plate, a
reinforcing member of open framework attached to the underside of
the composite plate to prevent warping thereof, a reflector pan
supporting the assembly, a releasable trim ring surrounding the
assembly and adapted to mount the heating unit in an apertured
support panel, and a temperature-limiting means sensing the
temperature of the composite plate and in circuit connection with
the heating element to deenergize the heating element at composite
plate temperatures somewhere above about 1,000.degree. F.
2. A surface-heating unit as recited in claim 1 wherein the
temperature-limiting means includes a temperature sensor in close
thermal contact with the underside of the composite plate and a
temperature responder that is supported from the said reflector
pan, and connecting means joining the temperature sensor to the
temperature responder.
3. A solid plate, surface-heating unit comprising a top plate of
glass ceramic material supported on a plate of composite metal
sheet material with a center core selected from a group of high
thermal conductivity metals and alloys such as copper, silver and
aluminum, and an outer skin selected from the group of oxidation-
and corrosion-resisting metals and alloys such as stainless steel,
nickel and chromium, an insulated electrical resistance heating
element of looped configuration attached to the underside of the
composite plate, reinforcing means braced beneath the composite
plate to prevent warping thereof, and fastening means for holding
the top glass ceramic plate and the composite metal plate firmly
together, and a temperature-limiting switch means carried by the
unit and having a temperature-sensing probe held in thermal
relation with the composite plate, said temperature-limiting switch
means being adapted to be connected in a power circuit for the
heating element to ensure that the glass ceramic plate does not
become overheated, whereby the heating element may be of higher
wattage than a heating unit without the temperature-limiting switch
so as to reduce the preheat time it takes for the glass ceramic
plate to reach a predetermined temperature.
4. A solid plate, surface-heating unit as recited in claim 3
wherein the composite metal plate distributes the heat from the
heating element evenly across the top glass ceramic plate, while in
the event of the presence of a warped cooking utensil on the unit,
the heat will flow from the hot spots on the top plate to the cold
spots by means of the composite metal plate.
5. A plate-type, surface-heating unit comprising a thin plate of
glass ceramic material, a thin flat heat spreader plate bearing
against the underside of the glass ceramic plate, said heat
spreader plate being of composite metal sheet material with a
center core selected from a group of high thermal conductivity
metals and alloys such as copper, silver and aluminum, and an outer
skin selected from the group of oxidation- and corrosion-resisting
metals and alloys such as stainless steel, nickel and chromium, a
metal-sheathed resistance heating element of looped configuration
attached to the underside of the heat spreader plate, reinforcing
means attached to the underside of the heat spreader plate to
prevent warpage at elevated temperatures, and temperature-limiting
means for controlling the maximum temperature of the glass ceramic
plate, said temperature-limiting means comprising a temperature
sensor associated with the heating element, and a single-point
temperature responder connected to the sensor, and circuit means
adapted to join the responder to the heating element such that when
the heat spreader plate rises to a predetermined temperature the
responder will open the power circuit to the heating element.
6. A glass ceramic surface unit as recited in claim 5 with
electrical grounding means joined to the heat spreader plate.
7. A glass ceramic surface unit as recited in claim 5 with a high
emissivity coating such as porcelain enamel on the top surface of
the said heat spreader plate.
8. A plate-type, surface-heating unit as recited in claim 5 wherein
the heating element is of a higher watts density for the diameter
of the heating unit than prior heating units that are not protected
with temperature-limiting means so as to obtain high heating
rates.
9. A plate-type, surface-heating unit as recited in claim 8 wherein
the said heat spreader plate has a high coefficient of thermal
conductivity and acts as a temperature equalizer for the glass
ceramic plate when the heat in the spreader plate flows from the
areas of high temperatures to the areas of lower temperatures
thereby obtaining substantially uniform temperature distribution
across the heated area of the glass ceramic plate.
10. A glass ceramic cook top having at least one heated area, each
heated area having a heat spreader plate positioned therebeneath
and in contact therewith, said heat spreader plate being of
composite metal sheet material with a center core selected from a
group of high thermal conductivity metals and alloys such as
copper, silver and aluminum, and an outer skin selected from the
group of oxidation- and corrosion-resisting metals and alloys such
as stainless steel, nickel and chromium, a metal-sheathed
resistance heating element attached to the underside of the heat
spreader plate, reinforcing means braced beneath the heat spreader
plate to prevent warping thereof, and temperature-limiting means
for restricting the maximum temperature of the glass ceramic plate,
said temperature-limiting means comprising a temperature sensor,
the heat spreader plate and a temperature responder associated with
the temperature sensor, said temperature responder being adapted to
be connected in a power circuit for the said heating element and
capable of deenergizing the heating element when the temperature of
the spreader plate is above a predetermined temperature, said
heating element being a high wattage unit for the size of the
spreader plate to obtain high heating rates, said heat spreader
plate also serving as a heat diffuser beneath the heating element
such that when the heating element is deenergized the spreader
plate serves to reduce the cooldown time.
Description
BACKGROUND OF THE INVENTION
The average housewife is becoming more interested in having
appliances of more pleasing appearance designs around her home, as
well as enjoying the improved ease of cleaning household kitchen
appliances. In the matter of cleaning the oven of the range, there
has been widespread acceptance of the pyrolytic, self-cleaning oven
such as is taught in U.S. Pat. no. 3,121,158 of the present
inventor. The solution of the problem of maintaining a clean oven
has turned the attention of the housewife to the difficulty of
keeping the cook top clean. A standard electric cook top is usually
provided with a plurality of metal-sheathed electrical resistance
heating elements which are each wound in the form of a spiral coil
and positioned in an opening formed in the cook top. Each heating
element is adapted to support a cooking utensil thereon. These
metal-sheathed heating elements are cleaned automatically of food
soil due to the high temperatures they are allowed to reach once
they are energized, but it is possible for spillovers to drain
through the heating element and accumulate in a collecting pan
located beneath the cook top from which they must be cleaned.
In order to reduce this cleaning problem, entire cook tops or in
some cases individual solid plate surface units have been proposed
in which the exposed surface is formed of glass ceramic plate. In
particular, a generally milk white, opaque, glass ceramic or
crystalline glass material has been suggested for this use.
Examples of such material are the types of crystalline glass sold
under such trademarks as "PYROCERAM," "CER-VIT," and "HERCUVIT."
The opaque crystalline glass, because of its smooth top surface of
almost ground glass appearance, not only presents a pleasing
appearance, but it is also readily cleanable and it does not permit
the drainage of spillovers therebeneath. There has developed a
rather serious problem in obtaining satisfactory heating rates as
compared with those available from the traditional, exposed,
metal-sheathed electrical resistance heating elements, or gas
surface burners. One difficulty encountered is the rather poor
quality of thermal conductivity through the glass ceramic material.
Such a material is used widely as a thermal- and
Electrical-insulating material rather than as in the present case
as a thermal conductor. Heat does not readily diffuse laterally
through the glass plate, and during a cooking operation heat will
flow to the utensil only near the points of contact. The rest of
the heated area will become very hot. Moreover, the glass ceramic
plate has a rather large heat capacity, such that when the power is
cut off to the heating element it takes a relatively long time to
cool the glass ceramic plate down to room temperature. Also, this
type of glass plate becomes increasingly more electrically
conductive at the higher temperatures so that this might create a
safety hazard in the event an open-coiled heater were employed as
the electrical heating means.
The principal object of the present invention is to provide a
high-speed, solid plate, surface heating unit with a glass ceramic
appearance plate that has an even temperature distribution by use
of a heat spreader plate and is still capable of efficient
operation when used with cooking utensils having irregular bottoms
supported on the surface unit.
A further object of the present invention is to provide a glass
surface heating plate of the class described with an insulated
electrical resistance heating element of high watts density in
combination with a temperture-limiting means so as to be able to
obtain fast heatup rates without overheating the glass surface
heating plate by controlling its maximum operating temperature.
A further object of the present invention is to provide a glass
surface heating plate that is supported on a heat spreader plate of
high thermal conductivity so as to create an even temperature
distribution, there being releasable spring means for holding the
two parts together.
A further object of the present invention is to provide a glass
plate surface heating unit with a grounding means to prevent
electrical current leakage.
A still further object of the present invention is to provide a
glass plate surface heating unit of the class described with an
even temperature distribution that permits a metal-sheathed heating
element of high watts density to operate at relatively lower sheath
temperatures due to the efficient thermal coupling between the
heating element and the glass plate.
SUMMARY OF THE INVENTION
The present invention, in accordance with one form thereof, relates
to a solid plate surface heating unit comprising a glass ceramic
plate supported on a heat spreader plate of high thermal
conductivity. A metal-sheathed electrical resistance heating
element is attached to the underside of the spreader plate. Means
are provided for limiting the maximum temperature to which the
glass ceramic plate is exposed. This temperature-limiting means
comprises a temperature sensor attached to the spreader plate and
communicating with temperature responder that is connected in a
power circuit for the heating element. Then when the heat spreader
plate rises to a predetermined temperature the responder will open
the power circuit of the heating element. This permits the use of a
heating element of high watts density which affords a fast heating
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will be better understood from the following
description taken in conjunction with the accompanying drawings and
its scope will be pointed out in the appended claims.
FIG. 1 is a fragmentary cross-sectional elevational view through
about one-half of a solid glass plate surface heating unit
embodying the present invention showing a temperature responder of
the temperature-limiting means suspended from the lower left side
of the heating unit.
FIG. 2 is a diagrammatic showing on a smaller scale of a glass
plate surface heating unit embodying the present invention and
showing the power circuit for the heating element of the surface
unit provided with a temperature-limiting means to control the
maximum operating temperature of the glass plate.
FIG. 3 is a fragmentary isometric view on a slightly enlarged scale
showing the side of the glass surface heating unit of FIG. 1, and
illustrating the manner of supporting the heating unit in
conjunction with a recessed flange that circumscribes an opening in
a cook top, as well as a trim ring and holddown means for clamping
the various elements of the surface unit together and closing and
sealing the edge of the heating unit from the passage of liquid
therebeneath.
FIG. 4 is a performance chart or temperature graph of the heating
coil for a standard metal-sheathed resistance heating unit held in
contact with a solid glass plate plotting temperature versus time
and showing a set of temperature curves for boiling a quart of
water; the first being for a flat pan, the second being for a
no-load condition and the third being for a warped pan.
FIG. 5 is another performance chart or graph of a glass plate
surface heating unit of the present invention for comparison with
the performance of a standard metal-sheathed resistance heating
element in FIG. 4, the first curve being for a flat pan, the second
being for a warped pan and the third being for a no-load
condition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to a consideration of the drawings and in particular to
FIG. 1, there is shown a cross-sectional elevational view of little
more than one-half of a solid plate surface heating unit 10 having
a top glass ceramic plate 12. This glass ceramic plate is
electrically insulating and thermally transmissive as well as being
highly wear and thermal shock resistant and resistant to the
physical and chemical attacks of foods and liquids which may come
into contact with the plate. While the term glass ceramic material
or crystalline glass material is used throughout, it should be
understood that this invention encompasses other materials with
similar characteristics such as quartz, high-silica glass,
high-temperature glasses and different ceramic materials. It would
be extremely difficult to maintain an even temperature distribution
across this glass plate 12 if it were heated directly by an open
coil heater or a metal-sheathed resistance heating element of
looped configuration. Heat diffuses very slowly laterally through
the glass plate, and hence hot spots would be created on the glass
surface nearest the areas of contact between the heater and the
glass. This type of glass cannot exceed an operational temperature
of about 1,300.degree. F. at any point, hence the total heat output
of a glass ceramic surface heating unit would be reduced if the
glass plate is provided with an uneven temperature distribution. In
the absence of a temperature-limiting means, the glass plate would
have to be underheated in order to avoid damaging the glass
plate.
The present invention contemplates the introduction of a heat
spreader plate 14 beneath the glass ceramic plate 12. This heat
spreader plate is of high thermal conductivity, preferably of thin
composite metal sheet material with a thin center core 16 for
distributing the heat rapidly over the entire plate so as to obtain
a generally uniform temperature distribution. Such a core would be
selected from metals and alloys such as copper, silver and
aluminum. Copper has very low strength at temperatures ranging in
the vicinity of 1,300.degree. F., and also it oxidizes very
readily. Since a copper core sheet 16 is of small thickness, on the
order of 0.040 inches, it would tend to warp or deform easily under
normal use conditions due to thermal stresses caused by temporary
uneven temperature distribution during the preheat period and also
due to the high temperatures to which it is exposed. Hence, the
core 16 is sandwiched or sealed between two thin, integral skins 17
and 18, each of the thickness of about 0.016 inches. Such skins
would be selected from metals and alloys such as stainless steel,
nickel and chromium. In any selection of materials it should be
borne in mind that the core and skin materials should have matched
coefficients of thermal expansion. In order to avoid exposure of
the copper on the peripheral edge of the plate, the two stainless
steel skins 17 and 18 will be sealed over the edge of the core with
a pinching action to protect against corrosion and oxidation. The
stainless steel skins 17 and 18 being on the outer surface of the
composite plate 14 provide strength to the plate and resist warpage
because it combines a high strength with high heat diffusivity,
which no single material plate can provide. This thin composite
sheet material 14 is of a central copper core 16 and two outer
stainless steel skins 17 and 18, and it may be formed of individual
sheets which are "area welded" as by a process of explosive
welding, which causes a bonding of the metal sheets along their
mating surfaces. See my copending U.S. Pat. No 3,569,672, which was
issued on Mar. 9, 1971.
A thin ceramic coating of porcelain enamel, silica or the like
surrounding the composite heat spreader plate 14 will further
improve the performance of the surface unit. Such a coating will
prevent a metallic contact between the composite plate 14 and the
glass plate 12, thus reducing the possibility of etching or the
creation of weak spots in the glass. Ceramic coatings have a high
emissivity as compared with a metallic surface. If the composite
plate does not make a good contact with the glass plate, the
ceramic coated plate will radiate heat towards the glass plate. The
ceramic coating on the bottom of the composite plate 14 will cause
the plate 14 to cool down faster for the same reason.
A metal-sheathed resistance heating element 20 shown of three-loop
configuration is brazed to the underside of the heat spreader plate
14. As is well understood by those skilled in this art, such a
metal-sheathed heating element 20 would include a central
electrical resistance, nichrome heating wire 21 of helical
formation that is inserted into a thin metal tube or sheath 23 of
inconel, stainless steel or the like. Then the sheath is filled
with a suitable electrical insulating and thermally conductive
material such as magnesium oxide (MgO) or the like to separate the
heater wire 21 from the metal sheath 23. The top surface of the
heating element 20 is flattened so as to obtain a good area contact
of the metal sheath 23 with the heat spreader plate 14. Moreover,
the heat spreader plate 14 serves in the manner similar to cooling
fins and keeps the heating element 20 at much lower operating
temperatures thus increasing the lifespan of the heating element.
Also, because of the high heat diffusivity of the heat spreader
plate 14, a shorter heating element 20 may be used. In other words,
standard heating elements of the same wattage would have more turns
or loops that is necessary in the practice of the present
invention. One of the two terminals 22 of the heating element is
shown extending down in a vertical direction beneath the heating
element 20. This terminal 22 is provided with a cold terminal end
having a spade connector 24 for receiving a slip-on connector (not
shown) for making an electrical connection therewith as is
conventional in this art.
In order to strengthen the heat spreader plate 14, the edge of the
plate is provided with a downturned flange 26 thereby giving the
heat spreader plate a configuration similar to an inverted shallow
pan. Another means of reinforcing the heat spreader plate 14 is to
provide a series of diagonal or radial struts 28 which are arranged
edgewise and fastened to the underside of the heat spreader plate
and possibly to the sheath of the heating element 20 as by brazing
or similar methods. Such strut members 28 may be of many different
configurations as there is no way to design them. The theory is to
give the heat spreader plate sufficient depth or beam action so
that it does not deflect readily under thermal or mechanical
stresses.
A reflector pan 30 of rather deep drawn configuration is positioned
beneath the heating unit 10. Looking at FIG. 1 it will be seen that
the plate-type surface heating unit 10 is positioned in a circular
opening 32 formed in a horizontal cook top 34. The edge of the cook
top which defines the opening 32 for the surface unit 10 is formed
with a recessed ledge 36. It is well to provide some means for
holding the surface unit 10 down in place, and this function is
provided by a trim ring 38 which has a transverse T-shaped cross
section with a first vertical shank portion 40 which is insertable
into the gap between the periphery of the heating unit 10 and the
vertical side 42 of the recessed ledge 36. The upper edge of the
shank 40 is provided with a folded-over crown 44 which overlies
both the edge of the cook top 34 and the edge of the glass ceramic
plate 12.
Releasable means must be provided for holding the trim ring 38 in
place. For this purpose a series of widely spaced clip members 46
are attached to the shank portion 40 of the trim ring 38 at widely
spaced positions around the trim ring. Each clip member 46 is of
thin spring material of narrow width, and at its upper end it is
provided with a pair of offset fingers 48 which are adapted to
extend through mating slots 50 formed in the shank portion of the
trim ring 38. The only way to insert the fingers 48 through the
slots 50 is to remove the heating unit 10 from the cook top and
insert the fingers at a generally perpendicular angle with respect
to the shank portion 40 of the trim ring, and then pivot the clip
member down against the shank portion 40 as is seen in both FIGS. 1
and 3. These fingers become captured in place due to the small
clearance between the shank portion 40 and the vertical side 42 of
the cook top edge. An asbestos gasket 51 is captured beneath the
crown 44 as a moisture barrier. Each clip member 46 is generally of
Z-shape in side view having a generally vertical upper flange 52, a
generally horizontal midportion 54 and a wide V-shaped lower
portion 56. This V-portion 56 has an apex 58 that is directed
generally toward the edge of the ledge 36 of the cook top 34 to
serve as a detent member such that when the surface unit 10 is
lowered onto the recessed ledge 36 of the cook top 34 the detent 58
tends to engage the innermost edge of the ledge 36 until additional
force causes the detent to spring away from the ledge and then snap
back beneath the ledge to serve as a tight holddown means. The
horizontal midportion 54 presses the reflector pan 30 and the heat
spreader plate 14 against the underside of the glass ceramic plate
12.
It is important to limit the operating temperature of the glass
ceramic plate 12 to a temperature below about 1,300.degree. F. This
can best be done by introducing a temperature-limiting limiting
means to the solid plate surface unit of the present invention such
that the power to the heating element 20 is cut off when the heat
spreader plate temperature reaches a predetermined limit. This
temperature-limiting means comprises a temperature sensor or
elongated bulb 62 which is brazed or otherwise attached to the
underside of the heat spreader plate 14 adjacent a turn or loop of
the heating element 20, as is best seen in FIG. 1. This sensor 62
would be filled with a high-temperature thermostatic fluid such as
sodium potassium (NaK) or the like which would communicate with a
temperature responder 64 by means of a capillary tube 66 that is
shown diagrammatically as a long dash line. This temperature
responder 64 is a single-point, temperature-limiting switch or
thermostat set at a critical temperature of about 1,250.degree. F.
This temperature responder is electrically connected in a power
circuit for the heating element 20 as is shown diagrammatically in
FIG. 2. The heating element 20 is shown simply as an electrical
resistance load connected in a power circuit across lines L1 and L2
by leads 68 and 70. Connected across the leads is a manual selector
switch 72 for controlling the power to the heating element. An
alternative would be to substitute for this switch 72 an infinite
heat switch that would govern the level of power to the heating
element. Notice the grounding circuit 73 in FIG. 2 connected to the
heat spreader 14. This is important because the glass ceramic plate
12 becomes conductive toward the upper end of the temperature range
and this creates a safety hazard to the user. The grounding of the
heat spreader plate 14 eliminates this safety hazard.
During the development of this solid plate surface heating unit 10
using the heat spreader plate 14 it was discovered that the surface
unit had a unique property due to the high heat diffusivity of the
heat spreader plate. The surface unit is able to operate
satisfactorily at considerably lower temperatures, and reference is
made here to the relative performance charts of FIGS. 4 and 5. FIG.
4 shows temperatures of a standard 900 watt spiral-coiled,
metal-sheathed resistance heating unit in contact with a glass
plate, while FIG. 5 relates to the heat spreader plate and glass
surface heating unit of 1,100-watts rating of the present
invention. Because in both cases the heaters are in contact with
glass ceramic plates, the glass at these contact points will
approach these temperatures after a prolonged operation time.
Looking at FIG. 4, the sheath temperature under "no-load"
conditions might run to above 1,550.degree. F. for the spiral unit,
while in the glass ceramic plate unit of FIG. 5 the maximum
"no-load" temperature condition would be about 1,300.degree. F. In
the event a quart of water is to be boiled in a pan with a warped
bottom on the heating unit of FIG. 4, the sheath temperature of the
spiral heating element would be about 1,700.degree. F., while in
the glass ceramic plate unit of FIG. 5 and of the present invention
the temperature of the heat spreader plate would be about
1,180.degree. F. Finally, if a quart of water were to be boiled in
a pan with a flat bottom, the maximum temperature of the spiral
unit would be about 1,290.degree. F. while in the plate unit of the
present invention the temperature of the spreader plate would be
about 850.degree. F. Also, notice that the times for boiling the
water are shorter in FIG. 5 than they are in FIG. 4, because a
higher wattage is used and the system of FIG. 5 is more efficient.
In other words, there is a much better thermal coupling between the
cooking utensil and the heat source of the present invention than
there would be between a glass ceramic unit if a spiral-coiled
heating element were used without the heat spreader plate. This
phenomenon permits the design of a low-temperature, high-speed
surface unit. For example in a 6-inch diameter glass plate unit
without a temperature-limiting means the wattage of the heating
element would be limited to approximately 800 watts in order not to
overheat the glass plate. On the contrary, with the incorporation
of the temperature-limiting means a 1,600 watt heater may be used
successfully. Such high wattage would provide much shorter time to
preheat the plate 12. The curves shown in FIG. 5 would have a much
steeper slope, than for a 1,100-watt unit as tested.
Having described above an important contribution in the art of
solid plate, surface-heatng units, it will readily be apparent to
those skilled in this art that the present glass ceramic plate
would be capable of operating with an even temperature distribution
and with more total heat output, and with a fast heating rate. The
glass plate would provide a better performance with a warped pan
than glass plate units of the prior art. The glass plate units of
the prior art would be subject to hot spots when used with a warped
pan. The heat tends to flow from the glass plate to the pan through
the points of contact and colder spots are created at these points.
However, the warped bottom acts as a reflector of heat turning the
heat back toward the glass plate and creating hotter areas. The
heat spreader plate functions as a temperature equalizer causing
heat to flow from the hotter areas to the colder spots. The use of
the temperature-limiting means permits the use of higher wattage
heaters which reduces the preheat time considerably, as well as the
time to boil a quart of water which is considered as a standard
heating load for testing purposes. The glass plate is supported by
the heat spreader plate and does not require an insulation pad for
support, therefore, the heat is free to flow downward. When the
power is cut off, the heat spreader plate 14 and the reinforcing
struts 28 act as cooling fins and heat radiates downward from the
glass plate. This cooling-down action would be even more effective,
if a forced cooling system were used.
The improved cooling-down performance causes a slight drop of the
surface unit efficiency because there will be higher heat losses.
However, this drop in efficiency is not very significant, because
of the nature of glass plate heaters. Glass operates at maximum
temperatures that are much lower than the sheath temperature of
metal-sheathed heating elements, and the heat losses are an
exponential function of temperature. Radiant heat losses are
directly proportional to T.sup.4. With a temperature-limiting
means, slightly higher heat losses do not have any effect on the
speed of the unit or time to boil because the heat transfer to the
utensil is a function of the glass plate temperature, and this
temperature will be maintained in the present invention by
supplying more power. It is true however that the power will be
applied longer during the cycling period.
This glass plate unit is adaptable to an automatic surface unit
because the even temperature distribution allows the plate to be
sensed effectively at any point. As an alternative, the glass plate
may be fitted with a central hole for insertion of a spring-biased
sensing element that is in direct contact with the bottom of a
utensil.
The heat spreader plate provides a solid support for the glass
plate so a thinner glass plate with a lower thermal mass is used.
This surface unit is adaptable to thin cook top units as the
surface unit requires no more than 1 1/2 inches of cook top depth.
A modification of this invention would be the substitution of a gas
burner means beneath the heat spreader plate 14 in place of
metal-sheathed electrical resistance heating element 20. However,
it is felt that this invention may have more importance when
incorporated with an electrical heater because the heat is so
concentrated in an electrical heater. The flame from a gas burner
means may be enlarged or reduced depending on the need.
Modifications of 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 embodiments disclosed, but that it is
intended to cover all modifications which are within the true
spirit and scope of this invention as claimed.
* * * * *