Abstrict
A novel radiant electric heater construction is provided employing
fused silica particulates as the thermal and electrical insulation
medium in said device. Said insulation means is prepared in a particular
manner providing a porous characteristic when employed together
with operatively associated metal heat shield means in the heater
device.
Claims
We claim as new and desire to secure by Letters Patent of the United
States is:
1. A radiant electric heater comprising a metal dish containing
a base layer of thermal and electric insulation material, a metal
heat shield interposed between said insulation material and said
metal dish and a resistance heating element disposed on top of said
insulation material, said insulation material comprising a deposited
solid foam mass of non-porous fused silica particulates having an
average particle size up to about minus 100 mesh United States screen
size and with the porous characteristic being formed by void spaces
between the deposited fused silica particulates.
2. The radiant heater of claim 1 wherein the base layer of insulation
layer is formed with two layers of the fused silica particulates
physically separated with an additional metal shield.
3. The radiant heater of claim 1 wherein the metal heat shield
is physically embedded in the bottom surface of said insulation
material.
4. The radiant heater of claim 1 wherein the metal heat shield
comprises aluminum foil.
5. The radiant heater of claim 1 wherein the insulation material
further contains titanium dioxide particulates.
6. The radiant heater of claim 1 wherein the insulation material
further contains fiberglass fibers.
7. The radiant heater of claim 1 wherein the insulation material
further contains titanium dioxide particulates and fiberglass fibers.
8. The radiant heater of claim 1 wherein the insulation material
is obtained upon drying a foamed liquid slurry of the fused silica
particulates.
9. The radiant heater of claim 8 wherein the foamed liquid slurry
is heat dried to an elevated temperature.
10. The radiant heater of claim 8 wherein the liquid slurry further
contains a foaming agent.
11. The radiant heater of claim 10 wherein the foaming agent is
an organic detergent.
Description BACKGROUND OF THE INVENTION
This invention relates to a novel radiant electric heater construction
especially adapted for use with top surface cooking appliances and
more particularly to a combination of novel component parts in said
type devices which can effectively serve to replace the customary
construction.
Radiant electric heaters intended for top surface cooking appliances
now employ "microporous" type thermal and electric insulation
which can be molded or otherwise formed to provide the base component
in said device when generally supported in a metal dish. Descriptions
of said customary radiant electric heater construction can be found
in U.S. Pat. No. 5,471,737, U.S. Pat. No. 5,512,731 and U.S. Pat.
No. 5,517,002 which all include a requirement for said type insulation
material in order to avoid overheating during device operation.
A still further description for preparation of a satisfactory microporous
insulation of this type can be found in U.S. Pat. No. 5,556,689
which includes a requirement for "a very high specific surface
area" of the finely divided metal oxides being employed in
order to achieve satisfactory thermal insulation.
The above described thermal insulation remains relatively expensive
due to costs of preparation including needed raw materials, processing
requirements and still other factors limiting availability. Thus,
a preparation of suitable microporous insulation according to the
foregoing references can involve elaborate processing techniques
such as gel formation, controlled precipitation and fume deposition
in order to control average particle size less than 100 nanometers.
It would be desirable, therefore, to replace conventional microporous
insulation now being used in radiant electric heaters by means which
are far less costly as well as far more readily available. In doing
so, it has now been discovered that fused silica of the type conventionally
produced in a rotary electric arc furnace (such as disclosed in
U.S. Pat. No. 4,217,462) and thereafter mechanically reduced by
conventional means to produce a suitable particle size provides
a satisfactory means of thermal and electric insulation for this
product application. It remains only required with said substituted
fused silica insulation to further include metal heat shield means
in a manner to be disclosed hereinafter for a satisfactory retention
of heat within the final device during its operation.
It is an important object of the present invention, therefore,
to provide a radiant electric heater employing a novel combination
of component parts.
It is another object of the present invention to provide a novel
radiant electric heater employing fused silica particulates as the
thermal and electrical insulation means in combination with operatively
associated heat shield means.
Still another object of the present invention is to provide a novel
preparation means for the particular fused silica material being
employed in the disclosed heater device.
These and still further objects of the present invention will become
more apparent upon considering the following detailed description
of the present invention.
SUMMARY OF THE INVENTION
Fused silica particulates have now been found to provide effective
thermal and electrical insulation means when disposed as a base
component in the radiant electric heater device. More particularly,
said novel insulation means can be deposited after preparation in
various ways to be more fully described hereinafter into the conventional
metal dish forming the bottom member in the present device. A metal
heat shield is interposed between the insulation and the metal dish
member for the purpose of reflecting heat energy back into the insulation
and thereby help prevent heat escape from the device. Aluminum foil
can provide a satisfactory heat shield material in the present device.
A conventional resistance heating element such as disclosed in the
previously cited prior art references is disposed atop the present
insulation material to complete a representative heater device of
the present invention.
The novel insulation means in the present device is produced in
a particular manner providing a foam type relatively open lattice
in the disposed fused silica particulates. The desired porous character
of said insulation can be achieved by drying a liquid slurry of
finely divided fused silica having an average particle size up to
about minus 100 mesh United States screen size which further contains
a suitable foaming agent such as an organic detergent, a conventional
phosphate soap or the like. The slurry is first agitated sufficiently
to form a frothy condition which can be produced with conventional
high speed rotary blender means. A commercially available colloidal
silica binder (Bindzel 30/A360) now being marketed by Akzo Nobel,
Atlanta, Ga. is also included in the present liquid slurry containing
the finely divided fused silica particulates for preparation of
a stable porous structure in the dried composition. Drying of the
agitated liquid slurry can be carried out in the customary manner
with heating of the slurry to elevated temperatures up to 1000.degree.
F. having been found helpful in maintaining the desired degree of
porosity in the final solid insulation. Other additives can also
be incorporated in the liquid slurry to include opacifiers such
as titanium dioxide particulates as well as chopped fiberglass fibers
which can also prove helpful in maintaining physical integrity of
the solid insulation. Freezing of the agitated slurry in a suitable
cooling medium such as liquid nitrogen before the drying procedure
takes place has also been found to improve heat retention in the
present insulation with measured temperatures at the bottom surface
of the present heater device not exceeding 350.degree. F. during
a half hour heating period.
In a different preferred embodiment, multiple dried cake layers
of insulation produced as above described are physically stacked
within the metal dish of the present heater device with additional
metal heat shield components being placed between adjoining layers
of insulation. By such means, heat retention can be modified in
the present heater device to accommodate operating conditions of
a particular cooking appliance without having to alter other components
in the heater device itself. Accordingly, a specific heater device
of the present invention having such multiple insulation layer configuration
can comprise a metal dish member containing a lower insulation pad
approximately 0.3 inch thick and an upper insulation pad approximately
0.4 inch thick with a first aluminum foil heat shield being placed
therebetween, a second aluminum foil heat shield being interposed
between the bottom of the lower insulation pad and the top surface
of the metal dish, and a conventional resistance heating element
being disposed on the top surface of the upper insulation pad. To
facilitate assembly of the final heater device having either a single
insulation pad or a multiple insulation pad configuration, the particular
heat shield component being selected can be physically embedded
in the bottom surface of the employed insulation pad or pads prior
to said final assembly. Additionally, lower bottom operating temperatures
have been measured in the present heater device when a multiple
insulation layer configuration is employed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view depicting a representative radiant
electrical heater device of the present invention.
FIG. 2 is a graph illustrating heat retention in radiant heater
devices of the present invention employing both single and dual
insulation layer configurations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, FIG. 1 depicts a representative radiant
electric heater of the present invention in cross-section which
can have a circular or rectangular shape when viewed from the top
direction. Said radiant heater 10 includes a bottom metal dish member
12, a base layer pad 14 of porous fused silica particulates according
to the present invention being deposited within said metal dish
member, a metal heat shield 16 being placed between the bottom surface
of said insulation pad and the top surface of the metal dish, and
a conventional resistance heating element 18 being disposed on the
top surface of said insulation pad. Preparation of the insulation
pad in said embodiment commenced with forming an aqueous slurry
containing 1380 grams of the fused silica particulates having an
average particle size of minus 100 mesh United States screen size
combined with 1060 grams of the aforementioned colloidal silica
binder and 20 grams of an organic detergent serving as the foaming
agent. The slurry was next agitated in a conventional high speed
rotary blender until foamed then frozen with liquid nitrogen to
produce pads having varying thickness ranging from 0.3 inch up to
0.7 inch in thickness. The frozen pads were next placed in a conventional
oven for drying up to 1000.degree. F. during a one hour drying time.
Various metals were employed as the heat shield component in the
illustrated heater embodiment to include stainless steel and aluminum
foil. Heating tests were conducted upon six inch diameter and eight
inch diameter circular heaters constructed in the foregoing manner
at power levels ranging from 785 watts being applied to the six
inch diameter units and 1396 watts being applied to the eight inch
diameter units.
The graph in FIG. 2 illustrates performance characteristics for
the present heater device when constructed with a single insulation
pad or dual insulation pads as hereinabove described. Curves 20
and 22 represent K type thermocouple measurements of temperature
at the bottom surface of the dish member while the heater units
were being operated at the above listed power levels. The temperatures
were measured at one minute intervals during a thirty minute heating
period for each of the different heating units. Curve 20 reports
the bottom temperature for a single insulation pad device with a
0.7 pad thickness during said test period. The comparable measurements
made upon the dual insulation pad construction having a 0.3 inch
thick bottom pad and a 0.4 inch thick top pad are shown in curve
22. These results clearly indicate the suitability of the present
radiant electric heater construction for general use in stove top
cooking appliances.
It will be apparent from the foregoing description that novel means
have been provided to construct a radiant electric heater having
versatile end product applications. It will be apparent, however,
that various structural modifications can be made in the present
heater device without departing from the spirit and scope of the
present invention. For example, still other additives than herein
specifically disclosed can be incorporated into the disclosed insulation
material without producing a deleterious effect such as colorants
and the like. Likewise, it is contemplated that minor amounts of
conventional additives to improve physical handling of the present
insulation during processing or assembly of the heater unit can
be employed. Consequently, it is intended to cover all modifications
of the disclosed radiant electric device which may be devised by
persons skilled in the art as falling within the true spirit and
scope of the present invention. |