Royalton natural air movement system

A holding oven designed for creating a natural convection current in the chamber of the oven, and constructed by fixing the heater element to a first material characterized by its high thermal conductivity for rapid build-up of thermal gradient into an insulated space, while that heater element is simultaneously in contact with a second material which is characterized by its high thermal capacitance for slowing thermal discharge upon repeated opening and closing of oven doors. The first material forms a heat sink and the second material forms a heat bank and the heat sink/bank is adapted to maintain a safe holding temperature over a more extended period of time in a power off mode.

FIELD OF THE INVENTION 
This invention relates to ovens for heating and keeping foods warm, and 
more particularly to an improved holding oven that uses natural convection 
derived from a novel stabilized heat source. 
BACKGROUND OF THE INVENTION 
Prior oven designs use heaters which are either fastened to the walls or 
floor, or may even totally encompass the entire interior of the cabinet. 
All of these systems tend to create stagnant heat near the top of the 
cooking or heating chamber which just continues to get hotter, resulting 
in a large thermal gradient within the cabinet. 
Positioning of heater elements and controls is generally dictated by 
whether the cabinet includes single or dual access doors as well as the 
fact that the heated air within the cabinet rises to the top. It has been 
found that convection heat flow is preferred for cooking purposes, since 
convection heat cooks the food more thoroughly and quickly, while radiant 
heat is used to store and tenderize food. Proofing is the process for 
adding dough and water during the cooking process to keep the dough from 
cracking when rising, and requires maintenance of a target temperature and 
humidity within the cabinet. 
These design considerations are complicated by frequent opening and closing 
of the doors during use. Thus an ideal design would enable natural heat 
circulation; would reduce temperature variation within the cabinet; would 
allow ready access to the heaters for enhanced serviceability with reduced 
maintenance costs; and, would achieve the foregoing at greater efficiency 
by reduction in wattage used. 
Prior art devices are not calculated to attain these goals or, if so 
designed, have not attained them. 
U.S. Pat. No. 2,718,854, to Michaelis, 1951, discloses a bake pan or oven 
deck to provide a diffuse heat to eliminate burned spots with decreased 
heat by conduction and increased heat by radiation. 
U.S. Pat. No. 3,197,185, to Beattie, 1963, is a heat furnace directed to 
heat treatment of glass. 
U.S. Pat. No. 3,282,578, to Ulbrich, 1966, represents a liner for a furnace 
or kiln to absorb thermal shock. 
U.S. Pat. No. 3,327,041, to Clune, 1964, is a heat shield pack with 
cylindrical heat shields of spaced-apart heat shield leaves of refractory 
material. 
U.S. Pat. No. 4,209,569, to Brugger, 1980, is an aluminum baking form 
coated with aluminum oxide and the method of making same. 
U.S. Pat. No. 4,648,377, to Van Camp, includes a gas fired blower and an 
improved, bifurcated heat exchanger. 
The firing chamber of U.S. Pat. No. 4,978,295, to Vukovich, Jr., 1990, 
includes an upper ventilation aperture and an exhaust fan. 
Accordingly, there is a need for commercial baking ovens and holding ovens 
with a chamber which can rapidly achieve a predetermined temperature with 
improved temperature maintenance capacity through repeated opening and 
closing of the cabinet in ordinary use and when the oven is in a 
thermostatically controlled power off mode. 
It is therefore a principal object of the invention to provide a holding 
oven incorporating a heat sink formed of a first material with high 
thermal capacitance for rapid build-up and slow discharge of heat into an 
insulated space and, further incorporating a heat sink formed of a 
material with lower thermal capacity but having a thermal conductivity a 
multiple of four or greater times that of the first material. 
Another object of the invention is to provide for natural heat circulation 
to diminish temperature variation and provide a radiant heat with 
convection heat flow within the cabinet. 
Yet another objective is construction of an oven having better efficiency 
with reduced wattage per hour of use. 
Still another objective of this invention is construction of a holding 
cabinet with heater elements positioned with consideration for either 
single or dual access doors, facilitating tear-down and serviceability of 
heaters, and resultant reduction in maintenance costs. 
Further objects and advantages of the invention will become apparent from 
the following detailed description of the preferred embodiments and from 
the accompanying drawings. 
SUMMARY OF THE INVENTION 
As such, the oven of the invention includes a thermal input means formed of 
one or more heater elements, generally a Nichrome wire in a mica jacket, 
with capability to achieve temperatures up to 1,000.degree. F. A 
mechanical attachment of heater element to cabinet is a clamp or holddown 
plate formed of a first material having a high thermal capacity, which may 
be copper, brass, a steel alloy or stainless steel. The clamp contacts a 
lower surface of the heater, while the clamp further communicates with the 
cabinet inner skin or inner casing, generally of stainless steel, to form 
a heat sink of the unit. A UL standard insulation, between inner and outer 
casings of the oven, provides thermal resistance and augments thermal 
retention. 
The clamp presses the upper surface of the heater element against a second 
material having relatively high thermal conductivity, copper, silver or 
aluminum, to form a heat conductor for rapidly conducting heat from the 
Nichrome wire to the chamber inner walls. The conductor is formed of two 
opposed "L-shaped" pieces in upright position, each "L-shaped" piece 
having an upright leg portion and a contiguous foot portion. The 
"L-shaped" pieces are brought together and the feet portions thereof 
welded together to form a "U-shaped" section, the heat conductor. 
A heater element, positioned under the juncture of the foot and the leg of 
each "L" draws heat up the sidewall of the sink, (leg of the "L"), but 
away from the central floor portion of the cabinet, since the foot 
portions of the "L-shaped" pieces are not one contiguous material although 
the foot portions are joined together in cabinet construction. This design 
results in creating a natural heat motion within the cabinet. 
The heater and clamp or holddown plate, so combined, prevent the heater 
from bowing away from the conductor, while simultaneously having a mirror 
effect with reflection of heat to rapidly achieve and maintain a 
predetermined temperature and serve as a heat sink.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
There are three different flow laws for heat, corresponding to three 
different heat processes. Associated with heat conduction is the name of 
Fourier, with heat convection is the name Newton, and with heat radiation 
are the names Stefan-Boltzmann. 
Heat Transfer by Radiation 
Several designs used the principle of heat transfer by radiation. FIG. 1 
shows a heated surface .theta..sub.1 that is losing heat by radiation to a 
region of lower temperature .theta..sub.2. The heat flow is given by the 
Stefan-Boltzmann law for surface radiation, which is: 
Q=.sigma.A.epsilon. (.theta..sub.1.sup.4 -.theta..sub.2.sup.4) joules/sec 
where 
Q=heat flow, joules/sec 
.sigma.=the Stefan-Boltzmann constant=5,672.times.10.sup.-8 joules/m.sup.2 
-sec-deg.sup.4 
.epsilon.=emissivity of the surface (between 0 and 1) 
A=surface area, m.sup.2 
.theta.=temperature, deg K 
In a practical application, designs using the principle of radiation heat 
transfer, have large internal thermal gradients because of the radiation 
resistance. The radiation resistance is written approximately 
##EQU1## 
where .theta..sub.a is the average of radiator and receiver temperatures. 
The radiation resistance varies inversely as the cube of the average 
temperature. The resistance as calculated from the above equation is not 
seriously in error when the source and receiver temperatures differ by a 
factor of two, but the error increases rapidly for greater differences. 
Heat Transfer by Convection 
To consider those systems that use convection heat transfer, refer to FIG. 
2 (which shows a fluid, either a liquid or gas, in either laminar or 
turbulent flow), .theta..sub.2, which flows across a heated surface 
.theta..sub.1, and thereby carries heat away from the heated surface. The 
region of the flowing liquid that absorbs or gives up the heat is the 
boundary layer. The convection heat transfer is given by Newton's law of 
"cooling," which is 
Q=h'A (.theta..sub.1 -.theta..sub.2) joules/sec 
where h' is the convection coefficient (joules/m.sup.2 -sec-deg K). For a 
given state, h' is nearly constant and is not dependent upon the state of 
the fluid flow, whether it is laminar or turbulent. It is important to 
realize that the process by which heat is carried from the heated surface 
to the fluid is molecular conduction, and for this reason the process of 
heat conduction by convection is essentially the same as that by 
conduction. One may, in fact, relate the Newton law to the Fourier law in 
the fluid at the surface of contact of the fluid and the wall. 
The thermal resistance due to convection is 
##EQU2## 
Heat Transfer by Conduction 
In considering a heat conducting system, where the ends of a conducting 
substance of which are maintained at different temperatures, .theta..sub.1 
and .theta..sub.2, by application of the Fourier law 
##EQU3## 
where 
Q=heat flow, joules/sec 
k=thermal conductivity, joules/meter-sec-deg 
A=area normal to the heat flow, m.sup.2 
L=length of conductor, m 
.sigma.=temperature, deg K. 
The thermal resistance for a conducting substance is 
##EQU4## 
The Royalton system is one that involves the transfer of heat from one 
substance to another. This is characterized by a system that has both 
resistance and capacitance. Substances that are characterized by 
resistance to heat flow have negligible storage of heat, and substances 
that are characterized by heat storage have negligible resistance to heat 
flow. There are many substances that satisfy the validity of such 
approximations. For example, substances as air, wood, cork, etc. possess 
high thermal resistance, but low thermal capacitance, whereas a block of 
aluminum or copper has a high thermal capacitance with a relatively low 
thermal resistance. 
Thermal Capacitance 
The thermal capacitance can be determined using the following relation 
##EQU5## 
where 
C=thermal capacitance, joules/deg 
t=time, sec 
The thermal capacitance is written directly as 
C=WC.sub.p 
where 
W=weight of substance, kg 
C.sub.p =specific heat at constant pressure, joules/deg-kg. 
The Royalton System 
The Royalton system can best be described in two electrically equivalent 
diagrams. The first diagram, FIG. 3, shows the application of a 
thermostatically controlled electrical energy source that provides the 
heat energy to the system. The second diagram, FIG. 4, shows the energy 
source removed with the heat load (food) placed within the chamber. 
The specific heat of aluminum is 
20.degree. C.=0.214 cal/gm 
100.degree. C.=0.225 cal/gm 
200.degree. C.=0.248 cal/gm 
The specific heat of iron is 
20.degree. C.=0.107 cal/gm 
100.degree. C.=0.115 cal/gm 
200.degree. C.=0.127 cal/gm 
The conductivity of aluminum is 
18.degree. C.=0.480 
100.degree. C.=0.492 
200.degree. C.=0.550 
The conductivity of steel is 
18.degree. C.=0.108 
100.degree. C.=0.107 
Turning now to FIG. 3, the heater means or element 3 is controlled by 
thermostat 2 from the 117 VAC energy source 1. The heater element 
transfers heat directly to the holddown plate 4, which plate is preferably 
stainless steel, such that the plate reflects heat back at the element 
with a mirror effect, whereby the sink for rapid increase of thermal 
energy. The heat sink of 3 and 4 further transfers heat to mass of 
aluminum or heat conductor, 6, via its thermal resistance 5. Because the 
aluminum is highly conductive and contains a relatively large mass, the 
temperature T.sub.2 is just slightly lower than the heater assembly 
temperature T.sub.1. The resistor 7 represents the thermal resistance of 
the stainless steel and is about 5 times greater in value. The value for 
capacitance 8 is about equal to that of the aluminum mass because the 
ratio of the specific heats is 2 to 1 and the weight of the steel cabinet 
is about twice the aluminum heat conductor. The value of resistor 9 is 
that of the glass wool insulation (R22), and is a very large value of 
resistance. 
Once the system is stabilized at its operating temperature, the thermal 
load 10, usually preheated trays of prepared food, is stored in the 
cabinet and the power removed. This is best shown by the equivalent 
thermal diagram of FIG. 4. All of the stored thermal energy in elements 6 
and 8 is transferred to the food via the natural convection currents as 
determined by the value assigned to resistor 11. As the temperature 
T.sub.4 of the load 10 tends to decrease, the stored energy is transferred 
maintaining the food at a relatively constant temperature for long periods 
of time. 
A target temperature within the cabinet may be designated by a food service 
company, standards and rules of the school, hospital or other institution 
using the oven or by the specific type of food within the oven itself. 
Most health department have minimum temperature requirements ranging from 
140.degree. F. to 160.degree. F. Thus the thermostat control preferably 
includes ON and OFF MODES to achieve a target temperature within the 
cabinet of about 200.degree. F. 
The chart of FIG. 5 graphs temperatures as a function of time to indicate 
test results on use of heater holddown plates of various materials. 
Temperatures were taken from a first lead on the heater element, (higher 
temperature or line set), and a second lead from within the cabinet, 
(lower line set shown with numbers primed), during both the automatic 
thermostatically controlled ON MODE, (time interval A--B, top of chart), 
OFF MODE, (B--C), ON MODE, (C--D). 
EXAMPLE I 
A heat sink was constructed using a heater element comprising a mica 
jacketed nichrome wire mechanically fastened to an enclosed heating 
cabinet by means of a holddom plate under a lower surface of the heater, 
that plate formed of aluminum having the same length and width dimensions 
as the heater to restrict the heater element from bowing away from the 
cabinet at maximum heater element temperatures. The upper surface of the 
heater element of the heat sink thus formed was placed against an L-shaped 
aluminum mass, at the juncture of the upright leg portion of L-shaped 
aluminum heat conductor and the foot portion of aluminum conductor. The 
cabinet was equipped with an automatic thermostat controller with an ON 
MODE and an OFF MODE, adapted to achieve a target temperature within the 
cabinet of 200.degree.. A first temperature lead was clamped between the 
aluminum holddown plate and the heater for determining temperatures of the 
heater element over time; while a second temperature lead was positioned 
within the cabinet to record cabinet temperatures over a corresponding 
time. As such, the test run for use of a heater holddown plate of aluminum 
produced the solid thin temperature line 55, for the heater element 
temperature and line 55' for the corresponding cabinet temperatures. The 
maximum heater element temperature was 850.degree. F. with a corresponding 
maximum cabinet temperature of 380.degree. F. 
EXAMPLE II 
The second test was run on the same cabinet, with the same temperature lead 
placements, using the same heater element under the same conditions of 
EXAMPLE I above, but with a holddown plate comprising a mild steel alloy. 
As such, the test run for use of a heater holddown plate formed of mild 
steel alloy produced the dashed temperature line 56, for the heater 
element temperature and line 56' for the corresponding cabinet 
temperatures. The maximum heater element temperature was 60.degree. F. 
with a corresponding maximum cabinet temperature of 325.degree. F. 
EXAMPLE III 
The third test was run on the same cabinet, with the same temperature lead 
placements, using the same heater element under the same conditions of 
EXAMPLES I and II above, but with a holddown plate comprised of copper. As 
such, the test run for use of the copper holddown plate produced the 
dotted temperature line 57, for the heater element temperature and line 
57' for the corresponding cabinet temperatures. The maximum heater element 
temperature during use of the copper holddown plate was 798.degree. F. 
with a corresponding maximum cabinet temperature of 375.degree. F. 
EXAMPLE IV 
The fourth test was run on the same cabinet, with the same temperature lead 
placements, using the same heater element under the same conditions of 
EXAMPLES I, II and III above, but with a holddown plate comprised of 
stainless steel. As such, the test run for use of the stainless steel 
holddown plate produced the heavy black temperature line 58, for the 
heater element temperature and line 58' for the corresponding cabinet 
temperatures. The maximum heater element temperature during use of the 
copper holddown plate was 775.degree. F. with a corresponding maximum 
cabinet temperature of 390.degree. F. 
Thus, FIG. 5 graphically demonstrates that heater element holddown plates 
comprising stainless steel are preferable in constructing the heat bank 
for two reasons: 1) Because stainless steel has higher thermal capacitance 
and resistance, plates of stainless steel were found to discharge thermal 
energy more slowly in the power off mode; and, 2) The stainless steel 
plates produced higher cabinet temperatures with at lower heater element 
temperatures for reduced thermal trauma to the heater elements. Stated 
otherwise, holddown plates made of stainless steel produced the highest 
cabinet temperature, 390.degree. F., shown by heavy black temperature 
line, 58', consequent to a relatively low, maximum heater element 
temperature of 775.degree., heavy black line, 58, than the plates of 
aluminum, 55', 55, mild steel alloy, 56',56, or copper, 57',57. 
The test results shown in the chart of FIG. 5 further indicate that the 
equipment having both ON and OFF MODES serves as a heat sink in the off 
mode, thus maintaining a safe holding temperature much longer than 
conventional heating methods while the heater element is off. When in use, 
the heat conductor/sink also serves to stabilize the equipment 
temperatures as the door or doors are constantly being opened and closed. 
FIG. 6 is a perspective view of a holding cabinet to indicate usual 
positioning generally prevalent for heater elements 30 in the prior art. 
FIG. 7 is a front cut-away view of the Royalton cabinet 22 demonstrating 
the heater element 30 which is retained against aluminum conductor 60 by 
holddown plate 32. The upright wall of the aluminum mass forming heat 
conductor 60 gives a natural convection heat flow 38 within the holding 
oven of the present invention. 
FIG. 7 further indicates that the Royalton Natural Air Movement System also 
incorporates the laws of physics that: 
a) A heated gas rises and cooler gases descend; 
b) Stainless steel is characterized by low conductivity and high 
capacitance and positioning the hold down plate immediately adjacent to 
the heating element results in a mirror effect creating a heat sink, 
wherein the steel plate reflects the heat back which is then drawn off by 
the aluminum conductor; 
c) Aluminum is characterized by high conductivity and more rapid thermal 
transmission which pulls heat from the heater element and thereby serves 
as a heat sink. 
Advantageous results are achieved by the heat repulsion or resistance of 
one material, the holddown plate preferably formed of stainless steel on 
one surface of the heater element, and the thermal conductivity of a 
second material, a mass of aluminum comprised of L-shaped pieces 61, 62 
fastened together, are positioned adjacent the second surface of the 
heater element to draw heat away from the heater element and up the 
sidewalls of the cabinet. The unique use of the highly conductive and less 
conductive materials and relative positioning of the heater elements at 
the juncture of the leg and foot portions of each L-shaped aluminum piece, 
gives resultant natural heat circulation 38, better efficiency by use of 
less wattage per time unit, reduced temperature variation within the 
cabinet and enhanced serviceability with resulting lowered maintenance 
costs. 
In FIG. 7, the oven of the invention 100 includes a thermal input means 
formed of one or more heater elements 30, generally a Nichrome wire in a 
mica jacket, with capability to achieve temperatures up to 1,000.degree. 
F. Heater means or element 30 is mechanically affixed to cabinet 22 by 
holddown plate 32 formed of a first material having a high thermal 
capacity, which may be a ceramic material, copper, brass, mild steel 
alloy, aluminum, or stainless steel. But in accord with the test results 
shown in FIG. 5 above, the holddown plate 32 is preferably formed of 
stainless steel. The holddown plate 32 contacts a lower surface of the 
heater element 30, to form heat bank 33. The heater means or heater 
element 30 and the holddown plate 32, formed of stainless steel, a ceramic 
material, copper, brass, or a mild steel alloy, is also shown as heat bank 
3,4 with resistance 7 for stainless steel, in FIG. 3. The holddown plate 
32 further communicates with the cabinet inner skin or inner casing 24, 
generally of stainless steel, to form a heat bank, (3,4 in FIG. 3), of the 
unit. 
Reference to FIG. 7 indicates a UL standard insulation 28, between inner 24 
and outer 26 casings of the oven, provides thermal resistance (11 in FIG. 
4) to augment thermal retention. 
The clamp or holddown plate 32 further presses the heater element 30 
against a second material having relatively high thermal conductivity, 
copper, silver or aluminum, (element 6 in FIG. 3), to form a heat 
conductor 60 for rapidly conducting heat from the Nichrome wire heater 
element 30 to the chamber inner walls 24. 
FIG. 7 further shows that heat conductor 60 is formed of two opposed 
"L-shaped" pieces 61,62 in upright position. A heater element 30, 
positioned under the juncture of the foot and the leg of each "L" draws 
heat up the sidewall of the conductor 60', (leg of the "L"), but away from 
the central floor portion 23 of the cabinet, since the foot portions of 
the "L-shaped" pieces are not one contiguous material although the foot 
portions are joined together in cabinet construction. The fact that the 
lateral floor portions, (foot of each L), and sides, (leg of each L), of 
the cabinet are hottest to propel the thermal energy upward along the 
cabinet sides and downward along a central portion 23. This design results 
in creating a natural heat convection within the cabinet. 
The heater and clamp, so combined, prevent the heater from bowing away from 
the sink, while simultaneously having a mirror effect with reflection of 
heat to rapidly achieve the predetermined temperature. 
FIG. 8 is a perspective elevation of a heater cabinet of the subject 
invention showing the heater element 30 positioned against the aluminum 
mass at the juncture of the upright leg portion 60 of the L-shaped piece 
and the contiguous foot portion of the L-shaped piece which form the heat 
conductor of the present invention. 
While there have been illustrated and described what are at present 
considered to be preferred embodiments of the present invention, it will 
be understood by those skilled in the art that various changes and 
modifications may be made, and equivalents may be substituted for elements 
thereof without departing from the true scope of the present invention. In 
addition, many modifications may be made to adapt a particular suggestion 
or material to the teaching of the present invention without departing 
from the central scope thereof. Therefore, it is intended that the present 
invention not be limited to the particular embodiment disclosed as the 
best mode contemplated for carrying out the present invention, but that 
the present invention include all embodiments falling within the scope of 
the appended claims.