Note: Descriptions are shown in the official language in which they were submitted.
CA 02454498 2006-05-10
MULTI-STAGE CATALYST FOR A COOKING APPLIANCE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to the art of cooking appliances and,
more particularly, to a multi-stage catalyst for a cooking appliance.
2. Discussion of the Prior Art
In the art of cooking appliances, it has been heretofore proposed to
enable an appliance to operate in a self-cleaning mode. For example, in a
conventional range having a cooking cavity which can be heated by one
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wn
or more cooking elements arranged within the cooking cavity to perform
at least baking and broiling functions, it is known to operate one or more
of the cooking elements to perform a pyrolytic self-cleaning operation in
order to cleanse the walls of the cavity from grease and other food soils
developed during normal cooking operations. In such a cooking
arrangement, the cooking elements used to perform the cleaning process
are located entirely within the cooking cavity.
In addition, it is known to provide a catalytic self-cleaning oven.
In such an arrangement, the walls of the oven are coated with a catalytic
io material which provides for self-cleaning of the oven cavity during
cooking operations. In performing any self-cleaning function,
byproducts, including smoke, gases and other odorous fumes, are
inherently produced. A typical oven cavity will be vented to permit the
escape of these byproducts to the ambient surroundings. In some cases, a
catalytic oxidation unit is provided in the vent to react with the flowing
byproducts.
In still other cooking appliance arrangements, a combination of
pyrolytic and catalytic cleaning is performed. Regardless of the fact that
various self-cleaning systems have been proposed in the art, there still
2o exists a need for an improved self-cleaning system for a cooking
appliance which maximizes the elimination of byproducts, while also
minimizing the necessary operating time for any self-cleaning mode.
Particular concerns are raised in connection with the necessary operating
time and byproduct elimination in a self-cleaning convection oven which
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essentially relies on a heated flow of recirculating air for raising the
temperature in an oven cavity. In particular, there exists a particular need
for an improved catalyst used to heat and vaporize grease and oil, while
starting the molecular breakdown of carbon chains in various stages for
cleaning purposes.
SUMMARY OF THE INVENTION
The present invention is directed to a catalyst used to heat and
vaporize grease and oil for cleaning of a convection cooking appliance,
particularly an appliance including an air channel assembly which is
io defined by ducting extending about portions of the oven cavity for
directing a recirculating flow of air into and out of the oven cavity. The
overall system utilizes various heating elements to enhance the heating of
the oven cavity, as well as the catalyst in order to enhance the efficient
elimination of developed smoke, odor and other byproducts, and to
effectively reduce the necessary cleaning cycle time for the appliance.
The convection cooking appliance preferably includes first, second
and third heating units which are individually controlled, along with a
blower unit, in performing a self-cleaning function for the appliance. The
first and third heating units are disposed in the air channel assembly,
while the second heating unit is positioned in the oven cavity. A
controller is provided for regulating the activation and deactivation state
of the various components in a manner which preheats the oven cavity in
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a relatively short time period, while assuring that initially developed
smoke, gases and other odorous fumes inherently produced as byproducts
of a self-cleaning operation are effectively eliminated.
During the preheat phase of the cleaning mode, the controller
initially activates the blower element in combination with each of the first
and third heating units in the air channel assembly, with the developed
flow of air through the oven cavity being directed to the catalyst for
elimination of the byproducts. Following the preheat phase, at least the
blower and the third heating element are deactivated and the second
i o heating unit is activated to rapidly heat the oven cavity through a
radiant
heating operation. A temperature sensor is linked to the controller to
efficiently determine the optimum time to switch between the various
heating sources for the oven cavity during the overall self-cleaning
operation.
is In accordance with a preferred self-cleaning method, an initial,
catalyst pre-heat stage, established mainly for smoke elimination
purposes, is followed by a moderately high-temperature presoak stage to
burn off various light molecular weight hydrocarbons and the like. For
the main cleaning operation, a high temperature stage is initiated in
20 combination with a high convection air flow to establish high oven
surface temperatures in a minimal time frame. This timed stage is
followed by a cool down period wherein both the temperature and the
convection speed are reduced. Thereafter, providing an intermediate
temperature heating stage with medium convective air flow, followed by
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a cooling stage, is provided for preset time periods. This combination of
intermediate temperature heating and subsequent cooling stages is
repeated until the total self-cleaning time is completed.
In accordance with the present invention, the convection cooking
appliance particularly employs a multi-stage catalyst. Specifically, the
catalyst is designed to heat and vaporize the grease and oils that are
produced from cooking food, with a first stage essentially constituting a
heating and vaporizing phase and a second or final stage operating to
break down carbon chains. The first stage constitutes a honeycomb
io structure of metal and ceramic that is heated to an elevated temperature
which prevents overloading of the second stage. The micro-particulate
grease and oils collect in various surfaces of the honeycomb and are
vaporized. The vaporized grease and oils are then blown off the
honeycomb structure onto a filter assembly which is part of the overall
catalyst.
A narrow gap or opening is formed between the first stage and the
fmal stage. The gap is designed to create a turbulence in order to
distribute the vapor molecules to the final stage of the catalyst. The final
stage of the catalyst preferably comprises a metallic or ceramic
2o honeycomb structure wherein the holes in the honeycomb are smaller
than in the first stage. This arrangement generates a larger surface area in
the final stage catalyst as compared to the first stage. One or more
portions of the catalyst can be treated with catalyst materials, such as
platinum, to convert the hot vaporized byproducts into carbon dioxide
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and water. As the cooking appliance preferably incorporates a
microwave energy source, the catalyst is covered with expanded metal or
a perforated sheet to block any microwave energy that may be used in
connection with the oven cavity.
Additional aspects, embodiments, features and advantages of the
present invention will become more fully apparent below with reference
to a preferred embodiment of the invention, when taken in conjunction
with the drawings wherein like reference numerals refer to corresponding
parts in the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective, partial sectional view of a convection
cooking appliance constructed in accordance with the present invention;
Figure 2 is a cross-sectional side view of the cooking appliance of
Figure 1;
Figure 3 is a schematic side view, similar to that of Figure 2, of the
cooking appliance;
Figure 4 is a block diagram illustrating a control arrangement used
in a self-cleaning system employed in the cooking appliance of Figure 1;
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Figure 5 is a perspective, partial cross-sectional view
corresponding to that of Figurel, while depicting a catalyst of the cooking
appliance in cross-section; and
Figure 6 is an enlarged cross-sectional view of the catalyst of
Figure 5.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENT
With initial reference to Figures 1-3, a cooking appliance 1 is
schematically shown in the form of a wall oven. Appliance 1 includes an
io oven cavity 5 generally defined by a bottom wa118, a top wall 9, a rear
wall 10 and a pair of side walls, one of which is indicated at 11. Oven
cavity 5 also has associated therewith an access opening 13 for food items
to be placed into or withdrawn from cavity 5. About access opening 13 is
provided a frontal plate 16. In a manner known in the art, frontal plate 16
is adapted to be mounted against a substantially vertical wall such as in
the kitchen of a residential home, and would have a door (not shown)
pivotally attached thereto for selectively sealing off access opening 13.
Extending generally along top, bottom and rear portions of cavity 5
is an air channel assembly 26 defined by ducting that leads into and out of
cavity 5. More specifically, air channel assembly 26 includes a lower air
return section 29, an upper air delivery section 30 and a rear air transfer
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section 31. Lower air return section 29 is open into cavity 5 through a
substantially central return air outlet 33 formed in bottom 8. In the most
preferred form of the invention, return air outlet 33 is constituted by a
generally circular insert provided with various spaced holes (not shown).
In a similar manner, upper air delivery section 30 includes a discharge or
delivery inlet 35 formed in top wal19. Although only partially shown in
Figure 1, inlet 35 is also preferably constituted by a generally circular-
shaped insert which is attached to the remainder of upper air delivery
section 30 and which is provided with a plurality of holes 37.
As will become more fully evident below, the particular
construction of cooking appliance 1 can significantly vary in accordance
with the present invention. However, as shown, cooking appliance 1
includes an air channel assembly, such as that discussed above with
reference to assembly 26, as well as a blower assembly, such as that
generally indicated at 40, for use in generating a circulating flow of air
through oven cavity 5. Although not considered a part of the present
invention, a preferred construction for oven cavity 5 and air channel
assembly 26 can be found in U.S. Patent No. 6,373,037 entitled "OVEN
CAVITY CONSTRUCTION".
In the preferred embodiment shown, cooking appliance 1
constitutes an electric appliance and, more specifically, a combination
convection, microwave and radiant cooking device. As shown in this
figure, cooking appliance 1 is provided with an annular filter basket 46,
having a multitude of circumferentially spaced holes 47, which is
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positioned within lower air return section 29 and through which the air
flowing from cavity 5 through return air outlet 33 is directed. Arranged
below filter basket 46 is a microwave generator unit 48 incorporating a
magnetron (not specifically shown).
Encircling at least a portion of filter basket 46 is a first electric
heating element 52. Heating unit 52 is shown as constituted by a
sheathed electric resistance heating element having upper and lower
interconnected legs 53 and 54. First electric heating unit 52 is preferably
provided to heat return air flowing from oven cavity 5, through outlet 33
lo and filter basket 56 prior to the air reaching a catalyst indicated at 57.
The present invention is particularly directed to the structure and
operation of catalyst 57 as will be detailed more fully below. However,
in general, catalyst 57 functions to eliminate smoke and the like from the
air stream. As shown, catalyst 57 extends partially within a rotatable
blower element 60 which forms part of blower assembly 40. Although
blower element 60 can take various forms while performing the desired
air flow generating function, blower element 60 preferably constitutes a
centrifugal unit arranged at the juncture of lower air return section 29 and
rear air transfer section 31. In general, blower element 60 is secured to a
shaft member 62 that is rotatably mounted through a bearing assembly
64. Shaft member 62 also has attached thereto, for non-relative rotation,
a sheave 66 which is adapted to receive a belt (not shown) for use in
rotating blower element 60 through shaft member 62 in combination with
an electric motor (also not shown). As illustrated, sheave 66 is preferably
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arranged within a housing extension 68 which projects from rear air
transfer section 31.
Preferably mounted in upper air delivery section 30 adjacent rear
transfer section 31 is a second electric heating element arrangement 70
that is preferably constituted by a bank of heating coils. Although not
pertinent to the present invention, second heating unit 70 can be defined
by a single open electric coil that runs back and forth across upper air
delivery section 30 or multiple, separately controllable coil elements. In
any event, second heating unit 70 functions to further heat the air flowing
io through channel assembly 26 prior to the air reaching discharge inlet 35.
Also shown in this figure is a third electric heating unit 72 which, in a
manner similar to first electric heating unit 52, is preferably constituted
by a sheathed, resistance-type heating element. Third electric heating
unit 72 preferably extends adjacent top wa119 and constitutes an
additional heat source for cavity 5 of cooking appliance 1. The particular
manner in which first, second and third electric heating units 52, 70 and
72 are utilized during operation of cooking appliance 1 for a cooking
mode of operation is not considered to constitute part of the present
invention. Instead, these details can be found in U.S. Patent No.
2o 6,291,808 entitled "HEATING SYSTEM FOR A COOKING
APPLIANCE".
As represented in Figure 4, each of blower assembly 40,
microwave generator 48 and first, second and third electric heating units
52, 70 and 72 are linked to an appliance controller or CPU 73. Controller
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73 also receives signals from operator input controls 74, as well as from a
temperature sensor 75 which is preferably arranged in upper air delivery
section 30, between heating unit 70 and delivery inlet 35. The present
invention is particularly directed to the manner in which cooking
appliance 1 is cleaned during operation and, more particularly, to the
configuration and operation of catalyst 57 which will be discussed fully
below after discussing the general operation of cooking appliance 1.
First of all, a user of cooking appliance 1 can select, through
operator input controls 74, a convection cooking mode wherein heating
io element 52 is activated, along with blower assembly 40 and heating unit
70, to direct a flow of recirculating air through oven cavity 5. With this
arrangement, heated air will be caused to flow within air channel
assembly 26 and through holes 37 in order to impinge on food items to be
cooked within oven cavity 5. During operation, blower assembly 40 can
is produce a certain degree of turbulence which is considered detrimental to
the uniform and consistent flow of air through channel assembly 26.
However, as indicated above, heating unit 70 is preferably constituted by
various rows of open coils, with six rows of coils being shown in the
preferred embodiment depicted in the drawings. Since the coils are open
2o and arranged perpendicular to the flow of air, any turbulence developed
by the operation of blower assembly 40 is transformed into a linear or
laminar flow which enhances a smooth and continuous flow through oven
cavity 5 for uniform heating.
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During a convection cooking mode of operation, heating unit 70
can be cycled on and off by controller 73 in dependence on the
temperature of the air as signaled by sensor 75. Within the spirit of the
invention, heating unit 70 can also be variably controlled, such as by
s establishing low, medium or high wattage settings. For purposes of the
invention, reference will be made to heating unit 70 and/or heating unit
52 being operable at varying power levels which is intended to
encompass various ways in which the heating capacity of each of these
units can be altered, e.g. through varying duty cycles and/or wattage
io settings. Although not shown, heating unit 70 is preferably, electrically
linked to controller 73 through the use of a triac. Regardless of the
particular operating status of heating unit 70, blower assembly 40 and
heating unit 52 are operated continuously throughout the convection
cooking mode in accordance with the most preferred embodiment of the
15 invention.
The user of cooking appliance 1 can also select a microwave
cooking mode wherein controller 73 activates generator 48. Again,
heating unit 52 is preferably, continuously operated whenever cooking
appliance 1 is operating in a cooking mode. Furthermore, in a cleaning
20 mode, each of heating units 52, 70 and 72 are controlled for effective
high temperature operation as covered by the above-referenced patent.
Upon initial activation of cooking appliance 1 for a convection
cooking operation, controller 73 energizes heating unit 70, preferably at
full power, and blower assembly 40 is run at a low to moderate speed.
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Once catalyst 57 reaches a critical temperature, e.g., 450 F, heating uziit
52 is initiated under full power, i.e., a high wattage setting and/or 100%
duty cycle. At this point, the greases, oils, other hydrocarbons, and the
like byproducts remaining from prior cooking operations will begin
s combusting, thereby generating some smoke, a majority of which is
forced out through catalyst 57. Because catalyst 57 has been sufficiently
heated, the fats, oils and other hydrocarbons can be completely
combusted and converted to carbon dioxide and water with very little or
no smoke. As catalyst 57 reaches a critical firing temperature, additional
io combustion of the grease, oils and other hydrocarbons will commence:
Therefore, in this manner, the combustion of the remaining byproducts is
performed in various, controlled stages. At this point, the speed of
blower assembly 40 is increased to draw additional oxygen into air
channel assembly 26 in order to maintain an oxidizing atmosphere.
15 If a door (not shown) adapted to extend across and substantially
seal oven cavity 5 is opened during preheating, blower assembly 40
remains ON in accordance with the invention, but is controlled to operate
at a lower speed, preferably in the order of 20%, to create a circulation in
oven cavity 5. Blower assembly 40 actually operates to create a negative
20 pressure differential in oven cavity 5 such that any smoke will be drawn
back into oven cavity 5 and through catalyst 57, instead of being released
into the ambient atmosphere. In addition, the creation of the pressure
differential actually functions to draw in more oxygen so as to enable
even further combustion.
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A main purpose of the preheat system as described above is to
clean oven cavity 5 prior to cooking and between self-cleaning
operations. In general, the self-cleaning sequence of the invention
preferably utilizes high velocity air to heat oven cavity 5 and air channel
assembly 26 to self-cleaning (pyrolitic) temperatures. In accordance with
a preferred embodiment of the invention, the self-cleaning sequence
includes four stages: a vent catalyst pre-heating stage; a low molecular
weight hydrocarbon burn-off stage; a high temperature cleaniing stage;
and a cool down stage, each of which will be detailed fully below.
During the vent catalyst pre-heating stage, the door for oven cavity
5 is locked and catalyst 57 is initially heated to facilitate conversion of
smoke and various hydrocarbons into carbon dioxide and water. In
addition, heating elements 52 and 70 are operated at 100% power during
this stage. At the same time, variable speed blower assembly 40 is
operated at a low range, preferably about 20% of maximum speed, to
direct a flow of heated air at catalyst 57. Heating of catalyst 57 and,
correspondingly, oven cavity 5, continues until catalyst 57 reaches a
predetermined temperature, preferably about 500 F.
In the next stage, catalyst 57 is fully energized as heating element
2o 70 is activated until the temperature of the catalyst is substantially
increased, preferably to about 750 F. At the same time, the speed of
blower assembly 40 is increased to about 70% of the maximum air flow
rate. Once catalyst 57 reaches the predetermined temperature, this
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temperature is maintained constant for a predetermined period of time,
preferably in the order of 20 minutes.
In the third stage, the speed of blower assembly 40 is increased to
about 90-100% of the maximum blower air flow rate. Heating element
52 is energized at 100% to substantially increase the temperature of
catalyst 57, preferably to between 900 F and 975 F and, most preferably,
to about 930 F. At the same time, heating element 70 and/or heating
element 72 is also controlled to raise the temperature of oven cavity 5 and
air channel assembly 40 to a minimum temperature above $40 F. This
io temperature of oven cavity 5 is maintained for a desired minimum time
period, preferably at least 60 minutes. However, the actual duration of
this high temperature cleaning stage can be adjusted by the operator in
order to improve the overall cleaning process.
In the final stage, a cool down sequence is initiated. During this
stage, all of heating elements 52, 70 and 72 are turned off, while blower
assembly 40 is maintained activated, preferably at about 70% of
maximum speed. After the temperature of oven cavity 5 drops below a
prescribed limit, preferably set at 500 F, the oven door will unlock. At
this point, the self-cleaning operation is terminated.
Whether in a dedicated cleaning cycle or simply operating in an
overall convection cooking operation, catalyst 57 functions to heat and
vaporize developed grease, oil and other byproducts produced from
cooking food. Particularly, as shown best in Figure 5, catalyst 57 defines
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a first stage essentially constituting a heating and vaporizing phase, a
turbulence creating gap, and a second or final stage operating to break
down carbon chains.
The first stage constitutes a first honeycomb structure 100 of metal
and ceramic that is heated to an elevated temperature which prevents
overloading of the second stage. The micro-particulate grease and oils
collect in the various surfaces of first honeycomb structure 100 and are
vaporized. The vaporized air is then blown off first honeycomb structure
100.
A narrow gap or opening 125 is formed between first honeycomb
structure 100 and a second or final catalyst stage. Gap 125 is designed to
create a turbulence of the vaporized grease and oil which causes the vapor
to be impinge loaded into the final stage of catalyst 57. The final stage of
the catalyst 57 preferably comprises a metallic or ceramic second
honeycomb structure 150 wherein holes 155 in second honeycomb
structure 150 are smaller than holes 165 in first honeycomb structure 100.
More specifically, holes 155 are preferably half the opening size than
holes 165. This arrangement generates a larger surface area in the final
stage, i.e., at second honeycomb structure 150, of catalyst 57 as compared
to the first stage defined by first honeycomb structure 100. This larger
surface area is preferably in the order of eight (8) times that of first
honeycomb structure 100, i.e. an approximately 8:1 surface area ratio is
established. In addition, an additional increase of about 4:1 in depth is
preferably employed. Therefore, an overall loaded surface area increase
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= ..
for second honeycomb structure 150 is increased by about thirty-two (32)
times.
One or more portions of catalyst 57 can be treated with catalyst
materials, such as platinum, to convert the hot vaporized greases and oils
into carbon dioxide and water. Most preferably, second honeycomb
structure 150 is treated in this manner to enhance the conversion of hot,
vaporized greases and oils to carbon dioxide and water inan oxidizing
atmosphere. As cooking appliance 1 preferably incorporates microwave
generator 48, the exhaust zone (not labeled) for catalyst 57 is preferably
io covered with expanded metal or a perforated sheet or layer 175 to block
microwave energy.
With this arrangement, catalyst 57 has been found to provide
enhanced operation, particularly when used in connection with a
convection, microwave appliance such as cooking appliance 1. As
is indicated in Figure 6, it is preferable to employ one or more control
temperature sensors 180 on or directly adjacent at least first honeycomb
structure 100 in order to regulate catalyst temperatures in order to ensure
that the greases and oils are suitably heated. In accordance with the most
preferred form of the invention, most of the thermal energy used to heat
20 catalyst 57, i.e., 75% to 80% of the energy, is applied to first honeycomb
structure 100 by heating element 52 and/or additional heating element(s)
190 on a conductor 200, preferably a relatively thick film ceramic
conductors, provided directly about catalyst 57. To this end, it should be
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noted that sensor(s) 180 can also be screened onto the same surface as
heating element 190 or even provided at heating element 52.
Although described with respect to a preferred embodiment of the
invention, it should be readily understood that various changes and/or
s modifications can be made to the invention without departing from the
spirit thereof. For instance, although the present invention has been
described with reference for use in connection with cooking appliance 1
which is designed for both convection and microwave cooking, it should
be readily apparent that the invention can also be applied to various types
io of convection cooking appliances, including ranges and other wall oven
anangements not including a microwave source. Particularly, although
catalyst 57 is considered to be particularly advantageous when used in
combination with convection and microwave cooking appliance 1,
catalyst 57 can be employed in other environrnents as well. In any event,
15 the invention is only intended to be limited by the scope of the following
claims.
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