Abstract:
A cooking appliance includes a recirculating airflow which is directed through particularly configured air plenum and nozzle structure that synergistically combine to enhance an overall cooking efficiency and effectiveness. In particular, the configuration of the air plenum, as well as the sizing and distribution of oven cavity entry nozzles, provides for an enhanced flow of heated air to the food product, thereby resulting in short cooking times due to high heat transfer while also providing an even browning and crisping of the food item being cooked. In addition, the cooking appliance incorporates an exhaust system wherein cooling air and a percentage of the recirculating airflow are mixed prior to being exhausted from an angled exhaust mixing tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents a National Stage application of PCT/US2008/079399 entitled “Air Circuit for Cooking Appliance Including Combination Heating System” filed Oct. 9, 2008, pending, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/960,662 entitled “Air Circuit for Cooking Appliance Including Combination Heating System” filed Oct. 9, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to the art of cooking appliances and, more particularly, to a compact cooking appliance capable of convection heating with other heating techniques to perform a cooking operation. 
     2. Discussion of the Prior Art 
     There exists a wide range of cooking appliances on the market. Many of these cooking appliances are designed for use in cooking various types of food products in different ways. For instance, where more conventional cooking appliances generally relied upon radiant energy as the sole heat source, more recent trends combine a radiant heat source with convection or microwave heating techniques, thereby increasing the versatility of the cooking appliance while potentially shortening required cook times. In particular, the prior art contains many examples of at least appliances that combine radiant and convection cooking, as well as combination convection and microwave cooking, techniques. 
     Regardless of the existence of these known arrangements, there still exists a need for a cooking appliance that employ multiple heating techniques in an efficient and effective manner to handle a wide range of food items. Particularly, there exists a need for a cooking appliance that can be used to rapidly prepare food products that require numerous different heat sources for full and complete cooking in an expedited manner. For example, the rapid preparation of commercially produced, open-faced grilled sandwiches raises various cooking concerns. Open-faced grilled sandwiches generally require, at the very least, that heat be directed both downward onto an upper portion of the sandwich and upward onto a lower bun portion of the sandwich. In most cases this is accomplished by passing the open-faced sandwich on a conveyor belt through an oven between opposing radiant heat sources. While effective to a degree, the process can be time consuming and really does not result in a uniform heating of the meat, cheese and/or other toppings on the bread, nor an even toasting of the bread itself. 
     In addition, a dual radiant oven of the type described above is simply not suitable for many other applications. For instance, an additional microwave oven or the like would typically be employed to heat soup or other liquid-based food items. To address this and other concerns, it has also been proposed in the prior art to combine each of radiant, convection and microwave energy sources in a compact, versatile cooking appliance. Such a prior art arrangement is disclosed, for example, in U.S. Pat. No. 7,235,763. 
     Regardless of the variety of known cooking appliances, there exists the need for a versatile cooking appliance that employs at least convection cooking and enhanced airflow features that enable a wide range of food items to be rapidly and effectively cooked. In connection with the airflow design, the manner in which the air is directed upon the food affects the browning, crispness and other characteristics of the food. Typical results with prior art yields food items with areas of increased browning compared to other areas of the food item. These areas of increased browning correspond to air nozzle location. Typical results yield a food item with a pattern of circular areas that visually have significantly more browning than the surrounding areas of the food item. The invention addresses this problem. 
     SUMMARY OF THE INVENTION 
     In its broadest aspects, the invention is directed to a convection cooking appliance. In accordance with the most preferred embodiment of the invention, the cooking appliance including an oven cavity having top, bottom and opposing side walls, a door for selectively accessing the oven cavity, at least one radiant heating element exposed to the oven cavity, a blower having a convection fan, a convection heating element directly exposed to an airflow generated by the convection fan and, preferably, multiple microwave heating devices. Behind a rear plate for the oven cavity is established an air return plenum portion for directing a return flow of air from the oven cavity to the blower. The cooking appliance also includes a discharge air plenum which extends above the oven cavity enabling a convection air stream to flow from the convection fan, across a catalyst and into the oven cavity through a plurality of downwardly exposed nozzles. The nozzles are sized, spaced and located relative to a load support in a manner that assures effective convection cooking. An exhaust tube opens to the discharge air plenum and projects into one end portion of an enlarged, downwardly and rearwardly extending exhaust duct. The air stream from the exhaust tube merges with a cooling air stream established in the cooking appliance about the oven cavity, while additional fresh intake air directly enters the oven cavity about the door. 
     The present invention is more specifically concerned with providing a combination cooking appliance including a recirculating airflow which is directed through particularly configured plenum and nozzle structure which synergistically combine to enhance the overall efficiency and effectiveness of the appliance. In particular, the configuration of the air plenum, as well as the sizing and distribution of air nozzles, provides for an enhanced flow of heated air to the food product, thereby resulting in short cooking times due to high heat transfer while also providing an even browning and crisping of the food item being cooked. Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an upper right perspective view of a cooking appliance incorporating a combination heating system constructed in accordance with the present invention; 
         FIG. 2  is a cross-sectional side view of the cooking appliance constructed in accordance with the present invention; 
         FIG. 3  is a partial cut-out side view of the cooking appliance of  FIG. 2 ; 
         FIG. 4  is a schematic side view of a nozzle and food support configuration in accordance with the invention; and 
         FIG. 5  is a top view of a nozzle pattern employed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With initial reference to  FIG. 1 , a cooking appliance constructed in accordance with the present invention is generally indicated at  2 . As will be discussed more fully below, cooking appliance  2  constitutes a combination cooking appliance which preferably employs radiant, convection and microwave heating techniques to rapidly and effectively cook a wide range of food items. As shown, cooking appliance  2  includes a cabinet  4  and a door  6  that is provided with a handle  8 . In the preferred embodiment shown, door  6  is pivotally mounted about a lower horizontal axis such that handle  8  can be utilized to open door  6  in order to access an internal oven cavity  12 . However, a vertical pivoting arrangement, or another type of door opening system, could be readily employed. Cooking appliance  2  also includes a programming interface  15  which is shown to include a control knob  18 , a key pad  20  and a touch pad display  22 . 
     As perhaps best illustrated in the schematic side view of  FIG. 2 , oven cavity  12  includes top, bottom, rear and opposing side walls  28 - 31  that collectively define a cooking chamber. Mounted within oven cavity  12  is an oven rack  35  which, in the preferred embodiment, constitutes a metal wire rack used to support a food item  36  during cooking. The use of a metal wire rack is desired in order to allow effective air flow around food item  36 , while also providing a fairly open support area such that the food item  36  can be radiantly heated from below. As depicted, oven rack  35  is spaced from bottom wall  29  to allow for a sufficient volume of air to flow past the to food item  36  such that convection heating can be utilized for both the top and bottom of the food item  36 , while also promoting even browning on both the top and bottom. Arranged between oven rack  35  and bottom wall  29  is a radiant heating element  40 . More specifically, a support  42  is provided to retain radiant heating element  40  at a substantially intermediate position between bottom wall  29  and oven rack  35 . In the most preferred embodiment shown, a single radiant heating element  40  is employed, with radiant heating element  40  being constituted by a sheathed, resistive-type heating element. However, at this point, it should be recognized that multiple radiant heating elements of varying designs can be employed in connection with the invention. 
     Extending about at least the rear and upper portions of oven cavity  12  is an air plenum  44 . More specifically, air plenum  44  includes a return plenum portion  46  which is arranged behind and at least partially defined by rear wall  30 . Rear plenum portion  46  has associated therewith an air intake  47  which is defined by a plurality of spaced openings located in rear wall  30  of oven cavity  12 . In the most preferred embodiment shown, the plurality of openings extend from above rack  35  to below radiant heating element  40 . Air plenum  44  also includes a blower plenum portion  49  and a discharge plenum portion  52 . Mounted in blower plenum portion  49  is a blower  55  that includes an electric motor  56  and a fan or impeller unit  57 . Blower plenum portion  49  is in fluid communication with return plenum portion  46  through a plurality of openings  60  which leads to a central intake portion of fan unit  57 . 
     Downstream of blower  55 , essentially at the juncture of blower plenum portion  49  and discharge plenum portion  52 , is a convection heating element  65 . Although convection heating element  65  can take various forms, it is preferred to employ a sheathed electric heating unit. Mounted directed adjacent convection heating element  65  is a catalyst unit  67 . In a manner known in the art, catalyst unit  67  functions to chemically breakdown airborne byproducts associated with cooking operations. Given the relative position between convection heating element  65  and catalyst unit  67 , convection heating element  65  advantageously functions to heat catalyst unit  67  and maintain its temperature at or above a minimum temperature required for proper function of the catalyst. 
     As clearly shown in these figures, discharge plenum portion  52  is provided with a plurality of nozzles  70  which are spaced along top wall  28  and fluidly interconnect discharge plenum portion  52  with oven cavity  12 . In the preferred embodiment shown, the plurality of nozzles  70  are essentially grouped in a first or foremost region  72 , a second or intermediate region  73 , and a third or aft region  74 . As depicted, a portion of the plurality of nozzles  70  located in third region  74  are located prior to catalyst  67 , while the remainder of the plurality of nozzles  70  at each of first, second and third regions  72 - 74  are arranged downstream of catalyst  67 . Further details of the nozzle arrangement will be provided below. For use in connection with controlling operating temperatures within oven cavity  12 , a first temperature sensor  76  is provided within return plenum portion  46  and a second temperature sensor  77  is provided within discharge plenum portion  52 . In the most preferred embodiment shown, second temperature sensor  77  is provided directly adjacent catalyst unit  67 . As the manner in which heating elements  40  and  65  are controlled to establish and maintain a desired temperature within oven cavity  12  is not part of the present invention, it will not be further discussed herein. Instead, reference is made to copending U.S. patent application entitled “Temperature Control for Cooking Appliance Including Combination Heating System” filed on even date herewith and incorporated herein by reference. 
     With the above arrangement, operation of blower  55  will create a circulating air flow into and out of oven cavity  12 . More specifically, a return flow constituted by a lower flow  79  going below oven rack  35  and an upper flow  80  going over food item  36  will be drawn in to return plenum portion  46  through the plurality of openings  47  of the air intake. This return air, generally indicated at  81 , will flow by first temperature sensor  76  and be directed into fan unit  57  through openings  60 . Through operation of fan unit  57 , the return air flow  81  will be redirected radially outwardly as convection air stream  82 . Air stream  82  will be heated upon flowing across convection heating element  65 , with a portion of the convection air stream  82  flowing directly into oven cavity  12  at the various nozzles  70  (see air streams  84 ) arranged in third region  74  prior to catalyst  67 , while a majority of the air flow will flow across catalyst  67  as indicated at  85 . The air stream  85  will then flow through the remainder of the plurality of nozzles  70  in the first-third regions  72 - 74  and back into oven cavity  12 , with the air flowing around food item  36  prior to the flow cycle being repeated. 
     In accordance with the most preferred embodiment of the invention, oven cavity  12  is not completely sealed. Instead, during operation of cooking appliance  2 , a low pressure is established within oven cavity  12  and a certain amount of ambient air is drawn into oven cavity  12  around door  6 , as indicated at  88 . In addition to providing a certain amount of fresh air for the cooking operation, the ambient air stream  88  advantageously functions to reduce door temperature. Based on the desire to allow ambient air  88  into oven cavity  12 , cooking appliance  2  also employs an exhaust system wherein a small amount of the generally recirculated air is exhausted. More specifically, the exhaust system includes an exhaust tube  92  having a first end  93  opening up into discharge plenum portion  52  and a second end  94  which terminates at a predetermined location within cavity  4 . With this arrangement, the recirculated air utilized for cooking has a portion thereof which is lead into exhaust tube  92  so as to define a cooking exhaust stream generally indicated at  95 . The overall exhaust system also includes an exhaust mixing tube  97  having a first end  99  which preferably encompasses and yet is radially spaced from second end  94  of exhaust tube  92 . That is, first end  99  of exhaust mixing tube  97  preferably has an enlarged diametric dimension relative to second end  94  of exhaust tube  92 , with exhaust mixing tube  97  also having associated therewith a second end  100  having an upwardly opening exit  102 . As will be discussed more fully below, exhaust mixing tube  97  preferably angles downwardly and rearwardly within cabinet  4  as clearly shown in  FIG. 2 . 
     Also included in connection with cooking appliance  2  is a cooling system arranged within cabinet  4 . More specifically, a cooling fan  105  is mounted behind return plenum portion  46 . Operation of cooling fan  105  functions to draw a flow of cooling air  106  into cabinet  4 , such as through cabinet openings  108 . The cooling air  106  is directed by cooling fan  105  through a rear chamber  111  to establish a cooling exhaust stream indicated at  114 . Cooling exhaust stream  114  flows about and cools motor  56 , as well as various electronic components (not shown), and then around second end  94  of exhaust tube  92  as indicated at  115 . This cooling exhaust stream  115  then flows into exhaust mixing tube  97  at first end  99  and mixes with cooking exhaust stream  95  to establish a mixed exhaust stream indicated at  117 . Actually, the provision for the cooling air flow creates an elevated pressure in cabinet  4 , about oven cavity  12 . The cooling exhaust stream  115  flows at a relatively high speed into exhaust mixing tube  97  such that a suction is created which draws cooking exhaust stream  95  into exhaust mixing tube  97 . Mixed exhaust stream  117  is then lead through exhaust mixing tube  97  and out upwardly opening exit  102  as a final exhaust  120 . 
     With the arrangement discussed above, cooking appliance  2  can be employed to cook food items utilizing both radiant and convection cooking techniques. However, it is also desired to further incorporate microwave cooking techniques. To this end, as best shown in  FIG. 3 , cooking appliance  2  is also provided with a pair of wave guides mounted along opposing side walls  31 , with one of the waveguides being indicated at  127 . Basically, the arrangement on each side of oven cavity  12  is preferably, identically constructed such that reference will be made in detailing one side arrangement. In any case, in connection with the microwave system employed, waveguide  127  includes a first end portion  129  and a second end portion  130 . Generically indicated at  133  is a magnetron for generating microwaves which propagate through waveguide  127  and are lead into oven cavity  12 . At second end portion  130  is provided a stirrer assembly generally indicated at  137 . In the preferred embodiment shown, stirrer assembly  137  includes a motor housing  140  to which is applied electric current through wires  141 . Motor housing  140  has associated therewith a rotatable output shaft  142  linked to a mechanism  147  for transferring the rotation of output shaft  142  to a drive member  164 . Drive member  164  is fixed for rotation to a rotatable stirrer  168  (see  FIG. 2 ) which is mounted behind a respective side wall  31 . Further details of the microwave supply and distribution arrangement is not part of the invention but rather can be found in copending U.S. patent application entitled “Combination Cooking Appliance Including Multiple Microwave Heating Units With Rotatable Antennae” filed on even date herewith and incorporated herein by reference. In the most preferred embodiment, the overall microwave system is designed to introduce microwaves from both of opposing side walls  31  at a position close to top wall  28 . This arrangement is considered to at least facilitate the use of metal pans by introducing the microwave energy into the oven cavity at a position wherein the metal pans will not block the microwave energy from reaching the food load. In any case, the stirrers  168  provided on side walls  131  further enhance the distribution of the microwaves for oven cavity  12 . 
     With the above construction, cooking appliance  2  can operate in various modes including: microwave only mode; bake mode; combination bake and convection mode; and full combination mode with microwave, radiant, and hot air convection for high speed food cooking. As the operational mode details are not part of the present invention, they will not be further discussed herein. Instead, reference is made to copending U.S. patent application entitled “Cooking Appliance Including Combination Heating System” filed on even date herewith and incorporated herein by reference. In any case, the present invention is particularly concerned with the air circuit features of cooking appliance  2 . More specifically, one aspect of the present invention is the location of fan unit  57  in combination with the angling and relative dimensions of blower plenum portion  49  as shown in  FIG. 2 . First of all, blower plenum portion  49  is angled at approximately 45°. This angling is also partly reflected at the juncture of top wall  28  and rear wall  30 . Second, blower plenum portion  49 , which is arranged downstream of return plenum portion  46  and almost entirely behind rear wall  30  of oven cavity  12  such that the plurality of openings  60  are rearward of the openings of air intake  47 . Third, blower plenum portion  49  preferably has a depth that is less than a width of return plenum portion  46  and in the order of about ⅔ of the height of air intake  47 , while approximately 20% of air intake  47  is arranged above rack  35 . This overall arrangement provides for an extremely efficient ingress of recirculated air which is heated downstream of fan unit  57  and directed to the plurality of nozzles  70 . 
     Another aspect of the present invention is concerned with the preferred geometry for the plurality of air nozzles  70  in cooking appliance  2 , with the specified geometry providing for effective heating and browning or crisping of food item  36 , while accelerating the cooking time. As particularly shown in  FIGS. 4 and 5 , rack  35 , or other food item support, is held in a fixed position with respect to the nozzles  70  during cooking. As indicated above, nozzles  70  are part of a forced air circuit which directs high speed heated air towards food item  36  placed on rack  35  or other surface located below nozzles  70 . The nozzles  70  are preferably arranged in a plane parallel to a top plane of rack  35  as best shown in  FIG. 4 , with a center axis of each of nozzles  70  being substantially perpendicular to the two planes. The distance from the exit of each nozzle  70  to the top plane of rack  35  is defined herein as H. The inner diameter of each nozzle  70  at the exit of the nozzle is defined herein as D. The distance between nozzle centers is defined herein as S. Based on the above: 
     D=nozzle inner diameter at nozzle exit 
     S=center to center distance between nozzles 
     H=distance from the nozzle exit to the top plane of the oven rack or other food load support structure. 
     In addition, the ratio of H divided by D is identified herein as the H/D ratio. The ratio of S divided by D is identified herein as the S/D ratio. With this data in mind, cooking appliance  2  has been specifically configured with an H/D ratio in the range of 19.0 to 22.0 and an S/D ratio in the range of 5.7 to 5.9. 
     As previously indicated, it is a further aspect of the invention that the spacing between the nozzles  70  increase from first region  72  to second region  73  and further increase to third region  74 . Basically, given the length and flow characteristics of discharge plenum portion  52 , decreasing the spacing between adjacent nozzles  70  as air stream  85  flows forward such that the greatest number of nozzles  70  is in the first or forwardmost region  72  provides for a more even distribution of the overall airflow exiting the plurality of nozzles  70  into oven cavity  12 , thereby enhancing the even heating and browning of food item  36 . In fact, with these overall design parameters, an efficient airflow and superior results are considered to be achieved in that uneven browning patterns are significantly less evident, while effective heat transfer is provided. Actually, the invention has surprisingly demonstrated a significant improvement in browning to consistency, providing extremely even browning of food products. 
     Although described with respect to a preferred embodiment of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. In general, the invention is only intended to be limited by the scope is, of the following claims.