Abstract:
Embodiments of the present invention can be used in situations in which heat and mass-transfer modified product, such as individual-sized pizzas, are to be prepared in relatively short periods of time, for example in one to five minutes. Through structural design features and air manipulation, embodiments of the invention provide distinct advantages over the prior art, including improving quality control by increasing uniformity of heating across the oven and within each particular product, decreasing required baking time by increasing the effectiveness of impinging air jets through air pulsation and fluctuation, and maintaining uniformity of air flow to contact the baked product substantially perpendicular to its surface. Tubes or pipes possessing impingement nozzles permit the use of compressed air instead of fans. Embodiments of the invention permit more compact oven design, better control of air jet parameters, and quieter oven operation, and other advantages.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The subject matter of this application is related to the subject matter of commonly assigned, co-pending Application No. 60/125,811, filed Mar. 23, 1999, priority to which is claimed under 35 U.S.C. §119(e) and which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to improving airflow in heat and mass transfer processes such as baking, freezing or drying and, more particularly, to improving impingement airflow by way of air fluctuation, increased airflow uniformity, and enhanced control of airflow. Embodiments of the invention are particularly applicable to use with ovens designed for baking pizza, especially restaurant ovens that can utilize the limited space available more efficiently and operate more quietly for a more compatible working environment. 
     2. Description of Related Art 
     Impingement processing that achieves heat and mass transfer modification of products is particularly attractive because impinging jets of air disrupt the layers of stagnant gas surrounding the surface of a product, thus increasing convection heat and mass transfer. The latter may also be increased by: vibration and pulsation of sound waves, rifling of air jets to be more cylindrical and less conical, rotating a jet about its axis, or swirling an air jet. See Huang, L. et al, “Heat Transfer and Flow Visualization Experiments of Swirling, Multi-Channel, and Conventional Impinging Jets,”  Int. J. Heat Mass Transfer , Vol. 41, No.3, pp.583-600 (1998), incorporated herein by reference, and placing a return air duct adjacent to air jet nozzles to make the jets more effective as described with respect to FIG. 1 of U.K. Patent No. 914,546, also incorporated herein by reference. 
     Many impingement ovens, for example, are hampered by a lack of uniformity of airflow due to design limitations. Nonuniform impingement airflow can cause irregularities within the oven, within baked products and between products being cooked at the same time. For example, the right-hand side of a pizza may be more baked than the left-hand side in some instances of nonuniform airflow, or identical raw pizzas may be baked to differing degrees in different zones of the same oven. Three important types of nonuniformity that adversely affect heat and mass transfer are: (1) differences in absolute air velocity at different nozzles due to irregular air flow into and out of the plenum chambers; (2) discharge of air from the nozzles in a non-perpendicular orientation to the surface of the heat and mass transfer modified product; and (3) tendency of the air jets to fan out into a widening cone more quickly than desired. 
     Past heat and mass transfer impingement devices have attempted to compensate for some of these deficiencies by passing air through long, thin, straight pipes which can impart a rifling effect to the discharged air. Air flowing through the pipes meets resistance to flow and experiences a pressure differential across the pipes. Expanding the pipe length aids in equilibrating the discharged air flowing from the pipes. Such processing equipment, however, tends to be very large and requires pipe lengths ranging from about 50-60 centimeters with pipe diameters of approximately two centimeters. 
     Impinging jets of gas in an oven can be expelled through nozzles in pipes through which gasflows under pressure, as described in U.K. Patent No. 985,443, incorporated herein by reference. Also, a high velocity jet of gas, so propelled, can be used to propel the flow of other gases by applying the Venturi effect, as exemplified by Newberry, Desmond, “Vacuum Cooling,” in  Proceedings of  72 nd    Annual Conference of the American Society of Bakery Engineers , Chicago, Ill., pp. 81-86, (March 1996), incorporated herein by reference. In both cases, airflow can be quieter and better controlled than with the use of plenum chambers into which air is propelled with the help of large fans and large air-circulation chambers. This is because the propulsion of gas under pressure can be regulated more conveniently by the adjustment of a valve opening than by the modulation of fan speed, without the inherent complexities of aerodynamic airflow within large chambers and the noise of the fan and said airflow within said chambers. Accordingly, an impingement system that uses compressed air in pipes with nozzles, or uses air propelled by the Venturi mechanism, promises to be quieter, more compact, and more easily controlled than circulating air with fans into plenum chambers. 
     One particular shortcoming of the prior art that is addressed by embodiments of the invention is the inability to employ impingement processing more effectively in applications where it is desirable to cook a baked product, such as pizza, traveling through the oven on a conveyor belt within the limiting space constraints of a restaurant kitchen. Even in those instances where a conveyor belt is used, only a single rack per oven would be feasible because of the blocking that would occur between vertically spaced baked products in typical ovens. Examples of ovens utilizing single-layer conveyor belts for cooking can be found in Ovadia, David Z., et al., “Impingement in Food Processing,”  Food Technology , Vol. 52, No.4, pp. 46-50 (April 1998), incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are contemplated for use in situations wherein heat and mass transfer modified product, such as individual-sized pizzas, are to be prepared in relatively short periods of time, for example in one to five minutes. Through e.g. structural design features and air manipulation, embodiments of the invention provide distinct advantages over the prior art, including improving quality control by increasing uniformity of heating across the oven and within each particular product, decreasing required baking time by increasing the effectiveness of impinging air jets through air pulsation and fluctuation, and maintaining uniformity of air flow to contact the baked product substantially perpendicular to its surface. Also, the use of tubes, or pipes possessing impingement nozzles described in the following, permits the use of compressed air instead of fans and the implementation of related advantages, permitting more compact oven design, better control of air jet parameters, and quieter oven operation, for example. 
     To overcome disadvantages of the prior art, embodiments of the invention utilize multiple short, thin, straight pipes to effect a columnar airflow which strikes perpendicularly to the surface of the heat and transfer modified product. The pipes are configured to achieve a predetermined ratio of length to diameter and desired pressure differentials across the respective pipes. A multiplicity of smaller adjacent pipes is used to facilitate a compact oven, while maintaining the appropriate length-to-diameter ratio necessary for the rifling of impinging jets. While said smaller pipes may be straight, they may also be spiraled to create swirled jet flow. Alternatively, pipes of substantially equal total length are configured to effect heating wherein one or a set of the pipes are folded back upon themselves or otherwise configured to facilitate a compact-sized heat and mass transfer processing device, such as an oven. 
     In another embodiment, variations in air fluctuation are effected with a shutter, flap, or other mechanism used to alternately route air through airflow conduits to different plenums or areas within the processing chamber to improve and accelerate heat and mass transfer. In one embodiment, a solenoid is employed to alternately open and close two channels through which air is alternately directed. In a further embodiment, a low-frequency whistle mechanism is placed opposite the flow of air being channeled into the processing cavity, but proximal to the plenums or other areas within the processing chamber to create low-frequency air vibration, thereby causing increased fluctuation in the airflow. 
     Although embodiments of the invention are especially well-suited to preparation of individual-sized pizzas, the invention is not limited to these embodiments. Larger pizzas, other pizza-type and pizza-related food products, and other food and non-food products (e.g., drying paper, textiles, wood and curing electronic circuit boards, etc.) also can be prepared according to embodiments of the invention. 
     More specifically, a nozzle insert according to an embodiment of the invention is for an impingement oven, the nozzle insert comprising an outer shell, connection structure, operably coupleable with the outer shell, constructed to position the nozzle insert to direct gasflow toward a processing chamber of an impingement oven, and a plurality of conduits disposed within the outer shell, the plurality of conduits being constructed to create a rifling effect on gasflow passing through the plurality of conduits, thereby creating a plurality of gas jets for impinging on a product in the processing chamber. The outer shell is generally cylindrical, according to one embodiment, and the plurality of conduits comprises a plurality of generally cylindrical tubes. The connection structure can be constructed to removably connect the nozzle insert in position within the oven. 
     According to one embodiment, the outer shell has an overall length of between about 2 cm and about 5 cm, and the plurality of conduits each have a diameter of between about 0.1 and 0.2 cm. The nozzle insert preferably creates a pressure differential between a processing-chamber end thereof and an opposite end thereof, the pressure differential being generally the same as that created by a single conduit having a length up to about 12 times longer than the outer shell and having a diameter up to about 10 times the diameter of the plurality of conduits. 
     The connection structure is constructed to removably connect the nozzle insert to a nozzle of the impingement oven, according to one embodiment, and the plurality of conduits are generally straight and provide a generally linear gasflow path. Alternatively, the plurality of conduits can be generally curved and provide a generally spiral gasflow path. Further, according to one embodiment the connection structure is constructed to connect the nozzle insert to a plenum of the impingement oven. 
     According to an alternative embodiment of the invention, an impingement oven includes a processing chamber and a plurality of nozzle inserts positioned to direct gasflow to the processing chamber, each nozzle insert comprising an outer shell, connection structure, operably coupleable with the outer shell, constructed to position the nozzle insert to direct gasflow toward the processing chamber of the impingement oven, and a plurality of conduits disposed within the outer shell, the plurality of conduits being constructed to create a rifling effect on gasflow passing through the plurality of conduits, thereby creating a plurality of gas jets for impinging on a product in the processing chamber. 
     According to an alternative embodiment of the invention, an impingement oven comprises a source of gas and a plurality of conduits constructed to direct heated gas from the source to a processing chamber within the oven, the plurality of conduits further being constructed to direct impinging gas to a product within the processing chamber, the plurality of conduits terminating at the processing chamber at different distances from the source; the plurality of conduits each having generally the same length. At least one of the conduits can fold back on itself between the source and the processing chamber. The impingement oven can further comprise a plurality of return conduits constructed to direct return gas from the processing chamber to the source, the return conduits originating at the processing chamber at different distances from the source, the return conduits each having generally the same length. At least one of the return conduits can fold back on itself between the processing chamber and the source. 
     The impingement oven can further include at least one nozzle insert, in fluid communication with at least one of the conduits, the at least one nozzle insert being positioned to direct gasflow to the processing chamber, the at least one nozzle insert comprising, an outer shell, connection structure, operably coupleable with the outer shell, constructed to position the nozzle insert to direct gasflow toward the processing chamber of the impingement oven, and a plurality of subconduits disposed within the outer shell, the plurality of subconduits being constructed to create a rifling effect on gasflow passing through the plurality of subconduits, thereby creating a plurality of gas jets for impinging on the product within the processing chamber. 
     According to an alternative embodiment, an impingement oven comprises a processing chamber, first structure at the processing chamber for directing impinging gas to a product in the processing chamber, second structure at the processing chamber for directing impinging gas to the product in the processing chamber, a gas source in fluid communication with the first and second structures, conduits for directing gas from the gas source to the first structure and the second structure and a gas fluctuation device, operably coupled with the conduits, for alternately channeling gas either to the first structure or to the second structure. 
     The first structure can comprise a plurality of nozzles of the impingement oven, and the plurality of nozzles can be a plurality of first nozzles. The second structure can comprise a plurality of second nozzles of the impingement oven, wherein the second nozzles are disposed on an opposite side of the processing chamber from the first nozzles. 
     The first structure comprises a plenum plate of the impingement oven, according to one embodiment, and the second structure comprises the plenum plate or an additional plenum plate of the impingement oven. The first structure can comprise a plurality of first nozzles and the second structure a plurality of second nozzles, the first nozzles being interspersed with the second nozzles. 
     The first and/or second structure comprises at least one nozzle insert, according to one embodiment, in fluid communication with at least one of the conduits, the at least one nozzle insert being positioned to direct gasflow to the processing chamber, the at least one nozzle insert comprising an outer shell, connection structure, operably coupleable with the outer shell, constructed to position the nozzle insert to direct gasflow toward the processing chamber of the impingement oven, and a plurality of subconduits disposed within the outer shell, the plurality of subconduits being constructed to create a rifling effect on gasflow passing through the plurality of subconduits, thereby creating a plurality of gas jets for impinging on the product within the processing chamber. 
     The gas source can comprise a source of compressed air, and the gas fluctuation device can comprise a shutter, the shutter moving to a first position to direct gas to the first structure and moving to a second position to direct gas to the second structure. The shutter can comprise a pivoted flap, and can define at least one aperture therethrough for allowing gas to flow alternately to the first structure and the second structure. The shutter can define a generally circular shape and define a plurality of apertures disposed therethrough, and the oven further can comprise a shutter plate operably connected to the shutter, rotation of the shutter plate with respect to the shutter allowing gas to flow through the apertures alternately to the first structure and the second structure. The shutter is constructed for linear movement between the first and second positions, according to one embodiment. 
     The gas fluctuation device also can comprise at least one reciprocating piston in fluid communication with at least one of the conduits. The conduits can comprise a first conduit in fluid communication with the first structure and a second conduit in fluid communication with the second structure, and the gas fluctuation device can comprise a first reciprocating piston in fluid communication with the first conduit, and a second reciprocating piston in fluid communication with the second conduit. The first and second structure can comprise a plurality of nozzles. The oven further can comprise a plurality of valves, in fluid communication with the first and second conduits, for aiding the gas fluctuation device. 
     The conduits comprise at least one first conduit constructed to direct gas from the source toward the processing chamber, according to one embodiment, and at least one second conduit constructed to direct gas from the processing chamber toward the source. The gas fluctuation device reverses gasflow within the oven such that the first conduit also directs gas from the processing chamber toward the source and such that the second conduit also directs gas from the source toward the processing chamber. The conduits terminate at the processing chamber at different distances from the source, according to one embodiment; the plurality of conduits each having generally the same length. Additionally, structure for producing low-frequency vibration also can be provided. 
     According to another embodiment of the invention, an impingement oven comprises a processing chamber, first means at the processing chamber for directing impinging gas to a product in the processing chamber, second means at the processing chamber for directing impinging gas to the product in the processing chamber, means for providing a source of gas, means for directing gas from the means for providing to the first means and the second means, and means for alternately channeling gas either to the first means or to the second means. 
     According to another embodiment of the invention, an impingement oven comprises a processing chamber, a gas source, an input channel for directing gas from the source toward the processing chamber, and a return channel for directing gas from the processing chamber toward the source, wherein at least one of the input channel and the output channel are constructed to support a product to be processed by the oven in the processing chamber. The processing chamber is constructed to accommodate multiple products top-to-bottom within the processing chamber, according to one embodiment, and the oven further comprises multiple input channels disposed directly above and directly below each product within the processing chamber. The product can be supported on the output channel. 
     The oven further can comprise structure to reverse the flow of gas in the input channel and the output channel, such that the input channel directs gas from the processing chamber toward the source and the return channel directs gas from the source toward the processing chamber. The gas source can comprise a compressed gas source, and the structure to reverse can comprise at least one valve associated with the compressed gas source. 
     According to another embodiment of the invention, an impingement oven comprises a processing chamber for accommodating a product to be processed by the oven, a gas source, an input channel for directing gas from the source toward the processing chamber, and a return channel for directing gas from the processing chamber toward the source, the input channel and the return channel comprising tubes disposed within the processing chamber in direct association with the product. 
     The oven can comprise structure to reverse the flow of gas in the input channel and the output channel, such that the input channel directs gas from the processing chamber toward the source and the return channel directs gas from the source toward the processing chamber. The input channel can comprise at least one nozzle aimed toward the product for directing gas toward the product, and the return channel can comprise at least one aperture aimed away from the product for receiving return air from the processing chamber. 
     Both the input channel and the return channel are disposed above the product, according to one embodiment, and the input channel can be disposed below the product and comprise at least one nozzle aimed away from the product. A second input channel can be disposed above the product, the second input channel comprising at least one nozzle aimed toward the product. A rebound plate for receiving and reflecting gas received from the at least one nozzle can also be provided. The gas source can be a compressed gas source. 
     The input channel can comprise at least one nozzle, and the oven further can comprise a heating element disposed around the at least one nozzle for improving heat transfer within the oven. The heating element can comprise an electric coil or an infrared heating element, for example. 
     The oven can comprise two input channels disposed within the processing chamber for directing gas toward the product at different distances from the source, and the two input channels can each have generally the same length. One of the input channels folds back upon itself, according to one embodiment, and at least one of the input channel and the return channel contacts the product. 
     Other features and advantages of the invention will be apparent from the following description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be described with respect to the figures, in which like reference numerals denote like elements, and in which: 
     FIG. 1 shows a typical rifled nozzle. 
     FIGS. 2-3 show an embodiment of the invention utilizing a pipe configuration to improve impingement airflow. 
     FIGS. 4-6 show insertion of the FIG. 2 embodiment into an existing nozzle according to an embodiment of the invention. 
     FIG. 7 shows an airflow path according to the FIG. 2 embodiment. 
     FIGS. 8-9 show airflow paths according to an alternative embodiment. 
     FIG. 10 shows an embodiment of the invention utilizing an alternative pipe configuration to improve impingement airflow. 
     FIG. 11 shows an embodiment of the invention utilizing air fluctuation to improve baking characteristics. 
     FIG. 12 shows an alternative embodiment of the invention utilizing air fluctuation to improve baking characteristics. 
     FIG. 13 shows an alternative embodiment of the invention utilizing air fluctuation to improve baking characteristics. 
     FIG. 14 shows an alternative embodiment of the invention utilizing air fluctuation to improve baking characteristics. 
     FIG. 15 shows an embodiment of the invention utilizing air fluctuation to improve baking characteristics. 
     FIG. 16 shows an embodiment of the invention using low-frequency sound waves to cause air fluctuation. 
     FIG. 17 shows an embodiment of the invention utilizing baking racks and impingement heating. 
     FIG. 18 is a side view of the embodiment depicted in FIG.  17 . 
     FIG. 19 is a view of the embodiment depicted in FIG. 17, utilizing an alternative method of redirecting impingement airflow. 
     FIGS. 20-21 are side and end views, respectively, of an embodiment of the invention utilizing baking racks to effect impingement heating. 
     FIG. 22 is a view of an embodiment of the invention depicting individual heating elements in association with heating conduits. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the invention provide devices and methods for improving impingement airflow in baking ovens by manipulation of the design and/or structure of heating conduits, piping and/or air nozzles, and/or or by utilization of air fluctuation techniques and designs. Embodiments of the invention are designed: (1) to equalize air speed from different nozzles into and out of the plenums or other chambers of an oven, (2) to ensure the maintenance of a substantially perpendicular orientation between the propelled jets of air from the nozzles and the surface of the baked good being heated, (3) to maintain columnar, as opposed to conical, bursts of air, (4) to generally improve heat and mass transfer and accelerate baking and other heat and mass transfer processes, (5) to make ovens more compact where space available for them is limited, and (6) to make oven operation quieter for a more compatible work environment. It is contemplated by the invention that different embodiments herein may be utilized in combination with one another to achieve and improve upon any of the above-stated objectives. 
     Referring to FIG. 1, it is known in the art to pass air through long pipes  10  having a proportionately sized diameter to create a “rifling effect” of air  20  into heating chamber  30 , that is, to create more perpendicular and columnar air flow. Such systems, however, require extensively sized ovens due to the overall dimensions (sometimes exceeding 60 centimeters in length and about 1.5 or 2 centimeters in diameter) of the heating conduits/pipes  10 . According to embodiments of the invention, on the other hand, effective pipe length is substantially reduced. 
     Referring to FIGS. 2-3, one such embodiment is depicted that is particularly, though not exclusively, useful in retrofit applications to nozzles in existing ovens. Nozzle inserts  40  are clipped, snapped, screwed, inserted, force-fit or otherwise placed in association with existing holes or nozzles  50  of an oven. The term “insert” should not be interpreted as limited to a retrofit application; use by original equipment manufacturers is contemplated as well, for example. Inserts  40  disperse and distribute heated air into plenums of an associated oven, or directly into heating chambers thereof. 
     As shown in FIGS. 4-6, inserts  40  can be snapped into or otherwise placed in association with existing oven nozzles having a defined shape. Composite nozzle insert  40  includes a generally cylindrical outer shell  52  to which is secured insertion clip  54 . As clip  54  approaches and engages existing plenum nozzle  56 , as indicated by arrow  57 , nozzle  56  flexes clip  54  inwardly, as at  58 . As shown, nozzle  56  can be considered to be a part of the plenum plate itself. As insertion is completed, clip  54  snaps or otherwise flexes back outwardly to its original position, as at  59  in FIG. 6, to lock composite nozzle insert  40  into place. 
     A multiplicity of much smaller conduits or tubules  60 , having a length of e.g. about 5 centimeters and a diameter of about 2 millimeters, are disposed within inserts  40 . Of course, other dimensions, such as a length of about 2 centimeters, for example, are contemplated according to the invention. Conduits  60  are together contained within a larger insert  40 , and together can create the same general pressure differential ΔP as the typical rifled nozzle of FIG. 1, but in a fraction of the length thereof. In effect, then, the multiplicity of smaller tubules or conduits  60  inserted within each nozzle comprise a compound or composite nozzle, thereby allowing for sufficient airflow to satisfy the above-mentioned and/or other objectives. 
     According to embodiments of the invention, the ratio of pipe length to diameter is optimized to facilitate the “rifling effect” when air is directed out of the nozzle, thereby maintaining a generally columnar arrangement to generally perpendicularly strike the surface of the baked good. Use of the appropriate ratio of length to diameter is necessary to create optimal flow of air jets toward the baked product. The striking of the surface of the baked product generally perpendicularly brushes away the stagnant layer of cooler gases residing at and/or near the surface. Engagement of the impinged air at a generally perpendicular orientation with the surface of the baked product is very important for optimal increase in heating and drying effects. If a generally perpendicular orientation is not maintained, the stagnant gas is not dispersed as well. 
     Small tubes  60  within each nozzle insert  40  are generally straight, in the previously described embodiments, and generally perpendicular to plenum plate  70 . Note airflow path  72  in the side view of FIG.  7 . In another embodiment of this invention, the small tubes may have a spiral formation, with the central axis of the spiral perpendicular to the plenum plate and/or baked product, consistent with airflow path  74  in the side view of FIG.  8 . This embodiment enables the air jets to be swirled, with the inherent advantages of improved heat and mass transfer and uniformity thereof. As shown in the top view of FIG. 9, the multiple spiral tubules of a composite nozzle insert according to this embodiment impart angular momentum to multiple air jets, providing multiple swirled airflow paths  76 . Of course, inserts  40  can be attached to one or more plenum plates within a particular oven or oven arrangement, with either the swirled or straight-line nozzle insert embodiment. Additionally, if desired, the number and/or pattern of holes or nozzles  50 ,  56  receiving inserts  40  can be varied, to create desired air flow characteristics, and both straight-line and swirled inserts can be used on the same plenum or in the same oven, as may be desired. 
     In a further embodiment, the invention contemplates the use of two or more heating pipes or ducts which have different configurations but which are of substantially the same overall length. As shown in FIG. 10, for example, one pipe represented schematically at  80  forms a generally L-shaped conduit while a second pipe represented schematically at  90  continually folds back against itself. Return air flow, illustrated schematically at  100 ,  110 , is generally parallel to flow within pipes  80 ,  90 . As shown, both pipes  80 ,  90  have substantially the same overall length from gas propulsion mechanism  120 , such as a compressed air source, fan and/or gas propelled by the Venturi effect, to the exit of the nozzle. The pressure differential from mechanism  120  to the exit of each nozzle is the same for each of pipes  80 ,  90 . In other words, if A represents mechanism  120 , B represents the end of pipe  90 , and C represents the end of pipe  80 , then P A −P B =P A −P C . Additionally, portions, e.g. generally equal portions, at the distal ends of each pipe or set of pipes, for example the last 10 centimeters or so, are arranged generally perpendicularly to the surface of the baked good to ensure the desired columnar air configuration and to achieve the desired heating effects attendant with impingement. Such an apparatus is particularly useful for new designs or redesigns of ovens. 
     While FIG.  10  and the above description discuss the use of two pipes  80 ,  90 , it should be understood that other numbers of pipes may be used to effect the desired heating, limited only by e.g. the size of the oven. Because all of the pipes are of generally the same length, the pressure drop across the pipes is substantially the same, as with the FIG. 10 embodiment. Accordingly, velocity of the impinged air exiting the nozzles is also substantially the same. Similar ducts or piping are present within the oven to facilitate return of air to the heater and the gas propulsion system. Exit orifices in the return air ducts should be spaced properly to encourage perpendicularity of air flowing out of the nozzles to the baked good. For example, use of multiple return air ducts would help to maintain pressure uniformity across the exit or return pipe orifices, as well as within the plenum. Conversely, utilization of too few return conduits or orifices would increase the pressure drop across the respective nozzles, thus affecting pressure within the plenum and causing nonuniform airflow. This nonuniformity will affect not only the return airflow out of different zones of the oven cavity, but likewise the ability of air flowing out of the nozzles into the oven cavity to strike the surface of the baked good at a perpendicular orientation and at uniform air speed, thus decreasing the desired uniformity of heat and mass transfer effects of impingement. As such, embodiments of the invention are utilized both for input and outtake of heated air to best improve heat and mass transfer efficiency on a system-wide basis. 
     In other embodiments of the invention, oven air flow conduits leading into the processing chamber are designed to utilize air fluctuation techniques to improve heat and mass transfer and additionally to accelerate, for example, baking of a product. Such utilization of air fluctuation techniques results in a marked improvement over the continuous flow of air typically used in impingement processing chambers. Air fluctuation results in marked improvement as a result of alternately channeling air into different conduits resulting in interruptions in airflow at different locations within the processing chamber. For example, if a first conduit supplies heated or cooled air to an upper plenum and a second conduit supplies heated or cooled air to a lower plenum of the processing chamber, a valve or other shutoff mechanism is utilized to channel the entirety of airflow first into one and then into the other conduit in an alternating fashion. Such manipulation of airflow will result in discrete bursts of air coming alternately from the upper and then the lower plenum into the impingement processing chamber. The result will be improved removal of the aforementioned layers of heat and mass transfer stagnant gases at the surface of the modified product. 
     In one embodiment, as depicted in FIG. 11, heating conduit or conduits  130  are separated to travel to top plenum  140  and base plenum  150 . As shown, air flow  160  is alternately directed completely into either top plenum  140 , as shown at  170 , or base plenum  150 , as shown at  180 , and then ultimately expressed through impinging jet holes or nozzles  190  into impingement baking or processing chamber  200 . Flap or other blocking device  210  is utilized to alternately block the respective plenums  140 ,  150 . As shown herein, flap  210  oscillates up and down about pivot  215  between positions  220  and  230 , at a predetermined frequency, to optimize the improved heating. Oscillation may be caused by rotating disk  240  attached to rod or cam  250  at pivot  260 , by a solenoid, or by any other suitable means of moving the flap. Impingement is improved by airflow fluctuation caused by the illustrated mechanism. 
     This particular embodiment is advantageous in that it can be readily adapted to many existing ovens without requiring much alteration, as flaps are presently positioned in many ovens. The existing flaps, however, are typically fixed at a given setting and are adjusted by a screw or other similar means, thereby being incapable of alternating back and forth between partially or completely closed and open positions during the heating/baking process, as hereinabove described. 
     Maximum air velocity is increased according to this embodiment because, at two stages of each cycle, as depicted at positions  220  and  230  in FIG. 11, the entire airflow is diverted to only one of plenums  140 ,  150 . Such diversion of air is typically not practiced in ovens of the prior art. It is further understood that during portions of each cycle, air is directed into both plenums  140 ,  150 . Because increasing air velocity increases heating in impingement ovens, the design as depicted in FIG. 11 seeks to optimize impingement heating. Flap  210  may continually move between positions  220  and  230  or, alternatively, may be programmed to momentarily stop at respective positions  220 ,  230  or at any point or points along its path of travel between positions  220 ,  230 . Microprocessor or other control is contemplated. 
     As shown in FIG. 12, a further embodiment of the invention contemplates the use of two or more channels  270 ,  280  positioned in a substantially parallel and adjacent manner. With the use of solenoid  290 , or other means of effecting movement, shutter  300  having at least one opening  310  is moved across or through the channels. Shutter  300  effectively blocks airflow through the channels except when the one or more shutter openings  310  are positioned across the particular corresponding channel or channels. Utilization of shutter  300  results in fluctuation of air alternately through each channel  270 ,  280  into respective top and base plenums of an oven, for example. One or more of the channels can be directed toward the top plenum, while the remaining channels can be directed toward a base plenum. Air fluctuation is effected by the constant up-and-down movement of solenoid  300  or other movement means, which continually allows air to pass into different conduits. This embodiment is particularly advantageous in that its design is compact and suitable for use over an extended period of time. 
     Thus, according to the illustrated embodiment, air flows from source  320 , such as an oven fan or compressed air source, into pre-channel area  330  and then into channels  270 ,  280 . Shutter  300  can be mounted on rollers  340  or other suitable mechanisms to permit stable and generally unhindered movement. The generally constant or other movement of shutter  300  “fires” hot air alternatively into the top or base plenum by the up-and-down movement of solenoid  290  or by operation of an alternative mechanism. 
     In a further embodiment of the invention, depicted in FIG. 13, air fluctuation is effected by a rotating shutter disk  350 , preferably partially circular, semicircular or hemispherical in shape. Movement of shutter disk  350  is effected through any suitable means to alternately block portions of air channels  360  respectively directed toward a base plenum and a top plenum. In the illustrated embodiment, for example, air channels  360  illustrated in dashed lines are blocked by shutter disk  350  and are directed to e.g. a top plenum. Air channels  360  illustrated in solid lines are not blocked and are thus free to release impingement airflow, e.g. to a base plenum. As disk  350  rotates, as indicated by arrow  370 , the distribution of blocked and open channels changes. Air channels  360  can be of a style similar to that previously described and depicted with respect to FIG. 10, or of a different style or configuration. Hence, any of the different embodiments of the invention may be combined to optimize heating effects, heat flow and pressure equalization throughout the oven system. 
     In a further embodiment of the invention, depicted in FIG. 14, an alternative means of effecting air fluctuation is shown. In this embodiment, shutter plate  380 , or a series of connected shutter plates, oscillate back and forth on rollers  385  above (and/or below) impingement nozzles  390 , thereby alternately blocking and opening the same to cause alternating impinging jets  393 ,  396 . In one embodiment, shutter plate  380  is connected to rotating motor  400 , which oscillates shutter plate  380 . In operation, shutter plate  380  and rotating motor  400  function much like a piston in a motor. Again, due to the alternate opening and closing of air channels/nozzles  390 , heat is directed to the respective base and top plenums of the oven to optimize air velocity, air flow, and the resulting effects of heat and mass transfer. Although only a top plenum is shown in FIG. 14, it will be understood that the illustrated mechanism also can be used in association with a base plenum, alone or simultaneously with the top plenum. As shown, shutter plate  380  may have multiple openings  410  which can be configured and designed with respect to associated nozzles to allow air to flow into the desired air channels going to top and/or base plenums. 
     In a further embodiment of the invention, depicted in FIG. 15, nozzles and return air ducts alternate their roles with the help of reciprocating pistons  420 ,  430  driven by wheel  440 , itself driven by e.g. a motor, and connectors  450 . With one stroke of piston  420 , nozzles N 5 -N 8  are inlet nozzles that inject air jets  455  onto processed item  460 . Correspondingly, piston  430  moves opposite to piston  420  and removes air  465  from processing chamber  470 , via nozzles N 1 -N 4  which here function as return nozzles. With the second of a two-stroke process, the direction of movement of pistons  420 ,  430  reverses. The nozzles of the first stroke become return air nozzles, and the return air ducts of the first stroke become nozzles injecting air jets into processing chamber  470 . For certain requirements, this may provide a preferred, compact embodiment of oven design and a more economic means of air jet fluctuation. 
     Thus, drive  440 , here a wheel, raises one piston and lowers another piston simultaneously. During 180 degrees of its rotation (half of its 360 total degrees of rotation), air is expelled through jet nozzles N 5 -N 8 , and nozzles N 1 -N 4  serve as return air ducts. During the second half, or, second 180 degrees, of rotation of the wheel, N 1 -N 4  expel jets of air, and N 5 -N 8  become return air ducts. 
     To further facilitate return airflow and reduce unnecessary strain on pistons  420 ,  430  and the wheel  440  that drives them, valve V 1  opens when nozzles N 1 -N 4  release return airflow, and valve V 2  opens to help return airflow in conjunction with nozzles N 5 -N 8 . It is to be understood that FIG. 15, like other figures herein, is a simplified drawing that aims to provide an example and demonstrate a principle, and is not necessarily a blueprint for the precise mechanism that would be used according to the invention. 
     In yet a further embodiment of the invention, shown in FIG. 16, a low-frequency whistle device  480  is placed in front of the heating fan prior to the branching of air channels  490 ,  500  to respective top and base plenums  510 ,  520 . As air  530  is blown across whistle device  480 , air flow fluctuation is created by low-frequency sound waves causing air vibration within plenums  510 ,  520 . Air is then directed from the top and base plenums  510 ,  520  into impingement baking chamber  540  at other than a continuous flow. The fluctuation caused by the low-frequency vibration of the air, represented by arrows  550 ,  560 , increases and decreases pressure at the surface of the baked good in chamber  540 , thereby dispersing the stagnant gases by imploding and exploding, respectively, the stagnant layer and increasing heat and mass transfer. This embodiment as described may be used in combination with other embodiments described herein, e.g. with flap  210  previously described and shown with respect to FIG. 11, for example, to alternately direct air into respective top and base plenum portions of an oven. 
     In further embodiments of the invention, depicted in FIGS. 17 through 21, air conduits  570  comprise perforated hollow tubes which optionally function as racks within the oven on which baked product, such as pizza  573 , is placed through door  574 . These embodiments are particularly useful in ovens where two or more racks are desired, such as in ovens containing conveyor- or batch-type systems where it is desirable to position the racks above and beneath one another, respectively, to increase the load in a given oven capacity of fixed size. Air conduit network  570  preferably includes hollow, perforated tubes, or “flutes,” and may be arranged alternately as input or impingement channels  575  with depending nozzles  580 , and return channels  585  with perforations  590 . 
     Additionally, the use of tubing instead of plenum chambers according to embodiments of the invention encourages the use of gas pressure, providing one or more of the following advantages or other advantages: 
     (a) more compact oven; 
     (b) quieter oven operation; and 
     (c) greater control because all that needs to be controlled is a pressure valve associated with e.g. a compressed gas source, not fan speed and complex airflow in large chambers. 
     As shown in the embodiments of FIGS. 17-19, baked product  573  rests on rack  595 , which is e.g. a typical metal rack forming a grid or other pattern. According to the embodiments of FIGS. 20-21, on the other hand, product  573  rests directly on one of the gas conduits forming flute network  600 . Thus, input or impingement channel  605  according to this embodiment has both ascending and descending nozzles  610  for providing impingement jets  615  to multiple products  573 , based on airflow  620 . Return channels  622  include perforations  624  for receiving return airflow streams  626 , which ultimately combine into return airflow streams  628 . Product  573  rests directly on return channels  622 , according to this embodiment. 
     With the embodiments of FIGS. 17-21, as well as with the other embodiments described in this application, it should be noted that the impingement and return air flutes or conduits can be formed a non-metallic substance such as silica, that not only can withstand the high temperatures of an impingement oven but also can be transparent to microwaves, radio waves, and other energy forms. Such transparency assures that the flutes or conduits will not reflect the microwave, radio wave or other energy, or otherwise render it less effective. 
     Also with the embodiments of FIGS. 17-21, a reciprocating piston can reverse the roles of the input/impingement conduits and the return conduits, e.g. on an oscillating two-cycle basis, in a manner akin to that described with respect to FIG.  15 . Alternatively, one or more fans or compressed air sources  630  can be used to cause impingement air flow  640  and return air flow  650 , generally illustrated in FIGS. 17-19 and  21 . 
     Embodiments of the invention enable simultaneous impingement heating of baked products on more than one rack. With previous ovens, such heating is generally considered impossible because air is directed in through a top plenum and base plenum, as previously described. If multiple racks are positioned in the oven, for example, the lower rack is shielded from the heat of the top plenum by the upper rack and vice versa. By configuring the racks to act as air conduits themselves, for example, or otherwise providing an oven as described herein, this disadvantage is overcome. 
     As illustrated, the baked product, e.g. personal-sized pan pizza  573 , can be on a middle rack and yet impingement heating is easily effected by the fluted conduit network  570  directing impingement air from beneath and above the product. Optionally, the lower impingement flutes may be turned downward to avoid clogging of the orifices with, for example, cheese or other product toppings. In such an instance, rebound plate  670  (FIG. 19) may be positioned beneath product  573  to redirect impingement air  640  toward product  573  in a generally perpendicular orientation. According to the illustrated embodiment, return air  650  flows to an upper portion of the oven and ultimately is returned to source  630 . 
     In a further embodiment of the invention, individual heat transfer elements  680  are affixed to each air pipe or conduit  690  in close association with jet nozzles  700 , as depicted in FIG.  22 . Heat transfer elements  680  may comprise electrical heating coils, for example. Heat transfer elements  680  may run the length of the tubing to improve heat transfer or, alternatively, may be placed inside, next to, or around a portion of the heat transfer conduit, for example, proximal the jet nozzle. In the example of FIG. 22, the same elements supply infrared heat directly to the baked product, increasing overall heat transfer. This embodiment can be used with other above-described embodiments, like that of FIG. 10, for example. 
     Embodiments described with respect to e.g. FIGS.  10  and  17 - 22  apply impingement using tubes. This is particularly adaptable to the use of compressed air as a propellant, as referenced above, instead of e.g. large and cumbersome fans and plenum chambers. Accordingly, advantages noted above of the use of compressed air impingement can be exploited to make ovens quieter, more compact, and airflow more uniform and controlled. 
     While the present invention has been described and illustrated with reference to particular preferred embodiments, the invention is not limited to the specific features and examples given. For example, baking, freezing or drying products other than pizzas or food items, such as drying paper, may benefit from the implementation of impingement heat and mass transfer as herein described. Although the term “air” or “airflow” is used herein, those of ordinary skill will understand that gasses other than air are equally contemplated according to embodiments of the invention. Additionally, combinations of the specified embodiments of the invention may be utilized to increase the advantages of the particular embodiments in any appropriate manner. More specifically, the embodiments shown and described with respect to any one figure can be used with and/or combined with the embodiments of any one or more additional figures. Various other modifications to the invention will occur to those of ordinary skill in the art, and other embodiments and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.