Balanced air return convection oven

Abstract: A method and apparatus for heating a product (P) which includes a plurality of tapered ducts (90; 290) in a cabinet (20) above and below a conveyor (15; 215) to form streams which are directed toward the product (P). Spent air is drawn through return ducts (81; 89) which have intake openings (83, 87) centered between entrance (24; 224) and exit (26) openings in the cabinet (20) and centered between lateral edges of a conveyor (15; 215) and between the tapered ducts (90; 290) to provide a balanced flow of spent air in the cabinet (20) to the return opening (83, 87). Temperature controlled gas is delivered at an angle through an array of openings (98; 298) adjacent opposite edges of an opening (24, 26; 224) through which a conveyor (15; 215) extends to cause most of the heated air to be drawn to the return duct opening (83, 87) and to maintain internal pressure in the cabinet (20) to prevent ingress and egress of air through the opening (24, 26; 224).

Description: Technical Field The apparatus for heating a product disclosed herein incorporates improved air circulation adapted to form air streams angled to exclude ambient air from entrance and exit openings into a cooking chamber, in combination with an air return duct having an air return passage configured to draw spent air to a central portion of the oven to balance air pressure across the openings. Background of Invention The oven disclosed herein relates to devices of the type disclosed in U.S. Patent No. 4,154,861, entitled "Heat Treatment of Food Products"; U.S. Patent No. 4,338,911, entitled "Cooking Apparatus"; U.S. Patent No. 4,377,109, entitled "Apparatus for Baking Food Products Such as Pizzas"; U.S. Patent No. 4,479,776, entitled "Thermal Treatment of Food Products"; U.S. Patent No. 4,679,542 entitled "Fan-Plenum Configuration"; U.S. Patent No. 4,462,383, entitled "Impingement Food Preparation Apparatus"; U.S. Patent No. 4,474,498, entitled "Multiple-Pass Impingement Heating and Cooling Device"; and U.S. Patent No. 4,757,800 entitled "Air Flow System for A Low Profile Impingement Oven." Ovens of the type disclosed in Smith Patent No. 4,679,542 have enjoyed substantial commercial success for food service applications and particularly in fast food operations such as cooking pizza and other food products in restaurants and food service establishments. Food products are rapidly heated by air streams which impinge against the surface of food products moved through the oven on a conveyor. Convection ovens employ high volumes of rapidly moving air which in some configurations tend to draw cool outside air through entrance and exit openings through which a conveyor extends and to permit the escape of heated air through the conveyor openings. Circulation of air into and out of the oven wastes energy required for heating the rapidly circulating air and increases the warmup time required to place the oven into service. Further, the escaping hot air may cause discomfort of nearby persons. The enhanced efficiency and rapid heating of food products in impingement ovens is dependent upon delivering defined air streams to impinge upon discrete areas of a moving food product. Blowers have been positioned either adjacent one side of the conveyorized oven or above or below the conveyor for circulating air. However, the suction toward the inlet of one side of the conveyor has caused air to be drawn into the sides of the inlet and exit openings nearest the blower and exhausted through sides of the inlet and exit openings on the opposite side of the conveyor. The conveyorized convection ovens described in the above identified patents use air ducts extending across the width of the conveyors to supply the air streams. Uniformity of the effective heat transfer across the width of the conveyor is very important for reproducible operation and uniform baking. Inspiration of outside air causes localized temperature differences; and, uneven intensity of an application across the width of the conveyor causes uneven baking. According to the invention, there is provided, apparatus for heating a product comprising: a cabinet; spaced walls on said cabinet, one of said walls having an entrance opening and a second of said walls having an exit opening; a conveyor in said cabinet for delivering products from said entrance opening to said exit opening; plenum means communicating with the interior of said cabinet; air dispensing means having a chamber communicating with said plenum, said air dispensing means extending transversely of said conveyor, said air dispensing means being adapted to deliver a stream of air toward said conveyor; means projecting into said interior of said cabinet for forming air return means communicating with said plenum, said means forming an air return passage positioned for drawing air from said air dispensing means toward a central portion of said conveyor, said means projecting into said interior of said cabinet for positioning said return passage to balance air pressure across the width of said conveyor adjacent said entrance and exit openings; blower means for moving air from said air return means through said plenum means and into said air dispensing means; and means for controlling temperature of air flowing through said plenum means. Improved air dispensing ducts disclosed in preferred embodiments herein include the air directing means for dispensing streams of gas toward the upper and/or lower surfaces of a conveyor and toward impact surfaces of the oven or products entering or leaving the oven. In some preferred embodiments, first and second arrays of air streams are advantageous. The first array of streams is angled or inclined relative to streams of the second array and the streams of the second array define a cooking area with maximum heating efficiency. Improved air flow into the dispensing ducts and improved uniformity of baking is preferably provided by a grid with multiple linear passages in the dispensing duct to form air streams which move longitudinally into the interior of the duct. A vane in the duct may be used to direct a portion of the streams of longitudinally moving air toward selected areas of the duct for balancing and substantially equalizing air flow from different areas of the dispensing duct. In this preferred arrangement, as air leaves the blower and turns into the dispensing ducts the rapidly moving air concentrates toward the outside of the turning radius leaving a low pressure area inside the turning radius. This low pressure area causes extreme turbulence wasting air velocity and also causes the air to apply reduced pressure to the duct openings near the inlet to the duct. The reduced pressure in these openings causes less heat transfer and uneven baking across the width of the conveyor. A multiple opening directing means with linear passages eliminates most of the turbulence in the low pressure area and reduces the starving of the openings nearest the inlet of the duct, thus providing improved uniformity of baking. Description of Drawings Drawings of a preferred embodiment of the conveyorized oven are annexed hereto so that the invention may be better and more fully understood; in which: Figure 1 is a perspective view illustrating the front and the entrance end of the conveyorized oven; Figure 2 is a perspective view illustrating the rear and the exit end thereof; Figure 3 is a cross sectional view taken substantially along line 3-3 of Figure 1; Figure 4 is a cross sectional view taken substantially along line 4-4 of Figure 3; Figure 5 is an exploded perspective view of a partition wall forming a plurality of plenums to which air dispensing ducts are connectable; Figure 6 is a perspective view of the rear of the partition wall illustrated in Figure 5; Figure 7 is a perspective view of the discharge end of the oven, the conveyor and one of the upper dispensing ducts being removed to more clearly illustrate details of construction; Figure 8 is a perspective view of a member having shaped passages communicating with the air return duct; Figure 9 is a perspective view of an upper air dispensing duct illustrating openings forming first and second arrays of air streams; Figure 10 is a perspective view of a lower air dispensing duct illustrating openings forming first and second arrays of air streams; Figure 11 is across sectional view similar to Figure 3 of a modified form of the oven; Figure 12 is a plan view of the perforated surface of dispensing ducts of the oven illustrated in Figure 11. Numeral references are employed to designate like parts throughout the various figures of the drawings. Description of a Preferred Embodiment Referring to Figure 1 of the drawing, the numeral 20 generally designates a cabinet for a conveyorized impingement oven. The cabinet comprises a front wall 21, rear wall 22 and spaced end walls 23 and 25. End wall 23 has an entrance opening 24 extending therethrough and end wall 25 has an exit opening 26 extending therethrough. Entrance opening 24 has top and bottom edges 24t and 24b and front and rear edges 24f and 24r. Exit opening 26 has a top wall 27 and bottom wall 28 extend between front wall 21 and rear wall 22 to form cabinet 20 through which a conveyor 15 extends. Conveyor 15 to support the food product P preferably comprises a continuous loop wire mesh conveyor of conventional design movably supported by rollers 15a adjacent each end of cabinet 20. The conveyor 15 passes through inlet opening 24 formed in the end wall 25 such that the product P may be deposited on the upper portion of conveyor 15 adjacent end wall 25, moved through the oven, discharged through exit opening 26 and deposited on shelf 20a detachably secured by hooks (not shown) to the end of conveyor 15. A front edge 15f of conveyor 15 extends adjacent front edges 24f and 26f of entrance opening 24 and exit opening 26 while a rear edge 15r extends through openings 24 and 26 adjacent rear edges 24r and 26r. Suitable means for driving the conveyor 15 comprises a variable speed electric motor 16 which is drivingly connected to one of the rollers 15a by a drive mechanism 19 Suitable speed control means such as a rheostat 15b is provided for controlling the speed of conveyor drive motor 16. The rheostat may be calibrated to indicate the number of minutes the food product is to remain in the oven depending upon the requirements for cooking a particular food product P. Conveyor drive motor 16 is mounted on the rear wall 22 of cabinet 20 and is drivingly connected to a gear box 17 which drives a flexible drive cable 18 connected to a conveyor gear drive box 19. The roller 15a on the end of conveyor 15 is driven by gear box 19 thorough a quick disconnect drive coupling on the end of drive cable 18 to permit removal of conveyor 15 for cleaning. As best illustrated in Figures 1 and 3 of the drawing, front wall 21, top wall 27 and bottom wall 28 are formed by outer pieces of sheet metal forming a cavity in which a generally u-shaped lining member 30 is mounted to form the interior of the oven and to provide hollow space containing insulation 32 to prevent thermal conduction through walls of the oven. As best illustrated in Figure 3 of the drawing, the layer of insulation 32a in front wall 21 is thinner than the space between front wall 21 and front panel 30a of oven liner 30. Further, sheet 32b of insulation between top panel 30b of liner 30 and top wall 27 is thinner than the space therebetween forming an air passage 35 which extends through the front wall 21 and across the top wall 20 of cabinet 20. Vent openings 21a are formed adjacent the lower edge of front wall 21 permitting air to be drawn by a transverse flow fan 35a through air passage 35 as indicated in the arrows in Figure 3 of the drawing to dissipate heat from the front wall 21 and top wall 27 of cabinet 20. Conveyor speed control rheostat 15b and temperature control thermostat 21a are mounted in the front wall 21 and cooled by the air stream drawn through air passage 35. End walls 23 and 25 of cabinet 20 are preferably detachably secured, as by screw fasteners 23a and 25a to entrance end surfaces 27i and 28i and exit end surfaces 27e and 28e of the top wall 27 and bottom wall 28 to permit removal of end walls 23 and 25, having entrance opening 24 and exit opening 26 formed therein, for cleaning the interior of the oven. A partition wall 40 extends between end walls 23 and 25 and is mounted adjacent rear wall 22 to form a plurality of plenums 40a, 40b, 40c, and 40d in cabinet 20, as illustrated in Figure 5. Referring to Figure 5 of the drawing, partition wall 40 includes a generally flat central portion 42 having a central opening 43 through which a shaft for a fan motor 36 extends. Inclined walls 44 and 46 are secured adjacent opposite edges of central portion 42. A flange 45 comprising an upper leg 45a, a lower leg 45b and spaced vertically extending legs 45c and 45d, extends about the periphery of partition 40. Baffles 47a and 47b support a vane 48 which extends around a portion of the periphery of a blower 60 as will hereinafter be more fully explained. Baffles 49a and 49b support a vane 50. A lower vane 52 has one end welded or otherwise secured to leg 45b of flange 45 and has another end supported by baffle 54 welded or otherwise secured to leg 45b of flange 45. A vane 56 has one end welded or otherwise secured to leg 45a of flange 45 and has another end supported by a baffle 58 secured to flange 45a. A heating element 62 in the form of an electrically energized coil is supported in notches formed in vanes 48 and 50 and baffles 54 and 58, as illustrated in Figures 3 and 5. The temperature may be adjusted by a thermostat 21a mounted on the front wall 21 of the cabinet which is controlled by the temperature sensor 21b located in plenum 40d. As will be hereinafter more fully explained, the baffles and vanes on partition 40, in the illustrated embodiment, form a blower housing 55 having four separate plenums 40a, 40b, 40c and 40d spaced around its periphery through which air is dispensed by a fan 60 into air dispensing ducts 90. Plenum outlet openings are formed by an x-shaped plenum wall member 70 having generally vertically extending legs 72 and 74, as illustrated in Figure 5, and transversely extending legs 76 and 78 which extend outwardly from a central opening 75. Leg 76 has a projection 77 extending outwardly from the end thereof which extends between baffles 47a and 47b on partition wall 40 while leg 78 has a protection 79 which extends between baffles 49a and 49b on partition 40. Legs 72 and 74 have projections 72a and 74a which are positionable adjacent baffles 54 and 56 for securing plenum wall member 70 to flange 45 which extends about the periphery of partition wall 40. As will be hereinafter more fully explained, hollow flanges 76a, 76b, 78a and 78b are formed on legs 76 and 78 and form entrance openings from plenums 40a, 40b, 40c and 40d into a pair of upper air dispensing ducts 90a and a pair of lower air dispensing ducts 90b. Air drawn by fan 60 from the upper and lower return ducts through openings 75 and 75a is projected from blower housing 55 generally radially into corners of plenums 40a, 40b, 40c and 40d. The air flowing into each plenum is turned by inclined walls 44 and 46 toward the passages 40a′, 40b′, 40c′, and 40d′ bounded by flanges 78a, 78b, 76b and 76a into dispensing ducts 90a and 90b. Thus, the direction of flow of air from fan 60 is turned 90 degrees from a plane parallel to back wall 42 into ducts 90a and 90b extending transversely above and below conveyor 15. As will be hereinafter more fully explained, a grid 100 of elongated passages 105 is employed to balance air flow into ducts 90a and 90b to attain uniform air flow through openings 94, 96 and 97 across conveyor 15 into the cooking compartment. To attain uniform cooking of products "P" spread across the width of conveyor 15, it is important that a uniform distribution of air pressure across the width of entrance opening 24 be maintained to prevent drawing ambient air through opening 24 on the blower side of the cabinet adjacent edge 24r and to prevent expelling heated air through opening 24 adjacent edge 24f. To achieve this, it is particularly important that the flow of spent air be returned such that there is no substantial differential in air pressure across the width of entrance opening 24 and exit opening 26 between front edges 24f and 26f and rear edges 24r and 26r. As will be hereinafter more fully explained, upper and lower air return ducts 81 and 89 collect spent air from an area substantially equal distances from lateral edges 24f and 24r of entrance opening 24 and substantially aligned with upper and lower edges 24t and 24b. In the illustrated embodiment of Figures 3 and 4, upper and lower air return ducts 81 and 89 are substantially bisected by a central transversely extending vertical plane C VT and are positioned substantially equal distances above a central horizontal plane C H which extends parallel to conveyor and through the center of entrance and exit openings 24 and 26. Plane C VT which extends transversely of the cooking compartment intersects plane C H along a line in Figure 4 which is parallel to and space equal distances between vertical planes in which entrance opening 24 and exit opening 26 lie. Planes C VL and C H intersect along a line in Figure 3 which is spaced equal distances from front edges 24f and 26f and rear edges 24r and 26r. Entrance opening 24 will preferably be formed to permit passage of a product P therethrough having preselected maximum dimensions. Return passages 82a and 83 into upper air return duct 81 are positioned near the upper surface of a product having the maximum preselected height and are substantially centered is between lateral edges 24f and 24r. The air stream 95 formed by an array of openings 98 is tilted at an angle "A" relative to a vertical plane to divert entrained outside air toward the outside of the cabinet and to direct heated air to the inside of the cabinet. It should be apparent that stream 95 is composed of air from two sources. Temperature controlled air is projected from ducts 90a and 90b to form air streams 95. Since the static pressure of air stream is less than that of ambient air in entrance opening 24, a thin boundary layer of ambient air is entrained and travels with jet 95. If a product P is not on conveyor 15 adjacent air streams 95, the streams will be projected through wire mesh conveyor 15 and impinge upon splash surfaces 99a and 99b on the opposing air dispensing duct 90a or 90b, in the configuration of the apparatus illustrated in Figure 4 of the drawing. As the product P is carried by conveyor 15 between air streams 95, the surface of product P is engaged by the inclined air streams 95 causing the boundary layer of ambient air to be recirculated forming a vortex in the area immediately adjacent inlet opening 24, the recirculating vortex of ambient air tends to reduce heat loss through opening 24. If a solid belt or band is employed for transporting products through the oven, the surface of the belt or band will function as a splash surface for separating the entrained boundary layer of ambient air from the mass of temperature controlled air to minimize ingress and egress of air through opening 24. It should be readily apparent that the provision of balanced air return through upper and lower air return ducts 91 relative to the entrance and exit openings 24 and 26 in combination with inclined air streams 95 which form a dynamic barrier or vortex of ambient air significantly reduces air flow through entrance and exit openings 24 and 26 for retaining heat in the oven and preventing uneven baking as a result of ambient air being drawn into openings in the cabinet nearest the blower and being discharged through ends of the openings on the opposite side of the cabinet. Referring to Figure 8 of the drawing, the numeral 80 generally designates an air return scoop comprising an upper cowling member 82 and a lower cowling member 84 deflected to form a transverse central partition 85 extending therebetween. Opposite edges of central portion 85 are deflected to provide generally vertically extending stand-off elements 86 to space partition 85 from the plane of the suction or intake opening 75 formed in plenum wall member 70. Air scoop 80 is provided with four outwardly extending legs, legs 80a and 80b extending generally horizontally outwardly from standoff members 86 and legs 80c and 80d projecting substantially vertically therefrom. Legs 80a-80d overlie legs 76, 74, 78 and 72 of x-shaped plenum wall member 70 when central portion 85 of air scoop 80 is positioned adjacent central opening 75 in wall member 70. Legs 80c and 80d have an opening 75a formed therein which is aligned with central opening 75 in plenum wall member through which return air is drawn by fan 60. As illustrated in Figure 8 of the drawing, cowling members 82 and 84 are provided with generally v-shaped openings 83 and 87, which as illustrated in Figure 7 of the drawing, extend above and below a central portion of the conveyor 15 extending through the cooking compartment. Passages 82a are formed in cowling 82 and central portion 85 to draw controlled quantities of fluid through the wall of the air scoop 80. Air return openings 82a, 83 and 87 are formed to balance the flow of spent air across the width of conveyor 15 between edges 15f and 15r to balance air pressure in cabinet 20 across the width of entrance opening 24 between edges 24f and 24r and across the width of exit opening 26 between edges 26f and 26r. Referring to Figures 9 and 10 of the drawingl air dispensing ducts 90a and 90b comprise hollow tapered fingers of the general type disclosed in U.S. Patent No. 4,338,911, the disclosure of which is incorporated herein by reference in its entirety for all purposes, having a perforated surface 92 in which stream forming openings are formed. As best illustrated in Fig. 4 of the drawing, upper dispensing ducts 90a and lower dispensing ducts 90b are spaced above and below conveyor 15 and are arranged to direct streams of heated air toward the surface of the product P on the conveyor 15, as will be hereinafter more fully explained. Ducts 90a and 90b preferably comprise channel-shaped body members having a web 92 extending between spaced flanges 93a and 93b which function as sidewalls of each duct. As illustrated in Figures 3 and 9, the cross-sectional area of each duct 90 is greater adjacent its inner end 91 which communicates with the respective plenums 40a, 40b, 40c, and 40d and decreases along the length of the duct toward the outer end 93 adjacent front wall 21. The decreasing cross-sectional area allows for even pressure distribution of the heated air along the length of the cavity through each duct 90. It should be apparent that the major dimension of each duct 90a and 90b is the length thereof which is the distance between ends 91 and 93 of each duct. Plates having a perforated surface 92 are secured across the open side of ducts 90a and 90b. A plurality of rows of perforations are formed in plate surface 92 and are arranged in the pattern as illustrated in Figures 9 and 10. In the illustrated embodiment of Figure 9, rows 94, 96 and 97 of passages form a first array of streams of temperature controlled gas which impinge against discrete areas on the surface of a food product while the row 98 of openings form a second array of air streams which recirculate any outside or ambient air drawn through the entrance and exit openings 24 and 26 back out of the cabinet. As will be hereinafter more fully explained, each air duct 90 has a splash surface 99a or 99b formed thereon against which streams 95 of heated gas of the second array formed by passages 98a or 98b impinge for separating heated air flowing through the duct from outside air which may be drawn into the air stream 95. As best illustrated in Figures 3 and 9 of the drawing, transversely extending inner-connected vertical partitions 102 and horizontal partitions 104 form a grid 100 of elongated passages 105, which project distinct generally parallel air streams, as indicated by arrows, which flow longitudinally into the interior of each air dispensing duct 90a and 90b. It is important to note that although baffles and vanes have been employed heretofore, for example as disclosed in U.S. Patent No. 4,757,800 in an effort to eliminate "cork-screw effect" of air delivered by axial fans, passages through the baffles have been relatively short compared to spacing between the baffles. Consequently, distinct parallel air streams were not formed as the air was moved through the passages between baffles. It is important that the length of each passage 105 be at least as long as the spacing between parallel surfaces of adjacent members 102 and adjacent members 104 to form streams which tend to flow substantially linearly into each duct 90a and 90b. As illustrated in Figure 3 of the drawing, a vane 108 is positioned to direct a portion of the parallel streams flowing through passages 105 in grid 100 toward the perforated surface 92 adjacent the inlet end 91 of duct 90 while permitting passage of a portion of the parallel streams toward the opposite end 93 of air dispensing duct (90 such that air flow through each opening 96 or 98 of each array of openings is substantially uniform longitudinally of each air dispensing duct 90. From the foregoing it should be apparent that the improved air circulating apparatus is particularly adapted for an oven in which a product P is moved in a generally horizontal plane through entrance and exit openings 24 and 26 lying in generally vertical planes into a chamber wherein fluid is moved by a blower 60 in heat exchange relation with the product P in the chamber. Each air dispensing duct 90, having a perforated surface 92 adapted to form and deliver a plurality of streams of air, is supported adjacent a first edge of the entrance opening 24 and exit opening 26 such that air streams 95 are projected across the opening. Each stream 95 is inclined toward the interior of the chamber at an angle indicated by "A" in Figure 11 in a range of 3° to 30° relative to the vertical plane of the opening 24 or 26 to form a stream of air across the opening. Splash surfaces 99a and 99b are formed adjacent an opposite edge of the opening. The air stream is inclined toward the interior of the oven so that most of the hot air returns to the interior of the oven and the entrained ambient air which has become part of the air stream is defected toward the outside of the oven. The splash surface may be opposing properly tilted air stream or a product passing through the oven opening. Perforated surface 92 has a first array of openings in row 94, 96 and 97 having a primary function of transferring heat to the product P and a second array of openings in a row 98 configured to recirculate ambient outside air adjacent the entrance and exit openings. Openings 94, 96, 97 and 98 each array preferably comprising circular apertures, centers of said apertures being spaced apart a distance not exceeding four times the diameter of the apertures. Air directing grid 100 in the air dispensing duct is adapted to form a plurality of generally parallel horizontally directed air streams in each air dispensing duct 90a and 90b as a volume of air is delivered through passages 105. Each of the passages 105 preferably has a length which is equal to or greater than the maximum dimension of any cross sectional area of the passage 105 and preferably at least two times the maximum width or height of the cross-section of the passage 105. Vane 108 mounted intermediate opposite ends 91 and 93 of each duct 90 is arranged to deflect a portion of the generally parallel air streams formed by air directing grid 100 toward openings 94, 96, 97 and 98 formed in the perforated plate 92. Air return ducts 81 and 89 formed by air scoop 80, adjacent air dispensing ducts 90, have inlet openings 83 and 87 positioned intermediate opposite ends of air dispensing ducts 90 for returning air flowing through the perforated surface 92 of each duct. The return ducts preferably have generally V-shaped entrance openings 83 and 87, said entrance openings lying in a plane generally parallel to the plane of the conveyor 15. Blower 60, between the air return ducts 81 and 89 and the air dispensing ducts 90a and 90b, is adapted to draw air from a central portion of the chamber through the V-shaped openings 83 and 87 and to deliver air into the dispensing ducts 90a and 90b through plenums 40a, 40b, 40c and 40d. Heater element 62 is in heat exchange relation with air circulated by blower 60. It should be appreciated that an improved method of controlling heat transfer to a surface of a product is performed by the apparatus hereinbefore described. A first array of a plurality of streams of high velocity gas is directed toward the surface of the product such that the streams of the first array of high velocity gas impinge against discrete areas on the surface of the product to wipe away a boundary layer of air and moisture from the discrete areas against which the streams of the first array impinge. The product P is moved by conveyor 15 relative to the streams of the first array such that heat is transferred, at areas upon which the streams of the first array impinge. A second array of streams of high velocity heated gas are directed at an angle "A" in a range of 3°to 30° relative to the generally vertical streams in the first array to prevent movement of spent air from the first array of streams from a confined area in the compartment. The fact that spent gas from streams of the first array is contained in the confined area by streams of the second array such that the return of spent gas is balanced relative to the entrance and exit openings 24 and 26 and travels between streams of the first array to prevent diffusion of gas in streams of the first array prior to impingement against the surface of the product is an important feature of the improved method. As hereinbefore described, the apparatus for heating a food product comprises a cabinet 21 having spaced walls, one of the walls 23 having an entrance opening 24 and a second of the walls 25 having an exit opening 26. Conveyor 15 extends through the chamber in cabinet 20 for delivering products from the entrance opening 23 to the exit opening 26. Air dispensing ducts 90 are mounted above and below conveyor 15 adjacent entrance opening 23 and exit opening 26. Air dispensing ducts 90 are adapted to direct streams of air toward a product on conveyor 15 and air return ducts 81 and 89 above and below conveyor 15 are provided with air return openings 83 and 87 above and below conveyor 15. Openings 83 and 89 lie intermediate opposite edges 15f and 15r of conveyor 15 for drawing air from air dispensing ducts 90 toward a central portion of cabinet 20 from areas above and below the conveyor 15. Directing streams of the second array to impinge against inclined splash surface 99 when a product P is not present on conveyor 15 causes outside air to be deflected away from the defined area while gas forming the streams of the second array is deflected into the defined area to significantly reduce the warm-up time of the oven. Second Embodiment A modified form of the invention is illustrated in Figures 11 and 12 of the drawing. The numeral 220 generally designates the second embodiment of the oven having a conveyor 215 extending through an entrance opening 224 formed in end wall 223. Conveyor 215 is preferably an open conveyor formed by intersecting wires or strands welded or otherwise secured together to form an open mesh surface through air streams 200a and 200b are allowed to pass. The air streams are formed by elongated upper tubes 192a and 192b which form a first array of air streams and tubes 198a which form a second array of air streams 200a which are projected from the upper air dispensing duct 290a. The lower air dispensing duct 290b is similarly constructed and includes a row of tubes 198b inclined at an angle "A" in a range between 3° and 30° relative to the vertical plane, as indicated in Figure 11, such that air streams 200a and 200b are projected toward a product P carried by conveyor 215 through the oven. A pair of splash plates 230a and 230b are mounted between the upper and lower flights of the open mesh conveyor 215, each of the splash plates having a generally triangular shaped outer end 232 slidably supported in support elements 234 for adjustment longitudinally of the oven relative to air streams 200a and 200b. The upper splash plate 230a is preferably positioned such that the outer edge of air stream 200a impinges against the apex of shaped surface 232 for causing the very thin outer layer of air of jet 200a to be directed to the right as viewed in Figure 11 of the drawing while the remainder of the air stream 200a is directed to the left as viewed in Figure 11. Air drawn through entrance opening 224 by the rapidly moving air stream 200a will be entrained in the outer layer of air which is separated by shaped surface 232 and redirected outwardly to cause a vortex or whirlpool of swirling air indicated by the arrows "V" to be repeatedly drawn to air stream 200, entrained in the rapidly moving air and then separated from the mass of the air stream by the shaped surface 232. This vortex of recirculating air "V" forms a dynamic barrier which substantially prevents ingress or egress of air through entrance opening 224. The temperature and humidity of the recirculating air in the oven are preferably controlled to provide optimum performance. The vortex "V" formed adjacent the entrance and exit openings enhances the control of temperature and humidity of the recirculating air in the oven. The enclosed chamber, formed in the oven through which controlled temperature and/or humidity of the rapidly circulating gas, is isolated from outside air adjacent entrance opening 224 by the dynamic barrier "B." The recirculated heated air is directed inwardly at an angle "A" so that the interior atmosphere is returned to the inside and the outside atmosphere which has been entrained by streams 220a and 220b is cast to the outside. As illustrated in Figure 12 of the drawing, tubes 192b and 194b are arranged to form multiple rows of streams of air forming a first array of streams while tubes 198b are arranged in a row adjacent inlet opening 224 to form a second array of openings through which the rapidly moving air is delivered. If it is deemed expedient to do so, tubes 198b may be replaced by a single slot inclined at an angle "A" relative to a vertical plane for forming a substantially continuous air stream which extends across the direction of travel of the conveyor 215. While the apparatus hereinbefore disclosed has been described as an oven equipped with heating elements for delivering heated air through the cooking is compartment, it should be readily apparent that the heating elements may be replace with cooling elements for circulating cold air for cooling or freezing products. In either event the heat transfer rate between the air streams and the product "P" is significantly increased by the rapidly moving streams which impinge against discrete areas on the surface of the product. It is important that the openings 83 and 87 into the air return ducts 81 and 89 be positioned to draw air from an area substantially aligned with the center of opening into the compartment to minimize the disruption of air streams delivered from the upper and lower air dispensing ducts in route to the air return duct and to balance air pressure in the cabinet across the width of openings 24 and 26. Perforated plates 92a and 92b may be replaced with elongated tubes 192 for forming air streams as hereinbefore described. Slots or other air dispensing openings may be formed in plate 92 if it is deemed expedient to do so. Further, the spaced air dispensing ducts 90a and 90b may be replaced with a plate 92 inclined at an angle relative to a horizontal plane such that all of the air streams of the first and second array of streams are inclined at an angle "A" relative to a vertical plane such that streams of the first and second arrays of streams are substantially parallel.

Claims: Apparatus for heating a product comprising: a cabinet (20); spaced walls (21,22,23,25,27,28) on said cabinet, one of said walls (23) having an entrance opening (24) and a second of said walls (25) having an exit opening (26); a conveyor (15) in said cabinet (20) for delivering products from said entrance opening (24) to said exit opening (26); plenum means (40a,40b,40c,40d) communicating with the interior of said cabinet; air dispensing means (90a,90b) having a chamber communicating with said plenum, said air dispensing means extending transversely of said conveyor (15), said air dispensing means (90a,90b) being adapted to deliver a stream of air toward said conveyor (15); means (82) projecting into said interior of said cabinet for forming air return means (81,89) communicating with said plenum, said means (82) forming an air return passage (83) positioned for drawing air from said air dispensing means (90a,90b) toward a central portion of said conveyor (15), said means (82) projecting into said interior of said cabinet for positioning said return passage (83) to balance air pressure across the width of said conveyor (15) adjacent said entrance (24) and exit (26) openings; blower means (60) for moving air from said air return means (81,89) through said plenum means (40a,40b,40c,40d) and into said air dispensing means (90a,90b); and means (21a,21b,62) for controlling temperature of air flowing through said plenum means. Apparatus for heating a product according to Claim 1, said air dispensing means comprising; a plurality of elongated air dispensing ducts (90a, 90b), each air dispensing duct having a perforated surface (92) adapted to form and direct a first array of spaced air streams to impinge against the surface of the product carried by said conveyor (15) and to form and direct a second array of air streams to form a vortex of recirculating air adjacent said entrance and exit openings to prevent ingress and egress of a substantial volume of air through said inlet (24) and outlet (26) openings. Apparatus for heating a product according to Claim 2, each of said air dispensing ducts comprising; a plurality of tapered ducts (90a, 90b) communicating with said plenum means, each said duct having an interior chamber; and a perforated grid (92) in said chamber adapted to form and direct a plurality of streams of air toward said perforated surface. Apparatus according to any one of Claims 1 to 3, said blower means (60) being between said air return means (81, 89) and said air dispensing means (90a, 90b) and being adapted to draw air from a central portion of said conveyor (15) through said air return opening and to deliver air into said air dispensing means. Apparatus according to any one of Claims 1 to 4, said means for controlling temperature of air flowing through said plenum means comprising: heater means (62) in heat exchange relation with air circulated by said blower means (60). Apparatus for heating a product according to any one of Claims 1 to 5, said air return means comprising: a return duct (81, 89) having an air return opening (83, 87), said air return opening and edges of said entrance and exit openings lying substantially in a common generally horizontal plane. Apparatus for heating a product according to Claim 6, said blower means (60) being between said air return duct (81, 89) and said air dispensing means (90a, 90b) said blower means being adapted to draw air from a central portion of said cabinet (20) through said air return opening and to deliver air to said dispensing ducts. Apparatus for heating a product according to any one of Claims 1 to 7, said air return means (81, 89) being positioned equally distant from edges of said entrance opening (24) adjacent opposite edges of said conveyor (15). Apparatus for heating a product according to any one of Claims 1 to 8 further comprising: means between said blower means and said air dispensing means adapted to substantially balance air pressure longitudinally of said chamber in said air dispensing means. Apparatus for heating a product according to Claim 1 further comprising: means adjacent said entrance opening (24) and said exit opening (26) to form a vortex of recirculating air adjacent said entrance and exit openings to prevent ingress and egress of a substantial volume of air through said entrance and exit openings. Apparatus for heating a product according to Claim 1, said air dispensing means comprising: upper and lower air dispensing ducts (90a, 90b) having passages adapted to form and deliver upper and lower streams of air: and means supporting said air dispensing ducts adjacent opposite edges of said entrance opening (24) and said exit opening (26) such that each of the air streams is projected across said entrance opening and said exit opening, each of said streams being inclined toward the interior of said cabinet at an angle in a range of 3° to 30° relative to the vertical plane to return a majority of the air to the interior of said cabinet while a minor portion of each air stream carries entrained ambient air toward the outside of said cabinet to reduce ingress or egress of air through said entrance and exit openings in the wall of said cabinet (20). Apparatus for heating a product according to Claim 11, said passages forming said inclined streams being formed in a first array of openings (94, 96, 97) in said upper and lower air dispensing ducts arranged to project an array of air streams toward said conveyor to heat products carried on said conveyor; and a second array (98) of openings inclined relative to said vertical plane adjacent opposite edges of said entrance opening and said exit opening. Apparatus for heating a product according to Claim 12, said first array of openings having centers spaced apart a distance in a range between two and four times the thickness of the air streams. Apparatus for heating a product according to Claim 12, said second array of openings comprising: circular apertures having a diameter, said apertures being spaced apart a distance of not more than four times said diameter. Apparatus for heating a product according to Claim 1, said air dispensing means comprising: a plurality of upper air dispensing ducts (90a) and a plurality of lower air dispensing ducts (90b); a blower housing (55) having an intake passage spaced substantially equal distance from each of said air dispensing ducts, said plenum means communicating with said blower housing (55) and each of said upper and lower air dispensing ducts (90a, 90b), said plenum means having a major dimension extending generally perpendicular to the major dimension of each of said upper and lower air dispensing ducts; and air directing means between said plenum means and each of said air dispensing ducts adapted to form a plurality of generally parallel air streams from a volume of air delivered through said plenum into said air dispensing ducts. Apparatus for heating a product according to Claim 15, said air directing means comprising: a grid (100) having air passages extending therethrough, each of said passages having a length which is greater than the maximin dimension of any cross sectional area of the passages. Apparatus for heating a product according to Claim 16, each of said air dispensing ducts having a vane (108) mounted intermediate opposite ends thereof, said vane being arranged to deflect portions of said generally horizontally directed air streams formed by said air directing means toward said openings. Apparatus for heating a product according to any one of Claims 15 to 17, said air return means having an air return opening adjacent each of said upper and lower air dispensing ducts positioned intermediate opposite ends of said air dispensing ducts for recirculating air flowing through said passages in surfaces of each said air dispensing duct. Apparatus for heating a product according to any one of Claims 15 to 18, said air return means comprising: a return duct (81, 89) having an air return opening, said air return opening and an edge of said exit opening lying in a common generally horizontal plane. Apparatus for heating a product according to Claim 19, said air return opening being positioned equal distance from edges of said entrance and exit openings, said blower means being adapted to draw air from a central portion of said conveyor through said air return opening and to deliver air into said dispensing ducts.

Inventors: DOBIE MICHAEL J, HIGH JARALD E, SMITH DONALD P, DOBIE, MICHAEL J., HIGH, JARALD E., SMITH, DONALD P.
Applicants: PATENTSMITH II INC, PATENTSMITH II, INC.
Application year: 1990
Main classification: F24C15
IPC codes: A47J37, F24C1, A21B1, F24C15