Abstract:
An improved airflow delivery system ( 1 ) comprising an air moving element ( 3 ) configured to move air in a flow path, a chamber ( 19 ) in the flow path configured to receive product ( 9 ), an air transfer chamber ( 11 ) comprising an inlet ( 10 ) of a selected area for receiving air in the flow path in a first direction (x-x) and an outlet ( 14 ) of a selected area greater than the area of the inlet for discharging air in the flow path in a second direction (y-y)different from the first direction, an airflow divider ( 33 ) extending across the air transfer outlet and configured to divide airflow in the flow path, an airflow directional ( 15 ) extending across the flow path downstream of the airflow divider and upstream of the chamber, the airflow directional having an upstream inlet face ( 28   a ) and a downstream outlet face ( 29   c ) and configured to receive airflow at the inlet face and split the airflow into multiple separated sub-paths ( 27 ) within the flow path and to discharge the airflow from the downstream outlet face substantially parallel to the flow path and without substantial reduction in static pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/364,071, filed Jul. 14, 2010. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to the field of ovens and dryers and, more particularly, to an improved oven or dryer airflow distribution system. 
       BACKGROUND ART 
       [0003]    Convection ovens and dryers that process continuous streams of product are in wide use in both industrial and baking applications. In many ovens the product moves horizontally on one or more levels, either carried on parallel moving conveyors or, in the case of textiles or webs, suspended under tension between external drives. A circulating hot air flow is brought in contact with the product for heating or drying. 
         [0004]    Certain means of providing airflow are known in the industry. U.S. Pat. No. 6,712,064 discloses an oven with multiple nozzles arranged both above and below the product conveyor, with the vertically directed airflow impinging the product at nearly a right angle. U.S. Pat. No. 6,539,934 and U.S. Pat. No. 6,684,875 describe impingement flow ovens with multiple parallel conveyors. These patents disclose that pressurized air from a fan discharge is distributed uniformly over the product by means of nozzles containing one or two perforated plates. Since the air flow contacts the product a very small distance from the nozzle discharge, it is not necessary that the nozzle provide airflow in a straight direction from the nozzle face. 
         [0005]    For delicate products that can be damaged by perpendicular impingement flow, there is an advantage to having the air flow parallel to the product. U.S. Pat. No. 4,515,561 discloses an oven with airflow parallel to and in the same direction as product travel, with sets of nozzles arranged above and below the product and connected to the fan discharge header at the side of the oven. In this oven configuration the air contacts the product for a large distance, essentially the length of the oven, after leaving the nozzle. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    With parenthetical reference to corresponding parts, portions or surfaces of the disclosed embodiment, merely for the purposes of illustration and not by way of limitation, the present invention provides an improved airflow delivery system ( 1 ) comprising an air moving element ( 3 ) configured to move air in a flow path, a chamber ( 19 ) in the flow path configured to receive product ( 9 ), an air transfer chamber ( 11 ) comprising an inlet ( 10 ) of a selected area for receiving air in the flow path in a first direction (x-x) and an outlet ( 14 ) of a selected area greater than the area of the inlet for discharging air in the flow path in a second direction (y-y) different from the first direction, an airflow divider ( 33 ) extending across the air transfer outlet and configured to divide airflow in the flow path, an airflow directional ( 15 ) extending across the flow path downstream of the airflow divider and upstream of the chamber, the airflow directional having an upstream inlet face ( 28   a ) and a downstream outlet face ( 29   c ) and configured to receive airflow at the inlet face and split the airflow into multiple separated sub-paths ( 27 ) within the flow path and to discharge the airflow from the downstream outlet face substantially parallel to the flow path and without substantial reduction in static pressure. 
         [0007]    The air moving element may comprise an eductor, a blower or a fan. The chamber may be a heating, cooling, curing or drying chamber. The air transfer chamber may comprise multiple turning vanes ( 13 ) in the flow path and the second flow path direction may be perpendicular to the first flow path direction. The airflow divider may comprise a perforated plate, wire mesh or a wire screen. 
         [0008]    The airflow directional may comprise a first layer ( 21 ) having an upstream inlet face ( 28   a ) and a downstream outlet face ( 29   a ) and multiple separated sub-paths ( 27   a ) within the flow path between the inlet face and the outlet face, the first layer configured to receive airflow at the inlet face and split the airflow into the multiple separated sub-paths within the flow path and to discharge the airflow from the downstream outlet face, a second layer ( 22 ) downstream from the first layer and having an upstream inlet face ( 28   b ), a downstream outlet face ( 29   b ) and multiple separated sub-paths ( 27   b ) within the flow path between the inlet face and the outlet face, the second layer configured to receive airflow at the inlet face of the second layer from the outlet face of the first layer and to discharge the airflow from the downstream outlet face of the second layer. The multiple separated sub-paths ( 27   b ) of the second layer may be configured relative to the multiple separated sub-paths ( 27   a ) of the first layer to split the airflow discharged from the multiple separated sub-paths of the first layer into the multiple separated sub-paths of the second layer and to discharge the airflow from the downstream outlet face of the second layer. At least a portion of the airflow discharged from at least two separated sub-paths of the first layer may be mixed together in at least one of the separated sub-paths of the second layer. 
         [0009]    The airflow delivery system may further comprise a second airflow divider ( 34 ) extending across the flow path downstream of the airflow directional and upstream of the chamber and configured to divide airflow in the flow path. The second airflow divider may comprise a perforated plate, wire mesh or a wire screen. 
         [0010]    The area of the outlet ( 36 × 35 ) of the air transfer chamber may be at least about four times greater than the area of the inlet ( 36 × 37 ) of the air transfer chamber. The airflow divider may comprise multiple airflow openings having an aggregate area between about 5% and about 35% of the area of the outlet of the air transfer chamber. The airflow divider may comprise multiple airflow openings each having a longest dimension perpendicular to the flow path of between about 0.1 and about 0.75 inches. 
         [0011]    The sub-paths may have an average depth ( 37 ) and may be defined at the inlet face by a pattern of repeated airflow openings ( 27 ), each of the openings having an area perpendicular to the flow path and characterized by a longest dimension ( 39 ) perpendicular to the flow path of between about 0.15 and 0.75 inches. The sub-paths may be defined at the inlet face by a pattern of repeated airflow openings, each of the openings having an area perpendicular to the flow path, and the pattern of repeated sub-paths may be a polygonal cellular pattern. The sub-paths may be formed from a thin-walled hexagonal honeycomb layer ( 21 ), or formed from multiple thin-walled hexagonal honeycomb layers ( 21 - 23 ), or formed from multiple off-set ( 40 ,  41 ) thin-walled hexagonal honeycomb layers. The sub-paths may have an average depth and may be defined at the inlet face by a pattern of repeated airflow openings, each of the openings having an area perpendicular to the flow path, and the sub-paths may have an average depth of between about 0.25 and about 3 inches, and the sub-path openings may have an area of between about 0.06 and about 1.5 square inches. 
         [0012]    In another aspect, the invention provides an airflow delivery system comprising an air moving element configured to move air in a flow path, a chamber in the flow path configured to receive product, an air transfer chamber comprising an inlet of a selected area for receiving air in the flow path in a first direction and an outlet of a selected area greater than the area of the inlet for discharging air in the flow path in a second direction different from the first direction, an airflow divider extending across the air transfer outlet and having multiple airflow openings, the airflow openings of the airflow divider having an average depth and an aggregate area perpendicular to the flow path, an airflow directional extending across the flow path downstream of the airflow divider and upstream of the chamber, the airflow directional having an upstream inlet face, a downstream outlet face, and multiple different sub-paths between the inlet face and the outlet face, the sub-paths having an average depth and defined at the inlet face by a pattern of repeated airflow openings, each of the openings having an area perpendicular to the flow path and characterized by a longest dimension perpendicular to the flow path, the average depth of the sub-paths being greater than the average depth of the openings in the air flow divider, the aggregate area of the airflow openings in the inlet face of the airflow directional being substantially greater than the aggregate area of the airflow openings in the airflow divider, and the average depth of the sub-paths being greater than the longest dimension of the openings perpendicular to the flow path of the sub-paths. 
         [0013]    The air moving element may comprise an eductor, a blower or a fan. The chamber may be a heating, cooling, curing, or drying chamber. The air transfer chamber may comprise multiple turning vanes in the flow path and the second flow path direction may be perpendicular to the first flow path direction. The air divider may comprise a perforated plate, wire mesh or a wire screen. 
         [0014]    The airflow directional may comprise a first layer having an upstream inlet face and a downstream outlet face and multiple separated sub-paths within the flow path between the inlet face and the outlet face, the first layer configured to receive airflow at the inlet face and split the airflow into the multiple separated sub-paths within the flow path and to discharge the airflow from the downstream outlet face, a second layer downstream from the first layer and having an upstream inlet face, a downstream outlet face and multiple separated sub-paths within the flow path between the inlet face and the outlet face, the second layer configured to receive airflow at the inlet face of the second layer from the outlet face of the first layer and to discharge the airflow from the downstream outlet face of the second layer. The multiple separated sub-paths of the second layer may be configured relative to the multiple separated sub-paths of the first layer to split the airflow discharged from the multiple separated sub-paths of the first layer into the multiple separated sub-paths of the second layer and to discharge the airflow from the downstream outlet face of the second layer. At least a portion of the airflow discharged from at least two separated sub-paths of the first layer may be mixed together in at least one of the separated sub-paths of the second layer. 
         [0015]    The sub-paths of the first layer may have an average depth and may be defined at the inlet face by a pattern of repeated airflow openings, the openings having an aggregate area perpendicular to the flow path and characterized by a longest dimension perpendicular to the flow path, the average depth of the sub-paths of the first layer being greater than the average depth of the openings in the air flow divider, the aggregate area of the airflow openings in the inlet face of the first layer being substantially greater than the aggregate area of the airflow openings in the airflow divider, and the average depth of the sub-paths of the first layer being greater than the longest dimension of the openings perpendicular to the flow path of the sub-paths. The sub-paths of the second layer may have an average depth and may be defined at the inlet face by a pattern of repeated airflow openings, the openings having an aggregate area perpendicular to the flow path and characterized by a longest dimension perpendicular to the flow path, the average depth of the sub-paths of the second layer being greater than the average depth of the openings in the air flow divider, the aggregate area of the airflow openings in the inlet face of the second layer being substantially greater than the aggregate area of the airflow openings in the airflow divider, and the average depth of the sub-paths of the second layer being greater than the longest dimension of the openings perpendicular to the flow path of the sub-paths. 
         [0016]    The pattern of repeated airflow openings of the sub-paths of the first layer may be substantially different from the pattern of repeated airflow openings of the sub-paths of the second layer. The average depth of the sub-paths of the first layer may be substantially different than the average depth of the sub-paths of the second layer. The aggregate area of the airflow openings in the inlet face of the first layer may be substantially different than the aggregate area of the airflow openings in the inlet face of the second layer. The longest dimension of the openings of the sub-paths of the first layer may be substantially different than the longest dimension of the openings of the sub-paths of the second layer. 
         [0017]    The area of the outlet of the air transfer chamber may be at least about four times greater than the area of the inlet of the air transfer chamber. The airflow openings of the airflow divider may have an aggregate area between about 5% and about 35% of the area of the outlet of the air transfer chamber. The airflow openings of the airflow divider may each have a diameter and the diameter may be between about 0.1 and about 0.75 inches. The longest dimension of each of the sub-paths may be between about 0.15 and 0.75 inches. 
         [0018]    The pattern of repeated sub-paths may be a hexagonal cellular pattern. The pattern of repeated sub-paths may be a polygonal cellular pattern. The sub-paths may be formed from a thin-walled hexagonal honeycomb layer. The sub-paths may be formed from multiple thin-walled hexagonal honeycomb layers, or formed from multiple off-set thin-walled hexagonal honeycomb layers. The sub-paths may have an average depth of between about 0.25 and about 3 inches. 
         [0019]    In another aspect, the invention provides an airflow delivery system comprising an air moving element configured to move air in a flow path, a chamber in the flow path configured to receive product, an air transfer chamber comprising an inlet of a selected area for receiving air in the flow path and an outlet of a selected area for discharging air in the flow path, an airflow divider extending across the air transfer outlet and configured to divide airflow in the flow path, an airflow directional extending across the flow path downstream of the airflow divider and upstream of the chamber, the airflow directional having an upstream inlet face and a downstream outlet face and configured to receive airflow at an inlet velocity at the inlet face and to discharge the airflow from the downstream outlet face at an outlet velocity, wherein the inlet velocity is greater than or equal to the outlet velocity, and wherein the outlet velocity is at least  4  meters per second. 
         [0020]    The area of the outlet of the air transfer chamber may be greater than the area of the inlet of the air transfer chamber, the inlet of the air transfer chamber may receive air in the flow path in a first direction and the outlet of the air transfer chamber may discharge air in the flow path in a second direction different from the first direction, and the airflow directional may be configured to receive airflow at the inlet face and split the airflow into multiple separated sub-paths within the flow path and to discharge the airflow from the downstream outlet face substantially parallel to the flow path. The air transfer chamber may comprise multiple turning vanes in the flow path and the second flow path direction may be perpendicular to the first flow path direction. The airflow divider may comprise a perforated plate, wire mesh or a wire screen. The airflow directional may comprise a first layer having an upstream inlet face and a downstream outlet face and multiple separated sub-paths within the flow path between the inlet face and the outlet face, the first layer configured to receive airflow at the inlet face and split the airflow into the multiple separated sub-paths within the flow path and to discharge the airflow from the downstream outlet face, a second layer downstream from the first layer and having an upstream inlet face, a downstream outlet face and multiple separated sub-paths within the flow path between the inlet face and the outlet face, the second layer configured to receive airflow at the inlet face of the second layer from the outlet face of the first layer and to discharge the airflow from the downstream outlet face of the second layer. The multiple separated sub-paths of the second layer may be configured relative to the multiple separated sub-paths of the first layer to split the airflow discharged from the multiple separated sub-paths of the first layer into the multiple separated sub-paths of the second layer and to discharge the airflow from the downstream outlet face of the second layer. At least a portion of the airflow discharged from at least two separated sub-paths of the first layer may be mixed together in at least one of the separated sub-paths of the second layer. The airflow delivery system may further comprise a second airflow divider extending across the flow path downstream of the airflow directional and upstream of the chamber and configured to divide airflow in the flow path. The area of the outlet of the air transfer chamber may be at least about four times greater than the area of the inlet of the air transfer chamber. 
         [0021]    One objective of the present invention is to provide a nozzle that achieves the desired flow properties with low pressure loss. Another objective is to provide a nozzle for parallel flow ovens and dryers that turns the airflow ninety degrees and has uniform air velocity across the nozzle outlet face as well as outflow that is straight and parallel to the normal vector of the nozzle face. Another objective is to achieve the desired airflow pattern in geometries where the length of the outlet face is much longer than the height or the depth. Another objective is to provide a nozzle readily scalable to different oven or dryer widths and different spacing between parallel layers of product. 
         [0022]    Another objective is to provide a nozzle for parallel flow, down flow or cross flow ovens and dryers that has uniform air velocity across the nozzle outlet face as well as outflow that is straight and parallel to the normal vector of the nozzle face. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a partial perspective view of an embodiment of the present invention configured in an oven that provides airflow parallel to the product. 
           [0024]      FIG. 2  is an exploded view of the nozzle shown in  FIG. 1 . 
           [0025]      FIG. 3  is a downstream facing view of one layer of the airflow directional shown in  FIG. 2 . 
           [0026]      FIG. 4  is an enlarged detailed view of the airflow directional layer shown in  FIG. 3 , taken within the indicated area A of  FIG. 3   
           [0027]      FIG. 5  is a downstream facing unexploded perspective view of the airflow directional shown in  FIG. 2 . 
           [0028]      FIG. 6  is an enlarged detailed view of the airflow directional shown in  FIG. 5 , taken within the indicated area B of  FIG. 5 . 
           [0029]      FIG. 7  is a graph of a set of measured velocity uniformity data and the corresponding dimensionless parameters for a comparative example or conventional design. 
           [0030]      FIG. 8  is a graph of a set of measured velocity uniformity data and the corresponding dimensionless parameters for an embodiment of the current invention. 
           [0031]      FIG. 9  is a graph of the variation of the velocity uniformity parameter as a function of the number and thickness of honeycombs included between parallel perforated plates. 
           [0032]      FIG. 10  is a graph of the variation of the velocity uniformity parameter as a function of the number of honeycomb interfaces in a fixed thickness arrangement. 
           [0033]      FIG. 11  is a graph of measured velocity straightness data for the comparative example and an embodiment of the present invention. 
           [0034]      FIG. 12  is a graph of the variation of the flow straightness as a function of the number and thickness of honeycombs included between parallel perforated plates. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
         [0036]    Referring to the drawings, and more particularly to  FIG. 1  thereof, this invention provides an improved airflow delivery system, of which a first embodiment is generally indicated at  1 . While this invention has many applications for providing a desired flow with reduced pressure loss, it is described with regard to its application to an oxidative stabilization oven for carbon fiber precursor. 
         [0037]      FIG. 1  shows a portion of oven  1  with internal chamber  19  and product layers  9  arranged and moving in parallel horizontal planes. Air is circulated to contact product  9  by fan  3 , which discharges into side plenum  2 , which in turn channels the air through filter  4  and heater  5  and into turning vanes  6 . From vanes  6  the heated air enters a set of nozzles  7 , which are arranged above and below product layers  9 , where the air is turned 90 degrees so that it is discharged in a direction parallel to the product  9  direction of travel. At the other end of oven  1 , the air enters collection plenums  8  through which it returns to the inlet of fan  3 . 
         [0038]    Referring now to  FIG. 2 , in this embodiment nozzle  7  is generally a rectangular cuboid made from sheet metal formed and welded using standard industrial practices. Nozzle  7  has a right open face  10  that serves as an air inlet and a front open face  14  that is perpendicular to inlet  10  and serves as an outlet. Solid face  17  is opposite air inlet  10  and top solid face  18   a,  bottom solid face  18   b  and rear solid face  18   c  are perpendicular to air inlet  10 . Solid face  17 , top face  18   a,  bottom face  18   b  and rear face  18   c  define transfer chamber  11 . A plurality of vanes  13  in chamber  11 , made from thin sheet metal that has been formed into 90 degree bends, are attached to both top and bottom solid faces  18   a  and  18   b  by discreet welds along their length. Vanes  13  are arranged so as to make a plurality of substantially equal size discharge channels that intersect perforated plate  33 . Thus, airflow enters chamber  11  through inlet  10  in direction x-x and is turned by vanes  13 , in this embodiment 90 degrees, to exit chamber  11  through outlet  14  generally in direction y-y. 
         [0039]    The air discharge face  20  of nozzle  7 , opposite solid face  18   c,  comprises upstream perforated plate  33 , airflow directional  15 , and downstream perforated plate  34 . As shown, perforated plates  33  and  34  are configured with a pattern of airflow holes  30  that allow air to flow from the upstream side  31  of the respective plate to the downstream side  32  of the respective plate. It is preferable, but not necessary, that perforated plates  33  and  34  have the same pattern of holes  30  and sizes of holes  30 . It is also preferable that the diameters of holes  30  in plates  33  and  34  be in the range of approximately 0.1 to 0.5 inches, and still more preferably in the range of approximately 0.2 to 0.4 inches. The open area of perforated plates  33  and  34  is preferably in the range of approximately 5 to 35% of the total area and more preferably in the range of approximately 15 to 25%. 
         [0040]    As shown in  FIG. 2 , airflow directional  15  is positioned between perforated plates  33  and  34  and, in this embodiment, comprises three sheets or layers  21 ,  22  and  23 . As shown in  FIG. 3 , each of sheets  21 - 23  has multiple separate flow sub-passages  27   a - c  defined by a hexagonal cross-sectional structure that is formed with repeated open hexagonal cells  25  of the same size, commonly referred to as honeycombs.  FIG. 3  shows an embodiment of layers  21 - 23  with a commercially available honeycomb shape. As shown, each of sheets  21 - 23  is formed by a repeated pattern of base cell  25 , shown in  FIG. 4 , having six sides forming an inner air passage  27 . Cell  25  preferably has no more than eight sides and more preferably has six sides. It is preferable, but not necessary, that all honeycomb layers  21 - 23  have the same cell  25  size, and that the cells have a longest transverse dimension  39  in the range of from approximately 0.15 to 0.75 inches, and more preferably in the range of approximately 0.3 to 0.6 inches. While a hexagonal cell is shown, other patterns may be used. For example, cell  25  could be any convex polygon or other shape. Sub-paths  27  have an average depth  37  of between approximately 0.25 and approximately 3 inches, and sub-path openings  27  have an area of between approximately 0.06 and approximately 1.5 square inches. 
         [0041]    As shown in  FIG. 5 , in this embodiment honeycomb layers  21 - 23  are arranged in multiple layers with their faces free to touch. The upstream face  28   a  of layer  21  is positioned against the downstream face  32   a  of plate  33 , layer  22  is positioned between the downstream face  29   a  of layer  21  and the upstream face  28   c  of layer  23 , and the downstream face  29   c  of layer  23  is positioned against the upstream face  31   b  of plate  34 . 
         [0042]    As shown in  FIG. 6 , layers  21 - 23  are arranged such that the leading edges  26  of their respective pattern of open cells  25 , and thus their multiple separate flow sub-paths  27 , are offset ( 40 ,  41 ) from one another, which is accomplished in this embodiment either by offsetting the cut at the ends or by using spacers on the boundaries of the respective layers.  FIG. 6  shows such offsetting on the upstream face  28   a  of directional  15 . As shown, the leading edge of layer  22  is offset from the leading edge of layer  21  by a distance  40   a  in a first dimension and  40   b  in a second dimension. Similarly, the leading edge of layer  23  is offset from the leading edge of layer  22  by a distance  41   a  in a first dimension and  42   b  in a second dimension. The distances are such that the leading edge of layer  23  is also offset from the leading edge of layer  21 . While all three layers are offset in this embodiment, one or more layers may be configured so they are not offset with respect to one or more other layers. 
         [0043]    The result of this offset is that the airflow path becomes tortuous due to the increased amount of leading edges encountered through the depth of airflow directional  15 . With three or more layers, the random nature of the alignment makes the entire assembly  15  more uniform over large areas. 
         [0044]    In addition, because first layer  21  has an upstream inlet face  28   a  and a downstream outlet face  29   a  and multiple separated sub-paths  27   a  within the flow path between the inlet face and the outlet face, layer  21  receives airflow at inlet face  28   a  and splits the airflow into multiple separated sub-paths  27   a  within the overall flow path and discharges the airflow from downstream outlet face  29   a.  Because second layer  22  downstream from first layer  21  is offset  40  from layer  21  and has an upstream inlet face  28   b,  a downstream outlet face  29   b  and multiple separated sub-paths  27   b  between inlet face  28   b  and outlet face  29   b,  the second layer receives airflow at inlet face  28   b  from outlet face  29   a  of first layer  21  and splits the airflow discharged from the multiple separated sub-paths  27   a  of the first layer into multiple separated sub-paths  27   b  of second layer  22  and discharges the airflow from downstream outlet face  29   b  of second layer  22 . Because of offset  40 , at least a portion of the airflow discharged from at least two separated sub-paths  27   a  of first layer  21  are mixed together in at least one of the separated sub-paths  27   b  of second layer  22 . Similarly, because third layer  23  downstream from first layer  22  is offset  41  from layer  22  and has an upstream inlet face  28   c,  a downstream outlet face  29   c  and multiple separated sub-paths  27   c  between inlet face  28   c  and outlet face  29   c,  the third layer receives airflow at inlet face  28   c  from outlet face  29   b  of second layer  21  and splits the airflow discharged from the multiple separated sub-paths  27   b  of the second layer into multiple separated sub-paths  27   c  of third layer  23  and discharges the airflow from downstream outlet face  29   c  of third layer  22 . Because of offset  41 , at least a portion of the airflow discharged from at least two separated sub-paths  27   b  of second layer  22  are mixed together in at least one of the separated sub-paths  27   c  of third layer  23 . 
         [0045]    Airflow directional  15  can be formed from one or more layers of any rigid shape that presents a substantially open cross-section and multiple airflow sub-paths to the air flowing between perforated plates  33  and  34 . It is preferable that the open area  27  of the face  28   a  of flow directional  15  be greater than approximately 80% of the total area, and more preferable that the open area  27  be greater than approximately 90% of the total area. 
         [0046]    In this embodiment, the aspect ratio of the nozzle is such that outlet face  20  is much wider or longer than inlet face  10 . Typically, horizontal product webs are spaced 150-300 mm apart and are 1500 to 4000 mm in width, which constrains the nozzle so that its outlet width  35  is 10 to 20 times more than its height  36 . Also, in this embodiment, the depth dimension  37  of the nozzle is kept no more than 2 times the product spacing  16 , which results in a higher fraction of product  9  in chamber  19  being exposed to the full air flow. 
         [0047]    In this embodiment, the area of outlet  14  of air transfer chamber  11  is at least approximately four times greater than the area of inlet  10  of chamber  11 . Multiple airflow openings  30  in perforated plates  33  and  34 , respectively, have an aggregate area between approximately 5% and approximately 35% of the area of outlet  14  of chamber  11 . 
         [0048]    The average depth  37  of sub-paths  27  for directional  15  is greater than the average depth of openings  30  in plate  33 , and the aggregate area of airflow openings  27  in inlet face  28   a  of airflow directional  15  is substantially greater than the aggregate area of airflow openings  30  in plate  33 , and the average depth of sub-paths  27  for directional  15  is greater than the longest dimension  39  of openings  27 . Furthermore, in this embodiment sub-paths  27   a  of first layer  21  themselves have an average depth greater than the average depth of openings  30  in airflow divider  33 , have an aggregate area of airflow openings  27   a  in inlet face  28   a  substantially greater than the aggregate area of airflow openings  30  in airflow divider  33 , and have an average depth greater than the longest dimension  39  of sub-paths  27   a.  Similarly, the sub-paths  27   b  and  27   c  of second layer  22  and third layer  23 , respectively, each have an average depth greater than the average depth of openings  30  in airflow divider  33 , have an aggregate area of substantially greater than the aggregate area of airflow openings  30  in airflow divider  33 , and have an average depth greater than the longest dimension  39 . 
         [0049]    Nozzle  7  provides airflow suitable for parallel flow ovens, coolers, curing chambers, and dryers. Nozzle  7  turns the airflow ninety degrees from an inlet  10  perpendicular to the nozzle outlet. Nozzle  7  also provides airflow into chamber  19  that has uniform air velocity across nozzle discharge face  20  and is substantially straight and normal to nozzle discharge face  20  and substantially parallel to axis y-y. 
         [0050]    In alternative embodiments, the pattern of repeated airflow openings of the sub-paths of first layer  21  may be substantially different from the pattern of repeated airflow openings of the sub-paths of second layer  22 . Similarly, the pattern of openings in third layer  23  may be substantially different from the pattern of openings in second layer  22  and the same or different from the pattern of openings in first layer  21 . The average depth of the sub-paths of first layer  21  may be substantially different than the average depth of the sub-paths of second layer  22  and/or third layer  23 . Also, the aggregate area of the airflow openings in inlet face  28   a  of first layer  21  may be substantially different than the aggregate area of the airflow openings in inlet face  28   b  and/or  28   c  of second layer  22  and/or third layer  23 , respectively. The longest dimension of the openings of the sub-paths of first layer  21  may be substantially different than the longest dimension of the openings of the sub-paths of second layer  22  and/or third layer  23 . In addition, as indicated below, more than three layers may be employed. Also, a single layer  21  or just two layers  21  and  22  may be employed. 
         [0051]    Because forced convection ovens, coolers, curing chambers, and dryers use fans that at large scale can draw several hundred kilowatts, it is a significant disadvantage if the airflow directing nozzle does not perform its function with low pressure losses, as the fan power required is directly proportional to the total pressure drop in the system, and the nozzle will typically cause the largest pressure drop in the circuit. Given the flow path created by plates  33  and  34  and airflow directional  15 , comprising multiple offset honeycomb layers, a high pressure loss was expected. However, when tested the pressure drop resulting from nozzle  7  was substantially lower than expected. 
         [0052]    A construction similar to the oven portion shown in  FIG. 1  was used to evaluate the performance of different embodiments of nozzle  7 , as compared to a conventional design. In the test setup, a 20 hp fan was used to supply air simultaneously to a set of three parallel nozzles in the same way as shown in  FIG. 1 . A dimensionless parameter that is useful as a criterion of airflow uniformity out of a nozzle is the ratio of the velocity standard deviation to the velocity mean from a set of measurements taken at different locations that cover the area of the nozzle.  FIG. 7  shows the variation of the velocity uniformity parameter for a conventional design, for four different mean gas flow rates, which correspond in affect to four different fan speeds. The nozzle for this example was 1600 mm wide, 200 mm high and 400 mm deep. The air inlet was on a face that was 200 mm×400 mm and the air outlet was on a face that was 200 mm×1600 mm. There were 7 vanes in the nozzle, evenly spaced both over the inlet and outlet, which were 200 mm high and were welded to the upper and lower faces. At the outlet were 2 parallel perforated plates, 76 mm apart, and having 15% open area and 6 mm diameter holes. There were 8 air velocity readings taken at each of the different fan speeds. These 8 points were selected to be 200 mm apart and cover essentially the entire width of the nozzle. Vertically, the data was collected in the center of the nozzle, which was 100 mm from the bottom. An Extech Instruments model 407113 thermo-anemometer was used for the velocity measurements. In all cases 20 separate velocity readings were averaged at each data location. In all cases the air temperature was between 20 and 24° C.  FIG. 7  shows the StDev/Mean velocity uniformity parameter for each of four constant fan speeds (shown as the mean velocity) which includes the readings from the 8 positions along the width, and also an overall value that takes into account all 32 readings. In  FIG. 7  the individual fan speed velocity parameters range from 6.1 to 7.4% and the overall value was 6.8%. 
         [0053]      FIG. 8  shows the velocity uniformity parameter for an embodiment of the present invention. The nozzle size was the same as the conventional design, namely 1600 mm wide, 200 mm high and 400 mm deep. There were 7 turning vanes and 2 parallel perforated plates that were also the same as the conventional design. In this embodiment of the improved nozzle, 6 layers of honeycomb material with 12 mm cell size and each 12.7 mm deep (for total depth of 76 mm) were inserted between the perforated plates. As shown in  FIG. 8 , the individual fan speed velocity parameters ranged from 0.9 to 1.1% and the overall value was 1.0%. As shown, the velocity uniformity in this embodiment was a substantial improvement over the conventional design with the addition of the structured flow directional  15 . 
         [0054]      FIGS. 7 and 8  also show the pressure drop (DP) for each of the four sets of data (which represent constant fan speeds). The pressure was measured just upstream of the nozzle using a Shortridge Instruments model ADM-860 electronic micromanometer. In all cases  12  separate readings were averaged at each fan speed. As shown, and unexpectedly, the pressure drops in this embodiment of the improved nozzle were effectively equal to the pressure drop in the conventional design. This was a surprising result in that the superior flow uniformity achieved from the addition of directional  15  did not come at the cost of imposing a higher pressure drop. 
         [0055]      FIG. 9  shows the variation in the velocity uniformity parameter as a function of the number and therefore overall thickness of the honeycomb material inserted between the perforated plates of this embodiment. Where there were multiple pieces of honeycomb material, they were arranged so that the adjacent faces were essentially touching with no gaps. The flow uniformity improves as the number of pieces or layers of 12.7 mm thick honeycomb material increases. After 6 pieces, or 76 mm total depth, the uniformity parameter was judged to be approaching the limit of resolution of the measurement system. 
         [0056]      FIG. 10  shows the variation of the velocity uniformity parameter as a function of the number of honeycombs at a fixed overall honeycomb depth or thickness of 76 mm. In this case, various different thickness pieces of honeycomb were used to achieve a constant total thickness, so that the variation between the trials was that there were a different number of interfaces between honeycombs. As with the other reported data, the honeycombs were essentially touching at their respective faces. This was a surprising result in that the velocity uniformity gets better as the number of individual layers, and therefore interfaces between layers, increases. So, for example, three layers of honeycomb 25.4 mm thick outperform a single layer 76 mm thick, and six layers 12.7 mm thick outperforms three layers 25.4 mm thick. 
         [0057]      FIG. 11  shows measured airflow straightness data for the conventional design or comparative example and an embodiment of the improved nozzle. The comparative example and an embodiment of the improved nozzle were the same as described previously with reference to  FIGS. 7 and 8 , respectively. There were 8 airflow angle readings taken at each of the different fan speeds. These 8 points were selected to be 200 mm apart and cover essentially the entire length of the nozzle. Vertically, the data was collected in the center of the nozzle, which was 100 mm from the bottom. The measurement was made by attaching a thread that extended 600 mm from the nozzle face at each of the 8 locations and then using a tape measure to measure the position of the thread at a point 400 mm from the nozzle face. The angle was then calculated from elementary geometry, with 0 degrees being defined as the angle normal to the face of the nozzle.  FIG. 11  shows that the comparative example or conventionally designed nozzle has a high degree of variation off the normal especially in the center region, with most individual readings off by over 10 degrees. The overall average angle variation from normal for the comparative example nozzle was 9.1 degrees. The data from this embodiment of the improved nozzle were substantially better with a large majority of the angle readings off normal by less than 3 degrees and an overall average angle variation from normal of only 1.6 degrees. 
         [0058]      FIG. 12  shows the variation in the airflow straightness as a function of the number and therefore overall depth or thickness of the honeycomb material inserted between the perforated plates of the preferred embodiment. The airflow straightness improves as the number of pieces of 12.7 mm honeycomb material increases. 
         [0059]    The present invention contemplates that many changes and modifications may be made. Therefore, while the presently-preferred form of the airflow delivery system has been shown and described, and several modifications and alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit and scope of the invention, as defined and differentiated by the following claims.