Patent Publication Number: US-6219937-B1

Title: Reheaters for kilns, reheater-like structures, and associated methods

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the drying of green lumber in a kiln and, more particularly, to reheaters in kilns for drying lumber. 
     BACKGROUND OF THE INVENTION 
     Lumber which has recently been cut contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried. Drying removes a large amount of water from the lumber and significantly reduces the potential for the lumber to become warped or cracked. Acceptable water content varies depending on the use of the lumber and type of wood; however, a moisture content of about nineteen percent, or less, is acceptable in many circumstances. 
     Although lumber may be dried in the ambient air, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. The charge of lumber typically consists of two or more rectangular stacks of lumber. A typical kiln chamber is a generally rectangular building that can be at least partially sealed to control the amount of air that is introduced to and exhausted from the kiln chamber. Hot air from a furnace is forced through an inlet duct to a plenum that is positioned in an upper portion of the kiln chamber, and the hot air is discharged from the plenum to the chamber through multiple openings defined in the top of the plenum. The heated air supplied to the chamber is circulated within the chamber by fans so that the heated air flows along a flow path that extends through one or more upstream stacks of lumber, and thereafter through one or more downstream stacks of lumber. 
     Hot air from the plenum also flows into and escapes from a row of reheaters. It is conventional for each of the reheaters to be a downwardly extending pipe-like structure that extends from the bottom of the plenum, is positioned between the upstream and downstream stacks of the lumber, and extends into the portion of the flow path that is between the upstream and downstream stacks of the lumber. In some kilns, each of the reheaters is rectangular in a horizontal section and defines elongate vertical slits through which hot air flows into the chamber. The hot air discharged from the reheaters serves to further heat the air circulating through the kiln, thereby at least somewhat compensating for the heat that has been lost in drying the upstream stack of lumber prior to introducing the reheated air into the downstream stack of lumber. Unfortunately, the reheaters contribute to the resistance to flow along the flow path since they extend into the flow path. 
     Each stack of lumber being dried also contributes to the resistance to flow along the flow path. As best understood with reference to FIGS. 16-17, it is conventional for each stack  248  of a charge of lumber to consist of a number of vertically stacked, horizontal rows  250  of lumber  252  that are arranged such that cross-sections of the stack are generally rectangular. The horizontal rows  250  are spaced apart with narrow wooden boards  254 , or the like, referred to as “stickers.” The stickers  254  are positioned between each horizontal row  250  to space the rows apart so that multiple passages  256  are defined between adjacent layers and are open at the opposite sides of the stack  248 . The heated air traveling along the flow path passes through the passages  256  and is in direct contact with both the upper and lower surfaces of individual pieces of lumber  252  so that the lumber is dried. For each of the passages  256 , airflow therethrough is such that layers of viscous air are developed proximate to the surfaces of the pieces of lumber that face and define the passage. Those viscous layers are referred to as boundary layers  260 . The boundary layers  260 , which are areas of retarded flow, are caused by the viscous interaction between the airflow through the passage  256  and the surfaces of the pieces of lumber  252  that define the passage, as well as interaction between the airflow and the lumber surfaces that are proximate to the inlet opening of the passage. 
     Each boundary layer  260  includes an initially protruding portion  262  (i.e., a separated region) at the entrance of its passage  256 . The protruding portion  262  tapers to a generally planar portion  264 . For each of the boundary layers  260 , the protruding portion  262  is a portion of the boundary layer that has become separated from the surface or surfaces of the one or more pieces of lumber  252  that define the passage. The separation occurs because of interaction between the airflow and an edge or edges of the one or more pieces of lumber  252  that define the inlet to the passage. 
     It is conventional for the edges of the layers of lumber  252  to be aligned so that they generally extend in a common plane. As a result, for each of the passages  256 , the protruding portions  262  of the boundary layers  260  are aligned in a manner that is very restrictive to flow, since the boundary layers are regions of retarded flow and thereby tend to block flow into the passage. More specifically, an unrestricted flow path exists only in that region between the boundary layers  260  of each of the passages  256 . Those unrestricted flow paths are characterized by generally fully developed two dimensional channel flow. Within each passage  256 , the protruding portions  262  are aligned in a manner that causes a significant reduction in the size of the unrestricted flow path at the entrance of the passage. It is generally characterized as a poor entrance, similar to a flanged pipe condition but for a two dimensional channel. 
     The resistance to flow along the flow path that is caused by the reheaters and the stacks of lumber reduces the speed at which the pieces of lumber can be dried, which can be disadvantageous since mill production depends upon the ability to dry lumber at a sufficient rate so that production need not be slowed to allow for the drying process. The resistance to flow along the flow path that is caused by the reheaters and the stacks of lumber also requires significant pressure increases to maintain the flowrate; therefore, the kiln fans, which force the heated air to flow along the flow path, must work excessively, which is disadvantageous. Operating the fans of a kiln consumes energy that adds to the cost of producing quality lumber. Of course it is advantageous to lower the cost of producing quality lumber. Whereas some conventional kilns can be characterized as being efficiently operated and able to dry lumber at a sufficient rate, there is always a demand for new kilns and kiln-related structures that can be even more efficiently operated, and that facilitate the drying of lumber at a sufficient rate. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above and other problems by providing improved reheater conduits, and the like. In accordance with one aspect of the present invention, the reheater conduits are elliptically shaped and extend downward from a plenum into a lower chamber interior space of a kiln chamber for supplying heated air from the plenum to the lower chamber interior space. One or more air moving devices circulate air in the lower chamber interior space along a flow path. The reheater conduits extend into a portion of the flow path and are generally perpendicular to the portion of the flow path. More specifically, each of the reheater conduits has opposite ends, defines a length that extends between the opposite ends and is generally perpendicular to the portion of the flow path, defines a first cross-dimension that is generally perpendicular to the length and parallel to the portion of the flow path, and defines a second cross-dimension that is perpendicular to both the length and the portion of the flow path. The second cross-dimension is less than the first cross-dimension so that the major axis of the elliptical reheater conduit is aligned with the flow path, thereby being less restrictive to flow along the flow path than if the first and second cross-dimensions were equal, in which case the cross section would be circular rather than elliptical. 
     In accordance with another aspect of the present invention, each of the reheater conduits defines outlets positioned along at least a section of the length of the reheater conduit. The reheater conduit defines a pair of vertices between which the second cross-dimension is defined. As such, the outlets of this embodiment are preferably proximate the vertices, which at least partially facilitates the aspects of the present invention that are described in the two immediately following paragraphs. 
     In accordance with another aspect of the present invention, the outlets of adjacent conduits are arranged so that jet-like flows from the outlets cooperate to produce turbulent whirling masses of air, known as vortices, that travel downstream along the flow path to eventually result in turbulence that interacts with a downstream stack of lumber. The vortices are formed by small jets issuing into the space between adjacent reheaters. The vortices break up as they flow toward the downstream lumber stack. The vortices naturally break up into turbulent eddies which then undergo decay from turbulent dissipation. The turbulence eventually decays to zero. However, the size of the initial vortices is selected so that the turbulence decays to a mean eddy size of approximately 0.05 inches. This is the mean integral scale. Eddies sized in this range interact with the downstream stack of lumber so as to restrict the separation of the boundary layers in the inlets of the passages extending through the downstream stack of lumber. This reduces the net resistance to flow through the downstream stack of lumber. That is, the whirling masses of air decrease the resistance to flow through the downstream stack(s) of lumber relative to the resistance to conventional flow through the downstream stack of lumber. More specifically, the turbulence resulting from the whirling masses reduces the size of the protruding separated regions of the boundary layers associated with the entrances to the passages defined through the downstream stack(s) of lumber and correspondingly increase the unrestricted flow path between the separated regions of the boundary layers. 
     In accordance with another aspect of the present invention, the jet-like flows from the outlets of the reheater conduits reach adjacent reheater conduits and interact with the boundary layers that extend generally around the adjacent reheater conduits so that separated regions of those boundary layers are smaller than they would be absent the jet-like flows. This enhances the convective heat transfer from the reheater conduits by increasing the airflow proximate the reheater conduits. 
     In accordance with another aspect of the present invention, movable dampers are respectively proximate the upper ends of the reheater conduits and are capable of being moved to respectively adjust the amount of heated air supplied to the reheater conduits. Each reheater conduit also preferably includes an internal converging/diverging section proximate its upper end for offsetting or balancing the effects of partially closing the inlet to the reheater conduit with the respective damper. 
     These and other aspects of the present invention are advantageous because they each pertain to either the efficient operation or timely operation of kilns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic, front end, partially cross-sectional view of a kiln, in accordance with one embodiment of the present invention. 
     FIG. 2 is a schematic, left side, cross-sectional view of a kiln chamber of the kiln of FIG. 1, wherein the view includes some of the items closely connected to or contained by the kiln chamber, and the cross-section is substantially along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a schematic, partial, cross-sectional view taken substantially along line  3 — 3  of FIG. 2, and illustrating portions of the kiln of FIG. 1, including portions of a composite plenum, a portion of a representative circulation passage extending through an intermediate plenum of the composite plenum, a portion of a representative fan, and representative nozzles-like outlets associated with the composite plenum. 
     FIG. 4 is a left elevation view of the circulation passage and fan illustrated in FIG. 3, and FIG. 4 also illustrates a portion of the intermediate plenum and some of the nozzle-like outlets carried by the intermediate plenum. 
     FIG. 5 is a partial and partially exploded schematic view taken along line  5 — 5  of FIG.  3 . 
     FIG. 6 is a schematic, partial, left elevation view of a portion of the composite plenum and two fans, and FIG. 6 further schematically and representatively illustrates nozzles that are carried by support plates, and holes in dampers that are moved by a damper control system to open and close the nozzles, in accordance with an alternative embodiment of the present invention. 
     FIG. 7 is a schematic, partial, cross-sectional view taken along line  7 — 7  of FIG. 6, in accordance with the embodiment illustrated in FIG.  6 . 
     FIG. 8 is a schematic exploded view of representative portions of a left wall of the intermediate plenum of the composite plenum of FIG. 6, a damper, a support plate, and associated attachment means, and a pair of representative nozzles, in accordance with the embodiment illustrated in FIGS. 6-7. 
     FIG. 9 is a schematic, partial, and side sectional view of a representative tee formed by return ducts, and FIG. 9 schematically illustrates a damper system within the tee in both open and closed configurations, in accordance with an alternative embodiment of the present invention. 
     FIG. 10 is an isolated, schematic, rear end elevation view illustrating a telescopic composite plenum that can be used in the kiln of FIG. 1, in accordance with one embodiment of the present invention, wherein the composite plenum is illustrated in both compacted and extended configurations. 
     FIG. 11 is a schematic right elevation view of a portion of a representative reheater conduit of the kiln of FIG. 1, a portion of a lower wall of the composite plenum to which the reheater conduit is attached, and a damper for throttling flow into the reheater conduit. 
     FIG. 12 is an isolated, schematic front elevation view of a portion of the reheater conduit of FIG.  11 . 
     FIG. 13 is a cross-sectional view of the portion of the reheater conduit of FIG. 12 taken along line  13 — 13  of FIG.  12 . 
     FIG. 14 is a schematic right elevation view of portions of a representative pair of adjacent reheater conduits of the kiln of FIG. 1, illustrating heated air being discharged therefrom. 
     FIG. 15 is a schematic right elevation view of portions of a representative pair of adjacent reheater conduits a kiln, in accordance with an alternative embodiment of the present invention. 
     FIG. 16 is a perspective view of a conventional stack of lumber that can be dried in a kiln. 
     FIG. 17 is a cross-sectional view of a portion of the stack of FIG. 12 taken along line  17 — 17  of FIG. 16, wherein boundary layers resulting from airflow through the stack are diagrammatically shown by dashed lines. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     A kiln  10  of one embodiment of the present invention is schematically illustrated in FIG. 1, which is a partially cross-sectional front view. The operation of the kiln  10  of the illustrated embodiment of the present invention will initially be very generally described. The very general description will be followed by separate sections that respectively describe details about structures of the kiln  10 , assembly of the kiln, and some exemplary operational aspects of the kiln. Some aspects of the present invention are described without regard to the sections, and the use of the sections is not intended to limit the scope of the present invention. 
     The kiln  10  includes a kiln chamber  12  that receives a charge  14  of lumber. The kiln  10  further includes a furnace, such as a suspension furnace  16 , or the like, and a communication system that routes heated air from the furnace to the kiln chamber  12  to dry the charge  14  of lumber. The communication system includes a plenum that can be characterized as a composite plenum  18  and a duct system  19  that communicates at least between the furnace  16  and the composite plenum. The kiln chamber  12  and some of the items closely connected to or contained by the kiln chamber are schematically illustrated in FIG. 2, which is a cross-sectional view taken substantially along line  2 — 2  of FIG.  1 . Multiple air moving devices, such as a series of fans  20 , are operated to circulate the heated air within the kiln chamber  12  and enhance the drying of the charge  14  of lumber. Only a few of the fans  20  are specifically identified by their reference numeral in FIG.  2 . 
     Structures of the Kiln 
     As best understood with reference to FIGS. 1 and 2, the kiln chamber  12  includes opposite front and rear ends  22 ,  24  and opposite right and left sides  26 ,  28 . The kiln chamber  12  defines a chamber interior space that receives the charge  14  of lumber and is heated by the furnace  16 . The kiln chamber  12  includes a lower chamber portion that defines a lower portion of the chamber interior space  30 , which can also be characterized as a lower chamber interior space. The lower chamber portion includes a slab  32  and load-bearing front and rear walls  34 ,  36  that extend generally vertically upward from and are carried by the slab. The front wall  34  defines a front door opening  38  therethrough and carries front doors  40 , typically in a pivotal or slideable fashion, that are used to open and close the front door opening. Similarly, the rear wall  36  defines a rear door opening  42  therethrough and carries rear doors  44 , also typically in a pivotal or slideable fashion, that that are used to open and close the rear door opening. The lower chamber portion further includes lower portions of right and left side walls  46 ,  48 . It should be apparent, however, that the lumber can be loaded and unloaded through the same set of doors such that only one of the front and rear walls includes doors, or alternatively the doors could be in one or both side walls, if so desired. 
     A transportation system is provided for moving a charge  14  of lumber into the lower portion of the chamber interior space  30 , such as through the front door opening  38 , for drying, and thereafter out of the lower portion of the chamber interior space, such as through the rear door opening  42 . As illustrated in FIG. 1, the transportation system includes two sets of tracks  50  upon which wheeled carriages  52  travel. The tracks  50  extend longitudinally across the slab  32  and through the lower portion of the chamber interior space  30 , the front door opening  38 , and the rear door opening  42 . Each wheeled carriage  52  carries a stack of lumber. The transportation system at least partially defines a charge-receiving space within the lower portion of the chamber interior space  30 . The charge-receiving space is the space that is occupied by the charge  14  of lumber in FIGS. 1 and 2. A distance “d 1 ” is defined between each of the right and left side walls  46 ,  48  and the charge-receiving space. In accordance with one particular example, the distances “d 1 ” are each preferably at least approximately 12.75 feet. 
     As is additionally illustrated in FIG. 1, the right and left stacks of lumber, which can be characterized as respectively occupying and defining a right stack-receiving space and a left stack-receiving space, are generally spaced apart, such as by a distance “d 3 ”. In accordance with one particular example, the distance “d 3 ” is approximately 4.5 feet. In accordance with one particular example, each of the right and left stack-receiving spaces defines a volume of approximately 5,341.25 cubic feet, such that the total volume of the lumber load is approximately 10,682.5 cubic feet. 
     In accordance with the illustrated embodiment of the present invention, a charge  14  includes six stacks of lumber. However, the kiln  10  is scaleable and in accordance with one embodiment of the present invention a smaller kiln is provided for which a charge includes a single stack of lumber. That is, kilns of various sizes are within the scope of the present invention. For example, kilns that are sufficiently small can include only a single fan and corresponding reduced numbers of other components of the illustrated embodiment. 
     The kiln chamber  12  also includes an upper chamber portion that is positioned above the lower chamber portion. The upper chamber portion defines an upper portion of the chamber interior space  54 , which can also be characterized as an upper chamber interior space. The upper portion of the chamber interior space  54  is positioned above the lower portion of the chamber interior space  30  and at least partially contains the composite plenum  18 . The upper chamber portion includes upper portions of the right and left side walls  46 ,  48 , an upper front wall  56 , an upper rear wall  58 , and a roof  60 . The boundary between the upper and lower chamber portions is not necessarily associated with a precise location, but rather the upper and lower chamber portions are described to provide a frame of reference that aids in the description of the kiln chamber  12 . Nonetheless, in accordance with the illustrated embodiment of the present invention, a generally horizontally extending lower wall  62  of the composite plenum  18  can be characterized as defining the boundary between the upper and lower portions of the chamber interior space  54 ,  30 . 
     The composite plenum  18  includes opposite front and rear ends respectively positioned at the front and rear ends  22 ,  24  of the kiln chamber. The composite plenum  18  extends in a longitudinal direction between its front and rear ends. The front and rear ends of the lower wall  62  of the composite plenum  18  are respectively positioned upon the load-bearing front and rear walls  34 ,  36 . The front and rear walls  34 ,  36  together bear the entire weight of the composite plenum  18  and the components carried by the composite plenum, in accordance with the illustrated embodiment of the present invention. 
     The composite plenum  18  is described herein as including an upper plenum  64 , a lower plenum  66 , and an intermediate plenum  68 , each of which can be characterized as being a distinct part or section of the composite plenum. It is within the scope of the present invention for the composite plenum  18  to be characterized as being a non-composite component. Nonetheless, for the sake of explanation is useful to identify the sum of the upper, lower, and intermediate plenums  64 ,  66 ,  68  as the composite plenum or as a plenum system, or the like. 
     The upper plenum  64  includes generally vertically extending, opposite front and rear walls  70 ,  72 , as well as upper and lower right walls  74 ,  76  that cooperate to define a deck-like right protrusion  78  that extends longitudinally between the front and rear walls of the upper plenum. Likewise, upper and lower left walls  80 ,  82  cooperate to define a deck-like left protrusion  84  that extends longitudinally between the front and rear walls  70 ,  72  of the upper plenum  64 . All of the walls  70 ,  72 ,  74 ,  76 ,  80 ,  82  of the upper plenum  64  at least partially bound and define an upper plenum cavity  86 . For example, the upper plenum cavity  86  extends into the right and left protrusions  78 ,  84  of the upper plenum  64 . Walls of the upper plenum  64  also define a longitudinally and horizontally extending, downward-oriented interplenum opening  88  that is open to the upper plenum cavity  86  and is illustrated by broken lines in FIG.  3 . The upper plenum cavity  86  and the downward-oriented interplenum opening  88  extend generally for the entire longitudinal length of the upper plenum  64 . The upper plenum  64 , including the upper plenum cavity  86  and the downward-oriented interplenum opening  88 , is generally uniform along the length of the upper plenum (that is, in the longitudinal direction). The upper plenum cavity  86  can contain one or more longitudinally extending baffle plates (not shown) that are operative to restrict any undesired flow characteristics of the heated air within the upper plenum  64 . 
     The lower plenum  66  includes generally vertically extending, opposite front and rear walls  90 ,  92 . The lower wall  62  that generally separates the lower and upper portions of the chamber interior space  30 ,  54  is part of the lower plenum  66  and extends longitudinally between the front and rear walls  90 ,  92  of the lower plenum. The lower plenum  66  further includes a right wall  94  that cooperates with the lower wall  62  to provide a front deck-like right protrusion  96  that extends longitudinally between the front and rear walls  90 ,  92  of the lower plenum. Likewise, a left wall  98  cooperates with the lower wall  62  to provide a deck-like left protrusion  100  that extends longitudinally between the front and rear walls  90 ,  92  of the lower plenum  66 . In an end elevation view the composite plenum  18  generally defines an I-like shape due to the protrusions  78 ,  84 ,  96 ,  100 . 
     All of the walls  62 ,  90 ,  92 ,  94 ,  98  of the lower plenum  66  at least partially bound and define a lower plenum cavity  102 . For example, the lower plenum cavity  102  extends into the right and left protrusions  96 ,  100 . The right wall  94  defines a right radius of curvature  104 , and the left wall  98  defines a left radius of curvature  106 . Walls of the lower plenum  66  also define a longitudinally and horizontally extending, upward-oriented interplenum opening  180  that is open to the lower plenum cavity  102  and is illustrated by broken lines in FIG.  3 . The lower plenum cavity  102  and the upward-oriented interplenum opening  108  extend generally for the entire longitudinal length of the lower plenum  66 . Further, the lower plenum  66 , including the lower plenum cavity  102  and upward-oriented interplenum opening  108 , is generally uniform along the longitudinal length of the lower plenum. The lower plenum cavity  102  can contain one or more longitudinally extending baffle plates (not shown) that are operative to restrict any undesired flow characteristics of the heated air within the lower plenum  66 . 
     The lower wall  62  of the lower plenum  66  includes longitudinally extending right and left edges  110 ,  112  that extend longitudinally between the front and rear walls  90 ,  92  of the lower plenum. The right and left edges  110 ,  112  are spaced apart from one another in a lateral direction that is generally perpendicular to the longitudinal direction. The right edge  110  of the lower wall  62  extends laterally beyond a right side  114  of the charge-receiving space by a distance “d 2 ”. Likewise, the left edge  112  of the lower wall  62  extends laterally beyond a left side  116  of the charge-receiving space by a distance “d 2 ”. The distances “d 2 ” are each preferably at least approximately one foot. A longitudinally extending right flange  118  is connected to the lower wall  62  proximate the right edge  110 . The right flange  118  hangs downward from the lower wall  62  and is generally concave when viewed from the charge-receiving space. Similarly, a longitudinally extending left flange  120  is connected to the lower wall  62  proximate the left edge  112 . The left flange  120  hangs downward from the lower wall  62  and is generally concave when viewed from the charge-receiving space. As shown in FIG. 1, the lower plenum  66  is typically larger than the upper plenum  64  since the lower plenum also serves to direct air about the upper right and left comers of the charge  14  of lumber, as will be discussed in greater detail below. However, the upper and lower plenums  64 ,  66  can have the same general size, if so desired. 
     As illustrated in FIGS. 1-2, multiple lower outlets, which are preferably in the form of reheater conduits  122 , are mounted to the lower wall  62  of the lower plenum  66 . Only a representative few of the reheater conduits  122  are identified by their reference numeral in FIG.  2 . The reheater conduits  122  direct heated air from the lower plenum cavity  102  to the lower portion of the chamber interior space  30 . Each reheater conduit  122  defines a series of vertically spaced apart apertures along its length that provide communication paths to the lower portion of the chamber interior space  30 , as will be discussed in greater detail below. As best understood with reference to FIG. 1, the reheater conduits  122  are typically centered between the right and left stack-receiving spaces; however, the reheater conduits can be disposed in other positions if so desired. The reheater conduits  122  will be described in greater detail below, with reference to FIGS. 11-17. 
     The intermediate plenum  68  includes generally vertically extending, opposite front and rear walls  124 ,  126 . The intermediate plenum  68  also includes generally vertically and longitudinally extending, opposite right and left walls  128 ,  130  that are laterally spaced apart from one another and extend between the front and rear walls  124 ,  126 . All of the walls  124 ,  126 ,  128 ,  130  of the intermediate plenum  68  at least partially bound and define an intermediate plenum cavity  132  (FIG.  3 ). Walls of the intermediate plenum also define horizontally and longitudinally extending upward-oriented and downward-oriented interplenum openings  134 ,  136 , both of which are illustrated by broken lines in FIG.  3 . The intermediate plenum cavity  132  and the interplenum openings  134 ,  136  extend generally for the entire longitudinal length of the intermediate plenum  68 . The interplenum openings  134 ,  136  are generally uniform along the length of the intermediate plenum  68 . In contrast, the intermediate plenum  68  varies in the longitudinal direction because the intermediate plenum  68  includes a series of generally cylindrical circulation passages  138 , which are discussed in greater detail below. 
     As best understood with reference to FIG. 3, the upward-oriented and downward-oriented interplenum openings  134 ,  136  of the intermediate plenum  68  are respectively contiguous with and open to the upward-oriented interplenum opening  108  of the lower plenum  66  and the downward-oriented interplenum opening  88  of the upper plenum  64 . As a result, the intermediate plenum cavity  132  is contiguous with and in direct communication with both the upper plenum cavity  86  and the lower plenum cavity  102  so that the plenum cavities  86 ,  102 ,  132  together constitute a single large interior space of the composite plenum  18 , and in accordance with one particular example that single large interior space has a volume of approximately 10,877 cubic feet. 
     As best understood with reference to FIG. 2, the circulation passages  138  of the intermediate plenum  68  are arranged in a horizontal row. Each of the circulation passages  138  extends generally laterally and horizontally through the intermediate plenum  68 . Only a few of the circulation passages  138  are identified by their reference numeral in FIG. 2. A representative one of the circulation passages  138  will now be described with reference to FIG. 3, which is a partial, cross-sectional view taken substantially along the line  3 — 3  of FIG.  2 . The circulation passage  138  includes an interior wall  140  extending around and defining an interior space  142  of the circulation passage, as well as defining opposite right and left openings  144 ,  146  to the circulation passage. The interior wall  140  isolates the interior space  142  of the circulation passage  138  from the intermediate cavity  132  defined within the intermediate plenum  68 . That is, the interior space  142  of the circulation passage  138  is discontiguous with the intermediate cavity  132 . Therefore, the circulation passage  138  does not function as an outlet from the intermediate cavity  132 . In contrast, the interior space  142  of the circulation passage  138  is in direct communication with/open to the upper portion of the chamber interior space  54  (FIG. 1) by way of the right and left openings  144 ,  146  of the circulation passage. The medial portion of the interior wall  140  that is between and distant from the right and left openings  144 ,  146  to the circulation passage  138  is cylindrical, and at the opposite ends of that cylindrical portion the interior wall tapers by forming larger and larger circles that are coaxial with the cylindrical portion. In addition to the foregoing, the interior wall  140  can be characterized as a fan shroud. 
     As illustrated in FIGS. 1-3, multiple right and left outlets, which are preferably in the form of right and left nozzles  148 ,  150  but are not required to be nozzle-like, are respectively mounted to the right and left walls  128 ,  130  of the intermediate plenum  68 . Only a few of the nozzles  148 ,  150  are specifically identified with their reference numerals in FIG. 1, and only a few of the left nozzles  150  are specifically identified with their reference numeral in FIG.  2 . All of the right and left nozzles  148 ,  150  are capable of providing a communication path between the intermediate cavity  132  and the upper portion of the chamber interior space  54  (FIG.  1 ). The arrangement and operation of the left nozzles  150  on the left wall  130  of the intermediate plenum  68  is representative of the arrangement and operation of the right nozzles  148  on the right wall  128  of the intermediate plenum. As illustrated in FIG. 2, respective upper and lower groups of the left nozzles  150  are arranged partially around the left opening  146  (FIG. 3) of each of the circulation passages  138 . Likewise, respective upper and lower groups of right nozzles  148  are arranged partially around the right opening  144  (FIG. 3) of each of the circulation passages  138 . 
     Representative groups of the nozzles  148 ,  150  will now be described with reference to FIG.  3  and FIG. 4, which is an isolated left elevation view of a section of the intermediate plenum  68  that includes the circulation passage  138  illustrated in FIG.  3 . Heated air within the intermediate plenum cavity  132  is capable of flowing into the upper portion of the chamber interior space  54  through the nozzles  148 ,  150 . It is within the scope of the present invention for the nozzles  148 ,  150  to be neither converging nor diverging. However, in accordance with the illustrated embodiment, each of the nozzles  148 ,  150  is preferably a converging nozzle, meaning that the interior diameter of the nozzle decreases in the direction of flow therethrough. As a result of the design of the kiln  10 , a jet-like flow of heated air is discharged from the nozzles  148 ,  150  that are open while the kiln is operated. In accordance with one acceptable example, the jet-like flow from each of the nozzles  148 ,  150  that is open is a flow of heated air with a circular cross section and a velocity of the order of 200 feet per second. During operation of the kiln  10 , the jet-like flow is approximately steady and of steady state. Accordingly, each nozzle  148 ,  150  can be characterized as defining a discharge axis  152  that generally dictates the direction in which the heated air discharged therefrom initially travels. Discharge axes are illustrated by broken lines in FIGS. 3-4. 
     Different arrangements can be utilized for opening and closing the nozzles  148 ,  150 . For example, one arrangement will be described with reference to FIG.  5 . Another example of an arrangement for opening and closing the nozzles  148 ,  150  will be subsequently described with reference to FIGS. 6-8, in accordance with an alternative embodiment of the present invention. 
     In accordance with one embodiment of the present invention, each of upper groups of right nozzles  148 , lower groups of right nozzles, upper groups of left nozzles  150 , and lower groups of left nozzles are respectively equipped with nozzle dampers  154  (FIG. 5) positioned in the intermediate plenum cavity  132  and operative for opening and closing the nozzles. Representative upper and lower nozzle dampers  154  will now be described with reference to FIG. 5, although other types of dampers can be employed. The nozzle dampers  154  illustrated in FIG. 5 are carried by the inside surface of the portion of left wall  130  of the intermediate plenum  68  that includes the representative circulation passage  138  and left nozzles  150  illustrated FIG.  4 . The nozzle dampers  154  illustrated in FIG. 5 are representative of the other nozzle dampers carried by the inside surface of the left wall  130  of the intermediate plenum  68 . Likewise, the nozzle dampers  154  illustrated in FIG. 5 are representative of the nozzle dampers carried by the inside surface of the right wall  128  of the intermediate plenum  68 . 
     The lower nozzle damper  150  illustrated in FIG. 5, which is representative of the upper nozzle damper illustrated in FIG. 5 except for orientation, is exploded away from its respective group of nozzles. Each nozzle damper  150  is arcuate in shape and includes openings  156  spaced along the length thereof, and those openings are sized and spaced in a manner corresponding to the sizing and spacing of the respective nozzles that are opened and closed by the nozzle damper. Brackets or bolting systems (not shown) movably hold the nozzle dampers  154  to the inside surface of the left wall  130  of the intermediate plenum  68 . 
     The operation of the upper nozzle damper  154  illustrated in FIG.  5  and the operation of a damper control system  157  illustrated in FIG. 5 are respectively representative of the operation of the other nozzle dampers and other damper control systems of the kiln  10  (FIG.  1 ). The upper nozzle damper  154  is illustrated in its open position by solid lines in FIG.  5 . In contrast, the upper nozzle damper  154  is illustrated in its closed position by broken lines in FIG.  5 . The nozzles  150  associated with the upper nozzle damper  154  are open while the upper nozzle damper is in the open configuration because those nozzles are respectively aligned with and communicating through the openings  156  of the nozzle damper. The nozzles  150  associated with the upper nozzle damper  154  are occluded by solid portions of the upper nozzle damper while the upper nozzle damper is in the closed configuration. 
     In accordance with the illustrated embodiment of the present invention, movement of the upper nozzle damper  154  between the open and closed configurations is facilitated by the damper control system  157 . The damper control system  157  includes a cylinder  158  that is mounted to be stationary and includes a movable push rod  159 . The push rod  159  is connected to and moves a control rod  160  that is connected to a clevis  161  that is mounted to the upper nozzle damper  154 . As a result, the cylinder  158  can be operated to move the upper nozzle damper  154  between its open and closed configurations. Multiple nozzle dampers  154  can be linked together through the use of additional control rods that are linked together and operated in unison by a single damper control system  157 . 
     The left-most nozzles  150  illustrated in FIG. 5 are not opened and closed by the dampers  154  illustrated in FIG.  5 . Rather, there are dampers  154  operative for opening and closing nozzles  150  extending around the circulation passage  138  adjacent to the circulation passage illustrated in FIG.  5 . The dampers  154  for that adjacent circulation passage  138  are respectively operative for opening and closing the left-most nozzles  150  illustrated in FIG.  5 . 
     The mounting of the nozzles  148 ,  150  and the opening and closing thereof will now be described with reference to FIGS. 6-8, in accordance with an alternative embodiment of the present invention that is identical to the embodiment described with reference to FIGS. 1-5, except for variations noted and variations that will be apparent to those of ordinary skill in the art. Only portions of the alternative kiln are illustrated in FIGS. 6-8, and it is to be understood that it is preferred for those representative portions illustrated in FIGS. 6-8 to be duplicated to provide a kiln like that disclosed with respect to FIGS. 1-5, except for the respective substitution of the components illustrated in FIGS. 6-8. 
     In accordance with the embodiment illustrated in FIGS. 6-8, the mounting of the left nozzles  150  and the arrangement and operation of their associated arcuate nozzle dampers  154 ′ (FIG. 8) and damper control systems  157  (FIG. 6) are representative of the mounting of the right nozzles  148  and the arrangement and operation of the nozzle dampers and damper control systems associated with the right nozzles. In accordance with the embodiment illustrated in FIGS. 6-8, the nozzles  150  are mounted, such as through the use of welding techniques or the like, to outside surfaces of respective arcuate support plates  162 . Only a representative few of the nozzles  150  are specifically identified by their reference numeral in FIG.  6 . The nozzles  150  are positioned to be coaxial with respective downstream openings  163  (FIG. 8) that are defined through the support plates  162 . The support plates  162  are mounted so that inside surfaces of the support plates are oriented toward the outside surface of the left wall  130  of the intermediate plenum  68 . The left wall  130  defines a plurality of upstream openings  164  (FIG. 8) therethrough that are open to the intermediate plenum cavity  134  (FIGS.  3  and  7 ). The support plates  162  are mounted so that the downstream openings  163  therethrough are capable of being generally coaxial with respective upstream openings  164 . 
     More specifically, and as best understood with reference to the exploded and representative nozzles  150  and portions of the left wall  130 , damper  154 ′, support plate  162 , and associated components illustrated in FIG. 8, each support plate is mounted to the left wall  130  by multiple bolts  165 . Referring to the representative components, or portions thereof, illustrated in FIG. 8, the support plate defines multiple slots  166 , and bolts  165  respectively extend through the slots. Each bolt  165  includes a threaded shaft that terminates at a head, and the threaded shafts are threaded into respective threaded bores  167  defined by the left wall  130 . 
     Referring to a representative one of the bolts  165  illustrated in FIG. 8, the shaft of the bolt receives a cylindrical washer  168  prior to the shaft being inserted through its respective slot  166 . The shaft of the bolt  165  receives a cylindrical bushing  169  after the shaft has been passed through its washer  168  and slot  166 , and prior to the shaft being threaded into its respective threaded bore  167 . Each of the washers  168  and bushings  168  has a major diameter that is sufficiently large to prevent the washers and bushings from passing through the respective slots  166  while assembled as described above. Accordingly, the support plate  162  is mounted to the left wall  130  by the bolts  165  and spaced apart from the left wall  130  by the bushings  168 . For example, the spacing of a support plate  162  with respect to the wall  130  is illustrated in FIG.  7 . 
     Further referring to the representative components, or portions thereof, illustrated in FIG. 8, a nozzle damper  154 ′ is positioned in the space between the support plate  162  and the left wall  130 . An inner edge  170  of the nozzle damper  154 ′ engages and is selectively movable relative to inner ones of the bushing  169  (that is, the upper bushings illustrated in FIG.  8 ). Likewise an outer edge  171  of the nozzle damper  154 ′ engages and is selectively moveable relative to outer ones of the bushings  169  (that is, the lower bushings illustrated in FIG.  8 ). The nozzle damper  154 ′ defines multiple intermediate openings  156 ′ therethrough and the nozzle damper is moveable between open and closed configurations. In the open configuration, the intermediate openings  156 ′ are generally respectively aligned with upstream openings  164 , downstream openings  163 , and nozzles  150 , as is generally illustrated in FIG. 8, so that heated air is supplied through the nozzles. In contrast and as illustrated in FIG. 6, in the closed configuration the intermediate openings  156 ′, which are illustrated by broken lines in FIG. 6, are offset from upstream openings  164 , downstream openings  163 , and nozzles  150  so that heated air is not supplied through the nozzles. Only a representative few of the intermediate openings  156 ′ are specifically identified by their reference numeral in FIG.  6 . 
     In accordance with the embodiment illustrated in FIGS. 6-8, movement of the nozzle dampers  154 ′ between the open and closed configurations is facilitated by the damper control systems  157  (FIG.  6 ). As best understood with reference to FIG. 6, each damper control system  157  includes a cylinder  158  that is mounted to be stationary and includes a movable push rod  159 . The push rod  159  is connected to and moves one or more control rods  160  that are respectively connected to devises  161  that are respectively mounted to the dampers  154 ′. As a result, the cylinder  158  can be operated to move multiple nozzle dampers  154 ′ between their open and closed configurations. 
     Further referring to the representative components, or portions thereof, illustrated in FIG. 8, the amount of flow through the nozzles  150  while the damper  154 ′ is in its open configuration can be adjusted by adjusting the alignment of the nozzles with the with upstream and intermediate openings  164 ,  156 ′. The alignment can be adjusted by loosening the bolts  165  so that the support plate  162  is movable relative to the wall  130 . Thereafter, the support plate  162 , which remains supported by the bolts  165 , is manually moved the desired amount so that the bolts are positioned differently in their respective slots  166 . Thereafter, the bolts  165  are tightened to secure the support plate  162  in its new position. This procedure can be used to increase or decrease the alignment between the nozzles  150  with their respective upstream and intermediate openings  164 ,  156 ′ so that the flow through the nozzles is respectively increased or decreased. 
     As best understood with reference to FIG. 1, in accordance with another alternative embodiment that is not illustrated, the nozzles  148 ,  150  are connected to the upper and lower plenums  64 ,  66  rather than being connected to the intermediate plenum  68 . More specifically, the upper right nozzles  148  are mounted to the lower right wall  76  of the upper plenum  64  and are capable of providing a communication path between the upper plenum cavity  86  (FIG. 3) and the upper portion of the chamber interior space  54 . Similarly, the upper left nozzles  150  are mounted to the lower left wall  82  of the upper plenum  64  and are capable of providing a communication path between the upper plenum cavity  86  and the upper portion of the chamber interior space  54 . Further, the lower right nozzles  148  are mounted to the right wall  94  of the lower plenum  66  and are capable of providing a communication path between the lower plenum cavity  102  (FIG. 3) and the lower portion of the chamber interior space  30 . Similarly, the lower left nozzles  150  are mounted to the left wall  98  of the lower plenum  66  and are capable of providing a communication path between the lower plenum cavity  102  and the lower portion of the chamber interior space  30 . In accordance with this alternative embodiment, the components for opening and closing the nozzles  148 ,  150  are relocated accordingly. 
     The suspension furnace  16  of the illustrated embodiment of the present invention is diagrammatically illustrated in FIG.  1 . The furnace  16  includes a mixing chamber  174  in which combustible fuel is burned to create fire  176 . The fire  176  creates combustion by-products that are mixed with heated air. The furnace  16  includes an air moving device  178  that moves the heated air and associated combustion by-products. Accordingly, for the portions of the Detailed Description of the Invention section of this disclosure that describe the embodiment of the present invention that is illustrated in FIGS. 1-6, “heated air” refers to the combination of the air heated by the furnace  16  and the combustion by-products carried by that heated air. In accordance with another embodiment of the present invention, the furnace  16  includes a heat exchanger and is operated so that the air heated by the furnace is substantially absent of the combustion by-products created by the fire  176 . Further, it is within the scope of the present invention for the furnace  16  to be of any type that is conventionally used to provide heated air to a plenum that distributes the heated air. 
     The duct system  19  that extends from the furnace  16  is schematically illustrated in FIG. 1 as including a hot duct assembly  180  and a cool duct assembly  182 . The hot duct assembly  180  directs heated air from the furnace  16  to the composite plenum  18 . The hot duct assembly  180  includes an upstream duct  184  having an upstream end connected to and in direct communication with the furnace  16 , and a bifurcated downstream end connected to and in communication with both an upper downstream duct  186  and a lower downstream duct  188 . An adjustable damper  190  is positioned within the upstream duct  184  at the juncture with the downstream ducts  186 ,  188  for balancing or adjusting the flows into the downstream ducts. The upper downstream duct includes an outlet end  192  (also see FIG. 2) that is mounted to the upper plenum  64  and is in direct communication with the upper plenum cavity  86 . The lower downstream duct  188  includes an outlet end  194  (also see FIG. 2) that is mounted to the lower plenum  66  and is in direct communication with the lower plenum cavity  102 . 
     The cool duct assembly  182  directs air from the upper portion of the chamber interior space  54  to the furnace  16 . The cool duct  182  assembly includes a pair of right return ducts  196  (also see FIG. 2) and a pair of left return ducts  198  (only one of which is shown) having upstream ends mounted to the roof  60  and capable of being in direct communication with the upper portion of the chamber interior space  54 . 
     Different arrangements can be utilized for opening and closing the return ducts  196 ,  198 . For example, one arrangement will be described with reference to FIG.  1 . Another example of an arrangement for opening and closing the return ducts  196 ′,  198 ′ will be described with reference to FIG. 9, in accordance with an alternative embodiment of the present invention. 
     In accordance with the embodiment illustrated in FIG. 1, each of the right return ducts  196  is equipped with a respective right return damper  200  (only one of which is shown) that is capable of being moved to open and close the duct. Likewise, each of the left return ducts  198  is equipped with a respective left return damper  202  (only one of which is shown) that is capable of being moved to open and close the duct. The right return damper  200  illustrated in FIG. 1 is positioned so that the right return duct  196  illustrated in FIG. 1 is open to the upper portion of the chamber interior space  54 . In contrast, the left return damper  202  illustrated in FIG. 1 is positioned so that the left return duct  198  illustrated in FIG. 1 is isolated from the upper portion of the chamber interior space  54 . 
     The opening and closing of return ducts  196 ′,  198 ′ will now be described with reference to FIG. 9, in accordance with an alternative embodiment of the present invention that is identical to the embodiment described with reference to FIGS. 1-5, except for variations noted and variations that will be apparent to those of ordinary skill in the art. In accordance with this alternative embodiment, one of the right return ducts  196 ′ joins one of the left return ducts  198 ′ and a downstream duct  193  to form a tee. There are preferably two separate tees (that is, two separate right return ducts  196 ′, two separate left return ducts  198 ′, and two downstream ducts  193 ) and associated components. Whereas only a single tee is illustrated in FIG. 9, the illustrated tee and its associated components are representative of the corresponding yet not illustrated tee and its associated components. 
     Referring to the representative components illustrated in FIG. 9, the downstream duct  193  provides the communication path from the right and left return ducts  196 ′,  198 ′ to the mixing chamber  174  (FIG.  1 ). As illustrated in FIG. 9, the right return damper  200 ′ is positioned in the right return duct  196 ′ at the tee. Similarly, the left return damper  202 ′ is positioned in the left return duct  198 ′ at the tee. Each of the dampers  200 ′,  202 ′ are respectively centrally pivotally mounted and moveable between the positions indicated by solid and broken lines in FIG.  9 . In addition, a linkage  199  is connected between and links the dampers  200 ′,  202 ′, and a piston assembly  197  is mounted within the tee and connected to the left return damper  202 ′. The piston assembly  197  is operated and the linkage  199  is operative so that the dampers  200 ′,  202 ′ move together between the positions illustrated by solid lines and the positions illustrated by broken lines in FIG.  9 . Accordingly, the right return duct  196 ′ is in communication with and the left return duct  198 ′ is not in communication with the mixing chamber  174  via the downstream duct  193  while the dampers  200 ′,  202 ′ are in the positions illustrated by solid lines in FIG.  9 . In contrast, the right return duct  196 ′ is not in communication with and the left return duct  198 ′ is in communication with the mixing chamber  174  via the downstream duct  193  while the dampers  200 ′,  202 ′ are in the positions illustrated by broken lines in FIG.  9 . 
     As best understood with reference to FIG. 2, air moving devices, which are fans  20  in accordance with the illustrated embodiment of the present invention, are positioned within the upper portion of the chamber interior space  54  in a parallel arrangement that extends in the longitudinal direction. The fans  20  are capable of providing a recirculating flow path  204  within the upper and lower portions of the chamber interior space  54 ,  30 . The general center of the recirculating flow path  204  is schematically illustrated in FIG. 1 by a line made up of a series of two short dashes alternating with one dash. The fans  20  are reversible and can be operated so that all of the air within the upper and lower portions of the chamber interior space  54 ,  30  moves either in a clockwise direction along the recirculating flow path  204  or a counterclockwise direction along the recirculating flow path. Throughout the Detailed Description of the Invention section of this disclosure, FIG. 1 is the frame of reference with respect to which flow in the clockwise and counterclockwise directions is defined. The direction of operation of the fans  20  is periodically reversed during the drying of a charge  14  of lumber because reversing the flow helps to uniformly dry the charge of lumber. 
     As shown in FIG. 2, each of the circulation passages  138  is equipped with a respective fan  20 . Only a few of the fans  20  are identified by their reference numeral in FIG. 2. A representative one of the fans  20  will now be described with reference to FIG. 1, in which a portion of the representative fan is hidden from view and therefore shown in broken lines. The fan  20  includes a motor  206  that rotates a drive shaft  208  by way of a drive belt  210 . An impeller  212  is mounted to the end of the drive shaft  208  and is positioned within the respective circulation passage  138 . Portions of a representative one of the fans  20  will now be described with reference to FIG.  3 . The motor  206  and drive belt  210  are not shown and the drive shaft  208  is partially cut away in FIG.  3 . Whereas FIG. 3 is a cross-sectional view taken substantially along line  3 — 3  of FIG. 2, the impeller  212  and drive shaft  208  are not cross-sectioned in FIG.  3 . The fan  20 , or more specifically the impeller  212 , has a rotational axis  214  that dictates the general direction in which the air moved by the fan initially travels. The interior wall  140  of the respective circulation passage  138  extends around and is coaxial with the rotational axis  214 . The impeller  212  includes multiple blades  216  that extend radially away from proximate the rotational axis  214  of the impeller, and each blade includes a blade tip  218  that is distant from the rotational axis. As best understood with reference to FIG. 2, the rotational axes (for example see the rotational axis  214  illustrated in FIG. 3) of all of the impellers  212  are parallel and extend in a common horizontal plane. 
     A representative one of the reheater conduits  122  will now be described with reference to FIG. 1, in accordance with one embodiment of the present invention. The reheater conduit  122  includes opposite top and bottom ends. More specifically, the top end of the reheater conduit  122  is in the form of an contraction fitting  222 , and the lower end of the reheater conduit  122  is in the form of a pipe-like structure  224  connected to and extending downward from the contraction fitting. The interior of the pipe-like structure  224  is open to the interior of the contraction fitting  222 . The pipe-like structure  224  is closed at its bottom end and includes a normally closed ash dump door  226  at its bottom end. In accordance with the illustrated embodiment of the present invention, ash is carried by the heated air supplied from the furnace  16  into the composite plenum  18 , and at least some of the ash settles into the bottom end of the pipe-like structure  224 . The ash dump door  226  is periodically opened while the kiln  10  is not operating so that ash can be removed from the pipe-like structure  224 , and thereafter the ash dump door is closed and remains closed during normal operation of the kiln. 
     A representative one of the reheater conduits  122  and an associated representative portion of the lower wall  62  of the composite plenum  18  will now be described with reference to FIG. 11, in accordance with one embodiment of the present invention. Whereas the reheater conduits  122  are described as being used in combination with the composite plenum  18 , the reheater conduits can be used in combination with a variety of different types of plenums, or the like. The top end of the contraction fitting  222  is mounted to the bottom surface of the lower wall  62  of the composite plenum  18 . The lower wall  62  of the composite plenum  18  defines an opening therethrough that is contiguous with an upper opening of the contraction fitting  222  so that the interior of the pipe-like structure  224  is in communication with the lower plenum cavity  102  via the contraction fitting. The communication path between the interior of the pipe-like structure  224  and the lower plenum cavity  102  can be opened and closed, or throttled, by manually operating a movable damper  228  that is held to the interior surface of the lower wall  62  by brackets (not shown). Each of the reheater conduits  122  is respectively equipped with a separate movable damper  228  so that each of the reheater conduits can be separately throttled. 
     A representative one of the reheater conduits  122  will now be described with reference to FIGS. 11-12, in accordance with one embodiment of the present invention. Adjacent internal portions of the contraction fitting  222  and the pipe-like structure  224  cooperate to define a contraction expansion that is shaped to advantageously offset or balance the effects of partially closing the inlet to the reheater conduit  122  with the respective damper  228 . More specifically, the contraction expansion includes an internal contraction  230  and an internal expansion  232 , both of which are illustrated by broken lines in FIGS. 11-12. The internal contraction  230  accelerates the flow into the contraction expansion to remove separation that may exist as the flow enters the reheater conduit  122  from the lower plenum  66 . In accordance with the illustrated embodiment, the internal contraction  230  is shorter than the internal expansion  232 . The internal expansion  232  is more gradual than the internal contraction  230  and functions to decelerate the flow therethrough to the predetermined lower velocity that is desired in the pipe-like structure  224 . The deceleration provided by the internal expansion  232  is gradual so as to avoid separation. In accordance with one embodiment of the present invention, the internal contraction  230  has a length of approximately 6 inches, the internal expansion  232  has a length of approximately 18 inches, the angle defined by the internal contraction  230  relative to the vertical is not critical, and the angle defined by the internal expansion  232  relative to the vertical is of the order of approximately 8 to 15 degrees. 
     A representative one of the pipe-like structures will now be described with reference to FIG. 13, which is a cross-sectional view taken along line  13 — 13  of FIG. 12, in accordance with one embodiment of the present invention. As illustrated in FIG. 13, the pipe-like structure  224  is preferably elliptical, or the like, although other shapes are also within the scope of the present invention, such as oblong shapes, and the like. More specifically, the pipe-like structure  224  is elliptical in a cross-section thereof taken perpendicular to the length thereof. The pipe-like structure  224  can be characterized as including a pair of major vertices  234  that define a major cross-dimension “d 4 ” therebetween. Further, the pipe-like structure can be characterized as including a pair of minor vertices  236  defining a minor cross-dimension “d 5 ” therebetween. The major cross-dimension “d 4 ” is preferably at least two times greater than the minor cross-dimension “d 5 ”. More specifically, in accordance with one specific example of the present invention, the major cross-dimension “d 4 ” is in the range of approximately 10 inches to approximately 18 inches, and most preferably approximately 14.6 inches; and the minor cross-dimension “d 5 ” is in the range of approximately 5 inches to approximately 12 inches, and is most preferably approximately 7.6 inches. In accordance with another specific example, the major cross-dimension “d 4 ” is approximately 18 inches and the minor cross-dimension “d 5 ” is approximately 9 inches. 
     Each of the reheater conduits  122  extends into the recirculating flow path  204  (FIG.  1 ). As a result, and as best understood with reference to FIG. 13, for each reheater  122  a flow-induced boundary layer  237  is formed generally therearound while the fans  20  (FIGS. 1-3) are operated to cause air to flow along the recirculating flow path  204 . A representative boundary layer  237  resulting from counterclockwise flow along the recirculating flow path  204 , from the frame of reference provided by FIG. 1, is illustrated by broken lines in FIG.  13 . Of course the orientation of the boundary layers  237  would be opposite from that illustrated for clockwise flow along the recirculating flow path  204 . As illustrated in FIG. 13, downstream portions of the boundary layers  237  can become separated from the trailing surfaces of the reheater conduits  122 . 
     As best understood with reference to FIGS. 11-14, each pipe-like structure  224  includes multiple outlets  238  spaced along its length from close to its top end to close to its bottom end. The outlets  238  are preferably closer to the minor vertices  236  than to the major vertices  234 , and most preferably the outlets are arranged along the minor vertices  236 . For each reheater conduit  122 , heated air within the lower plenum cavity  102  is capable of flowing along a flow path  240  (FIG. 11) into the reheater conduit, and out of reheater conduit and into the recirculating flow path  204  by way of outlets  238 . 
     Each of the outlets  238  is constructed to provide jet-like flow. More specifically, and as best understood with reference to FIG. 13, each outlet is preferably in the form of a cylindrical bore that extends through the wall of the respective pipe-like structure  224  to provide communication from the flow path  240  to the recirculating flow path  204  (FIG.  1 ). In accordance with the illustrated embodiment of the present invention, each outlet  238  has a length that extends in the direction of flow through the outlet, and the length is equal to the thickness of the wall of the pipe-like structure  224 ; and each outlet has a diameter that is perpendicular to the direction of flow through the outlet, and the diameter is in the range of approximately 0.25 inches to approximately 6 inches, and is most preferably approximately 0.625 inches. Whereas the outlet  238  are described above and below as being generally cylindrical in shape, it is within the scope of the present invention for them to be shaped differently. In accordance with one acceptable example, the jet-like flow from each of the outlets  238  is a flow of heated air with a circular cross section and a velocity of the order of 200 feet per second. During operation of the kiln  10  the jet-like flow is approximately steady and of steady state. 
     FIG. 14 is a schematic right elevation view of sections of a representative pair of adjacent reheater conduits  122  in accordance with one embodiment of the present invention. FIG. 14 illustrates heated air being discharged from the outlets  238  of the reheater conduits  122 . In FIG. 14, the jet-like flow being discharged by the outlets are represented by straight arrows. The interaction between the representative pair of adjacent reheater conduits  122  illustrated in FIG. 14 will now be described. The outlets  238  are arranged in groups, and the groups are staggered such that the jet-like flows from the adjacent reheater conduits cooperate to form eddy-like whirling masses of air  242 , which can also be characterized as vortices. The pattern of the outlets  238  illustrated in FIG. 14 repeats numerous times along the lengths of adjacent reheater conduits  122  so that for each pair of adjacent reheater conduits a series of the whirling masses of air  242  are contemporaneously and continuously produced while the kiln  10  is operating. Each of the formed whirling masses of air  242  travels generally along the recirculating flow path  204  (FIG. 1) and results in turbulence that interacts with a stack of lumber that is proximate to and downstream from the reheater conduits  122 , as will be discussed in greater detail below. In accordance with the illustrated embodiment of the present invention, the distance “d 6 ” between the adjacent reheater conduits  122  is in the range of approximately 8 inches to approximately 48 inches, and is most preferably approximately 25.5 inches. The center-to-center distance “d 7 ” between adjacent outlets  238  of the same reheater conduit  122  is preferably in the range of approximately 1 inch to approximately 10 inches, and is most preferably approximately 2.74 inches. The center-to-center distance “d 8 ” between the closest outlets  238  of adjacent groups of outlets of adjacent reheater conduits  122  is in the range of approximately 0.25 inches to approximately 30 inches, and is most preferably approximately 2.74 inches. 
     Whereas each group of adjacent outlets  238  for the same reheater conduit  122  is illustrated as including four outlets in FIG. 14, in accordance with an alternative embodiment (not shown) the reheater conduits are identical to and used identically to the reheater conduits illustrated in FIGS. 11-14, except for variations that are noted and variations that will be apparent to those of ordinary skill in the art. In accordance with this alternative embodiment, each group of adjacent outlets of the same reheater conduit includes two outlets, the center-to-center distance between the closest outlets of adjacent groups of outlets on the same side of the same reheater conduit is approximately 8.23 inches, the outlets begin on one side of the pipe-like structure  224  at a distance of approximately 14.75 inches from the base of the contraction fitting  222 , and the outlets begin on the other side of the pipe-like structure at a distance of approximately 20.23 inches from the base of the contraction fitting. In accordance with other embodiments, each group of adjacent outlets  238  of the same reheater conduit includes three outlets or more than four outlets. In accordance with other embodiments, the groups of adjacent outlets  238  are replaced with slots. 
     FIG. 15 is a schematic right elevation view of a representative pair of adjacent ones of reheater conduits  122 ′, in accordance with an alternative embodiment of the present invention. The reheater conduits  122 ′ of the alternative embodiment are identical to and used identically to the reheater conduits  122  illustrated in FIGS. 11-14, except for variations that are noted and variations that will be apparent to those of ordinary skill in the art. As will be noted, the adjacent reheater conduits included outlets that are staggered such that an outlet from one of the reheater conduits is vertically positioned between, typically in the middle of, a pair of adjacent outlets from the other reheater conduit. As described above in conjunction with the embodiment of FIG. 14, the staggered outlets serve to create whirling masses of air in the recirculating flow path. The center-to-center distance “d 7 ′” between adjacent outlets  238  of the same reheater conduit  122 ′ is preferably approximately in the range of approximately 1.0 inch to approximately 20 inches, and is most preferably approximately 8.23 inches. The center-to-center distance “d 8 ′” between adjacent outlets of adjacent reheater conduits  122  is approximately half of the distance “d 7 ′”. That is, the distance “d 8 ′” is in the range of approximately 0.5 inches to approximately 15 inches, and is most preferably approximately 2.74 inches. 
     As illustrated in both FIGS. 14 and 15, for a given reheater conduit, the outlets on one side thereof are preferably staggered with respect to the outlets on the other side thereof. 
     Construction of the Kiln 
     Some of the aspects relating to the efficient construction of the kiln  10  will now be described, in accordance with one embodiment of the present invention. The kiln  10  is preferably at least partially constructed and assembled using modular construction techniques. More specifically, the composite plenum  18  and other components of the kiln  10  are at least partially pre-manufactured remotely from the final construction site of the kiln and are trucked to the final construction site of the kiln. 
     In accordance with one embodiment of the present invention, the composite plenum  18  is in multiple different and separate pieces when shipped to the final construction site, and those pieces are welded or bolted together, or the like, at the construction site such that in isolation the assembled composite plenum is absent of movable parts. In contrast, in accordance with another embodiment of the present invention, the composite plenum  18  is constructed so that it can originally be transitioned between extended and collapsed configurations by moving (that is, telescoping) the intermediate plenum  68  into and out of the upward-oriented interplenum opening  108  (FIG. 3) of the lower plenum  66 . The extended configuration is illustrated by solid lines in FIGS. 1-3 and by the broken line in FIG. 10 that is in the form of alternating short and long dashes. In contrast, the collapsed configuration is illustrated by solid lines and by the broken line that is in the form of uniform dashes in FIG.  10 . As illustrated, the upper plenum  64  is mounted to the intermediate plenum  68  during both the compacted and extended configurations. Portions of the protrusions  96 ,  100  of the lower plenum  66  are cut away in FIG.  10 . 
     Further regarding the telescoping composite plenum  18  and as best understood with reference to FIG. 10, the walls  124 ,  126 ,  128 ,  130  (also see FIGS. 1-5) of the intermediate plenum  68  extend through the upward-oriented interplenum opening  108  (FIG. 3) of the lower plenum  66  and the lower ends of the walls of the intermediate plenum extend into the lower plenum cavity  102  and are proximate the lower wall  62  during the compacted configuration. As a result, the walls  90 ,  92 ,  94 ,  98  (also see FIGS. 1-3) of the lower plenum  66  that extend around and define the upward-oriented interplenum opening  108  of the lower plenum  66  overlap the walls  124 ,  126 ,  128 ,  130  of the intermediate plenum  68 , so that those walls of the intermediate plenum can be characterized as underlapping walls. At least lower ones of the nozzles  148 ,  150  (FIGS. 2-5) are not mounted to the intermediate plenum  68  during the compacted configuration, because at least some of the nozzles would interfere with the telescoping. 
     The telescoping capability is particularly advantageous when the kiln  10  is constructed and assembled using modular construction techniques. The composite plenum  18  is assembled and placed in the collapsed configuration at a location remote from the final site of the kiln  10  and is thereafter transported to the final site of the kiln, where the composite plenum is placed in the extended configuration. The extended configuration is achieved by telescopically lifting the combination of the upper and intermediate plenums  64 ,  68  with respect to the lower plenum  66 , such as through the use of a crane, or the like. The combination of the upper and intermediate plenums  64 ,  68  is lifted so that at least substantially less of the intermediate plenum extends into the lower cavity  102  of the lower plenum  66  during the extended configuration than during the compacted configuration. Lower portions of the intermediate plenum  68  are then immovably mounted to the lower plenum  66  to hold the composite plenum  18  in the extended configuration through the use of conventional mounting techniques, such as welding, bolting, or the like. Thereafter, the nozzles  148 ,  150  are mounted to the intermediate plenum  68  through the use of conventional mounting techniques, such as welding, bolting, or the like. 
     The slab  32  is poured at the final location of the kiln  10 . The load-bearing front and rear walls  34 ,  36  are positioned generally vertically upon the slab  32  and are spaced apart from one another in the longitudinal direction. Other walls of the kiln chamber  12  may be placed upon the slab  32  along with the load-bearing front and rear walls  34 ,  36  to stabilize the load-bearing front and rear walls. Thereafter, the composite plenum  18  is lifted, such as through the use of a crane, and the composite plenum is lowered so that the front and rear ends of the bottom wall  62  respectively rest upon the load-bearing front and rear walls  34 ,  36 , as is illustrated in FIG.  2 . The composite plenum  18  is secured to the load-bearing front and rear walls  34 ,  36  through the use of conventional construction techniques, such as welding, or bolting, or the like. Thereafter, the other walls  56 ,  58  and the roof  60  of the kiln chamber  12  are installed in a generally modular fashion to define the upper and lower portions of the chamber interior space  54 ,  30 . In accordance with the illustrated embodiment, the kiln chamber  12  is constructed so that the composite plenum  18  is suspended above the slab  32  solely by the load-bearing front and rear walls  34 ,  36 . In addition, the roof  60 , reheater conduits  122 , and at least some of the upper front and rear walls  56 ,  58  of the kiln chamber are mounted directly to and carried by the composite plenum  18 . As such, the composite plenum  18  and the load bearing portions of the front and rear walls  34 ,  36  are preferably formed of steel in order to support the kiln components carried thereby without additional load bearing structures. 
     In accordance with another embodiment of the present invention, the kiln  10  is more completely built at the final construction site of the kiln using construction techniques other than modular construction techniques. 
     Operation of the Kiln 
     The kiln  10  operates in a manner that efficiently dries a charge  14  of lumber. The basic operation of the kiln  10  will now be described, in accordance with one embodiment of the present invention, with occasional reference to exemplary advantageous aspects of the kiln. Advantageous aspects of the kiln  10  include, but are not limited to, those that promote the uniform drying of the charge  14  of lumber, that reduce flow-related losses within the kiln, that optimize heat utilization within the kiln, that enhance the operation of the fans  20 , that enhance the mixing of the heated air within the upper portion of the chamber interior space  54 , and that enhance mixing of heated air and efficient flow through the charge of lumber. Although some of the aspects of the kiln  10  are described in the context of a single advantage, those of ordinary skill in the art will appreciate that at least some of the recited advantages are not independent of one another. Further, this disclosure is not intended to provide an exhaustive list of all of the advantages provided by the present invention. 
     The kiln  10  is readied for operation by using the transportation system, which includes the tracks  50  and wheeled carriages  52 , to placing a charge  14  of green lumber within the charge-receiving space by way of the front door opening  38 . Thereafter, the front and rear doors  40 ,  44  are closed to respectively close the front and rear door openings  38 ,  42 . In addition, other openings (not shown) of the kiln chamber  12  are closed so that the interior space of the kiln chamber is generally enclosed. Some leakage of air into and out of the interior space of the kiln chamber  12  is desired, however, so that moisture escapes from the interior space of the kiln chamber and ambient air is drawn into the interior space of the kiln chamber. 
     After the interior space of the kiln chamber  12  is generally sealed with a charge  14  of green lumber in the charge-receiving space, the furnace  16  is operated so that heated air is supplied to the interior space of the kiln chamber  12  and the fans  20  are operated to move the heated air along the recirculating flow path  204 . In accordance with one aspect of the kiln  10 , the direction of operation of the fans is periodically reversed while a charge  14  of lumber is being dried, which promotes the uniform drying of the charge of lumber. Each fan  20  is operated in a manner that promotes clockwise flow along the recirculating flow path  204  during a clockwise mode. For each fan  20 , the right side thereof is the high-pressure or discharge side and the left side thereof is the low-pressure or intake side during the clockwise mode. Likewise, each fan  20  is operated in a manner that promotes counterclockwise flow along the recirculating flow path  204  during a counterclockwise mode. For each fan  20  the left side thereof is the high-pressure or discharge side and the right side thereof is the low-pressure or intake side during the counterclockwise mode. 
     The reheater conduits  122  are constructed and operate to provide heated air from the composite plenum  18  to the lower portion of the chamber interior space  30 . In addition, the reheater conduits  122  are constructed and operate to reduce flow-related losses associated with the flow along the portion of the recirculating flow path  204  that extends through the lower portion of the chamber interior space  30 . For example, for each of the reheater conduits  122  the major cross-dimension “d 4 ” is generally parallel to the portion of the recirculating flow path  204  through which the reheater conduits extend. As a result, the reheater conduits  122  define a relatively “low profile” with respect to flow along the recirculating flow path  204 . In addition, outlets of adjacent reheater conduits  122  are arranged to cooperate to produce whirling masses of air  242 . The whirling masses of air  242  travel generally along the recirculating flow path  204  and result in turbulence that interacts with a stack of lumber of the charge  14  that is proximate and downstream from the reheater conduits  122 . The whirling masses of air  242  form turbulent eddies with a characteristic size determined by the center-to-center distance “d 8 ” between the closest outlets  238  of adjacent groups of outlets of adjacent reheater conduits  122 . The turbulence decays in a natural process with time. As the whirling masses of air  242  (i.e., large vortices) decay they form smaller and smaller eddies. The small scale limit is the well known Kolmogorov Microscale, which is familiar to those knowledgeable in the art. During the time in which the vortices move from the reheater conduits  122  to the downstream stack(s) of lumber, the size of the vortices decays to a mean integral length scale of approximately 0.1 to 0.05 inches. The vortices of that size interact with the downstream stack(s) of lumber. Advantageously, this reduces the entrance loss associated with stack(s) of lumber by carrying the momentum into the separated region (discussed below). That is, the eddies interact with the downstream stack(s) of lumber in a manner that causes resistance to flow through the downstream stack(s) of lumber to be less than the resistance to flow through the downstream stack(s) of lumber absent the whirling masses of air  242 . This latter aspect of the reheater conduits  122  can best be understood by first understanding the dynamics of how air flows through a stack of lumber, which is described below. 
     FIG. 16 is a perspective view of a conventional stack of lumber  244  that is to be dried in the kiln chamber  12 . More specifically, the stack  244  includes a right side  246  and an opposite left side  248 , and multiple horizontally extending layers  250  of lumber that are arranged one above the other and extend between the right and left sides. Each layer  250  includes multiple pieces of lumber  252 . Multiple stickers or spacers  254 , which are typically in the form of narrow pieces of lumber, or the like, are positioned between the layers  250  and extend between the opposite sides  246 ,  248  so that multiple passages  256  are defined between adjacent layers  250  and are open at the opposite sides. Only a few of the layers  250 , pieces of lumber  252 , spacers  254 , and passages  256  are identified with a reference numeral in FIG. 16. A flow of heated air is forced through each of the passages  256  while the stack is dried by the kiln  10 . 
     A portion of a representative passage  256  is best seen in FIG. 17, which is a cross-sectional view of a portion of the stack  244  taken along line  17 — 17  of FIG.  16 . FIG. 17 diagrammatically illustrates boundary layers  260  that form while airflow is forced into the passages  256  via openings of the passages that are at the right side  246  of the stack  244 . The direction of the airflow is generally designated by the arrows  258  in FIG.  17 . 
     Each of the passages  256  of the stack  244  are generally identical; therefore, the flow into the passage  256  that is illustrated in FIG. 17 is generally representative of the flow into each of the passages  256  via the openings to the passages that are at the right side  246  of the stack  244 . Whereas FIG. 17 has been described heretofore as being illustrative of airflow into the passages  256  via openings at the right side  246  of the stack  244 , FIG. 17 is also illustrative of airflow into the passages via openings at the left side  248  of the stack, in which case FIG. 17 is a cross-sectional view of a portion of the stack taken along line A—A of FIG.  16 . 
     As best seen in FIG. 17, for each of the passages  256 , airflow therethrough is such that viscous layers of air are developed proximate to the surfaces of the pieces of lumber  252  that face and define the passage. Those viscous layers are referred to as boundary layers  260 , which are not visible but are generally illustrated by dashed lines in FIG.  17 . More specifically, the boundary layers  260 , which are areas of retarded flow, are caused by the viscous interaction between the airflow through the passage  256  and the surfaces of the pieces of lumber  252  that define the passage, as well as interaction between the airflow and the lumber surfaces that are proximate to the inlet opening of the passage. 
     Each boundary layer  260  includes a protruding separated region  262  that tapers to a generally planar tail portion  264 . For each of the boundary layers  260 , the protruding separated region  262  is a portion of the boundary layer that has become separated from the surface or surfaces of the one or more pieces of lumber  252  that define the passage. The separation occurs because of interaction between the airflow and an edge or edges of the one or more pieces of lumber  252  that define the inlet to the passage. 
     As illustrated in FIGS. 16-17, it is conventional for the edges of the layers  250  to be aligned so that they extend in a common plane. As a result, for each of the passages  256 , the protruding separated regions  262  of the boundary layers  260  are aligned in a manner that is very restrictive to flow, since the boundary layers are regions of retarded flow and thereby tend to block flow into the passage  256 . More specifically, an unrestricted flow path exists only in that region between the boundary layers  260  of each of the passages  256 . Those unrestricted flow paths are characterized by generally fully developed flow. Within each passage  256 , the protruding separated regions  262  are aligned in a manner that causes a significant reduction in the size of the unrestricted flow path, as designated by the arrow  266  in FIG.  17 . 
     In other words, at the entrance to each slot-like passage  256  there is a region of little or no flow moving along the slot-like passage. More specifically, that region is a region of separated flow that can be referred to as a separated region and is indicated by the numeral  262 . The separated region  262  is reduced along the length of the slot-like passage  256  and replaced by a shear flow that consists of thin boundary layers which quickly evolve into what is called fully developed flow. Inefficiencies are associated with the separated regions  262  because the separation effectively blocks the flow into the slot-like passage  256  and therefore increases the pressure required to move air through the slot-like passages. 
     As best understood with reference to FIGS. 1,  14 , and  17 , turbulence resulting from the whirling masses of air  242  that are respectively created by the reheater conduits  122  travels generally along the recirculating flow path  204  and interacts with the one or more stacks of lumber of the charge  14  that are proximate to and downstream from the reheater conduits. The turbulence resulting from the whirling masses of air  242  interacts with the downstream stack(s) of lumber in a manner that causes resistance to flow through the stack(s) of lumber to be less than the resistance to flow through the stack(s) of lumber absent the whirling masses. As best understood with reference to FIG. 17, the turbulence resulting from the whirling masses of air  242  interferes with the formation of the separated region  262  by impacting the surfaces of the pieces of lumber  252  that are adjacent to the openings of the passage  256  and impacting the protruding separated regions  262  of the boundary layers. Most specifically, the fine grain turbulence that results from the whirling masses of air  242  mixes momentum into the separated regions  262  thus reducing the size of the separation (i.e., reducing the size of the separated regions  262 ). That is, the turbulence resulting from the whirling masses of air  242 ,  242 ′ functions to at least increase the size of the unrestricted flow paths that are defined between the peaks of the protruding separated regions  262  and designated by the arrow  266  in FIG.  17 . 
     In addition to reducing flow related losses, the jet-like flow provided by the outlets  238  of the reheater conduits  122  enhances heat utilization within the lower portion of the chamber interior space  30 . More specifically, the distance “d 6 ” between adjacent reheater conduits  122  and the characteristics of the jet-like flows from the outlets  238  are selected so that the jet-like flows from of each of the reheater conduits  122  reach and interact with the boundary layer  237  (FIG. 13) around at least one adjacent reheater conduit, so that the boundary layer around the adjacent reheater conduit is smaller than it would be absent the jet-like flows, or more specifically so that the separated regions of the boundary layer associated with the adjacent reheater conduit is smaller than it would be absent the jet-like flows. That is, the jet-like flows from the outlets  238  of the reheater conduits  122  interfere with the formation of and thereby decrease the size of the separated regions of the boundary layers  237  that are proximate the downstream portions of the reheater conduits. As a result, convective heat transfer from the reheater conduits  122  is enhanced, because the separated regions of the boundary layers  237  formed generally around the reheater conduits tend to interfere with convective heat transfer from the reheater conduits. 
     The furnace  16  is operated so that the air moving device  178  of the furnace moves heated air from the mixing chamber  174  to the composite plenum  18  by way of the hot duct assembly  180 . In accordance with another aspect of the kiln  10 , the composite plenum  18  is sized and the kiln  10  is designed and operated so that the heated air within the interior space of the composite plenum is at a relatively high pressure and has a relatively low velocity, which reduces flow-related losses within the composite plenum and facilitates the balancing of flow from the composite plenum to the interior space of the kiln chamber  12 . More specifically, in accordance with one exemplary embodiment the interior space of the composite plenum  18  has a volume that is at least approximately as large as the total volume of the lumber load (i.e., the volume of the charge of lumber  14 ), and more specifically the volume of the composite plenum is approximately equal to the total volume of the lumber load, and most specifically the interior space of the composite plenum has a volume of approximately 10,877 cubic feet and the total volume of the lumber load (that is, the sum of the volume of the right and left stack receiving spaces) is approximately 10,682.5 cubic feet. 
     In accordance with another aspect of the kiln  10 , the right radius of curvature  104  defined by the right wall  94  of the lower plenum  66  provides for a smooth transition of the flow along the recirculation flow path  204  from the upper portion of the chamber interior space  54  to the lower portion of the chamber interior space  30  during the clockwise mode, which reduces flow-related losses within the kiln. In addition, the right radius of curvature provides for a smooth transition of the flow along the recirculation flow path  204  from the lower portion of the chamber interior space  30  to the upper portion of the chamber interior space  54  during the counterclockwise mode. Likewise, the left radius of curvature  106  defined by the left wall  98  of the lower plenum  66  provides for a smooth transition of the flow along the recirculation flow path  204  from the upper portion of the chamber interior space  54  to the lower portion of the chamber interior space  30  during the counterclockwise mode. In addition, the left radius of curvature  106  provides for a smooth transition of the flow from the lower portion of the chamber interior space  30  to the upper portion of the chamber interior space  54  during the clockwise mode. 
     In accordance with another aspect of the kiln  10 , the cool duct assembly  182  is operated so the air moving device  178  of the furnace  16  draws only relatively cool air from the interior space of the kiln chamber  12  to the mixing chamber  174 , which optimizes heat utilization within the kiln. More specifically, the return dampers  200 ,  202  are operated so that the left return ducts  198  are open and the right return ducts  196  are closed, or the return dampers  200 ′,  202 ′ are operated so that the left return ducts  198 ′ are open and the right return ducts  196 ′ are closed, during the clockwise mode. As a result, the air moving device  178  draws air into the mixing chamber  174  of the furnace  16  from the left portion of the upper portion of the chamber interior space  54  during the clockwise mode. In contrast, the return dampers  200 ,  202  are operated so that the right return ducts  196  are open and the left return ducts  198  are closed, or the return dampers  200 ′,  202 ′ are operated so that the right return ducts  196 ′ are open and the left return ducts  198 ′ are closed, during the counterclockwise mode. As a result, the air moving device  178  draws air into the mixing chamber  174  from the right portion of the upper portion of the chamber interior space  54  during the counterclockwise mode. 
     In accordance with another aspect of the kiln  10 , operation of the fans  20  is optimized by operating the control systems  150  that move the nozzle dampers  154 , or by operating the control systems  150  that move the nozzle dampers  154 ′, so that heated air is provided to the upper portion of the chamber interior space  54  substantially solely by either the right nozzles  148  or the left nozzles  150 . More specifically, the nozzle dampers  154  or the nozzle dampers  154 ′ carried by the left wall  130  of the intermediate plenum  68  are in their closed configurations and the nozzle dampers  154  or the nozzle dampers  154 ′ carried by the right wall  128  of the intermediate plenum are in their open configurations while the fans  20  operate in the clockwise mode. As a result, any amount of heated air supplied from the composite plenum  18  to the upper portion of the chamber interior space  54  through the left nozzles  150  is substantially less than the amount of heated air supplied to the upper portion of the chamber interior space through the right nozzles  148  during the clockwise mode. In contrast, the nozzle dampers  154  or the nozzle dampers  154 ′ carried by the right wall  128  of the intermediate plenum  68  are in their closed configurations and the nozzle dampers  154  or the nozzle dampers  154 ′ carried by the left wall  130  of the intermediate plenum are in their open configurations while the fans  20  operate in the counterclockwise mode. As a result, any amount of heated air supplied from the composite plenum  18  to the upper portion of the chamber interior space  54  through the right nozzles  148  is substantially less than the amount of heat supplied to the upper portion of the chamber interior space through the left nozzles  152  during the counterclockwise mode. 
     In accordance with another aspect of the kiln  10 , operation of the kiln  10  and, more particularly, operation of the fans  20  is optimized by the jet-like flow of heated air that is discharged by the nozzles  148 ,  150 . Due to the strategic opening and closing of the nozzle dampers  154  as described above, the jet-like flow always originates proximate the discharge side of the fans  20 , and the nozzles  148 ,  150  are oriented so that all of the discharge axes  152  of the nozzles are directed at least generally parallel to the rotational axes  214  of the fans  20 . Because the heated gas introduced into the upper portion of the chamber interior space  54  flows at least generally parallel to the rotational axes of the fans  20  and at least generally in the same direction as the flow being discharged by the fans  20 , the momentum of the flow along the recirculating flow path  204  is not sacrificed in order to accelerate the hot gas, which is supplied through the nozzles  148 ,  150 , in the desired direction. More specifically, in accordance with one embodiment of the present invention, the hot gas introduced through the nozzles augments the flow from the fans  20  and serves to increase the velocity along the recirculating flow path  204  so that the velocity along the recirculating flow path is greater while the fans are operating and hot air is introduced through the nozzles than when the fans are operating and hot air is not supplied through the nozzles. Stated differently, the jet-like flow from the nozzles  148 ,  150  that are open has momentum that is mostly parallel to the rotational axes  214 , and all of that momentum is in the downstream direction, which is the direction of flow defined by the exit velocity of the fans  20 . The jet-like flow from the nozzles  148 ,  150  that are open has a velocity greater than the component of the exit flow from the fans  20  that extends in the direction of the rotational axes  214 . As a result, any momentum exchange is such that the exit flow from the fans  20  experiences an increase in momentum in the downstream direction. More specifically, in accordance with one embodiment, the jet-like flow of heated air discharged from each of the nozzles  148 ,  150  that is open has a velocity at least as great as the velocity of the flow discharged from each of the fans  20 , and more preferably the jet-like flow of heated air discharged from each of the nozzles that is open has a velocity of the order of 200 feet per second, whereas the flow discharged from each of the fans has a velocity of the order of 25 feet per second. 
     In addition, the nozzles  148 ,  150  are preferably arranged generally around the fans  20  and/or are in close proximity to the fans  20 . This arrangement reduces the pressure near the exits of the fans  20  by means of Bernoulli&#39;s principle, thus further assisting the operation of the fans. More specifically, the static pressure near the jet-like flow is low because the velocity of the jet-like flow is high. That low pressure is proximate the exits of the fans  20  and provides a venturi effect at the exits of the fans. That venturi effect provides a slight suction to the exits of the fans  20  which enhances the operation of the fans  20 . 
     In accordance with another aspect of the kiln  10 , operation of the fans  20  is optimized because the blade tips  218  of the impellers  212  extend at least to, and preferably into, respective flow-induced boundary layers  220  (FIG.  3 ). This aspect of the kiln  10  will now be described with respect to the design and operation of the representative fan  20  and circulation passage  138  illustrated in FIG. 3, in accordance with one embodiment of the present invention. When the fan  20  is operated in the counterclockwise mode, the impeller  212  rotates about the rotational axis  214  and forces flow through the circulation passage  138 , resulting in the formation of a flow-induced boundary layer  220 . The flow-induced boundary layer  220  is schematically illustrated by dashed lines that are within the circulation passage  138  and adjacent the surface of the interior wall  140  that faces the impeller  212 . The flow-induced boundary layer related aspects associated with the operation the fan  20  in the counterclockwise mode are identical to the flow-induced boundary layer aspects associated with the operation of the fan in the clockwise mode, except that the impeller rotates in the opposite direction and the flow-induced boundary layer originates proximate the left opening  146  to the circulation passage  138  rather than the right opening  144 . 
     The fan  20  and the circulation passage  138  are constructed so that the blade tips  218  extend at least to, and preferably into, the flow-induced boundary layer  220  while the fan is operated, which restricts bypass flow proximate to the blade tips. The flow-induced boundary layer  220  extends generally uniformly for 360 degrees around the rotational axis  214  of the impeller  212 , and each of the blade tips  218  remain within the flow-induced boundary layer as they rotate 360 degrees around the rotational axis. The internal diameter and length of the circulation passage  138  and the design and rotational speed of the impeller  212  are selected so that the blade tips  218  extend at least to, and preferably into, the flow-induced boundary layer  220  while the fan  20  is operated. For example, the impeller  212  is designed so that the blade tips  218  are proximate the interior wall  140  and the interior wall is sufficiently lengthy in the lateral direction so that the boundary layer  220  is sufficiently thick to contact the blade tips. More specifically, the right and left walls  128 ,  130  of the intermediate plenum  68  respectively define a right and left inlet plane. Inlet distances “d 9 ” are respectively defined between the right and left inlet planes and the right-most and left-most leading edges of the blades  216 . In addition, the impeller  212  defines a diameter “d 10 ”, and in the vicinity of the impeller the surface of the interior wall  140  upon which the boundary layer  220  forms defines an internal diameter “d 11 ”. 
     The impeller  212  and the circulation passage are preferably coaxial, and the internal diameter “d 11 ” of the circulation passage  138  is preferably approximately 0.5 inches greater than the diameter “d 10 ” of the impeller  212 . Further, the inlet distance “d 9 ” divided by the impeller diameter “d 10 ” is preferably at least approximately 0.167, is more preferably in the range of approximately 0.167 to approximately 0.317, and is even more preferably approximately 0.317, and most preferably the inlet distance “d 9 ” is approximately 2 feet and the impeller diameter is approximately 6 feet. In addition to playing a role in facilitating the preferred formation of the boundary layer  220 , it is believed that the inlet distance “d 9 ” of approximately 2 feet will allow the flow entering the impeller  212  to align itself with the impeller and begin a small amount of pre-swirl before entering the impeller. 
     The velocity into the impeller  212  depends upon the design of the blades  216 , the pitch of the blades, and the rotational speed of the impeller. It is preferred for the blade tips  218  to have a velocity of approximately 298.5 ft/sec. The flow entering the impeller  212  travels along a spiral path because of the influence of the rotation of the impeller. The distance of the spiral path proximate the surface of the interior wall  140  upon which the boundary layer  200  forms may be estimated based upon the vector sum of the rotational and axial components of the velocity of the blades  216 . The magnitude of the velocity along the spiral path proximate the surface of the interior wall  140  upon which the boundary layer  200  forms is similarly the sum of the axial and circumferential components of the velocity of the blades  216 . The circumferential component increases as the flow approaches the leading edges of the blades  216 . The velocity also varies radially since the peak work region of each blade  216  occurs at approximately 70% of the blade radius. The velocity of interest is adjacent the surface of the interior wall  140  upon which the boundary layer  220  forms. At this location the velocity will be reduced according to the spanwise distribution along the blade. This distribution peaks near 70% of the tip radius and is zero at the tip. The resultant distance and velocity are calculated using a time step average. For this case, the pertinent length of the spiral travel path proximate the surface of the interior wall  140  upon which the boundary layer  200  forms, which is “L” in the following equation, is approximately 16.2 feet, and the pertinent velocity along that spiral travel path, which is “U” in the following equation, velocity is approximately 202 feet/sec. The Reynolds number, Re, is defined as 
     
       
         
           Re=ρUL/μ 
         
       
     
     where ρ is the fluid density and μ is the fluid viscosity. The Reynolds number provides the ratio of inertial and viscous effects in the flow. For this particular case, Re=1.4×10 7  at the standard operating temperature of the kiln  10 . The boundary layer  222  preferably grows along the interior wall  140  to a thickness such that the boundary layer fills the gap between the blade tips  218  and the interior wall  140 . 
     The important parameter for quantifying the thickness of the boundary layer  222  at the blade tips  218  is known as the momentum thickness, θ. A method to estimate the momentum thickness θ is provided by Schlichtings formula where the momentum thickness for a turbulent boundary layer is given as 
     
       
         θ=0.036 L ( Re ) −⅕   
       
     
     Using this estimate and the value for “L” and “Re” provided above, the momentum thickness θ, or more specifically the thickness of the boundary layer  220 , at the blade tips  218  is approximately 0.26 inches. As alluded to above, the gap between the blade tips  218  and the interior wall  140  is approximately 0.25 inches. That is, the inlet distance “d 9 ” has been selected in view of expected velocities to produce a boundary layer thickness that is approximately equal to, and not substantially larger than, the gap between the blade tips  218  and the interior wall  140 . 
     In accordance with another aspect of the kiln  10 , operation of the fans  20  is optimized by providing one or more constricting regions proximate the inlets of the fans and one or more expanding regions proximate the outlets of the fans. Stated differently, one or more constrictions to the recirculating flow path  204  are provided on the low-pressure sides of the fans  20 , and one or more expansions to the recirculating flow path are provided on the high-pressure sides of the fans. In accordance with the illustrated embodiment of the present invention, the protrusions  78 ,  84 ,  96 ,  100  of the upper and lower plenums  64 ,  66  and the right and left openings  144 ,  146  of the circulation passages  138  provide such constrictions and expansions. 
     As best understood with reference to FIG. 1, the front protrusions  78 ,  96  of the upper and lower plenums  64 ,  66  define a constriction to the recirculating flow path  204  proximate the inlets of the fans  20  so that airflow proximate the inlets of the fans is accelerated while the fans operate to provide counterclockwise flow along the recirculating flow path. In addition, the rear protrusions  84 ,  100  of the upper and lower plenums  64 ,  66  cooperate to define an expansion to the recirculating flow path  204  proximate the outlets of the fans  20  so that airflow proximate the outlets of the fans is decelerated while the fans operate to provide counterclockwise flow along the recirculating flow path. Likewise, the rear protrusions  84 ,  100  are constructed to define a constriction to the recirculating flow path  204  proximate the inlets of the fans  20  so that airflow proximate the inlets of the fans is accelerated while the fans are operated to cause clockwise flow along the recirculating flow path. The front protrusions  78 ,  96  are constructed to generally define an expansion to the recirculating flow path  204  proximate the outlets of the fans  20  so that airflow proximate the outlets of the fans is decelerated while the fans are operated to cause clockwise flow along the recirculating flow path. 
     As best understood with reference to the representative circulation passage  138  illustrated in FIG. 3, the right and left openings  144 ,  146  to the circulation passages are respectively shaped to provide constrictions to the recirculating flow path  204  proximate the inlets of the fans  20 , so that airflow proximate to the inlets is accelerated, and expansions to the recirculating flow path proximate the outlets of the fans, so that airflow proximate the outlets is decelerated, while the fans are operated to provide counterclockwise flow along the recirculating flow path. Likewise, the right and left openings  144 ,  146  are respectively shaped to provide expansions to the recirculating flow path  204  proximate the outlets of the fans  20 , so that airflow proximate the outlets is decelerated, and constrictions to the recirculating flow path proximate the inlets of the fans, so that airflow proximate the inlets is accelerated, while the fans are operated to provide clockwise flow along the recirculating flow path. 
     In accordance with another aspect of the kiln  10 , mixing of the heated air within the upper portion of the chamber interior space  54  is facilitated by the arrangement of the nozzles  148 ,  150 . The arrangement of the groups of left nozzles  150  illustrated in FIGS. 3-4 is generally representative of the arrangement of all of the right and left nozzles  148 ,  150  and will now be further described, in accordance with one embodiment of the present invention. The upper and lower groups of nozzles  150  include eight nozzles that are arranged in an arc. It is within the scope of the present invention for the groups to contain more or less nozzles. Further, for each of the groups of nozzles  150 , two of the nozzles can be characterized as being end nozzles because they are at the opposite ends of the group, and the other nozzles of the group can be characterized as being middle nozzles because they are between the end nozzles. The discharge axes  152  of the middle nozzles  150  are preferably directed at least partially toward, and most preferably they intersect, the rotational axis  214  of the impeller  212 . As best understood with reference to FIG. 4, the discharge axes of the end nozzles  150  do not intersect the rotational axis  214  of the impeller  212 , but they are preferably directed at least partially toward, and most preferably they intersect, the common horizontal plane in which all rotational axes  214  extend. A majority of the end nozzles  150  can be characterized as being “shared” by adjacent fans  20 . 
     Whereas the discharge axes  152  of the middle and end nozzles  150  respectively intersect the rotational axis  214  and the common horizontal plane in which the rotational axes  214  extend, those angles of intersection are preferably significantly less than 45 degrees in general and are preferably approximately 12 degrees. These inward angles enhance the mixing of the hot gas introduced into the upper portion of the chamber interior space  54 , but they also detract somewhat from the above described advantage of having the discharge axes  152  extend at least generally parallel to the rotational axes  214  of the fans  20 . Accordingly, an advantageous balance between the advantages has been determined to be achieved with the above mentioned angle of approximately 12 degrees. In accordance with another embodiment of the present invention, the discharge axes  152  are not oriented inwardly with respect to the rotational axes  214  or the like such that the discharge axes are horizontally extending and parallel to the rotational axes  214 . 
     In accordance with another aspect of the kiln  10 , mixing of the heated air within the upper portion of the chamber interior space  54  is facilitated by virtue of the blades  216  of different fans  20  being configured differently. That is, some of the impellers  212  are rotated clockwise about their respective axes  214  to provide clockwise flow along the flow path  204 , whereas other of the impellers are rotated counterclockwise about their respective axes to provide clockwise flow along the flow path. Likewise, some of the impellers  212  are rotated clockwise about their respective axes  214  to provide counterclockwise flow along the flow path  204 , whereas other of the impellers are rotated counterclockwise about their respective axes to provide counterclockwise flow along the flow path. 
     In accordance with another aspect of the kiln  10 , mixing of the heated air within the upper portion of the chamber interior space  54  is facilitated by virtue of elongate splitter plates (not shown) being positioned in the upper portion of the chamber interior space. The splitter plates are disclosed in U.S. Pat. No. 5,414,944, which is incorporated herein by reference. 
     In accordance with another aspect of the kiln  10 , the flow through the charge  14  of lumber is at least partially balanced by virtue of the right edge  110  of the lower wall  62  of the lower plenum  66  extending laterally beyond the charge-receiving area. More specifically, the overhang of the lower plenum  66  that is provided by the placement of the right edge  110  allows the clockwise flow from the upper portion of the chamber interior space  54  to the lower portion of the chamber interior space  30  to make an efficient turn so that entry of the airflow into the charge  14  of lumber is more generally “straight-on,” which promotes optimal airflow between the top layers of the charge of lumber. The right radius of curvature  104  and the right flange  118  also enhance this effect. In addition, the overhang of the lower plenum  66  that is provided by the placement of the right edge  110  functions to reduce a venturi-like effect that can be caused by upward airflow proximate the right-most top edge of the charge  14  of lumber. Left unchecked, the up-flow can draw a considerable flow through upper layers of the right-most stack of lumber, which can cause too rapid drying of those upper layers. The overhang provided by the right edge  110  reduces the venturi-like effect by moving the up-flow away from the charge  14  of lumber. Positioning the right side wall  46  the distance “d 1 ” from the charge  14  of lumber also decreases the speed of the up-flow, which correspondingly decreases the venturi-like effect. 
     In accordance with another aspect of the kiln  10 , the flow through the charge  14  of lumber is at least partially balanced by virtue of the left edge  112  of the lower wall  62  of the lower plenum  66  extending beyond the charge-receiving area. More specifically, the overhang of the lower plenum  66  that is provided by the placement of the left edge  112  allows the counterclockwise flow from the upper portion of the chamber interior space  54  to the lower portion of the chamber interior space  30  to make an efficient turn so that entry of the airflow into the charge  14  of lumber is more generally “straight-on,” which promotes optimal airflow between the top layers of the charge of lumber. The left radius of curvature  106  and the left flange  120  also enhance this effect. In addition, the overhang of the lower plenum  66  that is provided by the placement of the left edge  112  functions to reduce a disadvantageous venturi-like effect that can be caused by upward airflow proximate the left-most top edge of the charge  14  of lumber. Positioning the left side wall  48  the distance “d 1 ” from the charge  14  of lumber also decreases the venturi-like effect. 
     In accordance with one example, after a charge  14  of green lumber has been dried within the lower portion of the chamber interior space  30 , at least the rear doors  44  are opened and the dried charge of lumber is removed from the lower portion of the chamber interior space through the rear door opening  42 . 
     The above and other aspects of the kiln  10  are advantageous because they are pertinent to either the efficient construction, the efficient operation, or timely operation of the kiln. 
     This patent application incorporates by reference the U.S. patent application filed on Mar. 22, 2000 in the name of Robert T. Nagel et al. and entitled Improved Kiln and Kiln-Related Structures, and Associated Methods. 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.