Patent Publication Number: US-RE48227-E

Title: Uninterrupted alternating air circulation for use in lumber kilns

Description:
This application claims priority to co-pending U.S. patent application Ser. No. 13/831,361 filed Mar. 14, 2013 for Uninterrupted Alternating Air Circulation for Continuous Drying Lumber Kilns. The &#39;361 application is incorporated by reference. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     This disclosure applies to systems of the continuous drying kiln (CDK) design, (also referred to as dual path or triple length kilns), in which two paths of lumber travel in opposite directions through a sequence of chambers in which wood is pre-heated, dried, equalized and then conditioned. This disclosure also applies to unidirectional kilns where one or more sets of carriages on one or more sets of pathways to convey lumber through a first end of a kiln to a second end of a kiln. 
       FIG. 1  introduces a series of elements found in continuous drying kilns. Typically a continuous drying kiln will have a structure  104  with a first end  108  and a second end  112  at the opposite end of the structure  104 . Running through the structure  104 , is a first pathway  116  and a second pathway  120 . The pathways frequently use rails  124  to guide a first set of carriages  128  along the first pathway  116  and a second set of carriages  132  along the second pathway  120 . The carriages ( 128   132 ) may have wheels (not shown) much like those found on railroad cars. 
     If the first set of carriages  128  enters the structure  104  through the first end  108  and exits through the second end  112 , then the second set of carriages  132  enters the structure  104  through the second end  112  and exits through the first end  108 . Thus, when lumber  130  is stacked on the carriages ( 128  and  132 ) and exposed to heat in the main drying section  300 , the heated lumber  136  passes near lumber that has not yet been in the main drying section  300  (green lumber  140 ). Note the simplified drawing in  FIG. 1  shows the lumber as an essentially solid stack. This is not the case. Spacers (not shown) are placed across each layer of boards within each stack of lumber  130  to provide open area for air movement through the lumber stack  156 . Weights (not shown) on top of each lumber stack  156  compress the lumber  130  and spacers provide restraint, minimize warping, and prevent boards from falling off of the top of the lumber stack. To minimize the air flow that might otherwise go over the top of the lumber stack  156  within the structure  104 , structure  104  has longitudinal baffles ( 220   FIG. 2 ) that are aligned with the long axis of the structure  104  and thus aligned with the direction the lumber stacks travel through the kiln and orthogonal to the flow of air from the first side  144  to the second side  148  of the structure or to the flow of air from the second side  148  to the first side  144  of the structure  104 . These longitudinal baffles  220  are designed to minimize the leakage of air between the fan deck ( 224  discussed below) and the top of the lumber stack  152 , thus directing the air to flow through the air spaces between the layers of lumber  130  separated by spacers in the lumber stacks. 
     In the first end energy recovery section  310  and in the second end energy recovery section  340 , the heated lumber  136  passes heat to the green lumber  140  to partially heat and dry the green lumber  140  and the green lumber  140  cools the heated lumber  136  by absorbing heat and by evaporating the moisture content of the green lumber  140 . 
     Thus, lumber stack  156  starts as green lumber  140  stacked upon the first set of carriages  128  with spacers to allow for air flow amongst stacked lumber  136 . As the first set of carriages  128  moves along the first pathway  116 , the green lumber  140  is exposed to air that is circulating in the first end energy recovery section  310 .  FIG. 2  shows a cross section of the first end energy recovery section  310 , operating in a first circulation direction  204  as fans  200  push the air in the first circulation direction  204 . The fans  200  operate in openings in a center wall  228  that extends above the fan deck  224 . The center wall  228  helps promote circulation by having a high pressure side downstream of the fan  200  and a low pressure side upstream from the fan  200 . 
     Having an appropriate pressure gradient from the high pressure side of the center wall  228  to the low pressure side will cause a desired distribution of circulating air amongst the stacked lumber across the two sets of carriages ( 128  and  132 ). Heat from heated lumber  136  on the second pathway  120  partially dries and heats the green lumber  140 . Likewise the moisture from the green lumber  140  helps cool the heated lumber  136 . One of skill in the art will appreciate that the heating of the green lumber  140  is going to be most pronounced as the hot air reaches the green lumber  140  directly after leaving the heated lumber  136  and before the circulating air returns to the fans  200  above the fan deck  224 . Likewise, one of skill in the art will appreciate that the cooling of the heated lumber  136  is going to be most pronounced as the moist air reaches the heated lumber  136  directly after leaving the green lumber  140  and before the circulating air returns to the fans  200  above the fan deck  224 . 
     To reduce the variability between lumber  130  on the first side  144  and the second side  148  of the first set of carriages  132  or the second set of carriages  132 , the fans  200  are periodically stopped and allowed to coast to a full stop. Then the fans  200  are operated in the reverse direction to push air in the second circulation direction as shown in  FIG. 3 . Now air that has made a complete pass through the heated lumber  136  enters the green lumber  140  on the first side  144  of the green lumber  140  and the air that has passed through the green lumber  140  enters the heated lumber  136  on the first side  144 . 
     Normal practice is to reverse the fan direction about once every two to four hours. The period of running the fan in one direction is often called a fan cycle. The overall time to cure the lumber is frequently 40 hours although it may be longer for wood needing extra drying. As the first end energy recovery section  310 , main drying section  300 , and second end energy recovery section  340  all have fans that are periodically stopped and reversed (usually at the same time), a particular stack of lumber on a carriage should expect to have the fans stop approximately 10, 13, 20, or even more times during transit through the structure  104 . 
     When heated lumber  136  that has recently passed through the main drying section  300  and entered the first end energy recovery section  310  or the second end energy recovery section  340 , there is a risk that heavily dried and heated hot spots on the heated lumber  136  may be smoldering. Fire may be less likely in the main drying section if oxygen levels are reduced from exposure to an external direct fired burning furnace. However, even a momentary lack of circulation in an energy recover section can increase fire risk as the circulation of cooler moist air from the green lumber  140  abates and a hot spot may progress to an open fire. Thus, many structures include intermediate orthogonal baffles  320  within the energy recovery sections ( 310  and  340 ) to limit the travel of oxygen rich air from the first end  108  or the second end  112  towards the lumber in the energy recovery sections ( 310  or  340 ) that has recently emerged from the main drying section  300 . While first end energy recovery section  310  and second end energy recovery section  340  both are shown with a single set of intermediate orthogonal baffles  320 , there may be additional intermediate orthogonal baffles  320  to subdivide the first end energy recovery section  310  and second end energy recovery section  340  into additional energy recovery subsections ( 314 ,  318 ,  344 , and  348  in  FIG. 1 ). Additional orthogonal partitions  324  define the boundaries of the main drying section  300  although conventional structures do not currently have subsections within the main drying section  300 . 
     The first end  108 , and second end  112  may have some level of orthogonal baffles to limit the ingress of oxygen and loss of heat, but the structure  104  is typically far from hermetically sealed as there is a need for water vapor to leave the structure  104  at the first end  108  and second end  112  often as visible fog. 
     Returning to the processing of lumber stack  156  stacked upon the first set of carriages  128 , eventually, the lumber stack  156  progresses from the first end energy recovery section  310  through orthogonal partitions  324  to enter the main drying section  300 . 
     The main drying section  300  is much like the energy recovery sections  310  and  340  with a set of bi-directional fans  200  located above a fan deck  224  circulating air alternatively in the first circulation direction  204  and the second circulation direction  208 . Longitudinal baffles  220  keep the circulating air from passing between the top of the lumber stacks  152  and the fan deck  224 . A complication in the main drying section  300  for direct fired kilns is that an additional circulation path is needed to move air from the structure  104  to a mixing chamber where hot flue gas from a direct fired burner is mixed with the returning air from the structure  104  to create a mix within a prescribed temperature range. 
     This mix of heated air and flue gas is returned to the main drying section  300  to increase the temperature and decrease the humidity of the return air which is reintroduced to the main drying section  300 . A blower forces heated air leaving the mixing chamber into a distribution duct that extends the length of the main drying section  300 . The distribution duct may release heated air in an upward direction through apertures in the top surfaces of the fan deck  224  or it may also release heated air in a downward direction through slotted vertical ducts, which are called downcomers, that are located between the first pathway  116  and second pathway  120  below the fan deck  224 . The apertures and downcomers may be tuned to promote uniform distribution of the heated air. The flue gas leaving the direct fire burner may be near 2000 degrees Fahrenheit but after mixing with the return air from the structure  104 , may return to the main drying section  300  at 450 degrees Fahrenheit which is nearly twice the main drying section set point air temperature which is often between 240 degrees Fahrenheit and 260 degrees Fahrenheit. 
     As one can imagine, the process of stopping the fans  200  in the main drying section  300  poses special problems as circulation from the fans  200  is needed to avoid overheating the top of the lumber stacks  152 . Thus, while fans  200  are slowing, stopping, and coming back up to speed in the opposite direction, the blower continues to deliver additional air to the structure  104 . During this time period when fan direction is being reversed, the burner abort stack (not shown) opens momentarily and the direct fired burner ( 1534  in  FIG. 5  discussed below) is placed on idle in order to maintain the operating of the temperature in the direct fired burner ( 1534 ) while suspending heat energy delivery from the direct fired burner ( 1534 ) to the main drying section  300 . The opening of the abort stack allows ambient air into the mixing chamber ( 1538  below), during which time the opening of the return air damper acts to increase recirculation of air flow from the kiln structure  104  into the mixing chamber ( 1538 ) at the same time that the amount of heat being passed from the direct fired burner ( 1534 ) into the mixing chamber ( 2538 ) is reduced. 
     Eventually, lumber stack  156  stacked upon the first set of carriages  128  emerges from the main drying section  300  through orthogonal partitions  324  to enter the second end energy recovery section  340 . Now the lumber is heated lumber  136  giving off heat and drying green lumber  140  on carriages  132  on the second pathway  120 . The heated lumber  136  is exposed to air moving in the first circulation direction  204  and in the second circulation direction  208  as the bi-directional fans  200  are periodically turned off, allowed to coast to a stop, and then restarted in the opposite direction. 
     The lumber stack  156  emerges from the second end  112  and is eventually removed from the carriage  132 . 
     Lumber on carriages  132  on the second pathway  120  receive the same sequence of treatments but travel in the opposite direction from the second end  112  to the first end  108 . 
     The process of reversing from the first circulation direction  204  to the second circulation direction  208  may take fifteen minutes or more before the fully developed air flow pattern and dry bulb set point temperatures are regained. The sequence is as follows. First, the fans  200  are de-energized and allowed to coast to a full stop. After ample time elapses for all fans  200  in all sections of the structure  104  to reliably come to a full stop, the fans  200  are restarted in the opposite direction and eventually establish circulation at the desired speed. While the time to allow the fans to coast to a stop and restart may be as short as five minutes, some interruptions in the provision of heat may be in the 15 minute range as the heating system may be turned off before the fans are de-energized and heat may not be fully resumed for a few minutes after the fans have been re-energized. While the fans  200  are not energized and providing circulation at the desired rate, several things are not happening.
         1) Heat is not being added to the main drying section so the process of drying the lumber slows down. In the case of a steam radiator system, the loss of air flow will decrease the heat delivered to the main drying section  300  even if the steam is not isolated from the steam radiators.   2) Heat from a direct fired burner (if this is used rather than a steam system discussed below) turned down as direct fired burner goes to idle mode in order to avoid heating. After idling, the dynamics of the direct fired burner may require time to return to full operating levels of heat production.   3) Temperatures within the structure may develop local hot spots as circulation is needed to prevent hot spots.   4) Heated and now dry lumber does not receive the circulation from green lumber and may develop overheated sections.   5) The advancement of carriages will be slowed. Many structures use a periodic push of the carriages for movement rather than extremely slow continuous movement, but in either event, the push rate is selected to allow for the appropriate drying and curing of the lumber so that the lumber is within the structure  104  for an adequate time.       

     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 Number of 15 
                   
               
               
                   
                 minute transitions 
                   
               
               
                   
                 for 40 hour 
                 Percentage 
               
               
                 Length 
                 transit through 
                 of time that heat is NOT 
               
               
                 of fan cycle 
                 the structure 
                 being added to the structure. 
               
               
                   
               
             
            
               
                 2 hour fan cycle 
                 20 
                 1/9 - approximately 11 percent. 
               
               
                 3 hour fan cycle 
                 At least 13 
                 1/13 - approximately 7.7 percent. 
               
               
                 4 hour fan cycle 
                 10 
                 1/17 - approximately 5.9 percent. 
               
               
                   
               
            
           
         
       
     
     While there may not be a one to one relationship between the percentage of time that heat is not being delivered to the structure  104  and a reduction from optimal throughput for the structure, the loss in throughput should be proportion to the loss of time spent heating the structure  104   
     SUMMARY OF THE DISCLOSURE 
     The present disclosure teaches the use fans to circulate heated air to dry lumber in a kiln. The fans do not periodically stop and reverse directions as in prior art designs. Instead, the alternating direction of air flow is provided by moving the carriage of lumber from one subsection of the kiln with air always moving in a first direction to an adjacent subsection of the kiln where the air always moves in the opposite direction from the first direction. Elimination of fan reversals will enhance kiln fire safety and reduce the time and energy required to heat lumber in kilns, while improving the quality and uniformity of lumber being processed. Aspects of the teachings contained within this disclosure are addressed in the claims submitted with this application upon filing. Rather than adding redundant restatements of the contents of the claims, these claims should be considered incorporated by reference into this summary. 
     This summary is meant to provide an introduction to the concepts that are disclosed within the specification without being an exhaustive list of the many teachings and variations upon those teachings that are provided in the extended discussion within this disclosure. Thus, the contents of this summary should not be used to limit the scope of the claims that follow. 
     Inventive concepts are illustrated in a series of examples, some examples showing more than one inventive concept. Individual inventive concepts can be implemented without implementing all details provided in a particular example. It is not necessary to provide examples of every possible combination of the inventive concepts provided below as one of skill in the art will recognize that inventive concepts illustrated in various examples can be combined together in order to address a specific application. 
     Other systems, methods, features and advantages of the disclosed teachings will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within the scope of and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The disclosure can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a continuous kiln as exists in prior art 
         FIG. 2  is a diagram of clockwise rotation of heated air trough lumber stacks 
         FIG. 3  is a diagram of counter-clockwise rotation of heated air trough lumber stacks. 
         FIG. 4  shows a continuous kiln using teachings from the present disclosure that illustrates some of the teachings of the present disclosure with a main drying section shown pulled out of the structure in order to provide context for  FIG. 5 . 
         FIG. 5  provides an enlarged view of the main drying section used with a direct fired burner. 
         FIG. 6  shows an alternative main drying section that uses steam heat exchangers to provide heat to the main drying section. 
         FIG. 7  shows a unidirectional kiln that has all lumber traveling in a single direction that illustrates some teachings from the present disclosure with a main drying section shown pulled out of the structure in order to provide context for  FIG. 8 . 
         FIG. 8  provides an enlarged view of the main drying section from  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Use with Continuous Drying Kilns. 
       FIG. 4  shows a structure  1104  that illustrates some of the teachings of the present disclosure. Many elements present in  FIG. 4  were introduced during the discussion of prior art structure  104  in  FIG. 1 . Structure  1104  has a first end  108  and a second end  112  and a first side  144  and a second side  148 . Lumber  130  is stacked upon the first set of carriages  128  on rails  124  forming the first pathway  116  to traverse the structure  1104  from the first end  108  through the second end  112 . Lumber  130  is stacked upon the second set of carriages  132  to traverse the structure  1104  from the second end  112  through the first end  108 . The manner of stacking lumber  130  upon carriages with spacers (sometimes called “stickers”) and weights may be the same as discussed in connection with  FIG. 1 . As described in more detail below, the lumber  130  is exposed to periods of air movement in the first circulation direction  204  and to periods of air movement in the second circulation direction  208  as the relevant carriage passes through the structure  1104 . 
     Structure  1104  differs from structure  104  in that the main drying section  1300  has a number of orthogonal MD partitions  1504  to subdivide the main drying section  1300  which is bounded by orthogonal partitions  324 . 
     Thus main drying section  1300  has, in this instance, four subsections  1508 ,  1512 ,  1516 , and  1520 . The number of main drying section subsections does not need to be four but will be at least two and will usually be an even number of subsections as there is apt to be a desire to expose the lumber to equal ranges of the main drying section operated in the first circulation direction  204  and the second circulation direction  208  (as described below). 
       FIG. 5  provides an image of the main drying section  1300  in greater detail.  FIG. 4  shows the relationship between the details in  FIG. 5  and the structure  1104  by showing an image of the main drying section  1300  pulled out of the structure  1104 . 
     Turning to  FIG. 5 , a main drying section  1300  with subsections  1508 ,  1512 ,  1516 ,  1520  defined by orthogonal partitions  324  and orthogonal MD partitions  1504 . A return air duct  1530  draws air from one or more subsections  1508 ,  1512 ,  1516 , and  1520 . If not directly connected to all subsections, the return air duct  1530  is apt to be connected to the one or two subsections in the middle of the main drying section  1300  or to the two ends of the main drying section  1300  to promote movement of air across the length of the main drying section  1300 . Note that while the various orthogonal MD partitions  1504  impede the flow of air longitudinally, the seal is not perfect and air will flow based on pressure gradients. A direct fired burner  1534  (represented here by a flame) feeds burner exhaust at approximately 2000 degrees Fahrenheit into a mixing chamber  1538  to provide a mix of burner exhaust with return air from the return air duct  1530  to provide an output supplied to the main drying section  1300  above the main drying section set point which is often between 240 degrees Fahrenheit and 260 degrees Fahrenheit. The heated air is supplied via the supply duct  1546  and distributed to the space between the fan deck  224 , to the tops of the lumber stacks  156  and through down comers, located between the first pathway  116  and second pathway  120  below the fan deck  224 . The air moving to and from the mixing chamber  1538  may be moved by a blower  1542  located after the mixing chamber  1538 . 
     As lumber  130  on the first set of carriages  132  passes through the orthogonal partition  324  separating the main drying section  1300  from the first end energy recovery section  1310  ( FIG. 4 ), the lumber  130  is exposed to air moving in the first circulation direction  204 . The air in the first subsection  1508  of the main drying section  1300  always moves in the first circulation direction  204  as structure  1104  uses fans  1200  that are operated in a single direction. The fans  1200  may be bi-directional fans like fans  200  that are used in a retrofitted structure or they may be unidirectional fans that are optimized to push air in one direction only with blades designed for this purpose but lack the additional design features and components needed for a bi-directional fan. 
     The fans  1200  in subsections  1508  and  1516  push air in the first circulation direction  204 . But fans  1200  in subsections  1512  and  1520  push air in the second circulation direction  208 . Thus, lumber stack  156  on first set of carriages  128  or the second set of carriages  132  is subject to alternating circulation directions ( 208  and  204 ) without intermediate periods of no circulation as fans are de-energized, slowed to a stop, and started in the opposite direction. 
       FIG. 6  shows an alternative main drying section  2300  that uses steam heat exchangers  2530  to provide heat to the alternative main drying section  2300 . Analogous to the discussion of  FIG. 5 , in  FIG. 6 , the fans  1200  in subsections  2508  and  2516  push air in the first circulation direction  204 . But fans  1200  in subsections  2512  and  2520  push air in the second circulation direction  208 . Thus, lumber stacks  156  on first set of carriages  128  or the second set of carriages  132  is subject to alternating circulation directions ( 204  and  208 ) without intermediate periods of no circulation as fans are de-energized, slowed to a stop, and started in the opposite direction. 
     The steam supply to the steam heat exchangers  2530  may be regulated with control valves as is known in the art. While steam heat exchangers  2530  are shown on both sides of the fans  1200 , one of skill in the art will recognize that the steam heat exchangers  2530  could be on a single side of the fans  1200  or with additional heat exchangers between or besides the pathways ( 116  and  120 ). 
     Returning to  FIG. 4 , the intermediate orthogonal baffles  320  of  FIG. 1  may be termed intermediate orthogonal partitions  1320 . Thus, fans in subsection  1314  may continuously circulate air in the first circulation directions  204  and fans in subsection  1318  may continuously circulate air in the second circulation direction  208  so that movement of a carriage between subsection  1318  and subsection  1508  results in a change in air circulation direction from the second circulation direction  208  to the first circulation direction  204  or the reverse, depending on the direction of movement of the carriage  128  or  132 . Likewise, fans  1200  in subsection  1348  may continuously circulate air in the second circulation directions  208  and fans  1200  in subsection  1344  may continuously circulate air in the first circulation direction  204  so that movement of a carriage between subsection  1520  and subsection  1344  results in a change in air circulation direction. 
     One of skill in the art can appreciate that instead of using two subsections per energy recovery section ( 1310  and  1340 ) that one could use four or other even numbers of subsections. One could also use an odd number of subsections in the energy recovery sections ( 1310  and  1340 ) potentially by changing the lengths of the subsections so that the total amount of time subject to each circulation direction ( 204  and  208 ) is maintained equal even if done in a different number of segments. Alternatively, there may be a bias to pass heat from heated lumber to green lumber or moisture from green lumber to heated lumber. 
     While not absolutely required, it is expected that in most instances, there will be an even number of subsections in the alternative main drying section ( 1300  or  2300 ) and there will be the same number of subsections in the first end energy recovery section  1310  as in the second end energy recovery section  1340 . 
     The orthogonal partitions  324 ,  1320 , and  1504  use baffles created to allow passage of a carriage loaded as intended (with lumber, spacers, and weights) but substantially conform to that profile so that longitudinal flow of air is limited. However, as the stacking of lumber, spacers, and weights may have some small variation from carriage to carriage, the baffles must have a capacity to give way when a larger than expected profile attempts to cross an orthogonal partition. The baffles are intended to be easy to adjust or replace during maintenance outages so that longitudinal air flow continues to be effectively resisted. 
     Placing a set of baffles on a faux partition external to the structure  1104  for pathways heading toward the structure  1104  may be useful to allow adjustments to the green lumber  140 , spacers, and weights on a carriage to minimize the amount of contact with the baffles inside the structure  1104 . Working for conformity with the expected profile for a loaded carriage will reduce wear on the baffles inside the structure  1104  which will mean better resistance to longitudinal air flow over time and will reduce the risk that a grossly misaligned piece of lumber or weight will be knocked off the carriage by a baffle unable to move out of the way of such a misaligned stack. 
     As the direction of airflow in the energy recovery subsections adjacent to the main drying section is fixed, the structure may be optimized to provide the direction of airflow in these critical sections that is most useful for preventing an outbreak of fire on the recently heated lumber. For example, it may be prudent in these energy recovery subsections nearest the main drying section to always circulate air to push air from the green lumber directly onto the heated lumber to maximize the cooling effect on the heated lumber, especially as the lumber enters subsections with oxygen contents closer to atmospheric levels. Alternatively, some installations may want to design the structure with the concept that the hot air leaving the heated lumber is pushed directly onto the green lumber without going through a circulation fan to maximize the drying effect on the green lumber. With fixed flow directions per subsection, the designer has the opportunity to optimize a design as the flow confronting each carriage of lumber will be the same for that subsection, and the order of circulation flow directions encountered by the lumber will be the same for all carriages as they pass through the drying process. A structure  104  using mirror image energy recovery sections  314  and  318  will subject the first set of carriages  128  and the second set of carriages  128  to the same sequence and durations of first circulation direction  204  and second circulation direction  208 . In the event, a designer does not opt for mirror images, then the sequence will differ. 
     Use with Unidirectional Kilns. 
     The teachings of the present disclosure may be used with unidirectional kilns that have all lumber travel in a single direction. While the concept is expressed in connection with drawings that show a single set of carriages conveying lumber from a first end to the second end of a structure, the concept is applicable to structures having two or more sets of carriages conveying lumber on parallel pathways from the first end to the second end of the structure. Thus, if the drawings of  FIG. 4 ,  FIG. 5  and  FIG. 6  were assumed to show two parallel pathways  116  and  120  where all carriages  128  and  132  move from the first end  108  to the second end  112 , these drawings could be used to demonstrate the use with two parallel pathways both traveling in the same direction. The invention may be extended to three or more parallel pathways. 
     As the need for drying time in the structure may differ for one batch of lumber to another batch as the thickness, moisture content, and other parameters may differ from one batch of lumber to another batch, nothing in this disclosure should be interpreted to require that all carriages on all pathways move at the same speed or pattern so that the lumber spends the same amount of time within the structure no matter which pathway the lumber is on. 
       FIG. 7  shows a structure  3104  that illustrates some of the teachings of the present disclosure. Many elements present in  FIG. 7  were introduced during the discussion of prior art structure  104  in  FIG. 1 . Structure  3104  has a first end  108  and a second end  112  and a first side  144  and a second side  148 . Lumber  130  is stacked upon a set of carriages  3128  on rails  3124  forming the first pathway  116  to traverse the structure  3104  from the first end  108  through the second end  112 . The manner of stacking lumber  130  upon carriages with spacers (sometimes called “stickers”) and weights may be the same as discussed in connection with  FIG. 1 . As described in more detail below, the lumber  130  is exposed to periods of air movement in the first circulation direction  204  and to periods of air movement in the second circulation direction  208  as the relevant carriage passes through the structure  3104 . 
     Structure  4104  differs from structure  104  in that the main drying section  4300  has a number of orthogonal MD partitions  4504  (See  FIG. 8 ) to subdivide the main drying section  4300  which is bounded by orthogonal partitions  324  (See  FIG. 8 ) to separate the main drying section  4300  from first chamber  4310  and third chamber  4340 . 
     The first chamber  4310  and may or may not have a capacity to provide additional heat to the lumber as first chamber  4310  may simply allow heat from main drying section  4300  to pass into first chamber  4310  to preheat the lumber. The third chamber  4340  and may or may not have a capacity to provide additional heat to the lumber as third chamber  4340  may simply allow the heated lumber exiting from the main drying section  4300  to become uniformly heated as heat passes from the exterior of the lumber to the interior and to cool down before leaving the second side  148 . 
       FIG. 5  provides an image of the main drying section  4300  in greater detail.  FIG. 7  shows the relationship between the details in  FIG. 8  and the structure  3104  by showing an image of the main drying section  4300  pulled out of the structure  4104 . 
     Main drying section  4300  has, in this instance, four subsections  4508 ,  4512 ,  4516 , and  4520 . The number of main drying section subsections does not need to be four but will be at least two and will usually be an even number of subsections as there is apt to be a desire to expose the lumber to equal ranges of the main drying section operated in the first circulation direction  204  and the second circulation direction  208  (as described below). 
     Turning to  FIG. 8 , a main drying section  4300  with subsections  4508 ,  4512 ,  4516 ,  4520  defined by orthogonal partitions  324  and orthogonal MD partitions  4504 . 
     The main drying section  4300  may have a return duct that draws air from one or more subsections in the manner discussed in connection with  FIG. 5 . Note that while the various orthogonal MD partitions  4504  impede the flow of air longitudinally, the seal is not perfect and air will flow based on pressure gradients. As discussed in connection with  FIG. 5 , a direct fired burner feeds burner exhaust at approximately 2000 degrees Fahrenheit into a mixing chamber to provide a mix of burner exhaust with return air from the return air duct to provide an output supplied to the main drying section  4300  above the main drying section set point which is often between 240 degrees Fahrenheit and 260 degrees Fahrenheit. As discussed in connection with  FIG. 5 , the heated air is supplied via the supply duct and distributed to the space between the fan deck  224 , to the tops of the lumber stacks  3156 . As discussed in connection with  FIG. 5 , the air moving to and from the mixing chamber may be moved by a blower located after the mixing chamber. 
     As lumber  130  on the set of carriages  3128  passes through the orthogonal partition  324  separating the main drying section  4300  from the first chamber  4310  ( FIG. 7 ), the lumber  130  is exposed to air moving in the first circulation direction  204 . The air in the first subsection  4508  of the main drying section  4300  always moves in the first circulation direction  2084  as structure  4104  uses fans  1200  that are operated in a single direction. The fans  1200  may be bi-directional fans like fans  200  that are used in a retrofitted structure or they may be unidirectional fans that are optimized to push air in one direction only with blades designed for this purpose but lack the additional design features and components needed for a bi-directional fan. 
     The fans  1200  in subsections  4508  and  4516  push air in the first circulation direction  204 . But fans  1200  in subsections  1512  and  1520  push air in the second circulation direction  208 . Thus, lumber stack  3156  on the set of carriages  3128  is subject to alternating circulation directions ( 204  and  208 ) without intermediate periods of no circulation as fans are de-energized, slowed to a stop, and started in the opposite direction. 
     The teachings of the present disclosure may be applied to a structure that has all carriage pathways travel in a single direction and uses steam heat exchangers to provide heat to the main drying section as discussed in connection with  FIG. 6 . The steam supply to the heat exchangers may be regulated with control valves as is known in the art. 
     Returning to  FIG. 7 , the intermediate orthogonal baffles  320  of  FIG. 1  may be termed intermediate orthogonal partitions  4320 . Thus, fans in subsection  4314  may continuously circulate air in the first circulation directions  204  and fans in subsection  4318  may continuously circulate air in the second circulation direction  208  so that movement of a carriage between subsection  4318  and subsection  4508  ( FIG. 8 ) results in a change in air circulation direction from the second circulation direction  208  to the first circulation direction  204 . Likewise, fans  1200  in subsection  4348  may continuously circulate air in the first circulation directions  204  and fans  1200  in subsection  4348  may continuously circulate air in the second circulation direction  208  so that movement of a carriage between subsection  4520  ( FIG. 8 ) and subsection  4344  results in a change in air circulation direction. 
     One of skill in the art can appreciate that instead of using two subsections in the first chamber  4310  or the third chamber  4340  that one could use four or other even numbers of subsections. One could also use an odd number of subsections in the first chamber  4310  or the third chamber  4340  potentially by changing the lengths of the subsections so that the total amount of time subject to each circulation direction ( 204  and  208 ) is maintained equal even if done in a different number of segments. 
     The orthogonal partitions  324 ,  4320 , and  4504  use baffles created to allow passage of a carriage loaded as intended (with lumber, spacers, and weights) but substantially conform to that profile so that longitudinal flow of air is limited. However, as the stacking of lumber, spacers, and weights may have some small variation from carriage to carriage, the baffles may have a capacity to give way when a larger than expected profile attempts to cross an orthogonal partition. The baffles are intended to be easy to adjust or replace during maintenance outages so that longitudinal air flow continues to be effectively resisted. 
     Placing a set of baffles on a faux partition external to the structure  4104  for pathways heading toward the structure  4104  may be useful to allow adjustments to the green lumber  140 , spacers, and weights on a carriage to minimize the amount of contact with the baffles inside the structure  4104 . Working for conformity with the expected profile for a loaded carriage will reduce wear on the baffles inside the structure  4104  which will mean better resistance to longitudinal air flow over time and will reduce the risk that a grossly misaligned piece of lumber or weight will be knocked off the carriage by a baffle unable to move out of the way of such a misaligned stack. 
     Advantages from Using Continuous Fan Operation 
     One should expect that all other things being equal the push rate of a structure converted from reversing fan operation to alternating single direction fan operation should increase as heat will continue to be applied to the structure without interruption for fan direction reversals. As kilns of this type are frequently used continuously for extended periods and then serviced in a maintenance outage, an increase in push rate results in an increase in production capacity without decreasing quality. 
     Operation of heating systems of any type are usually easier at steady state and more difficult when there are transients since monitoring equipment set points must often be altered for transient conditions but may be set to closer tolerances during steady state operation as deviations are more meaningful during steady state operation. 
     One should expect reduced maintenance and operation costs from running fans in a constant direction as motors and other components receive additional strain during the effort to start the motor and accelerate the fan. 
     One should expect a reduced risk of fire in the structure  1304  or  4304  as continuous airflow over lumber in carriages will reduce the formation of hot spots within the structure which might have occurred during a cessation of air flow during a fan direction change. Hot spots during a period without air circulation may cause a portion of the structure to move from an operating temperature of approximately 250 degrees Fahrenheit to more than 300 degrees Fahrenheit. Given that fire suppression sprinkler heads are used with thermally activated fuse links that are often designed to open between 330 degrees 360 degrees Fahrenheit, there are risks that a thermal transient from a hot spot might trigger a sprinkler which would not be useful for drying wood. More importantly, triggering fused sprinkler heads also requires and immediate shutdown to replace the one-time activated fire suppression equipment, resulting in significant production delays and loss of production efficiency. With the use of single direction fans, the set points for fire protection equipment can be dropped to respond more quickly to true fires without the risk of responding to a transient thermal hot spot. 
     Fire Detection Instruments may be positioned and have alarm set-points optimized for a particular subsection. Knowing the direction of air flow will allow alarms to be placed in optimized locations. Tolerances for temperature or smoke detection may be tuned to be more proactive as the instruments will not have to compensate for the conditions associated with dead air disturbed only by natural thermal convection during the absence of forced air circulation. Thus, with tighter tolerances, the fire detection and suppression equipment can react quicker to any aberrant measurement that may indicate the onset of a fire. With the air largely precluded from longitudinal movement by the orthogonal partitions, smoke concentrations will rise faster in a particular subsection than would otherwise be the case which will further assist in the early detection of a fire. Fire suppression systems can be set to react to indications of a minor fire by only applying water to the specific subsection implicated as potentially having a fire. This avoids unnecessary spoilage of lumber that is not at risk of fire. The fire suppression systems may be automatically or manually activated so that instances of activation will not necessarily require replacing equipment. 
     Given that the direction of air flow within a subsection is known, the fire suppression systems can be optimized for the direction of air flow. For example, side mounted fog or water deluge nozzles maybe placed to envelope or soak the upwind side of a carriage enabling water droplets to be carried by the air flow through the lumber from the upwind to downwind side of the carriage. Side mounted fog, deluge, or other nozzle arrays could be mounted on the upwind side of each of the one or more first pathways to optimize fire suppression options and to make use of uninterrupted alternating air circulation. 
     A structure designed with the teachings of the present disclosure may be able to achieve air movement with less fan amps as fan blades designed for unidirectional operation may be more efficient than the compromise inherent in bi-directional fan blades. Typically, the delivered CFM per motor horse power is greater for unidirectional fan blades than it is for fan blades that must be shaped and pitched to equally propel air in opposite directions based on alternating rotation. 
     Alternatives and Variations 
     Those of skill in the art will recognize that the direction of travel of the first set of carriages  128  on the first pathway  116  and the second set of carriages  132  on the second pathway  120  may be reversed from the directions discussed above without deviating from the teachings of the present disclosure. 
     While it is anticipated that many that use the teachings of the present disclosure will use unidirectional fans or will perpetually use bi-directional fans in one direction, the option remains of using bi-directional fans and reversing the direction of all the fans during a maintenance overhaul if that is perceived to have a benefit of elongating the life of any fan component. 
     Those of skill in the art will recognize that the formation of partitions to form subsections may be facilitated by choosing places within the structure that have structural supports such as beams, pillars, and trusses. 
     A number of direct fire burners may be used to provide the heat if direct fire burners are used rather than steam. The burners used for wood kilns include biomass (such as green sawdust or wood waste) direct fired burners, fossil fuel (such as coal, natural gas, or petroleum products) heating units, or other direct fired burners. 
     The push rate for moving carriages and the widths of subsection widths may be selected so that a carriage enters one subsection with one circulation direction and then enters the next subsection to be subject to airflow of the opposite circulation direction every two to four hours. For some installations, a three hour interval may be optimal. Those of skill in the art will recognize that a kiln using lower temperatures or flow rates, a different carriage width, or a different amount of rows and spacers may find that a different time duration is suitable, perhaps less than two hours, perhaps more than four hours. 
     Sub-Sections of Different Lengths. 
     While the figures discussed above had uniform subsections lengths from one end of the structure to the other end, this is not a requirement. 
     Finally, there may be times when a structure originally designed for reversing fan operation is upgraded to uni-direction operation. As there are advantages to building the structures for partitions to coincide with existing steel supports, one may make some adjustments to sub-section length to take advantage of existing structure. An important criterion is limiting the maximum time duration exposed to any one circulation direction. A particularly long distance between existing structural steel may be further subdivided into two or three subsections to avoid an overly prolonged exposure to circulation in one direction. 
     Turning Off Fans During a Fire Incident. 
     While there are advantages to having fire detection and suppression equipment tuned for a single circulation direction rather than having to compromise to accommodate both circulation directions ( 204  and  208 ), the fire suppression scheme may call for de-energizing at least some fans in the structure to minimize the oxygen fed to the fire. Even in a system that anticipates using fire suppression with the fans de-energized, there will be advantages in early detection of a fire for a system that does not have alternating circulation directions within a single subsection. 
     One of skill in the art will recognize that some of the alternative implementations set forth above are not universally mutually exclusive and that in some cases additional implementations can be created that employ aspects of two or more of the variations described above. Likewise, the present disclosure is not limited to the specific examples or particular embodiments provided to promote understanding of the various teachings of the present disclosure. Moreover, the scope of the claims which follow covers the range of variations, modifications, and substitutes for the components described herein as would be known to those of skill in the art. 
     The legal limitations of the scope of the claimed invention are set forth in the claims that follow and extend to cover their legal equivalents. Those unfamiliar with the legal tests for equivalency should consult a person registered to practice before the patent authority which granted this patent such as the United States Patent and Trademark Office or its counterpart.