Abstract:
An improved method and apparatus for cooling the under-car channel of a tunnel kiln while minimizing migration of air between the above-car and under-car channels involves controlling or equalizing the mass flow of cooling air directed through the under-car channel and in particular through individual undercarriage cooling zones which can match individual heating zones of the tunnel kiln.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the prior filed, co-pending application Ser. No. 60/961,285 filed Jul. 20, 2007. 
    
    
     FIELD OF INVENTION 
     This invention is directed to a method and apparatus for cooling the undersides (under-car channels) of kiln cars carrying a load of material to be heated through a tunnel type kiln. The apparatus and method of the present invention focus on equalizing the mass flow rate of the supply air and exhaust air through the system and in particular through individual temperature control zones which match the individual heating zones of the tunnel kiln. 
     BACKGROUND OF THE INVENTION 
     Tunnel kilns are elongated kilns through which a train of kiln cars is advanced to heat or fire ceramic materials, such as bricks, supported on the kiln cars. The kiln car train typically travels on rails running through the tunnel. The material to be heated is supported on a flat deck which in turn is supported on an undercarriage with wheels which travel on the rails. It is known to cool the underside of tunnel kiln cars, such that the support and transport mechanisms are maintained in a relatively cooler atmosphere than the upper deck side. Cooling of the underside of the kiln cars is utilized to avoid overheating the undercarriage, wheels, bearings and the like located beneath the deck of the kiln cars. 
     The kiln tunnel is generally separated into an under-car tunnel area or channel and an above-car tunnel area or channel by the deck of the kiln cars and one or more mechanical seals connected to or associated with the kiln car decks. The seals function to attempt to keep heated and cooled air in their respective areas, such that heated air does not migrate from the above-car channel to the below-car channel and cooling air does not migrate from the under-car channel into the above-care channel and have to be heated to process temperatures. These mechanical seals are specifically necessary to prevent infiltration and ex-filtration of air into and out of the above-car channel as the primary kiln exhaust fan, typically located toward the kiln tunnel entrance, keeps a relatively negative pressure in the above-car channels, which are heated to process temperatures. 
     One conventional and sometimes additional method for sealing the moving kiln car sides to the kiln side walls is to provide aprons along the longitudinal car sides which dip into sand filled channels of the kiln side walls such that the sand forms a closed barrier extending the length of the kiln. Transverse joints between successive kiln cars may be sealed by means of conventional mechanical joints and elastic material cords. The purpose of such mechanical seals and sand barriers is to substantially prevent pressure equalization between the under-car channel and the heated above-car channels, the seals are far from perfect. For design and cost reasons, the depth to which aprons can dip into the sand must be relatively small. Additionally, the sand must be fairly coarse so that it will be heavy enough so as not to be blown out of the channel barrier area and entrained in the moving gas flows. As a result, the sand barrier actually is permeable to gas and does not provide a perfect seal. Mechanical and elastic material seals simply wear out and degrade from the excessive kiln temperatures and also do not provide a perfect seal. 
     An established method of cooling the under-car channel is forcing air through the under-car channel at each of the various heating zones in the tunnel kiln. A disadvantage of this method is that a portion of the forced air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures. A second method of cooling the under-car channel is forcing air into the under-car channel from the exit end of the tunnel kiln toward its entrance end, which may or may not be practiced with a secondary under-car exhaust fan located toward the kiln entrance which draws air from the under-car channel. This second method also has a disadvantage in that a portion of the forced air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures. 
     A third method is to use openings in the foundation or side walls of the under-car channel to allow natural cooling of the under-car channels. This third method also has a disadvantage in that a portion of the natural cooling air will penetrate the mechanical seals and then the cooling air will have to be heated to very high process temperatures. The cooling air penetration in all three cases of the known prior art is partially caused by imperfect and worn mechanical seals, misaligned seals caused by natural degradation of the tunnel kiln structure, and the negative pressure within the above-car channel caused by the kiln exhaust fan, i.e. a pressure imbalance between the under-car channel and above-car channel. 
     The above-car channel is typically filled with air, combustion products, and off gases (collectively gases) from the heating process and curing process from the ceramic materials. These gases are typically flowing the same direction as the under-car cooling air, such that a pressure gradient develops in both channels. Because there are different gas flow rates and resistances in the above-car and under-car channels, the pressure gradient is different as a function of distance along the tunnel thereby leading to “false” air flows between the two channels, usually in the form of air moving from the under-car channel to the above-car channel. The air flows between the two channels (infiltration into the above-car channel and ex-filtration from the under-car channel) must be avoided in order to avoid undue heating of the under-car channel or undue cooling of the above-car channel and the related excess energy usage to heat the infiltrated air from under-car channel. 
     In prior art it is also known that there may be multiple trains of kiln cars traveling parallel to one another in side-by-side fashion. Such a kiln may or may not have intermediate longitudinal walls located between adjacent kiln car trains. Such kiln cars are typically equipped with the same conventional sand seals, i.e. aprons described above which dip into sand filled channels disposed laterally of each train. The problems associated with this conventional “sand trough” sealing technique for multi-train kilns are simply an order of magnitude larger than those experienced with a single train kiln. For example, long lateral distances are needed between adjacent kiln car trains to accommodate the required volume of sand in the channels in order to seal each of the multiple trains. The long lateral distances and required structure disrupt the gas flow conditions existing in the firing channel. Also, the increased number of sand-sealed channels in multi-train kilns tunnels increases the infiltration into the above-car channels from the under-car channels, making it more difficult to heat the above-car channel and the material on the car. For these reasons, multi-train tunnels are not generally constructed for commercial use. 
     It would be advantageous if a kiln tunnel could be provided which minimizes “leaks” between the above-car channel and under-car channel while providing balanced under-car cooling. It would be further advantageous to develop such a system which could be utilized with multi-train or multi-track tunnels. 
     SUMMARY OF THE INVENTION 
     The improved method and apparatus for cooling the under-car channel of a tunnel kiln while minimizing migration of air between the above-car and under-car channels involves controlling or equalizing the mass flow of cooling air directed through the under-car channel and in particular through individual temperature control zones which match the individual heating zones of the tunnel kiln. 
     Cooling air is supplied to each section or zone of the under-car channel from a blower through a main supply duct or supply trunk and branched supply ducts connected to each zone. For each branch supply air duct, there is a branch exhaust air duct serving the same zone of the under-car channel and connected to an exhaust fan through an exhaust trunk or main exhaust duct. The supply air volume provided to each section or zone of the under-car channel is regulated with an adjustable air flow damper that is modulated according to the necessary air flow that is required to maintain the exhaust air from the under-car channel and zone at a given set point, such as 350° F. The branch exhaust air ducts are likewise equipped with adjustable air flow dampers. Temperature transducers in each zone or section communicate the detected temperatures to a controller which then controls the dampers to increase or decrease the mass flow rate of cooling air through the respective zones to achieve the desired level of cooling. 
     The methodology and apparatus disclosed herein are also applicable to multi-train kiln tunnels to provide a significantly improved seal between the above-car channel firing and under-car channel cooling without an intermediate continuous wall positioned between adjacent kiln car trains. Therefore, the distance between adjacent kiln car trains can be maintained at an acceptably small dimension. Also, the improved methodology is applicable to multi-train kiln tunnel kilns that use narrow kiln cars customarily associated with older multi-train tunnel kilns with minimal conversion thereof. The improved under car cooling system disclosed herein may also be used with a multilane tunnel kiln having a longitudinal pedestal positioned between adjacent kiln car trains. This pedestal may incorporate one or more mechanical seals described above. Therefore, the mass flow controlled under-car cooling can be accomplished in a multi-train tunnel kiln employing the techniques described herein. 
     Individual sub-chambers may be cooled differently as a function of prevailing temperature within that specific sub-chamber. For example, a first firing zone may have a different cooling air mass flow than a second firing zone and the mass flow of the under-car cooling air can be controlled for each specific sub-zone as required by the process parameters required in the respective above-car channels. With the balanced mass flow approach, there is no substantial gas flow into or out of the sub-chamber other than cooling air. Therefore, there is no substantial leakage gas flow between the cooling channel and the firing channel. As a result, the leakage rates of sand filled channels or other types of mechanically attempted “perfect” seals between the above-car firing channel and the under-car cooling channel are much more effective in that the leakage rates are substantially reduced, thereby increasing the net kiln energy efficiency. As an added advantage, the under-car channel individual zones can be cooled at different rates in different sections or sub-chambers as a function of position along the tunnel kiln. Kiln cars can be transported along a rail system in the same plane both inside and outside the tunnel kiln so that lifting or lowering devices are avoided when wet seals are used. This under-car cooling system substantially facilitates movement and circulation for the kiln cars with conventional apparatus and existing facilities which can be retrofitted or converted to practice this present invention. 
    
    
     
       BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS 
         FIG. 1  is a schematic longitudinal cross-sectional view through a tunnel kiln showing a kiln car train passing through an inlet zone, preheat zone, firing zone, transition zone and cooling zone and an improved under-carriage cooling system for kiln cars; 
         FIG. 2  is an enlarged and fragmentary schematic cross-sectional view through the tunnel kiln of  FIG. 1  showing kiln cars of the kiln car train supporting bricks and showing additional details of the under-carriage cooling system; 
         FIG. 3  is a schematic cross-sectional view taken along line  3 - 3  of  FIG. 1  showing the under-car cooling system; 
         FIG. 4  is a schematic cross-sectional view taken along line  4 - 4  of  FIG. 2 . 
         FIG. 5  is a schematic cross-sectional view taken along line  5 - 5  of  FIG. 2 . 
         FIG. 6  is a schematic view of a tunnel kiln showing an alternative embodiment of the undercarriage cooling system. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to the drawings in more detail, a schematic view of a tunnel kiln  1  with a kiln car train  3  passing therethrough is shown in  FIG. 1 . The kiln car train  3  comprises a plurality of kiln cars  5  coupled together in end to end alignment. Referring to  FIG. 2 , which is an enlarged and fragmentary view of the kiln car train  3  in the tunnel kiln  1 . Each of the kiln cars  5  includes a deck  7  supported by an undercarriage  9 . Articles to be cured, such as bricks  11 , are stacked on and supported by the deck  7  and the undercarriage facilitates passage of the kiln car  5  through the tunnel  1 . An undercarriage cooling system  13  is incorporated into the tunnel kiln  1  to cool the undercarriages  9  of the kiln cars  5  as the train  3  passes through the tunnel kiln  1 . It is to be understood that the drawings provided, including schematic views, are not drawn to scale and relative proportions have been modified to allow the drawings to fit the space provided. In addition, the relative proportions of individual elements in the drawings may vary from figure to figure. 
     As best seen in  FIG. 4 , the tunnel kiln  1  generally comprises a floor or base  15 , opposed sidewalls  16  and  17  and a roof or top  18 . A pair of rails  21  form a track extending through the tunnel kiln  1 . The kiln  1  shown in  FIG. 4 , includes a single track and the walls are spaced apart a distance to allow a single train  3  of kiln cars  5  to pass therethrough. It is to be understood however, that the undercarriage cooling system of the present invention could be adapted for use in kilns incorporating multiple tracks laid side to side. The undercarriage  9  of each kiln car  5  generally includes axle supports  23 , which include axle bearings, depending from the deck  7  and supporting a pair of axles  25 , each axle  25 , having a pair of flanged wheels  27  mounted thereon. 
     Referring again to  FIG. 4 , the tunnel kiln  1  may be of the type having recesses or grooves  31  formed in each sidewall  16  and  17  and sized to receive sides  33  of each deck  7  of the kiln cars  5 . Extension of the sides  33  of the kiln car decks  7  into the sidewall grooves forms a partially effective mechanical seal to prevent transfer of air across either side of the deck  7 . Flexible seals  35  are also preferably formed between each kiln car  5  and may or may not be formed between the sides  33  of the kiln car decks  7  and the kiln sidewalls  16  and  17 . The space in the tunnel  1  above the decks  7  may be referred to as the upper kiln tunnel channel  37  and the space in the tunnel  1  below the decks  7  may be referred to as the lower kiln tunnel channel  38  or the upper zone  37  and lower zone  38  respectively. 
     An additional sealing means for forming a seal between the upper and lower zones  37  and  38  is the use of a sand seal  40  extending along both sidewalls  16  and  17 . More specifically troughs  41  and  42  extend along the length of each sidewall  16  and  17  below the grooves  31  therein for receiving the deck sides  33 . The troughs  41  and  42  are filled with sand  44 . Aprons  45  and  46  depending from the deck  7  of each rail car on opposite sides thereof extend into the troughs  41  and  42  respectively and into the sand contained therein to create a seal to reduce airflow thereacross and between the upper and lower zones  37  and  38  of the tunnel kiln  1 . 
     Burners  48  are mounted in the upper zone  37  of the tunnel kiln  1  in the sidewalls  16  and  17  or the roof  18  or both. The burners  48  are operated or controlled to adjust the temperature in the upper zone  37  of the tunnel kiln  1  to affect the desired curing of the bricks  11  or other items to be cured. As is indicated schematically in  FIGS. 1 and 3 , the tunnel kiln  1  is divided into multiple temperature control zones. One end of the tunnel kiln  1  includes an inlet  51  through which the kiln cars  5  initially enter the tunnel kiln  1 . An outlet  52  is formed at the other end of the tunnel kiln  1 . The remainder of the tunnel kiln  1  is divided into a preheat or warmup zone  55 , a fire or firing zone  56 , a transition zone  57  and a cooling zone  58  just prior to the outlet  52 . It is to be understood that in some of the applications for the present invention, there may not be a zone referred to as the transition zone  57 . As is also shown in  FIG. 3 , each zone, and in particular the firing zone  56  and the preheat zone  55  may each be divided into multiple sub-zones. The tunnel kiln  1  shown in  FIG. 3  includes preheat zones  55   a ,  55   b  and  55   c  and firing zones  56   a ,  56   b ,  56   c  and  56   d . It is noted that more sub-zones for the firing zone  56  and preheat zone  55  are shown in  FIG. 3  than in  FIG. 2 . 
     The temperature control zones are generally distinguished by the presence or absence of burners  48  and the temperature to which the zone is heated and the effect of the resulting heating on the materials or bricks  11  passing therethrough. There generally are no burners  48  in the inlet zone. Burners  48  mounted in the preheat zones  55   a - c  are operated to increase the temperature of the bricks  11  as they travel toward the outlet  52 . The bricks  11  are heated in the preheat zone to a temperature approaching the curing temperature for the ceramic material forming the bricks  11 . Burners  48  mounted in the firing zones  56   a - d  are operated to maintain the temperature in the firing zones  56   a - d  at a temperature which results in curing of the ceramic material forming the bricks  11 . 
     A primary blower or fan  61  is positioned near the inlet  51  of the tunnel kiln  1  and draws combustion and curing gasses out of the upper zone  37  of the tunnel kiln  1 , along the length of the tunnel kiln  1  and out of the tunnel kiln  1  near the inlet  51 . The air drawn into the tunnel kiln  1  from the outlet  52  cools the bricks  11  leaving the firing zones  56   a - d . The hot combustion gasses also function to preheat the bricks  11  traveling through the preheat zones  55   a - c . Because the primary blower  61  is located near the inlet  51 , the negative pressure created thereby is greatest near the inlet  51  and decreases toward the outlet  52 . 
     A stream of cooling air is circulated through the upper kiln tunnel channel  37  of the cooling zone  58  by a cooling zone supply fan  62  and a cooling zone exhaust fan  63 . The transition zone  57  is formed between the cooling zone  58  and firing zone  56  to reduce the cross flow of heated air from the firing zone  56  to the cooling zone  58  or the cross flow of cooling air from the cooling zone  58  to the firing zone  56 . 
     As discussed previously, the undercarriage cooling system  13  is adapted to cool the undercarriages  9  of each kiln car  5  and the rails  21  to prevent damage thereto which would hamper the ability of the kiln cars  5  to pass through the kiln tunnel  1 . The undercarriage cooling system  13  functions to blow cooling air through portions of the lower kiln tunnel channel  38  corresponding to selected temperature control zones of the tunnel kiln  1  such as one or more of the firing zones  56   a - d  or the preheat zones  55   a - c  or both. 
     In the embodiment shown in  FIGS. 2 and 3 , cooling air flow paths or undercarriage cooling zones  65  are created in each of the firing zones  56   a - d  and the lattermost preheat zones  55   b - c . Each undercarriage cooling zone  65  is created or formed by a supply duct  67  and an exhaust or return duct  68 . Cooling air is directed out of the supply duct  67 , across the associated zone  56   a - d  or  55   b - c  and drawn away through the return duct  68 . As shown, a portion of the supply duct  67  may extend upward through the floor  15  of the kiln  1  and into the lower zone  38  thereof. A vent  69  is formed in the side of the supply duct  67  facing the return duct  68 . The return duct  68  opens through the floor  15  of the kiln  1  on a side of the respective zone opposite the supply duct  67  and is covered by a grate  71 . The supply duct  67  in each zone is located toward the outlet  52  of the kiln  1  and the exhaust duct  68  toward the inlet  51 , such that the cooling air flows counter to the path of travel of the train  5  and in the same direction as air drawn through the upper zone  37  of the kiln  1 . The path of travel of the cooling air is represented by arrows  72 . 
     Air dams  73  are positioned behind or upstream (relative to the direction of airflow out of the supply duct  67 ) of each supply duct  67  and downstream of each exhaust duct  68 . As shown, an air dam  73  extends between the exhaust duct  68  of one cooling air flow path  65  and the supply duct  67  of the adjacent cooling air flow path  65 . The air dams  73  function to generally maintain the air flow from each paired supply duct  67  and exhaust duct  68  within the corresponding firing zone  56   a - d  or preheat zone  55   b - c  to cool the undercarriages  9  of the kiln cars  5  passing through those zones. It is foreseen that the portion of the supply ducts extending above the floor  15  of the kiln  1  could also function as an air dam and a separate air dam would not be required. 
     Each supply duct  67  branches off of and is connected to a main cooling air supply duct or supply trunk line  75 . An air supply blower or fan  76  is connected to or mounted relative to the supply trunk line  75  to blow ambient air through the supply trunk line  75  and each supply duct  67  and toward the associated exhaust duct  68 . Each exhaust duct  68  is flow connected to a main exhaust duct or exhaust trunk line  79 . An exhaust fan  80  is connected to or mounted relative to the exhaust trunk line  79  to draw air through each of the exhaust ducts  68  and then the exhaust trunk line  79  and discharged through the fan  80 . 
     Referring again to  FIG. 3 , a remotely controlled, adjustable supply damper  83  is positioned in each supply duct  67  and a remotely controlled, adjustable exhaust damper  84  is positioned in each exhaust duct  68 . The degree of openness of the dampers  83  and  84  are is adjustable to vary the volumetric flow rate of air through the associated supply and exhaust ducts  67  and  68 . A supply duct mass flow meter  85  is mounted in each supply duct  67  proximate the supply damper  83  and an exhaust duct mass flow meter  86  is mounted in each exhaust duct  68  for measuring the mass flow of air therethrough. A thermocouple or temperature transducer  88  is mounted in each exhaust duct  68  to measure the temperature of the exhaust air. In addition, a thermometer  89  with a visually readable scale or output may also be mounted to each exhaust duct  68  with its probe extending into the duct to allow visual inspection of the temperature of the exhaust air in each exhaust duct  68 . 
     A pressure gauge or pressure transducer  90  is mounted or positioned in the main supply duct  75  to measure the pressure therein. The pressure transducer  90  communicates with a pressure PLC loop  92  which in turn controls the speed of the supply fan  76  in order to maintain a relatively constant air pressure in the main supply duct  75  and in each supply duct  67  up to the supply damper  83 . 
     The thermocouples  88  in each exhaust duct  68  communicate with a temperature responsive PLC loop  94  which controls the degree of openness of the supply damper  83  to adjust the flow of cooling air through the supply duct  67 , across the associated undercarriage cooling zone  65  and into the exhaust duct  68  to maintain the exhaust air from the undercarriage cooling zone  65  at a given set point, say 350° F. It is to be understood that the PLC could control the degree of openness of the supply damper  83  to adjust either the volumetric flow rate or mass flow rate of cooling air through the supply duct  67 . 
     In the embodiment shown in  FIG. 3 , a flow responsive control loop  96  communicates with the supply and exhaust duct mass flow meters  85  and  86  and compares the measured mass flow rate of the cooling air for each paired supply duct  67  and exhaust duct  68 . The mass flow meters  85  and  86  may comprise annubar or Pitot tube devices. If the supply and exhaust dampers  83  and  84  were maintained at the same degree of openness, the volumetric flow rate of cooling air through each would be the same. However, because the exhaust air is warmer than the supply air, the exhaust air is less dense and will have a lower mass flow rate than the supply air at the same volumetric flow rate. Therefore, the flow responsive control loop  96  adjusts the degree of openness of the associated exhaust damper  68  to equalize the mass flow rate of cooling air through the exhaust duct  68  with the measured mass flow rate of cooling air through the supply duct  67 . It is to be understood that each control loop  92 ,  94  and  96  may be incorporated into a single controller. 
     In an alternative embodiment as generally shown in  FIG. 6 , a single supply mass flow meter  85  is mounted on or positioned in the main supply line  75  and a single exhaust mass flow meter  86  is mounted on or positioned in the main exhaust line  79 . The degree of openness of the supply and exhaust dampers  83  and  84  is either maintained the same or at a specified proportionality. Instead of adjusting the degree of openness of the exhaust damper  84  relative to the associated supply damper  83 , the flow responsive control loop  96  adjusts the speed of the exhaust fan  80  which will approximately balance out the mass flow rates for each matched pair of supply ducts  83  and exhaust ducts  84 . 
     It is to be understood that a single supply damper and a single exhaust damper could be utilized to control the flow of air through multiple supply ducts and multiple exhaust ducts respectively. For example, in a modified version of the embodiment shown in  FIG. 3 , a single supply damper  83  could be positioned in a firing zone supply branch (not shown) to control the flow of cooling air to the supply ducts  67  associated with the air flow cooling paths  65  for each of the firing zones  56   a - d . A single exhaust damper  84  positioned in a firing zone exhaust branch (not shown) would then be used to control the flow of cooling air drawn out of these zones  56   a - d  by the exhaust ducts  68 . A mass flow meter would then be associated with both the firing zone supply branch and exhaust branch with a controller receiving measurements from these flow meters to adjust the degree of openness of the exhaust damper  84  to equalize the mass flow rate through the air flow cooling paths  65  of the firing zones  56   a - d . A similar configuration could then be utilized for the air flow cooling paths  65  associated with each of the preheat zones  55   b - c  if appropriate. 
     It is also to be understood that the supply ducts  67  and exhaust ducts  68  forming each air flow cooling path  65  for the associated firing zones  56   a - d  or preheat zones  55   b - c  could be arranged on opposite sides of the respective zone as generally represented in the schematic diagram of  FIG. 6 . In such a configuration, the supply air is directed across the path of travel of the kiln cars  5 . Air dams  73  would preferably still be utilized in such an embodiment positioned similarly to the arrangement shown in  FIG. 3 , to attempt to contain the cooling air for each air flow cooling path  65  within each of the respective zones  56   a - d  or  55   b - c . Moreover, the number of air flow cooling paths  65  utilized can vary from as few as one. It is preferable to have at least one air flow cooling path  65  per the overall firing zone or preheat zone, and in most cases there will be multiple air flow cooling paths  65  for the overall firing zone and preheat zone. 
     By controlling and attempting to equalize the mass flow rates of cooling air through the supply duct  67  and exhaust duct  68  of each air flow cooling path  65 , using the apparatus and methods described, the mass of the cooling air flowing into and out of each of the associated zones in the lower kiln tunnel channel  38  is generally equalized thereby avoiding the creation of an area of high or low pressure which would increase the amount of heated air or cooling air leaking between the upper kiln tunnel channel  37  and the lower kiln tunnel channel  38 . Standard PLC programming functions can be used to set bias flow rates in individual zones in the lower kiln tunnel channel  38  to compensate for upper kiln tunnel channel pressures along the length of the kiln. 
     It is to be further understood that the apparatus and methodologies disclosed herein for use with a single track tunnel kiln  1  can be utilized for multi-track tunnel kilns. For a multi-track tunnel kiln additional air flow cooling paths  65  extending in end to end alignment could be utilized for each track. In a cross-flow application, it is foreseeable that a single air flow cooling path  65  could be formed across multiple tracks for corresponding zones with the supply duct positioned outside of a first track and an associated exhaust duct positioned on an opposite side of an adjacent track or with additional tracks spaced therebetween. However, for cooling efficiency, it is anticipated that each air flow cooling path  65  would only extend across a single track. 
     As used in the claims, identification of an element with an indefinite article “a” or “an” or the phrase “at least one” is intended to cover any device assembly including one or more of the elements at issue. Similarly, references to first and second elements is not intended to limit the claims to such assemblies including only two of the elements, but rather is intended to cover two or more of the elements at issue. Only where limiting language such as “a single” or “only one” with reference to an element, is the language intended to be limited to one of the elements specified, or any other similarly limited number of elements. 
     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.