Abstract:
Certain embodiments of the present invention provide a cooling duct assembly for electronic equipment installed in a cabinet. The cooling duct assembly comprises at least one main duct in fluid communication with a front portion of the cabinet. The at least one main duct receives cold air from the front portion of the cabinet and routes the cold air toward a back portion of the cabinet. Additionally, the cooling duct assembly comprises at least one side duct in fluid communication with the at least one main duct. The at least one side duct receives the cold air from the at least one main duct and routes the cold air to at least one side air intake opening on the electronic equipment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/175,282, filed on May 4, 2009, and U.S. Provisional Patent Application No. 61/073,818, filed on Jun. 19, 2008, the subject matters of which are hereby incorporated by reference in their entireties. 
     
    
     SUMMARY OF THE INVENTION 
       [0002]    Certain embodiments of the present invention provide a cooling duct assembly for electronic equipment installed in a cabinet. The cooling duct assembly comprises at least one main duct in fluid communication with a front portion of the cabinet. The at least one main duct receives cold air from the front portion of the cabinet and routes the cold air toward a back portion of the cabinet. Additionally, the cooling duct assembly comprises at least one side duct in fluid communication with the at least one main duct. The at least one side duct receives the cold air from the at least one main duct and routes the cold air to at least one side air intake opening on the electronic equipment. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to network cabinets. More particularly, the present invention relates to passive cooling systems for network cabinets. 
         [0004]    Network cabinets store and secure network components, such as servers and switches, which generate heat. As network technology advances, network components will generate more and more heat as a byproduct of higher speeds and improved performance. Therefore, cooling systems for network cabinets are essential to the development and integration of advanced network technology, both now and in the future. 
         [0005]    Cooling systems for network cabinets are classified as passive or active. Passive cooling systems, such as perforated doors and chimneys, rely on ambient airflow to remove heat from the network cabinet. Conversely, active cooling systems rely on mechanical devices, such as fans and/or compressors, to cool the air and move it through the cabinet. Passive cooling systems are typically less expensive than active cooling systems, both to install and operate, but require more space than a comparable active cooling system. However, installing passive cooling systems in existing network cabinets is particularly difficult because existing network cabinets have limited space. 
         [0006]    As previously mentioned, one example of a passive cooling system is perforated cabinet doors, which allow air to flow in or out of the cabinet. One drawback to perforated doors if the exhaust flow is not controlled properly is that hot air exiting the cabinet may be recirculated into the surrounding environment, and eventually, back into the cabinet. Chimneys have been added to control the removal of hot air from the cabinet. However, most network cabinets do not have enough space to accommodate a properly sized chimney and corresponding exhaust plenum, at least not one that can effectively cool the cabinet. 
         [0007]    Network cabinets generate different amounts of heat at different times of day, depending on a number of factors, such as the type of network components installed in the cabinet and their particular usage requirements. Additionally, most data centers include more than one network cabinet. However, existing passive cooling systems are not designed to accommodate more than one cabinet. Therefore, passive cooling systems are designed to accommodate a maximum heat load in each cabinet, rather than an average heat load across the entire data center, resulting in wasted resources. 
         [0008]    Therefore, there is a need for a passive cooling system that is modular and can be easily installed in existing network cabinets. There is also a need for a passive cooling system that can be easily shared by more than one network cabinet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a passive cooling system for a network cabinet according to an embodiment of the present invention. 
           [0010]      FIG. 2  is an exploded perspective view of the passive cooling system of  FIG. 1 . 
           [0011]      FIG. 3  is a side view of the passive cooling system of  FIG. 1 . 
           [0012]      FIG. 4  is a back view of the passive cooling system of  FIG. 1  having a split back door and transparent windows. 
           [0013]      FIG. 5  is a back view of the passive cooling system of  FIG. 1  having a split back door and no windows. 
           [0014]      FIG. 6  is a top view of the passive cooling system of  FIG. 1 . 
           [0015]      FIG. 7  is a side view of the passive cooling system of  FIG. 1  showing the airflow pattern through the network cabinet. 
           [0016]      FIG. 8  is a top view of the passive cooling system of  FIG. 1  showing the airflow pattern through two network cabinets connected in a side-by-side arrangement. 
           [0017]      FIG. 9  is a top view of the passive cooling system of  FIG. 1  showing the airflow pattern through two network cabinets connected in a side-by-side arrangement when the side panels of the cabinet extension are removed therefrom. 
           [0018]      FIG. 10A  is a perspective view of a switch for the network cabinet of  FIG. 1 . 
           [0019]      FIG. 10B  is a perspective view of a duct for the switch of  FIG. 10A  according to an embodiment of the present invention. 
           [0020]      FIG. 10C  is a perspective view of a duct for the switch of  FIG. 10A  according to an alternative embodiment of the present invention. 
           [0021]      FIG. 10D  is a perspective view of an adjustable rail assembly for the cooling duct assembly of  FIG. 10C . 
           [0022]      FIG. 10E  is a perspective view of the adjustable rail assembly of  FIG. 10D , showing the outside of the adjustable rail assembly. 
           [0023]      FIG. 10F  is a perspective view of the adjustable rail assembly of  FIG. 10D , showing the inside of the adjustable rail assembly. 
           [0024]      FIG. 11  is a top back perspective view of a router, such as a Cisco 3845 Integrated Services Router. 
           [0025]      FIG. 12  is an exploded top back perspective view of a cooling duct assembly for the router of  FIG. 11 . 
           [0026]      FIG. 13  is a top back perspective view of the cooling duct assembly of  FIG. 12 , showing the cooling duct assembly installed in a network cabinet. 
           [0027]      FIG. 14  is a top front perspective view of the cooling duct assembly of  FIG. 12 , showing the cooling duct assembly installed in a network cabinet. 
           [0028]      FIG. 15  is an exploded top front perspective view of a main duct for the cooling duct assembly of  FIG. 12 . 
           [0029]      FIG. 16  is an exploded top back perspective view of a side duct for the cooling duct assembly of  FIG. 12 . 
           [0030]      FIG. 17  is an exploded top front perspective view of a side duct for the cooling duct assembly of  FIG. 12 . 
           [0031]      FIG. 18  is a top back perspective view of the cooling duct of  FIG. 12 , showing the airflow pattern through the main duct and one of the side ducts. 
           [0032]      FIG. 19  is a top back perspective view of a front access door assembly for the cooling duct assembly of  FIG. 12 , showing the front access door assembly installed in a network cabinet. 
           [0033]      FIG. 20  is a top front perspective view of the front access door assembly of  FIG. 19 . 
           [0034]      FIG. 21  is a top back perspective view of the cooling duct assembly of  FIG. 12 , showing the main ducts and the side ducts being installed in a network cabinet, the side ducts being in an inward position. 
           [0035]      FIG. 22  is a top back perspective view of the cooling duct assembly of  FIG. 12 , showing the main ducts and the side ducts being installed in a network cabinet, the side ducts being in an outward position. 
           [0036]      FIG. 23  is a top back perspective view of a pair of side brackets for the cooling duct assembly of  FIG. 12 , showing the side brackets installed in a router and being installed in a network cabinet. 
           [0037]      FIG. 24  is a top back perspective view of one of the side brackets of  FIG. 23 . 
           [0038]      FIG. 25  is a top back perspective view of a back access door assembly for the cooling duct assembly of  FIG. 12 , showing the back access door assembly being installed in a network cabinet. 
           [0039]      FIG. 26  is a top back perspective view of the back access door assembly of  FIG. 25 . 
           [0040]      FIG. 27  is a top back perspective view of the cooling duct assembly of  FIG. 12 , showing the airflow pattern through the cooling duct assembly. 
           [0041]      FIG. 28  is a side view of the cooling duct assembly of  FIG. 27 , further showing the airflow pattern through the cooling duct assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]      FIGS. 1-10  illustrate a passive cooling system  100  for a network cabinet  10 , such as the PANDUIT® NET-ACCESS™ Cabinets, according to several embodiments of the present invention. 
         [0043]    As best seen in  FIG. 1 , the passive cooling system  100  includes a chimney  110  and a cabinet extension  120 . The chimney  110  is positioned above the cabinet extension  120 , and overlaps a portion of the network cabinet  10 . 
         [0044]    As best seen in  FIG. 2 , the cabinet extension  120  includes a base  130 , two side panels  140 , and a back door  150 . The cabinet extension  120  is positioned behind the network cabinet  10 , and expands the volume of the network cabinet  10 . The cabinet extension  120  and a portion of the network cabinet  10  that is adjacent to the cabinet extension  120  define an exhaust plenum  160  for the passive cooling system  100 , as best seen in  FIG. 7 . Although the back door  150  is described with reference to the cabinet extension  120 , it is understood that the back door  150  may be the back door  150  that was provided with the network cabinet  10 . Moreover, the cabinet extension  120  is adapted to fit a variety of different network cabinets, for example, using optional mounting hardware (not shown). In certain embodiments of the present invention, the base  130  includes a blocking panel (not shown). The blocking panel limits airflow through the bottom of the cabinet extension  120 . 
         [0045]    As best seen in  FIG. 3 , the network cabinet  10  includes a top panel  20 , two side panels  40 , and a front door  50 , which have been removed from  FIG. 1  and  FIG. 2  for clarity. The network cabinet  10  also includes a base  30  and four side rails  60 , two in the front of the network cabinet  10  and two in the back of the network cabinet  10 , which provide structural support for the network cabinet  10  and its contents. In certain embodiments of the present invention, the base  30  includes a blocking panel (not shown). The blocking panel limits airflow through the bottom of the network cabinet  10 . 
         [0046]    In certain embodiments of the present invention, the front door  50  is perforated, which allows cold air from the surrounding environment to enter the network cabinet  10 . 
         [0047]    In certain embodiments of the present invention, the front door  50  and/or the back door  150  are split, which allows access to a smaller portion of the network cabinet  10  and reduces the amount of airflow to/from the network cabinet  10  when opened. Examples of split back doors  150  are provided in  FIG. 4  and  FIG. 5 . 
         [0048]    In certain embodiments of the present invention, the back door  150  of the network cabinet  10 , or portions thereof, may be transparent and/or translucent, as shown in  FIG. 4 , or opaque, as shown in  FIG. 5 . For example, as shown in  FIG. 4 , the back door  150  includes a plurality of windows  152 , which are preferably made of a transparent material, such as glass or plexiglass. Transparent doors are preferred because the interior of the network cabinet  10  is visible without opening the back door  150 . Opening the back door  150  allows hot air to exit the network cabinet  10  into the surrounding environment. The hot air is then recirculated into the network cabinet  10  and/or adjacent network cabinets  10 , thereby reducing the effectiveness of the passive cooling system  100 . 
         [0049]    As best seen in  FIG. 6 , the top panel  20  of the network cabinet  10  includes a plurality of openings  22 . Each of the openings  22  includes a knockout  24 . One or more of the knockouts  24  may be removed to accommodate additional airflow to the chimney  110 . For example, as shown in  FIG. 6 , one of the four side knockouts  22  and all of the rear knockouts  24  are removed. 
         [0050]    As described above, the chimney  110  is positioned above the cabinet extension  120  and overlaps a portion of the top panel  20 . The cabinet extension  120  extends beyond the two side rails  60  in the back of the network cabinet  10 , expanding the volume of the network cabinet  10 , and more particularly, the exhaust plenum  160 . A larger exhaust plenum  160  increases the capacity of the passive cooling system  100 , as compared to simply installing a chimney on an existing network cabinet. 
         [0051]    As best seen in  FIG. 7 , the network cabinet  10  stores and secures network components  70 , such as servers  72  and switches  74 , which generate heat. Cold air enters the network cabinet  10  through the perforated front doors  50 . Heat from the network components  70  is transferred to the cold air, transforming it into hot air, which flows into the exhaust plenum  160  and exits the network cabinet  10  through the chimney  110 . 
         [0052]    In certain embodiments of the present invention, two or more network cabinets  10  may be connected in a side-by-side arrangement, for example, in one or more rows and one or more columns, which is typical in a data center. The side panels  140  between adjacent network cabinets  10  may remain in place or be removed. For example, as shown in  FIG. 8 , the side panels  140  between adjacent network cabinets  10  remain in place, allowing the passive cooling systems  100  to act independently. That is, each passive cooling system  100  is responsible for cooling one, and only one, network cabinet  10 , and does not assist in cooling adjacent network cabinets  10 . Conversely, as shown in  FIG. 9 , the side panels  140  are removed, resulting in a shared exhaust plenum  160 . 
         [0053]    As shown in  FIG. 8  and  FIG. 9 , the network cabinet  10  on the left is generating a relatively high amount of heat, as represented by large arrows  92 , and the network cabinet  10  on the right is generating a relatively low amount of heat, as represented by small arrows  94 . Comparing  FIG. 8  and  FIG. 9 , the shared exhaust plenum  160  balances the heat generated by both network cabinets  10 , and evenly distributes it to each chimney  110 , as represented by medium arrows  96 . Therefore, the shared exhaust plenum  160  allows the network cabinets  10  to share passive cooling systems  100 , which is preferred because network cabinets  10  are dynamic, generating different amounts of heat at different times of day. 
         [0054]    As described above, the network cabinets  10  store and secure the network components  70 , such as servers  72  and switches  74 . Servers  72  are typically vented from front-to-back. That is, cold air enters through an inlet vent on the front of the server  72 , and hot air exits through an outlet vent on the back of the server  72 , as shown in  FIG. 7 . Switches  74 , on the other hand, are typically vented from side-to-back. That is, cold air enters through an inlet vent  76  on the side of the switch  74 , and hot air exits through an outlet vent  78  on the back of the switch  74 , as shown in  FIG. 10A . 
         [0055]    Switches  74  are typically installed at the top of the network cabinet  10 , with the back of the switch  74  facing the front of the network cabinet  10 . In such a configuration, hot air from the network cabinet  10  and the passive cooling system  100  enters through the inlet vents  76  on the sides of the switch  74 , and even hotter air exits through the outlet vents  78  on the back of the switch  74 , contaminating the cold air supply at the front of the cabinet. 
         [0056]    One solution to this problem is to reroute the airflow through the switch  74  using a duct  80 . Examples of ducts  80  and their corresponding airflow patterns are provided in  FIG. 10B  and  FIG. 10C . As shown in  FIG. 10B  and  FIG. 10C , cold air enters through the front of the duct  80 , which reroutes the cold air through the inlet vents  76  on the side of the switch  74 . Hot air exits through the outlet vent  78  on the back of the switch  76 . The duct  80  prevents the hot air, or at least a portion of it, from flowing below the switch  74  and into the network cabinet  10 , thereby rerouting the hot air into the exhaust plenum  160  and out the chimney  110  of the passive cooling system  100 . 
         [0057]    As best seen in  FIG. 10D , the cooling duct assembly  80  includes a main duct  80   a  and two side ducts  80   b . Preferably, the main duct  80   a  is disposed below the switch  74  to block hot air from the front air exhaust openings  78 , but it is likewise contemplated that the main duct  80   a  is disposed above the switch  74 . The side ducts  80   b  are disposed on opposite sides of the switch  74  and associated with the side air intake openings  76 . 
         [0058]    The cooling duct assembly  80  is mounted to the cabinet  10  (not shown) via an adjustable rail assembly  300 . As best seen in  FIG. 10D , the adjustable rail assembly  300  includes a first rail  310 , a second rail  320 , and a plurality of fasteners  330 . The first rail  310  and the second rail  320  are slidably connected, allowing the length of the adjustable rail assembly  300  in position. Tightening the fasteners  330  locks the adjustable rail assembly  300  at a desired length L. 
         [0059]    As best seen in  FIG. 10E , the first rail  310  includes a first end  311  and a second end  312  opposite the first end  311 . The first end  311  of the rail  310  includes a flange  313  for mounting the adjustable rail assembly  300  to the cabinet  10 . For example, the flange  313  includes a plurality of holes  314  for fastening the rail  310  to the cabinet  10 . Additionally, the first end  311  of the rail  310  includes an edge  315  for supporting the cooling duct assembly  80 , and more particularly, the main duct  80   a  (see  FIG. 10D ). The second end  312  of the rail  310  includes a slot  316 . 
         [0060]    As best seen in  FIG. 10F , the second rail  320  includes a first end  321  and a second end  322  opposite the first end  321 . The first end  321  of the rail  320  includes a flange  323  for mounting the rail  320  to the cabinet  10 . For example, the flange  323  includes a plurality of holes  324  for fastening the rail  320  to the cabinet  10 . In certain embodiments of the present invention, the flange  323  is rotatably connected to the rail  320 . Additionally, the first end  321  of the rail  320  includes a flange  325  for mounting the cooling duct assembly  80 , and more particularly, the main duct  80   b , to the adjustable rail assembly  300 . For example, the flange  325  includes an opening  326  for fastening the main duct  80   b  to the adjustable rail assembly  300 . Additionally, the first end  321  of the rail  320  includes a plurality of edges for supporting the cooling duct assembly  80 . For example, a first edge  327   a  supports the main duct  80   a  and a second edge  327   b  supports the side duct  80   b  (see  FIG. 10D ). The rail  320  includes a plurality of holes  328  for fastening the side ducts  80   b  to the adjustable rail assembly  300 . The second end  322  of the rail  320  includes a plurality of rollover edges  329  for receiving the second end  312  of the first rail  310 . Additionally, the second end  322  of the rail  320  includes a plurality of holes  331  for receiving the fasteners  330 . 
         [0061]    Most network components  20  are configured for front to back cooling, and therefore, most network cabinets  10 , which store the network components  20 , are similarly configured. That is, cold air, for example, from a cold aisle in a data center and/or under the floor, enters the front of the cabinet  10 , and hot air, for example, exhausted from one or more network components, exits the back of the cabinet  10 . Preferably, the hot air exits through a chimney, such as the chimney  110  of the passive cooling system  100  ( FIGS. 1-9 ), but it is likewise contemplated that the hot air exits the cabinet  10 , for example, through one or more openings in the back of the cabinet  10  and into a hot aisle and/or above the ceiling in the data center. 
         [0062]    Additionally or in the alternative, some network components  20  may be configured for side to front and/or back to front cooling. For example, as best seen in  FIG. 11 , a router  90 , such as a Cisco 3845 Integrated Services Router, is configured for side to front and back to front cooling. That is, cold air enters the sides of the router  90  via one or more side air intake openings  92 . Additionally, the cold air enters the back of the router  90  via one or more back air intake openings  94 . Hot air exits the front of the router  90  via one or more front air exhaust openings  96 . When the router  90  is installed in the cabinet  10 , hot air in the back of the cabinet  10  enters the side air intake ports  92  and the back air intake ports  94 , and even hotter air exits the front air exhaust openings  96 , contaminating the cold air in the front of the cabinet  10 . 
         [0063]    One solution to this problem is to change the airflow pattern of the router  90  to more closely match the airflow pattern of cabinet  10 . For example, as best seen in  FIG. 27  and  FIG. 28 , a cooling duct assembly  200  reroutes cold air from the front of the cabinet  10  to the sides and back of the router  90 , and hot air from the front of the router  90  to the sides and back of the cabinet  10 . 
         [0064]    As best seen in  FIG. 12 , the cooling duct assembly  200  includes a pair of main ducts  210 , such as top duct  210   a  and bottom duct  210   b , a pair of side ducts  220 , such as left side duct  220   a  and right side duct  220   b , a front access door assembly  230 , a back access door assembly  240 , and two side brackets  250 . 
         [0065]    As best seen in  FIG. 13  and  FIG. 14 , the main ducts  210  are disposed on the top and the bottom of the router  90 . That is, the top duct  210   a  is disposed on the top of the router  90  and the bottom duct  220   b  is disposed on the bottom of the router  90 . As best seen in  FIG. 28 , the main ducts  210  extend from the front of the router  90  and beyond the back of the router  90  to define a back air plenum  260  therebetween. The back air plenum  260  is aligned with the back air intake openings  94  of the router  90 . 
         [0066]    As best seen in  FIG. 15 , each of the main ducts  210  includes a perforated front panel  211 , which allows cold air to enter the main duct  210 , and a perforated bottom panel  212 , which allows cold air to exit the main duct  210  and enter the back air plenum  260 . Additionally, as best seen in  FIG. 15 , each of the main ducts  210  includes a pair of side duct openings  213 , which allows cold air to exit the main duct  210  and enter the side ducts  220 . The side duct openings  213  are adapted to receive the side ducts  220 . 
         [0067]    As best seen in  FIG. 13  and  FIG. 14 , the side ducts  220  are disposed on the sides of the router  90 . That is, the left side duct  220   a  is disposed on the left side of the router  90  and the right side duct  220   b  is disposed on the right side of the router  90 . The side ducts  220  are aligned with the side air intake openings  92  of the router  90 . As best seen in  FIG. 16  and  FIG. 17 , the side ducts  220  include one or more baffles  221 . 
         [0068]    As best seen in  FIG. 18 , cold air, for example, from a cold aisle in a data center, enters the perforated front panels  211  of the main ducts  210 . The cold air flows through the main ducts  210  and exits the perforated bottom panels  212  and the side openings  213 . The cold air from the perforated bottom panels  212  enters the back air plenum  260 . The cold air from the side openings  213  enters the side ducts  220  (only the left side duct  220   a  is shown). 
         [0069]    As best seen in  FIG. 14 , the front access door assembly  230  is associated with the front of the router  90 . As best seen in  FIG. 19 , the front access door assembly  230  is secured to the front of the cabinet  10 . As best seen in  FIG. 20 , the front access door assembly  230  includes a front access door  231  that rotates from an open position ( FIG. 20 ), which allows access to the front of the router  90 , to a closed position ( FIG. 14 ), which restricts access to the front of the router  90  and further separates the front air exhaust openings  96  of the router  90 , which is for hot air, from the front of the cabinet  10 , which is for cold air. Additionally, the front access door assembly  230  includes one or more guides  232  for securing the main ducts  210  to the front access door assembly  230 , and therefore, the cabinet  10 . 
         [0070]    As best seen in  FIG. 21 , the main ducts  210  slidably engage the guides  232  on the front access door assembly  230 , which secure the main ducts  210  to the cabinet  10 . Additionally, as best seen in  FIG. 21  and  FIG. 22 , the side ducts  220  slidably engage the side duct openings  213 . That is, the side ducts  220  slide from an inward or retracted position ( FIG. 21 ), which allows the main ducts  210  to be inserted into the cabinet  10 , to an outward or extended position ( FIG. 22 ), which allows cold air to flow more freely through the side ducts  220 . 
         [0071]    As best seen in  FIG. 23 , the side brackets  250  secure the router  90  to the cabinet  10 . As best seen in  FIG. 24 , the side brackets  250  include one or more windows  251 , which are aligned with the side air intake openings  92  of the router  90 . The windows  251  allow cold air to flow from the side ducts  220  and into the side air intake openings  92  of the router  90 . 
         [0072]    As best seen in  FIG. 25 , the back access door assembly  240  is associated with the back of the router  90 . The back access door assembly  240  encloses the back air plenum  260 . As best seen in  FIG. 26 , the back access door assembly  240  includes a back access door  241  that rotates from an open position ( FIG. 26 ), which allows access to the back of the router  90 , to a closed position ( FIG. 12 ), which restricts access to the back of the router  90  and further separates the back of the cabinet  10 , which is for hot air, from the back air intake openings  94  of the router  90  and the back air plenum  260 , which are for cold air. Additionally, as best seen in  FIG. 26 , the back access door assembly  240  includes one or more grommets  242  for routing cables to the back of the router  90 . 
         [0073]    As best seen in  FIG. 27  and  FIG. 28 , cold air, for example, from a cold aisle of a data center, enters the main ducts  210 . The cold air flows through the main ducts  210  and into the side ducts  220  and the back air plenum  260 . The cold air from the side ducts  220  enters the side air intake openings  92  of the router  90 . Additionally, the cold air from the back air plenum  260  enters the back air intake openings  94  of the router  90 . Hot air exits the front air exhaust openings  96  of the router  90 . The front access door assembly  230  prevents (or at least minimizes) mixing of the hot air with cold air in the front of the cabinet  10 . Rather, the hot air flows around the sides of the cabinet  10 . Preferably, the hot air exits the cabinet  10  through a chimney, such as the chimney  110  of the passive cooling system  100  ( FIGS. 1-9 ), but it is likewise contemplated that the hot air exits the cabinet  10  through one or more openings in the back of the cabinet  10 , for example, to a hot aisle in the data center. The back access door assembly  240  prevents (or at least minimizes) mixing of the hot air with cold air in the back air plenum  260 . 
         [0074]    While the particular preferred embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the teaching of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as limitation. The illustrated embodiments are examples only and should not be taken as limiting the scope of the present invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.