Abstract:
A system for cooling telecommunications device includes an air inducing mechanism which pushes air into an enclosure of the device and an air exhaust mechanism which exhausts the air from the enclosure. The rate at which the air is pushed into the enclosure is less than the rate at which the air is expelled from the enclosure and these two rates are controlled such that the pressure differential between the pressure within the enclosure and the ambient pressure outside the enclosure is minimized. By minimizing this pressure drop, the airflow mechanisms are able to operate more efficiently, that is, they are able to operate at substantially near their design speed.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/254,091, filed on Dec. 8, 2000. The entire teachings of the above application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     To protect telecommunications devices, such as computers, from extreme temperatures, cooling systems are used to maintain a desired temperature inside enclosures housing the electronic components of the devices. For example, conventional air apertures, such as openings in the enclosures, are employed to facilitate the circulation of air about the electronic components. Fans can also be used with the apertures, either integrated within or mounted to the enclosure, to exhaust air from the enclosure after the air has circulated about the electronic components. 
     SUMMARY OF THE INVENTION 
     Typically, conventional air apertures operate inefficiently. For instance, there may be an air filter covering the aperture that becomes blocked up with dust over time. In some devices, the energy release by the components is large enough to require the use of fans. Fans that only exhaust air from an enclosure or only push air into the enclosure operate inefficiently because of the large pressure differential between the pressure within the enclosure and the ambient pressure outside the enclosure. 
     The present invention implements a system for cooling telecommunications devices. Specifically, in one aspect of the invention, the cooling system includes an air inducing mechanism which pushes air into an enclosure of the device, and an air exhaust mechanism which exhausts the air from the enclosure. The rate at which the air is pushed into the enclosure is less than the rate at which the air is expelled from the enclosure and the two rates are optimized such that the pressure differential between the pressure within the enclosure and the ambient pressure outside the enclosure is minimized. By minimizing this pressure drop, the airflow mechanisms are able to operate more efficiently, that is, they are able to operate at substantially near their design speed. 
     Embodiments of this aspect can include one or more of the following features. The airflow mechanisms can be fans. There can be two fans mounted adjacent to each other for pushing air into the enclosure, and there can be two fans also mounted adjacent to each other for expelling the air from the enclosure. Each fan module may be removable, such that when one fan fails it may be removed while the other fans continue to operate to prevent thermal damage of the components within the enclosure. Each fan module, along with any replacement fan module, can be placed in the enclosure interchangeably, without modification, in any of the four locations. All of the fan modules are keyed or polarized to the full extent so as not to adversely affect the air flow direction or the performance of the heat transfer capabilities of the system within the enclosure. 
     In other embodiments of this aspect, the system can include a controller for variably controlling the speed of the airflow mechanisms or operating the airflow mechanism at a preset speed. The first airflow mechanism can be mounted in a lower half portion of the enclosure, while the second airflow mechanism is mounted in an upper half portion of the enclosure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is an isometric view of an enclosure for a telecommunications product with a cooling system in accordance with an embodiment of the present invention. 
     FIG. 2 a  is an exploded perspective view of a fan module of the cooling system of FIG.  1 . 
     FIG. 2 b  is a view of a connector cable of the fan module of FIG. 2 a.    
     FIG. 3 a  is an isometric view of a panel of the housing of FIG. 1 in which the module of FIG. 2 is mounted. 
     FIG. 3 b  is an exploded isometric view of the panel of FIG. 3 a.    
     FIG. 4 is a block diagram of a control system for the cooling system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. Referring to FIG. 1, there is illustrated a system for cooling a telecommunications enclosure. As shown in FIG. 1, an enclosure  10  houses electronic components (not shown) within the enclosure and includes a rear mounting panel  12  in which four fan modules  14   a ,  14   b ,  14   c , and  14   d , collectively referred to as modules  14 , are mounted. Enclosure  10  includes a front casing  11  that is provided with keying tabs  13  to ensure a proper fit between the front casing  11  and the rear mounting panel  12 . 
     Referring now to FIG. 2 a , each fan module  14  includes a front plate  30  and a back encasing  32  which encase a fan assembly  33 . The fan module  14  also includes a front handle  34  and a back handle  36  which can be grabbed by an operator to assist in placing and removing the module for the mounting panel  12  of the enclosure  10 . The back housing  32  is provided with a set of keying tabs  38  which fit into a set of slots  39  of the front plate  30 . The front plate  30 , the back housing  32  and the fan assembly  33  are held together by a set of screws  40 . The front plate  30  and the back housing  32  are provided with respective screened openings  42  and  44 . These openings have diameters of approximately the same size as a circular opening  45  of the fan assembly  33 . The fan assembly  33  holds a set of fan blades and a motor  48 . A power cable  50  (FIG. 2 b ) extends from each fan module  14  and includes a connector  52  which plugs into a connector housing  54  located inside the enclosure  10 . The connector  52  has two screws  56  attached to the connector housing  54 , used for attaching the connector  52  to the inside of the enclosure  10 . 
     Referring also to FIGS. 3 a  and  3   b , the rear mounting panel  12  is provided with a set of keying stubs  60  which engage with a set of keying holes  62  of the front plate  30  of the fan module  14 . These stubs  60  provide the keying and orientation for each fan module  14  as they relate (mount) to the rear panel  12 . Since all the fan modules  14  are identical, the top fan modules  14   c  and  14   d  can only go into the rear panel  12  in one direction and the bottom fan modules  14   a  and  14   b  can only be mounted in the opposite direction as allowed by the stubs  60 . This prevents placing the fan modules in the wrong locations. The rear mounting panel  12  is also provided with a set of keying slots  64  which engage with the keying tabs  13  of the front casing  11 . Each fan module  14 , as well as any replacement fan module, can be placed in the enclosure  10  without modifying the module in any of the four locations of the rear panel  12 . Thus the fan modules  14  are fully interchangeable. The fan modules  14  are keyed, or polarized, to ensure that they are properly oriented in the rear panel  12  so as not to adversely affect the air flow direction through the enclosure  10  or the performance of the heat transfer capabilities of the cooling system. 
     Although the front plate  30  and the back housing  32  of the fan module  14  are typically made of metal, they can be made from plastics. The stubs  60  are made of metal for durability. 
     In use, fan modules  14   a  and  14   b  push air into the enclosure, as indicated by arrow  20 . The air circulates about the components inside the enclosure thereby cooling the components. Fan modules  14   c  and  14   d  exhaust air from the enclosure in the direction as shown by arrow  22 . In the present embodiment, the fan modules operate at a predefined speed. For example, modules  14   a  and  14   b  each operate at approximately 240 cubic feet per minute (CFM), while each of modules  14   c  and  14   d  are designed to operate at about 300 CFM. The upper modules  14   c  and  14   d  must operate at a higher speed because of the thermal expansion of the air as it is heated while flowing about the heat generating components. Note, the push (modules  14   a  and  14   b )/pull (modules  14   c  and  14   d ) arrangement of the configuration shown in the drawings. If the upper modules operate alone, the pressure differential between that within the enclosure and the ambient outer pressure is larger than when these modules operate in series with the lower modules  14   a  and  14   b . In essence, the “pushing” of the air by modules  14   a  and  14   b  lowers the system impedance of the air flow through the enclosure. By doing so, both the upper and lower fan modules are able to operate more efficiently. 
     When a fan module fails, the module can easily be removed from the rear mounting panel  12 . The system is designed such that the fans that have not failed continue to operate at their predefined speed while the failed fan is replaced. Even in such circumstances, the system is able to adequately cool the electronic components within the enclosure to prevent thermal damage to the components. 
     To properly choose fans that able to dissipate the generated heat within the enclosure, the heat generated must be approximated and the system impedance must be calculated. 
     The major sources of resistance within the enclosure include the resistance of the channels between the boards, the boards themselves, and the openings for the air into and out of the board chassis. In the present example, the dimensions of the board channels are as follows: height, H, is about 18 inches, width, W, is about 1.11 inches, and the depth, D, is about 15 inches. 
     The resistance of the board channels is calculated to be          R                 b                 c     =         3.1   ×     10     -   4       ×   H         (     W   ×   D     )     2       =         3.1   ×     10     -   4       ×   18         (     1.11   ×   15     )     2       =     20.13   ×     10     -   6                                    
     in units of            in   ·     H   2          O         (       ft   3     min     )     2                            
     The total resistance of the boards in the systems is calculated to be        Rboards   =         (     1     14       R                 b                 c           )     2     =         (     1     14       20.13   ×     10     -   6               )     2     =     0.109   ×     10     -   6                                    
     And the resistance of the slots are found to be        Rslot   =         2.4   ×     10     -   3           Af                2         =         2.4   ×     10     -   3             (     18   ×   15   ×   0.5     )     2       =     0.132   ×     10     -   6                                    
     where Af is the open cross sectional area of a slot, with 50% of the cross section open. 
     The total system impedance is then 
     
       
           Rsystem=Rslot+Rboards= 2×0.132×10 −6 +0.106×10 −6    
       
     
     thus, 
     
       
           Rsystem= 0.373×10 −6    
       
     
     The heat generated, W, within the enclosure 10 in the present example is approximately 2000 Watts, and the airflow, Q (in units of CFM), needed to dissipate that heat is        Q   =       W       c   p        ρΔ                 T       =         1.76   ×   W       Δ                 T       =         1.76   ×   2000                 Watts       15      °                   C   .         =     235      C                 F                 M                                  
     where c p  is the specific heat of air at sea level, p is the density of air at sea level, and ΔT is the maximum allowable temperature rise in the system, which is about 15° C. 
     With the estimated airflow and system impedance calculated above, the pressure loss in the system can be calculated from the relation 
     
       
           Psystem=Rsystem×Q   2 =0.373×10 −6 ×235 2 =0.021  
       
     
     in units of in. H 2 O. Thus with a 235 CFM requirement across a pressure of 0.021 in H 2 O, appropriate fans were selected based on their static pressure curves. 
     In the embodiment discussed above, a controller coupled to the fans operates the fans at a preset speed. However, in alternative embodiments of the system, as illustrated in FIG. 4, a microprocessor-based controller  70  in conjunction with a temperature sensor  72 , such as a thermistor, detects high temperature conditions and varies the speed of the fans  14  to accommodate higher temperatures. When abnormally high temperatures are detected, for example, when one fan fails, the other fans operate at their maximum speed. The fans may also default to a maximum speed when the controller fails or when the controller is removed from the device for replacement. 
     Both the intake and exhaust fans can use 48 volts of DC nominal operating voltage. The fan module can also provide a tachometer output, which is referenced to a negative lead supply and produces two pulses per revolution. There can be a programmable speed control, which provides variable speed operation by pulse width modulation. The fan speed can be proportional to the duty cycle present on the fan input. Maximum speed is obtained when the lead is open and it drops down to a minimum speed when the lead connects to the negative lead of the fan supply voltage. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.