Abstract:
This rack ( 1 ) houses rackable electronic gear modules ( 2, 3 ) and has cooling by natural convection thanks to ventilation orifices ( 20, 21 ) provided in its bottom ( 10 ) and top ( 11 ) walls and forced air cooling thanks to internal air distribution ramps ( 30 ) fed with air under pressure through the intermediary of a distribution box ( 31 ) connected to a forced air circulation duct ( 37 ). The use of internal distribution ramps for the forced air makes it possible to produce a pulsed air circulation providing only a slight obstacle to the air circulation obtained by natural convection. Compared to usual configurations, this makes it possible to lower the operating temperature reached by the equipment in the event of loss of the forced ventilation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present Application is based on International Application No. PCT/EP2007/058872, filed on Aug. 27, 2007, which in turn corresponds to French Application No. 0607628, filed on Aug. 30, 2006, and priority is hereby claimed under 35 USC §119 based on these applications. Each of these applications are hereby incorporated by reference in their entirety into the present application. 
     
    
     DOMAIN OF THE INVENTION 
       [0002]    The invention relates to the cooling of electronic equipment housed in casings or racks placed on support platforms having forced cooling air circulation that can be subject to malfunctions. It relates notably to the casings or racks for electronic gear onboard aircraft. 
       BACKGROUND OF THE INVENTION 
       [0003]    The cooling of electronic equipment in a casing is usually obtained by evacuating the calories produced inside the casing to the external environment by means of a flow of forced air penetrating into the casing through ventilation orifices situated in the bottom part serving as nozzles, coming into contact with the components of the electronic equipment and being evacuated in the top part of the casing through ventilation orifices serving as vents. The calorie evacuation capabilities increase with the cooling air flow rate. In the case of the aeronautical standards such as Arinc 600 notably, the diameter and the number of the ventilation orifices are constrained so as to respect a headloss for a standardized air flow rate, which is in turn dependent on the dissipated power. This constraint makes it possible to distribute the flow of pulsed air between the different computers. 
         [0004]    When an electronic gear casing is also used as electromagnetic shielding, which is almost always the case for equipment onboard an aircraft, the effectiveness of the electromagnetic shielding imposes a smaller diameter on the ventilation holes, the number of which is then limited by the imposed headloss constraints. These limitations on the diameter and the number of the ventilation holes greatly restrict the capabilities of cooling by natural convection and very often make it necessary to limit the usable temperature range in case of loss of the forced ventilation. 
         [0005]    The electronic equipment designed to be onboard aircraft is usually placed in casings that are provided with ventilation orifices of a diameter and number that are inadequate to allow normal cooling by natural convection alone and that are placed on a pulsed air distribution box, a kind of organ windchest, distributing to them a flow of cooling air under pressure meeting precise specifications, for example those given in the Arinc 600 standard relating to the configuration of casings and subracks used in aircraft to house replaceable electronic equipment, also known as “rackable” equipment. 
         [0006]    Such an arrangement raises the difficult problem of an even distribution of the available pulsed air for the cooling between casings or disparate equipment that do not have the same cooling requirements and that, in addition, are not necessarily present. It also raises the problem, critical when it comes to safety, of the necessary continuity of certain functions handled by the electronic equipment in the case of loss of the forced cooling air flow. 
         [0007]    In case of loss of the forced air flow, the cooling is now provided only by natural convection that is ineffective because of the excessively small cross section that can be used of the ventilation orifices that are limited in diameter and in number: in diameter by the requirements of the electromagnetic shielding and in number by the headloss imposed by the standard, and because of the volume of external air available under the casings reduced to the capacity of the pulsed air distribution box. The temperature of the equipment then increases significantly, which degrades their temperature operating range. 
         [0008]    Control of the operating temperature of electronic equipment in the case of loss of the forced air flow is the main limitation encountered when seeking to reduce its bulk and increase its functionalities by increasing the density of the electronic circuitry because both are always accompanied by an increase in the production of calories by the liter. 
         [0009]    Now, it is not possible to improve the cooling by natural convection in case of loss of the forced cooling air flow, by providing additional air intake orifices in the bottom parts of the casings, outside the distribution box, because these additional intake orifices contravene compliance with the specified headloss requirement, notably that given in the Arinc 600 standard. 
       SUMMARY OF THE INVENTION 
       [0010]    The aim of the present invention is to remedy the abovementioned problems. Its aim is notably to improve the cooling by natural convection of an electronic gear casing with forced air cooling to limit the increase in temperature of the electronic gear in case of loss of ventilation and allow an increase in the compactness and the integration density of the electronic gear. 
         [0011]    Its subject is an electronic gear casing comprising orifices for ventilation by natural convection on its bottom and top walls and internal air distribution ramps fed with air under pressure through the intermediary of a distribution box connected to a forced air circulation duct. 
         [0012]    Advantageously, the internal air distribution ramps are fixed removably to pulsed air distribution orifices of the distribution box. 
         [0013]    Advantageously, the internal air distribution ramps are connected to pulsed air distribution orifices of the distribution box provided with isolating plugs. 
         [0014]    Advantageously, the internal air distribution ramps are positioned in the bottom part of the casing. 
         [0015]    Advantageously, the internal air distribution ramps are positioned horizontally. 
         [0016]    Advantageously, the internal air distribution ramps are fed with air under pressure by a distribution box placed at the base of the rear wall of the casing. 
         [0017]    Advantageously, the internal air distribution ramps are tubes that are closed at one end, provided with a nozzle at the other end and laterally pierced with a network of orifices serving as vents, distributed over their length and blowing in a substantially horizontal direction. 
         [0018]    Advantageously, when the electronic gear casing consists of a rack housing removable modules positioned side by side, on the edge, in an alignment perpendicular to the bottom of the rack, the internal air distribution ramps are placed between the modules and present lateral orifices serving as vents blowing toward the modules. 
         [0019]    Advantageously, when the electronic gear casing consists of a rack housing removable modules positioned side by side, on the edge, in an alignment perpendicular to the bottom of the rack, the internal air distribution ramps are placed horizontally, level with the bottom edges of the modules, and present lateral orifices serving as vents blowing upward, toward the modules. 
         [0020]    Advantageously, when the electronic gear casing consists of a rack housing removable modules positioned side by side, on the edge, in an alignment perpendicular to the bottom of the rack, the internal air distribution ramps are fixed to the modules perpendicularly to the rear wall of the rack and connected, removably, to pulsed air distribution orifices of the distribution box placed on the rear wall of the rack. 
         [0021]    Advantageously, when the electronic gear casing consists of a rack comprising backplane connectors on its rear wall and housing removable modules plugged into the backplane connectors, the distribution box is placed on the rear wall of the rack below the backplane connectors. 
         [0022]    Advantageously, when the electronic gear casing consists of a rack housing removable modules individually equipped with a finned cover serving as a heat sink and positioned side by side, on the edge, the internal air distribution ramps are fixed to the finned covers of the modules. 
         [0023]    Advantageously, when the electronic gear casing consists of a rack housing removable modules individually equipped with a finned cover serving as a heat sink and positioned side by side, on the edge, the internal air distribution ramps are fixed in grooves of the finned covers of the modules. 
         [0024]    Advantageously, when the electronic gear casing consists of a rack housing removable modules positioned side by side on the edge and individually equipped with a finned cover serving as a heat sink, the fins being configured to assume a vertical orientation once the modules are in place in the rack, the internal air distribution ramps are fixed in grooves of the covers, providing a free space relative to the bottoms of the grooves. 
         [0025]    Advantageously, when the electronic gear casing consists of a rack housing removable modules positioned side by side on the edge and individually equipped with a finned cover serving as a heat sink, the fins being configured to assume a vertical orientation once the modules are in place in the rack, the internal air distribution ramps are fixed in grooves, of the covers, providing a free space relative to the bottoms of the grooves and present lateral orifices serving as vents blowing upward, toward the fins. One beneficial effect is to speed up the flow of air originating from the natural convection and passing into the dividing space left free at the bottom of a groove by an internal air distribution ramp. 
         [0026]    Advantageously, the internal air distribution ramps can be prolonged by one or more capillaries directed toward the components requiring a more effective cooling. 
         [0027]    Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]    The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
           [0029]      FIG. 1  is a front lateral perspective view of an electronic rack for aircraft, conforming to the invention, shown partially dismantled with two modules or rackable electronic units in place, 
           [0030]      FIG. 2  is a front lateral perspective view of the electronic rack shown in  FIG. 1 , illustrating the movement to put in place a rackable module and the positioning on a module of an internal pulsed air distribution ramp, 
           [0031]      FIG. 3  is a rear lateral perspective view of the electronic rack shown in  FIG. 2 , illustrating the positioning of a forced air distribution box, and 
           [0032]      FIG. 4  is a view of the rack shown in  FIG. 1  represented in profile, in a vertical cross-section, detailing the paths of the cooling air flow along a wall of a module. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    The electronic rack  1  that will be described is intended to house a set of electronic modules  2 ,  3 , called rackable because they plug into the rack like drawers. 
         [0034]    The rackable electronic modules  2 ,  3  are of primarily flat rectangular shape. They comprise a printed circuit board  4  with visible longitudinal edges forming longitudinal guide edges  5 ,  6 . This printed circuit board  4  supports: 
         [0035]    on one of its faces or on both its faces, electronic and/or optical components that cannot be seen in the figures as well as wiring providing the electrical and/or optical interconnections for the supported components, 
         [0036]    on the component mounting face or faces, a finned cover  7  serving as a heat sink because it is in close thermal contact with the electronic and/or optical components releasing the most calories and, 
         [0037]    on one of its faces or on both its faces, a set  8  of electrical and/or optical connectors dedicated to connecting the modules to each other and to the environment outside the rack  1 . 
         [0038]    The rackable electronic modules  2 ,  3  can have other configurations provided that they observe the dimensioning constraints imposed by the rack  1 . Notably, their guide edges  5 ,  6  can be fitted to the finned cover  7  in order to evacuate a proportion of the calories into the subrack of the rack  1  or to a plate that can also have an electromagnetic shielding function fixed to the face of the printed circuit board  4  opposite to that covered by the finned cover  7 . 
         [0039]    The electronic rack  1  is provided with a metal subrack, usually comprising lateral walls that are not visible in the figures, an intermediate bottom wall  10 , a top wall  11  and a rear wall  12 . 
         [0040]    The intermediate bottom  10  and top  11  walls of the rack  1  support, facing each other, two networks  13  and  14  of parallel slides oriented perpendicularly to the rear wall  12  and used to guide the longitudinal edges  5 ,  6  of the modules  2 ,  3  when inserting, extracting and holding in place the modules  2 ,  3 , side by side, on the edge, with their printed circuit boards  4  oriented in vertical planes perpendicular to the rear wall  12  of the rack  1 . 
         [0041]    The rear wall  12  of the rack  1  supports, in its top part, a set  15  of backplane connectors that mate with the set or sets  8  of connectors on the printed circuit boards  4  of the modules  2 ,  3  to provide the connections of the modules  2 ,  3  to each other and to the environment outside the rack  1 . 
         [0042]    The fins of the covers  7  serving as heat sinks for the modules  2 ,  3  are oriented perpendicularly to the longitudinal edges of the modules  2 ,  3  so as to be restored to the vertical position once the modules  2 ,  3  are in place in the rack  1  and to facilitate their contact with an upward cooling air current obtained by natural convection and by forced circulation. 
         [0043]    The natural convection is due to the presence of ventilation orifices  20 ,  21  in the intermediate bottom  10  and top  11  walls of the rack  1 . 
         [0044]    The ventilation orifices  21  provided in the top wall  11  of the rack  1  consist of holes of small diameter, of the order of 3 mm, consistent with the requirements of electromagnetic shielding. Those  20  that are made in the intermediate bottom wall  10 , which is provided to separate two levels of modules, are wide longitudinal slots because the continuity of electromagnetic shielding is not provided at this level but at that of the bottom of the rack  1  which is not represented and which comprises ventilation orifices similar to those  21  of the top wall  11  with a small diameter, of the order of 3 mm. 
         [0045]    The ventilation orifices  21  distributed over the entire surface of the top wall  11  of the rack  1 , and over the entire surface of the bottom wall forming the true bottom of the rack  1 , form meshed areas between the slides of the networks  13 ,  14  supporting the modules  2 ,  3 , allowing a free circulation of the air with clearance spaces provided above and below the rack  1 . 
         [0046]    In addition to the natural convection ventilation, the fins of the covers  7  serving as heat sinks for the modules  2 ,  3  are subjected to a forced ventilation by means of pulsed air distribution ramps  30  fed through the intermediary of a distribution box  31  fixed to the outside of the rack  1 , in the bottom part of its rear wall  12 , below the set of backplane connectors  15  and connected by a nozzle  36  to a pressurized air duct made available to the rack  1 . 
         [0047]    The pulsed air distribution ramps  30  take the form of small “piccolos” with blocked mouths, the playing holes of which serve as lateral vents. They are positioned horizontally, at the base of the modules  2 ,  3  all along the finned covers  7 , perpendicularly to the rear wall  12  of the rack  1 , with their nozzles  32  fitted into pulsed air distribution orifices  33  of the distribution box  31  that are accessible from inside the rack  1  through the rear wall  12 , and with their lateral vents blowing cooling air between the fins of the covers  7  of the modules  2 ,  3 . 
         [0048]    As shown in the figures, the pulsed air distribution ramps  30  are fixed to the finned covers  7  of the modules  2 ,  3  in grooves  34  hollowed out perpendicularly to the fins, close to their bases. Their lateral vents, not visible in the figures, face toward the bottoms of the slots separating the fins, with a slight offset upward to initiate and maintain upward air currents. Preferably, the pulsed air distribution ramps  30  are spaced apart from the bottom of the grooves  34  so as not to provide an obstacle to the upward air currents resulting from the natural convection, an amplification, by the Venturi effect, of these upward air currents even being sought by means of an adequate orientation of the vents of the ramps  30 . 
         [0049]      FIG. 4  illustrates the distribution of the cooling air currents along a finned cover  7  of a module  2 ,  3 , resulting from the natural convention and from the pulsed air distribution by a ramp  30 . 
         [0050]    The natural convection provokes a layer of upward cooling air currents, directed from bottom to top all along the fins, in the directions identified by the arrows  35 . These upward currents originate from the clearance space provided under the rack  1  through the multiple ventilation orifices  20  provided in the bottom of the rack  1  and in the intermediate bottom walls  10 , and are evacuated toward the clearance space provided above the rack  1  through the multiple ventilation orifices  21  provided in the top wall  11  of the rack  1  and, possibly, in intermediate top walls providing separations between top levels. 
         [0051]    The pulsed air distribution from a duct connected from outside the rack  1  to the distribution box  31 , recalled in  FIG. 4  by the arrow  37 , provokes an additional layer of upward air currents from the distribution ramp  30  to the tops of the fins, directed in the directions of the arrows  38  and provoked by the breaths from the lateral vents of the distribution ramp  30 , which itself is passed through in the direction of the arrows  39  by the pulsed air leaving the distribution box  31 . 
         [0052]    The distribution orifices  33  of the distribution box  31  are fitted with isolating plugs pushed back by the nozzles  32  of the ramps  30  as they are inserted into the distribution orifices  33 , at the end of the installations in the rack  1  of the modules  2 ,  3  that support them. Thus, the distribution orifices  33  of the distribution box  31  assigned to unoccupied equipment cells remain blocked avoiding losses of pulsed cooling air. 
         [0053]    Furthermore, the ramps  30  as one with the modules  2 ,  3  can have lateral vents with flow rates customized according to the local cooling requirements appearing through the finned cover  7 . They can also be prolonged by one or more capillaries directed toward the components requiring a more effective cooling. 
         [0054]    Various configuration variants can be envisaged. The ramps  30  can be separated from the finned covers  7  of the modules  2 ,  3  and have their own mounting supports, for example clips fixed to the bottom wall  10  of the rack  1  between the sliders. They can even be joined to the distribution box  31 . Their lateral vents can take various forms, including slotted forms. 
         [0055]    It will be readily seen by one of ordinary skill in the art that the present invention fulfils all of the objects set forth above. After reading the foregoing specification, one of ordinary skill in the art will be able to affect various changes, substitutions of equivalents and various aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by definition contained in the appended claims and equivalents thereof.