Abstract:
An evaporator for a diphasic heat transfer system with capillary pumping, includes a base plate with a peripheral edge and an outer face for receiving heat receiving calories from a dissipative element, a body with a bottom, side portions and a border adjacent to the peripheral border of the base plate, a porous mass forming a layer with a capillary structure, interposed between the base plate and the bottom, which defines a first gas chamber and a second liquid chamber, the body being assembled on the base plate by crimping, such that there is no need for a gluing, screwing, riveting or welding operation to obtain a tight assembly of the evaporator.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to evaporators for two-phase thermal transfer systems with capillary pumping. 
         [0003]    More specifically, the invention relates to forming and assembling such evaporators, in particular those intended to remove heat produced generally by a dissipative element, in particular, but not exclusively, by any electronic processor or any electronic switching device. 
         [0004]    2. Description of the Related Art 
         [0005]    The expansion of data exchanged over the Internet network has led to an enormous need for calculating and storage power in servers serving to address the needs of users and in particular these servers are often gathered together in centers called “data centers”. 
         [0006]    The cooling needs of the electronic boards making up the servers in these data centers never stops growing because the density of processors in these boards is always higher. 
         [0007]    Using air or liquid fluid circulation for cooling the processor on a server board is known. The circulation of such a fluid requires the use of one or several fans or one or several pumps which can be subject to failure or at least require regular maintenance. 
         [0008]    It is therefore preferable to use a passive system, meaning one without an active pump, in order to extract the heat from processors; this type of system uses a capillary pumping phenomenon. Such a system is described in the patent application US2003/051859; however, the condenser function still calls for a fan and additionally the evaporator placed above each processor is complex to produce and consequently costly. Furthermore the bulk and reliability of operation of such an evaporator can be improved. 
         [0009]    A need therefore appeared for reducing the cost of such evaporators without impact on the performance thereof, in order to be able to offer two-phase thermal transfer systems with capillary pumping for various applications with large production volumes. 
       BRIEF SUMMARY 
       [0010]    For that purpose, the invention proposes especially an evaporator for a two-phase thermal transfer system with capillary pumping, including: 
         [0011]    a base plate with a peripheral edge and an outer surface for thermal reception intended to receive heat from a dissipative element, where the outer surface generally extends along an interface plane P adjacent to the dissipative element; 
         [0012]    a body with a back, lateral portions and a border adjacent to the peripheral edge of the base plate in order to delimit an inner space for the evaporator in that way; 
         [0013]    a porous mass forming a capillary structure layer, interposed between the base plate and the back, which defines a first chamber delimited by the base plate, the lateral portions and said porous mass and a second chamber delimited by the back, the lateral portions and said porous mass; 
         [0014]    where the first chamber is intended to house the working fluid in essentially gaseous phase, and includes an evaporator outlet for a tubular gas conduit, where the second chamber is intended to house the working fluid in essentially liquid phase, and includes an evaporator inlet for a tubular liquid conduit, characterized in that the body is assembled on the base plate by a crimping operation during which a hermetic seal between the base plate and the body is formed by material flow. 
         [0015]    In that way, a bonding, screwing, riveting or welding operation in order to obtain a sealed assembly of the evaporator is avoided. 
         [0016]    Because of these provisions, an evaporator which is particularly simple to assemble and therefore particularly low cost is obtained. In that way a controlled, potentially automatable and fully repeatable assembly process is achieved and mass production at a very attractive price can therefore be considered. 
         [0017]    In embodiments of the process according to the invention, it is additionally possible to use of one and/or another of the following provisions: 
         [0018]    Advantageously a joint projection integrally formed with the body and projecting towards the base plate is provided which projects into the porous mass such that a capillary seal is provided between the first and second chambers. In other words, the capillary seal avoids vapor short-circuiting the porous member and entering into the second chamber which is harmful to the performance of the system. In that way, with a single crimping operation one chamber is sealed from the other and the evaporator is globally sealed from the outside environment. 
         [0019]    The joint projection can be formed by a lip having a section with a pointed end directed towards the base plate by means of which the penetration of the joint lip into the porous mass is easier and the depth of penetration in that way gives a good seal between the first chamber and the second chamber. 
         [0020]    The outer surface of the base plate can be substantially flat and can be configured in order to press against a processor on an electronic board forming the dissipative element, such that this evaporator is particularly well-suited for cooling systems for electronic boards and in particular cooling processors or CPUs which are mounted thereon. 
         [0021]    The evaporator can have a thickness E, along a direction perpendicular to the plane P, less than 35 mm, such that such an evaporator can be easily integrated on a “1U” format server processor card, meaning a thickness of order 45 mm. 
         [0022]    The evaporator inlet and outlet are arranged parallel to the plane P in one of the lateral walls, such that the occupied volume perpendicular to the plane P is not increased by the fluid inlet, outlet and conduits. 
         [0023]    The base plate can include projections projecting inward, where these projections form vapor recovery channels and form spacers for holding the porous mass, such that it is possible to use a very simple porous layer without machining, where the vapor channels result from the shapes of the base plate. 
         [0024]    The porous mass can form a layer of constant thickness parallel to the interface plane P, such that a very low price porous element can be used, which is simply cut to the right length and right width without other preparation. 
         [0025]    The base plate has a rectangular shape and the body has a rectangular shape with four sides forming the lateral portions, such that the base plate can be perfectly suited to the geometry of a processor or CPU onto which it is installed and the body is a part with a geometry that is simple to produce. 
         [0026]    The base plate is made of copper and the body is stainless steel, by means of which the base plate has a very good thermal transfer coefficient and an adequate ductility for the crimping operation. Furthermore, the body has adequate features of durability and compatibility with fluids. 
         [0027]    The body includes fins arranged on the outer surface of the back of the body, such that if gas bubbles were to form in the second liquid chamber, they would re-condense under the cooling effect of the fins. 
         [0028]    The body includes means for attachment to the electronic card, for example in the form of lateral extensions parallel to the plane P, preferably screw holes. In this way the capacity for integration of the evaporator onto an electronics board is improved and the overall cost of the cooling function reduced. 
         [0029]    The border for the body includes a groove in which the material coming from deformation of the peripheral edge of the base plate comes to be lodged during crimping, by means of which the complementary shapes of the base plate and the body are optimized in order to obtain a quality crimping assuring the sealing of the inner space of the evaporator. 
         [0030]    The invention also targets a process for assembling and evaporator such as one of the type described above, where the process comprises the steps: 
         [0031]    providing a base plate; 
         [0032]    providing a cover; 
         [0033]    placing a porous material on top of the base plate; 
         [0034]    crimping the cover onto the base plate in order to form by material flowing/creeping a hermetic seal between the base plate and the body. 
         [0035]    Advantageously a capillary seal is simultaneously formed between the two chambers by penetration of the joint projection into the porous mass. 
         [0036]    Finally, the invention also targets a two-phase thermal transfer system with capillary pumping comprising an evaporator such as one of the type described above. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0037]    Other features and advantages of the invention will become apparent during the following description of one of the embodiments thereof, given as a nonlimiting example, with reference to the attached drawings. In the drawings: 
           [0038]      FIG. 1  is a general schematic view of a two-phase loop cooling system incorporating an evaporator according to the invention. 
           [0039]      FIG. 2  is a general perspective view of a cooling device incorporating an evaporator according to the invention. 
           [0040]      FIG. 3  is a transverse section view of an evaporator according to the invention. 
           [0041]      FIG. 4  shows a top view of the body of the evaporator from  FIG. 3 . 
           [0042]      FIGS. 5 and 6  illustrate the process for obtaining the evaporator by crimping; and 
           [0043]      FIG. 7  shows an implementation variant concerning the border of the base plate and of the body. 
       
    
    
       [0044]    In the various figures, the same references designate identical or similar items. 
       DETAILED DESCRIPTION 
       [0045]      FIG. 1  shows a sample closed-circuit two-phase loop cooling device which includes an evaporator  10 , a condenser module  8 , a first fluid pipe  40  referred to as “vapor” connecting the outlet of the evaporator to the inlet of the condenser module and a second fluid conduit  50  referred to as “liquid” connecting the outlet of the condenser module to the inlet of the evaporator. 
         [0046]    The circulation of a two-phase working fluid is assured via the capillary effect prevailing in a porous mass  3  disposed inside the evaporator  10 . Said porous mass  3  separates a first chamber  11  in thermal contact with the heat source and generally intended to contain the working fluid in gaseous phase, and a second chamber  12  arranged in the evaporator opposed from the heat source and intended to contain the heating fluid mainly in liquid phase. 
         [0047]    As is well known, the thermal energy supplied near the evaporator transforms the liquid into vapor during which it absorbs an energy equivalent to the latent heat of vaporization of the working fluid. The vapor created in that way is directed under pressure into the first conduit  40  towards the inlet  8   a  of the condenser module. In this area, the working fluid gives up the heat thereof either directly to the environment or to another liquid fluid  18  in forced circulation (as is shown: inlet  18   a , outlet  18   b ), and in this way the working fluid returns to liquid phase after having released energy corresponding to the latent heat. 
         [0048]    The liquid fluid of the outlet  8   b  of the condenser is aspirated by the pumping effect provided by the capillary structure of the porous mass, with the second chamber and the conduit  50  being filled with working fluid in liquid phase. 
         [0049]    Such capillary pumping systems are known under the acronyms LHP (Loop Heat Pipe) or CPL (Capillary Pumped Loop) and are used both in a gravity-less environment (space) and in terrestrial applications with gravity. 
         [0050]    It needs to be remarked that  FIG. 1  does not mention the vertical direction and consequently there is no specific constraint concerning the orientation and respective positions of the evaporator and the condenser module in so long as the capillary pumping effect remains greater than the effects of gravity considering the density of the fluid used. 
         [0051]    According to the present invention, the main target is terrestrial systems with high-volume or mass production applications, meaning very large numbers of parts to be produced. In this context, a goal is to reduce the material cost and the production cost of the components of the system and in particular the capillary evaporator  10 . 
         [0052]    One of the applications targeted by the present invention relates (not exclusively) to the cooling of electronic boards and especially processors or CPUs which are mounted on electronic boards in computers, conventional servers or even in racks of server boards.  FIG. 2  schematically shows an electronic board  19  equipped with at least one processor  9 . In the configuration shown, the board is shown in horizontal position, but it could just as well be in vertical position. 
         [0053]    The capillary evaporator  10  according to the invention configured for removing heat dissipated to the processor  9  according to a two-phase capillary loop process already described above is disposed on top of the processor  9 . 
         [0054]    The condenser module  8  is formed by a liquid/liquid type heat exchanger preferably with crossed flows, which is well known and therefore not described in detail here. The cooling fluid  18  will typically be water at the temperature at which it is available in the building or near the temperature of the surroundings or even slightly cooled. 
         [0055]    As shown in  FIG. 3 , the evaporator includes a base plate  1  intended to come into physical and thermal contact with the processor  9  along an interface plane P and a body  2  which complements the base plate in order to form an enclosure, corresponding to an inner space  20 , in which are disposed the elements of the evaporator which will be described below. 
         [0056]    The base plate  1  includes a peripheral edge  14  and an outer, thermal receiver, surface  17  intended to receive heat from a dissipative element such as the processor  9  in question. On the side opposite the outer surface, meaning the side referred to as “inner”, a plurality of projections  13  formed as ribs extending continuously or discontinuously and preferably in parallel with each other are arranged so as to create channels  41  otherwise stated grooves intended to convey the vapor formed in this area towards the vapor outlet  4 . The projections could also be pins arranged in a matrix or in staggered rows and separated from each other. Preferably the base plate is formed of copper, a material which has a very good coefficient of thermal conductivity and which furthermore has very good ductility whose interest will be seen later. 
         [0057]    There is provided, located above the projections  13 , a capillary structure layer which forms a porous mass  3 , which in the example shown looks like a layer of constant thickness without any specific secondary shape. It is therefore sufficient to cut out a porous layer with the correct length and width dimensions; there is no need to machine vapor channels inside of this porous mass. 
         [0058]    The makeup and the process for obtaining such a porous mass  3  is well known and are therefore not described in detail here. 
         [0059]    The aforementioned body  2  for the evaporator is disposed above the base plate and the porous mass and this body is preferably made of stainless steel. This body  2  comprises a back  21 , which forms a wall parallel to the plane P, lateral walls  22  which extend from the back all around the evaporator towards a border  24  which delimits the body  2  in the area of the interface plane P. 
         [0060]    The border  24  and the peripheral edge  14  of the base plate are joined hermetically as will be detailed below. 
         [0061]    In this way, a first chamber  11  referred to as “gaseous” is formed in the evaporator and delimited by the baseplate, lateral portions  22  and the porous mass  3 . A second chamber  12  referred to as “liquid” is formed in the evaporator and delimited by the back  21  of the body, the lateral portions  22  and the porous mass  3 . 
         [0062]    The hermetic separation between the first and second chambers is made by a joint projection  25  integrally coming from the body; this joint projection projects towards the baseplate and penetrates into the porous mass in order to form a sealed barrier for the working fluid in this area. 
         [0063]    A capillary seal is thus formed between the liquid chamber  12  and the vapor chamber  11 , meaning more precisely that there are no other possible passages between the two chambers except passing by the thickness of the porous mass  3 . In particular, this avoids any risk of hot vaporized liquid (vapor) passing into the liquid chamber, which would have the undesirable effect of increasing the temperature thereof and which would require re-condensing this vapor into liquid form by cooling it. The sealed closure formed by the joint projection  25  penetrating into the porous mass blocks any direct passage between the vapor chamber and the liquid chamber. 
         [0064]    A poor capillary seal results in a risk of leakage from the vapor chamber  11  into the liquid chamber  12 ; such a leakage could have an effect of degrading the thermal performance (reduction of the maximum flow density and of the maximum transferable power). 
         [0065]    More precisely, in the example illustrated here, the joint projection  25  has a sharp edge as a lip, meaning the lip has a generally triangular section with a pointed end. 
         [0066]    Furthermore, the body  2  includes lateral extensions  16  obtained integrally by the shaping of the body  2 ; these lateral extensions extend preferably parallel to the plane P and are provided with holes  66 . A well-known manner, spring-loaded screws (not shown) serve to press the evaporator against the processor. 
         [0067]    Optionally, the back  21  of the body can have on the outer surface thereof a plurality of fins  28  whose function is to limit the heating of the liquid in the second chamber  12  and which also serve to re-condense gas bubbles which could have formed near the porous mass and which rise towards the back  21 . 
         [0068]    As shown in  FIGS. 5 and 6 , in order to assemble the evaporator  10 , a base plate  1  is placed on a press base  60  and then a porous mass  3  layer is placed on the baseplate (as necessary on the projections  13  arranged on the baseplate as seen above) and then the body  2  is placed on top of the baseplate and the porous mass, where the border  24  of the body surrounds the peripheral edge  14  of the baseplate with a very reduced play. The end of the joint lip bears on the porous mass. 
         [0069]    Next, a press punch  61  is arranged above the body  2  and then pressure is applied downward on the punch  61  in order to crimp the body onto the baseplate. 
         [0070]    During the movement of the punch, there is both a ductile deformation of the peripheral edge of the baseplate (preferably of copper) and simultaneously the penetration of the end of the joint lip  25  into the porous mass  3 . 
         [0071]    Under the pressure exerted by the punch, the peripheral edge  14  of the baseplate comes to flow into a groove  27  provided for that purpose near the border  24  of the body  2  which results in a complementarity of shapes providing the seal. 
         [0072]    Because of the arrangement described above, a thin assembled capillary evaporator results; in the example shown, the thickness E of the evaporator is less than 35 mm, even 30 mm, which allows for an easy integration on server cards arranged one above the other in racks. 
         [0073]    As shown in  FIG. 7 , the border  24  the body  2  and the peripheral edge  14  of the baseplate can take various shapes, in particular the grooves in the body can be opened downward and the junction area between the two parts can be located away from the plane P. 
         [0074]    Advantageously, the thickness E of the evaporator  10  along the direction perpendicular to the plane P is less than 35 mm, even less than 30 mm, which can allow this evaporator to be used in very confined spaces and which can make the integration thereof in various environments easier; in particular this solution is advantageous for cooling electronic boards. 
         [0075]    In this respect, it should be remarked that the outlets  5 ,  4  for the liquid  50  and gas  40  conduits are arranged parallel to the plane P which does not increase the total thickness E of the evaporator  10 . 
         [0076]    It needs to be noted that the two-phase fluid capillary loop forms a passive system which does not require maintenance and does not generate fan or pump noise. 
         [0077]    It should also be noted that a thermal contact grease can be interposed between the baseplate and the processor. 
         [0078]    It should be noted that in order to improve the sealing performance of the closure the body  2  on the baseplate  1 , it may be advantageous to add a ribbon of glue or adhesive (not shown) in the rounded corners of the groove  27  laid out in the body. 
         [0079]    In order to improve the startup reliability of the two-phase loop, an anti-backflow device can be provided in the area of the discharge of the liquid inlet  5 , where this anti-backflow device prevents the liquid from flowing back towards the condenser via the liquid conduit  50 . 
         [0080]    Of course, the crimped capillary evaporator described above can be used for removing heat from any element of the device, not just from an electronic processor.