Patent Publication Number: US-2015059388-A1

Title: Information processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-181481 filed on Sep. 2, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present disclosure relates to an information processing apparatus equipped with a cooling device that cools a heating component mounted over a circuit board using liquid refrigerant. 
     BACKGROUND 
     Electronic elements such as a CPU or a control LSI are mounted on a main circuit board of a server device that is an information processing apparatus. These electronic elements generate heat at the time of being operated. Hence, in order to prevent the stable operation of the server device from being damaged by heat, it is necessary to cool the circuit board. As a cooling method, there has been proposed an air cooling system using a fan. However, the cooling method by a liquid cooling system using liquid refrigerant (“refrigerant”) is disclosed in patent document 1 and patent document 2, for example. 
     As the server devices require high performance as well as miniaturization and high-density packaging, the power consumption of the electronic elements is increased with the enhanced performance of the server devices. As a result, the electronic elements generate a large amount of heat and thus, the server devices are configured as follows so as to improve the cooling capacity of a cooling device that serves to cool the electronic elements. 
     In an air cooling system, it is necessary to send a lot of wind to the electronic elements so as to enhance the cooling capacity. Thus, in order to increase the flow of blown air, the air cooling system is configured to satisfy the following requirements.
         Increasing the number of rotations of fans, the size of the fans, and the number of the fans.   Providing a duct for efficiently conveying cooling air.   Increasing the size of a heat sink that is installed in an electronic element part.       

     However, when these measures are implemented, the following problems may occur.
         The increase of an air cooling space in the server device may negatively affect the high-density mounting of circuit components.   Since the electric power supplied to the fans increases, power capacity may increase and a power supply may be enlarged.   Securing of a heat sink space may inhibit miniaturization.   The increase of the heat sink space may suppress respective electronic elements from being disposed adjacent to each other.   Since wirings between the electronic elements are lengthened, high-speed signal transmission may be obstructed between CPUs or between a CPU and an interface.   Since the supply path between a power element and a CPU is lengthened, voltage drop may increase.   Since the increase of a power pattern and the installation of a bus bar or an electric wire are required, miniaturization and high-density mounting are obstructed.       

     In connection with the increase of heat quantity in a server device: a liquid cooling system having a relatively high cooling efficiency is used for an electronic element part generating a lot of heat, while an air cooling system is used for other parts. Further, in order to increase the cooling efficiency in the liquid cooling system, the refrigerant for a cooling body is supplied to a cooling plate (heat-exchange module) of a component via a pipe, and heated refrigerant is recovered via the pipe. 
     However, when the cooling plate is disposed above the electronic element generating a lot of heat and the refrigerant pipe is provided in the server device, the following problems may occur in terms of the miniaturization and high-density mounting in the server device.
         Securing of a pipe space may inhibit the miniaturization of the server device.   Arrangement of electronic elements in the vicinity of a CPU may be obstructed in the pipe space.   Since the wiring pattern length between electronic elements is increased, the high-speed transmission of a signal may be obstructed.   A power element may not be arranged in the vicinity of a CPU, and the voltage drop of power may increase.       

     Therefore, there has been proposed a combined cooling system that combines a liquid cooling system using the cooling plate together with an air cooling system.  FIG. 1A  illustrates a conventional stand-alone device  90  provided with an air cooling system and a liquid cooling system. A plurality of CPU units  91  is mounted in the front side of the stand-alone device  90 , and fans  92  and  93  for the air cooling system are provided in the rear side of the stand-alone device  90 . 
       FIG. 2A  illustrates an arrangement of an air cooling system  94  and a liquid cooling system  80  in the CPU unit  91  mounted in the stand-alone device  90  of  FIG. 1A . A memory element  95  or a hiding CPU and an interface element are provided on a circuit board  96  of the CPU unit  91 . The memory element  95  is cooled by cooling air CW of the air cooling system  94 . The liquid cooling system  80  is provided with a cooling plate  83  configured to cool the CPU and a cooling plate  84  configured to cool the interface element, and the cooling plates are coupled to a refrigerant entrance  81  and a refrigerant exit  85  via a refrigerant pipe  82 . The refrigerant entrance  81  and the refrigerant exit  85  are coupled to the cooling device  30  using the refrigerant illustrated in  FIG. 1A . 
       FIG. 2B  illustrates the cooling operation of the air cooling system  94  and the liquid cooling system  80  in the CPU unit  91  of  FIG. 2A . In the air cooling system  94 , the memory element  95  mounted over the circuit board  96  is cooled by the cooling air CW. The refrigerant pipe  82  of the liquid cooling system  80  is arranged in a direction orthogonal to the flowing direction of the cooling air CW. The refrigerant pipe  82  coupled to the refrigerant entrance  81  is provided with a refrigerant supply pipe  82 A extending from one end to the other end of the circuit board  96 , and a refrigerant recovery pipe  82 B bent at the other end and returning to the refrigerant exit  85 . In this example, the refrigerant supply pipe  82 A and the refrigerant recovery pipe  82 B have two systems, respectively. 
     A plurality of cooling plates  83  configured to cool the CPUs and a plurality of cooling plates  84  configured to cool the interface element are installed at predetermined positions on the refrigerant supply pipe  82 A, whereas nothing is installed on the refrigerant recovery pipe  82 B. After sequentially flowing through the plurality of cooling plates  83  to cool the CPUs and then sequentially flowing through the plurality of cooling plates  84  to cool the interface elements through the refrigerant supply pipe  82 A, the refrigerant returns to the refrigerant exit  85  through the refrigerant recovery pipe  83 B. 
     Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-095902 
     Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-266247 
     However, the cooling system using both the air cooling system  94  and the liquid cooling system  80  has the following problems.
         A pipe arrangement that does not disturb the flow of cooling air is required, and, when the refrigerant pipe is arranged to float from the circuit board to keep off the cooling air, an optimum piping route for the refrigerant may not be secured.   As for the electronic component to be cooled by the air cooling system, since an arrangement considering a duct is required, optimum mounting may be obstructed.       

     SUMMARY 
     According to an aspect of the embodiments, an information processing apparatus includes: a cooling device configured to cool, by using refrigerant, heating components mounted over a circuit board and having different use temperature conditions, wherein the circuit board is provided with a first area in which a first group of heating components having an operating condition of generating heat less than a given heat quantity and operating in a temperature range lower than a first temperature is arranged, a second area in which a second group of heating components having an operating condition of generating heat equal to or more than the given heat quantity and operating in a temperature range between the first temperature and a second temperature exceeding the first temperature is arranged, and a third area in which a third group of heating components having an operating condition of generating heat equal to or less than the given heat quantity and operating in a temperature range exceeding the second temperature is arranged. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a perspective view illustrating a conventional stand-alone device provided with an air cooling system and a liquid cooling system; 
         FIG. 1B  is a perspective view illustrating an external appearance of an information processing apparatus according to the present disclosure; 
         FIG. 2A  is a perspective view illustrating an arrangement of an air cooling system and a liquid cooling system in a CPU unit mounted in the stand-alone device of  FIG. 1A ; 
         FIG. 2B  is a plan view illustrating the operation of the air cooling system and the liquid cooling system in the CPU unit illustrated in  FIG. 2A ; 
         FIG. 3A  is a perspective view illustrating an internal configuration of a CPU module mounted in the information processing apparatus of  FIG. 1B ; 
         FIG. 3B  is an assembled perspective view illustrating an arrangement of three CPUs and a liquid cooling system corresponding thereto in one CPU device mounted in the CPU module of  FIG. 3A ; 
         FIG. 4A  is a plan view illustrating a state after the assembly of the CPU device illustrated in  FIG. 3B ; 
         FIG. 4B  is a sectional view taken along line A-A of  FIG. 4A ; 
         FIG. 5A  is a system view illustrating the flow of refrigerant in the liquid cooling system mounted in the CPU device of  FIG. 4A ; 
         FIGS. 5B to 5E  are sectional views illustrating exemplary embodiments of sectional shapes of a refrigerant supply pipe and a refrigerant recovery pipe that may be used in the cooling system of the information processing apparatus according to the present disclosure; 
         FIG. 6  is a system view illustrating the flow of refrigerant in a liquid cooling system mounted in the CPU device equipped with four CPUs, and a connection with the refrigerant cooling device that supplies the refrigerant to the CPU device; 
         FIG. 7A  is a system view illustrating a structure of a liquid cooling system corresponding to another CPU arrangement in the CPU device of  FIG. 3A ; 
         FIG. 7B  is a system view illustrating a structure of a liquid cooling system corresponding to a further CPU arrangement in the CPU device of  FIG. 3A ; 
         FIG. 8A  is a system view illustrating a modified embodiment of piping in the liquid cooling system of  FIG. 6 ; 
         FIG. 8B  is a system view illustrating a modified embodiment of piping in the liquid cooling system of  FIG. 7A ; 
         FIG. 9A  is a system view illustrating a first exemplary embodiment of a refrigerant stirring mechanism in the refrigerant supply pipe and the refrigerant recovery pipe of the liquid cooling system illustrated in  FIG. 5 ; 
         FIG. 9B  is a partially enlarged view illustrating the portion B in  FIG. 9A ; 
         FIG. 9C  is a partially enlarged view illustrating a modified embodiment of the stirring structure according to the first exemplary embodiment; 
         FIG. 10A  is a system view illustrating a second exemplary embodiment of a refrigerant stirring mechanism of a refrigerant conduit in the liquid cooling system of  FIG. 5 ; 
         FIG. 10B  is a partially enlarged perspective view illustrating the portion C in  FIG. 10A ; and 
         FIG. 10C  is a partially enlarged perspective view illustrating a modified embodiment of the stirring structure according to the second exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, aspects of the present disclosure will be described in detail on the basis of specific exemplary embodiments with reference to drawings. In the description of exemplary embodiments described below, an optical communication device provided with an optical interface element, a CPU and a power element will be described as an information processing apparatus. However, the information processing apparatus is not limited to the optical communication device and the present disclosure may be applied to other information processing apparatuses than the optical communication device. 
       FIG. 1B  illustrates an external appearance of an information processing apparatus  10  according to an exemplary embodiment of the present disclosure, and illustrates a server device for an optical communication. In the information processing apparatus  10  of the present exemplary embodiment, a plurality of CPU modules  2  is provided in a rack  1 . In the present exemplary embodiment, all of the CPU modules  2  provided in the rack  1  of the information processing apparatus  10  according to the present exemplary embodiment is cooled by a liquid cooling system. However, the information processing apparatus  10  is not equipped with the cooling device for cooling the refrigerant of the liquid cooling system. The refrigerant cooling device is installed in another place and supplies the refrigerant to the plurality of CPU modules  2 . 
       FIG. 3A  illustrates the internal configuration of the CPU module  2  mounted in the information processing apparatus of  FIG. 1B . According to the present exemplary embodiment, four CPU devices  3  are installed in the CPU module  2 .  FIG. 3B  illustrates the internal configuration of the CPU device  3  illustrated in  FIG. 3A . Since the information processing apparatus  10  of the present exemplary embodiment is an optical communication device, optical interface elements  11 , CPUs  12 , and power elements  13  are disposed on a circuit board  14  installed in the CPU device  3 . 
     Here, the temperature use conditions of the optical interface elements  11 , the CPUs  12  and the power elements  13  installed in the information processing apparatus  10  for optical communication are considered. The optical interface elements  11  are low heating and low temperature components having a temperature use condition in which the heating range is 15W to 25W and the use temperature condition is 20° C. to 40° C. The CPUs  12  are high heating and middle temperature components having a temperature use condition in which the heating range is 200W to 300W and the use temperature condition is 20° C. to 60° C. Further, the power elements  13  are low heating and high temperature components having a temperature use condition in which the heating range is 15W to 25W and the use temperature condition is 20° C. to 80° C. 
     According to the present disclosure, the component mounting area of the circuit board  14  is divided into, for example, three (3) areas, namely, a first area R 1 , a second area R 2  and a third area R 3  in a row form in the longitudinal direction of the circuit board  14 . Further, electronic elements are grouped according to the temperature use conditions and each group is arranged in one of the three divided areas R 1  to R 3 . For example, the optical interface elements  11  may be arranged in the vicinity of the CPUs  12  so as to reduce the length of a signal line, thus enabling high-speed transmission. Further, the power elements  13  may be arranged in the vicinity of the CPUs so as to minimize a voltage drop by power feeding. 
     In view of the above use conditions, for example, in the present exemplary embodiment, three CPUs  12  are arranged in the second area R 2  that is located in the center of the circuit board  14 , and a plurality of interface elements  11  and power elements  13  are arranged, respectively, in the first and third areas R 1  and R 3  adjacent to the second area R 2 . Further, according to the present disclosure, the air cooling system is not used but refrigerant piping of the liquid cooling system  20  is used so as to cool the optical interface elements  11 , the CPUs  12 , and the power elements  13  which are mounted over the circuit board  14 . Hereinafter, the cooling structure using the refrigerant piping of the liquid cooling system  20  will be described. 
     The liquid cooling system  20  includes: a refrigerant supply pipe  22  provided with a refrigerant entrance  21 ; connection pipes  23  provided with cooling plates  24  configured to cool the CPUs  12 , on predetermined portions thereof; and a refrigerant recovery pipe  25  configured to return refrigerant, which has been returned from the cooling plates  24  through the connection pipes  23 , to a refrigerant exit  26 . The refrigerant entrance  21  and the refrigerant exit  26  are coupled to a cooling device configured to recover, cool, and circulate the refrigerant whose temperature has risen. The refrigerant supply pipe  22  is disposed immediately above the optical interface element  11  along the first area R 1  of the circuit board  14 . The cooling plates  24  are heat-exchange modules, and are disposed immediately above the CPUs  12  mounted over the second area R 2  of the circuit board  14 . The refrigerant recovery pipe  25  is disposed immediately above the power element  13  along the third area R 3  of the circuit board  14 . The connection pipe  23  connects the refrigerant supply pipe  22  to each of the cooling plates  24 , and connects each of the cooling plates  24  to the refrigerant recovery pipe  25 . 
       FIG. 4A  illustrates a state where the liquid cooling system  20  is placed on the circuit board  14  illustrated in  FIG. 3B . In the liquid cooling system  20  of  FIG. 3B , the refrigerant entrance  21  and the refrigerant exit  26  are arranged to be adjacent to each other. However, in the exemplary embodiment of  FIG. 4A , the refrigerant entrance  21  and the refrigerant exit  26  are located to be spaced apart from each other.  FIG. 4B  illustrates a sectional view taken along line A-A of  FIG. 4A , and  FIG. 5A  illustrates the direction of the refrigerant flowing in the refrigerant supply pipe  22 , the connection pipes  23 , the cooling plates  24 , and the refrigerant recovery pipe  25  illustrated in  FIG. 4A . 
     As illustrated in  FIG. 4B , a cooling plate  24  is disposed immediately above a CPU, and is joined to the CPU  12  using a thermal sheet  15 . A plurality of fins  27  protrudes inside the cooling plate  24  so as to improve heat exchange efficiency when the refrigerant flows in the cooling plate  24  in a meandering manner. Lower surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  disposed immediately above the optical interface element  11  and the power element  13  are formed as flat surfaces so as to efficiently perform heat exchange with the optical interface element  11  and the power element  13 . The optical interface element  11  and the bottom surface  22 B of the refrigerant supply pipe  22  are joined to each other using the thermal sheet  15 , and the power element  13  and the bottom surface  25 B of the refrigerant recovery pipe  25  are also joined to each other using the thermal sheet  15 . 
     The conventional refrigerant supply pipe and refrigerant recovery pipe have the function of merely conveying the refrigerant. According to the present disclosure, the bottom surface  22 B of the refrigerant supply pipe  22  is formed as a flat surface to be disposed on the optical interface element  11  via, for example, the thermal sheet  15 , grease, and a spring such that the heat generated by the optical interface element  11  is cooled by the refrigerant supply pipe  22 . Likewise, the bottom surface  25 B of the refrigerant recovery pipe  25  is formed as a flat surface to be disposed on the power element  13  via, for example, the thermal sheet  15 , the grease, and the spring such that the heat generated by the power element  13  is cooled by the refrigerant recovery pipe  25 . The refrigerant supply pipe  22  and the refrigerant recovery pipe  25  are made of a material having high heat conductivity. Thus, in the information processing apparatus of the present disclosure, the air cooling system is not required. Further, when the thickness of the thermal sheet is adjusted, the height difference between the heating component and the refrigerant supply pipe  22  or the refrigerant recovery pipe  25  may be absorbed 
       FIGS. 5B and 5C  illustrate the cross-sections of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  in which the bottom surfaces  22 B and  25 B are formed as flat surfaces as illustrated in  FIG. 4B . The flat surfaces formed on the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  may be aligned with the top surfaces of heating elements.  FIGS. 5D and 5E  illustrate the cross-sectional shapes of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  according to other exemplary embodiments, in which the bottom surfaces  22 B and  25 B applicable to the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  are formed as flat surfaces. 
     When the liquid cooling system  20  is configured as described above, the optical interface element  11  located immediately below the refrigerant supply pipe  22  is cooled by the refrigerant that is supplied from the refrigerant entrance  21  and flows in the refrigerant supply pipe  22 , as illustrated in  FIG. 5A . The CPUs  12  located immediately below the cooling plates  24  are cooled by the refrigerant which is split into three branches by the connection pipes  23  and then flows in the cooling plates  24 . In addition, the power elopements located immediately below the refrigerant recovery pipe  25  are cooled by the refrigerant which returns to the connection pipes  23 , flows in the refrigerant recovery pipe  25  and then returns to the refrigerant exit  26 . Although the temperature of the refrigerant flowing in the refrigerant recovery pipe  25  rises, the power elements  13  located immediately below the refrigerant recovery pipe  25  may be cooled by the refrigerant of which the temperature has risen since the power elements  13  are electronic components having a use condition of a low heating and high temperature range. 
       FIG. 6  illustrates the flow of refrigerant in the liquid cooling system  20  installed in a CPU device  3  equipped with four CPUs  12  and a connection with the refrigerant cooling device  30  that supplies the refrigerant to the CPU device  3 . As described above, the refrigerant cooling device  30  is installed in a housing  35  provided outside the information processing apparatus  10 , and discharges the refrigerant having a low temperature from an outlet port  31  so as to supply the refrigerant through a distribution pipe  32  to each of the liquid cooling systems  20  of a plurality of CPU devices  3  installed in the information processing apparatus  10 . The refrigerant of which the temperature has risen in in each CPU device  3  is collected through a return pipe  33  and then returns to an inlet port  34 . The refrigerant cooling device  30  cools the refrigerant in a main body and then discharges the refrigerant from the outlet port  31  again. 
       FIG. 7A  is a system view illustrating the structure of an exemplary embodiment of a liquid cooling system  20 A corresponding to an additional CPU arrangement in the CPU device  3  of  FIG. 3A . In the present exemplary embodiment, the circuit board  14  is divided into a plurality of areas in a row form in the longitudinal direction thereof. Unlike the above-mentioned exemplary embodiment, five (5) areas exist in the present exemplary embodiment. In the present exemplary embodiment, the central area of the circuit board  14  is a first area R 1  in which a plurality of optical interface elements are disposed, and second areas R 2  are provided on both sides of the first area R 1  in which three CPUs are disposed. Third areas R 3  are provided outside the respective second areas R 2 , and the power element is disposed in each of the third area R 3 . 
     In the case of the liquid cooling system  20 A illustrated in  FIG. 7A , a refrigerant supply pipe  22  is disposed in the first area which is provided at the center of the circuit board  14 , and cooling plates  24  corresponding to the number of CPUs are disposed on the opposite sides of the refrigerant supply pipe  22 . Each cooling plate  24  is coupled to the refrigerant supply pipe  22  via the connection pipes  23  so that the refrigerant discharged from each cooling plate  24  returns to the two refrigerant recovery pipes  25  disposed at both ends of the circuit board  14  via the connection pipe  23 . Although the refrigerant exits  26  provided at two places are not illustrated in the drawing, refrigerant discharged therefrom meets and returns to the refrigerant cooling device  30  illustrated in  FIG. 6 . 
       FIG. 7B  is a system view illustrating the structure of an exemplary embodiment of a liquid cooling system  20 B corresponding to another CPU arrangement in the CPU device  3  of  FIG. 3A . In the present exemplary embodiment, the circuit board  14  is divided into three areas in a row form in the longitudinal direction thereof, similarly to the above-described exemplary embodiment. However, the present exemplary embodiment is different from the above-described exemplary embodiment in that the second area R 2  is wide and six (6) CPUs are aligned in two rows in the second area R 2 . The positions of the first area R 1  and the third area R 3  with respect to the second area R 2  are the same as those of the above-described exemplary embodiment. The first area R 1  is on one side of the second area R 2 , while the third area R 3  is on the other side of the second area R 2 . 
     In the liquid cooling system  20 B of  FIG. 7B , connection pipes  23  are coupled to twelve (12) cooling plates  24  from a refrigerant supply pipe  22 , respectively, so that the refrigerant discharged from the cooling plates  24  returns to the refrigerant recovery pipe  25  via the respective connection pipes  23 . The present exemplary-embodiment is the same as the above-described exemplary embodiment in that the optical interface elements are cooled on the bottom surface of the refrigerant supply pipe  22 , the CPUs are cooled by the cooling plates  24 , and the power elements are cooled on the bottom surface of the refrigerant recovery pipe  25 . Reference numeral  32  denotes a refrigerant distribution pipe, and reference numeral  33  denotes a refrigerant return pipe. 
       FIG. 8A  is a system view illustrating a liquid cooling system  20 C according to a modified embodiment where the connection of the connection pipe  23  in the liquid cooling system  20  of  FIG. 6  is changed. In the liquid cooling system  20  of  FIG. 6 , the connection pipes  23  connect the cooling plates  24  to the refrigerant recovery pipe  25  at the shortest distance. Meanwhile, in the liquid cooling system  20 C of the modified embodiment illustrated in  FIG. 8A , the length of the connection pipes  23  connecting the cooling plates  24  to the refrigerant recovery pipe  25  is increased so that the connection pipes  23  are coupled to an upstream side of the refrigerant flow of the refrigerant recovery pipe  25 . Consequently, the flow of the refrigerant in the refrigerant recovery pipe  25  is increased and, thus, the refrigerant is capable of cooling more power elements as compared with those on the circuit board  14 . 
       FIG. 8B  is a system view illustrating a liquid cooling system  20 D according to a modified embodiment where the connection of the connection pipe  23  in the liquid cooling system  20 A illustrated in  FIG. 7A  is changed. In the liquid cooling system  20 A of  FIG. 7A , the connection pipes  23  connect the cooling plates  24  to the refrigerant recovery pipe  25  at the shortest distance. Meanwhile, in the liquid cooling system  20 D of  FIG. 8B  according to the modified embodiment, the length of the connection pipes  23  connecting the cooling plates  24  to the refrigerant recovery pipe  25  is increased so that the connection pipes  23  are coupled to the upstream side of the refrigerant flow of the refrigerant recovery pipe  25 . Consequently, the flow of the refrigerant in the refrigerant recovery pipe  25  is increased and, thus, the refrigerant is capable of cooling more power elements as compared with those on the circuit board  14 . 
       FIG. 9A  illustrates a first exemplary embodiment of a refrigerant stirring mechanism in the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  of the liquid cooling system  20  illustrated in  FIG. 5A . As described above, the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  are flat surfaces, and absorb the heat generated by the optical interface elements and the power elements. Thus, the temperature of the refrigerant flowing in the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  gradually rises. Here, the temperature of the refrigerant near to the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  becomes higher than the temperature of the refrigerant distant from the bottom surfaces  22 B and  25 B. Consequently, the cooling efficiency of the optical interface elements and the power elements by the refrigerant is deteriorated. 
     As illustrated in  FIG. 9A , the refrigerant is stirred at predetermined positions in each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  so as to lower the temperature of the refrigerant near to the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25 .  FIG. 9B  is a partially enlarged view illustrating the portion B in  FIG. 9A  in which a convex portion  28  is provided at a predetermined position in the conduit. When the convex portion  28  is provided at a predetermined position in the conduit in this manner, the refrigerant CM flowing in the conduit is stirred by the convex portion  28 . Thus, the temperature of the refrigerant CM in the conduit becomes uniform, and the temperature of the refrigerant CM near to the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  decreases. Consequently, the cooling efficiency of the optical interface element and the power element by the refrigerant CM is enhanced. In the first exemplary embodiment, the convex portion  28  is provided on one side in the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25 . However, as in the modified embodiment of  FIG. 9C , convex portions  28  may be provided on both sides in the conduit. The convex portions  28  are not limited to a particular shape. 
       FIG. 10A  illustrates a second exemplary embodiment of a refrigerant stirring mechanism in the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  of the liquid cooling system  20  illustrated in  FIG. 5A . In the first exemplary embodiment, the convex portions  28  are provided at predetermined positions in the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  to stir the refrigerant CM so as to lower the temperature of the refrigerant CM near to the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25 . Meanwhile, in the second exemplary embodiment of  FIG. 10A , throttle portions  29  are provided at predetermined positions in the conduit to reduce the sectional area of the flow path in the conduit. 
     When the sectional area of the flow path is reduced by the throttle portions  29 , the refrigerant CM is stirred when the refrigerant CM has passed through the throttle portions  29 . Thus, when the throttle portions  29  are provided at predetermined positions in the conduit, the refrigerant CM flowing in the conduit is stirred after passing through the throttle portions  29 . Thus, the temperature of the refrigerant CM in the conduit becomes uniform, and the temperature of the refrigerant CM near to the bottom surfaces  22 B and  25 B of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  decreases. Consequently, the cooling efficiency of the optical interface element and the power element by the refrigerant CM is enhanced. 
       FIG. 10B  is a partially enlarged view illustrating the portion C in  FIG. 10A . In this exemplary embodiment, the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  has a rectangular cross-section. In this case, each side of the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  is narrowed and then widened to the original shape.  FIG. 10C  illustrates a modified embodiment of the stirring structure of the second exemplary embodiment. In this modified embodiment, the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  also has a rectangular cross-section. According to present modified embodiment, respective sides of the conduit of each of the refrigerant supply pipe  22  and the refrigerant recovery pipe  25  are twisted in the same direction by 90° so as to form a throttle portion  29 . In the modified embodiment of the second exemplary embodiment, since the conduit is turned by 90° in addition to being narrowed, the refrigerant may be well stirred after passing through the throttle portion  29 . 
     The stirring structures illustrated in  FIGS. 9A to 9C  and  FIGS. 10A to 10C  are merely examples. In addition to such structures, a propeller may be installed in the conduit, and a fin causing a flow to be disturbed may be provided. In the refrigerant supply pipe  22 , the stirring structures may be located at the upstream side of the connection portion of each connection pipe  23  in the refrigerant supply pipe  22 , and, in the refrigerant recovery pipe  25 , the structures may be located at the downstream side of the connection portion of each connection pipe  23  in the refrigerant recovery pipe  25 . 
     As described above, according to the present disclosure, a piping space of a circuit board is reduced, thereby enabling the miniaturization and high-density mounting of the board. Since an optical interface element or a power element may be located in the vicinity of a CPU, a wiring may be shortened such that high-speed communication may be realized, a voltage drop may be reduced, and the number of power components may be reduced. In addition, since it is not necessary to consider air flow, the flexibility in arranging components may be improved such that the optimum mounting of the components is enabled. Furthermore, since refrigerant piping may also be used for a cooling function, the optimum mounting of cooling components may be realized with a simple arrangement. 
     Hereinbefore, the present disclosure has been described in detail with reference to the exemplary embodiments. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a illustrating of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention has (have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.