Patent Publication Number: US-2007095087-A1

Title: Vapor compression cooling system for cooling electronics

Description:
FIELD OF THE INVENTION  
      This invention relates to cooling systems for electronics, and more particularly to vapor compression cooling systems for cooling at least one microprocessor.  
     BACKGROUND OF THE INVENTION  
      There is currently an ever-increasing demand to improve the processing speed, power, and memory of electronic devices, such as desktop computers, laptop or portable computers, hand-held computers, cellular phones, and such, while decreasing the overall size and weight of such devices. To this end, more powerful microprocessors are constantly being developed in smaller and smaller packages, but with increasing demands for heat rejection to remove the heat generated from the increased processing power and speed. To overcome challanges associated with heat rejection, a number of active cooling systems have been proposed for cooling microprocessors, and while many of these systems may prove adequate for this intended use, there is always room for improvement.  
     SUMMARY OF THE INVENTION  
      According to one feature of the invention, a vapor compression cooling system is provided for cooling at least one microprocessor. The cooling system includes a compressor to pressurize a refrigerant used in the cooling system, a condenser to condense pressurized refrigerant received from the compressor, an expansion device to expand pressurized refrigerant received from the condenser, and a cold plate. The cold plate includes a surface that mates with a heat rejecting surface of a corresponding microprocessor, and an evaporator to receive expanded refrigerant from the expansion device, transfer heat from the corresponding microprocessor to the expanded refrigerant, and return heated refrigerant back to the system with a quality of less than 100%. The cooling system further includes a suction line heat exchanger to receive heated refrigerant from the evaporator at a quality of less than 100% and transfer heat from the pressurized refrigerant to the heated refrigerant to provide refrigerant at a quality of at least 100% back to the compressor.  
      According to one feature, the suction line heat exchanger is located downstream from the condenser with respect to the refrigerant flow through the system to receive the pressurized refrigerant from the condenser.  
      In one feature, the suction line heat exchanger is located upstream from the condenser with respect to the refrigerant flow through the system to deliver the pressurized refrigerant to the condenser.  
      In accordance with one feature, the cooling system includes another cold plate including a surface that mates with a heat rejecting surface of a corresponding microprocessor and an evaporator to receive expanded refrigerant from the expansion device, transfer heat from the corresponding microprocessor to the expanded refrigerant, and return heated refrigerant back to the system with a quality of less than 100%.  
      According to one feature of the invention, a method is provided for operating a vapor compression cooling system to cool at least one microprocessor. The method includes the steps of compressing a refrigerant to provide pressurized refrigerant to the system, condensing the pressurized refrigerant to provide condensed refrigerant to the system, expanding the condensed refrigerant to provide cooled refrigerant to the system, transferring heat from a microprocessor to the cooled refrigerant to provide heated refrigerant with a quality of less than 100% to the system, and transferring additional heat from the pressurized refrigerant to the heated refrigerant to provide refrigerant with a quality of at least 100% to the system for use in the step of compressing.  
      In accordance with one feature of the invention, a method is provided for operating a vapor compression cooling system to cool at least one microprocessor. The method includes the steps of compressing a refrigerant to provide pressurized refrigerant to the system, condensing the pressurized refrigerant to provide condensed refrigerant to the system, expanding the condensed refrigerant to provide cooled refrigerant to the system, transferring heat from a microprocessor to the cooled refrigerant to provide heated refrigerant with a quality of less than 100% to the system, and transferring additional heat from the condensed refrigerant to the heated refrigerant to provide refrigerant with a quality of at least 100% to the system for use in the step of compressing.  
      Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagrammatic representation of a cooling system embodying the present invention;  
       FIG. 2  is a refrigerant pressure vs. enthalpy diagram representing operation of the system of  FIG. 1 ;  
       FIG. 3  is a diagrammatic representation of another vapor compression cooling system embodying the present invention; and  
       FIG. 4  is a refrigerant pressure vs. enthalpy diagram representing operation of the system of  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      With reference to  FIG. 1 , a vapor compression cooling system and method  10  are provided for cooling one or more microprocessors, such as the microprocessor shown schematically at  12  and the microprocessor shown schematically at  14 . The cooling system utilizes a suitable refrigerant, such as R 134   a , which is circulated through the cooling system via pressurization provided by a compressor  16 , which is preferably an electric motor driven compressor with speed control. The cooling system further includes a condenser  18 , an expansion device  20 , a pair of cold plates  22  and  24 , each associated with a corresponding one of the microprocessor  12  and  14 , and a suction line heat exchanger (SLHX)  26 . Each of the cold plates  22  and  24  includes a surface  28  and  30  that mates with the corresponding microprocessor  12  and  14 , and an evaporator, shown schematically at  32  and  34 , that receives heat generated by the corresponding microprocessor  12 ,  14 . One typical footprint for a microprocessor  12  or  14  would be a rectangular footprint with a width of 13 mm and a length of 9 mm, with a heat rejection of 175 watts. While each of the above components of the system  10  must be correctly sized for the required cooling of one or more of the microprocessors  12 ,  14 , which will typically mean miniaturization in comparison to conventional household or automotive type vapor compression cooling systems, there are many known and suitable forms for each of the components and the details of such components will depend greatly upon the parameters of each particular application for which the cooling system  10  is used. Accordingly, further details of the precise construction and/or form of each of the components will not be given herein.  
      Reference herein will be made to the “quality of the refrigerant” or just the “quality”. Quality is as conventionally defined, namely, the weight ratio % of the mass of refrigerant in the vapor phase to the total mass of the refrigerant, i.e., the combined mass of liquid refrigerant and vapor refrigerant, at a given point in the system. Thus, refrigerant wholly in the vapor phase will have a quality of 100%, while refrigerant wholly in the liquid phase will have a quality of 0%. Refrigerant that is both in the liquid and vapor phase will have a quality greater than 0% and less than 100%, the exact number being determined by the ratio of refrigerant vapor to total refrigerant.  
      The system  10  is designed to operate such that the quality of the refrigerant exiting the evaporator(s)  32 ,  34  is less than 100% so as to maximize the cooling ability of the cold plate(s)  22 , 24 , i.e., to avoid dry-out of the evaporator(s)  32 , 34 . The suction line heat exchanger  26  is provided to protect the compressor  16  by increasing the quality of the refrigerant from the evaporator to at least 100% (and preferably in a superheated state) so as to provide vapor phase refrigerant to the compressor  16 . As used herein, the phrase “a quality of at least 100%” is intended to mean that the refrigerant is at 100% quality or is in a superheated vapor state.  
      In operation, the compressor  16  compresses the refrigerant to provide pressurized refrigerant to the system  10 , as shown schematically by the line  40 . The condenser  18  receives the pressurized refrigerant from the compressor  20  and transfers heat to a coolant flow  36  (preferably an air flow) so as to provide condensed refrigerant to the system, as shown schematically by the lines  42 . The expansion device  20  expands the condensed, pressurized refrigerant received from the condenser  18  to provide cooled refrigerant to the system, as shown schematically by the lines  44 . The cooled refrigerant is directed to the evaporator(s)  32 , 34  wherein heat is transferred from the microprocessor(s)  12 , 14  to the cooled refrigerant to provide heated refrigerant with a quality of less than 100% to the system, as shown schematically by the lines  46 . The heated refrigerant is directed to the suction line heat exchanger  26  wherein heat is transferred from the condensed refrigerant (which has also been directed to the suction line heat exchanger  26  downstream from the condenser  18  and upstream from the expansion device  20 ) to the heated refrigerant so as to provide refrigerant at 100% quality back to the compressor  16 , as shown schematically by the lines  48 .  
      The pressure and enthalpy of the refrigerant as it moves through the system is illustrated in  FIG. 2  (assumes a ideal system) with the points A, A′; B, B′; C, C′: D, D′; E, E′; and F, F′ on the diagram corresponding to the like lettered locations in the system  10  of  FIG. 1 .  
      As a working example of the system  10  of  FIGS. 1 and 2 , the refrigerant provided from the expansion device  20  to the evaporator(s)  32 , 34  could have an entrance quality of around 16.2% and the refrigerant provided from the evaporator(s)  32 , 34  to the system could have an exit quality of around 65%, with the refrigerant provided from the suction line heat exchanger  26  back to the compressor  16  having a super heat of approximately 5° C.  
      The vapor compression cooling system  10  of  FIG. 3  is similar to the system  10  of  FIG. 1  with like numbers indicating like components, except for the location of the suction line heat exchanger  26 , which is shown located upstream of the condenser  18  in  FIG. 3  rather than downstream from the condenser  18  such as in the system  10  of  FIG. 1 . As with the system  10  of  FIG. 1 , the system  10  of  FIG. 3  is designed to operate such that the quality of the refrigerant exiting the evaporator(s)  32 ,  34  is less than 100% so as to maximize the cooling ability of the cold plate(s)  22 , 24 , i.e., to avoid dry-out of the evaporator(s)  32 , 34 . The suction line heat exchanger  26  is provided to protect the compressor  16  by increasing the quality of the refrigerant from the evaporator to 100% so as to provide vapor phase refrigerant to the compressor  16 .  
      Accordingly, for the system  10  of  FIG. 3 , the suction line heat exchanger  26  receives pressurized refrigerant from the compressor  16 , as shown schematically by the line  50 , and transfers heat from the pressurized refrigerant to the heated refrigerant received from the evaporator(s)  32 , 34  to provide refrigerant at 100% quality back to the compressor  16 , such as shown schematically by the line  52 . The pressurized refrigerant is then directed from the suction line heat exchanger  26  to the condenser  18 , such as shown schematically by the line  53 , so the condenser  18  can transfer heat from the pressurized refrigerant to a coolant flow  36 , such as an air flow, to provide condensed, pressurized refrigerant to the system  10 , such as shown schematically by the lines  54 . The expansion device  20  receives the condensed, pressurized refrigerant from the condenser  18  and expands the refrigerant to provide cooled refrigerant to the system  10 , such as shown schematically by the lines  36 . The expanded, cooled refrigerant is directed to the evaporator(s)  32 , 34  wherein heat is transferred from the microprocessor(s)  12 , 14  to the refrigerant to provide heated refrigerant back to the system with a quality of less than 100%, as shown schematically by the lines  58 .  
      Again,  FIG. 4  is a pressure-enthalpy diagram for the refrigerant as it passes through the system  10  of  FIG. 3  (assuming ideal system), with the letters A, A′; B, B′; C, C′; D, D′; E, E′; and F, F′ in the diagram of  FIG. 4  corresponding to the like lettered locations in the system  10  of  FIG. 3 .  
      As a working example of the system  10  of  FIGS. 3 and 4 , the refrigerant provided from the expansion device  20  to the evaporator(s)  32 , 34  could have an entrance quality of around 34.1% and the refrigerant provided from the evaporator(s)  32 , 34  to the system could have an exit quality of around 65%, with the refrigerant provided from the suction line heat exchanger  26  back to the compressor  16  having a super heat of approximately 5° C.  
      While there are many possible control schemes that could be utilized in the systems  10  of  FIGS. 1-4  to insure that the exit quality from the evaporator(s)  32 , 34  is less than 100%, in one preferred form, the speed of the compressor is controlled via the speed of an electric compressor drive motor (not shown) using any suitable motor speed control in response to selected system parameters that are monitored using suitable sensors or probes (not shown). For example, the exit temperatures of the refrigerant at points A and D in  FIGS. 1 and 2 , and points A and C in  FIGS. 3 and 4  could be monitored and compared to suitable set points that would yield the desired exit quality from the evaporator(s)  32 , 34 , with the speed of the compressor being increased or decreased to maintain the monitored temperatures within a suitable range of the set points. By way of another example, a temperature sensor  60  could be used to sense the exit temperature from the low pressure side of the suction line heat exchanger  26  with the sensed temperature then being used to control a thermal expansion valve, when the expansion device  20  is provided in that form. In such a system, it may be preferred to include a receiver in the high pressure flow path between the condenser  36  and the suction line heat exchanger  26 , with the potential for this receiver to be an integrated portion of the condenser  36 . As yet another example, when the expansion device  20  is provided in the form of a fixed orifice, it may be desirable for the suction line heat exchanger  26  to include a liquid accumulation function.  
      While the systems  10  of  FIGS. 1-4  have been shown herein as having two cold plates  22  and  24 , each cooling a corresponding one of the microprocessors  12  and  14 , it should be understood that the systems  10  could be configured with only a single cold plate or more than two cold plates, and that any given cold plate could be dedicated to cooling a single microprocessor or multiple microprocessors.  
      It should be appreciated that by providing a suction line heat exchanger  26  in a vapor compression cooling system  10  wherein the exit quality from the evaporator(s)  32 , 34  of the cooling plate(s)  22 , 24  is always maintained at less than 100%, the system  10  can provide optimal cooling of the microprocessor(s)  12 , 14  while protecting the compressor  16  from damage by providing refrigerant to the compressor with a quality of 100%.