Abstract:
A cooling technique involves: reducing a pressure of a cooling fluid to a subambient pressure at which the cooling fluid has a boiling temperature less than a temperature of a heat-generating structure; bringing the cooling fluid at the subambient pressure into thermal communication with the heat-generating structure, so that the coolant absorbs heat, boils and vaporizes; thereafter removing heat from the coolant so as to condense substantially all of the coolant to a liquid; and thereafter extracting a selected portion of the cooling fluid that has been cooled, the selected portion being a vapor that includes a non-condensable gas.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates in general to cooling techniques and, more particularly, to a method and apparatus for cooling a system which generates a substantial amount of heat.  
         BACKGROUND OF THE INVENTION  
         [0002]    Some types of electronic circuits use relatively little power, and produce little heat. Circuits of this type can usually be cooled satisfactorily through a passive approach, such as conduction cooling. In contrast, there are other circuits which consume large amounts of power, and produce large amounts of heat. One example is the circuitry used in a phased array antenna system.  
           [0003]    More specifically, a modern phased array antenna system can easily produce 25 to 30 kilowatts of heat, or even more, and thus requires about 25 to 30 kilowatts of cooling. Existing systems for cooling this type of circuitry utilize an active cooling approach, in which a fluid coolant is circulated. Existing cooling systems of this type will leak coolant at potential leakage sites, and leakage of coolant may be cause for the system to be shut down. A more recent approach, which can better handle newer circuitry that produces larger amounts of waste heat, involves a cooling system that uses boiling heat transfer, including a system where the pressure in the coolant loop is below the ambient pressure in order to promote boiling at lower temperatures. One advantage of this latter type of system is that, since the cooling loop is at a subambient pressure, the coolant does not have a tendency to leak out of the loop. Although existing units of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.  
           [0004]    For example, in the case of a subambient cooling system with a two-phase coolant, the coolant does not tend to leak out of the loop, but gases such as air from the ambient environment that may leak into the loop and become present in the coolant can decrease the cooling capability of the system. Existing systems of this type lack the capability, during system operation, to remove air that has leaked into the system&#39;s closed loop so as to ensure full capacity operation while eliminating the need to shut the system down for maintenance.  
         SUMMARY OF THE INVENTION  
         [0005]    From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for efficiently removing undesired gases from the coolant of a cooling system. One form of the invention involves: circulating through a flow loop a cooling fluid which includes a fluid coolant, the flow loop passing through heat-generating structure disposed in an environment having an ambient pressure; reducing a pressure of the cooling fluid at a selected location along the flow loop to a subambient pressure at which the cooling fluid has a boiling temperature less than a temperature of the heat-generating structure; bringing the cooling fluid at the subambient pressure into thermal communication with the heat-generating structure, so that the coolant boils and vaporizes to thereby absorb heat from the heat-generating structure; supplying the cooling fluid from the heat-generating structure to a device which removes heat from the coolant so as to condense substantially all of the coolant to a liquid; and thereafter extracting from the flow loop a selected portion of the cooling fluid that has been cooled by the device, the selected portion being a vapor that includes a non-condensable gas.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawing, which is a block diagram of an apparatus that includes a phased array antenna system, and an associated cooling arrangement which embodies aspects of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0007]    The drawing is a block diagram of an apparatus  10  which includes a phased array antenna system  12 . The antenna system  12  includes a plurality of identical modular parts that are commonly known as slats, two of which are depicted at  16  and  17 . A feature of the present invention involves techniques for cooling the slats  16  and  17 , so as to remove heat generated by electronic circuitry therein.  
         [0008]    The electronic circuitry within the antenna system  12  has a known configuration, and is therefore not illustrated and described here in detail. Instead, the circuitry is described only briefly here, to an extent which facilitates an understanding of the present invention. In particular, the antenna system  12  includes a two-dimensional array of not-illustrated antenna elements, each column of the antenna elements being provided on a respective one of the slats, including the slats  16  and  17 . Each slat includes separate and not-illustrated transmit/receive circuitry for each antenna element. It is the transmit/receive circuitry which generates most of the heat that needs to be withdrawn from the slats. The heat generated by the transmit/receive circuitry is shown diagrammatically in the drawing, for example by the arrows at  21  and  22 .  
         [0009]    Each of the slats  16  and  17  is configured so that the heat it generates is transferred to a tube  23  or  24  which extends through that slat. Each of the tubes  23  or  24  could alternatively be a channel or a passageway extending through the associated slat, instead of a physically separate tube. A fluid coolant flows through each of the tubes  23  and  24 . As discussed later, this fluid coolant is a two-phase coolant, which enters the slat in liquid form. Absorption of heat from the slat causes part or all of the liquid coolant to boil and vaporize, such that some or all of the coolant leaving the slats  16  and  17  is in its vapor phase. This departing coolant then flows successively through a heat exchanger  41 , a collection chamber  42 , a pump  46 , and a respective one of two orifices  47  and  48 , in order to again reach the inlet ends of the tubes  23  and  24 . The pump  46  causes the coolant to circulate around this endless loop. In the disclosed embodiment, the pump  46  consumes only about 0.5 kilowatts to 2.0 kilowatts of power.  
         [0010]    The orifices  47  and  48  facilitate proper partitioning of the coolant among the respective slats, and also help to create a large pressure drop between the output of the pump  46  and the tubes  23  and  24  in which the coolant vaporizes. It is possible for the orifices  47  and  48  to have the same size, or to have different sizes in order to partition the coolant in a proportional manner which facilities a desired cooling profile.  
         [0011]    Ambient air  56  is caused to flow through the heat exchanger  41 , for example by a not-illustrated fan of a known type. Alternatively, if the apparatus  10  was on a ship, the flow  56  could be ambient sea water. The heat exchanger  41  transfers heat from the coolant to the air flow  56 . The heat exchanger  41  thus cools the coolant, thereby causing most or all of the coolant which is in the vapor phase to condense back into its liquid phase.  
         [0012]    The liquid coolant exiting the heat exchanger  41  enters the collection chamber  42 . The pump  46  withdraws liquid coolant from the lower portion of the collection chamber  42 . An expansion reservoir  61  communicates with the conduit between the collection chamber  42  and the pump  46 . The expansion reservoir  61  is in turn coupled to a pressure controller  62 . In the disclosed embodiment, the pressure controller  62  is a vacuum pump. Since fluids typically take up more volume in their vapor phase than in their liquid phase, the expansion reservoir  61  is provided in order to take up the volume of liquid coolant that is displaced when some or all of the coolant in the system changes from its liquid phase to its vapor phase. The amount of coolant which is in its vapor phase can vary over time, due in part to the fact that the amount of heat being produced by the antenna system  12  will vary over time, as the antenna system operates in various operational modes.  
         [0013]    Typically, the ambient air pressure will be approximately that of atmospheric air, which at sea level is 14.7 pounds per square inch area (psia). In the portion of the cooling loop which is downstream of the orifices  47 - 48  and upstream of the pump  46 , the pressure controller  62  maintains the coolant at a subambient pressure, or in other words a pressure less than the ambient air pressure. In the disclosed embodiment, the pressure controller  62  maintains a subambient pressure within a range of about 2 psia to 8 psia, for example 3 psia.  
         [0014]    Turning now in more detail to the coolant, one highly efficient technique for removing heat from a surface is to boil and vaporize a liquid which is in contact with the surface. As the liquid vaporizes, it inherently absorbs heat. The amount of heat that can be absorbed per unit volume of a liquid is commonly known as the latent heat of vaporization of the liquid. The higher the latent heat of vaporization, the larger the amount of heat that can be absorbed per unit volume of liquid being vaporized.  
         [0015]    The coolant used in the disclosed embodiment is water. Water absorbs a substantial amount of heat as it vaporizes, and thus has a very high latent heat of vaporization. However, at atmospheric pressure of 14.7 psia, water boils at a temperature of 100° C. In order to provide suitable cooling for an electronic apparatus such as the phased array antenna system  12 , the coolant needs to boil at a temperature of approximately 60° C. When water is subjected to a subambient pressure of about 3 psia, its boiling temperature decreases to approximately 60° C. Thus, in the disclosed embodiment, the orifices  47  and  48  permit the coolant pressure downstream from them to be substantially less than the coolant pressure between the pump  46  and the orifices  47  and  48 . The pressure controller  62  maintains the water coolant at a pressure of approximately 3 psia along the portion of the loop which extends from the orifices  47  and  48  to the pump  46 , in particular through the tubes  23  and  24 , the heat exchanger  41 , and the collection chamber  42 .  
         [0016]    Water flowing from the pump  46  to the orifices  47  and  48  has a temperature of approximately 65° C. to 70° C., and a pressure in the range of approximately 15 psia to 100 psia. After passing through the orifices  47  and  48 , the water will still have a temperature of approximately 65° C. to 70° C., but will have a much lower pressure, in the range of about 2 psia to 8 psia. Due to this reduced pressure, some or all of the water will boil as it passes through and absorbs heat from the tubes  23  and  24 , and some or all of the water will thus vaporize. After exiting the slats  16  and  17 , the water vapor (and any remaining liquid water) will still have the reduced pressure of about 2 psia to 8 psia, but will have an increased temperature in the range of approximately 70° C. to 75° C.  
         [0017]    When this subambient coolant water reaches the heat exchanger  41 , heat will be transferred from the water to the forced air flow  56 . The air flow  56  has a temperature less than a specified maximum of 55° C., and typically has an ambient temperature below about 40° C. As heat is removed from the water coolant, any portion of the water which is in its vapor phase will condense, such that all of the coolant water will be in liquid form when it exits the heat exchanger  41  and enters the collection chamber  42 . This liquid will have a temperature of approximately 65° C. to 70° C., and will still be at the subambient pressure of approximately 2 psia to 8 psia. This liquid coolant will then flow through the pump  46 , and the pump will have the effect of increasing the pressure of the coolant water, to a value in the range of approximately 15 psia to 100 psia, as mentioned earlier.  
         [0018]    As mentioned above, the coolant used in the disclosed embodiment is water. However, it would alternatively be possible to use any of a variety of other coolants, including but not limited to methanol, a fluorinert, a mixture of water and methanol, or a mixture of water and ethylene glycol (WEGL). These alternative coolants each have a latent heat of vaporization less than that of water, which means that a larger volume of coolant must be flowing in order to obtain the same cooling effect that can be obtained with water. As one example, a fluorinert has a latent heat of vaporization which is typically about 5% of the latent heat of vaporization of water. Thus, in order for a fluorinert to achieve the same cooling effect as a given volume or flow rate of water, the volume or flow rate of the fluorinert would have to be approximately twenty times the given volume or flow rate of water.  
         [0019]    Despite the fact that these alternative coolants have a lower latent heat of vaporization than water, there are some applications where use of one of these other coolants can be advantageous, depending on various factors, including the amount of heat which needs to be dissipated. As one example, in an application where a pure water coolant may be subjected to low temperatures that might cause it to freeze when not in use, a mixture of water and ethylene glycol (WEGL) could be a more suitable coolant than pure water, even though the WEGL mixture has a latent heat of vaporization which is lower than that of pure water.  
         [0020]    Theoretically, the cooling loop discussed above should contain only coolant. As a practical matter, however, non-condensable gases such as external air may possibly leak into the cooling loop. Non-condensable gases can also originate from dissolved gases in the initial charge of liquid coolant, or in additional quantities of coolant added to the system from time to time to make up for coolant lost during normal operation. To the extent that non-condensable gases such as air accumulate within the system, they can significantly decrease the heat removal capability. Accordingly, the disclosed embodiment includes a reclamation section which is configured to remove non-condensable gases from the coolant. In more detail, the collection chamber  42  has an outlet  101  which is disposed above the highest permissible level for the liquid coolant within the chamber  42 . The outlet  101  is coupled to a pump  103 , which is selectively actuated and deactuated by a level switch  106 .  
         [0021]    The level switch  106  is disposed in the collection chamber  42  at approximately the level of the top surface of the liquid coolant in the lower portion of the chamber  42 . To the extent that non-condensable gases such as air may progressively leak into the system over time, they will take up a progressively increasing amount of room in the upper portion of the chamber  42 . As a result, the level of the liquid coolant in the lower portion of the collection chamber  42  will decrease, because the increasing amount of non-condensable gases will force some liquid coolant into the expansion reservoir  61 . When the top surface of the liquid coolant in the collection chamber  42  drops below the level switch  106 , the level switch  106  will activate the pump  103 . The pump  103  then withdraws a mixture of coolant vapor and non-condensable gases from the upper portion of the collection chamber  42 , while increasing the pressure of this mixture until it is higher than the ambient pressure.  
         [0022]    The mixture of coolant and non-condensable gases from the pump  103  then pass through a bypass valve  112 , which is discussed in more detail later, to an auxiliary heat exchanger  114 . Ambient air is caused to flow at  116  through the heat exchanger  114 , for example by a not-illustrated fan of a known type. Alternatively, if the apparatus  10  was on a ship, the flow  116  could be ambient sea water. The heat exchanger  114  transfers heat to the air flow  116  from the mixture of coolant and non-condensable gases, in order to condense substantially all coolant vapor in the mixture into liquid form, such that only the non-condensable gases remain.  
         [0023]    From the heat exchanger  14 , the vapor and liquid flow into a collection tank  126 . The tank  126  has a vent  128 , which provides fluid communication between the ambient environment and the upper portion of the tank. Due to the heat exchanger  14 , virtually all of the coolant will be in liquid form. Consequently, non-condensable gases such as air will exit the collection tank  126  through the vent  128 , but little or no coolant will be lost through the vent  128 . The gases exiting through the vent  128  will be saturated at the temperature of the tank  126 , which in turn will determine the required amount of make-up coolant needed for the system.  
         [0024]    The tank  126  also has an outlet  131  in a lower portion thereof, and the outlet  131  communicates through a reclamation fill valve  132  with the inlet to the pump  46 . The valve  132  is controlled by a level switch  134 , which is sensitive to the level of the liquid coolant within the tank  126 . When the top surface of the liquid coolant is respectively above and below the level switch  134 , the level switch  134  respectively opens and closes the valve  132 . As evident from the foregoing discussion, the pressure in the tank  126  is at or above ambient air pressure, and the pressure controller  62  maintains a subambient pressure at the inlet to the pump  46 . Consequently, when the valve  132  is open, the pressure differential on opposite sides of the valve  132  causes liquid coolant to readily flow from the tank  126  to the pump  46 . When the level of the top surface of the liquid coolant in the tank  126  drops below the level switch  134 , the level switch  134  closes the valve  132 .  
         [0025]    Turning now in more detail to the bypass valve  112 , the bypass valve  112  can be selectively operated in either of two operational modes. In one operational mode, the bypass valve  112  takes the mixture of coolant and non-condensable gases which it receives from the pump  103  and supplies this mixture to the heat exchanger  114 , in the manner discussed above. In the other mode of operation, the valve  112  takes the mixture which it receives from the pump  103  and supplies this mixture to a vent  141  that communicates with the ambient environment, such that all of the mixture is exhausted directly to the ambient environment, and none of the mixture reaches the heat exchanger  114 . The non-condensable gases in the collection chamber  42  are at 100% relative humidity, or in other words are saturated with respect to the coolant vapor. Where the ambient environment is humid, for example at 95% relative humidity, setting the bypass valve  112  to use the vent  141  results in a situation where the air leaking into the system is at 95% humidity, and the air expelled through the vent  141  is at 100% humidity. The difference of 5% relative humidity represents a very small volume of water being lost. There may be circumstances in which it is desirable to accept this relatively low rate of coolant loss, for example to permit use of the system even where the heat exchanger  114 , the level switch  134 , or the valve  132  is broken.  
         [0026]    In the disclosed embodiment, there is a not-illustrated sight glass, which is a vertical glass tube that is in fluid communication with the flow loop for the coolant. By looking at the level of coolant within the sight glass, a determination can be made of the extent to which the amount of coolant in the system has decreased, for example through loss of small amounts of coolant vapor through the vent  128  or the vent  141 . More liquid coolant can then be added to the system. Alternatively, it would be possible to calculate the required amount of make-up coolant with the aid of a psychometric chart, and with knowledge of the flow rate and temperature of the vapor-saturated gases leaving the tank  126  through the vent  128 . The provision of the heat exchanger  114  helps to convert as much of the coolant as possible to liquid form, thereby minimizing the amount of coolant lost through the vent  128 , which in turn reduces the amount of coolant which must be periodically added to replace lost coolant.  
         [0027]    The present invention provides a number of advantages. One such advantage is that non-condensable gases are removed from the coolant, through highly efficient separation of the non-condensable gases and the coolant, so as to avoid significant loss of coolant. This in turn reduces the amount of replacement coolant which must be periodically added to the system. Further, the efficient removal of the non-condensable gases ensures that the system continues to provide an optimum heat removal capability.  
         [0028]    Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.