Abstract:
An apparatus includes heat-generating structure disposed in an environment having an ambient pressure, and a cooling system for removing heat from the heat-generating structure. The cooling system includes a fluid coolant, structure which reduces a pressure of the coolant to a subambient pressure at which the coolant has a boiling temperature less than a temperature of the heat-generating structure; and structure which directs a flow of the liquid coolant at the subambient pressure so that it is brought into thermal communication with the heat-generating structure, the coolant then absorbing heat and changing to a vapor.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/192,891, filed Jul. 11, 2002, entitled “Method and Apparatus for Cooling With Coolant at a Subambient Pressure,” now U.S. Pat. No. 7,000,691 which issued on Feb. 21, 2006. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     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 
     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 convection 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. 
     More specifically, a modern phased array antenna system can easily produce 25 to 30 kilowatts of heat, or even more. One known approach for cooling this circuitry is to incorporate a refrigeration unit into the antenna system. However, suitable refrigeration units are large, heavy, and consume many kilowatts of power in order to provide adequate cooling. For example, a typical refrigeration unit may weigh about 200 pounds, and may consume about 25 to 30 kilowatts of power in order to provide about 25 to 30 kilowatts of cooling. Although refrigeration units of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
     In this regard, the size, weight and power consumption characteristics of these known refrigeration systems are all significantly larger than desirable for an apparatus such as a phased array antenna system. And given that there is an industry trend toward even greater power consumption and heat dissipation in phased array antenna systems, continued use of refrigeration-based cooling systems would involve refrigeration systems with even greater size, weight and power consumption, which is undesirable. 
     SUMMARY OF THE INVENTION 
     From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for efficiently cooling arrangements that generate substantial heat. According to the present invention, a method and apparatus are provided to address this need, and involve cooling of heat-generating structure disposed in an environment having an ambient pressure by: providing a fluid coolant; reducing a pressure of the coolant to a subambient pressure at which the coolant has a boiling temperature less than a temperature of the heat-generating structure; and bringing the coolant 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an apparatus which includes a phased array antenna system and an associated cooling arrangement that embodies aspects of the present invention; 
         FIG. 2  is a block diagram similar to  FIG. 1 , but showing an apparatus which is an alternative embodiment of the apparatus of  FIG. 1 ; and 
         FIG. 3  is a block diagram similar to  FIG. 1 , but showing an apparatus which is yet another alternative embodiment of the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  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. 
     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  FIG. 1 , for example by the arrows at  21  and  22 . 
     Each of the slats is configured so that the heat it generates is transferred to a tube  23  or  24  extending through that slat. Alternatively, the tube  23  or  24  could be a channel or passageway extending through the 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 , an expansion reservoir  42 , an air trap  43 , a pump  46 , and a respective one of two orifices  47  and  48 , in order to again to reach the inlet ends of the tubes  23  and  24 . The pump  46  causes the coolant to circulate around the endless loop shown in  FIG. 1 . In the embodiment of  FIG. 1 , the pump  46  consumes only about 0.5 kilowatts to 2.0 kilowatts of power. 
     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 facilitates a desired cooling profile. 
     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 seawater. The heat exchanger  41  transfers heat from the coolant to the air flow  56 . The heat exchanger  41  thus cools the coolant, thereby causing any portion of the coolant which is in the vapor phase to condense back into its liquid phase. 
     The liquid coolant exiting the heat exchanger  41  is supplied to the expansion reservoir  42 . Since fluids typically take up more volume in their vapor phase than in their liquid phase, the expansion reservoir  42  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 the 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. From the expansion reservoir  42 , liquid coolant flows to the air trap  43 . 
     Theoretically, the cooling loop shown in  FIG. 1  should contain only coolant. As a practical matter, however, external air may possibly leak into the cooling loop. When this occurs, air within the coolant circulates with the coolant, until it reaches the air trap  43 . The air trap  43  collects and retains the air. 
     The air trap  43  is operationally coupled to a pressure controller  51 , which is effectively a vacuum pump. In the portion of the cooling loop downstream of the orifices  47 - 48  and upstream of the pump  46 , the pressure controller  51  maintains the coolant at a subambient pressure, or in other words a pressure less than the ambient air pressure. Typically, the ambient air pressure will be that of atmospheric air, which at sea level is 14.7 pounds per square inch area (psia). In the event that the air trap  43  happens to collect some air from the cooling loop, the pressure controller  51  can remove this air from the air trap in association with its task of maintaining the coolant at a subambient pressure. 
     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. 
     The coolant used in the disclosed embodiment of  FIG. 1  is water. Water absorbs a substantial amount of heat as it vaporizes, and thus has a very high latent heat of vaporization. However, water boils at a temperature of 100° C. at atmospheric pressure of 14.7 psia. 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 the boiling temperature decreases to approximately 60° C. Thus, in the embodiment of  FIG. 1 , 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 air trap  43  and the pressure controller  51  maintain 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 , the expansion reservoir  42 , and the air trap  43 . 
     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 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, 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. 
     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 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 . 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 expansion reservoir  42  and the air trap  43  to the pump  46 . 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. 
     It will be noted that the embodiment of  FIG. 1  operates without any refrigeration system. In the context of high-power electronic circuitry, such as that utilized in the phased array antenna system  12 , the absence of a refrigeration system can result in a very significant reduction in the size, weight, and power consumption of the structure provided to cool the antenna system. 
     The system of  FIG. 1  is capable of cooling something from a temperature greater than that of ambient air or seawater to a temperature closer to that of ambient air or seawater. However, in the absence of a refrigeration system, the system of  FIG. 1  cannot cool something to a temperature less than that of the ambient air or sea water. Thus, while the disclosed cooling system is very advantageous for certain applications such as cooling the phased array antenna system shown at  12  in  FIG. 1 , it is not suitable for use in some other applications, such as the typical home or commercial air conditioning system that needs to be able to cool a room to a temperature less than the temperature of ambient air or water. 
     As mentioned above, the coolant used in the embodiment of  FIG. 1  is water. However, it would alternatively be possible to use 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 20 times the given volume or flow rate of water. 
     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 could be a more suitable coolant than pure water, even though the mixture has a latent heat of vaporization lower than that of pure water. 
       FIG. 2  is a block diagram of an apparatus  110  which is an alternative embodiment of the apparatus  10  of  FIG. 1 . Except for certain specific differences discussed below, the apparatus  110  of  FIG. 2  is effectively identical to the apparatus  10  of  FIG. 1 , and identical parts are identified with the same reference numerals. 
     The apparatus  110  of  FIG. 2  is configured for use in an aircraft, such as a reconnaissance plane or a military fighter jet. The aircraft would have an environmental control unit (ECU)  113 , and the ECU  113  would include a refrigeration system of a known type, which is provided within the plane for other purposes, and which causes a known polyalphaolefin (PAO) refrigerant to flow through a loop. In the embodiment of  FIG. 1 , the heat exchanger  41  transfers heat to a forced flow of air  56 . In the embodiment of  FIG. 2 , a portion of the PAO refrigerant from the refrigeration system of the ECU  113  is routed to the heat exchanger  41 . The heat exchanger  41  removes heat from the subambient water which cools the slat, and transfers this heat to the PAO refrigerant. 
       FIG. 3  is a block diagram of an apparatus  210  which is yet another alternative embodiment of the apparatus  10  of  FIG. 1 . Except for certain specific differences discussed below, the apparatus  210  of  FIG. 3  is effectively identical to the apparatus  10  of  FIG. 1 , and identical parts are identified with the same reference numerals. 
     The apparatus  210  of  FIG. 3  includes a phased array antenna system  212  having a plurality of slats, two of which are shown at  216  and  217 . The apparatus  210  of  FIG. 3  differs from the apparatus  10  of  FIG. 1  in that the slats  216 - 217  of  FIG. 3  have an internal configuration which is different from the internal configuration of the slats  16 - 17  of  FIG. 1 . 
     More specifically, each of the slats in the antenna system  212  has a spray chamber, for example as shown diagrammatically at  218  and  219  for the slats  216  and  217 . One side of each spray chamber is defined by a surface  221  or  222 , and heat  21 - 22  generated by the circuitry within the slats is supplied to the surface  221  or  222  of each slat for dissipation. Incoming coolant enters tubes  223  and  224 , which each have therealong a plurality of orifices that are oriented to spray coolant onto the associated surface  221  or  222 . The spray is shown diagrammatically in  FIG. 3 , for example at  226  and  227 . 
     When the coolant spray  226  and  227  contacts the associated surface  221  or  222 , it absorbs heat and then boils, and some or all the coolant vaporizes. The resulting vapor, along with any remaining liquid coolant, then exits the spray chamber  218  or  219  through a respective outlet conduit  228  or  229 . The pressure controller  51  ensures that coolant in the spray chambers  218  and  219  is at a subambient pressure which reduces the boiling point of the coolant, in the same manner as described above for the embodiment of  FIG. 1 . 
     Although the present invention has been disclosed in the context of a phased array antenna system, it will be recognized that it can be utilized in a variety of other contexts, including but not limited to a power converter assembly, or certain types of directed energy weapon (DEW) systems. 
     The present invention provides a number of technical advantages. One such technical advantage is that, through the use of a two-phase coolant at a subambient pressure, heat-generating structure such as a phased array antenna system can be efficiently cooled. A related advantage is that it is possible to effect cooling in this manner without any refrigeration system, thereby substantially reducing the weight, size and power consumption of the structure which effects cooling. In the context of a state-of-the-art phased array antenna system, the absence of a refrigeration system can reduce the system weight by approximately 200 pounds, and can reduce the system power consumption by 25 to 30 kilowatts, or more. In the absence of a refrigeration system, power consumption for cooling is basically limited to the power which is supplied to the pump in order to circulate the coolant, and the pump consumes only about 0.5 kilowatts to 2.0 kilowatts. 
     The cooling techniques according to the invention are particularly advantageous in a phased array antenna system, due in part to the use of a two-phase coolant. In particular, it is desirable that all of the circuitry in a phased array antenna system operate at substantially the same temperature, because temperature variations or gradients across the array can introduce unwanted phase shifts into signal components that are being transmitted or received, which in turn degrades the accuracy of the antenna system. The maximum permissible size for such temperature gradients decreases progressively as the antenna is operated at progressively higher frequencies. 
     In pre-existing systems, which use a single-phase coolant, temperature gradients are common, due in part to the fact that the coolant becomes progressively warmer as it moves across the array and absorbs progressively more heat. In contrast, since the invention uses a two-phase coolant that effects cooling primarily by virtue of the heat absorption which occurs as a result of coolant vaporization, and since vaporization occurs at a very precise and specific temperature for a given coolant pressure, the cooling effect is extremely uniform throughout the phased array antenna system, and is thus highly effective in minimizing temperature gradients. 
     Although selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from spirit and scope of the present invention, as defined by the following claims.