Abstract:
A system for providing coolant to a heat load includes a first coolant reservoir providing coolant to the heat load and a second coolant reservoir receiving used coolant after passing through the heat load. The used coolant is refreshed by a cooling apparatus which receives the used coolant from the second coolant reservoir, cools the used coolant, and supplies refreshed coolant to said first coolant reservoir. The resulting dual reservoir system offers significant reductions in the size, weight and power of the vapor cycle system (VCS) equipment while providing for accurate temperature control of the coolant delivered to the heat load.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to a coolant providing system having thermal reservoirs for reducing the requisite system power and size, and, more specifically, to a vapor cycle system (VCS) which provides apparatus and methods for providing a coolant to a heat load. The present invention is especially beneficial for applications having intermittent heat loads which require a high degree of accurate temperature control.  
           [0002]    Solid state lasers are known to use various cooling devices to prevent a thermal overload. In many applications, solid state lasers require a precisely controlled inlet coolant temperature. A high power solid state laser could potentially require in excess of 100 tons instantaneous cooling during laser firing. A conventional system would require significant space and weight consumption while also being extremely power intensive.  
           [0003]    U.S. Pat. No. 5,608,748 discloses a cooling liquid flowing from a reservoir, through the laser cavity, and back to the reservoir. The coolant is chilled within the reservoir with a cooling element. Precision cooling, however, is difficult, as the spent coolant is returned to the reservoir and mixed with the supply coolant. A large reservoir and/or a high power cooling element is required to approach the achievement of a suitable precision thermal control.  
           [0004]    U.S. Pat. No. 4,850,201 discloses a cooling liquid flowing from a reservoir, through the heat load (such as an industrial laser machine or an injection molding machine for plastic), and back to the reservoir. The disclosure describes controlling overcooling of the coolant by optionally removing a portion of the coolant from the loop and warming the liquid in a heat exchanger until the overcooling situation is corrected. In order to maintain precision thermal control, especially when used for intermittent high heat loads, a large reservoir and high cooling power is required.  
           [0005]    As can be seen, there is a need for an improved apparatus and method for a cooling system that provides precision thermally controlled coolant to a heat load. Furthermore, there exists a need to provide such a cooling system which is neither reliant upon an extraordinarily large coolant reservoir nor a large power supply.  
         SUMMARY OF THE INVENTION  
         [0006]    In one aspect of the present invention, a system for providing coolant to a heat load comprises a first coolant reservoir providing coolant to the heat load; a second coolant reservoir receiving used coolant after passing through the heat load; and a cooling means for receiving the used coolant from the second coolant reservoir, cooling the used coolant, and supplying coolant to the first coolant reservoir.  
           [0007]    In another aspect of the present invention, a method for providing coolant to a heat load comprises providing a first coolant reservoir and a second coolant reservoir; chilling coolant in the first coolant reservoir to provide a usable coolant; passing the usable coolant from the first coolant reservoir through the heat load to the second coolant reservoir, the usable coolant becoming used coolant after passing through the heat load; and passing the used coolant from the second coolant reservoir, through a cooling means, back to the first coolant reservoir, the used coolant becoming usable coolant after passing through the cooling means.  
           [0008]    In another aspect of the present invention, a vapor cycle system for cooling a laser heat load comprises a first coolant reservoir; a second coolant reservoir; a heat load coolant loop circulating coolant from the first coolant reservoir, to the laser heat load, and returning used coolant to the second coolant reservoir; a cooling means; and an evaporator coolant loop circulating used coolant from the second coolant reservoir, through the cooling means, cooling the coolant, and supplying coolant to the first coolant reservoir.  
           [0009]    In another aspect of the present invention, a vapor cycle system for cooling a laser heat load comprises a first water reservoir for storing coolant chilled to a predetermined temperature; a second water reservoir for receiving used coolant; a heat load coolant loop communicating the first water reservoir with the laser heat load, and the laser heat load with the second water reservoir; a first pump circulating coolant through the laser heat load in the heat load coolant loop; cooling means, the cooling means having a condenser, an evaporator, and at least one VCS pack; an evaporator coolant loop communicating the second water reservoir with the cooling means, and the cooling means with the first water reservoir; a second pump circulating coolant through the cooling means in the evaporator coolant loop, whereby the used coolant is chilled to the predetermined temperature and returned to the first water reservoir.  
           [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The FIGURE is a schematic diagram showing the VCS of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0013]    In general, the present invention is a coolant providing system having at least two thermal reservoirs. More particularly, the present invention relates to a vapor cycle system having a cold coolant reservoir and a hot coolant reservoir. Additionally, the present invention provides a method for providing a coolant to a heat load. The present invention is especially beneficial for applications having intermittent heat loads, such as solid state lasers, which require a high degree of accurate temperature control. In these cases the Vapor Cycle System Pack can be sized for the average heat load rather than the instantaneous heat load, resulting in a significant reduction in the size and weight of the Vapor Cycle System Pack. The lower the duty cycle of the heat load the greater the reduction in the size and weight of the pack.  
         [0014]    In conventional vapor cycle cooling systems, using a circulating liquid as the coolant, precision cooling, especially for intermittent high heat loads, is inadequate. Moreover, such conventional systems require a large coolant reservoir, require a high power cooling element, and/or have a considerable mass occupying a large volume.  
         [0015]    Referring to the FIGURE, a VCS system  52  may have a cold water reservoir  54  and a hot water reservoir  56 . Cold water reservoir  54  and hot water reservoir  56  may be thermally insulated reservoirs, thereby permitting minimal heat exchange between the stored coolant water and the environment. A cold water output line  58  may bring cold water from cold water reservoir  54  to a heat load. In one embodiment of the present invention, the heat load may include a first laser heat load  10  and a second laser heat load  12 . A first pump  14  may carry the used coolant, via a hot water reservoir input line  16 , to hot water reservoir  56 . A valve V 2  may control the flow of coolant through this heat load water loop as shown by the dotted-lined arrows.  
         [0016]    The coolant in hot water reservoir  56  may be driven, via a second pump  24 , through a hot water reservoir output line  18 , through cooling system  20 , returning to cold water reservoir  54  via a cold water reservoir input line  22 . Cooling system  20  may include a condenser  26 , an evaporator  28 , and a sufficient number of VCS packs  30  to provide for the average (not instantaneous) heat removal requirements. A valve V 3  may control the flow of coolant through this evaporator water loop as shown by the solid arrows.  
         [0017]    A temperature sensor  32  may be provided in cold water reservoir  54  for monitoring the coolant temperature and precisely controlling the inlet coolant to the correct temperature via a controls loop (not shown). A cooldown loop output line  34  circulates the coolant, via hot water reservoir output line  18 , through cooling system  20  as necessary to maintain the desired coolant output temperature. A valve V 1  may be provided in cooldown loop output line  34  to appropriately regulate the flow of coolant through cooldown loop output line  34 .  
         [0018]    A diaphragm  36  may be provided within each of cold water reservoir  54  and hot water reservoir  56 . Air contained within the coolant significantly limits the heat capacity of the coolant. Diaphragm  36  is designed to limit the amount of air contained in the coolant by isolating the coolant from the air at the head of the coolant reservoirs. Preferably, air is delivered via air pressure line  38  to supply adequate pressure from diaphragm  36  onto the surface of the coolant in the reservoirs.  
         [0019]    In one alternate embodiment of the present invention, at least one of pumps  14  and  24  may be removed. Pressure on the coolant via diaphragms  36  would then be used to move the coolant through VCS system  52 . Coolant pressure may be adjusted appropriately by regulating valves  42  and  44 .  
       EXAMPLE  
       [0020]    Referring still to the FIGURE, one embodiment of the present invention, uses VCS system  52  to provide precise thermal control to a first laser heat load  10  and a second laser heat load  12 . First laser heat load  10  requires a coolant controlled input temperature of approximately 40° F. The coolant output from first laser heat load  10  is fed to second laser heat load  12 .  
         [0021]    Initially, the cold water reservoir  54  is filled with about 170 lbs of ambient temperature water. While less water may be used in this example, excess water is preferred so that there is no chance of the system running dry. In this “cool down” operating condition, valve V 1  is opened and pump  24  feeds the ambient temperature water through cooldown loop output line  34 , hot water reservoir output line  18 , and cooling system  20 . The output coolant, having the precisely controlled inlet temperature, returns to cold water reservoir  54  via cold water reservoir input line  22 . Preferably, water flows through the system during this initial cool down phase at a flow rate of X lbm/sec, where X is a flow capable of achieving the desired cooling effect. When temperature sensor  32  detects the cold water reservoir coolant temperature to be controlled to the desired temperature, the VCS system  52  is operational and ready to cool a heat load.  
         [0022]    During the “heat load on” stage, valve V 1  is closed. Suppose the duty cycle (ratio of laser on-time to total cycle time) is 33 percent. Valve V 2  is opened and pump  14  feeds the coolant water, at a flow rate of 3X lbm/sec, from cold water reservoir  54  into first laser heat load  10  at the requisite controlled input temperature. The output coolant from first laser heat load  10 , is fed into second laser heat load  12 . The output coolant from second laser heat load  12 , flows, via hot water reservoir input line  16  to hot water reservoir  56 .  
         [0023]    At the same time, valve V 3  is opened and pump  24  feeds warm coolant from hot water reservoir  56  through hot water reservoir output line  18  and cooling system  20  to return chilled water, via cold water reservoir input line  22 , to cold water reservoir  54 . This evaporator water loop circulates at a flow rate of X lbm/sec. Thus, water is removed from cold water reservoir  54  at a net rate of 2X lbm/sec and water is added to hot water reservoir at a net rate of 2X lbm/sec. At the end of, say, a four second laser firing cycle, cold water reservoir  54  contains 170-8X lbm of water and hot water reservoir  56  contains 8X lbm of high temperature water.  
         [0024]    During the “heat load off” stage, valve V 2  is closed and warm coolant is removed from hot water reservoir  56  at a flow rate of X lbm/sec. The warm coolant is fed from hot water reservoir  56  through hot water reservoir output line  18  and cooling system  20  to return water, via cold water reservoir input line  22 , to cold water reservoir  54 . At the end of the cycle, hot water reservoir  56  contains no water and cold water reservoir  54  contains 170 lbs. of water. The water in cold water reservoir  54  is ready to act as coolant for another laser firing sequence.  
         [0025]    The table below summarized the above operations:  
                                                           Heat Load   Evaporator                   Water Loop   Water Loop       Operating   Valve   Pump   Flow   Flow       Condition   Positions   Operations   (lbm/sec)   (lbm/sec)                   Cool Down   V 1  open                       V 2  closed   Pump 24 on   0.0   X           V 3  closed   Pump 14 off       Heat Load On   V 1  closed           V 2  open   Pump 24 on   3X   X           V 3  open   Pump 14 on       Heat Load Off   V 1  closed           V 2  closed   Pump 24 on   0.0   X           V 3  open   Pump 14 off                  
 
         [0026]    In the above example, the average heat load requirements using the VCS system of the present invention, having water in a cold and hot reservoir for thermal storage and precise temperature control, is about one-third of the requisite instantaneous heat load at the laser.  
         [0027]    In the above example, water is used as the coolant. The present invention is not limited to water, as any conventional coolant may be used so long as it does not effect the operation of the heat load. When a laser is the heat load, water or a water/alcohol mixture is preferred. In a laser system, a coolant that has a different index of refraction may result in undesired effects to the laser pulse. However, in other systems, such as cooling systems for mission control avionics or wing-embedded sensors, any coolant may be used. For example, polyalphaolefin (PAO) is useful for its good dielectric properties as well as its low freezing point.  
         [0028]    The number of VCS packs required to cool the water is a function of the time interval between laser firings (off time) and the heat removal capacity of the VCS Pack. The longer the off time, the lower the average heat load and the fewer required VCS packs. Alternatively, the water reservoir weight penalty can be reduced by using a larger capacity VCS Pack.  
         [0029]    Further variations are within the scope of the present invention. For example, the heat load may be any heat source in need of cooling wherein cooling can be effected through a flowing liquid coolant. For example, the heat load may be an aviation-related heat load, such as mission control avionics or wing-embedded sensors. Other machines requiring cooling, such as injection molding machines, may also benefit from the VCS system of the present invention. The cooling system is not limited to using VCS packs, and may be any cooling means capable of cooling a flowing liquid coolant.  
         [0030]    The above example describes a VCS system using one cold water reservoir and one hot water reservoir. However, the present invention is not intended to be limited to such an embodiment. Any number of cold water reservoirs and any number of hot water reservoirs may prove useful in a VCS system of the present invention, depending on the desired functionality. For example, a plurality of cold and hot water reservoirs may be employed to create the most efficient use of available space and tubing.  
         [0031]    The above example describes a VCS system for cooling a first and a second laser heat load in series. However, the present invention is not intended to be limited to such an embodiment. Any number of head loads, either in series or in parallel, may be cooled by the cooling system of the present invention.  
         [0032]    The vapor cycle system of the present invention, having cold and hot thermal reservoirs, offers significant reductions in the size, weight and power of the VCS equipment while providing for accurate temperature control of the coolant delivered to the heat load. The cold water reservoir stores thermally controlled coolant, having it immediately available for a heat load. The hot water reservoir receives the used coolant, allowing the coolant to pass through the coolant cooling means before being returned to the cold water reservoir. Such a system is especially useful in systems having a high intermittent heat load, such as high powered solid state lasers.  
         [0033]    It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.