Abstract:
A refrigeration system that is capable of using carbon dioxide as a refrigerant and makes use of an evaporator that operates as a thermosyphon insensitive to orientation. The refrigeration system includes a condenser adapted to be wrapped around and physically contact a heat sink for conducting heat from a refrigerant within the condenser to the heat sink, a first line connected to the condenser through which the refrigerant is discharged from the condenser after being condensed to a liquid state, an evaporator coupled to the first fluid line and adapted for physical contact with a body so as to draw heat from the body to vaporize the refrigerant within the evaporator, and a second fluid line connected to the evaporator and through which the refrigerant is discharged from the evaporator after being vaporized and then delivered to the condenser. At least the evaporator is formed to have a multiport tube comprising a plurality of parallel passages with hydraulic diameters of less than 0.8 mm so as to enable refrigerant to be drawn into the passages regardless of orientations of the evaporator and the evaporator multiport tube.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/558,755, filed Apr. 1, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to refrigeration systems, and particularly refrigeration systems that employ a thermosyphon.  
         [0003]     Refrigeration and cooling systems have been proposed that employ a device known as a thermosyphon, which relies on thermodynamic properties to syphon fluid from one location to another. As with conventional refrigeration systems, a refrigeration system employing a thermosyphon generally requires a condenser where a refrigerant vapor is condensed to its liquid state and an evaporator where the refrigerant liquid is then evaporated, with the required heat of vaporization drawn from a body or space desired to be cooled. In a thermosyphon-based refrigeration system, at least the evaporator is in the form of a thermosyphon, by which the refrigerant liquid is drawn up into the evaporator via capillary action and thereby initiates the direction of flow of the refrigerant through the evaporator. Various heat sinks can be employed with the condenser to provide the required cooling, such as the heat acceptor of a Stirling engine.  
         [0004]     Refrigeration systems that employ a thermosyphon have the potential advantages of handling high and low heat flux conditions and lending themselves to cost efficient means of manufacturing. However, difficulties have been encountered when attempting to operate thermosyphon-based refrigeration systems with refrigerants other than conventional chlorofluorocarbon (CFC), such as when attempts are made to use carbon dioxide (CO 2 ) as the refrigerant to avoid the environmental concerns of CFC&#39;s. Furthermore, thermosyphons can be sensitive to orientation of the evaporator and condenser (horizontal or vertical), generally necessitating that the fluid lines coupled to the evaporator have different internal diameters. More particularly, the fluid line carrying the liquid refrigerant from the condenser to the evaporator is required to have a smaller internal diameter than the vapor line carrying the vaporized refrigerant from the evaporator to the condenser in order to create a pressure differential that will ensure the direction of refrigerant flow, namely, from the designated liquid inlet to the designated vapor outlet of the evaporator and from the designated vapor inlet to the designated liquid outlet of the condenser.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a refrigeration system capable of using carbon dioxide as a refrigerant and makes use of an evaporator that operates as a thermosyphon and is insensitive to orientation.  
         [0006]     The refrigeration system comprises a condenser configured to be wrapped around and physically contact a heat sink for conducting heat from a refrigerant within the condenser to the heat sink, a first line connected to the condenser through which the refrigerant is discharged from the condenser after being condensed to a liquid state, an evaporator coupled to the first fluid line and physically contacting a body for thermal communication therewith so as to draw heat from the body to vaporize the refrigerant within the evaporator, and a second fluid line connected to the evaporator and through which the refrigerant is discharged from the evaporator after being vaporized and then delivered to the condenser. The condenser comprises a condenser inlet manifold connected to the second fluid line, a condenser multiport tube comprising a plurality of parallel passages in fluidic communication with the condenser inlet manifold, and a condenser outlet manifold in fluidic communication with the parallel passages and connected to the first fluid line. The evaporator comprises an evaporator inlet manifold connected to the first fluid line, an evaporator multiport tube comprising a plurality of parallel passages in fluidic communication with the evaporator inlet manifold, and an evaporator outlet manifold in fluidic communication with the parallel passages of the evaporator multiport tube and connected to the second fluid line. According to a preferred aspect of the invention, the parallel passages of the evaporator multiport tube have hydraulic diameters of less than 0.8 mm so as to enable the refrigerant to be drawn into the parallel passages from the evaporator inlet manifold regardless of orientations of the evaporator and the evaporator multiport tube. In a particular embodiment of the invention, the first and second fluid lines have substantially equal and constant internal diameters, thereby permitting operation of the refrigeration system regardless of the flow direction of the refrigerant through the refrigeration system.  
         [0007]     In view of the above, the present invention provides a thermosyphon-based refrigeration system that can handle high and low heat flux conditions and lend itself for a cost efficient means of manufacturing, as well as operate insensitive to orientation (e.g., horizontal or vertical) of the evaporator and, optionally, the condenser. The refrigeration system can also have a modular and compact configuration that is advantageous for a variety of portable/stationary cooling applications, such as refrigeration cabinets. Using CO 2  as the refrigerant is environmentally friendly and eliminates the need for recycling of refrigerant when the final product reaches the end of its useful life.  
         [0008]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a perspective view of a refrigeration system in accordance with a first embodiment of this invention.  
         [0010]      FIG. 2  is a cross-sectional view of an evaporator manifold of the refrigeration system of  FIG. 1 .  
         [0011]      FIG. 3  is an end view of an evaporator multiport tube for use with refrigeration systems of this invention.  
         [0012]      FIGS. 4 and 5  are end and perspective views, respectively, of a condenser of the refrigeration system of  FIG. 1 .  
         [0013]      FIG. 6  is an end view of a condenser in accordance with an alternative embodiment of this invention.  
         [0014]      FIG. 7  is a perspective view of a refrigeration system in accordance with a second embodiment of this invention.  
         [0015]      FIG. 8  shows the refrigeration system of  FIG. 7  mounted on a tray for installation in a refrigeration cabinet in accordance with a preferred aspect of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIGS. 1 and 7  depict refrigeration systems  10  and  50  in accordance with this invention. With initial reference to  FIG. 1 , the refrigeration system  10  is shown comprising a condenser  12 , a liquid line  14  coupling the condenser  12  to an evaporator  16 , and a vapor line  18  coupling the evaporator  16  to the condenser  12 . The condenser  12  is configured for being thermally coupled to a heat sink, such as the heat acceptor  20  of a Stirling engine  22  as shown in  FIG. 8 . The evaporator  16  is configured to be thermally coupled to a body desired to be cooled, such as a “cold space”  82  of a refrigeration cabinet as shown in  FIG. 8 . In this manner, the refrigeration system  10  is adapted to transfer heat from a body to a heat sink through direct physically contact, as opposed to forced air or free convection, though such heat transfer mechanisms are also within the scope of this invention. While the invention will be described with reference to the use of a Stirling engine, other cooling devices could be used such as a Peltier-effect (thermoelectric) device.  
         [0017]     The refrigeration systems of this invention will also work with a variety of working fluids, which as used herein means all refrigerants capable of operating in liquid and gas (vapor) states within the refrigeration systems  10  and  50  and having the property of evaporating from liquid to vapor at temperatures lower than the required temperature of the space to be cooled. In practice, high vapor pressure fluids are believed to be preferred since higher vapor density allows for smaller vapor lines for a given vapor velocity. Furthermore, temperature distribution is extremely small since the liquid head is not a significant part of the system operating pressure. For most cooling applications, carbon dioxide (CO 2 ) is an excellent working fluid since it has all the above characteristics (at room temperature (about 25° C.), the system pressure is approximately 860 psi (about 60 bar)). In contrast, using a low vapor pressure fluid such as water would require an operating pressure of about 1.09 psi (about 0.075 bar) to operate the system at about 40° C., and a 100 mm liquid line would have a temperature differential of almost 3° C. just due to the pressure head of the column. Furthermore, most low pressure fluids freeze at relatively warm temperatures, thus forcing to run the system at higher temperatures than optimum.  
         [0018]     The condenser  12  and evaporator  16  shown in  FIG. 1  are both of a flat multiport tube design. In a preferred embodiment, the condenser  12  and evaporator  16  comprise multiport extruded (MPE) aluminum alloy tubes  24  and  26 , respectively, within which a plurality of parallel passages or ports  28  ( FIG. 3 ) are defined by the extrusion process. The condenser  12  of  FIG. 1  is represented in greater detail in  FIGS. 4 and 5  as comprising a fluidically parallel pair of MPE tubes  24 , each fluidically connected at one end to an inlet manifold  30  and at an opposite end to an outlet manifold  32 . Each manifold  30  and  32  is formed to have a slot  34  in which the adjacent ends of the tubes  24  are clamped, such as through the action of threaded fasteners (not shown). In this manner, the condenser  52  can be clamped around a heat sink (such as the heat acceptor  20  of the Stirling engine  22  of  FIG. 8 ) to provide intimate thermal contact with the heat sink. The inlet manifold  30  is shown in  FIG. 3  as being equipped with a charge port  36  through which the system  10  can be charged with the working fluid.  
         [0019]      FIG. 6  represents an alternative condenser design (and which is shown with the refrigeration system  50  of  FIG. 7 ). In  FIG. 6 , a condenser  52  is represented as comprising a single MPE tube  24  with its opposite ends fluidically connected to inlet and outlet manifolds  70  and  72 . The manifolds  70  and  72  are secured together, such as through the action of threaded fasteners (not shown), enabling the condenser  52  to be clamped around a heat sink (such as the heat acceptor  20  of the Stirling engine  22  of  FIG. 8 ). The condenser  52  has been shown to be superior to the condenser  12  of  FIG. 1  in terms of achieving a minimal temperature differential between the heat sink and the working fluid leaving the condenser  52 .  
         [0020]     The evaporator  16  of  FIG. 1  is represented as comprising a parallel pair of MPE tubes  26 , each fluidically connected at one end to an inlet manifold  38  and at an opposite end to an outlet manifold  40 . As shown in greater detail in  FIG. 2 , each manifold  38  and  40  is formed to have an internal channel  42  that fluidically communicates with the ends of the tubes  26 . The internal channel  42  is shown as having internal enhancements  44  that extend along the entire length of the channel  42  to facilitate flow of the working fluid along the length of each manifold  38  and  40  by acting as wicks (capillary action).  
         [0021]     The size of the tubes  24  and  26  will depend on the particular demands of the application as well as whether the tube  24  or  26  is installed with the condenser  12  or evaporator  16 , as evident from  FIG. 1 . In one example, the tubes  24  and  26  have widths of about 144 mm, thicknesses of about 2 mm, and contain one hundred twenty ports  28 . The flat surfaces of the tubes  24  and  26  promote thermal contact with their respect heat sink and cold space. According to the invention, each port  28  within at least the evaporator  16  (and optionally within the condenser  12 ) has a sufficiently small hydraulic diameter to enable the ports  28  to act as wicks (capillary action) to pick up the working fluid, thereby initiating the direction of flow of the working fluid through the tubes  24  and  26 . With particular reference to the evaporator  16 , and assuming the manifold represented in  FIG. 2  is the evaporator inlet manifold  38 , the small diameter ports  28  are able to draw the liquid working fluid from the manifold  38  and into the tube  26  entirely through capillary action. According to a preferred aspect of the invention, the ports  28  of at least the evaporator  16  have hydraulic diameters of less than 0.8 mm. When a heat load is applied to the evaporator  16  (such as the space to be cooled), vapor bubbles form in the liquid working fluid within the ports  28  of the evaporator  16 . It is believed that, as a result of the ports  28  having hydraulic diameters of less than 0.8 mm, vapor bubbles are prevented from flowing back through the liquid working fluid and are thereby forced to flow away from the liquid working fluid, i.e., toward the evaporator outlet manifold  40 , creating a siphoning affect that draws more liquid working fluid into the ports  28 . Ports with hydraulic diameters larger than 0.8 mm are believed to allow vapor bubbles to travel toward the evaporator inlet manifold  38 , interrupting the operation of the refrigeration system  10 . While the ports  28  of the condenser  12  also preferably have hydraulic diameters of less than 0.8 mm, larger hydraulic diameters are permissible in view of the fluid entering the condenser  12  being in the vapor instead of liquid state.  
         [0022]     Prior art thermosyphon refrigeration systems generally make use of liquid and vapor lines with different internal diameters, namely, the liquid line has a smaller internal diameter than the vapor line (large) to create a pressure differential to insure direction of flow in the evaporator (liquid inlet to vapor outlet) and in the condenser (vapor inlet to liquid outlet). In contrast, due to the wicking action in the small diameter ports  28  within the tubes  24  and  26 , it has been shown that the refrigeration system  10  of this invention is able to make use of liquid and vapor lines  14  and  18  that have substantially the same internal diameters along their entire lengths. As such, the refrigeration system  10  can operate in either direction, i.e., the flow of the working fluid within the system  10  can be intentionally reversed (e.g., based on the orientation of the evaporator  16 ) so that the line  14  (described as the liquid line with reference to  FIG. 1 ) carries vaporized working fluid from the evaporator  16  to the condenser  12 , and the line  18  (described as the vapor line with reference to  FIG. 1 ) carries the liquid working fluid from the evaporator  16  to the condenser  12 . By providing the liquid and vapor lines  14  and  18  with equal internal diameters, the refrigeration system  10  is insensitive to orientation because wicking of the working fluid through the condenser  12  and evaporator  16  occurs regardless of their orientation (horizontal and vertical). As such, refrigeration systems of this invention can be termed capillary loop thermosyphons. Lines  14  and  18  of equal size, and preferable a commonly available size, results in the refrigeration system  10  being less complex to manufacture.  
         [0023]     Because the condenser  12  and evaporator  16  can function horizontally or vertically and the working fluid can flow in either direction, depending on orientation, both lines  14  and  18  are preferably well insulated so that vapor bubbles do not form in the liquid line ( 14  or  18 , depending on flow direction). Such a condition would cause oscillation in the system (flow/no flow), which would adversely affect capacity. In addition, the condenser  12  is preferably well insulated to achieve the highest possible COP.  
         [0024]     The refrigeration system  10  operates ideally with approximately 20 to 40% liquid in the enclosed volume (defined by the combined internal volumes of the condenser  12 , evaporator  16 , and lines  14  and  18 ). Filling fractions are believed to be very important to the operation of the system  10 . A fill fraction of about 20 to 30% is preferred if the system  10  is operating below ambient conditions, while a fill fraction of about 30 to 40% is preferred if the system is operating above ambient applications. Another important aspect of the invention is to size the internal diameters of the liquid and vapor lines  14  and  18  to the smallest practical internal diameter suitable for the mass flow rate of the system  10 . Minimum internal diameters enable the system  10  to be less sensitive to insulation deficiencies, particularly for the liquid and vapor lines  14  and  18 .  
         [0025]     The refrigeration system  50  of  FIG. 7  differs in the construction of its condenser  52  and evaporator  56 , but is otherwise essentially identical to the system  10  of  FIG. 1 . As such, the refrigeration system  50  includes the condenser  52  and evaporator  56 , a liquid line  54  coupling a liquid-side manifold  72  of the condenser  52  to a liquid-side manifold  78  of the evaporator  56 , and a vapor line  58  coupling a vapor-side manifold  80  of the evaporator  56  to a vapor-side manifold  70  of the condenser  52 . As before, the condenser  52  is configured for being thermally coupled to a heat sink, such as the heat acceptor  20  of the Stirling engine  22  as shown in  FIG. 8 , and the evaporator  56  is configured to be thermally coupled to a body desired to be cooled, such as the cold space  82  ( FIG. 8 ) of a refrigeration cabinet. Furthermore, the condenser  52  and evaporator  56  are preferably both of a flat multiport tube design, such as the MPE aluminum alloy tubes  24  and  26  of the type shown in  FIG. 3 . As noted before, the condenser  52  of  FIG. 7  is configured in accordance with  FIG. 6 , while the evaporator  56  differs from that of  FIG. 1  by comprising multiple serpentine tubes  26 , each connected to the manifolds  78  and  80  so as to be in fluidic parallel and provide multiple passes for heat transfer to the body being cooled. This system  50  is shown in  FIG. 8  as installed on a tray  84  along with the Stirling engine  22  (e.g., 300 W) and a heat exchanger system  86  for transferring heat from the engine  22  to the environment. By mounting the system  50  on the tray  84 , the system  50  can be installed as a module into a refrigeration cabinet, such as a beverage refrigeration cabinet of the type commonly used to individually sell beverages in grocery and convenience stores.  
         [0026]     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the system could differ in appearance and construction from the embodiments shown in the drawings, and appropriate materials could be substituted for those noted. Furthermore, while insulation is not shown in the Figures, those skilled in the art will appreciate that insulation of all components of the condensers, evaporators, and fluid lines of the refrigeration systems is necessary for system performance. Therefore, the scope of the invention is to be limited only by the following claims.