CROSS REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
This application claims the benefit of U.S. Provisional Application No. 60/558,755, filed Apr. 1, 2004.
The present invention generally relates to refrigeration systems, and particularly refrigeration systems that employ a thermosyphon.
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.
- BRIEF SUMMARY OF THE INVENTION
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 (CO2) as the refrigerant to avoid the environmental concerns of CFC'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.
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.
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.
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 CO2 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.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of this invention will be better appreciated from the following detailed description.
FIG. 1 is a perspective view of a refrigeration system in accordance with a first embodiment of this invention.
FIG. 2 is a cross-sectional view of an evaporator manifold of the refrigeration system of FIG. 1.
FIG. 3 is an end view of an evaporator multiport tube for use with refrigeration systems of this invention.
FIGS. 4 and 5 are end and perspective views, respectively, of a condenser of the refrigeration system of FIG. 1.
FIG. 6 is an end view of a condenser in accordance with an alternative embodiment of this invention.
FIG. 7 is a perspective view of a refrigeration system in accordance with a second embodiment of this invention.
DETAILED DESCRIPTION OF THE INVENTION
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.
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.
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 (CO2) 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.
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.
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.
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).
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.
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.
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.
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.
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.
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.