Abstract:
A refrigeration system having a primary refrigerant path including a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein; and a bypass path coupled to an outlet of the condenser. The bypass path includes a secondary expansion device; and a heat exchanger thermally coupled to the primary refrigerant path between the condenser outlet and the primary expansion device inlet to remove heat from the refrigerant discharged from the condenser. The condenser is downsized such that lacks the heat transfer capacity to provide some or all of the required subcooling as provided according to conventional practice, and the heat exchanger provides some or all the required subcooling according to the capacity of the condenser. A pressure differential accommodating device operative to mix two vapors at different pressures may also be provided to connect the outlets of the evaporator and the heat exchanger to an inlet of the compressor. A method of operating a refrigeration system with a downsized condenser and an a bypass path including a heat exchanger to provide subcooling is also described.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/426,073, filed Nov. 11, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a high efficiency refrigeration system and more specifically, to a refrigeration system utilizing a bypass path for subcooling, in combination with selection of the sizes of the condenser, compressor and evaporator, to achieve increased overall system efficiency.  
         [0004]     2. Relevant Art  
         [0005]      FIG. 1  is a block diagram of a conventional refrigeration system, generally denoted at  10 . The system includes a compressor  12 , a condenser  14 , an expansion device  16  and an evaporator  18 . These components are connected together, typically by copper tubing such as indicated at  19  to form a closed loop system through which a refrigerant such as R-12, R-22, R-134a, R-407c, R-410a, ammonia, carbon dioxide or natural gas is cycled.  
         [0006]     The main steps in the refrigeration cycle are compression of the refrigerant by compressor  12 , heat extraction from the refrigerant to the environment by condenser  14 , throttling of the refrigerant in the expansion device  16 , and heat absorption by the refrigerant from the space being cooled in evaporator  18 . This process, sometimes referred to as a vapor-compression refrigeration cycle, is used in air-conditioning systems, which cool and dehumidify air in residential, commercial and industrial environments, in a moving vehicle (e.g., automobile, airplane, train, etc.), in refrigeration equipment, in heat pumps and in other applications.  
         [0007]     In the condenser  14 , heat is removed from the refrigerant so that the superheated refrigerant vapor from the compressor  12  becomes liquid refrigerant by the time it reaches the exit of the condenser. In  FIG. 1 , the condenser  14  is divided into two parts,  14   a  and  14   b . In the first portion,  14   a , superheated refrigerant vapor becomes saturated vapor, a process called desuperheating, and the saturated vapor undergoes phase change from vapor to liquid refrigerant. In the second portion,  14   b , the liquid refrigerant is further cooled below the saturation temperature at the condenser pressure, a process known as subcooling.  
         [0008]      FIG. 2  shows the temperature profiles inside the condenser. During the desuperheating process (from point A to point B), there is a rapid temperature drop. During the vapor-to-liquid phase change (from point B to point C), the temperature of the refrigerant remains constant. At the end of the condensation process (point C), 100 percent liquid refrigerant is present. The temperature of the liquid refrigerant is further decreased during the subcooling process (point C to point D) in the second portion of the condenser  14   b . In the subcooling process, the temperature difference between the refrigerant and cooling medium (e.g., air or water) decreases such that subcooling becomes an increasingly inefficient heat transfer process. Hence, for a given cooling capacity, one needs to have a relatively large-sized condenser to counteract the inefficient heat transfer which results from the small temperature difference.  
         [0009]     As is known, the required cooling capacity dictates the size of the evaporator, and this dictates the compressor capacity. While a larger compressor gives better cooling performance, cost and energy consumption must also be taken into account. Moreover, since the heat removal capacity of the condenser must equal the heat input due to the operation of the evaporator and the compressor, increasing the size of the compressor for a given cooling capacity means the condenser must be larger and more costly.  
         [0010]     Thus, a compromise is necessary, and according to conventional practice, in a a so-called optimized or balanced system, there is an accepted relationship between system cooling capacity (evaporator size) and compressor capacity. For example, in a conventional 1 ton system, the evaporator is designed to remove 12 KBTU/Hr. and the matched compressor size is 4 KBTU/Hr. The condenser must therefore be sized to handle 16 KBTU/Hr.  
         [0011]     Much effort has been directed find ways to improve the efficiency, size and cost of refrigeration systems. Because of the inefficiency of heat transfer during subcooling, this aspect of the refrigeration cycle has received considerable attention, but up to now, no suitably cost effective technique has been found to reduce the size of the subcooling section in the condenser or to eliminate it altogether.  
         [0012]     For example, it has been proposed to divert a portion of the high-pressure refrigerant exiting the condenser to expand through a secondary expansion device into a bypass circuit, and to employ the resulting cold refrigerant in a heat exchanger to subcool the main stream of the high-pressure refrigerant. The pressure at the bypass circuit is maintained to be the same as the pressure at the evaporator. Such an arrangement is shown in Kita et al. U.S. Pat. No. 6,164,086.  FIG. 3  shows a schematic diagram of a system of this kind.  
         [0013]     Kita et al. also propose an arrangement in which all the refrigerant is diverted to the bypass path, and when the refrigerant flows through the bypass, the main expansion valve in the main refrigeration path is shut off. The purpose of diverting the refrigerant to the bypass line is to produce ice in a heat storage container so that the ice can be used for subcooling the refrigerant. (Kita et al. use the term “supercooling” for the subcooling process.) To meet the normal subcooling operational requirements in Kita et al., the bypass line is shut off, and the main expansion valve is opened. Then, all of the refrigerant flows through the container filled with ice, and as the ice removes heat from the refrigerant, the refrigerant is subcooled. The subcooled refrigerant then flows to through the main expansion device and eventually to the evaporator.  
         [0014]     Kita et al., however, appear to suggest that their bypass methods are beneficial only for mixed (nonazetropic) refrigerant systems such as R-32/134a or R-407c due to the temperature gradient in the dual-phase region. For a single refrigerant (azeotripic) system such as R-22 or R-134a, the bypass method does not produce a temperature reduction at the inlet of the evaporator.  
         [0015]     Kim and Domanski, in Intracycle Evaporative Cooling in a Vapor Compression Cycle (NISTIR 5873), also investigated the first of the bypass methods described above, which they referred to as their “Method 2”. In addition, they also considered another method, referred to by them as “Method 1”, which is similar to a conventional liquid-line/suction line heat exchange where superheated vapor is used to subcool the high-pressure liquid, but which uses a liquid-vapor mixture from the evaporator instead of superheated vapor. This is shown schematically in  FIG. 4  herein.  
         [0016]     In neither instance, did they find beneficial results for a single refrigerant system, but with their first method, they did find some improvement for nonazetropic refrigerants. For their second method (the first method of the Kita et al. patent), however, they found no improvement with mixed or single refrigerant systems.  
         [0017]     Moreover, the reported improvements with mixed refrigerants are small, and in any case, are of limited current practical interest because mixed refrigerants are not in commercial use, and can not be used in current systems because they require higher pressure capability than systems using single refrigerants.  
         [0018]     An approach similar to the second method taught by Kita et al. appears to have been used in very large systems (e.g., 2,000 tons) but is of questionable use in small and intermediate size systems (less than about 1,000 tons).  
         [0019]     Cho and Bai, in U.S. Pat. No. 6,449,964, demonstrate a method and use of mixed refrigerant systems with higher bypass circuit pressure. They have also shown the use of a pressure differential accommodating device to mix the two vapors at two different pressures.  
         [0020]     Therefore, a need clearly still exists for a cost-effective way to achieve the subcooling without having a large subcooling section in the condenser, especially using current single refrigerants, and in systems having both small and large cooling capacities. The present invention seeks to meet this need.  
       SUMMARY OF THE INVENTION  
       [0021]     According to the present invention, it has been found that significant improvements can be obtained using a bypass circuit for subcooling in a system in which the conventional balanced or optimized relationship between the evaporator, compressor and condenser is abandoned, and in which a condenser is used which would not provide sufficient heat removal capacity according to normal practice. In other words, in an optimized system, the required capacity determines the evaporator size which then dictates the compressor size, and the heat input of these together dictate the condenser size. In contrast, according to the present invention, after the evaporator size has been determined, the condenser size is “de-optimized” by reducing or eliminating the subcooling capacity, and providing the lost subcooling through use of a heat exchanger driven by refrigerant diverted into a bypass circuit, e.g., from the main expansion valve. This allows use of smaller compressor, with consequent improved EER and system cost. The smaller condenser also reduces space requirements for the system.  
         [0022]     This surprising ability to achieve improved performance beyond that thought possible using bypass technology comes about because in a balanced system, the condenser is already large enough, and the system cannot utilize the additional subcooling. However, in a refrigeration system like the present invention where the condenser is substantially smaller than the optimum-sized condenser, the bypass method is able to show significant benefit as the increased subcooling makes the small condenser behave like an optimum-sized condenser or an oversized condenser. This increases both the cooling capacity and EER significantly.  
         [0023]     Similarly, the invention allows the evaporator to be made substantially larger than the optimum-sized evaporator, and the heat absorption is increased accordingly. Then, the bypass method is able to demonstrate significant benefit as the increased subcooling makes the proportionally smaller condenser behave like an optimum-sized condenser or an oversized condenser. In such an embodiment of the present invention, the condenser pressure is maintained constant despite the increased heat absorption at the evaporator, thus increasing both the cooling capacity and EER without increasing compressor work.  
         [0024]     Broadly stated, according to this invention, a portion of the liquid refrigerant exiting the condenser is diverted into a bypass line from which it is re-injected into the primary refrigerant path at a location between the evaporator outlet and compressor inlet. In the bypass line, a secondary expansion valve is used to throttle the liquid refrigerant that was diverted from the condenser, thus decreasing its temperature substantially below the condenser outlet temperature.  
         [0025]     The cooled refrigerant exiting the secondary expansion valve then passes through the heat exchanger which is thermally coupled to the primary refrigerant line between the condenser outlet and the primary expansion device inlet. The heat exchanger removes heat from the refrigerant vapor exiting the condenser, thus reducing its temperature. As a result, the refrigerant enters the primary expansion device at a substantially lower temperature than its saturation temperature. In other words, the level of subcooling is increased significantly, by 10-15 degree Celsius for example. Moreover this is achieved without devoting any portion of the condenser to subcooling.  
         [0026]     Because the refrigerant pressure in the bypass line at the outlet of the heat exchanger is greater than the pressure at the evaporator outlet, a pressure differential accommodating device is used at the intersection of bypass line outlet and the primary refrigerant line. A pressure differential accommodating device can be either a vacuum generating device or a pressure reducing device.  
         [0027]     According to a first aspect of the invention, there is provided a refrigeration system including refrigerant compressing means, refrigerant condensing means, expansion means and evaporation means connected together to form a closed-loop system with a refrigerant circulating therein, and a bypass line attached between the outlet of the condensing means and the inlet of the expansion means, the bypass line including a secondary expansion means, heat exchanging means to remove heat from the discharge liquid refrigerant from the condenser between the outlet of the condensing means and an inlet of the expansion means, and a pressure differential accommodating means for mixing two vapors at different pressures connecting the outlets of the evaporation means and the heat exchanging means to an inlet of the compressing means.  
         [0028]     According to a second aspect of the invention, there is provided a refrigeration system comprised of a primary refrigerant path including a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein, and a bypass line attached between the outlet of the condenser and the inlet of the compressor, the bypass line including a heat exchanger thermally coupled to the primary refrigerant path between the condenser outlet and the primary expansion device inlet to remove heat from the discharge vapor from the compressor, and a pressure differential accommodating device for mixing two vapors at two different pressures connecting the outlets of the evaporator and the heat exchanger to an inlet of the compressor.  
         [0029]     Further according to the second aspect of the invention, the pressure differential accommodating device may be a vacuum generating device with no moving parts such as a venturi tube, or a so-called “vortex tube” which is conventionally used to create two fluid steams of differing temperature from a single high pressure input stream.  
         [0030]     Also according to the second aspect of the invention, the pressure differential accommodating device may be a pressure reducing device with no moving parts such as a capillary tube, a restricted orifice, a valve, or a porous plug. The pressure reducing device is used in the bypass line which is maintained at a higher pressure than the evaporator. The pressure reducing device equalizes the pressure between the bypass line and the evaporator outlet, and includes suitable tubing connections to permit mixing of the pressure-equalized vapors before return to the compressor inlet.  
         [0031]     According to a third aspect of the invention, there is provided a method of increasing the efficiency of a refrigeration system comprised of a primary refrigerant path including a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein, the method comprising the steps of bypassing a portion of the refrigerant exiting the condenser into a secondary refrigerant line, passing the bypassed refrigerant through a heat exchanger thermally coupled to the primary refrigerant path between the condenser outlet and the primary expansion device inlet to remove heat from the discharge liquid refrigerant from the condenser, and passing the refrigerant exiting the heat exchanger and the refrigerant exiting the evaporator through a pressure differential accommodating device that mixes two vapors at different pressures and feeding the refrigerant exiting the pressure differential accommodating device to an inlet of the compressor.  
         [0032]     Providing a bypass path for performing subcooling makes the condenser more efficient, thereby reducing the condenser pressure, a phenomenon which decreases the pressure lift at compressor, and thus reduces the compressor work. Correspondingly, because subcooling does not have to be done inside the condenser, the condenser can be substantially smaller and becomes materially more efficient and cost-effective. The increased subcooling increases the amount of liquid refrigerant after the throttling process through the primary expansion valve. Thus, the heat absorption at the evaporator (often referred as the cooling capacity) increases.  
         [0033]     The above-described benefits of the subcooling bypass are achieved with diversion of 5-15% of the liquid refrigerant outflow from the condenser. At this level, reduced compressor work and increased cooling capacity are achieved. Since the EER (energy efficiency ratio) is defined as the ratio of the cooling capacity to compressor work, this increases the EER.  
         [0034]     According to a fourth aspect of the invention, when more than 15%, for example, 30%, of the liquid refrigerant from the condenser is diverted to the bypass path, the cooling capacity is reduced due to the substantial decrease in the refrigerant mass flow rate circulating through the evaporator. By use of an adjustable valve in the bypass path, the bypass mass flow rate, and thus, the cooling capacity, can be varied according to the thermal load, whereby it is possible to operate an air conditioning or refrigeration system without frequent, highly energy-inefficient, ON-OFF operations of the compressor. This results in an improved long-term seasonal energy efficiency ratio (SEER).  
         [0035]     According to a fifth aspect of the invention, multiple evaporators can be employed, e.g., in a zoned cooling system. Thus, several small evaporators could be provided for separate rooms, with one condenser and one compressor. When all the rooms require cooling, the system can be operated with a 5% bypass rate to provide the maximum cooling capacity and the maximum efficiency. If the thermal load decreases, as when fewer rooms need to be cooled, the bypass rate can be increased to reduce the cooling capacity without the need to cycle the compressor on and off. This is quite beneficial because the repeated ON-OFF cycling of the compressor is a very energy-inefficient process.  
         [0036]     In further contrast to conventional techniques, the concepts of this invention are applicable to conventional single-refrigerant systems, and also to mixed-refrigerant systems using a combination of refrigerants selected to provide the desired combination of thermal and flammability characteristics. Such mixed-refrigerant systems may also include regenerative features which provide higher evaporator efficiency by increasing the percentage of liquid in the refrigerant as it enters the evaporator. Regenerative mixed refrigerant systems are disclosed, for example, in U.S. Pat. Nos. 6,250,086 and 6,293,108, the contents of which are hereby incorporated by reference.  
         [0037]     According to a further aspect of the invention, even further reduction of condenser size can be achieved by employing the bypass circuit for de-superheating, as well as for subcooling. Use of de-superheating bypass is disclosed in my pending U.S. patent application Ser. No. 10/253,000, filed Sep. 23, 2002 (Atty Docket 3474-21), the contents of which are hereby incorporated by reference.  
         [0038]     It is accordingly an object of this invention to provide an apparatus and method that eliminates the subcooling section in the condenser of a refrigeration system.  
         [0039]     It is also an object of the invention to increase the efficiency of known refrigeration systems by providing a more cost-effective way of providing subcooling of the refrigerant in a refrigeration system.  
         [0040]     It is another object of the invention to increase the cooling capacity and EER of known refrigeration systems by providing a cost-effective way of providing subcooling of the refrigerant.  
         [0041]     A further object of the invention is to provide a system in increased cooling capacity and EER are achieved by use of bypass subcooling technology in combination with de-optimizing the size of the condenser used according to conventional practice for a given cooling capacity.  
         [0042]     A related object of the invention to allow use of smaller condensers in known refrigeration systems by providing a cost-effective way of providing subcooling of the refrigerant.  
         [0043]     It is another object of the invention to enable the use, without a degradation of EER or cooling capacity, of a condenser and a compressor of smaller sizes than the current optimum sizes and size ratios of components of known refrigeration systems without bypass subcooling technology.  
         [0044]     An additional object of the invention is to provide a method and apparatus for subcooling of the refrigerant, which may be used in single-refrigerant systems and also in mixed-refrigerant systems, with and without regenerative features.  
         [0045]     An additional object of the invention is to provide an improved refrigeration system with substantially lower evaporator pressure by use of a vacuum-generating device thereby boosting the evaporator capacity.  
         [0046]     An additional object of the invention is to provide an improved refrigeration system in which the mixing of refrigerant streams having two different pressures using a vacuum generating device increases the suction pressure of the compressor, whereby the required pressure rise over the compressor is reduced, and which, in turn, reduces the compressor work and increases the EER.  
         [0047]     An additional object of the invention is to provide an improved refrigeration system in which the mixing of two different pressure vapors is carried out using a vacuum generating device so that the pressure at the bypass line can be maintained at a higher pressure than the evaporator pressure.  
         [0048]     An additional object of the invention is to provide an improved refrigeration system in which the mixing of two different pressure vapors is carried out using a pressure-reducing device so that the pressure at the bypass line can be maintained at a higher pressure than the evaporator pressure.  
         [0049]     Yet another object of the invention is to provide an improved refrigeration system in which subcooling is performed outside the condenser in a bypass path to which refrigerant from the condenser outlet is diverted, into a bypass path, and in which the quantity of refrigerant diverted is controlled such that the cooling capacity can be adjusted to meet varying thermal requirements, whereby the system can be operated without the need for energy-inefficient repeated on and off cycling of the compressor.  
         [0050]     An additional object of the invention is to provide a method and apparatus for improving the cooling capacity and EER of a conventional refrigeration system by employing bypass technology in combination with de-optimizing condenser size both for subcooling and for de-superheating. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]      FIG. 1  shows a block diagram of a conventional refrigeration system.  
         [0052]      FIG. 2  shows an example of the temperature variation inside a condenser for the conventional refrigeration system of  FIG. 1 .  
         [0053]      FIG. 3  shows another prior art refrigeration system where a part of the high-pressure refrigerant expands through a secondary expansion device in a bypass line which is at the same pressure as that at the evaporator outlet.  
         [0054]      FIG. 4  shows an example of a bypass device using a conventional liquid-line/suction line heat exchanger.  
         [0055]      FIG. 5  shows a block diagram of an embodiment of the present invention in which subcooling bypass technology is used in combination with de-optimization of the condenser size, as dictated by conventional practice, and a pressure differential accommodating device is used to mix two refrigerant streams at two different pressures.  
         [0056]      FIG. 6  shows a block diagram of an embodiment of the present invention in which the evaporator is enlarged to take advantage of the additional subcooling.  
         [0057]      FIG. 7  shows a block diagram of an embodiment of the present invention using a vortex generator as a pressure differential accommodating device.  
         [0058]      FIG. 8  shows a block diagram of an embodiment of the present invention where the liquid refrigerant is diverted downstream of the secondary heat exchanger.  
         [0059]      FIG. 9  shows a block diagram of an embodiment of the present invention in which a thermostatic expansion valve (TXV) is used to maintain a constant suction temperature.  
         [0060]      FIGS. 10A and 10B  illustrate the construction of a vortex generator which may be used as a pressure differential accommodating device according to the invention.  
         [0061]      FIG. 11  is a block diagram showing application of the present invention to a zoned cooling system.  
         [0062]      FIG. 12  is a block diagram showing application of the present invention to a mixed-refrigerant system.  
         [0063]      FIG. 13  shows a block diagram of an embodiment of the present invention in which a small condenser and a large evaporator are used in combination with a TXV to take advantage of the additional subcooling in which the heat exchanger is driven by refrigerant exiting the evaporator.  
         [0064]      FIG. 14  shows a block diagram of an embodiment of the present invention in which a small condenser and a large evaporator are shown to take advantage of the additional subcooling in which the heat exchanger is driven by refrigerant diverted from the main expansion device. 
     
    
       [0065]     Throughout the drawings, like parts are given the same reference numerals.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0066]      FIG. 5  shows a bypass technology concept, where a portion of liquid refrigerant is bypassed through a bypass line or path  27 . The refrigerant in the bypass path goes through a secondary expansion device  23 , thus lowering its pressure and temperature. The cold refrigerant mixture after the secondary expansion device receives heat energy from the hot liquid refrigerant that has exited the condenser and is flowing through the primary refrigerant line, producing additional subcooling in the liquid refrigerant. The additional subcooling produced from this bypass method makes the subcooling process in the condenser unnecessary. Thus,  FIG. 5  shows a smaller condenser  14   b , where the subcooling section has been removed and is identified as a dotted rectangular box.  
         [0067]      FIG. 6  shows that the bypass technology enables the use of a larger evaporator than the evaporator in an optimized system without the bypass technology. The use of the larger evaporator is possible because of the increased subcooling produced by the bypass technology. The increased subcooling means more liquid refrigerant after the main expansion device is produced at a lower temperature thereby increasing the heat absorption at the evaporator. The increased size of the evaporator is indicated by a dotted rectangular box  18   a . The increased evaporator is identified as  28  in  FIG. 6 . The size of an evaporator directly reflects on the capacity of a refrigeration system. The use of an evaporator in this embodiment of the present invention that is larger than that of an optimized system without the bypass is very significant because it means that with the present bypass technology one can increase the capacity of a system without increasing the sizes of the condenser and compressor. An increase in evaporator size, with all other component sizes held equal, would represent a direct increase in cooling capacity, or from another perspective, by adding the bypass, the condenser and compressor sizes can be reduced and still meet the needs of a given evaporator capacity. For example, one can build a refrigeration system with a smaller condenser and a smaller compressor than the sizes in the optimized system, while maintaining evaporator size and cooling capacity. Since the cost of the compressor is currently about half of the total cost of a refrigeration system, the size reduction in the compressor is a particularly attractive option.  
         [0068]      FIG. 7  shows the bypass technology with a condenser  24  that is smaller and an evaporator  28  that is larger than those without the bypass. For example, in a 1 ton air-conditioning system without the bypass, one needs a condenser  14  corresponding to 1 ton (i.e., 15 KBtu/hr), an evaporator  18  corresponding to 1 ton (i.e., 12 KBtu/hr), as well as a compressor  12  designed for the 1 ton application as shown in  FIG. 1 . In a 1 ton air-conditioning system with the bypass, one needs a smaller condenser  24  (i.e., 10 KBtu/hr), a larger evaporator  28  (i.e., 15 KBtu/hr), and the same compressor  12  designed for 1 ton application as shown in  FIG. 6 .  
         [0069]      FIG. 6  shows the bypass technology using a pressure differential accommodating device  38 . The pressure at the bypass path  27  is greater than the pressure at the evaporator. Hence, one needs to have a pressure differential accommodating device to account for the vapors at two different pressures after the evaporator. The pressure differential accommodating device can be either a vacuum-generating device such as a vortex generator or a venturi tube or a pressure reducing device such as a capillary tube, a restricted orifice, a valve, or a porous plug. In the case of the pressure reducing device, friction reduces the pressure of the refrigerant stream coming from the bypass path to match the evaporator pressure. The pressure reducing device may also include suitable tubing or the like to permit mixing of the pressure-equalized vapors before return to the compressor inlet.  
         [0070]      FIG. 7  shows the bypass technology where a vortex generator  29  is used as a pressure differential accommodating device to generate a vacuum and achieve mixing for the two refrigerant streams at different pressures.  
         [0071]     In the implementations illustrated in  FIGS. 5-7 , the refrigerant is diverted to the secondary path before the primary refrigerant flow is subjected to subcooling in heat exchanger  22 .  FIG. 8  shows an alternative embodiment in which the diversion takes place after subcooling. Again, a pressure differential accommodating device  38  is used between the evaporator and the compressor to combine two vapors at different pressures.  
         [0072]      FIG. 9  shows an embodiment of the present invention which employs a thermostatic expansion valve (TXV)  16   a  together with the bypass technology. The TXV  16   a  meters the refrigerant flow to evaporator  28  using a thermal sensing element  41  to monitor the superheat. The TXV  16   a  opens or closes in response to the thermal element  41 . The TXV  16   a  maintains a constant superheat in the evaporator  28 . The use of the TXV  16   a  together with the bypass technology allows the use of a smaller evaporator than otherwise. When the heat absorption at the evaporator  28  increases, the superheat increases. Accordingly, the TXV  16   a  opens, increasing the circulating refrigerant mass flow rate so that the superheat remains constant. When one uses a larger evaporator with the TXV  16   a , the heat absorption at the evaporator can significantly increase as the TXV  16   a  can increase the circulating refrigerant mass flow rate.  
         [0073]     Without the bypass, the increased heat absorption resulting from use of a larger evaporator also increases the condenser pressure, thus increasing compressor work. Often the increase in the compressor work is greater than the increase in the heat absorption thereby decreasing the energy efficiency ratio (EER). However, in the present invention, the bypass technology creates enough subcooling at and after the condenser  24  so that the increased heat absorption at the evaporator  28  does not increase the condenser pressure, because the bypass enables the condenser  24  to behave as if it were oversized. Hence, the EER increases in the case with the bypass.  
         [0074]     The construction of a vortex generator is shown schematically in  FIGS. 10A and 10B . The design of the vortex generator, generally denoted at  40 , is derived from the so-called vortex tube, a known device which converts an incoming flow of compressed gas into two outlet streams-one stream hotter than and the other stream colder than the temperature of the gas supplied to the vortex tube. A vortex tube does not contain any moving parts. Such a device is illustrated in U.S. Pat. No. 6,250,086, which is incorporated herein for reference.  
         [0075]     As illustrated in  FIGS. 10A and 10B , vortex generator  40  is used to mix two vapors at different pressures into one stream. The present invention uses the vortex generator  40  as a mixing means. It is comprised of a tubular body  60 , with an axial inlet  52  and a tangential inlet  54  at an inlet end  62 , and an outlet  58  at an opposite outlet end  64 . The interior construction of tube  60  at the inlet end is such that a high-pressure gas stream entering tangential inlet  54  travels along a helical path toward the outlet  58 . This produces a strong vortex flow in tube  60 , and a radial pressure differential due to the centrifugal force created by the vortex flow forces the vapor radially outward and produces high pressure at the periphery and low pressure at the axis. The low pressure allows fluid drawn in through axial inlet  52  to mix with the high-pressure helical stream and to exit with it through outlet  58 .  
         [0076]     With reference to the system shown in  FIG. 7  and the construction of the vortex generator  40  as illustrated in  FIGS. 10A and 10B , the high-pressure tangential flow is provided through tube  54  from secondary heat exchanger  22  and the bypass path  27 , whereas the incoming stream at axial inlet  52  is provided from the outlet of evaporator  28 . Using a vacuum-generating device based on the vortex generator makes it possible to combine the refrigerant exiting from evaporator  28  and the higher pressure refrigerant exiting from the secondary heat exchanger  22  without the need for a costly pump having moving parts.  
         [0077]     Other devices which rely on geometry and fluid dynamics may also be used to generate a vacuum which permits mixing the refrigerant streams exiting from evaporator  18  and heat exchanger  22 . For example, a device operating on the principle of a venturi tube may also be used.  
         [0078]     Referring again to  FIG. 7 , in operation, a portion of the liquid refrigerant exiting from condenser  24  is diverted into bypass path  27 , for example, by a suitable valve (not shown). The diverted refrigerant passes through secondary expansion device  23  and then through heat exchanger  22  which performs the subcooling function conventionally performed by the downstream portion of the condenser. By proper selection of system parameters, in particular, the mass flow rate of refrigerant diverted to the bypass path, the refrigerant can be made to leave condenser  24  at or close to the saturation temperature, and the entire flow path through the condenser can be devoted to the phase-change operation by transfer of heat to the environment, whereby maximum condenser efficiency can be achieved. It has been found that this requires diversion of 5-15% of the liquid refrigerant outflow from the condenser to the bypass path.  
         [0079]     More particularly, providing a bypass path for subcooling makes the condenser  24  more efficient thereby reducing the condenser pressure, which, in turn, decreases the pressure lift at the compressor  12 , thus reducing the compressor work. The coefficient of performance (“COP”) of a refrigeration system, sometimes termed the energy-efficiency ratio (EER), is defined as Qv/Wc, where Qv is the heat absorption by the evaporator of the system and Wc is the work done by the compressor. As will be appreciated, a decrease in Wc increases the COP and the EER.  
         [0080]     Correspondingly, because subcooling does not have to be done inside condenser  24 , the condenser becomes more efficient, and subcooling prior to the main expansion device  16  is increased. This increases the amount of liquid refrigerant after the throttling process through the main expansion valve  16 . Thus, the heat absorption at evaporator  28  (often referred as the cooling capacity) increases.  
         [0081]     Referring still to  FIG. 7 , by proper design of the vacuum generating device such as vortex generator  40  illustrated in  FIGS. 10A and 10B , or venturi tube, the pressure at the low pressure inlet  52  can be made lower than the inlet pressure at main evaporator  28 . As a consequence, a pressure drop may be imposed across the evaporator  28 . This is advantageous in that the lower evaporator outlet pressure means that the evaporator temperature differential is greater, resulting in enhanced evaporator capacity.  
         [0082]     Of even more significance, after the mixing of the two vapor streams from heat exchanger  22  and evaporator  28 , the pressure of the combined stream can have a higher pressure than the evaporator inlet pressure. This means that the suction pressure at the compressor inlet is increased, which reduces the required pressure lift across the compressor  12 . The reduced compressor work can provide a beneficial increase in the EER.  
         [0083]      FIG. 11  illustrates a zoned air conditioning system embodying the principles of this invention, generally denoted at  110 . This differs from system  50  illustrated in  FIG. 5  in that bypass path  92  includes an adjustable control valve  94 , and the evaporator  96  is formed of several parallel-connected evaporator units  98   a  and  98   b  located to serve different rooms, and respectively connected to the primary expansion device  16  by ON-OFF valves  100   a  and  100   b . System  110  is thus configured to provide two separate cooling zones, but as will be appreciated, more zones can be provided if desired.  
         [0084]     The outlets of evaporator units  98   a  and  98   b  are at the same pressure, and are therefore connected in common to the input of pressure differential accommodating device  38 .  
         [0085]     In operation, when cooling in both zones is required, valves  100   a  and  100   b  are opened, and refrigerant flows through both evaporators  98   a  and  98   b . Valve  94  is adjusted to divert between 10 and 60 percent of the refrigerant from condenser  24  into bypass path  92  to achieve maximum cooling and efficiency. Thus, all of the benefits of the subcooling bypass described in connection with  FIGS. 5, 6  and  7  are also realized in system  110 .  
         [0086]     As an additional feature of system  110 , however, if cooling is required, e.g., only in the zone served by evaporator unit  98   a , valve  100   a  is opened, valve  100   b  is closed, and valve  94  is adjusted to divert the refrigerant which would otherwise flow through evaporator  98   b  into bypass path  92 , along with the refrigerant required for subcooling.  
         [0087]     To vary the bypass mass flow rate, valve  94  in bypass path  92  should be continuously adjustable or adjustable in steps, to provide the desired number of different flow rates. For example, 5% to 15% diversion could be provided for maximum performance, with 20%, 30%, 40%, 50%, and 60% diversion for reduced cooling capacity. Valves providing the above-described capability are commercially available, and any suitable or desired valve of this type may be employed.  
         [0088]     As previously indicated, maximum efficiency and cooling capacity are achieved by diversion of 5-15% of the refrigerant mass flow to bypass path  92 . As the amount of refrigerant diverted is increased beyond 15%, for example, up to 30% or more, the cooling capacity is reduced due to the substantial decrease in the refrigerant mass flow rate circulating through evaporator  96 . Thus, by diverting the refrigerant not needed in the idle evaporator, the cooling capacity can be made to vary according to the thermal load, without the need for repeated on-off cycling of the compressor or resort to costly variable speed compressors.  
         [0089]     This is particularly advantageous in that cycling the compressor on and off consumes a large quantity of energy. Eliminating this inefficiency results in significantly improved long-term energy efficiency, a parameter sometimes measured in terms of seasonal energy-efficiency ratio (SEER), which takes account of the ON/OFF operation of the compressor on the efficiency of the system. SEER is defined as the ratio of the sum of Qv (heat absorbed by the evaporator) times the hours of operation on one hand, to the sum of Wc (compressor work) times the hours of operation on the other.  
         [0090]     As will also be appreciated, a variable cooling capacity can be provided in single-zone systems such as illustrated in  FIGS. 5-9 . Here, additional refrigerant would be diverted to bypass path  27  through a suitable adjustable valve (not shown) to accommodate a decrease in required cooling capacity, and the system could operate without the need for frequent compressor on-off cycling.  
         [0091]     In the constructions described above, it has been assumed that a single refrigerant circulates through the system. Subcooling bypass can also be used in conjunction with mixed refrigerants in regenerative systems to achieve highly beneficial results.  
         [0092]      FIG. 12  illustrates an embodiment of the invention as applied to a simple mixed-refrigerant system, employing, for example, a mixture of refrigerants R-32, R-125, and R-134a. This is a commonly used beneficial combination, as the R-32 component is flammable but possesses excellent thermal characteristics, whereas the R-125 and R-134a components exhibit less desirable thermal characteristics than R-32 but are non-flammable and therefore safer. In the interest of simplicity, variations in the regenerative paths as illustrated in U.S. Pat. Nos. 6,293,108 and 6,449,964 have been omitted from the illustrative system of  FIG. 12 .  
         [0093]     The system, generally denoted at  120 , comprises of a compressor  12 , an expansion device  16   a , an evaporator  28 , a heat exchanger  22 , and a pressure differential accommodating device  38  in a bypass path  27  just as in system  50  (see  FIG. 5 ). The condenser in system  120  of  FIG. 12 , however, is split into two stages  24   a  and  24   b , and a liquid-vapor (LV) separator  108  of any suitable or desired type is provided between the two condenser stages.  
         [0094]     The LV separator  108  separates the incoming vapor stream exiting from condenser stage  24   a  into a first vapor component which passes to the inlet of condenser stage  24   b , and a second lower temperature liquid component a portion of which passes into the bypass path  27  through a valve  112  to the inlet of heat exchanger  22 .  
         [0095]     The second component exiting from LV separator  108  through the valve  112  is rich in R-134a refrigerant due to its high condensation and boiling point relative to the other refrigerant components. Aside from the advantages of performing the desuperheating step outside condenser stage  24   a  as described above, the R-134a-rich composition of the refrigerant allocated to the bypass path in liquid form has the added benefit of reducing the condenser pressure.  
         [0096]     As indicated above, the system illustrated in  FIG. 12  is representative of the application of the principles of this invention to mixed-refrigerant regenerative systems. It should be understood, however, that the bypass is applicable to other mixed-refrigerant regenerative system configurations as well.  
         [0097]      FIG. 13  illustrates the present invention as applied to the conventional liquid-line/suction line heat exchange where superheated vapor or liquid-vapor mixture exiting the evaporator is used to subcool the high-pressure liquid exiting the condenser combined with de-optimization of condenser size as dictated by conventional practice. As the suction temperature increases prior to the compressor  212 , the present invention increases the circulating mass flow rate of the refrigerant by using a thermostatic expansion device  216  together with a thermostatic bulb  241 , which monitors the suction temperature. The thermostatic expansion device  216  increases the mass flow rate of circulating refrigerant so that the suction temperature is maintained constant in the present invention. The present invention uses a condenser  214  whose size is much smaller than the condenser in an optimized system. Furthermore, the present invention uses an evaporator  218  whose size is much larger than the evaporator in an optimized system. In an optimized system, the conventional liquid-line/suction line heat exchange does not improve the efficiency of the system. The present invention using a large evaporator  218  allows a refrigeration system to be built with a smaller condenser and a smaller compressor than the sizes in an optimized system without the bypass method.  
         [0098]      FIG. 14  illustrates the present invention applied to a system configuration similar to the system shown in  FIG. 4 , again in combination with de-optimization of condenser size as dictated by conventional practice. Here, a portion of liquid refrigerant is bypassed through a secondary expansion device  223  and a heat exchanger  222  to subcool the high-pressure liquid exiting the condenser. The present invention uses a condenser  224  whose size is much smaller than the condenser in an optimized system. Furthermore, the present invention uses an evaporator  228  whose size is much larger than the evaporator in an optimized system.  
         [0099]     In describing the invention, specific terminology has been employed for the sake of clarity. However, the invention is not intended to be limited to the specific descriptive terms, and it is to be understood that each specific term includes all technical equivalents that operate in a similar manner to accomplish a similar purpose.  
         [0100]     Similarly, the embodiments described and illustrated are also intended to be exemplary, and various changes and modifications, and other embodiments within the scope of the invention will be apparent to those skilled in the art in light of the disclosure. The scope of the invention is therefore intended to be defined and limited only by the appended claims, and not by the description herein.