WO2004044503A2

WO2004044503A2 - Refrigeration system with bypass subcooling and component size de-optimization

Info

Publication number: WO2004044503A2
Application number: PCT/US2003/036424
Authority: WO
Inventors: Cheolho Bai
Original assignee: Vortex Aircon
Priority date: 2002-11-11
Filing date: 2003-11-12
Publication date: 2004-05-27
Also published as: AU2003295527A8; WO2004044503A3; JP2006505763A; KR20050103900A; EP1567814A2; CN1732365A; AU2003295527A1; US20070074536A1

Abstract

A refrigeration system having a primary refrigerant path including a compressor (12), a condenser (24), a primary expansion device (16), and an evaporator (28) connected together to form a closed loop system with a refrigerant circulating therein; and a bypass path (27) coupled to an outlet of the condenser. The bypass path includes a secondary expansion device (23); and a heat exchanger (22) 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 (38) 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.

Description

REFRIGERATION SYSTEM WITH BYPASS SUBCOOLING AND COMPONENT SIZE DE-OPTIMIZATION

CROSS-REFERENCE TO PRIOR APPLICATION

This application claims priority to U.S. Provisional Application Serial No. 60/426,073, filed November 11, 2002.

BACKGROUND OF THE INVENTION

Field of the Invention

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.

Relevant Art

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.

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 absoφtion 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, inheatpumps and in other applications. 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. InFig. 1, the condenser 14 is divided into two parts, 14aand l4b. In the first portion, 14a, 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, 14b, the liquid refrigerant is further cooled below the saturation temperature at the condenser pressure, a process known as subcooling.

Figure 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 14b. In the subcooling process, the temperature difference between the refrigerant and coolingmedium (e.g., air orwater) 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.

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. 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. 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.

For example, it has been proposed to divert aportionofthe 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. US Patent 6,164,086. Fig. 3 shows a schematic diagram of a system of this kind.

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 refiigerant then flows to through the main expansion device and eventually to the evaporator.

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.

Kim and Domanski, in Mracycle 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.

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.

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.

An approach similar to the second method taught by Kita et al. appears to have been used in very large sytems (e.g., 2,000 tons) but is of questionable use in small and intermediate size systems (less than about 1,000 tons).

Cho and Bai, in U.S. Patent 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.

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

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.

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.

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 absoφtion at the evaporator, thus increasing both the cooling capacity and EER without increasing compressor work.

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, hi 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.

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, h 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.

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.

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.

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.

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.

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. 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, aprimary 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 refiigerant line, passing the bypassed refiigerant througli 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.

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 absoφtion at the evaporator (often referred as the cooling capacity) increases. 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.

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).

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. hi 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. Patents 6,250,086 and 6,293,108, the contents of which are hereby incoφorated by reference.

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 September 23, 2002 (Atty Docket 3474-21), the contents of which are hereby incoφorated by reference.

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. It is also an obj ect 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.

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.

A further obj ect 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. 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.

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.

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. An additional obj ect 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.

An additional obj ect 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.

An additional obj ect 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.

An additional obj ect 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.

Yet another object of the invention is to provide an improved refrigeration system in which subcooling is performed outside the condenser in abypass 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.

An additional object of the invention is to provide a method and apparatus for improving the cooling capacity and EER of a conventional refrigeration ssytem by employing bypass technology in combination with de-optimizing condenser size both for subcooling and for de-superheating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a conventional refrigeration system. FIG. 2 shows an example of the temperature variation inside a condenser for the conventional refrigeration system of FIG. 1.

FIG.3 shows another prior art refrigeration system where apart 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.

FIG.4 shows an example of abypass device using a conventional liquid-line/suction line heat exchanger. 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. 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.

FIG. 7 shows a block diagram of an embodiment of the present invention using a vortex generator as a pressure differential accommodatmg device.

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.

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.

FIGS. 1QA and 10B illustrate the construction of a vortex generator which maybe used as a pressure differential accommodating device according to the invention.

FIG. 11 is a block diagram showing application of the present invention to a zoned cooling system.

FIG. 12 is a block diagram showing application of the present invention to a mixed-refrigerant system. FIG. 13 shows ablock diagram ofan embodiment ofthe present invention in which a small condenser and a large evaporator are used in combination with a TXV to take advantage ofthe additional subcooling in which the heat exchanger is driven by refrigerant exiting the evaporator.

FIG. 14 shows a block diagram ofan embodiment ofthe 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.

Throughout the drawings, like parts are given the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION Figure 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 14b, where the subcooling section has been removed and is identified as a dotted rectangular box. Figure 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 ofthe larger evaporator is possible because ofthe 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 absoφtion at the evaporator. The increased size ofthe evaporator is indicated by a dotted rectangular box 18a. The increased evaporator is identified as 28 in Fig. 6. The size ofan evaporator directly reflects on the capacity of a refrigeration system. The use of an evaporator in this embodiment ofthe present invention that is larger than that ofan 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.

Figure 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.l. 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.

Figure 6 shows the bypass technology using apressure 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 ofthe 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 ofthe pressure-equalized vapors before return to the compressor inlet.

Figure 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.

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. Figure 8 shows an alternative embodiment in which the diversion takes place after subcooling. Again, apressure differential accommodating device 38 is used between the evaporator and the compressor to combine two vapors at different pressures.

Figure 9 shows an embodiment of the present invention which employs a thermostatic expansion valve (TXV) 16a together with the bypass technology. The TXV 16a meters the refrigerant flow to evaporator 28 using a thermal sensing element 41 to monitor the superheat. The TXV 16a opens or closes in response to the thermal element 41. The TXV 16a maintains a constant superheat in the evaporator 28. The use of the TXV 16a together with the bypass technology allows the use of a smaller evaporator than otherwise. When the heat absoφtion at the evaporator 28 increases, the superheat increases. Accordingly, the TXV 16a opens, increasing the circulating refrigerant mass flow rate so that the superheat remains constant. When one uses a larger evaporator with the TXV 16a , the heat absoφtion at the evaporator can significantly increase as the TXV 16a can increase the circulating refrigerant mass flow rate.

Without the bypass, the increased heat absoφtion 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 absoφtion 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 absoφtion 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.

The construction of a vortex generator is shown schematically in Figs. lOAand 10B. The design ofthe 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. Patent 6,250,086, which is incoφorated herein for reference.

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.

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.

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. Referring again to Fig.7, in operation, a portion ofthe 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% ofthe liquid refrigerant outflow from the condenser to the bypass path.

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 absoφtion by the evaporator ofthe system and Wc is the work done by the compressor. As will be appreciated, a decrease in Wc increases the COP and the EER.

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 absoφtion at evaporator 28 (often referred as the cooling capacity) increases.

Referring still to Fig. 7, by proper design ofthe 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.

Of even more significance, after the mixing of the two vapor streams from heat exchanger 22 and evaporator 28, the pressure ofthe 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.

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 98a and 98b located to serve different rooms, and respectively connected to the primary expansion device 16 by ON-OFF valves 100a and 100b. System 110 is thus configured to provide two separate cooling zones, but as will be appreciated, more zones can be provided if desired.

The outlets of evaporator units 98a and 98b are at the same pressure, and are therefore connected in common to the input of pressure differential accommodating device 38. hi operation, when cooling in both zones is required, valves 100a and 100b are opened, and refrigerant flows through both evaporators 98a and 98b. Valve 94 is adjusted to divert between 10 and 60 percent ofthe refrigerant from condenser 24 into bypass path

92 to achieve maximum cooling and efficiency. Thus, all ofthe benefits ofthe subcooling bypass described in connection with Figs. 5, 6 and 7 are also realized in system 110.

As an additional feature of system 110, however, if cooling is required, e.g., only in the zone served by evaporator unit 98a, valve 100a is opened, valve 100b is closed, and valve 94 is adjusted to divert the refrigerant which would otherwise flow through evaporator 98b into bypass path 92, along with the refrigerant required for subcooling.

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.

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 refiigerant 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.

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 ofthe system. SEER is defined as the ratio ofthe 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.

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.

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. Figure 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, hi the interest of simplicity, variations in the regenerative paths as illustrated in U.S. Patents 6,293,108 and 6,449,964 have been omitted from the illustrative system of Fig. 12.

The system, generally denoted at 120, comprises of a compressor 12, an expansion device 16a, 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 24a and 24b, and a liquid- vapor (LV) separator 108 of any suitable or desired type is provided between the two condenser stages. The LV separator 108 separates the incoming vapor stream exiting from condenser stage 24a into a first vapor component which passes to the inlet of condenser stage 24b, 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.

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 24a 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. As indicated above, the system illustrated in Fig. 12 is representative of the application ofthe 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.

Figure 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 ofthe 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 refiigerant 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 ofthe 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.

Figure 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.

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 puφose.

Similarly, the embodiments described and illustrated are also intended to be exemplary, and various changes and modifications, and other embodiments within the scope ofthe invention will be apparent to those skilled in the art in light ofthe disclosure. The scope ofthe invention is therefore intended to be defined and limited only by the appended claims, and not by the description herein.

Claims

1. A refrigeration system comprising: a primary refrigerant path 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 path coupled to an outlet ofthe condensing means, the bypass path including: a secondary expansion means; heat exchanging means thermally coupled to the primary refrigerant path between the outlet ofthe condensing means and the inlet ofthe expansion means for removing heat from the liquid refiigerant discharged from the condenser; 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 ofthe compressing means.

2. A refrigeration system according to claim 1, wherein: the heat transfer capacity ofthe condensing means is insufficient to provide the required subcooling ofthe refrigerant; and the required subcooling is provided substantially entirely by the heat exchanging means.

3. A refiigeration system according to claim 1 , wherein the heat exchanging means and the condensing means are so constructed that the a substantial portion ofthe required subcooling ofthe refrigerant is provided by the heat exchanging means.

4. A refrigeration system comprising: aprimary refrigerant path including a compressor, a condenser, aprimary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein; and a bypass path coupled to the outlet ofthe condenser, the bypass path including: a secondary expansion device; 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 refiigerant discharged from the condenser; and a pressure differential accommodating device operative to mix two vapors at different pressures connecting the outlets of the evaporator and the heat exchanger to an inlet ofthe compressor.

5. A refrigeration system according to claim 4, wherein: the heat transfer capacity ofthe condenser is insufficient to provide the required subcooling ofthe refrigerant; and the required subcooling is provided substantially entirely by the heat exchanger.

6. A refrigeration system according to claim 4, wherein the heat exchanger and the condenser are so constructed that a substantial portion ofthe subcooling ofthe refrigerant is provided by the heat exchanger.

7. A refrigeration system according to claim 4, wherein the bypass path is connected to the outlet ofthe condenser downstream ofthe heat exchanger.

8. A refrigeration system according to claim 4, wherein: the evaporator is comprised of a plurality of parallel-connected evaporator elements located in respective portions ofthe space being cooled by the system; and the system further includes a plurality of on-off valves respectively connecting the primary expansion device to the evaporator elements, the on-off valves being operable to idle respective evaporator elements by shutting ofthe flow of refrigerant thereto when cooling of a particular location is not required at given time; and an adjustable valve in the bypass path, the adjustable valve being operative to control the flow of refrigerant in the bypass path such that refrigerant mass flow which is not required in the primary refrigerant path when a particular evaporator element is idle flows to the bypass path.

9. A refrigeration system according to claim 8, wherein the compressor is configured and controlled to run continuously when the system is in operation, independent of changes in thermal load.

10. A refrigeration system according to claim 8, wherein the pressure differential accommodating device includes: a low pressure inlet connected in common to outlets of he evaporator elements; a high pressure input connected to the bypass path; and an outlet connected to an inlet ofthe compressor.

11. A refrigeration system according to claim 8, wherein the valve in the bypass path is operable to divert about 5% to about 15% ofthe refrigerant to the bypass path when maximum cooling capacity is required due to operation of all the evaporator elements, and to divert up to about 60% ofthe refrigerant to the bypass path according to reductions in thermal load due to deactivation of particular evaporator elements.

12. A refrigeration system according to claim 4, further including a valve in the bypass path, the valve being operable to divert about 5% to about 15% ofthe refrigerant to the bypass path when maximum cooling capacity is required due to high thermal load, and to divert up to about 60% ofthe refrigerant to the bypass path according to reductions in thermal load.

13. A refrigeration system according to claim 12, wherein the compressor is configured and controlled to run continuously when the system is in operation, independent of changes in thermal load.

14. A refrigeration system according to claim 4, wherein the expansion device in the primary refrigeration path is thermostatically operated in response to a temperature sensor thermally coupled to the inlet of the compressor to maintain a constant superheat in the evaporator.

15. A refrigeration system according to claim 14, wherein the evaporator is downsized from that conventionally required to increase cooling capacity without increasing compressor work.

16. A refiigeration system according to claim 15, wherein the condenser is downsized from that conventionally required for the oversized evaporator selected to achieve a desired cooling capacity.

17. A refrigeration system according to claim 14, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

18. A refrigeration system according to claim 4, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

19. A refrigeration system according to claim 4, wherein the refrigerant circulated in the system consists of a single component.

20. A refrigeration system according to claim 4, wherein the refrigerant circulated in the system is a mixed-refrigerant comprising a plurality of components selected to provide a desired combination of thermal and flammability characteristics.

21. A refrigeration system according to claim 20, further including a liquid- vapor separator operable to selectively divert at least one component ofthe mixed refrigerant to the bypass path to increase the percentage of liquid in the refrigerant as it enters the evaporator, thereby improving evaporator efficiency.

22. A refrigeration system according to claim 21, wherein the diverted refiigerant has ahigh condensation temperature and a high boiling temperature relative to the remainder ofthe refrigerant.

23. A refrigeration system comprising: 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 attached between the outlet ofthe condenser and the inlet ofthe compressor, the bypass path including: 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 which provides subcooling of the refrigerant discharged from the condenser, the heat transfer capacity ofthe condenser being insufficient to provide the required subcooling.

24. A refrigeration system according to claim 23, wherein the heat exchanger and the condenser are so constructed that the required subcooling is provided substantially entirely by the heat exchanger.

25. A refrigeration system according to claim 23, wherein: the vapor pressure ofthe refrigerant exiting the heat exchanger is higher than that ofthe refrigerant exiting the evaporator, and the system further includes a pressure differential accommodating device connecting the outlets ofthe evaporator and the heat exchanger to an inlet ofthe compressor.

26. A refrigeration system according to claim 25, wherein the pressure differential accommodating device is a vacuum generating device having inputs thereof connected to outlets ofthe evaporator and the heat exchanger and an output connected to the inlet ofthe compressor.

27. A refrigeration system according to claim 25, wherein the pressure differential accommodating device is a venturi tube or a vortex tube.

28. A refrigeration system according to claim 25, wherein the pressure differential accommodating device includes a pressure reducing device connected to the outlet ofthe heat exchanger, and a mixing device connecting the pressure reducing device and the outlet ofthe evaporator to the inlet ofthe compressor..

29. A refrigeration system according to claim 28, wherein the pressure reducing device is a capillary tube, a restricted orifice, a valve, or a porous plug.

30. A refrigeration system according to claim 23, wherein the bypass path is comiected to the outlet ofthe condenser downstream ofthe heat exchanger.

31. A refrigeration system according to claim 23, wherein: the evaporator is comprised of aplurality of parallel-connected evaporator elements located in respective portions ofthe space being cooled by the system; and the system further includes a plurality of on-off valves respectively connecting the primary expansion device to the evaporator elements, the on-off valves being operable to idle respective evaporator elements by shutting ofthe flow of refrigerant thereto when cooling of a particular location is not required at given time; and an adjustable valve in the bypass path, the adjustable valve being operative to control the flow of refrigerant in the bypass path such that refrigerant mass flow which is not required in the primary refrigerant path when a particular evaporator element is idle flows to the bypass path.

32. A refrigeration system according to claim 31, wherein the compressor is configured and controlled to run continuously when the system is in operation, independent of changes in thermal load.

33. A refrigeration system according to claim 31, further including a pressure differential accommodating device having a low pressure inlet connected in connected in common to outlets of the evaporator elements, a high pressure input connected to the bypass path, and an outlet connected to an inlet ofthe compressor.

34. A refrigeration system according to claim 31, wherein the valve in the bypass path is operable to divert about 5% to about 15% ofthe refrigerant to the bypass path when maximum cooling capacity is required due to operation of all the evaporator elements, and to divert up to about 60% ofthe refrigerant to the bypass path according to reductions in thermal load due to deactivation of particular evaporator elements.

35. A refrigeration system according to claim 23, further including a valve in the bypass path, the valve being operable to divert about 5% to about 15% ofthe refrigerant to the bypass path when maximum cooling capacity is required due to high thermal load, and to divert up to about 60% ofthe refrigerant to the bypass path according to reductions in thermal load.

36. A refrigeration system according to claim 35, wherein the compressor is configured and controlled to run continuously when the system is in operation, independent of changes in thermal load.

37. A refrigeration system according to claim 23, wherein the expansion device in the primary refiigeration path is thermostatically operated in response to a temperature sensor thermally coupled to the inlet ofthe compressor to maintain a constant superheat in the evaporator.

38. A refrigeration system according to claim 37, wherein the evaporator is oversized from that conventionally required to increase cooling capacity without increasing compressor work.

39. A refiigeration system according to claim 38, wherein the condenser is downsized from that conventionally required for the oversized evaporator selected to achieve a desired cooling capacity.

40. A refrigeration system according to claim 37, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

41. A refrigeration system according to claim 23, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

42. A refrigeration system according to claim 23, wherein the heat exchanger is comiected to provide counter-flow of refrigerant in the heat exchanger and the thermally coupled refrigerant in the primary refiigerant path.

43. A refrigeration system according to claim 23 , wherein the refiigerant circulated in the system consists of a single component.

44. A refrigeration system according to claim 23, wherein the refiigerant circulated in the system is a mixed-refrigerant comprising a plurality of components selected to provide a desired combination of thermal and flammability characteristics.

45. A refrigeration system according to claim 44, further including a liquid- vapor separator operable to selectively divert at least one component ofthe mixed refrigerant to the bypass path to increase the percentage of liquid in the refrigerant as it enters the evaporator, thereby improving evaporator efficiency.

46. A refiigeration system according to claim 45, wherein the diverted refrigerant has a high condensation temperature and a high boiling temperature relative to the remainder of the refrigerant.

47. 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: diverting a portion ofthe refrigerant exiting the condenser into a secondary refrigerant path which includes a secondary expansion device and a heat exchanger thermally coupled to the primary refiigerant path between the condenser outlet and the primary expansion device inlet; passing the diverted refiigerant through the heat exchanger to remove heat from refiigerant flowing in the primary refrigerant path; 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 delivering the refrigerant exiting the pressure differential accommodating device to an inlet ofthe compressor.

48. A method according to claim 47, wherein the refrigerant is diverted to the bypass path at a location downstream ofthe heat exchanger.

49. A method according claim 47, wherein between about 5% and about 15% of the liquid refrigerant outflow from the condenser is diverted to the bypass path.

50. A method according claim 47, further including the steps of: controlling the quantity of refrigerant outflow from the condenser which is diverted to the bypass path to adjust the cooling capacity ofthe system according to the thermal load; and running the compressor substantially continuously when the system is in operation to provide improved long-term energy efficiency.

51. A method according claim 47, wherein: the primary refrigeration path includes a plurality of evaporators located in respective locations to be separately cooled; and the method further includes the steps of: diverting a predetermined minimum quantity of refrigerant to the bypass path when maximum cooling capacity is required to cool all ofthe locations; and diverting increasing quantities of refrigerant to the bypass path as thermal load decreases.

52. A method according claim 51, further includes the step of running the compressor continuously independent ofthe required cooling capacity when the system is in operation.

53. A method according claim 52, further includes the steps of: idling particular evaporators in locations which do not require cooling at a given time by blocking the flow of refrigerant thereto; diverting the refiigerant normally delivered to a particular evaporator to the bypass path what that evaporator is idle. ,

54. A method according to claim 47, wherein the refrigerant circulated in the system consists of a single component.

55. A method according to claim 47, wherein the refrigerant circulated in the system is a mixed-refrigerant comprising a plurality of components selected to provide a desired combination of thermal and flammability characteristics.

56. A method according to claim 55, further including the step of selectively diverting at least one component ofthe mixed refrigerant to the bypass path to increase the percentage of liquid in the refrigerant as it enters the evaporator, thereby improving evaporator efficiency.

57. A method according to claim 56, wherein the diverted refiigerant has a high condensation temperature and a high boiling temperature relative to the remainder ofthe refrigerant.

58. A method according to claim 47, further including the steps of: sensing the temperature ofthe refrigerant at the inlet ofthe compressor; and controlling the mass flow rate of refrigerant through the expansion device in the primary refrigeration path according to the sensed temperature to maintain the superheat of the refrigerant exiting the evaporator at a constant level.

59. A method according to claim 58, wherein the evaporator is oversized from that conventionally required to increase cooling capacity without increasing compressor work.

60. A method according to claim 59, wherein the condenser is downsized from that conventionally required for the oversized evaporator selected to achieve a desired cooling capacity.

61. A method according to claim 58 , wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

62. A method according to claim 47, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

63. A method of increasing the efficiency of a refrigeration system comprising the steps of: passing refrigerant through a primary refiigerant path which includes a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system wherein the heat transfer capacity ofthe condenser is insufficient to provide required subcooling for the circulating refrigerant; diverting a portion ofthe refrigerant exiting the condenser into a secondary refrigerant path which 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; and passing the diverted refrigerant through the heat exchanger to provide subcooling for refrigerant flowing in the primary refrigerant path.

64. A method according to claim 63, further including the steps of: 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 delivering the refrigerant exiting the pressure differential accommodating device to an inlet ofthe compressor.

65. A method according to claim 63, wherein the refrigerant is diverted to the bypass path at a location downstream ofthe heat exchanger.

66. A method according to claim 63, wherein substantially all ofthe subcooling required is provided by heat transfer in the heat exchanger.

67. A method according claim 63, wherein between about 5% and about 15% of the liquid refrigerant outflow from the condenser is diverted to the bypass path.

68. A method according claim 63, further including the steps of: controlling the quantity of refrigerant outflow from the condenser which is diverted to the bypass path to adjust the cooling capacity ofthe system according to the thermal load; and running the compressor substantially continuously when the system is in operation to provide improved long-term energy efficiency.

69. A method according claim 63, wherein: the primary refiigeration path includes a plurality of evaporators located in respective locations to be separately cooled; and the method further includes the steps of: diverting a predetermined minimum quantity of refrigerant to the bypass path when maximum cooling capacity is required to cool all ofthe locations; and diverting increasing quantities of refrigerant to the bypass path as thermal load decreases.

70. A method according claim 69, further includes the step of running the compressor continuously independent ofthe required cooling capacity when the system is in operation.

71. A method according claim 70, further includes the steps of: idling particular evaporators in locations which do not require cooling at a given time by blocking the flow of refrigerant thereto; diverting the refrigerant normally delivered to a particular evaporator to the bypass path what that evaporator is idle.

72. A method according to claim 63, wherein the refrigerant circulated in the system consists of a single component.

73. A method according to claim 63, wherein the refiigerant circulated in the system is a mixed-refrigerant comprising a plurality of components selected to provide a desired combination of thermal and flammability characteristics.

74. A method according to claim 73, further including the step of selectively diverting at least one component ofthe mixed refrigerant to the bypass path to increase the percentage of liquid in the refrigerant as it enters the evaporator, thereby improving evaporator efficiency.

75. A method according to claim 74, wherein the diverted refiigerant has a high condensation temperature and a high boiling temperature relative to the remainder ofthe refrigerant.

76. A method according to claim 63, further including the steps of: sensing the temperature ofthe refrigerant at the inlet ofthe compressor; and controlling the mass flow rate of refrigerant through the expansion device in the primary refrigeration path according to the sensed temperature to maintain the superheat of the refrigerant exiting the evaporator at a constant level.

77. A method according to claim 76, wherein the evaporator is oversized from that conventionally required to increase cooling capacity without increasing compressor work.

78. A method according to claim 77, wherein the condenser is downsized from that conventionally required for the oversized evaporator selected to achieve a desired cooling capacity.

79. A method according to claim 76, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.

80. A method according to claim 63, wherein the condenser is downsized from that conventionally required for an evaporator selected to achieve a desired cooling capacity.