Abstract:
A vapor compression system includes an evaporator, a compressor, and a condenser interconnected in a closed-loop system. In one embodiment, a multifunctional valve is configured to receive a liquefied heat transfer fluid from the condenser and a hot vapor from the compressor. A saturated vapor line connects the outlet of the multifunctional valve to the inlet of the evaporator and is sized so as to substantially convert the heat transfer fluid exiting the multifunctional valve into a saturated vapor prior to delivery to the evaporator. The multifunctional valve regulates the flow of heat transfer fluid through the valve by monitoring the temperature of the heat transfer fluid returning to the compressor through a suction line coupling the outlet of the evaporator to the inlet of the compressor. Separate gated passageways within the multifunctional valve permit the refrigeration system to be operated in defrost mode by flowing hot vapor through the saturated vapor line and the evaporator in a forward-flow process thereby reducing the amount of time necessary to defrost the system and improving the overall system performance. In one preferred embodiment of the invention, a heat source is applied to the heat transfer fluid after the heat transfer fluid passes through the expansion valve and before the heat transfer fluid enters the evaporator. The heat source converts the heat transfer fluid from a low quality liquid vapor mixture to a high quality liquid vapor mixture, or a saturated vapor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/431,830, filed Nov. 2, 1999, now U.S. Pat. No. 6,185,958. 
     This application is a continuation-in-part application of application Ser. No. 09/228,696, filed on Jan. 12, 1999, pending, which is hereby incorporated by reference. 
     Related subject matter is disclosed in commonly-owned, co-pending patent application entitled “VAPOR COMPRESSION SYSTEM AND METHOD” Ser. No. 09/228,696, filed on Jan. 12, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates, generally, to vapor compression systems, and more particularly, to mechanically-controlled refrigeration systems using forward-flow defrost cycles. 
     BACKGROUND OF THE INVENTION 
     In a closed-loop vapor compression cycle, the heat transfer fluid changes state from a vapor to a liquid in the condenser, giving off heat, and changes state from a liquid to a vapor in the evaporator, absorbing heat during vaporization. A typical vapor-compression refrigeration system includes a compressor for pumping a heat transfer fluid, such as a freon, to a condenser, where heat is given off as the vapor condenses into a liquid. The liquid flows through a liquid line to a thermostatic expansion valve, where the heat transfer fluid undergoes a volumetric expansion. The heat transfer fluid exiting the thermostatic expansion valve is a low quality liquid vapor mixture. As used herein, the term “low quality liquid vapor mixture” refers to a low pressure heat transfer fluid in a liquid state with a small presence of flash gas that cools off the remaining heat transfer fluid, as the heat transfer fluid continues on in a sub-cooled state. The expanded heat transfer fluid then flows into an evaporator, where the liquid refrigerant is vaporized at a low pressure absorbing heat while it undergoes a change of state from a liquid to a vapor. The heat transfer fluid, now in the vapor state, flows through a suction line back to the compressor. Sometimes, the heat transfer fluid exits the evaporator not in a vapor state, but rather in a superheated vapor state. 
     In one aspect, the efficiency of the vapor-compression cycle depends upon the ability of the system to maintain the heat transfer fluid as a high pressure liquid upon exiting the condenser. The cooled, high-pressure liquid must remain in the liquid state over the long refrigerant lines extending between the condenser and the thermostatic expansion valve. The proper operation of the thermostatic expansion valve depends upon a certain volume of liquid heat transfer fluid passing through the valve. As the high-pressure liquid passes through an orifice in the thermostatic expansion valve, the fluid undergoes a pressure drop as the fluid expands through the valve. At the lower pressure, the fluid cools an additional amount as a small amount of flash gas forms and cools of the bulk of the heat transfer fluid that is in liquid form. As used herein, the term “flash gas” is used to describe the pressure drop in an expansion device, such as a thermostatic expansion valve, when some of the liquid passing through the valve is changed quickly to a gas and cools the remaining heat transfer fluid that is in liquid form to the corresponding temperature. 
     This low quality liquid vapor mixture passes into the initial portion of cooling coils within the evaporator. As the fluid progresses through the coils, it initially absorbs a small amount of heat while it warms and approaches the point where it becomes a high quality liquid vapor mixture. As used herein, the term “high quality liquid vapor mixture” refers to a heat transfer fluid that resides in both a liquid state and a vapor state with matched enthalpy, indicating the pressure and temperature of the heat transfer fluid are in correlation with each other. A high quality liquid vapor mixture is able to absorb heat very efficiently since it is in a change of state condition. The heat transfer fluid then absorbs heat from the ambient surroundings and begins to boil. The boiling process within the evaporator coils produces a saturated vapor within the coils that continues to absorb heat from the ambient surroundings. Once the fluid is completely boiled-off, it exits through the final stages of the cooling coil as a cold vapor. Once the fluid is completely converted to a cold vapor, it absorbs very little heat. During the final stages of the cooling coil, the heat transfer fluid enters a superheated vapor state and becomes a superheated vapor. As defined herein, the heat transfer fluid becomes a “superheated vapor” when minimal heat is added to the heat transfer fluid while in the vapor state, thus raising the temperature of the heat transfer fluid above the point at which it entered the vapor state while still maintaining a similar pressure. The superheated vapor is then returned through a suction line to the compressor, where the vapor-compression cycle continues. 
     For high-efficiency operation, the heat transfer fluid should change state from a liquid to a vapor in a large portion of the cooling coils within the evaporator. As the heat transfer fluid changes state from a liquid to a vapor, it absorbs a great deal of energy as the molecules change from a liquid to a gas absorbing a latent heat of vaporization. In contrast, relatively little heat is absorbed while the fluid is in the liquid state or while the fluid is in the vapor state. Thus, optimum cooling efficiency depends on precise control of the heat transfer fluid by the thermostatic expansion valve to insure that the fluid undergoes a change of state in as large of cooling coil length as possible. When the heat transfer fluid enters the evaporator in a cooled liquid state and exits the evaporator in a vapor state or a superheated vapor state, the cooling efficiency of the evaporator is lowered since a substantial portion of the evaporator contains fluid that is in a state which absorbs very little heat. For optimal cooling efficiency, a substantial portion, or an entire portion, of the evaporator should contain fluid that is in both a liquid state and a vapor state. To insure optimal cooling efficiency, the heat transfer fluid entering and exiting from the evaporator should be a high quality liquid vapor mixture. 
     The thermostatic expansion valve plays an important role and regulating the flow of heat transfer fluid through the closed-loop system. Before any cooling effect can be produced in the evaporator, the heat transfer fluid has to be cooled from the high-temperature liquid exiting the condenser to a range suitable of an evaporating temperature by a drop in pressure. The flow of low pressure liquid to the evaporator is metered by the thermostatic expansion valve in an attempt to maintain maximum cooling efficiency in the evaporator. Typically, once operation has stabilized, a mechanical thermostatic expansion valve regulates the flow of heat transfer fluid by monitoring the temperature of the heat transfer fluid in the suction line near the outlet of the evaporator. The heat transfer fluid upon exiting the thermostatic expansion valve is in the form of a low pressure liquid having a small amount of flash gas. The presence of flash gas provides a cooling affect upon the balance of the heat transfer fluid in its liquid state, thus creating a low quality liquid vapor mixture. A temperature sensor is attached to the suction line to measure the amount of superheating experienced by the heat transfer fluid as it exits from the evaporator. Superheat is the amount of heat added to the vapor, after the heat transfer fluid has completely boiled-off and liquid no longer remains in the suction line. Since very little heat is absorbed by the superheated vapor, the thermostatic expansion valve meters the flow of heat transfer fluid to minimize the amount of superheated vapor formed in the evaporator. Accordingly, the thermostatic expansion valve determines the amount of low-pressure liquid flowing into the evaporator by monitoring the degree of superheating of the vapor exiting from the evaporator. 
     In addition to the need to regulate the flow of heat transfer fluid through the closed-loop system, the optimum operating efficiency of the refrigeration system depends upon periodic defrost of the evaporator. Periodic defrosting of the evaporator is needed to remove icing that develops on the evaporator coils during operation. As ice or frost develops over the evaporator, it impedes the passage of air over the evaporator coils reducing the heat transfer efficiency. In a commercial system, such as a refrigerated display cabinet, the build up of frost can reduce the rate of air flow to such an extent that an air curtain cannot form in the display cabinet. In commercial systems, such as food chillers, and the like, it is often necessary to defrost the evaporator every few hours. Various defrosting methods exist, such as off-cycle methods, where the refrigeration cycle is stopped and the evaporator is defrosted by air at ambient temperatures. Additionally, electrical defrost off-cycle methods are used, where electrical heating elements are provided around the evaporator and electrical current is passed through the heating coils to melt the frost. 
     In addition to off-cycle defrost systems, refrigeration systems have been developed that rely on the relatively high temperature of the heat transfer fluid exiting the compressor to defrost the evaporator. In these techniques, the high-temperature vapor is routed directly from the compressor to the evaporator. In one technique, the flow of high temperature vapor is dumped into the suction line and the system is essentially operated in reverse. In other techniques, the high-temperature vapor is pumped into a dedicated line that leads directly from the compressor to the evaporator for the sole purpose of conveying high-temperature vapor to periodically defrost the evaporator. Additionally, other complex methods have been developed that rely on numerous devices within the refrigeration system, such as bypass valves, bypass lines, heat exchangers, and the like. 
     In an attempt to obtain better operating efficiency from conventional vapor-compression refrigeration systems, the refrigeration industry is developing systems of growing complexity. Sophisticated computer-controlled thermostatic expansion valves have been developed in an attempt to obtain better control of the heat transfer fluid through the evaporator. Additionally, complex valves and piping systems have been developed to more rapidly defrost the evaporator in order to maintain high heat transfer rates. While these systems have achieved varying levels of success, the system cost rises dramatically as the complexity of the system increases. Accordingly, a need exists for an efficient refrigeration system that can be installed at low cost and operated at high efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention provides a refrigeration system that maintains high operating efficiency by feeding a saturated vapor into the inlet of an evaporator. As used herein, the term “saturated vapor” refers to a heat transfer fluid that resides in both a liquid state and a vapor state with matched enthalpy, indicating the pressure and temperature of the heat transfer fluid are in correlation with each other. Saturated vapor is a high quality liquid vapor mixture. By feeding saturated vapor to the evaporator, heat transfer fluid in both a liquid and a vapor state enters the evaporator coils. Thus, the heat transfer fluid is delivered to the evaporator in a physical state in which maximum heat can be absorbed by the fluid. In addition to high efficiency operation of the evaporator, in one preferred embodiment of the invention, the refrigeration system provides a simple means of defrosting the evaporator. A multifunctional valve is employed that contains separate passageways feeding into a common chamber. In operation, the multifunctional valve can transfer either a saturated vapor, for cooling, or a high temperature vapor, for defrosting, to the evaporator. 
     In one form, the vapor compression system includes an evaporator for evaporating a heat transfer fluid, a compressor for compressing the heat transfer fluid to a relatively high temperature and pressure, and a condenser for condensing the heat transfer fluid. A saturated vapor line is coupled from an expansion valve to the evaporator. In one preferred embodiment of the invention, the diameter and the length of the saturated vapor line is sufficient to insure substantial conversion of the heat transfer fluid into a saturated vapor prior to delivery of the fluid to the evaporator. In one preferred embodiment of the invention, a heat source is applied to the heat transfer fluid in the saturated vapor line sufficient to vaporize a portion of the heat transfer fluid before the heat transfer fluid enters the evaporator. In one preferred embodiment of the invention, a heat source is applied to the heat transfer fluid after the heat transfer fluid passes through the expansion valve and before the heat transfer fluid enters the evaporator. The heat source converts the heat transfer fluid from a low quality liquid vapor mixture to a high quality liquid vapor mixture, or a saturated vapor. Typically, at least about 5% of the heat transfer fluid is vaporized before entering the evaporator. In one embodiment of the invention, the expansion valve resides within a multifunctional valve that includes a first inlet for receiving the heat transfer fluid in the liquid state, and a second inlet for receiving the heat transfer fluid in the vapor state. The multifunctional valve further includes passageways coupling the first and second inlets to a common chamber. Gate valves position within the passageways enable the flow of heat transfer fluid to be independently interrupted in each passageway. The ability to independently control the flow of saturated vapor and high temperature vapor through the refrigeration system produces high operating efficiency by both increased heat transfer rates at the evaporator and by rapid defrosting of the evaporator. The increased operating efficiency enables the refrigeration system to be charged with relatively small amounts of heat transfer fluid, yet the refrigeration system can handle relatively large thermal loads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a vapor-compression system arranged in accordance with one embodiment of the invention; 
     FIG. 2 is a side view, in partial cross-section, of a first side of a multifunctional valve in accordance with one embodiment of the invention; 
     FIG. 3 is a side view, in partial cross-section, of a second side of the multifunctional valve illustrated in FIG. 2; 
     FIG. 4 is an exploded view of a multifunctional valve in accordance with one embodiment of the invention; 
     FIG. 5 is a schematic view of a vapor-compression system in accordance with another embodiment of the invention; 
     FIG. 6 is an exploded view of the multifunctional valve in accordance with another embodiment of the invention; 
     FIG. 7 is a schematic view of a vapor-compression system in accordance with yet another embodiment of the invention; 
     FIG. 8 is an enlarged cross-sectional view of a portion of the vapor compression system illustrated in FIG. 7; 
     FIG. 9 is a schematic view, in partial cross-section, of a recovery valve in accordance with one embodiment of this invention; and 
     FIG. 10 is a schematic view, in partial cross-section, of a recovery valve in accordance with yet another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a vapor-compression system  10  arranged in accordance with one embodiment of the invention is illustrated in FIG.  1 . Refrigeration system  10  includes a compressor  12 , a condenser  14 , an evaporator  16 , and a multifunctional valve  18 . Compressor  12  is coupled to condenser  14  by a discharge line  20 . Multifunctional valve  18  is coupled to condenser  14  by a liquid line  22  coupled to a first inlet  24  of multifunctional valve  18 . Additionally, multifunctional valve  18  is coupled to discharge line  20  at a second inlet  26 . A saturated vapor line  28  couples multifunctional valve  18  to evaporator  16 , and a suction line  30  couples the outlet of evaporator  16  to the inlet of compressor  12 . A temperature sensor  32  is mounted to suction line  30  and is operably connected to multifunctional valve  18 . In accordance with the invention, compressor  12 , condenser  14 , multifunctional valve  18  and temperature sensor  32  are located within a control unit  34 . Correspondingly, evaporator  16  is located within a refrigeration case  36 . In one preferred embodiment of the invention, compressor  12 , condenser  14 , multifunctional valve  18 , temperature sensor  32  and evaporator  16  are all located within a refrigeration case  36 . In another preferred embodiment of the invention, the vapor compression system comprises control unit  34  and refrigeration case  36 , wherein compressor  12  and condenser  14  are located within the control unit  34 , and wherein evaporator  16 , multifunctional valve  18 , and temperature sensor  32  are located within refrigeration case  36 . 
     The vapor compression system of the present invention can utilize essentially any commercially available heat transfer fluid including refrigerants such as, for example, chlorofluorocarbons such as R-12 which is a dicholordifluoromethane, R-22 which is a monochlorodifluoromethane, R-500 which is an azeotropic refrigerant consisting of R-12 and R-152a, R-503 which is an azeotropic refrigerant consisting of R-23 and R-13, and R-502 which is an azeotropic refrigerant consisting of R-22 and R-115. The vapor compression system of the present invention can also utilize refrigerants such as, but not limited to refrigerants R-13, R-113, 141b, 123a, 123, R-114, and R-11. Additionally, the vapor compression system of the present invention can utilize refrigerants such as, for example, hydrochlorofluorocarbons such as 141b, 123a, 123, and 124, hydrofluorocarbons such as R-134a, 134, 152, 143a, 125, 32, 23, and azeotropic HFCs such as AZ-20 and AZ-50 (which is commonly known as R-507). Blended refrigerants such as MP-39, HP-80, FC-14, R-717, and HP-62 (commonly known as R-404a), may also be used as refrigerants in the vapor compression system of the present invention. Accordingly, it should be appreciated that the particular refrigerant or combination of refrigerants utilized in the present invention is not deemed to be critical to the operation of the present invention since this invention is expected to operate with a greater system efficiency with virtually all refrigerants than is achievable by any previously known vapor compression system utilizing the same refrigerant. 
     In operation, compressor  12  compresses the heat transfer fluid, to a relatively high pressure and temperature. The temperature and pressure to which the heat transfer fluid is compressed by compressor  12  will depend upon the particular size of refrigeration system  10  and the cooling load requirements of the systems. Compressor  12  pumps the heat transfer fluid into discharge line  20  and into condenser  14 . As will be described in more detail below, during cooling operations, second inlet  26  is closed and the entire output of compressor  12  is pumped through condenser  14 . 
     In condenser  14 , a medium such as air, water, or a secondary refrigerant is blown past coils within the condenser causing the pressurized heat transfer fluid to change to the liquid state. The temperature of the heat transfer fluid drops about 10 to 40° F. (5.6 to 22.2° C.), depending on the particular heat transfer fluid, or glycol, or the like, as the latent heat within the fluid is expelled during the condensation process. Condenser  14  discharges the liquefied heat transfer fluid to liquid line  22 . As shown in FIG. 1, liquid line  22  immediately discharges into multifunctional valve  18 . Because liquid line  22  is relatively short, the pressurized liquid carried by liquid line  22  does not substantially increase in temperature as it passes from condenser  14  to multifunctional valve  18 . By configuring refrigeration system  10  to have a short liquid line, refrigeration system  10  advantageously delivers substantial amounts of heat transfer fluid to multifunctional valve  18  at a low temperature and high pressure. Since the fluid does not travel a great distance once it is converted to a high-pressure liquid, little heat absorbing capability is lost by the inadvertent warming of the liquid before it enters multifunctional valve  18 , or by a loss of in liquid pressure. While in the above embodiments of the invention, the refrigeration system uses a relatively short liquid line  22 , it is possible to implement the advantages of the present invention in a refrigeration system using a relatively long liquid line  22 , as will be described below. The heat transfer fluid discharged by condenser  14  enters multifunctional valve  18  at first inlet  22  and undergoes a volumetric expansion at a rate determined by the temperature of suction line  30  at temperature sensor  32 . Multifunctional valve  18  discharges the heat transfer fluid as a saturated vapor into saturated vapor line  28 . Temperature sensor  32  relays temperature information through a control line  33  to multifunctional valve  18 . 
     Those skilled in the art will recognize that refrigeration system  10  can be used in a wide variety of applications for controlling the temperature of an enclosure, such as a refrigeration case in which perishable food items are stored. For example, where refrigeration system  10  is employed to control the temperature of a refrigeration case having a cooling load of about 12000 Btu/hr (84 g cal/s), compressor  12  discharges about 3 to 5 lbs/min (1.36 to 2.27 kg/min) of R-12 at a temperature of about 110° F. (43.3° C.) to about 120° F. (48.9° C.) and a pressure of about 150 lbs/in 2  (1.03 E5 N/m 2 ) to about 180 lbs/in. 2  (1.25 E5 N/m 2 ) 
     In accordance with one preferred embodiment of the invention, saturated vapor line  28  is sized in such a way that the low pressure fluid discharged into saturated vapor line  28  substantially converts to a saturated vapor as it travels through saturated vapor line  28 . In one embodiment, saturated vapor line  28  is sized to handle about 2500 ft/min (76 m/min) to 3700 ft/min (1128 m/min) of a heat transfer fluid, such as R-12, and the like, and has a diameter of about 0.5 to 1.0 inches (1.27 to 2.54 cm), and a length of about 90 to 100 feet (27 to 30.5 m). As described in more detail below, multifunctional valve  18  includes a common chamber immediately before the outlet. The heat transfer fluid undergoes an additional volumetric expansion as it enters the common chamber. The additional volumetric expansion of the heat transfer fluid in the common chamber of multifunctional valve  18  is equivalent to an effective increase in the line size of saturated vapor line  28  by about 225%. 
     Those skilled in the art will further recognize that the positioning of a valve for volumetrically expanding of the heat transfer fluid in close proximity to the condenser, and the relatively great length of the fluid line between the point of volumetric expansion and the evaporator, differs considerably from systems of the prior art. In a typical prior art system, an expansion valve is positioned immediately adjacent to the inlet of the evaporator, and if a temperature sensing device is used, the device is mounted in close proximity to the outlet of the evaporator. As previously described, such system can suffer from poor efficiency because substantial amounts of the evaporator carry a liquid rather than a saturated vapor. Fluctuations in high side pressure, liquid temperature, heat load or other conditions can adversely effect the evaporator&#39;s efficiency. 
     In contrast to the prior art, the inventive refrigeration system described herein positions a saturated vapor line between the point of volumetric expansion and the inlet of the evaporator, such that portions of the heat transfer fluid are converted to a saturated vapor before the heat transfer fluid enters the evaporator. By charging evaporator  16  with a saturated vapor, the cooling efficiency is greatly increased. By increasing the cooling efficiency of an evaporator, such as evaporator  16 , numerous benefits are realized by the refrigeration system. For example, less heat transfer fluid is needed to control the air temperature of refrigeration case  36  at a desired level. Additionally, less electricity is needed to power compressor  12  resulting in lower operating cost. Further, compressor  12  can be sized smaller than a prior art system operating to handle a similar cooling load. Moreover, in one preferred embodiment of the invention, the refrigeration system avoids placing numerous components in proximity to the evaporator. By restricting the placement of components within refrigeration case  36  to a minimal number, the thermal loading of refrigeration case  36  is minimized. 
     While in the above embodiments of the invention, multifunctional valve  18  is positioned in close proximity to condenser  14 , thus creating a relatively short liquid line  22  and a relatively long saturated vapor line  28 , it is possible to implement the advantages of the present invention even if multifunctional valve  18  is positioned immediately adjacent to the inlet of the evaporator  16 , thus creating a relatively long liquid line  22  and a relatively short saturated vapor line  28 . For example, in one preferred embodiment of the invention, multifunctional valve  18  is positioned immediately adjacent to the inlet of the evaporator  16 , thus creating a relatively long liquid line  22  and a relatively short saturated vapor line  28 , as illustrated in FIG.  7 . In order to insure that the heat transfer fluid entering evaporator  16  is a saturated vapor, a heat source  25  is applied to saturated vapor line  28 , as illustrated in FIGS. 7-8. Temperature sensor  32  is mounted to suction line  30  and operatively connected to multifunctional valve  18 , wherein heat source  25  is of sufficient intensity so as to vaporize a portion of the heat transfer fluid before the heat transfer fluid enters evaporator  16 . The heat transfer fluid entering evaporator  16  is converted to a saturated vapor wherein a portion of the heat transfer fluids exists in a liquid state  29 , and another portion of the heat transfer fluid exists in a vapor state  31 , as illustrated in FIG.  8 . 
     Preferably heat source  25  used to vaporize a portion of the heat transfer fluid comprises heat transferred to the ambient surroundings from condenser  14 , however, heat source  25  can comprise any external or internal source of heat known to one of ordinary skill in the art, such as, for example, heat transferred to the ambient surroundings from the discharge line  20 , heat transferred to the ambient surroundings from a compressor, heat generated by the compressor, heat generated from an electrical heat source, heat generated using combustible materials, heat generated using solar energy, or any other source of heat. Heat source  25  can also comprise an active heat source, that is, any heat source that is intentionally applied to a part of refrigeration system  10 , such as saturated vapor line  28 . An active heat source includes but is not limited to source of heat such as heat generated from an electrical heat source, heat generated using combustible materials, heat generated using solar energy, or any other source of heat which is intentionally and actively applied to any part of refrigeration system  10 . A heat source that comprises heat which accidentally leaks into any part of refrigeration system  10  or heat which is unintentionally or unknowingly absorbed into any part of refrigeration system  10 , either due to poor insulation or other reasons, is not an active heat source. 
     In one preferred embodiment of the invention, temperature sensor  32  monitors the heat transfer fluid exiting evaporator  16  in order to insure that a portion of the heat transfer fluid is in a liquid state  29  upon exiting evaporator  16 , as illustrated in FIG.  8 . In one preferred embodiment of the invention, at least about 5% of the of the heat transfer fluid is vaporized before the heat transfer fluid enters the evaporator, and at least about 1% of the heat transfer fluid is in a liquid state upon exiting the evaporator. By insuring that a portion of the heat transfer fluid is in liquid state  29  and vapor state  31  upon entering and exiting the evaporator, the vapor compression system of the present invention allows evaporator  16  to operate with maximum efficiency. In one preferred embodiment of the invention, the heat transfer fluid is in at least about a 1% superheated state upon exiting evaporator  16 . In one preferred embodiment of the invention, the heat transfer fluid is between about a 1% liquid state and about a 1% superheated vapor state upon exiting evaporator  16 . 
     While the above embodiments rely on heat source  25  or the dimensions and length of saturated vapor line  28  to insure that the heat transfer fluid enters the evaporator  16  as a saturated vapor, any means known to one of ordinary skill in the art which can convert the heat transfer fluid to a saturated vapor upon entering evaporator  16  can be used. Additionally, while the above embodiments use temperature sensor  32  to monitor the state of the heat transfer fluid exiting the evaporator, any metering device known to one of ordinary skill in the art which can determine the state of the heat transfer fluid upon exiting the evaporator can be used, such as a pressure sensor, or a sensor which measures the density of the fluid. Additionally, while in the above embodiments, the metering device monitors the state of the heat transfer fluid exiting evaporator  16 , the metering device can also be placed at any point in or around evaporator  16  to monitor the state of the heat transfer fluid at any point in or around evaporator  16 . 
     Shown in FIG. 2 is a side view, in partial cross-section, of one embodiment of multifunctional valve  18 . Heat transfer fluid enters first inlet  24  and traverses a first passageway  38  to a common chamber  40 . An expansion valve  42  is positioned in first passageway  38  near first inlet  22 . Expansion valve  42  meters the flow of the heat transfer fluid through first passageway  38  by means of a diaphragm (not shown) enclosed within an upper valve housing  44 . Control line  33  is connected to an input  62  located on upper valve housing  44 . Signals relayed through control line  33  activate the diaphragm within upper valve housing  44 . The diaphragm actuates a valve assembly  54  (shown in FIG. 4) to control the amount of heat transfer fluid entering an expansion chamber  52  (shown in FIG. 4) from first inlet  24 . A gating valve  46  is positioned in first passageway  38  near common chamber  40 . In a preferred embodiment of the invention, gating valve  46  is a solenoid valve capable of terminating the flow of heat transfer fluid through first passageway  38  in response to an electrical signal. 
     Shown in FIG. 3 is a side view, in partial cross-section, of a second side of multifunctional valve  18 . A second passageway  48  couples second inlet  26  to common chamber  40 . A gating valve  50  is positioned in second passageway  48  near common chamber  40 . In a preferred embodiment of the invention, gating valve  50  is a solenoid valve capable of terminating the flow of heat transfer fluid through second passageway  48  upon receiving an electrical signal. Common chamber  40  discharges the heat transfer fluid from multifunctional valve  18  through an outlet  41 . 
     An exploded perspective view of multifunctional valve  18  is illustrated in FIG.  4 . Expansion valve  42  is seen to include expansion chamber  52  adjacent first inlet  22 , valve assembly  54 , and upper valve housing  44 . Valve assembly  54  is actuated by a diaphragm (not shown) contained within the upper valve housing  44 . First and second tubes  56  and  58  are located intermediate to expansion chamber  52  and a valve body  60 . Gating valves  46  and  50  are mounted on valve body  60 . In accordance with the invention, refrigeration system  10  can be operated in a defrost mode by closing gating valve  46  and opening gating valve  50 . In defrost mode, high temperature heat transfer fluid enters second inlet  26  and traverses second passageway  48  and enters common chamber  40 . The high temperature vapors are discharged through outlet  41  and traverse saturated vapor line  28  to evaporator  16 . The high temperature vapor has a temperature sufficient to raise the temperature of evaporator  16  by about 50 to 120° F. (27.8 to 66.7° C.). The temperature rise is sufficient to remove frost from evaporator  16  and restore the heat transfer rate to desired operational levels. 
     While the above embodiments use a multifunctional valve  18  for expanding the heat transfer fluid before entering evaporator  16 , any thermostatic expansion valve or throttling valve, such as expansion valve  42  or even recovery valve  19 , may be used to expand heat transfer fluid before entering evaporator  16 . 
     In one preferred embodiment of the invention heat source  25  is applied to the heat transfer fluid after the heat transfer fluid passes through expansion valve  42  and before the heat transfer fluid enters the inlet of evaporator  16  to convert the heat transfer fluid from a low quality liquid vapor mixture to a high quality liquid vapor mixture, or a saturated vapor. In one preferred embodiment of the invention, heat source  25  is applied to a multifunctional valve  18 . In another preferred embodiment of the invention heat source  25  is applied within recovery valve  19 , as illustrated in FIG.  9 . Recovery valve  19  comprises a first inlet  124  connected to liquid line  22  and a first outlet  159  connected to saturated vapor line  28 . Heat transfer fluid enters first inlet  124  of recovery valve  19  to a common chamber  140 . An expansion valve  142  is positioned near first inlet  124  to expand the heat transfer fluid entering first inlet  124  from a liquid state to a low quality liquid vapor mixture. Second inlet  127  is connected to discharge line  20 , and receives high temperature heat transfer fluid exiting compressor  12 . High temperature heat transfer fluid exiting compressor  12  enters second inlet  127  and traverses second passageway  123 . Second passageway  123  is connected to second inlet  127  and second outlet  130 . A portion of second passageway  123  is located adjacent to common chamber  140 . 
     As the high temperature heat transfer fluid nears common chamber  140 , heat from the high temperature heat transfer fluid is transferred from the second passageway  123  to the common chamber  140  in the form of heat source  125 . By applying heat from heat source  125  to the heat transfer fluid, the heat transfer fluid in common chamber  140  is converted from a low quality liquid vapor mixture to a high quality liquid vapor mixture, or saturated vapor, as the heat transfer fluid flows through common chamber  140 . Additionally, the high temperature heat transfer fluid in the second passageway  123  is cooled as the high temperature heat transfer fluid passes near common chamber  140 . Upon traversing second passageway  123 , the cooled high temperature heat transfer fluid exits second outlet  130  and enters condenser  14 . Heat transfer fluid in common chamber  140  exits recover valve  19  at first outlet  159  into saturated vapor line  28  as a high quality liquid vapor mixture, or saturated vapor. 
     While in the above preferred embodiment, heat source  125  comprises heat transferred to the ambient surroundings from a compressor, heat source  125  may comprise any external or internal source of heat known to one of ordinary skill in the art, such as, for example, heat generated from an electrical heat source, heat generated using combustible materials, heat generated using solar energy, or any other source of heat. Heat source  125  can also comprise any heat source  25  and any active heat source, as previously defined. 
     In one preferred embodiment of the invention, recovery valve  19  comprises third passageway  148  and third inlet  126 . Third inlet  126  is connected to discharge line  20 , and receives high temperature heat transfer fluid exiting compressor  12 . A first gating valve (not shown) capable of terminating the flow of heat transfer fluid through common chamber  140  is positioned near the first inlet  124  of common chamber  140 . Third passageway  148  connects third inlet  126  to common chamber  140 . A second gating valve (not shown) is positioned in third passageway  148  near common chamber  140 . In a preferred embodiment of the invention, the second gating valve is a solenoid valve capable of terminating the flow of heat transfer fluid through third passageway  148  upon receiving an electrical signal. 
     In accordance with the invention, refrigeration system  10  can be operated in a defrost mode by closing the first gating valve located near first inlet  124  of common chamber  140  and opening the second gating valve positioned in third passageway  148  near common chamber  140 . In defrost mode, high temperature heat transfer fluid from compressor  12  enters third inlet  126  and traverses third passageway  148  and enters common chamber  140 . The high temperature heat transfer fluid is discharged through first outlet  159  of recovery valve  19  and traverses saturated vapor line  28  to evaporator  16 . The high temperature heat transfer fluid has a temperature sufficient to raise the temperature of evaporator  16  by about 50 to 120° F. (27.8 to 66.7° C.). The temperature rise is sufficient to remove frost from evaporator  16  and restore the heat transfer rate to desired operational levels. 
     During the defrost cycle, any pockets of oil trapped in the system will be warmed and carried in the same direction of flow as the heat transfer fluid. By forcing hot gas through the system in a forward flow direction, the trapped oil will eventually be returned to the compressor. The hot gas will travel through the system at a relatively high velocity, giving the gas less time to cool thereby improving the defrosting efficiency. The forward flow defrost method of the invention offers numerous advantages to a reverse flow defrost method. For example, reverse flow defrost systems employ a small diameter check valve near the inlet of the evaporator. The check valve restricts the flow of hot gas in the reverse direction reducing its velocity and hence its defrosting efficiency. Furthermore, the forward flow defrost method of the invention avoids pressure build up in the system during the defrost system. Additionally, reverse flow methods tend to push oil trapped in the system back into the expansion valve. This is not desirable because excess oil in the expansion can cause gumming that restricts the operation of the valve. Also, with forward defrost, the liquid line pressure is not reduced in any additional refrigeration circuits being operated in addition to the defrost circuit. 
     It will be apparent to those skilled in the art that a vapor compression system arranged in accordance with the invention can be operated with less heat transfer fluid those comparable sized system of the prior art. By locating the multifunctional valve near the condenser, rather than near the evaporation, the saturated vapor line is filled with a relatively low-density vapor, rather than a relatively high-density liquid. Alternatively, by applying a heat source to the saturated vapor line, the saturated vapor line is also filled with a relatively low-density vapor, rather than a relatively high-density liquid. Additionally, prior art systems compensate for low temperature ambient operations (e.g. winter time) by flooding the evaporator in order to reinforce a proper head pressure at the expansion valve. In one preferred embodiment of the invention vapor compression system heat pressure is more readily maintained in cold weather, since the multifunctional value is positioned in close proximity to the condenser. 
     The forward flow defrost capability of the invention also offers numerous operating benefits as a result of improved defrosting efficiency. For example, by forcing trapped oil back into the compressor, liquid slugging is avoided, which has the effect of increasing the useful life of the equipment. Furthermore, reduced operating cost are realized because less time is required to defrost the system. Since the flow of hot gas can be quickly terminated, the system can be rapidly returned to normal cooling operation. When frost is removed from evaporator  16 , temperature sensor  32  detects a temperature increase in the heat transfer fluid in suction line  30 . When the temperature rises to a given set point, gating valve  50  and multifunctional valve  18  is closed. Once the flow of heat transfer fluid through first passageway  38  resumes, cold saturated vapor quickly returns to evaporator  16  to resume refrigeration operation. 
     Those skilled in the art will appreciate that numerous modifications can be made to enable the refrigeration system of the invention to address a variety of applications. For example, refrigeration systems operating in retail food outlets typically include a number of refrigeration cases that can be serviced by a common compressor system. Also, in applications requiring refrigeration operations with high thermal loads, multiple compressors can be used to increase the cooling capacity of the refrigeration system. 
     A vapor compression system  64  in accordance with another embodiment of the invention having multiple evaporators and multiple compressors is illustrated in FIG.  5 . In keeping with the operating efficiency and low-cost advantages of the invention, the multiple compressors, the condenser, and the multiple multifunctional valves are contained within a control unit  66 . Saturated vapor lines  68  and  70  feed saturated vapor from control unit  66  to evaporators  72  and  74 , respectively. Evaporator  72  is located in a first refrigeration case  76 , and evaporator  74  is located in a second refrigeration case  78 . First and second refrigeration cases  76  and  78  can be located adjacent to each other, or alternatively, at relatively great distance from each other. The exact location will depend upon the particular application. For example, in a retail food outlet, refrigeration cases are typically placed adjacent to each other along an isle way. Importantly, the refrigeration system of the invention is adaptable to a wide variety of operating environments. This advantage is obtained, in part, because the number of components within each refrigeration case is minimal. In one preferred embodiment of the invention, by avoiding the requirement of placing numerous system components in proximity to the evaporator, the refrigeration system can be used where space is at a minimum. This is especially advantageous to retail store operations, where floor space is often limited. 
     In operation, multiple compressors  80  feed heat transfer fluid into an output manifold  82  that is connected to a discharge line  84 . Discharge line  84  feeds a condenser  86  and has a first branch line  88  feeding a first multifunctional valve  90  and a second branch line  92  feeding a second multifunctional valve  94 . A bifurcated liquid line  96  feeds heat transfer fluid from condenser  86  to first and second multifunctional valves  90  and  94 . Saturated vapor line  68  couples first multifunctional valve  90  with evaporator  72 , and saturated vapor line  70  couples second multifunctional valve  94  with evaporator  74 . A bifurcated suction line  98  couples evaporators  72  and  74  to a collector manifold  100  feeding multiple compressors  80 . A temperature sensor  102  is located on a first segment  104  of bifurcated suction line  98  and relays signals to first multifunctional valve  90 . A temperature sensor  106  is located on a second segment  108  of bifurcated suction line  98  and relays signals to second multifunctional valve  94 . In one preferred embodiment of the invention, a heat source, such as heat source  25 , can be applied to saturated vapor lines  68  and  70  to insure that the heat transfer fluid enters evaporators  72  and  74  as a saturated vapor. 
     Those skilled in the art will appreciate that numerous modifications and variations of vapor compression system  64  can be made to address different refrigeration applications. For example, more than two evaporators can be added to the system in accordance with the general method illustrated in FIG.  5 . Additionally, more condensers and more compressors can also be included in the refrigeration system to further increase the cooling capability. 
     A multifunctional valve  110  arranged in accordance with another embodiment of the invention is illustrated in FIG.  6 . In similarity with the previous multifunctional valve embodiment, the heat transfer fluid exiting the condenser in the liquid state enters a first inlet  122  and expands in expansion chamber  152 . The flow of heat transfer fluid is metered by valve assembly  154 . In the present embodiment, a solenoid valve  112  has an armature  114  extending into a common seating area  116 . In refrigeration mode, armature  114  extends to the bottom of common seating area  116  and cold refrigerant flows through a passageway  118  to a common chamber  140 , then to an outlet  120 . In defrost mode, hot vapor enters second inlet  126  and travels through common seating area  116  to common chamber  140 , then to outlet  120 . Multifunctional valve  110  includes a reduced number of components, because the design is such as to allow a single gating valve to control the flow of hot vapor and cold vapor through the valve. 
     In yet another embodiment of the invention, the flow of liquefied heat transfer fluid from the liquid line through the multifunctional valve can be controlled by a check valve positioned in the first passageway to gate the flow of the liquefied heat transfer fluid into the saturated vapor line. The flow of heat transfer fluid through the refrigeration system is controlled by a pressure valve located in the suction line in proximity to the inlet of the compressor. Accordingly, the various functions of a multifunctional valve of the invention can be performed by separate components positioned at different locations within the refrigeration system. All such variations and modifications are contemplated by the present invention. 
     Those skilled in the art will recognize that the vapor compression system and method described herein can be implemented in a variety of configurations. For example, the compressor, condenser, multifunctional valve, and the evaporator can all be housed in a single unit and placed in a walk-in cooler. In this application, the condenser protrudes through the wall of the walk-in cooler and ambient air outside the cooler is used to condense the heat transfer fluid. 
     In another application, the vapor compression system and method of the invention can be configured for air-conditioning a home or business. In this application, a defrost cycle is unnecessary since icing of the evaporator is usually not a problem. 
     In yet another application, the vapor compression system and method of the invention can be used to chill water. In this application, the evaporator is immersed in water to be chilled. Alternatively, water can be pumped through tubes that are meshed with the evaporator coils. 
     In a further application, the vapor compression system and method of the invention can be cascaded together with another system for achieving extremely low refrigeration temperatures. For example, two systems using different heat transfer fluids can be coupled together such that the evaporator of a first system provide a low temperature ambient. A condenser of the second system is placed in the low temperature ambient and is used to condense the heat transfer fluid in the second system. 
     Without further elaboration it is believed that one skilled in the art can, using the preceding description, utilize the invention to its fullest extent. The following examples are merely illustrative of the invention and are not meant to limit the scope in any way whatsoever. 
     EXAMPLE I 
     A 5-ft (1.52 m) Tyler Chest Freezer was equipped with a multifunctional valve in a refrigeration circuit, and a standard expansion valve was plumbed into a bypass line so that the refrigeration circuit could be operated as a conventional refrigeration system and as an XDX refrigeration system arranged in accordance with the invention. The refrigeration circuit described above was equipped with a saturated vapor line having an outside tube diameter of about 0.375 inches (0.953 cm) and an effective tube length of about 10 ft (3.048 m). The refrigeration circuit was powered by a Copeland hermetic compressor having a capacity of about ⅓ ton (338 kg) of refrigeration. A sensing bulb was attached to the suction line about 18 inches from the compressor. The circuit was charged with about 28 oz. (792 g) of R-12 refrigerant available from The DuPont Company. The refrigeration circuit was also equipped with a bypass line extending from the compressor discharge line to the saturated vapor line for forward-flow defrosting (See FIG.  1 ). All refrigerated ambient air temperature measurements were made using a “CPS Date Logger” by CPS temperature sensor located in the center of the refrigeration case, about 4 inches (10 cm) above the floor. 
     XDX System—Medium Temperature Operation 
     The nominal operating temperature of the evaporator was 20° F. (−6.7° C.) and the nominal operating temperature of the condenser was 120° F. (48.9° C.). The evaporator handled a cooling load of about 3000 Btu/hr (21 g cal/s). The multifunctional valve metered refrigerant into the saturated vapor line at a temperature of about 20° F. (−6.7° C.). The sensing bulb was set to maintain about 25° F. (13.9° C.) superheating of the vapor flowing in the suction line. The compressor discharged pressurized refrigerant into the discharge line at a condensing temperature of about 120° F. (48.9° C.), and a pressure of about 172 lbs/in 2  (118,560 N/m 2 ). 
     XDX System—Low Temperature Operation 
     The nominal operating temperature of the evaporator was −5° F. (−20.5° C.) and the nominal operating temperature of the condenser was 115° F. (46.1° C.). The evaporator handled a cooling load of about 3000 Btu/hr (21 g cal/s). The multifunctional valve metered about 2975 ft/min (907 km/min) of refrigerant into the saturated vapor line at a temperature of about −5° F. (−20.5° C.). The sensing bulb was set to maintain about 20° F. (11.1° C.) superheating of the vapor flowing in the suction line. The compressor discharged about 2299 ft/min (701 m/min) of pressurized refrigerant into the discharge line at a condensing temperature of about 115° F. (46.1° C.), and a pressure of about 161 lbs/in 2  (110,977 N/m 2 ). The XDX system was operated substantially the same in low temperature operation as in medium temperature operation with the exception that the fans in the Tyler Chest Freezer were delayed for 4 minutes following defrost to remove heat from the evaporator coil and to allow water drainage from the coil. 
     The XDX refrigeration system was operated for a period of about 24 hours at medium temperature operation and about 18 hours at low temperature operation. The temperature of the ambient air within the Tyler Chest Freezer was measured about every minute during the 23 hour testing period. The air temperature was measured continuously during the testing period, while the refrigeration system was operated in both refrigeration mode and in defrost mode. During defrost cycles, the refrigeration circuit was operated in defrost mode until the sensing bulb temperature reached about 50° F. (10° C.). The temperature measurement statistics appear in Table I below. 
     Conventional System—Medium Temperature Operation With Electric Defrost 
     The Tyler Chest Freezer described above was equipped with a bypass line extending between the compressor discharge line and the suction line for defrosting. The bypass line was equipped with a solenoid valve to gate the flow of high temperature refrigerant in the line. An electric heat element was energized instead of the solenoid during this test. A standard expansion valve was installed immediately adjacent to the evaporator inlet and the temperature sensing bulb was attached to the suction line immediately adjacent to the evaporator outlet. The sensing bulb was set to maintain about 6° F. (3.33° C.) superheating of the vapor flowing in the suction line. Prior to operation, the system was charged with about 48 oz. (1.36 kg) of R-12 refrigerant. 
     The conventional refrigeration system was operated for a period of about 24 hours at medium temperature operation. The temperature of the ambient air within the Tyler Chest Freezer was measured about every minute during the 24 hour testing period. The air temperature was measured continuously during the testing period, while the refrigeration system was operated in both refrigeration mode and in reverse-flow defrost mode. During defrost cycles, the refrigeration circuit was operated in defrost mode until the sensing bulb temperature reached about 50° F. (10° C.). The temperature measurement statistics appear in Table I below. 
     Conventional System—Medium Temperature Operation With Air Defrost 
     The Tyler Chest Freezer described above was equipped with a receiver to provide proper liquid supply to the expansion valve and a liquid line dryer was installed to allow for additional refrigerant reserve. The expansion valve and the sensing bulb were positioned at the same locations as in the reverse-flow defrost system described above. The sensing bulb was set to maintain about 8° F. (4.4° C.) superheating of the vapor flowing in the suction line. Prior to operation, the system was charged with about 34 oz. (0.966 kg) of R-12 refrigerant. 
     The conventional refrigeration system was operated for a period of about 24½ hours at medium temperature operation. The temperature of the ambient air within the Tyler Chest Freezer was measured about every minute during the 24½ hour testing period. The air temperature was measured continuously during the testing period, while the refrigeration system was operated in both refrigeration mode and in air defrost mode. In accordance with conventional practice, four defrost cycles were programmed with each lasting for about 36 to 40 minutes. The temperature measurement statistics appear in Table I below. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 REFRIGERATION TEMPERATURES (° F./° C.) 
               
             
          
           
               
                   
                 XDX 1)   
                 XDX 1)   
                   
                   
               
               
                   
                 Medium 
                 Low 
                 Conventional 2)   
                 Conventional 2)   
               
               
                   
                 Temperature 
                 Temperature 
                 Electric Defrost 
                 Air Defrost 
               
               
                   
                   
               
             
          
           
               
                 Average 
                 38.7/3.7 
                 4.7/−15.2 
                 39.7/4.3 
                 39.6/4.2 
               
               
                 Standard 
                 0.8 
                 0.8 
                  4.1 
                  4.5 
               
               
                 Deviation 
               
               
                 Variance 
                 0.7 
                 0.6 
                 16.9 
                 20.4 
               
               
                 Range 
                 7.1 
                 7.1 
                 22.9 
                 26.0 
               
               
                   
               
               
                   1) one defrost cycle during 23 hour test period  
               
               
                   2) three defrost cycles during 24 hour test period  
               
             
          
         
       
     
     As illustrated above, the XDX refrigeration system arranged in accordance with the invention maintains a desired the temperature within the chest freezer with less temperature variation than the conventional systems. The standard deviation, the variance, and the range of the temperature measurements taken during the testing period are substantially less than the conventional systems. This result holds for operation of the XDX system at both medium and low temperatures. 
     During defrost cycles, the temperature rise in the chest freezer was monitored to determine the maximum temperature within the freezer. This temperature should be as close to the operating refrigeration temperature as possible to avoid spoilage of food products stored in the freezer. The maximum defrost temperature for the XDX system and for the conventional systems is shown in Table II below. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 MAXIMUM DEFROST TEMPERATURE (° F./° C.) 
               
             
          
           
               
                 XDX 
                 Conventional 
                 Conventional 
               
               
                 Medium Temperature 
                 Electric Defrost 
                 Air Defrost 
               
               
                   
               
               
                 44.4/6.9 
                 55.0/12.8 
                 58.4/14.7 
               
               
                   
               
             
          
         
       
     
     EXAMPLE II 
     The Tyler Chest Freezer was configured as described above and further equipped with electric defrosting circuits. The low temperature operating test was carried out as described above and the time needed for the refrigeration unit to return to refrigeration operating temperature was measured. A separate test was then carried out using the electric defrosting circuit to defrost the evaporator. The time needed for the XDX system and an electric defrost system to complete defrost and to return to the 5° F. (−15° C.) operating set point appears in Table III below. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 TIME NEEDED TO RETURN TO REFRIGERATION 
               
               
                 TEMPERATURE OF 5° F. (−15° C.) FOLLOWING 
               
             
          
           
               
                   
                 XDX 
                 Conventional System with Electric Defrost 
               
               
                   
                   
               
             
          
           
               
                 Defrost Duration (min) 
                 10 
                  36 
               
               
                 Recovery Time (min) 
                 24 
                 144 
               
               
                   
               
             
          
         
       
     
     As shown above, the XDX system using forward-flow defrost through the multifunctional valve needs less time to completely defrost the evaporator, and substantially less time to return to refrigeration temperature. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a vapor compression system that fully provides the advantages set forth above. Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. For example, non-halogenated refrigerants can be used, such as ammonia, and the like can also be used. It is therefore intended to include within the invention all such variations and modifications that fall within the scope of the appended claims and equivalents thereof.