an embodiment of a vapor - compression system 10 arranged in accordance with one embodiment of the invention is illustrated in fig1 . 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 inventions 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 fig1 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 fig7 . 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 fig7 - 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 fig8 . 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 fig8 . 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 fig2 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 fig4 ) to control the amount of heat transfer fluid entering in expansion chamber 52 ( shown in fig4 ) 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 fig3 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 fig4 . 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 fig9 . 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 inventive 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 fig5 . 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 fig5 . 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 fig6 . 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 . 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 ⅓ton ( 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 fig1 ). 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 . 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 ). 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 . 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 . 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 24½hours 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 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 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 . (− 29 ° 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 has fall within the scope of the appended claims and equivalents thereof .