Abstract:
A refrigeration system disposed within an outer cabinet having a freezer compartment and a fresh food compartment comprises a freezer evaporator and a fresh food evaporator each having an inlet and an outlet. A compressor is coupled to the freezer evaporator via a conduit and a control valve is disposed at the inlet of the freezer evaporator to control refrigerant flow therethrough. A first liquid line temperature sensor is disposed so as to detect refrigerant temperature at the inlet of the freezer evaporator and a second liquid line temperature sensor is disposed so as to detect refrigerant temperature at the inlet of the fresh food evaporator. A freezer compartment temperature sensor and a fresh food compartment temperature sensor are disposed within the freezer compartment and the fresh food compartment, respectively, to sense compartment a temperatures. A controller is coupled to the control valve and to the compressor so as to provide control signals thereto and to the liquid line temperature sensors and the compartment temperature sensors to receive temperature signals therefrom. The controller generates control signals to the control valve in response to temperature signals generated from the liquid line temperature sensors and the compartment temperature sensors to modify the control valves duty cycle of open-to-closed conditions so as to maintain evaporator dryness at a relatively fixed level.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to refrigeration systems and more particularly relates to energy saving refrigeration systems. 
     Household refrigerators typically operate on a simple vapor compression cycle. Such a cycle typically includes a compressor, a condenser, an expansion device, and an evaporator connected in series and charged with a refrigerant. The evaporator is a specific type of heat exchanger which transfers heat from air passing over the evaporator to refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is then used to refrigerate one or more freezer or fresh food compartments. 
     In conventional single-evaporator refrigerators, since the freezer compartment and the fresh food compartment are simultaneously cooled with one evaporator, the temperature of the evaporator must be maintained at a temperature lower than about −15° C., which is typically the temperature of the freezer compartment. Accordingly, an evaporator with a lower temperature than is necessary is used to cool the fresh food compartment, causing the efficiency of the overall system to be relatively low. 
     Other conventional refrigerators require at least two capillary tubes to control expansion. Each capillary tube is preceded in the refrigerant flow path by an electrically activated valve in order to control liquid discharge from the condenser to selectively flow through one of the capillary tubes. An air flow direction control scheme directs air flow to and from the sole evaporator to be either fresh food or freezer compartment air. When the air flow coupling is with the fresh food compartment (about 7° C.) the refrigerant operates at a relatively high evaporator saturation temperature and when the air flow coupling is with the freezer compartment (about −15° C.), the refrigerant operates at a relatively lower saturation temperature. 
     Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is substantially a saturated vapor. Accordingly, this is the purpose for two switchable capillary tubes. A higher evaporator saturation temperature produces a high pressure, thus a higher vapor density, thereby generating a greater compressor mass flow rate. To support a higher compressor mass flow rate, a less restrictive capillary tube is required. This system will work satisfactorily near operating conditions for which the capillary tubes were optimized. At off design conditions, however, the evaporator exit state will be either a vapor quality or superheat and cycle efficiency will be lower. 
     Additionally, during operation of conventional refrigeration systems, condensed moisture forms as frost or ice on the exposed surfaces of an evaporator. Since ice accumulation will eventually cause cycle efficiency degradation, the evaporator must periodically undergo a defrosting period. 
     Therefore, it is apparent from the above that there exists a need in the art for a simplified refrigeration expansion control and for improved defrosting within refrigeration systems. 
     SUMMARY OF THE INVENTION 
     A refrigeration system disposed within an outer cabinet having a freezer compartment and a fresh food compartment comprises a freezer evaporator and a fresh food evaporator each having an inlet and an outlet. A compressor is coupled to the freezer evaporator via a conduit and a control valve is disposed at the inlet of the freezer evaporator to control refrigerant flow therethrough. A first liquid line temperature sensor is disposed so as to detect refrigerant temperature at the inlet of the freezer evaporator and a second liquid line temperature sensor is disposed so as to detect refrigerant temperature at the inlet of the fresh food evaporator. A freezer compartment temperature sensor and a fresh food compartment temperature sensor are disposed within the freezer compartment and the fresh food compartment, respectively, to sense compartment temperatures. A controller is coupled to the control valve and to the compressor so as to provide control signals thereto and to the liquid line temperature sensors and the compartment temperature sensors to receive temperature signals therefrom. The controller generates control signals to the control valve in response to temperature signals generated from the liquid line temperature sensors and the compartment temperature sensors to modify the control valves duty cycle of open-to-closed conditions so as to maintain evaporator dryness at a relatively fixed level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional side elevation view of an illustrative embodiment of the instant invention; 
     FIG. 2 is an exemplary control logic flowchart in accordance with one embodiment of the instant invention; 
     FIG. 3 is another schematic cross-sectional side elevation view of an illustrative embodiment of the instant invention; 
     FIG. 4 is another exemplary control logic flowchart in accordance with an embodiment of the instant invention; 
     FIG. 5 is another schematic cross-sectional side elevation view of an illustrative embodiment of the instant invention; and 
     FIG. 6 is another exemplary control logic flowchart in accordance with an embodiment of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary embodiment of a refrigeration system  10  includes an outer cabinet  11  having a freezer compartment  12  separated from and substantially temperature independent of a fresh food compartment  14 , as shown in FIG. 1. A mullion  16  separates freezer compartment  12  and fresh food compartment  14 . Although the present invention is described herein in connection with refrigeration system  10 , the present invention is not limited to practice with refrigeration system  10 . In fact, the present invention can be implemented and utilized with many other configurations. 
     Freezer and fresh food compartments  12 ,  14  typically comprise a housing formed with thermally insulated walls provided with an opening or a door for placement or removal of articles. 
     Refrigeration system  10  further comprises a compressor  18 , a condenser  20 , a control valve  22 , a freezer evaporator  24  and a fresh food evaporator  26 . Control valve  22  and compressor  18  are each electrically coupled to a controller  27 . 
     Control valve  22 , typically a solenoid valve, an electronic expansion valve (EEV), or the like, is disposed within a liquid line  28 , which liquid line  28  connects condenser  20  and freezer evaporator  24 . Control valve  22  is typically positioned adjacent the inlet of freezer evaporator  24  and is the throttling or metering device that controls the operation of refrigeration system  10 . Control valve  22  is typically a closed valve, that is, control valve  22  closes when it is not energized. Control valve  22  may comprise a pulse width modulated valve and may be controlled by a method such as those described in commonly assigned U.S. Pat. Nos. 5,415,008, 5,425,246, 5,426,952 or 5,463,876, each of which is herein incorporated by reference. 
     A first liquid line temperature sensor  30  is disposed so as to detect the temperature of refrigerant prior to entering freezer evaporator  24  and a second liquid line temperature sensor  32  is disposed so as to detect the temperature of refrigerant prior to entering fresh food evaporator  26 . Additionally, a freezer compartment temperature sensor  34  and a fresh food compartment temperature sensor  36  are disposed within freezer compartment  12  and fresh food compartment  14 , respectively, to sense compartment temperatures. Temperature sensors  30 - 36  typically comprise solid state sensors such as resistance temperature detectors (RTD), thermocouples, thermistors or the like. Each temperature sensor  30 - 36  is electrically coupled to controller  27 . 
     For purposes of clarity, the operation of refrigeration system  10  will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE. Although the exemplary embodiments will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE, the invention is not limited to these modes. In fact, the present invention can be implemented and utilized with many other modes of operation. 
     During operation, freezer compartment temperature sensor  34  and fresh food compartment temperature sensor  36  generate temperature signals, which temperature signals are received by controller  27 . 
     If the temperature signals (T 1 ) generated by freezer compartment temperature sensor  34  rise above a preset freezer compartment temperature, typically in the range between about −20° C. to about −15° C., controller  27  enters FREEZER MODE. 
     During FREEZER MODE, controller  27  generates a compressor signal to activate compressor  18  and a condenser fan  39 , a control signal to energize control valve  22  and a fan signal to activate a freezer compartment fan  38 . High pressure gaseous refrigerant is discharged from compressor  18  and is condensed in condenser  20 . The now-liquid refrigerant is expanded through control valve  22  to a lower pressure and flows to freezer evaporator  24 . The refrigerant under low pressure, and correspondingly at a low temperature, enters freezer evaporator  24  where the refrigerant is evaporated in a conventional manner. The evaporation of the refrigerant lowers the temperature of the air surrounding freezer evaporator  24 . The cooled air is directed by the rotation of freezer compartment fan  38  into freezer compartment  12 . 
     If the temperature signals (T 1 ) generated by freezer compartment temperature sensor  34  drop below preset freezer compartment temperature cooling is no longer required and controller  27  exits FREEZER MODE. When controller  27  exits FREEZER MODE, controller  27  cuts the power to control valve  22  (unless simultaneously entering FRESH FOOD MODE) causing control valve  22  to move to a closed position, as discussed above. The closing of control valve  22  prevents refrigerant migration to freezer evaporator  24 , thereby conserving energy. Control valve  22  therefore acts as an energy valve eliminating the need for a separate valve to serve this function. 
     One current problem with some commercially available refrigerators is that the freezer compartment and the fresh food compartment are simultaneously cooled with a single evaporator. Accordingly, the temperature of the single evaporator must be maintained at a temperature lower than the temperature of the freezer compartment. Therefore, an evaporator having a lower temperature than is necessary is used to cool the fresh food compartment, causing the efficiency of the overall system to be relatively low. 
     In accordance with one embodiment of the instant invention, during FREEZER MODE, the average flow rate through control valve  22  is dependent upon the selected duty cycle. Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is close to a saturated vapor. A higher evaporator saturation temperature produces a higher pressure thus a higher vapor density thereby generating a greater compressor mass flow rate. 
     To support a higher compressor mass flow rate, controller  27  generates a signal to control valve  22  so that the time (t 1 ) control valve  22  is fully open is increased with respect to the time (t 2 ) control valve is fully closed, thereby producing a larger average flow rate. To support a lower compressor mass flow rate, controller  27  generates a signal to control valve  22  so that the time (t 1 ) control valve  22  is fully open is lessened with respect to the time (t 2 ) control valve  22  is fully closed, thereby producing a relatively lower average flow rate. 
     Controller  27  generates control signals to control valve  22  on the basis of evaporator dryness. The level of dryness (i.e., the amount of liquid refrigerant) indicates whether freezer evaporator  24  requires more refrigerant. The difference between the air temperature in freezer compartment  12  and the refrigerant temperature at the inlet of freezer evaporator  24  can be correlated with the evaporator dryness. Generally, as this temperature difference increases, control valve  22  should be opened more to increase the refrigerant flow rate. 
     In order to ensure that freezer evaporator  24  exit state is a substantially saturated vapor, controller  27  monitors the temperature signals generated by first liquid line temperature sensor  30  (T 2 ) and the temperature signals generated by freezer compartment temperature sensor  34  (T 1 ). Controller  27  ensures a substantially saturated vapor exit state in freezer evaporator  24  by generating control signals to control valve  22 . The control signals typically comprises a pulse width modulated frequency signal which causes control valve  22  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  22 . The pulse width is adjusted in accordance with the detected evaporator dryness to maintain dryness at a relatively fixed level. Preferably, the difference between the air temperature in freezer compartment  12  (T 1 ) and the refrigerant temperature at the inlet of freezer evaporator  24  (T 2 ) is held at a fixed delta in the range between about 5° C. to about 10° C. By controlling the duty cycle of control valve  22  to maintain the desired evaporator dryness, optimal system performance is obtained. 
     If the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  36  rise above a preset fresh food compartment temperature, typically in the range between about 3° C. to about 7° C., controller  27  enters FRESH FOOD MODE. 
     During FRESH FOOD MODE, controller  27  generates a compressor signal to activate compressor  18  and a condenser fan  39 , a control signal to energize control valve  22  and a fan signal to activate a fresh food compartment fan  40 . High pressure gaseous refrigerant is discharged from compressor  18  and is condensed in condenser  20 . The now-liquid refrigerant is expanded through control valve  22  to a lower pressure and flows to fresh food evaporator  26 . The refrigerant under low pressure, and correspondingly at a low temperature, enters fresh food evaporator  26  where the refrigerant is evaporated in a conventional manner. The evaporation of the refrigerant lowers the temperature of the air surrounding fresh food evaporator  26 . The cooled air is directed by the rotation of fresh food compartment fan  40  into fresh food compartment  14 . 
     If the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  36  drop below preset fresh food compartment temperature, cooling is no longer required and controller  27  exits FRESH FOOD MODE. When controller  27  exits FRESH FOOD MODE, controller  27  cuts the power to control valve  22  (unless simultaneously entering FREEZER MODE) causing control valve  22  to move to a closed position, as discussed above. 
     In accordance with one embodiment of the instant invention, during FRESH FOOD MODE, the average flow rate through control valve  22  is dependent upon the selected duty cycle. Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is close to a saturated vapor. A higher evaporator saturation temperature produces a higher pressure thus a higher vapor density thereby generating a greater compressor mass flow rate. 
     In order to ensure that fresh food evaporator  26  exit state is a substantially saturated vapor, controller  27  monitors the temperature signals (T 4 ) generated by second liquid line temperature sensor  33  and the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  36 . Controller  27  ensures a substantially saturated vapor exit state in fresh food evaporator  26  by generating signals to control valve  22 . This control signal typically comprises a pulse width modulated frequency signal which causes control valve  22  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  22 . The pulse width is adjusted in accordance with the evaporator dryness to maintain dryness predicted by T 3  and T 4  at a relatively fixed level. Preferably, the difference between the air temperature in fresh food compartment  14  (T 3 ) and the refrigerant temperature at the inlet of fresh food evaporator  24  (T 4 ) is held at a fixed delta in the range between about 5° C. to about 10° C. By controlling the duty cycle of control valve  22  to maintain the desired evaporator dryness, optimal system performance is obtained. 
     An exemplary control logic sequence  100  for refrigeration system  10  is shown in FIG.  2 . This control logic sequence is inputted into controller  27 , for example, by programming into memory of an application specific integrated circuit (ASIC) or other programmable memory device. 
     At block  102 , the freezer compartment temperature (T 1 ) is monitored by controller  27  through freezer compartment temperature sensor  34 . Controller  27  compares the temperature signals generated from freezer compartment temperature sensor  34  with the preset freezer temperature. 
     If T 1  is greater than the preset freezer temperature, the control sequence advances to block  104  and freezer compartment fan  38  is turned on. If, however, T 1  is less than or equal to the preset freezer temperature, the control sequence advances to block  104  and freezer compartment fan  38  remains off, or if previously on, freezer compartment fan  38  is turned off. 
     At block  106 , the fresh food compartment temperature T 3  is monitored by controller  27  through fresh food compartment temperature sensor  36 . Controller  27  compares the temperature signals generated from fresh food temperature sensor  36  with the preset fresh food temperature. 
     If T 3  is greater than the preset fresh food temperature, the control sequence advances to block  108  and fresh food compartment fan  40  is turned on. If, however, T 3  is less than or equal to the preset fresh food temperature, the control sequence advances to block  108  and fresh food compartment fan  40  remains off, or if previously on, fresh food compartment fan  40  is turned off. 
     If, at block  104 , freezer compartment fan  38  is turned on, the control sequence advances to block  110  where controller  27  determines if fresh food compartment fan  40  is currently on, indicating cooling is currently taking place within fresh food compartment  14 . If fresh food compartment fan  40  is not currently on, the control sequence advances to block  114  and the flow rate of control valve  22  is set for a fixed (T 1 −T 2 ) delta, as discussed above. 
     If, at block  104 , freezer fan  38  is turned off or remains off, the control sequence advances to block  118  where the controller determines if both fans are currently off. 
     If, at block  108 , fresh food fan  40  is turned on, the control sequence advances to block  112  where controller  27  determines if freezer compartment fan  38  is currently on, indicating cooling is currently taking place within freezer compartment  12 . If freezer compartment fan  38  is not currently on, the control sequence advances to block  116  and the flow rate of control valve  22  is set for a fixed (T 3 −T 4 ) delta, as discussed above. 
     If, at block  108 , fresh food fan  40  is turned off or remains off, the control sequence advances to block  118  wherein the controller determines if both fans are currently off. 
     At block  118 , controller  27  determines if both fans  38 ,  40  are currently off. If controller  27  determines that both fans  38 ,  40  are currently off, the control sequence advances to block  120 . At block  120 , controller  27  generates a signal to shut off, condenser fan  39 , compressor  18  and control valve  22 , or if currently off to maintain that status, as neither freezer compartment  12  nor fresh food compartment  14  needs cooling. 
     If, at block  118 , controller  27  determines that both fans  38 ,  40  are not currently off, the control sequence advances to block  120 . At block  120 , controller  27  generates a signal to turn on, condenser fan  39 , compressor  18  and control valve  22 , or if currently on to maintain that status, as either freezer compartment  12  or fresh food compartment  14  currently requires cooling. 
     FIG. 3 shows another embodiment of a refrigeration system  210  comprising compressor  18 , condenser  20 , control valve  22 , freezer evaporator  24 , and fresh food evaporator  26 . Refrigeration system  210  is similar to refrigeration system  10  of FIG. 1, except that refrigeration system  210  further comprises an accumulator  212  that capture excess liquid refrigerant that does not boil off in fresh food evaporator  26 . 
     For purposes of clarity, the operation of refrigeration system  210  will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE. Although the exemplary embodiments will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE, the invention is not limited to these modes. In fact, the present invention can be implemented and utilized with many other modes of operation. 
     Another current problem with some commercially available refrigerators is that frost builds up on the fresh food evaporator requiring a defrost heater, a timer and a terminator to eliminate this problem. This defrost event consumes energy, adds thermal load, and causes unwanted temperature fluctuations within a refrigeration system. 
     If the temperature signals (T 1 ) generated by freezer compartment temperature sensor  34  rise above a preset freezer compartment temperature, typically in the range between about −20° C. to about −15° C., controller  27  enters FREEZER MODE. 
     In accordance with one embodiment of the instant invention, during FREEZER MODE, the average flow rate through control valve  22  is dependent upon the selected duty cycle. Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is close to a saturated vapor. A higher evaporator saturation temperature produces a higher pressure thus a higher vapor density thereby generating a greater compressor mass flow rate. 
     Controller  27  generates control signals to control valve  22  on the basis of evaporator dryness. The level of dryness (i.e., the amount of liquid refrigerant) indicates whether freezer evaporator  24  requires more refrigerant. The difference between the air temperature in freezer compartment  12  (T 1 ) and the refrigerant temperature at the inlet of freezer evaporator  24  (T 2 ) can be correlated with the evaporator dryness. Generally, as this temperature difference increases, control valve  22  should be opened more to increase the refrigerant flow rate. 
     In order to ensure that freezer evaporator  24  exit state is a substantially saturated vapor, controller monitors the temperature signals (T 2 ) generated by first liquid line temperature sensor  30  and the temperature signals (T 1 ) generated by freezer compartment temperature sensor  34 . Controller  27  ensures a substantially saturated vapor exit state in freezer evaporator  24  by generating signals to control valve  22 . This control signal typically comprises a pulse width modulated frequency signal which causes control valve  22  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  22 . The pulse width is adjusted in accordance with the detected evaporator dryness to maintain dryness at a relatively fixed level. Preferably, the difference between the air temperature in freezer compartment  12  (T 1 ) and the refrigerant temperature at the inlet of freezer evaporator  24  (T 2 ) is held at a fixed delta in the range between about 5° C. to about 10° C. By controlling the duty cycle of control valve  22  to maintain the desired evaporator dryness, optimal system performance is obtained. 
     If the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  36  rise above a preset fresh food compartment temperature, typically in the range between about 3° C. to about 7° C., controller  27  enters FRESH FOOD MODE. 
     In order to ensure that fresh food evaporator  26  maintains frost-free conditions, controller  27  monitors the temperature signals generated by second liquid line temperature sensor  33  (T 4 ). Controller  27  ensures substantially frost-free conditions at fresh food evaporator  26  by generating control signals to control valve  22 . This control signal typically comprises a pulse width modulated frequency signal which causes control valve  22  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  22 . The pulse width of control valve  22  is adjusted in accordance with the detected temperature signals generated by second liquid line temperature sensor  33  (T 4 ) in order to maintain the refrigerant temperature at a relatively fixed level. Preferably, the refrigerant temperature (T 4 ) should be maintained at a temperature equal to or greater than 0° C. Because control valve  22  maintains the refrigerant temperature at a nominal value greater than or equal to 0° C., frost formation on fresh food evaporator  26  will be minimized. An additional system component, accumulator  212 , must be utilized, however, to avoid excess liquid refrigerant from draining to compressor  18 . 
     An exemplary control logic sequence  300  for refrigeration system  210  is shown in FIG.  4 . This control logic sequence is inputted into controller  27 , for example, by programming into memory of an application specific integrated circuit (ASIC) or other programmable memory device. 
     At block  302 , the freezer compartment temperature (T 1 ) is monitored by controller  27  through freezer compartment temperature sensor  34 . Controller  27  compares the temperature signals generated from freezer compartment temperature sensor  34  with the preset freezer temperature. 
     If T 1  is greater than the preset freezer temperature, the control sequence advances to block  304  and freezer compartment fan  38  is turned on. If, however, T 1  is less than or equal to the preset freezer temperature, the control sequence advances to block  304  and freezer compartment fan  38  remains off, or if previously on, freezer compartment fan  38  is turned off. 
     At block  306 , the fresh food compartment temperature T 3  is monitored by controller  27  through fresh food compartment temperature sensor  36 . Controller  27  compares the temperature signals generated from fresh food compartment temperature sensor  36  with the preset fresh food temperature. 
     If T 3  is greater than the preset fresh food temperature, the control sequence advances to block  308  and fresh food compartment fan  40  is turned on. If, however, T 3  is less than or equal to the preset fresh food temperature, the control sequence advances to block  308  and fresh food compartment fan  40  remains off, or if previously on, fresh food compartment fan  40  is turned off. 
     If, at block  304 , freezer compartment fan  38  is turned on, the control sequence advances to block  310  where controller  27  determines if fresh food compartment fan  40  is currently on, indicating cooling is currently taking place within fresh food compartment  14 . If fresh food compartment fan  40  is not currently on, the control sequence advances to block  314  and the flow rate of control valve  22  is set for a fixed (T 1 −T 2 ) delta as discussed above. 
     If, at block  304 , freezer fan  38  is turned off or remains off, the control sequence advances to block  318  where controller  27  determines if both fans are currently off. 
     If, at block  308 , fresh food compartment fan  40  is turned on, the control sequence advances to block  312  where controller  27  determines if freezer compartment fan  38  is currently on, indicating cooling is currently taking place within freezer compartment  12 . If freezer compartment fan  38  is not currently on, the control sequence advances to block  316  and the flow rate of control valve  22  is set for a fixed refrigerant temperature (T 4 ), typically greater than 0° C., as discussed above. 
     If, at block  308 , fresh food compartment fan  40  is turned off or remains off, the control sequence advances to block  318  wherein controller  27  determines if both fans are currently off. 
     At block  318 , controller  27  determines if both fans  38 ,  40  are currently off. If controller  27  determines that both fans  38 ,  40  are currently off, the control sequence advances to block  320 . At block  320 , controller  27  generates a signal to shut off, condenser fan  39 , compressor  18  and control valve  22 , or if currently off to maintain that status, as neither freezer compartment  12  nor fresh food compartment  14  needs cooling. 
     If, at block  318 , controller  27  determines that both fans are not currently off, the control sequence advances to block  320 . At block  320 , controller  27  generates a signal to turn on condenser fan  39 , compressor  18  and control valve  22 , or if currently on to maintain that status, as either freezer compartment  12  or fresh food compartment  14  currently needs cooling. 
     In an alternative embodiment of refrigeration system  210 , control logic system  300  may be a primary control logic sequence. If, however, the cooling of fresh food compartment  14  takes greater than a pre-determined amount of time, for example greater than about 20 minutes to about 30 minutes, control logic sequence may advance to a secondary control logic sequence, for example, control logic sequence  100 , as shown in FIG.  2 . In control logic sequence  100 , fresh food cooling utilizes a fixed T 3  −T 4  delta, that is capable of a faster rate of cooling than control logic sequence  300 . 
     An exemplary embodiment of a side-by-side refrigeration system  410  having a freezer compartment  412  separated from and substantially temperature independent of a fresh food compartment  414 , is shown in FIG. 5. A mullion  416  separates freezer compartment  412  and fresh food compartment  414 . Although the present invention is described herein in connection with refrigeration system  410 , the present invention is not limited to practice with refrigeration system  410 . In fact, the present invention can be implemented and utilized with many other configurations. 
     Refrigeration system  410  further comprises a compressor  418 , a condenser  420 , a control valve  422 , a freezer evaporator  424 , and a fresh food evaporator  426 . Control valve  422  and compressor  418  are each electrically coupled to a controller  427 . 
     Control valve  422  is disposed within a liquid line  428 , which liquid line  428  connects condenser  420  and freezer evaporator  424 . Control valve  422  is typically positioned adjacent the inlet of freezer evaporator  424  and is the throttling or metering device that controls the operation of refrigeration system  410 . Control valve  422  is typically a closed valve, that is, control valve  422  closes when it is not energized. 
     A first liquid line temperature sensor  430  is disposed so as to detect the temperature of refrigerant prior to entering freezer evaporator  424  and a second liquid line temperature sensor  432  is disposed so as to detect the temperature of refrigerant prior to entering fresh food evaporator  426 . Additionally, a freezer compartment temperature sensor  434  and a fresh food compartment temperature sensor  436  are disposed within freezer compartment  412  and fresh food compartment  414 , respectively, to sense compartment temperatures. Temperature sensors  430 - 436  typically comprise solid state sensors such as resistance temperature detectors (RTD), thermocouples, thermistors or the like. Each temperature sensor  430 - 436  is electrically coupled to controller  427 . 
     For purposes of clarity, the operation of refrigeration system  410  will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE. Although the exemplary embodiments will be discussed in terms of a FREEZER MODE and a FRESH FOOD MODE, the invention is not limited to these modes. In fact, the present invention can be implemented and utilized with many other modes of operation. 
     During operation, freezer compartment temperature sensor  434  and fresh food compartment temperature sensor  436  generate temperature signals, which temperature signals are received by controller  427 . 
     If the temperature signals (T 1 ) generated by freezer compartment temperature sensor  434  rise above a preset freezer compartment temperature, typically in the range between −15° C. to about −20° C., controller  427  enters FREEZER MODE. 
     During FREEZER MODE, controller  427  generates a compressor signal to activate compressor  418 , and a fan signal to activate an evaporator fan motor  438  and a condenser fan  439 . Evaporator fan motor  438  is disposed within mullion  416  and simultaneously powers both a freezer compartment fan  441  and a fresh food compartment fan  443  so as to circulate air within freezer compartment  412  and fresh food compartment  414 , respectively. Evaporator fan motor  438  may be a single speed, multi-speed or variable speed motor. 
     High pressure gaseous refrigerant is discharged from compressor  418  and is condensed in condenser  420 . The now-liquid refrigerant is expanded through control valve  422  to a lower pressure and flows to freezer evaporator  424 . The refrigerant under low pressure, and correspondingly at a low temperature, enters freezer evaporator  424  where the refrigerant is evaporated in a conventional manner. The evaporation of the refrigerant lowers the temperature of the air surrounding freezer evaporator  424 . The cooled air is directed by the rotation of freezer compartment fan  441  into freezer compartment  412 . 
     If the temperature signals (T 1 ) generated by freezer compartment temperature sensor  434  drops below preset freezer compartment temperature, cooling is no longer required and controller  427  exits FREEZER MODE. When controller  427  exits FREEZER MODE the controller cuts the power to control valve  422  (unless simultaneously entering FRESH FOOD MODE) causing control valve  422  to move to a closed position, as discussed above 
     If the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  36  rise above a preset fresh food compartment temperature, typically in the range between about 3° C. to about 7° C., controller  27  enters FRESH FOOD MODE. 
     During FRESH FOOD MODE, controller  427  generates a compressor signal to activate compressor  418  and a condenser fan  39 , a control signal to energize control valve  422 , and a fan signal to activate evaporator fan motor  438 . High pressure gaseous refrigerant is discharged from compressor  418  and is condensed in condenser  420 . The now-liquid refrigerant is expanded through control valve  422  to a lower pressure and flows to fresh food evaporator  426 . The refrigerant under low pressure, and correspondingly at a low temperature, enters fresh food evaporator  426  where the refrigerant is evaporated in a conventional manner. The evaporation of the refrigerant lowers the temperature of the air surrounding fresh food evaporator  426 . The cooled air is directed by the rotation of fresh food compartment fan  443  into fresh food compartment  414 . 
     If the temperature signals (T 3 ) generated by fresh food compartment temperature sensor  436  drop below preset fresh food compartment temperature, cooling is no longer required and controller  427  exits FRESH FOOD MODE. When controller  427  exits FRESH FOOD MODE the controller cuts the power to control valve  422  (unless simultaneously entering FREEZER MODE) causing control valve  422  to move to a closed position, as discussed above. 
     In accordance with one embodiment of the instant invention, during FREEZER MODE, the average flow rate through control valve  422  is dependent upon the selected duty cycle. Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is close to a saturated vapor. A higher evaporator saturation temperature produces a higher pressure thus a higher vapor density thereby generating a greater compressor mass flow rate. 
     To support a higher compressor mass flow rate, controller  427  generates a signal to control valve  422  so that the time (t 1 ) control valve  422  is fully open is increased with respect to the time (t 2 ) control valve is fully closed, thereby producing a larger average flow rate. To support a lower compressor mass flow rate, controller  427  generates a signal to control valve  422  so that the time (t 1 ) control valve  422  is fully open is lessened with respect to the time (t 2 ) control valve  422  is fully closed, thereby producing a relatively lower average flow rate. 
     Controller  427  generates control signals to control valve  422  on the basis of evaporator dryness. The level of dryness (i.e., the amount of liquid refrigerant) indicates whether freezer evaporator  424  requires more refrigerant. The difference between the air temperature in freezer compartment  412  and the refrigerant temperature at the inlet of freezer evaporator  424  can be correlated with evaporator dryness. Generally, as this temperature difference increases, control valve  422  should be opened more to increase the refrigerant flow rate. 
     In order to ensure that freezer evaporator  424  exit state is a substantially saturated vapor, controller  427  monitors the temperature signals generated by first liquid line temperature sensor  430  (T 2 ) and the temperature signals generated by freezer compartment temperature sensor  434  (T 1 ). Controller  427  ensures a substantially saturated vapor exit state in freezer evaporator  424  by generating control signals to control valve  422 . The control signals typically comprise a pulse width modulated frequency signal which causes control valve  422  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  422 . The pulse width is adjusted in accordance with the detected evaporator dryness to maintain dryness at a relatively fixed level. Preferably, the difference between the air temperature in freezer compartment  412  (T 1 ) and the refrigerant temperature at the inlet of freezer evaporator  424  (T 2 ) is held at a fixed delta in the range between about 5° C. to about 10° C. By controlling the duty cycle of control valve  422  to maintain the desired evaporator dryness, optimal system performance is obtained. 
     In accordance with one embodiment of the instant invention, during FRESH FOOD MODE, the average flow rate through control valve  422  is dependent upon the selected duty cycle. Higher evaporator refrigerant saturation temperature is desirable since the higher the saturation temperature, the greater the obtainable cycle efficiency. The cycle efficiency, however, will only be greater for higher temperatures if the evaporator exit state is such that the refrigerant is close to a saturated vapor. A higher evaporator saturation temperature produces a higher pressure thus a higher vapor density thereby generating a greater compressor mass flow rate. 
     In order to ensure that fresh food evaporator  426  exit state is a substantially saturated vapor, controller  427  monitors the temperature signals generated by second liquid line temperature sensor  433  (T 4 ) and the temperature signals generated by fresh food compartment temperature sensor  436  (T 3 ). Controller  427  ensures a substantially saturated vapor exit state in fresh food evaporator  426  by generating signals to control valve  422 . This control signal typically comprises a pulse width modulated frequency signal which causes control valve  422  to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through control valve  422 . The pulse width is adjusted in accordance with the detected evaporator dryness to maintain dryness at a relatively fixed level. Preferably, the difference between the air temperature in fresh food compartment  414  (T 3 ) and the refrigerant temperature at the inlet of fresh food evaporator  424  (T 4 ) is held at a fixed delta in the range between about 5° C. to about 15° C. By controlling the duty cycle of control valve  422  to maintain the desired evaporator dryness, optimal system performance is obtained. 
     An exemplary adaptive control logic sequence  500  for refrigeration system  410  is shown in FIG.  6 . This control logic sequence is inputted into controller  427 , for example, by programming into memory of an application specific integrated circuit (ASIC) or other programmable memory device. As used herein, the term “adaptive control” is defined as a control system that changes system parameters in a way so as to improve the performance of a system. 
     At block  502 , the freezer compartment temperature (T 1 ) is monitored by controller  427  through freezer compartment temperature sensor  434 . Controller  427  compares the temperature signals generated from freezer compartment temperature sensor  434  with the predetermined preset freezer temperature. 
     If T 1  is greater than the preset freezer temperature, the control sequence advances to block  508  and compressor  418  evaporator fan motor  438  and condenser fan  439  are turned on and control valve  422  is energized. If, however, T 1  is less than or equal to the preset freezer temperature, the control sequence advances to block  506 . 
     At block  506 , the fresh food compartment temperature T 3  is monitored by controller  427  through fresh food compartment temperature sensor  436 . Controller  427  compares the temperature signals generated from fresh food temperature sensor  436  with the preset fresh food temperature. 
     If T 3  is greater than the preset fresh food temperature, the control sequence advances to block  508  and compressor  418 , evaporator fan motor  438  and condenser fan  439  are turned on and control valve  422  is energized. If, however, T 3  is less than or equal to the preset freezer temperature, the control sequence advances to block  508  and compressor  418 , evaporator fan motor  438  and condenser fan  439  remain off and control valve  422  is not energized or if previously on, compressor  418 , evaporator fan motor  438  and condenser fan  439  are shut off and the power to control valve  422  is cut off. 
     If, at block  508 , compressor  418 , evaporator fan motor  438  and condenser fan  439  are turned on and control valve  422  is energized, the control sequence advances to block  510  where controller  427  determines if the freezer compartment temperature (T 1 ) minus the freezer set point is greater than the fresh food compartment temperature (T 3 ) minus the fresh food set point. 
     If, at block  510  controller  427  determines that freezer compartment temperature (T 1 ) minus the freezer set point is greater than the fresh food compartment temperature (T 3 ) minus the fresh food set point, the flow rate of control valve  422  is set for a fixed (T 1 −T 2 ) delta, as discussed above. 
     If, at block  510  controller  427  determines that freezer compartment temperature (T 1 ) minus the freezer set point is less than the fresh food compartment temperature (T 3 ) minus the fresh food set point, the flow rate of control valve  422  is set for a fixed (T 3 −T 4 ) delta, as discussed above. 
     The logic of block  510  is used to distribute the cooling capacity of refrigeration system  410  in such a way as to satisfy the cooling demand of both compartments within the same approximate time period. By adjusting the refrigerant flow rate with control valve  422 , the evaporator pressure and temperature, can be controlled. Increasing refrigerant flow raises both evaporator temperatures and provides more cooling to fresh food compartment  314 , by limiting the temperature difference between the freezer air and freezer evaporator. Decreasing refrigerant flow lowers both evaporator temperatures and provides more cooling to the freezer by limiting the amount of liquid refrigerant flowing into the fresh food evaporator. In this embodiment, there are two discrete selections of the temperature difference used to generate control signals to control valve  422  provided in blocks  512  and  514 . Alternatively, control valve  422  duty cycle is adjusted continuously to maintain a balanced cooling capacity between the two compartments using a proportional, differential, or integral value of the unbalance error, or any combination thereof, between the freezer demand (T 1 -freezer set point) and fresh food demand (T 3 -fresh food set point). 
     While only certain features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.