Abstract:
A cooling system including a pulse-width modulated variable capacity compressor operable between on-cycles and off-cycles, and in electrical communication with the compressor and operable to respectively synchronize opening and closing thereof with on- and off-cycles of the compressor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/195,839 filed on Jul. 15, 2002, now U.S. Pat. No. 6,672,090. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to refrigeration systems, compressor control systems and refrigerant regulating valve control systems. More particularly, the invention relates to liquid-side and vapor-side flow control strategies. 
     BACKGROUND OF THE INVENTION 
     Traditional refrigeration systems include a compressor, a condenser, an expansion valve, and an evaporator, all interconnected for establishing series fluid communication therebetween. Cooling is accomplished through evaporation of a liquid refrigerant under reduced temperature and pressure. Initially, vapor refrigerant is drawn into the compressor for compression therein. Compression of the vapor refrigerant results in a higher temperature and pressure thereof. From the compressor, the vapor refrigerant flows into the condenser. The condenser acts as a heat exchanger and is in heat exchange relationship with ambient. Heat is transferred from the vapor refrigerant to ambient, whereby the temperature is lowered. In this manner, a state change occurs, whereby the vapor refrigerant condenses to a liquid. 
     The liquid refrigerant exits an outlet of the condenser and flows into the expansion valve. As the liquid refrigerant flows through the expansion valve, its pressure is reduced prior to entering the evaporator. The evaporator acts as a heat exchanger, similar to the condenser, and is in heat exchange relationship with a cooled area (e.g., an interior of a refrigeration case). Heat is transferred from the cooled area to the liquid refrigerant, thereby increasing the temperature of the liquid refrigerant and resulting in boiling thereof. In this manner, a state change occurs, whereby the liquid refrigerant becomes a vapor. The vapor refrigerant then flows from the evaporators, back to the compressor. 
     The cooling capacity of the refrigeration system is generally achieved by varying the capacity of the compressor. One method of achieving capacity variation is continuously switching the compressor between on- and off-cycles using a pulse-width modulated signal. In this manner, a desired percent duty cycle for the compressor can be achieved. During the off-cycles, liquid refrigerant experiences “freewheel” flow, whereby the liquid refrigerant migrates into the evaporator. As the refrigerant migrates into the evaporator during the off-cycle, it is boiled therein, and becomes a vapor. This is detrimental to the performance of the refrigeration system in two ways: a significant reduction in the on-cycle evaporator temperature, and a decrease in flow recovery once switched back to the on-cycle. 
     Further, significant losses occur with traditional refrigeration systems when recently compressed vapor reverse migrates through the compressor, back toward the evaporator, during the off-cycle. These losses are compounded by reverse migration of liquid refrigerant back into the condenser during the off-cycle. 
     Therefore, it is desirable in the industry to provide a refrigeration system and flow control strategy for alleviating the deficiencies associated with traditional refrigeration systems. In particular, the refrigeration system should prohibit migration of liquid refrigerant into the evaporator during the off-cycle, prohibit reverse migration of vapor refrigerant through the compressor during the off-cycle, and prohibit reverse migration of liquid refrigerant through the condenser during the off-cycle. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a refrigeration system and control method thereof, for alleviating the deficiencies associated with traditional refrigeration systems. More particularly, the refrigeration system includes an evaporator, a variable capacity compressor coupled in fluid communication with the evaporator, a condenser coupled in fluid communication between the compressor and the evaporator, an expansion valve disposed intermediate the condenser and the evaporator, and an isolation valve disposed intermediate the condenser and the expansion valve. The isolation valve is in communication with the compressor for respectively synchronizing opening and closing thereof with on- and off-cycles of the compressor to prohibit migration of liquid refrigerant. In this manner, respective temperatures of the condenser and evaporator are better maintained during the off-cycle. 
     In accordance with an alternative embodiment, first and second check valves are respectively associated with the compressor and the condenser for prohibiting reverse migration of refrigerant during the off-cycle. In this manner, respective pressures of the refrigerant associated with the condenser and evaporator are decreased over a traditional refrigeration system. 
     The present invention further provides a method for controlling a refrigeration system having a compressor, a condenser and an evaporator connected in series flow communication. The method includes the steps of varying the compressor between on- and off-cycles to provide a percent duty cycle thereof, and synchronizing opening and closing of an isolation valve, respectively with the on- and off-cycles of the compressor, to prohibit migration of liquid refrigerant into the evaporator during the off-cycle. 
     In accordance with an alternative embodiment, the method further includes the steps of prohibiting reverse migration of the liquid refrigerant into the condenser, and prohibiting reverse migration of vapor refrigerant through the compressor, during the off-cycle. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of a refrigeration system implementing a closed expansion valve in accordance with the principles of the present invention; 
         FIG. 2  is a graph comparing a condenser temperature for the refrigeration system of  FIG. 1  to a condenser temperature for a traditional refrigeration system implementing a continuously open expansion valve; 
         FIG. 3  is a graph comparing an evaporator temperature for the refrigeration system of  FIG. 1  to a condenser temperature for a traditional refrigeration system implementing a continuously open expansion valve; 
         FIG. 4  is a schematic view of the refrigeration system of  FIG. 1 , implementing check valves in accordance with the principles of the present invention; 
         FIG. 5  is a graph depicting a pressure response for a traditional refrigeration system without the check valves; and 
         FIG. 6  is a graph depicting a pressure response for the refrigeration system of FIG.  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     With particular reference to  FIG. 1 , a refrigeration system  10  is schematically shown. Although the refrigeration system  10  is representative of a heat pump system, it will be appreciated that the implementation thereof, in accordance with the present invention, is for refrigeration. The refrigeration system  10  includes a compressor  12  having an associated pulse-width modulation (PWM) valve  14 , a four-way valve  16 , a condenser  18 , a liquid receiver  20 , an isolation valve  22 , dual evaporators  24  having respective expansion valves  26 , and a controller  28 . The controller  28  is in operable communication with the PWM valve  14  of the compressor  12 , a temperature sensor sensing  30  a temperature of a refrigerated area  32  (e.g. interior of a refrigeration case), and a pressure sensor  34  sensing a pressure of a refrigerant vapor discharged from the dual evaporators  24 , as explained in further detail hereinbelow. Although the present description includes dual evaporators, it is anticipated that the number of evaporators may vary, depending on particular system design requirements. Multiple maintenance valves  35  are also provided to enable maintenance and removal/addition of the various components. 
     The compressor  12 , and operation thereof, is similar to that disclosed in commonly assigned U.S. Pat. No. 6,047,557, entitled ADAPTIVE CONTROL FOR A REFRIGERATION SYSTEM USING PULSE WIDTH MODULATED DUTY CYCLE SCROLL COMPRESSOR, expressly incorporated herein by reference. A summary of the construction and operation of the compressor  12  is provided herein. 
     The compressor includes an outer shell and a pair of scroll members supported therein and drivingly connected to a motor-driven crankshaft. One scroll member orbits respective to the other, whereby suction gas is drawn into the shell via a suction inlet. Intermeshing wraps provided on the scroll members define moving fluid pockets that progressively decrease in size and move radially inwardly as a result of the orbiting motion of the scroll member. In this manner, the suction gas entering via the inlet is compressed. The compressed gas is then discharged into a discharge chamber. 
     In order to switch to an off-cycle (i.e., unload the PWM compressor  12 ), the PWM valve  14  is actuated in response to a signal from the controller  28 , thereby interrupting fluid communication to increase a pressure within the inlet to that of the discharge gas. The biasing force resulting from this discharge pressure causes the non-orbiting scroll member to move axially upwardly away from the orbiting scroll member. This axial movement will result in the creation of a leakage path between the scroll members, thereby substantially eliminating continued compression of the suction gas. When switching to an on-cycle (i.e., resuming compression of the suction gas), the PWM valve  14  is actuated so as to move the non-orbiting scroll member into sealing engagement with the orbiting scroll member. In this manner, the duty cycle of the compressor  12  can be varied between zero (0) and one hundred (100) percent via the PWM valve  14 , as directed by the controller  23 . 
     The controller  28  monitors the temperature of the refrigerated area  32  and pressure of the vapor refrigerant leaving the evaporators  24 . Based upon these two inputs, and implementing programmed algorithms, the controller  28  determines the percent duty cycle for the PWM compressor  12  and signals the PWM valve  14  for switching between the on- and off-cycles to achieve the desired percent duty cycle. 
     Operation of the refrigeration system  10  will now be described in detail. Cooling is accomplished through evaporation of a liquid refrigerant under reduced temperature and pressure. Initially, vapor refrigerant is drawn into the compressor  12  for compression therein. Compression of the vapor refrigerant results in a higher temperature and pressure thereof. From the compressor  12 , the vapor refrigerant flows into the condenser  18 . The condenser  18  acts as a heat exchanger and is in heat exchange relationship with ambient. Heat is transferred from the vapor refrigerant to ambient, whereby the temperature is lowered. In this manner, a state change occurs, whereby the vapor refrigerant condenses to a liquid. 
     The liquid refrigerant exits an outlet of the condenser  18  and is received into the receiver  20 , acting as a liquid refrigerant reservoir. As explained above, the isolation valve  22  is in communication with the controller  28 , whereby it switches between open and closed positions, respectively with the on-, and off-cycles of the PWM compressor  12 . With the isolation valve  22  in the open position, liquid refrigerant flows therethrough and is split, flowing into each of the expansion valves  26 . As the liquid refrigerant flows through the expansion valves  26 , its pressure is reduced prior to entering the evaporators  24 . 
     The evaporators  24  act as heat exchangers, similar to the condenser  18 , and are in heat exchange relationship with a refrigerated area  32 . Heat is transferred from the refrigerated area  32 , to the liquid refrigerant, thereby increasing the temperature of the liquid refrigerant resulting in boiling thereof. In this manner, a state change occurs, whereby the liquid refrigerant becomes a vapor. The vapor refrigerant then flows from the evaporators  24 , back to the compressor  12 . 
     The off-cycle occurs when the compressor  12  is essentially turned off by the controller  28 , or is otherwise operating at approximately zero (0) percent duty cycle. Pulse-width modulation results in periodic shifts between the on- and off-cycles to vary the capacity of the PWM compressor  12 . As discussed by way of background, when the refrigeration system  10  switches to the off-cycle from the on-cycle, the recovery of off-cycle flow (“flywheel” flow) is significantly decreased because the refrigerant temperature within the evaporators  24  quickly rises to the surface air temperature of the evaporator exteriors. To improve the recovery of off-cycle flow, the isolation valve  22  is closed during the off-cycle. In this manner, migration of liquid refrigerant into the evaporators  24  is prevented. 
     With particular reference to  FIGS. 2 and 3 , performance of the refrigeration system  10 , implementing the isolation valve  22 , can be compared to a traditional refrigeration system without such a valve, for a fifty (50) percent PWM duty cycle with a thirty (30) second cycle time. More particularly,  FIG. 2  provides a comparison of the condenser temperature between the present refrigeration system  10  and a conventional refrigeration system.  FIG. 3  provides a comparison of the evaporator temperature between the present refrigeration system  10  and a conventional refrigeration system. The flow recovery penalty of the conventional system can be seen, as the liquid refrigerant migration results in a lower on-cycle evaporator temperature and a correspondingly higher condenser temperature. Thus, more compressor power is required by a conventional refrigeration system to achieve an equivalent overall capacity when compared to the present refrigeration system  10 . The on-cycle condensing temperature of the conventional refrigeration system is higher because the condenser must do more liquid refrigerant sub-cooling to replenish the liquid refrigerant lost during the off-cycle. 
     The flow recovery penalty for the conventional refrigeration system will increase with longer off-cycles or lower percent PWM duty cycles. This is due to an increased refrigerant migration effect during longer off-cycles. 
     With particular reference to  FIG. 4 , the refrigeration system  10  is shown to further include first and second check valves  40 ,  42 , respectively. The first check valve is positioned at an outlet of the PWM compressor  12 , and the second check valve  42  is positioned at an outlet of the condenser  18 . The refrigeration system  10 , as shown in  FIG. 4 , operates similarly to that described above with reference to FIG.  1 . However, as the refrigeration system  10  switches from the on-cycle to the off-cycle, significant gas leaking through the compressor outlet side could produce a vapor refrigerant migration effect similar to that described above for the evaporators  24 . To minimize this effect, the first check valve  40  prevents vapor refrigerant migration back through the PWM compressor  12  to the evaporators  24 , and the second check valve  42  assures that the liquid refrigerant in the receiver  20  stays in the receiver  20 . 
     With particular reference to  FIGS. 4 and 5 , a performance comparison can be made between a traditional refrigeration system without check valves  40 ,  42  (FIG.  4 ), and the present refrigeration system  10  implementing the check valves  40 ,  42  (FIG.  5 ), for a fifty (50) percent PWM duty cycle with an approximately twelve (12) second cycle time. In particular, the refrigeration system pressure responses for the PWM compressor outlet (discharge), condenser outlet, and the PWM compressor inlet (suction) are shown. As can be seen, the pressure at the PWM compressor discharge is significantly increased, and a reduction in the pressure at the PWM compressor suction is also seen during the off-cycle. In this manner, the PWM compressor power penalty is significantly reduced, as compared to the traditional refrigeration system. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.