Abstract:
A method for controlling at least one electronic expansion valve coupled to an economizer in a refrigeration system in order to dynamically control the refrigeration system operating conditions and in order to accommodate more than one set of operating conditions. The system can also be used to control the capacity of the system. More specifically, the method can include determining the required capacity of the system and adjusting the flow of refrigerant through the electronic expansion valve to adjust the actual capacity of the system toward the required capacity of the system. In another aspect of the invention, the system is operated to maintain the power of the system below a threshold value (e.g., below a max rated horsepower). This method includes determining the power required to operate the compressor based on the measurement of system parameters; comparing the power required to a threshold value; and adjusting the flow of refrigerant through the electronic expansion valve in order to keep the power required below the threshold value. In yet another aspect of the invention, the system is operated in order to prevent overheating of the compressor. More specifically, the flow of refrigerant from the heat exchanger to the compressor can be adjusted so that some amount of liquid refrigerant is provided to quench the compressor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates in general to the operation of a refrigeration system, and more specifically to the control of at least one electronic expansion valve coupled to an economizer in a refrigeration system.  
           [0002]    Refrigeration systems generally include a refrigerant circuit including a compressor, a condenser, a main throttling device, and an evaporator. Vapor refrigerant is delivered to the compressor where the temperature and pressure of the vapor refrigerant is increased. The compressed vapor refrigerant is then delivered to the condenser where heat is removed from the vapor refrigerant in order to condense the vapor refrigerant into liquid form. The liquid refrigerant is then delivered from the condenser to a main throttling device, such as a mechanical thermostatic expansion valve. The main throttling device restricts the flow of the liquid refrigerant by forcing the liquid through a small orifice in order to decrease the pressure of the liquid and therefore decrease the boiling point of the liquid. Upon exiting the main throttling device, the liquid refrigerant is in the form of liquid refrigerant droplets. The liquid refrigerant droplets are delivered from the main throttling device to the evaporator, which is located within or in thermal communication with the space to be conditioned by the refrigeration system. As air passes over the evaporator, the liquid refrigerant droplets absorb heat from the air in order to cool the air. The cooled air is circulated through the conditioned space to cool the masses within the conditioned space. Once the liquid refrigerant droplets have absorbed sufficient heat, the liquid refrigerant droplets vaporize. To complete the refrigeration cycle, the vapor refrigerant is delivered from the evaporator back to the compressor.  
           [0003]    An additional heat exchanger in the form of an economizer may be added to the refrigeration system in order to enhance the efficiency of the cycle. The economizer is often coupled between the condenser and the main throttling device. Specifically, the economizer is coupled to the condenser by an economizer input line having a first branch and a second branch. The first branch delivers refrigerant through the economizer to the main throttling device. The second branch delivers refrigerant through a secondary throttling device, through an economizer chamber within the economizer, and back to the compressor. In an economizer system, the refrigerant flowing to the main throttling device is routed through the economizer to be sub-cooled, while some refrigerant is drawn off through the second branch of the economizer input line to a secondary throttling device. The drawn-off refrigerant passes through the secondary throttling device, where it is cooled by the throttling process, and into the economizer chamber. Once in the economizer chamber, the drawn-off refrigerant is in a heat transfer relationship with the refrigerant flowing through the first branch of the economizer input line to the main throttling device. The drawn-off refrigerant absorbs heat from the refrigerant flowing through the first branch to the main throttling device. Thus, the refrigerant flowing through the first branch is sub-cooled. Liquid refrigerant is sub-cooled when the temperature of the liquid is lower than the vaporization temperature for the refrigerant at a given pressure. The drawn-off refrigerant absorbs heat until it vaporizes.  
           [0004]    Before the drawn-off refrigerant is directed back to the compressor, the vaporized refrigerant has generally reached a superheat level. The refrigerant reaches a superheat level when all of the refrigerant has vaporized and the temperature of the refrigerant is above the vaporization temperature for the refrigerant at a given pressure. The refrigerant at the superheated level is then directed back to the compressor.  
           [0005]    The operating conditions of the refrigeration system are controlled, in part, by the operation of the economizer. The economizer is controlled by the secondary throttling device. Generally, the main and secondary throttling devices are mechanical thermostatic expansion valves (TXV), which operate based on the temperature and pressure of the refrigerant passing through the valve.  
         SUMMARY OF THE INVENTION  
         [0006]    The use of TXVs for the main and secondary throttling devices has several limitations. First, TXVs cannot be dynamically adjusted to control the operating conditions of the refrigeration system. TXVs are initially designed to optimize the operating conditions of the refrigeration system, but the TXVs cannot be dynamically adjusted to optimize the operating conditions at all times.  
           [0007]    Moreover, TXVs can only accommodate one set of operating conditions. A TXV in the economizer cycle is generally designed to maintain one set of primary operating conditions. However, extraordinary or secondary operating conditions may occur, which may demand the primary operating conditions to be overridden. A TXV cannot accommodate secondary operating conditions that may be desired to periodically override the primary operating conditions.  
           [0008]    Accordingly, the invention provides a method and apparatus for controlling at least one electronic expansion valve coupled to an economizer in a refrigeration system in order to dynamically control the refrigeration system operating conditions and in order to accommodate more than one set of operating conditions. The refrigeration system generally includes a compressor, a condenser coupled to the compressor, a heat exchanger coupled to both the condenser and the compressor, an evaporator coupled to both the heat exchanger and the compressor, and an electronic expansion valve, an input of the valve coupled between the condenser and the heat exchanger, an output of the valve coupled to the compressor.  
           [0009]    The above-described structure is normally operated under a set of primary operating conditions. One condition is that the state of the refrigerant flowing from the heat exchanger to the compressor is maintained above superheat temperature. More specifically, the pressure between the heat exchanger and the compressor is sensed. The sensed pressure is converted into a saturation temperature value. The temperature between the heat exchanger and the compressor is sensed. The saturated suction temperature value is compared to the sensed temperature. The flow of refrigerant through the EXV is adjusted until the sensed temperature is greater than the saturated suction temperature value.  
           [0010]    The above-described structure can also be used to control the capacity of the system. More specifically, the method can include determining the required capacity of the system and adjusting the flow of refrigerant through the electronic expansion valve to adjust the actual capacity of the system toward the required capacity of the system. For example, if the required capacity is less than the actual capacity, then the flow of refrigerant through the electronic expansion valve can be decreased. Likewise, if the required capacity is greater than the actual capacity, then the flow of refrigerant through the electronic expansion valve can be increased. In either event the method may require that the primary set of operating conditions be overridden.  
           [0011]    In another aspect of the invention, the system is operated to maintain the power of the system below a threshold value (e.g., below a max rated horsepower). This method includes determining the power required to operate the compressor based on the measurement of system parameters; comparing the power required to a threshold value; and adjusting the flow of refrigerant through the electronic expansion valve in order to keep the power required below the threshold value. There are many different ways to determine the required power (e.g., by sensing the pressure between the heat exchanger and the compressor, the pressure between the evaporator and the compressor, the pressure between the compressor and the condenser, and the flow rate of refrigerant). In this embodiment, if the horsepower required to operate the compressor is less than the threshold value, then there is no need to adjust the flow of refrigerant through the electronic expansion valve. However, if the power required to operate the compressor is greater than the threshold value, then the flow of refrigerant through the electronic expansion valve can be decreased to avoid operating the system above its rated power limit. In order to do this, the primary operating conditions may need to be overridden.  
           [0012]    In yet another aspect of the invention, the system is operated in order to prevent overheating of the compressor. More specifically, the flow of refrigerant from the heat exchanger to the compressor can be adjusted so that some amount of liquid refrigerant is provided to quench the compressor. The method includes measuring a system parameter corresponding with the temperature of the compressor; comparing the measured system parameter to a threshold value; and adjusting the flow of refrigerant into the compressor by adjusting the flow of refrigerant through the electronic expansion valve in order to keep the system parameter below the threshold value. The system parameter can be any parameter that corresponds with the temperature of the compressor (e.g., the temperature of the compressor, the temperature of refrigerant flowing from the compressor, etc.). In practice, if the system parameter exceeds the threshold value, the flow of refrigerant through the electronic expansion valve can be increased in order to provide a volume of liquid refrigerant to quench the compressor. In order to do this, the primary operating conditions may need to be overridden.  
           [0013]    Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following description, claims, and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic representation of a refrigeration system embodying the invention.  
         [0015]    [0015]FIG. 2 illustrates a method of controlling the superheat level of the refrigerant in the refrigeration system of FIG. 1.  
         [0016]    [0016]FIG. 3 illustrates a method of quenching the compressor of the refrigeration system of FIG. 1.  
         [0017]    [0017]FIGS. 4A and 4B illustrate a method of controlling the horsepower of the engine of the refrigeration system of FIG. 1.  
         [0018]    [0018]FIG. 5 illustrates the refrigeration system of FIG. 1 located within a refrigeration system-housing unit coupled to a transport container coupled to a tractor trailer. 
     
    
       [0019]    Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates a refrigeration system  10  embodying the invention. The refrigeration system  10  includes a refrigerant circuit  12  and a microprocessor circuit  100 . The refrigerant circuit  12  generally defines the flow of fluid refrigerant through the refrigeration system  10 . The refrigerant circuit  12  includes a first fluid path  14  and a second fluid path  40 .  
         [0021]    The first fluid path  14  is defined by a compressor  16 , a discharge line  18 , a condenser  20 , an economizer input line  22 , an economizer  24 , a first economizer output line  26 , a main electronic expansion valve (EXV)  28 , an evaporator input line  30 , an evaporator  32 , and a suction line  34 . The compressor  16  is fluidly coupled to the condenser  20  by the discharge line  18 . The condenser  20  is fluidly coupled to the economizer  24  by the economizer input line  22 . The economizer input line  22  includes a first branch  22   a  and a second branch  22   b . The first branch  22   a  defines part of the first fluid path  14 , while the second branch  22   b  defines part of the second fluid path  40 , as will be described below. The economizer  24  is fluidly coupled to the main EXV  28  by the first economizer output line  26 . The main EXV  28  is fluidly coupled to the evaporator  32  by the evaporator input line  30 . To complete the first fluid path  14 , the evaporator  32  is fluidly coupled to the compressor  16  by the suction line  34 .  
         [0022]    The second fluid path  40  is defined by some of the components of the first fluid path  14  and is also defined by some additional components. The second fluid path  40  passes through the compressor  16 , the discharge line  18 , the condenser  20 , the economizer input line  22  (via the second branch  22   b ), a secondary EXV  42 , an economizer chamber  44 , and a second economizer output line  46 . Similar to the first fluid path  14 , in the second fluid path  40 , the compressor  16  is fluidly coupled to the condenser  20  by discharge line  18 . Also, the condenser  20  is coupled to the economizer  24  by economizer input line  22 .  
         [0023]    The second branch  22   b  of the economizer input line  22  is fluidly coupled to the secondary EXV  42 . The secondary EXV  42  is coupled via the second branch  22   b  to the economizer chamber  44 , which is positioned within the economizer  24 . The refrigerant passing into the economizer chamber  44  via the second branch  22   b  is in a heat transfer relationship with the refrigerant passing through the economizer  24  via the first branch  22   a . To complete the second fluid path  40 , the economizer chamber  44  is fluidly coupled to the compressor  16  by the second economizer output line  46 .  
         [0024]    The refrigerant in its various states flows through the first fluid path  14  of the refrigerant circuit  12  as follows. Vaporized refrigerant is delivered to the compressor  16  by the suction line  34 . The compressor  16  compresses the vaporized refrigerant by increasing its temperature and pressure. The compressed, vaporized refrigerant is then delivered to the condenser  20  by the discharge line  18 . In a preferred embodiment of the invention, the compressor  16  is a screw-type compressor. However, the compressor  16  may be any appropriate type of compressor. Moreover, the refrigeration system  10  illustrated in FIG. 1 includes only a single compressor  16 . However, more than one compressor may be included in the refrigeration system  10 . If more than one compressor is included in the refrigeration system  10 , the compressors may be arranged in a series configuration or in a parallel configuration.  
         [0025]    The condenser  20  receives compressed, vaporized refrigerant from the compressor  16 . The condenser  20  is a heat exchanger apparatus used to remove heat from the refrigerant in order to condense the vaporized refrigerant into liquid refrigerant. In the condenser  20 , the compressed, vaporized refrigerant releases heat to the air in communication with the condenser  20  in order to cool the vaporized refrigerant. The cooling action of the condenser  20  causes the state of the refrigerant to change from vapor to liquid.  
         [0026]    While in the first fluid path  14 , the liquid refrigerant flows through the first branch  22   a  of economizer input line  22  to the economizer  24 . As the refrigerant flows through the first branch  22   a , the refrigerant is in a heat transfer relationship with the refrigerant in the economizer chamber  44 . The refrigerant flowing through the first branch  22   a  releases heat to the refrigerant in the economizer chamber  44 , thus sub-cooling the liquid refrigerant flowing through the first branch  22   a . Liquid refrigerant is sub-cooled when the temperature of the liquid is lower than its saturation or vaporization temperature at a given pressure. In general, both the condenser  20  and the economizer  24  sub-cool the liquid refrigerant, but the economizer  24  sub-cools the refrigerant more than the condenser  20 .  
         [0027]    The sub-cooled liquid refrigerant is then delivered to the main EXV  28  by the first economizer output line  26 . The main EXV  28  is a throttling device that restricts the flow of the liquid refrigerant by forcing the liquid refrigerant through a small orifice. Forcing the liquid refrigerant through a small orifice causes the pressure of the liquid refrigerant to decrease thereby lowering the boiling temperature of the refrigerant. Reducing the pressure on the liquid refrigerant lowers the boiling point of the refrigerant, making the refrigerant evaporate. As the liquid refrigerant passes through the small orifice of the main EXV  28 , the liquid refrigerant forms into liquid droplets.  
         [0028]    The liquid refrigerant droplets are delivered to the evaporator  32  by evaporator input line  30 . The liquid refrigerant droplets delivered to the evaporator  32  absorb heat from warm air flowing into the evaporator  32 . The evaporator  32  is located within or in thermal communication with the space being conditioned by the refrigeration system  10 . Air is generally circulated between the conditioned space and the evaporator  32  by one or more evaporator fans (not shown). Generally, warmer air flows into the evaporator  32 , the liquid refrigerant droplets absorb heat from the warmer air, and cooler air flows out of the evaporator  32 . The cooler air flowing out of the evaporator  32  cools the masses in the conditioned space by absorbing heat from the masses. Once the cooler air flowing out of the evaporator  32  absorbs heat from the masses within the conditioned space, the warmer air is circulated back to the evaporator  32  by the evaporator fans to be cooled again.  
         [0029]    The liquid refrigerant droplets vaporize once they have absorbed sufficient heat, i.e. once the liquid refrigerant droplets reach their saturation or vaporization temperature at a given pressure. The refrigerant, which has changed from liquid refrigerant droplets back to vaporized refrigerant, is then delivered by suction line  34  back to the compressor  16 . The delivery of the vaporized refrigerant back to the compressor  16  completes the flow of refrigerant through the first fluid path  14 .  
         [0030]    The refrigerant in its various states flows through the second fluid path  40  of the refrigerant circuit  12  as follows. Vaporized refrigerant is delivered to the compressor  16  by the second economizer output line  46 . Just as in the first fluid path  14 , the compressor  16  compresses the vaporized refrigerant by increasing the temperature and pressure of the vaporized refrigerant. The compressed, vaporized refrigerant is then delivered to the condenser  20  by discharge line  18 . In the condenser  20 , the compressed, vaporized refrigerant releases heat to the air in communication with the condenser  20 . The cooling action of the condenser  20  causes the state of the refrigerant to change from vapor to liquid. The liquid refrigerant exiting the condenser  20  is delivered to the economizer  24  by economizer input line  22 .  
         [0031]    Some of the liquid refrigerant exiting the condenser  20  may be drawn off and directed through the second branch  22   b  of the economizer input line  22 . The amount of liquid refrigerant drawn off and directed through the second branch  22   b  is determined by the position of the secondary EXV  42 , among other things. Similar to the main EXV  28 , the secondary EXV  42  is a throttling device used to reduce the pressure and lower the boiling point the refrigerant. As the liquid refrigerant passes through the small orifice of the secondary EXV  42 , the liquid refrigerant forms into liquid refrigerant droplets.  
         [0032]    The liquid refrigerant droplets from the secondary EXV  42  pass into the economizer chamber  44 , where the liquid refrigerant droplets are in a heat transfer relationship with the liquid refrigerant passing through the economizer  24  via the first branch  22   a . The liquid refrigerant droplets absorb heat from the liquid refrigerant passing through the first branch  22   a . The liquid refrigerant droplets vaporize once they have absorbed sufficient heat. The vaporization of the liquid refrigerant in the economizer compartment  44  further cools the liquid refrigerant passing through the first branch  22   a . Thus, the liquid refrigerant passing through the first branch  22   a  of the economizer input line  22  is sub-cooled. Liquid refrigerant is sub-cooled when the temperature of the liquid refrigerant is lower than the saturation or vaporization temperature of the refrigerant at a given pressure.  
         [0033]    Once all of the liquid refrigerant droplets in the economizer chamber  44  have vaporized, the vaporized refrigerant continues to absorb heat until the vaporized refrigerant is superheated. Refrigerant reaches a superheated level when the temperature of the refrigerant is above the vaporization or saturation temperature of the refrigerant at a given pressure. The vaporized refrigerant is then delivered to the compressor  16  via the second economizer output line  46 . The delivery of the vaporized refrigerant back to the compressor  16  completes the flow of refrigerant through the second fluid path  40 .  
         [0034]    The microprocessor circuit  100  includes a plurality of sensors  102  coupled to the refrigerant circuit  12  and coupled to a microprocessor  104 . The microprocessor circuit  100  also controls the main EXV  28  coupled to the microprocessor  104  and the secondary EXV  42  coupled to the microprocessor  104 .  
         [0035]    The plurality of sensors  102  includes a compressor discharge pressure (P D ) sensor  106 , a compressor discharge temperature (T D ) sensor  108 , a suction pressure (P S ) sensor  110 , a suction temperature sensor (T S )  112 , an economizer pressure (P E ) sensor  114 , an economizer temperature (T E ) sensor  116 , an evaporator input temperature (T air,in ) sensor  118 , an evaporator output temperature (T air,out ) sensor  120 , and at least one sensor  122  coupled to the compressor  16 . Each one of the plurality of sensors  102  is electrically coupled to an input to the microprocessor  104 . Moreover, the main EXV  28  and the secondary EXV  42  are each coupled to an output of the microprocessor  104 .  
         [0036]    In the preferred embodiment of the invention, as illustrated in FIG. 5, the above-described refrigeration system  10  is located within a refrigeration system housing unit  300  mounted on a transport container  302 . The transport container  302  is coupled to a tractor trailer  304 . Alternatively, the refrigeration system housing unit  300  may be coupled to any type of transport container unit coupled to any type of vehicle suitable for the transportation of goods, or to any type of vehicle (e.g. a truck or bus) that requires refrigeration.  
         [0037]    [0037]FIG. 2 illustrates a method of operating the refrigeration system  10  in order to maintain a set of primary operating conditions. Referring to FIG. 1, the purpose of the set of primary operating conditions is to ensure that the superheat level of the refrigerant flowing from the economizer  24  to the compressor  16  is maintained, while enhancing the capacity of the refrigeration system  10 .  
         [0038]    Referring to FIGS. 1 and 2, the microprocessor  104  reads  212  the economizer pressure (P E ) sensor  114 . The microprocessor  104  determines  214  a saturated temperature value (T sat ) from the P E  value. T sat  is determined from the P E  value by consulting a thermodynamic properties look-up table for the particular type of refrigerant being used in the refrigeration system  10 . The thermodynamic properties look-up table is provided by the refrigerant manufacturer. A suitable type of refrigerant for this system is R404A refrigerant, which is manufactured by several companies, including E. I. duPont de Nemours and Company, AlliedSignal, Inc., and Elf Atochem, Inc.  
         [0039]    Next, the microprocessor  104  reads  216  the economizer temperature (T E ) sensor  116 . The microprocessor  104  then determines  218  whether T E  is greater than T sat . If T E  is greater than T sat , the refrigerant being delivered from the economizer  24  to the compressor  16  is superheated. Thus, the refrigeration system is operating in a manner that ensures that liquid refrigerant will not be delivered from the economizer  24  through the second economizer output line  46  to the compressor  16 . As long as liquid is not currently being delivered to the compressor  16 , the flow of refrigerant through the secondary EXV  42  can be increased incrementally in order to increase the efficiency, and therefore the capacity, of the system. Accordingly, the microprocessor  104  sends a signal to the secondary EXV  42  to increase  220  the flow of refrigerant through the secondary EXV  42 . Once the microprocessor  104  sends the signal to increase the flow of refrigerant through the secondary EXV  42 , the microprocessor  104  begins the sequence again by performing act  200 .  
         [0040]    If T E  is less than T sat , the refrigerant being delivered from the economizer  24  to the compressor  16  is not superheated. In order to ensure that the superheat level of the refrigerant is maintained, the flow of refrigerant through the secondary EXV  42  can be decreased. Decreasing the flow through the secondary EXV  42  allows the refrigerant to absorb more heat while the refrigerant is in a heat exchange relationship with the refrigerant flowing through the first branch  22   a  of the economizer input line  22  to the main EXV  28 . The refrigerant absorbs more heat to ensure that all of the liquid refrigerant is vaporized. Decreasing the flow through the secondary EXV  42  also decreases the pressure of the refrigerant being delivered back to the compressor  16 . In order to perform this step, the microprocessor  104  sends a signal to the secondary EXV  42  to decrease  222  the flow through the secondary EXV  42 . Once the microprocessor  104  sends the signal to decrease the flow through the secondary EXV  42 , the microprocessor  104  begins the sequence again by performing act  200 .  
         [0041]    Typically, the capacity of a standard refrigeration system is controlled by either adjusting the speed of the compressor or by adjusting the position of the primary expansion valve (e.g., main EXV  28 ). In one aspect of the present invention, the capacity of the system is controlled by adjusting the position of the secondary EXV  42 . For example, if it is desired to reduce the capacity of the system, the secondary EXV  42  can be adjusted to a more closed position, thereby reducing the amount of refrigerant flowing through the economizer, which results in a reduction of the capacity of the system. Similarly, if there is a desire to increase the capacity of the system, the amount of refrigerant flowing through the secondary EXV  42  can be increased, thereby increasing the flow of refrigerant through the economizer, which increases the capacity of the system. It may be desirable to maintain feed back control on the system to ensure that the temperature of the refrigerant in the economizer output line  46  stays above the saturated temperature value for the given pressure to prevent delivery of liquid refrigerant to the compressor.  
         [0042]    [0042]FIG. 3 illustrates another method of operating the refrigeration system  10  embodying the invention. While the method shown in FIG. 2 illustrates the operation of the refrigeration system  10  in order to maintain a set of primary operating conditions, FIG. 3 illustrates the operation of the refrigeration system  10  in order to maintain a first set of secondary operating conditions. Referring to FIG. 1, the purpose of the first set of secondary operating conditions is to quench the compressor  16  with liquid refrigerant if the compressor  16  overheats. Referring to FIGS. 1 and 3, the microprocessor  104  reads  240  the compressor discharge temperature (T D ) sensor  108 . The compressor discharge temperature (T D ) sensor  108  may be physically located between the compressor  16  and the condenser  20  or on the compressor  16  itself. A compressor discharge temperature threshold value (T threshold ) is provided  242  to the microprocessor  104 . The T threshold  value is determined by the manufacturer of the particular compressor  16  being used in the refrigeration system  10 . A suitable compressor  16  for use in the refrigeration system  10  is a Thermo King Corporation double-screw compressor with a T threshold  value of approximately 310° F. The value for T threshold  may be stored in a memory location accessible by the microprocessor  104 .  
         [0043]    The microprocessor  104  determines  244  whether T D  is greater than T threshold . If T D  is not greater than T threshold , the compressor  16  is operating within its temperature range, i.e. the compressor is not overheating. Accordingly, the microprocessor  104  sends a signal to the secondary EXV  42  to maintain  246  the primary operating conditions of the refrigeration system  10  by maintaining the current flow of refrigerant through the secondary EXV  42 . Once the microprocessor  104  sends the signal to maintain  246  the primary operating conditions, the microprocessor  104  begins the sequence again by performing act  240 . However, if T D  is greater than T threshold , the compressor  16  may be overheating. The compressor  16  can be quenched by providing a combination of vapor and liquid refrigerant to the compressor  16  through the second economizer output line  46 . The refrigerant boils off of the compressor  16  in order to cool the compressor  16  to within its temperature operating range. In order to quench the compressor  16 , the primary operating conditions of the refrigeration system must first be overridden  248 , i.e. the flow of refrigerant to the compressor  16  must be increased even though the superheat level of the refrigerant flowing through the second economizer output line  46  will not be maintained while the compressor  16  is being quenched. Once the primary operating conditions are overridden  248 , the microprocessor  104  sends a signal to the secondary EXV  42  to increase  250  the flow of refrigerant through the secondary EXV  42 . Once the microprocessor  104  sends the signal to increase  250  the flow of refrigerant through the secondary EXV  42 , the microprocessor  104  begins the sequence again by performing act  240 . When T D  is returned to a level less than T threshold , the microprocessor  104  can return to the primary operating conditions.  
         [0044]    [0044]FIGS. 4A and 4B illustrate still another method of operating the refrigeration system  10  embodying the invention. FIGS. 4A and 4B illustrate the operation of the refrigeration system  10  in order to maintain a second set of secondary operating conditions. The purpose of the second set of secondary operating conditions is to prevent exceeding the horsepower output limit of the engine (not shown) that powers the compressor  16 . Referring to FIGS. 1 and 4A, the microprocessor  104  reads  270  the compressor discharge pressure (P D ) sensor  106 . The microprocessor  104  also reads  272  the compressor suction pressure (P S ) sensor  110 . Finally, the microprocessor  104  reads  274  the economizer pressure (P E ) sensor  114 .  
         [0045]    A compressor map is provided  276  to the microprocessor  104 . The compressor map may be stored in memory locations accessible by the microprocessor  104 . Using the values for P D , P S , and P E , the microprocessor  104  accesses the compressor map and determines  278  the horsepower required (HP required ) by the compressor  16  for the current sensed pressures, the current compressor speed, and the current mass flow of refrigerant through the refrigeration system  10 . In order to determine the current compressor speed and the current mass flow of refrigerant, the microprocessor  104  reads at least one sensor  122  coupled to the compressor  16 . It should be appreciated that there are other ways to determine the required power of the system, all of which fall within the scope of the present invention.  
         [0046]    Referring to FIG. 4B, an upper power limit in the form of a maximum horsepower output value (HP max ) is provided  280  to the microprocessor  104 . The HP max  value is based on the maximum horsepower available from the compressor engine or prime mover (not shown). The HP max  value for the compressor engine is provided by the manufacturer of the particular compressor engine and may be stored in memory accessible by the microprocessor  104 . The microprocessor  104  determines  282  whether HP required  is greater than HP max . If HP required  is not greater than HP max , enough horsepower is available from the engine powering the compressor  16  for the current mass flow of refrigerant through the refrigeration system  10 . Accordingly, the microprocessor  104  sends a signal to the secondary EXV  42  to maintain  284  the primary operating conditions by maintaining the current mass flow through the secondary EXV  42 . Once the microprocessor  104  sends the signal to maintain  284  the primary operating conditions, the microprocessor  104  begins the sequence again by performing act  270 .  
         [0047]    However, if HP required  is greater than HP max , the engine powering the compressor  16  will not be able to provide enough horsepower to the compressor  16  for the current flow of refrigerant through the refrigeration system  10 . In order to decrease the flow of refrigerant, the primary operating conditions must be overridden  286  and the flow through the secondary EXV  42  must be decreased  288 . Once the primary operating conditions are overridden  286 , the microprocessor  104  sends a signal to the secondary EXV  42  to decrease  288  the flow through the secondary EXV  42 . Once the microprocessor  104  sends the signal to decrease  288  the flow of refrigerant through the secondary EXV  42 , the microprocessor begins the sequence again by performing act  270 .  
         [0048]    Various features and advantages of the invention are set forth in the following claims.