Abstract:
A method is disclosed for controlling a discharge pressure of a compressor relative to a suction pressure of the compressor. The discharge pressure and suction pressure of the compressor are monitored and compared with a predetermined maximum pressure ratio of discharge pressure to suction pressure. If the pressure ratio of discharge pressure to suction pressure exceeds a predetermined pressure ratio limit less than the maximum pressure ratio, a controller unloads the compressor. The predetermined pressure ratio limit is determined relative to the measured suction pressure. The discharge pressure and suction pressure are further monitored and the compressor is inhibited from being reloaded for a predetermined time delay. After the predetermined time delay has elapsed, the compressor is reloaded only if i) the discharge pressure falls below a predetermined reload pressure and ii) the chiller system requires additional cooling capacity.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This patent application claims the benefit of U.S. Provisional Application No. 60/754,242 filed Dec. 28, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to a control system and method for a refrigeration or air conditioning system. Specifically, the present invention relates to a system and method for controlling the unloading and loading of a compressor system that attempts to operate above its maximum pressure ratio.  
       BACKGROUND OF THE INVENTION  
       [0003]     In a typical refrigeration system, the compressor operates within a range of acceptable pressure ratios. The pressure ratio is defined as the ratio of compressor discharge pressure to suction pressure. In a compressor operating in normal mode, i.e., with no capacity unloading devices activated, the relationship between discharge pressure and suction pressure is linear. The maximum pressure ratio is determined by the compressor manufacturer. Operating the compressor above the maximum pressure ratio can cause damage to the compressor and other system components, or may cause a safety shutdown resulting in a total loss of cooling provided by the system.  
         [0004]     Therefore, what is needed is a control algorithm that can determine when a compressor in a refrigeration system is operating at or above its maximum pressure ratio, and can promptly unload the compressor to avoid damage to the system, without shutting down the system. What is further needed is a control algorithm to unload a compressor that has reached a maximum pressure ratio, while continuing operation of the compressor and then to reload the compressor to respond to capacity demand only after the pressure ratio has stabilized.  
       SUMMARY OF THE INVENTION  
       [0005]     One embodiment of the present invention is directed to a method for controlling a discharge pressure of a compressor relative to a suction pressure of the compressor. The method includes the steps of measuring a discharge pressure and a suction pressure of the compressor; determining a compressor pressure ratio using the measured discharge pressure and the measured suction pressure; comparing the determined pressure ratio with a predetermined maximum pressure ratio of discharge pressure to suction pressure, wherein the predetermined maximum pressure ratio is less than a maximum rated pressure ratio for the compressor; and unloading the compressor in response to the determined pressure ratio exceeding the predetermined maximum pressure ratio.  
         [0006]     The disclosed method of the present invention further includes the steps of further monitoring the discharge pressure and suction pressure; inhibiting the compressor from being reloaded for a predetermined time delay; and reloading the compressor after the predetermined time delay has elapsed, in response to the discharge pressure falling below a predetermined reload pressure and the chiller system requiring additional cooling capacity.  
         [0007]     In another aspect of the invention, there is disclosed a method for controlling the discharge pressure of a compressor in a chiller system. The chiller system includes a compressor, a condenser, and an evaporator connected in a closed refrigerant loop. Also, a control panel is provided for controlling the operation of the system. The method of controlling the discharge pressure of the compressor includes the steps of: providing a set of maximum compressor discharge pressure values for corresponding suction pressure values, wherein the maximum compressor discharge pressure value is less than a maximum rated discharge pressure value for a corresponding suction pressure value; determining a compressor suction pressure; determining a compressor discharge pressure; unloading the compressor for at least a predetermined interval when the determined compressor discharge pressure is greater than the maximum compressor discharge pressure for the determined compressor suction pressure; initiating a compressor reloading process in response to unloading the compressor and receiving a demand for capacity. The compressor reloading process includes the steps of: providing a set of compressor reload discharge pressure values for corresponding compressor suction pressure values; waiting a predetermined interval; determining an unloaded compressor discharge pressure and an unloaded compressor suction pressure after unloading the compressor; comparing the compressor reload discharge pressure value with the determined unloaded compressor discharge pressure for the determined unloaded compressor suction pressure; waiting another predetermined interval when the determined unloaded compressor discharge pressure is greater than the reload pressure value for the determined unloaded compressor suction pressure; and reloading the compressor when the determined unloaded compressor discharge pressure is less than or equal to the reload pressure value for the determined unloaded compressor suction pressure.  
         [0008]     In another aspect of the invention, there is a computer program product implemented on a computer readable medium and executable by a microprocessor for determining when to unload a compressor in a chiller system. The computer program product includes computer instructions for executing the steps of: measuring a discharge pressure and a suction pressure of the compressor; determining a compressor pressure ratio using the measured discharge pressure and the measured suction pressure; comparing the determined pressure ratio with a predetermined maximum pressure ratio of discharge pressure to suction pressure, wherein the predetermined maximum pressure ratio is less than a maximum rated pressure ratio for the compressor; and unloading the compressor in response to the determined pressure ratio exceeding the predetermined maximum pressure ratio.  
         [0009]     Finally, the computer program product includes instructions for executing the steps of further monitoring the discharge pressure and suction pressure, inhibiting the compressor from being reloaded for a predetermined time delay, and reloading the compressor after the predetermined time delay has elapsed, in response to the discharge pressure falling below a predetermined reload pressure; and the chiller system requiring additional cooling capacity  
         [0010]     One advantage of the present invention is that the system automatically prevents the compressor from running above the maximum pressure ratio that is recommended by the compressor manufacturer, and prevents damage to the compressor that may be caused by exceeding the recommended maximum pressure ratio.  
         [0011]     Another advantage of the present invention is the reduction in safety shutdowns and in total loss of cooling in the chiller system, caused by exceeding the recommended maximum pressure ratio of the compressor.  
         [0012]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates schematically a refrigeration system of the present invention.  
         [0014]      FIG. 2  illustrates a flow chart of one embodiment of the present invention.  
         [0015]      FIG. 3  illustrates a graph of the discharge pressure with relative to suction pressure. 
     
    
       [0016]     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     A general refrigeration system to which the invention can be applied is illustrated, by means of example, in  FIG. 1 . As shown, the HVAC, refrigeration or liquid chiller system  100  has a single compressor, but it is to be understood that the system  100  can have more than one compressor for providing the desired system load. The system  100  includes a compressor  108 , a condenser  112 , a water chiller or evaporator  126 , and a control panel  140 . The control panel  140  can include an analog to digital (A/D) converter  148 , a microprocessor  150 , a non-volatile memory  144 , and an interface board  146 . The operation of the control panel  140  will be discussed in greater detail below. The conventional HVAC, refrigeration or liquid chiller system  100  includes many other features that are not shown in  FIG. 1 . These features have been purposely omitted to simplify the drawing for ease of illustration.  
         [0018]     In a single compressor embodiment, the compressor  108  compresses a refrigerant vapor and delivers it to the condenser  112 . The compressor  108  is preferably a scroll compressor, however the compressor can be any suitable type of compressor including screw compressors, reciprocating compressors, centrifugal compressors, rotary compressors or other type of compressor. The refrigerant vapor delivered to the condenser  112  enters into a heat exchange relationship with a fluid, preferably water, flowing through a heat-exchanger coil  116  connected to a cooling tower  122 . The refrigerant vapor in the condenser  112  undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid in the heat-exchanger coil  116 . The condensed liquid refrigerant from condenser  112  flows through an expansion device to an evaporator  126 .  
         [0019]     The evaporator  126  can include a heat-exchanger coil  128  having a supply line  128 S and a return line  128 R connected to a cooling load  130 . The heat-exchanger coil  128  can include a plurality of tube bundles within the evaporator  126 . A secondary liquid, which is preferably water, but can be any other suitable secondary liquid, e.g. ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator  126  via return line  128 R and exits the evaporator  126  via supply line  128 S. The liquid refrigerant in the evaporator  126  enters into a heat exchange relationship with the liquid in the heat-exchanger coil  128  to chill the temperature of the liquid in the heat-exchanger coil  128 . The refrigerant liquid in the evaporator  126  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the liquid in the heat-exchanger coil  128 . The vapor refrigerant in the evaporator  126  then returns to the compressor  108  to complete the cycle. While the above fluid flow configurations of the refrigerant and other fluids in the condenser  112  and evaporator  126  are preferred, it is to be understood that any suitable fluid flow configuration for the condenser  112  and evaporator  126  can be used for the exchange of heat with the refrigerant.  
         [0020]     To drive the compressor  108 , the system  100  includes a motor or drive mechanism  152 . While the term “motor” is used with respect to the drive mechanism for the compressor  108 , it is to be understood that the term “motor” is not limited to a motor but is intended to encompass any component that can be used in conjunction with the driving of the compressor  108 , such as a variable speed drive and a motor starter. In a preferred embodiment of the present invention, the motors or drive mechanism  152  is an electric motor and associated components. However, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive the compressor  108 .  
         [0021]     The microprocessor  150  of system  100  determines the maximum allowable discharge pressure for a compressor in response to a given suction pressure, by referencing a table or data map stored in the non-volatile memory  144 , or in the microprocessor  150  volatile memory or RAM (not shown). The table or data map  300 , which is illustrated in  FIG. 3 , contains a linear profile which defines the maximum pressure ratio between the discharge and suction pressure of a given compressor. The profile is a function of the specific compressor and is typically provided by the compressor manufacturer. The maximum pressure ratio line  302  indicates the compressor maximum rated discharge pressure P max , as a function of the compressor suction pressure P CS , over a range of suction pressure values  304 . The maximum operating discharge pressure (or the unloading pressure), P UN  is a pressure less than P max  that provides sufficient safety margin for the system to unload in response to rising pressure at the compressor discharge, before the compressor discharge pressure reaches the maximum pressure ratio line  302 . Preferably, P UN  is determined by subtracting a predetermined pressure, for example, 10 PSIG, from the maximum rated discharge pressure P max  specified by the compressor manufacturer for the compressor  108 . The unload line  306  is thus a line segment running parallel to the maximum pressure ratio line  302  on a graph of the table  300 . Alternately, P UN  may be calculated as a percentage of P max . Another line  308  on the table  300  indicates a reload compressor discharge pressure P RE  that is sufficiently below the unload pressure  306  corresponding to the measured parameter, the compressor suction pressure  304 , to allow reloading of the compressor. Preferably, the reload line pressure P RE  is a predetermined percentage, for example, about 85%, of the maximum operating compressor discharge pressure P UN , for the measured suction pressure P CS .  
         [0022]     When the discharge pressure P CD  of the compressor exceeds the maximum operating discharge pressure P UN  for the corresponding suction pressure P CS , the microprocessor  150  sends a signal via interface board  146  to unload the compressor. After a predetermined time period has elapsed, e.g., 10 minutes, the discharge pressure and the suction pressure of the compressor  108  are measured again. If after the time period has elapsed, the compressor discharge pressure P CD , is below the reload pressure P RE  for the compressor suction pressure P CS , the compressor may be reloaded. The system does not reload the compressor  108 , however, unless required to satisfy cooling demand.  
         [0023]     The system  100  includes a sensor  160  for sensing the discharge pressure of compressor  108 , and a sensor  164  for sensing the suction line pressure. The sensor  160  is preferably in the main refrigerant line  110  at the discharge outlet of the compressor  108 , or at the intake to the condenser  112 . However, the sensor  160  can be placed in any location that provides an accurate measurement of the discharge pressure. A signal, either analog or digital, corresponding to the discharge pressure is then transmitted via wireless transmission or signal cable  162  from the sensor  160  to the control panel  140 . In another embodiment of the present invention, the sensor  160  can measure any suitable parameter, e.g., the temperature of the refrigerant, that is related to the discharge pressure.  
         [0024]     If necessary, the signal input to control panel  140  over signal cable  162  is converted to a digital signal or word by A/D converter  148 . The digital signal (either from the A/D converter  148  or from the sensor  160 ) is then input into the control algorithm, which is described in more detail in the following paragraphs, to generate a control signal for the compressor. In another embodiment of the present invention, in lieu of the sensor  160  measuring the discharge pressure, another appropriate parameter is measured by the sensor  160 , such as condenser temperature or pressure, is input into the control algorithm. The control signal for unloading or loading the compressor  108  is provided to the interface board  146  of the control panel  140  by the microprocessor  150 , as appropriate, after executing the control algorithm. The interface board  146  then provides the control signal to the compressor  108  to unload or reload system capacity. In yet another embodiment of the present invention, a differential pressure sensor is used to measure the discharge pressure.  
         [0025]     Microprocessor  150  uses a control algorithm to determine when to unload or reload the compressor  108  by operation of inlet guide vanes or valves (not shown), such as capacity plug valves, slide valves, or other capacity valves in the system  100 . Unloading the compressor may involve turning off or on one of several other parallel compressors as well. In one embodiment, the control algorithm can be a computer program having a series of instructions executable by the microprocessor  150 . The control algorithm determines, after comparing the compressor discharge pressure ratio with the maximum operating pressure ratio, whether to unload the compressor of the system  100  or whether to keep the system  100  in its current operating state. While it is preferred that the control algorithm be embodied in a computer program(s) and executed by the microprocessor  150 , it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware by those skilled in the art. If hardware is used to execute the control algorithm, the corresponding configuration of the control panel  140  can be changed to incorporate the necessary components and to remove any components that may no longer be required, e.g., the A/D converter  148 .  
         [0026]     In addition to using the control algorithm to determine whether to unload the compressor  108  of the system  100  when the discharge pressure is approaching the maximum rated pressure ratio, the microprocessor  150  also executes additional control algorithms to control the “steady state” or normal operation of the system  100 , i.e., the discharge pressure or other related parameter is maintained in a range about a predetermined setpoint to satisfy load demands while staying below the maximum allowed discharge pressure.  
         [0027]      FIG. 2  illustrates a control algorithm of the present invention for determining whether to unload or reload the system based on the pressure ratio. The process for determining when to unload or reload a compressor  108  is described in the context of the refrigeration system  100  illustrated in  FIG. 1 , however, it is to be understood that the process can also be applied to a multiple compressor system, including a system with two or more compressors.  
         [0028]     In response to the activation or starting of the system  100  from an idle or off state, the process begins by activating or starting the compressor  108  at step  202 . It is to be understood that the steady state or normal loaded operating condition for the compressor  108  is different from the steady state operation of the system  100  discussed above. In one embodiment, the compressor  108  may be considered to be in a normal loaded operating state or condition upon the expiration of an optional “warm-up” time period. Regardless of whether a warm-up period is used, the pressure ratio of the compressor is controlled so as not to exceed the maximum pressure ratio. The warm-up time period for the compressor  108  can range up to 5 minutes if desired, but may be any suitable time period for the compressor  108  to reach a normal loaded operating state.  
         [0029]     In an alternate embodiment, if the compressor  108  has not reached a normal loaded operating state the process returns to the warm-up (possibly with a time delay) and the compressor  108  is again evaluated to determine if the compressor  108  has reached a normal loaded operating state.  
         [0030]     In another embodiment of the present invention, the compressor  108  can be determined to be in a normal loaded operating state by measuring an operating parameter of the compressor  108  instead of waiting for the expiration of the predetermined time period. For example, the amount of motor current used by the compressor motor or the positioning of any pre-rotation vanes of the compressor  108  can be measured and used to determine that the compressor  108  has reached a normal loaded operating state. The compressor can be considered to be operating in a normal loaded operating state when the measured motor current is equal to or greater than a predetermined current level, e.g. 100% of the full load current or the allowable motor current, or when the measured position of the pre-rotation vanes is equal to or more open than a predetermined position, e.g., a fully open position.  
         [0031]     Once the compressor  108  has reached a normal loaded operating state, the compressor discharge pressure P CD  and the compressor suction pressure P CS  are then measured in step  204 . While the measurement of P CD  and P CS  is preferred in step  204 , it is to be understood that other parameters can be measured instead of the discharge and suction pressures, e.g. the temperature or pressure of the refrigerant in the evaporator  126  or condenser  112 , and used to calculate P CD  and P CS .  
         [0032]     In still another embodiment of the present invention, steady state operation check can occur after the measurement of the P CD  and P CS  in step  204 . In this embodiment, if the compressor  108  is determined to be operating in a normal loaded operating state, the process would then continue or resume at the point immediately after where step  204  was performed. However, if the compressor  108  is not operating in a normal loaded operating state, the process would return to step  204  for another measurement of P CD  and P CS  and the process steps would be repeated until the compressor  108  is determined to be operating in a normal loaded operating state.  
         [0033]     Referring again to  FIG. 2 , the maximum pressure ratio profile  300  (see  FIG. 3 ) is stored in the system non-volatile memory  144  or loaded in the microprocessor  150  memory, and is accessed by the microprocessor  150 . At step  204 , compressor discharge pressure P CD  and suction pressure P CS  are input to the microprocessor  150  directly or after processing via A/D converter  148 . At step  208 , a maximum pressure P max  is determined based upon the measured P CS . The unloading pressure P UN  is determined based on P max . P CD  is then compared with P UN  in step  210 . If P CD  does not exceed P UN , then the system remains in normal operation or its current operating state through the next iteration of the process. If, however, P CD  exceeds P UN , at step  210  the microprocessor  150  transmits a signal to the system to unload in step  212 . After the unload signal is transmitted in step  212 , a delay period is initiated at step  214 . During the delay period, the pressure parameters may be updated, but no instructions to reload the system are processed. The delay period for reloading in the preferred embodiment is about 10 minutes, but may be longer or shorter, e.g., from 5 minutes to 25 minutes, depending on system response, system capacity and other requirements. At step  216 , after the delay period lapses, the compressor discharge pressure P CD  and suction pressure P CS  are again measured, and at step  218  the system determines whether the discharge pressure P CD  has dropped below an acceptable reloading pressure P RE  as described above with respect to  FIG. 3 .  
         [0034]     If P CD  is greater than P RE , then the system repeats step  216  as indicated by arrow  217 . During the iteration loop indicated by arrow  217 , the system may further unload the compressor, or if after a predetermined number of iterations P CD  continues to exceed P RE , shut down the system. If desired, a delay period (not shown) may be inserted by the microprocessor  150  during the iteration loop indicated by arrow  217 , but the delay in the iteration is not required. Otherwise, when P CD  is determined to be less than or equal to P RE , the system proceeds to step  220 . At step  220 , the microprocessor  150  sends a signal to the system  100  to permit reloading of the compressor, but does not reload the compressor. At step  222 , the system determines whether a capacity demand signal is present. If a capacity demand signal is received, the compressor  108  is reloaded at step  224  and the system  100  returns to normal operation. If no capacity demand signal is received at step  222 , the system returns to step  220 .  
         [0035]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.