Abstract:
A method and apparatus for refrigeration system control includes a control system operable to meet cooling demand and control suction pressure for a plurality of refrigeration circuits each including a variable valve and an expansion valve. The controller controls the variable valve independently of the expansion valves to meet cooling demand by determining a change in a measured parameter and controlling at least one of the variable valves based upon the change to an approximately fully open position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/621,625 filed on Jul. 17, 2003, which is a continuation of U.S. patent application Ser. No.10/146,848 filed on May 16, 2002 (now U.S. Pat. No. 6,601,398), which is a divisional of U.S. patent application Ser. No. 10/061,703 filed on Feb. 1, 2002 (now U.S. Pat. No. 6,449,968), which is a divisional of U.S. patent application Ser. No. 09/539,563 filed on Mar. 31, 2000 (now U.S. Pat. No. 6,360,553), which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method and apparatus for refrigeration system control and, more particularly, to a method and apparatus for refrigeration system control utilizing electronic evaporator pressure regulators and a floating suction pressure set point at a compressor rack.  
       BACKGROUND OF THE INVENTION  
       [0003]     A conventional refrigeration system includes a compressor that compresses refrigerant vapor. The refrigerant vapor from the compressor is directed into a condenser coil where the vapor is liquefied at high pressure. The high pressure liquid refrigerant is then generally delivered to a receiver tank. The high pressure liquid refrigerant from the receiver tank flows from the receiver tank to an evaporator coil after it is expanded by an expansion valve to a low pressure two-phase refrigerant. As the low pressure two-phase refrigerant flows through the evaporator coil, the refrigerant absorbs heat from the refrigeration case and boils off to a single phase low pressure vapor that finally returns to the compressor where the closed loop refrigeration process repeats itself.  
         [0004]     In some systems, the refrigeration system will include multiple compressors connected to multiple circuits where a circuit is defined as a physically plumbed series of cases operating at the same pressure/temperature. For example, in a grocery store, one set of cases within a circuit may be used for frozen food, another set used for meats, while another set is used for dairy. Each circuit having a group of cases will thus operate at different temperatures. These differences in temperature are generally achieved by using mechanical evaporator pressure regulators (EPR) or valves located in series with each circuit. Each mechanical evaporator pressure regulator regulates the pressure for all the cases connected within a given circuit. The pressure at which the evaporator pressure regulator controls the circuit is adjusted once during the system start-up using a mechanical pilot screw adjustment present in the valve. The pressure regulation point is selected based on case temperature requirements and pressure drop between the cases and the rack suction pressure.  
         [0005]     The multiple compressors are also piped together using suction and discharge gas headers to form a compressor rack consisting of the multiple compressors in parallel. The suction pressure for the compressor rack is controlled by modulating each of the compressors on and off in a controlled fashion. The suction pressure set point for the rack is generally set to a value that can meet the lowest evaporator circuit requirement. In other words, the circuit that operates at the lowest temperature generally controls the suction pressure set point which is fixed to support this circuit.  
         [0006]     There are, however, various disadvantages of running and controlling a system in this manner. For example, one disadvantage is that the requirement for the case temperature generally changes throughout the year. This requires a refrigeration mechanic to perform an in-situ change of evaporator pressure settings, via the pilot screw adjustment of each evaporator pressure regulator, thereby further requiring re-adjustment of the fixed suction pressure set point at the rack of compressors. Another disadvantage of this type of control system is that case loads change from winter to summer. Thus, in the winter, there is a lower case load which requires a higher suction pressure set point and in the summer there is a higher load requiring a lower suction pressure set point. However, in the real world, such adjustments are seldom done since they also require manual adjustment by way of a refrigeration mechanic.  
         [0007]     What is needed then is a method and apparatus for refrigeration system control which utilizes electronic evaporator pressure regulators and a floating suction pressure set point for the rack of compressors which does not suffer from the above mentioned disadvantages. This, in turn, will provide adaptive adjustment of the evaporator pressure for each circuit, adaptive adjustment of the rack suction pressure, enable changing evaporator pressure requirements remotely, enable adaptive changes in pressure settings for each circuit throughout its operation so that the rack suction pressure is operated at its highest possible value, enable floating circuit temperature based on a product simulator probe, and enable the use of case temperature information to control the evaporator pressure for the whole circuit and the suction pressure at the compressor rack. It is, therefore, an object of the present invention to provide such a method and apparatus for refrigeration system control using electronic evaporator pressure regulators and a floating suction pressure set point.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the teachings of the present invention, a method and apparatus for refrigeration system control utilizing electronic evaporator pressure regulators and a floating suction pressure set point is disclosed. To achieve the above objects of the present invention, the present method and apparatus employs electronic stepper regulators (ESR) instead of mechanical evaporator pressure regulators. The method and apparatus may also utilize temperature display modules at each case that can be configured to collect case temperature, product temperature and other temperatures. The display modules are daisy-chained together to form a communication network with a master controller that controls the electric stepper regulators and the suction pressure set point. The communication network utilized can either be a RS-485 or other protocol, such as LonWorks from Echelon.  
         [0009]     In this regard, the data is transferred to the master controller where the data is logged, analyzed and control decisions for the ESR valve position and suction pressure set points are made. The master controller collects the case temperature for all the cases in a given circuit, takes average/min/max (based on user configuration) and applies PI/PID/Fuzzy Logic algorithms to decide the ESR valve position for each circuit. Alternatively, the master controller may collect liquid sub-cooling or relative humidity information to control the ESR valve position for each circuit. The master controller also controls the suction pressure set point for the rack which is adaptively changed, such that the set point is adjusted in such a way that at least one ESR valve is always kept substantially 100% open.  
         [0010]     In one preferred embodiment, an apparatus for refrigeration system control includes a plurality of circuits with each of the circuits having at least one refrigeration case. An electronic evaporator pressure regulator is in communication with each circuit with each electronic evaporator pressure regulator operable to control the temperature of each circuit. A sensor is in communication with each circuit and is operable to measure a parameter from each circuit. A plurality of compressors is also provided with each compressor forming a part of a compressor rack. A controller controls each evaporator pressure regulator and a suction pressure of the compressor rack based upon the measured parameters from each of the circuits.  
         [0011]     In another preferred embodiment, a method for refrigeration system control is set forth. This method includes measuring a first parameter from a first circuit where the first circuit includes at least one refrigeration case, measuring a second parameter from a second circuit where the second circuit includes at least one refrigeration case, determining a first valve position for a first electronic evaporator pressure regulator associated with the first circuit based upon the first parameter, determining a second valve position for a second electronic evaporator pressure regulator associated with the second circuit based upon the second parameter, electronically controlling the first and the second evaporator pressure regulators to control the temperature in the first circuit and the second circuit.  
         [0012]     In another preferred embodiment, a method for refrigeration system control is set forth. This method includes a lead circuit having a lowest temperature set point from a plurality of circuits where each circuit has at least one refrigeration case, initializing a suction pressure set point for a compressor rack having at least one compressor based upon the identified lead circuit, determining a change in suction pressure set point based upon measured parameters from the lead circuit and updating the suction pressure based upon the change in suction pressure set point.  
         [0013]     In yet another preferred embodiment, a method for refrigeration system control is also set forth. This method includes setting a maximum allowable product temperature for a circuit having at least one refrigeration case, determining a product simulated temperature for the circuit, calculating the difference between the product simulated temperature and the maximum allowable product temperature, and adjusting the temperature set point of the circuit based upon the calculated difference.  
         [0014]     Use of the present invention provides a method and apparatus for refrigeration system control. As a result, the aforementioned disadvantages associated with the currently available refrigeration control systems have been substantially reduced or eliminated.  
         [0015]     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  
       [0016]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0017]      FIG. 1  is a block diagram of a refrigeration system employing a method and apparatus for refrigeration system control according to the teachings of the preferred embodiment in the present invention;  
         [0018]      FIG. 2  is a wiring diagram illustrating use of a display module according to the teachings of the preferred embodiment in the present invention;  
         [0019]      FIG. 3  is a flow chart illustrating circuit pressure control using an electronic pressure regulator;  
         [0020]      FIG. 4  is a flow chart illustrating circuit temperature control using an electronic pressure regulator;  
         [0021]      FIG. 5  is an adaptive flow chart to float the rack suction pressure set point according to the teachings of the preferred embodiment of the present invention;  
         [0022]      FIG. 6  is an illustration of the fuzzy logic utilized in methods  1  and  2  of  FIG. 5 ;  
         [0023]      FIG. 7  is an illustration of the fuzzy logic utilized in method  3  of  FIG. 5 ; and  
         [0024]      FIG. 8  is a flow chart illustrating floating circuit or case temperature control based upon a product simulator temperature probe; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0026]     Referring to  FIG. 1 , a detailed block diagram of a refrigeration system  10  according to the teachings of the preferred embodiment in the present invention is shown. The refrigeration system  10  includes a plurality of compressors  12  piped together with a common suction manifold  14  and a discharge header  16  all positioned within a compressor rack  18 . The compressor rack  18  compresses refrigerant vapor which is delivered to a condenser  20  where the refrigerant vapor is liquefied at high pressure. This high pressure liquid refrigerant is delivered to a plurality of refrigeration cases  22  by way of piping  24 . Each refrigeration case  22  is arranged in separate circuits  26  consisting of a plurality of refrigeration cases  22  which operate within a same temperature range.  FIG. 1  illustrates four (4) circuits  26  labeled circuit A, circuit B, circuit C and circuit D. Each circuit  26  is shown consisting of four (4) refrigeration cases  22 . However, those skilled in the art will recognize that any number of circuits  26 , as well as any number of refrigeration cases  22  may be employed within a circuit  26 . As indicated, each circuit  26  will generally operate within a certain temperature range. For example, circuit A may be for frozen food, circuit B may be for dairy, circuit C may be for meat, etc.  
         [0027]     Since the temperature requirement is different for each circuit  26 , each circuit  26  includes a pressure regulator  28  which is preferably an electronic stepper regulator (ESR) or valve  28  which acts to control the evaporator pressure and hence, the temperature of the refrigerated space in the refrigeration cases  22 . Each refrigeration case  22  also includes its own evaporator and its own expansion valve which may be either a mechanical or an electronic valve for controlling the superheat of the refrigerant. In this regard, refrigerant is delivered by piping  24  to the evaporator in each refrigeration case  22 . The refrigerant passes through an expansion valve where a pressure drop occurs to change the high pressure liquid refrigerant to a lower pressure combination of a liquid and a vapor. As the hot air from the refrigeration case  22  moves across the evaporator coil, the low pressure liquid turns into gas. This low pressure gas is delivered to the pressure regulator  28  associated with that particular circuit  26 . At the pressure regulator  28 , the pressure is dropped as the gas returns to the compressor rack  18 . At the compressor rack  18 , the low pressure gas is again compressed to a high pressure and delivered to the condenser  20  which again, creates a high pressure liquid to start the refrigeration cycle over.  
         [0028]     To control the various functions of the refrigeration system  10 , a main refrigeration controller  30  is used and configured or programmed to control the operation of each pressure regulator (ESR)  28 , as well as the suction pressure set point for the entire compressor rack  18 , further discussed herein. The refrigeration controller  30  is preferably an Einstein Area Controller offered by CPC, Inc. of Atlanta, Ga., or any other type of programmable controller which may be programmed, as discussed herein. The refrigeration controller  30  controls the bank of compressors  12  in the compressor rack  18 , via an input/output module  32 . The input/output module  32  has relay switches to turn the compressors  12  on an off to provide the desired suction pressure. A separate case controller, such as a CC- 100  case controller, also offered by CPC, Inc. of Atlanta, Ga. may be used to control the superheat of the refrigerant to each refrigeration case  22 , via an electronic expansion valve in each refrigeration case  22  by way of a communication network or bus  34 . Alternatively, a mechanical expansion valve may be used in place of the separate case controller. Should separate case controllers be utilized, the main refrigeration controller  30  may be used to configure each separate case controller, also via the communication bus  34 . The communication bus  34  may either be a RS-485 communication bus or a LonWorks Echelon bus which enables the main refrigeration controller  30  and the separate case controllers to receive information from each case  22 .  
         [0029]     In order to monitor the pressure in each circuit  26 , a pressure transducer  36  may be provided at each circuit  26  (see circuit A) and positioned at the output of the bank of refrigeration cases  22  or just prior to the pressure regulator  28 . Each pressure transducer  36  delivers an analog signal to an analog input board  38  which measures the analog signal and delivers this information to the main refrigeration controller  30 , via the communication bus  34 . The analog input board  38  may be a conventional analog input board utilized in the refrigeration control environment. A pressure transducer  40  is also utilized to measure the suction pressure for the compressor rack  18  which is also delivered to the analog input board  38 . The pressure transducer  40  enables adaptive control of the suction pressure for the compressor rack  18 , further discussed herein. In order to vary the openings in each pressure regulator  28 , an electronic stepper regulator (ESR) board  42  is utilized which is capable of driving up to eight (8) electronic stepper regulators  28 . The ESR board  42  is preferably an ESR  8  board offered by CPC, Inc. of Atlanta, Ga., which consists of eight (8) drivers capable of driving the stepper valves  28 , via control from the main refrigeration controller  30 .  
         [0030]     As opposed to using a pressure transducer  36  to control a pressure regulator  28 , ambient temperature inside the cases  22  may be also be used to control the opening of each pressure regulator  28 . In this regard, circuit B is shown having temperature sensors  44  associated with each individual refrigeration case  22 . Each refrigeration case  22  in the circuit B may have a separate temperature sensor  44  to take average/min/max temperatures used to control the pressure regulator  28  or a single temperature sensor  44  may be utilized in one refrigeration case  22  within circuit B, since all of the refrigeration cases in a circuit  26  operate at substantially the same temperature range. These temperature inputs are also provided to the analog input board  38  which returns the information to the main refrigeration controller  30 , via the communication bus  34 .  
         [0031]     As opposed to using an individual temperature sensor  44  to determine the temperature for a refrigeration case  22 , a temperature display module  46  may alternatively be used, as shown in circuit A. The temperature display module  46  is preferably a TD3 Case Temperature Display, also offered by CPC, Inc. of Atlanta, Ga. The connection of the temperature display  46  is shown in more detail in  FIG. 2 . In this regard, the display module  46  will be mounted in each refrigeration case  22 . Each module  46  is designed to measure up to three (3) temperature signals. These signals include the case discharge air temperature, via discharge temperature sensor  48 , the simulated product temperature, via the product simulator temperature probe  50  and a defrost termination temperature, via a defrost termination sensor  52 . These sensors may also be interchanged with other sensors, such as return air sensor, evaporator temperature or clean switch sensor. The display module  46  also includes an LED display  54  that can be configured to display any of the temperatures and/or case status (defrost/refrigeration/alarm).  
         [0032]     The product simulator temperature probe  50  is preferably the Product Probe, also offered by CPC, Inc. of Atlanta, Ga. The product probe  50  is a 16 oz. container filled with four percent (4%) salt water or with a material that has a thermal property similar to food products. The temperature sensing element is embedded in the center of the whole assembly so that the product probe  50  acts thermally like real food products, such as chicken, meat, etc. The display module  46  will measure the case discharge air temperature, via the discharge temperature sensor  48  and the product simulated temperature, via the product probe temperature sensor  50  and then transmit this data to the main refrigeration controller  30 , via the communication bus  34 . This information is logged and used for subsequent system control utilizing the novel methods discussed herein.  
         [0033]     Alarm limits for each sensor  48 ,  50  and  52  may also be set at the main refrigeration controller  30 , as well as defrosting parameters. The alarm and defrost information can be transmitted from the main refrigeration controller  30  to the display module  46  for displaying the status on the LED display  54 .  FIG. 2  also shows an alternative configuration for temperature sensing with the display module  46 . In this regard, the display module  46  is optionally shown connected to an individual case controller  56 , such as the CC- 100  Case Controller, offered by CPC, Inc. of Atlanta, Ga. The case controller  56  receives temperature information from the display module  46  to control the electronic expansion valve in the evaporator of the refrigeration case  22 , thereby regulating the flow of refrigerant into the evaporator coil and the resultant superheat. This case controller  56  may also control the alarm and defrost operations, as well as send this information back to the display module  46  and/or the refrigeration controller  30 .  
         [0034]     Briefly, the suction pressure at the compressor rack  18  is dependent in the temperature requirement for each circuit  26 . For example, assume circuit A operates at 10° F., circuit B operates at 15° F., circuit C operates at 20° F. and circuit D operates at 25° F. The suction pressure at the compressor rack  18 , which is sensed, via the pressure transducer  40 , requires a suction pressure set point based on the lowest temperature requirement for all the circuits  26  (i.e., circuit A) or the lead circuit  26 . Therefore, the suction pressure at the compressor rack  18  is set to achieve a 10° F. operating temperature for circuit A. This requires the pressure regulator  28  to be substantially opened 100% in circuit A. Thus, if the suction pressure is set for achieving 10° F. at circuit A and no pressure regulator valves  28  were used for each circuit  26 , each circuit  26  would operate at the same temperature. However, since each circuit  26  is operating at a different temperature, the electronic stepper regulators or valves  28  are closed a certain percentage for each circuit  26  to control the corresponding temperature for that particular circuit  26 . To raise the temperature to 15° F. for circuit B, the stepper regulator valve  28  in circuit B is closed slightly, the valve  28  in circuit C is closed further, and the valve  28  in circuit D is closed even further providing for the various required temperatures.  
         [0035]     Each electronic pressure regulator (ESR)  28  may be controlled in one of three (3) ways. Specifically, each pressure regulator  28  may be controlled based upon pressure readings from the pressure transducer  36 , based upon temperature readings, via the temperature sensor  44 , or based upon multiple temperature readings taken through the display module  46 .  
         [0036]     Referring to  FIG. 3 , a pressure control logic  60  is shown which controls the electronic pressure regulators (ESR)  28 . In this regard, the electronic pressure regulators  28  are controlled by measuring the pressure of a particular circuit  26  by way of the pressure transducer  36 . As shown in  FIG. 1 , circuit A includes a pressure transducer  36  which is coupled to the analog input board  38 . The analog input board  38  measures the evaporator pressure and transmits the data to the refrigeration controller  30  using the communication network  34 . The pressure control logic or algorithm  60  is programmed into the refrigeration controller  30 .  
         [0037]     The pressure control logic  60  includes a set point algorithm  62 . The set point algorithm  62  is used to adaptively change the desired circuit pressure set point value (SP_ct) for the particular circuit  26  being analyzed based on the level of liquid sub-cooling after the condenser  20  or based on relative humidity (RH) inside the store. The sub-cooling value is the amount of cooling in the liquid refrigerant out of the condenser  20  that is more than the boiling point of the liquid refrigerant. For example, assuming the liquid is water which boils at 212° F. and the temperature out of the condenser is 55° F., the difference between 212° F. and 55° F. is the sub-cooling value (i.e., sub-cooling equals difference between boiling point and liquid temperature). In use, a user will simply select a desired circuit pressure set point value (SP_ct) based on the desired temperature within the particular circuit  26  and the type of refrigerant used from known temperature look-up tables or charts. The set point algorithm  62  will adaptively vary this set point based on the level of liquid sub-cooling after the condenser  20  or based on the relative humidity (RH) inside the store. In this regard, if the circuit pressure set point (SP_ct) for a circuit  26  is chosen to be 30 psig for summer conditions at 80% RH, and 10° F. liquid refrigerant sub-cooling, then for 20% RH or 50° F. sub-cooling, the circuit pressure set point (SP_ct) will be adaptively changed to 33 psig. For other relative humidity (RH %) percentages or other liquid sub-cooling, the values can simply be interpolated from above to determine the corresponding circuit pressure set point (SP_ct). The resulting adaptive circuit pressure set point (SP_ct) is then forwarded to a valve opening control  64 .  
         [0038]     The valve opening control  64  includes an error detector  66  and a PI/PID/Fuzzy Logic algorithm  68 . The error detector  66  receives the circuit evaporator pressure (P_ct) which is measured by way of the pressure transducer  36  located at the output of the circuit  26 . The error detector  26  also receives the adaptive circuit pressure set point (SP_ct) from the set point algorithm  62  to determine the difference or error (E_ct) between the circuit evaporator pressure (P_ct) and the desired circuit pressure set point (SP_ct). This error (E_ct) is applied to the PI/PID/Fuzzy Logic algorithm  68 . The PI/PID/Fuzzy Logic algorithm  68  may be any conventional refrigeration control algorithm that can receive an error value and determine a percent (%) valve opening (VO_ct) value for the electronic evaporator pressure regulator  28 . It should be noted that in the winter, there is a lower load which therefore requires a higher circuit pressure set point (SP_ct), while in the summer there is a higher load requiring a lower circuit pressure set point (SP_ct). The valve opening (VO_ct) is then used by the refrigeration controller  30  to control the electronic pressure regulator (ESR)  28  for the particular circuit  26  being analyzed via the ESR board  42  and the communication bus  34 .  
         [0039]     Referring to  FIG. 4 , a temperature control logic  70  is shown which may be used in place of the pressure control logic  60  to control the electronic pressure regulator (ESR)  28  for the particular circuit  26  being analyzed. In this regard, each electronic pressure regulator  28  is controlled by measuring the case temperature with respect to the particular circuit  26 . As shown in  FIG. 1 , circuit B includes case temperature sensors  44  which are coupled to the analog input board  38 . The analog input board  38  measures the case temperature and transmits the data to the refrigeration controller  30  using the communication network  34 . The temperature control logic or algorithm  70  is programmed into the refrigeration controller  30 .  
         [0040]     The temperature control logic  70  may either receive case temperatures (T 1 , T 2 , T 3 , . . . T n ) from each case  22  in the particular circuit  26  or a single temperature from one case  22  in the circuit  26 . Should multiple temperatures be monitored, these temperatures (T 1 , T 2 , T 3 , . . . T n ) are manipulated by an average/min/max temperature block  72 . Block  72  can either be configured to take the average of each of the temperatures (T 1 , T 2 , T 3 , . . . T n ) received from each of the cases  22 . Alternatively, the average/min/max temperature block  72  may be configured to monitor the minimum and maximum temperatures from the cases  22  to select a mean value to be utilized or some other appropriate value. Selection of which option to use will generally be determined based upon the type of hardware utilized in the refrigeration control system  10 . From block  72 , the temperature (T_ct) is applied to an error detector  74 . The error detector  74  compares the desired circuit temperature set point (SP_ct) which is set by the user in the refrigeration controller  30  to the actual measured temperature (T_ct) to provide an error value (E_ct). Here again, this error value (E_ct) is applied to a PI/PID/Fuzzy Logic algorithm  76 , which is a conventional refrigeration control algorithm, to determine a particular percent (%) valve opening (VO_ct) for the particular electronic pressure regulator (ESR)  28  being controlled via the ESR board  42 .  
         [0041]     While the temperature control logic  70  is efficient to implement, it has inherent logistic disadvantages. For example, each case temperature sensor  44  requires connecting from each display case  22  to a motor room where the analog input board  38  is generally located. This creates a lot of wiring and installation costs. Therefore, an alternative to this configuration is to utilize the display module  46 , as shown in circuit A of  FIG. 1 . In this regard, a temperature sensor within each case  22  passes the temperature information to the display module  46  which is daisy-chained to the communication network  34 . This way, the discharge air temperature sensor  48  or the product probe  50  may be used to determine the case temperature (T 1 , T 2 , T 3 , . . . T n ). This information can then be transferred directly from the display module  46  to the refrigeration controller  30  without the need for the analog input board  38 , thereby substantially reducing wiring and installation costs.  
         [0042]     An adaptive suction pressure control logic  80  to control the rack suction pressure set point (P_SP) is shown in  FIG. 5 . In contrast, the suction pressure set point for a conventional rack is generally manually configured and fixed to a minimum of all the set points used for circuit pressure control. In other words, assume circuit A operates at 0° F., circuit B operates at 5° F., circuit C operates at 10° F. and circuit D operates at 20° F. A user would generally determine the required suction pressure set point based upon pressure/temperature tables and the lowest temperature circuit  26  (i.e., circuit A). In this example, for circuit A operating at 0° F., this would generally require a suction of 30 psig with R404A refrigerant. Therefore, pressure at the suction header  14  would be fixed slightly lower than 30 psig to support each of the circuits A-D. However, according to the teachings of the present invention, the suction pressure set point (P_SP) is not only chosen automatically but also it adaptively changed or floated during the regular control.  FIG. 5  illustrates the adaptive suction pressure control logic  80  to control the rack suction pressure set point according to the teachings of the present invention. This suction pressure set point control logic  80  is also generally programmed into the refrigeration controller  30  which adaptively changes the suction pressure, via turning the various compressors  12  on and off in the compressor rack  18 . The primary purpose of this adaptive suction pressure control logic  80  is to change the suction pressure set point in such a way that at least one electronic pressure regulator (ESR)  28  is substantially 100% open.  
         [0043]     The suction pressure set point control logic  80  begins at start block  82 . From start block  82 , the adaptive control logic  80  proceeds to locator block  84  which locates or identifies the lead circuit  26  based upon the lowest temperature set point circuit that is not in defrost. In other words, should circuit A be operating at −10° F., circuit B should be operating at 0° F., circuit C would be operating at 5° F. and circuit D would be operating at 10° F., circuit A would be identified as the lead circuit  26  in block  84 . From block  84 , the control logic  80  proceeds to decision block  86 . At decision block  86 , a determination is made whether or not the lead circuit  26  has changed from the previous lead circuit  26 . In this regard, upon initial start-up of the control logic  80 , the lead circuit  26  selected in block  84  which is not in defrost will be a new lead circuit  26 , therefore following the yes branch of decision block  86  to initialization block  88 .  
         [0044]     At initialization block  88 , the suction pressure set point P_SP for the lead circuit  26  is determined which is the saturation pressure of the lead circuit set point. For example, the initialized suction pressure set point (P_SP) is based upon the minimum set point from each of the circuits A-D (SP_ct 1 , SP_ct 2 , . . . SP_ctN) or the lead circuit  26 . Accordingly, if the electronic pressure regulators  28  are controlled based upon pressure, as set forth in  FIG. 3 , the known required circuit pressure set point (SP_ct) is selected from the lead circuit (i.e., circuit A) for this initialized suction pressure set point (P_SP). If the electronic pressure regulators  28  are controlled based on temperature, as set forth in  FIG. 4 , then pressure-temperature look-up tables or charts are used by the control logic  80  to convert the minimum circuit temperature set point (SP_ct) of the lead circuit  26  to the initialized suction pressure set point (P_SP). For example, for circuit A operating at −10°, the control logic  80  would determine the initialized suction pressure set point (P_SP) based upon pressure-temperature look-up tables or charts for the refrigerant used in the system. Since the suction pressure set point (P_SP) is taken from the lead circuit A, this is essentially a minimum of all the coolant saturation pressures of each of the circuits A-D.  
         [0045]     Once the minimum suction pressure set point (P_SP) is initialized in initialization block  88 , the adaptive control or algorithm  80  proceeds to sampling block  90 . At sampling block  90 , the adaptive control logic  80  samples the error value (E_ct) (difference between actual circuit pressure and corresponding circuit pressure set point if pressure based control is performed (see  FIG. 3 ), if temperature based control then E_ct is the difference between actual circuit temperature and corresponding circuit temperature set point (see  FIG. 4 )) and the valve opening percent (VO_ct) in the lead circuit every 10 seconds for 10 minutes. When the lead circuit A is in defrost, sampling is then performed on the next lead circuit (i.e., next higher temperature set point circuit) further discussed herein. This set of sixty samples of data from the lead circuit A is then used to calculate the percentage of error values (E_ct) and valve openings (VO_ct) that satisfy certain conditions in calculation block  92 .  
         [0046]     In calculation block  92 , the percentage of error values (E_ct) that are less than 0 (E0); the percent of error values (E_ct) which are greater than 0 and less than 1 (E1) and the valve openings (VO_ct) that are greater than ninety percent are determined in calculation block  92 , represented by VO as set forth in block  92 . For example, assuming the sample block  90  samples the following error data:  
                                                                                                 1   2   3   4   5   6                                    1     +0.5     [−1.0]     +0.1     +1.8   [−1.0]   [−1.0]         2     +1.0     [−1.5]   [−1.5]   +2.0   [−2.0]     0.1         3   +2.0   [−3.0]     +0.5     +6.0   [−2.5]     0.2         4   +3.0   [−7.0]   [−0.3]   +3.0   [−2.2]     0.5         5   +1.5   [−4.0]     +0.4     +1.5   [−2.8]     0.9         6     +0.7     [−2.0]     +0.7       +0.9     [−2.3]   1.2       7     +0.2     [−3.0]     +0.8       +0.8     [−5.5]   1.3       8      0.0     [−1.5]   +1.1     +0.1     [−6.0]   1.6       9   [−0.3]   [−0.5]   +1.7   [−0.3]   [−4.0]   1.8       10   [−0.8]   [−0.1]   +1.3   [−0.8]   [−2.0]   2.0                  
 
 where each column represents a measurement taken every ten seconds with six columns representing a total data set of 60 data points. There are 17 error values (E_ct) that are between 0 and 1 identified above by underlines, providing an E1 of 17/60×100%=28.3%. There are also  27  error values (E_ct) that are less than 0, identified above by brackets, providing an E0 of 27/60×100%=45%. Likewise, valve opening percentages are determined substantially in the same way based upon valve opening (VO_ct) measurements. 
 
         [0047]     From calculation block  92 , the control logic  80  proceeds to either method  1  branch  94 , method  2  branch  96 , or method  3  branch  98  with each of these methods providing a substantially similar final control result. Methods  1  and  2  utilize E0 and E1 data only, while method  3  utilizes E1 and VO data only. Methods  1  and  3  may be utilized with electronic pressure regulators  28 , while method  2  may be used with mechanical pressure regulators. A selection of which method to utilize is therefore generally determined based upon the type of hardware utilized in the refrigeration system  10 .  
         [0048]     From method  1  branch  94 , the control logic  80  proceeds to set block  100  which sets the electronic stepper regulator valve  28  for the lead circuit A at 100% open during refrigeration. Once the electronic stepper regulator valve  28  for circuit A is set at 100% open, the control logic  80  proceeds to fuzzy logic block  102 . Fuzzy logic block  102 , further discussed in detail, utilizes membership functions for E0 and E1 to determine a change in the suction pressure set point (dP). Once this change in suction pressure set point (dP) is determined based on the fuzzy logic block  102 , the control logic  80  proceeds to update block  104 . At update block  104 , a new suction pressure set point P_SP is determined based upon the change in pressure set point (dP) where new P_SP=old P_SP+dP.  
         [0049]     From the update block  104 , the control logic  80  returns to locator block  84  which locates or again identifies the lead circuit  26 . In this regard, should the current lead circuit A be put into defrost, the next lead circuit from the remaining circuits  26  in the system (circuit B-circuit D) is identified at locator block  84 . Here again, decision block  86  will identify that the lead circuit  26  has changed such that initialization block  88  will determine a new suction pressure set point (P_SP) based upon the new lead circuit  26  selected. Should circuit A not be in defrost and the temperatures for each circuit  26  have not been adjusted, the control logic will proceed to sample block  90  from decision block  86  to continue sampling data. In this way, should the lead circuit A be placed in defrost, the next leading circuit  26  will control the rack suction pressure and since this lead circuit  26  will have a temperature that is not as cold as the initial lead temperature, power is conserved based upon this power conserving loop formed by blocks  84 ,  86  and  88 .  
         [0050]     Referring to method  2  branch  96 , this method also proceeds to a fuzzy logic block  106  which determines the change in suction pressure set point (dP) based on E0 and E1, substantially similar to fuzzy logic block  102 . From block  106 , the control logic  80  proceeds to update block  108  which updates the suction pressure set point (P_SP) based on the change in suction pressure set point (dP). From update block  108 , the control logic  80  returns to locator block  84 .  
         [0051]     Referring to the method  3  branch  98 , this method utilizes fuzzy logic block  110  which determines a change in suction pressure set point (dP) based upon E1 and VO, further discussed herein. From fuzzy logic block  110 , the control logic  80  proceeds to update block  112  which again updates the suction pressure set point P_SP=old P_SP +dP. From the update block  112 , the control logic  80  returns again to locator block  84 . It should be noted that while method  1  branch  94  forces the lead circuit A to 100% open via block  100 , method branches  2  and  3  will eventually direct the electronic stepper regulator valve  28  of lead circuit A to substantially 100% open, based upon the controls shown in  FIGS. 3 and 4 .  
         [0052]     Turning to  FIG. 6 , the fuzzy logic utilized in method  1  branch  94  and method  2  branch  96  for fuzzy logic blocks  102  and  106  is further set forth in detail. In this regard, the membership function for E0 is shown in graph  6 A, while the membership function for E1 is shown in graph  6 B. Membership function E0 includes an E0_Lo function, an E0_Avg and an E0_Hi function. Likewise, the membership function for E1 also includes an E1_Lo function and E1_Avg function and an E1_Hi function, shown in graph  6 B. To determine the change in suction pressure set point (dP), a sample calculation is provided in  FIG. 6  for E0=40% and E1=30%.  
         [0053]     In step  1 , which is the fuzzification step, for E0=40%, we have both an E0_Lo of 0.25 and an E0_Avg of 0.75, as shown in graph  6 A. For E1 =30%, we have E1_Lo=0.5 and E1_Avg=0.5, as shown in graph  6 B. Once the fuzzification step  1  is performed, the calculation proceeds to step  2  which is a min/max step based upon the truth table  6 C. In this regard, each combination of the fuzzification step is reviewed in light of the truth table  6 C. These combinations include E0_Lo with E1_Lo; E0_Lo with E1_Avg; E0_Avg with E1_Lo; and E0_Avg with E1_Avg. Referring to the Truth Table  6 C, E0_Lo and E1_Lo provides for NBC which is a Negative Big Change. E0_Lo and E_Avg provides NSC which is a Negative Small Change. E0_Avg and E1_Lo provides for PSC or Positive Small Change. E0_Avg and E1_Avg provides for PSC or Positive Small Change. In the minimization step, a minimum of each of these combinations is determined, as shown in Step  2 . The maximum is also determined which provides a PSC=0.5; and NSC=0.25 and an NBC=0.25.  
         [0054]     From step  2 , the sample calculation proceeds to step  3  which is the defuzzification step. In step  3 , the net pressure set point change is calculated by using the following formula:  
             +   2     ⁢     (   PBC   )       +     1   ⁢     (   PSC   )       +     0   ⁢     (   NC   )       -     1   ⁢     (   NSC   )       -     2   ⁢     (   NBC   )           PBC   +   PSC   +   NC   +   NSC   +   NBC         
 
 By inserting the appropriate values for the variables, we obtain a net pressure set point change of −0.25, as shown in step  3  of the defuzzification step which equals dP. This value is then subtracted from the suction pressure set point in the corresponding update blocks  104  or  108 . 
 
         [0055]     Correspondingly for method  3  branch  98 , the membership function for VO and the membership function for E1 are shown in  FIG. 7 . Here again, the same three calculations from step  1  (fuzzification); step  2  (min/max) and step  3  (defuzzification) are performed to determine the net pressure set point change dP, based upon the membership function for VO shown in graph  7 A, the membership function for E1 shown in graph  7 B, and the Truth Table  7 C.  
         [0056]     Referring now to  FIG. 8 , a floating circuit temperature control logic  116  is illustrated. The floating circuit temperature control logic  116  is based upon taking temperature measurements from the product probe  50  shown in  FIG. 2  which simulates the product temperature for the particular product in the particular circuit  26  being monitored. The floating circuit temperature control logic  116  begins at start block  118 . From start block  118 , the control logic proceeds to differential block  120 . In differential block  120 , the average product simulation temperature for the past one hour or other appropriate time period is subtracted from a maximum allowable product temperature to determine a difference (diff). In this regard, measurements from the product probe  50  are preferably taken, for example, every ten seconds with a running average taken over a certain time period, such as one hour. The maximum allowable product temperature is generally controlled by the type of product being stored in the particular refrigeration case  22 . For example, for meat products, a limit of 41° F. is generally the maximum allowable temperature for maintaining meat in a refrigeration case  22 . To provide a further buffer, the maximum allowable product temperature can be set 5° F. lower than this maximum (i.e., 36° for meat).  
         [0057]     From differential block  120 , the control logic  116  proceeds to either determination block  122 , determination block  124  or determination block  126 . In determination block  122 , if the difference between the average product simulator temperature and the maximum allowable product temperature from differential block  120  is greater than 5° F., a decrease of the temperature set point for the particular circuit  26  by 5° F. is performed at change block  128 . From here, the control logic returns to start block  118 . This branch identifies that the average product temperature is too warm, and therefore, needs to be cooled down. At determination block  124 , if the difference is greater than −5° F. and less than 5° F., this indicates that the average product temperature is sufficiently near the maximum allowable product temperature and no change of the temperature set point is performed in block  130 . Should the difference be less than −5° F. as determined in determination block  126 , an increase in the temperature set point of the circuit by 5° F. is performed in block  132 .  
         [0058]     By floating the circuit temperature for the entire circuit  26  or the particular case  22  based upon the simulated product temperature, the refrigeration case  22  may be run in a more efficient manner since the control criteria is determined based upon the product temperature and not the case temperature which is a more accurate indication of desired temperatures. It should further be noted that while a differential of 5° F. has been identified in the control logic  116 , those skilled in the art would recognize that a higher or a lower temperature differential, may be utilized to provide even further fine tuning and all that is required is a high and low temperature differential limit to float the circuit temperature. It should further be noted that by using the floating circuit temperature control logic  116  in combination with the floating suction pressure control logic  80  further energy efficiencies can be realized.  
         [0059]     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.