Patent Publication Number: US-7716936-B2

Title: Method and apparatus for affecting defrost operations for a refrigeration system

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to control of defrost operations for refrigeration systems, including electrically operated heat pump systems, and especially to controlling scheduled defrost operations for a refrigeration system or an electrically operated heat pump system. 
     Many commercial refrigeration systems employ electro-mechanical relay timed control devices to schedule the start and control the termination of evaporator coil defrost operations. Many electrically operated heat pumps use similar timed control devices to defrost a heat exchanger outside of an air conditioned space when the heat pump is in a heating mode. For purposes of this disclosure the term “refrigeration systems” also is intended to include electrically operated heat pumps which use electric resistance heaters, microwave energy, or another electrically generated heat source to defrost a heat exchanger outside of an air conditioned space when operating in a heating mode. The timed control devices may be configured to be programmed to initiate a defrost operation at varied and multiple times throughout a day. The timing for defrost operations is typically specified by the needs of the application in which the particular refrigeration system is employed and by knowledge of the manufacturer or installer of the refrigeration system. The timed control devices may control the termination of a defrost process either upon receiving a signal from a temperature or pressure sensing device or upon lapsing of a maximum allowed time that may be pre-programmed in the timed control device. Once programmed, the timed control devices will typically activate the defrost operation in a consistent and repeating manner, regardless of the actual condition of the evaporator coil. 
     The manufacturer or installer must choose the appropriate number of defrosts, and the maximum allowed time for each defrost based upon knowledge of the application and type of equipment being used. Such design choices are sometimes based upon a worst-case scenario that the refrigeration system may be expected to encounter on a day-to-day basis. As a result of such a loose predictive selection method, the refrigeration system may defrost itself more times than is necessary on days not presenting the predicted worst-case scenario. Resulting additional defrosts in such environments are typically a waste of energy, and thus a waste of money. In addition, such additional defrost operations may put refrigerated products at risk of spoilage. 
     Redesigning a defrost control device for a refrigeration system may be expensive, especially in the case of already installed refrigeration systems. 
     There is a need for a defrost control method and apparatus that can be added to an existing refrigeration system to achieve control of defrost operations for a refrigeration system that is responsive to contemporaneous conditions rather than responsive to predicted environmental conditions. 
     There is a need for a method and apparatus for affecting defrost operations for a refrigeration system that is capable of analysis of performance of a refrigeration system and using results of the analysis to truncate a scheduled defrost operation when the method or apparatus determines that the defrost cycle is not required. 
     SUMMARY OF THE INVENTION 
     A method for affecting a scheduled defrost operation for a refrigeration system includes the steps of: (a) after an extant the scheduled defrost operation commences, evaluating at least one predetermined parameter relating to operation of the refrigeration system; (b) if the at least one predetermined parameter manifests a behavior of at least one first predetermined nature over at least one first time interval, continuing the extant scheduled defrost operation; and (c) if the at least one predetermined parameter manifests a behavior of at least one second predetermined nature over at least one second time interval, discontinuing the extant scheduled defrost operation. 
     An apparatus for affecting defrost operations for a refrigeration system includes: (a) A data collection and storage unit coupled with the refrigeration system. The data collection and storage unit acquires collected data from the refrigeration system during or after successive defrost operations of the refrigeration system. The data collection and storage unit stores at least a portion of the collected data as stored data. (b) An evaluation unit coupled with the data collection and storage unit. The evaluation unit operates after an extant scheduled defrost operation commences to effect evaluation of at least one predetermined aspect of at least a portion of the stored data relating to operation of the refrigeration system. (c) A control unit coupled with the evaluation unit and coupled with the refrigeration system. The control unit cooperates with the refrigeration system to effect continuing the extant scheduled defrost operation if the at least one predetermined aspect of the stored data manifests a behavior of at least one first predetermined nature over at least one first time interval. The control unit cooperates with the refrigeration system to effect discontinuing the extant scheduled defrost operation if the at least one predetermined aspect of the stored data manifests a behavior of at least one second predetermined nature over at least one second time interval. 
     It is, therefore, an object of the present invention to provide a defrost control method and apparatus that can be added to an existing refrigeration system to achieve control of defrost operations for a refrigeration system that is responsive to contemporaneous conditions rather than responsive to predicted environmental conditions. 
     It is a further object of the present invention to provide a method and apparatus for affecting defrost operations for a refrigeration system that is capable of analysis of performance of a refrigeration system and using results of the analysis to truncate a scheduled defrost operation when the method or apparatus determines that the defrost cycle is not required. 
     Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a refrigeration system installed for cooling a space. 
         FIG. 2  is a representation of timing for a representative refrigeration cycle. 
         FIG. 3  is a schematic diagram illustrating a representative connection of the apparatus of the present invention with an existing refrigeration system. 
         FIG. 4  is a schematic diagram illustrating the method of the present invention. 
         FIG. 5  is a flow chart illustrating details of a portion of the diagram of  FIG. 4 . 
         FIG. 6  is a flow chart illustrating a representative analysis of data useful for the method and apparatus of the present invention involving a multiple linear regression analysis. 
         FIG. 7  is a flow chart illustrating representative additional steps useful for the method and apparatus of the present invention. 
         FIG. 8  is a flow chart illustrating representative further steps useful for the method and apparatus of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The word “or’” is employed throughout this description to indicate that an inclusive relation applies between terms or among terms. For example, the expression “A or B” intends to describe the relationship (1) A, or (2) B or (3) A and B. 
       FIG. 1  is a schematic diagram of a refrigeration system installed for cooling a space. In  FIG. 1 , a refrigeration system  10  (sometimes referred to as a cooling system) includes equipment in an indoor space  12  and equipment in an outdoor space  14 . Equipment in indoor space  12  includes an indoor space heat exchanger or evaporator  20 . Equipment in outdoor space  14  includes an outdoor heat exchanger or condenser  40 . A circulating device or compressor  42  is coupled with or condenser  40  and with evaporator  20  to effect circulating of a heat transfer fluid or coolant (not shown in detail in  FIG. 1 ) between evaporator  20  and condenser  40 . A regulating device or expansion valve  22  is located between outlet side  41  of condenser  40  and inlet side  21  of evaporator  20  for regulating the rate of flow of coolant through evaporator  20 . A fan  24  is situated in indoor space  12  for directing ambient air in indoor space  12  across evaporator  20  to effect cooling of the ambient air. Evaporator  20  and condenser  40  are each configured for operation as heat transfer units, preferably in multiple pass coil structures. 
     A temperature control unit  26  is located within indoor space  12  for controlling operation of refrigeration or refrigeration system  10 . Temperature control unit  26  may be embodied in a thermostat, pressure switch, or another control mechanism. The control of refrigeration system  10  is preferably carried out as follows: when air temperature within indoor space  12  rises above a predetermined temperature set point, temperature control unit  26  activates a solenoid valve  28  to open and allow coolant to flow through expansion valve  22  and through evaporator  20 . Details of connections among various portions and units of refrigeration system  10  are known by those skilled in the art of cooling system design. In order to avoid unnecessarily cluttering the drawings, those well-known connection details are omitted from the drawings. A low pressure refrigerant or coolant fluid in gaseous form is returned to condenser  40  from evaporator  20  through a suction line  29 . A pressure switch  30  is coupled with suction line  29 . Flow of coolant within suction line  29  causes pressure in suction line  29  to rise. Pressure switch  30  is activated when pressure in suction line  29  reaches a predetermined pressure level. A compressor contactor unit  32  is coupled with pressure switch  30  (connection details are not included in  FIG. 1 ). When pressure switch  30  is activated, compressor contactor unit  32  is activated and a refrigeration process begins. When temperature in indoor space  12  (the refrigerated space) falls below a predetermined set point established by temperature control unit  26 , then temperature control unit  26  causes solenoid valve  28  to close, thereby blocking coolant from passing through expansion valve  22  and evaporator  20 . Compressor  42  continues to operate after solenoid valve  28  closes until pressure in suction line  29  drops low enough to cause pressure switch  30  to cause compressor contactor unit  32  to stop compressor  42 . Generally, solenoid valve  28  closes in response to de-energizing solenoid valve  28 . 
     A defrost time clock  44  is employed to control activation and termination of defrost operations for evaporator  20 . Defrost time clock  44  is typically embodied in an electro-mechanical relay time clock or an electronic controller located in an electrical panel  46  coupled with equipment located in outdoor space  14 . Defrost time clock  44  is sometimes referred to as the defrost timer. An evaporator fan contactor  48  is coupled with defrost time clock  44  and with fan  24  (connection details are not included in  FIG. 1 ). Defrost time clock  44  controls activation of an evaporator fan contactor  48 , compressor contactor unit  32  and a defrost heater contactor  49 . Defrost heater contactor  49  is coupled to control operation of al  50  defrost heater  34  (connection details are not included in  FIG. 1 ). 
     A temperature sensor  36  is coupled with evaporator  20  for sensing temperature of evaporator  20 . A pressure sensor  38  is coupled with evaporator  20  for sensing pressure of coolant passing through evaporator  20 . Either of temperature sensor  36  and pressure sensor  38  may provide a signal to defrost time clock  44  during a defrost process to indicate completion of the defrost process when temperature or pressure in evaporator  20  reaches a predetermined set point. A high temperature cutout switch  39  may be coupled with defrost heater  34  as an emergency back up sensor. Defrost heater  34  may be disconnected from power when high temperature cutout switch  39  senses a high temperature higher than a predetermined set point. Other parameters may also be employed, such as by way of example and not by way of limitation, rate of increase of temperature. Voltage is provided to operate defrost heater  34  when defrost time clock  44  activates defrost heater contactor unit  49 . Evaporator fan  24  is energized when defrost time clock  44  activates evaporator fan contactor  48 . As understood by those skilled in the art of refrigeration systems, an alternate control device such as a thermostat or time delay (not shown in  FIG. 1 ) may be employed to delay operation of fan  24  until temperature of evaporator  20  has been lowered to a predetermined set point. 
     Defrost heater  34  is typically embodied in an electrically resistive heating element. Defrost heater  34  is periodically energized to produce heat so as to melt and thereby remove frost or ice that may have deposited on coils, fins or other heat transfer structures of evaporator  20 . The process of periodically heating evaporator  20  is carried out to maintain effectiveness of heat transfer by evaporator  20 . Defrost time clock  44  operates to control application of voltage to defrost heater  34  by activating defrost heater contactor  49 . Defrost time clock  44  is pre-programmed to activate start of a defrost operation at specific times throughout a day. Pre-programming also often includes a maximum allowed defrost time in order to truncate a heating operation so as to avoid providing too much heat during a defrost cycle. Too much heat may damage defrost heater  34 , evaporator  20  or other elements of refrigeration system  10 . Pre-programming may be effected by a manufacturer, by an installing contractor or by other technical personnel familiar with operation and set-up of refrigeration system  10 . 
     Completion of a defrost operation (sometimes referred to as a defrost cycle) is accomplished by either an elapsing of the pre-programmed maximum allowable defrost time or by an input signal provided at a reset input locus of defrost time clock  44  (not shown in detail in  FIG. 1 ). The reset input locus is typically coupled for receiving signals from temperature sensor  36  indicating temperature of evaporator  20 . When temperature sensor  36  indicates that evaporator  20  has reached a predetermined temperature during a defrost operation, temperature sensor  36  will provide a signal at a reset input locus of defrost time clock  44  to effect termination of the extant defrost operation. Optional high-temperature cut out switch  39  located in proximity with defrost heater  34  provides additional protection by providing a signal at a reset input locus of defrost time clock  44  if defrost heater  34  reaches a predetermined temperature. A useful embodiment of refrigeration system  10  employs a defrost time clock  44  having a double-pole contact that controls defrost heater contactor  49  (and, thus, controls defrost heater  34 ) and also controls evaporator fan contactor  48 . In this double-pole configuration, when defrost time clock  44  is not activating a defrost operation, defrost time clock  44  is activating evaporator fan  24  and solenoid valve  28  to configure refrigeration system  10  for a cooling operation. 
     Defrost time clock  44  operates to carry out its pre-programmed cooling operation according to a refrigeration or cooling cycle. 
       FIG. 2  is a representation of timing for a representative refrigeration cycle. In  FIG. 2 , a graphic representation of a refrigeration cycle  60  is presented with respect to a horizontal axis  61  representing time. Refrigeration cycle  60  includes three main segments: a defrost cycle  62 , a pull-down cycle  64  and a run cycle  66 . Defrost cycle  66  commences at a time t 0  and spans a time interval t 0 -t 1 . Pull-down cycle  64  follows defrost cycle  62 ; pull-down cycle  64  begins at time t 1  and spans a time interval t 1 -t 2 . Duration of time interval t 1 -t 2  for completion of pull-down cycle  64  is the time required to remove heat introduced into evaporator  20  and air surrounding evaporator  20  by defrost heater  34 . This is an example of a continuous-run cycle that does not stop until the air temperature surrounding temperature control unit  26  ( FIG. 1 ) has fallen below the temperature control unit set point. 
     Run cycle  66  follows pull-down cycle  64 . Run cycle  66  begins at time t 2  and spans a time interval t 2 -t 3 . During time interval t 2 -t 3  (run cycle  66 ) compressor  42  cycles on and off based upon temperature control unit  26  becoming satisfied. That is, based upon temperature control unit  26  falls below a predetermined set point. A simple refrigeration cycle  60  substantially repeats the cycle indicated during time interval t 0 -t 3  so that refrigeration cycle  60  continues cyclically, as indicated by follow-on cycles: defrost cycle  70  spanning a time interval t 3 -t 4 , pull-down cycle  72  spanning a time interval t 4 -t 5  and run cycle  74  continuing after time t 5 . 
     Defrost cycle  62  is initiated by a defrost time clock  44  ( FIG. 1 ). By way of example and not by way of limitation, prior to powering up refrigeration system  10  ( FIG. 1 ), a manufacturer or an installing contractor places one or more pins onto pin positions of a timer wheel coupled with defrost time clock  44  (not shown in  FIGS. 1-2 ). Each pin position represents a respective time of a day, so the pin installer can select how many defrost cycles are to occur each day and can establish when each respective defrost cycle will begin. A defrost cycle ends either when evaporator  20  ( FIG. 1 ) reaches a predetermined temperature measured by temperature sensor  36  ( FIG. 1 ), or when coolant in evaporator  20  reaches a predetermined pressure as measured by a pressure sensor  38  ( FIG. 1 ) or after a predetermined maximum allowed time has elapsed since the start of the extant defrost cycle. If the total time of the extant defrost cycle is not determined by the elapsing of the maximum allowed time, variations in time of a respective defrost cycle duration may be attributed to differences in frost load of evaporator  20  prior to the start of the respective defrost cycle. An installing contractor or manufacturer typically takes into consideration a worst case day when programming a defrost schedule for a defrost time clock  44 . As a result, whatever schedule is programmed for a defrost time clock  44  (e.g., by positioning pins in defrost time clock  44  as described above), defrost time clock  44  will faithfully execute the same number of defrosts each day according to its programming. This faithful adherence to a pre-programmed defrost schedule, regardless of real-time conditions, establishes the need fulfilled by the present invention. 
     The apparatus of the present invention is embodied in a defrost control unit  50  ( FIG. 1 ) that may be coupled with defrost time clock  44 , compressor contactor unit  32 , a pressure transducer  52  and a temperature sensor  54  (connection details are not included in  FIG. 1 ). Temperature sensor  54  may advantageously be embodied in a thermistor unit (details not shown in  FIG. 1 ). Defrost time clock  44  operates according to its pre-programming to control starting and completion of defrost operations in refrigeration system  10 . Defrost control unit  50  cooperates with defrost time clock  44  to preempt a defrost operation when it is determined that an extant defrost operation is not necessary. In order to effect the desired cooperation between defrost control unit  50  and defrost time clock  44 , it is necessary to couple defrost control unit  50  with defrost time clock  44 , as illustrated in  FIG. 3 . 
       FIG. 3  is a schematic diagram illustrating a representative connection of the apparatus of the present invention with an existing refrigeration system. In  FIG. 3 , a defrost control unit  50  includes a power connection  80  and a ground connection  82  with defrost time clock  44  for providing power for defrost control unit  50 . Defrost control unit  50  also is coupled to receive a signal D from defrost time clock  44 . Signal D has a value greater than a predetermined signal level when refrigeration system  10  is providing power to evaporator fan  24 . By way of example and not by way of limitation, signal D may have a value of 230 volts when refrigeration system  10  is providing power to evaporator fan  24 . Defrost control unit  50  is still further coupled with defrost time clock  44  to provide a reset signal X to defrost time clock  44  to terminate a defrost operation. Defrost control unit  50  also is coupled with compressor contact unit  32  to receive compressor signals via signal lines  84 ,  86 . Compressor signals are provided to defrost control unit  50  from compressor contactor unit  32  when compressor contactor unit  32  is energized, indicating that contactor unit  32  is trying to turn on compressor  42 . 
     Defrost control unit  50  is also coupled with temperature sensor  54  to receive a signal indicating temperature in suction line  29 . Defrost control unit  50  may also coupled with pressure sensor  54  to receive a signal indicating pressure in suction line  29 . Defrost control unit  50  may also coupled with an ambient temperature sensor  56  (not shown in  FIG. 1 ) to receive a signal indicating ambient temperature in or around outdoor equipment  14 . 
     A microprocessor unit  88  is provided in defrost control unit  50  to control operation of defrost control unit  50 . It is preferred that microprocessors unit  88  include appropriate programming and memory necessary to make decisions whether to skip a defrost cycle as it is activated by defrost time clock  44 , as described below. 
     By way of example and not by way of limitation, defrost control unit  50  may be coupled electrically to coil voltage of compressor contact unit  32 . In such a connected arrangement, defrost control unit  50  may observe a voltage of 230 VAC (Volts, Alternating Current) via lines  80 ,  82  when compressor  42  is activated. Signals received from defrost time clock  44  and compressor contactor unit  32  may be employed to ascertain the operational mode of refrigeration system  10 , as indicated by way of example and not by way of limitation in Table 1 below: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Signal D 
                 COMPRESSOR 
                 SYSTEM MODE 
               
               
                   
                   
               
             
            
               
                   
                 230 Volts 
                 230 Volts 
                 COOLING 
               
               
                   
                 230 Volts 
                  0 Volts 
                 OFF 
               
               
                   
                  0 Volts 
                 — 
                 DEFROST 
               
               
                   
                   
               
            
           
         
       
     
     The third row of Table 1 indicates that when signal D is 0 Volts, refrigeration system  10  is in a defrost mode whatever the value of signals received at lines  84 ,  86  may be. 
     Temperature sensor  54  is coupled in refrigeration system  10  ( FIG. 1 ) at suction line  29 . The temperature in suction line  29  is employed to indicate refrigerant or coolant variations that may occur when evaporator  20  is iced and has lost control of superheat. Pressure sensor  52  is also coupled with suction line  29  and is used to indicate stability of pressure of coolant in suction line  29 . When evaporator  20  is iced and expansion valve  22  is unable to properly control superheat, modulation of expansion valve  22  may cause pressure of coolant in suction line  29  to become unstable. Such pressure variations may be detected and indicated by pressure sensor  52 . Ambient temperature sensor  56  indicates temperature of outside air which has a direct effect upon the capacity of refrigeration system  10 . At higher ambient temperatures a typical refrigeration system has less capacity and it will tend to have longer run cycles, which can increase icing of its evaporator. Conversely, lower ambient temperature can increase capacity of the refrigeration system. This occurrence may also increase the rate of evaporator icing. 
     Microprocessor unit  88  is connected within defrost controller unit  50  to monitor input signals received via lines  80 ,  82 ,  84 ,  86 ; signal D; sensors  52 ,  54 ,  56  and output signal X to operate a program which has a purpose of determining whether an extant defrost cycle initiated by defrost time clock  44  should be terminated or truncated or should be allowed to continue. If microprocessor unit  88  determines that an extant defrost operation (i.e. a defrost operation begun according to pre-programming of defrost time clock  44 ) should be terminated, signal X may be sent to defrost time clock  44  to reset defrost time clock  44 . This early resetting of defrost time clock  44  has the effect of “fooling” defrost time clock  44  into believing that the defrost termination temperature, or defrost termination pressure or another defrost termination criterion has been achieved. As a result, the defrost process is terminated substantially immediately as it begins. 
     The amount of time required to raise the temperature of evaporator  20  to a preset termination temperature (or pressure) is usually related to the amount of frost that has been deposited on the coil of evaporator  20  prior to the start of a defrost operation. By measuring and recording defrost times over a period of days and weeks, natural variations seen in the defrost elapsed times can give an indication when the evaporator  20  was iced and when evaporator  20  was not iced. If some specific input indicator variables are measured and recorded prior to the start of respective defrost cycles, one may be able to determine whether the measured input variables have some correlation to observed respective defrost times. Once a correlation is established and verified, the correlation can be used to predict a future defrost time just as the defrost cycle period is beginning. If the predicted defrost cycle time supports the conclusion that evaporator  20  is probably not iced, then that extant defrost cycle may be skipped. This is the basis of the control algorithm employed by the present invention. 
       FIG. 4  is a schematic diagram illustrating the method of the present invention. In  FIG. 4 , a method  100  for affecting defrost operations for a refrigeration system begins with the refrigeration system in a cooling mode as indicated by a block  102 . During the cooling mode indicated by block  102 , method  100  effects collection and recording or predetermined parameters, as indicated by a dotted line  103  and by a block  104 . The collected and recorded parameters may be saved in a parameter analysis data base  106  as indicated by a dotted line  105 . 
     Method  100  next enters a defrost operation, as indicated by a block  108 . The defrost operation indicated by block  108  is initiated externally of method  100 , such as by a pre-programmed schedule in a defrost time clock (e.g., defrost time clock  44 ;  FIG. 1 ). After initiation of the defrost operation indicated by block  108 , method  100  evaluates whether to skip the extant defrost operation indicated by block  108 , as indicated by a query block  110 . Evaluation is effected in cooperation with analysis carried out using collected and recorded parameters saved in parameter analysis data base  106 . Other parameters may be recorded and saved in parameter analysis data base  106  during the extant defrost operation indicated by block  108  in the evaluation, as indicated by dotted lines  112 ,  114 . Parameters collected during earlier defrost operations (as indicated by dotted lines  115 ,  117  and block  116 ) may also be recorded and saved in parameter analysis data base  106 . Still other parameters collected between earlier defrost operations (as indicated by a block  118  and dotted lines  120 ,  121 ) may be recorded and saved in parameter analysis data base  106 . Any of the recorded and saved parameters in parameter analysis data base  106  may be employed in the evaluation (indicated by block  110 ) whether to skip the extant defrost operation (indicated by block  108 ). 
     If the evaluation indicated by block  110  concludes that the extant defrost operation indicated by block  108  should be skipped, method  100  proceeds via YES response line  122  and deactivates or terminates the extant defrost operation, as indicated by a block  124 . Method  100  thereafter returns to a cooling mode, indicated by block  102 . If the evaluation indicated by block  110  concludes that the extant defrost operation indicated by block  108  should not be skipped, method  100  proceeds via NO response line  130  and continues in defrost mode to complete the extant defrost operation, as indicated by a block  132 . Method  100  thereafter effects post-cycle analysis to collect and record predetermined parameters, as indicated by block  118 . Method  100  then returns to a cooling mode, indicated by block  102 . 
     Evaluation effected pursuant to answering the query posed by query block  110  may, by way of example and not by way of limitation, involve determining whether the evaluated data manifests a behavior of at least one first predetermined nature over at least one first predetermined time interval, and if the data manifests a behavior of at least one first predetermined nature over at least one first predetermined time interval, continuing the extant defrost operation, as indicated by block  132 . Evaluation effected pursuant to answering the query posed by query block  110  may, by way of example and not by way of limitation, further involve determining whether the evaluated data manifests a behavior of at least one second predetermined nature over at least one second predetermined time interval, and if the evaluated data manifests a behavior of at least one second predetermined nature over at least one second predetermined time interval, discontinuing the extant defrost operation, as indicated by block  124 . 
       FIG. 5  is a flow chart illustrating details of a portion of the diagram of  FIG. 4 . In  FIG. 5 , a process  111  illustrates detailed steps relating to execution of method  100  ( FIG. 4 ), in particular indicating details of effecting query block  110  of method  100 . Process  111  may be first regarded while the refrigeration system is in a cooling mode, as indicated by a block  102  (also see  FIG. 4 ). A query is then posed whether a defrost cycle or operation has been activated externally, as indicated by a query block  140 . If no defrost cycle has been activated externally, process  111  proceeds via NO response line  142  and the refrigeration system remains in a cooling mode, as indicated by block  102 . If a defrost cycle has been activated externally, process  111  proceeds via YES response line  144  and a query whether to skip the extant defrost operation is posed, as indicated by query block  110  (also see  FIG. 4 ). 
     Pursuant to executing query block  110 , a query is posed whether a predetermined maximum time has elapsed since the last defrost operation was completed, as indicated by a query block  150 . If the predetermined maximum time has elapsed since the last defrost operation was completed, process  111  proceeds via a YES response line  152  and the defrost mode is continued, as indicated by block  132  (also see  FIG. 4 ). 
     A query is then posed whether the extant defrost cycle has been terminated externally, as represented by a query block  154 . If the extant defrost cycle has not been terminated externally, process  111  continues via NO response line  156  and the extant defrost cycle continues (block  132 ). If the extant defrost cycle has been terminated externally, the process continues via YES response line  158  and post-cycle analysis is carried out to collect and record predetermined parameters, as indicated by block  118  (also see  FIG. 4 ). The process then returns to a cooling mode, indicated by block  102 . 
     If the predetermined maximum time has not elapsed since the last defrost operation was completed, process  111  proceeds from query block  150  via a NO response line  160  and an evaluation of predetermined parameters is effected, as indicated by a block  162 . A query is then posed whether the parameter evaluation effected according to block  162  indicated the extant defrost operation should be terminated, as indicated by a query block  164 . If the parameter evaluation effected according to block  162  indicated the extant defrost operation should not be terminated, process  111  continues via a NO response line  166  and the extant defrost operation continues (block  132 ). The process continues thereafter from block  132  as described earlier herein in connection with  FIG. 5  until process  111  returns to a cooling mode, indicated by block  102 . If the parameter evaluation effected according to block  162  indicated the extant defrost operation should be terminated, process  111  continues via a YES response line  168  and the extant defrost operation is terminated (block  124 ; also see  FIG. 4 ). Thereafter, process  111  returns to a cooling mode, as indicated by block  102 . 
     By way of example and not by way of limitation, in a preferred embodiment, evaluation of defrost operations to evaluate whether to terminate an extant defrost operation or cycle employs a control algorithm using six input variables (X n ) in a multiple linear regression against the defrost cycle length (Y). Variables X n  are identified in  FIG. 2 . Each one of these variables X n , or variations of these variables X n  either independently or in combination with other variables may indicate evaporator frosting. Variable X 1  is the total cycle time from the start of pull-down to the start of the next defrost; represented by time interval t 1 -t 3  in  FIG. 2 . 
     The longer the time elapsed between defrost cycles, the more likely there will be frost deposited on evaporator  20  ( FIG. 1 ), especially if the defrost cycle start times are irregularly spaced. 
     Variable X 2  is the length of time it takes to pull down (pull down cycle  64 ) after a defrost cycle; represented by time interval t 1 -t 2  in  FIG. 2 . Variable X 2  could have an effect on the amount of frost deposited on evaporator  20 . When compared to other pull down times, a longer cycle could indicate a door left open to indoor space  14 , or a load introduced during a defrost cycle. 
     Variable X 3  is an on-off ratio during run cycle  66  (time interval t 2 -t 3  in  FIG. 2 ) that follows pull down cycle  64 . Refrigeration system  10  turns on and off based upon the set point established by temperature control unit  26  ( FIG. 1 ). The ratio of ‘On’ times to ‘Off’ times is recorded during this time period. A higher value indicates that compressor  40  had to operate longer to remove the heat within refrigerated indoor space  14 . This could be because evaporator  20  is iced. An iced evaporator would have less ability to transfer heat, and thus the ‘On’ times would become longer. Variable X 4  is the outside air temperature (ambient temperature). Variable X 4  can effect the operation of refrigeration system  10  because ambient temperature has a direct impact on the capacity of condenser  40 . With a higher ambient temperature, it would take longer to remove the same amount of heat out of refrigerated indoor space  14  then when the outside air is cooler. The additional run time could add more frost to evaporator  20 . Similarly, a much lower ambient air temperature could significantly increase the overall capacity of refrigeration system  10 , and cause evaporator  20  to ice more quickly. 
     Variable X 5  is the pressure measurement in suction line  29  ( FIG. 1 ) recorded during ‘On’ cycles of the refrigeration cycle. A statistical variance of the measurements is calculated during that On-time period. When evaporator  20  becomes iced, the pressures within suction line  29  become irregular due to expansion valve  22  being unable to properly maintain superheat at the outlet of evaporator  20 . This instability can be measured at condenser  40  on suction line  29  coming from evaporator  20 . 
     Variable X 6  is the temperature measurement in suction line  29  recorded during ‘On’ cycles of the refrigeration cycle. During each run cycle, the lowest measured temperature in suction line  29  is recorded. These measurements are used to calculate a temperature slope. When the resulting slope is slightly negative, evaporator  20  may be iced. When the slope has a large negative value, evaporator  20  is almost always iced up. 
     Upon powering up defrost control unit  50 , microprocessor  88  ( FIG. 3 ) begins recording the six variables X n . At the start of a defrost cycle (e.g., time t 0 ;  FIG. 2 ), extant values of variables X n  are saved in memory. When the defrost cycle is complete (e.g., time t 1 ;  FIG. 2 ), the defrost elapsed time (time interval t 0 -t 1 ) is added to the previous data set record. By way of example and not by way of limitation, when ten refrigeration cycles (e.g., from start of pull down cycle  64  to end of defrost  70 ; time interval t 1 -t 4 ;  FIG. 2 ) have been recorded, a multiple linear regression may be performed on the data. 
       FIG. 6  is a flow chart illustrating a representative analysis of data useful for the method and apparatus of the present invention involving a multiple linear regression analysis. In  FIG. 6 , a verifying process  200  for examining results of a multiple linear regression to determine if the results are valid begins at a START locus  202 . Process  200  continues by performing a preliminary regression, as indicated by a block  204 . Process  200  is carried out to determine whether X variables employed in the regression contain multi-colinearity. If multi-colinearity exists, the regression result is invalid. Process  200  continues by posing a query to individually examine X variables for a Variance Inflation Factor (VIF) of greater than a predetermined factor, such as by way of example and not by way of limitation a factor of 10, as indicated by a block  206 . A respective X variable&#39;s having a VIF&gt;10 would indicate that one of the other X variables has a correlation to the respective X variable by more than 90%. Once all of the X variables have been examined for VIF (block  206 ), if one or more has failed, process  200  proceeds according to NO response line  208  and a query is posed whether there are more than three X variables remaining, as indicated by a query block  210 . If there are at least four X variables left, process  200  proceeds via YES response line  211 , the respective X variable with the least statistical significance (using individual t statistics) is eliminated (as indicated by a block  212 ), process  200  returns to a process locus  213  and process  200  proceeds again as described in connection with blocks  204 ,  206 . If the regression fails the VIF test (block  206 ) and only three variables are remaining, process  200  proceeds via NO response line  214 , the regression test fails, as indicated by a block  216 , and process  200  ends at an EXIT locus  218 . 
     If all of the variables pass the VIF test (block  204 ), process  200  proceeds via YES response line  220  and re-performs the multiple linear regression with the remaining variables, as indicated by a block  222 . Process  200  continues thereafter to pose a query whether the regression passed the whole model test, as indicated by a query block  224 . The whole model test involves examining the F statistic for a minimum value. The minimum value is based upon an F statistic table that uses the number of variables and the number of observations to calculate a minimum value. If the regression result has an F statistic that is too low, process  200  proceeds via NO response line  226  and individual variables are examined to determine which has the least significance (using individual t statistics) to the resulting equation. A query is posed whether there are more than three variables left, as indicated by a query block  228 . If there are more than three variables left, process  200  proceeds via YES response line  230  and the least significant variable is eliminated, as indicated by a block  232 . Process  200  thereafter returns to a process locus  234  and process  200  proceeds again as described in connection with blocks  222 ,  224 . If there are three variables or fewer left, process  200  proceeds via NO response line  236 , the regression test fails, as indicated by a block  238 , and process  200  ends at an EXIT locus  240 . 
     If the regression result has an F statistic that is not too low, the whole model test passes, process  200  proceeds via YES response line  242  and the regression result is queried to determine whether the number of input variables being used in the regression is inflating the perceived percentage of variation accountability, as indicated by a query block  244 . An R 2  calculation is employed to express the percentage of input variable variation that is not considered error. Increasing the number of input variables can artificially increase this percentage. By modifying the R 2  calculation to include the effect of the degrees of freedom available, an adjusted R 2  calculation is achieved. If the R 2  and the adjusted R 2  values are compared, the results should be within 5%, as indicated by query block  244 . If the percentage difference between the R 2  and the adjusted R 2  values is greater than 5%, then one of the input variables is contributing too much error and must be eliminated, so process  200  proceeds via NO response line  246  to a process locus  247 . Process  200  proceeds thereafter as described in connection with blocks  228 ,  232 ,  238 ,  240 . If the percentage difference between the R 2  and the adjusted R 2  values is within 5%, then process  200  proceeds via YES response line, the regression test passes and the regression coefficients are recorded, as indicated by a block  250 . Process  200  ends at an EXIT locus  252 . 
       FIG. 7  is a flow chart illustrating representative additional steps useful for the method and apparatus of the present invention. In  FIG. 7 , a process  300  begins at a START locus  302  substantially at the end of each refrigeration cycle (from start of pull-down till end of defrost cycle; e.g., time interval t 1 -t 4 ;  FIG. 2 ). Process  300  continues by using the defrost controller unit  50  ( FIG. 1 ) to record the time interval of the just-completed defrost cycle (e.g., defrost cycle  70 ;  FIG. 2 ) and add the time interval to the data tables that are used to perform later regressions, as indicated by a block  304 . While recording the defrost time, the slope of the suction temperature measurement from the same refrigeration cycle is examined to determine whether there is any evidence of frost build up during the cooling cycle, as indicated by a query block  306 . Query block  306  poses a query whether slope of the suction temperature indicates frost buildup. Evidence of frost buildup is a negative slope value. If the suction temperature slope value for the refrigeration cycle is negative, then process  300  proceeds via YES response line  308  and process  300  ends at an EXIT locus  310 . If the suction temperature slope value for the refrigeration cycle is positive or zero, then process  300  proceeds via NO response line  312  and the defrost time is added to a running defrost time mean calculation, as indicated by a block  314 . A running defrost time standard deviation calculation is performed, as indicated by a block  316 . Process  300  terminates thereafter at an EXIT locus  318 . 
     The multiple linear regression calculations and the regression result testing ( FIG. 6 ) are performed at the start of each pull down cycle (e.g., at times t 1 , t 4 :  FIG. 2 ) for the data previously recorded. The exception to this is when the pull down cycle never completes. 
     After a predetermined number of refrigeration cycles have been observed and recorded (by way of example and not by way of limitation, it is preferred that at least ten refrigeration cycles be observed and recorded), a multiple linear regression is performed at the end of the refrigeration cycle (from start of pull-down until end of defrost cycle (e.g., time interval t 1 -t 4 ;  FIG. 2 ). When the next defrost cycle starts, a decision is made regarding whether or not to skip the defrost cycle.  FIG. 8  illustrates this decision process. 
       FIG. 8  is a flow chart illustrating representative further steps useful for the method and apparatus of the present invention. In  FIG. 8 , a process  400  begins at a START locus  402 . Process  400  continues by posing a query whether the last regression analysis passed all of the statistical tests, as indicated by a query block  404 . If the last regression analysis passed the tests, process  400  proceeds via YES response line  406  and queries are posed whether the adjusted R 2  value is greater than 0.5 and whether the mean value of the previous defrost times that did not indicate an iced evaporator is non-zero, as indicated by a block  408 . The queries are preferable posed serially so that if the adjusted R 2  value is greater than 0.5, then the mean of the previous defrost times that did not indicate an iced evaporator is examined. If the mean value is non-zero, process  400  proceeds via YES response line  410  and the data recorded during the extant refrigeration cycle is inserted into the regression calculation to determine the predicted time of the next defrost cycle, as indicated by a block  412 . 
     To correct for inaccuracies caused by data error, the standard error of the previous regression calculation is added to the prediction time. This corrected result is actually the largest value of a prediction range commonly referred to as the confidence interval. Process  400  continues by posing a query whether the corrected prediction time value is less than the previously calculated defrost cycle time mean (block  314 ;  FIG. 7 ) plus one standard deviation (block  316 ;  FIG. 7 ), as indicated by a query block  414 . If the corrected prediction time value is less than the previously calculated defrost cycle time mean plus one standard deviation, process  400  proceeds via YES response line  416  and a query is posed whether the suction temperature slope from the current data is less than −0.2, as indicated by a query block  418 . If the suction temperature slope from the current is not less than −0.2, then process  400  proceeds via NO response line  420 . Process  400  continues by posing a query whether the On-Off ratio is greater than the mean of the On-Off ratio plus two standard deviations, as indicated by a query block  422 . If the On-Off ratio is not greater than the mean of the On-Off ratio plus two standard deviations, process  400  proceeds via NO response line  424  and the extant defrost cycle is skipped, as indicated by a block  426 . Process  400  then terminates at an EXIT locus  428 . 
     If the last regression failed the statistical tests, process  400  proceeds from query block  404  via NO response line  430  to a process locus  433 . A negative response to the query posed by query block  408  proceeds via NO response line  431  to process locus  433 . Proceeding from process locus  433 , process  400  cannot calculate a defrost time prediction, as indicated by a block  432 . Process  400  continues by posing a query whether the range of the previous defrost times spans at least a 2-minute variation, as indicated by query block  434 . If there is at least a two-minute variation in defrost times, process  400  proceeds via YES response line  436  and a query is posed whether the suction pressure variance of the current refrigeration cycle is less than the mean of the suction pressure variances plus one standard deviation, as indicated by a query block  438 . If the current suction pressure variance is less than the mean plus one standard deviation, process  400  proceeds via YES response line  440  and a query is posed whether the current suction temperature slope is greater than −0.1, as indicated b a query block  442 . If the current suction temperature slope is greater than −0.1, process  400  proceeds via YES response line  444  and the extant defrost cycle is skipped or terminated or truncated, as indicated by a block  446 . Process  400  thereafter terminates at an EXIT locus  448 . 
     If the last regression failed the statistical tests and the range of the previous defrost times is less than 2 minutes, process  400  proceeds via NO response line  450  from query block  434  and a query is posed whether the suction pressure variance of the current refrigeration cycle is less than the mean of the suction pressure variances plus one standard deviation, as indicated by a query block  452 . If the current suction pressure variance is less than the mean of the suction pressure variances plus one standard deviation, process  400  proceeds via YES response line  454  and a query is posed whether the current suction temperature is greater than zero, as indicated by a query block  456 . If the current suction temperature is greater than zero, process  400  proceeds via YES response line  458  and the extant defrost cycle is skipped or terminated or truncated, as indicated by a block  460 . Process  400  thereafter terminates at an EXIT locus  462 . When a defrost cycle is skipped, the data recording continues. The data gathered from the thus-elongated cycle is used in the next succeeding regression calculation. 
     NO responses to queries posed by query blocks  414 ,  438 ,  442 ,  452 ,  456  will not skip or terminate or truncate an extant defrost cycle, as indicated by blocks  470 ,  472  and process  400  thereafter terminates at an exit locus  474 ,  476 . When a defrost cycle is skipped, the data recording continues. The data gathered from the elongated cycle is used in the next regression calculation. YES responses to queries posed by query blocks  418 ,  422  will not skip or terminate or truncate an extant defrost cycle, as indicated by block  470  and process  400  thereafter terminates at an exit locus  474 . 
     After a predetermined number of recorded cycles, (e.g., by way of example and not by way of limitation, thirty recorded cycles), the oldest data is discarded when a next data set becomes available. This provision leaves only the latest thirty cycles in each succeeding regression calculation data set. 
     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: