Abstract:
A method and apparatus for defrosting an evaporator of a refrigeration system including a defrost heater and a controller operatively connected to the evaporator and a defrost heater is provided. The method comprises initiating a defrost cycle to energize the defrost heater to defrost the evaporator, monitoring a temperature of the evaporator, terminating the defrost cycle by de-energizing the defrost heater when a low temperature termination point of the evaporator is reached when in a low temperature defrost cycle, and terminating the defrost cycle by de-energizing the defrost heater when a high temperature termination point of the evaporator is reached when in a high temperature defrost cycle.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates generally to refrigerators and, more particularly, to a method and apparatus for controlling refrigeration defrost cycles.  
           [0002]    Known frost free refrigerators include a refrigeration defrost system to limit frost buildup on evaporator coils. Conventionally, an electromechanical timer is used to energize a defrost heater after a pre-determined run time of the refrigerator compressor to melt frost buildup on the evaporator coils. To prevent overheating of the freezer compartment during defrost operations when the heater is energized, in at least one type of defrost system the compartment is pre-chilled. After defrost, the compressor is typically run for a predetermined time to lower the evaporator temperature and prevent food spoilage in the refrigerator and/or fresh food compartments of a refrigeration appliance.  
           [0003]    Such timer-based defrost systems, however are not as energy efficient as desired. For instance, they tend to operate regardless of whether ice or frost is initially present, and they often pre-chill the freezer compartment regardless of initial compartment temperature. In addition, the defrost heater is typically energized without temperature regulation in the freezer compartment, and the compressor typically runs after a defrost cycle regardless of the compartment temperature. Such open loop defrost control systems, and the accompanying inefficiencies are undesirable in light of increasing energy efficiency requirements.  
           [0004]    Recognizing the limitations of such timer-based defrost systems, efforts have been made to provide adaptive defrost systems employing limited feedback, such as door openings and compressor and evaporator conditions, for improved energy efficiency of defrost cycles. As such, unnecessary defrost cycles are avoided and the defrost heater is cycled on and only as necessary, such as until the evaporator reaches a fixed termination temperature. See, for example, U.S. Pat. No. 4,528,821. However, achieving some defrost goals, such as melting all of the frost off of the evaporator and melting ice out of an icemaker fill tube, are detrimental to achieving other defrost goals, such as maintaining freezer compartment temperatures at sufficient levels during defrost operations to prevent freezer burn and moisture formation/ice buildup in the freezer compartment. Known defrost systems have not resolved these difficulties.  
         SUMMARY OF INVENTION  
         [0005]    In one aspect, a method for defrosting an evaporator of a refrigeration system, the method utilizing a defrost heater and a controller operatively connected to the evaporator and a defrost heater, is provided. The method comprises initiating a defrost cycle to energize the defrost heater to defrost the evaporator, monitoring a temperature of the evaporator, terminating the defrost cycle by de-energizing the defrost heater when a low temperature termination point of the evaporator is reached when in a low temperature defrost cycle, and terminating the defrost cycle by de-energizing the defrost heater when a high temperature termination point of the evaporator is reached when in a high temperature defrost cycle.  
           [0006]    In another aspect, a method for defrosting a refrigeration unit including an evaporator, a defrost heater, and a controller operatively connected to the evaporator and the defrost heater is provided. The controller includes a defrost counter, and the method comprises initiating a defrost cycle to energize the defrost heater to defrost the evaporator, selecting a low temperature defrost cycle when the defrost counter is less than a predetermined value, and selecting a high temperature defrost cycle when the defrost counter equals said predetermined value.  
           [0007]    In still another aspect, a method for defrosting a refrigerator including a sealed system, an evaporator, a defrost heater, and a controller operatively connected to the evaporator and a defrost heater is provided. The controller includes a defrost counter and a defrost timer. The method comprises initiating a defrost cycle to energize the defrost heater to defrost the evaporator, selecting a low temperature defrost cycle when the defrost counter is less than a predetermined value, selecting a high temperature defrost cycle when the defrost counter equals the predetermined value, terminating the low temperature defrost cycle by de-energizing the defrost heater when a first temperature termination point of the evaporator is reached when the low temperature defrost cycle is selected, terminating the high temperature defrost cycle by de-energizing the defrost heater when a second temperature termination point of the evaporator is reached when the high temperature defrost cycle is selected, the second termination temperature higher than the first termination temperature, comparing an elapsed defrost time to a reference defrost time when either of the high temperature defrost and low temperature defrost are terminated, selecting a normal or abnormal defrost interval based upon the compared elapsed defrost time and reference defrost time, and operating the sealed system for the selected defrost interval.  
           [0008]    In still another aspect, a refrigeration defrost unit for an evaporator is provided. The defrost unit comprises a defrost heater, a controller operatively coupled to said defrost heater, and a thermistor adapted for sensing a temperature of the evaporator. The controller is configured to operate said defrost heater in a low temperature defrost mode de-energizing said defrost heater at a first temperature in response to said thermistor, and to operate said defrost heater in a high temperature defrost mode de-energizing said defrost heater at a second temperature in response to said thermistor, said second temperature higher than said first temperature.  
           [0009]    In another aspect a refrigeration unit is provided that comprises a compressor, an evaporator, a defrost heater, and a controller. The controller is operatively coupled to said compressor, said evaporator and said defrost heater, and the controller comprises a defrost timer and operates said compressor in a normal mode and an abnormal load in response to a value of the defrost timer. The controller further comprises a defrost counter and operates said defrost heater in a high temperature defrost mode and a low temperature defrost mode based upon a value of said counter.  
           [0010]    In a further aspect a refrigerator is provided which comprises a cabinet defining at least one refrigeration compartment, a sealed system for cooling said at least one refrigeration compartment, a defrost heater, and a controller operatively coupled to said sealed system and to the defrost heater. The controller is configured to adaptively control said defrost heater and said sealed system in a high temperature defrost mode and a low temperature defrost mode between normal and abnormal defrost intervals. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 is a perspective view of a refrigerator.  
         [0012]    [0012]FIG. 2 is a block diagram of a refrigerator controller in accordance with one embodiment of the present invention.  
         [0013]    [0013]FIG. 3 is a block diagram of the main control board shown in FIG. 2.  
         [0014]    [0014]FIG. 4 is a block diagram of the main control board shown in FIG. 2.  
         [0015]    [0015]FIG. 5 is a defrost state diagram executable by a state machine of the controller shown in FIG. 2.  
         [0016]    [0016]FIG. 6 is a method flow chart of an adaptive defrost algorithm executable by the controller shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 illustrates a side-by-side refrigerator  100  in which the present invention may be practiced. It is recognized, however, that the benefits of the present invention apply to other types of refrigerators, freezers, and refrigeration appliances wherein frost free operation is desirable. Consequently, the description set forth herein is for illustrative purposes only and is not intended to limit the invention in any aspect.  
         [0018]    Refrigerator  100  includes a fresh food storage compartment  102  and a freezer storage compartment  104  contained within an outer case  106  and inner liners  108  and  110 . A space between case  106  and liners  108  and  110 , and between liners  108  and  110 , is filled with foamed-in-place insulation. Outer case  106  normally is formed by folding a sheet of a suitable material, such as pre-painted steel, into an inverted shape to form top and side walls of case. A bottom wall of case  106  normally is formed separately and attached to the case side walls and to a bottom frame that provides support for refrigerator  100 . Inner liners  108  and  110  are molded from a suitable plastic material to form freezer compartment  104  and fresh food compartment  102 , respectively. Alternatively, liners  108 ,  110  may be formed by bending and welding a sheet of a suitable metal, such as steel. The illustrative embodiment includes two separate liners  108 ,  110  as it is a relatively large capacity unit and separate liners add strength and are easier to maintain within manufacturing tolerances. In smaller refrigerators, a single liner is formed and a mullion spans between opposite sides of the liner to divide it into a freezer compartment and a fresh food compartment.  
         [0019]    A breaker strip  112  extends between a case front flange and outer front edges of liners. Breaker strip  112  is formed from a suitable resilient material, such as an extruded acrylo-butadiene-styrene based material (commonly referred to as ABS).  
         [0020]    The insulation in the space between liners  108 ,  110  is covered by another strip of suitable resilient material, which also commonly is referred to as a mullion  114 . Mullion  114  also preferably is formed of an extruded ABS material. Breaker strip  112  and mullion  114  form a front face, and extend completely around inner peripheral edges of case  106  and vertically between liners  108 ,  110 . Mullion  114 , insulation between compartments, and a spaced wall of liners separating compartments, sometimes are collectively referred to herein as a center mullion wall  116 .  
         [0021]    Shelves  118  and slide-out drawers  120  normally are provided in fresh food compartment  102  to support items being stored therein. A bottom drawer or pan  122  partly forms a quick chill and thaw system (not shown) and selectively controlled, together with other refrigerator features, by a microprocessor (not shown in FIG. 1) according to user preference via manipulation of a control interface  124  mounted in an upper region of fresh food storage compartment  102  and coupled to the microprocessor. A shelf  126  and wire baskets  128  are also provided in freezer compartment  104 . In addition, an ice maker  130  may be provided in freezer compartment  104 .  
         [0022]    A freezer door  132  and a fresh food door  134  close access openings to fresh food and freezer compartments  102 ,  104 , respectively. Each door  132 ,  134  is mounted by a top hinge  136  and a bottom hinge (not shown) to rotate about its outer vertical edge between an open position, as shown in FIG. 1, and a closed position (not shown) closing the associated storage compartment. Freezer door  132  includes a plurality of storage shelves  138  and a sealing gasket  140 , and fresh food door  134  also includes a plurality of storage shelves  142  and a sealing gasket  144 .  
         [0023]    In accordance with known refrigerators, refrigerator  100  also includes a machinery compartment (not shown) that at least partially contains components for executing a known vapor compression cycle for cooling air. The components include a compressor (not shown in FIG. 1), a condenser (not shown in FIG. 1), an expansion device (not shown in FIG. 1), and an evaporator (not shown in FIG. 1) connected in series and charged with a refrigerant. The evaporator is a type of heat exchanger which transfers heat from air passing over the evaporator to a refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is used to refrigerate one or more refrigerator or freezer compartments via fans (not shown in FIG. 1). Collectively, the vapor compression cycle components in a refrigeration circuit, associated fans, and associated compartments are referred to herein as a sealed system. The construction of the sealed system is well known and therefore not described in detail herein, and the sealed system is operable to force cold air through the refrigerator subject to the following control scheme.  
         [0024]    [0024]FIG. 2 illustrates a controller  160  in accordance with one embodiment of the present invention. Controller  160  can be used, for example, in refrigerators, freezers and combinations thereof, such as, for example side-by-side refrigerator  100  (shown in FIG. 1).  
         [0025]    Controller  160  includes a diagnostic port  162  and a human machine interface (HMI) board  164  coupled to a main control board  166  by an asynchronous interprocessor communications bus  168 . An analog to digital converter (A/D converter)  170  is coupled to main control board  166 . A/D converter  170  converts analog signals from a plurality of sensors including one or more fresh food compartment temperature sensors  172 , a quick chill/thaw feature pan (i.e., pan  122  shown in FIG. 1) temperature sensors  174  (shown in FIG. 8), freezer temperature sensors  176 , external temperature sensors (not shown in FIG. 2), and evaporator temperature sensors  178  into digital signals for processing by main control board  166 .  
         [0026]    In an alternative embodiment (not shown), A/D converter  170  digitizes other input functions (not shown), such as a power supply current and voltage, brownout detection, compressor cycle adjustment, analog time and delay inputs (both use based and sensor based) where the analog input is coupled to an auxiliary device (e.g., clock or finger pressure activated switch), analog pressure sensing of the compressor sealed system for diagnostics and power/energy optimization. Further input functions include external communication via IR detectors or sound detectors, HMI display dimming based on ambient light, adjustment of the refrigerator to react to food loading and changing the air flow/pressure accordingly to ensure food load cooling or heating as desired, and altitude adjustment to ensure even food load cooling and enhance pull-down rate of various altitudes by changing fan speed and varying air flow.  
         [0027]    Digital input and relay outputs correspond to, but are not limited to, a condenser fan speed  180 , an evaporator fan speed  182 , a crusher solenoid  184 , an auger motor  186 , personality inputs  188 , a water dispenser valve  190 , encoders  192  for set points, a compressor control  194 , a defrost heater  196 , a door detector  198 , a mullion damper  200 , feature pan air handler dampers  202 ,  204 , and a quick chill/thaw feature pan heater  206 . Main control board  166  also is coupled to a pulse width modulator  208  for controlling the operating speed of a condenser fan  210 , a fresh food compartment fan  212 , an evaporator fan  214  associated with an evaporator  215  (shown in phantom in FIG. 3), and a quick chill system feature pan fan  216 .  
         [0028]    [0028]FIGS. 3 and 4 are more detailed block diagrams of main control board  166 . As shown in FIGS. 3 and 4, main control board  166  includes a processor  230 . Processor  230  performs temperature adjustments/dispenser communication, AC device control, signal conditioning, microprocessor hardware watchdog, and EEPROM read/write functions. In addition, processor  230  executes many control algorithms including sealed system control, evaporator fan control, defrost control, feature pan control, fresh food fan control, stepper motor damper control, water valve control, auger motor control, cube/crush solenoid control, timer control, and self-test operations.  
         [0029]    Processor  230  is coupled to a power supply  232  which receives an AC power signal from a line conditioning unit  234 . Line conditioning unit  234  filters a line voltage which is, for example, a 90-265 Volt AC, 50/60 Hz signal. Processor  230  also is coupled to an EEPROM  236  and a clock circuit  238 .  
         [0030]    A door switch input sensor  240  is coupled to fresh food and freezer door switches  242 , and senses a door switch state. A signal is supplied from door switch input sensor  240  to processor  230 , in digital form, indicative of the door switch state. Fresh food thermistors  244 , a freezer thermistor  246 , at least one evaporator thermistor  248 , a feature pan thermistor  250 , and an ambient thermistor  252  are coupled to processor  230  via a sensor signal conditioner  254 . Conditioner  254  receives a multiplex control signal from processor  230  and provides analog signals to processor  230  representative of the respective sensed temperatures. Processor  230  also is coupled to a dispenser board  256  and a temperature adjustment board  258  via a serial communications link  260 . Conditioner  254  also calibrates the above-described thermistors  244 ,  246 ,  248 ,  250 , and  252 .  
         [0031]    Processor  230  provides control outputs to a DC fan motor control  262 , a DC stepper motor control  264 , a DC motor control  266 , and a relay watchdog  268 . Watchdog  268  is coupled to an AC device controller  270  that provides power to AC loads, such as to water valve  190 , cube/crush solenoid  184 , a compressor  272 , auger motor  186 , a feature pan heater  206 , and defrost heater  196 . DC fan motor control  266  is coupled to evaporator fan  214 , condenser fan  210 , fresh food fan  212 , and feature pan fan  216 . DC stepper motor control  266  is coupled to mullion damper  200 , and DC motor control  266  is coupled to one of more sealed system dampers.  
         [0032]    Processor logic uses many inputs to make control decisions pertaining to the present invention, including but not limited to Freezer Door State via light switch detection using optoisolators, Fresh Food Door State via light switch detection using optoisolators, Freezer Compartment Temperature via a thermistor, Evaporator Temperature via a thermistor, Compressor On Time, Time to Complete a Defrost, and User Desired Set Points via electronic keyboard and display or encoders.  
         [0033]    The electronic controls activate many loads to control refrigerator functions and operation, many of which are beyond the scope of the present invention. Those loads having some effect on the defrost functions of the refrigerator include Multi-speed or variable speed (via PWM) fresh food fan, Multi-speed (via PWM) evaporator fan, Multi-speed (via PWM) condenser fan, Compressor Relay, Defrost Relay, and Drip pan heater Relay that activate the sealed system and defrost system components.  
         [0034]    These and other functions of the above-described electronic control system are performed under the control of firmware implemented as small independent state machines. As is described in detail below, the electronic controls facilitate an effective defrost scheme that, unlike known defrost systems, employs more than one defrost interval (normal and abnormal) and more than one defrost cycle (high and low temperature defrost) dependent upon actual operating conditions for improved defrost performance. Low temperature defrost cycles having a reduced effect on freezer compartment temperature are typically executed, while high temperature defrost cycles having a greater effect on freezer compartment temperature are selectively executed only at predetermined intervals. Instances of freezer burn and moisture buildup in the freezer compartment are thereby substantially avoided while still achieving an energy efficient, effective defrost system.  
         [0035]    [0035]FIG. 5 is a defrost state diagram  300  illustrating a state algorithm executable by a state machine of controller  160  (shown in FIGS.  2 - 4 ). As will be seen, controller  160  adaptively determines an optimal defrost state based upon effectiveness of defrost cycles as they occur.  
         [0036]    In an exemplary embodiment, by monitoring evaporator temperature over time, it is determined whether defrost cycles are deemed normal or abnormal. More specifically, when it is time to defrost, i.e. after an applicable defrost interval (explained below) has expired, the refrigerator sealed system is shut off, defrost heater  196  is turned on (at state  2 ), and a defrost timer is started. As the evaporator coils defrost, the temperature of the evaporator increases. When evaporator temperature reaches a predetermined termination temperature (dependant upon the high or low temperature defrost cycle explained below), the defrost heater  196  is shut off and the elapsed time defrost heater  196  was on (Δt de ) is recorded in system memory. Also, if the termination temperature is not reached within a predetermined maximum time, defrost heater  196  is shut off and the elapsed time the defrost heater was on is recorded in system memory.  
         [0037]    The elapsed defrost time Δt de  is then compared with a predetermined defrost de reference time (Δt dr ) representative of, for example, an empirically determined or calculated elapsed defrost heater time to remove a selected amount of frost buildup on the evaporator coils that is typically encountered in the applicable refrigerator platform under predetermined usage conditions. If elapsed defrost time Δt de  is greater than reference time Δt dr , thereby indicating excessive frost buildup, a first or abnormal defrost interval, or time until the next defrost cycle, is employed If elapsed defrost time Δt de  is less than reference time Δt dr , a second or normal defrost interval, or time until the next defrost cycle is employed. The normal and abnormal defrost intervals, as defined below, are selectively employed, using Δt dr  as a baseline, for more efficient defrost operation as refrigerator usage conditions change, thereby affecting frost buildup on the evaporator coils. In an exemplary embodiment, Δt dr  is twenty minutes, although it is appreciated that Δt dr  could be greater or lesser without departing from the scope of the present invention.  
         [0038]    In one embodiment, the following control scheme automatically cycles between the first or abnormal defrost interval and the second or normal defrost interval on demand. When usage conditions are heavy and refrigerator doors  132 ,  134  (shown in FIG. 1) are opened frequently, thereby introducing more humidity into the refrigeration compartment, the system tends to execute the first or abnormal defrost interval repeatedly. When usage conditions are light and the doors opened infrequently, thereby introducing less humidity into the refrigeration compartments, the system tends to execute the second or normal defrost interval repeatedly. In intermediate usage conditions the system alternates between one or more defrost cycles at the first or abnormal defrost interval and one or more defrost cycles at the second or normal defrost interval.  
         [0039]    Upon power up, controller  160  reads freezer thermistor  246  (shown in FIG. 3) over a predetermined period of time and averages temperature data from freezer thermistor  146  to reduce noise in the data. If the freezer temperature is determined to be substantially at or below a set temperature, thereby indicating a brief power loss, a defrost interval is read from EEPROM memory  236  (shown in FIG. 3) of controller  160 , and defrost continues from the point of power failure without resetting defrost parameters. Periodically, controller  160  saves a current time till defrost value in system memory in the event of power loss. Controller  160  therefore recovers from brief power loses and associated defrost cycles due to resetting of the system from momentary power failures are therefore avoided.  
         [0040]    If freezer temperature data indicates that freezer compartment  104  (shown in FIG. 1) is warm, i.e., at a temperature outside a normal operating range of freezer compartment, humid air is likely to be contained in freezer compartment  104 , either because of a sustained power outage or opened doors during a power outage. Because of the humid air, a defrost timer is initially set to the first or abnormal defrost interval. In one embodiment the first or abnormal defrost interval is set to, for example, eight hours of compressor run time. For each second of compressor run time, the first defrost interval is decremented by a predetermined amount, such as one second, and the first defrost interval is generally unaffected by any other event, such as opening and closing of fresh food and freezer compartment doors  134 ,  132 . In alternative embodiments, a first or abnormal defrost interval of greater or lesser than eight hours is employed, and decrement values of greater or lesser than one second are employed for optimal performance of a particular compressor system in a particular refrigerator platform.  
         [0041]    When the first defrost interval has expired, controller  160  runs compressor  272  (see FIG. 3) for a designated pre-chill period or until a designated pre-chill temperature is reached (at state  1 ). Defrost heater  196  (shown in FIGS.  2 - 4 ) is energized (at state  2 ) to defrost the evaporator coils. Defrost heater  196  is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired, and then a dwell state is entered (at state  3 ) wherein operation is suspended for a predetermined time period, which as described further below is dependent upon whether a high temperature or low temperature defrost cycle is executed.  
         [0042]    Upon completion of an abnormal defrost cycle after the first or abnormal defrost interval has expired, controller  160  (at state  0 ) sets the time till defrost to the second or normal pre-selected defrost interval that is different from the first or abnormal time to defrost. Therefore, using the second defrost interval, a normal defrost cycle is executed. For example, in one embodiment, the second defrost interval is set to about 60 hours of compressor run time. In alternative embodiments, a second defrost interval of greater or lesser than 60 hours is employed to accommodate different refrigerator platforms, e.g., top-mount versus side-by-side refrigerators or refrigerators of varying cabinet size.  
         [0043]    In one embodiment, the second defrost interval, unlike the first defrost interval, is decremented (at state  5 ) upon the occurrence of any one of several decrement events. For example, the second defrost interval is decremented (at state  5 ) by, for example, one second for each second of compressor run time. In addition, the second defrost interval is decremented by a predetermined amount, e.g., 143 seconds, for every second freezer door  132  (shown in FIG. 1) is open as determined by a freezer door switch or sensor  242  (shown in FIG. 3). Finally, the second defrost interval is decremented by a predetermined amount, such as 143 seconds in an exemplary embodiment, for every second fresh food door  134  (shown in FIG. 1) is open. In an alternative embodiment, greater or lesser decrement amounts are employed in place of the above-described one second decrement for each second of compressor run time and 143 second decrement per second of door opening. In a further alternative embodiment, the decrement values per unit time of opening of doors  132 ,  134  are unequal for respective door open events. In further alternative embodiments, greater or fewer than three decrement events are employed to accommodate refrigerators and refrigerator appliances having greater or fewer numbers of doors and to accommodate various compressor systems and speeds.  
         [0044]    When the second or normal defrost interval has expired, controller  160  runs compressor  272  for a designated pre-chill period or until a designated pre-chill temperature is reached (at state  1 ). Defrost heater  196  is energized (at state  2 ) to defrost the evaporator coils. Defrost heater  196  is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired. Defrost heater  196  is then shut off and the elapsed time defrost heater  196  was on (Δt de ) is recorded in system memory. A dwell state is then entered (at state  3 ) wherein sealed system operation is suspended for a predetermined time period. As will be seen further below, the duration of the dwell state is dependent upon the particular defrost cycle executed.  
         [0045]    The elapsed defrost time Δt de  is then compared with a predetermined defrost de reference time Δt dr . If elapsed defrost time Δt de  is greater than reference time Δt dr , thereby indicating excessive frost buildup, the first or abnormal defrost interval is employed for the next defrost cycle If elapsed defrost time Δt de  is less than reference time Δt dr , the second or normal defrost interval is employed for the next defrost cycle. The applicable defrost interval is applied and a defrost cycle is executed when the defrost interval expires. The elapsed defrost time Δt de  of the cycle is recorded and compared to reference time Δt dr  to determine the applicable defrost interval for the next cycle, and the process continues. Normal and abnormal defrost intervals are therefore selectively employed on demand in response to changing refrigerator conditions.  
         [0046]    It is recognized that that other known reference data may be employed in lieu of elapsed defrost time as indicative of evaporator frost buildup to distinguish between normal and abnormal defrost cycles. For example, compressor and evaporator loads may be monitored to determine effectiveness of the sealed system due frost buildup on the evaporator coils, and pressure and temperature sensors may be employed on the evaporator and/or compressor to sense performance parameters and changes over time that are indicative of defrost effectiveness. In addition, other reference values, such as elapsed time to cool a refrigeration compartment to a given temperature, or total elapsed door-open time may be employed to evaluate and demarcate a need for a normal or abnormal defrost cycle.  
         [0047]    [0047]FIG. 6 is a method flow chart of an adaptive defrost method  350  executable by controller  160  (shown in FIG. 2) for energy efficient effective defrost while minimizing the effect of freezer compartment temperature during defrost operations.  
         [0048]    As refrigerator controller  160  powers up  352 , controller  160  sets  354  a time till defrost interval X i  to a first or minimum length X min , which in an exemplary embodiment corresponds to the abnormal cycle described above, namely eight hours of compressor run time undecremented by door openings or external factors. In alternative embodiments, however, it is recognized that X min  may be greater or lesser than eight hours of compressor run time and further may be based or otherwise determined by other factors in lieu of or in addition to compressor run time.  
         [0049]    Additionally upon power up, a defrost counter N D  is set  356  to zero and controller  160  operates  358  the refrigerator sealed system to obtain set point temperatures in freezer compartment  104  and/or fresh food compartment  102  (shown in FIG. 1). Thus condenser fan speed  180 , evaporator fan speed  182 , compressor control  194 , mullion damper  200 , and pulse width modulator  208  for controlling the operating speed of condenser fan  210 , fresh food compartment fan  212 , and evaporator fan  214  (all shown in FIG. 2) are activated and regulated by controller  160  to cycle the appropriate components on and off to maintain refrigeration compartments  102 ,  104  at specified temperatures. As will be seen defrost counter N D  is employed to determine whether a high temperature or low temperature defrost cycle will be activated.  
         [0050]    As controller  160  operates the refrigerator sealed system, an elapsed sealed system time t ss  is compared  360  to defrost interval X i  set  354  by controller  160  upon power up. If elapsed sealed system time is less than the abnormal defrost time, i.e., if t ss &lt;X i , then controller  160  continues to operate  358  the sealed system. If elapsed sealed system time is equal to or exceeds the abnormal defrost time, i.e., if t ss ≧X i , then controller  160  initiates  362  defrost operations by pre-chilling freezer compartment  104  and turning off sealed system components to prepare for defrost. While pre-chilling of freezer compartment  104  is desirable in an illustrative embodiment, it is recognized that the low temperature defrost may partially, if not wholly, obviate the desirability of pre-chilling functions in alternative embodiments.  
         [0051]    When defrost is initiated  362 , controller  160  checks or compares  364  defrost counter N D  to a predetermined value N H  that corresponds to a high temperature defrost cycle. As will be seen further below, N D  is incremented with each low temperature defrost cycle executed and reset to zero at the completion of a high temperature defrost cycle. Thus, low temperature defrost cycles will be successively executed for a predetermined number of times before a high temperature defrost cycle is executed. In an illustrative embodiment, N D  equals five so that every fifth defrost is a high temperature defrost cycle. It is understood, however, that other values of N D  may be employed in alternative embodiments without departing from the scope of the present invention.  
         [0052]    If N D  does not equal N H  then a low temperature defrost is initiated and defrost heater  196  (shown in FIGS.  2 - 4 ) is energized  366  to heat the evaporator coils. Evaporator temperature is sensed or monitored and evaporator temperature (T e ) is compared  368  to a low defrost cycle termination temperature (T l ). In an illustrative embodiment T l  is set to a temperature (about 55° F. in a particular embodiment) sufficient to melt frost off of the evaporator but not necessarily to defrost other components, such as an icemaker fill tube. Further, T l  is selected to prevent freezer burn and moisture formation and ice buildup in freezer compartment  104  during the low temperature defrost cycle. In alternative embodiments it is appreciated that greater or lesser values for T l  may be employed in lieu of about 55° F.  
         [0053]    If actual evaporator temperature T e  is less than T l , controller  160  continues to energize  366  defrost heater  196 . If actual evaporator temperature T e  is not less than T l , controller  160  de-energizes  370  defrost heater  196 , sets  372  sealed system dwell time to a value corresponding to the low temperature defrost cycle, and also sets  374  a sealed system delay time to a value corresponding to the low temperature defrost cycle. As used herein, dwell refers to a period of time after defrost termination temperature is reached when the sealed system and evaporator fan are both off, and delay refers to time after the dwell period wherein the evaporator fan is off but the sealed system is on. The system will therefore remain in a dwell state for a certain time period and then in a delay state for another period of time. In the illustrative embodiment, the low temperature dwell time is set  372  to five minutes and the low temperature delay is set to zero (i.e., no delay). It is recognized that the foregoing low temperature dwell time and delay values are for illustrative purposes only and that other values may be employed in alternative embodiments.  
         [0054]    Once defrost heater  196  is de-energized and low temperature dwell and delay values are set  372 ,  374 , defrost counter ND is incremented  376  to its current value plus one for further use by controller  160 .  
         [0055]    When defrost operations are initiated  378 , if N D  does equal N H  when N D  and N H  are compared  364 , then a high temperature defrost is initiated and defrost heater  196  (shown in FIGS.  2 - 4 ) is energized  378  to heat the evaporator coils. Evaporator temperature is sensed or monitored and evaporator temperature (T e ) is compared  380  to a high defrost cycle termination temperature (T h ) that is different from low defrost cycle termination temperature T l . In an illustrative embodiment T h  is set to a temperature (about 65° F. in a particular embodiment) sufficient to melt frost off of the evaporator and to defrost other components, such as an icemaker fill tube, but without causing unacceptable temperature rises in freezer compartment  104 . It is appreciated, however, that greater or lesser values for T h  may be employed in lieu of about 65° F. in alternative embodiments.  
         [0056]    If actual evaporator temperature T e  is less than T h , controller  160  continues to energize  378  defrost heater  196 . If actual evaporator temperature T e  is not less than T h  controller  160  de-energizes  382  defrost heater  196 , sets  384  sealed system dwell time to a value corresponding to the high temperature defrost cycle, and also sets  386  a sealed system delay time to a value corresponding to the high temperature defrost cycle. In the illustrative embodiment, the high temperature dwell time is set  384  to twenty minutes and the high temperature delay is set to 10 minutes. It is recognized, however, that the foregoing high temperature dwell time and delay values are for illustrative purposes only and that other values may be employed in alternative embodiments.  
         [0057]    Once defrost heater  196  is de-energized  382  and high temperature dwell and delay values are set  384 ,  386 , defrost counter ND is reset  388  to zero for further use by controller  160 .  
         [0058]    After defrost counter N D  is reset  376 ,  388  upon completion of low temperature and high temperature defrosts, respectively, controller compares  390  elapsed defrost time Δt de  (explained above in relation to FIG. 5) to defrost reference time Δt dr  (also explained above in relation to FIG. 5). If elapsed defrost time Δt de  is greater than the reference defrost time Δt dr , defrost interval X i  is set  392  to the first or minimum length X min  corresponding to the abnormal defrost interval. Thus, in an illustrative embodiment defrost interval X min  is about eight hours of compressor run time unaffected by door open events. As noted previously, however, it is understood that other measures besides compressor run time may be utilized in alternative embodiments to define X min .  
         [0059]    If elapsed defrost time Δt de  is not greater than the reference defrost time Δt dr , defrost interval X i  is set  394  to the second or maximum length X max  corresponding to the normal defrost interval. Thus, in an illustrative embodiment defrost interval X max  is about sixty hours of compressor run time decremented by door open events as described above in relation to FIG. 5. It is understood, however, that other measures besides decremented compressor run time may be utilized in alternative embodiments to define X max .  
         [0060]    Once defrost counter has been incremented or reset  376 ,  378  and X i  has been determined as X min  or X max    392 ,  394  as described above, controller  160  returns to operate  358  the sealed system with the current values of defrost counter ND and defrost interval X i . The sealed system is operated and controller  160  compares  360  the sealed system time t ss  with defrost interval X i  until another defrost is initiated and the method repeats.  
         [0061]    It is believed that the above-described methodology could be programmed and implemented in control logic by those in the art without further explanation.  
         [0062]    A defrost system and method is therefore provided that utilizes a high termination temperature defrost at defined intervals in conjunction with a plurality of low temperature termination defrosts, and also employs normal and abnormal defrost intervals responsive to refrigerator usage through door open events. By using a low termination temperature defrost frequently and a high termination temperature defrost infrequently, freezer burn and moisture/ice buildup is substantially avoided and energy efficiency improved while providing satisfactory defrost performance.  
         [0063]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.