Abstract:
A refrigerator includes a refrigeration compartment having an upper region and a lower region, a first evaporator positioned in the upper region, a second evaporator, positioned in the lower region, and a fan disposed between the first and second evaporators. The fan is configured, such that air flows past first and second evaporators and is discharged into said upper and lower regions when said fan is energized.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to refrigerators, and more particularly, to control systems for refrigerators. 
   Some known refrigerators include a fresh food compartment and a freezer compartment. Such a refrigerator also typically includes a refrigeration sealed system circuit including a compressor, an evaporator, and a condenser connected in series. An evaporator fan is provided to blow air over the evaporator, and a condenser fan is provided to blow air over the condenser. 
   In operation, when an upper temperature limit is reached in the freezer compartment, the compressor, evaporator fan, and condenser fan are energized. Once the temperature in the freezer compartment reaches a lower temperature limit, the compressor, evaporator fan, and condenser fan are de-energized. 
   Some 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. After defrost, the compressor is typically run for a predetermined time to lower the evaporator temperature and reduce food spoilage in the refrigerator and/or fresh food compartments of a refrigeration appliance. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method of switching refrigerant flow between a path to a fresh food evaporator in a fresh food compartment and a path to a freezer evaporator in a freezer food compartment of a refrigerator using a three way valve includes providing the three way valve with a plurality of operation positions, the three way valve having a plurality of steps between each of the plurality of operation positions and moving the three way valve incrementally in steps with a time delay between consecutive steps between at least two operation positions such that the three way valve transitions between at least two operation positions gradually. 
   In another aspect, a method for operating a refrigerator having a fresh food compartment and a freezer food compartment, wherein both compartments include an evaporator, the method includes cooling the fresh food compartment using a control grid and cooling the freezer food compartment using a control grid. 
   In another aspect, a method for defrosting a refrigerator having a refrigerant path to a freezer evaporator and a refrigerant path to a fresh food evaporator, and a three way valve for controlling refrigerant flow from a compressor to each refrigerant path, the method including determining whether substantially all of the refrigerant is in at least one of the fresh food and freezer evaporators and returning the refrigerant to the compressor if substantially all of the refrigerant is not in at least one of the fresh food and freezer evaporators. 
   In a further aspect, a refrigerator includes a fresh food compartment having a fresh food evaporator, a fresh food door operable for opening and closing access to the fresh food compartment, and a fresh food defrosting assembly with a fresh food door counter for counting the number of fresh food door openings. The refrigerator also includes a freezer food compartment having a freezer evaporator, a freezer food door operable for opening and closing access to the freezer food compartment, a freezer food defrosting assembly with a freezer food door counter for counting the number of freezer food door openings. The refrigerator further includes a controller operationally coupled to the fresh food and freezer food defrosting assemblies and the fresh food and freezer food door counters. The controller is configured to adjusting the fresh food door counter when the fresh food door is opened, adjusting the freezer food door counter when the freezer food door is opened, updating the fresh food door counter when the fresh food compartment is cooled by the fresh food evaporator, and updating the freezer food door counter when the freezer food compartment is cooled by the freezer evaporator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a refrigerator; 
       FIG. 2  is a schematic illustration of the exemplary refrigerator; 
       FIG. 3  is a step diagram of a valve between an open position and a closed position for a refrigerant path to a fresh food evaporator and a path to a freezer evaporator; 
       FIG. 4  is a known time diagram of a valve between an open position and a closed position for a refrigerant path to a fresh food evaporator and a path to a freezer evaporator; 
       FIG. 5  is a time diagram of a valve between an open position and a closed position for a refrigerant path to a fresh food evaporator and a path to a freezer evaporator; 
       FIG. 6  is a flow diagram of a defrosting cycle; 
       FIG. 7  is a diagram of a control grid for operating a refrigerator; 
       FIG. 8  is a flow diagram of the control grid of  FIG. 7 ; 
       FIG. 9  is a flow diagram of a defrosting operation of a fresh food evaporator and a freezer evaporator; 
       FIG. 10  is a flow diagram of fresh food defrosting cycle; 
       FIG. 11  is a flow diagram of a freezer food compartment defrosting cycle; and 
       FIG. 12  is a flow diagram of a forced fresh food compartment defrosting cycle. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a side-by-side refrigerator  100  including a fresh food storage compartment  102  and freezer storage compartment  104 . Freezer compartment  104  and fresh food compartment  102  are arranged side-by-side. In one embodiment, refrigerator  100  is a commercially available refrigerator from General Electric Company, Appliance Park, Louisville, Ky. 40225, and is modified to incorporate the herein described methods and apparatus. 
   It is contemplated, however, that the teaching of the description set forth below is applicable to other types of refrigeration appliances, including but not limited to top and bottom mount refrigerators wherein undesirable temperature gradients exist. The present invention is therefore not intended to be limited to be limited to any particular type or configuration of a refrigerator, such as refrigerator  100 . 
   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 U-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. 
   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). 
   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 . 
   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 . 
   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 . 
   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. 
     FIG. 2  is schematic illustration of refrigerator  100 . During operation of refrigerators with a fresh food evaporator  172  and a freezer evaporator  174 , a three-way valve  192  with a step motor  194  is utilized to switch refrigerant flow from one evaporator to another depending on the temperatures in fresh food and freezer compartments  102  and  104 . A compressor  195  delivers refrigerant to fresh food evaporator  172  via a path to fresh food evaporator  196  and to freezer evaporator  174  via a path to freezer evaporator  198 . Three-way valve  192  has at least a first outlet (not shown) coupled to path to fresh food evaporator  196  and a second outlet coupled to the path to freezer evaporator  198 . In one embodiment, a heating unit  199  is coupled to at least one of fresh food evaporator  172  and freezer evaporator  174 . In another embodiment, heating unit  199  is positioned proximate to at least one of fresh food and freezer evaporators  172  and  174 . Each mode of the refrigeration system a operation requires different compressor pressure ratios. In known systems, there are considerable transition loses switching between modes because of the short time it takes for valve  192  to switch to various valve positions. 
   Step motor  194  of three-way valve  192  operates by a series of impulses that moves valve  192  incrementally in a plurality of steps between a plurality of operation modes or positions. These operation positions include position A, where only the first outlet port is open; position B, where the first outlet port is closed and the second outlet port is open; position C, where both the first and second outlet ports are open; and position D, where both outlet ports are closed. Because there is no time delay between the impulses, the time interval between the steps is short, such as hundreds or even thousands of a millisecond. Thus, valve  192  moves from one position to another for less than 1 to 10 seconds. To maintain smooth transition from one operation position to another of the sealed refrigeration system, an algorithm for the step motor valve  192  includes a delay time added to every operation position. In one embodiment, a delay time is an EEPROM valve and is different for each valve operation position. For example, when valve  192  moves from position A (first outlet port is open) to position C (both outlet ports are open) the time interval is a first time period t1. When valve  192  moves from position C to position B (second outlet port is open) the time interval is a second time period t2. When valve  192  moves from position B to position D (both outlet ports are closed) the time interval is a third time period t3, and so on. In one embodiment, first, second and third time periods t1, t2 and t3 are of different time duration. 
     FIG. 3  is a step diagram  200  for a method of operating valve positions for a refrigerant path to fresh food evaporator  202  and a refrigerant path to freezer evaporator  204 . From steps  0  to  4  the valve  192  (in position C) directs flow to both fresh food and freezer evaporators  172  and  174 . From step  4  to step  10 , valve  192  closes path to fresh food evaporator  202 . From step  10  to step  17 , valve  192  is in B position and only path to freezer evaporator  204  is open. From step  17  through step  23 , path to freezer evaporator  204  closes. From step  23  through step  27 , both paths  202  and  204  are closed and valve  192  is in position D. From step  27  through step  33 , valve  192  opens path to fresh food evaporator  202 . From step  33  to  40 , valve  192  is in position A, which results in opening path to fresh food evaporator  202  and closing path to freezer evaporator  204 . 
     FIG. 4  is a time diagram  220  for a known method of valve positioning. Time diagram  220  of  FIG. 3  has the same four valve operational positions of A, B, C, and D of FIG.  3 . For about 15 minutes, valve  192  is in position C (both paths  202  and  204  are open stepwise in any position between steps  0  and  4  (see FIG.  3 )) and then abruptly, (for less than 2 sec.) valve  192  goes into position B, where path to fresh food evaporator  202  is closed in any position between steps  10  and  17  (see FIG.  3 ). Then again abruptly, valve  192  is in position D in which both paths  202  and  204  are closed in any position between steps  23  and  27  (see FIG.  3 ). Then valve  192  is in position A and path to freezer evaporator  204  is closed and path to fresh food evaporator  202  is open, in any position between steps  33  and  40  (see FIG.  3 ). In one embodiment, the sequence of operations may be different. For example, immediately after position C, valve  192  may go to position D between steps  23  and  27 , then to position A (between step  33  and  40 ) or to position B (between steps  10  and  17 ), and so on. 
     FIG. 5  is a time diagram  240  for a method of valve positioning. Time diagram  240  of  FIG. 4  has four valve operation positions of A, B, C, and D. For about 13 minutes, valve  192  is in position C (both paths  202  and  204  are open stepwise in any position between steps  0  and  4 ). Then with time delay of, for example, 10 seconds per step, valve  192  goes to position B where path to freezer evaporator  204  is open and path to fresh food evaporator  202  is closed in any position between steps  10  and  17  (see FIG.  3 ). In one embodiment, the transition is between approximately 0 to 4 minutes. In another embodiment, the transition is about 15 steps or about 2.5 minutes. The transition from position C to position B with the time delay is very gradual. During the transition from position B to position A, the time delay between steps is, for example, about 20 seconds. The transition starts between steps  10  and  17  (see  FIG. 3 ) and finishes between steps  33  and  40  (see FIG.  3 ). In one embodiment, the transition is between approximately 2 to 10 minutes. In another embodiment, the transition is about 8 minutes. Because the transition from one valve position to another valve position is gradual, the amount of time valve  192  is at position D is only about a single step. When valve  192  changes between operation positions in the refrigerant circuit, the transition is long enough to provide the best energy efficiency of the system. 
   As discussed above, refrigerator  100  includes fresh food evaporator  172  located in fresh food compartment  102  and a separate freezer evaporator  174  in freezer food compartment  104 . Thus, refrigerant flows either through the fresh food evaporator  172  or through freezer evaporator  174 . When refrigerant flows through fresh food evaporator  172 , fresh food evaporator  172  is flooded with refrigerant. When refrigerant flows through freezer evaporator  174 , refrigerant floods freezer evaporator  174 . Thus, it takes some time or requires a special recovery mode to transmit refrigerant from one evaporator to compressor  195  and then to another evaporator. In addition, defrosting of either the fresh food or freezer evaporators  172  and  174  is enhanced with supplemental heating of refrigerant in evaporators  172  and  174 , such as heat from a defroster heater (not shown). 
     FIG. 6  shows a defrosting cycle  300  for fresh food and freezer evaporators  172  and  174 . Refrigerator  100  includes a defrost timer (not shown). When the defrost timer counts down to zero, a pre-chill cycle is started  305  and cools the fresh food and freezer compartments  102  and  104  to a certain temperature. As soon as temperatures in both compartments reach a predetermined level, valve  192  is set into position to flow refrigerant through the evaporator to be defrosted for a predetermined time, i.e. about 10 min. During defrost operation, valve  192  stays in the pre-defrost position. After this time expires, compressor  195  is switched off and the heater comes on until the evaporator reaches a fixed temperature. 
   In one embodiment, defrosting method  300  includes the step of checking or determining  310  the position of valve  192 . If valve  192  is in position B, compressor  195 runs or delivers  320  refrigerant to freezer evaporator  174  to be defrosted for predetermined first time (T1) before starting a defrost operation. If valve  192  is not in position B, defrosting cycle  300  performs a recovery mode. Recovery mode includes returning  330  refrigerant back to compressor  195  from fresh food evaporator  172 . After refrigerant is recovered from fresh food evaporator  172 , valve  192  is switched  340  to position B. Compressor  195  then runs or delivers  350  refrigerant to freezer evaporator  174  to be defrosted for a predetermined second time (T2) before starting a defrost operation, where T1&gt;T2. 
   In another embodiment, defrosting method  300  includes the step of checking whether or not valve  192  is in position A. If valve  192  is in position A, compressor  195  runs or delivers  320  refrigerant to fresh food evaporator  172  to be defrosted for predetermined first time (T1) before starting a defrost operation. If valve  192  is not in position recovery mode returns  330  refrigerant back to compressor  195  from freezer evaporator  174 . After refrigerant is recovered from freezer evaporator  174 , valve  192  is switched  340  to position A. Compressor  195  then runs or delivers  350  refrigerant to fresh food evaporator  172  to be defrosted for a predetermined second time (T2) before starting a defrost operation, where T1&gt;T2. 
   In one embodiment, the defroster heater heats  360  the lower portion of either fresh food or freezer evaporators  172  and  174 . The defroster heater heats the refrigerant in the evaporator until the refrigerant evaporates and migrates upward through the evaporator. As the refrigerant rises, the refrigerant cools until it liquefies, whereby the refrigerant returns (due to gravity) to the lower portion of the evaporator again to repeat the process. 
   In one embodiment, multiple speed compressor and fan logic is utilized to increase cooling efficiency and decrease energy consumption. Based on the temperatures of the cabinet, the position of the valve  192 , the speeds of compressor  195 , freezer fan  190 , the fresh food fan  182  and the condenser fan are all determined and compared with a control grid  380 , as shown in FIG.  7 . Control grid  380  includes fresh food compartment temperature on an x-axis and freezer food compartment temperature on a y-axis. 
   Control grid  380  is divided into 8 sections or areas numbering from Area 0 to Area 7, wherein some Areas are derivative sensitive. For example, in some areas, control grid  380  takes into account whether the previous area had a increase in temperature (ie. the temperature has a negative derivative). In other areas, control grid  380  takes into account whether the previous area had a decrease in temperature (ie. the temperature has a positive derivative). 
   Area 0 includes cells  24 Y,  25 Z, and  26 AA. Area 1 includes cells  2 C,  3 D,  4 E,  5 F,  11 L,  17 R and  23 ×. Area 2 includes cells  8 I,  9 J,  10 K,  16 Q, and  22 W. Area 3 includes cells  14 O,  15 P, and  21 V. Area 4 includes cell  20 V. Area 5 includes cells  0 A,  1 B,  6 G,  7 H,  12 M,  13 N,  18 S, and  19 T. Area 6 includes cells  27 AB,  28 AC, and  29 AD. Area 7 includes cells  32 AH,  33 AH,  34 AI, and  35 AJ. 
   In Area 0 of the control grid, all the fans and compressor  195  are shut down and valve  192  is in position A. When the system enters Area 1, which is far from a setpoint  390 , sealed system runs with a higher capacity in order to move towards the setpoint. Valve  192  is usually in position C thereby refrigerating both the evaporators. When the system is moving towards the setpoint in Area 2, the system maintains Area 1 settings in order to pull down efficiently. Otherwise, the sealed system and fans  182  and  190  run in medium speeds and valve  192  is in positions A or C depending on the distance from setpoint. 
   If the system is moving towards setpoint in Area 3 from Area 1, Area 2 settings come into effect in Area 3. Otherwise, sealed system and fans  182  and  190  run in low speeds. When the system enters Area 4, the system experiences no change. In Area 5, valve  192  is in position B (freezer evaporator only) and thus only freezer evaporator  174  is cooled until it reaches the setpoint. (However, in cell  19 T, when compressor  195  is not on, valve  192  is in position A). In Area 6, valve  192  is in position A (fresh food evaporator only) and only fresh food evaporator  172  is cooled until it reaches the setpoint. In Area 7, the sealed system and fans  182  and  190  run in middle speeds except the condenser fan which operates in a higher speed. This mode helps the system to be stable in high ambient conditions. 
     FIG. 8  is a flow diagram  400  of control grid  380 . If freezer temperature is high, step  410  runs compressor  195  and all the fans in medium speed except for condenser fan, which is run in super high speed. In step  410 , valve  192  is in position C. If the fresh food compartment temperature is higher than the fresh food compartment super high hysteresis or if freezer food compartment temperature is higher than the freezer food compartment super high hysteresis, then step  420  runs the compressor, the condenser freezer fan, and fresh food fan  182  at high speeds. Valve  192  in step  420  is in position C. If fresh food compartment temperature is greater than fresh food compartment extra high hysteresis or freezer food, compartment temperature is greater than freezer food compartment extra high hysteresis, then step  430  runs compressor, condenser, fresh food fan  182  and freezer fan  190  at medium speed. In step  430 , valve  192  is in position C, whereby path to fresh food evaporator  196  is opened first and then path to the freezer evaporator  198  is opened. If the fresh food compartment temperature is greater than the fresh food compartment high hysteresis or if freezer food compartment temperature is greater than freezer food compartment low hysteresis, then step  440  runs compressor, condenser, fresh food fan  182  and freezer fan  190  in low speed. In step  440 , valve  192  opens path to fresh food evaporator  196  first and then opens path to freezer evaporator  198 . If fresh food compartment temperature is less than fresh food low hysteresis and freezer food compartment temperature is less than freezer food low hysteresis, then step  450  turns off compressor  195  and all the fans. In step  450 , valve  192  only opens path to fresh food evaporator  196 . 
   Fresh food compartment  102  has a fresh food defrosting assembly (not shown) with a fresh food door counter (not shown) for counting the number of fresh food door openings before executing the defrosting operation. Freezer food compartment  104  has a freezer food defrosting assembly (not shown) with a freezer food door counter (not shown) for counting the number of freezer door openings before executing the defrosting operation. A controller (not shown) is operationally coupled to the fresh food and freezer defrost assemblies and the fresh food and freezer door counters. Once the respective door has been opened a specific number of times, the controller starts the defrost operation for that refrigerator compartment. Thus, door counter records the number of door opening by either incrementing or decrementing each door opening until the given number of door openings have been reached. 
     FIG. 9  is a flow diagram of a defrosting operation of fresh food and freezer evaporators  172  and  174  based on an adaptable defrost algorithm  500 , which incorporates door openings and the sealed system run time in freezer food compartment  104 . Fresh food evaporator  172  is defrosted using fresh food fan  182  that operates according to the adaptable defrost algorithm, shown in  FIG. 9 , which incorporates door openings and sealed system run time in fresh food compartment  102 . Three-way valve  192  is used to control the refrigerant flow between the fresh food and freezer food compartments  102  and  104 . In the defrost algorithm of  FIG. 9 , the fresh food door openings are counted only for fresh food evaporator  172 , and the freezer food door openings are only counted for freezer evaporator  174 . 
   In one embodiment, only fresh food door counter decrements  510  when the fresh food door is opened. In another embodiment, only fresh food door counter increments when the fresh food door is opened. In one embodiment, only freezer food door counter decrements  510  when the freezer door is opened. In another embodiment, only freezer food door counter increments when the fresh food door is opened. 
   If the fresh food door is not opened, then the controller updates  520  the fresh food door counter when the sealed system cools fresh food compartment  102 . If the freezer food door is not opened, then the controller updates  530  the freezer food door counter when the sealed system cools freezer food compartment  104 . 
   If fresh food compartment is not being cooled (off cycle), then the controller starts  540  fresh food normal defrost cycle.  FIG. 10  shows a fresh food defrost cycle  540 . Fresh food defrost cycle  540  sets  550  fresh food fan  182  in low speed and moves the valve  192  into position B. If the sealed system is off, then valve  192  is moved to position A. Fresh food defrost cycle runs  560  freezer food compartment  104  according to control grid  380  of FIG.  7 . If the temperature of the fresh food evaporator  172  is less than the defrosting temperature, then steps  550  and  560  are repeated. If the temperature of the fresh food evaporator  172 : is greater than the defrosting temperature, then fresh food defrost cycle runs  570  fresh food fan  182  in a low speed for the minimum time to ensure completion of the defrost operation. 
   The controller of adaptable defrost algorithm  500  does not count fresh food door openings for the freezer defrost decrement timer. As the freezer defrost timer expires (sealed system run time in freezer side, time corresponding to number of freezer door openings and duration of freezer door openings) for abnormal defrost timer or normal defrost timer, the controller starts a freezer defrost cycle  600 . 
     FIG. 11  shows freezer defrost cycle  600 . Freezer evaporator  174  pre-chills  610  and cools freezer compartment  104  to a certain temperature according to control grid  380  of FIG.  7 . Once the defrost timer counts down to zero or expires, the sealed system is switched away from freezer compartment  104 , valve  192  is moved to position A, and a defrost heater heats  620  freezer evaporator  174  until freezer evaporator  174  reaches a fixed temperature. If fresh food compartment  102  needs further cooling, the sealed system is switched on, otherwise the sealed system is off and freezer fan  190  is off. 
   During the dwell time of freezer defrost cycle  600 , valve  192  is in position A and the defrost heater is turned off  630 . No fans or sealed system are turned on in freezer food compartment  104 . If fresh food compartment  102  needs further cooling, the scaled system is switched on, otherwise the sealed system is off and freezer fan  190  is off. After the dwell time (post dwell time), step  640  cools freezer evaporator  174  by turning off the sealed system in freezer compartment  104  while freezer fan  190  remains off. In post dwell time, valve  192  moves to position C, if fresh food compartment  102  needs cooling. Otherwise, valve  192  is in position B. After the post dwell time, the controller goes back to the normal state and operates according to control grid  380 . During freezer defrost cycle  600 , fresh food compartment  102  runs according to control grid  380 . 
   After the fresh food defrost timer counts down to zero or expires, the controller starts forced defrost cycle  700 .  FIG. 12  shows forced defrost cycle  700 . In step  710 , valve  192  is moved to position B, the sealed system in fresh food compartment  102  is off, and fresh food fan  182  is running  710  in high speed until fresh food evaporator  172  reaches a certain temperature, after which fresh food fan  182  is kept running for a minimum time to ensure completion of the defrosting operation. During fresh food defrost cycle  700 , freezer food compartment  104  runs  720  according to control grid  380 . If the temperature of fresh food evaporator  172  is less than the defrost temperature, then steps  710  and  720  are repeated. If the temperature of fresh food evaporator  172  is greater than the defrost temperature, then fresh food fan  182  is run  730  in high speed for a minimum amount of, time to ensure completion of the defrost operation. 
   Exemplary embodiments of refrigerator systems are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each assembly may be utilized independently and separately from other components described herein. Each refrigerator component can also be used in combination with other refrigerator and evaporator components. 
   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.