Abstract:
A refrigeration appliance comprises a freezer compartment in which air cooled by an evaporator is circulated by a fan and a thermoelectric device having a hot side in heat exchange relationship with the evaporator or with the freezer compartment and a cold side in heat exchange relationship with air within at least a portion of the freezer compartment in order to reduce temperature oscillations in the freezer compartment or in said portion thereof during evaporator defrost cycle.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to EP 12198390.2, filed on Dec. 20, 2012, entitled “Refrigerator with no-frost freezer,” the disclosure of which is hereby incorporated herein by reference in its entirety. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    The present invention relates to a refrigeration appliance comprising a freezer compartment in which air cooled by an evaporator is circulated by a fan. These refrigeration appliances, known also as no-frost refrigerators, may have the evaporator within the freezer compartment itself or in a different “ad-hoc” compartment. 
         [0003]    It is well known in the art of freezers the problems related to defects of frozen food items. In particular, when spoiling caused by freezing occurs, food surface is characterized by opaque dehydrated areas or spots which show a decrease of original color. Moreover such changes in food appearance are linked to the presence of frost formation on food surface or internal food packaging. The main responsible for these changes is moisture migration occurring during storage. In fact, freezer temperature is not constant and such fluctuation causes changes into air freezer temperature and so air vapor pressure. In case of significant temperature fluctuations, when temperature decreases the water pressure of air surrounding food (that is packaging headspace) is lower than the one of the ice on food surface so that ice sublimates from food surface leading to opaque dehydrated areas on food called “freezing burns”. Freezer burns increase oxygen contact with the food surface area due to the formation of tiny cavities left behind the sublimated ice and raise oxidative reactions that alter color (discoloration that is brown spots on red meat), texture and flavor. After leaving the food surface, moisture condensates on food packaging turning into frost. When temperature increases, only part of the moisture diffuse back to the surface of food. In no frost refrigerators every defined period of time (for instance every 72 hours) it is planned an automatic evaporator defrost for about 15 minutes to a half hour. The cycle can be controlled by an electric or electronic timer, as well as by mechanical device such as a bi-metal plate. This procedure will support consumer in not performing manually defrost hence the energy consumption doesn&#39;t increase with time (due to the insulation effect of frost on evaporator). The defrost heater is switched off when the temperature on the evaporator is higher than 0° C., including a safety margin to ensure that the entire evaporator has been defrosted. During defrost “on” cycle, the temperature in the freezer chamber can reach temperatures significantly higher than the one at the start of the cycle, approximately in the range from −15° C. to −5° C., depending on the thermal inertia of the cavity. 
         [0004]    Locally, in regions of the compartment closer to the evaporator, the temperature increase is usually more severe. As a consequence of such temperature increase and subsequent decrease, moisture migration from and to food changes and could leave to surface defects. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    An object of the present invention is to provide a refrigeration appliance which does not present the above drawbacks and which improves the storing performances in storing frozen food items. 
         [0006]    Since the presence of freezing burns is also related to oscillations of relative humidity in the freezer compartment, a further object of the present invention is to provide a refrigeration appliance which can control the relative humidity level within the compartment. 
         [0007]    The above objects are reached thanks to features listed in the appended claims. 
         [0008]    One of the main innovative feature of the present invention is the use of a thermoelectric device inside the freezer compartment and preferably connected in parallel with the defrost heater when this latter is present. The thermoelectric device is preferably positioned in the freezer back wall in order to have one side thermally coupled to the evaporator and the other side thermally coupled to the freezer cavity, i.e. in heat transfer relationship with the air in the freezer cavity. The thermoelectric device, usually a thermoelectric Peltier generator, is switched ON during defrost phase when a current will pass through the Peltier electrical connection leading to the creation of two temperature conditions on the two sides. The side exposed to the evaporator is warmed up and drives or contributes to drive the defrost action carried out by the heater while the colder one is exposed to the refrigerator cavity. In particular, this cold side will be the key element in reducing temperature fluctuation of the air in the freezer cavity. In fact, thermoelectric device will cool down air working against the natural temperature increase given by the combination of compressor off phase and heating element action. Indeed the thermoelectric elements dimension should be designed and dimensioned in order to avoid air temperature to exceed a gradient higher than 2° C. in the whole freezer cavity or, at least, in a selected part of the freezer cavity, which will be the one dedicated to storage of food particularly sensitive to freezer burns. After the defrost cycle, the control system will switch off the current to the thermoelectric device (and to the heater as well, when present). 
         [0009]    Another embodiment of the invention is presented, to include the case in which the temperature difference between the two sides of the thermoelectric module results too high. Since the efficiency of the module is in inverse proportion to the difference in temperature among the two sides, and since the heater could require in some cases a significant heating effect from the hot side, this can drive to a decrease of efficiency. In this case (assessed according to the efficiency of the device used), the hot side can be placed inside the freezer compartment, outside from the area dedicated to freezing burns avoidance. The hot side will be cooled down by the surrounding, while the cold side will work as cooling booster for the no freezing-burns area. 
         [0010]    According to another feature of the invention, the relative humidity level within the freezer cavity can be maintained accurately by fine tuning of cooling/heating done with the help of the thermoelectric device which is used to heat water in a tray in order to melt ice and to maintain the desired relative humidity in the compartment. The Peltier hot side delivers heat that is used to melt the ice in the tray and provide a source of humidity. In some cases fan would be required to run to speed up the moisture addition process. 
         [0011]    Further features and advantages according to the present invention will be clear from the following detailed description with reference to the attached drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic representation of how the refrigeration appliance according to the invention works; 
           [0013]      FIG. 2  is a cross section of a portion of a refrigeration appliance according to a first embodiment of the present invention; 
           [0014]      FIG. 3  is a perspective view of a portion of a refrigeration appliance according to a second embodiment of the invention; and 
           [0015]      FIG. 4  is an exploded perspective view of the embodiment according to  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    With reference to the drawings, a refrigerator  10  comprises a freezer compartment FZ and a fresh food compartment FC. The freezer compartment FZ comprises an evaporator  12  which is part of a refrigerating circuit whose only a condenser  14  is shown. The evaporator  12  is of the no-frost type, i.e. it comprises a fan  16  and a defrosting heater  18 . Both the heater  18  and the fan  16  are controlled through a control unit  20 . 
         [0017]    According to the invention, a Peltier module  22  is mounted on the evaporator  12 . In detail, its hot side  22   a  is in contact with the evaporator  12  while its cold side  22   b  is in heat exchange relationship with the air flow inside the freezer compartment FZ. The Peltier module  22  is driven through the control unit  20 . 
         [0018]    The cyclic process according to the invention is shown in  FIG. 1 , where step A represents the defrost count down; step B represents the condition when the compressor (not shown) is switched off at the beginning of the defrost phase, in which the heater  18  is switched on and the Peltier module  22  is switched on too; step C represents the condition in which warm air from the heater  18  and Peltier module hot side  22   a  both carry out the defrost of the evaporator  12 , and in which cold air from Peltier module cold side  22   b  avoids a too high increase of air temperature in the freezer compartment FZ; and step D represents the end of the defrost phase (usually  30  minutes), when both heater  18  and Peltier module  22  are switched off. 
         [0019]    Tests carried out by the applicant have shown that by adopting the solution according to the invention freezer burns can be reduced or even eliminated. According to a second embodiment of the invention, the thermoelectric device can be used not only for heating the evaporator during the defrost phase, but also for keeping water contained in a tray in the liquid state in order to control relative humidity inside the freezer compartment. Of course the same thermoelectric device  22  used in the first embodiment can be also used for reaching the technical object according to the second embodiment, even if a second independent Peltier module is preferred in order to not interfere with the normal function of the evaporator  12 . 
         [0020]    According to  FIG. 3 , a sub-compartment  24  shaped as a metal box is preferably used, which is contained within the freezer compartment FZ. The sub-compartment  24  has a door  24   a  and it is also provided with fins  26  on the outside to enhance heat transfer. 
         [0021]    The sub-compartment  24  presents an internal wall  28  with an upper aperture  28   a  for an auxiliary fan  30 , and a bottom seat  28   b  for a Peltier module  32  having a hot side in contact with a bent end portion  34   a  of a metal tray  34  placed on the bottom of the sub-compartment  24 . The internal wall  28 , which is placed at a predetermined distance above the tray  34 , defines an air plenum P between an end wall of the sub-compartment  24  and the wall  28 . 
         [0022]    The tray  34  receives water from a water source  36  which can be a reservoir for defrost water from evaporator  12  or water for supplying ice maker. In order to maintain the proper humidity level in the sub-compartment  24 , water is fed to the tray  34 , which may have a level sensor in order to control water filling. The auxiliary fan  30  drives cold air over water in the tray  24  in order to increase relative humidity in the sub-compartment  24 . By switching on and off the auxiliary fan  30  the relative humidity can be controlled within a narrow band. 
         [0023]    Of course the tray  34  is also provided with a metal shelf (not shown) above the water level for placement of frozen food items; a plurality of metal shelves can be used as well in the sub-compartment  24 . The metal shelf may also be provided with an extension in contact with the cold side of the Peltier module  32  in order to cool down the whole shelf, while the hot side thereof is in contact with the tray  34  for transferring heat to the water contained therein and keeping it in a liquid state. 
         [0024]    The tray  34  is also provided with an overflow conduit (not shown). Arrows F in  FIG. 3  shows air circulation in the sub-compartment, while reference  38  shows a temperature and humidity sensor inside the sub-compartment  24 . 
         [0025]    Of course the second embodiment according to the present invention can be also implemented in the whole freezer compartment FZ and not only in a sub-compartment thereof. The use of a sub-compartment  24  is particularly advantageous for food items placed in the freezer compartment without any packaging or container, i.e. in cases where the food items are more subjected to freezing burns. 
         [0026]    According to a further embodiment (not shown), particularly when the sub-compartment  24  does occupy only part of the freezer compartment, in said sub-compartment  24  it is placed only the cold side  22   b  of the thermoelectric device  22 , while the hot side is outside the sub-compartment  24  (which is actually the area for avoiding freezing burns) and it is cooled down by air in the freezer compartment FZ. This embodiment has the advantage of increasing the efficiency of the thermoelectric device and of using the cold side thereof as a cooling booster for the sub-compartment  24 .