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(11) | EP 2 746 701 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Refrigerator with no-frost freezer |
| (57) 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.
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[0001] 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.
[0002] 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'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.
[0003] Locally, in regions of the compartment closer to the evaporator, the temperature increase is usually more severe.
[0004] As a consequence of such temperature increase and subsequent decrease, moisture migration from and to food changes and could leave to surface defects.
[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.
[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:
- Figure 1 is a schematic representation of how the refrigeration appliance according to the invention works;
- Figure 2 is a cross section of a portion of a refrigeration appliance according to a first embodiment of the present invention;
- Figure 3 is a perspective view of a portion of a refrigeration appliance according to a second embodiment of the invention; and
- Figure 4 is an exploded perspective view of the embodiment according to figure 3.
[0012] 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.
[0013] According to the invention, a Peltier module 22 is mounted on the evaporator 12. In detail, its hot side 22a is in contact with the evaporator 12 while its cold side 22b 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.
[0014] The cyclic process according to the invention is shown in figure 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 22a both carry out the defrost of the evaporator 12, and in which cold air from Peltier module cold side 22b 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.
[0015] 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.
[0016] 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.
[0017] According to figure 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 24a and it is also provided with fins 26 on the outside to enhance heat transfer. The sub-compartment 24 presents an internal wall 28 with an upper aperture 28a for an auxiliary fan 30, and a bottom seat 28b for a Peltier module 32 having a hot side in contact with a bent end portion 34a 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.
[0018] 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.
[0019] 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.
[0020] The tray 34 is also provided with an overflow conduit (not shown). Arrows F in figure 3 shows air circulation in the sub-compartment, while reference 38 shows a temperature and humidity sensor inside the sub-compartment 24.
[0021] 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.
[0022] 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 22b 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.
1. Refrigeration appliance comprising a freezer compartment (FZ) in which air cooled
by an evaporator (12) is circulated by a fan (16), characterized in that it further comprises at least one thermoelectric device (22, 32) having a hot side
(22a) in heat exchange relationship with the evaporator (12) or with the freezer compartment
(FZ) and a cold side (22b) in heat exchange relationship with air within at least
a portion of the freezer compartment (FZ) in order to reduce temperature oscillations
in the freezer compartment (FZ) or in said portion thereof, particularly during evaporator
defrost.
2. Refrigeration appliance according to claim 1, wherein said portion of the freezer
compartment (FZ) is defined by a box-shaped sub-compartment (24).
3. Refrigeration appliance according to claim 1, wherein the hot side (22a) of the thermoelectric
device (22, 32) is also in heat exchange relationship with a water tray (34) within
the compartment (FZ, 24) for keeping water in a liquid state in order to control relative
humidity within the freezer compartment (FZ, 24).
4. Refrigeration appliance according to claim 3, wherein the hot side (22a) associated
to the water tray (34) belongs to a second thermoelectric device (32).
5. Refrigeration appliance according to any of the preceding claims, wherein the evaporator
(12) is provided with an auxiliary heater (18) for defrost.
6. Refrigeration appliance according to claim 4, wherein the second thermoelectric device
(32) is placed in a sub-compartment (24) contained within the freezer compartment
(FZ) and containing said water tray (34).
7. Refrigeration appliance according to claim 6, wherein said sub-compartment (24) comprises
an auxiliary fan (30) in order to define an air flow (F) on water tray (34).
8. Refrigeration appliance according to claim 7, wherein said sub-compartment (24) comprises
an internal wall (28) defining an air plenum (P) with apertures (28a) for said air
flow (F).
9. Refrigeration appliance according to any of claims 6-8, wherein the second thermoelectric
device (32) has its cold side in heat exchange relationship with a metal shelf above
the water tray (34).
10. Method for decreasing freezing burns in food items contained in a freezer compartment
(FZ), characterized in that it comprises heating the evaporator (12) during a defrost phase with the help of
a hot side (22a) of a thermoelectric device (22), the cold side (22b) thereof reducing
temperature oscillations within freezer compartment (FZ).
11. Method according to claim 10, wherein the hot side of the thermoelectric device (32)
is used for maintaining water in a water tray (34) in a liquid state for controlling
relative humidity within the freezer compartment (FZ, 24).
12. Method according to claim 10, wherein the food items are placed within a sub-compartment
(24) which does not contain the hot side (22a) of the thermoelectric device (22) and
contains the cold side (22b) thereof.