Abstract:
A refrigerator has a fresh food compartment ( 3 ) adapted for maintaining the contents at a temperature above freezing. A vapour compression refrigeration system is operable in one mode so that an evaporator ( 7 ) is operated at a temperature within 10° C. of the temperature desired in the fresh food compartment ( 3 ) and a supply of air is maintained over the evaporator ( 7 ) and in to the compartment ( 3 ). The vapour compression system is used in a defrost mode in which the vapour compression refrigeration system is stopped or allowed to operate without significant heat extraction at the evaporator ( 7 ), and a supply of air above 0° C. is past over it. Operating the fresh food compartment evaporator ( 7 ) at such a high temperature (close to 0° C.) reduces the rate of frost build up and the under cooling of frost. Frost may be adequately removed by an ambient air flow without heater supplementation.

Description:
BACKGROUND TO THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to refrigerators and in particular to refrigerators having only a fresh food compartment or at least having a fresh food compartment and an associated evaporator dedicated to cooling the fresh food compartment.  
           [0003]    2. Summary of Prior Art  
           [0004]    Most domestic refrigerators operate using a vapour compression refrigeration system. In this system the refrigerant passes in turn from a compressor to a condenser, through an expansion device, through an evaporator and back to the compressor. Evaporation of the refrigerant in the evaporator draws heat from the surroundings of the evaporator. In early refrigerators (which may be referred to as ice box refrigerators) an evaporator was incorporated directly within the refrigeration compartment as the wall of an ice box, and cooling of fresh food compartment (surrounding the ice box) occurred by natural convection. More recently cooling has been achieved by forcing the flow of air across surfaces of the evaporator to cool the air and passing this cooled air into the refrigerated space. In each case the vapour compression refrigeration system has been operated such that the surface of the evaporator is held at a very low temperature, for example −18° C., some 25° C. lower than the desired temperature of the produce compartment. In the latter form this allows airflow to be utilised both in cooling the fresh food and frozen food compartments.  
           [0005]    Operating a refrigeration system with the surface of the evaporator at such low temperatures results in a rapid accumulation of frost on the evaporator from the passing airflow. This requires periodic defrosting by the user, or automatic heater assisted defrosting by the refrigeration device. In the case of a forced air evaporator, which generally has a compact arrangement of refrigerant tubing and cooling fins, removal of frost either takes considerable time, leaving the refrigerated compartments uncooled for considerable periods, or requires the inclusion of an auxiliary defrost heating or element in the evaporator structure, and associated wiring and controls.  
         SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to provide a refrigerator which at least goes some way towards overcoming the above disadvantages or which will at least provide the public with a useful choice.  
           [0007]    Accordingly the present invention consists in a refrigerator having a fresh food compartment adapted for maintaining the contents thereof at a temperature above freezing and a vapour compression refrigerator system, characterised in that in said vapour compression refrigeration system in one mode an evaporator is operated at a temperature within 10° C. of the temperature desired in said fresh food compartment and a supply of air is maintained over said evaporator and into said fresh food compartment, and in a second, defrost, mode said vapour compression refrigeration system is stopped or allowed to operate without significant heat extraction at said evaporator and a supply of air above 0° C. is passed over said evaporator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a side view in partial cutaway of a refrigerator according to a first embodiment of the present invention,  
         [0009]    [0009]FIG. 2 is a schematic representation of the refrigeration system according to the first embodiment of the present invention,  
         [0010]    [0010]FIG. 3 is a side view in partial cutaway of a refrigerator/freezer including a refrigeration system according to a second or third embodiment of the present invention,  
         [0011]    [0011]FIG. 4 is a schematic representation of the refrigeration system according to the second embodiment of the present invention, and  
         [0012]    [0012]FIG. 5 is a schematic representation of the refrigeration system according to the third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    In the present invention the refrigeration system at least of the produce or fresh food compartment is characterised in that the operating temperature of the evaporator of the vapour compression refrigeration system is not overly cold as has been the case in prior art systems. In particular the evaporator is preferably operated at a temperature within 10° C. of the temperature desired in the produce compartment. Generally the temperature desired in the produce compartment will range between 4° C. and 7° C. Consequently the temperature of the evaporator may in fact (if the temperature difference is say 6° C.) be above freezing.  
         [0014]    With the evaporator operating at a temperature of 0° C. or just below 0° C., say −5° C. to 0° C., the amount of condensation that freezes on the evaporator (frost) is dramatically less than where the evaporator operates at −18° C., and the amount of sub cooling of that frost (the difference between the temperature of the frost and its freezing temperature) is also greatly reduced. Furthermore the evaporator itself, and its components such as refrigerant tubing and fins, is at this higher evaporator temperature.  
         [0015]    In general operation of a refrigerator there are long periods when the heat load on a refrigeration space (the combination of heat migrating through the refrigerator walls, heat introduced by new produce placed within the refrigeration compartment, and heat entering the refrigerator while the door is open) is low. In these periods the refrigeration system generally cycles on and off to maintain the temperature within the refrigerator within a specified range. This may for example in a simplest form comprise an “on” temperature, being the upper limit of the temperature range and an “off” temperature being the lower limit of the temperature range. In refrigeration systems where the evaporator is maintained at a very low temperature (for efficiency of operation of the refrigeration system, or because the evaporator is also required to cool the air sufficiently to cool the freezing compartment which is at approximately −18° C. (some 25° C. less than desired temperature in the produce compartment)), the evaporator still experiences a significant frost build up and this frost is cooled to a very low temperature. Consequently during the “off” period of the compressor operation the built up frost does not usually rise to its melting temperature without the assistance of a defrosting heater. The process of defrosting is further hindered by the significant thermal mass of the evaporator at −18° C. absorbing substantial amounts of the heat which could otherwise be utilised in melting the ice frozen onto the evaporator surfaces.  
         [0016]    With the evaporator of the present invention operating at a temperature only slightly below freezing, the frost that forms is not significantly sub cooled and quickly rises to the melting temperature and drips off the evaporator. Furthermore, the evaporator, being maintained much closer to the melting temperature of the frost, does not significantly hinder defrosting.  
         [0017]    In the invention this is further assisted by operating the fan to continue to circulate air from the produce compartment over the evaporator during the defrost cycle. The air from the produce compartment is marginally above freezing and consequently will lose heat to the frost on the evaporator, at once cooling the air of the produce compartment and defrosting the evaporator.  
         [0018]    A further advantage of the present invention is the reduction in frost formation on the evaporator reduces the dehumidifying effect that the evaporator has on the refrigerator air flow.  
         [0019]    Particular embodiments of the invention are described now with reference to FIGS.  1 - 5 .  
         [0020]    Referring to FIG. 1 a refrigerator has a cabinet  1  and a door  2  enclosing the cabinet  1 . This is a single temperature refrigerator, having only a produce compartment  3 . A compressor  4  is mounted on a compressor tray  5  in a compressor enclosure  6  at the lower back portion of the refrigerator cabinet  1 . An evaporator  7  is mounted above a condensate collection guide  8 . The condensate collection guide  8  emits collected condensate onto a evaporation tray  9  mounted on the top of the compressor  4 . A fan  10  draws air flow over the evaporator  7 , and expels it into a vertically extending distribution duct  11  rearward of the produce compartment  3 . The distribution duct  11  has a series of air flow openings  12  into the produce compartment  3 . An opening  13  is provided at a lower part of the produce compartment for air intake to the evaporator chamber. Thus when air is circulated by the fan  10  it passes into the evaporator chamber as indicated by arrow  14  across the evaporator as indicated by arrow  15  out past the fan as indicated by arrow  16  and up through the distribution duct as indicated by arrows  18  to exit into the produce compartments base as indicated by arrows  17 .  
         [0021]    Referring then to FIG. 2 this represents the refrigeration system incorporated in the refrigerator of FIG. 1 in a diagrammatic form. The refrigeration system comprises a compressor  4 , a condenser  21 , a flow control valve  22  and evaporator  7  in series connection. These components are all well known in the art. They are connected by connecting tubes  23 ,  24 ,  25  and  26 , through which the gas or liquid refrigerant travels as indicated by arrows  20 . The refrigeration system is run in the conventional manner except that the system is configured such that the evaporator  7  is run as previously referred to at a temperature which is at only 0° C. or just below.  
         [0022]    The evaporator operating temperature requires greater evaporator operating effectiveness. A larger evaporator surface and/or higher air flow rates over the evaporator surface are two ways to achieve the higher evaporation temperature in accordance with the present invention. The lower heat transfer to the moving air (due to the reduced temperature difference) per unit area of evaporator surface or per unit volume for air flow than if the evaporator operated at say −18° C., is compensated by greater heat transfer surface and/or volume flow rate. Evaporator operating effectiveness may also be increased (where not already done) by providing boundary layer interrupters on the heat transfer surface.  
         [0023]    Air is thus supplied by the fan  10  through the duct  11  at the back of the refrigerator between the baffle  19  and the rear face of the cabinet  1  to reach the produce compartment  3 .  
         [0024]    Referring now to FIGS.  3 - 5  the present invention may also be incorporated into refrigerator/freezer combinations, such as depicted in FIG. 3 wherein a cabinet  29  has a freezer compartment  30  and a produce compartment  31  separated by a intermediary wall  32  with a door  33  and  34  to each of the compartments  30  and  31  respectively. In this embodiment, for example, each of the freezer and refrigerator compartments include a baffle  35 ,  36  respectively at the rear thereof to form respective ducts between the baffles and the rear wall  37  of the cabinet  29 . An evaporator enclosure  38  meets the baffle  36  to form an enclosure for the evaporator. An evaporator is separately provided in each of the two evaporator enclosures. Each of the evaporators supplies only its respective compartment with cooled or chilled air, and each has a corresponding and independently driven fan. The produce compartment evaporator  40  runs as indicated above at a temperature only just below 0° C. The freezer evaporator  39  however runs at a temperature which is much lower, for example −18° C. Thus the air supplied to the produce compartment  31  by the produce compartment fan  41  is at a temperature suitable for the produce compartment, while the air supplied by the freezer compartment fan  42  to the freezer compartment  30  is supplied at a temperature suitable for the freezer compartment. This air is circulated by the respective fans  41  or  42  as indicated by the arrows  43  and,  44  respectively. The produce compartment air flows through respective openings  45  in the baffle  36 . The circulating air for the freezer compartment flows through openings  46  and the freezer compartment baffle  35 .  
         [0025]    Supply of liquid refrigerant to the respective evaporators  39  and  40  are by a range of alternatives.  
         [0026]    In a first alternative, as depicted in FIG. 4, each of the evaporators are supplied by an entirely independent refrigeration system. Each refrigeration system has a respective compressor, condenser, flow control valve and evaporator. In FIG. 4 this is indicated by separate refrigeration circuits. Evaporator  39  of the freezer compartment  30  includes a first compressor  50 , first condenser  51  which extracts heat  52 , first flow control valve or capillary  53  and associated connecting conduits. A second refrigeration system for the produce compartment evaporator  40  includes a second compressor  55 , a second condenser  56  extracting heat  57 , a second flow control valve  58  and associated connecting conduit. The two refrigeration systems are essentially run independently with the produce compartment system being run entirely as described earlier. This arrangement allows for each of the systems to be ideally tuned to its respective compartment and may be particularly suitable with variable speed compressors where the compressor capacity can be made to accurately match the required heat pump capacity at any given time.  
         [0027]    Alternatively an arrangement, such as that depicted in FIG. 5, could be adopted where both refrigeration systems include common components, in particular a common compressor  60  and condenser  61 . One possible embodiment is depicted in FIG. 5 in which a selection valve  62  selectively incorporates either the freezer compartment evaporator or the produce compartment evaporator into the refrigeration system. In that case the flow control valves  63  or  64  for each of the respective evaporators  39  or  40  may be provided individually for each of the evaporator parts of the circuit or may be provided as shown such that the flow control valve for evaporator  39  is actually a combination of valves  63  and  64 , while for evaporator  40  is only the single flow control valve  64 . It is considered that, for this embodiment, to allow effective control of the refrigeration system the compressor  60  would be required to be of variable capacity, for example, a linear compressor operating at varying stroke or frequency. The return flow from the suction side of the evaporators  39  or  40  combine back into the main circuit at junction  65 . Junction  65  may include a further selection valve  62  operated in unison with valve  62 , if desired, to ensure that continuing suction is not provided to the evaporator not connected into the main refrigeration circuit.