WO 01/65190 PCT/NZO1/00028 A REFRIGERATOR BACKGROUND TO THE INVENTION Field of the Invention 5 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. Summary of Prior Art Most domestic refrigerators operate using a vapour compression refrigeration system. 0 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 5 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 0 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. 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 .5 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. 30 WO 01/65190 PCT/NZO1/00028 -2 SUMMARY OF THE INVENTION 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. 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 0 C is passed over said evaporator. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view in partial cutaway of a refrigerator according to a first embodiment of the present invention, Figure 2 is a schematic representation of the refrigeration system according to the first embodiment of the present invention, Figure 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, Figure 4 is a schematic representation of the refrigeration system according to the second embodiment of the present invention, and Figure 5 is a schematic representation of the refrigeration system according to the third embodiment of the present invention. 5 DETAILED DESCRIPTION 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. 0 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.
WO 01/65190 PCT/NZ0I/00028 -3 With the evaporator operating at a temperature of 0 C or just below 0 0 C, say -5'C to 0 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. 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 formcomprise 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. 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. 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.
WO 01/65190 PCT/NZO1/00028 -4 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. Particular embodiments of the invention are described now with reference to Figures 1-5. Referring to Figure 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. Referring then to Figure 2 this represents the refrigeration system incorporated in the refrigerator of Figure 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 0 C or just below. 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 WO 01/65190 PCT/NZO1/00028 -5 volume flow rate. Evaporator operating effectiveness may also be increased (where not already done) by providing boundary layer interrupters on the heat transfer surface. 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. Referring now to Figures 3 - 5 the present invention may also be incorporated into refrigerator/freezer combinations, such as depicted in Figure 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 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. Supply of liquid refrigerant to the respective evaporators 39 and 40 are by a range of alternatives. In a first alternative, as depicted in Figure 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 Figure 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 WO 01/65190 PCT/NZ01/00028 -6 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. Alternatively an arrangement, such as that depicted in Figure 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 Figure 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.