WO 2008/017130 PCT/AU2007/001139 EVAPORATOR DEFROST CYCLE WITH CONCURRENT REFRIGERATION 5 BACKGROUND The present invention relates to a method and assembly for defrosting evaporator coils of a refrigeration unit and more particularly though not limited to, commercial refrigeration units. More specifically, the invention relates to a 10 method and assembly for improved surface temperature control for goods displayed in an open display cabinet of commercial refrigeration units such as those having vertical or horizontal air curtain and which are usually found in supermarkets. The invention further provides an assembly allowing a defrost cycle in an evaporator of a refrigeration cabinet and which reduces or eliminates 15 spoilage of foodstuffs by controlling product surface temperature during the defrost cycle. More particularly, the invention relates to a method and apparatus for evaporator defrost cycle with continuous, at least partial refrigeration. Although the invention will be described with reference to its application in 20 commercial refrigeration and particularly open cabinet refrigeration, it will be appreciated that the assembly and method of defrost to be described, will have other applications such as in industrial open case refrigeration and domestic refrigeration. 25 PRIOR ART Evaporator coils of a refrigeration unit will in normal operation accumulate frost, by condensation and freezing of moisture vapor when the evaporator coils are operated at a temperature below the freezing point of water. This occurs for 30 instance, in open front display units found in supermarkets and the like where refrigerated goods are placed on display in an open well, which facilitates immediate customer access. Since the goods on display in such refrigeration units are capable of contact with the ambient air environment of the building in
I
WO 2008/017130 PCT/AU2007/001139 which the units are used, or in contact with a higher temperature air curtain when refrigeration is turned off. There may be contamination of refrigerated air in the cabinet by environmental air which during defrost cycles when refrigeration is temporarily suspended and when refrigeration to the air curtain 5 is off, will lead to an increase in surface temperature of the goods up to 8 0
C
10 C. In off cycle refrigeration, non-refrigerated air forming an air curtain is maintained by fans. The supermarket owner is left to bear the cost of food spoilage resulting from an unacceptable differential between core temperature and surface temperature of foodstuffs, the surface temperature rising due to a 10 temporary shut off of the refrigeration cycle. In typical refrigerated display cabinets, to maintain product temperature within cold chain requirements, the refrigeration system is set to cool circulating air below 0 0 C. Refrigerant evaporator temperature can be typically between -3'C to -40'C 15 depending upon the application. This sub-zero temperature of the evaporator causes water vapor in the surrounding air to freeze on the evaporator heat exchange surfaces and create obstruction to air flow. In an extreme case, frost may block the air flow altogether. Also, frost accumulations on the heat transfer surfaces increase the thermal resistance to heat flow therefore heat transfer and 20 efficiency of the evaporator is reduced. The evaporator thus needs to be defrosted in a regular basis. In medium temperature refrigeration cabinets the cold chain temperature is limited to -1*C to 5'C for which the evaporating temperature could be around -3'C to -15*C and delivering air curtain temperature of around -2*C to -5'C depending upon the cabinet design. The 25 temperature returning to the evaporator could be around +5'C to +10'C depending upon the design and performance of the air curtain. According to prior art methodology and one particular method, the frost accumulated on evaporator coils of conventional refrigeration units has been 30 removed by applying heat to the evaporator coils either from direct electrical heater or from other indirect sources for melting the frost. In a normal defrost 2 WO 2008/017130 PCT/AU2007/001139 cycle, the refrigeration operation is stopped. This may be effected by halting operation of the compressor and in addition, defrosting may be accelerated by the application of heat such as from a heating element to the evaporator coils. Another active defrost method employs heated air passing over the coils. 5 Refrigeration is again interrupted during defrost. It is preferable to run a refrigeration system in continuous operation while defrost is occurring so that cooling can be supplied to the refrigerated space provided energy is not over expended counteracting refrigeration during 10 defrosting. Although refrigeration can be continued even during a defrost cycle, extra energy would be consumed by the compressor working against heating of the evaporator coils to remove the frost. This is an inefficient and costly operation of a refrigeration system. 15 Some effort has been made in the past to address the disadvantages noted above in connection with the use of electrical heat for removing frost from evaporator coils. In an example of the prior art, US Patent 3665723 discloses a means of using 20 the compressor in a conventional refrigeration apparatus for defrosting the evaporator coils so that the frost may be removed without stopping the compressor. Broadly stated, the invention disclosed in that patent resides in the use of compressed refrigerant gas, which has been heated during compression, for supplying heat to evaporator coils for defrosting the evaporator coils. The 25 heated, compressed gas is passed through the evaporator coils in a direction opposite to that which the gas is passed through the evaporator coils during a cooling cycle. In another example of the prior art, United States patent application number 30 20050028544 describes a method where within a group of evaporators in a multi zone case or multiple cases at least two are defrosted at a time while 3 WO 2008/017130 PCT/AU2007/001139 others are providing refrigeration. One disadvantage of this method is that it requires special hardware components like a controller, multiple valves and connections and software such as a programming algorithm built with the special controller. 5 In a further example of the prior art, United States patent number 4945732 describes a method applicable for a display cabinet with dual air curtain plus use of an electric heater for quick defrosting to avoid a temperature rise during defrost. In this method an electric heater is placed in front of the evaporator 10 within a refrigerated air curtain within the refrigerated air duct. During defrosting, refrigeration is turned off and the heater is turned on to quickly defrost the frosted evaporator. During defrosting warmth created by the heater can also introduce heat rise to the refrigerated interior of the cabinet. This method applies a damper control to merge the inner air circuit with outer non 15 refrigeration air circuit to avoid warmth being circulated through the outer air passage. Obviously this method applies a significant complexity and additional cost of hardware for a dual duct and curtain, a damper and its drive, an electric heater as well as control means to control these functions. 20 In another example of the known prior art, United States Patent Number 5269151 describes a method of passive defrost of evaporator by circulating heat from a phase change material to the evaporator during defrost where heat was accumulated during normal refrigeration from a source within the refrigeration system, such as warm liquid refrigerant or part of condenser heat. A 25 disadvantage of this method is that it relies on additional expensive components like phase change material, container and circulating means. This method can however, assist in quick defrost aiming to reduce rise of air temperature of the refrigerated cabinet interior, but a large temperature rise may not reduce as much because heat is introduced . 30 4 WO 2008/017130 PCT/AU2007/001139 In yet a further example of the prior art, United States Patent Number 5887440 describes a method of passive defrost system where a heat exchanger with fan forcing ambient air is placed outside the refrigerated cabinet connected to evaporator using additional tubing inside the evaporator and a pumping and 5 solenoid valve to circulate and control the flow of a heat transfer fluid between that external heat exchanger and the evaporator. During defrost, the refrigeration to the evaporator is stopped and the passive defrost system is activated by turning on the pump, the external fan and opening the fluid solenoid. This allows the external heat exchanger to collect heat from ambient 10 carried by the heat transfer fluid and when passed through the frosted evaporator the evaporator gets defrosted. By applying external heat into the evaporator this method allows quicker defrost thus reducing peak rise of case interior temperature compared to off-cycle fan defrost. However, this method also suffers from the disadvantage that it requires a number of expensive 15 components to be built with the cabinet and additional energy costs due to pumping and fan requirements. Thus in general, known passive defrost systems, suffer from a disadvantage of costs of maintenance to pumps, fans and heat transfer fluid. 20 The most common and traditional method of defrosting a medium temperature refrigerated display cabinet evaporator is called "fan defrost" or "off cycle defrost" where refrigerant supply is stopped for a time period called "defrost duration" while a fan keeps circulating air through the evaporator. As the air 25 gets warmer due to lack of refrigeration, the frost from the heat exchange surfaces melts. This process is repeated a certain number of times per day called "defrost frequency". Off cycle defrost is very popular for the reason that it does not require any additional supply of energy or special expensive equipment other than a solenoid to stop the refrigeration flow and a time control system to 30 trigger this event in a regular manner at predetermined intervals. 5 WO 2008/017130 PCT/AU2007/001139 The conventional practice in the off cycle defrost method allows enough defrost time so that all parts of the evaporator are defrosted from the ice. The more the ambient humidity and air infiltration, more the frost growth thus longer the defrost time required. In practice, the actual defrosting does not start straight 5 after the closure of refrigerant line, rather when the remaining refrigerant in the evaporator pipeline is boiled off. Therefore, actual defrost duration is even longer than the theoretical time required for melting ice. This duration also includes a short period of "drain time" which allows molten water to run off the evaporator before the refrigeration re-starts to avoid re-freezing. This drain time 10 is particularly important for evaporators laid on a base where water may have tendency to stay longer between the fins and between fins and the base due to capillary and surface tension actions. Evaporators fitted vertically and hung on the side may have no drain time or very short drain time. Total defrost duration is also influenced by the oncoming air temperature which in turn has a relation 15 to the ambient air temperature. The colder the ambient temperature the longer the defrost duration should be. Although typical design and test setup is done to comply with standard ambient climate classes as appropriate with the target market, the defrost duration may also include an extended time which may be required in low ambient stores conditions. Sometimes defrost is terminated 20 when air temperature of the evaporator reaches a threshold temperature, a process called "temperature termination". The practice of selecting defrost frequency is dependent upon the speed of frost growth which in turn depends upon the design of the evaporator fin spacing, types of fin, case air infiltration rate, ambient humidity and velocity of air over the fins. The frequency is 25 determined by test watching the loss of heat transfer observed by rise of air temperature off the evaporator during refrigeration. When normal refrigeration is cut off, the air temperature off the evaporator starts rising but forms a plateau around the melting temperature of ice (0 0 C at 30 standard atmospheric pressure) due the fact that some cooling is still provided by the ice itself. There is also a residual refrigeration provided by the boiling off 6 WO 2008/017130 PCT/AU2007/001139 of gas remaining in the evaporator after refrigeration is shut off. When almost all ice has melted, the plateau tends to break and air temperature starts to rise sharply. As soon as refrigeration starts again, the air temperature off the evaporator starts dropping down almost immediately. Cabinet interior air 5 temperature and product temperature may take bit longer to cool down due to thermal inertia of the cabinet mass. S.A. Tassou and D. Datta (1999) have investigated the influence of ambient temperature and humidity on frosting and defrosting in vertical multideck 10 display cabinets. Their finding clearly indicates that frost accumulation is highly dependent on store environment humidity thus a fixed duration and low defrost frequency may offer a risk of product temperature rise too high during in an off cycle defrost. A test conducted at 22'C and 65% relative humidity suggests that temperature of the product at a worst location may swing between 5' to 12'C 15 through a full refrigeration and defrost cycle with only 4 defrosts per day. S.A. Tassou and D. Datta (1999) also explained that excessive variation of product temperature could be overcome with more frequent defrost but this may on the other hand increase the energy consumption, unless the duration of defrost is also controlled 20 Hussmann Corporation patented a technology in Australia as described in Australian patent No. 697909 that describes use only of an electronic evaporator pressure regulating (EEPR) valve to control the evaporator operating temperature to a preset value only during the drip time of an off cycle 25 defrost mode. Initiation and termination of defrost in this method is done with preset time frequency and time duration. At the start of the defrost cycle refrigeration is turned off. This method activates modulation of the electronic EPR after a specially designed and programmed controller identifies completion of melting of ice by measuring the rise of evaporator exit air temperature. This 30 method is not applicable with mechanical evaporator pressure regulating (EPR) 7 WO 2008/017130 PCT/AU2007/001139 valves and only relying on expensive electronic EPR valves and use of special controller. Although the aforesaid methods have some benefits, they suffer from a 5 number of disadvantages. For example, they are inefficient due to high energy consumption compensating for addressing temperature rise in the refrigeration cabinet during defrost which can result in food spoilage. INVENTION 10 The present invention seeks to ameliorate the problems of the prior art systems by providing an alternative method and assembly for defrosting evaporator coils for improved surface temperature control for goods displayed in an open display cabinet of an industrial refrigeration unit such as but not 15 limited to those found in supermarkets. The invention further provides an assembly allowing passive defrost in a refrigeration unit and which reduces or eliminates spoilage of foodstuffs by controlling product surface temperature during a defrost cycle. 20 The present invention provides an alternative to the known prior art and the shortcomings identified. Other objects and advantages will appear from the description to follow. In the description reference is made to the accompanying drawings and illustrations, which forms a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be 25 practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. In the accompanying illustrations, like reference characters designate the same or similar parts throughout the 30 several views. The following detailed description is, therefore, not to be taken as limiting the scope of the present invention. 8 WO 2008/017130 PCT/AU2007/001139 In its broadest form the present invention comprises: 5 an evaporator defrost system in a refrigeration system comprising: an insulated refrigeration cabinet, an evaporator within the cabinet having a refrigeration mode for cooling air and a defrost mode, control means for controlling the flow of high pressure liquid refrigerant gas 10 to an inlet side of said evaporator, means for circulation of air through the evaporator to a product storage space within the cabinet, means associated with the evaporator to control suction pressure to a compressor via an outlet line from the evaporator, 15 sensing means to determine when a refrigeration temperature has reached a predetermined level, the defrost system comprising, means for providing at least partial refrigeration during a defrost cycle. 20 In its broadest form the present invention comprises: an evaporator defrost system in a refrigeration system comprising: an insulated refrigeration cabinet, an evaporator within the cabinet having a refrigeration mode for cooling air and a defrost mode, 25 control means for controlling the flow of high pressure liquid refrigerant gas to an inlet side of said evaporator, means for circulation of air through the evaporator to a product storage space within the cabinet, means associated with the evaporator to control suction pressure to a 30 compressor via an outlet line from the evaporator, 9 WO 2008/017130 PCT/AU2007/001139 sensing means to determine when a refrigeration temperature has reached a predetermined level, the defrost system comprising, means for providing at least partial refrigeration during a defrost cycle. 5 Preferably, the defrost cycle allows melting of frost on said evaporator during said partial refrigeration. In another broad form of an apparatus aspect, the present invention 10 comprises: an evaporator defrost assembly in a refrigeration system comprising: an insulated refrigeration cabinet, an evaporator within the cabinet having a refrigeration mode for cooling air and a defrost mode, 15 control means for controlling the flow of high pressure liquid refrigerant gas to an inlet side of said evaporator, means for circulation of air through the evaporator to a product storage space within the cabinet, means associated with the evaporator to control suction pressure to a 20 compressor via an outlet line from the evaporator, sensing means to determine when a refrigeration temperature has reached a predetermined level, the defrost assembly including means for providing at least partial but continuous refrigeration during a defrost cycle. 25 Preferably, said means for providing at least partial continuous refrigeration comprises means to control that rate at which gas is removed from the evaporator during said partial refrigeration. The partial refrigeration is set at a temperature above the melting temperature of ice such that defrosting of coils and melting of ice on said evaporator occurs from inside and outside the coils. 30 Preferably, said control means forces evaporator to stay hot by blocking the gas keeping the evaporator temperature relatively high and at a level at about 2 10 WO 2008/017130 PCT/AU2007/001139 3 'C. During said partial refrigeration, cold air is supplied but not so cold that it causes ice to form on the evaporator coils. In one broad form the present invention comprises: 5 an evaporator defrost system in a refrigeration system comprising: an insulated refrigeration cabinet, an evaporator within the cabinet having a refrigeration mode for cooling air and a defrost mode, control means for controlling the flow of high pressure liquid refrigerant gas 10 to an inlet side of said evaporator, means for circulation of air through the evaporator to a product storage space within the cabinet, means associated with the evaporator to control suction pressure to a compressor via an outlet line from the evaporator, 15 sensing means to determine when a refrigeration temperature has reached a predetermined level, the defrost system comprising, by pass means to provide a passive defrost cycle and to maintain at least partial refrigeration during the defrost cycle and allowing melting of frost on 20 said evaporator during said partial refrigeration. According to a preferred embodiment, said means to enable passive defrost cycle comprises; an expansion valve at an inlet side of said evaporator capable of allowing or isolating refrigerant flow at varying flow rate to said evaporator via said inlet 25 side, a solenoid in said suction line to said compressor, a mechanical evaporator pressure-regulating valve (EPR) located in a bypass line from said suction gas return line of the evaporator; said valve being set to a predetermined defrost evaporating pressure; 11 WO 2008/017130 PCT/AU2007/001139 wherein, when said solenoid in said suction line, isolates said refrigerant flow, said compressor continues sucking the refrigerant via the suction line, the by pass line thereby allowing partial refrigeration. Preferably, said means for providing at least partial continuous refrigeration 5 comprises means to control that rate at which gas is removed from the evaporator during said partial refrigeration. The partial refrigeration is preferably set at a refrigerant evaporating temperature above the melting temperature of ice such that defrosting of coils and melting of ice on said evaporator occurs from inside and outside the coils. 10 Preferably, said control means forces evaporator to stay hot by blocking the gas keeping the evaporator temperature relatively high and at a level at about 2 3 'C. During said partial refrigeration, cold air is supplied but not so cold that it causes ice to form on the evaporator coils. 15 According to another embodiment, the means for partial refrigeration comprises a by pass line including a mechanical valve - Defrost EPRV- which controls flow of gas from the evaporator to a main suction line at a predetermined temperature higher than the melting temperature of ice. Another mechanical EPR valve- refrigeration EPRV, is fitted inline with the suction 20 solenoid in parallel arrangement with the defrost EPRV. This refrigeration EPRV is preset to the evaporating pressure suitable for the normal refrigeration mode. In this embodiment, the defrost EPRV starts working for providing partial refrigeration during defrost mode when the said solenoid is closed thus refrigeration EPRV is inoperative thus refrigerant flow switches between 25 refrigeration-EPRV and defrost-EPRV in a sequential manner switching between refrigeration mode and defrost mode thus providing continuous refrigeration. In another embodiment, the means for providing partial refrigeration comprises a controller in communication with a sensor fitted at the inlet side of an evaporator and an electronic evaporating pressure regulating 30 valve (EEPRV) on an outlet side of the evaporator. In this embodiment the EEPRV can be programmed to operate at an evaporating temperature higher 12 WO 2008/017130 PCT/AU2007/001139 than melting temperature of ice thus providing partial refrigeration during defrost mode and to operate at a suitable evaporating temperature for the refrigeration mode in a sequential manner switching between defrost mode and refrigeration mode providing continuous refrigeration. 5 In other arrangement, the said evaporator may be a bank of parallel evaporators operating simultaneously at the same operating conditions allowing one common passive defrost assembly providing partial refrigeration during 10 defrost of all said evaporators in the bank. BRIEF DESCRIPTION OF DRAWINGS 15 The present invention will now be described according to a preferred but non limiting embodiment and with reference to the accompanying illustrations wherein: 20 Figure 1: shows a front perspective view of a typically known medium temperature open multi deck cabinet construction refrigerator. Figure 2: shows a cross sectional view of the refrigeration cabinet of figure 1 showing a typical remote refrigeration plant including an evaporator, expansion 25 valve and liquid line solenoid valve using compression type refrigeration. Figure 3 shows a graphical illustration of a typical air curtain delivery temperature / time distribution for conventional refrigeration with off defrost cycle 30 Figure 4: shows a schematic view of a passive-defrost control assembly fitted with a mechanical defrost EPR valve bypass to suction line. 13 WO 2008/017130 PCT/AU2007/001139 Figure 5: shows a schematic view of a passive-defrost control assembly including two mechanical EPR valves one for defrost mode as bypass arrangement to the 2 nd one for refrigeration mode. 5 Figure 6. shows a schematic arrangement of a passive defrost refrigeration system using an electronic EPR valve and electronic control means. Figure 7 shows a temperature / time graph of Cabinet Air Temperatures at Top 10 of the Product Simulator in prior art off cycle defrost method. Figure 8 shows a temperature / time graph of Cabinet Air Temperatures at Top of the Product Simulator when partial refrigeration is provided during passive defrost cycle. 15 Figure 9 shows a time / temperature graph of air curtain temperature at an entry to the cabinet in a traditional prior art off cycle defrost. Figure 10 shows a time / temperature graph of air curtain temperature at an 20 entry to the cabinet with partial refrigeration passive defrost system. Figure 11 shows the results of product core temperature inside the cabinet for a typical off cycle defrost and 25 Figure 12 shows the results of product core temperature inside the cabinet for a partial refrigeration passive defrost cycle. Figure 13 shows the results of mass flow during defrost for a typical off cycle defrost system. 30 Figure 14 shows the results of mass flow during a partial refrigeration passive defrost cycle. 14 WO 2008/017130 PCT/AU2007/001139 Figure 15 shows cabinet evaporator temperature during an off cycle defrost cycle. Figure 16 shows cabinet evaporator temperature during a partial 5 refrigeration passive defrost cycle. Figure 17 show the results of pressure /time chart corresponding to evaporating temperature for an off cycle refrigeration cycle. 10 Figure 18 shows the results of pressure / time chart corresponding to evaporating temperature for a partial refrigeration passive defrost cycle. Figure 19 shows a schematic representation of temperature gradation through a typical coil cross section in a known off cycle defrost system 15 compared to a continuous refrigeration passive defrost system DETAILED DESCRIPTION 20 Figures 1 and 2 show general layouts of a known refrigeration system associated with an open display refrigeration cabinet. Throughout this description, a reference to at least partial refrigeration may be taken to mean that the refrigeration is partial during the defrost cycle over 25 . a range of refrigeration temperatures in that it may be set at any temperature within that range or range of temperatures within that range. Also, partial refrigeration may be taken to be related to the time range for a defrost cycle in that there may be refrigeration for all or part of the time taken for the defrost cycle. In addition at least partial refrigeration may be taken to relate 30 to a combination of the above scenarios. 15 WO 2008/017130 PCT/AU2007/001139 Figure 1 shows a front perspective view of a typical medium temperature open multi deck cabinet construction refrigerator 1. Refrigerator I comprises a base 2, rear wall 3 and overhang roof 4 which together define a space 5 in which products to be subject to refrigeration are located. Cabinet also has 5 side panels (not shown) those create the total volume of the cabinet. Shown in cutaway portion 6 is an evaporator 7 which provides heat collection for the typical refrigeration cycle. Base 2 further comprises fans 8 and 9 which blow refrigerated air throughout space 5 to refrigerate product in space 5 (not shown) on one or more product shelves (not shown). The aforesaid is a 10 typical arrangement for a known open display refrigerator. Figure 2 shows a cross sectional view of the refrigeration cabinet 1 of figure 1 with corresponding numbering of parts and showing a typical remote refrigeration plant 10 associated with evaporator 7. Product space 5 is shown 15 adapted with shelving trays 11, 12, 13, 14 and 15 which hold products to be refrigerated. The arrangement of the shelving trays shows the distribution of product throughout space 5. To maintain refrigerated air in space 5 at an optimal temperature refrigerated air is ducted through pathways 16, 17 and 18 until it reaches exit 19 whereupon the cooled air is delivered as an air 20 curtain as indicated by arrows 20. Air curtain 20 provides an invisible envelope of refrigerated air during the refrigeration cycle. Circulation of cooled air is fan assisted by means of fans 8 and 9. Refrigeration assembly 10 typically comprises a compressor 21 having a 25 suction line 22 and compressor discharge line 23 in communication with a condenser 24. Condenser 24 includes an outlet line 25 which delivers high pressure liquid to liquid receiver 26. From liquid receiver 26, high pressure liquid travels via outlet line 27 and liquid control solenoid 28 and refrigerant expansion valve 29 prior to entry into evaporator 7. Refrigerant gas is 30 pumped around the above circuit. Compressor 21 sucks heat laden gas vapour from the evaporator 7 pumping it at high pressure into condenser 24. 16 WO 2008/017130 PCT/AU2007/001139 Condenser 24 disposes of the heat from the compressed hot gas causing it to condense into high pressure liquid. The liquid returns to the evaporator via solenoid 28 and expansion valve 29 which are set to cause pressure buildup. As liquid is released by expansion valve 29 into evaporator 7 at lower 5 pressure it expands into a vapour again absorbing heat. When compressor 21 is running the above compression refrigeration cycle is continuous and usually thermostatically controlled. This activity keeps the temperature in space 5 to a predetermined range. Typically, in any open front refrigeration cabinet there will be controlled refrigeration and defrost cycle depending 10 upon system requirements. A most common practice to defrost a frosted evaporator is called "off cycle defrost" in which the refrigeration is shut off and fan keeps running as described earlier in the prior art section. 15 As soon as solenoid 28 (see figure 2) is closed, this isolates refrigerant flow to evaporator 7. Compressor 21 continues sucking the refrigerant gas, which eventually boils off and provides further refrigeration until all refrigerant is evaporated. As will be described in more detail with respect to Figure 3 below, this part of cooling can be called "residual refrigeration". At that 20 stage, the air temperature off the evaporator 7 starts rising but forms a plateau around the melting temperature of ice (0 0 C at standard atmospheric pressure) due the fact that some cooling is still provided by the ice itself. When all ice is nearly melted, the air temperature in space 5 starts to rise sharply above the temperature plateau. As soon as refrigeration starts again, the air temperature 25 off the evaporator starts dropping down almost immediately. Cabinet interior space 5 air temperature and product temperature may take bit longer to cool down due to thermal inertia of the cabinet mass. A typical disadvantage of the 'refrigeration off' cycle method is that during 30 the duration of the defrost period, the refrigeration cabinet air temperature rises to a peak level. The air temperature measured at the exit 19 of the air 17 WO 2008/017130 PCT/AU2007/001139 curtain nozzle could reach as high as I0 0 C and the average cabinet air temperature, particularly near the front of the shelves, can be higher than 1 0 0 C. This rise of cabinet temperature causes the surface of the food products to rise higher and sometimes may exceed the required cold chain 5 temperature. Core temperature of the products contained in the refrigeration unit is a guiding parameter in design standards and testing procedures for units of the type described in figures 1 and 2. In particular, for sensitive food products like fresh meat, poultry or fish, a rise of surface temperature can stimulate bacterial growth starting 10 from the surface then progressing to the other parts of the products. This in turn reduces the display life of these products or creates product spoilage with public health implications. Other disadvantages of the off cycle method is that the peak temperature after 15 defrost must be cooled down quickly requiring a high capacity refrigeration plant. Keeping the air temperature low during this defrost duration is a primary concern and key challenge for the display manufacturing industry in reducing the food products spoilage and increasing the shelf life. 20 Figure 3 shows a graphical illustration of a typical temperature / time distribution for a conventional refrigeration - off defrost cycle. Figure 3 graphically illustrates residual of normal refrigeration which is provided until all refrigerant gas has "boiled off' i.e. has evaporated. This part of cooling may be called "residual refrigeration". Following that, air 25 temperature off the evaporator starts rising but forms a plateau around the melting temperature of ice (0 0 C at standard atmospheric pressure) due to the fact that some cooling is still provided by the ice itself. When almost all ice has melted, the plateau tends to break and air temperature starts to rise sharply. As soon as refrigeration starts again, the air temperature of the 30 evaporator starts dropping down almost immediately. Cabinet interior air 18 WO 2008/017130 PCT/AU2007/001139 temperature and product temperature may take longer to cool down due to the thermal inertia of the cabinet mass. 5 The present invention to be discussed below with reference to particular embodiments, addresses the above problem and provides a passive defrost system in which instead of stopping the refrigeration as described above, partial refrigeration is provided during the defrost duration at an evaporating temperature set at higher than the melting temperature of ice but 10 limiting the air off temperature by a low threshold value so that a peak air temperature of refrigeration unit 1 and particularly within space 5 is minimized. This is enabled according to one embodiment, by use of a by pass arrangement in which refrigerant gas is diverted via a by pass line 51 (see figures 3 and 4) having a defrost EPRV. Throughout the specification a 15 reference to passive defrost can be taken to be a reference to use of continuous refrigeration during defrost. The continuous refrigeration may take place over a time period and at a prescribed temperature above the melting point of ice. 20 Having forced internal temperature higher than melting temperature of ice, the defrost actually starts immediately after the evaporating temperature is elevated to the predetermined defrost temperature allowing even shorter defrost duration compared to traditional (refrigeration off cycle) methods. This result in a significant decrease in temperature of air curtain 20 25 during defrost without having to drop the evaporating temperature in normal refrigeration. This results in reduced product surface temperature therefore less food product spoilage is expected. It was found that limiting the peak air temperature of space 5 to a minimum, the refrigeration capacity required to drop to the refrigeration temperature range after defrost is also reduced, 30 thereby providing a flatter continuously low product temperature. 19 WO 2008/017130 PCT/AU2007/001139 In typical off cycle defrost the air temperature difference between the inside rear of the cabinet and the air curtain at the front, forming a temperature band, is such that the product temperature variation between rear 5 and front is significantly high. Energy efficiency as well as product temperature safety margin can be achieved by reducing this variation. Having significantly reduced the peak air curtain temperature together with overall reduction of air curtain temperature with the current invention of passive defrost, it is now possible to achieve much smaller temperature band 10 variation between rear and front of the cabinet. Figure 4 shows a schematic view of a passive-defrost control assembly 60 with like numbering for like parts as for assembly. Assembly 60 shows the evaporator end of a typical refrigeration cycle and includes 15 evaporator 7. As described with reference to the assembly of figure 60, condenser 24 delivers liquid 41 through inlet line 27 with expansion valve 29 preset to cause liquid pressure buildup. As liquid is released by expansion valve 29 into evaporator 7 at lower pressure it expands into a vapour again absorbing heat. Outlet line 22 from evaporator 7 receives gas, which is 20 sucked out of evaporator 7. Upstream of solenoid 53 of suction line 22, there is provided a by pass line 51 which includes therein a defrost mechanical EPR valve 52. According to the embodiment shown, unlike the schematic arrangement of figure 5, refrigeration EPRV 42 is omitted. As with the arrangement of figure 4, mechanical evaporator pressure-regulating (EPR) 25 valve 52 in line 51 acts as a parallel by pass for suction gas return pipe 22 which takes gas from evaporator 7. The arrangement of figure 5 essentially differs from that shown in figure 4 in that valve 42 is not fitted to line 22 and in parallel with defrost valve EPRV 52. 30 According to one embodiment, the same compressor or a compressor rack may be connected to multiple evaporators or cases set to different 20 WO 2008/017130 PCT/AU2007/001139 evaporating temperatures. In a case where there exists higher evaporating temperature than the compressor suction pressure requires, an EPR valve 42 is connected to the evaporator outlet as in figure 5. 5 The new passive defrost by pass technology demonstrates that peak case air temperature rise during defrost could be limited down to 3C compared to typical up to 1 0 0 C giving a continuous colder environment for food products to be kept reducing the food product spoilage at no sacrifice to energy efficiency. In one embodiment, an electronic EPR valve (EEPRV) can 10 also be used for the same purposes where EEPRV can be programmed to step change the evaporating temperature from refrigeration to defrost set point without having to use of any solenoid valve and provide continuous refrigeration. EEPRV can use either discharge air off the evaporator or the coil inlet temperature as a control variable. 15 According to one embodiment, a refrigeration system with multiple cabinets running on same evaporating temperature may have one passive defrost control system for the group. 20 The additional cost of the components used for the passive defrost assembly described is negligible compared to the benefit it achieves over the traditional off-cycle defrost systems. The cabinet air temperature can be reduced, thus the product surface temperature to a minor scale with off-cycle defrost by supplying more cooling by dropping the evaporating temperature 25 even lower without having to changing any components, including the evaporator. This solution would in turn result in larger compressor capacity and cost as well as poorer energy performance of the refrigeration plant due to lower evaporating temperature. Note that for the same refrigeration system, increasing the temperature difference between the condenser and 30 evaporator would increase power consumption of the compressor for the same output thus reducing energy efficiency. Compared to other 21 WO 2008/017130 PCT/AU2007/001139 technologies used to reduce temperature rise during defrost or to improve defrost performance, this assembly and method of the present invention may be installed by current technicians. 5 As mentioned earlier, temperature termination is now possible to set at much lower temperature set point compared to traditional methods as defrost is done much faster due to evaporator inner temperature is forced to rise. Whereas termination is traditionally set 7-81C, a temperature termination at 2.5-3.0*C is possible with the passive defrost according to the invention. In a 10 test case set to a critical defrost setup to a frequency of 12 per day and 13 minutes duration each in traditional off cycle defrost, it was possible to shorten the defrost down to only 8 minutes with the same frequency in passive defrost setup. By critical defrost means, allowing absolutely no safety margin or drain time, the defrost duration is set to a minimum value to obtain 15 just defrost in a standard test room condition. In traditional off-cycle defrost, the differential peak to valley product temperature between refrigeration cycle and defrost cycle may vary between 1 to 3*C depending upon the frequency of defrost, whereas, with passive defrost this differential is reduced to only 0.2-0.3*C. 20 Figure 5 shows a schematic view of an assembly 50 which represents part of the refrigeration assembly 10 described with reference to figure 2 and with corresponding numbering for corresponding parts. Arrangement 50 shows the evaporator end of a typical refrigeration cycle and includes evaporator 7. 25 In this embodiment, there is one mechanical by pass EPR valve set to a Pe-d temperature of 2 *C. There is no liquid line solenoid but a suction line solenoid is included which is shut off during defrost. Evaporator 7 is under the influence of valve 29 during refrigeration and on defrost valve 52 during defrost when solenoid 53 is shut off. This is described in more detail below. 30 22 WO 2008/017130 PCT/AU2007/001139 Condenser 24 (figure 2) delivers refrigeration liquid 41 through inlet line 27 with expansion valve 29, preset to sense the difference between inlet line 27 and outlet line 22 set.to cause liquid pressure buildup. As liquid is released by expansion valve 29 into evaporator 7 at lower pressure, it expands into a 5 vapour again absorbing heat. Outlet line 22 from evaporator 7 receives gas which is sucked out of evaporator 7 and is returned to compressor 21 via EPR valve 42.. Refrigeration-EPRV 42 is set to case evaporating pressure, Per, 10 corresponding to the saturated evaporating temperature of the cabinet. In this case the defrost-EPRV 52 set to Pea is fitted parallel to the refrigeration EPRV 42. A suction side solenoid 53 is also required along with the refrigeration-EPRV 42 so that closing this solenoid can activate defrost EPRV 52. 15 A mechanical evaporator pressure-regulating (EPR) valve alternatively called as a defrost EPR valve 52, regulates passage of gas from the evaporator 7 via by pass line 51 eventually to compressor 21 (figure 2). Typically, refrigeration continues to be maintained at a predetermined pressure 20 corresponding to evaporating temperature for refrigeration mode until onset of a defrost mode when refrigeration is temporarily terminated. During the defrost mode (melting of frost on the evaporator coils) and while refrigeration is terminated, the temperature in cabinet space 5 can rise as high as + 3 to + 5C. 25 The defrost-EPRV valve 52 is set to a "defrost evaporating pressure", Ped corresponding to a saturated refrigerant temperature higher than the melting temperature of ice. The valve set point is typically between +2 to +3'C. No solenoid valve (such as valve 28 described in figure 2) is required 30 in the evaporator inlet 27 line to stop refrigeration. Instead, a solenoid valve 53 is fitted at the main suction pipe 22 parallel to the defrost-EPRV 52. 23 WO 2008/017130 PCT/AU2007/001139 During the duration of defrost, solenoid valve 53 is shut to stop gas returning to compressor 21 via valve 42, thus forcing the refrigerant gas to flow under suction generated by compressor 21 through line 51 to the defrost-EPRV 52. When defrost is terminated, solenoid 53 is opened to allow the normal 5 refrigeration cycle to occur at pre-determined "refrigeration evaporating pressure", Pe-r corresponding to saturated evaporating temperature for refrigeration. 10 Figure 6 shows a schematic view of an assembly 60 which represents part of the refrigeration assembly 10 described with reference to figure 2 and with corresponding numbering for corresponding parts. Arrangement 40 shows the evaporator end of a refrigeration assembly and includes evaporator 7 and includes electronic controller 40a. As before, condenser 24 (figure 2) 15 delivers refrigeration liquid 41 through inlet line 27 with expansion valve 29, preset to cause liquid pressure buildup. As liquid is released by expansion valve 29 into evaporator 7 at lower pressure, it expands into a vapour again absorbing heat. Outlet line 22 from evaporator 7 receives gas which is sucked out of evaporator 7 and is returned to compressor 21 via electronic EPR valve 20 42. . Under operation of controller 40a, the EEPRV can use either discharge air off the evaporator 7 or the evaporator inlet temperature as a control variable. The following description relates to performance testing carried out 25 on a prototype installation to determine performance results at each stage and in each component of the passive defrost cycle. TEST RESULTS 30 The result of tests conducted show that the above described apparatus and system of the invention works to reduce case temperature due to partial 24 WO 2008/017130 PCT/AU2007/001139 refrigeration maintained during a defrost cycle. Throughout the specification a reference to partial refrigeration should be taken to mean a reference to a range of refrigeration temperatures which are maintained during a defrost cycle and which have their lowest point above 0 degrees C. 5 An experiment was conducted with an objective to establish impacts of the passive (partial refrigeration) defrost technology on: 1. The reduction of rise of cabinet air temperature during defrost, 10 2. Consequential improvement of product temperature and 3. Energy consumption of the cabinet There were two test scenarios selected for product class 3M1 as per AS1731-2003: 15 The first was 3M1-Si: Baseline tests with typical off cycle defrost with possible minimum defrost duration having least safety margin (critical defrost). This would allow one to say that at the specified room condition the least possible case air temperature rise has been obtained without having to apply any new technology 20 The second was 3M1-S2 continuous refrigeration passive defrost. Passive defrost applied with possible optimisation of defrost duration by combination of time and temperature termination. The test was conducted on a typical open front refrigeration cabinet having dimension 25 approximately 2m high, with five tiers of angled internal product shelving. The test cabinet was tested in a calorimetric test room configured to comply with Australian Standard AS1731-2003 with horizontal air draft and room temperature and humidity maintained within the specified tolerance. The test room has a horizontal cross 30 air draught velocity of 0.2m/s as specified by the AS 1731-2003. Table 1 below indicates the measurement parameters taken, the instruments used and set up. TABLE 1: Measurements and Test instruments: 25 WO 2008/017130 PCT/AU2007/001139 Measurements Instruments Setup and locations Product core CPC product Loaded in locations specified in AS 1731 temperature temperature 2003. Simulators were also fitted to get simulator probes coldest product temperature, which usually calibrated against occurs, at the rear of shelves just above the M-packs supplied air off the evaporator. by UNSW as per AS1731-2003 Cabinet CPC temperature o At the inlet of the air curtain jet interior air probe o Top of the product simulators at the 1 /4th temperature and % horizontal locations on each shelf particularly in the front.. o At the air return grille Refrigeration CPC temperature a Liquid temperature entering the cabinet temperatures probes o Coil inlet temperature u TX valve bulb location u Cabinet exit suction temperature Refrigeration CPC pressure a Condensing pressure pressures probes u Cabinet exit evaporating pressure Refrigerant Siemens Instrument Liquid flow inlet to the cabinet mass flow rate Mass 2100 Room Visala HMD70U Room control location in front of the test Humidity cabinet as per AS 1731-2003 5 The cabinet was loaded with test packs made according to the standard AS 1731-2003. 10 Table 2 shows the cabinet operational setup during various test scenarios: TABLE 2 Test Scenarios and corresponding setup Evaporating Defrost frequency Defrost Test room Temperature and duration Termination temperature *C (Number/day x min) Temperature *C and humidity Tests *C/RH% 3M1-S1 -6 12 x 13 (critical) Not applicable 25/60 3M1-S2 -6 12 x 10 +3 25/60 15 In the passive defrost scenario 3M 1-S2, continuous refrigeration passive defrost the EPR valve was set to a position to obtain evaporating temperature within +2*C to +3'C range during defrost mode. 26 WO 2008/017130 PCT/AU2007/001139 Experimental Results: Results are primarily presented here by graphic demonstration of key 5 performance parameters showing the underlying benefit of the passive defrost technology according to one application of the invention. Figure 7 shows a temperature / time graph of Cabinet Air Temperatures at Top of the Product Simulator for the baseline off cycle defrost. Figure 8 shows a temperature / time graph of Cabinet Air Temperatures at Top of the Product 10 Simulator with partial refrigeration during passive defrost cycle. With the partial refrigeration passive defrost activated, the peak air temperature inside the cabinet was nearly 3*C colder than the baseline off cycle defrost as seen in the above graphs. The product surface temperature is highly influenced 15 by the cabinet air temperature thus the peak surface temperature variation is almost proportional to the cabinet air temperature rise during defrost. A lowered cabinet air temperature with passive defrost has reduced the peak product surface temperature by nearly 3C. 20 In the tests air temperature was monitored at the air entry to the air curtain. Figure 9 shows a time / temperature graph of air curtain temperature at an entry to the cabinet in the baseline off cycle defrost. Figure 10 shows a time / temperature graph of air curtain temperature at an 25 entry to the cabinet in the new continuous refrigeration passive defrost system. As may be seen from a comparison of the graphs in figures 9 and 10, the peak of the air curtain entry air temperature was nearly 6C colder in continuous 30 refrigeration passive defrost method (figure 10) than the baseline off cycle defrost (figure 9). This air curtain peak temperature directly influences the product surface temperature particularly at the front of the shelf. 27 WO 2008/017130 PCT/AU2007/001139 The testing includs a determination of product core temperatures inside the cabinet. Figure 11 shows the results of product core temperature inside the cabinet for the baseline off cycle defrost and Figure 12 shows the results of product core temperature inside the cabinet for a partial refrigeration passive 5 defrost cycle. As may be seen from the result graph in figure 13, the cabinet peak product core temperature is nearly O.7 0 C colder in passive (partial refrigeration) defrost than 10 the baseline off cycle defrost. Also the temperature variation during defrost has reduced from on average 0.7*C to only 0.25*C with passive defrost. This indicates a definite advantage of the stability of product temperature over the time. This advantage of colder product temperature including even colder surface temperature is predominantly achieved by reduced air temperature 15 around the front of the cabinet. The front of a typical cabinet is much warmer than the rear resulting in product temperature variation (temperature band) between from the rear to the front. This higher variation reduces the ability to lower the supply air temperature by lowering the evaporating temperature to achieve a colder air temperature in the front because the rear products may 20 freeze up. The reduction of temperature variation from rear to front is apparent with the passive defrost. This enables this continuous refrigeration passive defrost system to be applied for achieving even lower temperature class, for example, product temperature class 3MO that is currently used in Europe. Class 3MO specify a product temperature range of -1PC to +4'C at room climate 25 condition of 25'C and 60% relative humidity. Figure 13 shows the results of mass flow during defrost for the off cycle defrost system. Figure 14 shows the results of mass flow during a partial refrigeration passive defrost cycle 30 28 WO 2008/017130 PCT/AU2007/001139 The mass flow at the start of the refrigeration cycle after the completion of defrost indicates that with passive defrost the peak flow has reduced and thus the pull down refrigeration capacity is less allowing use of smaller safety margin for selecting correct size of the plant. Mass flow during the defrost time 5 indicates the cooling being supplied to the cabinet at the raised evaporating temperature for the passive defrost system. Figure 15 shows cabinet evaporator temperature during the baseline off cycle defrost cycle. Figure 16 shows cabinet evaporator temperature during a passive 10 (partial refrigeration) defrost cycle. As may be seen from figure 16, for the baseline off cycle defrost with critical defrost duration, the coil temperature has peaked just about 0.5'C at the test 15 condition. Therefore, duration could not be shortened any more to allow any reduction of cabinet peak air temperature during defrost by any temperature termination. The defrost duration was 13 minutes. Whereas in the new passive defrost results as shown in figure 17, although the defrost duration was set to 10 minutes, it actually lasted for only 8 minutes despite the coil temperature rising 20 to nearly 1.7'C indicating that defrosting has done clearly. It can also be seen that with passive defrost, the coil temperature starting rising very sharply immediately from the time of defrost started at 20:00hr whereas in the baseline off cycle defrost the residual refrigeration continued for nearly 1.5-2.0 minutes due to refrigerant being sucked by the compressor when the liquid line is closed. 25 Figure 17 show the results of pressure /time chart corresponding to evaporating temperature for the baseline off cycle refrigeration cycle. Figure 18 shows the results of pressure / time chart corresponding to evaporating temperature for a passive (partial refrigeration) defrost cycle. 30 Figure 19 shows a schematic representation of temperature gradation through a typical coil cross section in a known off cycle defrost system 29 WO 2008/017130 PCT/AU2007/001139 compared to a temperature gradation through a continuous refrigeration passive defrost system. At the left of the representation is a legend showing temperature gradation contours which may be compared with corresponding markings on the coil sections. It is clear form the off cycle defrost 5 representations that the temperatures in the corresponding regions of the coils are higher during each stage - i.e. cooling, defrost and drip time compared to temperatures in the same regions under the continuous refrigeration passive defrost. The maximum temperature reached at drip time during the continuous refrigeration passive defrost is substantially lower that the highest maximum 10 temperature reached at the same stage of the corresponding off cycle defrost. TABLE 3 Energy Extraction Rating Comparison: 15 Scenario *100% cycle *Last 75% **First 10% run ***Defrost time rating, cycle time time rating, period rating, Qr, kW rating, Qr75, Qr1O, kW Qd, kW kW 3M1-S1: Baseline off cycle defrost method 4.918 4.749 6.313 0 3M 1-S2: New passive defrost method 4.849 4.772 5.125 1.89 Difference in % -1.40% 0.48% -18.82% * Terminologies defined in AS]731-2003 as follows Qr-Average heat extraction rating during 100% time of the nominated refrigeration 20 period, kW Qr75=Average heat extraction rating during the last 75% time of the nominated refrigeration period after the defrost, kW 25 ** Qr I O=average heat extraction rating at the first 10% of the refrigeration cycle after the defrost indicating peak cooling capacity required for quick pull down, kW. Qd= average heat extraction rating during the passive defrost period, indicating cooling supplied to keep products cool, kW 30 WO 2008/017130 PCT/AU2007/001139 Given the fact that the measurement uncertainty can be within 1%, the variation of Qr75 cannot carry a value of significance when comparing the two scenarios. 5 Conclusions arising from comparative test analysis of the baseline off cycle refrigeration defrost and partial refrigeration passive defrost are set out below: 1) Reduced product surface temperature during defrost as well as during 10 refrigeration 2) Cabinet product core temperature also improved particular at the front of the cabinet 3) The temperature variation between the rear and the front of the cabinet is reduced with the new passive defrost thus reducing the temperature band, which would 15 allow cooling down of the cabinet further to a lower product temperature class without freezing at the rear of the cabinet. 4) The cabinet product temperature variation over time is smoother in passive defrost than the baseline off cycle defrost. 5) Particular to the parallel rack system and individual self contained cabinets the 20 rack sizing safety margin can be reduced by nearly 20% due to reduction of pull down capacity required after a defrost. Therefore plant cost reduction is possible with the new passive defrost 6) There has not been any significant change in the steady state run time heat extraction rating (Qr75) therefore; there is no requirement of increased capacity 25 plant for multiplex refrigeration system. 7) It was also evident from the product temperature graph that one could increase evaporating temperature with this passive defrost technology to achieve the same level of product temperature as off cycle defrost but the product surface temperature would be still colder than off cycle defrost method. This would lead 30 to gain in energy efficiency. Estimate suggests that the heat extraction rate could reduce around 5% requiring smaller plant size and overall energy consumption reduction around 2-3% despite continuous refrigeration with this passive defrost technology. 31 WO 2008/017130 PCT/AU2007/001139 8) All above comparison results are based on the best and critical defrost of the off cycle defrost. In reality, critical defrost is not applied in real application therefore, this passive defrost technology will give even better performance and energy efficiency in real applications. 5 As can be seen from the above description, the apparatus described can be changed from a cooling mode in the cooling unit to a defrosting mode in the cooling unit simply by changing the flow setting of a solenoid and 10 defrost EPRV and without any disruption of the compressor operation. The apparatus of the invention utilizes some conventional components and the invention resides in the combination of these conventional components and the manner of use thereof. Hence, any 15 conventional materials and designs of the individual components are acceptable to the present invention so long as the materials and designs function in the manner described. The above describes a method and apparatus for defrosting the 20 evaporator coil of a refrigeration cabinet while at the same time allowing continuous at least partial refrigeration to elevate the temperature of the evaporator to thereby accelerate defrosting. This is facilitated according to one embodiment, by adding a by pass line to an outlet from the evaporator and including in that line a valve which is preset to retain for a predetermined 25 period refrigerant gas causing heat exchange from the hot compressed refrigerant gas to melt the frost on the evaporator coil. While the principles of the invention were hereinbefore described with reference to the drawings and preferred embodiment, the specific 30 illustrations of the invention are intended to only exemplify, rather than limit, 32 WO 2008/017130 PCT/AU2007/001139 the invention, and the invention is applicable to the extent described above, and as defined in the statements of invention and the various embodiments. It will be recognised by persons skilled in the art that numerous 5 variations and modifications may be made to the invention broadly described herein without departing from the overall spirit and scope of the invention. 10 15 20 25 30 33