AU2013101170A4 - Improved defrosting method and system - Google Patents

Abstract

Apparatus (100) for defrosting a refrigerated space (112) including a compressor (102) for compressing refrigerant, an evaporator (104) and a condenser (106) linked in a circuit that provides continuous flow of the refrigerant, a metering device (114) located between 5 the evaporator (104) and the condenser (106), valve means (108) located between the compressor (102) and the evaporator (104) for reversing the direction of the refrigerant, wherein during a reverse cycle defrosting stage of the refrigerated space (112), heat is supplied to the evaporator (104) for a predetermined period of time. lg15124 134 144 131*. 106 --- q Fi G. i

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT ORIGINAL Applicant(s): GEMINI REFRIGERATION PRODUCTS PTY LTD & WALKER CELANO REFRIGERATION PTY LTD Actual Inventor(s): Ross Oxley and Lorenzo Mingerulli Address for Service: PATENT ATTORNEY SERVICES 26 Ellingworth Parade Box Hill Victoria 3128 Australia Title: IMPROVED DEFROSTING METHOD AND SYSTEM Associated Provisional Applications: No(s).: 2012903722 filed on 29 August 2012 The following statement is a full description of this invention, including the best method of performing it known to me/us:- IMPROVED DEFROSTING METHOD AND SYSTEM Field of the Invention This invention relates to a method and system of defrosting the cooling apparatus 5 that services a refrigerated space. Background of the Invention Refrigeration systems at some stage will need to be defrosted in order to maintain the efficiency of the evaporator which cools the refrigerated space. Frosting on the to evaporator is caused by water vapour existing in the refrigerated space. Such warm moist air enters the refrigerated space when doors to the space are opened and closed multiple times or is released from stored foodstuffs. If the temperature in the refrigerated space is being maintained at close to 0*C, then the evaporator cooling the refrigerated space operates at well below 0*C, This is due to the fact that in order to achieve a cooling 15 effect in the refrigerated space there must be a temperature difference (TD) or operating temperature difference (OTD) between the space and the evaporator. The temperature difference is often quoted in degrees Kelvin (*K). The TD results in the evaporator, which is a heat exchanger, having a temperature of below 0"C which causes condensation of the water vapour from the air inside the refrigerated space to form as frost on the cold 20 surface of the evaporator, This process of moisture condensation reduces the amount of water vapour in the air as well as potentially from any stored products within the space. Therefore the relative humidity within the space is reduced by this process and is called dehumidification. Frost formation will be continuous as long as the refrigeration equipment or plant 25 is operating. The rate at which the frost accumulates depends upon how much warm air 2 is allowed to enter the sealed space (that is how many times the door is opened and closed) or the moisture content of the stored products, and the TD. A larger or wider TD means that there is a much lower evaporator temperature than that of the sealed space and therefore a greater rate of dehumidification and a greater rate of frost accumulation will 5 occur. As the thickness of the frost increases it acts more and more like an insulator as it continues to accumulate on the surface of the evaporator. The heat transfer that takes place between the refrigerated space and the evaporator surface is slowed which in turn decreases the efficiency. Thus a defrosting process is needed in order to remove the 10 excess frost that has been built up on the surface of the evaporator. In a sealed space where the temperature is held above 0*C, the defrosting process is relatively straightforward. If the refrigeration unit is on a cycle to turn off for a preset time period, then a defrost process will occur over time as there is sufficient heat energy stored in the air inside the sealed space in order to melt the frost that has accumulated on is the evaporator. Most modern refrigeration evaporators are force ventilated with a fan in order to circulate air over the evaporator during defrosting. This decreases the time that elapses for the defrost period, which in turn improves efficiency. However where the refrigerated space is operating at a temperature lower than 0 0 C, difficulties start to occur. If a plant, for example, is operating with a refrigerated 20 space temperature of -20*C, set for freezing of food, and there is a temperature difference of 6* K, then the surface of the evaporator will be approximately -26*C. Due to the low temperature in the sealed space, frost accumulates on the surface of the evaporator at a higher rate than it would occur within a plant running above 0*C. Therefore in order to defrost the evaporator the surface temperature is required to be raised above 0*C. In a 25 sealed space of -20*C, there is no available heat in the air circulating in the space to 3 assist in the defrosting process. Therefore in order to achieve a defrost cycle, heat must be introduced to the surface of the evaporator. A number of existing defrost technologies are able to overcome frost accumulation. These include 'off-cycle defrost' where the refrigeration unit is turned off 5 and all stored product is moved from the sealed space which is then allowed to warm until the defrost is complete, This is a very slow process which is labour intensive and is generally limited to small domestic applications. A further technology is "water defrost" which involves warm water being pumped into the sealed space and circulated over the surface of the evaporator until the defrost process is complete. This requires complex 10 and expensive infrastructure in order to achieve an effective operation and is limited to very large commercial and industrial applications. The most common method is 'electric defrost' where electric elements are located against the surface of the evaporator in order to heat the surface of the evaporator. In the electric defrost method, the average cycle time for defrost is 25-30 minutes with between three and four cycles occurring each day. 15 However this method requires a large amount of energy to be inputted to the sealed space in order to complete defrosting. The heat dissipated from the electric elements moves slowly so that it takes a long time to warm the entire surface of the evaporator. This is a very inefficient and expensive process in terms of energy and dissipation. A further method involves 'hot gas defrost' which is where warm refrigerant gas 20 from a discharge outlet of the compressor, that is between the compressor and condenser, is piped directly into the evaporator through a control valve. This results in short cycling of the refrigeration process. The compressor must continue to operate during the defrost cycle and the gas pipe from the discharge outlet of the compressor to the evaporator can be directed through any necessary drip trays in order to warm these as well during the 25 defrost cycle. However a rapid loss of available heat within the refrigerant occurs due to 4 the short cycling effect which means that the defrosting is often difficult to achieve within the allotted timeframes. This is particularly the case when external ambient temperatures are low, In other words the defrosting will not work well when the external ambient temperature is low as the input of heat from compression alone is insufficient to 5 achieve effective defrosting on single compressor and evaporator plants. A last known method is 'reverse cycle defrost' which is applied in the air conditioning industry, In such reverse cycle systems, frosting occurs over the evaporator surface during heating cycles of the sealed space, The existing reverse cycle system can be briefly set to having its sealed space cooled in order to effect a defrost cycle, 10 However space heating in this format is limited as reverse cycle heat pumping becomes inefficient in conditions where the temperature of the evaporator falls below 0*C for lengthy periods. This is because during the heating cycle, the heat pumps shift the energy stored in the air around the evaporator into the sealed space. Therefore if the temperature of the evaporator air is already very low then there is much less heat energy 15 to be shifted. In the application of reverse cycle defrost in the freezing temperature range, due to the operating temperature range being much lower, many problems are created when attempting to balance a conventional reverse cycle system operation during the defrosting cycle. The present invention seeks to overcome the abovementioned problems by 20 providing a defrosting process which improves energy efficiency and the time required to complete the defrosting cycle in very low temperatures, The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art in Australia. 25 5 Summary of the Invention According to a first aspect of the invention, there is provided apparatus for defrosting a refrigerated space including: a compressor for compressing refrigerant, an evaporator and a condenser linked in a 5 circuit that provides continuous flow of the refrigerant; a defrost flow metering device located between the evaporator and the condenser; valve means located between the compressor and the evaporator for reversing the direction of the refrigerant; wherein during a reverse cycle defrosting stage of the refrigerated space, heat is 10 supplied to the evaporator and the flow rate of refrigerant around the circuit is optimised by the pumping capacity of the compressor and calibration and optimisation of the defrost flow metering device. According to a second aspect of the invention, there is provided a method of defrosting a refrigerated space including: 15 providing a compressor for compressing refrigerant, an evaporator and a condenser in a circuit that enables continuous flow of the refrigerant; locating a defrost flow metering device between the evaporator and the condenser; providing valve means between the compressor and the evaporator for reversing the direction of the refrigerant; and 20 supplying heat to the evaporator during a reverse cycle defrosting stage of the refrigerated space; optimising the flow rate of the refrigerant around the circuit by altering the pumping capacity of the compressor and by calibrating and optimising of the defrost flow metering device. The metering device is preferably a calibrated and optimised high refrigerant flow 25 metering device, designed to match the capacity of the compressor at its maximum 6 optimised displacement. Each plant will require attention to the appropriate optimised conditions. The apparatus may further include means for heating trays associated with the refrigerated space, said means located between the valve means and the evaporator. The means for heating the trays may be tubing that receives heat from the refrigerant 5 after the refrigerant has been compressed to heat the trays prior to heating the evaporator. The heating of the trays can occur during the reverse cycle defrost stage where the refrigerant is forced to pass through a one-way valve means. The reverse cycle defrost stage can be applied for a predetermined period of time or to cease at a preset evaporator surface temperature. 10 During a normal or standard cooling cycle, in which the refrigerant moves around the circuit in a direction opposite to that during the reverse cycle defrost stage, the defrost flow metering device and the means for heating the trays are preferably excluded from the circuit. One-way valves may be used to exclude the defrost flow metering device and the means for heating the trays from the circuit through which the refrigerant travels. is The defrost flow metering device is preferably a length of tube or a valve and the calibration of the defrost flow metering device involves matching the diameter and the length of the tube or capacity of the valve to provide said optimisation. The pumping capacity of the compressor is preferably set to near the optimised output of the compressor during the defrost cycle phase. A different metering device is active in the 20 circuit during the normal cooling cycle (with reference to Figure 2). According to a third aspect of the invention, there is provided apparatus for defrosting a refrigerated space including: a compressor for compressing refrigerant, an evaporator and a condenser linked in a circuit that provides continuous flow of the refrigerant; 25 a defrost flow metering device located between the evaporator and the condenser; 7 valve means located between the compressor and the evaporator for reversing the direction of the refrigerant; wherein during a reverse cycle defrosting stage of the refrigerated space, heat is supplied to the evaporator for a predetermined period of time or to cease at a preset surface 5 temperature of the evaporator. Brief Description of the Drawings A preferred embodiment of the invention will hereinafter be described, by way of example only, with reference to the drawings in which: 10 Figure 1 is a block diagram of a refrigeration circuit in a reverse cycle mode during defrosting; and Figure 2 is a block diagram of the refrigeration circuit shown in Figure 1 in a reverse cycle (normal) mode during cooling. 15 Detailed Description of the Preferred Embodiments In a standard refrigeration process there are essentially four components involved in the continuous circulation of refrigerant through the closed circuit. External energy is provided by the rotation of a compressor or a piston pump. Refrigerant flows to a condenser and then the refrigerant flows towards an evaporator, after passing through a 20 metering device usually in the form of a valve or a capillary tube. After exiting the evaporator, the refrigerant returns to the compressor, Refrigerant enters the compressor in a fully vaporised form which is then compressed by the compressor to a very high pressure and then discharged to the condenser, As the refrigerant travels through the condenser it changes state from a vapour to a liquid, in other words it condenses. The 25 condenser is located in an open environment and, as the condensation takes place heat is 8 rejected from the refrigerant and released into the open environment, assuming it is within the correct operating temperature range, The refrigerant exits the condenser and is now entirely in liquid form still under high pressure. The metering device, between the condenser and the evaporator allows the 5 refrigerant to enter the evaporator in its liquid form at a controlled rate of flow. As the refrigerant travels through the evaporator it changes from liquid to vapour and is now at a much lower pressure. The evaporator is contained within a sealed space and as evaporation takes place, heat is drawn away from the space and absorbed by the refrigerant resulting in the temperature in the sealed space falling. The operation of the 10 compressor as a piston pump draws refrigerant out of the evaporator and back into the compressor to repeat the cycle. In a reverse cycle vapour compression system, a reversing valve is used to control the flow of refrigerant in one direction or the opposite direction as it travels around the circuit. The evaporator and condenser swap or interchange functions when the reverse 15 cycle operates, Therefore in a heating cycle the evaporator becomes the condenser to reject heat and the sealed space results in a temperature rise. In the cooling cycle the evaporator remains in the sealed space to cool the sealed space in the normal manner. However as mentioned previously, conventional reverse cycle systems are generally limited to applications of temperatures down to only about -10*C (evaporator 20 temperature). Referring to Figure 1 there is shown a vapour compression system 100 which is in a reverse cycle defrost process. The system 100 includes a compressor 102, an evaporator 104 which is in a sealed space 112, such as a refrigerated space, and a condenser 106. Located between the compressor 102 and the evaporator 104 is a 25 reversing valve 108 that changes the direction of flow of the refrigerant between the 9 defrost and cooling phases of the cycle. An ancillary circuit I10 is located within the sealed space 112 adjacent the evaporator 104 and is linked to the reversing valve 108, through suction line cooling conduit 132 and conduit 130. Between the evaporator 104 and the condenser in conduit 134, there is located a one-way valve 116 and a high 5 refrigerant defrost flow metering device 114 through which the refrigerant flows. During a defrosting phase, when heat is directed into the sealed space 112 in order to heat the evaporator 104, refrigerant in the circuit that enters the compressor 102 through conduit 142 is then compressed and displaced at a rate which is optimised to match the maximum desired pumping capacity or displacement of the compressor 102, 10 The metering device 114 is also optimised to match the maximum desired flow. The metering device can be a valve or a capillary tube. The user will need to refer to relevant engineering tables, such as those supplied by Actrol in order to determine the particular dimensions, such as length and diameter, of the tubing to match the desired pumping capacity of the compressor. The resulting compressed high pressure high-temperature is refrigerant is outputted to conduit 126 which carries with it a large amount of heat, made up of heat from compression as well heat carried in the refrigerant. The compressed gas then enters the reversing valve 108 and is directed through port 144 into conduit 128. The gas is then forced through conduit 130 at junction 131 as one-way valve 122 restricts any flow of the gas through conduit 132, The refrigerant is directed through the one-way 20 valve 124 on its way to ancillary circuit 110 (otherwise known as a means for drip heating trays), Still in its hot compressed form, the gas exits the one-way valve 124 and enters the ancillary circuit 110 which is essentially a metal tube designed to heat the drip trays located at the bottom of the sealed space 112 and at the bottom of the evaporator 104. By heating the drip trays through the ancillary circuit 110, with the size of the metal 25 tubing being proportionate to the pumping capacity of compressor 102, it is possible in a 10 short period of time to quickly heat the drip trays first and still provide hot refrigerant to the tubing inside the evaporator (heat exchanger) 104 to heat all of the surfaces of the heat exchanger 104 and therefore perform a relatively quick defrost. Thus the rate of refrigerant gas which is displaced by the compressor 102 is much higher during defrost 5 as opposed to the cooling phase under normal operating conditions. The high displacement optimises the compressor's ability to create superheated refrigerant by maximising the flow of refrigerant at a high temperature as it enters the evaporator 104 and therefore maximises the heat released within the evaporator 104 to then undertake the defrosting process. 10 As the system 100 is still performing a refrigeration process, further energy is drawn from ambient air and converted into heat to provide further heat to the evaporator 104 in the sealed space 112, The condensed liquid refrigerant goes through conduit 134 at junction 133 as directed by one-way valve 116 and passes through a high refrigerant flow metering device 114. This is essentially a piece of tubing or valve which has a 15 diameter and length (or capacity in the case of a valve) chosen in accordance with a particular application of the defrost cycle and proportionate to the pumping capacity or displacement of gas by the compressor 102 at a point which is close to the optimised output of the compressor 102 during this application, Therefore a suitably optimised rate of flow of refrigerant is available using the metering device 114. As the superheated gas 20 flows through the evaporator 104, the gas is converted into liquid form but still at high pressure. The evaporator 104 essentially acts as a condenser in this modified reverse heating cycle. It then enters conduit 138 at junction 135 as a liquid at a much lower temperature and enters the condenser 106. The condenser 106 now acts as an evaporator which evaporates the liquid resulting in reduced pressure and all the liquid entering the 25 condenser is evaporated. The vapour flowing through the conduit 140, at the output to

II

the condenser 106, enters the reversing valve 108 at port 148, Due to the orientation of the reversing valve 108, the refrigerant is then redirected through middle port 146 of reversing valve 108 and through conduit 142 which leads to the input of the compressor 102 to start the cycle again when required. A defrost duration can vary, however tests 5 have confirmed that a 6 minute duration is achievable, This is a great improvement in efficiency and time, compared to traditional electric defrost times of 25-30 minutes, The flow of fluid around the entire circuit (shown by the arrows in Figure 1) can be therefore be described as a hot or heated fluid flowing through conduits 126, the conduit joining ports 145 and 144, conduits 128 and 130, one-way valve 116, ancillary 10 circuit 110, evaporator 104, conduit 134, one-way valve 116 and as far as the entry into the metering device 114. Thereafter, the fluid is cooled and flowing as such from the exit of the metering device 114, through conduit 138, condenser 106, conduit 140, through ports 148 and 146, through conduit 142 back to compressor 102 where the cycle then repeats as necessary. In this cycle, conduits 132 and 136 are bypassed or not used. In 15 Figure 1, there are labels "H" and "C" distributed around the circuit to indicate if the fluid passing through those conduits or components is hot/heated or cold/cooled, Referring to Figure 2 the apparatus 100 used as a vapour compression system shows the reverse cycle during a cooling phase (normal cooling cycle). The refrigerant moves in the opposite direction compared to the circuit shown in Figure 1 which is 20 possible through the reversing valve 108. This is done by opening port 148 through the slider piston 143 of the valve 108 and joining conduits 142 and 128 via ports 144 and 146 as well as joining conduits 126 and 140 through ports 145 and 148. Cool refrigerant gas enters the compressor 102 through conduit 142 and is then compressed by the compressor 102 which results in increased heat and pressure in the refrigerant exiting the 25 compressor 102 through conduit 126. The refrigerant gas now under pressure flows 12 through ports 145 and 148 of the reversing valve 108 and enters the condenser 106 through conduit 140. The condenser 106, being in an open environment, changes the state of the refrigerant from a vapour to a liquid and rejects heat at the same time. The liquid refrigerant exits the condenser 106 through conduit 138 as liquid under high 5 pressure. With the particular arrangement of the high flow metering device 114 and one way valve 116, it is not possible for the refrigerant to enter conduit 134 but instead is directed to go through conduit 136 after entering the inline filter 118 and a further metering device 120. In the evaporator 104 the refrigerant absorbs heat drawn from the sealed space 112 as it changes from a liquid to a vapour, This results in a much lower lo pressure in the refrigerant. The refrigerant exits the evaporator 104 and is directed through one-way valve 122 and conduit 132 and all of the refrigerant is evaporated by the evaporator 104 at this stage. It is not possible for the refrigerant to go through the ancillary circuit 110 (due to the existence of the single directional flow valve 124). Thus the refrigerant flows through the directional flow valve 122 and enters conduit 128 and 15 port 144 of the reversing valve 108. Due to the orientation of the reversing valve 108 the cooler gas is directed through port 146 in the reversing valve 108 into conduit 142 to enter the compressor 102 to start the cycle again. The flow of fluid around the entire circuit in this phase (shown by the arrows in Figure 2) can be therefore be described as a hot or heated fluid flowing through conduit 20 126, the conduit joining ports 145 and 148 of reversing valve 108, conduit 140, condenser 106, conduits 138 and 136 up to the entry of metering device 120. Thereafter, the fluid is cold or cooled and flowing as such from the exit of the metering device 120, through evaporator 104, one-way valve 122, conduits 132 and 128, through ports 144 and 146 and through conduit 142 back to compressor 102 where the cycle then repeats as 25 necessary. In this cycle, conduits 130 and 134 as well as one-way valves 124 and 116 13 and metering device 114 are bypassed or not used. In Figure 2, there are labels "H" and "C" distributed around the circuit to indicate if the fluid passing through those conduits or components is hot/heated or cold/cooled. Thus a complete cooling cycle is still maintained so that refrigerant is continually 5 flowing around the system. The bypass high flow metering device 114 does not require an inline filter and therefore the defrost cycle shown in Figure 1 bypasses inline filter 118 as well as cooling metering device 120. Provided below are some selection examples and comparisons of energy consumption between existing systems and the present invention, 10 Basis of engineering selections and operating conditions - Equipment condensing temperature (normal cooling cycle) is +55*C; - Equipment evaporating conditions (normal cooling cycle) is -24*C; - Refrigerated space operating temperature is - 18*C; - Equipment design run time of 18 hours in 24 hours. This is the standard 15 equipment run time factor used for selecting equipment in the freezing temperature range. - Number of daily defrost cycles is 4. Programmed duration of defrost cycles using electric defrost method in 30 minutes with temperature sensed early cycle termination option. (Experience shows that 30 minutes is usually 20 required to complete a defrost cycle) - Programmed duration of defrost cycles using reverse cycle defrost method is 10 minutes with temperature sensed early cycle termination option (experience shows average 6 minute cycles). - Equipment design evaporating condition during defrosting cycle of - 10*C 25 (reverse cycle defrost). 14 Selection example of standard equipment utilising electric defrost mode. Selection example 1: Electric defrost option. A refrigerated space has been engineered to require a cooling capacity of 1200 5 watts at * -18*C space temperature 9 -24 0 C evaporator temperature (TD=6K) 0 55*C condensing temperature * 18 hours in 24 run time. 10 A compressor has been selected from a manufacturer's tables to meet this duty.' Such a compressor is the L'Unite Hermetique compressor CAJ 2446Z rated at 1210 watts at these conditions on refrigerant gas R404 A. (Actrol parts catalogue 2005 page no 206.) An electric defrost evaporator has been selected from manufacturer's tables to 15 match this compressor. It is the Bufalo Trident BRLE series low temperature electric defrost evaporator model BRLE 12 which has a design capacity at - 24*C evaporator temperature of 1223 watts at TD=6K (Actrol parts catalogue 2005 page 250.) These two components are therefore considered to be a standard good practice design match, 20 The Buffalo Trident evaporator has electric defrosting elements installed with a consumption capacity of 1360 watts/hour. If this system requires 4 defrost cycles per day of 30 minutes duration then the daily energy input into the refrigerated space to achieve defrost will be 4.x1360 wafts xO.5 (1/2 hr) or a total of 2720 watts per day. (680 watts/cycle). 15 The refrigeration plant will then have to run for 2.248 hours @ 1210 watts displacement per hour to remove this heat and restore the refrigerated space to its correct holding conditions, 2.248h x 1210W/h = 2720W. 5 Therefore the total energy consumption of the four defrost cycles per day including post defrost space temperature recovery is 2720 watts (defrost input) + 2720 watts (equipment recovery) = 5440 watts. This equates to 4.49 hours of compressor running time per day. Heat infiltration into the sealed space during defrost is a factor but has not been considered in this calculation, 10 Selection example of standard equipment utilising super-efficient reverse cycle mode: Selection example 2: Reverse cycle defrost option. A refrigerated space has been engineered to require a cooling capacity of 1200 watts at: is -1 8 0 C space temperature; * -24*C evaporator temperature (TD=6K); * 55"C condensing temperature; * 18 hours in 24 run time. A compressor has been selected from manufacturers" tables to meet this duty. 20 That compressor is the L'Unite Hermetique compressor CAJ 2446Z rated at 1210 watts at these conditions on refrigerant gas R 404 A. The reverse cycle defrost condition will see the compressor delivering a governed capacity of approximately 2290 watts per/hour at -10 SST (Saturated Suction Temperature) (which is this unit's design maximum), (Actrol parts catalogue 2005 page no 206). 16 An electric defrost evaporator has been selected from manufacturers' tables to match this compressor. However, no electric elements will be utilised. The selected evaporator is the Buffalo Trident BRLE series low temperature electric defrost evaporator model BRLE 12 which has a design capacity at -24*C 5 evaporator temperature of 1223 watts at TD=6K (Actrol parts catalogue 2005 page 250). These two components are therefore considered to be a standard good practice design match, The Buffalo Trident evaporator has electric defrosting elements installed with a consumption capacity of 1360 watts/hour. These will NOT be utilised. The system will 10 operate on reverse cycle defrost and the evaporator will be so modified as to suit this application without loss of capacity. If this system requires 4 defrost cycles per day of 6 minutes duration then the daily energy input into the refrigerated space to achieve defrost will be 4 x 2290 watts/hr x factor THR 1.32 x 0,10 hr (6 minutes = 0.10 hours) or a total of 1209 watts per day. 15 (302 watts/cycle). The 2290 watts/hr is calculated with the compressor capacity at -10 SST. The refrigeration plant will then have to run for 1.0 hour at 1210 watts displacement per/hour to remove this heat and restore the refrigerated space to its correct holding conditions. 20 1.0h x 1210 W/h = 1210W, Therefore the total energy consumption of the four defrost cycles per day including post defrost space temperature recovery is 1209 watts (defrost input) + 1210 watts (equipment recovery) ==2419 watts. This equates to 2.0 hours of compressor running time per day. Heat infiltration into the sealed space during 25 defrost is a factor but has not been considered in this calculation. 17 It is accurate to state that each refrigeration plant has greatly variable load conditions related to usage, installation and environmental factors, Therefore these calculations are to be used as a guide only and will vary in actual application environments as some systems will be more efficient than others. 5 The THR (TOTAL HEAT OF REJECTION) is factor applied to enable the heat of compression input component of the displaced heat to be calculated into the total heat dissipated load on a condenser and varies with the application conditions of the plant. The present invention overcomes the problem of defrosting at particularly low 10 temperatures where the cooling must cease and heat must be introduced to effect the defrosting and the cooling process can resume and continue until all of the energy introduced during the defrosting stage has been removed, returning the sealed space to its original temperature condition. The present invention provides a shorter and more energy efficient defrost cycle that results in reduced energy input in order to provide a 15 defrost and a reduced plant running time after the cycle's completion which is required to return the plant to its original sealed space holding temperature. Therefore reducing the time to achieve the defrost also means that the amount of energy consumed by the process will be reduced over both the defrost phase and the defrost recovery phase. This is a large improvement on existing electric defrost systems which require an average 20 cycle time of 25-30 minutes per defrost and three or four cycles per day. The present invention uses a compressor at close to its maximum displacement to input close to its total possible energy output in order to achieve a defrost effect far more efficient than the electric element heating. I8

Claims (18)

1. Apparatus for defrosting a refrigerated space including: a compressor for compressing refrigerant, an evaporator and a condenser linked in a 5 circuit that provides continuous flow of the refrigerant; a defrost flow metering device located between the evaporator and the condenser; valve means located between the compressor and the evaporator for reversing the direction of the refrigerant; wherein during a reverse cycle defrosting stage of the refrigerated space, heat is 10 supplied to the evaporator and the flow rate of refrigerant around the circuit is optimised by the pumping capacity of the compressor and calibration and optimisation of the defrost flow metering device.

2. Apparatus according to claim I wherein the metering device is a calibrated and 15 optimised high refrigerant flow metering device designed to match the capacity of the compressor at its maximum optimised displacement.

3. Apparatus according to claim 1 or claim 2 further including means for heating trays associated with the refrigerated space, said means located between the valve means 20 and the evaporator.

4, Apparatus according to claim 3 wherein said means for heating the trays is tubing that receives heat from the refrigerant after the refrigerant has been compressed to heat the trays prior to heating the evaporator. 25 19

5. Apparatus according to claim 4 wherein the heating of the trays occurs during the reverse cycle defrost stage where the refrigerant passes through a one-way valve means.

6. Apparatus according to any one of the previous claims wherein the reverse cycle 5 defrost stage is applied for a predetermined period of time or to cease at a preset. surface temperature of the evaporator.

7. Apparatus according to any one of claims 2 to 6 wherein during a normal or standard cooling cycle, in which the refrigerant moves around the circuit in a direction to opposite to that during the reverse cycle defrost stage, the defrost flow metering device and the means for heating the trays are excluded from the circuit,

8. Apparatus according to claim 6 wherein one-way valves are used to exclude the defrost flow metering device and the means for heating the trays from the circuit through 15 which the refrigerant travels.

9. Apparatus according to any one of the preceding claims wherein the defrost flow metering device is a length of tube or a valve and the calibration of the defrost flow metering device involves matching the diameter and the length of the tube or capacity of 20 the valve to provide said optimisation.

10. Apparatus according to any one of the preceding claims wherein the pumping capacity of the compressor is set to near the optimised output of the compressor during the defrost cycle phase. 25 20

11, A method of defrosting a refrigerated space including: providing a compressor for compressing refrigerant, an evaporator and a condenser in a circuit that enables continuous flow of the refrigerant; locating a defrost flow metering device between the evaporator and the condenser; 5 providing valve means between the compressor and the evaporator for reversing the direction of the refrigerant; and supplying heat to the evaporator during a reverse cycle defrosting stage of the refrigerated space; optimising the flow rate of the refrigerant around the circuit by altering the pumping capacity of the compressor and by calibrating and optimising of 10 the defrost flov metering device.

12. A method according to claim 11 wherein the defrost flow metering device is a high refrigerant flow metering device, 15

13. A method according to claim 11 or claim 12 further including using the heated and compressed refrigerant to heat trays associated with the refrigerated space prior to heating the evaporator.

14. A method according to any one of claims 11 to 13 further including applying the 20 reverse cycle defrost stage for a predetermined period of time or to cease at a preset surface temperature of the evaporator.

15, A method according to any one of claims 11 to 14 further including, during a normal or standard cooling cycle in which the refrigerant moves around the circuit in a 21 direction opposite to that during the reverse cycle defrost stage, excluding the heating of the trays and the defrost flow metering means from the circuit.

16. A method according to any one of claims 11 to 15 wherein the defrost flow 5 metering device is a length of tube or a valve and the calibration of the defrost flow metering device involves matching the diameter and the length of the tube or the capacity of the valve to provide said optimisation of the rate of flow of refrigerant.

17. A method according to any one of claims 11 to 16 further including setting the 10 pumping capacity of the compressor to near the optimised output of the compressor during the defrost cycle through calibration of the defrost flow metering device.

18. Apparatus for defrosting a refrigerated space including: a compressor for compressing refrigerant, an evaporator and a condenser linked in a is circuit that provides continuous flow of the refrigerant; a defrost flow metering device located between the evaporator and the condenser; valve means located between the compressor and the evaporator for reversing the direction of the refrigerant; wherein during a reverse cycle defrosting stage of the refrigerated space, heat is 20 supplied to the evaporator for a predetermined period of time or to cease at a preset surface temperature of the evaporator. 22