Abstract:
A defrost refrigeration system of the type having a main refrigeration circuit operating a refrigeration cycle. The defrost refrigeration system comprises a first line extending from the first compressor to the evaporator stage and is adapted to receive a portion of discharged low-pressure refrigerant from the first compressor. Valves are provided for stopping a suction of cooling refrigerant in an evaporator of the evaporator stage and for directing a flow of defrost refrigerant to release heat to defrost the evaporator. A second line is provided for directing the refrigerant having released heat to the expansion stage of the refrigeration cycle. A pressure reducing device is optionally positioned downstream of the condensing stage for adjusting a pressure of the refrigerant in the high-pressure liquid state mixing with the defrost refrigerant having released heat.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This patent application is a continuation-in-part of U.S. patent application Ser. No. 10/863,495, filed on Jun. 9, 2004, by the present Applicant, which is a divisional of U.S. patent application Ser. No. 10/189,462, filed on Jul. 8, 2002, now U.S. Pat. No. 6,775,993. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to a high-speed evaporator defrost system for defrosting refrigeration coils of evaporators in a short period of time without having to increase compressor head pressure.  
       BACKGROUND ART  
       [0003]     In refrigeration systems found in the food industry to refrigerate fresh and frozen foods, it is necessary to defrost the refrigeration coils of the evaporators periodically, as the refrigeration systems working below the freezing point of water are gradually covered by a layer of frost which reduces the efficiency of evaporators. The evaporators become clogged up by the build-up of ice thereon during the refrigeration cycle, whereby the passage of air maintaining the foodstuff refrigerated is obstructed. Exposing foodstuff to warm temperatures during long defrost cycles may have adverse effects on their freshness and quality.  
         [0004]     One method known in the prior art for defrosting refrigeration coils uses an air defrost method wherein fans blow warm air against the clogged-up refrigeration coils while refrigerant supply is momentarily stopped from circulating through the coils. The resulting defrost cycles may last up to about 40 minutes, thereby possibly fouling the foodstuff.  
         [0005]     In another known method, gas is taken from the top of the reservoir of refrigerant at a temperature ranging from 80° F. to 90° F. and is passed through the refrigeration coils, whereby the latent heat of the gas is used to defrost the refrigeration coils. This also results in a fairly lengthy defrost cycle.  
         [0006]     U.S. Pat. No. 5,673,567, issued on Oct. 7, 1997 to the present inventor, discloses a system wherein hot gas from the compressor discharge line is fed to the refrigerant coil by a valve circuit and back into the liquid manifold to mix with the refrigerant liquid. This method of defrost usually takes about 12 minutes for defrosting evaporators associated with open display cases and about 22 minutes for defrosting frozen food enclosures. The compressors are affected by hot gas coming back through the suction header, thereby causing the compressors to overheat. Furthermore, the energy costs increases with the compressor head pressure increase.  
         [0007]     U.S. Pat. No. 6,089,033, published on Jul. 18, 2000 to the present inventor, introduces an evaporator defrost system operating at high speed (e.g., 1 to 2 minutes for refrigerated display cases, 4 to 6 minutes for frozen food enclosures) comprising a defrost conduit circuit connected to the discharge line of the compressors and back to the suction header through an auxiliary reservoir capable of storing the entire refrigerant load of the refrigeration system. The auxiliary reservoir is at low pressure and is automatically flushed into the main reservoir when liquid refrigerant accumulates to a predetermined level. The pressure difference between the low-pressure auxiliary reservoir and the typical high pressure of the discharge of the compressor creates a rapid flow of hot gas through the evaporator coils, thereby ensuring a quick defrost of the refrigeration coils. Furthermore, the suction header is fed with low-pressure gas to prevent the adverse effects of hot gas and high head pressure on the compressors.  
       SUMMARY OF INVENTION  
       [0008]     It is a feature of the present invention to provide a high-speed defrost refrigeration system that operates a defrost of evaporators at low pressure.  
         [0009]     It is a further feature of the present invention to provide a high-speed defrost refrigeration system having a compressor dedicated to defrost cycles.  
         [0010]     It is a still further feature of the present invention to provide a high-speed defrost refrigeration system having a low-pressure defrost loop.  
         [0011]     It is a still further feature of the present invention to provide a method for defrosting at high-speed refrigeration systems with low-pressure in the evaporators.  
         [0012]     It is a still further feature of the present invention to provide a method for operating a high-speed defrost refrigeration system having a compressor dedicated to defrost cycles.  
         [0013]     According to the above features, from a broad aspect, the present invention provides a defrost refrigeration system of the type having a main refrigeration circuit operating a refrigeration cycle, wherein a refrigerant goes through at least a compressing stage having at least a first and a second compressor, wherein said refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein said refrigerant in said high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein said refrigerant in said high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein said refrigerant in said first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to said compressing stage, said defrost refrigeration system comprising a first line extending from said first compressor to the evaporator stage and adapted to receive at least a portion of discharged refrigerant from said first compressor, a valve for stopping a suction by the compressing stage of said refrigerant in said first low-pressure liquid state in at least one evaporator of the evaporator stage and directing a flow of said discharged refrigerant to release heat to defrost the at least one evaporator and thereby changing phase at least partially to a second low-pressure liquid state, a second line for directing said refrigerant having released heat to the expansion stage of the refrigeration cycle, and a pressure reducing device downstream of the condensing stage for adjusting a pressure of the refrigerant in the high-pressure liquid state mixing with said refrigerant having released heat.  
         [0014]     Further in accordance with the present invention, there is provided a method for defrosting evaporators in a refrigeration system of the type having a cooling refrigerant circulating sequentially between a compression stage, a condensing stage, an expansion stage and an evaporation stage to then return to the compression stage, comprising the steps of: i) stopping a suction of the cooling refrigerant in a first evaporator of the evaporation stage; ii) directing defrost refrigerant from the compression stage to the first evaporator so as to defrost the first evaporator; iii) directing the defrost refrigerant from the first evaporator upstream of the expansion stage; and iv) mixing the cooling refrigerant from the condensing stage with the defrost refrigerant by controlling a cooling refrigerant pressure downstream of the condensing stage; whereby a second evaporator of the evaporation stage is cooled with the mixture of cooling refrigerant from the condensing stage with the defrost refrigerant.  
         [0015]     Still further in accordance with the present invention, there is provided a method for installing a defrost system in a refrigeration system of the type having a cooling refrigerant circulating sequentially between a compression stage, a condensing stage, an expansion stage and an evaporation stage to then return to the compression stage, comprising the steps of providing a valve to stop a suction of cooling refrigerant in at least a first evaporator of the evaporation stage, positioning a first line feeding the first evaporator with cooling refrigerant from the compression stage, positioning a second line between the first evaporator and a main line between the condensing stage and the expansion stage to direct the defrost refrigerant from the first evaporator to the main line, and providing a pressure reducing device in the main line to reduce the pressure of the cooling refrigerant for a subsequent mixing with the defrost refrigerant from the second line. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]     A preferred embodiment of the present invention will now be described with reference to the accompanying drawings in which:  
         [0017]      FIG. 1  is a block diagram showing a simplified refrigeration system constructed in accordance with a first embodiment of the present invention;  
         [0018]      FIG. 2  is a schematic view showing the refrigeration system of  FIG. 1 ;  
         [0019]      FIG. 3  is a block diagram showing a simplified refrigeration system constructed in accordance with a second embodiment of the present invention;  
         [0020]      FIG. 4  is a block diagram of the refrigeration system of  FIG. 1 , with additional sub-cooling features;  
         [0021]      FIG. 5A  is an enlarged block diagram showing an alternative sub-cooling system;  
         [0022]      FIG. 5B  is an enlarged block diagram showing a second alternative sub-cooling system;  
         [0023]      FIG. 5C  is an enlarged block diagram showing third and fourth alternative sub-cooling systems;  
         [0024]      FIG. 6A  is an enlarged block diagram showing a first embodiment of a line relating an evaporator in defrost to a main refrigeration line;  
         [0025]      FIG. 6B  is an enlarged block diagram showing a second embodiment of a line relating an evaporator in defrost to a main refrigeration line; and  
         [0026]      FIG. 6C  is an enlarged block diagram showing a third embodiment of a line relating an evaporator in defrost to a main refrigeration line; 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     Referring to the drawings, and more particularly to  FIG. 1 , a refrigeration system in accordance with a first embodiment of the present invention is generally shown at  10 . The refrigeration system  10  comprises the components found on typical refrigeration systems in which circulates a cooling refrigerant, at different states and pressures according to the stage of the refrigeration cycle, such as compressors  12  (one of which is  12 A, for reasons to be described hereinafter), a high-pressure reservoir  16 , expansion valves  18 , and evaporators  20 . The refrigeration system  10  is shown having a heat reclaim unit  22 , which is optional. In  FIG. 1 , the refrigeration system  10  is shown having only two sets of evaporator  20 /expansion valve  18  for the simplicity of the illustration. It is obvious that numerous other sets of evaporator  20 /expansion valve  18  may be added to the refrigeration system  10 .  
         [0028]     The compressors  12  are connected to the condenser units  14  by lines  28 . High-pressure gas refrigerant is discharged from the compressors  12  and flows to the condenser units  14  through the line  28 . A line  30  diverges from the line  28  by way of three-way valve  32 . The line  30  extends between the three-way valve  32  and the heat reclaim unit  22 . A line  34  connects the condenser units  14  to the high-pressure reservoir  16 , and a line  36  links the heat reclaim unit  22  to the high-pressure reservoir  16 . The condenser units  14  are typically rooftop condensers that are used to release energy of the high-pressure gas refrigerant discharged by the compressors  12  by a change to the liquid phase. Accordingly, refrigerant accumulates in the high-pressure reservoir  16  in a liquid state.  
         [0029]     Evaporator units  17  are connected between the high-pressure reservoir  16  and the compressors  12 / 12 A. Each of the evaporator units  17  has an evaporator  20  and an expansion valve  18 . The expansion valves  18  are connected to the high-pressure reservoir  16  by line  38 . As known in the art, the expansion valves  18  create a pressure differential so as to control the pressure of saturated liquid/gas refrigerant sent to the evaporators  20 . The outlet of the evaporators  20  are connected to the compressors  12  by lines  48 . The compressors  12  are supplied with low-pressure gas refrigerant via supply lines  48 . The expansion valves  18  control the pressure of the cooling refrigerant that is sent to the evaporators  20 , such that the cooling refrigerant changes phases in the evaporators  20  by a fluid, such as air, blown across the evaporators  20  to reach refrigerated display counters (e.g., refrigerators, freezers or the like) at low refrigerating temperatures.  
         [0030]     Refrigerant in the refrigeration system  10  is in a high-pressure gas state when discharged from the compressors  12 . For instance, a typical head pressure of the compressors is 200 Psi. The compressor head pressure changes as a function of the outdoor temperature to which the refrigerant in the condensing stage will be subjected. The high-pressure gas refrigerant is conveyed to the condenser units  14  and, if applicable, to the heat reclaim unit  22  via the line  28  and the line  30 , respectively.  
         [0031]     In the condenser units  14  and the heat reclaim unit  22 , the refrigerant releases heat so as to go from the gas state to a liquid state, with the pressure remaining generally the same. Accordingly, the high-pressure reservoir  16  accumulates high-pressure liquid refrigerant that flows thereto by the lines  34  and  36 , as previously described.  
         [0032]     The compressors  12  exert a suction on the evaporators  20  through the supply lines  48 . The expansion valves  18  control the pressure in the evaporators  20  as a function of the suction by the compressors  12 . Accordingly, high-pressure liquid refrigerant accumulates in the line  38  to thereafter exit through the expansion valves  18  to reach the evaporators  20  via the lines  43  in a low-pressure saturated liquid/gas state. During a refrigeration cycle, the refrigerant absorbs heat in the evaporators  20 , so as to change state to become a low-pressure gas refrigerant. Finally, the low-pressure gas refrigerant flows through the line  48  so as to be compressed once more by the compressors  12  to complete the refrigeration cycle.  
         [0033]     As frost and ice build-up are frequent on the evaporators, the evaporators  20  are provided with a defrost system for melting the frost and ice build-up. Only one of the evaporator units  17  is shown having defrost equipment, for simplicity of the drawings, but all evaporator units  17  can be provided with defrost equipment.  
         [0034]     Valves are provided in the evaporator units  17  so as to control the flow of refrigerant in the evaporators  20 . A valve  114  is typically provided in the line  38 . The valve  114  is normally open, but is closed during defrosting of its evaporator unit  17 . A valve  116  is positioned on the line  48  and is normally open. The line  106  merges with the line  48  between the valve  116  and the evaporator  20 . The line  106  has a valve  118  therein.  
         [0035]     In a normal refrigeration cycle, refrigerant flows in the line  38  through the valve  114 , to reach the expansion valves  18 . A pressure drop in refrigerant is caused at the expansion valve  18 . The resulting low-pressure liquid refrigerant reaches the evaporators  20 , wherein it will absorb heat to change state to gas. Thereafter, refrigerant flows through the low-pressure gas refrigerant line  48  and the valve  116  therein to the compressors  12 .  
         [0036]     During a defrost cycle of an evaporator  20 , valves  118  and  120  are open, whereas the valves  114  and  116  are closed. Accordingly, the expansion valve  18  and the evaporator  20  will not be supplied with low-pressure liquid refrigerant from the line  38 , as it is closed by valve  114 .  
         [0037]     The dedicated compressor  12 A collects low-pressure gas refrigerant from a suction header  204  that also supplies the other compressors  12  in refrigerant. However, the compressor  12 A is the only compressor supplying evaporators in defrost cycles, whereby its discharge pressure can be lowered. This is performed by having line  106  connected to the evaporators  20  by valve  116  closing to direct refrigerant via line  48  thereto (shown connected to only one line  48  in  FIG. 1  but connected to all lines  48  of all evaporators  20  requiring defrost). A portion of the refrigerant discharged by the compressor  12 A can be sent to the condensing stage, via line  106  that converges with the line  28 . A valve  200  (e.g., a three-way modulating valve), controls the portions of refrigerant discharge going to the lines  106  and  106 ″.  
         [0038]     Thereafter, the refrigerant exiting from the defrosted evaporators  20  is injected into the evaporators  20  in a refrigeration cycle. Line  112 ′ collects liquid refrigerant exiting from the evaporators  20  in defrost, and converges with the line  38  upstream of the expansion valves  18 , such that the liquid refrigerant can be injected in the evaporators  20  in the refrigeration cycle. A valve  202  (e.g., pressure regulating valve) ensures that a proper refrigerant pressure is provided to the line  38 , and compensates a lack of refrigerant pressure by transferring liquid refrigerant from the high-pressure reservoir  16  to the line  38 . The combination of the dedicated compressor  12 A (i.e., low-pressure refrigerant feed to the defrost evaporators, also achievable by a pressure regulator, as described for the refrigeration system of  FIG. 1  of U.S. Pat. No. 6,775,993) and the valve  202  enable the injection of low-pressure refrigerant, which exits from the defrost cycle, in the evaporator units  17 . Previously, reinjected defrost refrigerant had to be conveyed to the condensing stage to reach adequate conditions to be reinjected into the evaporation cycles.  
         [0039]     As seen in  FIG. 2 , a subcooling system  204  can be used to ensure the proper state of the refrigerant reaching the evaporator units  17 . With the refrigeration system  10  of  FIGS. 1 and 2 , the defrost refrigerant can be reinjected in the evaporator units  17  at pressures as low as 120 to 140 Psi for refrigerant  22 , and 140 to 160 Psi for refrigerant  507  and refrigerant  404 , even though the refrigerant  22  is up to about 220 to 260 Psi in the condenser units  14 , and the refrigerant  507  and the refrigerant  404  are up to about 250 to 340 Psi.  
         [0040]     A bypass line  134  and a check valve  136  therein are connected from the line  48  to the compressor  12 A. The check valve  136  enables a flow of refrigerant therethrough such that the inlet pressure at the compressors  12  and the dedicated compressor  12 A is generally the same.  
         [0041]     When the defrost cycle has been completed, the valves are reversed so as to return the defrosted evaporator  20  to the refrigeration cycle. More specifically, the valves  114  and  116  are opened, and the valves  118  and  120  are closed. It is preferred that the valve  116  be of the modulating type (e.g., Mueller modulating valve, www.muellerindustries.com), or a pulse valve. Accordingly, a pressure differential in the line  48  between upstream and downstream portions with respect to the valve  116  will not cause water hammer when the valve  116  is open. The pressure will gradually be decreased by the modulation of the valve  116 . Furthermore, the refrigerant reaching the compressors  12  via the line  48  will remain at advantageously low pressures.  
         [0042]     It is pointed out that line  112 ′ and valve  120  are generically illustrated in  FIG. 1  as connecting the evaporator  20  to the line  38 . This may be done in various configurations, using for instance existing lines. As shown in  FIGS. 6A and 6B , the line  112 ′ and the valve  120  may consist of a pair of lines and check valves that enable defrost refrigerant to surround the expansion valve  18  and the valve  114 , if applicable.  
         [0043]     It is also contemplated to operate defrost systems without the valve  114 , as shown in  FIG. 6C . More specifically, the valve  202  maintains the cooling refrigerant pressure lower than the pressure of the defrost refrigerant, so as to enable the mixing of both refrigerants.  
         [0044]     Accordingly, the pressure is greater downstream of the expansion valve  18  in defrost than upstream. The defrost refrigerant pressure therefore prevents circulation of cooling refrigerant through the expansion valve  18  associated with an evaporator  20  being defrosted.  
         [0045]     Referring to  FIG. 3 , a refrigeration system in accordance with another embodiment of the present invention is generally shown at  10 ′. The refrigeration system  10 ′ is generally similar to the refrigeration system  10  of  FIGS. 1 and 2 , and like reference numerals are therefore used to identify like elements.  
         [0046]     In the refrigeration system  10 ′ of  FIG. 3 , the compressions stage  12 ′ does not have any dedicated compressor outputting lower pressure refrigerant to feed evaporators in defrost. Instead, a pressure regulator  108  is provided in the line  106 , so as to lower a pressure of the cooling refrigerant, so as to produce defrost refrigerant of suitable lower pressure. It is pointed out that the refrigeration system  10 ′ of  FIG. 3  has been simplified for simplicity purposes. For instance, the condensation stage has simply been illustrated as  14 ′, but typically includes condenser units and/or heat reclaim units.  
         [0047]     In the refrigeration system  10 ′ of  FIG. 1 , the defrost of evaporators  20  is operated as follows. One of the evaporators  20  is supplied with refrigerant discharged from the compressor stage  12  by the line  106  having the pressure regulator  108  therein. The pressure regulator  108  creates a pressure differential in the line  106 , such that the high-pressure gas refrigerant (cooling refrigerant), typically around 200 Psi, is reduced to a low-pressure gas refrigerant thereafter (defrost refrigerant), for instance at about 110 Psi. The pressure regulator  108  may include a modulating valve in line  106 . In the event that the pressure in the evaporator  20  is lower than that of the refrigerant conveyed thereto by the line  106  in a defrost cycle, the modulating valve portion of the pressure regulator  108  will preclude the formation of water hammer by gradually increasing the pressure in the evaporator  20 . This feature of the pressure regulator  108  will allow the refrigeration system  10  to feed the evaporators  20  with high-pressure refrigerant, although it is preferred to defrost the evaporators  20  with low-pressure refrigerant. On the other hand, the modulating action can be effected by the valves  118 .  
         [0048]     Once the evaporator  20  has been defrosted with the defrost refrigerant, the defrost refrigerant is directed to the line  38 , thereby mixing with cooling refrigerant, for subsequently being fed to evaporator units  17  in defrost, as was described previously for the refrigeration system  10  of  FIGS. 1 and 2 .  
         [0049]     Referring to  FIG. 4 , a refrigeration system  10 ″ is shown that is essentially the refrigeration system  10  of  FIG. 1 , with alternative components, and with a sub-cooling loop  300 . In  FIG. 4 , a valve  200 ″ (e.g., a check valve or other two-way valve) is provided so as to enable refrigerant from the compressor  12 A to reach the line  28 . Also, no suction header, such as the suction header  204  of  FIG. 1 , is provided in the refrigeration system  10 ″ of  FIG. 4 . These are simple variations of refrigeration systems, provided for illustrative purposes.  
         [0050]     The sub-cooling system  300  is provided so as to reduce the amount of flash gas that is fed to the evaporators  20  in the refrigeration cycle. More specifically, due to the mixture of defrost refrigerant with cooling refrigerant for injection in the evaporators  20  in the evaporation stage, it is possible that some flash gas is present in the mixture of refrigerants. Therefore, the sub-cooling system  300  is provided so as to liquefy the cooling refrigerant prior to being mixed with the defrost refrigerant. Various sub-cooling systems may be used, and the sub-cooling system  300  is provided as two separate examples.  
         [0051]     Referring to  FIG. 4 , the sub-cooling system  300  has a line  308  that extends from the reservoir  16 . The sub-cooling refrigerant directed in the line  308  is expanded by expansion stage  304  such that its pressure is reduced. The sub-cooling refrigerant is then put in heat-exchange with the cooling refrigerant in heat-exchange stage  306 , so as to absorb heat from the cooling refrigerant and thus liquefy the cooling refrigerant, for its subsequent mixture with the defrost refrigerant. The sub-cooling refrigerant is then fed to the compression stage  12 .  
         [0052]     Also in  FIG. 4 , a valve  400  is shown at the outlet of the dedicated compressor  12 A. The valve  400  is provided so as to ensure that the line  106  at the outlet of the compressor  12 A maintains sufficient refrigerant pressure.  
         [0053]     In  FIG. 5A , a sub-cooling system  300 ′ is similar to the sub-cooling system  300  of  FIG. 5A , but with the valve  202  positioned upstream of the heat exchanger  306 . In  FIG. 5B , a sub-cooling system  300 ″ has the line  112 ′ mixing the defrost refrigerant to the cooling refrigerant upstream of the heat exchanger  306 . In  FIG. 5C , a sub-cooling system  300 ′″ collects sub-cooling refrigerant downstream of the heat exchanger  306 . It is pointed out that line  112 ′ can mix defrost refrigerant to the cooling refrigerant downstream or upstream of the heat exchanger  306 , as is illustrated. Other sub-cooling configurations are also possible.  
         [0054]     It is obvious that the control of valve operation is preferably fully automated. The valve operation for controlling the defrost of evaporators  20 , namely the control of valves  114 ,  116 ,  118  and  120 , is fully automated.  
         [0055]     The defrosting of one of the evaporators  20  can be stopped according to a time delay. More precisely, a defrost cycle of an evaporator  20  can be initiated periodically and have its duration predetermined. For instance, a typical defrost portion of a defrost cycle can last 8 minutes for low pressures of refrigerant fed to the evaporators  20  and can be even shorter for higher pressures. Thereafter, a period is required to have the defrosted evaporator  20  returned to its normal refrigeration operating temperature, and such a period is typically up to 7 minutes in duration. It is also possible to have a sensor positioned downstream of the evaporator  20  in a defrost cycle, that will control the duration of the defrost cycle of a respective evaporator  20  by monitoring the temperature of the refrigerant having defrosted the respective evaporator  20 . A predetermined low refrigerant temperature detected by the sensor could trigger an actuation of the valves  114 ,  116 ,  118  and  120 , to switch the respective evaporator  20  to a refrigeration cycle  20 .  
         [0056]     It is obvious that the various components enabling the defrost cycle can be regrouped in a pack so as to be provided on site as a defrost system ready to operate. This can simplify the installation of the defrost system to an existing refrigeration system, as the major step in the installation would be to connect the various lines to the defrost system.  
         [0057]     Although the refrigeration system  10  of the present invention enables the defrosting of the evaporators  20  at high pressure, it is preferable that the pressure regulator  108  or dedicated compressor  12 A reduce the pressure of the refrigerant fed to the evaporators  20  in defrost cycles. In such a case, less refrigerant is required to defrost an evaporator, whereby a plurality of evaporators  20  can be defrosted simultaneously. Moreover, the use of high-pressure refrigerant causes non-negligible thermal expansion of the refrigerant lines. This may result in damages to the lines, as well as rupture of insulating sleeves provided on the refrigerant lines. Accordingly, in an embodiment of the present invention, the refrigeration systems of FIGS.  1  to  5  overcome this disadvantage by using defrost refrigerant of a pressure that is closer to the pressure of the cooling refrigerant.  
         [0058]     It is within the ambit of the present invention to cover any obvious modifications of the embodiments described herein, provided such modifications fall within the scope of the appended claims.