Abstract:
A defrost refrigeration system having a main refrigeration system and comprising a first line extending from a compressing stage to an evaporator stage and adapted to receive refrigerant in high-pressure gas state from the compressing stage. A first pressure reducing device on the first line is provided for reducing a pressure of the refrigerant in the high-pressure gas state to a second low-pressure gas state. Valves are provided for stopping a flow of the refrigerant in a first low-pressure liquid state from a condensing stage to evaporators of the evaporator stage and directing a flow of the refrigerant in the second low-pressure gas state to release heat to defrost the evaporators and thereby changing phase at least partially to a second low-pressure liquid state. A second line is provided for directing the refrigerant having released heat to the compressing stage, the condensing stage or the evaporator stage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This patent application is a divisional of U.S. patent application Ser. No. 10/189,462, filed on Jul. 8, 2002, by the present Applicant. 
     
    
     
       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, wherein a refrigerant goes through at least a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the compressing stage to the evaporator stage and adapted to receive a portion of the refrigerant in the high-pressure gas state. A first pressure reducing device on the first line reduces a pressure of the portion of the refrigerant in the high-pressure gas state to a second low-pressure gas state. Valves stop a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage and direct a flow of the refrigerant in the second low-pressure gas state to release heat to defrost the at least one evaporator and thereby change phase at least partially to a second low-pressure liquid state. A second line directs the refrigerant having released heat to at least one of the compressing stage and the condensing stage.  
           [0014]    According to a further broad feature of the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a first compressor in a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage wherein the refrigerant in the high-pressure gas is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the compressing stage to the evaporator stage and is adapted to receive a portion of the refrigerant in the high-pressure gas state. Valves stop a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage and direct a flow of the portion of the refrigerant in the high-pressure gas state to release heat to defrost the at least one evaporator and thereby change phase to a second low-pressure liquid state. A dedicated compressor is adapted to receive an evaporated gas portion of the refrigerant in the second low-pressure liquid state. The dedicated compressor is connected to the condensing stage for directing a discharge thereof to the condensing stage.  
           [0015]    According to a still further broad feature of the present invention, there is provided a method for defrosting evaporators of a refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The method comprises the steps of i) stopping a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage; ii) reducing a pressure of a portion of the refrigerant in the high-pressure gas state to a second low-pressure gas state; and iii) directing the portion of the refrigerant in the second low-pressure gas state to the at least one evaporator to release heat to defrost the at least one evaporator and thereby changing phase at least partially to a second low-pressure liquid state.  
           [0016]    According to a still further broad feature of the present invention, there is provided a method for defrosting evaporators of a refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage having at least a first compressor, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The method comprises the steps of i) stopping a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator; ii) directing a portion of the refrigerant in the high-pressure gas state to the at least one evaporator to release heat to defrost the at least one evaporator and thereby changing phase at least partially to a second low-pressure liquid state; and iii) directing an evaporated gas portion of the refrigerant in the second low-pressure gas state to a dedicated compressor, the dedicated compressor being connected to the condensing stage for directing a discharge thereof to the condensing stage.  
           [0017]    According to a still further broad feature of the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the compressing stage to the evaporator stage and adapted to receive a portion of the refrigerant in the high-pressure gas state. Valves are provided for stopping a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage and directing a flow of the refrigerant in the high-pressure gas state 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 is provided for directing the refrigerant having released heat to the compressing stage, and pressure control means in the second line for controlling a pressure of the refrigerant reaching the compressing stage.  
           [0018]    According to a still further broad feature of the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the compressing stage to the evaporator stage and adapted to receive a portion of the refrigerant in the high-pressure gas state. Valves are provided for stopping a flow of the refrigerant in the first low-pressure liquid state to at least two evaporators of the evaporator stage and directing a flow of the refrigerant in the high-pressure gas state to release heat to defrost the at least two evaporators and thereby changing phase at least partially to a second low-pressure liquid state. A second line is provided for directing the refrigerant having released heat in the at least two evaporators to the compressing stage. Temperature monitor means are adapted to monitor an average temperature of the refrigerant in the second line and to reverse an action of the valves when the temperature reaches a predetermined value to re-establish the flow of the refrigerant in the first low-pressure liquid state to the at least two evaporators of the evaporator stage.  
           [0019]    According to a still further broad feature of the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded by an expansion valve to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the compressing stage to the expansion stage and adapted to receive a portion of the refrigerant in the high-pressure gas state. Valves are provided for stopping a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage and directing a flow of the refrigerant in the high-pressure gas state around the expansion valve to the at least one evaporator of the evaporator stage to release heat to defrost the at least one evaporator and thereby changing phase at least partially to a second low-pressure liquid state, to then be directed to the compressing stage.  
           [0020]    According to a still further broad feature of the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit, wherein a refrigerant goes through at least a compressing stage having at least a first and a second compressor, wherein the refrigerant is compressed to a high-pressure gas state to then reach a condensing stage, wherein the refrigerant in the high-pressure gas state is condensed at least partially to a high-pressure liquid state to then reach an expansion stage, wherein the refrigerant in the high-pressure liquid state is expanded to a first low-pressure liquid state to then reach an evaporator stage, wherein the refrigerant in the first low-pressure liquid state is evaporated at least partially to a first low-pressure gas state by absorbing heat, to then return to the compressing stage. The defrost refrigeration system comprises a first line extending from the first compressor to the evaporator stage and adapted to receive at least a portion of discharged low-pressure refrigerant from the first compressor. Valves are provided for stopping a flow of the refrigerant in the first low-pressure liquid state to at least one evaporator of the evaporator stage and directing a flow of the discharged low-pressure 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 is provided for directing the refrigerant having released heat to the evaporator stage. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    A preferred embodiment of the present invention will now be described with reference to the accompanying drawings in which:  
         [0022]    [0022]FIG. 1 is a block diagram showing a simplified refrigeration system constructed in accordance with the present invention;  
         [0023]    [0023]FIG. 2 is a schematic view showing a refrigeration system constructed in accordance with the present invention;  
         [0024]    [0024]FIG. 3 is an enlarged schematic view of an evaporator unit of the refrigeration system;  
         [0025]    [0025]FIG. 4 is an enlarged schematic view of an evaporator unit in accordance with another embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a block diagram showing a simplified refrigeration system constructed in accordance with another;  
         [0027]    [0027]FIG. 6 is a block diagram showing a simplified refrigeration system constructed in accordance with still another embodiment of the present invention; and  
         [0028]    [0028]FIG. 7 is a schematic view showing the refrigeration system of FIG. 6. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    Referring to the drawings, and more particularly to FIG. 1, a refrigeration system in accordance with the present invention is generally shown at  10 . The refrigeration system  10  comprises the components found on typical refrigeration systems, 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 .  
         [0030]    The compressors  12  are connected to the condenser units  14  by lines  28 . A pressure regulator  21  is in the line  28  but is not in operation during normal refrigeration cycles, and is thus normally open to enable refrigerant flow therethrough. 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.  
         [0031]    Evaporator units  17  are connected between the high-pressure reservoir  16  and the compressors  12 . 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 liquid 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 liquid refrigerant that is sent to the evaporators  20 , such that the liquid 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.  
         [0032]    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 obviously changes as a function of the outdoor temperature to which will be subject the refrigerant in the condensing stage. 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.  
         [0033]    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.  
         [0034]    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 liquid state. The typical pressure at an outlet of the expansion valve  18  is 35 Psi. 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.  
         [0035]    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. It is obvious that all evaporator units  17  can be provided with defrost equipment. One of the evaporators  20  is supplied with refrigerant discharged from the compressors  12  by a line  106  having a pressure regulator  108  therein. The pressure regulator  108  creates a pressure differential in the line  106 , such that the high-pressure gas refrigerant, typically around 200 Psi, is reduced to a low-pressure gas refrigerant thereafter, 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 .  
         [0036]    Valves are provided in the evaporator units  17  so as to control the flow of refrigerant in the evaporators  20 . A valve  114  is 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. A line  112 , connecting a low-pressure reservoir  100  to the evaporator  20 , has a valve  120  therein. The valves  118  and  120  are closed during a normal refrigeration cycle of their respective evaporators  20 .  
         [0037]    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 .  
         [0038]    During a defrost cycle of an evaporator  20 , the 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 . During the defrost cycle, low-pressure gas refrigerant accumulated in the line  106 , downstream of the pressure regulator  108 , is conveyed back into the evaporator  20  through the portion of line  48  between the valve  116  and the evaporator  20 . As the valve  116  is closed and the valve  118  is open. The closing of the valve  116  ensures that refrigerant will not flow from the line  106  to the compressors  12 . As the low-pressure gas refrigerant flows through the evaporator  20 , it releases heat to defrost and melt ice build-up on the evaporator  20 . This causes a change of phase to the low-pressure gas refrigerant, which changes to low-pressure liquid refrigerant. Thereafter, the low-pressure liquid refrigerant flows through the line  112  and the valve  120  to reach the low-pressure reservoir  100 . The low-pressure reservoir  100  accumulates liquid refrigerant at low pressure.  
         [0039]    The low-pressure reservoir  100  is connected to the compressors  12  by a line  126 . The line  126  is connected to a top portion of the reservoir  100  such that evaporated refrigerant exits therefrom. As the low-pressure reservoir  100  accumulates low-pressure liquid refrigerant, evaporation will normally occur such that a portion of the reservoir above the level of liquid refrigerant will comprise low-pressure gas refrigerant. The pressure in the low-pressure reservoir  100  is typically as low as 10 Psi.  
         [0040]    However, with the present invention a compressor is dedicated for discharging the low-pressure reservoir  100 , whereas the other compressors receive refrigerant exiting from the evaporators  20 . Reasons for the use of a dedicated compressor will be described hereinafter. Accordingly, as shown in FIG. 1, the compressor  12 A will be dedicated to discharging the low-pressure reservoir  100 . A line  128  diverges from the line  126  to reach the compressor  12 A. A valve  130  is in the line  128 , whereas a valve  132  is in the line  126 . During operation of the dedicated compressor  12 A, the valve  132  is closed, whereas the valve  130  is open.  
         [0041]    A bypass line  134  and a check valve  136  therein are connected from the line  48  to the compressor  12 A. The pressure in the lines  126  and  128  is generally lower than in the line  48 . The check valve  136  therefore 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.  
         [0042]    In order to flush the liquid refrigerant in the low-pressure reservoir  100  such that the latter does not overflow, a flushing arrangement is provided for the periodic flushing of the low-pressure reservoir  100 . The flushing arrangement has a line  140  having a valve  142  therein diverging from the line  28  and connecting to the low-pressure reservoir  100 . The line  140  diverges from the line  28  upstream of the pressure regulator  21 , such that high-pressure gas refrigerant can be directed from the compressors  12  directly to the low-pressure reservoir  100 .  
         [0043]    A line  144  having a valve  146  extends from the low-pressure reservoir  100  to the line  28  downstream of the pressure regulator  21 , and upstream of the three-way valve  32 . A line  148  having a valve  150  goes from the low-pressure reservoir  100  to the high-pressure reservoir  16 . A periodic flush of the low-pressure reservoir  100  is initiated by creating a pressure differential (e.g., 5 psi) in the line  28 .  
         [0044]    The valve  142  is opened while the valves  130  and  132  are simultaneously closed, if they were open. Accordingly, high-pressure gas refrigerant can be directed to the low-pressure reservoir  100 , but will be prevented from reaching the compressors  12  and  12 A. One of the valves  146  and  150  is opened, while the other remains closed. If the valve  146  is opened, a mixture of gas and liquid refrigerant will flow through the line  144  and to the line  28  downstream of the pressure regulator  21 . It is pointed out that the pressure differential caused by the pressure regulator  21  will create this flow. If the valve  150  is opened, the gas/liquid refrigerant will flow through the line  148  to reach the high-pressure reservoir  16 , in this case having a lower pressure than the low-pressure reservoir  100 , by the insertion of compressor discharge in the low-pressure reservoir  100  via line  140 , and by the pressure drop caused by the pressure regulator  21 .  
         [0045]    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. Although in the preferred embodiment of the present invention the refrigerant defrosting the evaporators  20  will be at generally low pressure because of the pressure regulator  108 , the refrigeration system  10  of the present invention may also provide high-pressure refrigerant to accelerate the defrosting of the evaporators  20 , whereby the modulation of the valve  116  is preferred when a defrosted evaporator  20  is returned to the refrigeration cycle. It is obvious that equivalents of the valve  116  can be used, and such equivalents will be discussed hereinafter.  
         [0046]    In the warmer periods, such as summer, the flushing is directed to the condenser units  14  via the line  144 , such that the liquid content of the flush cools the condenser units  14 . In the cooler periods, the flush is directed to the high-pressure reservoir  16 . When the flush is completed, for instance, when the liquid level in the low-pressure reservoir  100  reaches a predetermined low level, the flush is stopped by the closing of the valves  142  and  146  or  150  and the deactivation of the pressure regulator  21 . The valves  130  or  132  can also be opened if defrosting of one of the evaporators  20  is required.  
         [0047]    It is obvious that the control of valve operation is preferably fully automated. As mentioned above, the flushing of the low-pressure reservoir  100  can be stopped by the low-pressure reservoir  100  reaching a predetermined low level. Similarly, the flush of the low-pressure reservoir  100  can be initiated by the refrigerant level reaching a predetermined high level in the low-pressure reservoir  100 . Similarly, the valve operation for controlling the defrost of evaporators  20 , namely the control of valves  114 ,  116 ,  118 ,  120 ,  130  and  132 , is fully automated. For the flushing of the low-pressure reservoir  100 , and in the defrost cycles, an automation system may also be programmed to do periodic flushing or defrost cycles, respectively. It also has been thought to provide a pump (not shown) to pump the liquid refrigerant in the low-pressure reservoir  100  to the line  28  or to the high-pressure reservoir  16 .  
         [0048]    It is an advantageous feature to have a dedicated compressor  12 A. It is known that compressors are not adapted to receive liquids therein. However, as the defrost cycles produce a change of phase of gas refrigerant to liquid refrigerant, there is a risk that liquid refrigerant reaches the compressors  12 . It is thus important that the low-pressure reservoir  100  does not overflow, whereby the flushing can be actuated, as described above, upon the low-pressure reservoir&#39;s  100  reaching a predetermined high level of refrigerant. An alarm system (not shown) can also be provided in order to shut-off the compressors in the event of a low-pressure reservoir overflow. The alarm can be used to shut-off the compressors such that liquid refrigerant cannot affect the compressors. However, this involves a risk of fouling the foodstuff in the refrigeration display counters. The use of a dedicated compressor  12 A, isolated from the other compressors  12 , can prevent the shutting down of all compressors or the liquid from reaching the compressors. As described above, the valve  132  is shut during the use of the dedicated compressor  12 A such that the low-pressure reservoir  100  is isolated from the compressors  12 . On the other hand, the alarm (not shown) can be connected to the valve  130  in order to shut-off, the valve  130  when an overflow of the low-pressure reservoir  100  is detected. The compressor  12 A will then be supplied with gas refrigerant from the line  48  through the check valve  136 .  
         [0049]    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  152  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  152  could trigger an actuation of the valves  114 ,  116 ,  118  and  120 , to switch the respective evaporator  20  to a refrigeration cycle  20 .  
         [0050]    It is known to provide the sensor  152 . However, these sensors have been previously provided after each evaporator  20 . Accordingly, this proves to be a costly solution. Furthermore, in systems wherein defrost is effected for a few evaporators simultaneously, these evaporators are often synchronized to return back to refrigeration cycles only once all temperature sensors reach their predetermined low limit. This causes unnecessarily lengthy defrost cycles. The sensor  152  of the present invention is thus preferably positioned so as to measure an average temperature of the defrost refrigerant of all evaporators defrosted simultaneously. In consequence thereof, fewer sensors  52  are necessary and the operation of defrost cycles is more efficient.  
         [0051]    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.  
         [0052]    Now that the refrigeration system  10  has been described with reference to a simplified schematic figure, a refrigeration system  10 ′ is shown in FIGS. 2 and 3 in further detail. It is pointed out that like numerals will designate like elements. Furthermore, the refrigeration system  10 ′ illustrated in FIGS. 2 and 3 comprises additional elements to the refrigeration system  10 , and these additional elements are common to refrigeration systems but have been removed from FIG. 1 for clarity purposes.  
         [0053]    As seen in FIG. 2, the compressors  12  and  12 A are connected to the line  28 , which has a discharge header  24  to collect the discharge of all compressors  12  and  12 A. Although not shown, it is common to have an oil separator that will remove oil contents from the high-pressure gas refrigerant in the line  28 . The three-way valve  32  is preferably a motorized modulating valve that will prevent water hammer when stopping a supply of refrigerant to the heat reclaim unit  22 .  
         [0054]    The refrigeration system  10 ′ has a high-pressure liquid refrigerant header  40  and a suction header  44 . The high-pressure liquid refrigerant header  40  is in the line  38  and thus connected to the high-pressure reservoir  16  to supply refrigerant to the evaporators  20 . The suction header  44  is connected to inlets of the compressors  12  by the lines  48 . Refrigerant accumulates in the suction header  44  in a low-pressure gas state, and is conveyed through the lines  48  to the compressors  12  by the pressure drop at the inlets of the compressors  12 .  
         [0055]    Numerous evaporator units  17  extend between the high-pressure reservoir  16  and the suction header  44 , but only one is fully shown in FIG. 2 for clarify purposes. 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 liquid refrigerant header  40  by the lines  38 , and to the evaporators  20  by the lines  43 . As mentioned above, the expansion valves  18  create a pressure differential so as to control the pressure of liquid refrigerant sent to the evaporators  20 . The expansion valves  18  control the pressure of the liquid refrigerant that is sent to the evaporators  20  as a function of a fluid that is blown on the evaporators  20  (e.g., air), such that the liquid refrigerant changes phases in the evaporators  20  by the fluid, blown across the evaporators  20  to reach refrigerated display counters (e.g., refrigerators, freezers or the like) at low refrigerating temperatures.  
         [0056]    The compressors  12  exert a suction on the evaporators  20  through the suction header  44  and the 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  and the high-pressure liquid refrigerant header  40  to thereafter exit through the expansion valves  18  to reach the evaporators  20  in a low-pressure liquid state.  
         [0057]    In the refrigeration system  10 ′, the defrost system has a low-pressure gas header  102  and a low-pressure liquid header  104 . The low-pressure gas header  102  is supplied with refrigerant discharged from the compressors  12  by a defrost line  106 . As mentioned previously, the pressure regulator  108  creates a pressure differential, such that the high-pressure gas refrigerant is reduced to a low-pressure gas refrigerant thereafter. The low-pressure gas header  102  and the low-pressure liquid header  104  are connected by the evaporator units  17 . As seen in FIG. 3, the valve  114  is provided on the line  38 , with the line  112  connected to the line  38  between the expansion valve  18  and the valve  114 . The valve  114  is normally open, but is closed during defrosting of its evaporator unit  17 . The 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 the valve  118  therein, and the defrost outlet line  112  has the valve  120  therein. The valves  118  and  120  are closed during a normal refrigeration cycle of their respective evaporators  20 . A check valve  122  is provided parallel to the expansion valve  18 . It is pointed out that the check valve  122  is not shown in FIG. 1, yet the refrigeration system  10  of FIG. 1 and the refrigeration system  10 ′ of FIG. 2 operate in an equivalent fashion. The check valve  122  enables the use of the line  43  and a portion of the line  38  for defrost cycles, and this reduces the number of pipes going to the evaporators  20 . Furthermore, the check valves  122  will facilitate the adaptation of a defrost system to an existing refrigeration system.  
         [0058]    Although, as illustrated in FIG. 3, the line  106  is preferably connected to the line  48  to feed the evaporator  20  with refrigerant, whereas the line  112  is connected to the line  38  to provide an outlet for the refrigerant after having gone through the evaporator  20 , it is pointed out that the lines  106  and  112  can be appropriately connected. As shown in FIG. 4, the line  106  is connected to the line  38 , whereas the line  112  is connected to the line  48 . In doing so, the check valve  122  of FIG. 3 is replaced by a solenoid valve  122 ′ that will allow refrigerant to bypass the expansion valve  18  to reach the evaporator  20 .  
         [0059]    Therefore, as seen in FIGS. 2 and 3, in a normal refrigeration cycle, refrigerant flows in the line  38  through the valve  114 . The check valve  122  blocks flow therethrough in that direction of flow of refrigerant, such that refrigerant has to go through the expansion valve  18  to reach the evaporator  20  via the line  43 . Thereafter, refrigerant flows through the line  48 , including the valve  116  and the suction header  44 , to reach the compressors  12 .  
         [0060]    During a defrost cycle of one of the evaporators  20 , the 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 portion  38 , as it is closed by valve  114 . During the defrost cycle, low-pressure gas refrigerant is conveyed from the line  106  to the evaporator  20  through a portion of the line  48 . The valve  116  is closed and the valve  118  is open. As the valve  116  is closed, refrigerant will not flow from the line  106  to the suction header  44 . As the low-pressure gas refrigerant flows through the evaporator  20 , it releases heat to defrost and melt ice build on the evaporator  20 . This causes a change of phase to the low-pressure gas refrigerant, which changes to low-pressure liquid refrigerant. The check valve  122  will allow refrigerant to accumulate upstream thereof, such that the refrigerant in the evaporator  20  has time to release heat to melt the ice build-up on the evaporator  20 . The check valve  122  will open above a given pressure, such that low-pressure liquid refrigerant can flow through the line  38  to the line  112  and the valve  120  to reach the low-pressure liquid header  104  and the low-pressure reservoir  100 .  
         [0061]    The low-pressure reservoir  100  is connected to the suction header  144  by the line  126 . The line  126  is connected to a top portion of the reservoir  100  such that evaporated refrigerant exits therefrom.  
         [0062]    The compressor  12 A has its own portion  44 A of the header  44 . The portion  44 A is separated from the suction header  44 . The line  128  extends from the line  126  to the suction header portion  44 A. A valve  130  is in the line  128 , whereas the valve  132  is in the reservoir discharge line  126 . During operation of the dedicated compressor  12 A, the valve  132  is closed, whereas the valve  130  is open. The line  134  and the check valve  136  therein merge with the line  128  such that the dedicated compressor  12 A can be supplied with refrigerant from the suction header  44  to operate at a same pressure as the compressors  12 .  
         [0063]    A line  160  provides a valve  162  parallel to the valve  130 . The line  160  has a small diameter, and is used to lower the pressure of the gas refrigerant coming from the low-pressure reservoir  100  after a flush of the low-pressure reservoir  100  has been performed.  
         [0064]    A plurality of check valves  164  and manual valves  166  are provided through the refrigeration system  10 ′ to ensure the proper flow direction and allow maintenance of various parts of the refrigeration system  10 ′.  
         [0065]    The refrigeration system  10  of the present invention is advantageous, as it provides a defrost system that can readily be adapted to existing refrigeration systems. The valve configuration in the evaporator units  17 , as shown in FIG. 3, provides for the use of existing pipe of typical refrigeration systems for defrost cycles. Also, the evaporators  20  only receive low-pressure refrigerants therein, as opposed to known defrost systems, and this ensures that most types of evaporators are compatible with the present invention. For instance, aluminum coils of an evaporator may not be specified for high refrigerant pressures that are typical to known defrost systems. Finally, the dedicated compressor  12 A is a safety feature that will prevent costly failures and breakdown of all compressors  12 , and thus reduces the risks of fouling foodstuff.  
         [0066]    In FIG. 5, there is shown an alternative to the low-pressure reservoir  100 . In the refrigeration system  10 ′ of FIG. 5, the line  112  is connected to the line  48 , downstream of the valve  116 , for directing refrigerant directly to the compressors after having defrosted the evaporator  20 . The refrigeration system  10 ′ is similar to the refrigeration system  10  of FIG. 1, whereby like elements will bear like numerals. Pressure control means  180  are provided in the line  112 , downstream of the valve  120 . The pressure control means  180  will ensure that defrosting refrigerant reaching the compressors  12  is at a pressure generally similar to that of the refrigerant flowing to the compressors  12  after a refrigeration cycle. The pressure control means  180  may consist of any one of outlet regulating valves, modulating valves, pulse valves and a liquid accumulator, and may also consist in a circuit having heat exchangers (e.g., roof-top radiators) and expansion valves, that will reduce the refrigerant pressure and change the phase thereof. In the case where the pressure control means  180  are outlet regulating valves, these may be positioned directly after the evaporators  20 , or just before inlets of compressors  12 , to prevent liquid refrigerant from reaching the compressors  12  and to control the pressure of refrigerant supplied thereto. A liquid accumulator would preferably be positioned between suction headers (not shown) so as to ensure that no liquid refrigerant is fed to the compressors  12 . Considering that the refrigerant having defrosted an evaporator  20  will be generally liquid, the liquid accumulator prevents excessive liquid refrigerant from blocking the lines. The pressure control means  180  will enable the compressors  12  to operate at low pressures, i.e., independently from the pressure of refrigerant at the outlet of the defrost evaporators. Therefore, more evaporators can be defrosted at a same time as the compressor inlet pressure is generally independent from the number of evaporators in defrost, whereby such simultaneous defrosting will not substantially increase the energy costs of the compressors  12 .  
         [0067]    As mentioned previously, typical defrost periods with the refrigeration system  10  of the present invention are of 8 minutes for the evaporator  20  to reach the highest temperature, and 7 minutes for returning back to an operating temperature. Therefore, a total of 15 minutes is achievable from start to finish for a defrost period with the refrigeration system  10  of the present invention.  
         [0068]    Referring to FIGS. 6 and 7, another configuration of the refrigeration system  10 ″ is shown, wherein gas refrigerant is sent to defrost the evaporators  20  at a lower pressure than gas refrigerant sent to the condensing stage. 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. 6 but obviously 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 ″.  
         [0069]    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 the refrigeration system of FIG. 1) 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. As seen in FIG. 7, 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. 6 and 7, 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.  
         [0070]    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  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.  
         [0071]    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.