Abstract:
A defrost refrigeration system comprises first compressors and second compressors serially positioned with respect to one another such that refrigerant going through a compression stage in the refrigeration cycle passes sequentially through the first compressors and the second compressors. A first line extends from an exit of one of the first compressor and the second compressor of the compression stage, and is in fluid communication with the evaporator stage in a defrost cycle and is adapted to receive a defrost portion of refrigerant compressed in the compression stage. Valves switch evaporators between the refrigeration cycle and the defrost cycle, by stopping/allowing a flow of refrigerant from the condensation stage to the evaporators of the evaporation stage in the refrigeration cycle, and for allowing/stopping a flow of said defrost portion of refrigerant to defrost the evaporators in the defrost cycle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to refrigeration systems, and more particularly to defrost configurations for evaporators of industrial and commercial refrigeration cabinets, by which hot refrigerant is circulated in the evaporators for defrost. 
         [0003]    2. Background Art 
         [0004]    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. 
         [0005]    According to a method known in the art, 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. 
         [0008]    Such defrost refrigeration systems are efficient in defrosting evaporators. However, new compressor systems are now available, which compressor systems operate differently from existing compressors commonly used in the refrigeration systems. It is therefore desirable to adapt defrost configurations to such new compressor systems so as to optimize the defrost of evaporators. 
       SUMMARY OF INVENTION 
       [0009]    It is therefore an aim of the present invention to provide a novel defrost refrigeration system. 
         [0010]    Therefore, in accordance with the present invention, there is provided a defrost refrigeration system of the type having a main refrigeration circuit in which a refrigerant absorbs heat from an evaporator stage and releases heat in a condensation stage, with a compression stage sequentially between the evaporation stage and the condensation stage during a refrigeration cycle, said defrost refrigeration system comprising: at least a first compressor and a second compressor serially positioned with respect to one another such that refrigerant going through the compression stage in the refrigeration cycle passes sequentially through the first compressor and the second compressor; a first line extending from an exit of one of the first compressor and the second compressor of the compression stage, and in fluid communication with the evaporator stage in a defrost cycle and adapted to receive a defrost portion of refrigerant compressed in the compression stage; and valves for switching at least one evaporator between the refrigeration cycle and the defrost cycle, by stopping/allowing a flow of refrigerant from the condensation stage to at least one evaporator of the evaporation stage in the refrigeration cycle, and for allowing/stopping a flow of said defrost portion of refrigerant to defrost the at least one evaporator in the defrost cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof and in which: 
           [0012]      FIG. 1  is a block diagram of a defrost refrigeration system constructed in accordance with a preferred embodiment of the present invention; 
           [0013]      FIG. 2  is a schematic view of a defrost refrigeration system in accordance with a first embodiment of the present invention; 
           [0014]      FIG. 3  is a schematic view of a defrost refrigeration system in accordance with a second embodiment of the present invention; and 
           [0015]      FIG. 4  is a schematic view of the defrost refrigeration system of  FIG. 3 , with an alternative configuration at the exit of refrigeration of a defrost cycle. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring now to the drawings, and more particularly to  FIG. 1 , a defrost refrigeration system in accordance with a preferred embodiment is generally shown at  10 . The defrost refrigeration system  10  operates a refrigeration cycle and has a compression stage  12 , a condensation/heat reclaim stage  14  and an evaporation stage  16 . 
         [0017]    In the refrigeration cycle, the compression stage  12  performs a compression of a refrigerant to a high-pressure gas state. The compression stage  12  is in fluid communication with the condensation/heat reclaim stage  14  by way of line  13 . 
         [0018]    The condensation/heat reclaim stage  14  releases heat from the high-pressure gas refrigerant received from the compression stage  12 . The heat is released to the atmosphere, for instance using roof-top condensers. Alternatively, heat may be recuperated using heat reclaim systems in series or in parallel with condensers. Moreover, the condensation/heat reclaim stage  14  may have refrigerant tanks to accumulate refrigerant having released heat and ready to be fed to the evaporation stage  16 . 
         [0019]    The condensed refrigerant is directed to the evaporation stage  16  using line  15 . The evaporation stage  16  typically has numerous evaporators in refrigeration cabinets, as well as the necessary expansion valves if required to set the refrigerant to a suitable condition to absorb heat. In some instances, the evaporators may be flooded with liquid refrigerant such that expansion valves are optional. 
         [0020]    The refrigerant having absorbed heat is then directed to the compression stage  12  using line  18  to complete the refrigeration cycle. 
         [0021]    The compression stage  12  uses high-efficiency compressors. More specifically, the compressors used in the compression stage  12  are magnetic-bearing, variable-speed centrifugal compressors of the type manufactured by Turbocor. These compressors operate at high efficiency, but offer a compression ratio at a maximum of 4.5/5:1. 
         [0022]    Accordingly, as shown in  FIG. 1 , these compressors are cascaded in the compression stage  12  of  FIG. 1 . It is required to cascade the compressors so as to provide the required compression of refrigerant in view of the warmer periods of the year, during which high pressures of refrigerant must be reached for the effective release of heat. More specifically, one or more first compressors  20  compress refrigerant that is fed through line  21  to an accumulator  22 . One or more second compressors  24  are positioned downstream of the accumulator  22 , and compress refrigerant that is fed from the accumulator  22  through line  23 . The refrigerant then exits the compression stage  12  to be fed to the condensation/heat reclaim stage  14 . 
         [0023]    In order to proceed with the defrost of evaporators from the evaporation stage  16 , low pressure refrigerant is directed from the compression stage  12  to the at least one evaporator of the evaporation stage  16 . 
         [0024]    In a first embodiment, the low-pressure refrigerant is produced by the first compressor  20  and a portion of this refrigerant is fed directly to the evaporator of the evaporation group  16  that is to be defrosted. As the first compressors  20  compress the refrigerant to a relatively low pressure, the refrigerant may be fed directly to the evaporators for defrost. As is shown in  FIG. 1 , a line  25  directs a portion of refrigerant from the first compressor  20  to the evaporation stage  16  for defrost. 
         [0025]    In the first embodiment, the refrigerant that has released heat during defrost is returned to the compression stage  12  for compression. Depending on its condition, the defrost refrigerant uses either the lines  17  or  18 , through an appropriate network of valves, to be fed to the first compressor  20  or to the accumulator  22 . Moreover, the defrost refrigerant may also be re-injected in the evaporation stage  16  or directed to the condensation/heat reclaim stage  14 , depending on its state. 
         [0026]    Referring to  FIG. 2 , the first embodiment of the defrost refrigeration system  10  is illustrated in further details. Like elements bear like reference numerals in the  FIGS. 1 to 4 . In  FIG. 2 , the defrost refrigeration system  10 ′ has a roof-top condenser  14 ′ and a heat-reclaim loop  14 ″. The evaporation stage  16  is separated into a group of low-temperature evaporators  16 A (e.g., freezer applications), and a group of medium-temperature evaporators  16 B (e.g., refrigerator applications). Reference numerals affixed with an A pertain to low-temperature refrigeration in  FIGS. 2 to 4 , whereas reference numerals affixed with a B will relate to medium-temperature refrigeration in  FIGS. 2 to 4 . Other evaporators  16 C are typically provided in the defrost refrigeration system  10 ′, but are not illustrated to simplify  FIG. 2 . 
         [0027]    In the first embodiment illustrated in  FIG. 1 , some refrigerant is directed by the line  25  from the output of the first compressor/compressors  20  to the evaporator stage  16  for defrost. As shown in  FIG. 2 , the line  25  diverges into lines  25 A and  25 B to respectively feed the evaporators  16 A and  16 B, respectively, with defrost refrigerant. 
         [0028]    The lines  25 A and  25 B merge into the return lines  18 A and  18 B, using appropriate valves to prevent the defrost refrigerant to be sucked by the compressor stage  12 . More specifically, valves  30 A and  30 B are opened while valves  31 A and  31 B are closed in the defrost sequence. These valves are in opposite positions during a refrigeration cycle. 
         [0029]    Defrost refrigerant is therefore directed to the evaporators  16 A and/or  16 B in a defrost cycle. As is illustrated in  FIG. 2 , a bypass  32 A/ 32 B is provided for the defrost refrigerant to surround the expansion valves of the evaporation stage  16 . 
         [0030]    The defrost refrigerant having released heat during defrost in the evaporators  16 A/ 16 B is then directed to the accumulator  22  using lines  17 A/ 17 B, respectively, which merge into line  17 . Valves  33 A/ 33 B are opened during the defrost cycle, whereas the valves  34 A/ 34 B are closed. These valves are in opposite positions during a refrigeration cycle. 
         [0031]    In a second embodiment, the low-pressure refrigerant is produced by the second compressor  24  and a portion of this refrigerant is fed to the evaporator of the evaporation group  16  that is to be defrosted. As the first compressors  24  compress the refrigerant to a relatively high pressure, a pressure-reducing device  27  is provided to ensure that the refrigerant fed to defrost evaporators is at a suitable low pressure. As is shown in  FIG. 1 , a line  26  directs a portion of refrigerant from the second compressor  24  to the evaporation stage  16  for defrost, with the pressure-reducing device  27  being positioned in the line  26 . 
         [0032]    In the second embodiment, the refrigerant that has released heat during defrost is either returned to the compression stage  12  for compression, or re-injected into the evaporation stage  16  to be used in the refrigeration cycle. Depending on its condition, the defrost refrigerant uses either the lines  17  or  18 , through an appropriate network of valves, to be fed to the first compressor  20  or to the accumulator  22 . 
         [0033]    Referring to  FIG. 3 , the second embodiment of the defrost refrigeration system is illustrated in further detail. In  FIG. 3 , the defrost refrigeration system  10 ″ has stages similar to that of the defrost refrigeration system  10 ′ of  FIG. 2 , whereby like elements will bear like reference numerals. 
         [0034]    In the second embodiment illustrated in  FIG. 1 , some refrigerant is directed by the line  26  from the output of the second compressor/compressors  24  to the evaporator stage  16  for defrost, passing through a pressure-reducing device  27  or a suitable solenoid valve. As shown in  FIG. 3 , the line  26  diverges into lines  26 A and  26 B to respectively feed the evaporators  16 A and  16 B, respectively, with defrost refrigerant. Similarly to the defrost refrigeration system  10 ′ of  FIG. 2 , the defrost refrigeration system  10 ″ operates a defrost cycle using the return lines  18 A/ 18 B, using a network of valves for the defrost refrigerant to be directed to the evaporators. 
         [0035]    During the defrost of the evaporators, the valves  34 A and  34 B control the flow of defrost refrigerant in the evaporators, by releasing refrigerant into the line  15 . During a refrigeration cycle, the valves  34 A and  34 B are opened. 
         [0036]    Referring to  FIG. 3 , a heat exchanger  40  is provided between the lines  15  and  17 ′. The line  15  directs refrigerant in the refrigeration cycle from the condensation/heat reclaim stage  14  to the evaporator stage  16 . The line  17 ′ directs refrigerant from the condensation/heat reclaim stage  14  to the accumulator  22 , so as to ensure that the refrigerant in the accumulator  22  is in a suitable condition to be fed to the second compressor  24 . As such that lines  15  and  17 ′ converge downstream of the heat exchanger  40 . The heat exchanger  40 , along with expansion valve  41 , controls the conditions of the refrigerant being fed to the evaporator stage  16  for refrigerating purposes and to the accumulator  22 . 
         [0037]    In an alternative configuration of the second embodiment illustrated in  FIG. 4 , a defrost refrigeration system is illustrated as  10 ′″. In  FIG. 4 , the defrost refrigeration system  10 ′″ has stages similar to that of the defrost refrigeration system  10 ′ of FIGS.  2  and  10 ″ of  FIG. 4 , whereby like elements will bear like reference numerals. 
         [0038]    A portion of refrigerant is directed by the line  26  from the output of the second compressor/compressors  24  to the evaporator stage  16  for defrost, passing through a pressure-reducing device  27  or suitable solenoid valve. As shown in  FIG. 4 , the line  26  diverges into lines  26 A and  26 B to respectively feed the evaporators  16 A and  16 B, respectively, with defrost refrigerant. Similarly to the defrost refrigeration system  10 ′ of FIGS.  2  and  10 ″ of  FIG. 3 , the defrost refrigeration system  10 ′″ operates a defrost cycle using the return lines  18 A/ 18 B, using a network of valves for the defrost refrigerant to be directed to the evaporators. The defrost refrigerant is then directed to the accumulator  22  at the compression stage  12  using line  17 . 
         [0039]    The lines  25 A and  25 B merge into the return lines  18 A and  18 B, using appropriate valves to prevent the defrost refrigerant to be sucked by the compressor stage  12 . More specifically, valves  30 A and  30 B are opened while valves  31 A and  31 B are closed in the defrost sequence. These valves are in opposite positions during a refrigeration cycle. 
         [0040]    Defrost refrigerant is therefore directed to the evaporators  16 A and/or  16 B in a defrost cycle. As is illustrated in  FIG. 4 , a bypass  32 A/ 32 B is provided for the defrost refrigerant to surround the expansion valves of the evaporation stage  16 . 
         [0041]    The defrost refrigerant having released heat during defrost in the evaporators  16 A/ 16 B is then directed to the accumulator  22  using lines  17 A/ 17 B, respectively, which merge into line  17 . Valves  33 A/ 33 B are opened during the defrost cycle, whereas the valves  34 A/ 34 B are closed. These valves are in opposite positions during a refrigeration cycle. 
         [0042]    Although the choice of refrigerants has not been described, it is pointed out that any suitable refrigerant can be used taking into account the conditions at which the refrigeration system will operate.