Patent Publication Number: US-7913506-B2

Title: Free cooling cascade arrangement for refrigeration system

Description:
BACKGROUND 
     The present invention relates to a refrigeration system with a low temperature portion and a medium temperature portion. The present invention relates more particularly to a refrigeration system where the low temperature portion may receive condenser cooling from refrigerant in the medium temperature portion in a cascade arrangement, or may share condenser cooling directly with the medium temperature system. 
     Refrigeration systems typically include a refrigerant that circulates through a series of components in a closed system to maintain a cold region (e.g., a region with a temperature below the temperature of the surroundings). One exemplary refrigeration system is a vapor refrigeration system including a compressor. Such a refrigeration system may be used, for example, to maintain a desired temperature within a temperature controlled storage device, such as a refrigerated display case, coolers, freezers, etc. The refrigeration systems may have a first portion with equipment intended to maintain a first temperature (such as a low temperature) and a second temperature (such as a medium temperature). The refrigerant in the low temperature portion and the refrigerant in the medium temperature portion are condensed in condensers which require a source of a coolant. 
     If the outside temperature is cold enough, an outdoor heat exchanger such as a cooling tower or a fluid cooler may be used as a part of the refrigeration system to provide a source of cooling for the condensers. Such an arrangement is often called a “free cooling” arrangement because the system does not need to operate an additional compressor. However, if the exterior air is not sufficiently cold, an exterior heat exchanger may not provide sufficient cooling for some systems. 
     SUMMARY 
     One embodiment of the invention relates to a refrigeration system, including medium temperature compact chiller units arranged in parallel and configured to cool a medium temperature liquid coolant for circulation to medium temperature refrigerated display cases, and low temperature compact chiller units arranged in parallel and configured to cool a low temperature liquid coolant for circulation to low temperature refrigerated display cases. A coolant supply header supplies a coolant to the low and medium temperature compact chiller units. A coolant suction header receives the coolant from the low and medium temperature compact chiller units. A fluid cooler cools the coolant in the coolant supply header. A cascade heat exchanger receives a supply of the medium temperature liquid coolant from the medium temperature compact chiller units. A pump is configured to pump the coolant from the coolant suction header to the coolant supply header and through the fluid cooler. Another pump is configured to pump the coolant from the coolant suction header to the coolant supply header and through the cascade heat exchanger. A valve on the coolant supply header between the low temperature compact chiller units and the medium temperature compact chiller units is movable to a closed position to define one cooling flow path comprising the first pump and the fluid cooler and the medium temperature modular chiller units, and another cooling flow path comprising the second pump and the cascade heat exchanger and the low temperature compact chiller units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a refrigeration system according to an exemplary including an outside fluid cooler that may selectively provide cooling for a low temperature refrigeration loop. 
         FIG. 2  is a block diagram of chiller unit of the system of  FIG. 1  according to one exemplary embodiment. 
         FIG. 3  is a block diagram of an assembly of the chiller units of  FIG. 2  arranged in parallel. 
         FIG. 4  is a block diagram of a refrigeration system according to one exemplary embodiment in a normal or cascade cooling arrangement. 
         FIG. 5  is a block diagram of the refrigeration system of  FIG. 4  in a free cooling arrangement. 
         FIG. 6  is a block diagram of a refrigeration system according to another exemplary embodiment in a normal or cascade cooling arrangement. 
         FIG. 7  is a block diagram of the refrigeration system of  FIG. 6  in a free cooling arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a refrigeration system  10  is shown according to an exemplary embodiment. Refrigeration systems  10  typically include a refrigerant (e.g., a vapor compression/expansion type refrigerant, etc.) that circulates through a series of components in a closed system to maintain a cold region (e.g., a region with a temperature below the temperature of the surroundings). The refrigeration system  10  of  FIG. 1  includes several subsystems or loops. 
     A first or low temperature subsystem  20  includes a low temperature chiller  22 , one or more low temperature cases  24  (e.g., refrigerated display cases, etc.), and a pump  26 . Pump  26  circulates a low temperature liquid coolant (e.g., potassium formate at approximately minus (−) 20° F.) between chiller  22  and cases  24  to maintain cases  24  at a relatively low temperature. 
     A second or medium temperature subsystem  30  includes a medium temperature chiller  32 , one or more medium temperature cases  34  (e.g., refrigerated display cases), and a pump  36 . Pump  36  circulates a medium temperature liquid coolant (e.g., propylene glycol at approximately 20° F.) between chiller  32  and cases  34  to maintain cases  34  at a relatively medium temperature. 
     Medium temperature chiller  32  removes heat energy from medium temperature cases  34  and, in turn, gives the heat energy up to a heat exchanger, such as an outdoor fluid cooler  50  or outdoor cooling tower to be dissipated to the exterior environment. Medium temperature chiller  32  is further coupled to a cascade heat exchanger  40  to provide a source of coolant to the cascade heat exchanger. 
     Low temperature chiller  22  receives heat energy from low temperature cases  24 . Low temperature chiller  22  may be coupled to either cascade heat exchanger  40  or fluid cooler  50 . A valve  60  provided between low temperature subsystem  20  and fluid cooler  50  and a pump  42  provided between low temperature subsystem  20  and cascade heat exchanger  40  determine to which component low temperature chiller  22  is coupled. In a normal operation or cascade mode, valve  60  is closed and pump  42  is activated, coupling low temperature chiller  22  to cascade heat exchanger  40 . However, if the exterior temperature is low enough, system  10  may enter a free cooling mode. In a free cooling mode, pump  42  is turned off and valve  60  is opened, coupling low temperature chiller  22  to fluid cooler  50 . 
     The terms “low temperature” and “medium” temperature are used herein for convenience to differentiate between two subsystems of refrigeration system  10 . Low temperature system  20  maintains one or more cases  24  such as freezer display cases or other cooled areas at a temperature lower than the ambient temperature. Medium temperature system  30  maintains one or more cases  34  such as refrigerator cases or other cooled areas at a temperature lower than the ambient temperature but higher than low temperature cases  24 . According to one exemplary embodiment, low temperature cases  24  may be maintained at a temperature of approximately minus (−) 20° F. and medium temperature cases  34  may be maintained at a temperature of approximately 20° F. Although only two subsystems are shown in the exemplary embodiments described herein, according to other exemplary refrigeration system  10  may include more subsystems that may be selectively cooled in a cascade arrangement or in a free cooling arrangement. 
     One exemplary chiller unit  70  is shown in  FIG. 2  and may be either a low temperature chiller  22  or a medium temperature chiller  32 . Chiller unit  70  includes a refrigerant that is circulated through a vapor-compression refrigeration cycle including a first heat exchanger  72 , a compressor  74 , a second heat exchanger  76 , and an expansion valve  78 . In the first heat exchanger  72 , the refrigerant absorbs heat from an associated display case(s) or other cooled area via a coolant circulated by a pump (e.g. pump  26  for low temperature cases, pump  36  for medium temperature cases, etc.). In the second heat exchanger  76  (e.g. condenser, etc.), the refrigerant gives up heat to a second coolant. The second coolant, in turn, gives up heat to the exterior environment. Various elements of the chiller unit  70  may be combined. For example, heat exchangers  72  and  76  may comprise a single device in one exemplary chiller unit  70 . 
     According to one exemplary embodiment, chiller unit  70  is a compact modular chiller unit. As shown in  FIG. 3 , each of low temperature chiller  22  and medium temperature chiller  32  may include a multitude of chiller units  70  arranged in parallel. The number of chiller units  70  may be varied to accommodate various cooling loads associated with a particular system. 
     Referring now to  FIGS. 4 and 5 , a refrigeration system  10  is shown according to one exemplary embodiment in a cascade mode ( FIG. 4 ) and a free cooling mode ( FIG. 5 ). Refrigeration system  10  includes a low temperature subsystem  20 , a medium temperature subsystem  30 , a cascade heat exchanger  40 , a fluid cooler  50 , and a valve  60  that selectively couples low temperature subsystem  20  to fluid cooler  50 . 
     Fluid cooler  50  is shown to be provided outside a building where it is exposed to the outside air (e.g. at ambient temperature, etc.). Fluid cooler  50  uses the outside air to cool a coolant (e.g. a condenser coolant such as water, etc.) that flows through a condenser cooling circuit for refrigeration system  10 . Fluid cooler  50  is coupled to a condenser coolant supply header  54  and a condenser coolant suction header  56 . Flow through fluid cooler  50  is provided by a pump  52  located, for example, in-line with suction header  56 . Medium temperature subsystem  30  is cooled by fluid cooler  50  in all modes and fluid is circulated through medium temperature chiller  32  via supply header  54  and suction header  56 . Low temperature subsystem  20  is likewise coupled to supply header  54  and suction header  56  with valve  60  provided between low temperature chiller  22  and fluid cooler  50 . 
     Cascade heat exchanger  40  is coupled to both low temperature subsystem  20  and medium temperature subsystem  30 . According to an exemplary embodiment, one side of cascade heat exchanger  40  is connected to a first loop  46  that is coupled in parallel with medium temperature cases  34  to medium temperature chiller  32  (e.g., on the first heat exchanger  72  side of chiller  32 ). A second side of exchanger  40  is connected to a second loop  48  that is coupled to low temperature chiller  22  opposite of low temperature cases  24  (e.g., on the condenser or second heat exchanger  76  side of chiller  22 ). A pump  42  is provided to circulate fluid through second loop  48  and a check valve  44 . Fluid in first loop  46  is circulated by pump  36  of medium temperature subsystem  30 . 
     Referring to  FIG. 4 , in a normal operation or cascade mode, valve  60  is moved to a closed position that defines two flow paths, and pump  42  is activated. In the first flow path, low temperature chiller  22  is coupled to cascade heat exchanger  40  and pump  42  to provide a cascade condenser cooling loop for the low temperature chillers. In the second flow path, medium temperature chiller  32  is coupled to fluid cooler  50  and pump  52  to provide a condenser cooling loop for the medium temperature chillers. While valve  60  is closed, isolating low temperature chiller  22  from supply header  54 , a small amount of fluid may still mix with the fluid in suction header  56  (e.g. fluid flowing from medium temperature chiller  32  to condenser pumps  52 ). Fluid in second loop  48  passes through low temperature chiller  22  and is heated, carrying heat energy absorbed from low temperature cases  24  to cascade heat exchanger  40 . In heat exchanger  40  fluid in second loop  48  thus heats fluid in first loop  46 . Fluid in first loop  46  joins heated fluid from medium temperature cases  34  and is cooled by medium temperature chiller  32  before returning to cascade heat exchanger  40 . 
     If the outside temperature is sufficiently cold (e.g., below 60° F.), refrigeration system  10  may be converted to a “free cooling” mode as shown in  FIG. 5 . In the free-cooling mode valve  60  is moved to the open position to define a third flow path that provides condenser cooling for both the low and medium temperature chillers  22 ,  32  from fluid cooler  50  and bypasses the cascade heat exchanger  40  by turning pump  42  off and any back flow through second loop  48  is halted by check valve  44 . In the third flow path, pumps  52  circulate the fluid (e.g. condenser coolant) through the fluid cooler  50  and then to the heat exchangers (i.e. condensers) in both the low temperature chillers  22  and the medium temperature chillers  32 . The fluid passes through low temperature chiller  22  and is heated, carrying heat energy absorbed from low temperature cases  24  to suction header  56 . Pump  52  then pumps the fluid to fluid cooler  50  where it is cooled by the outside air before returning to supply header  54  and then to low temperature chiller  22 . Bypassing cascade heat exchanger  40  places a smaller load on medium temperature chillers  32  and takes advantage of the relatively low-cost cooling provided by outside fluid cooler  50 . 
     The operation of valve  60  and pump  42  is controlled by a control system  62 . Control system monitors the outside conditions (e.g., temperature, relative humidity, etc.) and determines whether refrigeration system  10  functions in a cascade mode or a free cooling mode by operating valve  60  and pump  42 . 
     Referring now to  FIGS. 6 and 7 , a refrigeration system  110  is shown according to another exemplary embodiment in a cascade mode ( FIG. 6 ) and a free cooling mode ( FIG. 7 ). Refrigeration system  110  may be, for example, an existing system that is retrofitted to incorporate the advantages described above. Refrigeration system  110  includes a low temperature subsystem  20 , a medium temperature subsystem  30 , a fluid cooler  50 , and a pump station  80 . Pump station  80  includes a cascade heat exchanger  40 , cascade pumps  42 , condenser pumps  52 , and a valve  60  that selectively couples low temperature subsystem  20  to fluid cooler  50  for operation in a free-cooling mode. 
     Fluid cooler  50  is typically provided outside a building (e.g. food retail outlet, etc.) where it is exposed to the outside air. Fluid cooler  50  uses the outside air to cool a coolant for refrigeration system  110 . Fluid cooler  50  is coupled to a common supply header  54  and a common suction header  56 . Flow through fluid cooler  50  is provided by a one or more condenser pumps  52 . As shown in  FIGS. 6 and 7 , two or more condenser pump  52  and check valve  58  pairs may be arranged in parallel and be coupled to common suction header  56 . Medium temperature subsystem  30  is cooled by fluid cooler  50  in all modes and fluid passes through medium temperature chiller  32  via supply header  54  and suction header  56 . Low temperature subsystem  20  is likewise coupled to supply header  54  and suction header  56  with valve  60  provided between low temperature chiller  22  and fluid cooler  50 . 
     Cascade heat exchanger  40  is coupled to both low temperature subsystem  20  and medium temperature subsystem  30 . According to an exemplary embodiment, one side of heat exchanger  40  is connected to a first loop  46  that is coupled in parallel with medium temperature cases  34  to medium temperature chiller  32  (e.g., on the first heat exchanger  72  side of chiller  32 ). A second side of cascade heat exchanger  40  is connected to a second loop  48  that is coupled to low temperature chiller  22  opposite of low temperature cases  24  (e.g., on the condenser or second heat exchanger  76  side of chiller  22 ). Cascade heat exchanger  40  includes one or more cascade pumps  42  to circulate fluid through second loop  48  and check valve  44 . As shown in  FIGS. 6 and 7 , two or more cascade pump  42  and check valve  44  pairs may be arranged in parallel and be coupled to common suction header  56 . Fluid in first loop  46  is circulated with pump  36  of medium temperature subsystem  30 . 
     Referring to  FIG. 6 , in a normal operation or cascade mode, valve  60  is closed and pumps  42  are activated, thus coupling low temperature chiller  22  to cascade heat exchanger  40 . While valve  60  is closed, isolating low temperature chiller  22  from supply header  54 , a small amount of fluid may still mix with the fluid in suction header  56  (e.g. fluid flowing from medium temperature chiller  32  to condenser pumps  52 ). Fluid in second loop  48  passes through low temperature chiller  22  and is heated, carrying heat energy absorbed from low temperature cases  24  to cascade heat exchanger  40 . In heat exchanger  40  fluid in second loop  48  heats the fluid in first loop  46 . Fluid in first loop  46  joins heated fluid from medium temperature cases  34  and is cooled by medium temperature chiller  32  before returning to cascade heat exchanger  40 . 
     If the outside temperature is sufficiently cold (e.g., below 60° F.), refrigeration system  110  may be converted to a free cooling mode as shown in  FIG. 7 . Valve  60  is opened, thus coupling low temperature chiller  22  to fluid cooler  50 . Pumps  42  are turned off and any back flow through second loop  48  is halted by check valves  44 . Fluid passes through low temperature chiller  22  and is heated, carrying heat energy absorbed from low temperature cases  24  to suction header  56 . Pumps  52  then circulate the fluid to fluid cooler  50  where it is cooled by the outside air before returning to supply header  56  and then to low temperature chiller  22 . Bypassing cascade heat exchanger  40  places a smaller load on medium temperature chillers  32  and takes advantage of the relatively low-cost cooling provided by outside fluid cooler  50 . 
     The operation of valve  60  and pump  42  is controlled by a control system  62 . Control system monitors the outside conditions (e.g., temperature, relative humidity, etc.) and determines whether refrigeration system  110  functions in a cascade mode or a free cooling mode by operating valve  60  and pump  42 . 
     It is important to note that the construction and arrangement of the elements of the refrigeration system provided herein are illustrative only. Although only a few exemplary embodiments of the present invention(s) have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible in these embodiments (such as variations in features such as connecting structure, components, materials, sequences, capacities, shapes, dimensions, proportions and configurations of the modular elements of the system, without materially departing from the novel teachings and advantages of the invention(s). For example, any number of chiller units may be provided in parallel to cool the low temperature and medium temperature cases, or more subsystems may be included in the refrigeration system (e.g., a very cold subsystem or additional cold or medium subsystems). Further, it is readily apparent that variations and modifications of the refrigeration system and its components and elements may be provided in a wide variety of materials, types, shapes, sizes and performance characteristics. Accordingly, all such variations and modifications are intended to be within the scope of the invention(s).