Abstract:
A refrigeration system ( 20 A) comprises an evaporator ( 27 ), a plurality of compressors ( 32, 34, 35 ) for compressing a refrigerant, a heat rejecting heat exchanger ( 24 ) for cooling the refrigerant, and a plurality of economizer heat exchangers ( 28 A,  28 B). Each of the economizer heat exchangers ( 28 A,  28 B) is configured to inject a portion of the refrigerant into a suction port ( 52, 56 ) of one of the compressors ( 34, 35 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to refrigerating systems used for cooling. More particularly, the present invention relates to a refrigerating system that incorporates economizer circuits to increase system efficiency. 
         [0002]    A typical refrigerating system includes an evaporator, a compressor, a condenser, and a throttle valve. A refrigerant, such as a hydrofluorocarbon (HFC), typically enters the evaporator as a two-phase liquid-vapor mixture. Within the evaporator, the liquid portion of the refrigerant changes phase from liquid to vapor as a result of heat transfer into the refrigerant. The refrigerant is then compressed within the compressor, thereby increasing the pressure of the refrigerant. Next, the refrigerant passes through the condenser, where it changes phase from a vapor to a liquid as it cools within the condenser. Finally, the refrigerant expands as it flows through the throttle valve, which results in a decrease in pressure and a change in phase from a liquid to a two-phase liquid-vapor mixture. 
         [0003]    While natural refrigerants such as carbon dioxide have recently been proposed as alternatives to the presently used HFCs, the high side pressure of carbon dioxide typically ends up in the supercritical region where there is no transition from vapor to liquid as the high pressure refrigerant is cooled. For a typical single stage vapor compression cycle, this leads to poor efficiency due to the loss of the subcritical constant temperature condensation process and to the relatively high residual enthalpy of supercritical carbon dioxide at normal high side temperatures. 
         [0004]    Thus, there exists a need for a refrigerating system that is capable of utilizing any refrigerant, including a transcritical refrigerant, while maintaining a high level of system efficiency. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention is a refrigeration system comprising an evaporator, a plurality of compressors for compressing a refrigerant, a heat rejecting heat exchanger for cooling the refrigerant, and a plurality of economizer heat exchangers. Each of the economizer heat exchangers is configured to inject a portion of the refrigerant into a suction port of one of the compressors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  illustrates a schematic diagram of a refrigeration system employing a pair of economizer circuits. 
           [0007]      FIG. 1B  illustrates a graph relating enthalpy to pressure for the refrigeration system of  FIG. 1A . 
           [0008]      FIG. 2A  illustrates a schematic diagram of a refrigeration system employing three economizer circuits. 
           [0009]      FIG. 2B  illustrates a graph relating enthalpy to pressure for the refrigeration system of  FIG. 2A . 
           [0010]      FIG. 3A  illustrates a schematic diagram of a refrigeration system employing four economizer circuits. 
           [0011]      FIG. 3B  illustrates a graph relating enthalpy to pressure for the refrigeration system of  FIG. 3A . 
           [0012]      FIG. 4A  illustrates a schematic diagram of a refrigeration system employing five economizer circuits. 
           [0013]      FIG. 4B  illustrates a graph relating enthalpy to pressure for the refrigeration system of  FIG. 4A . 
           [0014]      FIG. 5A  illustrates a schematic diagram of a second embodiment of a refrigeration system employing a pair of economizer circuits. 
           [0015]      FIG. 5B  illustrates a graph relating enthalpy to pressure for the refrigeration system of  FIG. 5A . 
           [0016]      FIG. 6  illustrates a schematic diagram of an alternative embodiment of the refrigeration system of  FIG. 1A . 
           [0017]      FIG. 7  illustrates a schematic diagram of another embodiment of the refrigeration system of  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1A  illustrates a schematic diagram of refrigeration system  20 A, which includes compressor unit  22 , heat rejecting heat exchanger  24 , first economizer circuit  25 A, second economizer circuit  25 B, main expansion valve  26 , evaporator  27 , and sensor  31 . First economizer circuit  25 A includes first economizer heat exchanger  28 A, expansion valve  30 A, and sensor  31 A, while second economizer circuit  25 B includes second economizer heat exchanger  28 B, expansion valve  30 B, and sensor  31 B. As shown in  FIG. 1A , first economizer heat exchanger  28 A and second economizer heat exchanger  28 B are parallel flow tube-in-tube heat exchangers. 
         [0019]    Compressor unit  22  includes two-stage compressor  32 , single-stage compressor  34 , and single-stage compressor  35 . Two-stage compressor  32  includes cylinders  36 A and  36 B connected in series, single-stage compressor  34  includes cylinder  36 C, and single-stage compressor  35  includes cylinder  36 D. Two-stage compressor  32 , single-stage compressor  34 , and single-stage compressor  35  may be stand-alone compressor units, or they may be part of a single, multi-cylinder compressor unit. In addition, two-stage compressor  32 , single-stage compressor  34 , and single-stage compressor  35  are preferably reciprocating compressors, although other types of compressors may be used including, but not limited to, scroll, screw, rotary vane, standing vane, variable speed, hermetically sealed, and open drive compressors. 
         [0020]    In refrigeration system  20 A, three distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is defined by the route between points  1 ,  2 ,  3 ,  4 ,  5 , and  6 . A first economized refrigerant path is defined by the route between points  5 A,  6 A,  7 A, and  8 A. Finally, a second economized refrigerant path is defined by the route between points  5 B,  6 B,  7 B, and  8 B. It should be understood that the paths are all closed paths that allow for continuous flow of refrigerant through refrigeration system  20 A. 
         [0021]    In reference to the main refrigerant path, after refrigerant exits two-stage compressor  32  at high pressure and enthalpy through discharge port  39  (point  4 ), the refrigerant loses heat in heat rejecting heat exchanger  24 , exiting heat rejecting heat exchanger  24  at low enthalpy and high pressure (point  5 A). The refrigerant then splits into two flow paths  40 A and  42 A prior to entering first economizer heat exchanger  28 A. The main path continues along paths  40 A and  40 B through first economizer heat exchanger  28 A (point  5 B) and second economizer heat exchanger  28 B (point  5 ), respectively. As the refrigerant in path  40 A flows through first economizer heat exchanger  28 A, it is cooled by the refrigerant in path  42 A of the first economized path. Similarly, as the refrigerant in path  40 B flows through second economizer heat exchanger  28 B, it is cooled by the refrigerant in path  42 B of the second economized path. Refrigerant from path  40 B is then throttled in main expansion valve  26 . Main expansion valve  26 , along with economizer expansion valves  30 A and  30 B, are preferably thermal expansion valves (TXV) or electronic expansion valves (EXV). After going through an expansion process within main expansion valve  26  (point  6 ), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator  27 . After evaporation of the remainder of the liquid (point  1 ), the refrigerant enters two-stage compressor  32  through suction port  37 . The refrigerant is compressed within cylinder  36 A, which is the first stage of two-stage compressor  32 , and is then directed out discharge port  50  (point  2 ), where it flows through intercooler  48  prior to a second stage of compression in cylinder  36 B. Intercooler  48  is configured to cool down the refrigerant discharged from cylinder  36 A prior to the second stage of compression within cylinder  36 B. After the second stage of compression, the refrigerant is discharged through discharge port  39  (point  4 ). 
         [0022]    In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger  24  at low enthalpy and high pressure (point  5 A) and splits into two flow paths  40 A and  42 A, the first economized path continues along path  42 A. In path  42 A, the refrigerant is throttled to a lower pressure by economizer expansion valve  30 A (point  6 A) prior to flowing through first economizer heat exchanger  28 A. The refrigerant from path  42 A that flowed through first economizer heat exchanger  28 A (point  7 A) is then directed along economizer return path  46 A and injected into suction port  52  of single-stage compressor  34  for compression in single-stage compressor  34 . After compression within single-stage compressor  34 , the refrigerant is discharged through discharge port  54  (point  8 A) where it merges with the refrigerant discharged from two-stage compressor  32  and single-stage compressor  35 . 
         [0023]    In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger  28 A (point  5 B), the refrigerant in path  40 A splits into two flow paths  40 B and  42 B. The second economized path continues along flow path  42 B where the refrigerant is throttled to a lower pressure by economizer expansion valve  30 B (point  6 B) prior to flowing through second economizer heat exchanger  28 B. The refrigerant from path  42 B that flowed through second economizer heat exchanger  28 B (point  7 B) is then directed along economizer return path  46 B and injected into suction port  56  of single-stage compressor  35  for compression in single-stage compressor  35 . After compression within single-stage compressor  35 , the refrigerant is discharged through discharge port  58  (point  8 B) where it merges with the refrigerant discharged from two-stage compressor  32  and single-stage compressor  34 . 
         [0024]    Refrigeration system  20 A also includes sensor  31  disposed between evaporator  27  and compressor unit  22  along the main refrigerant path. In general, sensor  31  acts with expansion valve  26  to sense the temperature of the refrigerant leaving evaporator  27  and the pressure of the refrigerant in evaporator  27  to regulate the flow of refrigerant into evaporator  27  to keep the combination of temperature and pressure within some specified bounds. In a preferred embodiment, expansion valve  26  is an electronic expansion valve and sensor  31  is a temperature transducer such as a thermocouple or thermistor. In another embodiment, expansion valve  26  is a mechanical thermal expansion valve and sensor  31  includes a small tube that terminates in a pressure vessel filled with a refrigerant that differs from the refrigerant running through refrigeration system  20 A. As refrigerant from evaporator  27  flows past sensor  31  on its way toward compressor unit  22 , the pressure vessel will either heat up or cool down, thereby changing the pressure within the pressure vessel. As the pressure in the pressure vessel changes, sensor  31  sends a signal to expansion valve  26  to modify the pressure drop caused by the valve. Similarly, in the case of the electronic expansion valve, sensor  31  sends an electrical signal to expansion valve  26  which responds in a similar manner to regulate refrigerant flow. For example, if a return gas coming from evaporator  27  is too hot, sensor  31  will then heat up and send a signal to expansion valve  26 , causing the valve to open further and allow more refrigerant per unit time to flow through evaporator  27 , thereby reducing the heat of the refrigerant exiting evaporator  27 . 
         [0025]    Economizer circuits  25 A and  25 B also include sensors  31 A and  31 B, respectively, that operate in a similar manner to sensor  31 . However, sensors  31 A and  31 B sense temperature along economizer return paths  46 A and  46 B and act with expansion valves  30 A and  30 B to control the pressure drops within expansion valves  30 A and  30 B instead. It should also be noted that various other sensors may be substituted for sensors  31 ,  31 A, and  31 B without departing from the spirit and scope of the present invention. 
         [0026]    By controlling the expansion valves  26 ,  30 A, and  30 B, the operation of refrigeration system  20 A can be adjusted to meet the cooling demands and achieve optimum efficiency. In addition to adjusting the pressures associated with expansion valves  26 ,  30 A, and  30 B, the displacements of cylinders  36 A,  36 B,  36 C, and  36 D may also be adjusted to help achieve optimum efficiency of refrigeration system  20 A. 
         [0027]      FIG. 1B  illustrates a graph relating enthalpy to pressure for the refrigeration system  20 A of  FIG. 1A . Vapor dome V is formed by a saturated liquid line and a saturated vapor line, and defines the state of the refrigerant at various points along the refrigeration cycle. Underneath vapor dome V, all states involve both liquid and vapor coexisting at the same time. At the very top of vapor dome V is the critical point. The critical point is defined by the highest pressure where saturated liquid and saturated vapor coexist. In general, compressed liquids are located to the left of vapor dome V, while superheated vapors are located to the right of vapor dome V. 
         [0028]    In  FIG. 1B , the main refrigerant path is defined by the route between points  1 ,  2 ,  3 ,  4 ,  5 , and  6 ; the first economized path is defined by the route between points  5 A,  6 A,  7 A, and  8 A; and the second economized path is defined by the route between points  5 B,  6 B,  7 B, and  8 B. The cycle begins in the main path at point  1 , where the refrigerant is at a low pressure and high enthalpy prior to entering compressor unit  22 . After a first stage of compression within cylinder  36 A of two-stage compressor  32 , both the enthalpy and pressure increase as shown by point  2 . Next, the refrigerant is cooled down as it flows through intercooler  48 , as shown by point  3 . After a second stage of compression within cylinder  36 B, the refrigerant exits compressor unit  22  at high pressure and even higher enthalpy, as shown by point  4 . Then, as the refrigerant flows through heat rejecting heat exchanger  24 , enthalpy decreases while pressure remains constant. Prior to entering first economizer heat exchanger  28 A, the refrigerant splits into a main portion and a first economized portion as shown by point  5 A. Similarly, prior to entering second economizer heat exchanger  28 B, a second economized portion is diverted from the main portion as shown by point  5 B. The first and second economized portions will be discussed in more detail below. The main portion is then throttled in main expansion valve  26 , decreasing pressure as shown by point  6 . Finally, the main portion of the refrigerant is evaporated, exiting evaporator  27  at a higher enthalpy as shown by point  1 . 
         [0029]    As stated previously, the first economized portion splits off of the main portion as indicated by point  5 A. The first economized portion is throttled to a lower pressure in expansion valve  30 A as shown by point  6 A. The first economized portion of the refrigerant then exchanges heat with the main portion in first economizer heat exchanger  28 A, cooling down the main portion of the refrigerant as indicated by point  5 B, and heating up the first economized portion of the refrigerant as indicated by point  7 A. The first economized portion is then compressed within single-stage compressor  34  and merged with the refrigerant discharged from two-stage compressor  32  and single-stage compressor  35 , as shown by point  8 A. 
         [0030]    As stated previously, the second economized portion splits off of the main portion as indicated by point  5 B. The second economized portion is throttled to a lower pressure in expansion valve  30 B as shown by point  6 B. The second economized portion of the refrigerant then exchanges heat with the main portion within second economizer heat exchanger  28 B, cooling down the main portion of the refrigerant to its lowest temperature as indicated by point  5 , and heating up the second economized portion of the refrigerant as indicated by point  7 B. The second economized portion is then compressed within single-stage compressor  35  and merged with the refrigerant discharged from two-stage compressor  32  and single-stage compressor  34 , as shown by point  8 B. 
         [0031]    In a refrigeration system, the specific cooling capacity, which is the measure of total cooling capacity divided by refrigerant mass flow, may typically be represented on a graph relating pressure to enthalpy by the length of the evaporation line. Furthermore, when the specific cooling capacity is divided by the specific power input to the compressor, the result is the system efficiency. In general, a high specific cooling capacity achieved by inputting a low specific power to the compressor will yield a high efficiency. 
         [0032]    As shown in  FIG. 1B , the specific cooling capacity of refrigeration system  20 A is represented by the length of evaporation line E 1  from point  6  to point  1 . Lines A 1  and A 2  represent the increased specific cooling capacity due to the addition of the first economizer circuit  25 A and second economizer circuit  25 B, respectively. This indicates that refrigeration system  20 A, which includes two economizer circuits, has a larger specific cooling capacity than a refrigeration system with no economizer circuits. Along with the increase in specific cooling capacity also comes an increase in specific power consumption. The increase in specific power consumption is a result of the additional compression of the economized flow shown between points  7 A and  8 A as well as between points  7 B and  8 B. However, since the economized vapor is compressed over a smaller pressure range than the main portion of refrigerant, the added compression power is less than the added capacity. Therefore, the ratio of capacity to power (the efficiency) is increased by the addition of the two economizer circuits. 
         [0033]      FIG. 2A  illustrates a schematic diagram of refrigeration system  20 B of the present invention employing three economizer circuits. Refrigeration system  20 B is similar to refrigeration system  20 A, except that single-stage compressor  70  is added to compressor unit  22 , and third economizer circuit  25 C is added to the system. Single-stage compressor  70  includes cylinder  36 E. 
         [0034]    In refrigeration system  20 B, four distinct refrigerant paths are formed by connection of the various elements in the system. The main refrigerant path, the first economized refrigerant path, and the second economized refrigerant path are similar to those described above in reference to  FIG. 1A . A third economized refrigerant path is defined by the route between points  5 C,  6 C,  7 C, and  8 C. 
         [0035]    In reference to the third economized path, after being cooled in the higher pressure second economizer heat exchanger  28 B, the refrigerant in path  40 B splits into two flow paths  40 C and  42 C (point  5 C). The third economized path continues along flow path  42 C where the refrigerant is throttled to a lower pressure by economizer expansion valve  30 C prior to flowing through third economizer heat exchanger  28 C (point  6 C). The refrigerant from path  42 C that flowed through third economizer heat exchanger  28 C (point  7 C) is then directed along economizer return path  46 C and injected into suction port  72  of single-stage compressor  70  for compression in single-stage compressor  70 . After compression within single-stage compressor  70 , the refrigerant is discharged through discharge port  74  (point  8 C) where it merges with the refrigerant discharged from two-stage compressor  32  and single-stage compressors  34  and  35 . 
         [0036]      FIG. 2B  illustrates a graph relating enthalpy to pressure for the refrigeration system  20 B of  FIG. 2A . In  FIG. 2B , the main refrigerant path is defined by the route between points  1 ,  2 ,  3 ,  4 ,  5 , and  6 ; the first economized path is defined by the route between points  5 A,  6 A,  7 A, and  8 A; the second economized path is defined by the route between points  5 B,  6 B,  7 B, and  8 B; and the third economized path is defined by the route between points  5 C,  6 C,  7 C, and  8 C. As shown in  FIG. 2B , evaporation line E 2  of refrigeration system  20 B is longer than evaporation line E 1  of refrigeration system  20 A ( FIG. 1B ). This indicates that refrigeration system  20 B, which includes three economizer circuits, has a larger specific cooling capacity than refrigeration system  20 A, which includes two economizer circuits. In particular, line A 3  represents the increased specific cooling capacity due to the addition of the third economizer circuit. 
         [0037]      FIG. 3A  illustrates a schematic diagram of refrigeration system  20 C of the present invention employing four economizer circuits. Refrigeration system  20 C is similar to refrigeration system  20 B, except that single-stage compressor  80  is added to compressor unit  22 , and fourth economizer circuit  25 D is added to the system. Single-stage compressor  80  includes cylinder  36 F. 
         [0038]    In refrigeration system  20 C, five distinct refrigerant paths are formed by connection of the various elements in the system. The main refrigerant path, the first economized refrigerant path, the second economized refrigerant path, and the third economized refrigerant path are similar to those described above in reference to  FIGS. 1A and 2A . A fourth economized refrigerant path is defined by the route between points  5 D,  6 D,  7 D, and  8 D. 
         [0039]    In reference to the fourth economized path, after being cooled in the higher pressure third economizer heat exchanger  28 C, the refrigerant in path  40 C splits into two flow paths  40 D and  42 D (point  5 D). The fourth economized path continues along flow path  42 D where the refrigerant is throttled to a lower pressure by economizer expansion valve  30 D prior to flowing through fourth economizer heat exchanger  28 D (point  6 D). The refrigerant from path  42 D that flowed through fourth economizer heat exchanger  28 D is then directed along economizer return path  46 D (point  7 D) and injected into suction port  82  of single-stage compressor  80  for compression in single-stage compressor  80 . After compression within single-stage compressor  80  (point  8 D), the refrigerant is discharged through discharge port  84  where it merges with the refrigerant discharged from two-stage compressor  32  and single-stage compressors  34 ,  35 , and  70 . 
         [0040]      FIG. 3B  illustrates a graph relating enthalpy to pressure for the refrigeration system  20 C of  FIG. 3A . In  FIG. 3B , the main refrigerant path is defined by the route between points  1 ,  2 ,  3 ,  4 ,  5 , and  6 ; the first economized path is defined by the route between points  5 A,  6 A,  7 A, and  8 A; the second economized path is defined by the route between points  5 B,  6 B,  7 B, and  8 B; the third economized path is defined by the route between points  5 C,  6 C,  7 C, and  8 C; and the fourth economized path is defined by the route between points  5 D,  6 D,  7 D, and  8 D. As shown in  FIG. 3B , evaporation line E 3  of refrigeration system  20 C is longer than evaporation line E 2  of refrigeration system  20 B ( FIG. 2B ). This indicates that refrigeration system  20 C, which includes four economizer circuits, has a larger specific cooling capacity than refrigeration system  20 B, which includes three economizer circuits. In particular, line A 4  represents the increased specific cooling capacity due to the addition of the fourth economizer circuit. 
         [0041]      FIG. 4A  illustrates a schematic diagram of refrigeration system  20 D of the present invention employing five economizer circuits. Refrigeration system  20 D is similar to refrigeration system  20 C, except that single-stage compressor  90  is added to compressor unit  22 , and fifth economizer circuit  25 E is added to the system. Single-stage compressor  90  includes cylinder  36 G. 
         [0042]    In refrigeration system  20 D, six distinct refrigerant paths are formed by connection of the various elements in the system. The main refrigerant path, the first economized refrigerant path, the second economized refrigerant path, the third economized refrigerant path, and the fourth economized refrigerant path are similar to those described above in reference to  FIGS. 1A ,  2 A, and  3 A. A fifth economized refrigerant path is defined by the route between points  5 E,  6 E,  7 E, and  8 E. 
         [0043]    In reference to the fifth economized path, after being cooled in the higher pressure fourth economizer heat exchanger  28 D, the refrigerant in path  40 D splits into two flow paths  40 E and  42 E (point  5 E). The fifth economized path continues along flow path  42 E where the refrigerant is throttled to a lower pressure by economizer expansion valve  30 E prior to flowing through fifth economizer heat exchanger  28 E (point  6 E). The refrigerant from path  42 E that flowed through fifth economizer heat exchanger  28 E is then directed along economizer return path  46 E (point  7 E) and injected into suction port  92  of single-stage compressor  90  for compression in single-stage compressor  90 . After compression within single-stage compressor  90 , the refrigerant is discharged through discharge port  94  (point  8 E) where it merges with the refrigerant discharged from two-stage compressor  32  and single-stage compressors  34 ,  35 ,  70 , and  80 . 
         [0044]      FIG. 4B  illustrates a graph relating enthalpy to pressure for the refrigeration system  20 D of  FIG. 4A . In  FIG. 4B , the main refrigerant path is defined by the route between points  1 ,  2 ,  3 ,  4 ,  5 , and  6 ; the first economized path is defined by the route between points  5 A,  6 A,  7 A, and  8 A; the second economized path is defined by the route between points  5 B,  6 B,  7 B, and  8 B; the third economized path is defined by the route between points  5 C,  6 C,  7 C, and  8 C; the fourth economized path is defined by the route between points  5 D,  6 D,  7 D, and  8 D; and the fifth economized path is defined by the route between points  5 E,  6 E,  7 E, and  8 E. As shown in  FIG. 4B , evaporation line E 4  of refrigeration system  20 D is longer than evaporation line E 3  of refrigeration system  20 C ( FIG. 3B ). This indicates that refrigeration system  20 D, which includes five economizer circuits, has a larger specific cooling capacity than refrigeration system  20 C, which includes four economizer circuits. In particular, line A 5  represents the increased specific cooling capacity due to the addition of the fifth economizer circuit. 
         [0045]      FIG. 5A  illustrates a schematic diagram of refrigeration system  20 E of the present invention employing two economizer circuits. Refrigeration system  20 E is similar to and an alternative embodiment of refrigeration system  20 A. In refrigeration system  20 E, intercooler  48  has been removed and two-stage compressor  32  has been replaced by single-stage compressor  100 . Single-stage compressor  100  includes cylinder  36 H. 
         [0046]    In refrigeration system  20 E, three distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is defined by the route between points  1 ,  2 ,  3 , and  4 . A first economized refrigerant path is defined by the route between points  3 A,  4 A,  5 A, and  6 A. Finally, a second economized refrigerant path is defined by the route between points  3 B,  4 B,  5 B, and  6 B. 
         [0047]    In reference to the main refrigerant path, after refrigerant exits single-stage compressor  100  at high pressure and enthalpy through discharge port  104  (point  2 ), the refrigerant loses heat in heat rejecting heat exchanger  24 , exiting heat rejecting heat exchanger  24  at low enthalpy and high pressure (point  3 A). The refrigerant then splits into two flow paths  40 A and  42 A prior to entering first economizer heat exchanger  28 A. The main path continues along paths  40 A and  40 B through first economizer heat exchanger  28 A (point  3 B) and second economizer heat exchanger  28 B (point  3 ), respectively. As the refrigerant in path  40 A flows through first economizer heat exchanger  28 A, it is cooled by the refrigerant in path  42 A of the first economized path. Similarly, as the refrigerant in path  40 B flows through second economizer heat exchanger  28 B, it is cooled by the refrigerant in path  42 B of the second economized path. 
         [0048]    Refrigerant from path  40 B is then throttled in main expansion valve  26 . After going through an expansion process within main expansion valve  26  (point  4 ), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator  27 . After evaporation of the remainder of the liquid (point  1 ), the refrigerant enters single-stage compressor  100  through suction port  102 . The refrigerant is then compressed within cylinder  36 H and discharged through discharge port  104  (point  2 ). 
         [0049]    In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger  24  at low enthalpy and high pressure (point  3 A) and splits into two flow paths  40 A and  42 A, the first economized path continues along path  42 A. In path  42 A, the refrigerant is throttled to a lower pressure by economizer expansion valve  30 A (point  4 A) prior to flowing through first economizer heat exchanger  28 A. The refrigerant from path  42 A that flowed through first economizer heat exchanger  28 A (point  5 A) is then directed along economizer return path  46 A and injected into suction port  52  of single-stage compressor  34  for compression in single-stage compressor  34 . After compression within single-stage compressor  34 , the refrigerant is discharged through discharge port  54  (point  6 A) where it merges with the refrigerant discharged from single-stage compressors  100  and  35 . 
         [0050]    In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger  28 A (point  3 B), the refrigerant in path  40 A splits into two flow paths  40 B and  42 B. The second economized path continues along flow path  42 B where the refrigerant is throttled to a lower pressure by economizer expansion valve  30 B (point  4 B) prior to flowing through second economizer heat exchanger  28 B. The refrigerant from path  42 B that flowed through second economizer heat exchanger  28 B (point  5 B) is then directed along economizer return path  46 B and injected into suction port  56  of single-stage compressor  35  for compression in single-stage compressor  35 . After compression within single-stage compressor  35 , the refrigerant is discharged through discharge port  58  (point  6 B) where it merges with the refrigerant discharged from single-stage compressors  34  and  100 . 
         [0051]      FIG. 5B  illustrates a graph relating enthalpy to pressure for the refrigeration system  20 E of  FIG. 5A . In  FIG. 5B , the main refrigerant path is defined by the route between points  1 ,  2 ,  3 , and  4 ; the first economized path is defined by the route between points  3 A,  4 A,  5 A, and  6 A; and the second economized path is defined by the route between points  3 B,  4 B,  5 B, and  6 B. 
         [0052]    As shown in  FIG. 5B , the specific cooling capacity of refrigeration system  20 E is represented by the length of evaporation line E 5  from point  4  to point  1 . Lines A 1 ′ and A 2 ′ represent the increased specific cooling capacity due to the addition of first economizer circuit  25 A and second economizer circuit  25 B, respectively. When compared with evaporation line E 1  of  FIG. 1B , evaporation line E 5  is substantially equivalent in length to evaporation line E 1 . This indicates that refrigeration system  20 E has a specific cooling capacity that is substantially equivalent to the specific cooling capacity of refrigeration system  20 A. Thus, a two-stage compressor and an intercooler may be replaced by a single-stage compressor in a refrigeration system such as that shown in  FIG. 1A  without a substantial change in specific cooling capacity. It should be noted that although refrigeration system  20 E is shown as a modified version of refrigeration system  20 A, refrigeration systems  20 B,  20 C, and  20 D may also be modified in the same manner without a substantial change in specific cooling capacity. 
         [0053]      FIG. 6  illustrates a schematic diagram of refrigeration system  20 A′, which is an alternative embodiment of refrigeration system  20 A. In the embodiment shown in  FIG. 6 , first economizer heat exchanger  28 A′ and second economizer heat exchanger  28 B′ comprise flash tanks. Thus, as used in refrigeration system  20 A′, flash tanks are an alternative type of heat exchanger. As stated previously, in the embodiment shown in  FIG. 1A , first and second economizer heat exchangers  28 A and  28 B are parallel flow tube-in-tube heat exchangers. However, parallel flow tube-in-tube heat exchangers may be replaced with flash tank type heat exchangers, as depicted in  FIG. 6 , without departing from the spirit and scope of the present invention. 
         [0054]      FIG. 7  illustrates a schematic diagram of refrigeration system  20 A″, which is another alternative embodiment of refrigeration system  20 A. In the embodiment shown in  FIG. 7 , first economizer heat exchanger  28 A″ and second economizer heat exchanger  28 B″ form a brazed plate heat exchanger. However, substituting a brazed plate heat exchanger for parallel flow tube-in-tube heat exchangers does not substantially affect the overall system efficiency. Thus, a refrigeration system using a brazed plate heat exchanger is also within the intended scope of the present invention. 
         [0055]    In addition to the parallel flow tube-in-tube heat exchangers, flash tanks, and brazed plate heat exchangers, numerous other heat exchangers may be used for the economizers without departing from the spirit and scope of the present invention. The list of alternative heat exchangers includes, but is not limited to, counter-flow tube-in-tube heat exchangers, parallel flow shell-in-tube heat exchangers, and counter-flow shell-in-tube heat exchangers. 
         [0056]    Although the refrigeration system of the present invention is useful to increase system efficiency in a system using any type of refrigerant, it is especially useful in refrigeration systems that utilize transcritical refrigerants, such as carbon dioxide. Because carbon dioxide is such a low critical temperature refrigerant, refrigeration systems using carbon dioxide typically run transcritical. Furthermore, because carbon dioxide is such a high pressure refrigerant, there is more opportunity to provide multiple pressure steps between the high and low pressure portions of the circuit to include multiple economizers, each of which contributes to increase the efficiency of the system. Thus, the present invention may be used to increase the efficiency of systems utilizing transcritical refrigerants such as carbon dioxide, making their efficiency comparable to that of typical refrigerants. However, the refrigeration system of the present invention is useful to increase the efficiency in systems using any refrigerant, including those that run subcritical as well as those that run transcritical. 
         [0057]    While the alternative embodiments of the present invention have been described as including a number of economizer circuits ranging from two to five, it should be understood that a refrigeration system with more than five economizer circuits is within the intended scope of the present invention. Furthermore, the economizer circuits may be connected to the compressors in various other combinations without decreasing system efficiency. Thus, refrigeration systems that utilize a greater number of economizer circuits or connect the economizer circuits in various other combinations are within the intended scope of the present invention. 
         [0058]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.