Patent Publication Number: US-2022228782-A1

Title: Refrigerant cycle system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a refrigerant cycle system. 
     BACKGROUND ART 
     As described in PTL 1 (Japanese Unexamined Patent Application Publication No. 2000-193339), a dual refrigerant circuit configured by a vapor compression refrigeration cycle is known. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Control of a state of refrigerant in the dual refrigerant circuit is not mentioned in PTL 1. 
     Solution to Problem 
     A refrigerant cycle system according to a first aspect includes a first refrigerant circuit, a second refrigerant circuit, and a first cascade heat exchanger. The first refrigerant circuit is a vapor compression refrigeration cycle. The second refrigerant circuit is a vapor compression refrigeration cycle. The first cascade heat exchanger exchanges heat between a first refrigerant that flows in the first refrigerant circuit and a second refrigerant that flows in the second refrigerant circuit. The refrigerant cycle system includes a switching mechanism. The switching mechanism switches a flow path of a refrigerant of at least either one of the first refrigerant circuit and the second refrigerant circuit. The first cascade heat exchanger includes a first main heat exchanging unit and a first sub heat exchanging unit. The first refrigerant that has passed through the first main heat exchanging unit passes through the first sub heat exchanging unit. 
     Consequently, it is possible to control the superheating state of the first refrigerant in the first sub heat exchanging unit. The superheating state of the first refrigerant is thus controlled easily. 
     A refrigerant cycle system according to a second aspect is the system according to the first aspect further including a first flow-rate regulating valve and a control unit. The first flow-rate regulating valve regulates the amount of the first refrigerant that flows in the first cascade heat exchanger in the first refrigerant circuit. The control unit regulates the opening degree of the first flow-rate regulating valve. When the first cascade heat exchanger of the first refrigerant circuit serves as an evaporator, the control unit regulates the opening degree of the first flow-rate regulating valve to cause the first refrigerant that exits the first sub heat exchanging unit to be in a superheating state. 
     A refrigerant cycle system according to the third aspect is the system according to the first aspect or the second aspect in which the first main heat exchanging unit is a plate heat exchanger or a heat exchanger that includes a plurality of stacked flat pipes. The first sub heat exchanging unit is a double pipe or a heat exchanging unit that has a structure in contact with a pipe. 
     Due to the sub heat exchanging unit  21   b  being the heat exchanger described above, it is possible to reduce an increase in costs caused by the provision of the sub heat exchanging unit  21   b.    
     A refrigerant cycle system according to a fourth aspect is the system according to any one of the first aspect to the third aspect further including a third refrigerant circuit and a second cascade heat exchanger. The third refrigerant circuit is a vapor compression refrigeration cycle. The second cascade heat exchanger exchanges heat between the first refrigerant that flows in the first refrigerant circuit and a third refrigerant that flows in the third refrigerant circuit. The second cascade heat exchanger includes a second main heat exchanging unit and a second sub heat exchanging unit. The second sub heat exchanging unit is configured to cause refrigerant that has passed through the second main heat exchanging unit to be in a superheating state. The first cascade heat exchanger and the second cascade heat exchanger are connected in parallel in the first refrigerant circuit. 
     Consequently, the number of connectable usage-side heat exchangers is increased, which increases flexibility in construction of a refrigerant cycle system. 
     A refrigerant cycle system according to a fifth aspect is the system according to the first aspect in which the first sub heat exchanging unit exchanges heat in the first refrigerant circuit between the first refrigerant that has not entered the first main heat exchanging unit yet and the first refrigerant that has exited the first main heat exchanging unit. 
     Consequently, it is possible to control the degree of superheating of the first refrigerant. 
     A refrigerant cycle system according to a sixth aspect is the system according to the fifth aspect in which the first refrigerant circuit further includes a first bypass circuit. When the first main heat exchanging unit serves as a condenser in the first refrigerant circuit, the first refrigerant that has exited the first main heat exchanging unit bypasses the first sub heat exchanging unit via the first bypass circuit and is sucked by a compressor included in the first refrigerant circuit. 
     The provision of the first bypass circuit enables the first refrigerant to bypass the first sub heat exchanging unit when the first refrigerant circuit performs heating operation. 
     A refrigerant cycle system according to a seventh aspect is the system according to the fifth aspect or the sixth aspect further including a third refrigerant circuit and a second cascade heat exchanger. The third refrigerant circuit is a vapor compression refrigeration cycle. The second cascade heat exchanger exchanges heat between the first refrigerant and a third refrigerant that flows in the third refrigerant circuit. The second cascade heat exchanger includes a second main heat exchanging unit and a second sub heat exchanging unit. The first refrigerant that has passed through the second main heat exchanging unit passes through the second sub heat exchanging unit. 
     Consequently, it is possible to connect a larger number of usage-side units with respect to one heat-source-side unit. 
     A refrigerant cycle system according to an eighth aspect is the system according to the seventh aspect in which the first refrigerant circuit further includes a second bypass circuit. When the second main heat exchanging unit serves as a condenser in the first refrigerant circuit, the second refrigerant that has exited the second main heat exchanging unit bypasses the second sub heat exchanging unit ( 241   b ) via the second bypass circuit and is sucked by a compressor included in the first refrigerant circuit. 
     The provision of the second bypass circuit enables the first refrigerant to bypass the second sub heat exchanging unit when the first refrigerant circuit performs heating operation. 
     A refrigerant cycle system according to a ninth aspect is the system according to the seventh aspect or the eighth aspect in which the second main heat exchanging unit has heat exchanging capacity larger than heat exchanging capacity of the second sub heat exchanging unit. 
     A refrigerant cycle system according to a tenth aspect is the system according to any one of the first aspect to the ninth aspect in which the first main heat exchanging unit has heat exchanging capacity larger than heat exchanging capacity of the first sub heat exchanging unit. 
     A refrigerant cycle system according to an eleventh aspect is the system according to any one of the first aspect to the tenth aspect in which each of the first refrigerant and the second refrigerant is any one of HFC refrigerant, HFO refrigerant, and natural refrigerant. Alternatively, each of the first refrigerant and the second refrigerant is a mixture refrigerant that contains any two or more of HFC refrigerant, HFO refrigerant, natural refrigerant, and CF 3 I. 
     A refrigerant cycle system according to a twelfth aspect is the system according to any one of the first aspect to the eleventh aspect in which each of the first refrigerant and the second refrigerant is R32. 
     Consequently, it is possible to divert an existing refrigerant cycle system. 
     A refrigerant cycle system according to a thirteenth aspect is the system according to any one of the first aspect to the twelfth aspect in which the first refrigerant is R32. The second refrigerant is carbon dioxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a refrigerant circuit of an air conditioning apparatus. 
         FIG. 2  is a view illustrating an outline of a control unit. 
         FIG. 3  is a view illustrating a refrigerant circuit of an air conditioning apparatus. 
         FIG. 4  is a view illustrating an outline of a control unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     (1) Configuration of Air Conditioning Apparatus 
     As illustrated in  FIG. 1 , an air conditioning apparatus  100  as one embodiment of a refrigerant cycle apparatus is an apparatus that cools and heats a room in a construction, such as a building, by a first refrigerant circuit  1 , a second refrigerant circuit  2 , and a third refrigerant circuit  3  that are vapor compression refrigeration cycles. 
     The air conditioning apparatus  100  mainly includes a heat-source-side unit  10  that belongs to the first refrigerant circuit  1 , a plurality of usage-side units  30 A and  30 B (two in the present embodiment) that belong to the second refrigerant circuit  2 , a plurality of usage-side units  50 A and  50 B (two in the present embodiment) that belong to the third refrigerant circuit, a first cascade unit  20  that is disposed between the heat-source-side unit  10  and the usage-side units  30 A and  30 B, a second cascade unit  40  that is disposed between the heat-source-side unit  10  and the usage-side units  50 A and  50 B, refrigerant connection pipes  4   a ,  4   b ,  5   a ,  5   b ,  6   a , and  6   b , and a control unit  60 . 
     The first cascade unit  20  and the second cascade unit  40  are connected in parallel to each other in the first refrigerant circuit  1 . The plurality of usage-side units  30 A and  30 B are connected in parallel to each other in the second refrigerant circuit  2 . The plurality of usage-side units  50 A and  50 B are connected in parallel to each other in the third refrigerant circuit  3 . 
     The control unit  60  is connected to a control unit of each unit via a transmission line and the like. The control unit  60  controls each constituent device included in the air conditioning apparatus  100  and controls the entirety of the air conditioning apparatus  100 . 
     R32 is charged as a first refrigerant, a second refrigerant, and a third refrigerant in the first refrigerant circuit  1 , the second refrigerant circuit  2 , and the third refrigerant circuit  3 , respectively. 
     (2) Detailed Configuration of Each Unit 
     (2-1) Usage-Side Unit 
     The usage-side units  30 A,  30 B,  50 A, and  50 B are installed inside a room of a building or the like. 
     The plurality of usage-side units  30 A and  30 B constituting part of the second refrigerant circuit  2  are connected to the first cascade unit  20  via the liquid-refrigerant connection pipe  5   a  and the gas-refrigerant connection pipe  5   b  that serve as refrigerant connection pipes. 
     The plurality of usage-side units  50 A and  50 B constituting part of the third refrigerant circuit  3  are connected to the second cascade unit  40  via the liquid-refrigerant connection pipe  6   a  and the gas-refrigerant connection pipe  6   b  that serve as refrigerant connection pipes. 
     Next, a configuration of the usage-side unit  30 A will be described. The usage-side unit  30 A and the usage-side units  30 B,  50 A, and  50 B have the same configuration. Thus, only the configuration of the usage-side unit  30 A will be described here, and description of the configurations of the usage-side units  30 B,  50 A, and  50 B is omitted. 
     The usage-side unit  30 A mainly includes a usage-side heat exchanger  31   a  and a flow-rate regulating valve  32   a.    
     The usage-side heat exchanger  31   a  is a heat exchanger that functions as an evaporator for the second refrigerant and cools indoor air or functions as a radiator for the second refrigerant and heats indoor air. Here, the usage-side unit  30 A includes a usage-side fan, which is not illustrated. The usage-side fan supplies indoor air as a cooling source or a heating source of the second refrigerant that flows in the usage-side heat exchanger  31   a  to the usage-side heat exchanger  31   a.    
     The flow-rate regulating valve  32   a  is an electric expansion valve capable of regulating, while decompressing the second refrigerant, the flow rate of the second refrigerant that flows in the usage-side heat exchanger  31   a . The opening degree of the flow-rate regulating valve  32   a  is regulated by the control unit  60  via a usage-side control unit  64 . 
     The usage-side unit  30 A is provided with various types of sensors, which are not illustrated. Values detected by each of the sensors are sent to the control unit  60  via the usage-side control unit  64 . 
     (2-2) Heat-Source-Side Unit 
     The heat-source-side unit  10  constituting part of the first refrigerant circuit  1  is installed outside a room of a construction, such as a building, for example, on a rooftop or on the ground. The heat-source-side unit  10  is connected to the first cascade unit  20  or the second cascade unit  40  via the liquid-refrigerant connection pipe  4   a  and the gas-refrigerant connection pipe  4   b.    
     The heat-source-side unit  10  mainly includes a compressor  11  and a heat-source-side heat exchanger  12 . The heat-source-side unit  10  includes a switching mechanism  13  as a cooling-heating switching mechanism that switches between a cooling operation state in which the heat-source-side heat exchanger  12  functions as a radiator for refrigerant and a heating operation state in which the heat-source-side heat exchanger  12  functions as an evaporator for refrigerant. 
     The compressor  11  is a device for compressing the first refrigerant and is, for example, a compressor having a hermetic structure and in which a compression element of a positive displacement type, such as a rotary type of scroll type, is driven to rotate by a compression motor. 
     The heat-source-side heat exchanger  12  is a heat exchanger that functions as a radiator for the first refrigerant or functions as an evaporator for the first refrigerant. Here, the heat-source-side unit  10  includes a heat-source-side fan, which is not illustrated. The heat-source-side fan takes outdoor air into the heat-source-side unit  10  and discharges the outdoor air to the outside after causing heat to be exchanged between the outdoor air and the first refrigerant in the heat-source-side heat exchanger  12 . 
     The first refrigerant circuit  1  is provided with an expansion valve  14  near the liquid-side end of the heat-source-side heat exchanger  12 . The expansion valve  14  is an electric expansion valve that decompresses the first refrigerant in a heating operation state. The opening degree of the expansion valve  14  is regulated by the control unit  60  via a heat-source-side control unit  61 . 
     The heat-source-side unit  10  is provided with various types of sensors, which are not illustrated. Values detected by each of the sensors are sent to the control unit  60  via the heat-source-side control unit  61 . 
     (2-3) Cascade Unit 
     The first cascade unit  20  and the second cascade unit  40  are installed in a space of, for example, an attic of a room of a construction, such as a building. 
     The first cascade unit  20  is interposed between the usage-side units  30 A and  30 B and the heat-source-side unit  10  and constitutes part of the first refrigerant circuit  1  and part of the second refrigerant circuit  2 . 
     The second cascade unit  40  is interposed between the usage-side units  50 A and  50 B and the heat-source-side unit  10  and constitutes part of the first refrigerant circuit  1  and part of the third refrigerant circuit  3 . 
     Next, a configuration of the first cascade unit  20  will be described. The first cascade unit  20  and the second cascade unit  40  have the same configuration. Thus, only the configuration of the first cascade unit  20  will be described here, and description of the configuration of the second cascade unit  40  is omitted. 
     The first cascade unit  20  mainly includes a first cascade heat exchanger  21 , a first flow-rate regulating valve  22 , a compressor  24 , and an expansion valve  26 . The first cascade unit  20  includes a switching mechanism  25  as a cooling-heating switching mechanism that switches between a cooling operation state in which the first cascade heat exchanger  21  functions as a radiator for refrigerant and a heating operation state in which the first cascade heat exchanger  21  functions as an evaporator for refrigerant. 
     The first cascade heat exchanger  21  functions as an evaporator for the second refrigerant in the second refrigerant circuit  2  when functioning as a radiator for the first refrigerant in the first refrigerant circuit  1 . The first cascade heat exchanger  21  functions as a radiator for the second refrigerant in the second refrigerant circuit  2  when functioning as an evaporator for the first refrigerant in the first refrigerant circuit  1 . The first cascade heat exchanger  21  is a heat exchanger that exchanges heat between the first refrigerant that flows in the first refrigerant circuit  1  and the second refrigerant that flows in the second refrigerant circuit  2 . 
     Here, the first cascade heat exchanger  21  includes a first main heat exchanging unit  21   a  and a first sub heat exchanging unit  21   b . The first sub heat exchanging unit  21   b  is configured to cause the first refrigerant that has passed through the first main heat exchanging unit  21   a  to be in a superheating state. The superheating state is a state in which a degree of superheating has been given to the first refrigerant. A degree of superheating to be given is not limited as long as some degree of superheating is given. 
     The first main heat exchanging unit  21   a  is a heat exchanger having heat exchanging capacity larger than that of the first sub heat exchanging unit  21   b . For example, the first main heat exchanging unit  21   a  is a plate heat exchanger, and the first sub heat exchanging unit  21   b  is a double pipe. 
     The heat exchanging capacity of a heat exchanger can be calculated by a heat transfer rate and the like. The heat exchanging capacity of a plate heat exchanger used as the first main heat exchanging unit  21   a  is generally larger than the heat exchanging capacity of a double pipe used as the first sub heat exchanging unit  21   b.    
     A method of calculating the heat exchanging capacity of a heat exchanger is not specially limited. 
     The first refrigerant circuit  1  is provided with the first flow-rate regulating valve  22  near the liquid-side end of the first main heat exchanging unit  21   a . The first flow-rate regulating valve  22  is an electric expansion valve that decompresses refrigerant during cooling operation. The valve opening degree of the first flow-rate regulating valve  22  is regulated by the control unit  60  via a first cascade control unit  62  to cause the first refrigerant that exits the first sub heat exchanging unit  21   b  to be in a superheating state. 
     The compressor  24  is a device for compressing the second refrigerant. For example, a compressor having a hermetic structure and in which a compression element of a positive displacement type, such as a rotary type or scroll type, is driven to rotate by a compression motor is used. 
     The switching mechanism  25  is a device capable of switching the flow of the second refrigerant in the second refrigerant circuit  2  and is constituted by, for example, a four-way switching valve. 
     The second refrigerant circuit  2  is provided with the expansion valve  26  near the liquid-side end of the first main heat exchanging unit  21   a . The expansion valve  26  is an electric expansion valve that decompresses refrigerant during heating operation. The opening degree of the expansion valve  26  is regulated by the control unit  60  via the first cascade control unit  62 . 
     As illustrated in  FIG. 1 , the first cascade unit  20  is provided with an inlet temperature sensor  23   a  and an outlet temperature sensor  23   b . The inlet temperature sensor  23   a  detects a temperature (inlet temperature) of the first refrigerant at the liquid-side end of the first main heat exchanging unit  21   a  in the first refrigerant circuit  1 . The outlet temperature sensor  23   b  detects a temperature (outlet temperature) of the first refrigerant at the gas-side end of the first sub heat exchanging unit  21   b  in the first refrigerant circuit  1 . Values detected by the inlet temperature sensor  23   a  and the outlet temperature sensor  23   b  are sent to the control unit  60  via the first cascade control unit  62 . 
     The first cascade unit  20  is also provided with various types of sensors, which are not illustrated, other than the aforementioned sensors. 
     (2-4) Control Unit 
     As illustrated in  FIG. 2 , the control unit  60  includes the heat-source-side control unit  61 , the first cascade control unit  62 , a second cascade control unit  63 , and usage-side control units  64 ,  65 ,  66 , and  67 . Each of the control units  60 ,  61 ,  62 ,  63 ,  64 ,  65 ,  66 , and  67  includes a processor, such as a CPU or a GPU, a memory, and the like. The processor is capable of reading a program stored in the memory and performing predetermined processing in accordance with the program. 
     The heat-source-side control unit  61  is disposed at the heat-source-side unit  10 . The heat-source-side control unit  61  controls the entirety of the heat-source-side unit  10  and the opening degree of the expansion valve  14 . The first cascade control unit  62  is disposed at the first cascade unit  20 . The first cascade control unit  62  controls the entirety of the first cascade unit  20  and the opening degrees of the first flow-rate regulating valve  22  and the expansion valve  26 . The second cascade control unit  63  is disposed at the second cascade unit  40 . The second cascade control unit  63  controls the entirety of the second cascade unit  40  and the opening degrees of a second flow-rate regulating valve  42  and an expansion valve  46 . The usage-side control unit  64  is disposed at the usage-side unit  30 A. The usage-side control unit  64  controls the entirety of the usage-side unit  30 A and the opening degree of the flow-rate regulating valve  32   a . The usage-side control unit  65  is disposed at the usage-side unit  30 B. The usage-side control unit  65  controls the entirety of the usage-side unit  30 B and the opening degree of a flow-rate regulating valve  32   b . The usage-side control unit  66  is disposed at the usage-side unit  50 A. The usage-side control unit  66  controls the entirety of the usage-side unit  50 A and the opening degree of a flow-rate regulating valve  52   a . The usage-side control unit  67  is disposed at the usage-side unit  50 B. The usage-side control unit  67  controls the entirety of the usage-side unit  50 B and the opening degree of a flow-rate regulating valve  52   b.    
     The control unit  60  and the control units  61 ,  62 ,  63 ,  64 ,  65 ,  66 , and  67  each include a control board on which electric components, such as a microcomputer and a memory, are mounted. The control unit  60  controls the entirety of the air conditioning apparatus  100  via the control units  61 ,  62 ,  63 ,  64 ,  65 ,  66 , and  67 . The control unit  60  is capable of receiving values detected by each of the sensors provided at the air conditioning apparatus  100  via the control units and sending control signals and the like to each constituent device. 
     Specifically, the control unit  60  receives via the first cascade control unit  62  an inlet temperature that is detected by the inlet temperature sensor  23   a  provided at the first cascade unit  20  and an outlet temperature that is detected by the outlet temperature sensor  23   b . The control unit  60  is in advance provided with opening-degree regulating algorithm for regulating the opening degree of the first flow-rate regulating valve  22 . The control unit  60  uses the opening-degree regulating algorithm and generates from the inlet temperature and the outlet temperature a control signal for causing the first refrigerant that exits the first sub heat exchanging unit  21   b  to have an appropriate degree of superheating. The first flow-rate regulating valve  22  is capable of causing the first refrigerant that exits the first sub heat exchanging unit  21   b  to have an appropriate degree of superheating by regulating the opening degree of the first flow-rate regulating valve  22  on the basis of the control signal. 
     Regulation of the opening degree of the second flow-rate regulating valve  42  included in the second cascade unit  40  is the same as that described above. The control unit  60  receives via the second cascade control unit  63  an inlet temperature that is detected by an inlet temperature sensor  43   a  of the second cascade unit  40  and an outlet temperature that is detected by an outlet temperature sensor  43   b . The control unit  60  uses the opening-degree regulating algorithm and sends a control signal that regulates the opening degree of the second flow-rate regulating valve  42  to the second flow-rate regulating valve  42 . The second flow-rate regulating valve  42  regulates the opening degree on the basis of the control signal. 
     A method by which the control unit  60  regulates the opening degree of the first flow-rate regulating valve  22  or the second flow-rate regulating valve  42  is not limited thereto. 
     (3) Basic Operation of Air Conditioning Apparatus 
     Next, basic operation of the air conditioning apparatus  100  will be described. The basic operation of the air conditioning apparatus  100  includes cooling operation and heating operation. The basic operation of the air conditioning apparatus  100  described below is performed by the control unit  60  that controls constituent devices of the air conditioning apparatus  100  (the heat-source-side unit  10 , the usage-side units  30 A,  30 B,  50 A, and  50 B, the first cascade unit  20 , and the second cascade unit  40 ). 
     (3-1) Cooling Operation 
     For example, when all of the usage-side units  30 A,  30 B,  50 A, and  50 B perform cooling operation (operation in which all of the usage-side heat exchangers  31   a ,  31   b ,  51   a , and  51   b  function as evaporators for refrigerant and the heat-source-side heat exchanger  12  functions as a radiator for refrigerant), the switching mechanisms  13 ,  25 , and  45  are switched to a cooling operation state (the state indicated by the solid lines in  FIG. 1 ). 
     (3-1-1) First Refrigerant Circuit 
     During cooling operation, the first refrigerant discharged from the compressor  11  and having a high pressure is sent to the heat-source-side heat exchanger  12  through the switching mechanism  13  in the first refrigerant circuit  1 . In the heat-source-side heat exchanger  12  that functions as a radiator for the first refrigerant, the first refrigerant sent to the heat-source-side heat exchanger  12  condenses by exchanging heat with outdoor air supplied by the heat-source-side fan and being cooled. The first refrigerant flows out from the heat-source-side unit  10  through the expansion valve  14 . 
     The first refrigerant that has flowed out from the heat-source-side unit  10  is sent to the first cascade unit  20  or the second cascade unit  40 . 
     After being decompressed by the first flow-rate regulating valve  22  to an appropriate pressure, the first refrigerant sent to the first cascade unit  20  flows into the first cascade heat exchanger  21 . In the first main heat exchanging unit  21   a  and the first sub heat exchanging unit  21   b  that function as evaporators for the first refrigerant, the first refrigerant sent to the first cascade heat exchanger  21  evaporates by exchanging heat with the second refrigerant that flows in the second refrigerant circuit  2  and being heated. An appropriate degree of superheating has been given to the first refrigerant that has exited the first sub heat exchanging unit  21   b . The first refrigerant flows out from the first cascade unit  20  and in a state of merging with the first refrigerant that has flowed out from the second cascade unit  40  is sucked by the compressor  11 . 
     After being decompressed by the second flow-rate regulating valve  42  to an appropriate pressure, the first refrigerant sent to the second cascade unit  40  flows into a second cascade heat exchanger  41 . In a second main heat exchanging unit  41   a  and a second sub heat exchanging unit  41   b  that function as evaporators for the first refrigerant, the first refrigerant sent to the second cascade heat exchanger  41  evaporates by exchanging heat with the third refrigerant that flows in the third refrigerant circuit  3  and being heated. An appropriate degree of superheating has been given to the first refrigerant that has exited the second sub heat exchanging unit  41   b . The first refrigerant flows out from the second cascade unit  40  and is in a state of merging with the first refrigerant that has flowed out from the first cascade unit  20  sucked by the compressor  11 . 
     (3-1-2) Second Refrigerant Circuit 
     In the second refrigerant circuit  2 , the second refrigerant discharged from the compressor  24  and having a high pressure is sent to the first cascade heat exchanger  21  through the switching mechanism  25 . In the first sub heat exchanging unit  21   b  and the first main heat exchanging unit  21   a  that function as radiators for the second refrigerant, the second refrigerant sent to the first cascade heat exchanger  21  condenses by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  1  and being cooled. The second refrigerant flows out from the first cascade unit  20  through the expansion valve  26 . The second refrigerant that has flowed out from the first cascade unit  20  is sent to each of the usage-side units  30 A and  30 B. 
     After being decompressed by the flow-rate regulating valve  32   a  to an appropriate pressure, the second refrigerant sent to the usage-side unit  30 A evaporates in the usage-side heat exchanger  31   a  that functions as an evaporator for the second refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant flows out from the usage-side unit  30 A and in a state of merging with the second refrigerant that has flowed out from the usage-side unit  30 B is suck by the compressor  24 . 
     After being decompressed by the flow-rate regulating valve  32   b  to an appropriate pressure, the second refrigerant sent to the usage-side unit  30 B evaporates in the usage-side heat exchanger  31   b  that functions as an evaporator for the second refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant flows out from the usage-side unit  30 B and in a state of merging with the second refrigerant that has flowed out from the usage-side unit  30 A is sucked by the compressor  24 . 
     On the other hand, indoor air cooled in the usage-side heat exchangers  31   a  and  31   b  is sent to the inside of a room, thereby cooling the inside of the room. 
     (3-1-3) Third Refrigerant Circuit 
     In the third refrigerant circuit  3 , the third refrigerant discharged from a compressor  44  and having a high pressure is sent to the second cascade heat exchanger  41  through the switching mechanism  45 . In the second sub heat exchanging unit  41   b  and the second main heat exchanging unit  41   a  that function as radiators for the third refrigerant, the third refrigerant sent to the second cascade heat exchanger  41  condenses by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  1  and being cooled. The third refrigerant flows out from the second cascade unit  40  through the expansion valve  46 . The third refrigerant that has flowed out from the second cascade unit  40  is sent to each of the usage-side units  50 A and  50 B. 
     After being decompressed by the flow-rate regulating valve  52   a  to an appropriate pressure, the third refrigerant sent to the usage-side unit  50 A evaporates in the usage-side heat exchanger  51   a  that functions as an evaporator for the third refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant flows out from the usage-side unit  50 A and in a state of merging with the third refrigerant that has flowed out from the usage-side unit  50 B is sucked by the compressor  44 . 
     After being decompressed by the flow-rate regulating valve  52   b  to an appropriate pressure, the third refrigerant sent to the usage-side unit  50 B evaporates in the usage-side heat exchanger  51   b  that functions as an evaporator for the third refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant flows out from the usage-side unit  50 A and in a state of merging with the third refrigerant that has flowed out from the usage-side unit  50 B is sucked by the compressor  44 . 
     On the other hand, indoor air cooled in the usage-side heat exchangers  51   a  and  51   b  is sent to the inside of a room, thereby cooling the inside of the room. 
     (3-2) Heating Operation 
     For example, when all of the usage-side units  30 A,  30 B,  50 A, and  50 B perform heating operation (operation in which all of the usage-side heat exchangers  31   a ,  31   b ,  51   a , and  51   b  function as radiators for refrigerant and the heat-source-side heat exchanger  12  functions as an evaporator for refrigerant), the switching mechanisms  13 ,  25 , and  45  are switched to a heating operation state (the state indicated by the broken lines in  FIG. 1 ). 
     (3-2-1) First Refrigerant Circuit 
     During heating operation, in the first refrigerant circuit  1 , the first refrigerant discharged from the compressor  11  and having a high pressure flows out from the heat-source-side unit  10  through the switching mechanism  13 . 
     The first refrigerant that has flowed out from the heat-source-side unit  10  is sent to the first cascade unit  20  or the second cascade unit  40 . 
     In the first sub heat exchanging unit  21   b  and the first main heat exchanging unit  21   a  that function as radiators for refrigerant, the first refrigerant sent to the first cascade unit  20  condenses by exchanging heat with the second refrigerant that flows in the second refrigerant circuit  2  and being cooled. The first refrigerant that has condensed passes through the first flow-rate regulating valve  22  and flows out from the first cascade unit  20 . In a state of merging with the first refrigerant that has flowed out from the second cascade unit  40 , the first refrigerant that has flowed out from the first cascade unit  20  is sent to the heat-source-side unit  10 . 
     In the second sub heat exchanging unit  41   b  and the second main heat exchanging unit  41   a  that function as radiators for refrigerant, the first refrigerant sent to the second cascade unit  40  condenses by exchanging heat with the third refrigerant that flows in the third refrigerant circuit  3  and being cooled. The third refrigerant that has condensed passes through the second flow-rate regulating valve  42  and flows out from the first cascade unit  20 . In a state of merging with the first refrigerant that has flowed out from the first cascade unit  20 , the first refrigerant that has flowed out from the second cascade unit  40  is sent to the heat-source-side unit  10 . 
     The first refrigerant sent to the heat-source-side unit  10  is sent to the expansion valve  14 . The first refrigerant sent to the expansion valve  14  is sent to the heat-source-side heat exchanger  12  after being decompressed by the expansion valve  14 . The first refrigerant sent to the heat-source-side heat exchanger  12  evaporates by exchanging heat with outdoor air supplied by the heat-source-side fan and being heated. The first refrigerant that has evaporated is sucked by the compressor  11  through the switching mechanism  13 . 
     (3-2-2) Second Refrigerant Circuit 
     In the second refrigerant circuit  2 , during heating operation, the second refrigerant discharged from the compressor  24  and having a high pressure flows out from the first cascade unit  20  through the switching mechanism  25 . 
     The second refrigerant that has flowed out from the first cascade unit  20  is sent to each of the usage-side units  30 A and  30 B. 
     In the usage-side heat exchanger  31   a  that functions as a radiator for refrigerant, the second refrigerant sent to the usage-side unit  30 A condenses by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant that has condensed passes through the flow-rate regulating valve  32   a  and flows out from the usage-side unit  30 A. In a state of merging with the second refrigerant that has flowed out from the usage-side unit  30 B, the second refrigerant that has flowed out from the usage-side unit  30 A is sent to the first cascade unit  20 . 
     In the usage-side heat exchanger  31   b  that functions as a radiator for refrigerant, the second refrigerant sent to the usage-side unit  30 B condenses by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant that has condensed passes through the flow-rate regulating valve  32   b  and flows out from the usage-side unit  30 B. In a state of merging with the second refrigerant that has flowed out from the usage-side unit  30 A, the second refrigerant that has flowed out from the usage-side unit  30 B is sent to the first cascade unit  20 . 
     On the other hand, indoor air heated in the usage-side heat exchangers  31   a  and  31   b  is sent to the inside of a room, thereby heating the inside of the room. 
     The second refrigerant that has flowed into the first cascade unit  20  flows into the expansion valve  26 . The second refrigerant that has flowed into the expansion valve  26  is sent to the first cascade heat exchanger  21  after being decompressed by the expansion valve  26 . In the first main heat exchanging unit  21   a  and the first sub heat exchanging unit  21   b  that function as evaporators for the second refrigerant, the second refrigerant that has flowed into the first cascade heat exchanger  21  evaporates by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  1  and being heated. The second refrigerant that has evaporated is sucked by the compressor  24  through the switching mechanism  25 . 
     (3-2-3) Third Refrigerant Circuit 
     In the third refrigerant circuit  3 , the third refrigerant discharged from the compressor  44  and having a high pressure flows out from the second cascade unit  40  through the switching mechanism  45 . 
     The third refrigerant that has flowed out from the second cascade unit  40  is sent to each of the usage-side units  50 A and  50 B. 
     In the usage-side heat exchanger  51   a  that functions as a radiator for refrigerant, the third refrigerant sent to the usage-side unit  50 A condenses by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant that has condensed passes through the flow-rate regulating valve  52   a  and flows out from the usage-side unit  50 A. In a state of merging with the third that has flowed out from the usage-side unit  50 B, the third refrigerant that has flowed out from the usage-side unit  50 A is sent to the second cascade unit  40 . 
     In the usage-side heat exchanger  51   b  that functions as a radiator for refrigerant, the third refrigerant sent to the usage-side unit  50 B condenses by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant that has condensed passes through the flow-rate regulating valve  52   b  and flows out from the usage-side unit  50 B. In a state of merging with the third refrigerant that has flowed out from the usage-side unit  50 A, the third refrigerant that has flowed out from the usage-side unit  50 B is sent to the second cascade unit  40 . 
     On the other hand, indoor air heated in the usage-side heat exchangers  51   a  and  51   b  is sent to the inside of a room, thereby heating the inside of the room. 
     The third refrigerant that has flowed into the second cascade unit  40  is sent to the expansion valve  46 . The third refrigerant that has flowed into the expansion valve  46  is sent to the second cascade heat exchanger  41  after being decompressed by the expansion valve  46 . In the second main heat exchanging unit  41   a  and the second sub heat exchanging unit  41   b  that function as evaporators for the third refrigerant, the third refrigerant that has flowed into the second cascade heat exchanger  41  evaporates by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  1  and being heated. The third refrigerant that has evaporated is sucked by the compressor  44  through the switching mechanism  45 . 
     (4) Modifications 
     (4-1) 
     The first main heat exchanging unit  21   a  and the second main heat exchanging unit  41   a  of the air conditioning apparatus  100  are plate heat exchangers. The first sub heat exchanging unit  21   b  and the second sub heat exchanging unit  41   b  are double pipes. Each heat exchanging unit is not limited thereto. 
     For example, the first main heat exchanging unit  21   a  and the second main heat exchanging unit  41   a  may be heat exchangers each including a plurality of stacked flat pipes. The first sub heat exchanging unit  21   b  and the second sub heat exchanging unit  41   b  may be heat exchangers each having a structure in contact with a pipe. 
     The first main heat exchanging unit  21   a  has heat exchanging capacity larger than that of the first sub heat exchanging unit  21   b . The second main heat exchanging unit  41   a  has heat exchanging capacity larger than that of the second sub heat exchanging unit  41   b . The heat exchanging capacity of a plate heat exchanger or a heat exchanger including a plurality of stacked flat pipes is generally larger than the heat exchanging capacity of a double pipe or a heat exchanger having a structure in contact with a pipe. 
     (4-2) 
     In the first refrigerant circuit  1 , the second refrigerant circuit  2 , and the third refrigerant circuit  3  of the air conditioning apparatus  100 , R32 having high stability of refrigerant and high performance is charged as the first refrigerant, the second refrigerant, and the third refrigerant, respectively. 
     (4-3) 
     The refrigerant cycle system presented in the present disclosure has been described by using the air conditioning apparatus  100  as a specific example of the refrigerant cycle system. The form of the refrigerant cycle system is, however, not limited thereto. For example, the refrigerant cycle system may be a heat-pump hot water supply apparatus, or the like. 
     (5) Features 
     (5-1) 
     The air conditioning apparatus  100  as a refrigerant cycle system includes the first refrigerant circuit  1 , the second refrigerant circuit  2 , and the first cascade heat exchanger  21 . The first refrigerant circuit  1  is a vapor compression refrigeration cycle. The second refrigerant circuit  2  is a vapor compression refrigeration cycle. The first cascade heat exchanger  21  exchanges heat between the first refrigerant that flows in the first refrigerant circuit  1  and the second refrigerant that flows in the second refrigerant circuit  2 . The air conditioning apparatus  100  includes the switching mechanisms  13  and  25 . The switching mechanisms  13  and  25  are each present in at least either one of the first refrigerant circuit  1  and the second refrigerant circuit  2  and switch a flow path of a refrigerant of the circuit. The first cascade heat exchanger  21  includes the first main heat exchanging unit  21   a  and the first sub heat exchanging unit  21   b . The first sub heat exchanging unit  21   b  is configured to cause the first refrigerant that has passed through the first main heat exchanging unit  21   a  to be in a superheating state. 
     The air conditioning apparatus  100  further includes the first flow-rate regulating valve  22  and the control unit  60 . The first flow-rate regulating valve  22  regulates the amount of the first refrigerant that flows in the first cascade heat exchanger  21  in the first refrigerant circuit  1 . The control unit  60  regulates the opening degree of the first flow-rate regulating valve  22 . When the first cascade heat exchanger  21  of the first refrigerant circuit  1  serves as an evaporator, the control unit  60  regulates the opening degree of the first flow-rate regulating valve  22  to cause the first refrigerant that exits the first sub heat exchanging unit  21   b  to be in a superheating state. Conventionally, a plate heat exchanger or a heat exchanger including a plurality of stacked flat pipes may be used in a dual refrigerant circuit configured by a vapor compression refrigeration cycle. These heat exchangers have high heat exchanging capacity and high performance and are also excellent in compact characteristics. However, controlling the degree of superheating of refrigerant in a plate heat exchanger or a heat exchanger including a plurality of stacked flat pipes decreases heat exchanging capacity and increases a pressure loss. Consequently, the performance of the plate heat exchanger or the heat exchanger including the plurality of stacked flat pipes is impaired. 
     However, in the air conditioning apparatus  100  according to the present embodiment, it is possible due to the aforementioned configuration to control the superheating state of the first refrigerant in the first sub heat exchanging unit  21   b . It is thus possible to control the superheating state of the first refrigerant without impairing the high heat exchanging capacity of the first main heat exchanging unit  21   a.    
     (5-2) 
     The first main heat exchanging unit  21   a  of the air conditioning apparatus  100  has heat exchanging capacity larger than that of the first sub heat exchanging unit  21   b.    
     The first main heat exchanging unit  21   a  of the air conditioning apparatus  100  is a plate heat exchanger or a heat exchanger including a plurality of stacked flat pipes. The first sub heat exchanging unit  21   b  is a double pipe or a heat exchanger having a structure in contact with a pipe. 
     Due to the first sub heat exchanging unit  21   b  being a double pipe or a heat exchanger having a structure in contact with a pipe, it is possible to provide a cascade heat exchanger that does not impair the compact characteristics of the main heat exchangers. In addition, due to the first sub heat exchanging unit  21   b  being a double pipe or a heat exchanger having a structure in contact with a pipe, it is possible to reduce an increase in costs caused by the provision of the first sub heat exchanging unit  21   b.    
     (5-3) 
     The air conditioning apparatus  100  further includes the third refrigerant circuit  3  and the second cascade heat exchanger  41 . The third refrigerant circuit  3  is a vapor compression refrigeration cycle. The second cascade heat exchanger  41  exchanges heat between the first refrigerant that flows in the first refrigerant circuit  1  and the third refrigerant that flows in the third refrigerant circuit  3 . The second cascade heat exchanger  41  includes the second main heat exchanging unit  41   a  and the second sub heat exchanging unit  42   b . The second sub heat exchanging unit  42   b  is configured to cause refrigerant that has passed through the second main heat exchanging unit  41   a  to be in a superheating state. The first cascade heat exchanger  21  and the second cascade heat exchanger  41  are connected in parallel in the first refrigerant circuit  1 . 
     The air conditioning apparatus  100  according to the present embodiment is also applicable to a multi-dual refrigerant circuit including a plurality of cascade units. Consequently, the number of connectable usage-side heat exchangers is increased, which increases flexibility in construction of the air conditioning apparatus  100 . 
     (5-4) 
     In the first refrigerant circuit  1 , the second refrigerant circuit  2 , and the third refrigerant circuit  3  of the air conditioning apparatus  100 , R32 having high stability is charged as the first refrigerant, the second refrigerant, and the third refrigerant, respectively. However, refrigerant other than R32 may be charged in the refrigerant cycle system presented in the present disclosure. For example, it is preferable that the first refrigerant be R32 and that the second refrigerant and the third refrigerant be carbon dioxide. 
     R32 has high stability of refrigerant and thus is widely used in existing refrigerant cycle systems. An existing refrigerant cycle system can be diverted into the refrigerant cycle system presented in the present disclosure. 
     Each of the first refrigerant, the second refrigerant, and the third refrigerant charged in the refrigerant cycle system is preferably any one of HFC refrigerant, HFO refrigerant, and natural refrigerant. Alternatively, each of the first refrigerant and the second refrigerant is preferably a mixture refrigerant that contains any two or more of HFC refrigerant, HFO refrigerant, natural refrigerant, and CF 3 I. Specifically, the HFC refrigerant is R32, R125, R134a, R143a, R245fa, or the like. The HFO refrigerant is R1234yf, R1234zd, R1123, R1132(E), or the like. The natural refrigerant is R744, R717, R290, R600a, R1270, or the like. 
     Second Embodiment 
     (1) Overall Configuration 
     As illustrated in  FIG. 3 , an air conditioning apparatus  200  as one embodiment of a refrigerant cycle apparatus is an apparatus that cools and heats a room in a construction, such as a building, by a first refrigerant circuit  201 , a second refrigerant circuit  202 , and a third refrigerant circuit  203  that are vapor compression refrigeration cycles. 
     The air conditioning apparatus  200  mainly includes a heat-source-side unit  210  that belongs to the first refrigerant circuit  201 , a plurality of usage-side units  230 A and  230 B (two in the present embodiment) that belong to the second refrigerant circuit  202 , a plurality of usage-side units  250 A and  250 B (two in the present embodiment) that belong to the third refrigerant circuit  203 , a first cascade unit  220  that is disposed between the heat-source-side unit  210  and the usage-side units  230 A and  230 B, a second cascade unit  240  that is disposed between the heat-source-side unit  210  and the usage-side units  250 A and  250 B, refrigerant connection pipes  204   a ,  204   b ,  205   a ,  205   b ,  206   a , and  206   b , and a control unit  260 . 
     The first cascade unit  220  and the second cascade unit  240  are connected in parallel to each other in the first refrigerant circuit  201 . The plurality of usage-side units  230 A and  230 B are connected in parallel to each other in the second refrigerant circuit  202 . The plurality of usage-side units  250 A and  250 B are connected in parallel to each other in the third refrigerant circuit  203 . 
     The control unit  260  is connected to a control unit of each unit via a transmission line and the like. The control unit  260  controls each constituent device included in the air conditioning apparatus  200  and controls the entirety of the air conditioning apparatus  200 . 
     R32 is charged as a first refrigerant, a second refrigerant, and a third refrigerant in the first refrigerant circuit  201 , the second refrigerant circuit  202 , and the third refrigerant circuit  203 , respectively. 
     (2) Detailed Configuration of Each Unit 
     (2-1) Usage-Side Unit 
     The usage-side units  230 A,  230 B,  250 A, and  250 B are installed inside a room of a building or the like. 
     The plurality of usage-side units  230 A and  230 B constituting part of the second refrigerant circuit  202  are connected to the first cascade unit  220  via a liquid-refrigerant connection pipe  205   a  and a gas-refrigerant connection pipe  205   b  that serve as refrigerant connection pipes. 
     The plurality of usage-side units  250 A and  250 B constituting part of the third refrigerant circuit  203  are connected to the second cascade unit  240  via a liquid-refrigerant connection pipe  206   a  and a gas-refrigerant connection pipe  206   b  that serve as refrigerant connection pipes. 
     Next, a configuration of the usage-side unit  230 A will be described. The usage-side unit  230 A and the usage-side units  230 B,  250 A, and  250 B have the same configuration. Thus, only the configuration of the usage-side unit  230 A will be described here, and description of the configurations of the usage-side units  230 B,  250 A, and  250 B is omitted. 
     The usage-side unit  230 A mainly includes a usage-side heat exchanger  231   a  and a flow-rate regulating valve  232   a . Each constituent device of the usage-side unit  230 A is controlled by the control unit  260  via a usage-side control unit  264 . 
     The usage-side heat exchanger  231   a  is a heat exchanger that functions as an evaporator for the second refrigerant and cools indoor air or functions as a radiator for the second refrigerant and heats indoor air. Here, the usage-side unit  230 A includes a usage-side fan, which is not illustrated. The usage-side fan supplies indoor air as a cooling source or a heating source of the second refrigerant that flows in the usage-side heat exchanger  231   a  to the usage-side heat exchanger  231   a.    
     The flow-rate regulating valve  232   a  is an electric expansion valve capable of regulating, while decompressing the second refrigerant, the flow rate of the second refrigerant that flows in the usage-side heat exchanger  231   a . The opening degree of the flow-rate regulating valve  232   a  is regulated by the control unit  260  via the usage-side control unit  264 . 
     The usage-side unit  230 A is provided with various types of sensors, which are not illustrated. Values detected by each of the sensors are sent to the control unit  260  via the usage-side control unit  264 . 
     (2-2) Heat-Source-Side Unit 
     The heat-source-side unit  210  constituting part of the first refrigerant circuit  201  is installed outside a room of a construction, such as a building, for example, on a rooftop or on the ground. The heat-source-side unit  210  is connected to the first cascade unit  220  or the second cascade unit  240  via the liquid-refrigerant connection pipe  204   a  and the gas-refrigerant connection pipe  204   b.    
     The heat-source-side unit  210  mainly includes a compressor  211  and a heat-source-side heat exchanger  212 . The heat-source-side unit  210  includes a switching mechanism  213  as a cooling-heating switching mechanism that switches between a cooling operation state in which the heat-source-side heat exchanger  212  functions as a radiator for refrigerant and a heating operation state in which the heat-source-side heat exchanger  212  functions as an evaporator for refrigerant. Each constituent device of the heat-source-side unit  210  is controlled by the control unit  260  via a heat-source-side control unit  261 . 
     The compressor  211  is a device for compressing the first refrigerant and is, for example, a compressor having a hermetic structure and in which a compression element of a positive-displacement type, such as a rotary type or scroll type, is driven to rotate by a compression motor. 
     The heat-source-side heat exchanger  212  is a heat exchanger that functions as a radiator for the first refrigerant or functions as an evaporator for the first refrigerant. Here, the heat-source-side unit  210  includes a heat-source-side fan, which is not illustrated. The heat-source-side fan takes outdoor air into the heat-source-side unit  210  and discharges the outdoor air to the outside after causing heat to be exchanged between the outdoor air and the first refrigerant in the heat-source-side heat exchanger  212 . 
     The first refrigerant circuit  201  is provided with an expansion valve  214  near the liquid-side end of the heat-source-side heat exchanger  212 . The expansion valve  214  is an electric expansion valve that decompresses the first refrigerant in a heating operation state. The opening degree of the expansion valve  214  is regulated by the control unit  260  via the heat-source-side control unit  261 . 
     The heat-source-side unit  210  is provided with various types of sensors, which are not illustrated. Values detected by each of the sensors are sent to the control unit  260  via the heat-source-side control unit  261 . 
     (2-3) Cascade Unit 
     The first cascade unit  220  and the second cascade unit  240  are installed in a space of, for example, an attic of a room of a construction, such as a building. 
     The first cascade unit  220  is interposed between the usage-side units  230 A and  230 B and the heat-source-side unit  210  and constitutes part of the first refrigerant circuit  201  and part of the second refrigerant circuit  202 . 
     The second cascade unit  240  is interposed between the usage-side units  250 A and  250 B and the heat-source-side unit  210  and constitutes part of the first refrigerant circuit  201  and part of the third refrigerant circuit  203 . 
     Next, a configuration of the first cascade unit  220  will be described. The first cascade unit  220  and the second cascade unit  240  have the same configuration. Thus, only the configuration of the first cascade unit  220  will be described here, and description of the configuration of the second cascade unit  240  is omitted. 
     The first cascade unit  220  mainly includes a first main heat exchanging unit  221   a , a first sub heat exchanging unit  221   b , a first flow-rate regulating valve  222 , a first bypass circuit  225 , a first bypass valve  223 , a compressor  226 , and an expansion valve  228 . The first cascade unit  220  includes a switching mechanism  227  that serves as a cooling-heating switching mechanism. Each constituent device of the first cascade unit  220  is controlled by the control unit  260  via a first cascade control unit  262 . 
     When functioning as a radiator for the first refrigerant in the first refrigerant circuit  201 , the first main heat exchanging unit  221   a  functions as an evaporator for the second refrigerant in the second refrigerant circuit  202 . When functioning as an evaporator for the first refrigerant in the first refrigerant circuit  201 , the first main heat exchanging unit  221   a  functions as a radiator for the second refrigerant in the second refrigerant circuit  202 . The first main heat exchanging unit  221   a  is a heat exchanger that exchanges heat between the first refrigerant that flows in the first refrigerant circuit  201  and the second refrigerant that flows in the second refrigerant circuit  202 . 
     The first sub heat exchanging unit  221   b  is configured to cause the first refrigerant that has passed through the first main heat exchanging unit  221   a  to be in a superheating state. The superheating state is a state in which a degree of superheating has been given to the first refrigerant. A degree of superheating to be given is not limited as long as some degree of superheating is given. In a cooling operation state, the first sub heat exchanging unit  221   b  exchanges heat between the first refrigerant that has not entered the first main heat exchanging unit  221   a  yet and the first refrigerant that has exited the first main heat exchanging unit  221   a . In a heating operation state, the first bypass valve  223 , which will be described later, is fully closed. Consequently, the first refrigerant that has exited the first main heat exchanging unit  221   a  flows out from the first cascade unit  220  via the first bypass circuit  225 , which will be described later. The first sub heat exchanging unit  221   b  thus does not exchange heat. 
     The first main heat exchanging unit  221   a  is a heat exchanger having heat exchanging capacity larger than that of the first sub heat exchanging unit  221   b . For example, the first main heat exchanging unit  221   a  is a plate heat exchanger, and the first sub heat exchanging unit  221   b  is a double pipe. 
     The heat exchanging capacity of a heat exchanger can be calculated by a heat transfer rate and the like. The heat exchanging capacity of a plate heat exchanger used as the first main heat exchanging unit  221   a  is generally larger than the heat exchanging capacity of a double pipe used as the first sub heat exchanging unit  221   b.    
     A method of calculating the heat exchanging capacity of a heat exchanger is not limited. 
     The first refrigerant circuit  201  is provided with the first flow-rate regulating valve  222  near the liquid-side end of the first main heat exchanging unit  221   a . The first flow-rate regulating valve  222  is an electric expansion valve that decompresses the first refrigerant in a cooling operation state. The valve opening degree of the first flow-rate regulating valve  222  is regulated by the control unit  260  via the first cascade control unit  262  to cause the first refrigerant that exits the first sub heat exchanging unit  221   b  to be in a superheating state. 
     The first bypass circuit  225  is, for example, a capillary. In the first refrigerant circuit  201  in a heating operation state, the first refrigerant that has exited the first main heat exchanging unit  221   a  bypasses the first sub heat exchanging unit  221   b  via the first bypass circuit  225 . The first refrigerant that has bypassed the first sub heat exchanging unit  221   b  flows out from the first cascade unit  220 . 
     In the first refrigerant circuit  201  in a heating operation state, the first bypass valve  223  is provided on the upstream side of the first sub heat exchanging unit  221   b . The first bypass valve  223  is fully closed in a heating operation state. Consequently, the first refrigerant that has exited the first main heat exchanging unit  221   a  flows out from the first cascade unit  220  via the first bypass circuit  225 , and the first sub heat exchanging unit  221   b  does not exchange heat. The first bypass valve  223  is an electric expansion valve, and the valve opening degree of the first bypass valve  223  is regulated by the control unit  260  via the first cascade control unit  262 . 
     The compressor  226  is a device for compressing the second refrigerant. For example, a compressor having a hermetic structure and in which a compression element of a positive displacement type, such as a rotary type or scroll type, is driven to rotate by a compression motor is used. 
     The second refrigerant circuit  202  is provided with the expansion valve  228  near the liquid-side end of the first main heat exchanging unit  221   a . The expansion valve  228  is an electric expansion valve that decompresses refrigerant in a heating operation state. The opening degree of the expansion valve  228  is regulated by the control unit  260  via the first cascade control unit  262 . 
     As illustrated in  FIG. 3 , the first cascade unit  220  is further provided with an inlet temperature sensor  224   a  and an outlet temperature sensor  224   b . The inlet temperature sensor  224   a  detects a temperature (inlet temperature) of the first refrigerant at the liquid-side end of the first main heat exchanging unit  221   a  in the first refrigerant circuit  201 . The outlet temperature sensor  224   b  detects a temperature (outlet temperature) of the first refrigerant at the gas-side end of the first sub heat exchanging unit  221   b  in the first refrigerant circuit  201 . Values detected by the inlet temperature sensor  224   a  and the outlet temperature sensor  224   b  are sent to the control unit  260  via the first cascade control unit  262 . 
     The first cascade unit  220  is also provided with various types of sensors, which are not illustrated, other than the aforementioned sensors. 
     (2-4) Control Unit 
     As illustrated in  FIG. 4 , the control unit  260  includes the heat-source-side control unit  261 , the first cascade control unit  262 , a 2 second cascade control unit  263 , and usage-side control units  264 ,  265 ,  266 , and  267 . Each of the control units  260 ,  261 ,  262 ,  263 ,  264 ,  265 ,  266 , and  267  includes a processor, such as a CPU or a GPU, a memory, and the like. The processor is capable of reading a program stored in the memory and performing predetermined processing in accordance with the program. 
     The heat-source-side control unit  261  is disposed at the heat-source-side unit  210 . The heat-source-side control unit  261  controls the entirety of the heat-source-side unit  210  and the opening degree of the expansion valve  214 . The first cascade control unit  262  is disposed at the first cascade unit  220 . The first cascade control unit  262  controls the entirety of the first cascade unit  220  and the opening degrees of the first flow-rate regulating valve  222 , the first bypass valve  223 , and the expansion valve  228 . The second cascade control unit  263  is disposed at the second cascade unit  240 . The second cascade control unit  263  controls the entirety of the second cascade unit  240  and the opening degrees of a second flow-rate regulating valve  242 , a second bypass valve  243 , and an expansion valve  248 . The usage-side control unit  264  is disposed at the usage-side unit  230 A. The usage-side control unit  264  controls the entirety of the usage-side unit  230 A and the opening degree of the flow-rate regulating valve  232   a . The usage-side control unit  265  is disposed at the usage-side unit  230 B. The usage-side control unit  265  controls the entirety of the usage-side unit  230 B and the opening degree of a flow-rate regulating valve  232   b . The usage-side control unit  266  is disposed at the usage-side unit  250 A. The usage-side control unit  266  controls the entirety of the usage-side unit  250 A and the opening degree of a flow-rate regulating valve  252   a . The usage-side control unit  267  is disposed at the usage-side unit  250 B. The usage-side control unit  267  controls the entirety of the usage-side unit  250 B and the opening degree of a flow-rate regulating valve  252   b.    
     The control unit  260  and the control units  261 ,  262 ,  263 ,  264 ,  265 ,  266 , and  267  each include a control board on which electric components, such as a microcomputer and a memory, are mounted. The control unit  260  controls the entirety of the air conditioning apparatus  200  via the control units  261 ,  262 ,  263 ,  264 ,  265 ,  266 , and  267 . The control unit  260  is capable of receiving values detected by the sensors provided at the air conditioning apparatus  200  via the control units  261 ,  262 ,  263 ,  264 ,  265 ,  266 , and  267  and sending control signals and the like to each constituent device. 
     Specifically, for example, the control unit  260  receives via the first cascade control unit  262  an inlet temperature that is detected by the inlet temperature sensor  224   a  provided at the first cascade unit  220  and an outlet temperature that is detected by the outlet temperature sensor  224   b . The control unit  260  is in advance provided with opening-degree regulating algorithm for regulating the opening degree of the first flow-rate regulating valve  222 . The control unit  260  uses the opening-degree regulating algorithm and generates from the inlet temperature and the outlet temperature a control signal for causing the first refrigerant that exits the first sub heat exchanging unit  221   b  to have an appropriate degree of superheating. The first flow-rate regulating valve  222  is capable of causing the first refrigerant that exits the first sub heat exchanging unit  221   b  to have an appropriate degree of superheating by regulating the opening degree of the first flow-rate regulating valve  222  on the basis of the control signal. 
     Regulation of the opening degree of the second flow-rate regulating valve  242  included in the second cascade unit  240  is the same as that described above. The control unit  260  receives via the second cascade control unit  263  an inlet temperature that is detected by an inlet temperature sensor  244   a  of the second cascade unit  240  and an outlet temperature that is detected by an outlet temperature sensor  244   b . The control unit  260  uses the opening-degree regulating algorithm and sends a control signal that regulates the opening degree of the second flow-rate regulating valve  242  to the second flow-rate regulating valve  242 . The second flow-rate regulating valve  242  regulates the opening degree on the basis of the control signal. 
     A method by which the control unit  260  regulates the opening degree of the first flow-rate regulating valve  222  or the second flow-rate regulating valve  242  is not limited thereto. 
     (3) Basic Operation of Air Conditioning Apparatus 
     Next, basic operation of the air conditioning apparatus  200  will be described. The basic operation of the air conditioning apparatus  200  includes cooling operation and heating operation. The basic operation of the air conditioning apparatus  200  described below is performed by the control unit  260  that controls constituent devices of the air conditioning apparatus  200  (the heat-source-side unit  210 , the usage-side units  230 A,  230 B,  250 A, and  250 B, the first cascade unit  220 , and the second cascade unit  240 ). 
     (3-1) Cooling Operation 
     For example, when all of the usage-side units  230 A,  230 B,  250 A, and  250 B perform cooling operation (operation in which all of the usage-side heat exchangers  231   a ,  231   b ,  251   a , and  251   b  function as evaporators for refrigerant and the heat-source-side heat exchanger  212  functions as a radiator for refrigerant), the switching mechanisms  213 ,  227 , and  247  are switched to a cooling operation state (the state indicated by the solid lines in  FIG. 3 ). 
     (3-1-1) First Refrigerant Circuit 
     During cooling operation, the first refrigerant discharged from the compressor  211  and having a high pressure is sent to the heat-source-side heat exchanger  212  through the switching mechanism  213  in the first refrigerant circuit  201 . The first refrigerant sent to the heat-source-side heat exchanger  212  condenses in the heat-source-side heat exchanger  212  that functions as a radiator for the first refrigerant by exchanging heat with outdoor air supplied by the heat-source-side fan and being cooled. The first refrigerant flows out from the heat-source-side unit  210  through the expansion valve  214 . 
     The first refrigerant that has flowed out from the heat-source-side unit  210  is sent to the first cascade unit  220  and the second cascade unit  240 . 
     The first refrigerant that has flowed into the first cascade unit  220  enters the first sub heat exchanging unit  221   b . The first refrigerant that has entered the first sub heat exchanging unit  221   b  exchanges heat with the first refrigerant that has exited the first main heat exchanging unit  221   a . The first refrigerant exits the first sub heat exchanging unit  221   b  and passes through the first bypass valve  223 . The first refrigerant then enters the first flow-rate regulating valve  222  whose opening degree is appropriately regulated by the control unit  260  and is decompressed. The decompressed first refrigerant enters the first main heat exchanging unit  221   a . In the first main heat exchanging unit  221   a  that functions as an evaporator for the first refrigerant, the first refrigerant evaporates by exchanging heat with the second refrigerant that flows in the second refrigerant circuit  202  and being heated. The first refrigerant that has exited the first sub heat exchanging unit  221   b  enters the first sub heat exchanging unit  221   b  and exchanges heat with the first refrigerant that has not entered the first main heat exchanging unit  221   a  yet. An appropriate degree of superheating has been given to the first refrigerant that has exchanged heat here and exited the first sub heat exchanging unit  221   b . The first refrigerant flows out from the first cascade unit  220  and is sucked in a state of merging with the first refrigerant that has flowed out from the second cascade unit  240  by the compressor  211 . 
     The first refrigerant that has flowed into the second cascade unit  240  enters a second sub heat exchanging unit  241   b . The first refrigerant that has entered the second sub heat exchanging unit  241   b  exchanges heat with the first refrigerant that has exited a second main heat exchanging unit  241   a . The first refrigerant exits the second sub heat exchanging unit  241   b  and passes through the second bypass valve  243 . The first refrigerant then enters the second flow-rate regulating valve  242  whose opening degree is appropriately regulated by the control unit  260  and is decompressed. The decompressed first refrigerant enters a second heat exchanger  241 . The first refrigerant that has entered the second heat exchanger  241  evaporates in the second main heat exchanging unit  241   a  that functions as an evaporator for the first refrigerant by exchanging heat with the third refrigerant that flows in the third refrigerant circuit  203  and being heated. The first refrigerant that has exited the second sub heat exchanging unit  241   b  enters the second sub heat exchanging unit  241   b  and exchanges heat with the first refrigerant that has not entered the second main heat exchanging unit  241   a  yet. An appropriate degree of superheating has been given to the first refrigerant that has exchanged heat here and exited the second sub heat exchanging unit  241   b . The first refrigerant flows out from the second cascade unit  240  and in a state of merging with the first refrigerant that has flowed out from the first cascade unit  220  is sucked by the compressor  211 . 
     (3-1-2) Second Refrigerant Circuit 
     In the second refrigerant circuit  202 , the second refrigerant discharged from the compressor  226  and having a high pressure is sent to the first main heat exchanging unit  221   a  through the switching mechanism  227  in the second refrigerant circuit  202 . In the first main heat exchanging unit  221   a  that functions as a radiator for the second refrigerant, the second refrigerant condenses by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  201  and being cooled. The second refrigerant flows out from the first cascade unit  220  through the expansion valve  228 . The second refrigerant that has flowed out from the first cascade unit  220  is sent to each of the usage-side units  230 A and  230 B. 
     After being decompressed by the flow-rate regulating valve  232   a  to an appropriate pressure, the second refrigerant sent to the usage-side unit  230 A evaporates in the usage-side heat exchanger  231   a  that functions as an evaporator for the second refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant flows out from the usage-side unit  230 A and in a state of merging with the second refrigerant that has flowed out from the usage-side unit  230 B is sucked by the compressor  226 . 
     After being decompressed by the flow-rate regulating valve  232   b  to an appropriate pressure, the second refrigerant sent to the usage-side unit  230 B evaporates in the usage-side heat exchanger  231   b  that functions as an evaporator for the second refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant flows out from the usage-side unit  230 B and is in a state of merging with the second refrigerant that has flowed out from the usage-side unit  230 A sucked by the compressor  226 . 
     Indoor air cooled in the usage-side heat exchangers  231   a  and  231   b  is sent to the inside of a room, thereby cooling the inside of the room. 
     (3-1-3) Third Refrigerant Circuit 
     In the third refrigerant circuit  203 , the third refrigerant discharged from a compressor  244  and having a high pressure is sent to the second main heat exchanging unit  241   a  through the switching mechanism  247 . The third refrigerant sent to the second main heat exchanging unit  241   a  condenses in the second main heat exchanging unit  241   a  that functions as a radiator for the third refrigerant by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  201  and being cooled. The third refrigerant flows out from the second cascade unit  240  through an expansion valve  246 . The third refrigerant that has flowed out from the second cascade unit  240  is sent to each of the usage-side units  250 A and  250 B. 
     After being decompressed by the flow-rate regulating valve  252   a  to an appropriate pressure, the third refrigerant sent to the usage-side unit  250 A evaporates in the usage-side heat exchanger  251   a  that functions as an evaporator for the third refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant flows out from the usage-side unit  250 A and in a state of merging with the third refrigerant that has flowed out from the usage-side unit  250 B is sucked by the compressor  244 . 
     After being decompressed by the flow-rate regulating valve  252   b  to an appropriate pressure, the third refrigerant sent to the usage-side unit  250 B evaporates in the usage-side heat exchanger  251   b  that functions as an evaporator for the third refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant flows out from the usage-side unit  250 A and in a state of merging with the third refrigerant that has flowed out from the usage-side unit  250 B is sucked by the compressor  244 . 
     Indoor air cooled in the usage-side heat exchangers  251   a  and  251   b  is sent to the inside of a room, thereby cooling the inside of the room. 
     (3-2) Heating Operation 
     For example, when all of the usage-side units  230 A,  230 B,  250 A, and  250 B perform heating operation (operation in which all of the usage-side heat exchangers  231   a ,  231   b ,  251   a , and  251   b  function as radiators for refrigerant and the heat-source-side heat exchanger  212  functions as an evaporator for refrigerant), the switching mechanisms  213 ,  227 , and  247  are switched to a heating operation state (the state indicated by the broken lines in  FIG. 3 ). 
     (3-2-1) First Refrigerant Circuit 
     During heating operation, in the first refrigerant circuit  201 , the first refrigerant discharged from the compressor  211  and having a high pressure flows out from the heat-source-side unit  210  through the switching mechanism  213 . 
     The first refrigerant that has flowed out from the heat-source-side unit  210  is sent to the first cascade unit  220  and the second cascade unit  240 . 
     The first refrigerant sent to the first cascade unit  220  passes through the first sub heat exchanging unit  221   b  and enters the first main heat exchanging unit  221   a . At this time, the first refrigerant does not exchange heat in the first sub heat exchanging unit  221   b . The first refrigerant that has entered the first main heat exchanging unit  221   a  condenses by exchanging heat with the second refrigerant that flows in the second refrigerant circuit  202  and being cooled. The first refrigerant that has condensed passes through the first flow-rate regulating valve  222 . Here, the first bypass valve  223  has an opening degree regulated by the control unit  260  and is in a fully closed state. The first refrigerant that has passed through the first flow-rate regulating valve  222  bypasses the first sub heat exchanging unit  221   b  via the first bypass circuit  225 . The first refrigerant flows out from the first cascade unit  220 . In a state of merging with the first refrigerant that has flowed out from the second cascade unit  240 , the first refrigerant that has flowed out from the first cascade unit  220  is sent to the heat-source-side unit  210 . 
     The first refrigerant sent to the second cascade unit  240  passes through the second sub heat exchanging unit  241   b  and enters the second main heat exchanging unit  241   a . At this time, the first refrigerant does not exchange heat in the second sub heat exchanging unit  241   b . The first refrigerant that has entered the second main heat exchanging unit  241   a  condenses by exchanging heat with the third refrigerant that flows in the third refrigerant circuit  203  and being cooled. The first refrigerant that has condensed passes through the second flow-rate regulating valve  242 . Here, the second bypass valve  243  has an opening degree regulated by the control unit  260  and is in a fully closed state. The first refrigerant that has passed through the second flow-rate regulating valve  242  bypasses the second sub heat exchanging unit  241   b  via a second bypass circuit  245 . The first refrigerant flows out from the second cascade unit  240 . In a state of merging with the first refrigerant that has flowed out from the first cascade unit  220 , the first refrigerant that has flowed out from the second cascade unit  240  is sent to the heat-source-side unit  210 . 
     The first refrigerant sent to the heat-source-side unit  210  is sent to the expansion valve  214 . After being decompressed by the expansion valve  214  whose opening degree is regulated by the control unit  260 , the first refrigerant sent to the expansion valve  214  is sent to the heat-source-side heat exchanger  212 . The first refrigerant that has entered the heat-source-side heat exchanger  212  evaporates by exchanging heat with outdoor air supplied by the heat-source-side fan and being heated. The first refrigerant that has evaporated is sucked by the compressor  211  through the switching mechanism  213 . 
     (3-2-2) Second Refrigerant Circuit 
     In the second refrigerant circuit  202 , during heating operation, the second refrigerant discharged from the compressor  226  and having a high pressure flows out from the first cascade unit  220  through the switching mechanism  227 . 
     The second refrigerant that has flowed out from the first cascade unit  220  is sent to each of the usage-side units  230 A and  230 B. 
     The second refrigerant sent to the usage-side unit  230 A condenses in the usage-side heat exchanger  231   a  that functions as a radiator for refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant that has condensed passes through the flow-rate regulating valve  232   a  and flows out from the usage-side unit  230 A. In a state of merging with the second refrigerant that has flowed out from the usage-side unit  230 B, the second refrigerant that has flowed out from the usage-side unit  230 A is sent to the first cascade unit  220 . 
     The second refrigerant sent to the usage-side unit  230 B condenses in the usage-side heat exchanger  231   b  that functions as a radiator for refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The second refrigerant that has condensed passes through the flow-rate regulating valve  232   b  and flows out from the usage-side unit  230 B. In a state of merging with the second refrigerant that has flowed out from the usage-side unit  230 A, the second refrigerant that has flowed out from the usage-side unit  230 B is sent to the first cascade unit  220 . 
     Indoor air heated in the usage-side heat exchangers  231   a  and  231   b  is sent to the inside of a room, thereby heating the inside of the room. 
     The second refrigerant that has flowed into the first cascade unit  220  flows into the expansion valve  228 . The second refrigerant that has flowed into the expansion valve  228  is sent to the first main heat exchanging unit  221   a  after being decompressed by the expansion valve  228 . In the first main heat exchanging unit  221   a  that functions as an evaporator for the second refrigerant, the second refrigerant evaporates by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  201  and being heated. The second refrigerant that has evaporated is sucked by the compressor  226  through the switching mechanism  227 . 
     (3-2-3) Third Refrigerant Circuit 
     In the third refrigerant circuit  203 , the third refrigerant discharged from the compressor  244  and having a high pressure flows out from the second cascade unit  240  through the switching mechanism  247 . 
     The third refrigerant that has flowed out from the second cascade unit  240  is sent to each of the usage-side units  250 A and  250 B. 
     The third refrigerant sent to the usage-side unit  250 A condenses in the usage-side heat exchanger  251   a  that functions as a radiator for refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant that has condensed passes through the flow-rate regulating valve  252   a  and flows out from the usage-side unit  250 A. In a state of merging with the third refrigerant that has flowed out from the usage-side unit  250 B, the third refrigerant that has flowed out from the usage-side unit  250 A is sent to the second cascade unit  240 . 
     The third refrigerant sent to the usage-side unit  250 B condenses in the usage-side heat exchanger  251   b  that functions as a radiator for refrigerant by exchanging heat with outdoor air supplied by the usage-side fan. The third refrigerant that has condensed passes through the flow-rate regulating valve  252   b  and flows out from the usage-side unit  250 B. In a state of merging with the third refrigerant that has flowed out from the usage-side unit  250 A, the third refrigerant that has flowed out from the usage-side unit  250 B is sent to the second cascade unit  240 . 
     Indoor air heated in the usage-side heat exchangers  251   a  and  251   b  is sent to the inside of a room, thereby heating the inside of the room. 
     The third refrigerant that has flowed into the second cascade unit  240  is sent to the expansion valve  246 . The third refrigerant that has flowed into the expansion valve  246  is sent to the second main heat exchanging unit  241   a  after being decompressed by the expansion valve  246 . The third refrigerant that has flowed into the second main heat exchanging unit  241   a  evaporates in the second main heat exchanging unit  241   a  that functions as an evaporator for the third refrigerant by exchanging heat with the first refrigerant that flows in the first refrigerant circuit  201  and being heated. The third refrigerant that has evaporated is sucked by the compressor  244  through the switching mechanism  247 . 
     (4) Modifications 
     (4-1) 
     The first main heat exchanging unit  221   a  and the second main heat exchanging unit  241   a  of the air conditioning apparatus  200  are plate heat exchangers, and the first sub heat exchanging unit  221   b  and the second sub heat exchanging unit  241   b  are double pipes. Each heat exchanging unit is not limited thereto. 
     For example, the first main heat exchanging unit  221   a  and the second main heat exchanging unit  241   a  may be heat exchangers each including a plurality of stacked flat pipes, and the first sub heat exchanging unit  221   b  and the second sub heat exchanging unit  241   b  may be heat exchangers each having a structure in contact with a pipe. 
     The first main heat exchanging unit  221   a  is at least a heat exchanger having heat exchanging capacity larger than that of the first sub heat exchanging unit  221   b . The second main heat exchanging unit  241   a  is at least a heat exchanger having heat exchanging capacity larger than that of the second sub heat exchanging unit  241   b.    
     The heat exchanging capacity of a plate heat exchanger or a heat exchanger including a plurality of stacked flat pipes is generally larger than the heat exchanging capacity of a double pipe or a heat exchanger having a structure in contact with a pipe. 
     (4-2) 
     As illustrated in  FIG. 3 , the first cascade unit  220  of the air conditioning apparatus  200  is provided with the inlet temperature sensor  224   a  at the liquid-side end of the first main heat exchanging unit  221   a  and provided with the outlet temperature sensor  224   b  at the gas-side end of the first sub heat exchanging unit  221   b . Arrangements of the inlet temperature sensor and the outlet temperature sensor are, however, not limited thereto. 
     For example, when the first refrigerant circuit  201  performs cooling operation, the outlet temperature sensor  224   b  may be provided at an outlet of the first main heat exchanging unit  221   a.    
     (4-3) 
     The first bypass circuit  225  and the second bypass circuit  245  of the air conditioning apparatus  200  are capillaries. The forms of the first bypass circuit  225  and the second bypass circuit  245  are, however, not limited thereto. For example, the first bypass circuit  225  and the second bypass circuit  245  may be electric expansion valves, electric on-off valves, or check valves. 
     The first bypass valve  223  and the second bypass valve  243  are electric expansion valves. Forms of the first bypass valve  223  and the second bypass valve  243  are, however, not limited thereto. For example, the first bypass valve  223  and the second bypass valve  243  may be electric on-off valves or check valves. 
     (4-4) 
     In the first refrigerant circuit  201 , the second refrigerant circuit  202 , and the third refrigerant circuit  203  of the air conditioning apparatus  200 , R32 having high stability is charged as the first refrigerant, the second refrigerant, and the third refrigerant, respectively. However, refrigerant other than R32 may be charged in the refrigerant cycle system presented in the present disclosure. For example, it is preferable that the first refrigerant be R32 and that the second refrigerant and the third refrigerant be carbon dioxide. 
     Each of the first refrigerant, the second refrigerant, and the third refrigerant charged in the refrigerant cycle system is preferably any one of HFC refrigerant, HFO refrigerant, and a natural refrigerant. Alternatively, each of the first refrigerant and the second refrigerant is preferably a mixture refrigerant that contains any two or more of HFC refrigerant, HFO refrigerant, natural refrigerant, and CF 3 I. Specifically, the HFC refrigerant is R32, R125, R134a, R143a, R245fa, or the like. The HFO refrigerant is R1234yf, R1234zd, R1123, R1132(E), or the like. The natural refrigerant is R744, R717, R290, R600a, R1270, or the like. 
     (4-5) 
     The refrigerant cycle system presented in the present disclosure has been described by using the air conditioning apparatus  200  as a specific example of the refrigerant cycle system. The form of the refrigerant cycle system is, however, not limited thereto. For example, the refrigerant cycle system may be a heat-pump hot water supply apparatus, or the like. 
     (5) Features 
     (5-1) 
     The air conditioning apparatus  200  as the refrigerant cycle system presented in the present disclosure includes the first refrigerant circuit  201 , the second refrigerant circuit  202 , the third refrigerant circuit  203 , the first main heat exchanging unit  221   a , the second main heat exchanging unit  241   a , the first flow-rate regulating valve  222 , and the second flow-rate regulating valve  242 . The first refrigerant circuit  201  is a vapor compression refrigeration cycle. The second refrigerant circuit  202  is a vapor compression refrigeration cycle. The third refrigerant circuit  203  is a vapor compression refrigeration cycle. The first main heat exchanging unit  221   a  exchanges heat between the first refrigerant and the second refrigerant. The first refrigerant is refrigerant that flows in the first refrigerant circuit  201 . The second refrigerant is refrigerant that flows in the second refrigerant circuit  202 . The second main heat exchanging unit  241   a  exchanges heat between the first refrigerant and the third refrigerant. The third refrigerant is refrigerant that flows in the third refrigerant circuit  203 . The first flow-rate regulating valve  222  regulates the amount of the first refrigerant that enters the first main heat exchanging unit  221   a  in the first refrigerant circuit  201 . The second flow-rate regulating valve  242  regulates the amount of the first refrigerant that enters the second main heat exchanging unit  241   a  in the first refrigerant circuit  201 . The first main heat exchanging unit  221   a  and the second main heat exchanging unit  241   a  are connected in parallel in the first refrigerant circuit  201 . 
     A dual refrigeration cycle to which a vapor compression refrigeration cycle is connected via a cascade heat exchanger is known in the art. It is preferable in the dual refrigeration cycle that a large number of usage-side units be connectable to one heat source unit, for example, when an air conditioner is constructed in large-scale commercial facility or a construction, such as a building. Consequently, it is possible to reduce a space and costs required for the installation of the air conditioner. 
     In the air conditioning apparatus  200  presented in the present embodiment, with respect to the first refrigerant circuit  201  that is a vapor compression refrigeration cycle, the second refrigerant circuit  202  including the first main heat exchanging unit  221   a  and the third refrigerant circuit  203  including the second main heat exchanging unit  241   a  are connected in parallel. Consequently, it is possible to connect a larger number of usage units with respect to one heat source unit. 
     (5-2) 
     The air conditioning apparatus  200  further includes the control unit  260 . The control unit  260  regulates the opening degrees of the first flow-rate regulating valve  222  and the second flow-rate regulating valve  242 . When the first main heat exchanging unit  221   a  of the first refrigerant circuit  201  serves as an evaporator, the control unit  260  regulates the opening degree of the first flow-rate regulating valve  222  to cause the first refrigerant that exits the first main heat exchanging unit  221   a  to be in a superheating state. When the second main heat exchanging unit  241   a  of the first refrigerant circuit  201  serves as an evaporator, the control unit  260  regulates the opening degree of the second flow-rate regulating valve  242  to cause the first refrigerant that exits a second main heat exchanging unit  242   a  to be in a superheating state. 
     The control unit  260  is capable of regulating the opening degrees of the first flow-rate regulating valve  222  and the second flow-rate regulating valve  242 . Consequently, the control unit  260  is capable of controlling the degree of superheating of the first refrigerant that exits the first main heat exchanging unit  221   a  or the second main heat exchanging unit  242   a  and capable of driving the air conditioning apparatus  200  efficiently. 
     (5-3) 
     The air conditioning apparatus  200  further includes the first sub heat exchanging unit  221   b . The first sub heat exchanging unit  221   b  exchanges heat in the first refrigerant circuit  201  between the first refrigerant that has not entered the first main heat exchanging unit  221   a  yet and the first refrigerant that has exited the first main heat exchanging unit  221   a.    
     The first refrigerant circuit  201  of the air conditioning apparatus  200  further includes the first bypass circuit  225 . In the first refrigerant circuit  201 , when the first main heat exchanging unit  221   a  serves as a condenser, the first refrigerant that has exited the first main heat exchanging unit  221   a  bypasses the first sub heat exchanging unit  221   b  via the first bypass circuit  225 . The first refrigerant that has bypassed the first sub heat exchanging unit  221   b  is sucked by the compressor  211  included in the first refrigerant circuit  201 . 
     The first main heat exchanging unit  221   a  of the air conditioning apparatus  200  has heat exchanging capacity larger than that of the first sub heat exchanging unit  221   b.    
     Due to the first sub heat exchanging unit  221   b  included in the air conditioning apparatus  200 , it is possible to reduce a decrease in the heat exchanging capacity of the first main heat exchanging unit  221   a  caused by control of the degree of superheating of the first refrigerant. In addition, the first bypass circuit  225  included in the air conditioning apparatus  200  enables the first refrigerant to bypass the first sub heat exchanging unit  221   b  when the first refrigerant circuit  201  performs heating operation. The first main heat exchanging unit  221   a  is preferably a high-performance heat exchanger having large heat exchanging capacity. The first sub heat exchanging unit  221   b  is a heat exchanger capable of giving a degree of superheating to the first refrigerant without impairing the heat exchanging capacity of the first main heat exchanging unit  221   a.    
     (5-4) 
     The air conditioning apparatus  200  further includes the second sub heat exchanging unit  241   b . The second sub heat exchanging unit  241   b  exchanges heat in the second refrigerant circuit  202  between the first refrigerant that has not entered the second main heat exchanging unit  241   a  yet and the first refrigerant that has exited the second main heat exchanging unit  241   a.    
     In the air conditioning apparatus  200 , the first refrigerant circuit  201  further includes the second bypass circuit  245 . In the first refrigerant circuit  201 , when the second main heat exchanging unit  241   a  serves as a condenser, the first refrigerant that has exited the second main heat exchanging unit  241   a  bypasses the second sub heat exchanging unit  241   b  via the second bypass circuit  245 . The first refrigerant that has bypassed the second sub heat exchanging unit  241   b  is sucked by the compressor  211  included in the first refrigerant circuit  201 . 
     The second main heat exchanging unit  241   a  of the air conditioning apparatus  200  has heat exchanging capacity larger than that of the second sub heat exchanging unit  241   b.    
     Consequently, it is also possible in the third refrigerant circuit  203  to obtain the same effects as those in the second refrigerant circuit  202 . 
     (5-5) 
     In the first refrigerant circuit  201 , the second refrigerant circuit  202 , and the third refrigerant circuit  203  of the air conditioning apparatus  200 , R32 having high stability is charged as the first refrigerant, the second refrigerant, and the third refrigerant, respectively. However, refrigerant other than R32 may be charged in the refrigerant cycle system presented in the present disclosure. For example, it is preferable that the first refrigerant be R32 and that the second refrigerant and the third refrigerant be carbon dioxide. 
     Each of the first refrigerant, the second refrigerant, and the third refrigerant charged in the refrigerant cycle system is preferably any one of HFC refrigerant, HFO refrigerant, and natural refrigerant. Alternatively, each of the first refrigerant and the second refrigerant is preferably a mixture refrigerant that contains any two or more of HFC refrigerant, HFO refrigerant, natural refrigerant, and CF 3 I. 
     For example, the aforementioned refrigerants can be employed as refrigerant charged in the air conditioning apparatus  200  according to the present embodiment. 
     An embodiment of the present disclosure has been described above; however, it should be understood that various changes in the forms and details are possible without departing from the gist and the scope of the present disclosure described in the claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  201  first refrigerant circuit 
               2 ,  202  second refrigerant circuit 
               3 ,  203  third refrigerant circuit 
               13 ,  25 ,  213 ,  225  switching mechanism 
               21 ,  221  first cascade heat exchanger 
               21   a ,  221   a  first main cascade heat exchanging unit 
               21   b ,  221   b  first sub cascade heat exchanging unit 
               22 ,  222  first flow-rate regulating valve 
               41 ,  241  second cascade heat exchanger 
               41   a ,  241   a  second main cascade heat exchanging unit 
               41   b ,  241   b  second sub cascade heat exchanging unit 
               60 ,  260  control unit 
               100 ,  200  refrigerant cycle system 
               211  compressor 
               225  first bypass circuit 
               245  second bypass circuit 
           
         
       
    
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2000-193339