Patent Publication Number: US-2022214056-A1

Title: Air conditioner

Description:
TECHNICAL FIELD 
     The present disclosure relates to an air conditioner. 
     BACKGROUND 
     As disclosed in PTL 1 (Japanese Unexamined Patent Application Publication No. 2010-156493), multi-split air conditioners exist in the art that include plural heat-source-side heat exchangers and plural use-side units and are designed such that whether to perform a cooling operation or a heating operation can be freely selected for each individual use-side unit. One conceivable way to improve the operating efficiency of such an air conditioner is to provide the air conditioner with an economizer heat exchanger. 
     SUMMARY 
     An air conditioner according to one or more embodiments includes a plurality of use-side units, and a heat-source-side unit. The heat-source-side unit includes a compressor, a discharge pipe, a first main heat-source-side flow path, a second main heat-source-side flow path, a first heat-source-side heat exchanger, a second heat-source-side heat exchanger, a first economizer heat exchanger, and a second economizer heat exchanger. Each of the use-side units is switchable between a cooling operation and a heating operation. The discharge pipe is a pipe through which a refrigerant discharged from the compressor flows. The first main heat-source-side flow path and the second main heat-source-side flow path branch off from the discharge pipe. The first heat-source-side heat exchanger and the first economizer heat exchanger are connected in series in the first main heat-source-side flow path. The second heat-source-side heat exchanger and the second economizer heat exchanger are connected in series in the second main heat-source-side flow path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an air conditioner  1  according to one or more embodiments of the present disclosure. 
         FIG. 2  is a block diagram of a control unit (used interchangeably herein with “controller”) of a refrigeration cycle apparatus illustrated in  FIG. 1 . 
         FIG. 3  is a schematic diagram illustrating how the air conditioner  1  performs a first operation. 
         FIG. 4  is a schematic diagram illustrating how the air conditioner  1  performs a second operation. 
         FIG. 5  is a schematic diagram illustrating how the air conditioner  1  performs a third operation A. 
         FIG. 6  is a schematic diagram illustrating how the air conditioner  1  performs the third operation A if the overall evaporation load on use-side heat exchangers is small. 
         FIG. 7  is a schematic diagram illustrating how the air conditioner  1  performs a third operation B. 
         FIG. 8  is a schematic diagram illustrating how the air conditioner  1  performs a third operation C. 
         FIG. 9  is a schematic diagram illustrating an example of the related art related to an air conditioner. 
         FIG. 10  is a schematic diagram of the air conditioner  1  according to a modification B. 
         FIG. 11  is a schematic diagram of the air conditioner  1  according to a modification D. 
     
    
    
     DETAILED DESCRIPTION 
     (1) General Configuration of Air Conditioner 
       FIG. 1  is a schematic diagram of an air conditioner  1  according to one or more embodiments of the present disclosure. The air conditioner  1  includes the following components that constitute a refrigerant circuit  30 : plural use-side units  101   a,    101   b,  and  101   c , a heat-source-side unit  110 , a control unit  120 , and branch units  70   a,    70   b,  and  70   c.  The air conditioner  1  is designed such that whether to perform a cooling operation (first operation) or a heating operation (second operation) can be freely selected for each individual use-side unit. The air conditioner  1  performs a two-stage compression refrigeration cycle by use of a refrigerant that works in the supercritical region (which in this example is a CO2 refrigerant or a CO2 refrigerant mixture that comprises a CO2 refrigerant). 
     (2) Detailed Configuration 
     (2-1) Use-Side Units 
     The use-side units  101   a,    101   b,  and  101   c  are installed on the indoor ceiling of a building or other structure such as by being embedded in or suspended from the ceiling. Alternatively, the use-side units  101   a,    101   b,  and  101   c  are installed on the indoor wall such as by being mounted on the wall. The use-side units  101   a,    101   b,  and  101   c  are connected to the heat-source-side unit  110  via the following components: a liquid-refrigerant connection pipe  2 , a high/low pressure gas-refrigerant connection pipe  3 , a low pressure gas-refrigerant connection pipe  4 , the branch units  70   a,    70   b,  and  70   c,  a first shutoff valve  90 , a second shutoff valve  91 , and a third shutoff valve  92 . The use-side units  101   a,    101   b,  and  101   c  constitute a part of the refrigerant circuit  30 . 
     The first use-side unit  101   a  includes a first use-side heat exchanger  102   a,  and a first use-side expansion mechanism  103   a.  The second use-side unit  101   b  includes a second use-side heat exchanger  102   b,  and a second use-side expansion mechanism  103   b.  The third use-side unit  101   c  includes a third use-side heat exchanger  102   c,  and a third use-side expansion mechanism  103   c.  The use-side heat exchangers  102   a,    102   b,  and  102   c  are heat exchangers that exchange heat between the refrigerant and indoor air to thereby handle an indoor air-conditioning load (thermal load). The use-side expansion mechanisms  103   a,    103   b,  and  103   c  are mechanisms for causing the refrigerant to expand. The use-side expansion mechanisms  103   a,    103   b,  and  103   c  are each implemented by an electric expansion valve. 
     The use-side units  101   a,    101   b,  and  101   c  each include a use-side control unit  104  that controls operations of individual components constituting the use-side units  101   a ,  101   b,  and  101   c.  The use-side control unit  104  includes a microcomputer, and various electrical components. The microcomputer includes a central processing unit (CPU), a memory, and other components provided for controlling the use-side units  101   a,    101   b,  and  101   c.  The CPU reads a program stored in the memory or other storage device, and performs a predetermined computational process in accordance with the program. Further, the CPU is capable of performing an operation in accordance with the program, such as writing the results of computation into the memory or reading information stored in the memory. The use-side control unit  104  is capable of exchanging a control signal or other information with the heat-source-side unit  110  via a communications line. The use-side control unit  104  is also capable of receiving a signal related to activation or deactivation of the air conditioner  1 , a signal related to various settings, or other information transmitted from a remote control (not illustrated) used for operating the use-side units  101   a,    101   b,  and  101   c.    
     Although the following description of the embodiments is directed to the air conditioner  1  including three use-side units  101   a,    101   b,  and  101   c,  the present disclosure is also applicable to an air conditioner including more than three use-side units. 
     (2-2) Heat-Source-Side Unit 
     The heat-source-side unit  110  is installed on the rooftop of a building or other structure, or around a building or other structure. The heat-source-side unit  110  is connected to the use-side units  101   a,    101   b,  and  101   c,  and constitutes a part of the refrigerant circuit  30 . 
     The heat-source-side unit  110  mainly includes the following components: a first compressor  11 , a second compressor  12 , a discharge pipe  10 , a first main heat-source-side flow path  21 , a second main heat-source-side flow path  22 , a first heat-source-side heat exchanger  81 , a second heat-source-side heat exchanger  82 , a first economizer heat exchanger  61 , a second economizer heat exchanger  62 , a first economizer pipe  31 , a second economizer pipe  32 , a fourth shutoff valve  93 , and an accumulator  95 . 
     The heat-source-side unit  110  also includes a heat-source-side control unit  111  that controls operations of individual components constituting the heat-source-side unit  110 . The heat-source-side control unit  111  includes a microcomputer, and various electrical components. The microcomputer includes a central processing unit (CPU), a memory, and other components provided for controlling the heat-source-side unit  110 . The CPU reads a program stored in the memory or other storage device, and performs a predetermined computational process in accordance with the program. Further, the CPU is capable of performing an operation in accordance with the program, such as writing the results of computation into the memory or reading information stored in the memory. The heat-source-side control unit  111  is capable of exchanging a control signal or other information with the use-side control unit  104  of each of the use-side units  101   a,    101   b,  and  101   c  via a communications line. 
     (2-2-1) Compressors 
     The compressors  11  and  12  include the first compressor  11 , which is the compressor of the lower stage, and the second compressor  12 , which is the compressor of the higher stage. 
     The compressors  11  and  12  include the first compressor  11 , which is a single-stage compressor that compresses low pressure refrigerant in the refrigeration cycle to an intermediate pressure in the refrigeration cycle, and the second compressor  12 , which is a single-stage compressor that compresses intermediate-pressure refrigerant in the refrigeration cycle to a high pressure in the refrigeration cycle. Low-pressure refrigerant in the refrigeration cycle is sucked via a suction pipe  8  into the first compressor  11  of the lower stage, and compressed by the first compressor  11  to an intermediate pressure in the refrigeration cycle. After being compressed by the first compressor  11  to an intermediate pressure in the refrigeration cycle, the intermediate-pressure refrigerant in the refrigeration cycle is discharged to an intermediate refrigerant pipe  9  and then sucked into the second compressor  12  of the higher stage. After being sucked into the second compressor  12  of the higher stage, the intermediate-pressure refrigerant in the refrigeration cycle is compressed by the second compressor  12  to a high pressure in the refrigeration cycle before being discharged to the discharge pipe  10 . 
     (2-2-2) Discharge Pipe 
     The discharge pipe  10  is a pipe to which refrigerant is discharged after being compressed by the second compressor  12  of the higher stage to a high pressure in the refrigeration cycle. As illustrated in  FIG. 1 , the discharge pipe  10  branches off into the first main heat-source-side flow path  21 , the second main heat-source-side flow path  22 , and the high/low pressure gas-refrigerant connection pipe  3 . 
     (2-2-3) First Main Heat-Source-Side Flow Path and Second Main Heat-Source-Side Flow Path 
     The first main heat-source-side flow path  21  is a pipe that branches off from the discharge pipe  10  and connects to the liquid-refrigerant connection pipe  2 . The first main heat-source-side flow path  21  connects the first heat-source-side heat exchanger  81  and the first economizer heat exchanger  61  in series. The first main heat-source-side flow path  21  branches off to the first economizer pipe  31  at a point between the first heat-source-side heat exchanger  81  and the first economizer heat exchanger  61 . The first main heat-source-side flow path  21  is provided with a first heat-source-side expansion mechanism  24   a.    
     The second main heat-source-side flow path  22  is a pipe that branches off from the discharge pipe  10  and connects to the liquid-refrigerant connection pipe  2 . The second main heat-source-side flow path  22  connects the second heat-source-side heat exchanger  82  and the second economizer heat exchanger  62  in series. The second main heat-source-side flow path  22  branches off to the second economizer pipe  32  at a point between the second heat-source-side heat exchanger  82  and the second economizer heat exchanger  62 . The second main heat-source-side flow path  22  is provided with a second heat-source-side expansion mechanism  24   b.    
     The first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b  are each implemented by an electric expansion valve in this case. 
     (2-2-4) First Economizer Pipe and Second Economizer Pipe 
     The first economizer pipe  31  is a pipe that branches off from the first main heat-source-side flow path  21  at a point between the first heat-source-side heat exchanger  81  and the first economizer heat exchanger  61 , and extends toward the compressors  11  and  12 . 
     The second economizer pipe  32  is a pipe that branches off from the second main heat-source-side flow path  22  at a point between the second heat-source-side heat exchanger  82  and the second economizer heat exchanger  62 , and extends toward the compressors  11  and  12 . 
     The first economizer pipe  31  and the second economizer pipe  32  have a common part  35 . 
     The common part  35  is a pipe disposed between the location of branching from the first main heat-source-side flow path  21 , and the first economizer heat exchanger  61 , and between the location of branching from the second main heat-source-side flow path  22 , and the second economizer heat exchanger  62 . The common part  35  is provided with an expansion mechanism (i.e., expansion valve)  36 . The refrigerant passing through the common part  35  is decompressed by the expansion mechanism  36  to an intermediate pressure in the refrigeration cycle. 
     (2-2-5) First Heat-Source-Side Heat Exchanger and Second Heat-Source-Side Heat Exchanger 
     Each of the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  is a heat exchanger that functions as either a radiator or condenser for refrigerant. The liquid side of the first heat-source-side heat exchanger  81 , and the liquid side of the second heat-source-side heat exchanger  82  are connected by the first main heat-source-side flow path  21  and the second main heat-source-side flow path  22 . 
     The first heat-source-side heat exchanger  81  is connected in series with the first economizer heat exchanger  61  by the first main heat-source-side flow path  21 . The second heat-source-side heat exchanger  82  is connected in series with the second economizer heat exchanger  62  by the second main heat-source-side flow path  22 . 
     (2-2-6) First Economizer Heat Exchanger and Second Economizer Heat Exchanger 
     The first economizer heat exchanger  61  and the second economizer heat exchanger  62  are double-pipe heat exchangers or plate heat exchangers in this case. After refrigerant rejects heat in the first heat-source-side heat exchanger  81  or the second heat-source-side heat exchanger  82 , the refrigerant is subcooled by further rejecting heat in the first economizer heat exchanger  61  or the second economizer heat exchanger  62 . 
     In the first economizer heat exchanger  61 , the refrigerant flowing in the first main heat-source-side flow path  21 , and the refrigerant flowing in the first economizer pipe  31  exchange heat. The first economizer heat exchanger  61  is connected in series with the first heat-source-side heat exchanger  81  via the first main heat-source-side flow path  21 . 
     In the second economizer heat exchanger  62 , the refrigerant flowing in the second main heat-source-side flow path  22 , and the refrigerant flowing in the second economizer pipe  32  exchange heat. The second economizer heat exchanger  62  is connected in series with the second heat-source-side heat exchanger  82  via the second main heat-source-side flow path  22 . 
     (2-3) Control Unit  120   
     The control unit  120  controls the operations of individual devices constituting the air conditioner  1 . The air conditioner  1  can be controlled by the control unit  120  to switch between a first operation, a second operation, and a third operation, which will be described later. 
     The control unit  120  includes the following components coupled to each other via a communications line (see  FIG. 2 ): the use-side control unit  104  mentioned above, the heat-source-side control unit  111  mentioned above, and a branch-side control unit  74  described later. 
     Exemplary devices constituting the air conditioner  1  and controlled by the control unit  120  include the compressors  11  and  12 , a first heat-source-side switching mechanism  5 , a second heat-source-side switching mechanism  6 , a third heat-source-side switching mechanism  7 , the heat-source-side expansion mechanisms  24   a  and  24   b,  the use-side expansion mechanisms  103   a,    103   b,  and  103   c,  and the branch units  70   a,    70   b,  and  70   c.    
     The first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are mechanisms for switching the directions of refrigerant flow in the refrigerant circuit  30 . More specifically, these switching mechanisms are used to switch between a radiating operation state and an evaporating operation state. In the radiating operation state, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as radiators for refrigerant. In the evaporating operation state, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as evaporators for refrigerant. 
     The first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are four-way switching valves in this case. A fourth port  5   d  of the first heat-source-side switching mechanism  5 , a fourth port  6   d  of the second heat-source-side switching mechanism  6 , and a fourth port  7   d  of the third heat-source-side switching mechanism  7  are closed, and thus the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  function as three-way valves. 
     (2-4) Branch Units 
     The branch units  70   a,    70   b,  and  70   c  are respectively installed, for example, near the use-side units  101   a,    101   b,  and  101   c  in an indoor space of a building or other structure. The branch units  70   a,    70   b,  and  70   c  are respectively interposed between the use-side units  101   a ,  101   b,  and  101   c  and the heat-source-side unit  110  and each constitute a part of the refrigerant circuit  30 , together with the liquid-refrigerant connection pipe  2 , the high/low pressure gas-refrigerant connection pipe  3 , and the low pressure gas-refrigerant connection pipe  4 . The branch units  70   a,    70   b,  and  70   c  are respectively installed for the three use-side units  101   a,    101   b , and  101   c  in a one-to-one relationship. Alternatively, plural use-side units that are switched between cooling and heating at the same timing are connected to a single branch unit. The branch units  70   a,    70   b,  and  70   c  may be respectively incorporated in the use-side units  101   a ,  101   b,  and  101   c.  In this case, the branch units  70   a,    70   b,  and  70   c  can be respectively regarded as constituting portions of the use-side units  101   a,    101   b,  and  101   c.    
     The branch units  70   a,    70   b,  and  70   c  each mainly include a first branch path, and a second branch path. The respective first branch paths of the branch units  70   a,    70   b,  and  70   c  include first branch-unit switching valves  71   a,    72   a,  and  73   a,  and the respective second branch paths of the branch units  70   a,    70   b,  and  70   c  include second branch-unit switching valves  71   b ,  72   b,  and  73   b.  The first branch-unit switching valves  71   a,    72   a,  and  73   a  are electromagnetic valves for switching whether to allow communication between the high/low pressure gas-refrigerant connection pipe  3  and the use-side heat exchangers  102   a,    102   b,  and  102   c , respectively. The second branch-unit switching valves  71   b,    72   b,  and  73   b  are electromagnetic valves for switching whether to allow communication between the low pressure gas-refrigerant connection pipe  4  and the use-side heat exchangers  102   a,    102   b,  and  102   c,  respectively. 
     The branch units  70   a,    70   b,  and  70   c  each include the branch-side control unit  74  that controls operations of individual components constituting the branch units  70   a,    70   b,  and  70   c.  The branch-side control unit  74  includes a microcomputer, and various electrical components. The microcomputer includes a central processing unit (CPU), a memory, and other components provided for controlling the branch units  70   a,    70   b,  and  70   c.  The CPU reads a program stored in the memory or other storage device, and performs a predetermined computational process in accordance with the program. Further, the CPU is capable of performing an operation in accordance with the program, such as writing the results of computation into the memory or reading information stored in the memory. The branch-side control unit  74  is capable of exchanging a control signal or other information with the use-side control unit  104  of each of the use-side units  101   a,    101   b,  and  101   c.    
     (3) Operation of Air Conditioner 
     Reference is now made to how the air conditioner  1  according to one or more embodiments operates. The air conditioner  1  according to one or more embodiments is switched between the first operation, the second operation, and the third operation by the control unit  120  to thereby provide air conditioning. 
     The first operation is an operational state (cooling only operation) in which only use-side heat exchangers serving as evaporators for refrigerant (use-side units that perform cooling) exist. 
     The second operation is an operational state (heating only operation) in which only use-side heat exchangers serving as radiators for refrigerant (use-side units that perform heating) exist. 
     The third operation is an operation in which both a use-side unit that performs cooling and a use-side unit that performs heating exist (cooling and heating simultaneous operation). The third operation includes a third operation A, a third operation B, and a third operation C. 
     The third operation A is an operational state (cooling main operation) in which although both a use-side heat exchanger serving as an evaporator for refrigerant and a use-side heat exchanger serving as a radiator for refrigerant exist, the load on the evaporation side is greater as a whole. 
     The third operation B is an operational state (heating main operation) in which although both a use-side heat exchanger serving as a radiator for refrigerant and a use-side heat exchanger serving as an evaporator for refrigerant exist, the load on the radiation side is greater as a whole. 
     The third operation C is an operational state (cooling and heating balanced operation) in which both a use-side heat exchanger serving as an evaporator for refrigerant and a use-side heat exchanger serving as a radiator for refrigerant exist, and the evaporation load and the radiation load are balanced as a whole. 
     (3-1) First Operation 
     Reference is now made to how the first operation is performed, by way of an example case where the control unit  120  causes the first use-side heat exchanger  102   a  and the third use-side heat exchanger  102   c  to function as evaporators for refrigerant to perform cooling, and deactivates the second use-side heat exchanger  102   b  (see  FIG. 3 ). 
     In the first operation, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as radiators for refrigerant. The control unit  120  switches the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  to a radiating operation state (in which the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are in the state shown by solid lines in  FIG. 3 ). The control unit  120  closes the first branch-unit switching valves  71   a,    72   a,  and  73   a  and the second branch-unit switching valve  72   b,  and opens the second branch-unit switching valves  71   b  and  73   b.    
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see arrows attached to the refrigerant circuit  30  in  FIG. 3 ), low pressure refrigerant in the refrigeration cycle is sucked from the suction pipe  8  into the first compressor  11  of the lower stage. After being sucked into the first compressor  11  of the lower stage, the low pressure refrigerant in the refrigeration cycle is compressed in the first compressor  11  of the lower stage to an intermediate pressure in the refrigeration cycle before being discharged to the intermediate refrigerant pipe  9 . After being discharged from the first compressor  11  of the lower stage to the intermediate refrigerant pipe  9 , the intermediate-pressure refrigerant in the refrigeration cycle is sucked into the second compressor  12  of the higher stage, and compressed in the second compressor  12  to a high pressure in the refrigeration cycle before being discharged to the discharge pipe  10 . At this time, the high pressure refrigerant in the refrigeration cycle discharged from the second compressor  12  of the higher stage has been compressed through the two-stage compression action of the compressors  11  and  12  to a pressure exceeding the critical pressure of the refrigerant. After the high pressure refrigerant in the refrigeration cycle is discharged to the discharge pipe  10  from the second compressor  12  of the higher stage, a part of the high pressure refrigerant flows to the first main heat-source-side flow path  21 , and the remainder flows to the second main heat-source-side flow path  22 . 
     The refrigerant that has flown from the discharge pipe  10  to the first main heat-source-side flow path  21  is routed via the first heat-source-side switching mechanism  5  to the first heat-source-side heat exchanger  81 . The high pressure refrigerant in the refrigeration cycle routed to the first heat-source-side heat exchanger  81  rejects heat through heat exchange with outdoor air or other medium in the first heat-source-side heat exchanger  81  serving as a radiator for refrigerant. After rejecting heat in the first heat-source-side heat exchanger  81 , the high pressure refrigerant in the refrigeration cycle is decompressed in the first heat-source-side expansion mechanism  24   a.  The refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  is routed to the first economizer heat exchanger  61 . At this time, a part of the refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  and flowing in the first main heat-source-side flow path  21  branches off to the first economizer pipe  31 . 
     The refrigerant that has been decompressed in the first heat-source-side expansion mechanism  24   a  and has branched off from the first main heat-source-side flow path  21  to the first economizer pipe  31  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the first economizer pipe  31  again, and then flows to the first economizer heat exchanger  61 . Upon entering the first economizer heat exchanger  61 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 . After exchanging heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant flowing in the first main heat-source-side flow path  21  that has been decompressed in the first heat-source-side expansion mechanism  24   a  and routed to the first economizer heat exchanger  61  is cooled in the first economizer heat exchanger  61  through heat exchange with the refrigerant flowing in the first economizer pipe  31 . After being cooled in the first economizer heat exchanger  61 , the refrigerant flowing in the first main heat-source-side flow path  21  is routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   c.    
     The refrigerant that has flown from the discharge pipe  10  to the second main heat-source-side flow path  22  is routed via the second heat-source-side switching mechanism  6  to the second heat-source-side heat exchanger  82 . The high pressure refrigerant in the refrigeration cycle routed to the second heat-source-side heat exchanger  82  rejects heat through heat exchange with outdoor air or other medium in the second heat-source-side heat exchanger  82  serving as a radiator for refrigerant. After rejecting heat in the second heat-source-side heat exchanger  82 , the high pressure refrigerant in the refrigeration cycle is decompressed in the second heat-source-side expansion mechanism  24   b.  The refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  is routed to the second economizer heat exchanger  62 . At this time, a part of the refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  and flowing in the second main heat-source-side flow path  22  branches off to the second economizer pipe  32 . 
     The refrigerant that has been decompressed in the second heat-source-side expansion mechanism  24   b  and has branched off from the second main heat-source-side flow path  22  to the second economizer pipe  32  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the second economizer pipe  32  again, and then flows to the second economizer heat exchanger  62 . After branching off to the second economizer pipe  32  and entering the second economizer heat exchanger  62 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the second economizer heat exchanger  62  with the refrigerant flowing in the second main heat-source-side flow path  22 . After exchanging heat in the second economizer heat exchanger  62  with the refrigerant flowing in the second main heat-source-side flow path  22 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  and routed to the second economizer heat exchanger  62  is cooled in the second economizer heat exchanger  62  through heat exchange with the refrigerant flowing in the second economizer pipe  32 . After being cooled in the second economizer heat exchanger  62 , the refrigerant is routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   c.    
     The refrigerant routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   c  after undergoing heat exchange in the first economizer heat exchanger  61  and the second economizer heat exchanger  62  is decompressed in the use-side expansion mechanisms  103   a  and  103   c  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the use-side expansion mechanisms  103   a  and  103   c,  the low pressure refrigerant in the refrigeration cycle is routed to the use-side heat exchangers  102   a  and  102   c  respectively corresponding to the use-side expansion mechanisms  103   a  and  103   c.  The low pressure refrigerant in the refrigeration cycle routed to the use-side heat exchangers  102   a  and  102   c  evaporates through heat exchange with indoor air or other medium in the use-side heat exchangers  102   a  and  102   c  serving as evaporators for refrigerant. After evaporating in the use-side heat exchangers  102   a  and  102   c,  the low pressure refrigerant in the refrigeration cycle is passed through the low pressure gas-refrigerant connection pipe  4 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. In this way, the first operation is performed. 
     (3-2) Second Operation 
     Reference is now made to how the second operation is performed, by way of an example case where the control unit  120  causes the first use-side heat exchanger  102   a  and the third use-side heat exchanger  102   c  to function as radiators for refrigerant to perform heating, and deactivates the second use-side heat exchanger  102   b  (see  FIG. 4 ). 
     In the second operation, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as evaporators for refrigerant. The control unit  120  switches the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  to an evaporating operation state (in which the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are in the state shown by solid lines in  FIG. 4 ). The control unit  120  closes the first branch-unit switching valve  72   a  and the second branch-unit switching valves  71   b,    72   b,  and  73   b,  and opens the first branch-unit switching valves  71   a  and  73   a.    
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see arrows attached to the refrigerant circuit  30  in  FIG. 4 ), low pressure refrigerant in the refrigeration cycle is sucked from the suction pipe  8  into the first compressor  11  of the lower stage. After being sucked into the first compressor  11  of the lower stage, the low pressure refrigerant in the refrigeration cycle is compressed in the first compressor  11  of the lower stage to an intermediate pressure in the refrigeration cycle before being discharged to the intermediate refrigerant pipe  9 . After being discharged from the first compressor  11  of the lower stage to the intermediate refrigerant pipe  9 , the intermediate-pressure refrigerant in the refrigeration cycle is sucked into the second compressor  12  of the higher stage, and compressed in the second compressor  12  to a high pressure in the refrigeration cycle before being discharged to the discharge pipe  10 . At this time, the high pressure refrigerant in the refrigeration cycle discharged from the second compressor  12  of the higher stage has been compressed through the two-stage compression action of the compressors  11  and  12  to a pressure exceeding the critical pressure of the refrigerant. After being discharged from the second compressor  12  of the higher stage, the high pressure refrigerant in the refrigeration cycle is routed via the high/low pressure gas-refrigerant connection pipe  3  and the third heat-source-side switching mechanism  7  to the use-side heat exchangers  102   a  and  102   c.  The high pressure refrigerant in the refrigeration cycle routed to the use-side heat exchangers  102   a  and  102   c  rejects heat through heat exchange with indoor air or other medium in the use-side heat exchangers  102   a  and  102   c  serving as radiators for refrigerant. After rejecting heat in the use-side heat exchangers  102   a  and  102   c,  the high pressure refrigerant in the refrigeration cycle is routed to the use-side expansion mechanisms  103   a  and  103   c.  The high pressure refrigerant in the refrigeration cycle routed to the use-side expansion mechanisms  103   a  and  103   c  is decompressed in the use-side expansion mechanisms  103   a  and  103   c.  After being decompressed in the use-side expansion mechanisms  103   a  and  103   c,  the resulting refrigerant is routed via the liquid-refrigerant connection pipe  2  to the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b.  The refrigerant routed to the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b  is decompressed in the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b,  the low pressure refrigerant in the refrigeration cycle is routed to the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82 . The low pressure refrigerant in the refrigeration cycle routed to the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  evaporates through heat exchange with outdoor air or other medium in the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  serving evaporators for refrigerant. The low pressure refrigerant in the refrigeration cycle that has evaporated in the first heat-source-side heat exchanger  81  is passed through the first heat-source-side switching mechanism  5 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. The low pressure refrigerant in the refrigeration cycle that has evaporated in the second heat-source-side heat exchanger  82  is passed through the second heat-source-side switching mechanism  6 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. In this way, the second operation is performed. 
     (3-3) Third Operation 
     The third operation is now described separately for the following three types of operations: the third operation A, the third operation B, and the third operation C. 
     (3-3-1) Third Operation A 
     Reference is now made to how the third operation A is performed, by way of an example case where the control unit  120  causes the first use-side heat exchanger  102   a  and the second use-side heat exchanger  102   b  to function as evaporators for refrigerant to perform cooling, and causes the third use-side heat exchanger  102   c  to function as a radiator for refrigerant to perform heating (see  FIG. 5 ). 
     In the third operation A, as with the first operation, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as radiators for refrigerant. Further, the control unit  120  determines to cause the third use-side heat exchanger  102   c  to function as a radiator for refrigerant. The control unit  120  switches the first heat-source-side switching mechanism  5  and the second heat-source-side switching mechanism  6  to a radiating operation state (in which the first heat-source-side switching mechanism  5  and the second heat-source-side switching mechanism  6  are in the state shown by solid lines in  FIG. 5 ), and switches the third heat-source-side switching mechanism  7  to an evaporating operation state (in which the third heat-source-side switching mechanism  7  is in the state shown by solid lines in  FIG. 5 ). The control unit  120  closes the first branch-unit switching valves  71   a  and  72   a  and the second branch-unit switching valve  73   b,  and opens the first branch-unit switching valve  73   a  and the second branch-unit switching valves  71   b  and  72   b.    
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see arrows attached to the refrigerant circuit  30  in  FIG. 5 ), low pressure refrigerant in the refrigeration cycle is sucked from the suction pipe  8  into the first compressor  11  of the lower stage. After being sucked into the first compressor  11  of the lower stage, the low pressure refrigerant in the refrigeration cycle is compressed in the first compressor  11  of the lower stage to an intermediate pressure in the refrigeration cycle before being discharged to the intermediate refrigerant pipe  9 . After being discharged from the first compressor  11  of the lower stage to the intermediate refrigerant pipe  9 , the intermediate-pressure refrigerant in the refrigeration cycle is sucked into the second compressor  12  of the higher stage, and compressed in the second compressor  12  to a high pressure in the refrigeration cycle before being discharged to the discharge pipe  10 . At this time, the high pressure refrigerant in the refrigeration cycle discharged from the second compressor  12  of the higher stage has been compressed through the two-stage compression action of the compressors  11  and  12  to a pressure exceeding the critical pressure of the refrigerant. After the high pressure refrigerant in the refrigeration cycle is discharged from the second compressor  12  of the higher stage, a part of the high pressure refrigerant flows from the discharge pipe  10  to the first main heat-source-side flow path  21  or the second main heat-source-side flow path  22 , and the remainder is routed via the high/low pressure gas-refrigerant connection pipe  3  and the third heat-source-side switching mechanism  7  to the third use-side heat exchanger  102   c.    
     The refrigerant that has flown from the discharge pipe  10  to the first main heat-source-side flow path  21  is routed via the first heat-source-side switching mechanism  5  to the first heat-source-side heat exchanger  81 . The high pressure refrigerant in the refrigeration cycle routed to the first heat-source-side heat exchanger  81  rejects heat through heat exchange with outdoor air or other medium in the first heat-source-side heat exchanger  81  serving as a radiator for refrigerant. After rejecting heat in the first heat-source-side heat exchanger  81 , the high pressure refrigerant in the refrigeration cycle is decompressed in the first heat-source-side expansion mechanism  24   a.  The refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  is routed to the first economizer heat exchanger  61 . At this time, a part of the refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  and flowing in the first main heat-source-side flow path  21  branches off to the first economizer pipe  31 . 
     The refrigerant that has been decompressed in the first heat-source-side expansion mechanism  24   a  and has branched off from the first main heat-source-side flow path  21  to the first economizer pipe  31  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the first economizer pipe  31  again, and then flows to the first economizer heat exchanger  61 . After branching off from the common part  35  to the first economizer pipe  31  and then flowing to the first economizer heat exchanger  61 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 . After exchanging heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant flowing in the first main heat-source-side flow path  21  that has been decompressed in the first heat-source-side expansion mechanism  24   a  and routed to the first economizer heat exchanger  61  is cooled in the first economizer heat exchanger  61  through heat exchange with the refrigerant flowing in the first economizer pipe  31 . After being cooled in the first economizer heat exchanger  61 , the refrigerant flowing in the first main heat-source-side flow path  21  is routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   b.    
     The refrigerant that has flown from the discharge pipe  10  to the second main heat-source-side flow path  22  is routed via the second heat-source-side switching mechanism  6  to the second heat-source-side heat exchanger  82 . The high pressure refrigerant in the refrigeration cycle passed to the second main heat-source-side flow path  22  and then routed to the second heat-source-side heat exchanger  82  rejects heat through heat exchange with outdoor air or other medium in the second heat-source-side heat exchanger  82  serving as a radiator for refrigerant. After rejecting heat in the second heat-source-side heat exchanger  82 , the high pressure refrigerant in the refrigeration cycle is decompressed in the second heat-source-side expansion mechanism  24   b.  The refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  is routed to the second economizer heat exchanger  62 . At this time, a part of the refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  and flowing in the second main heat-source-side flow path  22  branches off to the second economizer pipe  32 . 
     The refrigerant that has been decompressed in the second heat-source-side expansion mechanism  24   b  and has branched off from the second main heat-source-side flow path  22  to the second economizer pipe  32  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the second economizer pipe  32  again, and then flows to the second economizer heat exchanger  62 . After branching off from the common part  35  to the second economizer pipe  32  again and then flowing to the second economizer heat exchanger  62 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the second economizer heat exchanger  62  with the refrigerant flowing in the second main heat-source-side flow path  22 . After exchanging heat in the second economizer heat exchanger  62  with the refrigerant flowing in the second main heat-source-side flow path  22 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant decompressed in the second heat-source-side expansion mechanism  24   b  and routed to the second economizer heat exchanger  62  is cooled in the second economizer heat exchanger  62  through heat exchange with the refrigerant flowing in the second economizer pipe  32 . After being cooled in the second economizer heat exchanger  62 , the refrigerant is routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   b.    
     Meanwhile, the high pressure refrigerant in the refrigeration cycle routed to the third use-side heat exchanger  102   c  rejects heat through heat exchange with indoor air or other medium in the third use-side heat exchanger  102   c  serving as a radiator for refrigerant. After rejecting heat in the third use-side heat exchanger  102   c,  the high pressure refrigerant in the refrigeration cycle is routed to the third use-side expansion mechanism  103   c.  The high pressure refrigerant in the refrigeration cycle routed to the third use-side expansion mechanism  103   c  is decompressed in the third use-side expansion mechanism  103   c.  The refrigerant decompressed in the third use-side expansion mechanism  103   c  is merged in the liquid-refrigerant connection pipe  2  with the refrigerant that has undergone heat exchange in each of the first economizer heat exchanger  61  and the second economizer heat exchanger  62 . After these streams of refrigerant are merged in the liquid-refrigerant connection pipe  2 , the resulting merged refrigerant is routed to the use-side expansion mechanisms  103   a  and  103   b.    
     The refrigerant routed to the use-side expansion mechanisms  103   a  and  103   b  is decompressed in the use-side expansion mechanisms  103   a  and  103   b  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the use-side expansion mechanisms  103   a  and  103   b,  the low pressure refrigerant in the refrigeration cycle is routed to the use-side heat exchangers  102   a  and  102   b  respectively corresponding to the use-side expansion mechanisms  103   a  and  103   b.  The low pressure refrigerant in the refrigeration cycle routed to the use-side heat exchangers  102   a  and  102   b  evaporates through heat exchange with indoor air or other medium in the use-side heat exchangers  102   a  and  102   b  serving as evaporators for refrigerant. After evaporating in the use-side heat exchangers  102   a  and  102   b,  the low pressure refrigerant in the refrigeration cycle is passed through the low pressure gas-refrigerant connection pipe  4 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. 
     (3-3-1-1) 
     In performing the third operation A, the control unit  120  may in some cases determine that the overall evaporation load on the use-side heat exchangers is small, due to reasons such as a small number of use-side heat exchangers that are acting as evaporators for refrigerant. In such cases, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  to function as a radiator for refrigerant, and to cause the second heat-source-side heat exchanger  82  to function as an evaporator for refrigerant. As the control unit  120  performs such control, the radiation load on the first heat-source-side heat exchanger  81  and the evaporation load on the second heat-source-side heat exchanger  82  are balanced out, which allows for reduced overall radiation load on the heat-source-side heat exchangers (see  FIG. 6 ). 
     When performing the above-mentioned operation, the control unit  120  switches the first heat-source-side switching mechanism  5  to a radiating operation state (in which the first heat-source-side switching mechanism  5  is in the state shown by solid lines in  FIG. 6 ), and switches the second heat-source-side switching mechanism  6  and the third heat-source-side switching mechanism  7  to an evaporating operation state (in which the second heat-source-side switching mechanism  6  and the third heat-source-side switching mechanism  7  are in the state shown by solid lines in  FIG. 6 ). 
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see the arrows attached to the refrigerant circuit  30  in  FIG. 6 ), the refrigerant passed to the first main heat-source-side flow path  21  is routed to the first heat-source-side heat exchanger  81  serving as a radiator for refrigerant, and undergoes heat exchange in the first heat-source-side heat exchanger  81 . After undergoing heat exchange in the first heat-source-side heat exchanger  81 , the refrigerant is routed to the first heat-source-side expansion mechanism  24   a,  and decompressed in the first heat-source-side expansion mechanism  24   a.  At this time, a part of the refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  flows to the first economizer pipe  31 , and the remainder is routed to the first economizer heat exchanger  61 . 
     The refrigerant that has been decompressed in the first heat-source-side expansion mechanism  24   a  and has branched off from the first main heat-source-side flow path  21  to the first economizer pipe  31  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the first economizer pipe  31  again, and then flows to the first economizer heat exchanger  61 . After branching off from the common part  35  to the first economizer pipe  31  and then flowing to the first economizer heat exchanger  61 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 . After exchanging heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant flowing in the first main heat-source-side flow path  21  that has been decompressed in the first heat-source-side expansion mechanism  24   a  and routed to the first economizer heat exchanger  61  is cooled in the first economizer heat exchanger  61  through heat exchange with the refrigerant flowing in the first economizer pipe  31 . A part of the refrigerant flowing in the first main heat-source-side flow path  21  after undergoing heat exchange in the first economizer heat exchanger  61  is routed via the liquid-refrigerant connection pipe  2  to the use-side expansion mechanisms  103   a  and  103   b,  and the remainder flows to the second main heat-source-side flow path  22 . 
     The refrigerant that has flown to the second main heat-source-side flow path  22  is decompressed in the second heat-source-side expansion mechanism  24   b  before being routed to the second heat-source-side heat exchanger  82 . After being decompressed in the second heat-source-side expansion mechanism  24   b,  the resulting low pressure refrigerant in the refrigeration cycle evaporates through heat exchange with outdoor air or other medium in the second heat-source-side heat exchanger  82  serving as an evaporator for refrigerant. The low pressure refrigerant in the refrigeration cycle that has evaporated in the second heat-source-side heat exchanger  82  is passed through the second heat-source-side switching mechanism  6 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. 
     Meanwhile, the high pressure refrigerant routed from the discharge pipe  10  to the third use-side heat exchanger  102   c  rejects heat through heat exchange with indoor air or other medium in the third use-side heat exchanger  102   c  serving as a radiator for refrigerant. After rejecting heat in the third use-side heat exchanger  102   c,  the high pressure refrigerant in the refrigeration cycle is routed to the third use-side expansion mechanism  103   c.  The high pressure refrigerant in the refrigeration cycle routed to the third use-side expansion mechanism  103   c  is decompressed in the third use-side expansion mechanism  103   c.  The refrigerant decompressed in the third use-side expansion mechanism  103   c  is merged in the liquid-refrigerant connection pipe  2  with the refrigerant that has undergone heat exchange in the first economizer heat exchanger  61 . After these streams of refrigerant are merged in the liquid-refrigerant connection pipe  2 , the resulting merged refrigerant is routed to the use-side expansion mechanisms  103   a  and  103   b.    
     The refrigerant routed to the use-side expansion mechanisms  103   a  and  103   b  is decompressed in the use-side expansion mechanisms  103   a  and  103   b  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the use-side expansion mechanisms  103   a  and  103   b,  the low pressure refrigerant in the refrigeration cycle is routed to the use-side heat exchangers  102   a  and  102   b  respectively corresponding to the use-side expansion mechanisms  103   a  and  103   b.  The low pressure refrigerant in the refrigeration cycle routed to the use-side heat exchangers  102   a  and  102   b  evaporates through heat exchange with indoor air or other medium in the use-side heat exchangers  102   a  and  102   b  serving as evaporators for refrigerant. After evaporating in the use-side heat exchangers  102   a  and  102   b,  the low pressure refrigerant in the refrigeration cycle is passed through the low pressure gas-refrigerant connection pipe  4 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. In this way, the third operation A is performed. 
     (3-3-2) Third Operation B 
     Reference is now made to how the third operation B is performed, by way of an example case where the control unit  120  causes the first use-side heat exchanger  102   a  and the second use-side heat exchanger  102   b  to function as radiators for refrigerant to perform heating, and causes the third use-side heat exchanger  102   c  to function as an evaporator for refrigerant to perform cooling (see  FIG. 7 ). 
     In the third operation B, as with the second operation, the control unit  120  determines to cause the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  to function as evaporators for refrigerant. The control unit  120  switches the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  to an evaporating operation state (in which the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are in the state shown by solid lines in  FIG. 7 ). The control unit  120  closes the first branch-unit switching valve  73   a  and the second branch-unit switching valves  71   b  and  72   b,  and opens the first branch-unit switching valves  71   a  and  72   a  and the second branch-unit switching valve  73   b.    
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see arrows attached to the refrigerant circuit  30  in  FIG. 7 ), low pressure refrigerant in the refrigeration cycle is sucked from the suction pipe  8  into the first compressor  11  of the lower stage. After being sucked into the first compressor  11  of the lower stage, the low pressure refrigerant in the refrigeration cycle is compressed in the first compressor  11  of the lower stage to an intermediate pressure in the refrigeration cycle before being discharged to the intermediate refrigerant pipe  9 . After being discharged from the first compressor  11  of the lower stage to the intermediate refrigerant pipe  9 , the intermediate-pressure refrigerant in the refrigeration cycle is sucked into the second compressor  12  of the higher stage, and compressed in the second compressor  12  to a high pressure in the refrigeration cycle before being discharged to the discharge pipe  10 . At this time, the high pressure refrigerant in the refrigeration cycle discharged from the second compressor  12  of the higher stage has been compressed through the two-stage compression action of the compressors  11  and  12  to a pressure exceeding the critical pressure of the refrigerant. After being discharged from the second compressor  12  of the higher stage, the high pressure refrigerant in the refrigeration cycle is routed via the high/low pressure gas-refrigerant connection pipe  3  and the third heat-source-side switching mechanism  7  to the use-side heat exchangers  102   a  and  102   b.  The high pressure refrigerant in the refrigeration cycle routed to the use-side heat exchangers  102   a  and  102   b  rejects heat through heat exchange with indoor air or other medium in the use-side heat exchangers  102   a  and  102   b  serving as radiators for refrigerant. After rejecting heat in the use-side heat exchangers  102   a  and  102   b,  the high pressure refrigerant in the refrigeration cycle is routed to the use-side expansion mechanisms  103   a  and  103   b.  The high pressure refrigerant in the refrigeration cycle routed to the use-side expansion mechanisms  103   a  and  103   b  is decompressed in the use-side expansion mechanisms  103   a  and  103   b.  After being decompressed in the use-side expansion mechanisms  103   a  and  103   b,  a part of the refrigerant is routed via the liquid-refrigerant connection pipe  2  to the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b,  and the remainder branches off from the liquid-refrigerant connection pipe  2  and is routed to the third use-side expansion mechanism  103   c.    
     The refrigerant routed to the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b  is decompressed in the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the first heat-source-side expansion mechanism  24   a  and the second heat-source-side expansion mechanism  24   b,  the low pressure refrigerant in the refrigeration cycle is routed to the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82 . The low pressure refrigerant in the refrigeration cycle that has evaporated in the first heat-source-side heat exchanger  81  is passed through the first heat-source-side switching mechanism  5 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. The low pressure refrigerant in the refrigeration cycle that has evaporated in the second heat-source-side heat exchanger  82  is passed through the second heat-source-side switching mechanism  6 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11  again. 
     Meanwhile, the refrigerant routed to the third use-side expansion mechanism  103   c  is decompressed in the third use-side expansion mechanism  103   c  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the third use-side expansion mechanism  103   c,  the low pressure refrigerant in the refrigeration cycle is routed to the third use-side heat exchanger  102   c  corresponding to the third use-side expansion mechanism  103   c.  The low pressure refrigerant in the refrigeration cycle routed to the third use-side heat exchanger  102   c  evaporates through heat exchange with indoor air or other medium in the third use-side heat exchanger  102   c  serving as an evaporator for refrigerant. After evaporating in the third use-side heat exchanger  102   c,  the low pressure refrigerant in the refrigeration cycle is routed via the low pressure gas-refrigerant connection pipe  4 , the accumulator  95 , and the suction pipe  8  to the first compressor  11 . 
     (3-3-3) Third Operation C 
     Reference is now made to how the third operation C is performed, by way of an example case where the control unit  120  causes the first use-side heat exchanger  102   a  to function as a radiator for refrigerant to perform heating, deactivates the second use-side heat exchanger  102   b,  and causes the third use-side heat exchanger  102   c  to function as an evaporator for refrigerant to perform cooling (see  FIG. 8 ). 
     In the third operation C, the control unit  120  determines that the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  respectively have a small radiation load and a small evaporation load. The control unit  120  switches the first heat-source-side switching mechanism  5  to a radiating operation state shown by solid lines in  FIG. 8 , and switches the second heat-source-side switching mechanism  6  and the third heat-source-side switching mechanism  7  to an evaporating operation state shown by solid lines in  FIG. 8 . The control unit  120  closes the first branch-unit switching valves  72   a  and  73   a  and the second branch-unit switching valves  71   b  and  72   b,  and opens the first branch-unit switching valve  71   a  and the second branch-unit switching valve  73   b.    
     With the refrigerant circuit  30  in the above-mentioned state (for the flow of refrigerant in this state, see arrows attached to the refrigerant circuit  30  in  FIG. 8 ), low pressure refrigerant in the refrigeration cycle is sucked from the suction pipe  8  into the first compressor  11  of the lower stage. After being sucked into the first compressor  11  of the lower stage, the low pressure refrigerant in the refrigeration cycle is compressed in the first compressor  11  of the lower stage to an intermediate pressure in the refrigeration cycle before being discharged to the intermediate refrigerant pipe  9 . The intermediate-pressure refrigerant in the refrigeration cycle discharged from the first compressor  11  of the lower stage is compressed in the second compressor  12  of the higher stage to a high pressure in the refrigeration cycle, and then discharged from the second compressor  12  of the higher stage to the discharge pipe  10 . At this time, the high pressure refrigerant in the refrigeration cycle discharged from the second compressor  12  of the higher stage has been compressed through the two-stage compression action of the compressors  11  and  12  to a pressure exceeding the critical pressure of the refrigerant. After the high pressure refrigerant in the refrigeration cycle is discharged to the discharge pipe  10  from the second compressor  12  of the higher stage, a part of the high pressure refrigerant is routed to the first heat-source-side heat exchanger  81 , and the remainder is routed to the first use-side heat exchanger  102   a.    
     The high pressure refrigerant in the refrigeration cycle routed to the first heat-source-side heat exchanger  81  rejects heat through heat exchange with outdoor air or other medium in the first heat-source-side heat exchanger  81  serving as a radiator for refrigerant. After rejecting heat in the first heat-source-side heat exchanger  81 , the high pressure refrigerant in the refrigeration cycle is decompressed in the first heat-source-side expansion mechanism  24   a.  The refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  is routed to the first economizer heat exchanger  61 . At this time, a part of the refrigerant decompressed in the first heat-source-side expansion mechanism  24   a  and flowing in the first main heat-source-side flow path  21  branches off to the first economizer pipe  31 . 
     The refrigerant that has been decompressed in the first heat-source-side expansion mechanism  24   a  and has branched off from the first main heat-source-side flow path  21  to the first economizer pipe  31  flows to the common part  35 . Upon entering the common part  35 , the refrigerant is decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle. After being decompressed by the expansion mechanism  36  of the common part  35  to an intermediate pressure in the refrigeration cycle, the refrigerant branches off from the common part  35  to the first economizer pipe  31  again, and then flows to the first economizer heat exchanger  61 . After branching off from the common part  35  to the first economizer pipe  31  and then flowing to the first economizer heat exchanger  61 , the intermediate-pressure refrigerant in the refrigeration cycle exchanges heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 . After exchanging heat in the first economizer heat exchanger  61  with the refrigerant flowing in the first main heat-source-side flow path  21 , the intermediate-pressure refrigerant in the refrigeration cycle is routed via the intermediate refrigerant pipe  9  to the second compressor  12  of the higher stage. 
     The refrigerant flowing in the first main heat-source-side flow path  21  that has been decompressed in the first heat-source-side expansion mechanism  24   a  and routed to the first economizer heat exchanger  61  is cooled in the first economizer heat exchanger  61  through heat exchange with the refrigerant flowing in the first economizer pipe  31 . The refrigerant flowing in the first main heat-source-side flow path  21  after being cooled in the first economizer heat exchanger  61  flows to the second main heat-source-side flow path  22 , and is routed to the second heat-source-side expansion mechanism  24   b.  The refrigerant routed to the second heat-source-side expansion mechanism  24   b  is decompressed in the second heat-source-side expansion mechanism  24   b  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the second heat-source-side expansion mechanism  24   b,  the low pressure refrigerant in the refrigeration cycle is routed to the second heat-source-side heat exchanger  82 . The low pressure refrigerant routed to the second heat-source-side heat exchanger  82  evaporates through heat exchange with outdoor air or other medium in the second heat-source-side heat exchanger  82  serving as an evaporator for refrigerant. The low pressure refrigerant in the refrigeration cycle that has evaporated in the second heat-source-side heat exchanger  82  is passed through the second heat-source-side switching mechanism  6 , the accumulator  95 , and the suction pipe  8  before being sucked into the first compressor  11 . 
     Meanwhile, the high pressure refrigerant routed from the discharge pipe  10  to the first use-side heat exchanger  102   a  rejects heat through heat exchange with indoor air or other medium in the first use-side heat exchanger  102   a  serving as a radiator for refrigerant. After rejecting heat in the first use-side heat exchanger  102   a,  the high pressure refrigerant in the refrigeration cycle is routed to the first use-side expansion mechanism  103   a.  The high pressure refrigerant in the refrigeration cycle routed to the first use-side expansion mechanism  103   a  is decompressed in the first use-side expansion mechanism  103   a.  After being decompressed in the first use-side expansion mechanism  103   a,  the refrigerant is routed via the liquid-refrigerant connection pipe  2  to the third use-side expansion mechanism  103   c.  The refrigerant routed to the third use-side expansion mechanism  103   c  is decompressed in the third use-side expansion mechanism  103   c  and turns into low-pressure refrigerant in the refrigeration cycle that is in a two-phase gas-liquid state. After being decompressed in the third use-side expansion mechanism  103   c,  the low pressure refrigerant in the refrigeration cycle is routed to the third use-side heat exchanger  102   c.  The low pressure refrigerant in the refrigeration cycle routed to the third use-side heat exchanger  102   c  evaporates through heat exchange with indoor air or other medium in the third use-side heat exchanger  102   c  serving as an evaporator for refrigerant. After evaporating in the third use-side heat exchanger  102   c,  the low pressure refrigerant in the refrigeration cycle is passed through the low pressure gas-refrigerant connection pipe  4 , the accumulator  95 , and the suction pipe  8  and sucked into the first compressor  11 . In this way, the third operation C is performed. 
     (4) Characteristic Features 
     (4-1) 
     As described above in the section (3-3-1-1), in performing the third operation A, the control unit  120  may in some cases determine that the overall evaporation load on the use-side heat exchangers is small, due to reasons such as a small number of use-side heat exchangers that are acting as evaporators for refrigerant. In such cases, the control unit  120  causes the first heat-source-side heat exchanger  81  to function as a radiator for refrigerant, and causes the second heat-source-side heat exchanger  82  to function as an evaporator for refrigerant so that the radiation load on the first heat-source-side heat exchanger  81  and the evaporation load on the second heat-source-side heat exchanger  82  are balanced out. In this way, the control unit  120  performs an operation for reducing the overall radiation load on the heat-source-side heat exchangers. 
     As described above in the section (3-3-3), in performing the third operation C, the control unit  120  determines that the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  respectively have a small radiation load and a small evaporation load. In this case, the control unit  120  causes the first heat-source-side heat exchanger  81  to function as a radiator for refrigerant, and causes the second heat-source-side heat exchanger  82  to function as an evaporator for refrigerant so that the radiation load on the first heat-source-side heat exchanger  81  and the evaporation load on the second heat-source-side heat exchanger  82  are balanced out. 
     As described above, when an air conditioner with plural heat-source-side heat exchangers is to perform a cooling and heating simultaneous operation, the air conditioner may sometimes operate such that a part or all of refrigerant that has passed through one heat-source-side heat exchanger serving as a radiator flows to another heat-source-side heat exchanger serving as an evaporator, and the remainder of the refrigerant flows to a use-side unit. By operating in this way, the air conditioner with plural heat-source-side heat exchangers is able to handle a small thermal load for the heat-source-side heat exchangers as a whole during the cooling and heating simultaneous operation. 
     Some multi-split air conditioners with plural heat-source-side heat exchangers and plural use-side units in the related art are designed such that whether to perform a cooling operation or a heating operation can be freely selected for each individual use-side unit. One conceivable way to improve the operating efficiency of such an air conditioner is to employ a configuration in which separate streams of refrigerant that have undergone heat exchange in plural heat-source-side heat exchangers  181  and  182  are merged before undergoing heat exchange in a single economizer heat exchanger  161  (see  FIG. 9 ). 
     If an air conditioner employing the above-mentioned configuration is to perform the operation described above in the section (3-3-1-1), a part of the refrigerant that passes through one heat-source-side heat exchanger serving as a radiator for refrigerant and is then routed to a use-side unit flows through an economizer heat exchanger. However, the refrigerant that passes through one heat-source-side heat exchanger serving as a radiator for refrigerant and is then routed to another heat-source-side heat exchanger does not flow through an economizer heat exchanger. 
     If the operation described above in the section (3-3-3) is to be performed, the refrigerant having passed through one heat-source-side heat exchanger serving as a radiator for refrigerant is routed to another heat-source-side heat exchanger serving as an evaporator for refrigerant. Consequently, such refrigerant does not flow through an economizer heat exchanger. 
     In the case of an air conditioner employing the above-mentioned configuration in which separate streams of refrigerant that have undergone heat exchange in plural heat-source-side heat exchangers are merged before undergoing heat exchange in a single economizer heat exchanger, such an air conditioner is subject to situations where, during cooling and heating simultaneous operation, sufficient heat exchange does not take place as only a part of the refrigerant flows through the economizer heat exchanger. 
     In the air conditioner  1  according to the present disclosure, the first economizer heat exchanger  61  is connected in series with the first heat-source-side heat exchanger  81 , and the second economizer heat exchanger  62  is connected in series with the second heat-source-side heat exchanger  82 . 
     The air conditioner  1  according to the present disclosure employs the above-mentioned configuration so that the refrigerant flowing in the first main heat-source-side flow path  21  passes through the first heat-source-side heat exchanger  81  and the first economizer heat exchanger  61  before flowing to the use-side units  101   a  and  101   b  or to the second heat-source-side heat exchanger  82 . This ensures that in performing the cooling and heating simultaneous operation as described above in the section (3-3-1-1) or (3-3-3), sufficient heat exchange takes place in the economizer heat exchangers  61  and  62 . 
     (4-2) 
     In performing the first operation or the third operation A, the first heat-source-side heat exchanger  81  and the second heat-source-side heat exchanger  82  are caused to function as radiators. In the air conditioner  1  according to the present disclosure, the first economizer heat exchanger  61  is connected in series with the first heat-source-side heat exchanger  81 , and the second economizer heat exchanger  62  is connected in series with the second heat-source-side heat exchanger  82 . The air conditioner  1  according to the present disclosure employs the above-mentioned configuration to ensure that in performing the first operation or the third operation A, the refrigerant that has rejected heat in the first heat-source-side heat exchanger  81  or the second heat-source-side heat exchanger  82  passes through the first economizer heat exchanger  61  or the second economizer heat exchanger  62 . As a result, sufficient heat exchange takes place in the economizer heat exchangers  61  and  62 . 
     (4-3) 
     The air conditioner  1  according to the present disclosure performs a supercritical refrigeration cycle. In performing the supercritical refrigeration cycle, two-stage compression may be performed by using plural compressors. The two-stage compression may involve injecting cooled refrigerant to each compressor. In the air conditioner  1  according to the present disclosure, the first economizer heat exchanger  61  is connected in series with the first heat-source-side heat exchanger  81 , and the second economizer heat exchanger  62  is connected in series with the second heat-source-side heat exchanger  82 . Further, the common part  35  is disposed between the location of branching from the first main heat-source-side flow path  21 , and the first economizer heat exchanger  61 , and between the location of branching from the second main heat-source-side flow path  22 , and the second economizer heat exchanger  62 . This allows two-stage compression to be efficiently performed in the compressors  11  and  12  of the air conditioner  1  that performs a supercritical refrigeration cycle. 
     Further, the common part  35  is positioned as described above, and the common part  35  is provided with the expansion mechanism  36 . This configuration allows for cost reduction compared to a configuration in which each of the first economizer pipe  31  and the second economizer pipe  32  individually has an expansion mechanism and individually returns to the compressors  11  and  12 . 
     (5) Modifications 
     Reference is now made to modifications of the air conditioner  1  according to the above-described embodiments. Features similar to those in the embodiments mentioned above are denoted by like reference signs and not described in further detail below. 
     (5-1) Modification A 
     In the foregoing description of the embodiments, the compressors  11  and  12  are two compressors with a single-stage compression structure that are connected in series. However, the compressors according to the present disclosure may not necessarily have the above-mentioned configuration. Alternatively, for example, the compressors according to the present disclosure may have a two-stage compression structure such that the two compressors  11  and  12  are incorporated in a single casing. 
     (5-2) Modification B 
     In the foregoing description of the embodiments, the compressors  11  and  12  are two compressors with a single-stage compression structure that are connected in series. However, the compressors according to the present disclosure may not necessarily have the above-mentioned configuration. Alternatively, for example, a single compressor  11   a  with a single-stage compression structure may be used that has an injection port through which intermediate-pressure refrigerant can be introduced to some point in the compression process. When an air conditioner  1   a  employing this configuration is to perform a cooling only operation, a cooling main operation, or a cooling and heating simultaneous operation, the intermediate-pressure refrigerant in the refrigeration cycle flowing in the first economizer pipe  31  and the second economizer pipe  32  undergoes heat exchange in the first economizer heat exchanger  61  and the second economizer heat exchanger  62  before being routed via the injection port to the single compressor  11   a  with a single-stage compression structure (see  FIG. 10 ). 
     (5-3) Modification C 
     In the foregoing description of the embodiments, the heat-source-side unit  110  includes two heat-source-side heat exchanger  81  and  82 , and two economizer heat exchangers  61  and  62  respectively corresponding to the heat-source-side heat exchangers  81  and  82 . However, the heat-source-side unit  110  according to the present disclosure may not necessarily include two heat-source-side heat exchangers and two economizer heat exchangers. Alternatively, the heat-source-side unit  110  may include a greater number of heat-source-side heat exchangers, and a number of economizer heat exchangers corresponding to the number of heat-source-side heat exchangers. 
     (5-4) Modification D 
     In the foregoing description of the embodiments, the heat-source-side unit  110  of an air conditioner  1  includes two heat-source-side heat exchanger  81  and  82 , and two economizer heat exchangers  61  and  62  respectively corresponding to the heat-source-side heat exchangers  81  and  82 . However, the heat-source-side heat exchangers and the economizer heat exchangers according to the present disclosure may not necessarily be configured as described above. Alternatively, a single economizer heat exchanger  63  may have a number of high-pressure flow paths equal to the number of heat-source-side heat exchangers, and a single low-pressure flow path. For example, if the heat-source-side unit  110  includes two heat-source-side heat exchangers  81  and  82 , the single economizer heat exchanger  63  has two high-pressure flow paths, and a single low-pressure flow path (see  FIG. 11 ). In this case, the single economizer heat exchanger  63  serves as a first economizer heat exchanger  63   a  and a second economizer heat exchanger  63   b.  Further, in this case, the first economizer pipe  31  and the second economizer pipe  32  are merged in the common part  35 , and the resulting merged economizer pipe returns to the compressors  11  and  12 . 
     (5-5) Modification E 
     In the foregoing description of the embodiments, the first heat-source-side switching mechanism  5 , the second heat-source-side switching mechanism  6 , and the third heat-source-side switching mechanism  7  are four-way switching valves. However, according to the present disclosure, four-way switching valves may not necessarily be used as flow switching valves. For example, other switching valves, such as electromagnetic valves, electric valves, three-way valves, or five-way valves may be used as flow switching valves. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present disclosure. Accordingly, the scope of the disclosure should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
       1 ,  1   a ,  1   b  air conditioner 
       2  liquid-refrigerant connection pipe 
       3  high/low pressure gas-refrigerant connection pipe 
       4  low pressure gas-refrigerant connection pipe 
       10  discharge pipe 
       11 ,  11   a,    12  compressor 
       21  first main heat-source-side flow path 
       22  second main heat-source-side flow path 
       31  first economizer pipe 
       32  second economizer pipe 
       35  common part 
       36  expansion mechanism 
       61 ,  63   a  first economizer heat exchanger 
       62 ,  63   b  second economizer heat exchanger 
       70   a,    70   b,    70   c  branch unit 
       81  first heat-source-side heat exchanger 
       82  second heat-source-side heat exchanger 
       90  first shutoff valve 
       90   a  high pressure refrigerant pipe 
       91  second shutoff valve 
       91   a  high/low pressure pipe 
       92  third shutoff valve 
       92   a  low pressure refrigerant pipe 
       110  heat-source-side unit 
       101   a,    101   b,    101   c  use-side unit 
       120  control unit 
     PATENT LITERATURE 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2010-156493