Patent Publication Number: US-11047590-B2

Title: Air conditioner

Description:
TECHNICAL FIELD 
     The present invention relates to air conditioners. The present invention more particularly relates to an air conditioner including an outdoor unit having a compressor and an outdoor heat exchanger, a plurality of indoor units having an indoor heat exchanger, and a liquid-refrigerant connection pipe that connects the outdoor unit to the plurality of indoor units, in which a liquid-pressure adjustment expansion valve that decompresses a refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe is in a gas-liquid two-phase state is provided in an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger to the liquid-refrigerant connection pipe. 
     BACKGROUND ART 
     An air conditioner of related art includes an outdoor unit having a compressor and an outdoor heat exchanger, a plurality of indoor units having an indoor heat exchanger, and a liquid-refrigerant connection pipe that connects the outdoor unit to the plurality of indoor units, and performs an operation of sending a refrigerant, which has been discharged from the compressor, to the outdoor heat exchanger, the liquid-refrigerant connection pipe, and the indoor heat exchanger in that order. An example of such an air conditioner may be one, as disclosed in PTL 1 (International Publication No. 2015/029160), in which a liquid-pressure adjustment expansion valve that decompresses the refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe is in a gas-liquid two-phase state is provided in an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger to the liquid-refrigerant connection pipe. That is, when the air conditioner performs the operation of sending the refrigerant, which has been discharged from the compressor, to the outdoor heat exchanger, the liquid-refrigerant connection pipe, and the indoor heat exchanger in that order, the air conditioner transports the refrigerant in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe and feeding the refrigerant from the outdoor unit to the indoor units through the decompression using the liquid-pressure adjustment expansion valve. 
     SUMMARY OF THE INVENTION 
     In the air conditioner of PTL 1, if the discharge temperature of the compressor excessively increases, for example, the opening degree of an indoor expansion valve that is provided in each indoor unit may be temporarily increased for protective control to decrease the discharge temperature. 
     However, with the temporary increase in the opening degree of the indoor expansion valve, the state of the refrigerant flowing through the liquid-refrigerant connection pipe may vary, and a desirable gas-liquid two-phase state may not be obtained, possibly causing a trouble in transporting the refrigerant in the two-phase state using the liquid-pressure adjustment expansion valve. 
     The present invention aims at an air conditioner including an outdoor unit having a compressor and an outdoor heat exchanger, a plurality of indoor units having an indoor heat exchanger, and a liquid-refrigerant connection pipe that connects both the units to each other, in which a liquid-pressure adjustment expansion valve that decompresses a refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe is in a gas-liquid two-phase state is provided in an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger to the liquid-refrigerant connection pipe. An object of the present invention is that the air conditioner properly transports the refrigerant in a two-phase state while suppressing an increase in a discharge temperature of the compressor. 
     An air conditioner according to a first aspect of the present invention is an air conditioner including an outdoor unit having a compressor and an outdoor heat exchanger, a plurality of indoor units having an indoor heat exchanger, and a liquid-refrigerant connection pipe that connects the outdoor unit to the plurality of indoor units, the air conditioner being configured to perform an operation of sending a refrigerant, which has been discharged from the compressor, to the outdoor heat exchanger, the liquid-refrigerant connection pipe, and the indoor heat exchanger in that order. A liquid-pressure adjustment expansion valve that decompresses the refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe is in a gas-liquid two-phase state is provided in an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger to the liquid-refrigerant connection pipe. A liquid injection pipe that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe and feeds the branched refrigerant to the compressor is connected to a portion of the outdoor liquid-refrigerant pipe on a side of the outdoor heat exchanger with respect to the liquid-pressure adjustment expansion valve. 
     In the configuration in which the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe and feeding the refrigerant from the outdoor unit to the indoor units using the liquid-pressure adjustment expansion valve, as described above, the liquid injection pipe is further provided in the portion of the outdoor liquid-refrigerant pipe on the side of the outdoor heat exchanger with respect to the liquid-pressure adjustment expansion valve to branch part of the refrigerant flowing through the outdoor liquid-refrigerant pipe and feed the refrigerant to the compressor. Since the liquid injection pipe is provided, the refrigerant can be fed to the compressor while a variation in the temperature of the refrigerant flowing through the outdoor liquid-refrigerant pipe is suppressed. Thus, an increase in the discharge temperature of the compressor can be suppressed while a variation in the state of the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve is suppressed. Thus, the refrigerant flowing through the liquid-refrigerant connection pipe can be reliably maintained in a desirable gas-liquid two-phase state while an increase in the discharge temperature of the compressor is suppressed. 
     That is, in the configuration having the liquid-pressure adjustment expansion valve, since the liquid injection pipe is provided, the refrigerant can be properly transported in the two-phase state while an increase in the discharge temperature of the compressor is suppressed. 
     An air conditioner according to a second aspect of the present invention is the air conditioner according to the first aspect of the present invention, in which the liquid injection pipe is connected to a suction refrigerant pipe through which the refrigerant to be sucked into the compressor flows. 
     In this case, as described above, since the refrigerant branched from the outdoor liquid-refrigerant pipe can be fed to the suction side of the compressor, the temperature of the refrigerant to be sucked into the compressor can be decreased. 
     An air conditioner according to a third aspect of the present invention is the air conditioner according to the second aspect of the present invention, in which an accumulator that temporarily stores the refrigerant is provided in the suction refrigerant pipe; and the liquid injection pipe is connected to a portion of the suction refrigerant pipe on an outlet side of the accumulator. 
     In this case, as described above, since the liquid injection pipe is connected to the outlet side of the accumulator, the refrigerant flowing through the liquid injection pipe can be joined to the refrigerant to be sucked into the compressor without via the accumulator. Thus, the effect of decreasing the temperature of the refrigerant to be sucked into the compressor can be increased as compared with a case where the liquid injection pipe is connected to an inlet side of the accumulator. 
     An air conditioner according to a fourth aspect of the present invention is the air conditioner according to the second aspect of the present invention, in which the liquid injection pipe is bifurcated; and the liquid injection pipe is connected to both a portion of the suction refrigerant pipe on an inlet side of the accumulator and a portion of the suction refrigerant pipe on an outlet side of the accumulator. 
     In this case, as described above, since the liquid injection pipe is connected to both the inlet side and the outlet side of the accumulator, the refrigerant flowing through the liquid injection pipe may be fed to the outlet side of the accumulator to increase the effect of decreasing the temperature of the refrigerant to be sucked into the compressor, and the refrigerant flowing through the liquid injection pipe may be fed to the inlet side of the accumulator to reduce the liquid so that the pressure of the refrigerant discharged from the compressor does not exceed a predetermined discharge pressure threshold. 
     An air conditioner according to a fifth aspect of the present invention is the air conditioner according to the first aspect of the present invention, in which the liquid injection pipe is connected to an intermediate portion of a compression stroke of the compressor. 
     In this case, as described above, since the refrigerant branched from the outdoor liquid-refrigerant pipe can be fed to the intermediate portion of the compression stroke of the compressor, the temperature of the refrigerant compressed to an intermediate pressure in the compressor can be decreased. 
     An air conditioner according to a sixth aspect of the present invention is the air conditioner according to the fifth aspect of the present invention, in which an accumulator that temporarily stores the refrigerant is provided in the suction refrigerant pipe through which the refrigerant to be sucked into the compressor flows; the liquid injection pipe is bifurcated; and the liquid injection pipe is connected to both a portion of the suction refrigerant pipe on an inlet side of the accumulator and the intermediate portion of the compression stroke of the compressor. 
     In this case, as described above, since the liquid injection pipe is connected to both the inlet side of the accumulator and the intermediate portion of the compression stroke of the compressor, the refrigerant flowing through the liquid injection pipe may be fed to the intermediate portion of the compression stroke of the compressor to decrease the temperature of the refrigerant compressed to the intermediate pressure in the compressor, and the refrigerant flowing through the liquid injection pipe may be fed to the inlet side of the accumulator to reduce the liquid so that the pressure of the refrigerant discharged from the compressor does not exceed the predetermined discharge pressure threshold. 
     An air conditioner according to a seventh aspect of the present invention is the air conditioner according to the first aspect of the present invention, in which a refrigerant return pipe that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe and feeds the branched refrigerant to the compressor is connected to the outdoor liquid-refrigerant pipe, and a refrigerant cooler that cools the refrigerant flowing through a portion of the outdoor liquid-refrigerant pipe on the side of the outdoor heat exchanger with respect to the liquid-pressure adjustment expansion valve using the refrigerant flowing through the refrigerant return pipe is provided in the outdoor liquid-refrigerant pipe. 
     In this configuration in which the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe and feeding the refrigerant from the outdoor unit to the indoor units using the liquid-pressure adjustment expansion valve, as described above, the refrigerant cooler is further provided that cools the refrigerant flowing through the portion of the outdoor liquid-refrigerant pipe on the side of the outdoor heat exchanger with respect to the liquid-pressure adjustment expansion valve using the refrigerant return pipe and the refrigerant flowing through the refrigerant return pipe. 
     If the liquid injection pipe is not provided and the refrigerant return pipe and the refrigerant cooler are provided, the refrigerant flowing through the refrigerant return pipe cools the refrigerant flowing through the refrigerant cooler and then is fed to the compressor, and hence an increase in the discharge temperature of the compressor can be suppressed. However, since the refrigerant flowing through the refrigerant return pipe is fed to the compressor after cooling the refrigerant flowing through the outdoor liquid-refrigerant pipe in the refrigerant cooler, the temperature of the refrigerant flowing through the outdoor liquid-refrigerant pipe after the refrigerant has passed through the refrigerant cooler varies depending on the flow rate of the refrigerant flowing through the refrigerant return pipe. Consequently, the state of the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve also varies. For example, if the flow rate of the refrigerant flowing through the refrigerant return pipe excessively increases, an increase in the discharge temperature of the compressor can be suppressed to a certain extent; however, the temperature of the refrigerant flowing through the outdoor liquid-refrigerant pipe after the refrigerant has passed through the refrigerant cooler excessively decreases. Consequently, the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve is in a gas-liquid two-phase state containing more liquid component. 
     That is, in the configuration having the liquid-pressure adjustment expansion valve, merely providing the refrigerant return pipe and the refrigerant cooler may not maintain a desirable gas-liquid two-phase state. It is difficult to properly transport the refrigerant in the two-phase state while an increase in the discharge temperature of the compressor is suppressed. 
     Owing to this, the liquid injection pipe is provided in addition to the refrigerant return pipe and the refrigerant cooler. Since the liquid injection pipe is provided, the refrigerant can be fed to the compressor while a variation in the temperature of the refrigerant flowing through the outdoor liquid-refrigerant pipe is suppressed. Thus, an increase in the discharge temperature of the compressor can be suppressed without increasing the flow rate of the refrigerant flowing through the refrigerant return pipe. If the flow rate of the refrigerant flowing through the refrigerant return pipe is not excessively large, the temperature of the refrigerant flowing through the outdoor liquid-refrigerant pipe after the refrigerant has passed through the refrigerant cooler does not excessively decrease. Consequently, the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant is decompressed by the liquid-pressure adjustment expansion valve does not become the refrigerant in the gas-liquid two-phase state containing more liquid component. Thus, the refrigerant flowing through the liquid-refrigerant connection pipe can be maintained in a desirable gas-liquid two-phase state while an increase in the discharge temperature of the compressor is suppressed. 
     That is, in the configuration having the liquid-pressure adjustment expansion valve, since the liquid injection pipe is provided in addition to the refrigerant return pipe and the refrigerant cooler, the refrigerant can be properly transported in the two-phase state while an increase in the discharge temperature of the compressor is suppressed. 
     An air conditioner according to an eighth aspect of the present invention is the air conditioner according to the seventh aspect of the present invention, in which the liquid injection pipe and/or the refrigerant return pipe is connected to a suction refrigerant pipe through which the refrigerant to be sucked into the compressor flows. 
     In this case, as described above, since the refrigerant branched from the outdoor liquid-refrigerant pipe can be fed to the suction side of the compressor, the temperature of the refrigerant to be sucked into the compressor can be decreased. 
     An air conditioner according to a ninth aspect of the present invention is the air conditioner according to the eighth aspect of the present invention, in which an accumulator that temporarily stores the refrigerant is provided in the suction refrigerant pipe; and the liquid injection pipe and/or the refrigerant return pipe is connected to a portion of the suction refrigerant pipe on an outlet side of the accumulator. 
     In this case, as described above, since the liquid injection pipe and/or the refrigerant return pipe is connected to the outlet side of the accumulator, the refrigerant flowing through the liquid injection pipe and/or the refrigerant return pipe can be joined to the refrigerant to be sucked into the compressor without via the accumulator. Thus, the effect of decreasing the temperature of the refrigerant to be sucked into the compressor can be increased as compared with a case where the liquid injection pipe and/or the refrigerant return pipe is connected to an inlet side of the accumulator. 
     An air conditioner according to a tenth aspect of the present invention is the air conditioner according to the eighth aspect of the present invention, in which the liquid injection pipe and/or the refrigerant return pipe is connected to a portion of the suction refrigerant pipe on an inlet side of the accumulator. 
     In this case, as described above, since the liquid injection pipe and/or the refrigerant return pipe is connected to the inlet side of the accumulator, the refrigerant flowing through the liquid injection pipe and/or the refrigerant return pipe can be joined to the refrigerant to be sucked into the compressor via the accumulator. Thus, for example, occurrence of liquid compression in the compressor can be prevented as compared with a case where the liquid injection pipe and/or the refrigerant return pipe is connected to an outlet side of the accumulator. 
     An air conditioner according to an eleventh aspect of the present invention is the air conditioner according to the eighth aspect of the present invention, in which the liquid injection pipe or the refrigerant return pipe is bifurcated; and the liquid injection pipe or the refrigerant return pipe is connected to both a portion of the suction refrigerant pipe on an inlet side of the accumulator and a portion of the suction refrigerant pipe on an outlet side of the accumulator. 
     In this case, as described above, since the liquid injection pipe or the refrigerant return pipe is connected to both the inlet side and the outlet side of the accumulator, the refrigerant flowing through the liquid injection pipe or the refrigerant return pipe may be fed to the outlet side of the accumulator to increase the effect of decreasing the temperature of the refrigerant to be sucked into the compressor, and the refrigerant flowing through the liquid injection pipe or the refrigerant return pipe may be fed to the inlet side of the accumulator to reduce the liquid so that the pressure of the refrigerant discharged from the compressor does not exceed a predetermined discharge pressure threshold. 
     An air conditioner according to a twelfth aspect of the present invention is the air conditioner according to the seventh aspect of the present invention, in which the liquid injection pipe and/or the refrigerant return pipe is connected to an intermediate portion of a compression stroke of the compressor. 
     In this case, as described above, since the refrigerant branched from the outdoor liquid-refrigerant pipe can be fed to the intermediate portion of the compression stroke of the compressor, the temperature of the refrigerant compressed to an intermediate pressure in the compressor can be decreased. 
     An air conditioner according to a thirteenth aspect of the present invention is the air conditioner according to the twelfth aspect of the present invention, in which an accumulator that temporarily stores the refrigerant is provided in a suction refrigerant pipe through which the refrigerant to be sucked into the compressor flows; the liquid injection pipe or the refrigerant return pipe is bifurcated; and the liquid injection pipe or the refrigerant return pipe is connected to both a portion of the suction refrigerant pipe on an inlet side of the accumulator and the intermediate portion of the compression stroke of the compressor. 
     In this case, as described above, since the liquid injection pipe or the refrigerant return pipe is connected to both the inlet side of the accumulator and the intermediate portion of the compression stroke of the compressor, the refrigerant flowing through the liquid injection pipe or the refrigerant return pipe may be fed to the intermediate portion of the compression stroke of the compressor to decrease the temperature of the refrigerant compressed to the intermediate pressure in the compressor, and the refrigerant flowing through the liquid injection pipe or the refrigerant return pipe may be fed to the inlet side of the accumulator to reduce the liquid so that the pressure of the refrigerant discharged from the compressor does not exceed a predetermined discharge pressure threshold. 
     An air conditioner according to a fourteenth aspect of the present invention is the air conditioner according to any one of the first to sixth aspects of the present invention, in which a liquid-injection expansion valve that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe is provided in the liquid injection pipe. A controller that controls a component of the outdoor unit and the indoor units controls an opening degree of the liquid-injection expansion valve so that a temperature of the refrigerant discharged from the compressor does not exceed a predetermined discharge temperature threshold. 
     In this case, as described above, since the flow rate of the refrigerant that is fed from the outdoor liquid-refrigerant pipe to the compressor can be adjusted through the liquid injection pipe by controlling the opening degree of the liquid-injection expansion valve provided in the liquid injection pipe, an increase in the temperature of the refrigerant discharged from the compressor (a discharge temperature of the compressor) can be reliably suppressed. 
     An air conditioner according to a fifteenth aspect of the present invention is the air conditioner according to the fourth or sixth aspect of the present invention, in which a liquid relief valve that feeds the refrigerant branched from the outdoor liquid-refrigerant pipe to the accumulator is provided in a portion of the liquid injection pipe that is connected to the portion of the suction refrigerant pipe on the inlet side of the accumulator. A controller that controls a component of the outdoor unit and the indoor units controls the liquid relief valve so that a pressure of the refrigerant discharged from the compressor does not exceed a predetermined discharge pressure threshold. 
     In this case, as described above, since the refrigerant can be fed from the outdoor liquid-refrigerant pipe to the accumulator via the liquid injection pipe by controlling the liquid relief valve provided in the portion of the liquid injection pipe connected to the inlet side of the accumulator, an increase in the pressure of the refrigerant discharged from the compressor (a discharge pressure of the compressor) can be suppressed. 
     An air conditioner according to a sixteenth aspect of the present invention is the air conditioner according to any one of the seventh to thirteenth aspects of the present invention, in which a liquid-injection expansion valve that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe is provided in the liquid injection pipe, and a refrigerant-return expansion valve that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe is provided in the refrigerant return pipe. A controller that controls a component of the outdoor unit and the indoor units controls an opening degree of the liquid-injection expansion valve so that a temperature of the refrigerant discharged from the compressor does not exceed a predetermined discharge temperature threshold and controls an opening degree of the refrigerant-return expansion valve so that a temperature of the refrigerant in a portion of the outdoor liquid-refrigerant pipe between the refrigerant cooler and the liquid-pressure adjustment expansion valve becomes a target liquid-pipe temperature. 
     In this case, as described above, since the flow rate of the refrigerant that is fed from the outdoor liquid-refrigerant pipe to the compressor can be adjusted through the liquid injection pipe by controlling the opening degree of the liquid-injection expansion valve provided in the liquid injection pipe, an increase in the temperature of the refrigerant discharged from the compressor (a discharge temperature of the compressor) can be reliably suppressed. In addition, since the flow rate of the refrigerant that exchanges heat with the refrigerant flowing through the outdoor liquid-refrigerant pipe in the refrigerant cooler can be adjusted by controlling the opening degree of the refrigerant-return expansion valve provided in the refrigerant return pipe, the temperature of the refrigerant in the portion of the outdoor liquid-refrigerant pipe between the refrigerant cooler and the liquid-pressure adjustment expansion valve (a liquid-pipe temperature) can be constant at a target liquid-pipe temperature. Since the liquid-pipe temperature is constant, the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve can be reliably maintained in a desirable gas-liquid two-phase state. When the refrigerant is transported in the two-phase state using the liquid-pressure adjustment expansion valve, the refrigerant return pipe and the refrigerant cooler are used to maintain the liquid-pipe temperature to be constant, and the liquid injection pipe is used to suppress an increase in the discharge temperature of the compressor. 
     An air conditioner according to a seventeenth aspect of the present invention is the air conditioner according to the eleventh or thirteenth aspect of the present invention, in which a liquid relief valve that feeds the refrigerant branched from the outdoor liquid-refrigerant pipe to the accumulator is provided in a portion of the liquid injection pipe or the refrigerant return pipe that is connected to the portion of the suction refrigerant pipe on the inlet side of the accumulator. A controller that controls a component of the outdoor unit and the indoor units controls the liquid relief valve so that a pressure of the refrigerant discharged from the compressor does not exceed a predetermined discharge pressure threshold. 
     In this case, as described above, since the refrigerant can be fed from the outdoor liquid-refrigerant pipe to the accumulator via the liquid injection pipe or the refrigerant return pipe by controlling the liquid relief valve provided in the portion of the liquid injection pipe or the refrigerant return pipe connected to the inlet side of the accumulator, an increase in the pressure of the refrigerant discharged from the compressor (a discharge pressure of the compressor) can be suppressed. 
     An air conditioner according to an eighteenth aspect of the present invention is the air conditioner according to any one of the fourteenth to seventeenth aspect of the present invention, in which the control unit controls an opening degree of the liquid-pressure adjustment expansion valve so that a degree of subcooling of the refrigerant at the liquid-side end of the outdoor heat exchanger becomes a target degree of subcooling, to cause the liquid-pressure adjustment expansion valve to decompress the refrigerant flowing through the liquid-refrigerant connection pipe to be in the gas-liquid two-phase state. 
     In this case, as described above, since the opening degree of the liquid-pressure adjustment expansion valve is controlled so that the degree of subcooling of the refrigerant at the liquid-side end of the outdoor heat exchanger becomes the target degree of subcooling, the holding refrigerant amount of the outdoor heat exchanger can be easily maintained in a desirable state, and consequently, the refrigerant flowing through the liquid-refrigerant connection pipe after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve can be easily maintained in a desirable gas-liquid two-phase state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an air conditioner according to a first embodiment of the present invention. 
         FIG. 2  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner according to the first embodiment of the present invention. 
         FIG. 3  is a schematic configuration diagram of an air conditioner according to a first modification of the first embodiment of the present invention. 
         FIG. 4  is a schematic configuration diagram of an air conditioner according to a second modification of the first embodiment of the present invention. 
         FIG. 5  is a schematic configuration diagram of an air conditioner according to a third modification of the first embodiment of the present invention. 
         FIG. 6  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner according to the third modification of the first embodiment of the present invention. 
         FIG. 7  is a schematic configuration diagram of an air conditioner according to a fourth modification of the first embodiment of the present invention. 
         FIG. 8  is a schematic configuration diagram of an air conditioner according to a second embodiment of the present invention. 
         FIG. 9  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner according to the second embodiment of the present invention. 
         FIG. 10  is a schematic configuration diagram of an air conditioner according to a first modification of the second embodiment of the present invention. 
         FIG. 11  is a schematic configuration diagram of the air conditioner according to a second modification of the second embodiment of the present invention. 
         FIG. 12  is a schematic configuration diagram of the air conditioner according to the second modification of the second embodiment of the present invention. 
         FIG. 13  is a schematic configuration diagram of the air conditioner according to a third modification of the second embodiment of the present invention. 
         FIG. 14  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner according to the third modification of the second embodiment of the present invention. 
         FIG. 15  is a schematic configuration diagram of the air conditioner according to the third modification of the second embodiment of the present invention. 
         FIG. 16  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner according to the third modification of the second embodiment of the present invention. 
         FIG. 17  is a schematic configuration diagram of an air conditioner according to a fourth modification of the second embodiment of the present invention. 
         FIG. 18  is a schematic configuration diagram of the air conditioner according to the fourth modification of the second embodiment of the present invention. 
         FIG. 19  is a schematic configuration diagram of an air conditioner according to a third embodiment of the present invention. 
         FIG. 20  is a schematic configuration diagram (only for the periphery of an outdoor liquid-refrigerant pipe) of an air conditioner according to another embodiment of the present invention. 
         FIG. 21  is a schematic configuration diagram (only for the periphery of an outdoor liquid-refrigerant pipe) of an air conditioner according to still another embodiment of the present invention. 
         FIG. 22  is a schematic configuration diagram (only for the periphery of an outdoor liquid-refrigerant pipe) of an air conditioner according to yet another embodiment of the present invention. 
         FIG. 23  is a schematic configuration diagram (only for the periphery of an outdoor liquid-refrigerant pipe) of an air conditioner according to a further embodiment of the present invention. 
         FIG. 24  is a schematic configuration diagram (only for the periphery of an outdoor liquid-refrigerant pipe) of an air conditioner according to a still further embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Air conditioners according to embodiments of the present invention are described below with reference to the drawings. Note that specific configurations of the air conditioners according to embodiments of the present invention are not limited to those described in the following embodiments and modifications thereof, and may be changed within the scope of the present invention. 
     (1) First Embodiment 
     &lt;Configuration&gt; 
       FIG. 1  is a schematic configuration diagram of an air conditioner  1  according to a first embodiment of the present invention. The air conditioner  1  is an apparatus that performs cooling and heating in a room of a building or the like through a vapor compression refrigeration cycle. The air conditioner  1  mainly includes an outdoor unit  2 , a plurality of (in this case, two) indoor units  3   a  and  3   b  that are mutually connected in parallel, a liquid-refrigerant connection pipe  5  and a gas-refrigerant connection pipe  6  that connect the outdoor unit  2  to the indoor units  3   a  and  3   b , and a control unit  19  that controls components of the outdoor unit  2  and the indoor units  3   a  and  3   b . A vapor compression refrigerant circuit  10  of the air conditioner  1  is defined by connecting the outdoor unit  2  to the plurality of indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6 . The refrigerant circuit  10  is filled with a refrigerant such as R32. 
     —Connection Pipes— 
     The liquid-refrigerant connection pipe  5  mainly includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  5   a  and  5   b  that are a plurality of (in this case, two) branched pipe portions branched before the indoor units  3   a  and  3   b . The gas-refrigerant connection pipe  6  mainly includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  6   a  and  6   b  that are a plurality of (in this case, two) branched pipe portions branched before the indoor units  3   a  and  3   b.    
     —Indoor Units— 
     The indoor units  3   a  and  3   b  are installed in rooms of the building or the like. The indoor units  3   a  and  3   b  are connected to the outdoor unit  2  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6  and constitute part of the refrigerant circuit  10  as described above. 
     Configurations of the indoor units  3   a  and  3   b  are described next. Since the configurations of the indoor units  3   a  and  3   b  are similar to each other, the configuration of the indoor unit  3   a  is described here. For the configuration of the indoor unit  3   b , a letter “b” is applied to each component of the indoor unit  3   b  instead of a letter “a” indicative of each component of the indoor unit  3   a , and the description of each component of the indoor unit  3   b  is omitted. 
     The indoor unit  3   a  mainly includes an indoor expansion valve  51   a  and an indoor heat exchanger  52   a . The indoor unit  3   a  also includes an indoor liquid-refrigerant pipe  53   a  that connects a liquid-side end of the indoor heat exchanger  52   a  to the liquid-refrigerant connection pipe  5 , and an indoor gas-refrigerant pipe  54   a  that connects a gas-side end of the indoor heat exchanger  52   a  to the gas-refrigerant connection pipe  6 . 
     The indoor expansion valve  51   a  is an electrically powered expansion valve that adjusts the flow rate of the refrigerant flowing in the indoor heat exchanger  52   a  while decompressing the refrigerant, and is provided in the indoor liquid-refrigerant pipe  53   a.    
     The indoor heat exchanger  52   a  is a heat exchanger that functions as an evaporator of the refrigerant to cool indoor air, or that functions as a radiator of the refrigerant to heat the indoor air. The indoor unit  3   a  includes an indoor fan  55   a  that sucks the indoor air into the indoor unit  3   a , that allows the indoor heat exchanger  52   a  to exchange heat with the refrigerant, and then that supplies the indoor air as supply air into the room. That is, the indoor unit  3   a  includes the indoor fan  55   a  as a fan that supplies the indoor air, which serves as a cooling source or a heating source of the refrigerant flowing in the indoor heat exchanger  52   a , to the indoor heat exchanger  52   a . The indoor fan  55   a  is driven by an indoor fan motor  56   a.    
     The indoor unit  3   a  is provided with various sensors. More specifically, the indoor unit  3   a  is provided with an indoor heat-exchanger liquid-side sensor  57   a  that detects a temperature Trl of the refrigerant at the liquid-side end of the indoor heat exchanger  52   a , an indoor heat-exchanger gas-side sensor  58   a  that detects a temperature Trg of the refrigerant at the gas-side end of the indoor heat exchanger  52   a , and an indoor air sensor  59   a  that detects a temperature Tra of the indoor air which is sucked into the indoor unit  3   a.    
     —Outdoor Unit— 
     The outdoor unit  2  is installed outside the rooms of the building or the like. The outdoor unit  2  is connected to the indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6  and constitutes part of the refrigerant circuit  10  as described above. 
     A configuration of the outdoor unit  2  is described next. 
     The outdoor unit  2  mainly includes a compressor  21  and an outdoor heat exchanger  23 . The outdoor unit  2  also includes a switching mechanism  22  that switches between a radiation operation state in which the outdoor heat exchanger  23  functions as a radiator of the refrigerant and an evaporation operation state in which the outdoor heat exchanger  23  functions as an evaporator of the refrigerant. The switching mechanism  22  and a suction side of the compressor  21  are connected by a suction refrigerant pipe  31 . The suction refrigerant pipe  31  is provided with an accumulator  29  that temporarily stores the refrigerant to be sucked into the compressor  21 . A discharge side of the compressor  21  and the switching mechanism  22  are connected by a discharge refrigerant pipe  32 . The switching mechanism  22  and a gas-side end of the outdoor heat exchanger  23  are connected by a first outdoor gas-refrigerant pipe  33 . A liquid-side end of the outdoor heat exchanger  23  and the liquid-refrigerant connection pipe  5  are connected by an outdoor liquid-refrigerant pipe  34 . A liquid-side shutoff valve  27  is provided at a connection portion between the outdoor liquid-refrigerant pipe  34  and the liquid-refrigerant connection pipe  5 . The switching mechanism  22  and the gas-refrigerant connection pipe  6  are connected by a second outdoor gas-refrigerant pipe  35 . A gas-side shutoff valve  28  is provided at a connection portion between the second outdoor gas-refrigerant pipe  35  and the gas-refrigerant connection pipe  6 . The liquid-side shutoff valve  27  and the gas-side shutoff valve  28  are valves that are manually opened and closed. 
     The compressor  21  is a device that compresses a refrigerant. For example, a closed-structure compressor in which a rotary or scroll positive-displacement compression element (not illustrated) is rotationally driven by a compressor motor  21   a  is used. 
     The switching mechanism  22  is a device capable of switching the flow of the refrigerant in the refrigerant circuit  10  to connect the discharge side of the compressor  21  and the gas side of the outdoor heat exchanger  23  (see solid lines of the switching mechanism  22  in  FIG. 1 ) when the outdoor heat exchanger  23  functions as a radiator of the refrigerant (referred to as “outdoor radiation state” hereinafter) and to connect the suction side of the compressor  21  and the gas side of the outdoor heat exchanger  23  (see broken lines of the switching mechanism  22  in  FIG. 1 ) when the outdoor heat exchanger  23  functions as an evaporator of the refrigerant (referred to as “outdoor evaporation state” hereinafter). The switching mechanism  22  is, for example, a four-way switching valve. 
     The outdoor heat exchanger  23  is a heat exchanger that functions as a radiator of the refrigerant or an evaporator of the refrigerant. The outdoor unit  2  includes an outdoor fan  24  that sucks outdoor air into the outdoor unit  2 , that allows the outdoor air to exchange heat with the refrigerant in the outdoor heat exchanger  23 , and then that discharges the outdoor air to the outside. That is, the outdoor unit  2  includes the outdoor fan  24  as a fan that supplies the outdoor air, which serves as a cooling source or a heating source of the refrigerant flowing in the outdoor heat exchanger  23 , to the outdoor heat exchanger  23 . The outdoor fan  24  is driven by an outdoor fan motor  24   a.    
     Focusing only on the compressor  21 , the outdoor heat exchanger  23 , the liquid-refrigerant connection pipe  5 , and the indoor heat exchangers  52   a  and  52   b , the air conditioner  1  performs an operation (cooling operation) of sending the refrigerant, which has been discharged from the compressor  21 , to the outdoor heat exchanger  23 , the liquid-refrigerant connection pipe  5 , and the indoor heat exchangers  52   a  and  52   b  in that order. Focusing only on the compressor  21 , the gas-refrigerant connection pipe  6 , the indoor heat exchangers  52   a  and  52   b , the liquid-refrigerant connection pipe  5 , and the outdoor heat exchanger  23 , the air conditioner  1  performs an operation (heating operation) of sending the refrigerant, which has been discharged from the compressor  21 , to the gas-refrigerant connection pipe  6 , the indoor heat exchangers  52   a  and  52   b , the liquid-refrigerant connection pipe  5 , and the outdoor heat exchanger  23  in that order. The switching mechanism  22  is switched to the outdoor radiation state during cooling operation, and the switching mechanism  22  is switched to the outdoor evaporation state during heating operation. 
     The outdoor liquid-refrigerant pipe  34  is provided with an outdoor expansion valve  25  and a liquid-pressure adjustment expansion valve  26 . The outdoor expansion valve  25  is an electrically powered expansion valve that decompresses the refrigerant during heating operation, and is provided in a portion of the outdoor liquid-refrigerant pipe  34  close to the liquid-side end of the outdoor heat exchanger  23 . The liquid-pressure adjustment expansion valve  26  is an electrically powered expansion valve that decompresses the refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe  5  is in a gas-liquid two-phase state during cooling operation, and is provided in a portion of the outdoor liquid-refrigerant pipe  34  close to the liquid-refrigerant connection pipe  5 . That is, the liquid-pressure adjustment expansion valve  26  is provided in a portion of the outdoor liquid-refrigerant pipe  34  close to the liquid-refrigerant connection pipe  5  with respect to the outdoor expansion valve  25 . 
     During cooling operation, the air conditioner  1  transports the refrigerant in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26 . 
     In addition, a liquid injection pipe  46  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  and feeds the branched refrigerant to the compressor  21  is connected to the outdoor liquid-refrigerant pipe  34 . The liquid injection pipe  46  is connected to a portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26 . More specifically, the liquid injection pipe  46  is connected to a portion of the outdoor liquid-refrigerant pipe  34  between the outdoor expansion valve  25  and the liquid-pressure adjustment expansion valve  26 . The liquid injection pipe  46  is connected to the suction refrigerant pipe  31  through which the refrigerant to be sucked into the compressor  21  flows. The liquid injection pipe  46  is connected to a portion of the suction refrigerant pipe  31  on an outlet side of the accumulator  29 . The liquid injection pipe  46  is provided with a liquid-injection expansion valve  47  that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe  34 . The liquid-injection expansion valve  47  is an electrically powered expansion valve. 
     The outdoor unit  2  is provided with various sensors. More specifically, the outdoor unit  2  is provided with a discharge pressure sensor  36  that detects a pressure (discharge pressure Pd) of the refrigerant discharged from the compressor  21 , a discharge temperature sensor  37  that detects a temperature (discharge temperature Td) of the refrigerant discharged from the compressor  21 , a suction pressure sensor  39  that detects a pressure (suction pressure Ps) of the refrigerant to be sucked into the compressor  21 , and a suction temperature sensor  40  that detects a temperature (suction temperature Ts) of the refrigerant to be sucked into the compressor  21 . In addition, the outdoor unit  2  is provided with an outdoor heat-exchanger liquid-side sensor  38  that detects a temperature Tol (outdoor heat-exchanger outlet temperature Tol) of the refrigerant at the liquid-side end of the outdoor heat exchanger  23 , and a liquid-pipe temperature sensor  49  that detects a temperature (liquid-pipe temperature Tlp) of the refrigerant in a portion of the outdoor liquid-refrigerant pipe  34  between the outdoor expansion valve  25  and the liquid-pressure adjustment expansion valve  26 . 
     —Control Unit— 
     The control unit  19  is connected to control boards or the like (not illustrated) that are provided in the outdoor unit  2  and the indoor units  3   a  and  3   b  for communication. In  FIG. 1 , the control unit  19  is illustrated at a position separated from the outdoor unit  2  and the indoor units  3   a  and  3   b  for the convenience of illustration. The control unit  19  controls the various components  21 ,  22 ,  24 ,  25 ,  26 ,  47 ,  51   a ,  51   b ,  55   a , and  55   b  of the air conditioner  1  (in this case, the outdoor unit  2  and the indoor units  3   a  and  3   b ) on the basis of detection signals or the like of the above-described various sensors  36 ,  37 ,  38 ,  39 ,  40 ,  49 ,  57   a ,  57   b ,  58   a ,  58   b ,  59   a , and  59   b . That is, the control unit  19  controls the entire operation of the air conditioner  1 . 
     &lt;Operations and Features of Air Conditioner&gt; 
     Operations and features of the air conditioner  1  are described next with reference to  FIGS. 1 and 2 .  FIG. 2  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner  1  according to the first embodiment of the present invention. 
     The air conditioner  1  performs cooling operation and heating operation as described above. During cooling operation, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26  provided in the outdoor liquid-refrigerant pipe  34 . Further, during cooling operation, an operation is performed to feed the refrigerant to the compressor  21  while suppressing a variation in the temperature (liquid-pipe temperature Tlp) of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  using the liquid injection pipe  46  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  from the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  and feeds the branched refrigerant to the compressor  21 . Note that the control unit  19  that controls the components of the air conditioner  1  performs the operations of the air conditioner  1  which will be described below. 
     —Cooling Operation— 
     For cooling operation, for example, when all the indoor units  3   a  and  3   b  perform cooling operation (that is, operation in which all the indoor heat exchangers  52   a  and  52   b  function as evaporators of the refrigerant and the outdoor heat exchanger  23  functions as a radiator of the refrigerant), the switching mechanism  22  is switched to the outdoor radiation state (the state indicated by solid lines of the switching mechanism  22  in  FIG. 1 ), and the compressor  21 , the outdoor fan  24 , and the indoor fans  55   a  and  55   b  are driven. 
     Then, the refrigerant at a high pressure discharged from the compressor  21  is fed to the outdoor heat exchanger  23  via the switching mechanism  22  (see point B in  FIGS. 1 and 2 ). The refrigerant fed to the outdoor heat exchanger  23  exchanges heat with the outdoor air supplied by the outdoor fan  24 , and hence is cooled and condensed in the outdoor heat exchanger  23  that functions as the radiator of the refrigerant (see point C in  FIGS. 1 and 2 ). The refrigerant flows out from the outdoor unit  2  via the outdoor expansion valve  25 , the liquid-pressure adjustment expansion valve  26 , and the liquid-side shutoff valve  27  (see point D in  FIGS. 1 and 2 ). 
     The refrigerant flowing out from the outdoor unit  2  is branched and fed to the indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  (see point E in  FIGS. 1 and 2 ). The refrigerants fed to the indoor units  3   a  and  3   b  are decompressed to a low pressure by the indoor expansion valves  51   a  and  51   b  and then are fed to the indoor heat exchangers  52   a  and  52   b  (see point F in  FIGS. 1 and 2 ). The refrigerants fed to the indoor heat exchangers  52   a  and  52   b  exchange heat with the indoor air supplied from the inside of the rooms by the indoor fans  55   a  and  55   b , and hence are heated and evaporated in the indoor heat exchangers  52   a  and  52   b  that function as the evaporators of the refrigerant (see point G in  FIGS. 1 and 2 ). The refrigerants flow out from the indoor units  3   a  and  3   b . In contrast, the indoor air cooled in the indoor heat exchangers  52   a  and  52   b  is fed into the rooms, and thus the rooms are cooled. 
     The refrigerants flowing out from the indoor units  3   a  and  3   b  are joined and fed to the outdoor unit  2  via the gas-refrigerant connection pipe  6  (see point H in  FIGS. 1 and 2 ). The refrigerant fed to the outdoor unit  2  is sucked into the compressor  21  via the gas-side shutoff valve  28 , the switching mechanism  22 , and the accumulator  29  (see point A in  FIGS. 1 and 2 ). 
     During the above-described cooling operation, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26 . In addition, as described below, when the refrigerant is to be transported in the two-phase state, the refrigerant is properly transported in the two-phase state while suppressing an increase in the discharge temperature Td of the compressor  21  using the liquid injection pipe  46 . 
     First, the control unit  19  controls the liquid-pressure adjustment expansion valve  26  to decompress the refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe  5  is in the gas-liquid two-phase state (see points C and D in  FIGS. 1 and 2 ). The refrigerant decompressed by the liquid-pressure adjustment expansion valve  26  is a refrigerant at an intermediate pressure that is lower than the pressure of a refrigerant at a high pressure and is higher than the pressure of a refrigerant at a low pressure (see point D in  FIGS. 1 and 2 ). The control unit  19  controls the opening degree of the liquid-pressure adjustment expansion valve  26  so that a degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  becomes a target degree of subcooling SCot. More specifically, the control unit  19  obtains the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  from the outdoor heat-exchanger outlet temperature Tol. The control unit  19  obtains the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  by subtracting the outdoor heat-exchanger outlet temperature Tol from a temperature Toc of the refrigerant obtained by converting the discharge pressure Pd into a saturation temperature. The control unit  19  performs control to increase the opening degree of the liquid-pressure adjustment expansion valve  26  if the degree of subcooling SCo is larger than the target degree of subcooling SCot, and performs control to decrease the opening degree of the liquid-pressure adjustment expansion valve  26  if the degree of subcooling SCo is smaller than the target degree of subcooling SCot. Note that, in this case, the control unit  19  performs control to fix the opening degree of the outdoor expansion valve  25  in a full-open state. 
     With this control, the state of the refrigerant flowing through the liquid-refrigerant connection pipe  5  becomes the gas-liquid two-phase state. The liquid-refrigerant connection pipe  5  is not filled with the refrigerant in a liquid state unlike a case where the refrigerant flowing through the liquid-refrigerant connection pipe  5  is in a liquid state. The amount of the refrigerant existing in the liquid-refrigerant connection pipe  5  can be decreased by that amount. 
     In addition, the control unit  19  branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  to suppress an increase in the discharge temperature Td of the compressor  21  and feeds the branched refrigerant to the compressor  21  (in this case, the suction refrigerant pipe  31  connected to the suction side of the compressor  21 ). The control unit  19  controls the opening degree of the liquid-injection expansion valve  47  so that the discharge temperature Td of the compressor  21  does not exceed a predetermined discharge temperature threshold Tdx (for example, upper-limit discharge temperature). More specifically, when the discharge temperature Td has increased above the discharge temperature threshold Tdx, the control unit  19  performs control to increase the opening degree of the liquid-injection expansion valve  47  until the discharge temperature Td becomes the discharge temperature threshold Tdx or lower. 
     With this control, the refrigerants fed from the indoor units  3   a  and  3   b  to the outdoor unit  2  (point H in  FIGS. 1 and 2 ) are joined to the refrigerant to be fed to the compressor  21  via the liquid injection pipe  46  and is cooled (see points H and A in  FIGS. 1 and 2 ). Thus, an increase in the discharge temperature Td of the compressor  21  can be suppressed by that cooled amount (see point B in  FIGS. 1 and 2 ). 
     —Heating Operation— 
     For heating operation, for example, when all the indoor units  3   a  and  3   b  perform heating operation (that is, operation in which all the indoor heat exchangers  52   a  and  52   b  function as radiators of the refrigerant and the outdoor heat exchanger  23  functions as an evaporator of the refrigerant), the switching mechanism  22  is switched to the outdoor evaporation state (the state indicated by broken lines of the switching mechanism  22  in  FIG. 1 ), and the compressor  21 , the outdoor fan  24 , and the indoor fans  55   a  and  55   b  are driven. 
     The refrigerant at a high pressure discharged from the compressor  21  flows out from the outdoor unit  2  via the switching mechanism  22  and the gas-side shutoff valve  28 . 
     The refrigerant flowing out from the outdoor unit  2  is branched and fed to the indoor units  3   a  and  3   b  via the gas-refrigerant connection pipe  6 . The refrigerants fed to the indoor units  3   a  and  3   b  are fed to the indoor heat exchangers  52   a  and  52   b . The refrigerants at a high pressure fed to the indoor heat exchangers  52   a  and  52   b  exchange heat with the indoor air supplied from the inside of the rooms by the indoor fans  55   a  and  55   b , and hence are cooled and condensed in the indoor heat exchangers  52   a  and  52   b  that function as the radiators of the refrigerant. The refrigerants flow out from the indoor units  3   a  and  3   b  via the indoor expansion valves  51   a  and  51   b . In contrast, the indoor air heated in the indoor heat exchangers  52   a  and  52   b  is fed into the rooms, and thus the rooms are heated. 
     The refrigerants flowing out from the indoor units  3   a  and  3   b  are joined and fed to the outdoor unit  2  via the liquid-refrigerant connection pipe  5 . The refrigerant fed to the outdoor unit  2  is fed to the outdoor expansion valve  25  via the liquid-side shutoff valve  27  and the liquid-pressure adjustment expansion valve  26 . The refrigerant fed to the outdoor expansion valve  25  is decompressed to a low pressure by the outdoor expansion valve  25  and then is fed to the outdoor heat exchanger  23 . The refrigerant fed to the outdoor heat exchanger  23  exchanges heat with the outdoor air supplied by the outdoor fan  24 , and hence is heated and evaporated. The refrigerant is sucked into the compressor  21  via the switching mechanism  22  and the accumulator  29 . 
     In this case, the control unit  19  performs control to fix the opening degree of the liquid-pressure adjustment expansion valve  26  in a full-open state. With this control, the opening degree of the liquid-injection expansion valve  47  is in a full-closed state so as not to send the refrigerant to the liquid injection pipe  46 . 
     —Features— 
     In the configuration in which the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26 , as described above, the liquid injection pipe  46  is further provided in the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  to branch part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  and feed the refrigerant to the compressor  21 . Since the liquid injection pipe  46  is provided, the refrigerant can be fed to the compressor  21  (see point C in  FIG. 2 ) while a variation in the temperature (liquid-pipe temperature Tlp) of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  is suppressed. Thus, an increase in the discharge temperature Td of the compressor  21  can be suppressed (see point B in  FIG. 2 ) while a variation in the state of the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  is suppressed (see point D in  FIG. 2 ). Thus, the refrigerant flowing through the liquid-refrigerant connection pipe  5  can be reliably maintained in a desirable gas-liquid two-phase state while an increase in the discharge temperature Td of the compressor  21  is suppressed. 
     That is, in the configuration having the liquid-pressure adjustment expansion valve  26 , since the liquid injection pipe  46  is provided, the refrigerant can be properly transported in the two-phase state while an increase in the discharge temperature Td of the compressor  21  is suppressed. 
     In addition, the control unit  19  controls the opening degree of the liquid-pressure adjustment expansion valve  26  so that the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  becomes the target degree of subcooling SCot, and thus the refrigerant flowing through the liquid-refrigerant connection pipe  5  is decompressed using the liquid-pressure adjustment expansion valve  26  to be in the gas-liquid two-phase state. With such control, the holding refrigerant amount of the outdoor heat exchanger  23  can be easily maintained in a desirable state (see point C in  FIG. 2 ), and consequently, the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  can be easily maintained in a desirable gas-liquid two-phase state (see point D in  FIG. 2 ). 
     In addition, the liquid-injection expansion valve  47  that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe  34  is provided in the liquid injection pipe  46 . The control unit  19  controls the opening degree of the liquid-injection expansion valve  47  so that the discharge temperature Td of the compressor  21  does not exceed the predetermined discharge temperature threshold Tdx (for example, upper-limit discharge temperature). Since the flow rate of the refrigerant that is fed from the outdoor liquid-refrigerant pipe  34  to the compressor  21  can be adjusted through the liquid injection pipe  46 , an increase in the discharge temperature Td of the compressor  21  can be reliably suppressed (see point B in  FIG. 2 ). 
     In addition, the liquid injection pipe  46  is connected to the suction refrigerant pipe  31  through which the refrigerant to be sucked into the compressor  21  flows. Thus, since the refrigerant branched from the outdoor liquid-refrigerant pipe  34  can be fed to the suction side of the compressor  21 , the temperature of the refrigerant to be sucked into the compressor  21  can be decreased (see points H and A in  FIG. 2 ). In this case, the liquid injection pipe  46  is connected to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , and hence the refrigerant flowing through the liquid injection pipe  46  can be joined to the refrigerant to be sucked into the compressor  21  without via the accumulator  29 . Thus, the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21  can be increased as compared with a case where the liquid injection pipe  46  is connected to an inlet side of the accumulator  29 . 
     &lt;First Modification&gt; 
     In the air conditioner  1  of the above-described first embodiment (see  FIG. 1 ), the liquid injection pipe  46  is connected to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , and hence the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21  is increased. However, the connection position of the liquid injection pipe  46  to the suction refrigerant pipe  31  is not limited thereto. 
     As illustrated in  FIG. 3 , the liquid injection pipe  46  is connected to a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29 , and hence the refrigerant flowing through the liquid injection pipe  46  can be joined to the refrigerant to be sucked into the compressor  21  via the accumulator  29 . Thus, for example, occurrence of liquid compression in the compressor  21  can be prevented as compared with a case where the liquid injection pipe  46  is connected to the outlet side of the accumulator  29 . In this configuration, a return liquid pipe  31   a  that feeds the refrigerant from a bottom portion of the accumulator  29  to a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29  may be provided, and the control unit  19  may control a return liquid valve  31   b  provided in the return liquid pipe  31   a  to return the liquid refrigerant stored in the accumulator  29  to the compressor  21 . 
     &lt;Second Modification&gt; 
     In the air conditioner  1  of the first embodiment and the first modification (see  FIGS. 1 and 3 ), the liquid injection pipe  46  is connected to the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  or the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . However, the connection position of the liquid injection pipe  46  to the suction refrigerant pipe  31  is not limited thereto. 
     As illustrated in  FIG. 4 , the liquid injection pipe  46  is bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . One of the branched pipes of the liquid injection pipe  46  connected to the portion on the outlet side of the accumulator  29  is referred to as the first liquid injection pipe  46   a , and the other of the branched pipes of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29  is referred to as the second liquid injection pipe  46   b . Since the liquid injection pipe  46  is connected to both the inlet side and the outlet side of the accumulator  29 , the refrigerant flowing through the liquid injection pipe  46  may be fed to the outlet side of the accumulator  29  to increase the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21 , and the refrigerant flowing through the liquid injection pipe  46  may be fed to the inlet side of the accumulator  29 , for example, to prevent occurrence of liquid compression in the compressor  21 . 
     In addition, by using the configuration in which the liquid injection pipe  46  in  FIG. 4  is bifurcated and connected to both the portions of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and on the outlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. More specifically, a liquid relief valve  46   d  is provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  46   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx (for example, upper-limit discharge pressure). More specifically, when the discharge pressure Pd has increased above the discharge pressure threshold Pdx, the control unit  19  performs control to open the liquid relief valve  46   d  until the discharge pressure Pd becomes the discharge pressure threshold Pdx or lower. Thus, the liquid refrigerant existing in the outdoor heat exchanger  23  can be fed to and retracted in the accumulator  29  via the liquid injection pipe  46 . Consequently, an increase in the discharge pressure Pd can be suppressed. Since the refrigerant can be fed from the outdoor liquid-refrigerant pipe  34  to the accumulator  29  via the liquid injection pipe  46  by controlling the liquid relief valve  46   d  provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. In this configuration, a capillary tube  46   c  serving as a flow resistance is provided in the first liquid injection pipe  46   a  so as to send the refrigerant to the second liquid injection pipe  46   b  more during the liquid relief control. 
     &lt;Third Modification&gt; 
     In the air conditioner  1  of the first embodiment (see  FIG. 1 ), since the liquid injection pipe  46  is connected to the suction refrigerant pipe  31 , the refrigerant branched from the outdoor liquid-refrigerant pipe  34  is fed to the suction side of the compressor  21  to decrease the temperature of the refrigerant to be sucked into the compressor  21  (see points H and A in  FIG. 2 ) and thus suppress an increase in the discharge temperature Td of the compressor  21 . However, the feeding destination of the refrigerant flowing through the liquid injection pipe  46  to the compressor  21  is not limited thereto. 
     As illustrated in  FIG. 5 , the liquid injection pipe  46  may be connected to an intermediate portion of a compression stroke of the compressor  21 . 
     With the configuration, unlike the first embodiment (see points H and A in  FIG. 2 ), as illustrated in  FIGS. 5 and 6 , an increase in the discharge temperature Td of the compressor  21  can be suppressed by feeding the refrigerant, which has been branched from the outdoor liquid-refrigerant pipe  34  to the liquid injection pipe  46 , to the intermediate portion of the compression stroke of the compressor  21  and decreasing the temperature of the refrigerant, which has been compressed to an intermediate pressure in the compressor  21  (see point I in  FIG. 6 ). Even in this case, the control and so forth on the liquid-pressure adjustment expansion valve  26  for transporting the refrigerant in the two-phase state is similar to that of the first embodiment, and hence the description is omitted here. 
     &lt;Fourth Modification&gt; 
     Also in the air conditioner  1  of the third modification (see  FIG. 5 ) of the first embodiment, similarly to the second modification (see  FIG. 4 ), as illustrated in  FIG. 7 , the liquid injection pipe  46  may be bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and an intermediate portion of the compression stroke of the compressor  21 . One of the branched pipes of the liquid injection pipe  46  connected to the intermediate portion of the compression stroke of the compressor  21  is referred to as the first liquid injection pipe  46   a , and the other of the branched pipes of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29  is referred to as the second liquid injection pipe  46   b . Since the liquid injection pipe  46  is connected to both the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and the intermediate portion of the compression stroke of the compressor  21 , the refrigerant flowing through the liquid injection pipe  46  can be fed to the intermediate portion of the compression stroke of the compressor  21  to decrease the temperature of the refrigerant compressed to the intermediate pressure in the compressor  21 , and the refrigerant flowing through the liquid injection pipe  46  can be fed to the inlet side of the accumulator  29 , for example, to prevent occurrence of liquid compression in the compressor  21 . 
     In addition, by using the configuration in which the liquid injection pipe  46  in  FIG. 7  is bifurcated and connected to both the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and the intermediate portion of the compression stroke of the compressor  21 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. More specifically, a liquid relief valve  46   d  is provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  46   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx (for example, upper-limit discharge pressure). More specifically, when the discharge pressure Pd has increased above the discharge pressure threshold Pdx, the control unit  19  performs control to open the liquid relief valve  46   d  until the discharge pressure Pd becomes the discharge pressure threshold Pdx or lower. Thus, the liquid refrigerant existing in the outdoor heat exchanger  23  can be fed to and retracted in the accumulator  29  via the liquid injection pipe  46 . Consequently, an increase in the discharge pressure Pd can be suppressed. Since the refrigerant can be fed from the outdoor liquid-refrigerant pipe  34  to the accumulator  29  via the liquid injection pipe  46  by controlling the liquid relief valve  46   d  provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. 
     (2) Second Embodiment 
     &lt;Configuration&gt; 
       FIG. 8  is a schematic configuration diagram of an air conditioner  1  according to a second embodiment of the present invention. The air conditioner  1  is an apparatus that performs cooling and heating in a room of a building or the like through a vapor compression refrigeration cycle. The air conditioner  1  mainly includes an outdoor unit  2 , a plurality of (in this case, two) indoor units  3   a  and  3   b  that are mutually connected in parallel, a liquid-refrigerant connection pipe  5  and a gas-refrigerant connection pipe  6  that connect the outdoor unit  2  to the indoor units  3   a  and  3   b , and a control unit  19  that controls components of the outdoor unit  2  and the indoor units  3   a  and  3   b . A vapor compression refrigerant circuit  10  of the air conditioner  1  is defined by connecting the outdoor unit  2  to the plurality of indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6 . The refrigerant circuit  10  is filled with a refrigerant such as R32. 
     —Connection Pipes— 
     The liquid-refrigerant connection pipe  5  mainly includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  5   a  and  5   b  that are a plurality of (in this case, two) branched pipe portions branched before the indoor units  3   a  and  3   b . The gas-refrigerant connection pipe  6  mainly includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  6   a  and  6   b  that are a plurality of (in this case, two) branched pipe portions branched before the indoor units  3   a  and  3   b.    
     —Indoor Units— 
     The indoor units  3   a  and  3   b  are installed in rooms of the building or the like. The indoor units  3   a  and  3   b  are connected to the outdoor unit  2  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6  and constitute part of the refrigerant circuit  10  as described above. 
     The configurations of the indoor units  3   a  and  3   b  are similar to the configurations of the indoor units  3   a  and  3   b  of the first embodiment, and the description is omitted here. 
     —Outdoor Unit— 
     The outdoor unit  2  is installed outside the rooms of the building or the like. The outdoor unit  2  is connected to the indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6  and constitutes part of the refrigerant circuit  10  as described above. 
     The configuration of the outdoor unit  2  differs from the configuration of the outdoor unit  2  of the first embodiment only in that a refrigerant return pipe  41  and a refrigerant cooler  45  are provided. Thus, the configurations of the refrigerant return pipe  41  and the refrigerant cooler  45  are mainly described. 
     The refrigerant return pipe  41  is connected to the outdoor liquid-refrigerant pipe  34 . The refrigerant cooler  45  is provided in the outdoor liquid-refrigerant pipe  34 . The refrigerant return pipe  41  is a refrigerant pipe that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  and feeds the branched refrigerant to the compressor  21 . The refrigerant cooler  45  is a heat exchanger that cools the refrigerant flowing through a portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  using the refrigerant flowing through the refrigerant return pipe  41 . The outdoor expansion valve  25  is provided in a portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the refrigerant cooler  45 . The liquid-pressure adjustment expansion valve  26  is provided in a portion of the outdoor liquid-refrigerant pipe  34  on the side of the liquid-refrigerant connection pipe  5  with respect to the portion to which the refrigerant cooler  45  is connected (in this case, a portion between the refrigerant cooler  45  and the liquid-side shutoff valve  27 ). In addition, the liquid injection pipe  46  is connected to a portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26 . 
     The refrigerant return pipe  41  is a refrigerant pipe that feeds the refrigerant branched from the outdoor liquid-refrigerant pipe  34  to the suction side of the compressor  21 . The refrigerant return pipe  41  mainly has a refrigerant-return inlet pipe  42  and a refrigerant-return outlet pipe  43 . The refrigerant-return inlet pipe  42  is a refrigerant pipe that branches the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  from a portion between the liquid-side end of the outdoor heat exchanger  23  and the liquid-pressure adjustment expansion valve  26  (in this case, a portion between the outdoor expansion valve  25  and the refrigerant cooler  45 ) and feeds the branched refrigerant to the inlet of the refrigerant cooler  45  on the side of the refrigerant return pipe  41 . The refrigerant-return inlet pipe  42  is provided with a refrigerant-return expansion valve  44  that adjusts the flow rate of the refrigerant flowing through the refrigerant cooler  45  while decompressing the refrigerant flowing through the refrigerant return pipe  41 . The refrigerant-return expansion valve  44  is an electrically powered expansion valve. The refrigerant-return outlet pipe  43  is a refrigerant pipe that feeds the refrigerant from the outlet of the refrigerant cooler  45  on the side of the refrigerant return pipe  41  to the suction refrigerant pipe  31 . The refrigerant-return outlet pipe  43  of the refrigerant return pipe  41  is connected to a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . The refrigerant cooler  45  cools the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  using the refrigerant flowing through the refrigerant return pipe  41 . During cooling operation, the refrigerant cooler  45  is a heat exchanger of a type in which the refrigerant flowing through the refrigerant return pipe  41  and the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  form counter-current flows. 
     In addition, the liquid-pipe temperature sensor  49  is provided in a portion of the outdoor liquid-refrigerant pipe  34  between the outlet of the refrigerant cooler  45  and the portion to which the liquid injection pipe  46  is connected so as to detect the temperature of the refrigerant at the outlet of the refrigerant cooler  45  as the liquid-pipe temperature Tlp. 
     —Control Unit— 
     The control unit  19  is connected to control boards or the like (not illustrated) that are provided in the outdoor unit  2  and the indoor units  3   a  and  3   b  for communication. In  FIG. 8 , the control unit  19  is illustrated at a position separated from the outdoor unit  2  and the indoor units  3   a  and  3   b  for the convenience of illustration. The control unit  19  controls the various components  21 ,  22 ,  24 ,  25 ,  26 ,  41 ,  47 ,  51   a ,  51   b ,  55   a , and  55   b  of the air conditioner  1  (in this case, the outdoor unit  2  and the indoor units  3   a  and  3   b ) on the basis of detection signals or the like of the above-described various sensors  36 ,  37 ,  38 ,  39 ,  40 ,  49 ,  57   a ,  57   b ,  58   a ,  58   b ,  59   a , and  59   b . That is, the control unit  19  controls the entire operation of the air conditioner  1 . 
     &lt;Operations and Features of Air Conditioner&gt; 
     Operations and features of the air conditioner  1  are described next with reference to  FIGS. 8 and 9 .  FIG. 9  is a pressure-enthalpy chart showing a refrigeration cycle during cooling operation in the air conditioner  1  according to the second embodiment of the present invention. 
     The air conditioner  1  performs cooling operation and heating operation as described above. During cooling operation, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26  provided in the outdoor liquid-refrigerant pipe  34 . Furthermore, during cooling operation, an operation is performed to cool the refrigerant in the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  using the refrigerant return pipe  41  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  and feeds the branched refrigerant to the compressor  21 , and the refrigerant cooler  45  that cools the refrigerant flowing through the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  with the refrigerant flowing through the refrigerant return pipe  41 . Further, during cooling operation, an operation is performed to feed the refrigerant to the compressor  21  while suppressing a variation in the temperature (liquid-pipe temperature Tlp) of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  using the liquid injection pipe  46  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  from the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  and feeds the branched refrigerant to the compressor  21 . Note that the control unit  19  that controls the components of the air conditioner  1  performs the operations of the air conditioner  1  which will be described below. 
     —Cooling Operation— 
     For cooling operation, for example, when all the indoor units  3   a  and  3   b  perform cooling operation (that is, operation in which all the indoor heat exchangers  52   a  and  52   b  function as evaporators of the refrigerant and the outdoor heat exchanger  23  functions as a radiator of the refrigerant), the switching mechanism  22  is switched to the outdoor radiation state (the state indicated by solid lines of the switching mechanism  22  in  FIG. 8 ), and the compressor  21 , the outdoor fan  24 , and the indoor fans  55   a  and  55   b  are driven. 
     Then, the refrigerant at a high pressure discharged from the compressor  21  is fed to the outdoor heat exchanger  23  via the switching mechanism  22  (see point B in  FIGS. 8 and 9 ). The refrigerant fed to the outdoor heat exchanger  23  exchanges heat with the outdoor air supplied by the outdoor fan  24 , and hence is cooled and condensed in the outdoor heat exchanger  23  that functions as the radiator of the refrigerant (see point C in  FIGS. 8 and 9 ). The refrigerant flows out from the outdoor unit  2  via the outdoor expansion valve  25 , the refrigerant cooler  45 , the liquid-pressure adjustment expansion valve  26 , and the liquid-side shutoff valve  27  (see point D in  FIGS. 8 and 9 ). 
     The refrigerant flowing out from the outdoor unit  2  is branched and fed to the indoor units  3   a  and  3   b  via the liquid-refrigerant connection pipe  5  (see point E in  FIGS. 8 and 9 ). The refrigerants fed to the indoor units  3   a  and  3   b  are decompressed to a low pressure by the indoor expansion valves  51   a  and  51   b  and then are fed to the indoor heat exchangers  52   a  and  52   b  (see point F in  FIGS. 8 and 9 ). The refrigerants fed to the indoor heat exchangers  52   a  and  52   b  exchange heat with the indoor air supplied from the inside of the rooms by the indoor fans  55   a  and  55   b , and hence are heated and evaporated in the indoor heat exchangers  52   a  and  52   b  that function as the evaporators of the refrigerant (see point G in  FIGS. 8 and 9 ). The refrigerants flow out from the indoor units  3   a  and  3   b . In contrast, the indoor air cooled in the indoor heat exchangers  52   a  and  52   b  is fed into the rooms, and thus the rooms are cooled. 
     The refrigerants flowing out from the indoor units  3   a  and  3   b  are joined and fed to the outdoor unit  2  via the gas-refrigerant connection pipe  6  (see point H in  FIGS. 8 and 9 ). The refrigerant fed to the outdoor unit  2  is sucked into the compressor  21  via the gas-side shutoff valve  28 , the switching mechanism  22 , and the accumulator  29  (see point A in  FIGS. 8 and 9 ). 
     During the above-described cooling operation, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26 . In addition, as described below, when the refrigerant is to be transported in the two-phase state, the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  is cooled using the refrigerant return pipe  41  and the refrigerant cooler  45 , and the refrigerant is properly transported in the two-phase state while suppressing an increase in the discharge temperature Td of the compressor  21  using the liquid injection pipe  46 . 
     First, the control unit  19  controls the liquid-pressure adjustment expansion valve  26  to decompress the refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe  5  is in the gas-liquid two-phase state (see points J and D in  FIGS. 8 and 9 ). The refrigerant decompressed by the liquid-pressure adjustment expansion valve  26  is a refrigerant at an intermediate pressure that is lower than the pressure of a refrigerant at a high pressure and is higher than the pressure of a refrigerant at a low pressure (see point D in  FIGS. 8 and 9 ). The control unit  19  controls the opening degree of the liquid-pressure adjustment expansion valve  26  so that the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  becomes the target degree of subcooling SCot. More specifically, the control unit  19  obtains the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  from the outdoor heat-exchanger outlet temperature Tol. The control unit  19  obtains the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  by subtracting the outdoor heat-exchanger outlet temperature Tol from the temperature Toc of the refrigerant obtained by converting the discharge pressure Pd into a saturation temperature. The control unit  19  performs control to increase the opening degree of the liquid-pressure adjustment expansion valve  26  if the degree of subcooling SCo is larger than the target degree of subcooling SCot, and performs control to decrease the opening degree of the liquid-pressure adjustment expansion valve  26  if the degree of subcooling SCo is smaller than the target degree of subcooling SCot. Note that, in this case, the control unit  19  performs control to fix the opening degree of the outdoor expansion valve  25  in a full-open state. 
     With this control, the state of the refrigerant flowing through the liquid-refrigerant connection pipe  5  becomes the gas-liquid two-phase state. The refrigerant connection pipe  5  is not filled with the refrigerant in a liquid state unlike a case where the refrigerant flowing through the liquid-refrigerant connection pipe  5  is in a liquid state. The amount of the refrigerant existing in the liquid-refrigerant connection pipe  5  can be decreased by that amount. 
     Furthermore, the control unit  19  controls the temperature of the refrigerant (liquid-pipe temperature Tlp) in the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  to be constant by cooling the refrigerant flowing through the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  by the refrigerant cooler  45  using the refrigerant flowing through the refrigerant return pipe  41 . The control unit  19  controls the opening degree of the refrigerant-return expansion valve  44  so that the temperature of the refrigerant (liquid-pipe temperature Tlp) in the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  becomes a target liquid-pipe temperature Tlpt. More specifically, the control unit  19  performs control to increase the opening degree of the refrigerant-return expansion valve  44  if the liquid-pipe temperature Tlp is higher than the target liquid-pipe temperature Tlpt, and performs control to decrease the opening degree of the refrigerant-return expansion valve  44  if the liquid-pipe temperature Tlp is lower than the target liquid-pipe temperature Tlpt. 
     With this control, the temperature of the refrigerant (liquid-pipe temperature Tlp) in the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  can be maintained to be constant at the target liquid-pipe temperature Tlpt (see point J in  FIGS. 8 and 9 ). 
     Furthermore, the control unit  19  branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  to suppress an increase in the discharge temperature Td of the compressor  21  and feeds the branched refrigerant to the compressor  21  (in this case, the suction refrigerant pipe  31  connected to the suction side of the compressor  21 ). The control unit  19  controls the opening degree of the liquid-injection expansion valve  47  so that the discharge temperature Td of the compressor  21  does not exceed a predetermined discharge temperature threshold Tdx (for example, upper-limit discharge temperature). More specifically, when the discharge temperature Td has increased above the discharge temperature threshold Tdx, the control unit  19  performs control to increase the opening degree of the liquid-injection expansion valve  47  until the discharge temperature Td becomes the discharge temperature threshold Tdx or lower. 
     With this control, the refrigerants fed from the indoor units  3   a  and  3   b  to the outdoor unit  2  (point H in  FIGS. 8 and 9 ) are joined to the refrigerant to be fed to the compressor  21  via the liquid injection pipe  46  and is cooled (see points H and A in  FIGS. 8 and 9 ). Thus, an increase in the discharge temperature Td of the compressor  21  can be suppressed by that cooled amount (see point B in  FIGS. 8 and 9 ). 
     —Heating Operation— 
     For heating operation, for example, when all the indoor units  3   a  and  3   b  perform heating operation (that is, operation in which all the indoor heat exchangers  52   a  and  52   b  function as radiators of the refrigerant and the outdoor heat exchanger  23  functions as an evaporator of the refrigerant), the switching mechanism  22  is switched to the outdoor evaporation state (the state indicated by broken lines of the switching mechanism  22  in  FIG. 8 ), and the compressor  21 , the outdoor fan  24 , and the indoor fans  55   a  and  55   b  are driven. 
     The refrigerant at a high pressure discharged from the compressor  21  flows out from the outdoor unit  2  via the switching mechanism  22  and the gas-side shutoff valve  28 . 
     The refrigerant flowing out from the outdoor unit  2  is branched and fed to the indoor units  3   a  and  3   b  via the gas-refrigerant connection pipe  6 . The refrigerants fed to the indoor units  3   a  and  3   b  are fed to the indoor heat exchangers  52   a  and  52   b . The refrigerants at a high pressure fed to the indoor heat exchangers  52   a  and  52   b  exchange heat with the indoor air supplied from the inside of the rooms by the indoor fans  55   a  and  55   b , and hence are cooled and condensed in the indoor heat exchangers  52   a  and  52   b  that function as the radiators of the refrigerant. The refrigerants flow out from the indoor units  3   a  and  3   b  via the indoor expansion valves  51   a  and  51   b . In contrast, the indoor air heated in the indoor heat exchangers  52   a  and  52   b  is fed into the rooms, and thus the rooms are heated. 
     The refrigerants flowing out from the indoor units  3   a  and  3   b  are joined and fed to the outdoor unit  2  via the liquid-refrigerant connection pipe  5 . The refrigerant fed to the outdoor unit  2  is fed to the outdoor expansion valve  25  via the liquid-side shutoff valve  27 , the liquid-pressure adjustment expansion valve  26 , and the refrigerant cooler  45 . The refrigerant fed to the outdoor expansion valve  25  is decompressed to a low pressure by the outdoor expansion valve  25  and then is fed to the outdoor heat exchanger  23 . The refrigerant fed to the outdoor heat exchanger  23  exchanges heat with the outdoor air supplied by the outdoor fan  24 , and hence is heated and evaporated. The refrigerant is sucked into the compressor  21  via the switching mechanism  22  and the accumulator  29 . 
     In this case, the control unit  19  performs control to fix the opening degree of the liquid-pressure adjustment expansion valve  26  in a full-open state. With this control, the opening degrees of the refrigerant-return expansion valve  44  and the liquid-injection expansion valve  47  are in a full-closed state so as not to send the refrigerant to the refrigerant return pipe  41  and the liquid injection pipe  46 . 
     —Features— 
     In the configuration in which the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a  and  3   b  using the liquid-pressure adjustment expansion valve  26 , as described above, the refrigerant cooler  45  is further provided that cools the refrigerant flowing through the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  using the refrigerant return pipe  41  and the refrigerant flowing through the refrigerant return pipe  41 . 
     If the liquid injection pipe  46  is not provided and the refrigerant return pipe  41  and the refrigerant cooler  45  are provided, the refrigerant flowing through the refrigerant return pipe  41  cools the refrigerant flowing through the refrigerant cooler  45  and then is fed to the compressor  21 , and hence an increase in the discharge temperature Td of the compressor  21  can be suppressed. However, since the refrigerant flowing through the refrigerant return pipe  41  is fed to the compressor  21  after cooling the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  in the refrigerant cooler  45 , the temperature of the refrigerant (liquid-pipe temperature Tlp) flowing through the outdoor liquid-refrigerant pipe  34  after the refrigerant has passed through the refrigerant cooler  45  varies depending on the flow rate of the refrigerant flowing through the refrigerant return pipe  41 . Consequently, the state of the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  also varies. For example, if the flow rate of the refrigerant flowing through the refrigerant return pipe  41  excessively increases, an increase in the discharge temperature Td of the compressor  21  can be suppressed to a certain extent; however, the liquid-pipe temperature Tlp excessively decreases. Consequently, the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  is in a gas-liquid two-phase state containing more liquid component. 
     That is, in the configuration having the liquid-pressure adjustment expansion valve  26 , merely providing the refrigerant return pipe  41  and the refrigerant cooler  45  may not maintain a desirable gas-liquid two-phase state. It is difficult to properly transport the refrigerant in the two-phase state while an increase in the discharge temperature Td of the compressor  21  is suppressed. 
     Owing to this, the liquid injection pipe  46  is provided in addition to the refrigerant return pipe  41  and the refrigerant cooler  45 . Since the liquid injection pipe  46  is provided, the refrigerant can be fed to the compressor  21  while a variation in the liquid-pipe temperature Tlp is suppressed (see point J in  FIG. 9 ). Thus, an increase in the discharge temperature Td of the compressor  21  can be suppressed without increasing the flow rate of the refrigerant flowing through the refrigerant return pipe  41 . If the flow rate of the refrigerant flowing through the refrigerant return pipe  41  is not excessively large, the liquid-pipe temperature Tlp does not excessively decrease. Consequently, the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  does not become the refrigerant in the gas-liquid two-phase state containing more liquid component. Thus, the refrigerant flowing through the liquid-refrigerant connection pipe  5  can be maintained in a desirable gas-liquid two-phase state (see point D in  FIG. 9 ) while an increase in the discharge temperature Td of the compressor  21  is suppressed (see point B in  FIG. 9 ). 
     That is, in the configuration having the liquid-pressure adjustment expansion valve  26 , since the liquid injection pipe  46  is provided in addition to the refrigerant return pipe  41  and the refrigerant cooler  45 , the refrigerant can be properly transported in the two-phase state while an increase in the discharge temperature Td of the compressor  21  is suppressed. 
     In addition, the control unit  19  controls the opening degree of the liquid-pressure adjustment expansion valve  26  so that the degree of subcooling SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger  23  becomes the target degree of subcooling SCot, and thus the refrigerant flowing through the liquid-refrigerant connection pipe  5  is decompressed using the liquid-pressure adjustment expansion valve  26  to be in the gas-liquid two-phase state. With such control, the holding refrigerant amount of the outdoor heat exchanger  23  can be easily maintained in a desirable state (see point C in  FIG. 9 ), and consequently, the state of the refrigerant to be fed to the refrigerant cooler  45  can be stable. 
     In addition, the liquid-injection expansion valve  47  that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe  34  is provided in the liquid injection pipe  46 . The refrigerant-return expansion valve  44  that decompresses the refrigerant branched from the outdoor liquid-refrigerant pipe  34  is provided in the refrigerant return pipe  41 . The control unit  19  controls the opening degree of the liquid-injection expansion valve  47  so that the discharge temperature Td of the compressor  21  does not exceed the predetermined discharge temperature threshold Tdx (for example, upper-limit discharge temperature), and controls the opening degree of the refrigerant-return expansion valve  44  so that the temperature of the refrigerant (liquid-pipe temperature Tlp) in the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  becomes the target liquid-pipe temperature Tlpt. Since the flow rate of the refrigerant that is fed from the outdoor liquid-refrigerant pipe  34  to the compressor  21  can be adjusted through the liquid injection pipe  46  by controlling the opening degree of the liquid-injection expansion valve  47  provided in the liquid injection pipe  46 , an increase in the discharge temperature Td of the compressor  21  can be reliably suppressed (see point B in  FIG. 9 ). In addition, since the flow rate of the refrigerant that exchanges heat with the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  can be adjusted in the refrigerant cooler  45  by controlling the opening degree of the refrigerant-return expansion valve  44  provided in the refrigerant return pipe  41 , the liquid-pipe temperature Tlp can be constant at the target liquid-pipe temperature Tlpt (see point J in  FIG. 9 ). Since the liquid-pipe temperature Tlp is constant, the refrigerant flowing through the liquid-refrigerant connection pipe  5  after the refrigerant has been decompressed by the liquid-pressure adjustment expansion valve  26  can be reliably maintained in a desirable gas-liquid two-phase state (see point D in  FIG. 9 ). When the refrigerant is transported in the two-phase state using the liquid-pressure adjustment expansion valve  26 , the refrigerant return pipe  41  and the refrigerant cooler  45  are used to maintain the liquid-pipe temperature Tlp to be constant, and the liquid injection pipe  46  is used to suppress an increase in the discharge temperature Td of the compressor  21 . 
     In addition, the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the suction refrigerant pipe  31  through which the refrigerant to be sucked into the compressor  21  flows. Thus, since the refrigerant branched from the outdoor liquid-refrigerant pipe  34  can be fed to the suction side of the compressor  21 , the temperature of the refrigerant to be sucked into the compressor  21  can be decreased (see points H and A in  FIG. 9 ). In this case, the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , and hence the refrigerant flowing through the liquid injection pipe  46  can be joined to the refrigerant to be sucked into the compressor  21  without via the accumulator  29 . Thus, the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21  can be increased as compared with a case where the liquid injection pipe  46  and/or the refrigerant return pipe  41  is connected to the inlet side of the accumulator  29 . 
     &lt;First Modification&gt; 
     In the air conditioner  1  of the above-described second embodiment (see  FIG. 8 ), the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , and hence the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21  is increased. However, the connection positions of the liquid injection pipe  46  and the refrigerant return pipe  41  to the suction refrigerant pipe  31  are not limited thereto. 
     As illustrated in  FIG. 10 , the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29 , and hence the refrigerant flowing through the liquid injection pipe  46  can be joined to the refrigerant to be sucked into the compressor  21  via the accumulator  29 . Thus, for example, occurrence of liquid compression in the compressor  21  can be prevented as compared with a case where the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the outlet side of the accumulator  29 . In this configuration, the return liquid pipe  31   a  that feeds the refrigerant from the bottom portion of the accumulator  29  to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29  may be provided, and the control unit  19  may control the return liquid valve  31   b  provided in the return liquid pipe  31   a  to return the liquid refrigerant stored in the accumulator  29  to the compressor  21 . 
     Although not illustrated, the liquid injection pipe  46  may be connected to a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , and the refrigerant return pipe  41  may be connected to a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29 . Alternatively, the liquid injection pipe  46  may be connected to a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29 , and the refrigerant return pipe  41  may be connected to a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . 
     &lt;Second Modification&gt; 
     In the air conditioner  1  of the second embodiment and the first modification (see  FIGS. 8 and 10 ), the liquid injection pipe  46  and/or the refrigerant return pipe  41  is connected to the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  or the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . However, the connection position of the liquid injection pipe  46  and/or the refrigerant return pipe  41  to the suction refrigerant pipe  31  is not limited thereto. 
     As illustrated in  FIG. 11 , the liquid injection pipe  46  is bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . One of the branched pipes of the liquid injection pipe  46  connected to the portion on the outlet side of the accumulator  29  is referred to as the first liquid injection pipe  46   a , and the other of the branched pipes of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29  is referred to as the second liquid injection pipe  46   b . Since the liquid injection pipe  46  is connected to both the inlet side and the outlet side of the accumulator  29 , the refrigerant flowing through the liquid injection pipe  46  may be fed to the outlet side of the accumulator  29  to increase the effect of decreasing the temperature of the refrigerant to be sucked into the compressor  21 , and the refrigerant flowing through the liquid injection pipe  46  may be fed to the inlet side of the accumulator  29 , for example, to prevent occurrence of liquid compression in the compressor  21 . As illustrated in  FIG. 12 , the refrigerant return pipe  41  may be bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and a portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 . One of the branched pipes of the refrigerant return pipe  41  connected to the portion on the outlet side of the accumulator  29  is referred to as the first refrigerant return pipe  41   a , and the other of the branched pipes of the refrigerant return pipe  41  connected to the portion on the inlet side of the accumulator  29  is referred to as the second refrigerant return pipe  41   b.    
     In addition, by using the configuration in which the liquid injection pipe  46  in  FIG. 11  is bifurcated and connected to both the portions of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and on the outlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. More specifically, a liquid relief valve  46   d  is provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  46   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx (for example, upper-limit discharge pressure). More specifically, when the discharge pressure Pd has increased above the discharge pressure threshold Pdx, the control unit  19  performs control to open the liquid relief valve  46   d  until the discharge pressure Pd becomes the discharge pressure threshold Pdx or lower. Thus, the liquid refrigerant existing in the outdoor heat exchanger  23  can be fed to and retracted in the accumulator  29  via the liquid injection pipe  46 . Consequently, an increase in the discharge pressure Pd can be suppressed. Since the refrigerant can be fed from the outdoor liquid-refrigerant pipe  34  to the accumulator  29  via the liquid injection pipe  46  by controlling the liquid relief valve  46   d  provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. In this configuration, a capillary tube  46   c  serving as a flow resistance is provided in the first liquid injection pipe  46   a  so as to send the refrigerant to the second liquid injection pipe  46   b  more during the liquid relief control. In addition, by using the configuration in which the refrigerant return pipe  41  in  FIG. 12  is bifurcated and connected to both the portions of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and on the outlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. More specifically, similarly to the liquid injection pipe  46 , a liquid relief valve  41   d  is provided in the second refrigerant return pipe  41   b  of the refrigerant return pipe  41  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  41   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx. Even in this configuration, a capillary tube  41   c  serving as a flow resistance may be provided in the first refrigerant return pipe  41   a  so as to send the refrigerant to the second refrigerant return pipe  41   b  more during the liquid relief control. 
     &lt;Third Modification&gt; 
     In the air conditioner  1  of the second embodiment (see  FIG. 8 ), since the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the suction refrigerant pipe  31 , the refrigerant branched from the outdoor liquid-refrigerant pipe  34  is fed to the suction side of the compressor  21  to decrease the temperature of the refrigerant to be sucked into the compressor  21  (see points H and A in  FIG. 2 ) and thus suppress an increase in the discharge temperature Td of the compressor  21 . However, the feeding destination of the refrigerant flowing through the liquid injection pipe  46  and the refrigerant return pipe  41  to the compressor  21  is not limited thereto. 
     As illustrated in  FIG. 13 , the refrigerant return pipe  41  may be connected to an intermediate portion of the compression stroke of the compressor  21 . 
     The configuration differs from the second embodiment in that, as illustrated in  FIGS. 13 and 14 , the refrigerant, which has been branched from the outdoor liquid-refrigerant pipe  34  to the refrigerant return pipe  41 , is fed to the intermediate portion of the compression stroke of the compressor  21 , and the temperature of the refrigerant, which has been compressed to the intermediate pressure in the compressor  21 , is decreased (see point K in  FIG. 14 ). Even in this case, the control and so forth on the liquid-pressure adjustment expansion valve  26  for transporting the refrigerant in the two-phase state is similar to that of the second embodiment, and hence the description is omitted here. 
     As illustrated in  FIG. 15 , the liquid injection pipe  46  may be connected to an intermediate portion of a compression stroke of the compressor  21 . 
     With the configuration, as illustrated in  FIGS. 15 and 16 , unlike the second embodiment (see points H and A in  FIG. 9 ), an increase in the discharge temperature Td of the compressor  21  can be suppressed by feeding the refrigerant, which has been branched from the outdoor liquid-refrigerant pipe  34  to the liquid injection pipe  46 , to the intermediate portion of the compression stroke of the compressor  21  and decreasing the temperature of the refrigerant, which has been compressed to an intermediate pressure in the compressor  21  (see point L in  FIG. 16 ). Even in this case, the control and so forth on the liquid-pressure adjustment expansion valve  26  for transporting the refrigerant in the two-phase state is similar to that of the second embodiment, and hence the description is omitted here. 
     Furthermore, although not illustrated, the refrigerant, which has been branched from the outdoor liquid-refrigerant pipe  34  to the liquid injection pipe  46 , and the refrigerant, which has been branched from the outdoor liquid-refrigerant pipe  34  to the refrigerant return pipe  41 , both may be fed to the intermediate portion of the compression stroke of the compressor  21  and the temperature of the refrigerant, which has been compressed to the intermediate pressure in the compressor  21 , may be decreased similarly to  FIG. 16 . 
     &lt;Fourth Modification&gt; 
     Also in the air conditioner  1  of the third modification (see  FIG. 13 ) of the second embodiment, similarly to the second modification (see  FIG. 11 ), as illustrated in  FIG. 17 , the liquid injection pipe  46  may be bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and an intermediate portion of the compression stroke of the compressor  21 . One of the branched pipes of the liquid injection pipe  46  connected to the intermediate portion of the compression stroke of the compressor  21  is referred to as the first liquid injection pipe  46   a , and the other of the branched pipes of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29  is referred to as the second liquid injection pipe  46   b . Since the liquid injection pipe  46  is connected to both the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and the intermediate portion of the compression stroke of the compressor  21 , the refrigerant flowing through the liquid injection pipe  46  can be fed to the intermediate portion of the compression stroke of the compressor  21  to decrease the temperature of the refrigerant compressed to the intermediate pressure in the compressor  21 , and the refrigerant flowing through the liquid injection pipe  46  can be fed to the inlet side of the accumulator, for example, to prevent occurrence of liquid compression in the compressor  21 . In addition, as illustrated in  FIG. 18 , the refrigerant return pipe  41  may be bifurcated and connected to both a portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and an intermediate portion of the compression stroke of the compressor  21 . One of the branched pipes of the refrigerant return pipe  41  connected to the portion on the outlet side of the accumulator  29  is referred to as the first refrigerant return pipe  41   a , and the other of the branched pipes of the refrigerant return pipe  41  connected to the portion on the inlet side of the accumulator  29  is referred to as the second refrigerant return pipe  41   b.    
     In addition, by using the configuration in which the liquid injection pipe  46  in  FIG. 17  is bifurcated and connected to both the portions of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and the intermediate portion of the compression stroke of the compressor  21 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. More specifically, a liquid relief valve  46   d  is provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  46   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx (for example, upper-limit discharge pressure). More specifically, when the discharge pressure Pd has increased above the discharge pressure threshold Pdx, the control unit  19  performs control to open the liquid relief valve  46   d  until the discharge pressure Pd becomes the discharge pressure threshold Pdx or lower. Thus, the liquid refrigerant existing in the outdoor heat exchanger  23  can be fed to and retracted in the accumulator  29  via the liquid injection pipe  46 . Consequently, an increase in the discharge pressure Pd can be suppressed. Since the refrigerant can be fed from the outdoor liquid-refrigerant pipe  34  to the accumulator  29  via the liquid injection pipe  46  by controlling the liquid relief valve  46   d  provided in the second liquid injection pipe  46   b  of the liquid injection pipe  46  connected to the portion on the inlet side of the accumulator  29 , an increase in the discharge pressure Pd of the compressor  21  can be suppressed. In addition, by using the configuration in which the refrigerant return pipe  41  in  FIG. 18  is bifurcated and connected to both the portion of the suction refrigerant pipe  31  on the inlet side of the accumulator  29  and the intermediate portion of the compression stroke of the compressor  21 , an increase in the discharge pressure Pd of the compressor  21  may be suppressed. More specifically, similarly to the liquid injection pipe  46 , a liquid relief valve  41   d  is provided in the second refrigerant return pipe  41   b  of the refrigerant return pipe  41  connected to the portion on the inlet side of the accumulator  29 . The liquid relief valve  41   d  is controlled so that the discharge pressure Pd of the compressor  21  does not exceed a predetermined discharge pressure threshold Pdx. 
     (3) Third Embodiment 
     &lt;Configuration&gt; 
       FIG. 19  is a schematic configuration diagram of an air conditioner  1  according to a third embodiment of the present invention. The air conditioner  1  is an apparatus that performs cooling and heating in a room of a building or the like through a vapor compression refrigeration cycle. The air conditioner  1  mainly includes an outdoor unit  2 , a plurality of (in this case, four) indoor units  3   a ,  3   b ,  3   c , and  3   d  that are mutually connected in parallel, relay units  4   a ,  4   b ,  4   c , and  4   d  that are connected to the indoor units  3   a ,  3   b ,  3   c , and  3   d , respectively, refrigerant connection pipes  5  and  6  that connect the outdoor unit  2  to the indoor units  3   a ,  3   b ,  3   c , and  3   d  via the relay units  4   a ,  4   b ,  4   c , and  4   d , and a control unit  19  that controls components of the outdoor unit  2 , the indoor units  3   a ,  3   b ,  3   c , and  3   d , and the relay units  4   a ,  4   b ,  4   c , and  4   d . A vapor compression refrigerant circuit  10  of the air conditioner  1  is defined by connecting the outdoor unit  2 , the indoor units  3   a ,  3   b ,  3   c , and  3   d , the relay units  4   a ,  4   b ,  4   c , and  4   d , and the refrigerant connection pipes  5  and  6  to one another. The refrigerant circuit  10  is filled with a refrigerant such as R32. The air conditioner  1  allows the indoor units  3   a ,  3   b ,  3   c , and  3   d  to individually perform cooling operation or heating operation via the relay units  4   a ,  4   b ,  4   c , and  4   d . By feeding the refrigerant from the indoor unit that performs heating operation to the indoor unit that performs cooling operation, heat can be recovered among the indoor units (in this case, cooling operation and heating operation can be simultaneously performed, i.e., cooling and heating simultaneous operation can be performed). 
     —Connection Pipes— 
     The liquid-refrigerant connection pipe  5  mainly includes a joint pipe portion extending from the outdoor unit  2 , first branch pipe portions  5   a ,  5   b ,  5   c , and  5   d  that are a plurality of (in this case, four) branched pipe portions branched before the relay units  4   a ,  4   b ,  4   c , and  4   d , and second branch pipe portions  5   aa ,  5   bb ,  5   cc , and  5   dd  that connect the relay units  4   a ,  4   b ,  4   c , and  4   d  to the indoor units  3   a ,  3   b ,  3   c , and  3   d . The gas-refrigerant connection pipe  6  mainly includes a high-and-low-pressure gas-refrigerant connection pipe  7 , a low-pressure gas-refrigerant connection pipe  8 , and branch pipe portions  6   a ,  6   b ,  6   c , and  6   d  that connect the relay units  4   a ,  4   b ,  4   c , and  4   d  to the indoor units  3   a ,  3   b ,  3   c , and  3   d . The high-and-low-pressure gas-refrigerant connection pipe  7  includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  7   a ,  7   b ,  7   c , and  7   d  that are a plurality of (in this case, four) branched pipe portions branched before the relay units  4   a ,  4   b ,  4   c , and  4   d . The low-pressure gas-refrigerant connection pipe  8  includes a joint pipe portion extending from the outdoor unit  2  and branch pipe portions  8   a ,  8   b ,  8   c , and  8   d  that are a plurality of (in this case, four) branched pipe portions branched before the relay units  4   a ,  4   b ,  4   c , and  4   d.    
     —Indoor Units— 
     The indoor units  3   a ,  3   b ,  3   c , and  3   d  are installed in rooms of the building or the like. The indoor units  3   a ,  3   b ,  3   c , and  3   d  are connected to the outdoor unit  2  via the liquid-refrigerant connection pipe  5 , the gas-refrigerant connection pipe  6  (the high-and-low-pressure gas-refrigerant connection pipe  7 , the low-pressure gas-refrigerant connection pipe  8 , and the branch pipe portions  6   a ,  6   b ,  6   c , and  6   d ), and the relay units  4   a ,  4   b ,  4   c , and  4   d ; and constitute part of the refrigerant circuit  10  as described above. 
     The configurations of the indoor units  3   a ,  3   b ,  3   c , and  3   d  are similar to the configurations of the indoor units  3   a  and  3   b  of the first and second embodiments, and the description is omitted here. 
     —Relay Units— 
     The relay units  4   a ,  4   b ,  4   c , and  4   d  are installed in the rooms of the building or the like together with the indoor units  3   a ,  3   b ,  3   c , and  3   d . The relay units  4   a ,  4   b ,  4   c , and  4   d , together with the liquid-refrigerant connection pipe  5  and the gas-refrigerant connection pipe  6  (the high-and-low-pressure gas-refrigerant connection pipe  7 , the low-pressure gas-refrigerant connection pipe  8 , and the branch pipe portions  6   a ,  6   b ,  6   c , and  6   d ), are arranged between the indoor units  3   a ,  3   b ,  3   c , and  3   d  and the outdoor unit  2 ; and constitute part of the refrigerant circuit  10 . 
     Configurations of the relay units  4   a ,  4   b ,  4   c , and  4   d  are described next. Since the configuration of the relay units  4   a  is similar to the configurations of the relay units  4   b ,  4   c , and  4   d , only the configuration of the relay unit  4   a  is described here. For each of the configurations of the relay units  4   b ,  4   c , and  4   d , a letter “b”, “c”, or “d” is applied to each component of the relay unit  4   b ,  4   c , or  4   d  instead of a letter “a” indicative of each component of the relay unit  4   a , and the description of each component of the relay unit  4   b ,  4   c , or  4   d  is omitted. 
     The relay unit  4   a  mainly includes a liquid connection pipe  61   a  and a gas connection pipe  62   a.    
     The liquid connection pipe  61   a  has one end that is connected to the first branch pipe portion  5   a  of the liquid-refrigerant connection pipe  5 , and the other end that is connected to the second branch pipe portion  5   aa  of the liquid-refrigerant connection pipe  5 . 
     The gas connection pipe  62   a  includes a high-pressure gas connection pipe  63   a  that is connected to the branch pipe portion  7   a  of the high-and-low-pressure gas-refrigerant connection pipe  7 , a low-pressure gas connection pipe  64   a  that is connected to the branch pipe portion  8   a  of the low-pressure gas-refrigerant connection pipe  8 , and a joint gas connection pipe  65   a  that joins the high-pressure gas connection pipe  63   a  and the low-pressure gas connection pipe  64   a  to each other. The joint gas connection pipe  65   a  is connected to the branch pipe portion  6   a  of the gas-refrigerant connection pipe  6 . The high-pressure gas connection pipe  63   a  is provided with a high-pressure gas valve  66   a . The low-pressure gas connection pipe  64   a  is provided with a low-pressure gas valve  67   a . The high-pressure gas valve  66   a  and the low-pressure gas valve  67   a  are electrically powered expansion valves. 
     When the indoor unit  3   a  performs cooling operation, the relay unit  4   a  can function to open the low-pressure gas valve  67   a , thereby allowing the refrigerant to flow into the liquid connection pipe  61   a  via the first branch pipe portion  5   a  of the liquid-refrigerant connection pipe  5 , to feed the flowing-in refrigerant to the indoor unit  3   a  via the second branch pipe portion  5   aa  of the liquid-refrigerant connection pipe  5 , and then to return the refrigerant evaporated through heat exchange with the indoor air in the indoor heat exchanger  52   a  to the branch pipe portion  8   a  of the low-pressure gas-refrigerant connection pipe  8  via the branch pipe portion  6   a  of the gas-refrigerant connection pipe  6 , the joint gas connection pipe  65   a , and the low-pressure gas connection pipe  64   a . When the indoor unit  3   a  performs heating operation, the relay unit  4   a  can function to close the low-pressure gas valve  67   a  and open the high-pressure gas valve  66   a , thereby allowing the refrigerant to flow into the high-pressure gas connection pipe  63   a  and the joint gas connection pipe  65   a  via the branch pipe portion  7   a  of the high-and-low-pressure gas-refrigerant connection pipe  7 , to feed the flowing-in refrigerant to the indoor unit  3   a  via the branch pipe portion  6   a  of the gas-refrigerant connection pipe  6 , and then to return the refrigerant radiated through heat exchange with the indoor air in the indoor heat exchanger  52   a  to the first branch pipe portion  5   a  of the liquid-refrigerant connection pipe  5  via the second branch pipe portion  5   aa  of the liquid-refrigerant connection pipe  5 , and the liquid connection pipe  61   a . The functions of the relay units  4   a ,  4   b , and  4   c  are similar to the function of the relay unit  4   a . The relay units  4   a ,  4   b ,  4   c , and  4   d  can individually switch the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  between evaporators or radiators of the refrigerant. 
     —Outdoor Unit— 
     The outdoor unit  2  is installed outside the rooms of the building or the like. The outdoor unit  2  is connected to the indoor units  3   a ,  3   b ,  3   c , and  3   d  via the liquid-refrigerant connection pipe  5 , the gas-refrigerant connection pipe  6  (the high-and-low-pressure gas-refrigerant connection pipe  7 , the low-pressure gas-refrigerant connection pipe  8 , and the branch pipe portions  6   a ,  6   b ,  6   c , and  6   d ), and the relay units  4   a ,  4   b ,  4   c , and  4   d ; and constitutes part of the refrigerant circuit  10  as described above. 
     The outdoor unit  2  mainly includes a compressor  21  and a plurality of (in this case, two) outdoor heat exchangers  23   a  and  23   b . The outdoor unit  2  also includes switching mechanisms  22   a  and  22   b  that each switch between a radiation operation state in which each of the outdoor heat exchangers  23   a  and  23   b  functions as a radiator of the refrigerant and an evaporation operation state in which each of the outdoor heat exchangers  23   a  and  23   b  functions as an evaporator of the refrigerant. The switching mechanisms  22   a  and  22   b  and a suction side of the compressor  21  are connected by a suction refrigerant pipe  31 . The suction refrigerant pipe  31  is provided with an accumulator  29  that temporarily stores the refrigerant to be sucked into the compressor  21 . A discharge side of the compressor  21  and the switching mechanisms  22   a  and  22   b  are connected by a discharge refrigerant pipe  32 . The switching mechanism  22   a  and gas-side ends of the outdoor heat exchangers  23   a  and  23   b  are connected by first outdoor gas-refrigerant pipes  33   a  and  33   b . Liquid-side ends of the outdoor heat exchangers  23   a  and  23   b  and the liquid-refrigerant connection pipe  5  are connected by an outdoor liquid-refrigerant pipe  34 . A liquid-side shutoff valve  27  is provided at a connection portion between the outdoor liquid-refrigerant pipe  34  and the liquid-refrigerant connection pipe  5 . In addition, the outdoor unit  2  includes a third switching mechanism  22   c  that switches between a refrigerant outflow state in which the refrigerant, which has been discharged from the compressor  21 , to the high-and-low-pressure gas-refrigerant connection pipe  7 , and a refrigerant inflow state in which the refrigerant flowing through the high-and-low-pressure gas-refrigerant connection pipe  7  to the suction refrigerant pipe  31 . The third switching mechanism  22   c  and the high-and-low-pressure gas-refrigerant connection pipe  7  are connected by a second outdoor gas-refrigerant pipe  35 . The third switching mechanism  22   c  and the suction side of the compressor  21  are connected by the suction refrigerant pipe  31 . The discharge side of the compressor  21  and the third switching mechanism  22   c  are connected by the discharge refrigerant pipe  32 . A high-and-low-pressure gas-side shutoff valve  28   a  is provided at a connection portion between the second outdoor gas-refrigerant pipe  35  and the high-and-low-pressure gas-refrigerant connection pipe  7 . The suction refrigerant pipe  31  is connected to the low-pressure gas-refrigerant connection pipe  8 . A low-pressure gas-side shutoff valve  28   b  is provided at a connection portion between the suction refrigerant pipe  31  and the low-pressure gas-refrigerant connection pipe  8 . The liquid-side shutoff valve  27  and the gas-side shutoff valves  28   a  and  28   b  are valves that are manually opened and closed. 
     The compressor  21  is a device that compresses a refrigerant. For example, a hermetic compressor in which a rotary or scroll positive-displacement compression element (not illustrated) is rotationally driven by a compressor motor  21   a  is used. 
     The first switching mechanism  22   a  is a device capable of switching the flow of the refrigerant in the refrigerant circuit  10  to connect the discharge side of the compressor  21  and the gas side of the first outdoor heat exchanger  23   a  (see solid lines of the first switching mechanism  22   a  in  FIG. 19 ) when the first outdoor heat exchanger  23   a  functions as a radiator of the refrigerant (referred to as “outdoor radiation state” hereinafter) and to connect the suction side of the compressor  21  and the gas side of the first outdoor heat exchanger  23   a  (see broken lines of the first switching mechanism  22   a  in  FIG. 19 ) when the first outdoor heat exchanger  23   a  functions as an evaporator of the refrigerant (referred to as “outdoor evaporation state” hereinafter). The first switching mechanism  22   a  is, for example, a four-way switching valve. The second switching mechanism  22   b  is a device capable of switching the flow of the refrigerant in the refrigerant circuit  10  to connect the discharge side of the compressor  21  and the gas side of the second outdoor heat exchanger  23   b  (see solid lines of the second switching mechanism  22   b  in  FIG. 19 ) when the second outdoor heat exchanger  23   b  functions as a radiator of the refrigerant (referred to as “outdoor radiation state” hereinafter) and to connect the suction side of the compressor  21  and the gas side of the second outdoor heat exchanger  23   b  (see broken lines of the second switching mechanism  22   b  in  FIG. 19 ) when the second outdoor heat exchanger  23   b  functions as an evaporator of the refrigerant (referred to as “outdoor evaporation state” hereinafter). The second switching mechanism  22   b  is, for example, a four-way switching valve. By changing the switching states of the switching mechanisms  22   a  and  22   b , the functions of the outdoor heat exchangers  23   a  and  23   b  can be individually switched between evaporators or radiators of the refrigerant. 
     The first outdoor heat exchanger  23   a  is a heat exchanger that functions as a radiator of the refrigerant or an evaporator of the refrigerant. The second outdoor heat exchanger  23   b  is a heat exchanger that functions as a radiator of the refrigerant or an evaporator of the refrigerant. The outdoor unit  2  includes an outdoor fan  24  that sucks the outdoor air into the outdoor unit  2 , that allows the outdoor air to exchange heat with the refrigerant in the outdoor heat exchangers  23   a  and  23   b , and then that discharges the outdoor air to the outside. That is, the outdoor unit  2  includes the outdoor fan  24  as a fan that supplies the outdoor air, which serves as a cooling source or a heating source of the refrigerant flowing in the outdoor heat exchangers  23   a  and  23   b , to the outdoor heat exchangers  23   a  and  23   b . The outdoor fan  24  is driven by an outdoor fan motor  24   a.    
     The third switching mechanism  22   c  is a device capable of switching the flow of the refrigerant in the refrigerant circuit  10  to connect the discharge side of the compressor  21  and the high-and-low-pressure gas-refrigerant connection pipe  7  (see broken lines of the third switching mechanism  22   c  in  FIG. 19 ) when the refrigerant discharged from the compressor  21  is fed to the high-and-low-pressure gas-refrigerant connection pipe  7  (referred to as “refrigerant outflow state” hereinafter) and to connect the suction side of the compressor  21  and the high-and-low-pressure gas-refrigerant connection pipe  7  (see solid lines of the third switching mechanism  22   c  in  FIG. 19 ) when the refrigerant flowing through the high-and-low-pressure gas-refrigerant connection pipe  7  is fed to the suction refrigerant pipe  31  (referred to as “refrigerant inflow state” hereinafter). The third switching mechanism  22   c  is, for example, a four-way switching valve. 
     Focusing only on the compressor  21 , the outdoor heat exchangers  23   a  and  23   b , the liquid-refrigerant connection pipe  5 , and the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d , the air conditioner  1  performs an operation (cooling only operation and cooling main operation) of sending the refrigerant, which has been discharged from the compressor  21 , to the outdoor heat exchangers  23   a  and  23   b , the liquid-refrigerant connection pipe  5 , and the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  in that order. In this case, cooling only operation represents an operation state in which only an indoor heat exchanger that functions as an evaporator of the refrigerant exists, and cooling main operation represents a state in which both an indoor heat exchanger that functions as an evaporator of the refrigerant and an indoor heat exchanger that functions as a radiator of the refrigerant exist; however, a load on the evaporation side is relatively large as a whole. Focusing only on the compressor  21 , the gas-refrigerant connection pipe  6 , the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d , the liquid-refrigerant connection pipe  5 , and the outdoor heat exchangers  23   a  and  23   b , the air conditioner  1  performs an operation (heating only operation and heating main operation) of sending the refrigerant, which has been discharged from the compressor  21 , to the gas-refrigerant connection pipe  6 , the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d , the liquid-refrigerant connection pipe  5 , and the outdoor heat exchangers  23   a  and  23   b  in that order. In this case, heating only operation represents an operation state in which only an indoor heat exchanger that functions as a radiator of the refrigerant exists, and heating main operation represents a state in which both an indoor heat exchanger that functions as an evaporator of the refrigerant and an indoor heat exchanger that functions as a radiator of the refrigerant exist; however, a load on the radiation side is relatively large as a whole. During cooling only operation and cooling main operation, at least one of the switching mechanisms  22   a  and  22   b  is switched to the outdoor radiation state and the outdoor heat exchangers  23   a  and  23   b  function as a radiator of the refrigerant as a whole. In this state, the refrigerant flows from the outdoor unit  2  to the indoor units  3   a ,  3   b ,  3   c , and  3   d  via the liquid-refrigerant connection pipe  5 . During heating only operation and heating main operation, at least one of the switching mechanisms  22   a  and  22   b  is switched to the outdoor evaporation state, the third switching mechanism  22   c  is switched to the refrigerant outflow state, and the outdoor heat exchangers  23   a  and  23   b  function as an evaporator of the refrigerant as a whole. In this state, the refrigerant flows from the indoor units  3   a ,  3   b ,  3   c , and  3   d  to the outdoor unit  2  via the liquid-refrigerant connection pipe  5 . 
     In addition, a liquid-pressure adjustment expansion valve  26 , a liquid injection pipe  46 , a refrigerant return pipe  41 , and a refrigerant cooler  45  are provided in the outdoor liquid-refrigerant pipe  34  similarly to the outdoor unit  2  of the second embodiment. The configurations of the liquid-pressure adjustment expansion valve  26 , the liquid injection pipe  46 , the refrigerant return pipe  41 , and the refrigerant cooler  45  are similar to those of the second embodiment, and the description is omitted here. 
     —Control Unit— 
     The control unit  19  is connected to control boards or the like (not illustrated) that are provided in the outdoor unit  2 , the indoor units  3   a ,  3   b ,  3   c , and  3   d , and the relay units  4   a ,  4   b ,  4   c , and  4   d  for communication. In  FIG. 19 , the control unit  19  is illustrated at a position separated from the outdoor unit  2 , the indoor units  3   a ,  3   b ,  3   c  and  3   d , and the relay units  4   a ,  4   b ,  4   c , and  4   d  for the convenience of illustration. The control unit  19  controls the various components  21 ,  22   a  to  22   c ,  24 ,  25   a ,  25   b ,  26 ,  41 ,  47 ,  51   a  to  51   d ,  55   a  to  55   d ,  66   a  to  66   d , and  67   a  to  67   d  of the air conditioner  1  (in this case, the outdoor unit  2 , the indoor units  3   a ,  3   b ,  3   c , and  3   d , and the relay units  4   a ,  4   b ,  4   c , and  4   d ) on the basis of detection signals or the like of the above-described various sensors  36 ,  37 ,  38 ,  39 ,  40 ,  49 ,  57   a  to  57   d ,  58   a  to  58   d , and  59   a  to  59   d . That is, the control unit  19  controls the entire operation of the air conditioner  1 . 
     &lt;Operations and Features of Air Conditioner&gt; 
     Operations and features of the air conditioner  1  are described next with reference to  FIGS. 19 and 9 . 
     The air conditioner  1  performs cooling only operation, cooling main operation, heating only operation, and heating main operation as described above. During cooling only operation and cooling main operation, similarly to the first and second embodiments, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a ,  3   b ,  3   c , and  3   d  using the liquid-pressure adjustment expansion valve  26  provided in the outdoor liquid-refrigerant pipe  34 . Furthermore, during cooling only operation and cooling main operation, an operation is performed to cool the refrigerant in a portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26  using the refrigerant return pipe  41  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  and feeds the branched refrigerant to the compressor  21 , and the refrigerant cooler  45  that cools the refrigerant flowing through a portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  with the refrigerant flowing through the refrigerant return pipe  41 . Further, during cooling only operation and cooling main operation, an operation is performed to feed the refrigerant to the compressor  21  while suppressing a variation in the temperature (liquid-pipe temperature Tlp) of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  using the liquid injection pipe  46  that branches part of the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  from the portion of the outdoor liquid-refrigerant pipe  34  on the side of the outdoor heat exchanger  23  with respect to the liquid-pressure adjustment expansion valve  26  and feeds the branched refrigerant to the compressor  21 . Note that the control unit  19  that controls the components of the air conditioner  1  performs the operations of the air conditioner  1 . Moreover, cooling only operation is representatively described below for an operation accompanied by the control on the liquid-pressure adjustment expansion valve  26  and so forth, and the description of cooling main operation is omitted. 
     For cooling only operation, for example, when all the indoor units  3   a ,  3   b ,  3   c , and  3   d  perform cooling operation (that is, operation in which all the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  function as evaporators of the refrigerant and the outdoor heat exchangers  23   a  and  23   b  function as radiators of the refrigerant), the switching mechanisms  22   a  and  22   b  are switched to the outdoor radiation states (the states indicated by solid lines of the switching mechanisms  22   a  and  22   b  in  FIG. 19 ), and the compressor  21 , the outdoor fan  24 , and the indoor fans  55   a  and  55   b  are driven. In addition, the third switching mechanism  22   c  is switched to the refrigerant inflow state (the state indicated by solid lines of the switching mechanism  22   c  in  FIG. 19 ), and the high-pressure gas valves  66   a ,  66   b ,  66   c , and  66   d  and the low-pressure gas valves  67   a ,  67   b ,  67   c , and  67   d  of the relay units  4   a ,  4   b ,  4   c , and  4   d  are opened. 
     Then, the refrigerant at a high pressure discharged from the compressor  21  is fed to the outdoor heat exchangers  23   a  and  23   b  via the switching mechanisms  22   a  and  22   b  (see point B in  FIGS. 19 and 9 ). The refrigerant fed to the outdoor heat exchangers  23   a  and  23   b  exchanges heat with the outdoor air supplied by the outdoor fan  24 , and hence is cooled and condensed in the outdoor heat exchangers  23   a  and  23   b  that function as the radiators of the refrigerant (see point C in  FIGS. 19 and 9 ). The refrigerant flows out from the outdoor unit  2  via the outdoor expansion valves  25   a  and  25   b , the refrigerant cooler  45 , the liquid-pressure adjustment expansion valve  26 , and the liquid-side shutoff valve  27  (see point D in  FIGS. 19 and 9 ). 
     The refrigerant flowing out from the outdoor unit  2  is branched and fed to the indoor units  3   a ,  3   b ,  3   c , and  3   d  via the liquid-refrigerant connection pipe  5  and the relay units  4   a ,  4   b ,  4   c , and  4   d  (see point E in  FIGS. 19 and 9 ). The refrigerants fed to the indoor units  3   a ,  3   b ,  3   c , and  3   d  are decompressed to a low pressure by the indoor expansion valves  51   a ,  51   b ,  51   c , and  51   d  and then are fed to the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  (see point F in  FIGS. 19 and 9 ). The refrigerants fed to the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  exchange heat with the indoor air supplied from the inside of the rooms by the indoor fans  55   a  and  55   b , and hence are heated and evaporated in the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  that function as the evaporators of the refrigerant (see point Gin  FIGS. 19 and 9 ). The refrigerants flow out from the indoor units  3   a ,  3   b ,  3   c , and  3   d . In contrast, the indoor air cooled in the indoor heat exchangers  52   a ,  52   b ,  52   c , and  52   d  is fed into the rooms, and thus the rooms are cooled. 
     The refrigerants flowing out from the indoor units  3   a ,  3   b ,  3   c , and  3   d  are joined and fed to the outdoor unit  2  via the gas-refrigerant connection pipe  6  and the relay units  4   a ,  4   b ,  4   c , and  4   d  (see point H in  FIGS. 19 and 9 ). The refrigerant fed to the outdoor unit  2  is sucked into the compressor  21  via the gas-side shutoff valve  28  and the accumulator  29  (see point A in  FIGS. 19 and 9 ). 
     During the above-described cooling only operation, the refrigerant is transported in the two-phase state by sending the refrigerant in the gas-liquid two-phase state to the liquid-refrigerant connection pipe  5  and feeding the refrigerant from the outdoor unit  2  to the indoor units  3   a ,  3   b ,  3   c , and  3   d  using the liquid-pressure adjustment expansion valve  26  similarly to cooling operation of the first and second embodiments. In addition, when the refrigerant is to be transported in the two-phase state, similarly to cooling operation of the second embodiment, the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  is cooled using the refrigerant return pipe  41  and the refrigerant cooler  45 , and the refrigerant is properly transported in the two-phase state while suppressing an increase in the discharge temperature Td of the compressor  21  using the liquid injection pipe  46 . The details of the operation are similar to the operation and control relating to the transport of the refrigerant in the two-phase state in cooling operation of the second embodiment, and hence the description is omitted here. The operation and control relating to the transport of the refrigerant in the two-phase state in cooling main operation are similar to those of cooling only operation. 
     &lt;Modifications&gt; 
     In the air conditioner  1  of the above-described third embodiment (see  FIG. 19 ), while the liquid injection pipe  46  and the refrigerant return pipe  41  are connected to the portion of the suction refrigerant pipe  31  on the outlet side of the accumulator  29 , the connection positions are not limited thereto. The connection positions of the liquid injection pipe  46  and the refrigerant return pipe  41  may be changed similarly to the second embodiment and the first to fourth modifications. 
     In the air conditioner  1  of the above-described third embodiment (see  FIG. 19 ), while the refrigerant return pipe  41  and the refrigerant cooler  45  are provided, it is not limited thereto. The refrigerant return pipe  41  and the refrigerant cooler  45  may be omitted similarly to the first embodiment and the first to fourth modifications. 
     (4) Other Embodiments 
     &lt;A&gt; 
     In the air conditioner  1  of the second and third embodiments and the modifications, while the liquid injection pipe  46  is connected to the portion of the outdoor liquid-refrigerant pipe  34  between the refrigerant cooler  45  and the liquid-pressure adjustment expansion valve  26 , it is not limited thereto. 
     For example, as illustrated in  FIG. 20 , the liquid injection pipe  46  may be connected to a position in the outdoor liquid-refrigerant pipe  34  close to the outdoor heat exchanger  23  with respect to the branch position of the refrigerant return pipe  41 . Alternatively, as illustrated in  FIG. 21 , the liquid injection pipe  46  may be connected to a position in the outdoor liquid-refrigerant pipe  34  between the branch position of the refrigerant return pipe  41  and the refrigerant cooler  45 . 
     &lt;B&gt; 
     In the air conditioner  1  of the second and third embodiments and the modifications, during cooling operation (including cooling only operation and cooling main operation), the refrigerant cooler  45  is a heat exchanger of a type in which the refrigerant flowing through the refrigerant return pipe  41  and the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  form counter-current flows, and the refrigerant return pipe  41  is branched from the outdoor liquid-refrigerant pipe  34  at the position located upstream of the refrigerant cooler  45 . However, it is not limited thereto. 
     For example, as illustrated in  FIG. 22 , during cooling operation (including cooling only operation and cooling main operation), the refrigerant cooler  45  may be a heat exchanger of a type in which the refrigerant flowing through the refrigerant return pipe  41  and the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  form parallel-current (co-current) flows, and the refrigerant return pipe  41  may be branched from the outdoor liquid-refrigerant pipe  34  at a position located upstream of the refrigerant cooler  45 . Alternatively, for example, as illustrated in  FIG. 23 , during cooling operation (including cooling only operation and cooling main operation), the refrigerant cooler  45  may be a heat exchanger of a type in which the refrigerant flowing through the refrigerant return pipe  41  and the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  form counter-current flows, and the refrigerant return pipe  41  may be branched from the outdoor liquid-refrigerant pipe  34  at a position located downstream of the refrigerant cooler  45 . Still alternatively, for example, as illustrated in  FIG. 24 , during cooling operation (including cooling only operation and cooling main operation), the refrigerant cooler  45  may be a heat exchanger of a type in which the refrigerant flowing through the refrigerant return pipe  41  and the refrigerant flowing through the outdoor liquid-refrigerant pipe  34  form parallel-current (co-current) flows, and the refrigerant return pipe  41  may be branched from the outdoor liquid-refrigerant pipe  34  at a position located downstream of the refrigerant cooler  45 . 
     &lt;C&gt; 
     The air conditioner  1  of the first and second embodiments and the modifications can switch between cooling operation and heating operation; however, it is not limited thereto. An air conditioner dedicated for cooling that can perform only cooling operation may be provided. 
     INDUSTRIAL APPLICABILITY 
     The present invention is widely applicable to air conditioners each including an outdoor unit having a compressor and an outdoor heat exchanger, a plurality of indoor units having an indoor heat exchanger, and a liquid-refrigerant connection pipe that connects the outdoor unit to the plurality of indoor units, in which a liquid-pressure adjustment expansion valve that decompresses a refrigerant so that the refrigerant flowing through the liquid-refrigerant connection pipe is in a gas-liquid two-phase state is provided in an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger to the liquid-refrigerant connection pipe. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  air conditioner 
               2  outdoor unit 
               3   a ,  3   b ,  3   c ,  3   d  indoor unit 
               5  liquid-refrigerant connection pipe 
               19  control unit 
               21  compressor 
               23 ,  23   a ,  23   b  outdoor heat exchanger 
               26  liquid-pressure adjustment expansion valve 
               29  accumulator 
               31  suction refrigerant pipe 
               34  outdoor liquid-refrigerant pipe 
               41  refrigerant return pipe 
               41   d  liquid relief valve 
               44  refrigerant-return expansion valve 
               45  refrigerant cooler 
               46  liquid injection pipe 
               46   d  liquid relief valve 
               47  liquid-injection expansion valve 
               52   a ,  52   b ,  52   c ,  52   d  indoor heat exchanger 
           
         
       
    
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: International Publication No. 2015/029160