Patent Publication Number: US-2022214055-A1

Title: Outdoor unit and air-conditioning apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Application No. PCT/JP2019/027278 filed on Jul. 10, 2019, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an outdoor unit and an air-conditioning apparatus, and particularly to an outdoor unit and an air-conditioning apparatus in which an inlet and an outlet are unchanged regardless of on which of operations is performed. 
     BACKGROUND 
     An air-conditioning apparatus includes, for example, a refrigerant circuit in which an outdoor unit that is a heat source unit and installed outside a building and an indoor unit installed in the building are connected by pipes. In the refrigerant circuit, refrigerant is circulated to heat or cool air by transferring or receiving heat to or from the air, thereby heating or cooling an air-conditioned space that is a load. 
     An existing air-conditioning apparatus includes an outdoor unit and a relay unit that are connected by two pipes, and is capable of performing a cooling and heating mixed operation (for example, see Patent Literature 1). In this air-conditioning apparatus, check valves that are backflow prevention devices are provided at a plurality of refrigerant pipes in the outdoor unit. Thus, in both a cooling operation and a heating operation, a pipe through which refrigerant flows out of the outdoor unit is used as a dedicated outflow pipe and a pipe through which the refrigerant flows into the outdoor unit is used as a dedicated inflow pipe; that is, the outflow pipe and the inflow pipe are not interchanged. Thus, the air-conditioning apparatus can achieve a stable operation. 
     PATENT LITERATURE 
     Patent Literature 1: Japanese Patent No. 2757584 
     In an air-conditioning apparatus disclosed in Patent Literature 1, however, low-temperature and low-pressure liquid refrigerant and gas refrigerant both pass through an inflow pipe. Thus, in particular, in the cooling operation, when the gas refrigerant passes through a backflow prevention device, a pressure loss increases, thus reducing a cooling performance. 
     SUMMARY 
     The present disclosure is applied to solve the above problem, and relates to an outdoor unit and an air-conditioning apparatus that can achieve a stable operation without reducing the performance. 
     An outdoor unit according to an embodiment of the present disclosure includes: a compressor that sucks refrigerant, compresses the sucked refrigerant, and discharges the compressed refrigerant; a first refrigerant flow switching device that switches a flow passage for he refrigerant between a flow passage for a cooling operation and a flow passage for a heating operation; a heat-source-side heat exchanger that causes heat exchange to be performed between the refrigerant and external fluid; a heat-source-side backflow prevention device and a connection pipe that are included in a flow passage for the refrigerant in which an outlet from which the refrigerant flows out to an outside region and an inlet into which the refrigerant flows from the outside region are unchanged regardless of which of the cooling operation and the heating operation is performed; and a flow passage pipe through which part of the refrigerant having flowed from the inlet passes in the cooling operation.  
     Furthermore, an air-conditioning apparatus according to another embodiment of the present disclosure includes the above outdoor unit and an indoor unit that receives heat transferred from the outdoor unit and conditions air in an air-conditioned space. 
     In the outdoor unit according to the embodiment of the present disclosure, because of provision of the flow passage pipe that allows, in the cooling operation, part of refrigerant having flowed from the inlet to pass, the amount of gas refrigerant or two-phase gas-liquid refrigerant that passes through the heat-source-side backflow prevention device can be reduced, and a pressure loss of the refrigerant can be reduced. Furthermore, since a larger number of flow passages through which the refrigerant passes are provided, the pressure loss of the refrigerant can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration of an air-conditioning apparatus  100  according to Embodiment 1. 
         FIG. 2  is an explanatory view for the flow of refrigerant in a cooling only operation mode of the air-conditioning apparatus  100  according to Embodiment 1. 
         FIG. 3  is an explanatory view for the flow of refrigerant in a cooling main operation mode of the air-conditioning apparatus  100  according to Embodiment 1. 
         FIG. 4  is an explanatory view for the flow of refrigerant in a heating only operation mode of the air-conditioning apparatus  100  according to Embodiment 1. 
         FIG. 5  is an explanatory view for the flow of refrigerant in a heating main operation mode of the air-conditioning apparatus  100  according to Embodiment 1. 
         FIG. 6  illustrates a configuration of the air-conditioning apparatus  100  according to Embodiment 2. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described with reference to the drawings. In each of figures that will be referred to, components which are the same as or equivalent to those in a previous figure or figures are denoted by the same reference signs. The same is true of the entire text of the “Description of Embodiments” section. Also, in the entire text, the configurations of components are described by way of example; that is, the configuration of the components are not limited to those described in the specification. In particular, in the case where components are combined, it is not limited to the case where components according to the same embodiment are combined. A component in an embodiment can be applied to another embodiment as appropriate. Also, the levels of temperature, pressure, etc., are not determined in relation to absolute values, that is, they are relatively determined in accordance with the state and operation of a system or an apparatus, for example. 
     In addition, with respect to a plurality of devices that are of the same type and distinguished from each other by suffixes, in the case where they do not particularly need to be identified or distinguished from each other, the suffixes may be omitted. Additionally, in some cases, the relationship between sizes of components in the figures may differ from the relationship between the actual sizes of the components. 
     Embodiment 1 
     &lt;Configuration of Air-Conditioning Apparatus  100 &gt; 
       FIG. 1  illustrates a configuration of an air-conditioning apparatus  100  according to Embodiment 1. As illustrated in  FIG. 1 , the air-conditioning apparatus  100  includes a single outdoor unit  1  that serves as a heat source unit, four indoor units  2  that are an indoor unit  2   a , an indoor unit  2   b , an indoor unit  2   c , and an indoor unit  2   d , and a relay unit  3  provided between the outdoor unit  1  and the indoor units  2   a  to  2   d . In the air-conditioning apparatus  100  according to Embodiment 1, the outdoor unit  1 , the indoor units  2 , and the relay unit  3  are connected by pipes to form a refrigerant circuit. It should be noted that although  FIG. 1  illustrates four indoor units  2  that are the indoor units  2   a  to  2   d , the number of indoor units  2  connected together may be two, three, or five or more.  
     The outdoor unit  1  and the relay unit  3  are connected by an outflow pipe  5   b  and an inflow pipe  5   a  that are two pipes through which refrigerant passes. Furthermore, the relay unit  3  and each of the indoor units  2   a  to  2   d  are connected by branch pipes  8   a  and  8   b  through which refrigerant flows. Cooling energy or heating energy generated in the outdoor unit  1  is supplied to the indoor units  2   a  to  2   d  via the relay unit  3 . Each of the indoor units  2   a  to  2   d  can select and perform a cooling operation or a heating operation. 
     The outflow pipe  5   b  and the inflow pipe  5   a  are pipes that connect the outdoor unit  1  and the relay unit  3 . In particular, in the outdoor unit  1 , the inflow pipe  5   a  is connected to an inlet  1   a  of the outdoor unit  1 , and the outflow pipe  5   b  is connected to an outlet  1   b  of the outdoor unit  1 . The outflow pipe  5   b  is a high-pressure side pipe through which high-pressure refrigerant flows out from the outlet  1   b  of the outdoor unit  1 . The inflow pipe  5   a  is a low-pressure side pipe into which low-pressure refrigerant whose pressure is lower than the pressure of refrigerant in the outflow pipe  5   b  flows via the inlet  1   a  of the outdoor unit  1 . The relay unit  3  and each of the indoor units  2   a  to  2   d  are connected by two branch pipes  8   a  and  8   b . Thus, the outdoor unit  1  and the relay unit  3  are connected by using two pipes, and the relay unit  3  and each of the indoor units  2   a  to  2   d  are also connected by two pipes, whereby the air-conditioning apparatus  100  can be easily installed. 
     &lt;Configuration of Outdoor Unit  1 &gt; 
     The outdoor unit  1  is a heat source unit that generates heat to be supplied to a load. The outdoor unit  1  includes a compressor  10 , refrigerant flow switching devices  11 , heat-source-side heat exchangers  12 , heat-source-side expansion devices  17 , a heat-source-side fan  18 , and an accumulator  19  that are all housed in a housing of the outdoor unit  1 . The compressor  10 , the refrigerant flow switching devices  11 , the heat-source-side heat exchangers  12 , the heat-source-side expansion devices  17 , and the accumulator  19  are connected by refrigerant pipes  4 . The compressor  10  sucks refrigerant, compresses the refrigerant to change into high-temperature and high-pressure refrigerant, and then discharges the high-temperature and high-pressure refrigerant. The compressor  10  may be, for example, an inverter compressor whose capacity can be controlled. 
     As described later, the refrigerant flow switching devices  11  each switch the flow of refrigerant in the refrigerant circuit between the flow of refrigerant in the refrigerant circuit in the heating operation, for example, in a heating only operation mode or a heating main operation mode and the flow of refrigerant in the refrigerant circuit in the cooling operation, for example, in a cooling only operation mode or a cooling main operation mode. During the heating operation, gas refrigerant flows out of the outdoor unit  1 , and liquid refrigerant or two-phase gas-liquid refrigerant flows into the outdoor unit  1 . Furthermore, during the cooling operation, liquid refrigerant or two-phase gas-liquid refrigerant flows out of the outdoor unit  1 , and gas refrigerant or two-phase gas-liquid refrigerant flows into the outdoor unit  1 . It should be noted that the outdoor unit  1  according to Embodiment 1 includes three refrigerant flow switching devices  11  that are a first refrigerant flow switching device  11   a , a first refrigerant flow switching device  11   b , and a second refrigerant flow switching device  11   c . The refrigerant flow switching devices  11  according to Embodiment 1 are four-way valves. The refrigerant flow switching devices  11  will be described in detail later. 
     The heat-source-side heat exchangers  12  causes heat exchange to be performed between refrigerant and outside air, that is, outdoor air that is supplied from the heat-source-side fan  18  which will be described later. The heat-source-side heat exchangers  12  each operate as an evaporator in the heating operation to cause the refrigerant to absorb heat. Furthermore, the heat-source-side heat exchangers  12  each operate as a condenser or a radiator in the cooling operation to cause refrigerant to transfer heat. Although the heat-source-side heat exchangers  12  causes heat exchange to be performed between the refrigerant and outside air, the heat-source-side heat exchangers  12  may cause heat exchange to be performed between the refrigerant and other external fluid. In the air-conditioning apparatus  100  according to Embodiment 1, the outdoor unit  1  includes two heat-source-side heat exchangers  12   a  and  12   b  that are connected in parallel by pipes. The heat-source-side expansion devices  17  adjust the flow rate and pressure of refrigerant that passes through the heat-source-side heat exchangers  12 . In the air-conditioning apparatus  100  according to Embodiment 1, the outdoor unit  1  includes heat-source-side expansion devices  17   b  and  17   a  that are provided in association with the two heat-source-side heat exchangers  12   a  and  12   b , respectively. The heat-source-side fan  18  makes a flow of air along which outside air is supplied to the heat-source-side heat exchangers  12 . 
     The accumulator  19  is provided on a suction side of the compressor  10 . The accumulator  19  stores surplus refrigerant the amount of which corresponds to the difference between the amount of refrigerant that flows during the heating operation and the amount of refrigerant that flows during the cooling operation, or the amount of which corresponds to the difference between the amount of refrigerant that flows after a transient change of the operation and the amount of refrigerant that flows before the transient change of the operation. 
     Furthermore, the outdoor unit  1  according to Embodiment 1 includes a first connection pipe  6 , a second connection pipe  7 , a heat-source-side backflow prevention device  13 , a heat-source-side backflow prevention device  14 , heat-source-side backflow prevention devices  15 , and a heat-source-side backflow prevention device  16 . Additionally, the outdoor unit  1  according to Embodiment 1 includes a flow passage pipe  9  that allows part of refrigerant that has flowed therein from the inflow pipe  5   a  to flow to the accumulator  19  provided on the suction side of the compressor  10 . The outdoor unit  1  includes the first connection pipe  6 , the second connection pipe  7 , and the heat-source-side backflow prevention devices  13  to  16 , thereby enabling refrigerant to flow out from the outdoor unit  1  to the outflow pipe  5   b  and to flow from the inflow pipe  5   a  into the outdoor unit  1 , regardless of which of operations, such as heating and cooling operations, is performed. Therefore, the outlet  1   b  and the inlet  1   a  in the outdoor unit  1  are not changed depending on which of operations is performed; that is, the outlet  1   b  and the inlet  1   a  in the outdoor unit  1  are used as a dedicated outlet and a dedicated inlet, respectively. Although the heat-source-side backflow prevention devices  13  to  16  will be described as check valves, the heat-source-side backflow prevention devices  13  to  16  may be on-off valves or other valves. 
     In the cooling only operation mode and the cooling main operation mode, the heat-source-side backflow prevention device  13  allows refrigerant from a heat-source-side heat exchanger  12  to flow to the outflow pipe  5   b . Furthermore, in the heating only operation mode and the heating main operation mode, the heat-source-side backflow prevention device  13  prevents the backflow of refrigerant in the second connection pipe  7 , that is, prevents the refrigerant in the second connection pipe  7  from flowing toward the heat-source-side heat exchanger  12 . In the heating only operation mode and the heating main operation mode, the heat-source-side backflow prevention device  14  allows refrigerant in the inflow pipe  5   a  to flow toward the heat-source-side heat exchanger  12 . Furthermore, in the cooling only operation mode and the cooling main operation mode, the heat-source-side backflow prevention device  14  prevents the backflow of refrigerant in the first connection pipe  6 , that is, prevents the refrigerant in the first connection pipe  6  from flowing toward to the inflow pipe  5   a . In the heating only operation mode and the heating main operation mode, the heat-source-side backflow prevention device  16  allows refrigerant in a flow passage on a discharge side of the compressor  10  to flow toward the outflow pipe  5   b . Furthermore, in the cooling only operation mode and the cooling main operation mode, the heat-source-side backflow prevention device  16  prevents the backflow of refrigerant in the second connection pipe  7 , that is, prevents the refrigerant in the second connection pipe  7  from flowing toward the accumulator  19 .  
     In the cooling only operation mode and the cooling main operation mode, the heat-source-side backflow prevention devices  15  according to Embodiment 1 allows refrigerant in the inflow pipe  5   a  to flow toward the accumulator  19  provided on the suction side of the compressor  10  via the refrigerant flow switching devices  11 . The air-conditioning apparatus  100  according to Embodiment 1 includes, as the heat-source-side backflow prevention devices  15 , a heat-source-side backflow prevention device  15   a , a heat-source-side backflow prevention device  15   b , and a heat-source-side backflow prevention device  15   c . In particular, it should be noted that the heat-source-side backflow prevention device  15   b  is a flow passage pipe backflow prevention device. In the air-conditioning apparatus  100  according to Embodiment 1, flow passages are provided through which refrigerant having flowed from the inflow pipe  5   a  into the outdoor unit  1  passes through the heat-source-side backflow prevention device  15   a  and the heat-source-side backflow prevention device  15   b  and flows to the accumulator  19  in the cooling only operation mode and the cooling main operation mode. Furthermore, the refrigerant that has passed through the heat-source-side backflow prevention device  15   a  further branches into refrigerant streams. In the air-conditioning apparatus  100  according to Embodiment 1, a flow passage is provided through which one of the refrigerant streams passes through the heat-source-side backflow prevention device  15   c  and flows to the accumulator  19 . 
     As described above, the outdoor unit  1  includes the refrigerant flow switching devices  11  each of which switches a refrigerant flow passage therein between a plurality of refrigerant flow passages, depending on which of the operations is performed. The first refrigerant flow switching device  11   a  switches, in the cooling operation, a refrigerant flow passage therein to a refrigerant flow passage through which refrigerant discharged from the compressor  10  flows to the heat-source-side heat exchanger  12   a  and refrigerant having flowed from the inflow pipe  5   a  flows to the accumulator  19 . Furthermore, in the heating operation, the first refrigerant flow switching device  11  a switches the flow passage for the refrigerant to a flow passage through which the refrigerant discharged from the compressor  10  flows to the outflow pipe  5   b  and the refrigerant having flowed from the inflow pipe  5   a  passes through the heat-source-side heat exchanger  12   a  and flows to the accumulator  19 . The first refrigerant flow switching device  11   b  switches, in the cooling operation, a refrigerant flow passage therein to a refrigerant flow passage through which the refrigerant discharged from the compressor  10  flows to the heat-source-side heat exchanger  12   b  and the refrigerant in the inflow pipe  5   a  flows to the accumulator  19 . Furthermore, the first refrigerant flow switching device  11   b  switches, in the heating operation, the flow passage to a flow passage through which refrigerant discharged from the compressor  10  does not flow and the refrigerant having flowed from the inflow pipe  5   a  flows through the heat-source-side heat exchanger  12   b  and flows to the accumulator  19 . The second refrigerant flow switching device  11   c  is provided at the flow passage pipe  9 . The second refrigerant flow switching device  11   c  switches, in the cooling operation a refrigerant flow passage therein to a refrigerant flow passage through which the flow of the refrigerant discharged from the compressor  10  is prevented by the heat-source-side backflow prevention device  15   a  and the refrigerant having flowed from the inflow pipe  5   a  flows to the accumulator  19 . Furthermore, the second refrigerant flow switching device  11   c  inhibits, in the heating operation, refrigerant from flowing. It should be noted that flow passages in the refrigerant flow switching devices  11  are switched by power supplied by a controller  50  and a differential pressure between a high-pressure side and a low-pressure side in the refrigerant circuit. Therefore, in the air-conditioning apparatus  100  according to Embodiment 1, the second refrigerant flow switching device  11   c  and the discharge side of the compressor  10  are connected by pipes, whereby a high pressure of the refrigerant discharged from the compressor  10  is transferred to the second refrigerant flow switching device  11 c. 
     In the air-conditioning apparatus  100  according to Embodiment 1, in the cooling only operation and the cooling main operation, refrigerant that has flowed from the inflow pipe  5   a  flows through three flow passages connected to the accumulator  19  and flows to the accumulator  19  via the respective refrigerant flow switching devices  11 . In the cooling only operation and the cooling main operation, the refrigerant that flows from the inflow pipe  5   a  is low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant. Therefore, when the refrigerant passes through the heat-source-side backflow prevention devices  15 , a pressure loss increases. Thus, in the air-conditioning apparatus  100  according to Embodiment 1, in the cooling operation, low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant is made to branch off to flow through the plurality of heat-source-side backflow prevention devices  15 , whereby the air-conditioning apparatus  100  according to Embodiment 1 reduces the pressure loss caused by the heat-source-side backflow prevention devices  15  as a whole. Furthermore, in the air-conditioning apparatus  100  according to Embodiment 1, by increasing the number of flow passages that extend from the inflow pipe  5   a  to the accumulator  19 , the total cross-sectional area of all the flow passages is increased, thereby reducing the pressure loss of the refrigerant. 
     The outdoor unit  1  includes a discharge temperature sensor  43 , a discharge pressure sensor  40 , and an outside-air temperature sensor  46 . The discharge temperature sensor  43  detects the temperature of refrigerant discharged from the compressor  10  and outputs a discharge temperature detection signal. The discharge pressure sensor  40  detects the pressure of the refrigerant discharged by the compressor  10  and outputs a discharge pressure detection signal. The outside-air temperature sensor  46  detects, for example, an outside air temperature that is an ambient temperature of the outdoor unit  1 , and outputs an outside-air temperature detection signal. In the outdoor unit  1 , the outside-air temperature sensor  46  is provided at air-inflow part of the heat-source-side heat exchangers  12  into which air flows. 
     Furthermore, the outdoor unit  1  includes the controller  50  that controls various devices. The controller  50  is a device that controls each of the devices in the air-conditioning apparatus  100  and performs the entire air-conditioning apparatus  100 . The controller  50  is, for example, an analog circuit, a digital circuit, a CPU, or a combination of two or more of these elements. The controller  50  controls various devices or various apparatuses, for example, based on data on a physical quantity detected by each of the above sensors and an instruction from an input device, such as a remote control unit, and causes the air-conditioning apparatus  100  to operate in each of operation modes which will be described later. Although  FIG. 1  illustrates the case where the controller  50  is provided in the outdoor unit  1 , it is not limiting. In the outdoor unit  1 , the relay unit  3 , and the indoor units  2 , respective controllers  50  may be provided. Furthermore, in the indoor units  2 , respective controllers  50  may be provided. 
     &lt;Configuration of Relay Unit  3 &gt; 
     The relay unit  3  transfers heat supplied from the outdoor unit  1  that is a heat source unit to another unit. The relay unit  3  includes a gas-liquid separator  29 , a first relay expansion device  30 , and a second relay expansion device  27 . Furthermore, the relay unit  3  includes a plurality of first opening and closing devices  23   a  to  23   d , a plurality of second opening and closing devices  24   a  to  24   d , a plurality of relay-side first backflow prevention devices  21   a  to  21   d , and a plurality of relay-side second backflow prevention devices  22   a  to  22   d.    
     In a cooling and heating mixed operation in which an air-cooling load is high, the gas-liquid separator  29  separates high-pressure two-phase gas-liquid refrigerant generated in the outdoor unit  1  into liquid refrigerant and gas refrigerant. The gas-liquid separator  29  causes the liquid refrigerant to flow into a pipe on a lower side of the figure to supply cooling energy to one or more of the indoor units  2 , and also causes the gas refrigerant to flow into a pipe on an upper side of the figure to supply heating energy to another one or others of the indoor units  2 . The gas-liquid separator  29  is provided at an inlet portion of the relay unit  3  in the flow of refrigerant.  
     The first relay expansion device  30  has functions of a pressure reducing valve and an on-off valve. The first relay expansion device  30  reduces the pressure of liquid refrigerant to a predetermined pressure and also opens and closes a flow passage for the liquid refrigerant. The opening degree of the first relay expansion device  30  can be adjusted, for example, continuously or by stages. As the first relay expansion device  30 , for example, an electronic expansion valve is used. The first relay expansion device  30  is provided in at a pipe that allows the liquid refrigerant to flow out of the gas-liquid separator  29 . 
     The second relay expansion device  27  has functions of a pressure reducing valve and an on-off valve. In the heating only operation, the second relay expansion device  27  opens a refrigerant flow passage therein to adjust the flow rate of the refrigerant. In the heating main operation, the second relay expansion device  27  adjusts the flow rate of liquid refrigerant that flows through a bypass, depending on an indoor-side load. The opening degree of the second relay expansion device  27  can be adjusted, for example, continuously or by stages. As the second relay expansion device  27 , for example, an electronic expansion valve is used. 
     The first opening and closing devices  23   a  to  23   d  are provided for the indoor units  2   a  to  2   d , respectively. The first opening and closing devices  23   a  to  23   d  open and close respective flow passages for high-temperature and high-pressure gas refrigerant that is supplied to the respective indoor units  2   a  to  2   d . The first opening and closing devices  23   a  to  23   d  are, for example, solenoid valves or other valves. The first opening and closing devices  23   a  to  23   d  are each connected to a gas-side pipe of the gas-liquid separator  29 . It should be noted that the first opening and closing devices  23   a  to  23   d  have only to open and close respective flow passages, and may each be an expansion device having a function of fully closing the flow passage.  
     The second opening and closing devices  24   a  to  24   d  are provided for the indoor units  2   a  to  2   d , respectively. The second opening and closing devices  24   a  to  24   d  open and close respective flow passages for low-pressure and low-temperature gas refrigerant that has flowed out of the respective indoor units  2   a  to  2   d . The second opening and closing devices  24   a  to  24   d  are, for example, solenoid valves or other valves. The second opening and closing devices  24   a  to  24   d  are connected to respective low-pressure pipes that extend to an outlet side of the relay unit  3 . It should be noted that the second opening and closing devices  24   a  to  24   d  have only to open and close respective flow passages, and may each be an expansion device having a function of fully closing the flow passage. 
     The relay-side first backflow prevention devices  21   a  to  21   d  are provided for the indoor units  2   a  to  2   d , respectively. Each of the relay-side first backflow prevention devices  21   a  to  21   d  allows high-pressure liquid refrigerant to flow into an associated one of the indoor units  2  when the associated indoor unit  2  is in the cooling operation. The relay-side first backflow prevention devices  21   a  to  21   d  are connected to a pipe on an outlet side of the first relay expansion device  30 . In the cooling main operation mode and the heating main operation mode, each of the relay-side first backflow prevention devices  21   a  to  21   d  prevents intermediate-temperature and intermediate-pressure liquid refrigerant or two-phase gas-liquid refrigerant that flows out of the load-side expansion device  25  of an associated one of the indoor units  2  when the associated indoor unit  2  is in the heating operation, from flowing into a load-side expansion device  25  of another one or others of the indoor units  2  that are in the cooling operation. As the relay-side first backflow prevention devices  21   a  to  21   d , for example, check valves are used. The relay-side first backflow prevention devices  21   a  to  21   d  have only to prevent the backflow of refrigerant, and, for example, an opening and closing device, or an expansion device having a function of fully closing the flow passage.  
     The relay-side second backflow prevention devices  22   a  to  22   d  are provided for the indoor units  2   a  to  2   d , respectively. Each of the relay-side second backflow prevention devices  22   a  to  22   d  allows low-pressure gas refrigerant to flow into the relay-side second backflow prevention device from an associated one of the indoor units  2  when the associated indoor unit  2  is in the heating operation. The relay-side second backflow prevention devices  22   a  to  22   d  are connected to a pipe on the outlet side of the first relay expansion device  30 . In the cooling main operation mode and the heating main operation mode, each of the relay-side second backflow prevention devices  22   a  to  22   d  inhibits intermediate-temperature and intermediate-pressure liquid or two-phase refrigerant that passes through the first relay expansion device  30 , from flowing into a load-side expansion device  25  of an associated one of the indoor units  2  when the associated indoor unit  2  is in the cooling operation. As the relay-side second backflow prevention devices  22   a  to  22   d , check valves are used. The relay-side second backflow prevention devices  22   a  to  22   d  have only to prevent the backflow of the refrigerant, and may be, for example, an opening and closing device or an expansion device having a function of fully closing the valve. 
     In the relay unit  3 , a first relay-expansion-device inlet-side pressure sensor  33  is provided on an inlet side of the first relay expansion device  30 . The first relay-expansion-device inlet-side pressure sensor  33  detects the pressure of high-pressure refrigerant. On the outlet side of the first relay expansion device  30 , a first relay-expansion-device outlet-side pressure sensor  34  is provided. The first relay-expansion-device outlet-side pressure sensor  34  detects, in the cooling main operation mode, an intermediate pressure of liquid refrigerant on the outlet side of the first relay expansion device  30 . 
     &lt;Configuration of Indoor Units  2   a  to  2   d&gt;   
     The indoor units  2  receive heat transferred from the heat source unit and condition air of an air-conditioned space that is a load. The indoor units  2   a  to  2   d  are included in the refrigerant circuit. The indoor units  2   a  to  2   d  have, for example, the same configuration. The indoor unit  2   a  includes a load-side heat exchanger  26   a  and a load-side expansion device  25   a ; the indoor unit  2   b  includes a load-side heat exchanger  26   b  and a load-side expansion device  25   b ; the indoor unit  2   c  includes a load-side heat exchanger  26   c  and a load-side expansion device  25   c ; and The indoor unit  2   d  includes a load-side heat exchanger  26   d  and a load-side expansion device  25   d . Each of the load-side heat exchangers  26   a  to  26   d  is connected, by associated branch pipes  8   a  and  8   b , to the relay unit  3  connected with a refrigerant pipe  4 . In each of the load-side heat exchangers  26   a  to  26   d , heat exchange is performed between air supplied by a load-side fan (not illustrated) and refrigerant, and air for cooling or heating that is to be supplied to an indoor space is generated. The opening degrees of the load-side expansion devices  25   a  to  25   d  can be adjusted, for example, continuously or by steps. As the load-side expansion devices  25   a  to  25   d , for example, electronic expansion valves are used. The load-side expansion devices  25   a  to  25   d  each have functions of a pressure reducing valve and an expansion valve. The load-side expansion devices  25   a  to  25   d  reduce the pressure of refrigerant to expand the refrigerant. In the flow of refrigerant in the cooling only operation mode, the load-side expansion devices  25   a  to  25   d  are provided upstream of the respective load-side heat exchangers  26   a  to  26   d.    
     The indoor units  2   a  to  2   d  include a plurality of inlet-side temperature sensors  31   a  to  31   d  that detect the temperatures of refrigerant that flow into the load-side heat exchangers  26   a  to  26   d , respectively. The indoor units  2   a  to  2   d  include a plurality of outlet-side temperature sensors  32   a  to  32   d  that detect the temperatures of refrigerant that has flowed out of the load-side heat exchangers  26   a  to  26   d , respectively. The inlet-side temperature sensors  31   a  to  31   d  and the outlet-side temperature sensors  32   a  to  32   d  are, for example, thermistors or other devices. Each of the inlet-side temperature sensors  31   a  to  31   d  and the outlet-side temperature sensors  32   a  to  32   d  outputs a detection signal to the controller  50 .  
     &lt;Cooling Only Operation Mode&gt; 
       FIG. 2  illustrates the flow of refrigerant in the cooling only operation mode of the air-conditioning apparatus  100  according to Embodiment 1. In  FIG. 2 , the flow directions of the refrigerant are indicated by solid arrows. It is assumed that a cooling load is generated in the load-side heat exchangers  26   a  and  26   b . In the cooling only operation mode, the controller  50 , for example, causes the first refrigerant flow switching devices  11   a  and  11   b  included in the outdoor unit  1  to switch the respective flow passage to flow passages through which refrigerant discharged by the compressor  10  flows into the heat-source-side heat exchangers  12 . Furthermore, the controller  50 , for example, causes the second refrigerant flow switching device  11   c  to switch the flow passage to a flow passage through which refrigerant having flowed from the inflow pipe  5   a  into the outdoor unit  1  flow into the accumulator  19  through the flow passage pipe  9 . 
     As illustrated in  FIG. 2 , low-temperature and low-pressure refrigerant is sucked into the compressor  10  and compressed by the compressor  10  to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is discharged from the compressor  10 . The high-temperature and high-pressure gas refrigerant discharged from the compressor  10  flows into the heat-source-side heat exchangers  12   a  and  12   b  via the first refrigerant flow switching devices  11   a  and  11   b . Then, the refrigerant that has flowed into the heat-source-side heat exchangers  12  changes into high-pressure liquid refrigerant while transferring heat to outdoor air. The high-pressure liquid refrigerant that has flowed out of the heat-source-side heat exchangers  12   a  and  12   b  passes through the heat-source-side expansion devices  17   a  and  17   b  and the heat-source-side backflow prevention device  13  and flows out of the outdoor unit  1 . The refrigerant that has flowed out of the outdoor unit  1  flows into the relay unit  3  through the outflow pipe  5 b. 
     The high-pressure liquid refrigerant that has flowed into the relay unit  3  passes through the gas-liquid separator  29  and the first relay expansion device  30 , and most of the refrigerant passes through the relay-side first backflow prevention devices  21   a  and  21   b  and the branch pipes  8   b  and is expanded by the load-side expansion devices  25   a  and  25   b  to change into low-temperature and low-pressure two-phase gas-liquid refrigerant. Remaining part of the high-pressure refrigerant is expanded by the second relay expansion device  27  to change into low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant. Then, the low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant flows into the low-pressure pipe on the outlet side of the relay unit  3 . At this time, the opening degree of the second relay expansion device  27  is controlled to cause the degree of subcooling of the refrigerant to be constant. 
     The two-phase gas-liquid refrigerant expanded by the load-side expansion devices  25   a  and  25   b  flows into the respective load-side heat exchangers  26   a  and  26   b  each of which operates as an evaporator, and receives heat from indoor air to cool the indoor air, thereby changing into low-temperature and low-pressure gas refrigerant. At this time, the opening degree of the load-side expansion device  25   a  is controlled to cause superheat that is the degree of superheat obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   a  and a temperature detected by the outlet-side temperature sensor  32   a  to be constant. Similarly, the opening degree of the load-side expansion device  25   b  is controlled to cause superheat obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   b  and a temperature detected by the outlet-side temperature sensor  32   b  to be constant. 
     The gas refrigerant that has flowed out of the load-side heat exchangers  26   a  and  26   b  passes through the respective branch pipes  8   a  and the respective second opening and closing devices  24   a  and  24   b  and flows out of the relay unit  3 . The refrigerant that has flowed out of the relay unit  3  re-flows into the outdoor unit  1  though the inflow pipe  5   a.    
     The refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  branches off to pass through the heat-source-side backflow prevention devices  15   a  and  15   b . The refrigerant that has passed through the heat-source-side backflow prevention device  15   b  passes through the second refrigerant flow switching device  11   c  and flows into the accumulator  19 . Furthermore, the refrigerant that has passed through the heat-source-side backflow prevention device  15   a  further branches into refrigerant streams that are divided refrigerant. One of the refrigerant streams, i.e., the divided refrigerant, passes through the first refrigerant flow switching device  11   a  and flows into the accumulator  19 , and the other passes through the heat-source-side backflow prevention device  15   c  and the first refrigerant flow switching device  11   b  and flows into the accumulator  19 . The refrigerant that has passed through the accumulator  19  is re-sucked into the compressor  10 . 
     In the case where no heating load is generated in the load-side heat exchanger  26   c  or  26   d , refrigerant does not need to be made to flow therein, and the load-side expansion device  25   c  and  25   d  which are associated with the load-side heat exchangers  26   c  and  26   d , respectively, are in a closed state. By contrast, in the case where a cooling load is generated in the load-side heat exchanger  26   c  or  26   d , the load-side expansion device  25   c  or  25   d  is opened, and the refrigerant is circulated. At this time, the opening degree of the load-side expansion device  25   c  or the load-side expansion device  25   d  is controlled in the same manner as in the load-side expansion device  25   a  or  25   b . Also, superheat obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   c  or  31   d  and a temperature detected by the outlet-side temperature sensor  32   c  or  32   d  is caused to be constant. 
     &lt;Cooling Main Operation Mode&gt; 
       FIG. 3  illustrates the flow direction of refrigerant in the cooling main operation mode of the air-conditioning apparatus  100  according to Embodiment 1. In  FIG. 3 , flow directions of refrigerant are indicated by solid arrows. It is assumed that a cooling load is generated in the load-side heat exchanger  26   a  and a heating load is generated in the load-side heat exchanger  26   b . In the cooling main operation mode, the controller  50 , for example, causes the first refrigerant flow switching devices  11   a  and  11   b  included in the outdoor unit  1  to switch respective flow passages through which the refrigerant discharged from the compressor  10  flows into the heat-source-side heat exchangers  12 . Furthermore, the controller  50 , for example, causes the second refrigerant flow switching device  11   c  to switch the refrigerant flow passage to a refrigerant flow passage through which refrigerant having flowed from the inflow pipe  5   a  into the outdoor unit  1  passes through the flow passage pipe  9  and flows into the accumulator  19 . 
     As illustrated in  FIG. 3 , low-temperature and low-pressure refrigerant is sucked into the compressor  10  and compressed by the compressor  10  to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is discharged from the compressor  10 . The high-temperature and high-pressure gas refrigerant discharged from the compressor  10  flows into the heat-source-side heat exchangers  12   a  and  12   b  via the first refrigerant flow switching devices  11   a  and  11   b . Then, the refrigerant that has flowed into the heat-source-side heat exchangers  12  transfers heat to outdoor air to change into high-pressure two-phase gas-liquid refrigerant. After flowing out of the heat-source-side heat exchangers  12   a  and  12   b , the refrigerant passes through the heat-source-side expansion devices  17   a  and  17   b  and the heat-source-side backflow prevention device  13  and flows out of the outdoor unit  1 . The refrigerant that has flowed out of the outdoor unit  1  flows into the relay unit  3  through the outflow pipe  5   b.    
     The two-phase gas-liquid refrigerant that has flowed into the relay unit  3  is separated into high-pressure gas refrigerant and high-pressure liquid refrigerant by the gas-liquid separator  29 . The high-pressure gas refrigerant passes through the first opening and closing device  23   b  and the branch pipe  8   a  and then flows into the load-side heat exchanger  26   b  that operates as a condenser. The high-pressure gas refrigerant transfers heat to indoor air to heat the indoor air, thereby changing into liquid refrigerant. At this time, the opening degree of the load-side expansion device  25   b  is controlled to cause subcooling that is the degree of subcooling obtained as a difference between a value obtained by converting a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  into a saturation temperature and a temperature detected by the inlet-side temperature sensor  31   b  to be constant. After flowing out of the load-side heat exchanger  26   b , the liquid refrigerant is expanded by the load-side expansion device  25   b  and flows through the branch pipe  8   b  and the relay-side second backflow prevention device  22   b.    
     Subsequently, intermediate-pressure liquid refrigerant into which the refrigerant is changed through expansion by the first relay expansion device  30  that causes the pressure of the refrigerant to reach an intermediate pressure, after being subjected to separation by the gas-liquid separator  29 , joins the liquid refrigerant that has flowed through the relay-side second backflow prevention device  22   b . At this time, the opening degree of the first relay expansion device  30  is controlled to cause a pressure difference between a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  and a pressure detected by the first relay-expansion-device outlet-side pressure sensor  34  to reach a predetermined pressure difference (for example, 0.3 MPa). 
     Most of the liquid refrigerant obtained by the above joining passes through the relay-side first backflow prevention device  21   a  and the branch pipe  8   b  and is expanded by the load-side expansion device  25   a  to change into low-temperature and low-pressure two-phase gas-liquid refrigerant. The remaining part of the liquid refrigerant is expanded by the second relay expansion device  27  to change into low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant. At this time, the opening degree of the second relay expansion device  27  is controlled to cause subcooling of the refrigerant to be constant. Then, the low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant flows into the low-pressure pipe on the outlet side of the relay unit  3 . 
     On the other hand, the high-pressure liquid refrigerant that has been separated by the gas-liquid separator  29  passes through the relay-side first backflow prevention device  21   a  and flows into the indoor unit  2   a . The two-phase gas-liquid refrigerant that has been obtained through expansion by the load-side expansion device  25   a  of the indoor unit  2   a  flows into the load-side heat exchanger  26   a  that operates as an evaporator, and receives heat from indoor air to cool the indoor air, thereby changing into low-temperature and low-pressure gas refrigerant. At this time, the opening degree of the load-side expansion device  25   a  is controlled to cause superheat obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   a  and a temperature detected by the outlet-side temperature sensor  32   a  to be constant. After flowing out of the load-side heat exchanger  26   a , the gas refrigerant passes through the branch pipe  8   a  and the second opening and closing device  24   a  and flows out of the relay unit  3 . The refrigerant that has flowed out of the relay unit  3  re-flows into the outdoor unit  1  through the inflow pipe  5   a.    
     The refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  branches off to flow through the heat-source-side backflow prevention devices  15   a  and  15   b . The refrigerant that has flowed through the heat-source-side backflow prevention device  15   b  passes through the second refrigerant flow switching device  11   c  and flows into the accumulator  19 . Furthermore, the refrigerant that has flowed through the heat-source-side backflow prevention device  15   a  further branches into refrigerant streams that are divided refrigerant. One of the refrigerant streams, i.e., the divided refrigerant, passes through the first refrigerant flow switching device  11   a  and flows into the accumulator  19 , and the other passes through the heat-source-side backflow prevention device  15   c  and the first refrigerant flow switching device  11   b  and flows into the accumulator  19 . The refrigerant that has passed through the accumulator  19  is re-sucked into the compressor  10 . 
     In the case where no heating load is generated in the load-side heat exchanger  26   c  or the load-side heat exchanger  26   d , refrigerant does not need to be flow therein, and the load-side expansion devices  25   c  and  25   d  which are associated with the load-side heat exchangers  26   c  and  26   d , respectively, are made to be in a closed state. By contrast, in the case where a cooling load is generated in the load-side heat exchanger  26   c  or  26   d , the load-side expansion device  25   c  or  25   d  is opened to allow the refrigerant to circulate. At this time, the opening degree of the load-side expansion device  25   c  or the load-side expansion device  25   d  is controlled to cause superheat to be constant as in the load-side expansion device  25   a . The superheat is a difference between a temperature detected by the inlet-side temperature sensor  31   c  or  31   d  and a temperature detected by the outlet-side temperature sensor  32   c  or  32   d.    
     &lt;Heating Only Operation Mode&gt; 
       FIG. 4  is an explanatory view for the flow of refrigerant in the heating only operation mode of the air-conditioning apparatus  100  according to Embodiment 1. In  FIG. 4 , flow directions of the refrigerant flows are indicated by solid arrows. It is assumed that a heating load is generated in the load-side heat exchanger  26   a  and the load-side heat exchanger  26   b . In the heating only operation mode, the controller  50 , for example, causes the first refrigerant flow switching devices  11   a  and  11   b  included in the outdoor unit  1  to switch respective flow passages to flow passages through which the refrigerant discharged by the compressor  10  directly passes through the outflow pipe  5   b  and flows into the relay unit  3 . Furthermore, the controller  50  performs, for example, switching of the flow passage in the second refrigerant flow switching device  11   c  to inhibit refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  from passing through the flow passage pipe  9 .  
     As illustrated in  FIG. 4 , low-temperature and low-pressure refrigerant is sucked into the compressor  10  and compressed by the compressor  10  to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is discharged from the compressor  10 . The refrigerant discharged from the compressor  10  passes through the first refrigerant flow switching device  11   a  and the heat-source-side backflow prevention device  16  and flows out of the outdoor unit  1 . The refrigerant that has flowed out of the outdoor unit  1  passes through the outflow pipe  5   b  and flows into the relay unit  3 . 
     The high-temperature and high-pressure gas refrigerant that has flowed into the relay unit  3  passes through the gas-liquid separator  29 , the first opening and closing devices  23   a  and  23   b , and the branch pipes  8   a  and then flows into the respective load-side heat exchangers  26   a  and  26   b  each of which operates as a condenser. The refrigerant that has flowed into the load-side heat exchangers  26   a  and  26   b  transfers heat to indoor air to heat the indoor air, thereby changing into liquid refrigerant. After flowing out of the load-side heat exchangers  26   a  and  26   b , the liquid refrigerant is expanded by the respective load-side expansion devices  25   a  and  25   b . Then, the expanded refrigerant passes through the branch pipes  8   b , the relay-side second backflow prevention devices  22   a  and  22   b , the second relay expansion device  27  controlled to be in an opened state, and the inflow pipe  5   a , and re-flows into the outdoor unit  1 . At this time, the opening degree of the load-side expansion device  25   a  is controlled to cause subcooling obtained as a difference between a value obtained by converting a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  into a saturation temperature and a temperature detected by the inlet-side temperature sensor  31   a  to be constant. Similarly, the opening degree of the load-side expansion device  25   b  is controlled to cause subcooling obtained as a difference between a value obtained by converting a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  into a saturation temperature and a temperature detected by the inlet-side temperature sensor  31   b  to be constant. 
     The refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  passes through the heat-source-side backflow prevention device  13  and the heat-source-side expansion devices  17   a  and  17   b  and flows into the heat-source-side heat exchangers  12   a  and  12   b . The refrigerant that has flowed into the heat-source-side heat exchangers  12   a  and  12   b  receives heat from outdoor air to change into low-temperature and low-pressure gas refrigerant, and the low-temperature and low-pressure gas refrigerant flows out of the heat-source-side heat exchangers  12   a  and  12   b . The refrigerant that has flowed out of the heat-source-side heat exchangers  12   a  and  12   b  passes through the first refrigerant flow switching devices  11   a  and  11   b  and flows into the accumulator  19 . The refrigerant that has passed through the accumulator  19  is re-sucked into the compressor  10 . 
     In the case where no heating load is generated in the load-side heat exchanger  26   c  or the load-side heat exchanger  26   d , refrigerant does not need to be made to flow therein, and the load-side expansion devices  25   c  and  25   d  which are associated with the load-side heat exchangers  26   c  and  26   d , respectively, are in a closed state. By contrast, in the case where a heating load is generated in the load-side heat exchanger  26   c  or  26   d , the load-side expansion device  25   c  or  25   d  is opened to allow the refrigerant to circulate. At this time, the opening degree of the load-side expansion device  25   c  or the load-side expansion device  25   d  is controlled to cause subcooling to be constant as in the above load-side expansion device  25   a  or  25   b . The subcooling is obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   c  or  31   d  and a temperature detected by the outlet-side temperature sensor  32   c  or  32   d.    
     &lt;Heating Main Operation Mode&gt; 
       FIG. 5  is an explanatory view for the flow of refrigerant in the heating main operation mode of the air-conditioning apparatus  100  according to Embodiment 1. In  FIG. 5 , flow directions of the refrigerant flows are indicated by solid arrows. It is assumed that a cooling load is generated in the load-side heat exchanger  26   a  and a heating load is generated in the load-side heat exchanger  26   b . In the heating main operation mode, the controller  50 , for example, causes the first refrigerant flow switching devices  11   a  and  11   b  included in the outdoor unit  1  to switch respective flow passages to flow passages through which refrigerant discharged by the compressor  10  directly passes through the outflow pipe  5   b  and flows into the relay unit  3 . Furthermore, the controller  50  performs, for example, switching of the flow passage in the second refrigerant flow switching device  11   c  to inhibit refrigerant that has flowed form the inflow pipe  5   a  into the outdoor unit  1  from passing through the flow passage pipe  9 . 
     As illustrated in  FIG. 5 , low-temperature and low-pressure refrigerant is sucked into the compressor  10  and compressed by the compressor  10  to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is discharged from the compressor  10 . The refrigerant discharged from the compressor  10  passes through the first refrigerant flow switching device  11   a  and the heat-source-side backflow prevention device  16  and flows out of the outdoor unit  1 . The refrigerant that has flowed out of the outdoor unit  1  flows into the relay unit  3  through the outflow pipe  5   b.    
     The high-temperature and high-pressure gas refrigerant that has flowed into the relay unit  3  passes through the gas-liquid separator  29 , the first opening and closing device  23   b , and the branch pipe  8   a  and then flows into the load-side heat exchanger  26   b  that operates as a condenser. The refrigerant that has flowed into the load-side heat exchanger  26   b  transfers heat to indoor air to heat the indoor air, thereby changing into liquid refrigerant. After flowing out of the load-side heat exchanger  26   b , the liquid refrigerant is expanded by the load-side expansion device  25   b  and passes through the branch pipe  8   b  and the relay-side second backflow prevention device  22   b . Subsequently, most of the liquid refrigerant passes through the relay-side first backflow prevention device  21   a  and the branch pipe  8   b  and is then expanded by the load-side expansion device  25   a  to change into low-temperature and low-pressure two-phase gas-liquid refrigerant. Remaining part of the liquid refrigerant is expanded by the second relay expansion device  27  that is also provided in a bypass to change into intermediate-temperature and intermediate-pressure liquid or two-phase gas-liquid refrigerant. The liquid or two-phase gas-liquid refrigerant flows into a low-pressure pipe on the outlet side of the relay unit  3 . 
     The two-phase gas-liquid refrigerant expanded by the load-side expansion device  25   a  flows into the load-side heat exchanger  26   a  that operates as an evaporator and receives heat from indoor air to cool the indoor air, thereby changing into low-temperature and intermediate-pressure two-phase gas-liquid refrigerant. After flowing out of the load-side heat exchanger  26   a , the two-phase gas-liquid refrigerant passes through the branch pipe  8   a  and the second opening and closing device  24   a  and flows out of the relay unit  3 . The refrigerant that has flowed out of the relay unit  3  re-flows into the outdoor unit  1  through the inflow pipe  5   a.    
     The refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  passes through the heat-source-side backflow prevention device  13  and the heat-source-side expansion devices  17   a  and  17   b  and flows into the heat-source-side heat exchangers  12   a  and  12   b . The refrigerant that has flowed into the heat-source-side heat exchangers  12   a  and  12   b  receives heat from outdoor air to change into low-temperature and low-pressure gas refrigerant, and the low-temperature and low-pressure gas refrigerant flows out of the heat-source-side heat exchangers  12   a  and  12   b . The refrigerant that has flowed out of the heat-source-side heat exchangers  12   a  and  12   b  passes through the first refrigerant flow switching devices  11   a  and  11   b  and flows into the accumulator  19 . The refrigerant that has passed through the accumulator  19  is re-sucked into the compressor  10 .  
     At this time, the opening degree of the load-side expansion device  25   b  is controlled to cause subcooling obtained as a difference between a value obtained by converting a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  into a saturation temperature and a temperature detected by the inlet-side temperature sensor  31   b  to be constant. On the other hand, the opening degree of the load-side expansion device  25   a  is controlled to cause superheat obtained as a difference between a temperature detected by the inlet-side temperature sensor  31   a  and a temperature detected by the outlet-side temperature sensor  32   a  to be constant. 
     The opening degree of the second relay expansion device  27  is controlled to cause subcooling of the refrigerant to be constant. For example, the opening degree of the second relay expansion device  27  is controlled to cause a pressure difference between a pressure detected by the first relay-expansion-device inlet-side pressure sensor  33  and a pressure detected by the first relay-expansion-device outlet-side pressure sensor  34  to reach a predetermined pressure difference (for example, 0.3 MPa). 
     In the case where no heating load is generated in the load-side heat exchanger  26   c  or  26   d , refrigerant does not need to be made to flow therein, and the load-side expansion devices  25   c  and  25   d  which are associated with the load-side heat exchangers  26   c  and  26   d , respectively, are in a closed state. By contrast, in the case where a heating load is generated in the load-side heat exchanger  26   c  or  26   d , the load-side expansion device  25   c  or  25   d  is opened to allow the refrigerant to circulate. 
     As described above, the air-conditioning apparatus  100  according to Embodiment 1 includes the flow passage pipe  9 , the heat-source-side backflow prevention device  15   b , which is a flow rate pipe backflow prevention device, and the second refrigerant flow switching device  11   c . During the cooling operation, for example, in the cooling only operation mode and the cooling main operation mode, in the air-conditioning apparatus  100 , refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  is made to branch off to flow into the accumulator  19  through the flow passage pipe  9 . Thus, in the air-conditioning apparatus  100  according to Embodiment 1, the amount of low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant that passes through the heat-source-side backflow prevention device  15   a  can be reduced, and the pressure loss of the refrigerant can be reduced. Furthermore, in the air-conditioning apparatus  100  according to Embodiment 1, the number of flow passages through which the low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  passes is increased, and as a result the pressure loss of the refrigerant can thus be reduced. 
     Embodiment 2 
       FIG. 6  illustrates a configuration of the air-conditioning apparatus  100  according to Embodiment 2. In  FIG. 6 , for example, devices, etc., that are denoted by the same reference signs as those in  FIG. 1  or other figures fulfill and perform functions and operations similar to those described regarding Embodiment 1. 
     As illustrated in  FIG. 6 , in the air-conditioning apparatus  100  according to Embodiment 2, the outdoor unit  1  includes an opening and closing switching valve  20  that is an opening and closing device. When being opened, the opening and closing switching valve  20  allows the refrigerant to flow through the flow passage pipe  9 . On the other hand, when being closed, the opening and closing switching valve  20  does not allow the refrigerant to flow through the flow passage pipe  9 . The controller  50  performs a control to open/close the opening and closing switching valve  20 . 
     In the air-conditioning apparatus  100  according to Embodiment 2, the opening and closing switching valve  20  is provided at the flow passage pipe  9  in place of the second refrigerant flow switching device  11   c  and the heat-source-side backflow prevention device  15   b  which is a flow rate pipe backflow prevention device. During the cooling operation, for example, in the cooling only operation mode and the cooling main operation mode, the controller  50  performs a control to open the opening and closing switching valve  20 , thereby allowing low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  to flow through the flow passage pipe  9 . Therefore, as in Embodiment 1, in the air-conditioning apparatus  100  according to Embodiment 2, the amount of refrigerant that passes through the heat-source-side backflow prevention device  15   a  can be reduced, and the pressure loss of the refrigerant can be reduced. Furthermore, the number of flow passages through which the low-temperature and low-pressure gas refrigerant or two-phase gas-liquid refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  passes is increased, and as a result the pressure loss of the refrigerant can be reduced. 
     Embodiment 3 
     Although each of the above air-conditioning apparatuses  100  according to Embodiments  1  and  2  is an air-conditioning apparatus in which refrigerant is made to flow through an indoor unit  2  to cool or heat an air-conditioned space, each air-conditioning apparatus  100  is not limited to such an air-conditioning apparatus. For example, the air-conditioning apparatus  100  can be used, for example, as an air-conditioning apparatus or chiller system that includes a heat medium circuit through which a heat medium, such as water, circulates and that heats or cools the heat medium by using heat supplied from the outdoor unit  1  and causes heat exchange to be performed between the heated or cooled heat medium and air of an air-conditioned space to condition the air. 
     Furthermore, although it is described above that in each of the above air-conditioning apparatuses  100  according to Embodiments 1 and 2, two heat-source-side heat exchangers  12   a  and  12   b  are provided in parallel, it is not limiting. The number of heat-source-side heat exchangers  12  may be one. In this case, the first refrigerant flow switching device  11   b  and the heat-source-side backflow prevention device  15   c  need not be provided. Furthermore, the number of the heat-source-side heat exchangers  12  may be three or more. 
     Additionally, although regarding Embodiments 1 and 2, the air-conditioning apparatus  100  is described as an air-conditioning apparatus that is capable of performing the simultaneous cooling and heating operation in which the cooling main operation and the heating main operation can be performed, the air-conditioning apparatus  100  is not limited to such an air-conditioning apparatus. The air-conditioning apparatus  100  can be used as an air-conditioning apparatus  100  including the outdoor unit  1  in which a pipe through which refrigerant flows out of the outdoor unit  1  is used as a dedicated outflow pipe and a pipe through which refrigerant flows into the outdoor unit  1  is used as a dedicated inflow pipe; that is, the pipe used as the outflow pipe and the pipe used as the inflow pipe are not changed. 
     Furthermore, in Embodiments 1 and 2 described above, for example, a combination of a plurality of check valves may be provided to enable, during the cooling operation, refrigerant to pass through the flow passage pipe  9 , for example, because of a differential pressure. In this case, the controller  50  need not perform control. 
     In the above air-conditioning apparatus  100  according to Embodiment 1, refrigerant that has passed through the heat-source-side backflow prevention device  15   a  is made to further branch off to flow toward the first refrigerant flow switching device  11   a  and toward the heat-source-side backflow prevention device  15   c  and the first refrigerant flow switching device  11   b . The amount of refrigerant that passes through the heat-source-side backflow prevention device  15   c  is smaller than the amount of refrigerant that passes through the heat-source-side backflow prevention device  15   a , whereby as the heat-source-side backflow prevention device  15   c , a check valve smaller than the heat-source-side backflow prevention device  15   a  can be used. However, the air-conditioning apparatus  100  is not limited to such an air-conditioning apparatus. In the air-conditioning apparatus  100 , for example, the heat-source-side backflow prevention devices  15   a ,  15   b , and  15   c  may be connected in parallel. Thus, refrigerant that has flowed from the inflow pipe  5   a  into the outdoor unit  1  can be made to branch into three refrigerant streams.