Patent Publication Number: US-2023148118-A1

Title: Heat exchanger and air-conditioning apparatus including the heat exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a U.S. National Stage Application of International Application No. PCT/JP2020/020345 filed on May 22, 2020, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a heat exchanger including a plurality of flat tubes, and fins provided between the flat tubes adjacent to each other, and also relates to an air-conditioning apparatus including the heat exchanger. 
     BACKGROUND 
     As disclosed in Patent Literature 1, some heat exchanger used in an outdoor unit of an air-conditioning apparatus has been known that includes a plurality of flat tubes spaced from and parallel to each other, a plurality of fins provided between the flat tubes adjacent to each other, an upper header, and a lower header. Inside each of the flat tubes, refrigerant flow passages are formed through which refrigerant flows in an up-down direction. Respective upper end portions of the plurality of flat tubes are connected to the upper header. Respective lower end portion of the plurality of flat tubes are connected to the lower header. 
     PATENT LITERATURE 
     
         
         Patent Literature 1: International Publication No. WO 2015/005352 
       
    
     The heat exchanger having the above configuration allows refrigerant to flow vertically upward inside the flat tubes. Thus, the refrigerant flow velocity needs to be increased. For example, a variable-capacity air-conditioning apparatus may sometimes perform part-load operation with a reduced operational frequency of a compressor when the air-conditioning load is low. In this case, in the heat exchanger disclosed in Patent Literature 1, refrigerant in all the flat tubes is equally affected by gravity. This may generate an area with a lower refrigerant flow velocity than the required refrigerant flow velocity for refrigerant to flow upward inside the flat tubes. There is thus a risk that heat exchange performance may be degraded. 
     SUMMARY 
     The present disclosure has been achieved to solve the above problem, and it is an object of the present disclosure to provide a heat exchanger and an air-conditioning apparatus including the heat exchanger, in which even when the air-conditioning apparatus performs part-load operation with a reduced operational frequency of a compressor, the heat exchanger still obtains a refrigerant flow velocity required for refrigerant to flow upward inside flat tubes, and still reduces degradation in heat exchange performance. 
     A heat exchanger according to one embodiment of the present disclosure is a heat exchanger including a plurality of heat exchange units configured to exchange heat between refrigerant and air. Each of the plurality of heat exchange units includes a plurality of flat tubes spaced from and parallel to each other and in which a refrigerant flow passage is formed through which refrigerant flows in an up-down direction, a plurality of fins provided between the plurality of flat tubes adjacent to each other, an upper header to which respective upper end portions of the plurality of flat tubes are connected, and a lower header to which respective lower end portions of the plurality of flat tubes are connected. In the plurality of heat exchange units, the upper headers are connected such that the upper headers communicate with each other, and the lower headers are connected such that the lower headers communicate with each other through an opening-closing valve. The heat exchanger has a configuration in which when the heat exchanger serves as a condenser, the opening-closing valve is controlled such that refrigerant in at least one of the plurality of heat exchange units flows in an upward direction, and refrigerant in the other one or the other ones of the plurality of heat exchange units flows in a downward direction. 
     An air-conditioning apparatus according to another embodiment of the present disclosure includes a compressor and the above heat exchanger through which refrigerant discharged from the compressor flows, and the opening-closing valve is controlled in response to an operational frequency of the compressor set in advance. 
     In the heat exchanger according to an embodiment of the present disclosure, and the air-conditioning apparatus including this heat exchanger, when, for example, the air-conditioning apparatus performs part-load operation with a reduced operational frequency of the compressor, the heat exchanger allows only refrigerant in some of the heat exchange units to flow upward inside the flat tubes, a refrigerant flow velocity required for refrigerant to flow upward inside the flat tubes is thus obtained, and consequently degradation in the heat exchange performance is reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a refrigerant circuit diagram of an air-conditioning apparatus according to the present Embodiment 1. 
         FIG.  2    is a perspective view of the cross-section of the portion II illustrated in  FIG.  1    when the cross-section is viewed from above. 
         FIG.  3    is a graph that illustrates the relationship between the height of flat tube of a heat exchanger according to the present Embodiment 1 and the flow velocity required for refrigerant to flow vertically upward. 
         FIG.  4    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 when the air-conditioning load is low during cooling operation. 
         FIG.  5    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 when the air-conditioning load is high during cooling operation. 
         FIG.  6    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 during heating operation. 
         FIG.  7    is an explanatory diagram that illustrates operation of a heat exchanger according to the present Embodiment 2 when the air-conditioning load is low during cooling operation. 
         FIG.  8    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 2 when the air-conditioning load is high during cooling operation. 
         FIG.  9    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 2 during heating operation. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described hereinafter with reference to the drawings. Note that in each of the drawings, the same or equivalent components are denoted by the same reference signs, and their descriptions are appropriately omitted or simplified. The shape, size, location, and other properties of the components described in each of the drawings may be appropriately changed. 
     Embodiment 1 
       FIG.  1    is a refrigerant circuit diagram of an air-conditioning apparatus according to the present Embodiment 1.  FIG.  2    is a perspective view of the cross-section of the portion II illustrated in  FIG.  1    when the cross-section is viewed from above. Note that the arrows illustrated in  FIG.  1    represent the flow direction of refrigerant. Every opening-closing valve in an opened state is represented by a white valve symbol. Every opening-closing valve in a closed state is represented by a black valve symbol. 
     As illustrated in  FIG.  1   , an air-conditioning apparatus  300  according to the present Embodiment 1 is made up of an outdoor unit  100  and an indoor unit  200 . The air-conditioning apparatus  300  includes a refrigerant circuit in which a compressor  101 , first flow switching means  102 , an indoor heat exchanger  201 , an expansion mechanism  103 , an outdoor heat exchanger  104 , and a refrigerant container  105  are connected by a refrigerant pipe  107  to allow refrigerant to circulate in the refrigerant circuit. The outdoor unit  100  includes the compressor  101 , the first flow switching means  102 , the expansion mechanism  103 , the outdoor heat exchanger  104 , and the refrigerant container  105 . The indoor unit  200  includes the indoor heat exchanger  201 . Note that the air-conditioning apparatus  300  is not limited to the one made up of the constituent elements illustrated in  FIG.  1   , but may include other constituent element. 
     Operation of the air-conditioning apparatus  300  is controlled by a controller  109 . The controller  109  is made up of a computation device such as a microcomputer and a CPU, and software to be executed by the computation device. Note that the controller  109  may be made up of hardware such as a circuit device that implements the functions of the controller  109 . 
     The compressor  101  compresses sucked refrigerant into a high-temperature and high-pressure state, and discharges the compressed refrigerant. The compressor  101  is, for example, a positive-displacement compressor configured to vary the operational capacity (frequency), and driven by a motor that is controlled by the inverter. 
     The first flow switching means  102  is, for example, a four-way valve, and is configured to switch the flow passages of refrigerant. During cooling operation, the first flow switching means  102  changes the refrigerant flow passage such that the refrigerant discharge port of the compressor  101  is connected to the gas portion of the outdoor heat exchanger  104 , and the refrigerant suction port of the compressor  101  is connected to the gas portion of the indoor heat exchanger  201 . In contrast, during heating operation, the first flow switching means  102  changes the refrigerant flow passage such that the refrigerant discharge port of the compressor  101  is connected to the gas portion of the indoor heat exchanger  201 , and the refrigerant suction port of the compressor  101  is connected to the gas portion of the outdoor heat exchanger  104 . Note that the first flow switching means  102  may be formed in combination with a two-way valve or a three-way valve. 
     The indoor heat exchanger  201  serves as an evaporator during cooling operation, and exchanges heat between air and refrigerant flowing out of the expansion mechanism  103 . The indoor heat exchanger  201  serves as a condenser during heating operation, and exchanges heat between air and refrigerant discharged from the compressor  101 . The indoor heat exchanger  201  sucks room air delivered by an indoor fan, and supplies the air having exchanged heat with refrigerant into the room. 
     The expansion mechanism  103  reduces the pressure of refrigerant flowing in the refrigerant circuit to expand the refrigerant. The expansion mechanism  103  is, for example, an electronic expansion valve whose opening degree is variably controlled. The refrigerant container  105  is, for example, a receiver or an accumulator. The refrigerant container  105  stores in its inside surplus liquid refrigerant during the operation. 
     The outdoor heat exchanger  104  serves as a condenser during cooling operation, and exchanges heat between air and refrigerant discharged from the compressor  101 . The outdoor heat exchanger  104  serves as an evaporator during heating operation, and exchanges heat between air and refrigerant flowing out of the expansion mechanism  103 . The outdoor heat exchanger  104  sucks outdoor air delivered by an outdoor fan, and discharges the air having exchanged heat with refrigerant to the outside. 
     The outdoor heat exchanger  104  according to the present Embodiment 1 includes a first heat exchange unit  104 A and a second heat exchange unit  104 B, each of which exchanges heat between refrigerant and air. As illustrated in  FIGS.  1  and  2   , each of the first heat exchange unit  104 A and the second heat exchange unit  1048  includes a plurality of flat tubes  1  spaced from and parallel to each other and in which refrigerant flow passages  10  are formed through which refrigerant flows in an up-down direction Y, a plurality of fins  2  provided between the flat tubes  1  adjacent to each other, an upper header  3  to which respective upper end portions of the plurality of flat tubes  1  are connected, and a lower header  4  to which respective lower end portions of the plurality of flat tubes  1  are connected. 
     The flat tubes  1  are made of, for example, aluminum. The flat tubes  1  are spaced from each other in a left-right direction X, which is perpendicular to an airflow direction Z, and located parallel to each other. The flat tubes  1  are located with their flat faces extending substantially parallel to the airflow direction Z. Inside each of the flat tubes  1 , a plurality of refrigerant flow passages  10 , through which refrigerant flows in the up-down direction Y, are formed in line along the airflow direction Z. Note that the up-down direction Y not only refers to the vertical direction, but also includes a direction inclined to the vertical direction. In addition, the left-right direction X not only refers to the horizontal direction, but also includes a direction inclined to the horizontal direction. 
     The fins  2  are, for example, aluminum parts to transfer heat of refrigerant flowing inside the flat tubes  1 . Each of the fins  2  is a corrugated fin formed by bending a thin plate into a wavy shape. The fin  2  is provided between two of the plurality of flat tubes  1  adjacent to each other. Each of the bent tip portions of the fin  2  is joined to the flat face of either of the two flat tubes  1 . A space defined by the fin  2  and the flat tubes  1  serves as an air flow path. Note that the fin  2  may be provided with drain holes, louvers, or other portion, through which condensed water is drained, on the inclined surfaces of the fin  2 , although the drain holes, louvers, and other portion are not illustrated. The fin  2  is not limited to a corrugated fin, and may be, for example, a plate fin. 
     As illustrated in  FIG.  1   , the first heat exchange unit  104 A and the second heat exchange unit  1046  are located next to each other. One end of the upper header  3  of the first heat exchange unit  104 A is connected to one end of the upper header  3  of the second heat exchange unit  1046  by a first connection pipe  5  such that these upper headers  3  communicate with each other. One end of the lower header  4  of the first heat exchange unit  104 A is connected to one end of the lower header  4  of the second heat exchange unit  1046  by a second connection pipe  6  such that these lower headers  4  communicate with each other. The second connection pipe  6  is provided with an opening-closing valve  6   a , which is controlled by the controller  109 . The opening-closing valve  6   a  is, for example, a solenoid valve. 
     Note that instead of connecting the upper header  3  of the first heat exchange unit  104 A to the upper header  3  of the second heat exchange unit  1046  by the first connection pipe  5 , these upper headers  3  may be made up of a single upper header, although the single upper header is not illustrated. In addition, instead of connecting the lower header  4  of the first heat exchange unit  104 A to the lower header  4  of the second heat exchange unit  1046  by the second connection pipe  6 , these lower headers  4  may be made up of a single lower header. In this case, inside the single lower header, an opening-closing valve is provided to control communication between the first heat exchange unit  104 A and the second heat exchange unit  1046 . 
     The other end of the lower header  4  of the first heat exchange unit  104 A is connected to a first flow pipe  7 . The first flow pipe  7  branches off from a portion of the refrigerant pipe  107  connected between the first flow switching means  102  and the outdoor heat exchanger  104 . The refrigerant pipe  107  is provided with second flow switching means  106  at a position where the first flow pipe  7  branches off from the refrigerant pipe  107 . The second flow switching means  106  is, for example, a three-way valve and is controlled by the controller  109 . 
     The other end of the upper header  3  of the second heat exchange unit  104 B is connected through a second flow pipe  8  to a portion of the refrigerant pipe  107  connected between the second flow switching means  106  and the expansion mechanism  103 . The second flow pipe  8  is provided with an opening-closing valve  8   a , which is controlled by the controller  109 . The opening-closing valve  8   a  is, for example, a solenoid valve. 
     The other end of the lower header  4  of the second heat exchange unit  104 B is connected through a third flow pipe  9  to a portion of the refrigerant pipe  107  connected between the expansion mechanism  103  and the connection point with the second flow pipe  8 . The third flow pipe  9  is provided with an opening-closing valve  9   a , which is controlled by the controller  109 . The opening-closing valve  9   a  is, for example, a solenoid valve. 
     A valve body  108  is provided in a portion of the refrigerant pipe  107  connected between the connection point with the second flow pipe  8  and the connection point with the third flow pipe  9 . By use of the valve body  108 , refrigerant flows only in one direction between the connection point with the second flow pipe  8  and the connection point with the third flow pipe  9 . 
     Next, cooling operation of the air-conditioning apparatus  300  is described. High-temperature and high-pressure gas refrigerant discharged from the compressor  101  passes through the first flow switching means  102 , and then flows through the first flow pipe  7  to the outdoor heat exchanger  104  to exchange heat with air and become liquid refrigerant. The liquid refrigerant flows out to the refrigerant pipe  107  through the second flow pipe  8  or the third flow pipe  9 , and is reduced in the pressure by the expansion mechanism  103  into low-pressure two-phase gas-liquid refrigerant. The low-pressure two-phase gas-liquid refrigerant flows to the indoor heat exchanger  201  to exchange heat with air and become gas refrigerant. The gas refrigerant passes through the first flow switching means  102 , and is sucked into the compressor  101  through the refrigerant container  105 . 
     Next, heating operation of the air-conditioning apparatus  300  is described. High-temperature and high-pressure gas refrigerant discharged from the compressor  101  passes through the first flow switching means  102 , and then flows to the indoor heat exchanger  201  to exchange heat with air and become liquid refrigerant. The liquid refrigerant is reduced in the pressure by the expansion mechanism  103  into low-pressure two-phase gas-liquid refrigerant. The low-pressure two-phase gas-liquid refrigerant flows through the third flow pipe  9  to the outdoor heat exchanger  104  to exchange heat with air and become gas refrigerant. The gas refrigerant flows out to the refrigerant pipe  107  through the second flow pipe  8 , and then passes through the first flow switching means  102 . Thereafter, the gas refrigerant is sucked into the compressor  101  through the refrigerant container  105 . 
       FIG.  3    is a graph that illustrates the relationship between the height of flat tube of the heat exchanger according to the present Embodiment 1 and the flow velocity required for refrigerant to flow vertically upward. The horizontal axis represents the height of flat tube. The vertical axis represents the refrigerant flow velocity required for refrigerant to flow vertically upward. 
     The above outdoor heat exchanger  104  allows refrigerant to flow vertically upward inside the flat tubes  1 . Thus, as illustrated in  FIG.  3   , as the height of the flat tube  1  increases, the refrigerant flow velocity, required for refrigerant to flow vertically upward, increases. For example, a variable-capacity air-conditioning apparatus  300  may sometimes perform part-load operation with a reduced operational frequency of the compressor  101  when the air-conditioning load is low. In this case, in the outdoor heat exchanger  104 , refrigerant in all the flat tubes  1  is equally affected by gravity. This may generate an area with a lower refrigerant flow velocity than the required refrigerant flow velocity for refrigerant to flow upward inside the flat tubes  1 . There is thus a risk that heat exchange performance may be degraded. 
     To solve this problem, in the outdoor heat exchanger  104  according to the present Embodiment 1, the opening-closing valve  6   a  is controlled in response to the operational frequency of the compressor  101  set in advance, in the manner as illustrated in  FIGS.  4  to  6    to change the flow direction of refrigerant flowing through the first heat exchange unit  104 A and the second heat exchange unit  1046 . Note that the arrows illustrated in  FIGS.  4  to  6    represent the flow direction of refrigerant. Every opening-closing valve in an opened state is represented by a white valve symbol. Every opening-closing valve in a closed state is represented by a black valve symbol. 
     First,  FIG.  4    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 when the air-conditioning load is low during cooling operation. As illustrated in  FIG.  4   , the outdoor heat exchanger  104  has a configuration in which when the air-conditioning load is low during cooling operation, the opening-closing valve  6   a  of the second connection pipe  6  is brought into a closed state, such that refrigerant in the first heat exchange unit  104 A flows in the upward direction, while refrigerant in the second heat exchange unit  104 B flows in the downward direction. 
     Gas refrigerant flowing into the lower header  4  of the first heat exchange unit  104 A from the first flow pipe  7  is distributed to the flat tubes  1  of the first heat exchange unit  104 A. At this time, the opening-closing valve  6   a  of the second connection pipe  6  is in a closed state. This prevents the gas refrigerant having flowed into the lower header  4  of the first heat exchange unit  104 A from flowing into the lower header  4  of the second heat exchange unit  1046 . The gas refrigerant flows upward inside the flat tubes  1  to condense and liquefy into liquid refrigerant. The liquid refrigerant in the flat tubes  1  joins together in the upper header  3 , and flows through the first connection pipe  5  into the upper header  3  of the second heat exchange unit  1046 . The liquid refrigerant flowing into the upper header  3  of the second heat exchange unit  1046  is distributed to the flat tubes  1  of the second heat exchange unit  104 B, and flows downward inside the flat tubes  1 . This allows the liquid refrigerant to be subcooled. At this time, the opening-closing valve  8   a  of the second flow pipe  8  is in a closed state. This prevents the liquid refrigerant having flowed into the upper header  3  of the second heat exchange unit  1046  from flowing out to the refrigerant pipe  107  through the second flow pipe  8 . The liquid refrigerant, flowing downward inside the flat tubes  1  of the second heat exchange unit  1046 , joins together in the lower header  4 , then flows out to the refrigerant pipe  107  through the third flow pipe  9  with the opening-closing valve  9   a  brought into an opened state, and thereafter flows to the expansion mechanism  103 . 
     As described above, the part-load operation with a reduced operational frequency of the compressor  101  is performed when the air-conditioning load is low during cooling operation. In this part-load operation, only refrigerant in the first heat exchange unit  104 A is allowed to flow upward inside the flat tubes  1 , the refrigerant flow velocity required for refrigerant to flow upward inside the flat tubes  1  is thus obtained, and consequently degradation in the heat exchange performance is reduced. 
       FIG.  5    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 when the air-conditioning load is high during cooling operation. As illustrated in  FIG.  5   , the outdoor heat exchanger  104  has a configuration in which when the air-conditioning load is high during cooling operation, the opening-closing valve  6   a  of the second connection pipe  6  is brought into an opened state, such that refrigerant in both the first heat exchange unit  104 A and the second heat exchange unit  104 B flows in the upward direction. 
     Gas refrigerant flowing into the lower header  4  of the first heat exchange unit  104 A from the first flow pipe  7  is distributed to the flat tubes  1  of the first heat exchange unit  104 A, while flowing into the lower header  4  of the second heat exchange unit  104 B through the second connection pipe  6 . At this time, the opening-closing valve  9   a  of the third flow pipe  9  is in a closed state. This prevents the gas refrigerant having flowed into the lower header  4  of the second heat exchange unit  1046  from flowing out to the refrigerant pipe  107  through the third flow pipe  9 . The gas refrigerant flows upward inside the flat tubes  1  of both the first heat exchange unit  104 A and the second heat exchange unit  104 B to condense and liquefy into liquid refrigerant. The liquid refrigerant in the flat tubes  1  joins together in each upper header  3 . The liquid refrigerant in the upper header  3  of the first heat exchange unit  104 A flows into the upper header  3  of the second heat exchange unit  1046  through the first connection pipe  5 , and then joins with the liquid refrigerant in the upper header  3  of the second heat exchange unit  1046 . The liquid refrigerant having joined together flows out to the refrigerant pipe  107  through the second flow pipe  8  with the opening-closing valve  8   a  brought into an opened state, then passes through the valve body  108 , and flows to the expansion mechanism  103 . 
     There is a case where the air-conditioning load is so high during cooling operation that it is unnecessary to reduce the operational frequency of the compressor  101 . In this case, the heat exchange efficiency is increased by allowing refrigerant in both the first heat exchange unit  104 A and the second heat exchange unit  104 B to flow in the upward direction as described above. 
       FIG.  6    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 1 during heating operation. As illustrated in  FIG.  6   , the outdoor heat exchanger  104  has a configuration in which the opening-closing valve  6   a  of the second connection pipe  6  is brought into an opened state during heating operation, such that refrigerant in both the first heat exchange unit  104 A and the second heat exchange unit  1046  flows in the upward direction. 
     Two-phase gas-liquid refrigerant flows into the lower header  4  of the second heat exchange unit  104 B from the third flow pipe  9  with the opening-closing valve  9   a  brought into an opened state. This two-phase gas-liquid refrigerant is distributed to the flat tubes  1  of the second heat exchange unit  1046 , while flowing into the lower header  4  of the first heat exchange unit  104 A through the second connection pipe  6 . At this time, the flow direction of the second flow switching means  106  is controlled such that the two-phase gas-liquid refrigerant having flowed into the lower header  4  of the first heat exchange unit  104 A is prevented from flowing out to the first flow pipe  7 . The two-phase gas-liquid refrigerant flows upward inside the flat tubes  1  of both the first heat exchange unit  104 A and the second heat exchange unit  1046  to evaporate and vaporize into gas refrigerant. The gas refrigerant in the flat tubes  1  joins together in each upper header  3 . The gas refrigerant in the upper header  3  of the first heat exchange unit  104 A flows into the upper header  3  of the second heat exchange unit  1046  through the first connection pipe  5 , and then joins with the gas refrigerant in the upper header  3  of the second heat exchange unit  1046 . The gas refrigerant having joined together flows out to the refrigerant pipe  107  through the second flow pipe  8  with the opening-closing valve  8   a  brought into an opened state, and then flows to the compressor  101 . Note that the pressure in the third flow pipe  9  is higher than the pressure in the second flow pipe  8  during heating operation, the gas refrigerant flowing out of the second flow pipe  8  is thus prevented from flowing toward the valve body  108 . 
     There is a case where the air-conditioning load is so high during heating operation that it is unnecessary to reduce the operational frequency of the compressor. In this case, the heat exchange efficiency is increased by allowing refrigerant in both the first heat exchange unit  104 A and the second heat exchange unit  104 B to flow in the upward direction as described above. 
     Embodiment 2 
     Next, the heat exchanger  104  according to the present Embodiment 2 and the air-conditioning apparatus  300  including this heat exchanger  104  are described on the basis of  FIGS.  7  to  9    with reference to  FIGS.  1  and  2   .  FIG.  7    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 2 when the air-conditioning load is low during cooling operation.  FIG.  8    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 2 when the air-conditioning load is high during cooling operation.  FIG.  9    is an explanatory diagram that illustrates operation of the heat exchanger according to the present Embodiment 2 during heating operation. Note that the constituent elements that are the same as those of the heat exchanger  104  described in Embodiment 1, and the constituent elements that are the same as those of the air-conditioning apparatus  300  including this heat exchanger  104  are denoted by the same reference signs, and descriptions of such constituent elements are appropriately omitted. 
     As illustrated in  FIGS.  7  to  9   , the outdoor heat exchanger  104  according to the present Embodiment 2 includes the first heat exchange unit  104 A, the second heat exchange unit  1048 , and a third heat exchange unit  104 C, each of which exchanges heat between refrigerant and air. As illustrated in  FIG.  2   , each of the first heat exchange unit  104 A, the second heat exchange unit  1048 , and the third heat exchange unit  104 C includes a plurality of flat tubes  1  spaced from and parallel to each other and in which refrigerant flow passages  10  are formed through which refrigerant flows in an up-down direction Y, a plurality of fins  2  provided between the flat tubes  1  adjacent to each other, an upper header  3  to which respective upper end portions of the plurality of flat tubes  1  are connected, and a lower header  4  to which respective lower end portions of the plurality of flat tubes  1  are connected. 
     As illustrated in  FIG.  7   , the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C are located next to each other. One end of the upper header  3  of the first heat exchange unit  104 A is connected to one end of the upper header  3  of the second heat exchange unit  1048  by the first connection pipe  5  such that these upper headers  3  communicate with each other. The other end of the upper header  3  of the second heat exchange unit  104 B is connected to one end of the upper header  3  of the third heat exchange unit  104 C by a third connection pipe  50  such that these upper headers  3  communicate with each other. 
     One end of the lower header  4  of the first heat exchange unit  104 A is connected to one end of the lower header  4  of the second heat exchange unit  104 B by the second connection pipe  6  such that these lower headers  4  communicate with each other. The second connection pipe  6  is provided with the opening-closing valve  6   a , which is controlled by the controller  109 . The opening-closing valve  6   a  is, for example, a solenoid valve. The other end of the lower header  4  of the second heat exchange unit  104 B is connected to one end of the lower header  4  of the third heat exchange unit  104 C by a fourth connection pipe  60  such that these lower headers  4  communicate with each other. The fourth connection pipe  60  is provided with an opening-closing valve  60   a , which is controlled by the controller  109 . The opening-closing valve  60   a  is, for example, a solenoid valve. 
     Note that instead of connecting the upper header  3  of the first heat exchange unit  104 A, the upper header  3  of the second heat exchange unit  1046 , and the upper header  3  of the third heat exchange unit  104 C to each other by the first connection pipe  5  and the third connection pipe  50 , these upper headers  3  may be made up of a single upper header, although the single upper header is not illustrated. In addition, instead of connecting the lower header  4  of the first heat exchange unit  104 A, the lower header  4  of the second heat exchange unit  1046 , and the lower header  4  of the third heat exchange unit  104 C to each other by the second connection pipe  6  and the fourth connection pipe  60 , these lower headers  4  may be made up of a single lower header. In this case, inside the single lower header, an opening-closing valve is provided to control communication between the first heat exchange unit  104 A and the second heat exchange unit  1046 , and an opening-closing valve is also provided to control communication between the second heat exchange unit  104 B and the third heat exchange unit  104 C. 
     The other end of the lower header  4  of the first heat exchange unit  104 A is connected to the first flow pipe  7 . The first flow pipe  7  branches off from a portion of the refrigerant pipe  107  connected between the first flow switching means  102  and the outdoor heat exchanger  104 . The other end of the upper header  3  of the third heat exchange unit  104 C is connected through the second flow pipe  8  to a portion of the refrigerant pipe  107  connected between the second flow switching means  106  and the expansion mechanism  103 . The second flow pipe  8  is provided with the opening-closing valve  8   a , which is controlled by the controller  109 . The opening-closing valve  8   a  is, for example, a solenoid valve. 
     The other end of the lower header  4  of the third heat exchange unit  104 C is connected through the third flow pipe  9  to a portion of the refrigerant pipe  107  connected between the expansion mechanism  103  and the connection point with the second flow pipe  8 . The third flow pipe  9  is provided with the opening-closing valve  9   a , which is controlled by the controller  109 . The opening-closing valve  9   a  is, for example, a solenoid valve. 
     In the outdoor heat exchanger  104  according to the present Embodiment 2, the opening-closing valves  6   a  and  60   a  are controlled in response to the operational frequency of the compressor  101  set in advance, in the manner as illustrated in  FIGS.  7  to  9    to change the flow direction of refrigerant flowing through the first heat exchange unit  104 A, the second heat exchange unit  1046 , and the third heat exchange unit  104 C. Note that the arrows illustrated in  FIGS.  7  to  9    represent the flow direction of refrigerant. Every opening-closing valve in an opened state is represented by a white valve symbol. Every opening-closing valve in a closed state is represented by a black valve symbol. 
     As illustrated in  FIG.  7   , in the outdoor heat exchanger  104 , when the air-conditioning load is low during cooling operation, first the opening-closing valve  6   a  of the second connection pipe  6  is brought into an opened state, while the opening-closing valve  60   a  of the fourth connection pipe  60  is brought into a closed state. That is, refrigerant in both the first heat exchange unit  104 A and the second heat exchange unit  104 B flows in the upward direction, while refrigerant in the third heat exchange unit  104 C only flows in the downward direction. 
     Gas refrigerant flowing into the lower header  4  of the first heat exchange unit  104 A from the first flow pipe  7  is distributed to the flat tubes  1  of the first heat exchange unit  104 A. Simultaneously, the gas refrigerant flows into the lower header  4  of the second heat exchange unit  1046  through the second connection pipe  6 , and is then distributed to the flat tubes  1  of the second heat exchange unit  1046 . At this time, the opening-closing valve  60   a  of the fourth connection pipe  60  is in a closed state. This prevents the gas refrigerant having flowed into the lower header  4  of the second heat exchange unit  1046  from flowing into the lower header  4  of the third heat exchange unit  104 C. The gas refrigerant flows upward inside the flat tubes  1  to condense and liquefy into liquid refrigerant. The liquid refrigerant in the flat tubes  1  joins together in each upper header  3 . The liquid refrigerant in the upper header  3  of the first heat exchange unit  104 A flows into the upper header  3  of the second heat exchange unit  104 B through the first connection pipe  5 , and then joins with the liquid refrigerant in the upper header  3  of the second heat exchange unit  104 B. The liquid refrigerant having joined together flows into the upper header  3  of the third heat exchange unit  104 C through the third connection pipe  50 . The liquid refrigerant flowing into the upper header  3  of the third heat exchange unit  104 C is distributed to the flat tubes  1  of the third heat exchange unit  104 C, and flows downward inside the flat tubes  1 . This allows the liquid refrigerant to be subcooled. At this time, the opening-closing valve  8   a  of the second flow pipe  8  is in a closed state. This prevents the liquid refrigerant having flowed into the upper header  3  of the third heat exchange unit  104 C from flowing out to the refrigerant pipe  107  through the second flow pipe  8 . The liquid refrigerant, flowing downward inside the flat tubes  1  of the third heat exchange unit  104 C, joins together in the lower header  4 , then flows out to the refrigerant pipe  107  through the third flow pipe  9  with the opening-closing valve  9   a  brought into an opened state, and thereafter flows to the expansion mechanism  103 . 
     As described above, the part-load operation with a reduced operational frequency of the compressor  101  is performed when the air-conditioning load is low during cooling operation. In this part-load operation, only refrigerant in the first heat exchange unit  104 A and the second heat exchange unit  1046  is allowed to flow upward inside the flat tubes  1 , the refrigerant flow velocity required for refrigerant to flow upward inside the flat tubes  1  is thus obtained, and consequently degradation in the heat exchange performance is reduced. 
     Note that the outdoor heat exchanger  104  may have a configuration in which the opening-closing valve  6   a  of the second connection pipe  6  is brought into a closed state, and the opening-closing valve  60   a  of the fourth connection pipe  60  is brought into a closed state to allow only refrigerant in the first heat exchange unit  104 A to flow upward inside the flat tubes  1 , although this configuration is not illustrated. 
     As illustrated in  FIG.  8   , the outdoor heat exchanger  104  has a configuration in which when the air-conditioning load is high during cooling operation, the opening-closing valve  6   a  of the second connection pipe  6  is brought into an opened state, and the opening-closing valve  60   a  of the fourth connection pipe  60  is brought into an opened state, such that refrigerant in the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C all flows in the upward direction. 
     Gas refrigerant flowing into the lower header  4  of the first heat exchange unit  104 A from the first flow pipe  7  is distributed to the flat tubes  1  of the first heat exchange unit  104 A. Simultaneously, this gas refrigerant flows into the lower header  4  of the second heat exchange unit  1046  through the second connection pipe  6 , and further flows into the lower header  4  of the third heat exchange unit  104 C through the fourth connection pipe  60 . At this time, the opening-closing valve  9   a  of the third flow pipe  9  is in a closed state. This prevents the gas refrigerant having flowed into the lower header  4  of the third heat exchange unit  104 C from flowing out to the refrigerant pipe  107  through the third flow pipe  9 . The gas refrigerant flows upward inside the flat tubes  1  of all the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C to condense and liquefy into liquid refrigerant. The liquid refrigerant in the flat tubes  1  joins together in each upper header  3 . The liquid refrigerant in the upper header  3  of the first heat exchange unit  104 A flows into the upper header  3  of the second heat exchange unit  1046  through the first connection pipe  5 , and then joins with the liquid refrigerant in the upper header  3  of the second heat exchange unit  1046 . The liquid refrigerant having joined together flows into the upper header  3  of the third heat exchange unit  104 C through the third connection pipe  50 , and then joins with the liquid refrigerant in the upper header  3  of the third heat exchange unit  104 C. This liquid refrigerant having joined together flows out to the refrigerant pipe  107  through the second flow pipe  8  with the opening-closing valve  8   a  brought into an opened state, then passes through the valve body  108 , and flows to the expansion mechanism  103 . 
     There is a case where the air-conditioning load is so high during cooling operation that it is unnecessary to reduce the operational frequency of the compressor  101 . In this case, the heat exchange efficiency is increased by allowing refrigerant in the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C to all flow in the upward direction as described above. 
     As illustrated in  FIG.  9   , the outdoor heat exchanger  104  has a configuration in which during heating operation, the opening-closing valve  6   a  of the second connection pipe  6  is brought into an opened state, and the opening-closing valve  60   a  of the fourth connection pipe  60  is brought into an opened state, such that refrigerant in the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C all flows in the upward direction. 
     Two-phase gas-liquid refrigerant flowing into the lower header  4  of the third heat exchange unit  104 C from the third flow pipe  9  is distributed to the flat tubes  1  of the third heat exchange unit  104 C. Simultaneously, this two-phase gas-liquid refrigerant flows into the lower header  4  of the second heat exchange unit  1046  through the fourth connection pipe  60 , and further flows into the lower header  4  of the first heat exchange unit  104 A through the second connection pipe  6 . At this time, the flow direction of the second flow switching means  106  is controlled such that the two-phase gas-liquid refrigerant having flowed into the lower header  4  of the first heat exchange unit  104 A is prevented from flowing out to the first flow pipe  7 . The two-phase gas-liquid refrigerant flows upward inside the flat tubes  1  of all the first heat exchange unit  104 A, the second heat exchange unit  1046 , and the third heat exchange unit  104 C to evaporate and vaporize into gas refrigerant. The gas refrigerant in the flat tubes  1  joins together in each upper header  3 . The gas refrigerant in the upper header  3  of the first heat exchange unit  104 A flows into the upper header  3  of the second heat exchange unit  104 B through the first connection pipe  5 , and then joins with the gas refrigerant in the upper header  3  of the second heat exchange unit  1046 . This gas refrigerant having joined together flows into the upper header  3  of the third heat exchange unit  104 C through the third connection pipe  50 , and then joins with the gas refrigerant in the upper header  3  of the third heat exchange unit  104 C. This gas refrigerant having joined together flows out to the refrigerant pipe  107  through the second flow pipe  8  with the opening-closing valve  8   a  brought into an opened state, and then flows to the compressor  101 . Note that the pressure in the third flow pipe  9  is higher than the pressure in the second flow pipe  8  during heating operation, the gas refrigerant flowing out of the second flow pipe  8  is thus prevented from flowing toward the valve body  108 . 
     There is a case where the air-conditioning load is so high during heating operation that it is unnecessary to reduce the operational frequency of the compressor  101 . In this case, the heat exchange efficiency is increased by allowing refrigerant in the first heat exchange unit  104 A, the second heat exchange unit  104 B, and the third heat exchange unit  104 C to all flow in the upward direction as described above. 
     The heat exchanger  104 , and the air-conditioning apparatus  300  including this heat exchanger  104  have been described above on the basis of the embodiments. However, the heat exchanger  104  and the air-conditioning apparatus  300  are not limited to the configurations described in the above embodiments. For example, the heat exchanger  104  is made up of two or three heat exchange units in the above embodiments, but may be made up of four or more heat exchange units. In addition, for example, the heat exchanger  104  and the air-conditioning apparatus  300  are not limited to the ones made up of the constituent elements described above, but may include other constituent element. To sum up, the heat exchanger  104  and the air-conditioning apparatus  300  include a range of design changes and application variations usually made by a person skilled in the art without departing from the technical spirit of the heat exchanger  104  and the air-conditioning apparatus  300 .