Patent Publication Number: US-9903625-B2

Title: Air-conditioning apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of PCT/JP2012/072848 filed on Sep. 7, 2012, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an air-conditioning apparatus that performs air-conditioning by using a refrigeration cycle (heat pump cycle). 
     BACKGROUND 
     In, for example, an air-conditioning apparatus that uses a refrigeration cycle (heat pump cycle), a refrigerant circuit that circulates a refrigerant is formed by connecting a heat-source-device-side unit (to be also referred to as a heat source device or an outdoor unit hereinafter) including a compressor and a heat-source-device-side heat exchanger, and load-side units (to be also referred to as indoor units hereinafter) including flow control devices (for example, expansion valves) and indoor-unit-side heat exchangers to one another by refrigerant pipes. In the indoor-unit-side heat exchanger, air-conditioning is performed while changing, for example, the pressure and temperature of the refrigerant in the refrigerant circuit, using the fact that the refrigerant receives or transfers heat from or to the air in an air-conditioned space, which is to undergo heat exchange, when the refrigerant evaporates or condenses. 
     An exemplary air-conditioning apparatus has conventionally been available which is capable of performing a simultaneous cooling and heating operation (cooling and heating mixed operation) in which cooling or heating is performed in each of a plurality of indoor units by automatically determining whether to perform cooling or heating for each indoor unit, in accordance with the temperature set on a remote controller provided to a corresponding indoor unit, and the temperature of the environment surrounding this indoor unit. 
     In an air-conditioning apparatus to be installed in, for example, a cold climate area, if the temperature of the air on the outside (to be referred to as the outdoor air hereinafter) is low, the refrigerant is guided via an injection pipe into an intermediate portion of a compression stroke of a compressor, provided in a heat source device, so as to improve the heating capacity (see, for example, Patent Literature 1). Such a configuration improves the capacity by increasing the density of refrigerant discharged from the compressor. 
     Guiding the refrigerant via the injection pipe into the intermediate portion of the compression stroke of the compressor will be referred to as “injection” hereinafter. The heating capacity refers to the amount of heat supplied to the indoor unit per unit time by refrigerant circulation during heating. Likewise, the cooling capacity refers to the amount of heat supplied to the indoor unit per unit time by refrigerant circulation during cooling. The heating capacity and the cooling capacity will sometimes be collectively referred to as “the capacity” hereinafter. 
     PATENT LITERATURE 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-198099 
     On the low-pressure side of the refrigeration cycle (to be simply referred to as the low-pressure side hereinafter), the refrigerant is susceptible, for example, to the temperature of the outdoor air, and the mode of operation. Therefore, if the refrigerant on the low-pressure side is injected into the compressor via a bypass during a heating operation performed in an environment in which the temperature of the outdoor air is low, a sufficient differential pressure cannot sometimes be obtained with respect to the pressure of the refrigerant that is being compressed. 
     Hence, the amount of refrigerant to be injected may become insufficient. Consequently, the temperature of the refrigerant as discharged from the compressor (to be referred to as the discharge temperature hereinafter) may rise excessively. 
     If the other end of the injection pipe is connected to a portion at which the refrigerant discharged from the compressing device is divided and flows into the injection pipe while the refrigerant, as condensed by coming into contact with and exchanging heat with a part of the heat-source-device-side heat exchanger, flows into the intermediate portion of the compression stroke, the heated gas refrigerant as injected cannot be supplied to those indoor units that are performing heating. Therefore, to ensure a satisfactory capacity, the total amount of circulation needs to be increased correspondingly. 
     SUMMARY 
     The present invention has been made in order to solve the above-described problems, and has as its object to provide an air-conditioning apparatus which can suppress a rise in discharge temperature of a compressing device and ensure a satisfactory capacity even if the temperature of the outdoor air is low. 
     An air-conditioning apparatus according to the present invention includes a heat source device including a compressing device that compresses a refrigerant, and a heat-source-side heat exchanger that exchanges heat between the refrigerant and outdoor air; an indoor unit including an indoor heat exchanger that exchanges heat between conditioned air and the refrigerant, and expansion means; a refrigerant pipe that connects the heat source device and the indoor unit to each other and forms a refrigerant circuit; an injection pipe allowing a part of the refrigerant, as discharged from the compressing device, to flow into an intermediate portion of a compression stroke of the compressing device; and an injection internal heat exchanger that exchanges heat between a refrigerant stream that flows through the injection pipe and a refrigerant stream that flows into the heat-source-side heat exchanger after passing through the indoor unit. 
     According to the present invention, an excessive rise in discharge temperature of the compressing device can be suppressed, and a satisfactory capacity can be ensured even if the temperature of the outdoor air is low. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 1 in a cooling only operation. 
         FIG. 2  is a diagram illustrating an exemplary configuration of a compressing device in the air-conditioning apparatus according to Embodiment 1. 
         FIG. 3  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in a heating only operation. 
         FIG. 4  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in a heating main operation. 
         FIG. 5  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in a cooling main operation. 
         FIG. 6  is a flowchart illustrating an exemplary operation of the air-conditioning apparatus according to Embodiment 1. 
         FIG. 7  is a diagram illustrating an exemplary configuration of a refrigerant circuit of an air-conditioning apparatus according to Embodiment 3. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 1 in a cooling only operation. 
     An exemplary configuration of a refrigerant circuit of an air-conditioning apparatus  1  will now be described with reference to  FIG. 1 . 
     The air-conditioning apparatus  1  is installed in a building such as an office building, an apartment, or a condominium. 
     The air-conditioning apparatus  1  performs a cooling/heating operation by using a refrigeration cycle (heat pump cycle) in which a refrigerant (a refrigerant for air-conditioning) circulates. 
     The air-conditioning apparatus  1  is capable of performing a simultaneous cooling and heating operation in which cooling and heating are performed simultaneously in combination in a plurality of indoor units. 
     An operation in which all indoor units that are in operation perform cooling will be referred to as a cooling only operation. 
     An operation in which all indoor units that are in operation perform heating will be referred to as a heating only operation. 
     An operation in which some indoor units perform cooling while others perform heating and cooling involves a higher load will be referred to as a cooling main operation. 
     An operation in which some indoor units perform cooling while others perform heating and heating involves a higher load will be referred to as a heating main operation. 
     [Overall Configuration] 
     The air-conditioning apparatus  1  includes a heat source device A, a plurality of indoor units B and C, and a relay device D. 
     The relay device D is provided between the heat source device A and the indoor units B and C. 
     The relay device D controls the flow of the refrigerant. 
     The relay device D is connected to the heat source device A by a first main pipe  107  and a second main pipe  106 . 
     The plurality of indoor units B and C are connected in parallel to the relay device D. The indoor unit B is connected by connection pipes  133  and  134 . The indoor unit C is connected by connection pipes  135  and  136 . 
     A controller  200  controls the operation of the air-conditioning apparatus  1 . 
     The heat source device A and the relay device D are connected to each other by the first main pipe  107  and the second main pipe  106 . 
     The first main pipe  107  has a diameter larger than that of the second main pipe  106 . 
     The second main pipe  106  guides the refrigerant from the heat source device A to the relay device D. 
     The first main pipe  107  guides the refrigerant from the relay device D to the heat source device A. 
     The refrigerant flowing through the first main pipe  107  has a pressure lower than that of the refrigerant flowing through the second main pipe  106 . 
     Note that a high or low pressure and a high or low stage do not refer to those defined by the relationship with a reference pressure (numerical value). 
     A high or low pressure refers to that relative to pressures (including an intermediate pressure) in the refrigerant circuit when pressurization is done by a compressing device  101 , the open/closed states (opening degrees) of respective flow control devices are controlled, or other operations are done. 
     The refrigerant has a highest pressure when it is discharged from the compressing device  101 . 
     Since the flow control devices or other devices reduce the pressure of the refrigerant, the refrigerant has a lowest pressure when it is drawn into the compressing device  101  by suction. 
     The relay device D and the indoor unit B are connected to each other by the connection pipes  134  and  133 . 
     The relay device D and the indoor unit C are connected to each other by the connection pipes  136  and  135 . 
     By connection using the first main pipe  107 , the second main pipe  106 , the connection pipes  134  and  133 , the refrigerant circulates through the heat source device A, the relay device D, and the indoor unit B. 
     By connection using the first main pipe  107 , the second main pipe  106 , the connection pipes  136  and  135 , the refrigerant circulates through the heat source device A, the relay device D, and the indoor unit C. 
     [Heat Source Device A] 
     The heat source device A includes the compressing device  101 , a four-way switching valve  102 , a heat-source-side heat exchanger  103 , an accumulator  104 , check valves  105   a ,  105   b ,  105   c , and  105   d , and an injection internal heat exchanger  122 . 
     The heat source device A also includes injection pipes  120   a  and  120   b , injection flow control devices  121   a  and  121   b , and a gas-liquid separating device  123 . 
     The “injection pipe  120   a ” corresponds to the “injection pipe” according to the present invention. 
     The “injection pipe  120   b ” corresponds to the “second injection pipe” according to the present invention. 
     The “injection flow control device  121   a ” corresponds to the “injection flow control device” according to the present invention. 
     The “injection flow control device  121   b ” corresponds to the “second injection flow control device” according to the present invention. 
       FIG. 2  is a diagram illustrating an exemplary configuration of the compressing device in the air-conditioning apparatus according to Embodiment 1. 
     The compressing device  101  pressurizes the refrigerant as drawn by suction, and discharges (delivers) the refrigerant. 
     As illustrated in  FIG. 2 , the compressing device  101  has a two-stage configuration including a low-stage-side compressor  101   a  and a high-stage-side compressor  101   b.    
     The driving frequencies of the low-stage-side compressor  101   a  and the high-stage-side compressor  101   b  can be arbitrarily changed. 
     The driving frequencies of the low-stage-side compressor  101   a  and the high-stage-side compressor  101   b  are controlled by an inverter circuit (not illustrated) on the basis of instructions sent by the controller  200 . 
     The compressing device  101  as a whole is capable of changing the amount of discharge (the amount of refrigerant discharged per unit time), and the capacity in correspondence with the amount of discharge. 
     The driving frequencies of the low-stage-side compressor  101   a  and the high-stage-side compressor  101   b  may be determined in advance at a predetermined ratio in accordance with the stroke volumes of the respective compressors. 
     The predetermined ratio refers to that when the suction pressure of the high-stage-side compressor  101   b  is equal to a predetermined value. 
     An injection port  101   c  is provided in an intermediate portion of the compression stroke between the low-stage-side compressor  101   a  and the high-stage-side compressor  101   b.    
     The injection port  101   c  allows the refrigerant flowing from the injection pipes  120   a  and  120   b  to be drawn into the high-stage-side compressor  101   b  by suction. 
     For example, in an environment in which the temperature of the outdoor air is low, if the pressure on the low-pressure side of the refrigerant circuit reduces and the density of refrigerant drawn into the low-stage-side compressor  101   a  by suction reduces, the controller  200  increases the rotation speed of the compressing device  101  by using the inverter circuit. Thus, a reduction in flow rate of the refrigerant is prevented, and a certain heating capacity is maintained. 
     When the pressure on the low-pressure side of the refrigerant circuit reduces, the compressing device  101  operates at a high compression ratio, leading to a high discharge temperature. In such a case, the controller  200  supplies the refrigerant, as cooled in the heat-source-side heat exchanger  103 , into the compressing device  101  via the injection port  101   c . Thus, a rise (an excessive rise) in temperature of the refrigerant, as discharged from the compressing device  101 , is prevented. 
     The four-way switching valve  102  switches the passage of the refrigerant on the basis of instructions sent by the controller  200 . 
     The four-way switching valve  102  switches the passage of the refrigerant among that for the cooling only operation, that for the heating only operation, that for the cooling main operation, and that for the heating main operation. 
     The heat-source-side heat exchanger  103  includes heat transfer tubes which pass the refrigerant, and fins provided for increasing the area of heat transfer between the refrigerant flowing through the heat transfer tubes and the air (the outdoor air). 
     The heat-source-side heat exchanger  103  exchanges heat between the refrigerant and the air (the outdoor air). 
     The heat-source-side heat exchanger  103  functions as an evaporator in the heating only operation and the heating main operation, and evaporates the refrigerant into a gas. 
     The heat-source-side heat exchanger  103  functions as a condenser in the cooling only operation and the cooling main operation, and condenses the refrigerant into a liquid. 
     In, for example, the cooling main operation, the heat-source-side heat exchanger  103  does not completely gasify or liquefy the refrigerant but may control the refrigerant state such that, for example, the refrigerant condenses into a two-phase mixture composed of a liquid and a gas (two-phase gas-liquid refrigerant). 
     An air-sending device  140  is provided near the heat-source-side heat exchanger  103 . 
     The air-sending device  140  sends air to the heat-source-side heat exchanger  103  so as to efficiently exchange heat between the refrigerant and the air. 
     The air-sending device  140  changes the volume of airflow on the basis of instructions sent by the controller  200 . 
     With a change in volume of airflow produced by the air-sending device  140 , the heat exchange capacity of the heat-source-side heat exchanger  103  can be changed. 
     The accumulator  104  is provided between the compressing device  101  and the four-way switching valve  102 . 
     The accumulator  104  stores an excess amount of refrigerant in the refrigerant circuit. 
     The check valve  105   a  is provided in a pipe extending between the heat-source-side heat exchanger  103  and the second main pipe  106 . 
     The check valve  105   a  supplies the refrigerant only in one direction from the heat-source-side heat exchanger  103  toward the second main pipe  106 . 
     The check valve  105   b  is provided in a pipe extending between the four-way switching valve  102  and the first main pipe  107 . 
     The check valve  105   b  supplies the refrigerant only in one direction from the first main pipe  107  toward the four-way switching valve  102 . 
     The second main pipe  106  and the first main pipe  107  are connected to each other by a connection pipe  130  that connects the upstream ends of the check valves  105   a  and  105   b  to each other. 
     The second main pipe  106  and the first main pipe  107  are also connected to each other by a connection pipe  131  that connects the downstream ends of the check valves  105   a  and  105   b  to each other. 
     That is, a connection portion “a” between the second main pipe  106  and the connection pipe  130  is located upstream of a connection portion “b” between the second main pipe  106  and the connection pipe  131 , and opposed to the connection portion “b” across the check valve  105   a.    
     A connection portion “c” between the first main pipe  107  and the connection pipe  130  is located upstream of a connection portion “d” between the first main pipe  107  and the connection pipe  131 , and opposed to the connection portion “d” across the check valve  105   b.    
     The connection pipe  130  is provided with the check valve  105   d.    
     The check valve  105   d  supplies the refrigerant only in one direction from the first main pipe  107  toward the second main pipe  106 . 
     The connection pipe  131  is provided with the check valve  105   c.    
     The check valve  105   c  supplies the refrigerant only in one direction from the first main pipe  107  toward the second main pipe  106 . 
     Referring to  FIG. 1 , among the check valves  105   a  to  105   d , open check valves are represented by open marks, and closed check valves are represented by filled marks. The same applies to refrigerant circuit diagrams to be described below, in which among the check valves  105   a  to  105   d , open check valves are represented by open marks, and closed check valves are represented by filled marks. 
     One end of the injection pipe  120   a  is connected to a pipe extending between the check valve  105   a  and the second main pipe  106 . 
     The other end of the injection pipe  120   a  is connected to the injection port  101   c.    
     The injection pipe  120   a  passes the refrigerant that is to flow into the high-stage-side compressor  101   b  of the compressing device  101 . 
     The injection pipe  120   a  is provided with the injection flow control device  121   a.    
     The injection flow control device  121   a  controls, on the basis of instructions sent by the controller  200 , the flow rate and pressure of the refrigerant that passes through the injection pipe  120   a.    
     The injection internal heat exchanger  122  is provided in a pipe extending between the check valve  105   a  and a flow control device  124 . 
     The injection internal heat exchanger  122  exchanges heat between a refrigerant stream that flows through the injection pipe  120   a  and a refrigerant stream that flows through the heat-source-side heat exchanger  103 . 
     The heat-source-side heat exchanger  103  includes an injection heat exchanging portion  103   a  that exchanges heat between a refrigerant stream that flows through the heat-source-side heat exchanger  103  and a refrigerant stream that flows through the injection pipe  120   a  when the heat-source-side heat exchanger  103  functions as an evaporator. 
     The injection heat exchanging portion  103   a  may be omitted. 
     One end of the injection pipe  120   b  is connected to the gas-liquid separating device  123 . 
     The other end of the injection pipe  120   b  is connected to the injection port  101   c.    
     The injection pipe  120   b  passes the refrigerant that is to flow (to be supplied) into the high-stage-side compressor  101   b  of the compressing device  101 . 
     The injection pipe  120   b  is provided with the injection flow control device  121   b.    
     The injection flow control device  121   b  controls, on the basis of instructions sent by the controller  200 , the flow rate and pressure of the refrigerant that passes through the injection pipe  120   b.    
     The gas-liquid separating device  123  separates the refrigerant that has passed through the first main pipe  107  into gas and liquid refrigerants. 
     The gas-liquid separating device  123  supplies at least a part of the separated liquid refrigerant into the injection pipe  120   b.    
     The gas-liquid separating device  123  may have a simple arrangement in which the refrigerant is drawn by suction in the lateral direction from a pipe extending vertically, and is thereby separated into a liquid refrigerant that flows downwards and a gas refrigerant that flows upwards. 
     In the cooling only operation or the cooling main operation, a high-pressure liquid refrigerant or a two-phase gas-liquid refrigerant flows through the first main pipe  107 . Since the gas-liquid separating device  123  is provided, the cooling capacity can be kept as high as possible, free from the influence of a significant pressure loss. 
     The heat source device A is provided with pressure detectors  125  and  126 , and an outdoor air temperature detector  127 . 
     The pressure detector  125  is provided to a pipe connected to the discharge end of the compressing device  101 . 
     The pressure detector  125  detects the pressure of the refrigerant as discharged from the compressing device  101 . 
     The pressure detector  125  may be implemented using a pressure sensor. 
     The controller  200  obtains a signal detected by the pressure detector  125 . 
     On the basis of the signal detected by the pressure detector  125 , the controller  200  detects, for example, a pressure Pd and a temperature Td of the refrigerant as discharged from the compressing device  101 . 
     On the basis of the pressure Pd, the controller  200  calculates, for example, a condensing temperature Tc. 
     The pressure detector  126  is provided to a pipe that connects the heat source device A and the first main pipe  107  to each other. 
     The pressure detector  126  detects the pressure of the refrigerant that flows from the relay device D (the indoor units B and C) into the heat source device A. 
     The outdoor air temperature detector  127  detects the temperature of the outdoor air (the outdoor air temperature). 
     [Relay Device D] 
     The relay device D includes a gas-liquid separating device  108 , a first branch portion  109 , a second branch portion  110 , a first heat exchanger  111 , and a second heat exchanger  113 . 
     The gas-liquid separating device  108  separates the refrigerant, upon flowing from the second main pipe  106  into the relay device D, into gas and liquid refrigerants. 
     The gas-liquid separating device  108  includes a gas-phase portion out of which the gas refrigerant flows, and a liquid-phase portion out of which the liquid refrigerant flows. 
     The gas-phase portion of the gas-liquid separating device  108  is connected to the first branch portion  109 . 
     The liquid-phase portion of the gas-liquid separating device  108  is connected to the second branch portion  110  via the first heat exchanger  111  and the second heat exchanger  113 . 
     In the first branch portion  109 , connection pipe  133  includes two branched connection pipes  133   a  and  133   b.    
     The branched connection pipe  133   a  is connected to the first main pipe  107 . 
     The branched connection pipe  133   b  is connected to a connection pipe  132 . 
     In the first branch portion  109 , connection pipe  135  includes two branched connection pipes  135   a  and  135   b.    
     The branched connection pipe  135   a  is connected to the first main pipe  107 . 
     The branched connection pipe  135   b  is connected to a connection pipe  132 . 
     The connection pipe  132  connects the gas-liquid separating device  108  and the first branch portion  109  to each other. 
     The connection pipe  133   a  that is connected to the indoor unit B is provided with a switching valve  109   a   1 . 
     The connection pipe  135   a  that is connected to the indoor unit C is provided with a switching valve  109   b   2 . 
     The connection pipe  133   b  that is connected to the indoor unit B is provided with a switching valve  109   b   1 . 
     The connection pipe  135   b  that is connected to the indoor unit C is provided with a switching valve  109   a   2 . 
     The switching valves  109   a   1 ,  109   a   2 ,  109   b   1 , and  109   b   2  are set open or closed under the control of the controller  200 , whereby the refrigerant is enabled or disabled to pass through them. 
     Referring to  FIG. 1 , among the switching valves  109   a   1 ,  109   a   2 ,  109   b   1 , and  109   b   2 , open switching valves are represented by open marks, and closed switching valves are represented by filled marks. The same applies to refrigerant circuit diagrams to be described below, in which among the switching valves  109   a   1 ,  109   a   2 ,  109   b   1 , and  109   b   2 , open switching valves are represented by open marks, and closed switching valves are represented by filled marks. 
     In the second branch portion  110 , the connection pipe  134  includes two branched connection pipes  134   a  and  134   b.    
     The branched connection pipe  134   b  is connected via a first merging portion  115  to a pipe extending between a first flow control device  112  (to be described later) and the second heat exchanger  113 . 
     The branched connection pipes  134   a  is connected via a second merging portion  116  to a pipe extending between a second flow control device  114  (to be described later) and the second heat exchanger  113 . 
     In the second branch portion  110 , the connection pipe  136  includes two branched connection pipes  136   a  and  136   b.    
     The branched connection pipe  136   b  is connected via a first merging portion  115  to a pipe extending between a first flow control device  112  (to be described later) and the second heat exchanger  113 . 
     The branched connection pipes  136   a  is connected via a second merging portion  116  to a pipe extending between a second flow control device  114  (to be described later) and the second heat exchanger  113 . 
     The connection pipe  134   a  that is connected to the indoor unit B is provided with a check valve  110   a   1 . 
     The connection pipe  136   a  that is connected to the indoor unit C is provided with a check valve  110   b   2 . 
     The connection pipe  134   b  that is connected to the indoor unit B is provided with a check valve  110   b   1 . 
     The connection pipe  136   b  that is connected to the indoor unit C is provided with another check valve  110   a   2 . 
     Each of the check valves  110   a   1 ,  110   a   2 ,  110   b   1 , and  110   b   2  supplies the refrigerant only in one direction. 
     Referring to  FIG. 1 , among the check valves  110   a   1 ,  110   a   2 ,  110   b   1 , and  110   b   2 , open check valves are represented by open marks, and closed check valves are represented by filled marks. The same applies to refrigerant circuit diagrams to be described below, in which among the check valves  110   a   1 ,  110   a   2 ,  110   b   1 , and  110   b   2 , open check valves are represented by open marks, and closed check valves are represented by filled marks. 
     The first merging portion  115  connects the gas-liquid separating device  108  and the second branch portion  110  to each other via the first flow control device  112  and the first heat exchanger  111 . 
     The second merging portion  116  provides branches each extending between the second branch portion  110  and the second heat exchanger  113 . 
     One branch is connected to the first merging portion  115  via the second heat exchanger  113 . 
     The other branch that forms a first bypass pipe  116   a  is connected to the first main pipe  107  via the second flow control device  114 , the second heat exchanger  113 , and the first heat exchanger  111 . 
     The first heat exchanger  111  is provided between the gas-liquid separating device  108  and the first flow control device  112 . 
     The first heat exchanger  111  exchanges heat between a refrigerant stream that flows from the gas-liquid separating device  108  toward the first merging portion  115 , and a refrigerant stream that flows from the second heat exchanger  113  to the first main pipe  107 . 
     In, for example, the cooling only operation, the first heat exchanger  111  supercools and supplies the liquid refrigerant to the indoor units B and C. 
     The first heat exchanger  111  is connected to the first main pipe  107  by a pipe, and supplies, into the first main pipe  107 , the refrigerant streams that flow from the indoor units B and C and the refrigerant stream used for supercooling. 
     The second heat exchanger  113  is provided between the first merging portion  115  and the second merging portion  116 . 
     The second heat exchanger  113  exchanges heat between a refrigerant stream that flows from the first merging portion  115  to the second merging portion  116 , and a refrigerant stream that branches off at the second merging portion  116  and flows through the first bypass pipe  116   a.    
     In, for example, the cooling only operation, the second heat exchanger  113  supercools and supplies the liquid refrigerant to the indoor units B and C. The second heat exchanger  113  is connected to the first main pipe  107  by a pipe, and supplies, into the first main pipe  107 , the refrigerant streams that flow from the indoor units B and C and the refrigerant stream used for supercooling. 
     The first flow control device  112  is provided between the first heat exchanger  111  and the second heat exchanger  113 . 
     The first flow control device  112  has its opening degree controlled on the basis of instructions sent by the controller  200 . 
     The first flow control device  112  controls the flow rate and pressure of the refrigerant flowing from the gas-liquid separating device  108  to the first heat exchanger  111 . 
     The second flow control device  114  is provided in the first bypass pipe  116   a  extending between the second merging portion  116  and the second heat exchanger  113 . 
     The second flow control device  114  has its opening degree controlled on the basis of instructions sent by the controller  200 . 
     The second flow control device  114  controls the flow rate and pressure of the refrigerant flowing through the first bypass pipe  116   a.    
     The relay device D is provided with pressure detectors  128  and  129 . 
     The pressure detector  128  is provided to a pipe extending between the first heat exchanger  111  and the first flow control device  112 . 
     The pressure detector  128  detects the pressure of the refrigerant flowing from the first heat exchanger  111  to the first flow control device  112 . 
     The pressure detector  129  is provided to a pipe extending between the first flow control device  112  and the first merging portion  115 . 
     The pressure detector  129  detects the pressure of the refrigerant flowing from the first flow control device  112  to the first merging portion  115 . 
     The controller  200  obtains signals detected by the pressure detectors  128  and  129 . 
     On the basis of the difference between the pressures detected by the pressure detectors  128  and  129 , the controller  200  determines the opening degree of the second flow control device  114 . 
     The refrigerant having flowed through the second flow control device  114  and the first bypass pipe  116   a  supercools the refrigerant pools in, for example, the second heat exchanger  113  and the first heat exchanger  111 , and flows into the first main pipe  107 . 
     The second heat exchanger  113  exchanges heat between a refrigerant stream which passes through the second flow control device  114  and flows through the first bypass pipe  116   a , and a refrigerant stream that flows from the first flow control device  112 . 
     The first heat exchanger  111  exchanges heat between a refrigerant stream having passed through the first bypass pipe  116   a  and the second heat exchanger  113 , and a refrigerant stream that flows from the gas-liquid separating device  108  to the first flow control device  112 . 
     A second bypass pipe  116   b  supplies the refrigerant which passes through the second heat exchanger  113  and flows into the indoor unit B via the check valve  110   a   1   
     The second bypass pipe  116   b  supplies the refrigerant which passes through the second heat exchanger  113  and flows into the indoor unit C via the check valve  110   b   2   
     In the cooling main operation and the heating main operation, the refrigerant having passed through the second bypass pipe  116   b  flows through the second heat exchanger  113 . Subsequently, the refrigerant partially or wholly flows into either of the indoor units B and C that is performing cooling. 
     In, for example, the heating only operation, the refrigerant wholly passes through the second flow control device  114  and the first bypass pipe  116   a  and flows into the first main pipe  107 . 
     [Indoor Units B and C] 
     The indoor unit B includes an expansion means  117   a  and an indoor heat exchanger  118   a  that are connected in series to each other. 
     The indoor unit C includes an expansion means  117   b  and an indoor heat exchanger  118   b  that are connected in series to each other. 
     According to Embodiment 1, in the cooling main operation and the heating main operation, the indoor unit B receives cooling energy supplied from the heat source device A and takes charge of a cooling load, while the indoor unit C receives heating energy supplied from the heat source device A and takes charge of a heating load. 
     In the cooling only operation, both the indoor units B and C receive cooling energy supplied from the heat source device A and take charge of a cooling load. 
     In the heating only operation, both the indoor units B and C receive heating energy supplied from the heat source device A and take charge of a heating load. 
     The indoor heat exchangers  118   a  and  118   b  include heat transfer tubes which pass the refrigerant, and fins provided to increase the area of heat transfer between the refrigerant flowing through the heat transfer tubes and the indoor air. The indoor heat exchangers  118   a  and  118   b  exchange heat between the refrigerant and the indoor air. 
     The indoor heat exchangers  118   a  and  118   b  function as a radiator (condenser) or an evaporator. 
     The indoor heat exchangers  118   a  and  118   b  condense the refrigerant into a liquid or evaporates it into a gas. 
     Air-sending devices  141   a  and  141   b  are provided near the indoor heat exchangers  118   a  and  118   b , respectively. 
     The air-sending devices  141   a  and  141   b  send the air to the indoor heat exchangers  118   a  and  118   b  so that heat is efficiently exchanged between the refrigerant and the air, respectively. 
     The air-sending devices  141   a  and  141   b  change the volume of airflow on the basis of instructions sent by the controller  200 . With a change in volume of airflow caused by the air-sending devices  141   a  and  141   b , the heat exchange capacity of the indoor heat exchangers  118   a  and  118   b  can be changed respectively. 
     The expansion means  117   a  and  117   b  function as a pressure reducing valve or an expansion valve. 
     The expansion means  117   a  and  117   b  decompress and expand the refrigerant. The opening degree of the expansion means  117   a  and  117   b  is variable. 
     [Controller  200  and Storage Means  201 ] 
     The controller  200  performs, for example, determination processes on the basis of signals transmitted from various detectors (sensors) provided inside and outside the air-conditioning apparatus  1  and from the devices (means) in the air-conditioning apparatus  1 . 
     The controller  200  operates the devices on the basis of the results obtained by the determination processes or other processes. 
     The controller  200  systematically controls the operation of the overall air-conditioning apparatus  1 . 
     Specifically, the controller  200  controls, for example, the driving frequency of the compressing device  101 , the opening degrees of flow control devices including the flow control device  124 , and the switching of the four-way switching valve  102 , the switching valves  109   a   1 ,  109   a   2 ,  109   b   1 , and  109   b   2 , and the expansion means  117   a  and  117   b.    
     A storage means  201  temporarily or for a long period of time stores various types of data, programs, and other types of information which are necessary for the above-mentioned processes of the controller  200 . 
     While Embodiment 1 assumes that the controller  200  and the storage means  201  are provided independently of the heat source device A, the present invention is not limited to such a case. For example, the controller  200  and the storage means  201  may be included in the heat source device A. 
     While Embodiment 1 assumes that the controller  200  and the storage means  201  are included in the air-conditioning apparatus  1 , the present invention is not limited to such a case. For example, the controller  200  and the storage means  201  may be provided outside the air-conditioning apparatus  1 , and the air-conditioning apparatus  1  may be remotely controlled by signal communication over a telecommunications network or the like. 
     [Operation] 
     The air-conditioning apparatus  1  according to Embodiment 1 performs any of the cooling only operation, the heating only operation, the cooling main operation, and the heating main operation. 
     The heat-source-side heat exchanger  103  functions as a condenser in the cooling only operation and the cooling main operation. 
     The heat-source-side heat exchanger  103  functions as an evaporator in the heating only operation and the heating main operation. 
     The operations of the devices and the flow of the refrigerant in each operation will now be described. 
     [Cooling Only Operation] 
     The operations of the devices and the flow of the refrigerant in the cooling only operation will be described with reference to  FIG. 1 . 
     The following description assumes that all indoor units are performing cooling without interruption. 
     The compressing device  101  compresses the refrigerant drawn by suction and discharges a high-pressure gas refrigerant. 
     The high-pressure gas refrigerant discharged from the compressing device  101  flows through the four-way switching valve  102  into the heat-source-side heat exchanger  103 . 
     While passing through the heat-source-side heat exchanger  103 , the high-pressure gas refrigerant is condensed by exchanging heat with the outdoor air, and turns into a high-pressure liquid refrigerant. 
     The high-pressure liquid refrigerant flows through the check valve  105   a.    
     In this process, the high-pressure liquid refrigerant does not flow toward the check valve  105   c  or  105   d  because of factors associated with the relationship of pressure of the refrigerant. 
     The high-pressure liquid refrigerant then flows through the second main pipe  106  into the relay device D. 
     The gas-liquid separating device  108  separates the refrigerant having flowed into the relay device D into gas and liquid refrigerants. 
     The refrigerant that flows into the relay device D in the cooling only operation is a liquid refrigerant. 
     The controller  200  switches the switching valve  109   a   1  that is provided in the connection pipe  133   a  to an open state. 
     The controller  200  switches the switching valve  109   b   2  that is provided in the connection pipe  135   a  to an open state. 
     The controller  200  switches the switching valve  109   b   1  that is provided in the connection pipe  133   b  to a closed state. 
     The controller  200  switches the switching valve  109   a   2  that is provided in the connection pipe  135   b  to a closed state. 
     Therefore, the gas refrigerant separated by the gas-liquid separating device  108  does not flow from the gas-liquid separating device  108  to the indoor units B and C. 
     The liquid refrigerant separated by the gas-liquid separating device  108  flows through the first heat exchanger  111 , the first flow control device  112 , and the second heat exchanger  113 , and a part of the liquid refrigerant flows into the second branch portion  110 . 
     The refrigerant having flowed into the second branch portion  110  is divided into refrigerant streams that flow through the check valve  110   a   1  connected to the connection pipe  134   a  and the check valve  110   b   2  connected to the connection pipe  136   a , flow through the connection pipes  134  and  136 , and flow into the indoor units B and C, respectively. 
     The controller  200  controls the opening degrees of the expansion means  117   a . In the indoor unit, the expansion means  117   a  controls the pressure of the liquid refrigerant having flowed into it from the connection pipe  134 . 
     The opening degree of the expansion means  117   a  is controlled on the basis of the degree of superheat of the refrigerant at the outlet of the indoor heat exchanger  118   a.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   a  flows into the indoor heat exchanger  118   a.    
     The low-pressure liquid refrigerant or the two-phase gas-liquid refrigerant evaporates by exchanging heat with the indoor air in the air-conditioned space while passing through the indoor heat exchanger  118   a.    
     In this process, the refrigerant exchanges heat with the indoor air and cools it, whereby the indoor space is cooled. 
     The refrigerant having passed through the indoor heat exchanger  118   a  turns into a low-pressure gas refrigerant and flows into a corresponding connection pipe  133 . 
     The controller  200  controls the opening degrees of the expansion means  117   b . In the indoor unit C, the expansion means  117   b  controls the pressure of the liquid refrigerant having flowed into it from the connection pipe  136 . 
     The opening degree of the expansion means  117   b  is controlled on the basis of the degree of superheat of the refrigerant at the outlet of the indoor heat exchanger  118   b.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   b  flows into the indoor heat exchanger  118   b.    
     The low-pressure liquid refrigerant or the two-phase gas-liquid refrigerant evaporates by exchanging heat with the indoor air in the air-conditioned space while passing through the indoor heat exchanger  118   b.    
     In this process, the refrigerant exchanges heat with the indoor air and cools it, whereby the indoor space is cooled. 
     The refrigerant having passed through the indoor heat exchanger  118   b  turns into a low-pressure gas refrigerant and flows into a corresponding connection pipe  135 . 
     The refrigerant having passed through the indoor heat exchangers  118   a  and  118   b  may turn into a two-phase gas-liquid refrigerant. 
     If, for example, the air-conditioning load of at least one of the indoor units B and C is small or shifting because, for example, operation has just started, the refrigerant is not completely gasified in at least one of the indoor heat exchangers  118   a  and  118   b , and turns into a two-phase gas-liquid refrigerant. 
     The air-conditioning load refers to the amount of heat necessary for each of the indoor units B and C, and will also be simply referred to as the load hereinafter. 
     The low-pressure gas refrigerant or the two-phase gas-liquid refrigerant (low-pressure refrigerant) having flowed through the connection pipe  133  flows through the switching valve  109   a   1  connected to the connection pipe  133   a , and flows into the first main pipe  107 . 
     The low-pressure gas refrigerant or the two-phase gas-liquid refrigerant (low-pressure refrigerant) having flowed through each connection pipe  135  flows through a corresponding one of the switching valve  109   b   2  connected to the connection pipe  135   a , and flows into the first main pipe  107 . 
     The refrigerant having passed through the first main pipe  107  into the heat source device A flows through the check valve  105   b , the four-way switching valve  102 , and the accumulator  104 , and returns to the compressing device  101 . 
     The above-mentioned arrangement corresponds to a basic circulation passage of the refrigerant in the cooling only operation. 
     In the cooling only operation, the controller  200  sets the opening degrees of the injection flow control devices  121   a  and  121   b  to zero. 
     The injection flow control device  121   a  is set to zero opening degree, and does not supply the refrigerant into the injection pipe  120   a.    
     The injection flow control device  121   b  is set to zero opening degree, and does not supply the refrigerant into the injection pipe  120   b.    
     The flow of the refrigerant in the first heat exchanger  111  and the second heat exchanger  113  will now be described. 
     The liquid refrigerant separated by the gas-liquid separating device  108  passes through the first heat exchanger  111 , the first flow control device  112 , and the second heat exchanger  113 . Then, a part of the liquid refrigerant flows into the second branch portion  110 , while the remaining part of the liquid refrigerant flows into the second flow control device  114 . 
     The refrigerant having flowed into the second flow control device  114  passes through the first bypass pipe  116   a , supercools the refrigerant stream flowing from the gas-liquid separating device  108  in the second heat exchanger  113  and the first heat exchanger  111 , and flows into the first main pipe  107 . 
     By supercooling and supplying the refrigerant toward the second branch portion  110 , the enthalpy of the refrigerant on the inlet side (the side of the connection pipes  134  and  136 ) can be reduced. Hence, the amount of heat exchanged with the air in the indoor heat exchangers  118   a  and  118   b  can be increased. 
     If the opening degree of the second flow control device  114  is large, and the amount of refrigerant flowing through the first bypass pipe  116   a  (the refrigerant to be used for supercooling) is thus relatively large, too little refrigerant may evaporate in the indoor heat exchangers  118   a  and  118   b.    
     Therefore, the controller  200  controls the opening degree of the second flow control device  114  such that the difference between pressures detected by the pressure detectors  128  and  129  reaches a predetermined value, thereby controlling the degree of superheat of the refrigerant at the outlet of the first flow control device  112 . 
     The controller  200  controls the discharge capacity of the compressing device  101  and the volume of airflow produced by the air-sending devices  140 ,  141   a  and  141 , and provides a capacity corresponding to the load imposed on the indoor units B and C. 
     With this operation, the controller  200  controls the evaporating temperatures of the refrigerant in the indoor heat exchangers  118   a  and  118   b  and the condensing temperature of the refrigerant in the heat-source-side heat exchanger  103  to reach predetermined target temperatures. 
     [Heating Only Operation] 
       FIG. 3  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in the heating only operation. 
     The operations of the devices and the flow of the refrigerant in the heating only operation will now be described with reference to  FIG. 3 . 
     The following description assumes that all indoor units are performing cooling without interruption. 
     The compressing device  101  compresses the refrigerant drawn by suction and discharges a high-pressure gas refrigerant. 
     The high-pressure gas refrigerant discharged from the compressing device  101  flows through the four-way switching valve  102  into the check valve  105   c.    
     In this process, the high-pressure gas refrigerant does not flow toward the check valve  105   b  or  105   a  because of factors associated with the relationship of pressure of the refrigerant. 
     The high-pressure gas refrigerant then flows through the second main pipe  106  into the relay device D. 
     The controller  200  switches the switching valve  109   a   1  that is provided in the connection pipe  133   a  to a closed state. 
     The controller  200  switches the switching valve  109   b   2  that is provided in the connection pipe  135   a  to a closed state. 
     The controller  200  switches the switching valve  109   b   1  that is provided in the connection pipe  133   b  to an open state. 
     The controller  200  switches the switching valve  109   a   2  that is provided in the connection pipe  135   b  to an open state. 
     Hence, the gas refrigerant separated by the gas-liquid separating device  108  flows from the first branch portion  109 , flows through the connection pipes  133  and  135 , and flows toward the indoor units B and C, respectively. 
     While passing through the indoor heat exchangers  118   a  and  118   b , the high-pressure gas refrigerant is condensed by exchanging heat with the indoor air in a corresponding air-conditioned space. 
     In this process, the refrigerant exchanges heat with the indoor air and heats it, whereby the indoor space is heated. 
     The refrigerant having passed through the indoor heat exchangers  118   a  and  118   b  turns into a liquid refrigerant, and further passes through the expansion means  117   a  and  117   b.    
     The controller  200  controls the opening degrees of the expansion means  117   a  and  117   b.    
     In the indoor unit B, the expansion means  117   a  controls the pressure of the liquid refrigerant having flowed out of a corresponding indoor heat exchanger  118   a.    
     The opening degree of the expansion means  117   a  is controlled on the basis of the degree of supercooling of the refrigerant at the outlet of the indoor heat exchanger  118   a.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   a  flows through the connection pipe  134  into the second branch portion  110 . 
     The refrigerant having flowed into the second branch portion  110  flows into the first merging portion  115  through the check valve  110   b   1  that is connected to the connection pipes  134   b.    
     In the indoor unit C, the expansion means  117   b  controls the pressure of the liquid refrigerant having flowed out of a corresponding indoor heat exchanger  118   b.    
     The opening degree of the expansion means  117   b  is controlled on the basis of the degree of supercooling of the refrigerant at the outlet of the indoor heat exchanger  118   b.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   b  flows through the connection pipe  136  into the second branch portion  110 . 
     The refrigerant having flowed into the second branch portion  110  flows into the first merging portion  115  through the check valve  110   a   2  that is connected to the connection pipe  136   b.    
     The refrigerant having flowed from the first merging portion  115  into the second heat exchanger  113  flows from the second merging portion  116  into the second flow control device  114 . 
     Then, the refrigerant having flowed out of the second flow control device  114  passes through the first bypass pipe  116   a , the second heat exchanger  113 , and the first heat exchanger  111 , and flows into the first main pipe  107 . 
     In this process, the opening degree of the second flow control device  114  is controlled, whereby the low-pressure two-phase gas-liquid refrigerant flows into the first main pipe  107 . 
     The refrigerant having flowed through the first main pipe  107  into the heat source device A flows through the check valve  105   d  into the heat-source-side heat exchanger  103 . 
     While the refrigerant having flowed into the heat-source-side heat exchanger  103  passes through the heat-source-side heat exchanger  103 , the refrigerant exchanges heat with the outdoor air and evaporates, thereby turning into a gas refrigerant. 
     The gas refrigerant flows through the four-way switching valve  102  and the accumulator  104 , and returns to the compressing device  101 . 
     The above-mentioned arrangement corresponds to a circulation passage of the refrigerant in the heating only operation. 
     The controller  200  controls the discharge capacity of the compressing device  101  and the volume of airflow produced by the air-sending devices  140 ,  141   a , and  141   b , and provides a capacity corresponding to the load imposed on the indoor units B and C. 
     With this operation, the controller  200  controls the condensing temperatures of the refrigerant in the indoor heat exchangers  118   a  and  118   b  and the evaporating temperature of the refrigerant in the heat-source-side heat exchanger  103  to reach predetermined target temperatures. 
     In the heating only operation, the controller  200  controls the opening degrees of the injection flow control devices  121   a  and  121   b  on the basis of the temperature of the outdoor air. 
     That is, the controller  200  controls the opening degree of the injection flow control device  121   a  on the basis of the temperature of the outdoor air, supplies the high-pressure gas refrigerant into the injection pipe  120   a , and supplies the high-pressure gas refrigerant from the injection port  101   c  into the suction end of the high-stage-side compressor  101   b.    
     Furthermore, the controller  200  controls the opening degree of the injection flow control device  121   b , supplies the liquid refrigerant into the injection pipe  120   b , and further supplies the liquid refrigerant from the injection port  101   c  into the suction side of the high-stage-side compressor  101   b.    
     Details of the injection operation will be described later. 
     The capacity provided by the compressing device  101  is maintained by, for example, increasing the driving frequency. 
     While the above description assumes that in the cooling only operation and the heating only operation, both the indoor units B and C are in operation, one of the indoor units B and C may be kept stopped, for example. 
     If, for example, one of the indoor units is kept stopped, and the overall load of the air-conditioning apparatus  1  is small, the capacity to be provided by the compressing device  101  may be changed while the low-stage-side compressor  101   a  or the high-stage-side compressor  101   b  is kept stopped. 
     [Heating Main Operation] 
       FIG. 4  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in the heating main operation. 
     The operations of the devices and the flow of the refrigerant in the heating main operation will now be described with reference to  FIG. 4 . 
     The following description assumes that the indoor unit C performs heating while the indoor unit B performs cooling. 
     The operations of the devices and the flow of the refrigerant in the heat source device A are the same as in the heating only operation that has been described with reference to  FIG. 3 . 
     The controller  200  switches the switching valve  109   a   1  connected to the connection pipe  133   a  to an open state. 
     The controller  200  switches the switching valve  109   a   2  connected to the connection pipe  135   b  to an open state. 
     The controller  200  switches the switching valve  109   b   1  connected to the connection pipe  133   b  to a closed state. 
     The controller  200  switches the switching valve  109   b   2  connected to the connection pipe  135   a  to a closed state. 
     Therefore, the gas refrigerant separated by the gas-liquid separating device  108  flows only toward the indoor unit C from the first branch portion  109  via the connection pipe  135 . 
     The flow of the refrigerant in the indoor unit C that is performing heating is the same as in the heating only operation that has been described with reference to  FIG. 3 . 
     On the other hand, the flow of the refrigerant in the indoor unit B that is performing cooling is different from that in the indoor unit C that is performing heating. 
     In the indoor unit C, a low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   b  flows through the connection pipe  136  into the second branch portion  110 . 
     The refrigerant having flowed into the second branch portion  110  flows through the check valve  110   a   2  connected to the connection pipe  136   b  into the first merging portion  115 . 
     The controller  200  closes the first flow control device  112 , thereby blocking the flow of the refrigerant between the gas-liquid separating device  108  and the first merging portion  115 . 
     Therefore, the refrigerant flows from the first merging portion  115  into the second merging portion  116  through the second heat exchanger  113 . 
     A part of the refrigerant having flowed into the second merging portion  116  flows into the second bypass pipe  116   b , and flows through the check valve  110   a   1  connected to the connection pipe  134   a  and through the connection pipe  134  into the indoor unit B. 
     The controller  200  controls the opening degree of the expansion means  117   a.    
     In the indoor unit B, the expansion means  117   a  controls the pressure of the liquid refrigerant having flowed into it from the connection pipe  134 . 
     The opening degree of the expansion means  117   a  is controlled on the basis of the degree of superheat of the refrigerant at the outlet of a corresponding indoor heat exchanger  118   a.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   a  flows into the indoor heat exchanger  118   a  of the indoor unit B. 
     While passing through the indoor heat exchanger  118   a , the low-pressure liquid refrigerant or the two-phase gas-liquid refrigerant exchanges heat with the indoor air in the air-conditioned space and thus evaporates. 
     In this process, the refrigerant exchanges heat with the indoor air and cools it, whereby the indoor space is cooled. 
     The refrigerant having passed through the indoor heat exchanger  118   a  turns into a low-pressure gas refrigerant, and flows into a corresponding connection pipe  133 . 
     The low-pressure gas refrigerant or the two-phase gas-liquid refrigerant (low-pressure refrigerant) having flowed through the connection pipe  133  passes through the switching valve  109   a   1  connected to the connection pipe  133   a , and flows into the first main pipe  107 . 
     On the other hand, a part of the refrigerant having flowed through the second heat exchanger  113  into the second merging portion  116  flows into the second flow control device  114 . 
     The refrigerant having flowed out of the second flow control device  114  passes through the first bypass pipe  116   a , the second heat exchanger  113 , and the first heat exchanger  111 , and flows into the first main pipe  107 . 
     In this process, the controller  200  controls the opening degree of the second flow control device  114 , whereby an amount of refrigerant necessary for the indoor unit C is supplied, and the remaining amount of refrigerant flows into the first main pipe  107  via the first bypass pipe  116   a.    
     As in the heating only operation described above, in the heating main operation, the controller  200  controls the opening degrees of the injection flow control devices  121   a  and  121   b  on the basis of the temperature of the outdoor air. Details of the injection operation will be described later. 
     In the heating main operation, the refrigerant having flowed out of the indoor unit that is performing heating (in this case, the indoor unit C) flows into the indoor unit that is performing cooling (in this case, the indoor unit B). 
     Therefore, when the indoor unit B that is performing cooling stops its operation, the amount of two-phase gas-liquid refrigerant which flows through the first bypass pipe  116   a  increases. 
     On the other hand, as the load imposed on the indoor unit B that is performing cooling increases, the amount of two-phase gas-liquid refrigerant flowing through the first bypass pipe  116   a  decreases. 
     Therefore, while the amount of refrigerant necessary in the indoor unit C that is performing heating remains the same, the load imposed on the indoor heat exchanger  118   a  (evaporator) of the indoor unit B that is performing cooling changes. 
     In the heating main operation as well, the controller  200  controls the discharge capacity of the compressing device  101  and the volume of airflow produced by the air-sending devices  140 ,  141   a , and  141   b , and provides a capacity corresponding to the load imposed on the indoor units B and C. 
     [Cooling Main Operation] 
       FIG. 5  is a refrigerant circuit diagram of the air-conditioning apparatus according to Embodiment 1 in the cooling main operation. 
     The operations of the devices and the flow of the refrigerant in the cooling main operation will now be described with reference to  FIG. 5 . 
     The following description assumes that the indoor unit C performs heating while the indoor unit B performs cooling. 
     The operations of the devices and the flow of the refrigerant in the heat source device A are the same as in the cooling only operation that has been described with reference to  FIG. 1 . 
     However, in the cooling main operation, the condensing capacity of the refrigerant in the heat-source-side heat exchanger  103  is controlled such that the refrigerant flowing through the second main pipe  106  into the relay device D becomes a two-phase gas-liquid refrigerant. 
     That is, the controller  200  controls the discharge capacity of the compressing device  101  and the volume of airflow produced by the air-sending device  140 , thereby controlling the condensing capacity of the refrigerant in the heat-source-side heat exchanger  103 . 
     The gas-liquid separating device  108  separates the refrigerant having flowed into the relay device D into gas and liquid refrigerants. 
     The refrigerant flowing into the relay device D in the cooling main operation is a two-phase gas-liquid refrigerant. 
     The controller  200  switches the switching valve  109   a   1  connected to the connection pipe  133   a  to an open state. 
     The controller  200  switches the switching valve  109   a   2  connected to the connection pipe  135   b  to an open state. 
     The controller  200  switches the switching valve  109   b   1  connected to the connection pipe  133   b  to a closed state. 
     The controller  200  switches the switching valve  109   b   2  connected to the connection pipe  135   a  to a closed state. 
     Therefore, the gas refrigerant separated by the gas-liquid separating device  108  flows only toward the indoor unit C from the first branch portion  109  via the connection pipe  135 . 
     In the indoor unit C, while passing through the indoor heat exchanger  118   b , the high-pressure gas refrigerant is condensed by heat exchange and turns into a liquid refrigerant. The liquid refrigerant passes through the expansion means  117   b.    
     In this process, the refrigerant exchanges heat with the indoor air and heats it, whereby the indoor space is heated. 
     The refrigerant having passed through the expansion means  117   b  turns into a liquid refrigerant whose pressure has been slightly reduced. The liquid refrigerant flows through the connection pipe  136  into the second branch portion  110 . 
     The refrigerant having flowed into the second branch portion  110  flows through the check valve  110   a   2  connected to the connection pipe  136   b  into the first merging portion  115 . 
     The controller  200  controls the opening degree of the first flow control device  112 , and supplies the liquid refrigerant separated by the gas-liquid separating device  108  into the first merging portion  115 . 
     Therefore, the liquid refrigerant having flowed from the gas-liquid separating device  108  and the liquid refrigerant having flowed from the second branch portion  110  merge in the first merging portion  115 . 
     The merged liquid refrigerant flows from the first merging portion  115  into the second merging portion  116  through the second heat exchanger  113 . 
     A part of the refrigerant having flowed into the second merging portion  116  flows into the second bypass pipe  116   b , and further flows into the indoor unit B through the check valve  110   a   1  connected to the connection pipe  134   a  and through the connection pipe  134 . 
     The controller  200  controls the opening degree of the expansion means  117   a . In the indoor unit B, the expansion means  117   a  controls the pressure of the liquid refrigerant having flowed into it from the connection pipe  134 . 
     The opening degree of the expansion means  117   a  is controlled on the basis of the degree of superheat of the refrigerant at the outlet of the indoor heat exchanger  118   a.    
     A low-pressure liquid refrigerant or a two-phase gas-liquid refrigerant generated by controlling the opening degree of the expansion means  117   a  flows into the indoor heat exchanger  118   a  of the indoor unit B. 
     While passing through the indoor heat exchanger  118   a , the low-pressure liquid refrigerant or the two-phase gas-liquid refrigerant exchanges heat with the indoor air in the air-conditioned space and thus evaporates. 
     In this process, the refrigerant exchanges heat with the indoor air and cools it, whereby the indoor space is cooled. 
     The refrigerant having passed through the indoor heat exchanger  118   a  turns into a low-pressure gas refrigerant, and flows into a corresponding connection pipe  133 . 
     The low-pressure gas refrigerant or the two-phase gas-liquid refrigerant (low-pressure refrigerant) having flowed through the connection pipe  133  flows through the switching valve  109   a   1  connected to the connection pipe  133   a  into the first main pipe  107 . 
     As described above, in the cooling main operation, the heat-source-side heat exchanger  103  functions as a condenser. 
     The refrigerant having passed through the indoor unit C that is performing heating is used as a refrigerant for the indoor unit B that is performing cooling. 
     In this process, if, for example, the load imposed on the indoor unit B is small and the amount of refrigerant flowing through the indoor unit B is kept small, the controller  200  increases the opening degree of the second flow control device  114 . 
     With this operation, the refrigerant can be supplied through the first bypass pipe  116   a  into the first main pipe  107  without supplying an excess amount of refrigerant to the indoor unit B that is performing cooling. 
     In the cooling main operation as well, the controller  200  controls the discharge capacity of the compressing device  101  and the volume of airflow produced by the air-sending devices  140 ,  141   a , and  141   b , and provides a capacity corresponding to the load imposed on the indoor units B and C. 
     In the cooling main operation, the controller  200  sets the opening degrees of the injection flow control devices  121   a  and  121   b  to zero. 
     The injection flow control device  121   a  is set to zero opening degree, and does not supply the refrigerant into the injection pipe  120   a.    
     The injection flow control device  121   b  is set to zero opening degree, and does not supply the refrigerant into the injection pipe  120   b.    
     [Control Operation for Injection] 
     When the temperature of the outdoor air lowers, the pressure of the refrigerant in the heat-source-side heat exchanger  103  that functions as an evaporator in the heating only operation and the heating main operation also lowers. That is, the pressure of the refrigerant on the suction side of the compressing device  101  lowers. 
     Therefore, the amount of refrigerant drawn into the compressing device  101  by suction (the refrigerant in circulation) reduces (the density of refrigerant reduces). 
     As the amount of refrigerant drawn into the compressing device  101  by suction reduces, the compression ratio increases, whereby the temperature of the refrigerant discharged from the compressing device  101  (discharge temperature), in turn, increases. 
     Hence, the controller  200  changes the opening degree of at least one of the injection flow control devices  121   a  and  121   b.    
     Thus, some refrigerant is supplied from the injection port  101   c , whereby the density of refrigerant is increased. 
     Furthermore, the temperature of the refrigerant drawn into the high-stage-side compressor  101   b  by suction is reduced so that the temperature of the refrigerant discharged from the compressing device  101  does not rise excessively. 
     According to Embodiment 1, in the heating only operation and the heating main operation, the high-pressure gas refrigerant, as discharged from the compressing device  101 , is divided at one end of the injection pipe  120   a.    
     The other end of the injection pipe  120   a  is connected to the injection port  101   c  of the compressing device  101 . 
     The controller  200  reduces the pressure of the refrigerant passing through the injection pipe  120   a  by using the injection flow control device  121   a.    
     A part of the injection pipe  120   a  extends through the injection internal heat exchanger  122 . 
     In the injection internal heat exchanger  122 , a refrigerant stream which flows through the injection pipe  120   a  and a refrigerant stream which flows into the heat-source-side heat exchanger  103  exchange heat with each other, whereby the refrigerant is condensed. 
     The refrigerant, as condensed in the injection internal heat exchanger  122 , flows from the injection port  101   c  of the compressing device  101  into the high-stage-side compressor  101   b.    
     Thus, the pressure of the high-pressure refrigerant, as discharged from the compressing device  101  and stabilized, is reduced by the injection flow control device  121   a , whereby a satisfactory differential pressure is produced. Consequently, a predetermined amount of refrigerant stably flows from the injection port  101   c  into the compressing device  101 . 
     According to Embodiment 1, in the heating only operation and the heating main operation, the low-pressure, two-phase gas-liquid refrigerant having passed through the indoor units B and C and the relay device D is separated into liquid and gas refrigerants. The gas refrigerant is divided at one end of the injection pipe  120   b . The other end of the injection pipe  120   b  is connected to the injection port  101   c  of the compressing device  101 . 
     The controller  200  reduces the pressure of the refrigerant which passes through the injection pipe  120   b  by using the injection flow control device  121   b.    
     With this operation, the refrigerant having passed through those indoor units that are performing heating is injected. Therefore, a large amount of refrigerant is allowed to flow through the indoor units that are performing heating. 
     Hence, when only a small amount of refrigerant needs to be injected, for example, when a sufficient differential pressure can be produced in the heating only operation; when the temperature of the outdoor air is relatively high; or when the heating load is small, a certain heating capacity can be provided and the operation efficiency can be increased mainly by utilizing injection from the injection pipe  120   b.    
     With the injection internal heat exchanger  122 , the high-pressure refrigerant passing through the injection pipe  120   a  exchanges heat with the low-pressure, two-phase gas-liquid refrigerant having passed through those indoor units that are performing cooling and the relay device D. 
     Thus, the enthalpy of the refrigerant to be injected can be reduced. 
     The enthalpy of the low-pressure, two-phase gas-liquid refrigerant having passed through the indoor units that are performing cooling and the relay device D is increased. Therefore, the load on the heat-source-side heat exchanger  103  can be reduced. Consequently, the low-side pressure can be raised, and the heating capacity can be increased. 
       FIG. 6  is a flowchart illustrating an exemplary operation of the air-conditioning apparatus according to Embodiment 1. 
     Details of the control operation associated with injection will now be described with reference to  FIG. 6 . 
     (STEP 1) 
     The controller  200  determines whether the outdoor air temperature is lower than a predetermined outdoor air temperature on the basis of a signal transmitted from the outdoor air temperature detector  127  (determination as to whether the outdoor air temperature is sufficiently low). 
     If the outdoor air temperature is not lower than the predetermined outdoor air temperature, the process proceeds to STEP 8. 
     (STEP 2) 
     In contrast, if the outdoor air temperature is lower than the predetermined outdoor air temperature, the controller  200  controls the opening degree of the flow control device  124  such that the pressure detected by the pressure detector  126  reaches a predetermined target intermediate pressure. 
     (STEP 3) On the basis of the value detected by the pressure detector  125 , the controller  200  detects the pressure Pd and the temperature Td of the refrigerant as discharged from the compressing device  101 . 
     On the basis of the pressure Pd, the controller  200  calculates the condensing temperature Tc. 
     The controller  200  calculates a discharge degree of superheat TdSH, which is the difference between the temperature Td and the condensing temperature Tc. 
     (STEP 4) 
     The controller  200  determines whether the discharge degree of superheat TdSH calculated in STEP 3 is higher than a predetermined target discharge degree of superheat TdSHm. 
     If the discharge degree of superheat TdSH is higher than the target discharge degree of superheat TdSHm, the process returns to STEP 1. 
     (STEP 5) 
     In contrast, if the discharge degree of superheat TdSH is not higher than the target discharge degree of superheat TdSHm, the controller  200  controls the opening degree of the injection flow control device  121   b  such that the discharge degree of superheat TdSH reaches the target discharge degree of superheat TdSHm. 
     (STEP 6) 
     The controller  200  determines whether the opening degree of the injection flow control device  121   b  takes a maximum value. 
     If the opening degree of the injection flow control device  121   b  does not take a maximum value, the process returns to STEP 1. 
     (STEP 7) 
     If the opening degree of the injection flow control device  121   b  takes a maximum value, the controller  200  controls the opening degree of the injection flow control device  121   a  such that the discharge degree of superheat TdSH reaches the target discharge degree of superheat TdSHm. 
     (STEP 8) 
     If it is determined in STEP 1 that the outdoor air temperature is not lower than the predetermined outdoor air temperature, the controller  200  closes the injection flow control devices  121   a  and  121   b . Then, the process returns to STEP 1. If the injection flow control devices  121   a  and  121   b  are closed, they remain the same. 
     Thus, the refrigerant is prevented from flowing into the injection pipes  120   a  and  120   b , and control is performed by normal operation. 
     As described above, according to Embodiment 1, in the heating only operation in which a certain differential pressure can be produced and a stable flow rate of injection can be provided, and in the heating main operation in which the outdoor air temperature is relatively high and the flow rate of injection need not be high, the refrigerant having passed through the indoor units is injected. If a sufficient flow rate of injection is required, control is performed so that the high-pressure gas refrigerant having been discharged from the compressing device  101  and the two-phase refrigerant having passed through the indoor units are made to exchange heat with each other for condensation, and the condensed refrigerant is injected into the compressing device  101 . 
     Hence, if the pressure of the refrigerant discharged from one of the indoor heat exchangers  118   a  and  118   b  functioning as an evaporator is controlled while a certain heating capacity provided to those indoor units that are performing heating is ensured (maintained), a certain cooling capacity provided to those indoor units that are performing cooling can be ensured (maintained). 
     Thus, an efficient operation can be implemented utilizing injection, and the aforementioned pipe connection configuration employed in such a system. 
     Embodiment 2 
     Embodiment 2 assumes an evaporating operation performed in the heating main operation in the heat source device A to prevent the freezing of those indoor units that are performing cooling. 
     The flow of the refrigerant in the heating main operation according to Embodiment 2 is the same as in the heating main operation that has been described above in Embodiment 1 with reference to  FIG. 4 . 
     The controller  200  according to Embodiment 2 performs not only the operations described in Embodiment 1 but also an evaporating operation for preventing the freezing of those indoor units that are performing cooling. 
     In the heating main operation, the controller  200  controls the opening degree of the flow control device  124  such that the intermediate pressure detected by the pressure detector  126  reaches a predetermined pressure (a pressure that makes the saturation temperature 0 degrees C. or higher). 
     In such a control operation, the evaporating temperature of the indoor heat exchanger  118   a  of the indoor unit B that is performing cooling can be maintained at 0 degrees C. or higher, and the freezing of the indoor unit B that is performing cooling can be prevented. 
     Embodiment 3 
     While Embodiments 1 and 2 have been described assuming that the air-conditioning apparatus  1  includes the relay device D and is capable of a simultaneous cooling and heating operation, the present invention is not limited to such a configuration. 
     For example, as illustrated in  FIG. 7 , the heat source device A may be connected to the indoor units B and C without the relay device D. 
     The present invention is applicable, for example, to an air-conditioning apparatus  1  that switches the operation between cooling and heating without the relay device D. 
     The present invention is also applicable, for example, to an air-conditioning apparatus  1  including indoor units (load-side units) provided exclusively for heating.