Patent Publication Number: US-2021190402-A1

Title: Air-conditioning apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to an air-conditioning apparatus that exchanges heat between refrigerant circulating through a refrigerant circulation circuit and a heat medium circulating through a heat medium circulation circuit. 
     BACKGROUND ART 
     Conventionally, direct expansion air-conditioning apparatuses have been used where an outdoor unit and indoor units are connected with each other, and refrigerant is caused to circulate between the outdoor unit and the indoor units to air-condition an indoor space being a space to be air-conditioned (see Patent Literature 1, for example). There are also some air-conditioning apparatuses that include a plurality of outdoor units and a plurality of indoor units, the plurality of indoor units being connected in parallel to the plurality of outdoor units connected in series to perform air conditioning of a plurality of indoor spaces. 
     In such an air-conditioning apparatus, a defrosting operation is performed to remove frost when the frost forms on an outdoor heat exchanger provided to any of the plurality of outdoor units during the heating operation where a heat exchanger provided to the outdoor unit serves as an evaporator. During the defrosting operation, the outdoor heat exchanger serves as a condenser, and refrigerant with s high temperature is supplied to the outdoor heat exchanger, so that frost on the outdoor heat exchanger is removed by heat of the refrigerant. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: International Publication No. WO 2015/140885 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the air-conditioning apparatus where the plurality of outdoor units are connected in series, the defrosting operation for even a single outdoor unit requires the defrosting operations performed on the other outdoor units in the same manner. Therefore, during the defrosting operation, the operation of all indoor units is stopped, so that the temperature of the indoor space decreases. 
     The present disclosure has been made in view of the above-mentioned problem in the conventional technique, and it is an object of the present disclosure to provide an air-conditioning apparatus that can continue a heating operation without stopping the operation of the indoor unit even during the defrosting operation. 
     Solution to Problem 
     An air-conditioning apparatus of an embodiment of the present disclosure includes: a plurality of outdoor units each including a compressor and an outdoor heat exchanger, refrigerant flowing through the plurality of outdoor units; an indoor unit including an indoor heat exchanger, a heat medium flowing through the indoor unit; a plurality of relay devices to which the plurality of outdoor units are connected independently, and to which the indoor unit is connected, each of the plurality of relay devices including a heat medium heat exchanger configured to exchange heat between the refrigerant and the heat medium; and a controller configured to control action of the plurality of outdoor units, the indoor unit, and the plurality of relay devices, wherein the controller includes a defrost determination unit configured to determine necessity for a defrosting operation for each of the plurality of outdoor units, a load determination unit configured to compare an indoor unit total load with an outdoor unit total capacity, in a case where the defrosting operation is necessary, the indoor unit total load indicating an air conditioning load during a heating operation, the outdoor unit total capacity indicating a capacity of an other outdoor unit excluding a target outdoor unit where the defrosting operation is necessary, and an equipment control unit configured to control an operating frequency of the compressor of the other outdoor unit to increase the outdoor unit total capacity in a case where the indoor unit total load is greater than the outdoor unit total capacity as a result of a comparison made by the load determination unit. 
     Advantageous Effects of Invention 
     According to the embodiment of the present disclosure, in the case where the indoor unit total load during the heating operation is greater than the outdoor unit total capacity, the outdoor unit total capacity of the outdoor units excluding the outdoor unit that is the target of the defrosting operation is increased to compensate for the outdoor unit total capacity reduced due to the defrosting operation. With such a compensation, the outdoor unit total capacity required during the heating operation can be ensured and hence, it is possible to continue the heating operation without stopping the operation of the indoor unit even during the defrosting operation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an example of the configuration of an air-conditioning apparatus according to Embodiment 1. 
         FIG. 2  is a schematic view showing an example of the configuration of an outdoor unit shown in  FIG. 1 . 
         FIG. 3  is a schematic view showing an example of the configuration of an indoor unit shown in  FIG. 1 . 
         FIG. 4  is a function block diagram showing an example of the configuration of a controller shown in  FIG. 1 . 
         FIG. 5  is a hardware configuration diagram showing an example of the configuration of the controller shown in  FIG. 4 . 
         FIG. 6  is a hardware configuration diagram showing another example of the configuration of the controller shown in  FIG. 4 . 
         FIG. 7  is a flowchart showing an example of the flow of a processing of defrost control according to Embodiment 1. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     Hereinafter, an air-conditioning apparatus according to Embodiment 1 of the present disclosure will be described.  FIG. 1  is a schematic view showing an example of the configuration of an air-conditioning apparatus  100  according to Embodiment 1. As shown in  FIG. 1 , the air-conditioning apparatus  100  includes outdoor units  1 A to  1 C, relay devices  2 A to  2 C, indoor units  3 A to  3 C, and a controller  4 . 
     [Configuration of Air-Conditioning Apparatus  100 ] 
     The relay devices  2 A to  2 C are independently provided, and the outdoor units  1 A to  1 C are connected to the relay devices  2 A to  2 C. Specifically, the outdoor unit  1 A and the relay device  2 A are connected by a refrigerant pipe, thus forming a refrigerant circulation circuit through which refrigerant circulates. The outdoor unit  1 B and the relay device  2 B are connected by a refrigerant pipe, thus forming a refrigerant circulation circuit through which refrigerant circulates. The outdoor unit  1 C and the relay device  2 C are connected by a refrigerant pipe, thus forming a refrigerant circulation circuit through which refrigerant circulates. In this example, the outdoor unit and the relay device are connected with each other on a one to one basis. However, the configuration is not limited to the above. For example, provided that a plurality of relay devices are provided independently, a plurality of outdoor units may be connected to one relay device. 
     The relay devices  2 A to  2 C and the indoor units  3 A to  3 C are connected by heat medium pipes, thus forming a heat medium circulation circuit through which a heat medium circulates. As the heat medium, for example, water, brine (antifreeze), mixed liquid of water and brine, or the like may be used. Hereinafter, the description will be made with reference to the example of the case where water is used as the heat medium. The indoor units  3 A to  3 C are connected in parallel to the relay devices  2 A to  2 C. In this example, three outdoor units  1 A to  1 C, three relay devices  2 A to  2 C, and three indoor units  3 A to  3 C are connected with each other. However, the number of outdoor units, the number of relay devices, and the number of indoor units are not limited to the numbers in this example. For example, one, two, or four or more indoor units may be used. Further, provided that a plurality of outdoor units and a plurality of relay devices are used, the number of outdoor units and the number of relay devices are not particularly limited. 
     The heat medium pipes connected to the indoor units  3 A to  3 C are provided with flow control valves  5 A to  5 C, pressure sensors  6 A to  6 C, and pressure sensors  7 A to  7 C. The flow control valves  5 A to  5 C control flow rates of water flowing through the indoor units  3 A to  3 C. The opening degrees of the flow control valves  5 A to  5 C are controlled by the controller  4 . The pressure sensors  6 A to  6 C are provided at positions close to the water inflow sides of the flow control valves  5 A to  5 C, and detect pressures of water flowing into the flow control valves  5 A to  5 C. The pressure sensors  7 A to  7 C are provided at positions close to the water outflow side of the flow control valves  5 A to  5 C, and detect pressures of water flowing out from the flow control valves  5 A to  5 C. 
     (Outdoor Units  1 A to  1 C) 
       FIG. 2  is a schematic view showing an example of the configuration of the outdoor unit  1 A shown in  FIG. 1 . The outdoor units  1 A to  1 C have substantially the same configuration and hence, the description will be made by taking the outdoor unit  1 A as an example hereinafter. As shown in  FIG. 2 , the outdoor unit  1 A includes a compressor  11 , a refrigerant flow passage switching device  12 , an outdoor heat exchanger  13 , and an outdoor fan  14 . 
     The compressor  11  suctions refrigerant with low temperature and low pressure, compresses the suctioned refrigerant, and then discharges the refrigerant with high temperature and high pressure. The compressor  11  may be, for example, an inverter compressor or other compressor where a capacity, which is a feeding amount per unit time, can be controlled by changing the operating frequency of the compressor  11 . The operating frequency of the compressor  11  is controlled by the controller  4  described later. 
     The refrigerant flow passage switching device  12  may be a four-way valve, for example. The refrigerant flow passage switching device  12  switches between a cooling operation and a heating operation by switching the flow direction of the refrigerant. At the time of performing the cooling operation, the refrigerant flow passage switching device  12  is switched such that, as shown by a solid line in  FIG. 2 , the discharge side of the compressor  11  and the outdoor heat exchanger  13  are connected with each other. At the time of performing the heating operation, the refrigerant flow passage switching device  12  is switched such that, as shown by a broken line in  FIG. 2 , the discharge side of the compressor  11  and the relay device are connected with each other. Switching of the flow passages in the refrigerant flow passage switching device  12  is controlled by the controller  4 . 
     The outdoor heat exchanger  13  exchanges heat between refrigerant and outdoor air supplied by the outdoor fan  14 . During the cooling operation, the outdoor heat exchanger  13  serves as a condenser that transfers heat of refrigerant to outdoor air to condense the refrigerant. During the heating operation, the outdoor heat exchanger  13  serves as an evaporator that evaporates refrigerant to cool outdoor air by heat of vaporization generated when the refrigerant is evaporated. 
     The outdoor fan  14  supplies air to the outdoor heat exchanger  13 . The rotation speed of the outdoor fan  14  is controlled by the controller  4 . The amount of air sent to the outdoor heat exchanger  13  is adjusted by controlling the rotation speed of the outdoor fan  14 . An expansion device  15  may be an expansion valve, for example. The expansion device  15  causes refrigerant to expand. The expansion device  15  is a valve whose opening degree can be controlled, such as an electronic expansion valve, for example. The opening degree of the expansion device  15  is controlled by the controller  4 . 
     The outdoor unit  1 A also includes an outdoor-side outlet temperature sensor  16 . The outdoor-side outlet temperature sensor  16  is provided at a position close to the refrigerant outflow side of the outdoor heat exchanger  13  during the heating operation, and detects a refrigerant outlet temperature that is the temperature of refrigerant flowing out from the outdoor heat exchanger  13  during the heating operation. 
     (Relay Devices  2 A to  2 C) 
     Each of the relay devices  2 A to  2 C in  FIG. 1  includes a heat medium heat exchanger  21 , a pump  22 , and a bypass valve  23 . 
     The heat medium heat exchanger  21  serves as a condenser or an evaporator, so exchanges heat between refrigerant flowing through the refrigerant circulation circuit connected to a refrigerant-side flow passage and a heat medium flowing through the heat medium circulation circuit connected to a heat-medium-side flow passage. During the cooling operation, the heat medium heat exchanger  21  serves as an evaporator that evaporates refrigerant to cool the heat medium by heat of vaporization generated when the refrigerant is evaporated. During the heating operation, the heat medium heat exchanger  21  serves as a condenser that transfers heat of refrigerant to the heat medium to condense the refrigerant. 
     The pump  22  is driven by a motor not shown in the drawing to circulate water flowing through the heat medium pipe and serving as a heat medium. The pump  22  may be a pump whose capacity can be controlled, for example. The flow rate of each pump  22  can be controlled depending on the magnitude of the load on the indoor unit  3 A to  3 C. The driving of the pump  22  is controlled by the controller  4 . Specifically, the pump  22  is controlled by the controller  4  such that the pump  22  has a higher flow rate of water when the indoor unit has a larger load, and the pump  22  has a lower flow rate of water when the indoor unit has a smaller load. 
     The bypass valve  23  is provided to a bypass  20  that bypasses the outlet and the inlet of the refrigerant-side flow passage of the heat medium heat exchanger  21 . When the bypass valve  23  is brought into an open state, refrigerant flowing through the refrigerant circulation circuit does not flow through the heat medium heat exchanger  21 , but flows through the bypass  20  provided with the bypass valve  23 . The opening and closing of the bypass valve  23  is controlled by the controller  4 . 
     (Indoor units  3 A to  3 C) 
       FIG. 3  is a schematic view showing an example of the configuration of the indoor unit  3 A shown in  FIG. 1 . The indoor units  3 A to  3 C have substantially the same configuration and hence, the description will be made by taking the indoor unit  3 A as an example hereinafter. As shown in  FIG. 3 , the indoor unit  3 A includes an indoor heat exchanger  31  and an indoor fan  32 . 
     The indoor heat exchanger  31  exchanges heat between water (including hot water) and indoor air supplied by the indoor fan  32 . Such heat exchange generates air for cooling or air for heating being conditioned air to be supplied to the indoor space. The indoor fan  32  supplies air to the indoor heat exchanger  31 . The rotation speed of the indoor fan  32  is controlled by the controller  4 . The amount of air sent to the indoor heat exchanger  31  is adjusted by controlling the rotation speed of the indoor fan  32 . 
     The indoor unit  3 A also includes an indoor-side inlet temperature sensor  33 , an indoor-side outlet temperature sensor  34 , and a suction temperature sensor  35 . The indoor-side inlet temperature sensor  33  is provided at a position close to the water inflow side of the indoor unit  3 A, and detects a heat medium inlet temperature being the temperature of water flowing into the indoor unit  3 A. The indoor-side outlet temperature sensor  34  is provided at a position close to the water outflow side of the indoor unit  3 A, and detects the heat medium outlet temperature being the temperature of water flowing out from the indoor unit  3 A. The suction temperature sensor  35  is provided at a position close to the air suction side of the indoor unit  3 A, and detects the suction air temperature of air suctioned into the indoor unit  3 A. 
     (Controller  4 ) 
     The controller  4  controls the action of the entire air-conditioning apparatus  100  that includes the outdoor units  1 A to  1 C, the relay devices  2 A to  2 C, and the indoor units  3 A to  3 C based on various information received from the various sensors provided to the units of the air-conditioning apparatus  100 . Particularly, in Embodiment 1, the controller  4  controls the operating frequency of the compressor  11 , the driving of the pump  22 , the opening and closing of the bypass valve  23 , the driving of the indoor fan  32  and the like based on the magnitudes of the loads on the indoor units  3 A to  3 C. 
     The various functions of the controller  4  are implemented by executing software in an arithmetic unit, such as a microcomputer. Alternatively, the controller  4  is hardware or the like, such as a circuit device, that implements various functions. In Embodiment 1, the controller  4  is provided separately from each equipment. However, the configuration is not limited to the above. For example, the controller  4  may be provided to any of the outdoor units  1 A to  1 C, the relay devices  2 A to  2 C, and the indoor units  3 A to  3 C. 
       FIG. 4  is a function block diagram showing an example of the configuration of the controller  4  shown in  FIG. 1 . As shown in  FIG. 4 , the controller  4  includes a defrost determination unit  41 , a priority order determination unit  42 , a defrosting time determination unit  43 , a load determination unit  44 , an equipment control unit  45 , and a memory unit  46 . 
     The defrost determination unit  41  determines the necessity for a defrosting operation based on the refrigerant outlet temperature of the outdoor heat exchanger  13  in each of the outdoor units  1 A to  1 C, and based on a set temperature set in advance and stored in the memory unit  46 . The set temperature is a threshold set for the refrigerant outlet temperature to determine the necessity for the defrosting operation. The defrost determination unit  41  determines the necessity for the defrosting operation for each of the outdoor units  1 A to  1 C. 
     In the case where the defrosting operation is necessary for all of the outdoor units  1 A to  1 C, the priority order determination unit  42  determines the order of priority of the defrosting operations for all of the outdoor units  1 A to  1 C, based on the determination result from the defrost determination unit  41 . The order of priority is determined for performing the defrosting operations on the outdoor units in order of decreasing necessity for the defrosting operation. 
     The defrosting time determination unit  43  determines a defrosting time, meaning the time for the defrosting operation, for an outdoor unit on which the defrosting operation is to be performed. The defrosting time determination unit  43  determines the defrosting time based on the refrigerant outlet temperature in the outdoor unit on which the defrosting operation is to be performed and a defrosting time determination table stored in advance in the memory unit  46 . The defrosting time determination table is a table where refrigerant outlet temperatures and defrosting times are associated with each other, and a defrosting time is associated in a stepwise manner with every set range of a refrigerant outlet temperature. 
     The load determination unit  44  compares an indoor unit total load, being the sum of air conditioning loads on the indoor units  3 A to  3 C during the heating operation, with an outdoor unit total capacity, being the sum of the capacities of outdoor units other than the outdoor unit on which the defrosting operation is to be performed. With such a comparison, the load determination unit  44  determines the magnitude of the indoor unit total load relative to the outdoor unit total capacity. In the case where the indoor unit total load is greater than the outdoor unit total capacity, the load determination unit  44  further determines the magnitude of the indoor unit total load using a water temperature threshold Tv stored in advance in the memory unit  46 . The water temperature threshold Tv is a threshold set in relation to the temperature of water in the relay device corresponding to the outdoor unit on which the defrosting operation is to be performed. For example, the water temperature threshold Tv is a set temperature for the indoor unit  3 A to  3 C, or a temperature specified based on the set temperature, such as “2 degrees C. below the set temperature”. 
     The equipment control unit  45  controls the outdoor units  1 A to  1 C, the relay devices  2 A to  2 C, and the indoor units  3 A to  3 C based on the processing results from the units of the controller  4 . Particularly, In Embodiment 1, the equipment control unit  45  controls the outdoor units  1 A to  1 C and the relay devices  2 A to  2 C when the defrosting operation is performed. The equipment control unit  45  also controls the outdoor units  1 A to  1 C, the relay devices  2 A to  2 C, and the indoor units  3 A to  3 C according to the determination result from the load determination unit  44 . 
     The memory unit  46  stores in advance the set temperature used by the defrost determination unit  41 , the defrosting time determination table used by the defrosting time determination unit  43 , and the water temperature threshold Tv used by the load determination unit  44 . 
       FIG. 5  is a hardware configuration diagram showing an example of the configuration of the controller  4  shown in  FIG. 4 . In the case where the various functions of the controller  4  are executed by hardware, as shown in  FIG. 5 , the controller  4  shown in  FIG. 4  is a processing circuit  51 . Each of functions of the defrost determination unit  41 , the priority order determination unit  42 , the defrosting time determination unit  43 , the load determination unit  44 , the equipment control unit  45 , and the memory unit  46  shown in  FIG. 4  is implemented by the processing circuit  51 . 
     In the case where each of the functions is executed by hardware, for example, the processing circuit  51  corresponds to a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the combination of these. Each of the functions of the defrost determination unit  41 , the priority order determination unit  42 , the defrosting time determination unit  43 , the load determination unit  44 , the equipment control unit  45 , and the memory unit  46  may be implemented by the processing circuit  51 , or the functions of the units may be implemented by one processing circuit  51 . 
       FIG. 6  is a hardware configuration diagram showing another example of the configuration of the controller  4  shown in  FIG. 4 . In the case where the various functions of the controller  4  are executed by software, as shown in  FIG. 6 , the controller  4  shown in  FIG. 4  includes a processor  61  and a memory  62 . The functions of the defrost determination unit  41 , the priority order determination unit  42 , the defrosting time determination unit  43 , the load determination unit  44 , the equipment control unit  45 , and the memory unit  46  shown in  FIG. 4  are implemented by the processor  61  and the memory  62 . 
     In the case where each of the functions is executed by software, the functions of the defrost determination unit  41 , the priority order determination unit  42 , the defrosting time determination unit  43 , the load determination unit  44 , and the equipment control unit  45  are implemented by software, firmware, or the combination of the software and the firmware. The software or the firmware is described as a program, and is stored in the memory  62 . The processor  61  reads and executes the program stored in the memory  62  to implement the functions of the respective units. 
     As the memory  62 , for example, a nonvolatile or volatile semiconductor memory may be used, such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable and programmable ROM (EPROM), or an electrically erasable and programmable ROM (EEPROM). Further, as the memory  62 , for example, a detachable recording medium may be used, such as a magnetic disk, a flexible disk, an optical disc, a compact disc (CD), a mini disc (MD) or a digital versatile disc (DVD). 
     [Defrost Control] 
     The defrost control performed by the air-conditioning apparatus  100  according to Embodiment 1 will be described. In this defrost control, the operations of the outdoor units  1 A to  1 C are controlled to prevent all of the outdoor units  1 A to  1 C from performing the defrosting operation simultaneously during the heating operation. At the same time, the defrost control allows the heating operation to be continuously performed. 
       FIG. 7  is a flowchart showing an example of the flow of a processing of defrost control according to Embodiment 1. This defrost control is performed when the defrosting operation becomes necessary during the heating operation. First, in step S 1 , the outdoor-side outlet temperature sensor  16  provided to each of the outdoor units  1 A to  1 C detects the refrigerant outlet temperature of refrigerant flowing out from the outdoor heat exchanger  13  during the heating operation. 
     In step S 2 , the controller  4  determines whether or not the defrosting operation is necessary for each of the outdoor units  1 A to  1 C. In this case, the controller  4  determines the necessity for the defrosting operation based on the refrigerant outlet temperature of the outdoor heat exchanger  13  of each of the outdoor units  1 A to  1 C and the set temperature for determining the necessity for the defrosting operation. 
     The defrost determination unit  41  reads the set temperature from the memory unit  46 , and compares the refrigerant outlet temperature detected by the outdoor-side outlet temperature sensor  16  with the set temperature. When the refrigerant outlet temperature is equal to or below the set temperature, the defrost determination unit  41  determines that the defrosting operation is necessary (step S 2 ; Yes), so that the processing advances to step S 3 . In contrast, when the refrigerant outlet temperature is above the set temperature, the defrost determination unit  41  determines that the defrosting operation is not necessary (step S 2 ; No), so that the processing returns to step S 1 . 
     In step S 3 , the defrost determination unit  41  determines whether or not the defrosting operation is necessary for all of the outdoor units  1 A to  1 C. When the refrigerant outlet temperatures in all of the outdoor units  1 A to  1 C are equal to or below the set temperature, so that it is determined that the defrosting operation is necessary for all of the outdoor units  1 A to  1 C (step S 3 ; Yes), the processing advances to step S 4 . In contrast, when the refrigerant outlet temperature in any one of the outdoor units  1 A to  1 C is above the set temperature, so that it is determined that the defrosting operation is not necessary for all of the outdoor units  1 A to  1 C (step S 3 ; No), the processing advances to step S 5 . 
     In step S 4 , the priority order determination unit  42  determines the order of priority of the defrosting operations for all of the outdoor units  1 A to  1 C where the defrosting operation is necessary. In Embodiment 1, the order of priority for the outdoor units  1 A to  1 C is set such that the defrosting operation is preferentially performed on an outdoor unit with a high possibility of frost formed on the outdoor heat exchanger  13 , or on an outdoor unit with a larger amount of frost already formed on the outdoor heat exchanger  13 . 
     The outdoor unit with a high possibility of frost formed on the outdoor heat exchanger  13  or the outdoor unit with a large amount of formed frost has a lower refrigerant outlet temperature than an outdoor unit having a low possibility of frost or an outdoor unit with a small amount of formed frost. Therefore, based on the refrigerant outlet temperatures in the outdoor units  1 A to  1 C, the priority order determination unit  42  determines the order of priority of the defrosting operations for the outdoor units  1 A to  1 C such that an outdoor unit with a lower refrigerant outlet temperature has a higher order of priority. 
     In step S 5 , the defrosting time determination unit  43  determines a defrosting time for an outdoor unit that is the target of the defrosting operation (hereinafter, referred to as “target outdoor unit” when appropriate). A time required for defrosting the outdoor heat exchanger  13  increases as the amount of formed frost increases. Therefore, it is preferable to increase the defrosting time as the amount of formed frost increases. However, as described above, the outdoor unit with a larger amount of frost formed on the outdoor heat exchanger  13  has a lower refrigerant outlet temperature. Therefore, the defrosting time determination unit  43  sets a defrosting time such that an outdoor unit with a lower refrigerant outlet temperature has a longer defrosting time. Hereinafter, to facilitate the understanding of the defrost control, the description will be made with reference to the example of the case where the outdoor unit  1 B acts as an outdoor unit that is the target of the defrosting operation, and outdoor units  1 A and  1 C excluding the outdoor unit  1 B act as outdoor units that are not the target of the defrosting operation. 
     Defrosting times are set in a stepwise manner according to the refrigerant outlet temperatures. In Embodiment 1, a defrosting time determination table is prepared where the defrosting time is associated in a stepwise manner with every set range of the refrigerant outlet temperature. The defrosting time determination table is stored in advance in the memory unit  46 . The defrosting time determination unit  43  determines a defrosting time by referencing to the set temperature stored in the memory unit  46  based on the refrigerant outlet temperature in the outdoor unit  1 B on which the defrosting operation is to be performed. 
     In step S 6 , the equipment control unit  45  controls the outdoor units  1 A to  1 C and the relay devices  2 A to  2 C to start the defrosting operation for the outdoor unit  1 B that is the target of the defrosting operation. The defrosting operation is performed only for the defrosting time determined in step S 5 . In the case where the order of priority for the outdoor units  1 A to  1 C is determined in step S 4 , the equipment control unit  45  starts the defrosting operation for the outdoor units  1 A to  1 C in order according to the determined order of priority and the defrosting time set in step S 5 . 
     Next, in step S 7 , the load determination unit  44  compares an indoor unit total load, being the sum of loads on the indoor units  3 A to  3 C during the heating operation, with an outdoor unit total capacity, being the sum of the capacities of the outdoor units excluding the target outdoor unit  1 B (hereinafter, referred to as “the other outdoor units” when appropriate)  1 A and  1 C. The load determination unit  44  determines whether or not the indoor unit total load is greater than the outdoor unit total capacity. 
     The indoor unit total load can be obtained based on the difference between a suction air temperature detected by the suction temperature sensor  35  and the set temperature for the indoor space. The set temperature is a target temperature of the indoor space set by using a remote control or other device not shown in the drawing. The indoor unit total load is not limited to the above, and may be obtained based on the difference between a heat medium inlet temperature detected by the indoor-side inlet temperature sensor  33  and a heat medium outlet temperature detected by the indoor-side outlet temperature sensor  34 . The outdoor unit total capacity is a capacity that the outdoor units  1 A to  1 C can exhibit during the operation, and can be obtained based on the operating frequencies of the compressors  11 . 
     When the indoor unit total load is equal to or lower than the outdoor unit total capacity in step S 7  (step S 7 ; No), the other outdoor units  1 A and  1 C can handle loads on the indoor units  3 A to  3 C during the heating operation with normal capacities. Therefore, in step S 8 , the equipment control unit  45  controls units of the other outdoor units  1 A and  1 C such that the other outdoor units  1 A and  1 C continue the heating operation with capacities substantially equal to the normal capacities. 
     In contrast, when the indoor unit total load is higher than the outdoor unit total capacity (step S 7 ; Yes), the processing advances to step S 9 . In step S 9 , the load determination unit  44  determines whether or not the indoor unit total load is excessively greater than the outdoor unit total capacity. In this case, the load determination unit  44  reads the water temperature threshold Tv from the memory unit  46 , and compares the temperature of water in the relay device  2 B corresponding to the target outdoor unit  1 B with the read water temperature threshold Tv. When the water temperature in the relay device  2 B is equal to or higher than the water temperature threshold Tv as a result of the comparison, the load determination unit  44  determines that the indoor unit total load is not significantly larger than the outdoor unit total capacity (step S 9 ; No). 
     In this case, the target outdoor unit  1 B is in the defrosting operation, so that the heat medium heat exchanger  21  of the relay device  2 B corresponding to the target outdoor unit  1 B serves as an evaporator. In other words, water flowing into the heat medium heat exchanger  21  of the relay device  2 B exchanges heat with refrigerant, thus being cooled, and then flows out from the heat medium heat exchanger  21  with a temperature lower than the temperature at which the water flows into the heat medium heat exchanger  21 . Therefore, for water obtained by the merge of water flowing out from the heat medium heat exchangers  21  of the relay devices  2 A and  2 C corresponding to the other outdoor units  1 A and  1 C and water flowing out from the heat medium heat exchanger  21  of the relay device  2 B, the temperature of the water obtained by the merge when the defrosting operation is performed is lower than the temperature of the water obtained by the merge when the defrosting operation is not performed. When such water having a decreased temperature flows into the indoor units  3 A to  3 C, the temperature of indoor air decreases during the heating operation, so that comfort may be impaired. 
     In view of the above, in such a case, the decrease in the temperature of water flowing out from the relay device  2 B is compensated for by increasing the temperature of water flowing out from the relay devices  2 A and  2 C. Specifically, in step S 10 , the equipment control unit  45  performs control such that the operating frequencies of the compressors  11  of the other outdoor units  1 A and  1 C that are not in the defrosting operation are increased to increase the capacities of the other outdoor units  1 A and  1 C. With such control, the temperature of water flowing out from the relay devices  2 A and  2 C rises and hence, it is possible to compensate for the decrease in the temperature of water flowing out from the relay device  2 B, so that the merged water is allowed to have a temperature substantially equal to a temperature of water when the defrosting operation is not performed. Therefore, a decrease in the temperature of indoor air can be suppressed, and a heating operation substantially equal to the normal heating operation can be continued. 
     In contrast, when a water temperature in the relay device  2 B is lower than the water temperature threshold Tv in step S 9 , the load determination unit  44  determines that the indoor unit total load is extremely greater than the outdoor unit total capacity (step S 9 ; Yes). Also in this case, the target outdoor unit  1 B is in the defrosting operation, and the heat medium heat exchanger  21  of the relay device  2 B serves as an evaporator. Further, a water temperature in the relay device  2 B is below the set temperature for the indoor units  3 A to  3 C. If the heating operation is performed in this state, it is difficult to allow indoor air to have the set temperature. Accordingly, it is necessary to perform operations to cause the water temperature to be above the set temperature. 
     In view of the above, when the indoor unit total load is extremely greater than the outdoor unit total capacity, the controller  4  stops the heating operation, and controls the units of the relay devices  2 A to  2 C and the units of the indoor units  3 A to  3 C such that the water temperature is above the set temperature. 
     Specifically, in step S 11 , the equipment control unit  45  brings the bypass valve  23  of the relay device  2 B corresponding to the target outdoor unit  1 B into an open state. With such an operation, refrigerant flowing out from the outdoor unit  1 B flows through the bypass  20  without flowing into the heat medium heat exchanger  21  of the relay device  2 B, and then flows into the outdoor unit  1 B again. Further, with such a flow of refrigerant, heat exchange between refrigerant and water is not performed in the heat medium heat exchanger  21  serving as an evaporator. Accordingly, it is possible to suppress a decrease in the temperature of water flowing out from the relay device  2 B. 
     The equipment control unit  45  also reduces the wind speeds of the indoor fans  32  of all of the indoor units  3 A to  3 C. With such a reduction, it is possible to reduce the amount of heat exchange performed by the indoor heat exchanger  31  between indoor air and water with a low temperature and hence, a decrease in the temperature of indoor air can be suppressed. In this case, the equipment control unit  45  may perform control to stop the indoor fans  32 . 
     In addition to the above, the equipment control unit  45  increases flow rates in the pumps  22  of all of the relay devices  2 A to  2 C. Such an increase promotes a rise in the temperature of water brought about by the other outdoor units  1 A and  1 C and hence, the temperature of water is allowed to rapidly rise. 
     In the case where the temperature of water flowing out from the relay devices  2 A to  2 C is below the set temperature, the starting of the heating operation is delayed. However, controlling the actions of the units as described above allows the heating operation to be rapidly restarted. 
     Next, in step S 12 , the equipment control unit  45  outputs a defrost inhibit signal for inhibiting the defrosting operation to the other outdoor units  1 A and  1 C. With such an operation, it is possible to prevent the plurality of outdoor units from performing the defrosting operation simultaneously. 
     As described above, in the air-conditioning apparatus  100  according to Embodiment 1, when the indoor unit total load during the heating operation is greater than the outdoor unit total capacity, the outdoor unit total capacities of the outdoor units  1 A and  1 C, excluding the outdoor unit that is the target of the defrosting operation, are increased. With such an increase, the outdoor unit total capacity reduced due to the defrosting operation is compensated for and hence, an outdoor unit total capacity required during the heating operation can be ensured. Accordingly, it is possible to continue the heating operation without stopping the operation of the indoor units  3 A to  3 C even during the defrosting operation. 
     In the air-conditioning apparatus  100 , the defrost determination unit  41  determines that the defrosting operation is necessary when the refrigerant outlet temperature is equal to or below the set temperature. With such a determination, it is possible to easily determine the necessity for the defrosting operation for the outdoor units  1 A to  1 C. 
     In the air-conditioning apparatus  100 , the load determination unit  44  obtains an indoor unit total load based on a suction temperature and a set temperature, and obtains an outdoor unit total capacity based on the operating frequencies of the compressors  11  of the other outdoor units  1 A and  1 C. With such operations, in the air-conditioning apparatus  100 , control is performed during the heating operation according to the indoor unit total load and the outdoor unit total capacity and hence, it is possible to continue the heating operation in a state where the defrosting operation is being performed for the outdoor unit  1 B. In the air-conditioning apparatus  100 , the load determination unit  44  may obtain the indoor unit total load based on a heat medium inlet temperature and a heat medium outlet temperature. Also with such operation, it is possible to continue the heating operation in a state where the defrosting operation is being performed for the outdoor unit  1 B. 
     In the air-conditioning apparatus  100 , when the temperature of the heat medium flowing through the relay device  2 B connected to the target outdoor unit  1 B is lower than the water temperature threshold Tv, the equipment control unit  45  brings the bypass valve  23  of the relay device  2 B connected to the target outdoor unit  1 B into an open state, reduces the wind speeds of the indoor fans  32  of all of the indoor units  3 A to  3 C or stops the indoor fans  32 , and increases the flow rates in the pumps  22  of all of the relay devices  2 A to  2 C. With such operations, the temperature of water being a heat medium is allowed to rapidly rise while a decrease in the temperature of the indoor space is suppressed and hence, the heating operation can be restarted at an early stage. 
     In the air-conditioning apparatus  100 , when the defrost determination unit  41  determines that the defrosting operation is necessary for all of the outdoor units  1 A to  1 C, the priority order determination unit  42  determines the order of priority in the case of performing the defrosting operation for all of the outdoor units  1 A to  1 C. At this point of operation, the priority order determination unit  42  determines the order of priority such that an outdoor unit with a lower refrigerant outlet temperature has a higher order of priority. With such a determination, it is possible to prevent all of the outdoor units  1 A to  1 C from performing the defrosting operation simultaneously and hence, it is possible to continue the heating operation even in a state where the defrosting operation is being performed. 
     In the air-conditioning apparatus  100 , the defrosting time determination unit  43  determines defrosting times in the case of performing the defrosting operation for all of the outdoor units  1 A to  1 C. At this point of operation, the defrosting time determination unit  43  determines the defrosting times such that an outdoor unit with a lower refrigerant outlet temperature has a longer defrosting time. With such a determination, it is possible to surely defrost the outdoor heat exchanger  13  on which frost is formed. Also in the case where frost is not formed on the outdoor heat exchanger  13 , it is possible to surely prevent frost from forming on the outdoor heat exchanger  13 . 
     In the air-conditioning apparatus  100 , the defrosting time determination unit  43  determines defrosting times, using the defrosting time determination table where the defrosting time is associated in a stepwise manner with every set range of the refrigerant outlet temperature, such that an outdoor unit with a lower refrigerant outlet temperature has a longer defrosting time increased in a stepwise manner. With such a determination, it is possible to surely defrost the outdoor heat exchanger  13  on which frost is formed. Also in the case where frost is not formed on the outdoor heat exchanger  13 , it is possible to surely prevent frost from forming on the outdoor heat exchanger  13 . Further, the defrosting time is associated in a stepwise manner with every set range of the refrigerant outlet temperature and hence, it is possible to easily set a defrosting time according to the amount of formed frost or a possibility of frost. 
     Embodiment 1 of the present disclosure has been described heretofore. However, the present disclosure is not limited to the above-mentioned Embodiment 1 of the present disclosure, and various modifications and applications are conceivable without departing from the gist of the present disclosure. The necessity for the defrosting operation is determined by comparing the refrigerant outlet temperature of the outdoor heat exchanger  13  with the set temperature. However, the method of determining the necessity for the defrosting operation is not limited to the above. For example, the necessity for the defrosting operation may be determined by comparing an evaporating temperature with a specified temperature. 
     REFERENCE SIGNS LIST 
       1 A,  1 B,  1 C outdoor unit  2 A,  2 B,  2 C relay device  3 A,  3 B,  3 C indoor unit  4  controller  5 A,  5 B,  5 C flow control valve  6 A,  6 B,  6 C pressure sensor  7 A,  7 B,  7 C pressure sensor  11  compressor  12  refrigerant flow passage switching device  13  outdoor heat exchanger  14  outdoor fan  15  expansion device  16  outdoor-side outlet temperature sensor  20  bypass  21  heat medium heat exchanger  22  pump  23  bypass valve  31  indoor heat exchanger  32  indoor fan  33  indoor-side inlet temperature sensor  34  indoor-side outlet temperature sensor  35  suction temperature sensor  41  defrost determination unit  42  priority order determination unit  43  defrosting time determination unit  44  load determination unit  45  equipment control unit  46  memory unit  51  processing circuit  61  processor  62  memory  100  air-conditioning apparatus.