Technical Field
The present invention relates to an air-conditioning apparatus formed by connecting a plurality of outdoor units and at least one indoor unit by common gas pipes and common liquid pipes, especially to a method for transferring refrigerant existing in a closed circuit when at least one of the plurality of outdoor units is in a stopped state during a cooling operation. Background Art
Conventionally, in order to increase the capacity of air-conditioning apparatuses, air-conditioning apparatuses each of which is formed by connecting a plurality of outdoor units and a plurality of indoor units with common gas pipes and common liquid pipes have been provided (see, for example, Patent Literature 1).
In an air-conditioning apparatus described in Patent Literature 1, an accumulator is provided in each of outdoor units, and usually stores surplus refrigerant. Furthermore, in general, liquid equalization and surplus refrigerant treatment is often performed by controlling the rotation speed of a compressor, the rotation speed of a fan, the opening degree of a flow control valve, etc., thereby adjusting the amount of refrigerant circulating in each of the outdoor units. By virtue of such a configuration, the accumulators provided in the outdoor units are do not connected to each other by liquid equalization pipes or pressure equalization pipes, and as a result, a simple circuit configuration can be achieved. Citation List Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2001-201199 Summary of Invention Technical Problem [0005]
In the air-conditioning apparatus described in Patent Literature 1, there is a possibility that refrigerant may be kept accumulated in a closed circuit. It is therefore necessary to remove the refrigerant from the closed circuit to prevent the internal pressure of the closed circuit from exceeding an allowable value. Therefore, in general, a bypass pipe is provided to connect outdoor units in order to transfer refrigerant from the closed circuit. However, if such a specific bypass pipe is provided, the manufacturing cost is increased. [0006]
The present invention has been made in order to solve the above problems, and an object of the invention is to provide an air-conditioning apparatus capable of causing the internal pressure of a closed circuit to fall within an allowable value range, without increasing the manufacturing cost. Solution to Problem [0007]
An air-conditioning apparatus according to an embodiment of the present invention includes: an indoor unit and a plurality of outdoor units, which forms a refrigerant circuit that a compressor, an outdoor heat exchanger, an expansion valve, and an indoor heat exchanger are connected to circulate refrigerant, and a controller, and the plurality of outdoor units are connected in parallel to the indoor unit by gas pipes and liquid pipes. Each of the outdoor units has a flow passage closing device which closes a passage for refrigerant to form a closed circuit in a part of the refrigerant circuit, a bypass pipe connecting a side of the outdoor heat exchanger which is located on a downstream side thereof in a cooling operation, and a low pressure side of the compressor, a bypass flow control valve provided at the bypass pipe, and a pressure sensor which detects a pressure of a low pressure side of the outdoor unit. The

bypass pipe is connected to the closed circuit. The controller includes a pressure calculation unit which calculates, when at least one of the plurality of outdoor units is in a stopped state during a cooling operation, a pressure difference between an internal pressure of the closed circuit formed in the at least one outdoor unit being in the stopped state and the pressure of a low pressure side of the other one or ones of the outdoor units which are in operation, and a valve opening-degree control unit which determines an opening degree of the bypass flow control valve based on the pressure difference and controlling the bypass flow control valve to set the opening degree thereof to the determined opening degree. Advantageous Effects of Invention [0008]
The air-conditioning apparatus of an embodiment of the present invention includes: a pressure calculation unit which calculates a pressure difference between an internal pressure of the closed circuit formed in an outdoor unit which is in a stopped state and the pressure of a low pressure side of an outdoor unit which is in operation; and a valve opening-degree control unit which determines an opening degree of the bypass flow control valve based on the pressure difference, and controls the bypass flow control valve to set the opening degree thereof the determined opening degree. Since refrigerant in the closed circuit of the outdoor unit being in the stopped state is transferred to the low pressure side of the outdoor unit being in operation via the bypass pipe connected to the closed circuit, the internal pressure of the closed circuit can be controlled to fall within an allowable value range, even when the internal pressure of the closed circuit increases. In addition, it is not necessary to provide a specific bypass pipe, and it is therefore to prevent the manufacturing cost from being increased. Brief Description of Drawings [0009]
[Fig. 1] Fig. 1 is a refrigerant circuit diagram illustrating the configuration of a refrigerant circuit of an air-conditioning apparatus according to an embodiment of the present invention.

[Fig. 2] Fig. 2 is a functional block diagram of a controller of the air-conditioning apparatus according to the embodiment.
[Fig. 3] Fig. 3 is a flowchart illustrating a flow of control processes of the air-conditioning apparatus according to the embodiment of the present invention. Description of embodiment [0010]
An embodiment of the present invention will be described with reference to the drawings. It should be noted that the present invention is not limited to the embodiment. Also, in the drawings to be referred to below, there is a case where relationships in size between components are different from those between actual components. [0011] Embodiment
Fig. 1 is a refrigerant circuit diagram illustrating the configuration of a refrigerant circuit of an air-conditioning apparatus 300 of an embodiment of the present invention.
Circuit configurations and operations of the air-conditioning apparatus 300 according to the embodiment will be described with reference to Fig. 1.
The air-conditioning apparatus 300 performs a cooling operation or a heating operation, using a refrigeration cycle (heat pump cycle) which circulates refrigerant. [0012]
The air-conditioning apparatus 300 is formed by connecting two outdoor units 10a and 10b and two indoor units 50a and 50b by refrigerant pipes. To be more specific, two use-side units are connected in parallel to two heat-source units to communicate with the two heat-source units. That is, in the air-conditioning apparatus 300, components provided in the two heat-source units are connected to components provided in the two use-side units by the refrigerant pipes, thereby forming a refrigerant circuit, and refrigerant is circulated in the refrigerant circuit, whereby a cooling operation or a heating operation can be performed. [0013]

The refrigerant pipes of the air-conditioning apparatus 300 includes: gas branching pipes connected to the respective outdoor units, that is, a gas branching pipe 202a connected to the outdoor unit 10a and a gas branching pipe 202b connected to the outdoor unit 10b; gas branch pipes connected to the respective indoor units, that is, a gas branch pipe 206a connected to the indoor unit 50a a and a gas branch pipe 206b connected to the indoor unit 50b; a common gas pipe 204 connecting the gas branching pipes and the gas branch pipes; liquid branching pipes connected to the respective outdoor units, that is, a liquid branching pipe 203a connected to the outdoor unit 10a and a liquid branching pipe 203b connected to the outdoor unit 10b; liquid branch pipes connected to the respective indoor units, that is, a liquid branch pipe 207a connected to the indoor unit 50a and a liquid branch pipe 207b connected to the indoor unit 50b; and a common liquid pipe 205 connecting the liquid branching pipes and the liquid branch pipes. [0014]
A gas distributor 200 is provided between the gas branching pipes 202a and 202b and the gas pipe 204 to connect these pipes together. Also, a liquid distributor 201 is provided between the liquid branching pipes 203a and 203b and the liquid pipe 205 to connect these pipes together. It should be noted that although Fig. 1 illustrates an example in which the gas distributor 200 and the liquid distributor 201 are provided in the air-conditioning apparatus 300, the invention is not limited to the example in which the gas distributor 200 and the liquid distributor 201 are provided. Furthermore, the gas branching pipe 202a, the gas branching pipe 202b and the gas pipe 204 are applied as gas pipes, and the liquid branching pipe 203a, the liquid branching pipe 203b and the liquid pipe 205 are applied as liquid pipes. [0015]
The outdoor unit 10a and the indoor unit 50a are connected to each other by the gas branching pipe 202a, the gas pipe 204, the gas branch pipe 206a, the liquid branch pipe 207a, the liquid pipe 205 and the liquid branching pipe 203a. The outdoor unit 10a and the indoor unit 50b are connected to each other by the gas branching pipe

202a, the gas pipe 204, the gas branch pipe 206b, the liquid branch pipe 207b, the
liquid pipe 205 and the liquid branching pipe 203a.
[0016]
Similarly, the outdoor unit 10b and the indoor unit 50a are connected to each other by the gas branching pipe 202b, the gas pipe 204, the gas branch pipe 206a, the liquid branch pipe 207a, the liquid pipe 205 and the liquid branching pipe 203b. The outdoor unit 10b and the indoor unit 50b are connected to each other by the gas branching pipe 202b, the gas pipe 204, the gas branch pipe 206b, the liquid branch pipe 207b, the liquid pipe 205 and the liquid branching pipe 203b. [0017]
In the outdoor unit 10a, a compressor 1a, an oil separator 2a, a check valve 3a, a four-way valve 4a, an outdoor heat exchanger 5a, a high and low pressure heat exchanger 6a, a flow control valve 8a, a liquid-side opening and closing valve 9a, a gas-side opening and closing valve 11a, an accumulator 12a, an oil returning bypass capillary 13a, a solenoid valve 14a for oil returning bypass, and a bypass flow control valve 7a are provided. The compressor 1a, the oil separator 2a, the check valve 3a, the four-way valve 4a, the outdoor heat exchanger 5a, the high and low pressure heat exchanger 6a, the flow control valve 8a, the liquid-side opening and closing valve 9a, the gas-side opening and closing valve 11a, and the accumulator 12a are provided to be connected in series by refrigerant pipes. [0018]
The high and low pressure heat exchanger 6a is provided at part of a liquid pipe 26a which is located between the outdoor heat exchanger 5a and the flow control valve 8a. To the high and low pressure heat exchanger 6a, the liquid pipe 26a and a bypass pipe 23a are connected. The bypass pipe 23a is branched off from part of the liquid pipe 26a which is located between the high and low pressure heat exchanger 6a and the flow control valve 8a, and is connected to an upstream side of the accumulator 12a which is located on a low-pressure side of the compressor 1. In addition, the bypass flow control valve 7a is provided at part of the bypass pipe 23a which is located between the high and low pressure heat exchanger 6a and the flow control valve 8a.

[0019]
Furthermore, the oil returning bypass capillary 13a and the solenoid valve 14a for oil returning bypass are provided in an oil returning bypass circuit 29a which connects the oil separator 2a and a refrigerant pipe connecting the accumulator 12a and the compressor 1a. The oil returning bypass capillary 13a is provided to connect an upstream side and a downstream side of the solenoid valve 14a for oil returning bypass, and is located in such a way as to bypass the solenoid valve 14a for oil returning bypass. [0020]
It should be noted that in the following description, a point at which the liquid pipe 26a and the bypass pipe 23a are connected is referred to as a connection point 25a, and a point at which the bypass pipe 23a and an upstream side pipe between the four-way valve 4a and the accumulator 12a are connected is referred to as a connection point 24a. [0021]
In addition, the outdoor unit 10a is provided with a controller 27a which controls actuators provided in the outdoor unit 10a, such as the compressor 1a, the four-way valve 4a and an outdoor fan not shown. Furthermore, the outdoor unit 10a is provided with a first pressure sensor 15a, a second pressure sensor 16a, a first temperature sensor 17a, a second temperature sensor 18a, a third temperature sensor 19a, a fourth temperature sensor 20a, a fifth temperature sensor 21a, a sixth temperature sensor 22a and a seventh temperature sensor 28a. [0022]
The compressor 1a includes an inverter circuit, and controls the rotation speed of the compressor 1a by converting a power source frequency with the inverter circuit, thereby controlling the capacity of the compressor 1a, and compresses refrigerant sucked in the compressor 1a to make the refrigerant to be in a high-temperature high-pressure state. The oil separator 2a is provided on a discharge side of the compressor 1a, and has a function of separating a refrigerating machine oil component from refrigerant gas which is discharged from the compressor 1a and which contains a

refrigerating machine oil. The check valve 3a is provided at a refrigerant pipe between the oil separator 2a and the four-way valve 4a, and prevents the refrigerant from flowing back to a discharge portion of the compressor 1a when the compressor 1a is in a stopped state. [0023]
The four-way valve 4a functions as a flow switching device, and switches the flow of refrigerant between the flow of refrigerant in the cooling operation and that in the heating operation. However, in the case where the refrigerant circuit is used only for the cooling operation, the four-way valve 4a is not required. Alternatively, instead of using the four-way valve 4a, two-way or three-way valves may be combined to form a flow switching device. [0024]
The outdoor heat exchanger 5a functions as a condenser or a radiator in the cooling operation, and functions as an evaporator in the heating operation, and causes heat exchange to be performed between refrigerant and air supplied from the outdoor fan not shown. The high and low pressure heat exchanger 6a causes heat exchange to be performed between refrigerant flowing in the liquid pipe 26a and refrigerant flowing in the bypass pipe 23a. The flow control valve 8a is provided downstream of the connection point 25a in a cooling circuit, and functions as a pressure reducing valve or an expansion valve to expand refrigerant by reducing the pressure. As the flow control valve 8a, a valve whose opening degree can be variably controlled, for example, an electronic expansion valve, can be applied. [0025]
The liquid-side opening and closing valve 9a is opened or closed manually or by the controller 27a to allow refrigerant to pass through it or shut out the refrigerant. Similarly, the gas-side opening and closing valve 11a is opened or closed manually or by controller 27a to allow refrigerant to pass through it or shut out the refrigerant. The liquid-side opening and closing valve 9a and the gas-side opening and closing valve 11a are provided to adjust variation of the internal pressure of the refrigeration cycle by

opening and closing. The accumulator 12a is provided on a suction side of the compressor 1a, and stores surplus refrigerant circulating in the refrigerant circuit. [0026]
The bypass flow control valve 7a is provided at part of the bypass pipe 23a which is located between the connection point 25a and the high and low pressure heat exchanger 6a, and functions as a pressure reducing valve or an expansion valve to expand refrigerant by reducing the pressure. As the bypass flow control valve 7a, a valve whose opening degree can be variably controlled, for example, an electronic expansion valve, can be applied. The oil returning bypass circuit 29a returns the refrigerating machine oil separated by the oil separator 2a to the suction side of the compressor 1a. The oil returning bypass capillary 13a adjusts the flow rate of the refrigerating machine oil which passes through the oil returning bypass circuit 29a. The solenoid valve 14a for oil returning bypass is controlled to be opened and closed to adjust the flow rate of the refrigerating machine oil in conjunction with the oil returning bypass capillary 13a. [0027]
The first pressure sensor 15a is provided between the oil separator 2a and the four-way valve 4a to detect the pressure of refrigerant discharged from the compressor 1a. The second pressure sensor 16a is provided upstream of the accumulator 12a to detect the pressure of refrigerant to be sucked into the compressor 1a. The first temperature sensor 17a is provided between the compressor 1a and the oil separator 2a to detect temperature of the refrigerant discharged from the compressor 1a. The second temperature sensor 18a detects the ambient temperature of the outdoor unit 10a. The third temperature sensor 19a is provided between the outdoor heat exchanger 5a and the high and low pressure heat exchanger 6a to detect the temperature of refrigerant passing between the outdoor heat exchanger 5a and the high and low pressure heat exchanger 6a. [0028]
The fourth temperature sensor 20a is provided on part of the bypass pipe 23a which is located downstream of the high and low pressure heat exchanger 6a, to detect

the temperature of refrigerant passing through the bypass pipe 23a after passing through the high and low pressure heat exchanger 6a. The fifth temperature sensor 21a is provided between the connection point 25a and the flow control valve 8a to detect the temperature of refrigerant passing through part of the liquid pipe 26a which is located between the connection point 25a and the flow control valve 8a. The sixth temperature sensor 22a is provided between the connection point 24a and the accumulator 12a to detect the temperature of refrigerant passing between the connection point 24a and the accumulator 12a. The seventh temperature sensor 28a is provided between the accumulator 12a and the compressor 1a to detect the temperature of the refrigerant to be sucked into the compressor 1a. [0029]
Pressure information on pressures detected by the pressure sensors and the temperature information on temperatures detected by the temperature sensors are transmitted as signals to the controller 27a. As described in detail later, the controller 27a controls each of actuators on the basis of the signals transmitted from the pressure sensors and the temperature sensors. Although the type of the controller 27a is not limited, the controller 27a can be formed of, for example, specific hardware capable of controlling each of actuators provided in the outdoor unit 10a, or a microcomputer which executes a program stored in a memory. [0030]
Fig. 2 is a functional block diagram of the controller 27a of the air-conditioning apparatus 300 according to the embodiment of the present invention.
As illustrated in Fig. 2, the controller 27a includes a compressor control unit 30, an outside-air temperature acquisition unit 31, a first determination unit 32, a pressure calculation unit 33, a pressure acquisition unit 34, a valve opening-degree control unit 35, a heat energy calculation unit 36, an inflow-gas heat-quantity calculation unit 37, a time calculation unit 38, and a second determination unit 39. [0031]
Incidentally, the outdoor unit 10b has the same configuration as the outdoor unit 10a. The reference signs denoting the components of the outdoor unit 10a are

different from those denoting the components of the outdoor unit 10b in suffix only, and if the suffix “a” in the reference signs denoting the components of the outdoor unit 10a is changed to “b”, the references signs denote the components of the outdoor unit 10b. Also, in the following description, with respect to the components of the outdoor units, the names of components denoted by reference signs including no suffix are general terms for the components. [0032]
It should be noted that although Fig. 1 illustrates an example in which the outdoor units 10a and 10b are provided with respective controllers, both the outdoor units 10a and 10b may be controlled by a single controller. Furthermore, in the outdoor units 10a and 10b provided with the respective controllers, the controllers can communicate with each other wirelessly or through wire. [0033]
In the indoor unit 50a, an indoor heat exchanger 100a and an expansion valve 101a are provided, and connected in series by the gas branch pipe 206a and the liquid branch pipe 207a. Also, the indoor unit 50a is provided with a controller 102a which controls driving of actuators provided in the indoor unit 50a, such as an expansion valve 101a and an indoor fan not shown. Furthermore, the indoor unit 50a is provided with an eighth temperature sensor 103 and a ninth temperature sensor 104a. [0034]
The indoor heat exchanger 100a functions as an evaporator in the cooling operation and functions as a condenser (or a radiator) in the heating operation, and causes heat exchange to be performed between refrigerant and air. The expansion valve 101a functions as a pressure reducing valve or an expansion valve to expand refrigerant by reducing the pressure. As the expansion valve 101a, a valve whose opening degree can be variably controlled, for example, an electronic expansion valve, can be applied. The eighth temperature sensor 103a is provided at the gas branch pipe 206a connected to the indoor heat exchanger 100a to detect the temperature of refrigerant at a gas-side outlet of the indoor heat exchanger 100a. The ninth temperature sensor 104a is provided on the liquid branch pipe 207a connected to the

indoor heat exchanger 100a to detect the temperature of refrigerant at a liquid-side
outlet of the indoor heat exchanger 100a.
[0035]
Information items on the temperatures detected by the temperature sensors are transmitted as signals to the controller 102a. As described later, the controller 102a controls the actuators on the basis of the signals transmitted from the temperature sensors. Although the type of the controller 102a is not limited, the controller 12a can be formed of, for example, specific hardware capable of controlling each of the actuators installed in the indoor unit 50a, or a microcomputer which executes a program stored in a memory. [0036]
Incidentally, the indoor unit 50b has the same configuration as the indoor unit 50a. The reference signs denoting the components of the indoor unit 50a are different from those denoting the components of the indoor unit 50b, and if the suffix “a” in the reference signs denoting the components of the indoor unit 50a is changed to “b”, the reference signs denote the components of the indoor unit 50b. Also, in the following description, with respect to the indoor units, the names of the components denoted by reference signs including no suffix are general terms for the components. [0037]
Although Fig. 1 illustrates an example in which the indoor units 50a and 50b are provided with respective controllers, both the indoor units 50a and 50b may be controlled by a single controller. Also, in the indoor units 50a and 50 provided with the respective controllers, the controllers can communicate with each other wirelessly or through wire. Furthermore, the controllers provided in the indoor units are made capable of communicating with the controllers provided in the outdoor units wirelessly or through wire. [0038]
In the cooling circuit of the air-conditioning apparatus 300, the compressor 1, the oil separator 2, the check valve 3, the four-way valve 4, the outdoor heat exchanger 5, the high and low pressure heat exchanger 6, the flow control valve 8, the liquid-side

opening and closing valve 9, the expansion valve 101, the indoor heat exchanger 100, the gas-side opening and closing valve 11, the four-way valve 4 and the accumulator 12 are successively connected by pipes, whereby refrigerant flows in the cooling circuit. [0039]
Next, operations of the air-conditioning apparatus 300 according to the embodiment will be described.
First, operations of the air-conditioning apparatus 300 in the cooling operation will be described.
In this case, the four-way valve 4 is switched to cause refrigerant discharged from the compressor 1 to flow into the outdoor heat exchanger 5. To be more specific, in the four-way valve 4, the directions of connection of pipes are directions indicated solid lines in Fig. 1. Also, in addition, operation is started after the flow control valve 8 is set to be fully opened or almost fully opened, the opening degree of the bypass flow control valve 7 is set to an appropriate opening degree, and the opening degree of the expansion valve 101 is also set to an appropriate opening degree. In this case, refrigerant will flow as follow. [0040]
High-temperature high-pressure gas refrigerant discharged from the compressor 1 first passes the oil separator 2. At this time, most of refrigerating machine oil contained in the refrigerant is separated from the refrigerant and stored at the inner bottom of the oil separator 2, and then passes through the oil returning bypass circuit 29. It should be noted that in the case where the solenoid valve 14 for oil returning bypass is opened, the refrigerating machine oil stored at an inner bottom of the oil separator 2 also passes though the solenoid valve 14 for oil returning bypass. Then, this refrigerating machine oil is returned to a suction pipe of the compressor 1. Thereby, the amount of refrigerating machine oil flowing out to the outside of the outdoor unit 10 can be reduced, and the reliability of the compressor 1 can be improved. [0041]
After the ratio of the refrigerating machine oil to the high-temperature high-pressure refrigerant is decreased in the above manner, the high-temperature high-

pressure refrigerant passes through the four-way valve 4, and is condensed and liquefied at the outdoor heat exchanger 5, and then passes through the high and low pressure heat exchanger 6. After flowing from the high and low pressure heat exchanger 6, the refrigerant is separated into refrigerant which will flow into the bypass pipe 23 and refrigerant which will flowing into the liquid pipe 26. The refrigerant flowing in the bypass pipe 23 is appropriately adjusted in flow rate by the bypass flow control valve 7 to change into low-pressure low-temperature refrigerant, and then exchanges heat with the refrigerant flowing out from the outdoor heat exchanger 5, in the high and low pressure heat exchanger 6. As a result, the state of the refrigerant at the outlet of the high and low pressure heat exchanger 6 has lower enthalpy than the state of the refrigerant at the outlet of the outdoor heat exchanger 5. [0042]
After passing through the bypass flow control valve 7 and being discharged from the high and low pressure heat exchanger 6, the low-pressure refrigerant flows into the bypass pipe 23 and reaches the connection point 24a at which the bypass pipe 23a and an upstream-side pipe of the accumulator 12a are connected to each other. As a result, an enthalpy difference is increased, and as a result, the refrigerant flow rate required to obtain the same capacity can be reduced, and the performance is improved because of reduction of a pressure loss. It should be noted that the terms “high pressure” and “low pressure” used in the description are each intended to express a relatively high or low pressure in the refrigerant circuit, and the same is true of “high temperature” and “low temperature”. [0043]
By contrast, high-pressure refrigerant which flows from the high and low pressure heat exchanger 6 passes through the flow control valve 8, and the pressure of the refrigerant is not greatly reduced, since the flow control valve 8 is fully opened or almost fully opened. Then, the refrigerant is supplied to the liquid pipe 205 as high pressure liquid refrigerant. Thereafter, the liquid refrigerant flows into the indoor unit 50, and changes into low-pressure two-phase refrigerant after its pressure is reduced by the expansion valve 101, and the low-pressure two-phase refrigerant is evaporated and

gasified in the indoor heat exchanger 100. At this time, cooling air is supplied to a to-be-air-conditioned space such as an indoor space, and as a result, a cooling operation for the to-be-air-conditioned space is carried out. The refrigerant flowing out from the indoor heat exchanger 100 passes through the gas pipe 204, the four-way valve 4 and the accumulator 12, and then is re-sucked into the compressor 1. [0044]
In this case, since the accumulator 12 is provided with a U-shaped tube as illustrated in Fig. 1, when two-phase gas-liquid refrigerant flows into the accumulator 12, liquid refrigerant stays at a lower part of the accumulator 12, and gas-rich refrigerant which flows from an upper opening portion of the U-shaped tube flows out from the accumulator 12. By virtue of provision of the accumulator 12, the gas-rich refrigerant is sucked into the compressor 1. Also, since the accumulator 12 can hold transient liquid or two-phase gas-liquid refrigerant, it can temporarily prevent the liquid from returning to the compressor 1 until it overflows the liquid, and the reliability of the compressor 1 can be therefore maintained. [0045]
Fig. 3 is a flowchart illustrating a flow of control processes of the air-conditioning apparatus 300 according to the embodiment of the present invention.
A flow of control processes to be performed by the controller 27, which is a feature of the air-conditioning apparatus 300 according to the embodiment, will be described with reference to Fig. 3.
First, when a remote control for the indoor unit 50 is turned on by a user, the compressor control unit 30 of the controller 27 starts driving of the compressor 1, and the air-conditioning apparatus 300 starts the cooling operation (step S1). At this time, one of the plurality of outdoor units, for example, the outdoor unit 10b, is in a thermo-off state, that is, the compressor 1 b is in the stopped state. It should be noted that it is assumed that after the outdoor units 10 and the indoor unit 50 are turned on, each of control units forming the controller 27 sets a fixed value in accordance with an initial setting according with an initial state detected by each of the sensors to complete start¬up processing.

[0046]
While the outdoor unit 10a is being operated, if the compressor 1b of the outdoor unit 10b is in a stopped state, in the refrigerant circuit forming the outdoor unit 10b, a closed circuit from and into which refrigerant does not flow is formed by the check valve 3b, the bypass flow control valve 7b and the flow control valve 8b. That is, the check valve 3b, the bypass flow control valve 7b and the flow control valve 8b are flow passage closing devices which close a passage for refrigerant, and form a closed circuit in part of the refrigerant circuit. [0047]
After step S1, the outside-air temperature acquisition unit 31 of the controller 27 acquires information on an outside-air temperature Tb which the second temperature sensor 18b detects immediately after the outdoor unit 10b is stopped or after a previous refrigerant transferring control is performed, which will be explained later (step S2). [0048]
After step S2, the outside-air temperature acquisition unit 31 of the controller 27 acquires information on an outside air temperature Tbn which the second temperature sensor 18b detects after a given period of time elapses from time when the outside air temperature Tb is detected (step S3). The first determination unit 32 of the controller 27 estimates an increasing degree of the internal pressure of the closed circuit of the outdoor unit 10b which is in the stopped state, on the basis of a temperature difference between the outside air temperature Tbn and the outside air temperature Tb. Then, the first determination unit 32 determines that the pressure of a pipe in the closed circuit of the outdoor unit 10b being in the stopped state exceeds an allowable pressure when the temperature difference is greater than or equal to a threshold AT. For this reason, the first determination unit 32 of the controller 27 determines whether Tbn-Tb>AT (>0 degrees) is satisfied (step S4). [0049]
When the relationship “Tbn-Tb>AT” is satisfied (Yes in step S4), the first determination unit 32 of the controller 27 determines that the internal pressure of the

closed circuit of the outdoor unit 10b being in the stopped state exceeds the allowable pressure, and the process to be executed proceeds to a subsequent step.
By contrast, when Tbn-Tb>AT is not satisfied (No in step S4), the first determination unit 32 of the controller 27 determines at regular intervals whether Tbn-Tb>AT is satisfied or not, until this relationship is satisfied, that is, until the first determination unit 32 determines that the internal pressure of the closed circuit of the outdoor unit 10b being in the stopped state exceeds the allowable pressure (step S4). [0050]
After step S4, the pressure calculation unit 33 of the controller 27 calculates based on the outside air temperature Tbn, an internal pressure Pm of the closed circuit of the outdoor unit 10b being in the stopped state. [0051]
It should be noted that an actual increasing degree of the internal pressure Pm of the closed circuit varies in accordance with a filling ratio (= the volume of refrigerant in the closed circuit / the volume of pipes of the closed circuit) of liquid refrigerant existing in the closed circuit of the outdoor unit 10b being in the stopped state. For this reason, the pressure calculation unit 33 of the controller 27 calculates the internal pressure Pm of the closed circuit based on the outside air temperature Tbn, on the assumption that foreign matters are temporarily caught in the bypass flow control valve 7b and the flow control valve 8b of the outdoor unit 10b being in the stopped state, and the filling ratio of liquid refrigerant in the closed circuit reaches 1 (step S5). Furthermore, the internal pressure of an accumulator in an outdoor unit being in operation, to which refrigerant is to be transferred, is determined to be nearly equal to an operating-side low pressure Ps which is a pressure detected by the second pressure sensor 16a, and the pressure acquisition unit 34 of the controller 27 acquires information on the pressure detected by the second pressure sensor 16a (step S6). Then, the pressure calculation unit 33 of the controller 27 calculates a pressure difference AP=Pm-Ps (step S7). [0052]
After step S7, the valve opening-degree control unit 35 of the controller 27 determines an opening degree of the bypass flow control valve 7b on the basis of the

pressure difference AP calculated in step S7, and controls the bypass flow control valve 7b to set the opening degree thereof to the determined opening degree (step S8) to transfer an appropriate amount of refrigerant. The flow of refrigerant at this time is indicated by arrows in the refrigerant circuit diagram of Fig. 1. First, by opening the bypass flow control valve 7b, refrigerant existing in the closed circuit of the outdoor unit 10b passes through the high and low pressure heat exchanger 6b, then flows, via the bypass pipe 23b, toward a four-way valve 4b where the pressure is low, and flows into the outdoor unit 10a via the gas branching pipe 202b and the gas branching pipe 202a in this order. Eventually, the refrigerant flows into the accumulator 12a which stores surplus refrigerant. In such a manner, since an appropriate amount of refrigerant can be transferred, even when the internal pressure of the closed circuit in the outdoor unit 10b being in the stopped state increases, the internal pressure of the closed circuit in the outdoor unit 10b being in the stopped state can be controlled within an allowable value range. [0053]
After the refrigerant is transferred to the accumulator 12a, it is necessary to determine whether in subsequent control, surplus refrigerant in the accumulator 12a can be re-transferred or not, based on the latent heat quantity of the refrigerant in the accumulator 12a and the heat quantity of low-pressure gas flowing into the accumulator 12a. [0054]
Next, the amount of refrigerant distributed in the accumulator 12a will be explained. In the cooling operation, a superheat-degree constant control is performed at the outlets of the outdoor unit 10a and the outdoor unit 10b, using the eighth temperature sensor 103 and the ninth temperature sensor 104. In addition, the superheat-degree constant control is performed at the outlet of the bypass pipe 23a for the refrigerant that has passed through the high and low pressure heat exchanger 6a, using the fourth temperature sensor 20a and the second pressure sensor 16a. [0055]

Therefore, no liquid refrigerant flows into the accumulator 12a, and only the amount of transferred refrigerant in step S8 is determined as the amount of refrigerant distributed in the accumulator 12a. The amount of transferred refrigerant is calculated using the fifth temperature sensor 21b (determination of a liquid density), the pressure difference AP=Pm-Ps and the opening degree of the bypass flow control valve 7b. It should be noted that the amount of refrigerant to be transferred is set to be smaller than or equal to an effective capacity of the accumulator 12a in the outdoor unit 10a which is in operation, to which refrigerant is to be transferred. That is, the valve opening-degree control unit 35 determines the opening degree of the bypass flow control valve 7 based on the effective capacity of the accumulator 12a of the outdoor unit 10 being in operation, in addition to the pressure difference AP. [0056]
Furthermore, a saturation temperature of the accumulator 12a is calculated based on the pressure detected by the second pressure sensor 16a. In addition, a saturated liquid enthalpy and saturated gas enthalpy are calculated based on the pressure detected by the second pressure sensor 16a, and a latent heat amount of the refrigerant is calculated from the saturated liquid enthalpy and the saturated gas enthalpy. [0057]
Then, a heat energy required to evaporate the liquid refrigerant in the accumulator 12a is calculated from the above refrigerant distribution amount and the latent heat quantity of the refrigerant in the accumulator 12a. [0058]
The enthalpy of gas refrigerant which flows into the accumulator 12a during the cooling operation is calculated based on a temperature detected by the sixth temperature sensor 22b and the pressure detected by the second pressure sensor 16a. In addition, the circulation amount of refrigerant flowing into the accumulator 12a is calculated based on operation states of the compressor 1a (which are an operation frequency, a pressure detected by the first pressure sensor 15a, the pressure detected by the second pressure sensor 16a, and a temperature detected by the seventh

temperature sensor 28b). Then, a heat quantity of gas flowing into the accumulator 12a is calculated based on the enthalpy of the gas refrigerant which flows into the accumulator 12a during the cooling operation and the circulation amount of the refrigerant which flows into the accumulator 12a. [0059]
As described above, based on the refrigerant distribution amount and the latent heat amount of the refrigerant in the accumulator 12a, the heat energy calculation unit 36 of the controller 27 calculates a heat energy required to evaporate the liquid refrigerant in the accumulator 12a (step S9). Furthermore, based on the enthalpy of gas refrigerant which flows into the accumulator 12a during the cooling operation and the circulation amount of refrigerant which flows into the accumulator 12a during the cooling operation, the inflow-gas heat-quantity calculation unit 37 of the controller 27 calculates a heat quantity of the gas flowing into the accumulator 12a (step S10). Then, the time calculation unit 38 of the controller 27 calculates time t_ac required to evaporate the liquid refrigerant in the accumulator 12a, based on the required heat energy and the heat quantity of inflow gas (step S11). [0060]
After step S11, the second determination unit 39 of the controller 27 determines whether or not the time t_ac required to evaporate the liquid refrigerant exceeds start time ts2 of a subsequent control of the steps from step S2 onward (t_ac>ts2) (step S12).
When the second determination unit 39 of the controller 27 determines that t_ac>ts2 is satisfied (Yes in step S12), the compressor control unit 30 of the controller 27 increases a compressor frequency F of the compressor 1a in accordance with the time t_ac (step S13), to thereby increase the heat quantity of gas flowing into the accumulator 12a so that the time t_ac is shortened. Then, the controller 27 causes steps S12 and S13 to be repeatedly carried out until it is determined that t_ac>ts2 is not satisfied. By doing so, it allows restart of the control of the steps from step 2 onward after the surplus refrigerant in the accumulator 12a is sufficiently evaporated (step S12).

By contrast, when the second determination unit 39 of the controller 27 determines that t_ac>ts2 is not satisfied (No in step S12), the step to be carried out re-returns to step S2.
It should be noted that in step S12, the refrigerant is prevented from overflowing the accumulator 12a, and being sucked into the compressor 1a. [0061]
As described above, the air-conditioning apparatus 300 according to the embodiment includes: the indoor unit 50 and the plurality of outdoor units 10, which form a refrigerant circuit that the compressor 1, the outdoor heat exchanger 5, the expansion valve 101 and the indoor heat exchanger 100 are connected to circulate refrigerant; and the controller 27, and the plurality of outdoor units 10 are connected in parallel to the indoor unit 50 by gas pipes and liquid pipes. Each of the outdoor units 10 includes the flow passage closing device which closes a passage for refrigerant to form a closed circuit in a part of the refrigerant circuit, the bypass pipe 23 connecting a side of the outdoor heat exchanger 5, which is located on a downstream side thereof in a cooling operation, and a low pressure side of the compressor 1, the bypass flow control valve 7 provided at the bypass pipe 23, and a pressure sensor which detects pressure. The bypass pipe 23 is connected to the closed circuit. The controller 27 includes the pressure calculation unit 33 which calculates, when at least one of the plurality of outdoor units 10 is in the stopped state during the cooling operation, a pressure difference between the internal pressure of the closed circuit formed in the at least one outdoor unit 10 being in the stopped state and the low pressure side pressure of the other one or ones of the outdoor units 10 which are in operation, and the valve opening-degree control unit 35 which determines an opening degree of the bypass flow control valve 7 based on the pressure difference, and controls the bypass flow control valve 7 to set the opening degree thereof to the determined opening degree. [0062]
According to the air-conditioning apparatus 300 of the present embodiment, the internal pressure of the closed circuit can be controlled to fall within an allowable value range, even when the internal pressure of the closed circuit increases. Furthermore,

since it is not necessary to provide a specific bypass pipe, it is possible to prevent the
manufacturing cost from being increased.
[0063]
Moreover, the air-conditioning apparatus 300 of the present embodiment is provided with a temperature sensor which detects the pressure of outside air, and the pressure calculation unit 33 calculates the internal pressure of a closed circuit formed in an outdoor unit 10 being in the stopped state, based on a temperature of outside air temperature which is detected after a given period of time elapses from the time when the outdoor unit 10 is stopped. [0064]
In a conventional air-conditioning apparatus, in order that the internal pressure of a closed circuit should not exceed an allowable value, the internal pressure of the closed circuit is directly detected by pressure sensors, and refrigerant is transferred via, for example, a bypass pipe, thereby to adjust the internal pressure of the closed circuit. Also, in another conventional air-conditioning apparatus, a pressure relief port or the like which is opened when the internal pressure of a closed circuit becomes greater than or equal to a predetermined primary side pressure is provided in parallel with the closed circuit, to thereby adjust the internal pressure of the closed circuit. [0065]
The pressure calculation unit 33 calculates the internal pressure of the closed circuit formed in the outdoor unit 10 being in the stopped state, based on the temperature of outside air which is detected after a given period of time elapses from the time when the outdoor unit 10 is stopped. Therefore, it is not necessary to provide a unit which directly detects a pressure such as a pressure sensor or installation, in parallel with the closed circuit, or a pressure relief port or the like that is opened when the internal pressure becomes greater than or equal to the predetermined primary side pressure. It is therefore possible to prevent the cost from being increased. [0066]
Furthermore, in the air-conditioning apparatus 300 of the embodiment, the outdoor unit 10 includes the accumulator 12 which is connected to the low pressure side

of the compressor 1 and to the bypass pipe 23, and the valve opening-degree control unit 35 determines the opening degree of the bypass flow control valve 7 on the basis of the effective capacity of the accumulator 12 of the outdoor unit 10 being in operation, in addition to the pressure difference. [0067]
In a further conventional air-conditioning apparatuses, in the case where an outdoor unit is provided with an accumulator, a liquid level detector provided in the accumulator is used to control the flow rate of refrigerant. However, in terms of cost, productivity and reliability, it is more advantageous and realistic than provision of the liquid level detector that the capacity of the accumulator is sufficiently increased to prevent overflow of refrigerant. However, if the capacity of the accumulator is increased to prevent overflow, it does not satisfy a demand for reduction of the size and cost. [0068]
According to the air-conditioning apparatus 300 of the present embodiment, the opening degree of the bypass flow control valve 7 is determined on the basis of the effective capacity of the accumulator 12 of the outdoor unit 10 being in operation, in addition to the pressure difference. It is therefore possible to prevent the above overflow without increasing the capacity of the accumulator 12, and satisfy the demand for reduction of the size and cost. [0069]
Furthermore, in the air-conditioning apparatus 300 of the present embodiment, the controller 27 includes the time calculation unit 38which calculates time required to evaporate refrigerant in the accumulator 12 after refrigerant in the closed circuit of the outdoor unit 10 being in the stopped state is transferred to the low pressure side of the outdoor unit 10 being in operation, via the bypass pipe 23 connected to the closed circuit, and the determination unit that determines, on the basis of the calculated time, timing at which refrigerant in the closed circuit of the outdoor unit 10 being in the stopped state is re-transferred to the low pressure side of the outdoor unit 10 being in operation, via the bypass pipe 23 connected to the closed circuit.

[0070]
In a still another air-conditioning apparatus in which two or more outdoor units are combined, an outdoor unit to be restarted is determined at intervals of a given time period to equalize the operation loads on the outdoor units. However, the outdoor unit to be started is not switched from one outdoor unit to another unless the given time period elapses, and as a result an outdoor unit may be started, with an excessive amount of refrigerant distributed in the accumulator. After restarting of the outdoor unit, if surplus refrigerant treatment cannot be controlled appropriately, there is a possibility that overflow will occur repeatedly, and as a result, the load on the compressor, that is, the load on the outdoor unit, will be increased, it may adversely affect the product life, and the quality may be greatly worsened. [0071]
According to the air-conditioning apparatus 300 of the embodiment, the controller 27 includes the time calculation unit 38 which calculates time required to evaporate refrigerant in the accumulator 12 after refrigerant in the closed circuit of the outdoor unit 10 being in the stopped state is transferred to the low pressure side of the outdoor unit 10 being in operation, via the bypass pipe 23 connected to the closed circuit, and the determination unit that determines, on the basis of the calculated time, timing at which refrigerant in the closed circuit of the outdoor unit 10 being in the stopped state is re-transferred to the low pressure side of the outdoor unit 10 being in operation, via the bypass pipe 23 connected to the closed circuit. Therefore, surplus refrigerant treatment after restart can be controlled appropriately, and the above overflow can be prevented. In addition, it is possible to prevent increasing of the load on the compressor 1, that is, the load on the outdoor unit 10, and prevent lowering of the quality.
Reference Signs List [0072]
1 compressor 1a compressor 1b compressor 2 oil separator 2a oil separator 3 check valve 3a check valve 3b check valve 4 four-way valve 4a four-way valve 4b four-way valve 5 outdoor heat exchanger 5a outdoor

heat exchanger 6 high and low pressure heat exchanger 6a high and low pressure heat exchanger 6b high and low pressure heat exchanger 7 bypass flow control valve 7a bypass flow control valve 7b bypass flow control valve 8 flow control valve 8a flow control valve 8b flow control valve 9 liquid-side opening and closing valve 9a liquid-side opening and closing valve 10 outdoor unit 10a outdoor unit 10b outdoor unit 11 gas-side opening and closing valve 11a gas-side opening and closing valve 12 accumulator 12a accumulator 13a oil returning bypass capillary 14 solenoid valve for oil returning bypass 14a solenoid valve for oil returning bypass 15a first pressure sensor 16a second pressure sensor 17a first temperature sensor 18a second temperature sensor 18b second temperature sensor 19a third temperature sensor 20a fourth temperature sensor 21a fifth temperature sensor 21b fifth temperature sensor 22a sixth temperature sensor 22b sixth temperature sensor 23 bypass pipe 23a bypass pipe 23b bypass pipe 24a connection point 25a connection point 26 liquid pipe 26a liquid pipe 27 controller 27a controller 28a seventh temperature sensor 28b seventh temperature sensor 29 oil returning bypass 29a oil returning bypass 30 compressor control unit 31 outside-air temperature acquisition unit 32 first determination unit 33 pressure calculation unit 34 pressure acquisition unit 35 valve opening-degree control unit 36 heat energy calculation unit 37 inflow-gas heat-quantity calculation unit 38 time calculation unit 39 second determination unit 50 indoor unit 50a indoor unit 50b indoor unit 100 indoor heat exchanger 100a indoor heat exchanger 101 expansion valve 101a expansion valve 102a controller 103 eighth temperature sensor 103a eighth temperature sensor 104 ninth temperature sensor 104a ninth temperature sensor 200 gas distributor 201 liquid distributor 202a gas branching pipe 202b gas branching pipe 203a liquid branching pipe 203b liquid branching pipe 204 gas pipe 205 liquid pipe 206a gas branch pipe 206b gas branch pipe 207a liquid branch pipe 207b liquid branch pipe 300 air-conditioning apparatus

We claim:
[Claim 1]
An air-conditioning apparatus comprising:
an indoor unit and a plurality of outdoor units, which form a refrigerant circuit that a compressor, an outdoor heat exchanger, an expansion valve and an indoor heat exchanger are connected to circulate refrigerant; and
a controller,
wherein the plurality of outdoor units are connected in parallel to the indoor unit by gas pipes and liquid pipes,
wherein each of the outdoor units includes
a flow passage closing device configured to close a passage for refrigerant, thereby forming a closed circuit in a part of the refrigerant circuit,
a bypass pipe connecting a side of the outdoor heat exchanger, which is located on a downstream side thereof in a cooling operation, and a low pressure side of the compressor,
a bypass flow control valve provided at the bypass pipe, and
a pressure sensor configured to detect a pressure of a low-pressure side of the each of the outdoor units,
wherein the bypass pipe is connected to the closed circuit, and
wherein the controller includes
a pressure calculation unit configured to calculate, when at least one of the plurality of outdoor units is in a stopped state during the cooling operation, a pressure difference between an internal pressure of the closed circuit formed in the at least one outdoor unit being in the stopped state and a pressure of a low pressure side of the other one or ones of the outdoor units which are in operation, and
a valve opening-degree control unit configured to determine an opening degree of the bypass flow control valve based on the pressure difference, and control the bypass flow control valve to set the opening degree thereof to the determined opening degree. [Claim 2]
The air-conditioning apparatus of claim 1, further comprising

a temperature sensor configured to detect a temperature of outside air,
wherein the pressure calculation unit is configured to calculate the internal pressure of the closed circuit formed in the at least one outdoor unit being in the stopped state, based on a temperature of the outside air which is detected after a given period of time elapses from time at which the at least one outdoor unit is stopped. [Claim 3]
The air-conditioning apparatus of claim 1 or 2,
wherein the each of the outdoor units further includes an accumulator connected to the low pressure side of the compressor and also to the bypass pipe, and
the valve opening-degree control unit is configured to determine the opening degree of the bypass flow control valve based on an effective capacity of the accumulator of the other one or ones of the outdoor units which are in operation, in addition to the pressure difference. [Claim 4]
The air-conditioning apparatus of claim 3,
wherein the controller further includes
a time calculation unit configured to calculate time required to evaporate refrigerant in the accumulator after refrigerant in the closed circuit of the at least one of the outdoor units which is in the stopped state is transferred to the low pressure side of the other one or ones of the outdoor units which are in operation, via the bypass pipe connected to the closed circuit, and
a determination unit configured to determine, based on the calculated time, timing at which refrigerant in the closed circuit of the at least one outdoor unit being in the stopped state is re-transferred to the low pressure side of the other outdoor unit or units which are in operation, via the bypass pipe connected to the closed circuit is performed.