Patent Publication Number: US-2022214080-A1

Title: Refrigeration cycle apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a refrigeration cycle apparatus capable of performing a heating operation, a defrosting operation, and a simultaneous heating-defrosting operation. 
     BACKGROUND ART 
     An air-conditioning apparatus is disclosed in FIG. 1 of Patent Literature 1. This air-conditioning apparatus includes an outdoor heat exchanger including a first heat exchanger and a second heat exchanger. In this air-conditioning apparatus, the outdoor heat exchanger can be defrosted without stopping heating by defrosting the first heat exchanger and the second heat exchanger alternately. This air-conditioning apparatus is provided with two flow switch valves to enable high-temperature high-pressure refrigerant from a compressor to flow to the first heat exchanger and the second heat exchanger. The flow switch valves are constituted by three-way valves using four-way valves. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: WO 2017/094148 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, a valve operated by differential pressure is used as a flow switch valve in an air-conditioning apparatus. A valve operated by differential pressure has, for example, a high pressure port connected to a discharge side of a compressor and a low pressure port connected to a suction side of the compressor and operates using the difference in pressure between high pressure and low pressure. For a valve operated by differential pressure, a minimum operating differential pressure for assuredly operating the valve is defined. In a case where the difference in pressure between high pressure and low pressure becomes less than or equal to the minimum operating differential pressure, the valve does not normally operate and does not perform flow path switching. 
     The present disclosure has been made to solve problems as described above, and an object of the present disclosure is to provide a refrigeration cycle apparatus that enables a flow switch valve operated by differential pressure to normally operate in a refrigerant circuit that can perform a heating operation, a defrosting operation, and a simultaneous heating-defrosting operation. 
     Solution to Problem 
     A refrigeration cycle apparatus according to an embodiment of the present disclosure includes a first flow switch valve including a first port, a second port, a third port, and a fourth port, a second flow switch valve and a third flow switch valve each including a fifth port, a sixth port, and a seventh port, the second flow switch valve and the third flow switch valve operating by differential pressure, a compressor including a suction port for sucking refrigerant and a discharge port for discharging the refrigerant, a discharge pipe connecting between the discharge port and the first port, a suction pipe connecting between the suction port and the second port, a first high pressure pipe connecting between the discharge pipe and the fifth port of the second flow switch valve and the fifth port of the third flow switch valve, a second high pressure pipe connecting the third port and a bifurcation arranged at the first high pressure pipe, a bypass expansion valve provided at a part of the first high pressure pipe, the part extending between the discharge pipe and the bifurcation, a valve provided at the second high pressure pipe, a low pressure pipe connecting between the suction pipe and the sixth port of the second flow switch valve and the sixth port of the third flow switch valve, a first outdoor heat exchanger connected to the seventh port of the second flow switch valve, a second outdoor heat exchanger connected to the seventh port of the third flow switch valve, an indoor heat exchanger connected to the fourth port, and a controller configured to control operation frequency of the compressor and opening degree of the bypass expansion valve. The controller is configured to perform a differential pressure ensuring process, when switching the second flow switch valve or the third flow switch valve. In the differential pressure ensuring process, the controller is configured to set the operation frequency of the compressor to a first frequency and set the opening degree of the bypass expansion valve to a first degree if a first condition is not met, and set the operation frequency of the compressor to a second frequency which is higher than the first frequency or set the opening degree of the bypass expansion valve to a second degree which is larger than the first degree if the first condition is met. 
     A refrigeration cycle apparatus according to an embodiment of the present disclosure includes a first flow switch valve including a first port, a second port, a third port, and a fourth port, a second flow switch valve and a third flow switch valve each including a fifth port, a sixth port, and a seventh port, the second flow switch valve and the third flow switch valve operating by differential pressure, a compressor including a suction port for sucking refrigerant and a discharge port for discharging the refrigerant, a discharge pipe connecting between the discharge port and the first port, a suction pipe connecting between the suction port and the second port, a first high pressure pipe connecting between the discharge pipe and the fifth port of the second flow switch valve and the fifth port of the third flow switch valve, a second high pressure pipe connecting the third port and a bifurcation arranged at the first high pressure pipe, a bypass expansion valve provided at a part of the first high pressure pipe, the part extending between the discharge pipe and the bifurcation, a valve provided at the second high pressure pipe, a low pressure pipe connecting between the suction pipe and the sixth port of the second flow switch valve and the sixth port of the third flow switch valve, a first outdoor heat exchanger connected to the seventh port of the second flow switch valve, a second outdoor heat exchanger connected to the seventh port of the third flow switch valve, and an indoor heat exchanger connected to the fourth port. The bypass expansion valve has a flow path allowing the refrigerant to flow therethrough even in a case where the bypass expansion valve is in a closed state. 
     Advantageous Effects of Invention 
     According to an embodiment of the present disclosure, the minimum operating differential pressures of the second flow switch valve and the third flow switch valve, which are operated by differential pressure, can be ensured by performing the differential pressure ensuring process when switching of the second flow switch valve or the third flow switch valve is performed. Consequently, the second flow switch valve and the third flow switch valve can be normally operated in any environment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a refrigerant circuit diagram illustrating the configuration of a refrigeration cycle apparatus according to Embodiment 1. 
         FIG. 2  is a functional block diagram of a controller according to Embodiment 1. 
         FIG. 3  is a cross-sectional view illustrating a schematic configuration of a four-way valve of the refrigeration cycle apparatus according to Embodiment 1. 
         FIG. 4  is a diagram illustrating the operation of the refrigeration cycle apparatus according to Embodiment 1 at the time of a heating operation. 
         FIG. 5  is a diagram illustrating the operation of the refrigeration cycle apparatus according to Embodiment 1 at the time of a defrosting operation. 
         FIG. 6  is a diagram illustrating the operation of the refrigeration cycle apparatus according to Embodiment 1 at the time of a first operation during a simultaneous heating-defrosting operation. 
         FIG. 7  is a diagram illustrating the operation of the refrigeration cycle apparatus according to Embodiment 1 at the time of a second operation during the simultaneous heating-defrosting operation. 
         FIG. 8  is a flow chart illustrating the procedure of the operation of the refrigeration cycle apparatus according to Embodiment 1. 
         FIG. 9  is a table illustrating an example of a relationship between pressure at a second flow switch valve and outdoor air temperature in Embodiment 1. 
         FIG. 10  is a table illustrating an example of a relationship between pressure at the second flow switch valve and operation frequency of a compressor in Embodiment 1. 
         FIG. 11  is a flow chart illustrating the procedure of a differential pressure ensuring process of Embodiment 1. 
         FIG. 12  is a table illustrating an example of a relationship between pressure at a second flow switch valve and outdoor air temperature in Embodiment 2. 
         FIG. 13  is a table illustrating an example of a relationship between pressure at the second flow switch valve and operation frequency of a compressor in Embodiment 2. 
         FIG. 14  is a flow chart illustrating the procedure of a simultaneous heating-defrosting operation of Embodiment 2. 
         FIG. 15  is a schematic configuration diagram of a bypass expansion valve of a refrigeration cycle apparatus according to Embodiment 3. 
         FIG. 16  is a plan view of a base of the bypass expansion valve according to Embodiment 3. 
         FIG. 17  is a cross-sectional view of a restriction portion of the bypass expansion valve according to Embodiment 3. 
         FIG. 18  is a cross-sectional view of the restriction portion in a case where the bypass expansion valve according to Embodiment 3 is in a closed state. 
         FIG. 19  is a cross-sectional view of a restriction portion of a bypass expansion valve according to a modification of Embodiment 3. 
         FIG. 20  is a cross-sectional view of a restriction portion of a bypass expansion valve according to another modification of Embodiment 3. 
         FIG. 21  is a graph illustrating a relationship between opening degree and Cv value at the bypass expansion valve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     A refrigeration cycle apparatus  1  according to Embodiment 1 will be described.  FIG. 1  is a refrigerant circuit diagram illustrating the configuration of the refrigeration cycle apparatus  1  according to Embodiment 1. In Embodiment 1, an air-conditioning apparatus is illustrated by example as the refrigeration cycle apparatus  1 . As illustrated in  FIG. 1 , the refrigeration cycle apparatus  1  according to Embodiment 1 includes a refrigerant circuit  10  and a controller  50 , which controls the refrigerant circuit  10 . The refrigerant circuit  10  according to Embodiment 1 includes a compressor  11 , a first flow switch valve  12 , an indoor heat exchanger  13 , an expansion valve  14 , a first outdoor heat exchanger  15   a , a second outdoor heat exchanger  15   b , a second flow switch valve  21   a , and a third flow switch valve  21   b . As will be described later, the refrigerant circuit  10  is configured to be able to perform at least a heating operation, a reverse cycle defrosting operation (hereinafter simply referred to as “defrosting operation”), and a simultaneous heating-defrosting operation. The refrigerant circuit  10  may also be configured to be able to perform a cooling operation. At the time of the cooling operation, the first flow switch valve  12 , the second flow switch valve  21   a , and the third flow switch valve  21   b  are set to be in a state that is substantially the same as that for the defrosting operation. 
     The refrigeration cycle apparatus  1  includes an outdoor unit installed in an outdoor space and an indoor unit installed in an indoor space. The outdoor unit includes the compressor  11 , the first flow switch valve  12 , the expansion valve  14 , the first outdoor heat exchanger  15   a , the second outdoor heat exchanger  15   b , the second flow switch valve  21   a , and the third flow switch valve  21   b . The indoor unit includes the indoor heat exchanger  13 . 
     The compressor  11  is a fluid machine that sucks and compresses low-pressure gas refrigerant to discharge high-pressure gas refrigerant. An inverter-driven compressor capable of adjusting operation frequency is used as the compressor  11 . An operation frequency range is preset in the compressor  11 . The compressor  11  is configured to operate under control performed by the controller  50  at an operation frequency that can be changed within the operation frequency range. The compressor  11  includes a suction port  11   a  for sucking refrigerant and a discharge port  11   b  for discharging compressed refrigerant. The suction port  11   a  is maintained at suction pressure, namely; low pressure. The discharge port  11   b  is maintained at discharge pressure, namely, high pressure. 
     The first flow switch valve  12  is a four-way valve and has four ports E, F, G, and H. In the following description, the port G, the port E, the port F, and the port H may also be referred to as “first port G”, “second port E”, “third port F”, and “fourth port H”, respectively. The first port G is a high pressure port whose pressure is maintained at high pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The second port E is a low pressure port whose pressure is maintained at low pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The first flow switch valve  12  can be in a first state indicated by a solid line in  FIG. 1  and a second state indicated by a broken line in  FIG. 1 . In the first state, the first port G communicates with the fourth port H, and the second port E communicates with the third port F. In the second state, the first port G communicates with the third port F, and the second port E communicates with the fourth port H. The first flow switch valve  12  is set to be in the first state at the time of the heating operation and at the time of the simultaneous heating-defrosting operation and to be in the second state at the time of the defrosting operation under control performed by the controller  50 . 
     The indoor heat exchanger  13  is a heat exchanger that exchanges heat between refrigerant flowing thereinside and air sent by an indoor fan (not illustrated) included in the indoor unit. The indoor heat exchanger  13  serves as a condenser at the time of the heating operation and as an evaporator at the time of the cooling operation. 
     The expansion valve  14  is a valve that reduces the pressure of refrigerant. As the expansion valve  14 , an electronic expansion valve is used that can change the opening degree thereof under control performed by the controller  50 . 
     The first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are each a heat exchanger that exchanges heat flowing thereinside and air sent by an outdoor fan (not illustrated) included in the outdoor unit. The first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  serve as an evaporator at the time of the heating operation and as a condenser at the time of the cooling operation. The first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are connected in parallel with each other in the refrigerant circuit  10 . The first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are configured by, for example, dividing one heat exchanger into two portions, which are upper and lower portions. For example, the first outdoor heat exchanger  15   a  is arranged under the second outdoor heat exchanger  15   b . In this case, the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are also arranged in parallel with each other with respect to airflow. 
     The second flow switch valve  21   a  is a four-way valve and has four ports I, J, K, and L. In the following description, the port K, the port I, the port L, and the port J may also be referred to as “fifth port K”, “sixth port I”, “seventh port L”, and “eighth port J”, respectively. The fifth port K is a high pressure port whose pressure is maintained at high pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The sixth port I is a low pressure port whose pressure is maintained at low pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The eighth port J is closed such that refrigerant does not leak from the eighth port J. The second flow switch valve  21   a  can be in a first state indicated by a solid line in  FIG. 1  and a second state indicated by a broken line in  FIG. 1 . In the first state, the fifth port K communicates with the eighth port J, and the sixth port I communicates with the seventh port L. In the second state, the fifth port K communicates with the seventh port L, and the sixth port communicates with the eighth port J. The second flow switch valve  21   a  is set to be in the first state at the time of the heating operation, to be in the second state at the time of the defrosting operation, and to be in the first state or the second state at the time of the simultaneous heating-defrosting operation under control performed by the controller  50 . 
     The third flow switch valve  21   b  is a four-way valve and has four ports M, N, O, and P. In the following description, the port O, the port M, the port P, and the port N may also be referred to as “fifth port O”, “sixth port M”, “seventh port P”, and “eighth port N”, respectively. The fifth port O is a high pressure port whose pressure is maintained at high pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The sixth port M is a low pressure port whose pressure is maintained at low pressure in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The eighth port N is closed such that refrigerant does not leak from the eighth port N. The third flow switch valve  21   b  can be in a first state indicated by a solid line in  FIG. 1  and a second state indicated by a broken line in  FIG. 1 . In the first state, the fifth port O communicates with the eighth port N, and the sixth port M communicates with the seventh port P. In the second state, the fifth port O communicates with the seventh port P, and the sixth port M communicates with the eighth port N. The third flow switch valve  21   b  is set to be in the first state at the time of the heating operation, to be in the second state at the time of the defrosting operation, and to be in the first state or the second state at the time of the simultaneous heating-defrosting operation under control performed by the controller  50 . 
     The first flow switch valve  12 , the second flow switch valve  21   a , and the third flow switch valve  21   b  are each a four-way valve operated by differential pressure, the four-way valve operating using the difference in pressure between discharge-side pressure and suction-side pressure of the compressor  11 . As the first flow switch valve  12 , the second flow switch valve  21   a , and the third flow switch valve  21   b , four-way valves having the same configuration can be used. 
     The discharge port  11   b  of the compressor  11  and the first port G of the first flow switch valve  12  are connected by a discharge pipe  61 . High-pressure refrigerant discharged from the discharge port  11   b  of the compressor  11  flows through the discharge pipe  61  in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. The suction port  11   a  of the compressor  11  and the second port E of the first flow switch valve  12  are connected by a suction pipe  62 . Low-pressure refrigerant to be sucked into the suction port  11   a  of the compressor  11  flows through the suction pipe  62  in any of the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. 
     One end of a first high pressure pipe  67  is connected to a bifurcation  63  arranged partway along the discharge pipe  61 . The other end of the first high pressure pipe  67  divides at a bifurcation  68  into a first high pressure pipe  67   a  and a first high pressure pipe  67   b . The first high pressure pipe  67   a  is connected to the fifth port K, which is a high pressure port, of the second flow switch valve  21   a . The first high pressure pipe  67   b  is connected to the fifth port O, which is a high pressure port, of the third flow switch valve  21   b.    
     The first high pressure pipe  67  has another bifurcation  65  arranged between the bifurcation  63  and the bifurcation  68 . The bifurcation  65  of the first high pressure pipe  67  and the third port F of the first flow switch valve  12  are connected by a second high pressure pipe  64 . 
     The first high pressure pipe  67  has a bypass expansion valve  18  provided between the bifurcation  63  and the bifurcation  65 . The bypass expansion valve  18  is an electronic expansion valve whose opening degree is controlled under control performed by the controller  50 . The bypass expansion valve  18  also has the function of reducing the pressure of refrigerant. The operation of the bypass expansion valve  18  will be described later. 
     The second high pressure pipe  64  has a check valve  22 . The check valve  22  is configured to allow refrigerant to flow in the direction from the third port F of the first flow switch valve  12  toward the first high pressure pipe  67  and prevent refrigerant from flowing in the direction from the first high pressure pipe  67  toward the third port F. Instead of the check valve  22 , an on-off valve such as a solenoid valve or a motor operated valve that opens and closes under control performed by the controller  50  can be used. An operation performed in a case where an on-off valve is used instead of the check valve  22  will be described later. 
     One end of a low pressure pipe  70  is connected to a bifurcation  69  arranged partway along the suction pipe  62 . The other end of the low pressure pipe  70  divides at a bifurcation  71  into a low pressure pipe  70   a  and a low pressure pipe  70   b . The low pressure pipe  70   a  is connected to the sixth port I, which is a low pressure port, of the second flow switch valve  21   a . The low pressure pipe  70   b  is connected to the sixth port M, which is a low pressure port, of the third flow switch valve  21   b.    
     The fourth port H of the first flow switch valve  12  is connected to one inlet-outlet of the indoor heat exchanger  13  with a refrigerant pipe  80  interposed therebetween. A portion of the refrigerant pipe  80  is constituted by an extension pipe connecting the outdoor unit and the indoor unit. The refrigerant pipe  80  has a stop valve, which is not illustrated, at a position between the extension pipe and the outdoor unit. 
     The other inlet-outlet of the indoor heat exchanger  13  is connected to one inlet-outlet of the expansion valve  14  with a refrigerant pipe  81  interposed therebetween. A portion of the refrigerant pipe  81  is constituted by an extension pipe connecting the outdoor unit and the indoor unit. The refrigerant pipe  81  has a stop valve, which is not illustrated, at a position between the extension pipe and the outdoor unit. 
     One end of a refrigerant pipe  82  is connected to the other inlet-outlet of the expansion valve  14 . The other end of the refrigerant pipe  82  divides at a bifurcation  84  into a refrigerant pipe  82   a  and a refrigerant pipe  82   b . The refrigerant pipe  82   a  has a pressure reducing device such as a capillary tube  17   a . The refrigerant pipe  82   a  is connected to one inlet-outlet of the first outdoor heat exchanger  15   a . The refrigerant pipe  82   b  has a pressure reducing device such as a capillary tube  17   b . The refrigerant pipe  82   b  is connected to one inlet-outlet of the second outdoor heat exchanger  15   b . That is, the other inlet-outlet of the expansion valve  14  is connected to the one inlet-outlet of the first outdoor heat exchanger  15   a  and the one inlet-outlet of the second outdoor heat exchanger  15   b  with the refrigerant pipe  82  interposed therebetween. The one inlet-outlet of the first outdoor heat exchanger  15   a  is connected to the one inlet-outlet of the second outdoor heat exchanger  15   b  with the refrigerant pipe  82   a  and the refrigerant pipe  82   b  interposed therebetween. 
     The other inlet-outlet of the first outdoor heat exchanger  15   a  is connected to the seventh port L of the second flow switch valve  21   a  with a refrigerant pipe  83   a  interposed therebetween. The other inlet-outlet of the second outdoor heat exchanger  15   b  is connected to the seventh port P of the third flow switch valve  21   b  with a refrigerant pipe  83   b  interposed therebetween. At least at the time of the heating operation and at the time of the defrosting operation, the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are connected in parallel with each other in the refrigerant circuit  10 . 
     The casing of the outdoor unit is provided with an outdoor air temperature sensor  91 , which is configured to detect the temperature of outside air around the outdoor unit. In a case where the outdoor unit is installed indoors, the temperature of outside air means the ambient temperature around the outdoor unit. The first outdoor heat exchanger  15   a  is provided with a heat exchanger temperature sensor  92   a , which is configured to detect the temperature of the first outdoor heat exchanger  15   a . The second outdoor heat exchanger  15   b  is provided with a heat exchanger temperature sensor  92   b , which is configured to detect the temperature of the second outdoor heat exchanger  15   b . The outdoor air temperature sensor  91 , the heat exchanger temperature sensor  92   a , and the heat exchanger temperature sensor  92   b  are, for example, thermistors. Detection results from the outdoor air temperature sensor  91 , the heat exchanger temperature sensor  92   a , and the heat exchanger temperature sensor  92   b  are transmitted to the controller  50  and are used by the controller  50  to perform control. The refrigerant circuit  10  may include a temperature sensor or a pressure sensor other than the sensors described above. 
     The controller  50  includes a microcomputer including, for example, a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input/output (I/O) port.  FIG. 2  is a functional block diagram of the controller  50  according to Embodiment 1. As illustrated in  FIG. 2 , the controller  50  receives detection results from the outdoor air temperature sensor  91 , the heat exchanger temperature sensor  92   a , and the heat exchanger temperature sensor  92   b . The controller  50  may also receive a detection signal from the other temperature sensor and the other pressure sensor provided in the refrigerant circuit  10  and an operation signal from an operation unit that receives an operation performed by the user. 
     The controller  50  includes an operation control unit  501 , a condition determination unit  502 , and a differential pressure ensuring unit  503  as functional units realized by executing programs. The controller  50  further includes a storage unit  504  including a memory such as a ROM or a RAM. The operation control unit  501  control the operation of the entire refrigeration cycle apparatus  1  on the basis of input signals and performs the heating operation, the defrosting operation, and the simultaneous heating-defrosting operation. Specifically, the operation control unit  501  controls the operation frequency of the compressor  11 , the opening degree of the expansion valve  14 , switching of the first flow switch valve  12 , switching of the second flow switch valve  21   a , and switching of the third flow switch valve  21   b , the opening degree of the bypass expansion valve  18 , and a fan. 
     The condition determination unit  502  determines whether a first condition is met on the basis of detection results from the outdoor air temperature sensor  91 , the heat exchanger temperature sensor  92   a , and the heat exchanger temperature sensor  92   b  when switching of the second flow switch valve  21   a  and switching of the third flow switch valve  21   b  are performed. In a case where the condition determination unit  502  determines that the first condition is met, the differential pressure ensuring unit  503  controls the opening degree of the bypass expansion valve  18  to ensure a certain difference in pressure. The condition determination unit  502  and the differential pressure ensuring unit  503  will be described in detail below. The storage unit  504  stores various types of data and programs used in processing performed by the operation control unit  501 , the condition determination unit  502 , and the differential pressure ensuring unit  503 . 
       FIG. 3  is a cross-sectional view illustrating a schematic configuration of the second flow switch valve  21   a  of the refrigeration cycle apparatus  1  according to Embodiment 1. As illustrated in  FIG. 3 , the second flow switch valve  21   a  includes a valve body  100  and a pilot solenoid valve  120 . The second flow switch valve  21   a  is a four-way valve operated by differential pressure. 
     The valve body  100  includes a cylinder  101 , a slide base  102  formed on a portion of the inner wall of the cylinder  101 , and a slide valve  103  that slides on the slide base  102  along the central axis direction of the cylinder  101 . The central part of the slide base  102  in the central axis direction of the cylinder  101  is provided with the sixth port I, which is a low pressure port. The seventh port L and the eighth port J are provided on both sides of the sixth port I such that the seventh port L and the eighth port J sandwich the sixth port I in the central axis direction of the cylinder  101 . The fifth port K, which is a high pressure port, is provided at a position facing the sixth port across the central axis of the cylinder  101 . 
     The slide valve  103  has a dome shape that is open toward the slide base  102 . On one end side of the slide valve  103  in the central axis direction of the cylinder  101 , a piston  104  coupled to the slide valve  103  is provided. A first space  106  is formed between one end of the cylinder  101  and the piston  104 . On the other end side of the slide valve  103  in the central axis direction of the cylinder  101 , a piston  105  coupled to the slide valve  103  is provided. A second space  107  is formed between the other end of the cylinder  101  and the piston  105 . The pistons  104  and  105  are provided such that the pistons  104  and  105  are slidable along the inner wall surface of the cylinder  101 , The pistons  104  and  105  move together with the slide valve  103  along the central axis direction of the cylinder  101 . 
     The pilot solenoid valve  120  is connected to the valve body  100  with four individual pilot tubes  110 ,  111 ,  112 , and  113  interposed therebetween. The pilot tube  110  is connected to the fifth port K of the valve body  100 . The pilot tube  111  is connected to the sixth port I of the valve body  100 . The pilot tube  112  is connected to the first space  106  of the valve body  100 . The pilot tube  113  is connected to the second space  107  of the valve body  100 . 
     The pilot solenoid valve  120  is switched between a first state and a second state under control performed by the controller  50 . In the first state, the pilot tube  110  communicates with the pilot tube  113  in the inside of the pilot solenoid valve  120 , and the pilot tube  111  communicates with the pilot tube  112  in the inside of the pilot solenoid valve  120 . Thus, in the first state, the fifth port K communicates with the second space  107  so that the pressure in the second space  107  becomes high, and the sixth port I communicates with the first space  106  so that the pressure in the first space  106  becomes low. The slide valve  103  moves toward the first space  106  side due to the difference in pressure between the first space  106  and the second space  107  and enters the state illustrated in  FIG. 3 . As a result, the sixth port I communicates with the seventh port L, and the fifth port K communicates with the eighth port J. 
     In the second state, the pilot tube  110  communicates with the pilot tube  112  in the inside of the pilot solenoid valve  120 , and the pilot tube  111  communicates with the pilot tube  113  in the inside of the pilot solenoid valve  120 . Thus, in the second state, the fifth port K communicates with the first space  106  so that the pressure in the first space  106  becomes high, and the sixth port I communicates with the second space  107  so that the pressure in the second space  107  becomes low. The slide valve  103  moves toward the second space  107  side due to the difference in pressure between the first space  106  and the second space  107 . As a result, the sixth port I communicates with the eighth port J, and the fifth port K communicates with the seventh port L. 
     In any of the first state and the second state, the pressure of the fifth port K is higher than that of the sixth port I, and thus the slide valve  103  is pressed against the slide base  102  due to the difference in pressure. As a result, the leakage of refrigerant from the slide valve  103  is suppressed. 
     Although not illustrated and not described, the third flow switch valve  21   b  and the first flow switch valve  12  each have substantially the same configuration as that of the second flow switch valve  21   a.    
     Next, the operation of the refrigeration cycle apparatus  1  at the time of the heating operation will be described.  FIG. 4  is a diagram illustrating the operation of the refrigeration cycle apparatus  1  according to Embodiment 1 at the time of the heating operation. As illustrated in  FIG. 4 , at the time of the heating operation, the first flow switch valve  12  is set to be in the first state, in which the first port G communicates with the fourth port H, and the second port E communicates with the third port F. The second flow switch valve  21   a  is set to be in the first state, in which the fifth port K communicates with the eighth port J, and the sixth port I communicates with the seventh port L. The third flow switch valve  21   b  is set to be in the first state, in which the fifth port O communicates with the eighth port N, and the sixth port M communicates with the seventh port P. 
     The bypass expansion valve  18  is set to be in an open state. In this case, the opening degree of the bypass expansion valve  18  is set to “fully open”, By setting the bypass expansion valve  18  to be in the open state, the pressure of the fifth port K of the second flow switch valve  21   a  and that of the fifth port O of the third flow switch valve  21   b  are maintained at high pressure or intermediate pressure. In this case, the intermediate pressure is pressure higher than the suction pressure of the compressor  11  and lower than the discharge pressure of the compressor  11 . In a case where the bypass expansion valve  18  is set to be in the open state, an end side of the first high pressure pipe  67  is closed by the eighth port J of the second flow switch valve  21   a  and the eighth port N of the third flow switch valve  21   b . Thus, refrigerant does not flow out from the other ports of the second flow switch valve  21   a  and the third flow switch valve  21   b . The bypass expansion valve  18  may be set to be in a closed state. The pressure of the sixth port I of the second flow switch valve  21   a  and that of the sixth port M of the third flow switch valve  21   b  are maintained at low pressure. Thus, even when the bypass expansion valve  18  is set to be in the closed state, the pressure of the fifth port K of the second flow switch valve  21   a  is maintained at a higher pressure than that of the sixth port I, and the pressure of the fifth port O of the third flow switch valve  21   b  is maintained at a higher pressure than that of the sixth port M. 
     The flow of refrigerant in the direction from the first high pressure pipe  67  toward the third port F of the first flow switch valve  12  is blocked by the check valve  22 . In a case where an on-off valve is used instead of the check valve  22 , the on-off valve is set to be in a closed state. As a result, the flow of refrigerant in the direction from the first high pressure pipe  67  toward the third port F of the first flow switch valve  12  is blocked by the on-off valve. 
     High-pressure gas refrigerant discharged from the compressor  11  flows into the indoor heat exchanger  13  via the discharge pipe  61 , the first flow switch valve  12 , and the refrigerant pipe  80 . The indoor heat exchanger  13  serves as a condenser at the time of the heating operation. That is, in the indoor heat exchanger  13 , heat is exchanged between refrigerant flowing inside the indoor heat exchanger  13  and indoor air sent by the indoor fan, and the heat of condensation of the refrigerant is transferred to the indoor air. As a result, the gas refrigerant that has flowed into the indoor heat exchanger  13  condenses to high-pressure liquid refrigerant. The indoor air sent by the indoor fan is heated by heat transferred from the refrigerant. 
     Liquid refrigerant that has flowed out from the indoor heat exchanger  13  flows into the expansion valve  14  via the refrigerant pipe  81 . Liquid refrigerant that has flowed into the expansion valve  14  is decompressed to low-pressure two-phase refrigerant. Two-phase refrigerant that has flowed out from the expansion valve  14  flows through the refrigerant pipe  82  and divides into the refrigerant pipe  82   a  and the refrigerant pipe  82   b . Two-phase refrigerant diverted into the refrigerant pipe  82   a  is further decompressed at the capillary tube  17   a  and then flows into the first outdoor heat exchanger  15   a . Two-phase refrigerant diverted into the refrigerant pipe  82   b  is further decompressed at the capillary tube  17   b  and then flows into the second outdoor heat exchanger  15   b.    
     Both of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  serve as an evaporator at the time of the heating operation. That is, in each of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b , heat is exchanged between refrigerant flowing inside the outdoor heat exchanger and outdoor air sent by the outdoor fan, and the heat of evaporation of the refrigerant is absorbed from the outdoor air. As a result, two-phase refrigerant that has flowed into each of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  evaporates into low-pressure gas refrigerant. 
     Gas refrigerant that has flowed out from the first outdoor heat exchanger  15   a  flows through the refrigerant pipe  83   a , the second flow switch valve  21   a , the low pressure pipe  70   a , the low pressure pipe  70 , and the suction pipe  62  and is taken into the compressor  11 . Gas refrigerant that has flowed out from the second outdoor heat exchanger  15   b  flows through the refrigerant pipe  83   b , the third flow switch valve  21   b , and the low pressure pipe  70   b , merges with gas refrigerant that has flowed out from the first outdoor heat exchanger  15   a , and is taken into the compressor  11 . That is, gas refrigerant that has flowed out from the first outdoor heat exchanger  15   a  and gas refrigerant that has flowed out from the second outdoor heat exchanger  15   b  are taken into the compressor  11  without flowing through the first flow switch valve  12 . Gas refrigerant taken into the compressor  11  is compressed into high-pressure gas refrigerant. At the time of the heating operation, the cycle described above is repeated continuously. 
     At the time of the heating operation, all the first port G of the first flow switch valve  12 , the fifth port K of the second flow switch valve  21   a , and the fifth port O of the third flow switch valve  21   b  are maintained at high pressure or intermediate pressure. At the time of the heating operation, all the second port E of the first flow switch valve  12 , the sixth port I of the second flow switch valve  21   a , and the sixth port M of the third flow switch valve  21   b  are maintained at low pressure. 
     Next, the operation of the refrigeration cycle apparatus  1  at the time of the defrosting operation will be described.  FIG. 5  is a diagram illustrating the operation of the refrigeration cycle apparatus  1  according to Embodiment 1 at the time of the defrosting operation. As illustrated in  FIG. 5 , at the time of the defrosting operation, the first flow switch valve  12  is set to be in the second state, in which the first port G communicates with the third port F, and the second port E communicates with the fourth port H. The second flow switch valve  21   a  is set to be in the second state, in which the fifth port K communicates with the seventh port L, and the sixth port I communicates with the eighth port J. The third flow switch valve  21   b  is set to be in the second state, in which the fifth port O communicates with the seventh port P, and the sixth port M communicates with the eighth port N. 
     The bypass expansion valve  18  is set to be in, for example, the closed state. The flow of refrigerant in the direction from the third port F of the first flow switch valve  12  toward the first high pressure pipe  67  is allowed by the check valve  22 . In a case where an on-off valve is used instead of the check valve  22 , the on-off valve is set to be in the open state. As a result, the flow of refrigerant in the direction from the third port F of the first flow switch valve  12  toward the first high pressure pipe  67  is allowed by the on-off valve. 
     High-pressure gas refrigerant discharged from the compressor  11  flows through the discharge pipe  61 , the first flow switch valve  12 , the second high pressure pipe  64 , and the first high pressure pipe  67  and divides into the first high pressure pipe  67   a  and the first high pressure pipe  67   b . Gas refrigerant diverted into the first high pressure pipe  67   a  flows into the first outdoor heat exchanger  15   a  via the second flow switch valve  21   a  and the refrigerant pipe  83   a . Gas refrigerant diverted into the first high pressure pipe  67   b  flows into the second outdoor heat exchanger  15   b  via the third flow switch valve  21   b  and the refrigerant pipe  83   b . Both of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  serve as a condenser at the time of the defrosting operation. That is, at the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b , heat transferred from refrigerant flowing inside each of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  melts frost formed on a corresponding one of the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b . As a result, the first outdoor heat exchanger  15   a  and the second outdoor heat exchanger  15   b  are defrosted. Gas refrigerant that has flowed into the first outdoor heat exchanger  15   a  and gas refrigerant that has flowed into the second outdoor heat exchanger  15   b  condense to liquid refrigerant. 
     Liquid refrigerant that has flowed out from the first outdoor heat exchanger  15   a  is decompressed at the capillary tube  17   a  and flows into the expansion valve  14  via the refrigerant pipe  82   a  and the refrigerant pipe  82 . Liquid refrigerant that has flowed out from the second outdoor heat exchanger  15   b  is decompressed at the capillary tube  17   b , flows through the refrigerant pipe  82   b , merges with liquid refrigerant that has flowed out from the first outdoor heat exchanger  15   a , and flows into the expansion valve  14 . Liquid refrigerant that has flowed into the expansion valve  14  is decompressed to low-pressure two-phase refrigerant. Two-phase refrigerant that has flowed out from the expansion valve  14  flows into the indoor heat exchanger  13  via the refrigerant pipe  81 . The indoor heat exchanger  13  serves as an evaporator at the time of the defrosting operation. That is, in the indoor heat exchanger  13 , the heat of evaporation of refrigerant flowing inside the indoor heat exchanger  13  is absorbed from indoor air. As a result, two-phase refrigerant that has flowed into the indoor heat exchanger  13  evaporates into low-pressure gas refrigerant. Gas refrigerant that has flowed out from the indoor heat exchanger  13  flows through the refrigerant pipe  80 , the first flow switch valve  12 , and the suction pipe  62 , and is taken into the compressor  11 . Gas refrigerant taken into the compressor  11  is compressed into high-pressure gas refrigerant. At the time of the defrosting operation, the cycle described above is repeated continuously. 
     At the time of the defrosting operation, all the first port G of the first flow switch valve  12 , the fifth port K of the second flow switch valve  21   a , and the fifth port O of the third flow switch valve  21   b  are maintained at high pressure. At the time of the defrosting operation, all the second port E of the first flow switch valve  12 , the sixth port I of the second flow switch valve  21   a , and the sixth port M of the third flow switch valve  21   b  are maintained at low pressure. 
     Next, the operation of the refrigeration cycle apparatus  1  at the time of the simultaneous heating-defrosting operation will be described. The simultaneous heating-defrosting operation includes a first operation and a second operation. At the time of the first operation, the first outdoor heat exchanger  15   a  and the indoor heat exchanger  13  serve as a condenser, and the second outdoor heat exchanger  15   b  serves as an evaporator. As a result, the first outdoor heat exchanger  15   a  is defrosted while continuing heating. At the time of the second operation, the second outdoor heat exchanger  15   b  and the indoor heat exchanger  13  serve as a condenser, and the first outdoor heat exchanger  15   a  serves as an evaporator. As a result, the second outdoor heat exchanger  15   b  is defrosted while continuing heating.  FIG. 6  is a diagram illustrating the operation of the refrigeration cycle apparatus  1  according to Embodiment 1 at the time of the first operation during the simultaneous heating-defrosting operation. 
     As illustrated in  FIG. 6 , at the time of the first operation, the first flow switch valve  12  is set to be in the first state, in which the first port G communicates with the fourth port H, and the second port E communicates with the third port F. The second flow switch valve  21   a  is set to be in the second state, in which the fifth port K communicates with the seventh port L, and the sixth port I communicates with the eighth port J. The third flow switch valve  21   b  is set to be in the first state, in which the fifth port O communicates with the eighth port N, and the sixth port M communicates with the seventh port P. 
     The bypass expansion valve  18  is set to be in the open state at a predetermined opening degree. The flow of refrigerant in the direction from the first high pressure pipe  67  toward the third port F of the first flow switch valve  12  is blocked by the check valve  22 . In a case where an on-off valve is used instead of the check valve  22  the on-off valve is set to be in the closed state. As a result, the flow of refrigerant in the direction from the first high pressure pipe  67  toward the third port F of the first flow switch valve  12  is blocked by the on-off valve. 
     A portion of high-pressure gas refrigerant discharged from the compressor  11  is diverted into the first high pressure pipe  67  from the discharge pipe  61 . Gas refrigerant that has diverted into the first high pressure pipe  67  is decompressed at the bypass expansion valve  18  to be at intermediate pressure and flows into the first outdoor heat exchanger  15   a  via the first high pressure pipe  67   a , the second flow switch valve  21   a , and the refrigerant pipe  83   a . At the first outdoor heat exchanger  15   a , heat transferred from refrigerant flowing inside the first outdoor heat exchanger  15   a  melts frost formed on the first outdoor heat exchanger  15   a . As a result, the first outdoor heat exchanger  15   a  is defrosted. Gas refrigerant that has flowed into the first outdoor heat exchanger  15   a  condenses to intermediate-pressure liquid refrigerant or two-phase refrigerant, flows out from the first outdoor heat exchanger  15   a , and is decompressed at the capillary tube  17   a.    
     Out of high-pressure gas refrigerant discharged from the compressor  11 , gas refrigerant other than the high-pressure gas refrigerant that has been diverted into the first high pressure pipe  67  flows into the indoor heat exchanger  13  via the first flow switch valve  12  and the refrigerant pipe  80 . In the indoor heat exchanger  13 , heat is exchanged between refrigerant flowing inside the indoor heat exchanger  13  and indoor air sent by the indoor fan, and the heat of condensation of the refrigerant is transferred to the indoor air. As a result, the gas refrigerant that has flowed into the indoor heat exchanger  13  condenses to high-pressure liquid refrigerant. The indoor air sent by the indoor fan is heated by heat transferred from the refrigerant. 
     Liquid refrigerant that has flowed out from the indoor heat exchanger  13  flows into the expansion valve  14  via the refrigerant pipe  81 . Liquid refrigerant that has flowed into the expansion valve  14  is decompressed to low-pressure two-phase refrigerant. Two-phase refrigerant that has flowed out from the expansion valve  14  flows through the refrigerant pipe  82 , merges with liquid refrigerant or two-phase refrigerant decompressed at the capillary tube  17   a , is further decompressed at the capillary tube  17   b , and flows into the second outdoor heat exchanger  15   b . In the second outdoor heat exchanger  15   b , heat is exchanged between refrigerant flowing inside the second outdoor heat exchanger  15   b  and outdoor air sent by the outdoor fan, and the heat of evaporation of the refrigerant is absorbed from the outdoor air, As a result, two-phase refrigerant that has flowed into the second outdoor heat exchanger  15   b  evaporates into low-pressure gas refrigerant. Gas refrigerant that has flowed out from the second outdoor heat exchanger  15   b  flows through the refrigerant pipe  83   b , the third flow switch valve  21   b , the low pressure pipe  70   b , the low pressure pipe  70 , and the suction pipe  62 , and is taken into the compressor  11 . That is, gas refrigerant that has flowed out from the second outdoor heat exchanger  15   b  is taken into the compressor  11  without flowing through the first flow switch valve  12 . Gas refrigerant taken into the compressor  11  is compressed into high-pressure gas refrigerant. At the time of the first operation during the simultaneous heating-defrosting operation, the first outdoor heat exchanger  15   a  is defrosted and heating is continued by continuously repeating the cycle described above. 
     At the time of the first operation during the simultaneous heating-defrosting operation, all the first port G of the first flow switch valve  12 , the fifth port K of the second flow switch valve  21   a , and the fifth port O of the third flow switch valve  21   b  are maintained at high pressure or intermediate pressure. At the time of the first operation, all the second port E of the first flow switch valve  12 , the sixth port I of the second flow switch valve  21   a , and the sixth port M of the third flow switch valve  21   b  are maintained at low pressure. 
       FIG. 7  is a diagram illustrating the operation of the refrigeration cycle apparatus  1  according to Embodiment 1 at the time of the second operation during the simultaneous heating-defrosting operation. As illustrated in  FIG. 7 , at the time of the second operation during the simultaneous heating-defrosting operation, unlike at the time of the first operation, the second flow switch valve  21   a  is set to be in the first state, and the third flow switch valve  21   b  is set to be in the second state. The first flow switch valve  12  and the bypass expansion valve  18  are set to be in the same states as those used in the first operation. As a result, at the time of the second operation, the second outdoor heat exchanger  15   b  is defrosted while continuing heating. At the time of the second operation, all the first port G of the first flow switch valve  12 , the fifth port K of the second flow switch valve  21   a , and the fifth port O of the third flow switch valve  21   b  are maintained at high pressure or intermediate pressure. At the time of the second operation, all the second port E of the first flow switch valve  12 , the sixth port of the second flow switch valve  21   a , and the sixth port M of the third flow switch valve  21   b  are maintained at low pressure. 
       FIG. 8  is a flow chart illustrating the procedure of the operation of the refrigeration cycle apparatus  1  according to Embodiment 1. The operation control unit  501  of the controller  50  starts the heating operation on the basis of, for example, a heating operation start signal from the operation unit (S 1 ). When the heating operation is started, the operation control unit  501  determines whether a defrosting determination condition is met ( 32 ). The defrosting determination condition is that, for example, a period of time elapsed from the start of the heating operation exceeds a threshold time period (for example, 20 minutes). In a case where it is determined that the defrosting determination condition is met (S 2 : YES), the process proceeds to processing in step S 3 . In a case where it is determined that the defrosting determination condition is not met (S 2 : NO), processing in step S 2  is periodically repeated. 
     In step S 3 , the operation control unit  501  acquires, as an operation frequency f, the value of the operation frequency of the compressor  11  at this point in time or the average value of the operation frequencies of the compressor  11  from the start of the heating operation to this point in time. Thereafter, the controller  50  determines whether a frequency difference value (fmax−f) is greater than or equal to a threshold fth, the frequency difference value being obtained by subtracting the operation frequency f from a maximum operation frequency fmax of the compressor  11  (S 3 ). In this case, the maximum operation frequency fmax is the upper limit of the operation frequency range of the compressor  11 . The value of the maximum operation frequency fmax and that of the threshold fth are prestored in the ROM of the controller  50 . The compressor  11  is controlled such that the heavier the heating load, the higher the operation frequency, and thus the operation frequency of the compressor  11  is generally in proportion to the heating load. 
     In a case where the value obtained by subtracting the operation frequency f from the maximum operation frequency fmax is greater than or equal to the threshold fth (fmax−f≥fth) (S 3 : YES), the process proceeds to processing in step S 4 . In contrast, in a case where the value obtained by subtracting the operation frequency f from the maximum operation frequency fmax is smaller than the threshold fth (fmax−f&lt;fth) (S 3 : NO), the process proceeds to processing in step S 7 . 
     In step S 4 , before switching from the heating operation to the simultaneous heating-defrosting operation, the condition determination unit  502  and the differential pressure ensuring unit  503  perform a differential pressure ensuring process (S 4 ). The content of the differential pressure ensuring process will be described below in detail. After the differential pressure ensuring process, the operation control unit  501  performs the simultaneous heating-defrosting operation for a predetermined time period (S 5 ). In the simultaneous heating-defrosting operation, the first operation, in which the first outdoor heat exchanger  15   a  is defrosted, is performed first, and thereafter the second operation, in which the second outdoor heat exchanger  15   b  is defrosted, is performed. In this case, the operation control unit  501  has a counter that stores the number of times N the simultaneous heating-defrosting operation is performed. The initial value of the counter is zero. In a case where the simultaneous heating-defrosting operation is performed, the operation control unit  501  adds one to the number of times N stored in the counter. 
     Next, the operation control unit  501  determines whether the number of times N the simultaneous heating-defrosting operation is performed is greater than or equal to a threshold Nth corresponding to the number of times (S 6 ). In a case where the number of times N is greater than or equal to the threshold Nth corresponding to the number of times (N≥Nth) (S 6 : YES), the process proceeds to processing in step S 8 . Before proceeding to processing in step S 8 , the heating operation may be performed. In contrast, in a case where the number of times N is smaller than the threshold Nth corresponding to the number of times (N&lt;Nth) (S 6 : NO), the process returns to step S 1 , and the heating operation is restarted. 
     In step S 7 , the operation control unit  501  further continues the heating operation for a predetermined time period. Thereafter, the process proceeds to processing in step S 8 . In step S 8 , the controller  50  ends the heating operation or the simultaneous heating-defrosting operation and performs the defrosting operation for a predetermined time period. Generally, a period during which the defrosting operation is performed is shorter than a period during which the simultaneous heating-defrosting operation is performed. Moreover, in a case where the operation control unit  501  has performed the defrosting operation, the operation control unit  501  initializes the counter and sets the number of times N the simultaneous heating-defrosting operation is performed to zero. After the operation control unit  501  has ended the defrosting operation, the process returns to step S 1 , and the operation control unit  501  restarts the heating operation. 
     Next, the differential pressure ensuring process in Embodiment 1 will be described. In the following, the second flow switch valve  21   a  will be described as an example of a valve operated by differential pressure; however, the first flow switch valve  12  and the third flow switch valve  21   b  also have the same configuration as that of the second flow switch valve  21   a . For the second flow switch valve  21   a , a minimum operating differential pressure necessary for operation is defined. When the difference between the pressure of refrigerant at the fifth port K, which is the high pressure port of the second flow switch valve  21   a , and the pressure of refrigerant at the sixth port which is the low pressure port of the second flow switch valve  21   a , becomes less than or equal to the minimum operating differential pressure, the second flow switch valve  21   a  does not operate. The meaning of “the second flow switch valve  21   a  does not operate” includes port switching not being performed or occurrence of intermediate stopping in which port switching stops partway. The minimum operating differential pressure varies depending on the specifications of the second flow switch valve  21   a  and is 0.1 Mpa to 0.2 Mpa, for example. The minimum operating differential pressure of the second flow switch valve  21   a  is ensured in normal operation environments; however, there is a case where the minimum operating differential pressure cannot be ensured under specific circumstances. 
       FIG. 9  is a table illustrating an example of a relationship between pressure at the second flow switch valve  21   a  and outdoor air temperature in Embodiment 1. The example in  FIG. 9  illustrates pressures at the second flow switch valve  21   a  at the time of the heating operation in a state where the bypass expansion valve  18  is closed. Assume that the minimum operating differential pressure of the second flow switch valve  21   a  is 0.20 Mpa. Outdoor air saturation pressure is saturation pressure corresponding to outdoor air temperature and corresponds to the pressure of refrigerant at the high pressure port (the fifth port K) of the second flow switch valve  21   a . Suction pressure is pressure corresponding to suction saturation temperature obtained by subtracting the temperature difference between outdoor air temperature and heat exchanger temperature from the outdoor air temperature and corresponds to the pressure of refrigerant at the low pressure port (the sixth port I) of the second flow switch valve  21   a . Limit bypass pressure is obtained by adding minimum operating differential pressure to suction pressure and is the pressure of the high pressure port necessary for the second flow switch valve  21   a  to operate. Error corresponds to values obtained by subtracting outdoor air saturation pressure from limit bypass pressure. In other words, an error is the difference between the pressure of the high pressure port necessary to ensure the minimum operating differential pressure of the second flow switch valve  21   a  and an actual pressure of the high pressure port. In a case where the error has a negative value, the minimum operating differential pressure can be ensured. In a case where the error has a positive value, the minimum operating differential pressure cannot be ensured. 
     As illustrated in  FIG. 9 , it is clear that the lower the outdoor air temperature, the greater the error. Specifically, in a case where the outdoor air temperature is 5 degrees C., the error is 0.062 Mpa. In contrast, in a case where the outdoor air temperature is −15 degrees C., the error is 0.118 Mpa. That is, in a case where the outdoor air temperature is −15 degrees C., compared with the case where the outdoor air temperature is 5 degrees C., if the pressure of the high pressure port is not increased, the minimum operating differential pressure of the second flow switch valve  21   a  cannot be ensured. 
       FIG. 10  is a table illustrating an example of a relationship between pressure at the second flow switch valve  21   a  and operation frequency of the compressor  11  in Embodiment 1. The example in  FIG. 10  illustrates pressures at the second flow switch valve  21   a  at the time of the heating operation in a state where the bypass expansion valve  18  is closed. Assume that the minimum operating differential pressure of the second flow switch valve  21   a  is 0.20 Mpa. The compressor  11  is controlled such that the heavier the heating load, the higher the operation frequency, and thus the operation frequency of the compressor  11  is generally in proportion to a heating load. A heating load is generally in proportion to the temperature difference between outdoor air temperature and a heat exchanger temperature. A heat exchange temperature is, for example, the average value of the temperature of the first outdoor heat exchanger  15   a  detected by the heat exchanger temperature sensor  92   a  and the temperature of the second outdoor heat exchanger  15   b  detected by the heat exchanger temperature sensor  92   b . Note that the heat exchanger temperature may be either the temperature of the first outdoor heat exchanger  15   a  or the temperature of the second outdoor heat exchanger  15   b . In  FIG. 10 , assume that the temperature difference between outdoor air temperature and a heat exchanger temperature represents the operation frequency of the compressor  11 . 
     Outdoor air saturation pressure is saturation pressure corresponding to outdoor air temperature and corresponds to the pressure of refrigerant at the high pressure port (the fifth port K) of the second flow switch valve  21   a . Suction pressure is pressure corresponding to suction saturation temperature obtained by subtracting the temperature difference between outdoor air temperature and heat exchanger temperature from the outdoor air temperature and corresponds to the pressure of refrigerant at the low pressure port (the sixth port I) of the second flow switch valve  21   a . Limit bypass pressure is obtained by adding minimum operating differential pressure to suction pressure and is the pressure of the high pressure port necessary for the second flow switch valve  21   a  to operate. Error corresponds to the difference between limit bypass pressure and outdoor air saturation pressure. In other words, an error represents the difference between the pressure of the high pressure port necessary to ensure the minimum operating differential pressure of the second flow switch valve  21   a  and an actual pressure of the high pressure port. In a case where the error has a negative value, the minimum operating differential pressure can be ensured. In a case where the error has a positive value, the minimum operating differential pressure cannot be ensured. 
     As illustrated in  FIG. 10 , it is clear that the smaller the temperature difference between outdoor air temperature and heat exchanger temperature, that is, the lower the operation frequency of the compressor  11 , the greater the error. Specifically, in a case where the outdoor air temperature is 2 degrees C. and the temperature difference is 15 degrees C., the error is −0.142 Mpa, and the minimum operating differential pressure can be ensured. In contrast, in a case where the temperature difference is 5 degrees C., the error is 0.072 Mpa, and the minimum operating differential pressure cannot be ensured. That is, in a case where the outdoor air temperature is 2 degrees C. and the temperature difference is 5 degrees C., compared with the case where the temperature difference is 15 degrees C., if the pressure of the high pressure port is not increased, the minimum operating differential pressure of the second flow switch valve  21   a  cannot be ensured. 
     On the basis of the above, in a case where outdoor air temperature is low or a case where the temperature difference between outdoor air temperature and heat exchanger temperature is small, that is, in a case where the operation frequency of the compressor  11  is low, there may be a case where the minimum operating differential pressure of the second flow switch valve  21   a  cannot be ensured. In Embodiment 1, a case where outdoor air temperature is low or a case where the temperature difference between outdoor air temperature and heat exchanger temperature is small, that is, a case where the operation frequency of the compressor  11  is low is treated as a first condition. In a case where the first condition is met, the opening degree of the bypass expansion valve  18  is controlled to increase the pressure of the high pressure port. 
       FIG. 11  is a flow chart illustrating the procedure of the differential pressure ensuring process of Embodiment 1. This process is performed by the condition determination unit  502  and the differential pressure ensuring unit  503  of the controller  50 . In this process, first, outdoor air temperature is detected by the outdoor air temperature sensor  91  (S 101 ). The temperature difference between the outdoor air temperature and a heat exchanger temperature is calculated by the condition determination unit  502  (S 102 ). Specifically, the temperature of the first outdoor heat exchanger  15   a  is detected by the heat exchanger temperature sensor  92   a , and the temperature of the second outdoor heat exchanger  15   b  is detected by the heat exchanger temperature sensor  92   b . The average value of these temperatures is calculated as the heat exchanger temperature. Thereafter, the temperature difference between the outdoor air temperature and the heat exchanger temperature is calculated. 
     Subsequently, the condition determination unit  502  determines whether the first condition is met (S 103 ). The first condition corresponds to, for example, a case where the outdoor air temperature is lower than or equal to a first threshold or a case where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to a second threshold. The first threshold and the second threshold are set in advance and are stored in the storage unit  504  of the controller  50 . For example, the first threshold is 0 degrees C. As illustrated in  FIG. 9 , in a case where the outdoor air temperature is lower than or equal to 5 degrees C., the error has a positive value. Thus, a temperature that is freely set and less than or equal to 5 degrees C. may be treated as the first threshold. The second threshold is, for example, 8.05 degrees C. As illustrated in  FIG. 10 , in a case where the outdoor air temperature is 2 degrees C., when the temperature difference between the outdoor air temperature and the heat exchanger temperature is 8.05 degrees C., the error is 0. As a result, even in a case where the outdoor air temperature is higher than 0 degrees C., when the temperature difference is less than 8.05 degrees C., it is estimated that the minimum operating differential pressure cannot be ensured. Thus, the second threshold is set to 8.05 degrees C. 
     Note that the first condition is not limited to the above. For example, the first condition may correspond only to a case where the outdoor air temperature is lower than or equal to the first threshold. Alternatively, the first condition may correspond only to a case where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the second threshold. Furthermore, the first condition may correspond to a case where the outdoor air temperature is lower than or equal to the first threshold and the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the second threshold. Moreover, when the first condition includes a case where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the second threshold, a plurality of second thresholds may be set in accordance with outdoor air temperature. For example, in the case of the example in  FIG. 10 , the second threshold may be 8 degrees C. when the outdoor air temperature is 0 to 5 degrees C., the second threshold may be 9.85 degrees C. when the outdoor air temperature is 0 degrees C. to −5 degrees C., and the second threshold may be 11.55 degrees C. when the outdoor air temperature is −5 degrees C. to −10 degrees C. Any of the second thresholds may be selected in accordance with the outdoor air temperature, The first threshold and the second threshold are set as appropriate in accordance with the minimum operating differential pressure of the second flow switch valve  21   a  and the specifications and operation environment of the refrigeration cycle apparatus  1  and are not limited to the examples above. 
     In a case where the first condition is not met (S 103 : NO), the opening degree of the bypass expansion valve  18  is set to a predetermined first degree (S 104 ). In contrast, in a case where the first condition is met (S 103 : YES), the opening degree of the bypass expansion valve  18  is set to a second degree, which is larger than the first degree, by the differential pressure ensuring unit  503  (S 105 ). For example, the second degree is 1.5 times larger than the first degree. As a result, the differential pressure ensuring process ends, the second flow switch valve  21   a  is switched, and the first operation of the simultaneous heating-defrosting operation is performed in step S 6  of  FIG. 8 . In this case, under circumstances where the first condition is met, that is, under circumstances where the minimum operating differential pressure of the second flow switch valve  21   a  is not ensured, the opening degree of the bypass expansion valve  18  is set to the second degree, which is larger than the first degree. As a result, the pressure of the high pressure port (the fifth port K) of the second flow switch valve  21   a  increases, and a pressure difference greater than or equal to the minimum operating differential pressure is ensured between the high pressure port of the second flow switch valve  21   a  and the low pressure port (the sixth port I) of the second flow switch valve  21   a.    
     As above, according to Embodiment 1, when the heating operation is switched to the simultaneous heating-defrosting operation, the minimum operating differential pressure of the second flow switch valve  21   a  can be ensured by performing the differential pressure ensuring process. Consequently, the second flow switch valve  21   a  can be normally operated in any environment. 
     Embodiment 2 
     Next, Embodiment 2 will be described. Embodiment 2 differs from Embodiment 1 in that the differential pressure ensuring process is performed when the first operation is switched to the second operation in the simultaneous heating-defrosting operation. The other configuration of and control performed on the refrigeration cycle apparatus  1  are the same as those of Embodiment 1. 
       FIG. 12  is a table illustrating an example of a relationship between pressure at the second flow switch valve  21   a  and outdoor air temperature in Embodiment 2. The example in  FIG. 12  illustrates pressures at the second flow switch valve  21   a  at the time of the first operation in the simultaneous heating-defrosting operation in a state where the bypass expansion valve  18  is closed. The minimum operating differential pressure of the second flow switch valve  21   a  is 0.20 Mpa. Suction pressure is pressure corresponding to suction saturation temperature obtained by subtracting the temperature difference between outdoor air temperature and heat exchanger temperature from the outdoor air temperature and corresponds to the pressure of refrigerant at the low pressure port (the sixth port I) of the second flow switch valve  21   a . Note that a heat exchanger temperature according to Embodiment 2 is the temperature of the second outdoor heat exchanger  15   b  detected by the heat exchanger temperature sensor  92   b . Limit bypass pressure is obtained by adding minimum operating differential pressure to suction pressure and is the pressure of the high pressure port necessary for the second flow switch valve  21   a  to operate. 
     Bypass pressure is the pressure of the high pressure port (the fifth port K) of the second flow switch valve  21   a  and is the pressure of the first outdoor heat exchanger  15   a  in Embodiment 2. In Embodiment 2, the first outdoor heat exchanger  15   a  is operating the defrosting operation, and thus the saturation temperature of refrigerant is 0 degrees C. to 5 degrees C., and the pressure of the refrigerant is 0.813 Mpa to 0.951 Mpa in the first outdoor heat exchanger  15   a . In the example in  FIG. 12 , the bypass pressure is set to 0.813 Mpa (saturation temperature=0 degrees C.). Error corresponds to values obtained by subtracting bypass pressure from limit bypass pressure. In other words, an error is the difference between the pressure of the high pressure port necessary to ensure the minimum operating differential pressure of the second flow switch valve  21   a  and an actual pressure of the high pressure port. In a case where the error has a negative value, the minimum operating differential pressure can be ensured. In a case where the error has a positive value, the minimum operating differential pressure cannot be ensured. 
     As illustrated in  FIG. 12 , in Embodiment 2, it is clear that the higher the outdoor air temperature, the greater the error. Specifically, when the outdoor air temperature becomes higher than 1.45 degrees C., the error has a positive value, and the minimum operating differential pressure cannot be ensured. Note that in a case where the bypass pressure is set to 0.951 Mpa (saturation temperature=5 degrees C.), even when the outdoor air temperature is 7 degrees C., the error has a negative value (0.938 Mpa−0.951 Mpa=−0.013 Mpa), and the minimum operating differential pressure can be ensured. Thus, by setting the bypass pressure to 0.951 Mpa or lower, the second flow switch valve  21   a  and the third flow switch valve  21   b  operate assuredly at the outdoor air temperature at which frost forms. Thus, in the following, description will be made using 0.813 Mpa as the bypass pressure. 
       FIG. 13  is a table illustrating an example of a relationship between pressure at the second flow switch valve  21   a  and operation frequency of the compressor  11  in Embodiment 2. The example in  FIG. 13  illustrates pressures at the second flow switch valve  21   a  at the time of the first operation in the simultaneous heating-defrosting operation in a state where the bypass expansion valve  18  is closed. The minimum operating differential pressure of the second flow switch valve  21   a  is 0.20 Mpa. Similarly to as in Embodiment 1, assume that the temperature difference between outdoor air temperature and heat exchanger temperature represents the operation frequency of the compressor  11 . Note that a heat exchanger temperature according to Embodiment 2 is the temperature of the second outdoor heat exchanger  15   b  detected by the heat exchanger temperature sensor  92   b . Suction pressure is pressure corresponding to suction saturation temperature obtained by subtracting the temperature difference between outdoor air temperature and heat exchanger temperature from the outdoor air temperature and corresponds to the pressure of refrigerant at the low pressure port (the sixth port I) of the second flow switch valve  21   a . Limit bypass pressure is obtained by adding minimum operating differential pressure to suction pressure and is the pressure of the high pressure port necessary for the second flow switch valve  21   a  to operate. Bypass pressure is the pressure of the high pressure port (the fifth port K) of the second flow switch valve  21   a  and is the pressure of the first outdoor heat exchanger  15   a  in Embodiment 2. In the example in  FIG. 12 , the bypass pressure is set to 0.813 Mpa (saturation temperature=0 degrees C.). Error corresponds to values obtained by subtracting bypass pressure from limit bypass pressure. In other words, an error is the difference between the pressure of the high pressure port necessary to ensure the minimum operating differential pressure of the second flow switch valve  21   a  and an actual pressure of the high pressure port. In a case where the error has a negative value, the minimum operating differential pressure can be ensured. In a case where the error has a positive value, the minimum operating differential pressure cannot be ensured. 
     As illustrated in  FIG. 13 , it is clear that the smaller the temperature difference between outdoor air temperature and heat exchanger temperature, that is, the lower the frequency of the compressor  11 , the greater the error. Specifically, in a case where the outdoor air temperature is 5 degrees C. and the temperature difference is 15 degrees C., the error is −0.03 Mpa, and the minimum operating differential pressure can be ensured. In contrast, in a case where the temperature difference is 10 degrees C., the error is 0.078 Mpa, and the minimum operating differential pressure cannot be ensured. That is, in a case where the outdoor air temperature is 5 degrees C. and the temperature difference is 10 degrees C., compared with the case where the temperature difference is 15 degrees C., if the pressure of the high pressure port is not increased, the minimum operating differential pressure of the second flow switch valve  21   a  cannot be ensured. 
     On the basis of the above, in Embodiment 2, in a case where outdoor air temperature is high or a case where the temperature difference between outdoor air temperature and heat exchanger temperature is small, that is, in a case where the operation frequency of the compressor  11  is low, there may be a case where the minimum operating differential pressure cannot be ensured. In Embodiment 2, a case where outdoor air temperature is high or a case where the temperature difference between outdoor air temperature and heat exchanger temperature is small, that is, a case where the operation frequency of the compressor  11  is low is treated as a first condition. In a case where the first condition is met, the opening degree of the bypass expansion valve  18  is controlled to increase the pressure of the high pressure port. 
       FIG. 14  is a flow chart illustrating the procedure of a simultaneous heating-defrosting operation of Embodiment 2. This process is performed by the operation control unit  501 , the condition determination unit  502 , and the differential pressure ensuring unit  503  of the controller  50 . In Embodiment 2, steps  3205  to  3209  of  FIG. 14  correspond to the differential pressure ensuring process. In the simultaneous heating-defrosting operation, first, the first operation is performed. Specifically, the second flow switch valve  21   a  is switched to be in the second state by the operation control unit  501  ( 3201 ). In this case, the third flow switch valve  21   b  is maintained at the first state as in the case of the heating operation. In accordance with a result from the differential pressure ensuring process, the bypass expansion valve  18  is set to be in the open state at the first degree or the second degree, and defrosting of the first outdoor heat exchanger  15   a  is started ( 3202 ). It is then determined whether a predetermined time has elapsed from the start of defrosting of the first outdoor heat exchanger  15   a  (S 203 ). The predetermined time is an estimated time to complete defrosting of the first outdoor heat exchanger  15   a , and a freely set time is set. In a case where the predetermined time has elapsed (S 203 : YES), the process proceeds to processing in step  3204 . In a case where the predetermined time has not elapsed (S 203 : NO), processing in step S 203  is periodically repeated. 
     In step  3204 , the condition determination unit  502  determines whether the temperature of the first outdoor heat exchanger  15   a  detected by the heat exchanger temperature sensor  92   a  is higher than 5 degrees C. (S 204 ). In a case where the temperature of the first outdoor heat exchanger  15   a  is higher than 5 degrees C. ( 3204 : YES), the process proceeds to processing in step  3208 . In a case where the temperature of the first outdoor heat exchanger  15   a  is higher than 5 degrees C., the bypass pressure is 0.951 Mpa, and the minimum operating differential pressure can be ensured even when the outdoor air temperature is 7 degrees C. Thus, in a case where the temperature of the first outdoor heat exchanger  15   a  is higher than 5 degrees C., the differential pressure ensuring process is not performed, and the process proceeds to processing in step S 208 . 
     In contrast, in a case where the temperature of the first outdoor heat exchanger  15   a  is less than or equal to 5 degrees C. ( 3204 : NO), the process proceeds to processing in step S 205 . In step S 205 , outdoor air temperature is detected by the outdoor air temperature sensor  91  (S 205 ). The temperature difference between the outdoor air temperature and a heat exchanger temperature is then calculated (S 206 ). Specifically, the temperature difference between the outdoor air temperature and the temperature of the second outdoor heat exchanger  15   b  detected by the heat exchanger temperature sensor  92   b  is calculated. 
     Subsequently, the condition determination unit  502  determines whether the first condition is met (S 207 ). The first condition corresponds to, for example, a case where the outdoor air temperature is higher than or equal to a third threshold and the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to a fourth threshold. The third threshold and the fourth threshold are set in advance and are stored in the memory of the controller  50 . For example, the third threshold is 1.45 degrees C. The fourth threshold is, for example, 15.6 degrees C. As illustrated in  FIG. 13 , in a case where the outdoor air temperature is 7 degrees C., when the temperature difference is greater than 15.6 degrees C., the error has a negative value. As a result, even in a case where the outdoor air temperature is higher than or equal to 1.45 degrees C., when the temperature difference is greater than 15.6 degrees C., it is clear that the minimum operating differential pressure can be ensured. Thus, by treating, as the first condition, a case where the outdoor air temperature is higher than or equal to the third threshold and the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the fourth threshold, control for the differential pressure ensuring process can be prevented from being performed unnecessarily in a case where the differential pressure ensuring process is unnecessary. 
     Note that the first condition is not limited to the above. For example, the first condition may correspond only to a case where the outdoor air temperature is higher than or equal to the third threshold. Alternatively, the first condition may correspond only to a case where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the fourth threshold. Furthermore, the first condition may correspond to a case where the outdoor air temperature is higher than or equal to the third threshold or where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the fourth threshold. Moreover, when the first condition includes a case where the temperature difference between the outdoor air temperature and the heat exchanger temperature is lower than or equal to the fourth threshold, a plurality of fourth thresholds may be set in accordance with outdoor air temperature. For example, in the case of the example in  FIG. 13 , the fourth threshold may be 15.6 degrees C. when the outdoor air temperature is 7 degrees C., and the fourth threshold may be 13.6 degrees C. when the outdoor air temperature is 5 degrees C. Any one of the fourth thresholds may be selected in accordance with the outdoor air temperature. The third threshold and the fourth threshold are set as appropriate in accordance with the minimum operating differential pressure of the second flow switch valve  21   a  and the specifications and operation environment of the refrigeration cycle apparatus  1  and are not limited to the examples above. 
     In a case where the first condition is not met (S 207 : NO), the opening degree of the bypass expansion valve  18  is set to a predetermined first degree (S 208 ). In contrast, in a case where the first condition is met (S 207 : YES), the opening degree of the bypass expansion valve  18  is set to a second degree, which is larger than the first degree, by the differential pressure ensuring unit  503  (S 209 ). For example, the second degree is 1.5 times larger than the first degree. As a result, the differential pressure ensuring process ends, and the second operation of the simultaneous heating-defrosting operation is performed. Specifically, the second flow switch valve  21   a  is switched to be in the first state, and the third flow switch valve  21   b  is switched to be in the second state (S 210 ). The bypass expansion valve  18  is set to be in the open state at the first degree or the second degree, and defrosting of the second outdoor heat exchanger  15   b  is started (S 211 ). In this case, under circumstances where the first condition is met, that is, under circumstances where the minimum operating differential pressure of the second flow switch valve  21   a  is not ensured, the opening degree of the bypass expansion valve  18  is set to the second degree, which is larger than the first degree. As a result, the pressure of the high pressure port (the fifth port K) of the second flow switch valve  21   a  increases, and a pressure difference greater than or equal to the minimum operating differential pressure is ensured between the high pressure port of the second flow switch valve  21   a  and the low pressure port (the sixth port I) of the second flow switch valve  21   a . Note that, similarly to as in the case of the second flow switch valve  21   a , the minimum operating differential pressure of the third flow switch valve  21   b  can also be ensured by performing the differential pressure ensuring process. 
     As described above, in Embodiment 2, when the first operation of the simultaneous heating-defrosting operation is switched to the second operation of the simultaneous heating-defrosting operation, the minimum operating differential pressure of the second flow switch valve  21   a  and that of the third flow switch valve  21   b  can be ensured by performing the differential pressure ensuring process. Consequently, the second flow switch valve  21   a  and the third flow switch valve  21   b  can be normally operated in any environment. 
     Embodiment 3 
     Next, Embodiment 3 will be described. Embodiment 3 differs from Embodiment 1 in that the bypass expansion valve  18  has a flow path for ensuring a minimum operating differential pressure. The other configuration of a refrigeration cycle apparatus  1  is the same as that of Embodiment 1. 
       FIG. 15  is a schematic configuration diagram of the bypass expansion valve  18  of the refrigeration cycle apparatus  1  according to Embodiment 3. The bypass expansion valve  18  is an electronic expansion valve whose opening degree is controlled by the controller  50 . As illustrated in  FIG. 15 , the bypass expansion valve  18  has a main body  180 , a base  181  and a needle  182  arranged in the main body  180 , and a drive device  183 , which drives the needle  182 . 
     The main body  180  is formed by, for example, cutting and processing a brass casting. A valve chamber  184 , into which refrigerant flows, is formed inside the main body  180 . A refrigerant inlet  185 , which is for causing refrigerant to flow into the valve chamber  184 , is formed in the side surface of the main body  180 . A portion of the first high pressure pipe  67  on the bifurcation  63  side ( FIG. 1 ) is connected to the refrigerant inlet  185 . 
     The base  181  is arranged such that the base  181  penetrates through the bottom of the main body  180 . The base  181  has a tubular shape, and a refrigerant outlet  186 , which penetrates through the base  181  in the axial direction of the base  181 , is formed in the center of the base  181 . A portion of the first high pressure pipe  67  on the bifurcation  65  side ( FIG. 1 ) is connected to the refrigerant outlet  186 , and refrigerant whose pressure is reduced flows out from the refrigerant outlet  186 . A slope portion  181   a , which widens upward, is formed at the upstream end of the refrigerant outlet  186  in the base  181 . 
     The needle  182  has a tip portion  182   a  having a conical shape and is arranged in the valve chamber  184 . The needle  182  is arranged such that the tip portion  182   a  faces the refrigerant outlet  186  formed in the base  181 , and is configured to move in the direction toward the base  181  and in the direction away from the base  181 . The tip portion  182   a  of the needle  182  has a shape matching the slope portion  181   a  of the base  181 , and the refrigerant outlet  186  is blocked by fitting the tip portion  182   a  of the needle  182  into the slope portion  181   a  of the base  181 . By moving the needle  182  to change the distance to the base  181 , the opening degree of the refrigerant outlet  186  can be changed, and the flow rate of flowing-out refrigerant can be changed. That is, the base  181  and the needle  182  form a restriction portion of the bypass expansion valve  18 . 
     The drive device  183  is provided on the top of the main body  180 . The drive device  183  includes, for example, a stepping motor or an electromagnetic coil and causes the needle  182  to reciprocate in the direction toward the base  181  and in the direction away from the base  181  in accordance with a control signal from the controller  50 . 
       FIG. 16  is a plan view of the base  181  of the bypass expansion valve  18  according to Embodiment 3.  FIG. 17  is a cross-sectional view of the restriction portion of the bypass expansion valve  18  according to Embodiment 3.  FIG. 17  is an enlarged view of a portion of the base  181  and that of the needle  182  included in the restriction portion, and is a cross-sectional view taken along A-A of  FIG. 16  from an arrow direction. As illustrated in  FIGS. 16 and 17 , recess  187  are formed at the slope portion  181   a  of the base  181  of Embodiment 3. The recess  187  are each a recess formed by cutting away a portion of the slope portion  181   a  from the top end to the bottom end of the slope portion  181   a . In Embodiment four recess  187  are arranged at equal intervals in the circumferential direction. 
       FIG. 18  is a cross-sectional view of the restriction portion in a case where the bypass expansion valve  18  according to Embodiment 3 is in the closed state. The case where the bypass expansion valve  18  is in the closed state corresponds to a state where the tip portion  182   a  of the needle  182  abuts against the slope portion  181   a  of the base  181 . In the existing technology, in the case where the bypass expansion valve  18  is in the closed state, a gap is not present between the base and the needle, and refrigerant does not flow to the refrigerant outlet  186 . In contrast, in Embodiment 3, the slope portion  181   a  of the base  181  has the recess  187 , and thus even when the bypass expansion valve  18  is in the closed state, flow paths as indicated by broken lines in  FIG. 18  are formed between the base  181  and the needle  182 . As a result, even in a case where the bypass expansion valve  18  is in the closed state, refrigerant inside the valve chamber  184  can be caused to flow out from the refrigerant outlet  186 . Consequently, a predetermined amount of high-pressure refrigerant can always be caused to flow to the fifth port K of the second flow switch valve  21   a  and the fifth port O of the third flow switch valve  21   b  through the first high pressure pipe  67  connected to the refrigerant outlet  186 . 
     As a result, the fifth port K of the second flow switch valve  21   a  and the fifth port O of the third flow switch valve  21   b  can be maintained at high pressure, so that the minimum operating differential pressure can be ensured between the fifth port K and the sixth port I, which is a low pressure port, and the minimum operating differential pressure can be ensured between the fifth port O and the sixth port M, which is a low pressure port. Thus, the second flow switch valve  21   a  and the third flow switch valve  21   b  can be normally operated in any environment. 
     For example, even in a case where the bypass expansion valve  18  is set to be in the open state but is then caused to be in the closed state due to a failure of the bypass expansion valve  18  while performing the simultaneous heating-defrosting operation, the predetermined amount of high-pressure refrigerant flows to the second flow switch valve  21   a  and the third flow switch valve  21   b  through the recess  187 . As a result, the minimum operating differential pressure of the second flow switch valve  21   a  and that of the third flow switch valve  21   b  can be ensured, which enables switching of the second flow switch valve  21   a  and that of the third flow switch valve  21   b  as needed. 
     Even in a case where the bypass expansion valve  18  is fixed in the closed state due to a failure thereof when the heating operation is switched to the simultaneous heating-defrosting operation, the predetermined amount of high-pressure refrigerant flows to the second flow switch valve  21   a  and the third flow switch valve  21   b  through the recess  187 . As a result, the minimum operating differential pressure of the second flow switch valve  21   a  and that of the third flow switch valve  21   b  can be ensured, which enables switching of the second flow switch valve  21   a  and that of the third flow switch valve  21   b . Thereafter, since the bypass expansion valve  18  remains in the closed state even after switching of the second flow switch valve  21   a  and that of the third flow switch valve  21   b , flow rate control cannot be performed in the simultaneous heating-defrosting operation, so that a malfunction is detected. In this case, switching of the second flow switch valve  21   a  and that of the third flow switch valve  21   b  have been normally performed, and thus it is easy to identify that a malfunction has occurred at the bypass expansion valve  18 . 
     Note that, in Embodiment 3, the differential pressure ensuring process in Embodiment 1 or 2 may be performed or does not have to be performed. Moreover, flow paths at the bypass expansion valve  18  for ensuring a minimum operating differential pressure are not limited to paths formed by the recess  187 . A modification of Embodiment 3 will be described below. 
       FIG. 19  is a cross-sectional view of a restriction portion of a bypass expansion valve  18  according to the modification of Embodiment 3. As illustrated in  FIG. 19 , a recess  187 A may be formed not on the base  181  but on the tip portion  182   a  of the needle  182 . The recess  187 A is a recess formed by cutting away a portion of the tip portion  182   a  from the top end to the bottom end of the tip portion  182   a . Even in this modification, even when the bypass expansion valve  18  is in the closed state, a flow path along which refrigerant flows is formed between the base  181  and the needle  182 . As a result, even in a case where the bypass expansion valve  18  is in the closed state, a predetermined amount of high-pressure refrigerant can be caused to flow to the second flow switch valve  21   a  and the third flow switch valve  21   b , and the minimum operating differential pressures can be ensured. 
       FIG. 20  is a cross-sectional view of a restriction portion of a bypass expansion valve  18  according to another modification of Embodiment 3. As illustrated in  FIG. 20 ; protrusions  188  may be provided instead of the recess  187  on the slope portion  181   a  of the base  181 . The tip portion  182   a  of the needle  182  may be provided with protrusions  188 A. The protrusions  188  and the protrusions  188 A are provided to be spaced apart from each other in the circumferential direction such that refrigerant flows. Note that, in  FIG. 20 , the base  181  is provided with the protrusions  188 , and the needle  182  is provided with the protrusions  188 A; however, it is sufficient that at least one of the base  181  and the needle  182  have the corresponding protrusions. 
     In a case where the protrusions  188  or the protrusions  188 A are provided, the state in which the base  181  or the needle  182  abuts against the protrusions  188 A or the protrusions  188  corresponds to the state in which the bypass expansion valve  18  is closed. In this case, gaps are also formed between the base  181  and the needle  182 ; and the gaps become flow paths and refrigerant flows along the flow paths. As a result, even in a case where the bypass expansion valve  18  is in the closed state, a predetermined amount of high-pressure refrigerant can be caused to flow to the second flow switch valve  21   a  and the third flow switch valve  21   b , and the minimum operating differential pressures can be ensured. Note that the protrusions  188  or the protrusions  188 A may be integrally formed with the base  181  or the needle  182 , or another member such as a spacer may be disposed on the slope portion  181   a  of the base  181  or on the tip portion  182   a  of the needle  182  and treated as the protrusions  188  or the protrusions  188 A. 
     Regarding the bypass expansion valve  18  according to Embodiment 3, the sizes of the recess  187  and  187 A and those of the protrusions  188  and  188 A may each be any size and the number of recess  187  and  187 A and that of protrusions  188  and  188 A may each be any number as long as the minimum operating differential pressures (for example, 0.1 Mpa to 0.2 Mpa) can be ensured, and these sizes and numbers are not limited to the examples illustrated in  FIGS. 16 to 20 . For example, it is sufficient that the number of recess  187  and  187 A and the number of protrusions  188  and  188 A be greater than or equal to one. Note that in a case where the number of recess  187  and  187 A and the number of protrusions  188  and  188 A are too large or a case where the sizes of the recess  187  and  187 A and the sizes of the protrusions  188  and  188 A are too large, the bypass expansion valve  18  cannot be controlled at a Cv value, at which control of the bypass expansion valve  18  is desired. 
       FIG. 21  is a graph illustrating a relationship between opening degree and Cv value for the bypass expansion valve  18 . As illustrated in  FIG. 21 , in a case where the opening degree of the bypass expansion valve  18  is small, the Cv value remains constant. That is, in the case where the opening degree of the bypass expansion valve  18  is small, it is not possible to control the Cv value. Here, in a case where the bypass expansion valve  18  is provided with the recess  187  and  187 A and the protrusions  188  and  188 A and flow paths are formed, a range in which the Cv value cannot be controlled becomes wider as illustrated by a broken line in  FIG. 21 . Thus, the sizes of the recess  187  and  187 A, the number of recess  187  and  187 A, the sizes of the protrusions  188  and  188 A, and the number of protrusions  188  and  188 A are set to be in a range in which the minimum operating differential pressures can be ensured and the Cv value can be controlled. 
     The configuration of the bypass expansion valve  18  is not limited to that illustrated in  FIG. 15 , and the shapes of the main body  180 , the base  181 , and the needle  182  and the positions of the refrigerant inlet  185  and the refrigerant outlet  186 , for example, may be changed. 
     Embodiments have been described above. The present disclosure is not limited to Embodiments described above and may be modified in various manners without departing from the gist of the present disclosure. For example, the refrigeration cycle apparatus  1  may perform either the differential pressure ensuring process of Embodiment 1 described above or the differential pressure ensuring process of Embodiment 2 described above only or may perform both the differential pressure ensuring processes. Moreover, the second flow switch valve  21   a  and the third flow switch valve  21   b  are not limited to four-way valves and may be other valves or combinations of valves such as three-way valves operated by differential pressure. 
     In the differential pressure ensuring processes according to Embodiments described above, the temperature difference between outdoor air temperature and heat exchanger temperature is used as an indicator of the operation frequency of the compressor  11 ; however, this does not have to be used. For example, the operation frequency of the compressor  11  is measured, and a case where the operation frequency of the compressor  11  instead of the temperature difference is less than or equal to a fifth threshold may be treated as the first condition. The fifth threshold is set in advance on the basis of, for example, the minimum operating differential pressures, the specifications of the compressor  11 , and the use environment of the refrigeration cycle apparatus  1  and is stored in the memory of the controller  50 . 
     In Embodiments described above, the configurations are used with which the pressures of the high pressure ports are increased by setting the opening degree of the bypass expansion valve  18  to the second degree, which is larger than the predetermined first degree, in a case where the first condition is met; however, what is performed with these configurations is not limited to this. Instead of or in addition to changing the opening degree of the bypass expansion valve  18 , the operation frequency of the compressor  11  may be changed. Specifically, in step S 103  of  FIG. 11  and step S 207  of  FIG. 14 , in a case where the first condition is not met, the operation frequency of the compressor  11  is set to a predetermined first frequency. In contrast, in a case where the first condition is met, the operation frequency of the compressor  11  is set to a second frequency, which is higher than the first frequency. Even in this case, the pressure of the high pressure port (the fifth port K) of the second flow switch valve  21   a  increases, and a pressure difference greater than or equal to the minimum operating differential pressure can be ensured between the high pressure port of the second flow switch valve  21   a  and the low pressure port (the sixth port I) of the second flow switch valve  21   a.    
     Furthermore, in Embodiments described above or the modifications, in a case where the first condition is met, the opening degree of the bypass expansion valve  18  is uniformly set to the second degree; however, the opening degree is not limited to this. For example, in a case where the first condition corresponds to a case where the outdoor air temperature is lower than or equal to the first threshold, the opening degree of the bypass expansion valve  18  may be set in accordance with the outdoor air temperature. Specifically, in a case where the first threshold is set to 0 degrees C., the opening degree of the bypass expansion valve  18  may be set to the second degree when the outdoor air temperature is 0 degrees C. to 15 degrees C., and the opening degree of the bypass expansion valve  18  may be set to a third opening degree, which is larger than the second degree, when the outdoor air temperature is −5 degrees C. to −10 degrees C. Alternatively, a table or an equation in which a relationship between outdoor air temperature and the opening degree of the bypass expansion valve  18  is defined is stored in a memory or the like, and the opening degree of the bypass expansion valve  18  may be determined using the table or equation. Even in a case where the first condition corresponds to other examples, the opening degree of the bypass expansion valve  18  can be determined in a similar manner. Even in a case where the operation frequency of the compressor  11  is changed instead of the opening degree of the bypass expansion valve  18 , the operation frequency of the compressor  11  is not necessarily uniformly set to the second frequency and can be set in accordance with, for example, the outdoor air temperature. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : refrigeration cycle apparatus,  10 : refrigerant circuit,  11 : compressor,  11   a : suction port,  11   b : discharge port,  12 : first flow switch valve,  13 : indoor heat exchanger,  14 : expansion valve,  15   a : first outdoor heat exchanger,  15   b : second outdoor heat exchanger,  17   a : capillary tube,  17   b : capillary tube,  18 : bypass expansion valve,  21   a : second flow switch valve,  21   b : third flow switch valve,  22 : check valve,  50 : controller,  61 : discharge pipe,  62 : suction pipe,  63 ,  65 ,  68 ,  69 ,  71 ,  84 : bifurcation,  64 : second high pressure pipe,  67 ,  67   a ,  67   b : first high pressure pipe,  70 ,  70   a ,  70   b : low pressure pipe,  80 ,  81 ,  82 ,  82   a ,  82   b ,  83   a ,  83   b : refrigerant pipe,  91 : outdoor air temperature sensor,  92   a ,  92   b : heat exchanger temperature sensor,  100 : valve body,  101 : cylinder,  102 : slide base,  103 : slide valve,  104 ,  105 : piston,  106 : first space,  107 : second space,  110 ,  111 ,  112 ,  113 : pilot tube,  120 : pilot solenoid valve,  180 : main body,  181 : base,  181   a : slope portion,  182 : needle,  182   a : tip portion,  183 : drive device,  184 : valve chamber,  185 : refrigerant inlet,  186 : refrigerant outlet,  187 ,  187 A: recess,  188 ,  188 A: protrusion,  501 : operation control unit,  502 : condition determination unit,  503 : differential pressure ensuring unit,  504 : storage unit.