Patent Publication Number: US-2022221210-A1

Title: Refrigeration apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2020/039918, filed on Oct. 23, 2020, which claims priority under 35 U.S.C. 119(a) to Patent Application No. 2019-199258, filed in Japan on Oct. 31, 2019, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a refrigeration apparatus, in particular to a refrigeration apparatus in which a container is disposed between an evaporator and a compressor. 
     BACKGROUND ART 
     Conventionally, there has been a refrigeration apparatus including a container that temporarily stores a refrigerant returning from an evaporator to a compressor. Refrigerating machine oil is sealed in a refrigerant circuit of the refrigeration apparatus together with the refrigerant, and the refrigerant and the refrigerating machine oil may separate in the container depending on temperature and pressure conditions. For this problem, Patent Literature 1 (Japanese Laid-Open Patent Publication No. 2016-211774) discloses an apparatus that executes an operation of stirring the separated refrigerant and refrigerating machine oil to solve the separation state. 
     SUMMARY 
     A refrigeration apparatus of a first aspect includes a refrigerant circuit, a detection unit, and a control unit. In the refrigerant circuit, a compressor, a radiator, an expansion valve, an evaporator, and a container are connected in order. The refrigerant flows inside the refrigerant circuit. The detection unit detects the temperature or pressure of the refrigerant. The control unit controls the number of revolutions of the compressor and the opening degree of the expansion valve. On determination that the refrigerant and a lubricating oil are separated inside the container based on a detection result of the detection unit, the control unit executes first control and second control. The first control is control to decrease the number of revolutions of the compressor. The second control sets the opening degree of the expansion valve to a predetermined opening degree. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an air conditioning apparatus. 
         FIG. 2  is a schematic configuration diagram of an accumulator. 
         FIG. 3  is a control block diagram of the air conditioning apparatus. 
         FIG. 4  is a flowchart of separation solution control of a refrigerant and refrigerating machine oil in the accumulator. 
         FIG. 5  is a graph showing the relationship between oil concentration and a two-layer separation temperature. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an air conditioning apparatus as a refrigeration apparatus will be described with reference to the drawings. 
     (1) Overall Configuration 
       FIG. 1  is a schematic configuration diagram of an air conditioning apparatus  1  (refrigeration apparatus). The air conditioning apparatus  1  is an apparatus capable of cooling and heating a room of a building or the like by a vapor compression refrigeration cycle. The air conditioning apparatus  1  includes an outdoor unit  2  and an indoor unit  4 . The outdoor unit  2  and the indoor unit  4  are connected via a liquid-refrigerant connection pipe  5  and a gas-refrigerant connection pipe  6 . A refrigerant circuit  10  that constitutes the vapor compression refrigeration cycle of the air conditioning apparatus  1  is configured by the outdoor unit  2  and the indoor unit  4  being connected via the refrigerant connection pipes  5  and  6 . Difluoromethane (R32), which is a refrigerant, is charged in the refrigerant circuit  10 . Refrigerating machine oil, which is immiscible with the refrigerant, is also charged in the refrigerant circuit  10  together with the refrigerant. 
     (2) Detailed Configuration 
     (2-1) Indoor Unit 
     The indoor unit  4  is installed indoors and constitutes part of the refrigerant circuit  10 . The indoor unit  4  includes an indoor heat exchanger  41 . 
     In a cooling operation, the indoor heat exchanger  41  functions as an evaporator for a refrigerant to cool indoor air, and in a heating operation, the indoor heat exchanger  41  functions as a radiator for a refrigerant to heat indoor air. A first end of the indoor heat exchanger  41  is connected to the liquid-refrigerant connection pipe  5 . A second end of the indoor heat exchanger  41  is connected to the gas-refrigerant connection pipe  6 . 
     The indoor unit  4  includes an indoor fan  42 . The indoor fan  42  sucks indoor air into the indoor unit  4 , exchanges heat with the refrigerant in the indoor heat exchanger  41 , and then supplies the air indoors as supply air. The indoor fan  42  is, for example, a centrifugal fan, a multi-blade fan, or the like driven by an indoor fan motor  43 . The indoor fan motor  43  can change a frequency (number of revolutions) by an inverter. 
     The indoor unit  4  includes various sensors. The indoor unit  4  includes a liquid pipe temperature sensor  56 , an intermediate temperature sensor  57 , and an indoor temperature sensor  58 . The liquid pipe temperature sensor  56  detects a temperature Trl of the refrigerant in the liquid side refrigerant pipe of the indoor heat exchanger  41 . The intermediate temperature sensor  57  detects a temperature Trm of the refrigerant in an intermediate portion of the indoor heat exchanger  41 . The indoor temperature sensor  58  detects a temperature Tra of the indoor air sucked into the indoor unit  4 . 
     (2-2) Outdoor Unit 
     The outdoor unit  2  is installed outdoors and constitutes part of the refrigerant circuit  10 . The outdoor unit  2  includes a compressor  21 , a four-way switching valve  22 , an outdoor heat exchanger  23 , an expansion valve  24 , a liquid-side shutoff valve  26 , a gas-side shutoff valve  27 , and an accumulator  28 . The outdoor unit  2  includes an outdoor fan  36 . 
     (2-2-1) Compressor 
     The compressor  21  compresses a low-pressure refrigerant in the refrigeration cycle until the refrigerant turns into a high-pressure refrigerant. The compressor  21  drives a positive-displacement compression element (not shown), such as a rotary type or scroll type, to rotate by a compressor motor  21   a . Here, as the compressor  21 , a rotary compressor with closed structure is used. The compressor motor  21   a  can change a frequency (number of revolutions) by an inverter. A suction pipe  31  is connected to a suction side of the compressor  21 , and a discharge pipe  32  is connected to a discharge side. The suction pipe  31  connects the suction side of the compressor  21  to a first port  22   a  of the four-way switching valve  22 . The suction pipe  31  is provided with the accumulator  28 . The suction pipe  31  is divided into a first pipe  31   a  and a second pipe  31   b  before and after the accumulator  28 . The accumulator  28  is a container that temporarily stores the refrigerant sucked into the compressor  21 . The accumulator  28  will be described in detail later with reference to  FIG. 2 . The discharge pipe  32  is a refrigerant pipe connecting the discharge side of the compressor  21  to a second port  22   b  of the four-way switching valve  22 . 
     (2-2-2) Four-Way Switching Valve 
     The four-way switching valve  22  switches a refrigerant flow direction in the refrigerant circuit  10 . 
     When starting the cooling operation, the four-way switching valve  22  switches to the cooling cycle state in which the outdoor heat exchanger  23  functions as a radiator for the refrigerant compressed in the compressor  21 , and the indoor heat exchanger  41  functions as an evaporator for the refrigerant that has radiated heat in the outdoor heat exchanger  23 . When starting the cooling operation, the four-way switching valve  22  switches such that the second port  22   b  and a third port  22   c  communicate with each other, and the first port  22   a  and a fourth port  22   d  communicate with each other. Accordingly, the discharge side of the compressor  21  (discharge pipe  32 ) is connected to a gas side of the outdoor heat exchanger  23  (first gas refrigerant pipe  33 ) (see the solid line in the four-way switching valve  22  in  FIG. 1 ). Furthermore, the suction side of the compressor  21  (suction pipe  31 ) is connected to the gas-refrigerant connection pipe  6  side (second gas refrigerant pipe  34 ) (see the solid line in the four-way switching valve  22  in  FIG. 1 ). 
     When starting the heating operation, the four-way switching valve  22  switches to the heating cycle state in which the outdoor heat exchanger  23  functions as an evaporator for the refrigerant that has radiated heat in the indoor heat exchanger  41 , and the indoor heat exchanger  41  functions as a radiator for the refrigerant compressed in the compressor  21 . When starting the heating operation, the four-way switching valve  22  switches such that the second port  22   b  and the fourth port  22   d  communicate with each other, and the first port  22   a  and the third port  22   c  communicate with each other. Accordingly, the discharge side of the compressor  21  (discharge pipe  32 ) is connected to the gas-refrigerant connection pipe  6  side (second gas refrigerant pipe  34 ) (see the broken line in the four-way switching valve  22  in  FIG. 1 ). Furthermore, the suction side of the compressor  21  (suction pipe  31 ) is connected to the gas side of the outdoor heat exchanger  23  (first gas refrigerant pipe  33 ) (see the broken line in the four-way switching valve  22  in  FIG. 1 ). The first gas refrigerant pipe  33  is a refrigerant pipe that connects the third port  22   c  of the four-way switching valve  22  to the gas side of the outdoor heat exchanger  23 . The second gas refrigerant pipe  34  is a refrigerant pipe that connects the fourth port  22   d  of the four-way switching valve  22  to the gas-refrigerant connection pipe  6  side. 
     (2-2-3) Outdoor Heat Exchanger 
     In the cooling operation, the outdoor heat exchanger  23  functions as a radiator for the refrigerant whose cooling source is outdoor air. In the heating operation, the outdoor heat exchanger  23  functions as an evaporator for the refrigerant whose heating source is outdoor air. A first end on the liquid side of the outdoor heat exchanger  23  is connected to a liquid refrigerant pipe  35 , and a second end on the gas side is connected to the first gas refrigerant pipe  33 . The liquid refrigerant pipe  35  is a refrigerant pipe that connects the first end on the liquid side of the outdoor heat exchanger  23  to the liquid-refrigerant connection pipe  5 . 
     (2-2-4) Expansion Valve 
     In the cooling operation, the expansion valve  24  decompresses the high-pressure refrigerant that has radiated heat in the outdoor heat exchanger  23  in the refrigeration cycle to low pressure in the refrigeration cycle. In the heating operation, the expansion valve  24  decompresses the high-pressure refrigerant that has radiated heat in the indoor heat exchanger  41  in the refrigeration cycle to low pressure in the refrigeration cycle. The expansion valve  24  is provided in the liquid refrigerant pipe  35 . The expansion valve  24  is an electric expansion valve with a changeable opening degree. 
     (2-2-5) Liquid-Side Shutoff Valve and Gas-Side Shutoff Valve 
     The liquid-side shutoff valve  26  and the gas-side shutoff valve  27  are provided in connecting ports with external devices and pipes (specifically, liquid-refrigerant connection pipe  5  and gas-refrigerant connection pipe  6 ). The liquid-side shutoff valve  26  is provided at an end of the liquid refrigerant pipe  35 . The gas-side shutoff valve  27  is provided at an end of the second gas refrigerant pipe  34 . The liquid-side shutoff valve  26  and the gas-side shutoff valve  27  are manual valves that are opened and closed by hand. 
     (2-2-6) Outdoor Fan 
     The outdoor fan  36  plays a role of sucking outdoor air into the outdoor unit  2  to exchange heat with the refrigerant in the outdoor heat exchanger  23 , and then discharging the air to the outside. The outdoor fan  36  is a propeller fan or the like driven by an outdoor fan motor  37 . The outdoor fan motor  37  can change a frequency (number of revolutions) by an inverter. 
     (2-2-7) Various Sensors 
     The outdoor unit  2  includes various sensors. The outdoor unit  2  includes a suction temperature sensor  51 , a discharge temperature sensor  52 , an intermediate temperature sensor  53 , a liquid pipe temperature sensor  54 , and an outside air temperature sensor  55 . The suction temperature sensor  51  detects a temperature Ts of the low-pressure refrigerant sucked into the compressor  21  in the refrigeration cycle. The discharge temperature sensor  52  detects a temperature Td of the high-pressure refrigerant discharged from the compressor  21  in the refrigeration cycle. The intermediate temperature sensor  53  detects a temperature Tom of the refrigerant in the intermediate portion of the outdoor heat exchanger  23 . The liquid pipe temperature sensor  54  detects a temperature Tol of the refrigerant on the liquid side of the outdoor heat exchanger  23 . The outside air temperature sensor  55  detects a temperature Toa of the outdoor air sucked into the outdoor unit  2 . 
     (2-2-8) Accumulator 
     As described above, the accumulator  28  of the outdoor unit  2  is disposed between the suction side of the compressor  21  and the first port  22   a  of the four-way switching valve  22 . The accumulator  28  has a function of separating the refrigerant into gas and liquid, and storing excess refrigerant on the suction side of the compressor  21 . The accumulator  28  separates, into gas and liquid, the refrigerant returned from the indoor heat exchanger  41  or the outdoor heat exchanger  23  serving as an evaporator through the first pipe  31   a  of the suction pipe  31  connected to the four-way switching valve  22 . Out of the refrigerant separated into gas and liquid, the gas refrigerant is sent to the compressor  21 . As shown in  FIG. 2 , the accumulator  28  includes a casing  71  forming an internal space IS, an inlet pipe  72 , and an outlet pipe  73 . 
     The casing  71  mainly includes a cylindrical body  71   a , a bowl-shaped upper lid  71   b  closing an opening above the body  71   a , and a bowl-shaped lower lid  71   c  closing an opening below the body  71   a . The inlet pipe  72  introduces the refrigerant that has passed through the first pipe  31   a  of the suction pipe  31  into the internal space IS. The inlet pipe  72  penetrates a periphery of the upper lid  71   b . A tip opening  72   a  of the inlet pipe  72  is disposed in an upper portion of the internal space IS. 
     The outlet pipe  73  of the accumulator  70  guides the gas refrigerant separated in the internal space IS to the second pipe  31   b  of the suction pipe  31  connected to the compressor  21 . The outlet pipe  73  is a J-shaped pipe. The outlet pipe  73  penetrates the upper lid  71   b  and makes a U-turn in a lower portion of the internal space IS. The height position of an opening  73   a  at an upper end (tip) of the outlet pipe  73  is located in an upper portion of the internal space IS. An oil return hole  73   b  is formed in the U-turn portion of the outlet pipe  73  in the lower portion of the internal space IS. The oil return hole  73   b  is provided to return the refrigerating machine oil accumulated together with the liquid refrigerant in the lower portion of the internal space IS of the casing  71  to the compressor  21 . A pressure equalizing hole  73   c  is formed in a portion of the outlet pipe  73  near the upper lid  71   b.    
     The outlet pipe  73  of the accumulator  70  is connected to the compressor  21  by the second pipe  31   b  of the suction pipe  31 . 
     (3) Refrigerant Connection Pipe 
     The refrigerant connection pipes  5  and  6  are refrigerant pipes constructed on the spot when the air conditioning apparatus  1  is installed at an installation location such as a building. The length and pipe diameter of the refrigerant connection pipes  5  and  6  are selected according to installation conditions such as the installation location and a combination of the outdoor unit  2  and the indoor unit  4 . 
     As described above, part of the refrigerant circuit  10  of the indoor unit  4  is connected to part of the refrigerant circuit  10  of the outdoor unit  2  by the refrigerant connection pipes  5  and  6 , constituting the refrigerant circuit  10  as a whole. In the refrigerant circuit  10 , mainly, the compressor  21 , the outdoor heat exchanger  23  which functions as a radiator or evaporator for the refrigerant, the expansion valve  24 , the indoor heat exchanger  41  which functions as an evaporator or radiator for the refrigerant, and the accumulator (container)  28  are connected in order. 
     (4) Control Configuration 
       FIG. 3  is a control block diagram of the air conditioning apparatus  1  (refrigeration apparatus). The air conditioning apparatus  1  includes a control unit  8  that controls constituent devices. The control unit  8  is configured by connecting an outdoor control unit  38 , an indoor control unit  44 , and a remote control device  9  via a transmission line or a communication line. The outdoor control unit  38  is provided in the outdoor unit  2 . The indoor control unit  44  is provided in the indoor unit  4 . The remote control device  9  is provided indoors. Here, the control units  38  and  44  and the remote control device  9  are connected by wire via a transmission line or a communication line, but may be wirelessly connected. 
     (4-1) Outdoor Control Unit 
     The outdoor control unit  38  is provided in the outdoor unit  2  as described above, and mainly includes an outdoor CPU  38   a , an outdoor transmission unit  38   b , and an outdoor storage unit  38   c . The outdoor control unit  38  receives detection signals such as signals from the temperature sensors  51  to  55 . 
     The outdoor CPU  38   a  is connected to the outdoor transmission unit  38   b  and the outdoor storage unit  38   c . The outdoor transmission unit  38   b  transmits control data and the like to and from the indoor control unit  44 . The outdoor storage unit  38   c  stores control data and the like. The outdoor CPU  38   a  controls constituent devices provided in the outdoor unit  2  (compressor  21 , four-way switching valve  22 , expansion valve  24 , outdoor fan  36 , and the like) while transmitting, reading, and writing control data and the like via the outdoor transmission unit  38   b  and the outdoor storage unit  38   c.    
     (4-2) Indoor Control Unit 
     The indoor control unit  44  is provided in the indoor unit  4  as described above, and mainly includes an indoor CPU  44   a , an indoor transmission unit  44   b , an indoor storage unit  44   c , and an indoor communication unit  44   d . The indoor control unit  44  receives detection signals such as signals from the temperature sensors  56  to  58 . 
     The indoor CPU  44   a  is connected to the indoor transmission unit  44   b , the indoor storage unit  44   c , and the indoor communication unit  44   d . The indoor transmission unit  44   b  transmits control data and the like to and from the outdoor control unit  38 . The indoor storage unit  44   c  stores control data and the like. The indoor communication unit  44   d  sends and receives control data and the like to and from the remote control device  9 . The indoor CPU  44   a  controls constituent devices provided in the indoor unit  4  (indoor fan  42  and the like) while transmitting, reading, writing, sending, and receiving control data and the like via the indoor transmission unit  44   b , the indoor storage unit  44   c , and the indoor communication unit  44   d.    
     (4-3) Remote Control Device 
     The remote control device  9  is provided indoors as described above, and mainly includes a remote control CPU  91 , a remote control communication unit  93 , a remote control manipulation unit  94 , and a remote control display unit  95 . 
     The remote control CPU  91  is connected to the remote control communication unit  93 , the remote control manipulation unit  94 , and the remote control display unit  95 . The remote control communication unit  93  sends and receives control data and the like to and from the indoor communication unit  44   d . The remote control manipulation unit  94  receives input such as a control command from a user. The remote control display unit  95  displays the operation and the like. The remote control CPU  91  receives input such as operation commands and control commands via the remote control manipulation unit  94 , and issues control commands and the like to the indoor control unit  44  via the remote control communication unit  93  while displaying the operating state, control state, and the like on the remote control display unit  95 . 
     (5) Basic Operation 
     Next, the basic operation of the air conditioning apparatus  1  (refrigeration apparatus) will be described with reference to  FIGS. 1 and 3 . As the basic operation, the air conditioning apparatus  1  executes the cooling operation and the heating operation. 
     (5-1) Cooling Operation 
     When a cooling operation command is received via the remote control manipulation unit  94  of the remote control device  9  or the like, the control unit  8  sets the operating mode of the air conditioning apparatus  1  to the cooling operation. Then, the control unit  8  switches the four-way switching valve  22  to the cooling cycle state (state shown by the solid line in  FIG. 1 ), drives the compressor  21  and the fans  36  and  42 , and opens the expansion valve  24 . 
     Then, the low-pressure refrigerant in the refrigeration cycle in the refrigerant circuit  10  is sucked into the compressor  21 , compressed to high pressure in the refrigeration cycle, and then discharged. 
     The high-pressure gas refrigerant discharged from the compressor  21  is sent to the outdoor heat exchanger  23  through the four-way switching valve  22 . 
     The high-pressure gas refrigerant sent to the outdoor heat exchanger  23  radiates heat by heat exchange with outdoor air supplied as a cooling source by the outdoor fan  36  in the outdoor heat exchanger  23 , and becomes a high-pressure liquid refrigerant. 
     The high-pressure liquid refrigerant that has radiated heat in the outdoor heat exchanger  23  is sent to the expansion valve  24 . The high-pressure liquid refrigerant sent to the expansion valve  24  is decompressed by the expansion valve  24  to low pressure in the refrigeration cycle. 
     The low-pressure refrigerant decompressed in the expansion valve  24  is sent to the indoor heat exchanger  41  through the liquid-side shutoff valve  26  and the liquid-refrigerant connection pipe  5 . 
     The low-pressure refrigerant sent to the indoor heat exchanger  41  exchanges heat with the indoor air supplied by the indoor fan  42  as a heating source to evaporate in the indoor heat exchanger  41 . The indoor air is thus cooled and then supplied into the room, thereby cooling the room. 
     The low-pressure refrigerant evaporated in the indoor heat exchanger  41  is sent to the suction pipe  31  through the gas-refrigerant connection pipe  6 , the gas-side shutoff valve  27 , and the four-way switching valve  22 . Thereafter, the refrigerant is sucked into the compressor  21  again through the accumulator  28 . 
     (5-2) Heating Operation 
     When a heating operation command is received via the remote control manipulation unit  94  of the remote control device  9  or the like, the control unit  8  sets the operating mode of the air conditioning apparatus  1  to the heating operation. Then, the control unit  8  switches the four-way switching valve  22  to the heating cycle state (state shown by the broken line in  FIG. 1 ), drives the compressor  21  and the fans  36  and  42 , and opens the expansion valve  24 . 
     Then, the low-pressure refrigerant in the refrigeration cycle in the refrigerant circuit  10  is sucked into the compressor  21 , compressed to high pressure in the refrigeration cycle, and then discharged. 
     The high-pressure gas refrigerant discharged from the compressor  21  is sent to the indoor heat exchanger  41  via the four-way switching valve  22 , the gas-side shutoff valve  27 , and the gas-refrigerant connection pipe  6 . 
     The high-pressure gas refrigerant sent to the indoor heat exchanger  41  radiates heat by heat exchange with indoor air supplied as a cooling source by the indoor fan  42  in the indoor heat exchanger  41 , and becomes a high-pressure liquid refrigerant. The indoor air is thus heated and then supplied into the room, thereby heating the room. 
     The high-pressure liquid refrigerant that has radiated heat in the indoor heat exchanger  41  is sent to the expansion valve  24  through the liquid-refrigerant connection pipe  5  and the liquid-side shutoff valve  26 . 
     The high-pressure liquid refrigerant sent to the expansion valve  24  is decompressed by the expansion valve  24  to low pressure in the refrigeration cycle. The low-pressure refrigerant decompressed in the expansion valve  24  is sent to the outdoor heat exchanger  23 . The low-pressure liquid refrigerant sent to the outdoor heat exchanger  23  exchanges heat with the outdoor air supplied as a heating source by the outdoor fan  36  to evaporate in the outdoor heat exchanger  23 . 
     The low-pressure refrigerant evaporated in the outdoor heat exchanger  23  is sent to the suction pipe  31  through the four-way switching valve  22 , and sucked again into the compressor  21  through the accumulator  28 . 
     (5-3) Basic Control 
     In the above-described basic operation (cooling operation and heating operation), the control unit  8  executes compressor capacity control and expansion valve degree of subcooling control as basic control. 
     (5-3-1) Compressor Capacity Control 
     The compressor capacity control is control to change the frequency F of the compressor  21  based on the temperature difference ΔTra between the indoor temperature Tra and the indoor set temperature Trat. The set temperature Trat is a temperature value set via the remote control manipulation unit  94  of the remote control device  9 , or the like. 
     In the cooling operation, the control unit  8  obtains the temperature difference ΔTra by subtracting the set temperature Trat from the indoor temperature Tra. In the heating operation, the control unit  8  obtains the temperature difference ΔTra by subtracting the indoor temperature Tra from the set temperature Trat. 
     Since it is required to increase the air conditioning capacity (cooling capacity or heating capacity) as refrigerating capacity when the temperature difference ΔTra is positive (in other words, when the indoor temperature Tra does not reach the set temperature Trat), the control unit  8  increases the frequency F of the compressor  21 . Specifically, the control unit  8  determines the change width ΔF of the frequency F of the compressor  21  according to the magnitude of the temperature difference ΔTra to increase the frequency F of the compressor  21  by the change width ΔF. Since it is required to decrease the air conditioning capacity (cooling capacity or heating capacity) when the temperature difference ΔTra is negative (in other words, when the indoor temperature Tra reaches the set temperature Trat), the control unit  8  decreases the frequency F of the compressor  21 . Specifically, the control unit  8  determines the change width ΔF of the frequency F of the compressor  21  according to the magnitude of the temperature difference ΔTra to decrease the frequency F of the compressor  21  by the change width ΔF. 
     (5-3-2) Expansion Valve Degree of Subcooling Control 
     The expansion valve degree of subcooling control is control to change the opening degree MV of the expansion valve  24  based on the degree of subcooling SC of the refrigerant at an outlet of the radiator for the refrigerant. Specifically, the control unit  8  changes the opening degree MV of the expansion valve  24  such that the degree of subcooling SC becomes the target degree of subcooling SCt. The degree of subcooling SC is the degree of subcooling at the outlet of the outdoor heat exchanger  23  that functions as a radiator for the refrigerant in the cooling operation, and is the degree of subcooling at the outlet of the indoor heat exchanger  41  that functions as a radiator for the refrigerant in the heating operation. 
     In the cooling operation, the control unit  8  subtracts the refrigerant temperature Tol on the liquid side of the outdoor heat exchanger  23  from the refrigerant temperature Tom in the intermediate portion of the outdoor heat exchanger  23  to obtain the degree of subcooling SC. In the heating operation, the control unit  8  subtracts the temperature Trl from the temperature Trm of the indoor heat exchanger  41  to obtain the degree of subcooling SC. 
     When the degree of subcooling SC is greater than the target degree of subcooling SCt, the control unit  8  increases the opening degree MV of the expansion valve  24  in order to decrease the degree of subcooling SC. Specifically, the control unit  8  determines the change width ΔMV of the opening degree MV of the expansion valve  24  according to the degree of subcooling difference ΔSC between the degree of subcooling SC and the target degree of subcooling SCt, and increases the opening degree MV of the expansion valve  24  by the change width ΔMV. When the degree of subcooling SC is smaller than the target degree of subcooling SCt, the control unit  8  decreases the opening degree MV of the expansion valve  24  in order to increase the degree of subcooling SC. Specifically, the control unit  8  determines the change width ΔMV of the opening degree MV of the expansion valve  24  according to the degree of subcooling difference ΔSC between the target degree of subcooling SCt and the degree of subcooling SC, and decreases the opening degree MV of the expansion valve  24  by the change width ΔMV. 
     (5-4) Oil Return Control 
     The oil return control is control in an oil return operation for returning the refrigerating machine oil that has flowed out from the compressor  21  to the refrigerant circuit  10  (except compressor  21 ) to the compressor  21 . In the oil return operation, the compressor  21  is driven at a predetermined number of oil return revolutions for a predetermined time. 
     Note that the predetermined number of oil return revolutions is required at least to be set to the number of revolutions at which the desired amount of refrigerating machine oil out of the refrigerating machine oil that has flowed out to the refrigerant circuit  10  except the compressor  21  returns to the compressor  21  by driving the compressor  21  for a predetermined time, and to be determined as appropriate by simulation, experiment, calculation on paper, or the like. The predetermined number of oil return revolutions is usually set to some relatively high number of revolutions. This is to efficiently return the refrigerating machine oil in the refrigerant circuit  10  to the compressor  21 . 
     When the condition that the amount of refrigerant circulating in the refrigerant circuit  10  exceeds a threshold value is satisfied, the amount being integrated after the previous oil return operation, the control unit  8  executes the oil return operation. The threshold value of the integrated value of the refrigerant is set near the upper limit of the amount of discharged oil allowed for reliability of the compressor  21 . 
     (5-5) Separation Solution Control to Solve the Separation State of the Refrigerant and the Refrigerating Machine Oil in the Accumulator 
     Since the air conditioning apparatus  1  uses difluoromethane (R32) as a refrigerant, when the outside air temperature is low, the degree of miscibility between the refrigerant and the refrigerating machine oil, which is sealed with the refrigerant for lubrication of the compressor  21 , is very small. Therefore, on the low-pressure side in the refrigeration cycle, because of a decrease in the refrigerant temperature, the degree of miscibility between the refrigerating machine oil and the refrigerant greatly decreases. The refrigerant and the refrigerating machine oil are separated into two layers in the accumulator  28  that becomes low pressure in the refrigeration cycle, and it becomes difficult for the refrigerating machine oil to return to the compressor  21 . For example, in the heating operation when the outside air temperature is low, as shown in  FIG. 2 , the lower portion of the internal space IS of the casing  71  tends to be filled with the liquid refrigerant and the refrigerating machine oil separated from the liquid refrigerant tends to gather in the upper portion of the internal space IS. Then, the oil return hole  73   b  of the outlet pipe  73  of the accumulator  28  is separated from the refrigerating machine oil, and therefore the refrigerating machine oil that has accumulated in the internal space IS of the accumulator  28  cannot be returned to the compressor  21 . In other words, since the amount of liquid refrigerant increases around the oil return hole  73   b  of the outlet pipe  73 , the amount of refrigerating machine oil sucked from the oil return hole  73   b  decreases, and a sufficient amount of refrigerating machine oil cannot be returned to the compressor  21 . 
     (5-5-1) Separation Solution Control Including Separation Solution Operation 
     In view of this, when the refrigerant and the refrigerating machine oil are separated in the accumulator  28 , the control unit  8  executes a separation solution operation to solve the separation state. Hereinafter, the separation solution control including the separation solution operation will be described with reference to the control flowchart shown in  FIG. 4 . 
     In step S 1 , the control unit  8  determines whether there is an operation stop signal. The operation stop signal is a signal sent from the remote control device  9  to the indoor control unit  44  when a manipulation of stopping the operation of the air conditioning apparatus  1  is executed with the remote control manipulation unit  94  of the remote control device  9 . The operation stop signal is, for example, a thermo-off signal sent from the indoor control unit  44  to the outdoor control unit  38  when the room temperature becomes higher than the indoor heating set temperature by 1° C. or more. 
     On determination in step S 1  that there is an operation stop signal, the process proceeds to step S 12 , and the control unit  8  determines whether the suction temperature Ts is lower than a first threshold temperature T 1 . The suction temperature Ts is a temperature of the refrigerant in front of the accumulator  28 , the temperature being detected by the suction temperature sensor  51 . 
     On determination in step S 12  that the suction temperature Ts is equal to or higher than the first threshold temperature T 1 , the degree of separation between the refrigerant and the refrigerating machine oil in the accumulator  28  is within a permissible range while the compressor is stopped, and the control unit  8  stops the compressor  21  as it is (step S 13 ). 
     On determination in step S 1  that there is no operation stop signal, the process proceeds to step S 2 , and the control unit  8  determines whether the suction temperature Ts is lower than a second threshold temperature T 2 . 
     On determination in step S 2  that the suction temperature Ts is equal to or higher than the second threshold temperature T 2 , the degree of separation between the refrigerant and the refrigerating machine oil in the accumulator  28  is within the permissible range while the compressor is operating, and thus the control unit  8  maintains normal control of the number of revolutions of the compressor  21  and control of the opening degree of the expansion valve  24  at that time, and returns to step S 1 . 
     Note that regarding the degree of separation between the refrigerant and the refrigerating machine oil in the accumulator  28 , the permissible range while the compressor is stopped is different from the permissible range while the compressor is operating. Since it is preferable to continue normal control as much as possible while the compressor is operating, the permissible range while the compressor is operating is set widely. The permissible range while the compressor is stopped is set narrower than the permissible range while the compressor is operating in order to ensure that the refrigerating machine oil in the compressor  21  is sufficient when restarting the compressor  21 . Therefore, the second threshold temperature T 2  is lower than the first threshold temperature T 1 . 
     On determination in step S 2  that the suction temperature Ts is below the second threshold temperature T 2  or on determination in step S 12  that the suction temperature Ts is below the first threshold temperature T 1 , the control unit  8  proceeds to steps S 3  and S 4 . In steps S 3  and S 4 , in order to alleviate and solve the separation state between the refrigerant and the refrigerating machine oil in the accumulator  28 , the number of revolutions of the compressor  21  is decreased to a predetermined number of revolutions, and the opening degree of the expansion valve  24  is increased until fully opened. The control unit  8  executes each of the operations of steps S 3  and S 4  in parallel. 
     Thereafter, after waiting for a certain period of time (step S 5 ), the process proceeds to step S 6 , and the control unit  8  returns to normal control before executing steps S 3  and S 4  by which the opening degree of the expansion valve  24  and the number of revolutions of the compressor  21  are adjusted. The number of revolutions of the compressor  21  and the opening degree of the expansion valve  24  in normal control are determined as described in (5-3-1) and (5-3-2). 
     Note that the certain period of time in step S 5  can be selected from the range from 1 minute to 10 minutes, and is set in advance when the air conditioning apparatus  1  is manufactured. 
     As described above, the control unit  8  determines whether the refrigerant and the refrigerating machine oil are separated in the accumulator  28  based on the temperature Ts detected by the suction temperature sensor  51  (steps S 2  and S 12 ). Then, when it is detected that the refrigerant and the refrigerating machine oil are separated in the accumulator  28 , the control unit  8  executes the separation solution operation (steps S 3 , S 4 , S 5 ). In the separation solution operation, the compressor  21  is driven at a predetermined number of revolutions lower than in the oil return operation. Accordingly, the separation state of the refrigerant and the refrigerating machine oil in the internal space IS of the accumulator  28  is alleviated and solved. 
     (5-5-2) Determination of Degree of Separation Between Refrigerant and Refrigerating Machine Oil in Accumulator 
     In steps S 12  and S 2 , it is determined whether the refrigerant and the refrigerating machine oil are separated in the accumulator  28  by using respective threshold values (first threshold temperature T 1  and second threshold temperature T 2 ). This determination is made by the control unit  8  based on the temperature inside the accumulator  28 , here, the suction temperature Ts corresponding to the temperature. 
     The control unit  8  determines whether the refrigerant and the refrigerating machine oil are separated in the accumulator  28  with reference to the graph shown in  FIG. 5 . The graph shown in  FIG. 5  is divided into a region A in an environment where the refrigerant and the refrigerating machine oil are separated, and a region B in an environment where the refrigerant and the refrigerating machine oil are not separated. The graph shown in  FIG. 5  is a graph showing the relationship between oil concentration and two-layer separation temperature when the refrigerant is difluoromethane (R32) and the refrigerating machine oil is polyvinyl ether (PVE). For example, when the oil concentration is 25 wt %, the two-layer separation temperature is about 0° C. and each threshold value is set near 0° C. For example, the second threshold temperature T 2  is set to −3° C. and the first threshold temperature T 1  is set to 0° C. 
     Note that in the separation solution operation, a decrease in the number of revolutions of the compressor  21  and an increase in the opening degree of the expansion valve  24  lead to an increase in the pressure in the accumulator  28  and an increase in the temperature of the refrigerant. With this configuration, even if the refrigerant and the refrigerating machine oil are separated in the accumulator  28 , the temperature of the refrigerant increases to exceed the two-layer separation temperature shown in  FIG. 5 , alleviating and solving the separation state. 
     (6) Features 
     Next, features of the air conditioning apparatus  1  (refrigeration apparatus) will be described. 
     (6-1) 
     In the air conditioning apparatus  1 , the suction temperature sensor  51  detects the temperature of the refrigerant flowing into the accumulator  28 . The control unit  8  controls the number of revolutions of the compressor  21  and the opening degree of the expansion valve  24 . On determination that the refrigerant and the refrigerating machine oil (lubricating oil) are separated inside the accumulator  28  based on the detection result of the suction temperature sensor  51 , the control unit  8  executes the separation solution operation including steps S 3  and S 4 . In the control of step S 3 , the number of revolutions of the compressor  21  is decreased. In the control of step S 4 , the opening degree of the expansion valve  24  is set to the predetermined opening degree (fully open). 
     Here, the separation solution operation of decreasing the number of revolutions of the compressor  21  and increasing the opening degree of the expansion valve  24  is executed when the refrigerant and the refrigerating machine oil are separated inside the accumulator  28 . Therefore, the pressure (low pressure value) on the suction side of the compressor  21  including the accumulator  28  can be increased. This makes it possible to change the pressure and temperature in the accumulator  28  to solve the separation state between the refrigerant and the refrigerating machine oil. 
     (6-2) 
     In the air conditioning apparatus  1 , the control unit  8  fully opens the opening degree of the expansion valve  24  in the control of step S 4 . Therefore, since the separation solution operation is executed to fully open the opening degree of the expansion valve  24  when the refrigerant and the refrigerating machine oil are separated inside the accumulator  28 , a large amount of high-temperature refrigerant flows into the accumulator  28 . This allows the separation solution operation to solve the separation state between the refrigerant and the refrigerating machine oil at an early stage. 
     (6-3) 
     In the air conditioning apparatus  1 , the control unit  8  decreases the number of revolutions of the compressor  21  in the control of step S 3  to set the number of revolutions of the compressor  21  to a predetermined number of revolutions. Here, the control to decrease the number of revolutions of the compressor  21  to the predetermined number of revolutions is adopted instead of the control to decrease the number of revolutions a little. Therefore, the separation state between the refrigerant and the refrigerating machine oil is solved in a short time. Note that as one example, in the control of step S 3 , the number of revolutions of the compressor  21  is decreased to a predetermined number of revolutions in the range from 20 to 30 rpm. 
     (6-4) 
     In the air conditioning apparatus  1 , the control unit  8  executes the oil return operation separately from the separation solution operation. As described above, the oil return operation is an operation of returning the refrigerating machine oil staying in the refrigerant circuit  10  except the compressor  21  to the compressor  21 . 
     Some conventional refrigeration apparatus, such as the air conditioning apparatus, also executes the oil return operation similar to the present embodiment. However, the oil return operation, in which the motor of the compressor is turned at a relatively high number of revolutions, may not be preferable as an operation to solve the separation state between the refrigerant and the refrigerating machine oil inside the container such as the accumulator. Therefore, the control unit  8  of the air conditioning apparatus  1  executes the separation solution operation shown in  FIG. 4 , in addition to the oil return operation, to alleviate and solve the separation between the refrigerant and the refrigerating machine oil in the accumulator  28 . 
     Note that, in contrast to the oil return operation of turning the compressor  21  at a relatively high number of revolutions, in the separation solution operation to solve the separation between the refrigerant and the refrigerating machine oil in the accumulator  28 , the number of revolutions of the compressor  21  is decreased to the predetermined number of revolutions. Since the compressor  21  is turned at a lower number of revolutions (predetermined number of revolutions) unlike the oil return operation, the pressure in the accumulator  28  increases and the separation state between the refrigerant and the refrigerating machine oil in the accumulator  28  is alleviated and solved at an early stage. 
     (6-5) 
     In the air conditioning apparatus  1 , when the request to stop the compressor  21  is received, the control unit  8  determines whether to execute the separation solution operation before stopping the compressor  21 , based on the detection result of the suction temperature sensor  51  (see step S 12  in  FIG. 4 ). If the suction temperature Ts is so low that stopping the compressor  21  as it is may lead to a situation where the refrigerating machine oil in the compressor  21  is insufficient when restarting, control is executed to stop the compressor (step S 13  in  FIG. 4 ) after the separation solution operation is executed. When the suction temperature Ts is lower than the first threshold temperature T 1  in step S 12  and the separation solution operation is performed, the suction temperature Ts increases accordingly. When the determination is made again in step S 12  after the separation solution operation is finished, it is determined in step S 12  that the suction temperature Ts is higher than the first threshold temperature T 1 , and the process proceeds to step S 13  to stop the compressor  21 . 
     Here, the situation in which the compressor  21  is stopped while the refrigerant and the refrigerating machine oil are separated in the accumulator  28  and the compressor  21  runs out of refrigerating machine oil when the compressor  21  is started again is inhibited. 
     (6-6) 
     In the air conditioning apparatus  1 , when the request to stop the compressor  21  is received, the control unit  8  determines whether the refrigerant and the refrigerating machine oil are separated inside the accumulator  28  by a first criterion (first threshold temperature T 1 ) based on the detection result of the suction temperature sensor  51 . Meanwhile, when the request to stop the compressor  21  is not received, the control unit  8  determines whether the refrigerant and the refrigerating machine oil are separated inside the accumulator  28  by a second criterion (second threshold temperature T 2 ) different from the first criterion (first threshold temperature T 1 ) based on the detection result of the suction temperature sensor  51 . 
     Here, both when the request to stop the compressor  21  is received and not received, it is determined whether the refrigerant and the refrigerating machine oil are separated inside the accumulator  28 . Therefore, both when the compressor  21  is operating and when the compressor  21  is stopped, the separation solution operation for solving the separation state between the refrigerant and the refrigerating machine oil can be executed. The criterion for determining whether the refrigerant and the refrigerating machine oil are separated inside the accumulator  28  is changed depending on whether the request to stop the compressor  21  is received or not. This makes it possible, for example, to decrease the frequency at which the first and second control is executed when the compressor  21  is operating, and to increase the frequency at which the first and second control is executed when the compressor  21  stops. 
     (7) Modifications 
     (7-1) 
     The embodiment determines the degree of separation between the refrigerant and the refrigerating machine oil in the accumulator  28  by using the measured value of the suction temperature sensor  51  that detects the temperature of the refrigerant flowing into the accumulator  28 . 
     However, instead of this, it is also possible to install a sensor that can directly measure the temperature inside the accumulator  28  and use the measured value of the sensor. 
     It is also possible to attach a temperature sensor to the outer peripheral surface of the accumulator  28 , or to attach a temperature sensor to a pipe downstream of the accumulator  28 . 
     Furthermore, it is possible to install a pressure sensor that measures the pressure of the refrigerant in the accumulator  28  or around the accumulator  28  instead of the temperature sensor, and to calculate the temperature of the refrigerant in the accumulator  28  from the measured value. 
     Instead of determining the degree of separation of the refrigerant and refrigerating machine oil in the accumulator  28  from the measured values of one sensor alone, the separation may be determined based on a plurality of parameters such as the measured value of the suction temperature sensor  51  and the evaporation temperature. 
     (7-2) 
     The air conditioning apparatus  1  of the embodiment is an air conditioning apparatus that can switch between the cooling operation and the heating operation, but is not limited to this apparatus. The above-described separation solution operation is also effective for an air conditioning apparatus that executes only the cooling operation. When the refrigerant and the refrigerating machine oil are separated in the accumulator  28  in both the cooling operation and the heating operation, the separation solution operation is effective. 
     (7-3) 
     In the embodiment, the expansion valve  24  is fully opened in the separation solution operation (step S 4  in  FIG. 4 ), but is not necessarily required to be fully opened. This is because when the expansion valve  24  is fully opened, there is a disadvantage that it takes a little time to return to normal control after the separation solution operation. However, the opening degree of the expansion valve  24  in the separation solution operation is preferably 90% or more of the fully open position. This is because the liquid refrigerant held inside the heat exchanger by the expansion valve degree of subcooling control finally flows into the accumulator  28 . 
     (7-4) 
     The embodiment has described the air conditioning apparatus  1  that uses difluoromethane (R32) alone as a refrigerant. However, even if a mixed refrigerant containing difluoromethane is used, the above-described separation solution operation is effective as long as the mixed refrigerant separates from the refrigerating machine oil when the temperature is low. Even if a refrigerant that does not contain difluoromethane is used, the above-described separation solution operation is effective as long as the mixed refrigerant separates from the refrigerating machine oil when the temperature is low. 
     (7-5) 
     The embodiment of the present disclosure has been described above. It will be understood that various changes to modes and details can be made without departing from the spirit and scope of the present disclosure recited in the claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : air conditioning apparatus (refrigeration apparatus) 
               8 : control unit 
               10 : refrigerant circuit 
               21 : compressor 
               23 : outdoor heat exchanger 
               24 : expansion valve 
               28 : accumulator (container) 
               41 : indoor heat exchanger 
               51 : suction temperature sensor (detection unit) 
             S 3 : control step of separation solution operation (first control) 
             S 4 : control step of separation solution operation (second control) 
           
         
       
    
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2016-211774 A