Patent Publication Number: US-2022214089-A1

Title: Refrigeration apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Application No. PCT/JP2020/036922, filed Sep. 29, 2020, which claims priority to Japanese Patent Application No. 2019-180814, filed Sep. 30, 2019. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a refrigeration apparatus that uses an HFO (hydrofluoroolefin) refrigerant or a refrigerant mixture including an HFO refrigerant. 
     BACKGROUND 
     As described in, for example, Background Art in PTL 1 (Japanese Unexamined Patent Application Publication No. 2012-077983), the volume of an outdoor heat exchanger is generally set to be larger than the volume of an indoor heat exchanger in an air conditioner in which one indoor unit is operated by one outdoor unit. The larger the volume difference between the outdoor heat exchanger and the indoor heat exchanger, the more the refrigerant insufficiency or the refrigerant excess occurs in cooling operation and heating operation and the COP decreases. Since the volume of the outdoor heat exchanger is larger than the volume of the indoor heat exchanger in the aforementioned air conditioner, a refrigerant becomes slightly excessive in heating operation when the amount of the refrigerant in the system is suitably regulated in cooling operation. Meanwhile, when the amount of the refrigerant is suitably regulated during heating operation, the refrigerant becomes insufficient during cooling operation. 
     SUMMARY 
     A refrigeration apparatus according to one or more embodiments is a refrigeration apparatus in which a refrigerant flows during cooling operation in a compressor, an outdoor heat exchanger, an expansion mechanism, and an indoor heat exchanger sequentially and in which the refrigerant flows during heating operation in the compressor, the indoor heat exchanger, the expansion mechanism, and the outdoor heat exchanger sequentially. The refrigerant is an HFO (hydrofluoroolefin) refrigerant or a refrigerant mixture that includes an HFO refrigerant. Both of the indoor heat exchanger and the outdoor heat exchanger are cross-fin-type heat exchangers or stack-type heat exchangers. When the outdoor heat exchanger has a one-row configuration, a volume ratio S [Vo] of the outdoor heat exchanger to the indoor heat exchanger satisfies a relational expression of 
       100≤ S≤ 1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2472 E+ 02
 
     where ρ [kg/m{circumflex over ( )}3]: an average density of a saturated liquid gas at a condensation temperature of 45° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an air conditioning apparatus as a refrigeration apparatus according to one or more embodiments of the present disclosure. 
         FIG. 2  is a schematic front view of a cross-fin-type heat exchanger. 
         FIG. 3  is an external perspective view of a stack-type heat exchanger. 
         FIG. 4  is a table of target models of upper limit setting of the volume ratio. 
         FIG. 5A  is a table showing a relationship between the liquid-gas average density of an HFO-based refrigerant and the upper limit of a volume ratio S of an outdoor heat exchanger to an indoor heat exchanger when the outdoor heat exchanger is a one-row heat exchanger. 
         FIG. 5B  is a graph showing a relationship between the liquid-gas average density of an HFO-based refrigerant and the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger when the outdoor heat exchanger is a one-row heat exchanger. 
         FIG. 6A  is a table showing a relationship between the liquid-gas average density of an HFO-based refrigerant and the upper limit of the volume ratio of the outdoor heat exchanger to the indoor heat exchanger when the outdoor heat exchanger is a two-row heat exchanger. 
         FIG. 6B  is a graph showing a relationship between the liquid-gas average density of an HFO-based refrigerant and the upper limit of the volume ratio of the outdoor heat exchanger to the indoor heat exchanger when the outdoor heat exchanger is a two-row heat exchanger. 
         FIG. 7  is a schematic diagram of an air conditioning apparatus as a refrigeration apparatus according to a modification of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiments 
     (1) Configuration of Air Conditioning Apparatus  1   
       FIG. 1  is a schematic diagram of an air conditioning apparatus  1  according to one or more embodiments of the present disclosure. In  FIG. 1 , the air conditioning apparatus  1  is a refrigeration apparatus that performs cooling operation and heating operation by a vapor compression refrigeration cycle. 
     A refrigerant circuit  10  of the air conditioning apparatus  1  is constituted by an outdoor unit  2  and an indoor unit  4  that are connected to each other via a liquid-refrigerant connection pipe  5  and a gas-refrigerant connection pipe  6 . 
     A refrigerant enclosed in the refrigerant circuit  10  is an HFO (hydrofluoroolefin) refrigerant or a refrigerant mixture that includes an HFO refrigerant. An HFO refrigerant is any of R1132(E), R1123, R1234yf, and R1234ze. 
     (1-1) Indoor Unit  4   
     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 , an indoor fan  42 , and an indoor-side control unit  44 . 
     (1-1-1) Indoor Heat Exchanger  41   
     The indoor heat exchanger  41  functions as an evaporator for the refrigerant during cooling operation and cools indoor air. The indoor heat exchanger  41  functions as a radiator for the refrigerant during heating operation and heats indoor air. The refrigerant inlet side of the indoor heat exchanger  41  during cooling operation is connected to the liquid-refrigerant connection pipe  5 , and the refrigerant outlet side thereof is connected to the gas-refrigerant connection pipe  6 . 
     As the indoor heat exchanger  41 , a cross-fin-type heat exchanger or a stack-type heat exchanger is employed. The cross-fin-type heat exchanger and the stack-type heat exchanger will be described in “(3) Selection of Indoor Heat Exchanger  41  and Outdoor Heat Exchanger  23 ”. 
     (1-1-2) Indoor Fan  42   
     The indoor fan  42  takes indoor air into the indoor unit  4 , causes the indoor air to exchange heat with the refrigerant in the indoor heat exchanger  41 , and then supplies the air to the inside of a room. As the indoor fan  42 , a centrifugal fan, a multi-blade fan, or the like is employed. The indoor fan  42  is driven by an indoor fan motor  43 . 
     (1-1-3) Indoor-Side Control Unit  44   
     The indoor-side control unit  44  controls operation of each portion that constitutes the indoor unit  4 . The indoor-side control unit  44  has a microcomputer and a memory that are for controlling the indoor unit  4 . 
     The indoor-side control unit  44  transmits and receives a control signal and the like to and from a remote controller (not illustrated). In addition, the indoor-side control unit  44  transmits and receives a control signal and the like to and from an outdoor-side control unit  38  of the outdoor unit  2  via a transmission line  8   a.    
     (1-2) Outdoor Unit  2   
     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  26 , a liquid-side shutoff valve  27 , and a gas-side shutoff valve  28 . 
     (1-2-1) Compressor  21   
     The compressor  21  is a device that compresses a low-pressure refrigerant of the refrigeration cycle. The compressor  21  drives and rotates a positive-displacement compression element (not illustrated) of a rotary type, a scroll type, or the like by a compression motor  21   a.    
     A suction pipe  31  is connected to the suction side of the compressor  21 , and a discharge pipe  32  is connected to the discharge side thereof. The suction pipe  31  is a refrigerant pipe that connects the suction side of the compressor  21  and the four-way switching valve  22  to each other. The discharge pipe  32  is a refrigerant pipe that connects the discharge side of the compressor  21  and the four-way switching valve  22  to each other. 
     An accumulator  29  is connected to the suction pipe  31 . The accumulator  29  separates a flowed-in refrigerant into a liquid refrigerant and a gas refrigerant and causes only the gas refrigerant to flow to the suction side of the compressor  21 . 
     (1-2-2) Four-Way Switching Valve  22   
     The four-way switching valve  22  switches the direction of the flow of the refrigerant in the refrigerant circuit  10 . During cooling operation, the four-way switching valve  22  causes the outdoor heat exchanger  23  to function as a radiator for the refrigerant and causes the indoor heat exchanger  41  to function as an evaporator for the refrigerant. 
     During cooling operation, the four-way switching valve  22  connects the discharge pipe  32  of the compressor  21  and a first gas refrigerant pipe  33  of the outdoor heat exchanger  23  to each other (refer to the solid lines of the four-way switching valve  22  in  FIG. 1 ) and connects the suction pipe  31  of the compressor  21  and a second gas refrigerant pipe  34  to each other (refer to the solid lines of the four-way switching valve  22  in  FIG. 1 ). 
     During heating operation, the four-way switching valve  22  is switched to a heating cycle state in which the outdoor heat exchanger  23  functions as an evaporator for the refrigerant and in which the indoor heat exchanger  41  functions as a radiator for the refrigerant. 
     During heating operation, the four-way switching valve  22  connects the discharge pipe  32  of the compressor  21  and the second gas refrigerant pipe  34  to each other (refer to the broken lines of the four-way switching valve  22  in  FIG. 1 ) and connects the suction pipe  31  of the compressor  21  and the first gas refrigerant pipe  33  of the outdoor heat exchanger  23  to each other (refer to the broken lines of the four-way switching valve  22  in  FIG. 1 ). 
     Here, the first gas refrigerant pipe  33  is a refrigerant pipe that connects the four-way switching valve  22  and the refrigerant inlet of the outdoor heat exchanger  23  during cooling operation to each other. The second gas refrigerant pipe  34  is a refrigerant pipe that connects the four-way switching valve  22  and the gas-side shutoff valve  28  to each other. 
     (1-2-3) Outdoor Heat Exchanger  23   
     The outdoor heat exchanger  23  functions as a radiator for the refrigerant during cooling operation. In addition, the outdoor heat exchanger  23  functions as an evaporator for the refrigerant during heating operation. One end of a liquid refrigerant pipe  35  is connected to the refrigerant outlet of the outdoor heat exchanger  23  during cooling operation. The other end of the liquid refrigerant pipe  35  is connected to the expansion valve  26 . 
     As the outdoor heat exchanger  23 , a cross-fin-type heat exchanger or a stack-type heat exchanger is employed. The cross-fin-type heat exchanger and the stack-type heat exchanger will be described in “(3) Selection of Indoor Heat Exchanger  41  and Outdoor Heat Exchanger  23 ”. 
     (1-2-4) Expansion Valve  26   
     The expansion valve  26  is an electric expansion valve. During cooling operation, the expansion valve  26  decompresses a high-pressure refrigerant that is sent from the outdoor heat exchanger  23  to a low pressure. During heating operation, the expansion valve  26  decompresses a high-pressure refrigerant that is sent from the indoor heat exchanger  41  to a low pressure. 
     (1-2-5) Liquid-Side Shutoff Valve  27  and Gas-Side Shutoff Valve  28   
     The liquid-side shutoff valve  27  is connected to the liquid-refrigerant connection pipe  5 . The gas-side shutoff valve  28  is connected the gas-refrigerant connection pipe  6 . The liquid-side shutoff valve  27  is positioned downstream the expansion valve  26  in a refrigerant circulation direction during cooling operation. The gas-side shutoff valve  28  is positioned upstream the four-way switching valve  22  in a refrigerant circulation direction during cooling operation. 
     (1-2-6) Outdoor Fan  36   
     The outdoor unit  2  includes an outdoor fan  36 . The outdoor fan  36  takes outdoor air into the outdoor unit  2 , causes the outdoor air to exchange heat with the refrigerant in the outdoor heat exchanger  23 , and then discharges the air to the outside. As the outdoor fan  36 , a propeller fan or the like is employed. The outdoor fan  36  is driven by an outdoor-fan motor  37 . 
     (1-2-7) Outdoor-Side Control Unit  38   
     The outdoor-side control unit  38  controls operation of each portion that constitutes the outdoor unit  2 . The outdoor-side control unit  38  has a microcomputer and a memory that are for controlling the outdoor unit  2 . 
     The outdoor-side control unit  38  transmits and receives a control signal and the like to and from the indoor-side control unit  44  of the indoor unit  4  via the transmission line  8   a.    
     (1-3) Connection Pipes  5  and  6   
     The connection pipes  5  and  6  are refrigerant pipes that are constructed locally during installation of the air conditioning apparatus  1  in an installation location at a building or the like. As each of the connection pipes  5  and  6 , a pipe having an appropriate length and an appropriate diameter is employed in accordance with installation conditions such as an installation location, a combination of the outdoor unit  2  and the indoor unit  4 , and the like. 
     (2) Basic Operation of Air Conditioning Apparatus  1   
     Next, a basic operation of the air conditioning apparatus  1  will be described with reference to  FIG. 1 . The air conditioning apparatus  1  is capable of performing cooling operation and heating operation as basic operation. 
     (2-1) Cooling Operation 
     During cooling operation, the four-way switching valve  22  is switched to a cooling cycle state (the state indicated by the solid lines in  FIG. 1 ). In the refrigerant circuit  10 , a low-pressure gas refrigerant of the refrigeration cycle is sucked by the compressor  21  and discharged after compressed. 
     The high-pressure gas refrigerant discharged from the compressor  21  is sent to the outdoor heat exchanger  23  via the four-way switching valve  22 . 
     In the outdoor heat exchanger  23  that functions as a radiator, the high-pressure gas refrigerant sent to the outdoor heat exchanger  23  radiates heat by exchanging heat with outdoor air supplied from the outdoor fan  36 , and becomes a high-pressure liquid refrigerant. The high-pressure liquid refrigerant is sent to the expansion valve  26 . 
     The high-pressure liquid refrigerant sent to the expansion valve  26  is decompressed to a low pressure of the refrigeration cycle by the expansion valve  26  and becomes a low-pressure gas-liquid two-phase refrigerant. The low-pressure gas-liquid two-phase refrigerant decompressed in the expansion valve  26  is sent to the indoor heat exchanger  41  via the liquid-side shutoff valve  27  and the liquid-refrigerant connection pipe  5 . 
     The low-pressure gas-liquid two-phase refrigerant sent to the indoor heat exchanger  41  evaporates in the indoor heat exchanger  41  by exchanging heat with indoor air supplied from the indoor fan  42 . Consequently, the indoor air is cooled. Then, the cooled air is supplied to the inside of a room, thereby cooling the inside of the room. 
     The low-pressure gas refrigerant that has evaporated in the indoor heat exchanger  41  is sucked again by the compressor  21  via the gas-refrigerant connection pipe  6 , the gas-side shutoff valve  28 , and the four-way switching valve  22 . 
     (2-2) Heating Operation 
     During heating operation, the four-way switching valve  22  is switched to the heating cycle state (the state indicated by the broken lines in  FIG. 1 ). In the refrigerant circuit  10 , a low-pressure gas refrigerant of the refrigeration cycle is sucked by the compressor  21  and discharged after compressed. 
     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  28 , and the gas-refrigerant connection pipe  6 . 
     The high-pressure gas refrigerant sent to the indoor heat exchanger  41  radiates heat in the indoor heat exchanger  41  by exchanging heat with indoor air supplied from the indoor fan  42 , and becomes a high-pressure liquid refrigerant. Consequently, the indoor air is heated. Then, the heated air is supplied to the inside of a room, thereby heating the inside of the room. 
     The high-pressure liquid refrigerant that has radiated heat in the indoor heat exchanger  41  is sent to the expansion valve  26  via the liquid-refrigerant connection pipe  5  and the liquid-side shutoff valve  27 . 
     The high-pressure liquid refrigerant sent to the expansion valve  26  is decompressed to a low pressure of the refrigeration cycle by the expansion valve  26  and becomes a low-pressure gas-liquid two-phase refrigerant. The low-pressure gas-liquid two-phase refrigerant decompressed in the expansion valve  26  is sent to the outdoor heat exchanger  23 . 
     The low-pressure gas-liquid two-phase refrigerant sent to the outdoor heat exchanger  23  evaporates in the outdoor heat exchanger  23  by exchanging heat with outdoor air supplied from the outdoor fan  36 , and becomes a low-pressure gas refrigerant. 
     The low-pressure refrigerant that has evaporated in the outdoor heat exchanger  23  is sucked again by the compressor  21  through the four-way switching valve  22 . 
     (3) Selection of Heat Exchangers 
     Here, an outline of the cross-fin-type heat exchanger or the stack-type heat exchanger employed in the indoor heat exchanger  41  and the outdoor heat exchanger  23  will be described. 
     (3-1) Outline of Cross-Fin-Type Heat Exchanger 
       FIG. 2  is a front view of a cross-fin-type heat exchanger  51 . In  FIG. 2 , the cross-fin-type heat exchanger  51  has a heat transfer fin  512  and a heat transfer tube  511 . 
     The heat transfer fin  512  is a thin aluminum flat plate. The heat transfer fin  512  has a plurality of through holes. The heat transfer tube  511  has a straight tube  511   a  inserted into the through holes of the heat transfer fin  512 , and U-shaped tubes  511   b  and  511   c  that couple end portions of mutually adjacent straight tubes  511   a  to each other. 
     The straight tube  511   a  is in close contact with the heat transfer fin  512  by being subjected to tube expansion processing after inserted into the through holes of the heat transfer fin  512 . The straight tube  511   a  and the first U-shaped tube  511   b  are formed integrally with each other. The second U-shaped tube  511   c  is coupled to an end portion of the straight tube  511   a  by welding, brazing, or the like after the straight tube  511   a  is inserted into the through holes of the heat transfer fin  512  and subjected to tube expansion processing. 
     (3-2) Outline of Stack-Type Heat Exchanger 
       FIG. 3  is an external perspective view of a stack-type heat exchanger  53 . In  FIG. 3 , the stack-type heat exchanger  53  includes a plurality of flat pipes  531  and a plurality of heat transfer fins  532 . 
     (3-2-1) Flat Pipe  531   
     Each flat pipe  531  is a multi-hole pipe. The flat pipe  531  is formed of aluminum or an aluminum alloy and has a flat portion  531   a  that serves as a heat transfer surface, and a plurality of internal flow paths  531   b  in which the refrigerant flows. 
     The flat pipes  531  are arrayed in a plurality of stages to be stacked with a gap (ventilation space) therebetween in a state in which respective flat portions  531   a  are directed upward/downward. 
     (3-2-2) Heat Transfer Fin  532   
     Each heat transfer fin  532  is a fin made of aluminum or an aluminum alloy. The heat transfer fin  532  is disposed in a ventilation space between the flat pipes  531  vertically adjacent to each other and is in contact with the flat portions  531   a  of the flat pipes  531 . 
     The heat transfer fin  532  has a cutout  532   c  into which the flat pipes  531  are inserted. After the flat pipes  531  are inserted into the cutouts  532   c  of the heat transfer fins  532 , the heat transfer fins  532  and the flat portions  531   a  of the flat pipes  531  are joined to each other by brazing or the like. 
     (3-2-3) Headers  533   a  and  533   b    
     The headers  533   a  and  533   b  are coupled to both ends of the flat pipes  531  arrayed in the plurality of stages in the up-down direction. The headers  533   a  and  533   b  have a function of supporting the flat pipes  531 , a function of guiding the refrigerant to the internal flow paths of the flat pipes  531 , and a function of gathering the refrigerant that has flowed out from the internal flow paths. 
     When the stack-type heat exchanger  53  functions as an evaporator for the refrigerant, the refrigerant flows into the first header  533   a . The refrigerant that has flowed into the first header  533   a  is distributed to the internal flow paths  531   b  of the flat pipes  531  of the stages substantially evenly and flows toward the second header  533   b . The refrigerant that flows in the internal flow paths of the flat pipes  531  of the stages absorbs via the heat transfer fins  532  heat from an air flow that flows in the ventilation spaces. The refrigerant that has flowed in the internal flow paths of the flat pipes  531  of the stages gathers at the second header  533   b  and flows out from the second header  533   b.    
     When the stack-type heat exchanger  53  functions as a radiator for the refrigerant, the refrigerant flows into the second header  533   b . The refrigerant that has flowed into the second header  533   b  is distributed to the internal flow paths  531   b  of the flat pipes  531  of the stages substantially evenly and flows toward the first header  533   a . The refrigerant that flows in the internal flow paths of the flat pipes  531  of the stages radiates via the heat transfer fins  532  heat into an air flow that flows in the ventilation space. The refrigerant that has flowed in the internal flow paths of the flat pipes  531  of the stages gathers at the first header  533   a  and flows out from the first header  533   a.    
     (4) Volume Ratio S of Outdoor Heat Exchanger to Indoor Heat Exchanger 
     (4-1) Influence of HFO-based Refrigerant 
     In the air conditioning apparatus  1  according to one or more embodiments, one indoor unit  4  is operated by one outdoor unit  2 . 
     In such an air conditioning apparatus, generally, the volume of the outdoor heat exchanger is larger than the volume of the indoor heat exchanger. 
     This is because a temperature difference between an ambient temperature and an evaporation temperature or a condensation temperature is set to be smaller for the outdoor heat exchanger. In particular, in the outdoor heat exchanger during heating operation, a difference between the evaporation temperature and the ambient temperature is extremely small to suppress frost as much as possible. 
     Due to the circumstance described above, a volume difference between the indoor heat exchanger and the outdoor heat exchanger is generated. For example, even when an amount of the refrigerant is determined such that the amount of the refrigerant does not become insufficient during heating operation in which the indoor heat exchanger having a smaller volume than the outdoor heat exchanger serves as a condenser, the refrigerant becomes insufficient during cooling operation, and subcooling tends to be not performed at the outlet of the condenser. 
     Therefore, generally, a proper refrigerant amount is determined in consideration of an influence of excess and insufficiency of the refrigerant amount. 
     However, when an HFO-based refrigerant is enclosed instead of the refrigerant of an air conditioning apparatus (hereinafter referred to as the HFC machine) using a HFC-based refrigerant, the refrigerant amount becomes more excessive in heating operation when the refrigerant amount is regulated such that the degree of subcooling is the same as that in the HFC machine in cooling operation. When the refrigerant amount is regulated such that the degree of subcooling is the same as that in the HFC machine in heating operation, the refrigerant amount becomes more insufficient in cooling operation, in particular, under a low load condition. 
     This is due to that the liquid-gas average density, in particular, the gas density at a pressure higher than the HFC refrigerant is larger in the HFO-based refrigerant than in the HFC refrigerant. When the degree of subcooling is the same, the holding amount of the refrigerant in the heat exchanger is large. 
     As described above, when an HFO-based refrigerant is used in an air conditioning apparatus, the apparatus easily, compared with an existing machine, enters a state in which the refrigerant amount is excessive or insufficient due to the “difference in the volume ratio between the indoor heat exchanger and the outdoor heat exchanger” and the “difference in the liquid-gas average density at a high pressure”. 
     (4-2) Target Model 
     Here, the upper limit of the volume ratio S is set to avoid incorrect design of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  when an HFO-based refrigerant is used in the air conditioning apparatus. 
       FIG. 4  is a table of target models of upper limit setting of the volume ratio S. In  FIG. 4 , first, the outdoor heat exchanger is generally categorized into a type in which the outdoor heat exchanger  23  is a one-row heat exchanger and a type in which the outdoor heat exchanger  23  is a two-row heat exchanger. 
     (4-2-1) Types A1, B1, and C1 
     Next, on the premise that the outdoor heat exchanger  23  is a one-row heat exchanger, a combination of types of heat exchangers is categorized for the indoor heat exchanger  41  and the outdoor heat exchanger  23  into three types of a type A1, a type B1, and a type C1. 
     In the type A1, both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type. In the type B1, the indoor heat exchanger  41  is of the cross-fin type, and the outdoor heat exchanger  23  is of the stack type. In Type C1, the indoor heat exchanger  41  is of the stack type, and the outdoor heat exchanger  23  is of the cross-fin type. 
     (4-2-2) Types A2, B2, and C2 
     Next, on the premise that the outdoor heat exchanger  23  is a two-row heat exchanger, a combination of types of heat exchangers is categorized for the indoor heat exchanger  41  and the outdoor heat exchanger  23  into three types of a type A2, a type B2, and a type C2. 
     In the type A2, both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type. In the type B2, the indoor heat exchanger  41  is of the cross-fin type, and the outdoor heat exchanger  23  is of the stack type. In the type C2, the indoor heat exchanger  41  is of the stack type, and the outdoor heat exchanger  23  is of the cross-fin type. 
     (4-3) Upper limit of Volume Ratio S of Types A1, B1, and C1 
     To determine the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  in a refrigerant circuit that uses an HFO-based refrigerant, the volume ratio of an outdoor heat exchanger to an indoor heat exchanger in a refrigerant circuit that uses HFC-32 (hereinafter referred to as R32), which is an existing refrigerant serving as a comparative reference will be described. 
     (4-3-1) Type A1 
     As a premise, both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type, the outdoor heat exchanger  23  is a one-row heat exchanger, the indoor heat exchanger  41  is a two-row heat exchanger, and the operating mode is heating operation. 
     A heat exchanging capacity Qc of the indoor heat exchanger  41  and a heat exchanging capacity Qe of the outdoor heat exchanger  23  during heating operation are generally expressed by the following expressions. 
     Qc=K×Ac×ΔTc 
     Qe=K×Ae×ΔTe 
     K: heat-transfer coefficient (frontal area reference) 
     Ac: frontal area of indoor heat exchanger  41   
     Ae: frontal area of outdoor heat exchanger  23   
     ΔTc: difference between condensation temperature and heating reference indoor temperature 
     ΔTe: difference between heating reference outdoor temperature and evaporation temperature 
     If the condensation temperature is approximately 43° C. and the heating reference indoor temperature is 20° C., ΔTc=23 is satisfied. If the heating reference outdoor temperature is 6° C. and the evaporation temperature is approximately 0° C., ΔTe=6 is satisfied. 
     Therefore, the following expressions are satisfied. 
         Qc=K×Ac× 23  [1]
 
         Qe=K×Ae× 6  [2]
 
     When the coefficient of performance (COP) during heating operation is 5, the following expression is satisfied. 
         Qc/Qe≈  5/4  [3]
 
     From the expressions [1], [2], and [3], the following expression is satisfied. 
         Ac/Ae≈ ⅓
 
     This is the upper limit of the volume ratio S. 
     Therefore, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger in the refrigerant circuit that uses R32 is 300%. 
     The liquid-gas average density ρ of an HFO-based refrigerant in a condensation region is larger than that of R32. Here, the liquid-gas average density ρ is defined as an average value of a saturated liquid density and a saturated gas density at a temperature of 45° C. 
     When the liquid-gas average density of R32 is ρR32, and the liquid-gas average density of an HFO-based refrigerant is ρHFO, a required volume of the outdoor heat exchanger  23  is [ρR32/ρHFO] times the volume of the outdoor heat exchanger that uses R32. 
     Therefore, based on the liquid-gas average density of HFO-based refrigerants of several types, the upper limit of the volume ratio S corresponding to each of the HFO-based refrigerants can be calculated. 
       FIG. 5A  is a table showing a relationship between the liquid-gas average density ρ of an HFO-based refrigerant and the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  when the outdoor heat exchanger  23  is a one-row heat exchanger. In  FIG. 5A , a refrigerant mixture A, a refrigerant mixture B, R1123, R1234yf, and R1234ze are selected as HFO-based refrigerants for comparison with the liquid-gas average density of a HFC-based refrigerant of R32. Then, the liquid-gas average density, ρR32/ρHFO, and the upper limit of the volume ratio S for each refrigerant are obtained and described. 
     Both of the refrigerant mixture A and the refrigerant mixture B are refrigerant mixtures of R1132(E), R1123, and R1234yf but are different from each other in a three-component composition diagram. However, details will not be described in the present application. 
     (4-3-2) Type B1 
     As a premise, the indoor heat exchanger  41  is of the cross-fin type, the outdoor heat exchanger  23  is of the stack type, the outdoor heat exchanger  23  is a one-row heat exchanger, the indoor heat exchanger  41  is a two-row heat exchanger, and the operating mode is heating operation. 
     As described for the type A1, when both the indoor heat exchanger and the outdoor heat exchanger are of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 300%. 
     In general, the volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger is α (α&lt;1; or, α=0.6). Therefore, when the indoor heat exchanger is of the cross-fin type and the outdoor heat exchanger is of the stack type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 300×α%. 
     Therefore, the upper limit of the volume ratio S corresponding to an HFO-based refrigerant can be calculated by multiplying 300×α% by [ρR32/ρHFO]. 
     In  FIG. 5A , the upper limit of the volume ratio S in the type B1 is obtained for each of the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234ze. The calculations are based on that α=0.6. 
     (4-3-3) Type C1 
     As a premise, the indoor heat exchanger  41  is of the stack type, the outdoor heat exchanger  23  is of the cross-fin type, the outdoor heat exchanger  23  is a one-row heat exchanger, the indoor heat exchanger  41  is a two-row heat exchanger, and the operating mode is heating operation. 
     As described for the type A1, when both the indoor heat exchanger and the outdoor heat exchanger are of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 300%. 
     In general, the volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger is α (α&lt;1; or, α=0.6). Therefore, when the indoor heat exchanger is of the stack type and the outdoor heat exchanger is of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 300/a %. 
     Therefore, the upper limit of the volume ratio S corresponding to an HFO-based refrigerant can be calculated by multiplying 300/α % by [ρR32/ρHFO]. 
     In  FIG. 5A , the upper limit of the volume ratio S in the type C1 is obtained for each of the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234ze. The calculations are based on that α=0.6. 
       FIG. 5B  is a graph showing a relationship between the liquid-gas average density ρ of an HFO-based refrigerant and the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  when the outdoor heat exchanger  23  is a one-row heat exchanger. In  FIG. 5B , the horizontal axis of the graph indicates a liquid-gas average density [kg/m 3 ]. The vertical axis of the graph indicates the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41 . 
     A curve SA1 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type A1. The curve SA1 is expressed by “1.0112E−03×ρ{circumflex over ( )}2−1.5836E+00×ρ+8.2472E+02”, and a relationship to the volume ratio S is as follows. 
       100≤ S≤ 1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2472 E+ 02
 
     Similarly, a curve SB1 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type B1. 
     The curve SB1 is expressed by “(1.0112E−03×ρ{circumflex over ( )}2−1.5836E+00×ρ+8.2427E+02)× α”, and a relationship to the volume ratio S is thus as follows. 
       100×α≤ S ≤(1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2427 E+ 02)×α
 
     Similarly, a curve SC1 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type C1. 
     The curve SC1 is expressed by “(1.0112E−03×ρ{circumflex over ( )}2−1.5836E+00×ρ+8.2472E+02)/α”, and a relationship to the volume ratio S is thus as follows. 
       100/α≤ S ≤(1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2472 E+ 02)/α
 
     (4-4) Upper Limits of Volume Ratios S of Types A2, B2, and C2 
     (4-4-1) Type A2 
     As a premise, both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type, the outdoor heat exchanger  23  is a two-row heat exchanger, the indoor heat exchanger  41  is a three-row heat exchanger, and the operating mode is heating operation. 
     In such a case, the frontal area Ae of the outdoor heat exchanger is two times the frontal area Ae in the type A1, and the frontal area Ac of the indoor heat exchanger is 1.5 times the frontal area Ac in the type A1. Since “Ac/Ae≈⅓” is satisfied in the type A1, Ac/Ae≈(⅓)×(1.5/2)=¼ is satisfied in the type A2. 
     Therefore, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger in the refrigerant circuit that uses R32 is 400%. 
     The liquid-gas average density ρ of an HFO-based refrigerant in a condensation region is larger than that of R32. Here, the liquid-gas average density ρ is defined as an average value of a saturated liquid density and a saturated gas density at a temperature of 45° C. 
     When the liquid-gas average density of R32 is ρR32, and the liquid-gas average density of an HFO-based refrigerant is ρHFO, a required volume of the outdoor heat exchanger is [ρR32/ρHFO] times the volume of the outdoor heat exchanger that uses R32. 
     Therefore, based on the liquid-gas average density of HFO-based refrigerants of several types, the upper limit of the volume ratio S corresponding to each of the HFO-based refrigerants can be calculated. 
       FIG. 6A  is a table showing a relationship between the liquid-gas average density ρ of an HFO-based refrigerant and the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  when the outdoor heat exchanger  23  is a two-row heat exchanger. In  FIG. 6A , the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234ze are selected as HFO-based refrigerants for comparison with the liquid-gas average density of a HFC-based refrigerant of R32. Then, the liquid-gas average density, ρR32/ρHFO, and the upper limit of the volume ratio S for each refrigerant are obtained and described. 
     Both of the refrigerant mixture A and the refrigerant mixture B are refrigerant mixtures of R1132(E), R1123, and R1234yf but are different from each other in a three-component composition diagram. However, details will not be described in the present application. 
     (4-4-2) Type B2 
     As a premise, the indoor heat exchanger  41  is of the cross-fin type, the outdoor heat exchanger  23  is of the stack type, the outdoor heat exchanger  23  is a two-row heat exchanger, the indoor heat exchanger  41  is a three-row heat exchanger, and the operating mode is heating operation. 
     As described for type A2, when both the indoor heat exchanger and the outdoor heat exchanger are of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 400%. 
     In general, the volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger is α (α&lt;1; or, α=0.6). Therefore, when the indoor heat exchanger is of the cross-fin type and the outdoor heat exchanger is of the stack type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 400×α%. 
     Therefore, the upper limit of the volume ratio S corresponding to an HFO-based refrigerant can be calculated by multiplying 400×α % by [ρR32/ρHFO]. 
     In  FIG. 6A , the upper limit of the volume ratio S in the type B1 is obtained for each of the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234ze. The calculations are based on that α=0.6. 
     (4-4-3) Type C2 
     As a premise, the indoor heat exchanger  41  is of the stack type, the outdoor heat exchanger  23  is of the cross-fin type, the outdoor heat exchanger  23  is a two-row heat exchanger, the indoor heat exchanger  41  is a three-row heat exchanger, and the operating mode is heating operation. 
     As described for type A2, when both the indoor heat exchanger and the outdoor heat exchanger are of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 400%. 
     In general, the volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger is α (α&lt;1; or, α=0.6). Therefore, when the indoor heat exchanger is of the stack type and the outdoor heat exchanger is of the cross-fin type in the refrigerant circuit that uses R32, the upper limit of the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger is 400/α %. 
     Therefore, the upper limit of the volume ratio S corresponding to an HFO-based refrigerant can be calculated by multiplying 400/α % by [ρR32/ρHFO]. 
     In  FIG. 6A , the upper limit of the volume ratio S in the type C1 is obtained for each of the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234ze. The calculations are based on that α=0.6. 
       FIG. 6B  is a graph showing a relationship between the liquid-gas average density ρ of an HFO-based refrigerant and the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41  when the outdoor heat exchanger  23  is a two-row heat exchanger. In  FIG. 6B , the horizontal axis of the graph indicates the liquid-gas average density ρ [kg/m 3 ]. The vertical axis of the graph indicates the upper limit of the volume ratio S of the outdoor heat exchanger  23  to the indoor heat exchanger  41 . 
     A curve SA2 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type A2. The curve SA2 is expressed by “1.3483E−03×ρ{circumflex over ( )}2−2.1115E+00×ρ+1.0996E+03”, and a relationship to the volume ratio S is as follows. 
       130≤ S≤ 1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03
 
     Similarly, a curve SB2 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type B2. 
     The curve SB1 is expressed by (1.3483E−03×ρ{circumflex over ( )}2−2.1115E+00×ρ+1.0996E+03)×α, and a relationship to the volume ratio S is thus as follows. 
       130×α≤ S ≤(1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03)×α
 
     Similarly, a curve SC2 is a curve that is obtained by plotting the upper limits of the volume ratios S of R32, the refrigerant mixture A, the refrigerant mixture B, R1123, R1234yf, and R1234z for the type C2. 
     The curve SC1 is expressed by (1.3483E−03×ρ{circumflex over ( )}2-2.1115E+00×ρ+1.0996E+03)/α, and a relationship to the volume ratio S is thus as follows. 
       130/α≤ S ≤(1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03)/α
 
     (5) Features 
     (5-1) 
     Regarding a refrigeration apparatus that uses an HFO refrigerant or a refrigerant mixture including an HFO refrigerant, when a model in which the outdoor heat exchanger  23  is a one-row heat exchanger, the indoor heat exchanger  41  is a two-row heat exchanger, the operating mode is heating operation, and both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type is the type A1, a model in which the indoor heat exchanger  41  is of the cross-fin type and the outdoor heat exchanger  23  is of the stack type is the type B1, and a model in which the indoor heat exchanger  41  is of the stack type and the outdoor heat exchanger  23  is of the cross-fin type is the type C1, 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type A1 is 
       100≤ S≤ 1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2472 E+ 02,
 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type B1 is 
       100×α≤ S ≤(1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2427 E+ 02)×α, and
 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type C1 is 
       100/α≤ S ≤(1.0112 E −03×ρ{circumflex over ( )}2−1.5836 E+ 00×ρ+8.2472 E+ 02)/α.
 
     Note that a is a volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger when the cross-fin-type heat exchanger and the stack-type heat exchanger have identical heat exchange performance, and p [kg/m{circumflex over ( )}3] is the average density of a saturated liquid gas at a condensation temperature of 45° C. 
     In this refrigeration apparatus, due to the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger being set to a value that satisfies the aforementioned relational expression, excess and insufficiency of the refrigerant amount during cooling operation and heating operation are suppressed. 
     (5-2) 
     Regarding a refrigeration apparatus in which an HFO refrigerant or a refrigerant mixture including an HFO refrigerant flows, when a model in which the outdoor heat exchanger  23  is a two-row heat exchanger, the indoor heat exchanger  41  is a three-row heat exchanger, the operating mode is heating operation, and both the indoor heat exchanger  41  and the outdoor heat exchanger  23  are of the cross-fin type or the stack type is the type A2, a model in which the indoor heat exchanger  41  is of the cross-fin type and the outdoor heat exchanger  23  is of the stack type is the type B2, and a model in which the indoor heat exchanger  41  is of the stack type and the outdoor heat exchanger  23  is of the cross-fin type is the type C2, 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type A2 is 
       130≤ S≤ 1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03,
 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type B2 is 
       130×α≤ S ≤(1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03)×α, and
 
     a relationship between the liquid-gas average density ρ and the volume ratio S in the type C2 is 
       130/α≤ S ≤(1.3483 E −03×ρ{circumflex over ( )}2−2.1115 E+ 00×ρ+1.0996 E+ 03)/α.
 
     Note that a is a volume ratio of a stack-type heat exchanger to a cross-fin-type heat exchanger when the cross-fin-type heat exchanger and the stack-type heat exchanger have identical heat exchange performance, and ρ [kg/m{circumflex over ( )}3] is the average density of a saturated liquid gas at a condensation temperature of 45° C. 
     In this refrigeration apparatus, due to the volume ratio S of the outdoor heat exchanger to the indoor heat exchanger being set to a value that satisfies the aforementioned relational expression, excess and insufficiency of the refrigerant amount during cooling operation and heating operation are suppressed. 
     (5-3) 
     The expression of 476.1&lt;ρ is satisfied. 
     (5-4) 
     An HFO refrigerant is any of R1132(E), R1123, R1234yf, and R1234ze. 
     (6) Modifications 
       FIG. 7  is a schematic diagram of an air conditioning apparatus as a refrigeration apparatus according to a modification of the present disclosure. In  FIG. 7 , a difference from one or more embodiments in  FIG. 1  is a feature of including a receiver  24  that is connected between the outdoor heat exchanger  23  of the refrigerant circuit  10  and the expansion valve  26 ; a bypass pipe  30  that connects the receiver  24  and the suction pipe  31  to each other; and a flow-rate regulating valve  25  that is connected to an intermediate portion of the bypass pipe  30 . Features other than the aforementioned feature are identical to those in one or more embodiments in  FIG. 1 . Thus, description thereof is omitted. 
     The receiver  24  is a container capable of storing an excess refrigerant. Generally, in a refrigerant circuit of an air conditioning apparatus, a refrigerant amount suitable during cooling operation differs from a refrigerant amount suitable during heating operation. Therefore, a proper volume of an outdoor heat exchanger that functions as a condenser during cooling operation differs from a proper volume of an indoor heat exchanger that functions as a condenser during heating operation. 
     Normally, the volume of the outdoor heat exchanger is larger than the volume of the indoor heat exchanger, and a refrigerant that is not possible to be accommodated in the indoor heat exchanger during heating operation is temporarily stored in the accumulator  29  or the like. An excess liquid refrigerant that is not possible to be accommodated in the accumulator  29  is accommodated in the receiver  24 . 
     During heating operation, the refrigerant immediately before entering the receiver  24  includes a gas component generated when passing through the expansion valve  26 . After entering the receiver  24 , the refrigerant is separated into a liquid refrigerant and a gas refrigerant, and the liquid refrigerant and the gas refrigerant are stored on the lower side and the upper side, respectively. 
     The gas refrigerant separated in the receiver  24  flows into the suction pipe  31  through the bypass pipe  30 . The liquid refrigerant separated in the receiver  24  flows into the outdoor heat exchanger  23 . The flow-rate regulating valve  25  is connected to an intermediate portion of the bypass pipe  30 . In the present modification, the flow-rate regulating valve  25  is an electric expansion valve. 
     When the indoor heat exchanger  41  is of the cross-fin type and the outdoor heat exchanger  23  is of the stack type, the volume of the outdoor heat exchanger  23  is smaller than the volume of the indoor heat exchanger, and a refrigerant (excess refrigerant) that is not possible to be accommodated in the outdoor heat exchanger  23  during cooling operation is generated. The amount of the refrigerant is more than an amount that can be stored in the accumulator  29  or the like. 
     In such a case, the receiver  24  accommodates an excess liquid refrigerant that is not possible to be accommodated in the outdoor heat exchanger  23  during cooling operation. 
     In the present modification, the volume of the outdoor heat exchanger  23  is considered a volume including the volume of the receiver  24  since the receiver  24  is provided between the outdoor heat exchanger  23  and the expansion valve  26 . 
     (7) Definition of Terms 
     The volume of a heat exchanger denotes an internal volume that is from the refrigerant inlet to the refrigerant outlet of the heat exchanger itself and that can be filled with a refrigerant. 
     When the receiver  24  for storing a refrigerant is included, as in the aforementioned modification, the volume of the outdoor heat exchanger  23  denotes an internal volume that is from the refrigerant inlet of the outdoor heat exchanger  23  itself to the refrigerant outlet of the receiver  24  itself and that can be filled with a refrigerant. 
     Embodiments of the present disclosure have been described above; however, it should be understood that various changes in the forms and details are possible without departing from the gist and the scope of the present disclosure described in the claims. 
     The present disclosure is widely applicable to a refrigeration apparatus (for example, a low-temperature apparatus) capable of performing cooling operation and heating operation. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present disclosure. Accordingly, the scope of the disclosure should be limited only by the attached claims. 
     REFERENCES SIGNS LIST 
     
         
         
           
               1  air conditioning apparatus (refrigeration apparatus) 
               21  compressor 
               23  outdoor heat exchanger 
               24  receiver (high-pressure receiver) 
               26  expansion valve (expansion mechanism) 
               41  indoor heat exchanger 
           
         
       
    
     PATENT LITERATURE 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2012-077983