Refrigeration device and container for transportation

Abstract

A refrigerant circuit (30) of this refrigeration device (10) includes a defrosting pipe (32) and a fourth expansion valve (EV4). The refrigeration device (10) performs a defrosting operation for melting frost adhering to an internal heat exchanger (57). In the defrosting operation, a refrigerant discharged by a compressor (50) flows through the defrosting pipe (32), is decompressed when passing through the fourth expansion valve (EV4), and then flows into the internal heat exchanger (57). In the defrosting operation, a controller (90) controls the opening degree of the fourth expansion valve (EV4) on the basis of one or both of the pressure and the temperature of the refrigerant discharged by the compressor (50).

Description

Refrigeration units and shipping containers

[0001] The present disclosure relates to refrigeration devices and shipping containers.

[0002] Patent Document 1 discloses a heat pump device that operates in a refrigeration cycle. The refrigerant circuit of this heat pump device includes a high-pressure side defrosting circuit. This high-pressure side defrosting circuit is a passage through which refrigerant flows, bypassing the radiant heat exchanger and the pressure reducing means. In a defrosting operation to melt frost adhering to the atmospheric heat exchanger, refrigerant discharged from the compressor passes through the high-pressure side defrosting circuit and is supplied to the atmospheric heat exchanger. The refrigerant warms and melts the frost adhering to the atmospheric heat exchanger.

[0003] In the defrosting operation performed by the heat pump apparatus of Patent Document 1, the refrigerant passes through a high-pressure defrosting circuit and circulates between the compressor and the atmospheric heat exchanger. This defrosting operation utilizes heat imparted to the refrigerant in the compressor to melt frost that has adhered to the atmospheric heat exchanger.

[0004] Japanese Patent Application Laid-Open No. 2004-347185

[0005] In the defrosting operation of the heat pump apparatus of Patent Document 1, the refrigerant discharged from the compressor flows into the atmospheric heat exchanger without being substantially depressurized. As a result, the difference between the pressure of the refrigerant drawn into the compressor and the pressure of the refrigerant discharged from the compressor becomes small, which may result in a decrease in the amount of heat imparted to the refrigerant by the compressor. If the amount of heat imparted to the refrigerant by the compressor during the defrosting operation is small, the amount of heat available to melt the frost on the atmospheric heat exchanger decreases, which may result in a longer defrosting time.

[0006] An object of the present disclosure is to reduce the time required to defrost a heat exchanger in a refrigeration device that performs a refrigeration cycle.

[0007] A first aspect of the present disclosure is a refrigeration system (10) for air-conditioning a target space (5), comprising a refrigerant circuit (30) having a compressor (50), a heat source side heat exchanger (56), an expansion valve (65), and a utilization side heat exchanger (57), wherein the refrigerant circuit (30) has a bypass passage (32) for sending refrigerant discharged from the compressor (50) to the utilization side heat exchanger (57) bypassing the heat source side heat exchanger (56) and the expansion valve (65), and a pressure reducing valve (EV4) with a variable opening that reduces the pressure of the refrigerant flowing through the bypass passage (32), and the refrigeration system (10) is configured such that the heat source side heat exchanger (56) The refrigeration cycle includes a cooling operation in which the compressor (50) functions as a radiator and the utilization side heat exchanger (57) functions as an evaporator, and the utilization side heat exchanger (57) functions as an evaporator, and the refrigeration cycle includes a cooling operation in which air cooled in the utilization side heat exchanger (57) is blown into the target space (5), and a defrosting operation in which frost adhering to the utilization side heat exchanger (57) is melted by supplying the refrigerant discharged from the compressor (50) to the utilization side heat exchanger (57) through the bypass passage (32), and a controller (90) that controls the opening of the pressure reducing valve (EV4) based on either or both of the pressure and temperature of the refrigerant discharged from the compressor (50) in the defrosting operation.

[0008] In a first aspect, the refrigeration system (10) performs a defrosting operation. During the defrosting operation, the controller (90) controls the opening degree of the pressure reducing valve (EV4). During the defrosting operation, the refrigerant supplied to the utilization side heat exchanger (57) through the bypass passage (32) is reduced in pressure when passing through the pressure reducing valve (EV4). Therefore, compared to when the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) without being reduced in pressure, the difference between the pressure of the refrigerant drawn into the compressor (50) (suction pressure) and the pressure of the refrigerant discharged from the compressor (50) (discharge pressure) becomes larger. The larger the difference between the suction pressure and the discharge pressure, the greater the amount of heat imparted to the refrigerant during the process of compression by the compressor (50). Therefore, according to this embodiment, the amount of heat available for melting the frost adhering to the utilization side heat exchanger (57) is increased and the time required to defrost the utilization side heat exchanger (57) is shortened, compared to when the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) without being reduced in pressure.

[0009] A second aspect of the present disclosure is related to the first aspect, wherein the refrigerant circuit (30) has a receiver (62) arranged between the heat source side heat exchanger (56) and the utilization side heat exchanger (57), and the refrigeration device (10) performs, during the defrosting operation, a normal operation of circulating refrigerant between the compressor (50) and the utilization side heat exchanger (57) while blocking the inflow and outflow of refrigerant to and from the receiver (62), an outflow operation of causing refrigerant to flow out of the receiver (62) to reduce the amount of refrigerant stored in the receiver (62), and an inflow operation of causing refrigerant to flow into the receiver (62) to increase the amount of refrigerant stored in the receiver (62).

[0010] The refrigeration system (10) of the second aspect performs a normal operation, an outflow operation, and an inflow operation during defrosting operation. The outflow operation and the inflow operation are operations for adjusting the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) during defrosting operation. In the outflow operation, the amount of refrigerant stored in the receiver (62) decreases, and the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) increases. In the inflow operation, the amount of refrigerant stored in the receiver (62) increases, and the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) decreases.

[0011] A third aspect of the present disclosure is the second aspect, wherein the outflow operation is started when the temperature of the refrigerant discharged from the compressor (50) becomes higher than a reference temperature during the normal operation.

[0012] If the temperature of the refrigerant discharged from the compressor (50) becomes high during normal operation, it can be assumed that the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) is insufficient. Therefore, in the refrigeration system (10) of the third aspect, if the temperature of the refrigerant discharged from the compressor (50) becomes higher than a reference temperature during normal operation, the refrigeration system (10) ends the normal operation and starts the outflow operation.

[0013] In a fourth aspect of the present disclosure, in the second or third aspect, when the degree of superheat of the refrigerant flowing out of the utilization side heat exchanger (57) becomes lower than a reference degree of superheat during the outflow operation, the normal operation is started.

[0014] When the degree of superheat of the refrigerant flowing out of the utilization side heat exchanger (57) becomes low during the outflow operation, it can be assumed that the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) is appropriate. Therefore, the refrigeration system (10) of the fourth aspect ends the outflow operation and starts normal operation when the degree of superheat of the refrigerant flowing out of the utilization side heat exchanger (57) becomes lower than the reference degree of superheat during the outflow operation.

[0015] A fifth aspect of the present disclosure is any one of the second to fourth aspects, wherein the outflow operation includes a gas outflow operation for causing gas refrigerant to flow out of the receiver (62) and a liquid outflow operation for causing liquid refrigerant to flow out of the receiver (62).

[0016] The refrigeration system (10) of the fifth aspect performs a gas outflow operation or a liquid outflow operation as the outflow operation.

[0017] A sixth aspect of the present disclosure is any one of the second to fourth aspects, wherein in the outflow operation, the compressor (50) sucks the refrigerant that has flowed out of the receiver (62).

[0018] In the outflow operation performed by the refrigeration system (10) of the sixth aspect, the refrigerant flowing out from the receiver (62) is sucked into the compressor (50). As a result, the amount of refrigerant circulating between the compressor (50) and the utilization side heat exchanger (57) during the defrosting operation increases.

[0019] A seventh aspect of the present disclosure is the sixth aspect, wherein the outflow operation includes a first outflow operation in which the compressor (50) draws refrigerant from both the receiver (62) and the utilization side heat exchanger (57), and a second outflow operation in which the compressor (50) draws refrigerant from the receiver (62) but does not draw refrigerant from the utilization side heat exchanger (57).

[0020] The refrigeration system (10) of the seventh aspect performs a first outflow operation or a second outflow operation as an outflow operation. In the first outflow operation, the compressor (50) draws in the refrigerant flowing out of the receiver (62) and the refrigerant flowing out of the utilization side heat exchanger (57). In the second outflow operation, the compressor (50) draws in the refrigerant flowing out of the receiver (62) but does not draw in the refrigerant flowing out of the utilization side heat exchanger (57).

[0021] An eighth aspect of the present disclosure is any one of the second to seventh aspects, wherein when the pressure of the refrigerant discharged from the compressor (50) becomes higher than a reference pressure during the normal operation, the inflow operation is started.

[0022] If the pressure of the refrigerant discharged from the compressor (50) increases during normal operation, it can be assumed that the amount of refrigerant circulating between the compressor (50) and the utilization-side heat exchanger (57) is excessive. Therefore, the refrigeration system (10) of the eighth aspect ends the normal operation and starts the inflow operation when the pressure of the refrigerant discharged from the compressor (50) increases above the reference pressure during normal operation.

[0023] A ninth aspect of the present disclosure is any one of the second to eighth aspects, wherein, in the inflow operation, a portion of the refrigerant discharged from the compressor (50) passes through the heat source side heat exchanger (56) and flows into the receiver (62), and the remainder of the refrigerant discharged from the compressor (50) passes through the bypass passage (32) and is supplied to the utilization side heat exchanger (57).

[0024] In the inflow operation performed by the refrigeration system (10) of the ninth aspect, a portion of the refrigerant discharged from the compressor (50) flows into the receiver (62), increasing the amount of refrigerant stored in the receiver (62). In addition, in this inflow operation, the remainder of the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32), melting frost adhering to the utilization side heat exchanger (57).

[0025] A tenth aspect of the present disclosure is any one of the first to ninth aspects, wherein the refrigerant circuit (30) has a path change mechanism (67) that changes a flow path of the refrigerant in the refrigerant circuit (30), and the controller (90) switches between the cooling operation and the defrosting operation by controlling the path change mechanism (67).

[0026] In the tenth aspect, the controller (90) controls the path change mechanism (67). The path change mechanism (67) changes the flow path of the refrigerant, thereby switching between the cooling operation and the defrosting operation.

[0027] An eleventh aspect of the present disclosure is any one of the second to ninth aspects, wherein the refrigerant circuit (30) has a path changing mechanism (67) that changes a flow path of the refrigerant in the refrigerant circuit (30), and the controller (90) controls the path changing mechanism (67) to switch between the cooling operation and the defrosting operation, and to switch between the normal operation, the outflow operation, and the inflow operation in the defrosting operation.

[0028] In an eleventh aspect, the controller (90) controls the path change mechanism (67). The path change mechanism (67) changes the refrigerant flow path, thereby switching between a cooling operation and a defrosting operation. Furthermore, during the defrosting operation, the path change mechanism (67) changes the refrigerant flow path, thereby switching between a normal operation, an outflow operation, and an inflow operation.

[0029] A twelfth aspect of the present disclosure is the tenth or eleventh aspect, wherein the path changing mechanism (67) includes a plurality of control valves controlled by the controller (90).

[0030] In a twelfth aspect, the controller (90) controls a plurality of control valves included in the path change mechanism (67).

[0031] A thirteenth aspect of the present disclosure is any one of the first to twelfth aspects, wherein the pressure reducing valve includes a first pressure reducing valve (EV4-1) and a second pressure reducing valve (EV4-2) connected in parallel.

[0032] In the bypass passage (32) of the refrigeration system (10) of the thirteenth aspect, the first pressure reducing valve (EV4-1) and the second pressure reducing valve (EV4-2) are connected in parallel.

[0033] A fourteenth aspect of the present disclosure is any one of the first to thirteenth aspects, wherein a heating operation is performed in which the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32), and the air heated in the utilization side heat exchanger (57) is blown into the target space (5).

[0034] In the fourteenth aspect, the refrigeration system (10) performs a heating operation. In the heating operation, the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32) and exchanges heat with air passing through the utilization side heat exchanger (57). The air heated in the utilization side heat exchanger (57) is blown into the target space (5).

[0035] A fifteenth aspect of the present disclosure is any one of the first to fourteenth aspects, wherein the refrigerant circuit (30) includes a reheat heat exchanger (58) that is arranged upstream of the pressure reducing valve (EV4) in the bypass passage (32) and exchanges heat between the air that has passed through the utilization side heat exchanger (57) and the refrigerant.

[0036] In the fifteenth aspect, the refrigerant circuit (30) is provided with a reheat heat exchanger (58). The reheat heat exchanger (58) exchanges heat between the refrigerant supplied through the bypass passage (32) and air that has passed through the utilization side heat exchanger (57). The refrigerant that has passed through the reheat heat exchanger (58) passes through a pressure reducing valve (EV4) and is then sent to the utilization side heat exchanger (57).

[0037] A sixteenth aspect of the present disclosure is any one of the first to fifteenth aspects, wherein the refrigerant circuit (30) is filled with carbon dioxide as the refrigerant.

[0038] The refrigerant circuit (30) of the sixteenth aspect performs a refrigeration cycle by circulating carbon dioxide filled as a refrigerant.

[0039] A seventeenth aspect of the present disclosure is a transport container (1) including the refrigeration system (10) of any one of the first to sixteenth aspects and a container body (2) that forms the target space (5) that is air-conditioned by the refrigeration system (10).

[0040] A transport container (1) according to a seventeenth aspect includes a refrigeration unit (10) and a container body (2). The container body (2) defines a target space (5). The refrigeration unit (10) conditions the air in the target space (5).

[0041] FIG. 1 is a schematic perspective view of a refrigeration apparatus according to a first embodiment. FIG. 2 is a schematic cross-sectional view of a transport container according to the first embodiment. FIG. 3 is a piping diagram illustrating the configuration of a refrigeration apparatus according to the first embodiment. FIG. 4 is a block diagram illustrating the configuration of a controller according to the first embodiment. FIG. 5 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a cooling operation. FIG. 6 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a dehumidifying operation. FIG. 7 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a heating operation and a normal defrosting operation. FIG. 8 is a Mollier diagram (pressure-enthalpy diagram) illustrating changes in the state of refrigerant in a refrigerant circuit during a defrosting operation. FIG. 9 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in an inflow operation. FIG. 10 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a gas outflow operation of the first outflow operation. FIG. 11 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a liquid outflow operation of the first outflow operation. FIG. 12 is a piping diagram corresponding to FIG. 3, showing the flow of refrigerant in a gas outflow operation of the second outflow operation. Fig. 13 is a state transition diagram showing the operation of the controller in defrosting operation. Fig. 14 is a piping diagram showing the configuration of a refrigeration apparatus of embodiment 2. Fig. 15 is a piping diagram showing the configuration of a refrigeration apparatus of embodiment 3. Fig. 16 is a flow diagram showing the operation performed by the controller during defrosting operation of a refrigeration apparatus of embodiment 4. Fig. 17 is a piping diagram showing the configuration of a refrigeration apparatus of a first modified example of another embodiment. Fig. 18 is a piping diagram showing the configuration of a refrigeration apparatus of a second modified example of another embodiment.

[0042] First Embodiment A first embodiment will be described. This embodiment is a transport container (1) equipped with a refrigeration unit (10).

[0043] 1, the transport container 1 includes a container body 2 and a refrigeration unit 10. The transport container 1 is a reefer container capable of controlling the temperature inside.

[0044] The transport container (1) of this embodiment is mainly used for marine transportation. This transport container (1) is loaded onto a ship or the like for transport. However, the use of the transport container (1) is not limited to marine transportation. The transport container (1) may also be used for land transportation. In this case, the transport container (1) is transported by automobile such as a truck, or by rail.

[0045] - Container Body - As shown in Figure 2, the container body (2) is formed in the shape of a hollow box. The container body (2) is formed horizontally long. An opening is formed at one longitudinal end of the container body (2). The opening of the container body (2) is closed by a refrigeration unit (10). The container body (2) forms an interior space (5) for storing cargo. The interior space (5) is a target space that is air-conditioned by the refrigeration unit.

[0046] 2, the refrigeration unit (10) is attached to the opening of the container body (2). The refrigeration unit (10) of this embodiment is a refrigeration unit for transport. The refrigeration unit (10) includes a casing (11), a refrigerant circuit (30), and a controller (80). The refrigeration unit (10) adjusts the temperature of the air in the interior space (5) (interior air).

[0047] <Casing> The casing (11) includes a partition wall (12) and a partition plate (15).

[0048] An internal flow path (20) is formed inside the partition wall (12). An external chamber (23) is formed outside the partition wall (12). The internal flow path (20) and the external chamber (23) are separated by the partition wall (12).

[0049] The partition wall (12) includes an outer wall (13) and an inner wall (14). The outer wall (13) is located outside the container body (2). The inner wall (14) is located inside the container body (2).

[0050] The exterior wall (13) closes the opening of the container body (2). The exterior wall (13) is attached to the periphery of the opening of the container body (2). The lower part of the exterior wall (13) bulges outward toward the inside of the container body (2). The exterior chamber (23) is formed by the lower part of the exterior wall (13).

[0051] The interior wall (14) faces the exterior wall (13). The interior wall (14) has a shape that conforms to the exterior wall (13). The interior wall (14) is disposed at a distance from the exterior wall (13). A heat insulating material (16) is provided between the interior wall (14) and the exterior wall (13).

[0052] The partition plate (15) is disposed inside the container body (2) relative to the interior wall (14). An internal flow path (20) is formed between the partition wall (12) and the partition plate (15). An air suction port (21) is formed between the upper end of the partition plate (15) and the top plate of the container body (2). An air outlet (22) is formed between the lower end of the partition plate (15) and the lower end of the partition wall (12). The internal flow path (20) is formed from the air suction port (21) to the air outlet (22).

[0053] <Refrigerant Circuit> The refrigerant circuit (30) is a closed circuit filled with a refrigerant. The refrigerant circuit (30) circulates the refrigerant to perform a vapor compression refrigeration cycle. The refrigerant circuit (30) includes an external heat exchanger (56), an internal heat exchanger (57), and a reheat heat exchanger (58). The refrigerant circuit (30) will be described in detail later.

[0054] Each of the external heat exchanger (56), the internal heat exchanger (57), and the reheat heat exchanger (58) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and air.

[0055] The external heat exchanger (56) is disposed in an upper portion of the external chamber (23). The external heat exchanger (56) is a heat source side heat exchanger that exchanges heat between the refrigerant and the external air. The external heat exchanger (56) has a generally rectangular cylindrical shape.

[0056] The internal heat exchanger (57) is arranged in the internal flow path (20). The internal heat exchanger (57) is a utilization side heat exchanger that exchanges heat between the refrigerant and the internal air.

[0057] The reheat heat exchanger (58) is arranged downstream of the internal heat exchanger (57) in the internal flow path (20). The reheat heat exchanger (58) is a heat exchanger that exchanges heat between the refrigerant and the internal air.

[0058] Although not shown, a drain pan is provided below the internal heat exchanger (57). The drain pan receives drain water generated in the internal heat exchanger (57). The drain water that falls into the drain pan is discharged to the outside of the container.

[0059] <External Fan> The refrigeration system (10) includes an external fan (26). The external fan (26) is a propeller fan. The external fan (26) is disposed in the external chamber (23). The external fan (26) is disposed inside an external heat exchanger (56) formed in a cylindrical shape. The external fan (26) sends external air to the external heat exchanger (56).

[0060] <In-compartment fan> The refrigeration system (10) includes an internal fan (27). The internal fan (27) is a propeller fan. The internal fan (27) is disposed in the internal flow path (20). The internal fan (27) is disposed above the internal heat exchanger (57). The internal fan (27) sends internal air to the internal heat exchanger (57).

[0061] <Sensors> The refrigeration system (10) includes a first air temperature sensor (86), a second air temperature sensor (87), and a humidity sensor (88).

[0062] The first air temperature sensor (86) is arranged in the internal flow path (20) upstream of the internal fan (27). The first air temperature sensor (86) measures the temperature of the air that has flowed into the internal flow path (20) through the air inlet (21).

[0063] The second air temperature sensor (87) and the humidity sensor (88) are arranged downstream of the reheat heat exchanger (58) in the internal flow path (20). The second air temperature sensor (87) measures the temperature of the air that has passed through the reheat heat exchanger (58). The humidity sensor (88) measures the relative humidity of the air that has passed through the reheat heat exchanger (58).

[0064] <Electrical Component Box> As shown in Fig. 1 , the refrigeration system (10) has an electrical component box (28). The electrical component box (28) is disposed in an upper portion of the exterior chamber (23). Electrical components such as an inverter board and a control board are accommodated inside the electrical component box (28).

[0065] Refrigerant Circuit As shown in Fig. 3, the refrigerant circuit (30) is a closed circuit filled with a refrigerant. The refrigerant filled in the refrigerant circuit (30) of this embodiment is carbon dioxide.

[0066] The refrigerant circuit (30) includes a main circuit (31), a defrosting pipe (32), and a reheating pipe (33). The refrigerant circuit (30) also includes a gas side connecting pipe (41), a liquid side connecting pipe (42), an intermediate connecting pipe (43), a low stage connecting pipe (44), and a high stage connecting pipe (45).

[0067] <Main Circuit> The main circuit (31) includes a low-stage compressor (51), a high-stage compressor (52), an external heat exchanger (56), a receiver (62), and an internal heat exchanger (57). In the main circuit (31), the low-stage compressor (51), the high-stage compressor (52), the external heat exchanger (56), the receiver (62), and the internal heat exchanger (57) are connected in this order by pipes.

[0068] The discharge pipe of the low-stage compressor (51) is connected to the suction pipe of the high-stage compressor (52). A first check valve (CV1) and a first motor-operated valve (MV1) are provided in the piping connecting the discharge pipe of the low-stage compressor (51) and the suction pipe of the high-stage compressor (52). The first motor-operated valve (MV1) is disposed downstream of the first check valve (CV1). The first check valve (CV1) allows the flow of refrigerant in the direction outflowing from the low-stage compressor (51) and prevents the flow of refrigerant in the opposite direction.

[0069] A discharge pipe of the high-stage compressor (52) is connected to one end of the external heat exchanger (56). A second check valve (CV2) and a second motor-operated valve (MV2) are provided in a pipe connecting the discharge pipe of the high-stage compressor (52) and one end of the external heat exchanger (56). The second motor-operated valve (MV2) is disposed downstream of the second check valve (CV2). The second check valve (CV2) allows the refrigerant to flow in the direction outflowing from the high-stage compressor (52) and prevents the refrigerant from flowing in the opposite direction.

[0070] The other end of the external heat exchanger (56) is connected to an inlet of the receiver (62). A first flow path (61a) of the internal heat exchanger (61) and a first expansion valve (EV1) are provided in a pipe connecting the other end of the external heat exchanger (56) and the inlet of the receiver (62). The first expansion valve (EV1) is disposed downstream of the internal heat exchanger (61).

[0071] A liquid outlet of the receiver (62) is connected to one end of the internal heat exchanger (57). A first solenoid valve (SV1) and a second expansion valve (EV2) are provided in a pipe connecting the liquid outlet of the receiver (62) and one end of the internal heat exchanger (57). The second expansion valve (EV2) is arranged downstream of the first solenoid valve (SV1).

[0072] The other end of the internal heat exchanger (57) is connected to the suction pipe of the low stage compressor (51).

[0073] In the main circuit (31), a first expansion valve (EV1) is arranged upstream of the receiver (62), and a second expansion valve (EV2) is arranged downstream of the receiver (62). The first expansion valve (EV1) and the second expansion valve (EV2) are expansion valves (65) of the refrigerant circuit (30).

[0074] <Gas Side Connecting Pipe> One end of the gas side connecting pipe (41) is connected to a gas outlet of the receiver (62). The other end of the gas side connecting pipe (41) is connected to one end of the intermediate connecting pipe (43). The gas side connecting pipe (41) is provided with a second solenoid valve (SV2) and a second flow path (61b) of the internal heat exchanger (61). The second flow path (61b) of the internal heat exchanger (61) is disposed downstream of the second solenoid valve (SV2).

[0075] <Liquid Side Connecting Pipe> One end of the liquid side connecting pipe (42) is connected to a position between the first solenoid valve (SV1) and the second expansion valve (EV2) in the main circuit (31). The other end of the liquid side connecting pipe (42) is connected to one end of the intermediate connecting pipe (43). The liquid side connecting pipe (42) is provided with a third expansion valve (EV3).

[0076] <Intermediate Connecting Pipe> As described above, one end of the intermediate connecting pipe (43) is connected to the other end of the gas side connecting pipe (41) and the other end of the liquid side connecting pipe (42). The other end of the intermediate connecting pipe (43) is connected to a position in the main circuit (31) between the first electric valve (MV1) and the high-stage compressor (52). The intermediate connecting pipe (43) is provided with a third check valve (CV3). The third check valve (CV3) allows refrigerant to flow from one end of the intermediate connecting pipe (43) to the other end and prevents refrigerant from flowing in the opposite direction.

[0077] <Low-stage connecting pipe> The low-stage connecting pipe (44) is a pipe through which refrigerant flows, bypassing the low-stage compressor (51). One end of the low-stage connecting pipe (44) is connected to the main circuit (31) between the suction pipe of the low-stage compressor (51) and the in-compartment heat exchanger (57). The other end of the low-stage connecting pipe (44) is connected to the main circuit (31) between the first check valve (CV1) and the first motor-operated valve (MV1). The low-stage connecting pipe (44) is provided with a fourth check valve (CV4). The fourth check valve (CV4) allows refrigerant to flow from one end of the low-stage connecting pipe (44) to the other end and prevents refrigerant from flowing in the opposite direction.

[0078] <High-stage connecting pipe> The high-stage connecting pipe (45) is a pipe through which refrigerant flows, bypassing the high-stage compressor (52). One end of the high-stage connecting pipe (45) is connected to a portion between the first check valve (CV1) and the first electric valve (MV1) in the main circuit (31). The other end of the high-stage connecting pipe (45) is connected to a portion between the second check valve (CV2) and the second electric valve (MV2) in the main circuit (31). The high-stage connecting pipe (45) is provided with a fifth check valve (CV5). The fifth check valve (CV5) allows refrigerant to flow from one end of the high-stage connecting pipe (45) to the other end and prevents refrigerant from flowing in the opposite direction.

[0079] <Defrosting Pipe> One end of the defrosting pipe (32) is connected to the main circuit (31) between the second check valve (CV2) and the second motor-operated valve (MV2). In the main circuit (31), one end of the defrosting pipe (32) is located downstream of the other end of the high-stage connecting pipe (45). The other end of the defrosting pipe (32) is connected to the main circuit (31) between the second expansion valve (EV2) and the in-compartment heat exchanger (57). The defrosting pipe (32) forms a bypass passage that sends the refrigerant discharged from the high-stage compressor (52) to the in-compartment heat exchanger (57) bypassing the external heat exchanger (56), the first expansion valve (EV1), and the second expansion valve (EV2).

[0080] The defrosting pipe (32) is provided with a fourth expansion valve (EV4). The fourth expansion valve (EV4) is a pressure reducing valve with a variable opening that reduces the pressure of the refrigerant flowing through the defrosting pipe (32).

[0081] <Reheat Pipe> One end of the reheat pipe (33) is connected to the main circuit (31) between the second check valve (CV2) and the second motor-operated valve (MV2). In the main circuit (31), one end of the reheat pipe (33) is located downstream of the other end of the high-stage connecting pipe (45). The other end of the reheat pipe (33) is connected to the main circuit (31) between the second expansion valve (EV2) and the internal heat exchanger (57).

[0082] The reheat pipe (33) is provided with a third solenoid valve (SV3), a reheat heat exchanger (58), and a fifth expansion valve (EV5). The reheat heat exchanger (58) is disposed downstream of the third solenoid valve (SV3). The fifth expansion valve (EV5) is disposed downstream of the reheat heat exchanger (58).

[0083] <Expansion Valves> The first expansion valve (EV1), second expansion valve (EV2), third expansion valve (EV3), fourth expansion valve (EV4), and fifth expansion valve (EV5) are each so-called electronic expansion valves. Each expansion valve (EV1 to EV5) includes a valve disc and a stepping motor that drives the valve disc. When the valve disc is moved by the stepping motor, the opening of the expansion valve (EV1 to EV5) changes continuously.

[0084] <Motor-operated valve> The first motor-operated valve (MV1) and the second motor-operated valve (MV2) are each variable-opening valves. Each motor-operated valve (MV1, MV2) includes a valve element and a stepping motor that drives the valve element. When the valve element is moved by the stepping motor, the opening of the motor-operated valve (MV1, MV2) changes continuously.

[0085] <Solenoid valves> The first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) are each an on-off valve. Each solenoid valve (SV1 to SV3) has a valve element and a solenoid that drives the valve element. When the valve element is moved by the solenoid, the solenoid valve (SV1 to SV3) opens or closes.

[0086] <Path Changing Mechanism> The second expansion valve (EV2), the third expansion valve (EV3), the fourth expansion valve (EV4), the second electric valve (MV2), the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) constitute a path changing mechanism (67). The path changing mechanism (67) is a mechanism that changes the flow path of the refrigerant in the refrigerant circuit (30).

[0087] The second expansion valve (EV2), the third expansion valve (EV3), the fourth expansion valve (EV4), the second electric valve (MV2), the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) constituting the path changing mechanism (67) are control valves controlled by a controller (90). When the controller (90) changes the state of one or more of these valves constituting the path changing mechanism (67), the flow path of the refrigerant in the refrigerant circuit (30) changes, and as a result, the operating state of the refrigeration system (10) changes.

[0088] <Low-stage compressor, high-stage compressor> The low-stage compressor (51) and the high-stage compressor (52) are each a hermetic scroll compressor. Although not shown, each of the low-stage compressor (51) and the high-stage compressor (52) includes a compression mechanism, an electric motor that drives the compression mechanism, and a casing that houses the compression mechanism and the electric motor. The compression mechanism is a scroll-type fluid machine that draws in and compresses refrigerant.

[0089] An accumulator (51a) is provided in the suction pipe of the low-stage compressor (51). An accumulator (52a) is provided in the suction pipe of the high-stage compressor (52). Each of the low-stage compressor (51) and the high-stage compressor (52) compresses the refrigerant sucked through the suction pipe and discharges the compressed refrigerant from a discharge pipe. The low-stage compressor (51) and the high-stage compressor (52) are compressors (50) provided in the refrigerant circuit (30).

[0090] It should be noted that each of the low-stage compressor (51) and the high-stage compressor (52) is not limited to a scroll compressor, and each of the low-stage compressor (51) and the high-stage compressor (52) may be, for example, a rotary compressor or a reciprocating compressor.

[0091] <External Heat Exchanger, Internal Heat Exchanger> As described above, each of the external heat exchanger (56), the internal heat exchanger (57), and the reheat heat exchanger (58) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and air. The external heat exchanger (56) exchanges heat between the refrigerant and external air (outside air). The internal heat exchanger (57) and the reheat heat exchanger (58) exchange heat between the refrigerant and internal air.

[0092] <Internal Heat Exchanger> The internal heat exchanger (61) is a heat exchanger that exchanges heat between refrigerants. In this embodiment, the internal heat exchanger (61) is a plate-type heat exchanger. The internal heat exchanger (61) is formed with a first flow path (61a) and a second flow path (61b). The first flow path (61a) of the internal heat exchanger (61) is disposed between the external heat exchanger (56) and the first expansion valve (EV1) in the main circuit (31). The second flow path (61b) of the internal heat exchanger (61) is disposed downstream of the second solenoid valve (SV2) in the gas side connecting pipe (41). The internal heat exchanger (61) exchanges heat between the refrigerant flowing through the first flow path (61a) and the refrigerant flowing through the second flow path (61b).

[0093] <Receiver> The receiver (62) is a container-like member for storing refrigerant. The receiver (62) also functions as a gas-liquid separator. The receiver (62) separates the gas-liquid two-phase refrigerant that has flowed in through the inlet into a liquid refrigerant and a gas refrigerant. In the receiver (62), the liquid refrigerant accumulates in the lower part of the receiver (62) and flows out through a liquid outlet formed in the bottom of the receiver (62). In the receiver (62), the gas refrigerant accumulates in the upper part of the receiver (62) and flows out through a gas outlet formed in the upper part of the receiver (62).

[0094] <Sensors Related to Low-Stage Compressor> In the main circuit (31), a low-stage suction temperature sensor (70) and a low-stage suction pressure sensor (75) are provided in a pipe connected to a suction pipe of the low-stage compressor (51). The low-stage suction temperature sensor (70) measures the temperature of the refrigerant sucked into the low-stage compressor (51). The low-stage suction pressure sensor (75) measures the pressure of the refrigerant sucked into the low-stage compressor (51).

[0095] In the main circuit (31), a low-stage discharge temperature sensor (71) and a low-stage discharge pressure sensor (76) are provided in a piping between the discharge pipe of the low-stage compressor (51) and the first check valve (CV1). The low-stage discharge temperature sensor (71) measures the temperature of the refrigerant discharged from the low-stage compressor (51). The low-stage discharge pressure sensor (76) measures the pressure of the refrigerant discharged from the low-stage compressor (51).

[0096] <Sensors Related to High-Stage Compressor> In the main circuit (31), a high-stage suction temperature sensor (72) and a high-stage suction pressure sensor (77) are provided in the piping between the suction pipe of the high-stage compressor (52) and the first motor-operated valve (MV1). The high-stage suction temperature sensor (72) measures the temperature of the refrigerant sucked into the high-stage compressor (52). The high-stage suction pressure sensor (77) measures the pressure of the refrigerant sucked into the high-stage compressor (52).

[0097] In the main circuit (31), a high-stage discharge temperature sensor (73) and a high-stage discharge pressure sensor (78) are provided in a piping between the discharge pipe of the high-stage compressor (52) and the second check valve (CV2). The high-stage discharge temperature sensor (73) measures the temperature of the refrigerant discharged from the high-stage compressor (52). The high-stage discharge pressure sensor (78) measures the pressure of the refrigerant discharged from the high-stage compressor (52).

[0098] <Other Sensors> The refrigerant circuit (30) is provided with a receiver pressure sensor (79), first to fourth refrigerant temperature sensors (81 to 84), and a heat exchanger temperature sensor (85).

[0099] The receiver pressure sensor (79) is connected to the gas side connecting pipe (41) between the receiver (62) and the second solenoid valve (SV2). The receiver pressure sensor (79) measures the pressure of the refrigerant stored in the receiver (62).

[0100] The first refrigerant temperature sensor (81) is provided on a pipe in the main circuit (31) between the external heat exchanger (56) and the internal heat exchanger (61). The first refrigerant temperature sensor (81) measures the temperature of the refrigerant flowing into the first flow path (61a) of the internal heat exchanger (61).

[0101] The second refrigerant temperature sensor (82) is provided in a pipe between the receiver (62) and the first solenoid valve (SV1) in the main circuit (31). The second refrigerant temperature sensor (82) measures the temperature of the refrigerant flowing out from the liquid outlet of the receiver (62).

[0102] The third refrigerant temperature sensor (83) is provided on a pipe in the main circuit (31) between the second expansion valve (EV2) and the internal heat exchanger (57). The third refrigerant temperature sensor (83) is disposed in the vicinity of one end of the internal heat exchanger (57). The third refrigerant temperature sensor (83) measures the temperature of the refrigerant at the inlet of the internal heat exchanger (57).

[0103] The fourth refrigerant temperature sensor (84) is provided in a pipe in the main circuit (31) between the internal heat exchanger (57) and the low-stage compressor (51). The fourth refrigerant temperature sensor (84) is disposed near the other end of the internal heat exchanger (57). The fourth refrigerant temperature sensor (84) measures the temperature of the refrigerant at the outlet of the internal heat exchanger (57).

[0104] The heat exchanger temperature sensor (85) is attached to the internal heat exchanger (57) and measures the temperature of the internal heat exchanger (57).

[0105] -Controller- As shown in Fig. 4, the controller (90) includes a microcomputer (91) and a memory device (92). The memory device (92) is a semiconductor memory. The memory device (92) stores software for operating the microcomputer (91). The controller (90) is housed in the electrical component box (28).

[0106] The controller (90) receives measurement values ​​of sensors provided in the refrigeration system (10). The controller (90) controls devices provided in the refrigeration system (10) based on the received measurement values ​​of the sensors. For example, the controller (90) controls the rotation speed of the low-stage compressor (51), the rotation speed of the high-stage compressor (52), the apertures of the first to fifth expansion valves (EV1 to EV5), the apertures of the first to second motor-operated valves (MV1, MV2), the rotation speed of the external fan (26), the rotation speed of the internal fan (27), etc.

[0107] --Operation of the Refrigeration Unit-- The operation of the refrigeration unit (10) will be described. The refrigeration unit (10) performs cooling operation, dehumidifying operation, heating operation, and defrosting operation. The controller (90) controls a plurality of valves constituting the path change mechanism (67) to switch between the cooling operation, dehumidifying operation, heating operation, and defrosting operation.

[0108] <Cooling Operation> The cooling operation is an operation for cooling the internal air. In the cooling operation, the refrigeration system (10) blows the air cooled in the internal heat exchanger (57) into the internal space (5).

[0109] In the cooling operation of the refrigeration system (10), the refrigerant circuit (30) performs a refrigeration cycle. In the cooling operation, the refrigerant circulates in the main circuit (31) of the refrigerant circuit (30), the external heat exchanger (56) functions as a radiator, and the internal heat exchanger (57) functions as an evaporator. The internal heat exchanger (57) cools the air flowing through the internal flow path (20). In the cooling operation, the refrigerant flows through the gas side connecting pipe (41) and the intermediate connecting pipe (43).

[0110] In the cooling operation of the refrigeration unit (10), the controller (90) operates the external fan (26) and the internal fan (27). In the transport container (1), air circulates between the internal flow path (20) of the refrigeration unit (10) and the internal space (5) of the container body (2). The internal air in the internal space (5) flows into the internal flow path (20) through the air inlet (21). The internal air flowing through the internal flow path (20) is cooled by the internal heat exchanger (57). The internal air cooled by the internal heat exchanger (57) is supplied to the internal space (5) through the air outlet (22).

[0111] During cooling operation, the controller (90) controls the rotational speeds of the low-stage compressor (51) and the high-stage compressor (52) so that the temperature of the air blown out through the air outlet (22) into the interior space (5) (specifically, the measured value of the second air temperature sensor (87)) becomes the set temperature.

[0112] The cooling operation will be described with reference to FIG.

[0113] In the cooling operation, the controller (90) controls the rotation speeds of the low-stage compressor (51) and the high-stage compressor (52). The controller (90) also controls the openings of the first expansion valve (EV1) and the second expansion valve (EV2) to keep the third expansion valve (EV3), the fourth expansion valve (EV4), and the fifth expansion valve (EV5) in a fully closed state. The controller (90) also keeps the first electric valve (MV1) and the second electric valve (MV2) in a fully open state, keeps the first solenoid valve (SV1) and the second solenoid valve (SV2) in an open state, and keeps the third solenoid valve (SV3) in a closed state.

[0114] The high-stage compressor (52) compresses the refrigerant it draws in to a pressure higher than the critical pressure of the refrigerant and discharges the refrigerant. The refrigerant discharged from the high-stage compressor (52) exchanges heat with outside air in the external heat exchanger (56) and dissipates heat to the outside air. The refrigerant that has passed through the external heat exchanger (56) flows into the first flow path (61a) of the internal heat exchanger (61) and is cooled by heat exchange with refrigerant flowing through the second flow path (61b) of the internal heat exchanger (61). The refrigerant that has passed through the first flow path (61a) of the internal heat exchanger (61) is reduced in pressure as it passes through the first expansion valve (EV1) and becomes a gas-liquid two-phase state. Thereafter, the refrigerant flows into the receiver (62) and is separated into liquid refrigerant and gas refrigerant.

[0115] The liquid refrigerant flowing out of the receiver (62) is reduced in pressure when passing through the second expansion valve (EV2) and then flows into the internal heat exchanger (57). The refrigerant flowing into the internal heat exchanger (57) absorbs heat from the air passing through the internal heat exchanger (57) and evaporates. The refrigerant flowing out of the internal heat exchanger (57) is sucked into the low-stage compressor (51). The low-stage compressor (51) compresses and discharges the sucked refrigerant. The refrigerant discharged from the low-stage compressor (51) is sucked into the high-stage compressor (52).

[0116] The gas refrigerant flowing out of the receiver (62) flows through the gas side connecting pipe (41), enters the second flow path (61b) of the internal heat exchanger (61), and absorbs heat from the refrigerant flowing through the first flow path (61a) of the internal heat exchanger (61). The refrigerant flowing out of the second flow path (61b) of the internal heat exchanger (61) passes through the intermediate connecting pipe (43), and is sucked into the high-stage compressor (52) together with the refrigerant discharged from the low-stage compressor (51).

[0117] <Dehumidifying Operation> The dehumidifying operation is an operation for dehumidifying the air inside the refrigerator while maintaining the temperature of the refrigerator space (5). In the dehumidifying operation, the refrigeration system (10) blows air that has been dehumidified in the refrigerator heat exchanger (57) and then heated in the reheat heat exchanger (58) into the refrigerator space (5).

[0118] In the dehumidifying operation of the refrigeration system (10), the controller (90) operates the external fan (26) and the internal fan (27), similarly to the cooling operation. In the transport container (1), air circulates between the internal flow path (20) of the refrigeration system (10) and the internal space (5) of the container body (2). The internal air in the internal space (5) flows into the internal flow path (20) through the air inlet (21). The internal air flowing through the internal flow path (20) passes through the internal heat exchanger (57) and the reheat heat exchanger (58) in this order, and is then supplied to the internal space (5) through the air outlet (22).

[0119] The dehumidifying operation will be described with reference to FIG.

[0120] In the dehumidification operation, the controller (90) controls the rotation speeds of the low-stage compressor (51) and the high-stage compressor (52). The controller (90) also controls the apertures of the first expansion valve (EV1), the second expansion valve (EV2), and the fifth expansion valve (EV5), and keeps the third expansion valve (EV3) and the fourth expansion valve (EV4) in a fully closed state. The controller (90) also keeps the first electric valve (MV1) and the second electric valve (MV2) in a fully open state, and keeps the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) in an open state.

[0121] The dehumidifying operation differs from the cooling operation in that the controller (90) controls the aperture of the fifth expansion valve (EV5) and keeps the third solenoid valve (SV3) open. The dehumidifying operation also differs from the cooling operation in that the refrigerant flows not only through the main circuit (31), the gas side connecting pipe (41), and the intermediate connecting pipe (43) but also through the reheat pipe (33).

[0122] In the dehumidifying operation of the refrigeration system (10), the refrigerant circuit (30) performs a refrigeration cycle. In the dehumidifying operation, as in the cooling operation, the refrigerant circulates in the main circuit (31) of the refrigerant circuit (30), the external heat exchanger (56) functions as a radiator, and the internal heat exchanger (57) functions as an evaporator. The internal heat exchanger (57) cools the air flowing through the internal flow path (20).

[0123] In the dehumidifying operation, the evaporation temperature of the refrigerant in the internal heat exchanger (57) is set to a value lower than the dew point temperature of the air flowing into the internal heat exchanger (57). Therefore, in the internal heat exchanger (57), moisture contained in the air condenses to form drain water. The drain water generated in the internal heat exchanger (57) flows down into a drain pan and is discharged to the outside of the cabinet. Therefore, the temperature and absolute humidity of the air flowing through the internal flow path (20) decrease while passing through the internal heat exchanger (57).

[0124] In the dehumidifying operation, as in the cooling operation, the refrigerant flows through the gas side connecting pipe (41) and the intermediate connecting pipe (43). The gas refrigerant flowing out of the receiver (62) passes through the gas side connecting pipe (41) and the intermediate connecting pipe (43) in this order, and is sucked into the high-stage compressor (52) together with the refrigerant discharged from the low-stage compressor (51).

[0125] In the dehumidifying operation, a portion of the refrigerant discharged from the high-stage compressor (52) is supplied to the reheat heat exchanger (58) through the reheat pipe (33). In the reheat heat exchanger (58), the air that has passed through the internal heat exchanger (57) is heated by the refrigerant. The refrigerant that has dissipated heat in the reheat heat exchanger (58) is reduced in pressure when passing through the fifth expansion valve (EV5). The refrigerant that has passed through the fifth expansion valve (EV5) flows into the main circuit (31) and then flows into the internal heat exchanger (57) together with the refrigerant that has passed through the second expansion valve (EV2).

[0126] In the dehumidification operation, the controller (90) controls the rotation speed of the low-stage compressor (51) so that the humidity of the air passing through the reheat heat exchanger (58) becomes a target humidity. Specifically, the controller (90) controls the rotation speed of the low-stage compressor (51) so that the measurement value of the humidity sensor (88) becomes a set humidity. When the measurement value of the humidity sensor (88) is higher than the set humidity, the controller (90) increases the rotation speed of the low-stage compressor (51) to decrease the evaporation temperature of the refrigerant in the internal heat exchanger (57). When the measurement value of the humidity sensor (88) is lower than the set humidity, the controller (90) decreases the rotation speed of the low-stage compressor (51) to increase the evaporation temperature of the refrigerant in the internal heat exchanger (57).

[0127] Furthermore, in the dehumidifying operation, the controller (90) controls the aperture of the fifth expansion valve (EV5) so that the temperature of the air passing through the reheat heat exchanger (58) becomes a set temperature. Specifically, the controller (90) controls the aperture of the fifth expansion valve (EV5) so that the measurement value of the second air temperature sensor (87) becomes the set temperature. When the measurement value of the second air temperature sensor (87) is higher than the set temperature, the controller (90) reduces the aperture of the fifth expansion valve (EV5) to reduce the flow rate of refrigerant in the reheat heat exchanger (58). When the measurement value of the second air temperature sensor (87) is lower than the set temperature, the controller (90) increases the aperture of the fifth expansion valve (EV5) to increase the flow rate of refrigerant in the reheat heat exchanger (58).

[0128] In the dehumidifying operation, the temperature and absolute humidity of the air flowing through the internal flow path (20) decrease as it passes through the internal heat exchanger (57), and the temperature increases as it passes through the reheat heat exchanger (58). Therefore, by performing the dehumidifying operation of the refrigeration system (10), it is possible to decrease the humidity of the internal air while maintaining the temperature of the internal space (5).

[0129] <Heating Operation> The heating operation is an operation for heating the air inside the refrigerator. In the heating operation, the refrigeration system (10) blows out the air heated in the internal heat exchanger (57) into the internal space (5). The heating operation is performed, for example, when the outside air temperature is lower than the set temperature of the internal space (5), in order to maintain the air temperature of the internal space (5) at the set temperature.

[0130] During the heating operation of the refrigeration system (10), the controller (90) stops the external fan (26) and activates the internal fan (27). In the transport container (1), air circulates between the internal flow path (20) of the refrigeration system (10) and the internal space (5) of the container body (2). The internal air in the internal space (5) flows into the internal flow path (20) through the air inlet (21). The internal air flowing through the internal flow path (20) passes through the internal heat exchanger (57) and is then supplied to the internal space (5) through the air outlet (22).

[0131] The heating operation will be described with reference to FIG.

[0132] In the heating operation, the controller (90) keeps the low-stage compressor (51) stopped and controls the rotation speed of the high-stage compressor (52). The controller (90) also keeps the first expansion valve (EV1) open, the second expansion valve (EV2), the third expansion valve (EV3), and the fifth expansion valve (EV5) closed, and controls the opening degree of the fourth expansion valve (EV4). The controller (90) also keeps the first electric valve (MV1) fully open, the second electric valve (MV2) fully closed, and the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) closed.

[0133] In the refrigerant circuit (30) during the heating operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57) without passing through the external heat exchanger (56). The refrigerant discharged from the high-stage compressor (52) flows through the defrosting pipe (32) and is reduced in pressure when passing through the fourth expansion valve (EV4). The refrigerant that has passed through the fourth expansion valve (EV4) flows into the internal heat exchanger (57) and dissipates heat to the air passing through the internal heat exchanger (57). The refrigerant that has flowed out from the internal heat exchanger (57) passes through the low-stage connecting pipe (44) and is drawn into the high-stage compressor (52). The high-stage compressor (52) compresses the drawn refrigerant and discharges it.

[0134] In the heating operation, the controller (90) controls the aperture of the fourth expansion valve (EV4) so ​​that the temperature of the air passing through the internal heat exchanger (57) becomes a set temperature. Specifically, the controller (90) controls the aperture of the fourth expansion valve (EV4) so ​​that the measurement value of the second air temperature sensor (87) becomes the set temperature. When the measurement value of the second air temperature sensor (87) is higher than the set temperature, the controller (90) reduces the aperture of the fourth expansion valve (EV4) to reduce the flow rate of refrigerant in the internal heat exchanger (57). When the measurement value of the second air temperature sensor (87) is lower than the set temperature, the controller (90) increases the aperture of the fourth expansion valve (EV4) to increase the flow rate of refrigerant in the internal heat exchanger (57).

[0135] - Defrosting Operation of Refrigeration Apparatus - The defrosting operation is an operation for melting frost that has formed on the internal heat exchanger (57) during the cooling operation.

[0136] In the cooling operation, the evaporation temperature of the refrigerant in the internal heat exchanger (57) may fall below 0° C. In this case, moisture in the air freezes and turns into frost, which adheres to the internal heat exchanger (57). When frost adheres to the internal heat exchanger (57), the frost obstructs the flow of air passing through the internal heat exchanger (57) and also obstructs heat exchange between the refrigerant and the air.

[0137] Therefore, when a defrosting start condition is satisfied during the cooling operation, the refrigeration system (10) temporarily suspends the cooling operation and performs the defrosting operation. The defrosting start condition is, for example, a condition that "the cumulative value of the execution time of the cooling operation during which the evaporation temperature of the refrigerant in the internal heat exchanger is lower than 0°C reaches a predetermined time (for example, 2 hours)."

[0138] Furthermore, when a defrost termination condition is satisfied during the defrosting operation, the refrigeration system (10) terminates the defrosting operation and resumes the cooling operation. The defrost termination condition is, for example, that the temperature of the internal heat exchanger (57) (specifically, the value measured by the heat exchanger temperature sensor (85)) reaches a predetermined temperature (for example, 10°C).

[0139] In the defrosting operation, the refrigeration system (10) mainly performs normal operation. If the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) becomes too large during normal operation, the refrigeration system (10) temporarily suspends normal operation and performs an inflow operation. If the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) becomes too small during normal operation, the refrigeration system (10) temporarily suspends normal operation and performs an outflow operation. The controller (90) controls a plurality of valves constituting the path change mechanism (67) to switch between the normal operation, the inflow operation, and the outflow operation.

[0140] In each of the normal operation, the inflow operation, and the outflow operation, the controller (90) keeps the internal fan (27) stopped, so that no air flows through the internal flow path (20). In addition, in each of the normal operation, the inflow operation, and the outflow operation, the controller (90) keeps the low-stage compressor (51) stopped and operates the high-stage compressor (52).

[0141] <Normal Operation> The normal operation of the defrosting operation will be described with reference to FIG.

[0142] In normal operation, the controller (90) keeps the external fan (26) and the internal fan (27) stopped. The controller (90) also keeps the low-stage compressor (51) stopped and controls the rotation speed of the high-stage compressor (52). The controller (90) also keeps the first expansion valve (EV1) open, the second expansion valve (EV2), the third expansion valve (EV3), and the fifth expansion valve (EV5) closed, and controls the opening of the fourth expansion valve (EV4). The controller (90) also keeps the first motor-operated valve (MV1) fully open, the second motor-operated valve (MV2) fully closed, and the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) closed.

[0143] In the refrigerant circuit (30) during normal operation, the refrigerant flows in the same manner as in the heating operation. In the refrigerant circuit (30) during normal operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57) while the inflow and outflow of the refrigerant to the receiver (62) is blocked.

[0144] The state of the refrigerant in the refrigerant circuit (30) during normal operation will be described with reference to the Mollier diagram (pressure-enthalpy diagram) of FIG.

[0145] In the refrigerant circuit (30), refrigerant in a state of point A is sucked into the high-stage compressor (52) and compressed by the high-stage compressor (52) to reach a state of point B. In the process of compressing the refrigerant by the high-stage compressor (52), the pressure of the refrigerant increases and the enthalpy of the refrigerant also increases.

[0146] The refrigerant discharged from the high-stage compressor (52) flows through the defrosting pipe (32) into the fourth expansion valve (EV4). In the process from the high-stage compressor (52) to the fourth expansion valve (EV4), the refrigerant releases a small amount of heat, and the state of the refrigerant changes from point B to point C. The refrigerant in the state of point C is decompressed while passing through the fourth expansion valve (EV4) and reaches the state of point D.

[0147] The refrigerant in the state of point D flows into the internal heat exchanger (57) and releases heat. In the internal heat exchanger (57), the frost adhering to the internal heat exchanger (57) is warmed by the refrigerant and melts. In addition, in the internal heat exchanger (57), the pressure of the refrigerant decreases due to pressure loss while the refrigerant passes through the internal heat exchanger (57). Therefore, the refrigerant reaches the state of point E at the outlet of the internal heat exchanger (57). The refrigerant flowing out of the internal heat exchanger (57) releases some heat while flowing toward the high-stage compressor (52) and reaches the state of point A.

[0148] <Inflow Operation> The inflow operation is an operation for causing refrigerant to flow into the receiver (62) and increasing the mass of the refrigerant stored in the receiver (62). The refrigeration system (10) performs the inflow operation in order to reduce the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0149] The inflow operation of the defrosting operation will be described with reference to FIG.

[0150] In the inflow operation, the controller (90) operates the external fan (26) and opens the second motor-operated valve (MV2). The difference between the inflow operation and the normal operation is that the external fan (26) operates and the second motor-operated valve (MV2) opens. In addition, in the inflow operation, the controller (90) controls the opening of the first expansion valve (EV1) as necessary.

[0151] In the inflow operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57), as in the normal operation. In the inflow operation, a portion of the refrigerant discharged from the high-stage compressor (52) flows into the external heat exchanger (56). The refrigerant that has flowed into the external heat exchanger (56) dissipates heat into the outside air, and then passes through the first expansion valve (EV1) and flows into the receiver (62).

[0152] During the inflow operation, the first solenoid valve (SV1) and the second solenoid valve (SV2) are closed, and therefore, no refrigerant flows out of the receiver (62). Therefore, during the inflow operation, the mass of the refrigerant stored in the receiver (62) increases, and as a result, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) decreases.

[0153] In addition, when the pressure of the refrigerant discharged from the high-stage compressor (52) during the inflow operation is equal to or higher than the critical pressure of the refrigerant, the controller (90) controls the opening degree of the first expansion valve (EV1) so that the pressure of the refrigerant flowing into the receiver (62) becomes lower than the critical pressure of the refrigerant.

[0154] <Outflow Operation> The outflow operation is an operation for causing the refrigerant to flow out of the receiver (62) and reducing the mass of the refrigerant stored in the receiver (62). The refrigeration system (10) performs the outflow operation in order to increase the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0155] The refrigeration system (10) performs either the first outflow operation or the second outflow operation as the outflow operation. In each of the first outflow operation and the second outflow operation, the refrigeration system (10) performs either the gas outflow operation or the liquid outflow operation.

[0156] <First Outflow Operation / Gas Outflow Operation> The gas outflow operation of the first outflow operation will be described with reference to FIG.

[0157] In the gas outflow operation of the first outflow operation, the controller (90) opens the second solenoid valve (SV2). The difference between the gas outflow operation of the first outflow operation and the normal operation is that the second solenoid valve (SV2) is opened.

[0158] In the gas outflow operation of the first outflow operation, as in the normal operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57). In the gas outflow operation of the first outflow operation, the gas refrigerant in the receiver (62) flows into the gas side connecting pipe (41). The gas refrigerant that has flowed into the gas side connecting pipe (41) passes through the intermediate connecting pipe (43) and is sucked into the high-stage compressor (52) together with the refrigerant that has flowed out of the internal heat exchanger (57).

[0159] In the gas outflow operation of the first outflow operation, the second motor-operated valve (MV2) is closed, and therefore, no refrigerant flows into the receiver (62). Therefore, in the gas outflow operation of the first outflow operation, the mass of the refrigerant stored in the receiver (62) decreases, and as a result, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0160] <First Outflow Operation / Liquid Outflow Operation> For example, when the outdoor air temperature is relatively low, the pressure of the refrigerant in the receiver (62) may be relatively low, and the difference between the refrigerant pressure in the receiver (62) and the refrigerant pressure in the internal heat exchanger (57) during the outflow operation may be small. If the gas outflow operation is performed in this case, the flow rate of gas refrigerant flowing out of the receiver (62) may be reduced, and it may take a long time to increase the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57). Therefore, in such a case, the refrigeration system (10) performs a liquid outflow operation instead of a gas outflow operation.

[0161] The liquid outflow operation of the first outflow operation will be described with reference to FIG.

[0162] In the liquid outflow operation of the first outflow operation, the controller (90) opens the first solenoid valve (SV1), closes the second solenoid valve (SV2), and opens the third expansion valve (EV3). The difference between the liquid outflow operation and the gas outflow operation of the first outflow operation is that the second solenoid valve (SV2) is closed, and the first solenoid valve (SV1) and the third expansion valve (EV3) are open.

[0163] In the liquid outflow operation of the first outflow operation, similar to the gas outflow operation of the first outflow operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57). In the liquid outflow operation of the first outflow operation, the liquid refrigerant in the receiver (62) flows into the liquid side connecting pipe (42). The liquid refrigerant that has flowed into the liquid side connecting pipe (42) passes through the intermediate connecting pipe (43) and is sucked into the high-stage compressor (52) together with the refrigerant that has flowed out of the internal heat exchanger (57).

[0164] In the liquid outflow operation of the first outflow operation, the second motor-operated valve (MV2) is closed, and therefore, no refrigerant flows into the receiver (62). Therefore, in the liquid outflow operation of the first outflow operation, the mass of the refrigerant stored in the receiver (62) decreases, and as a result, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0165] Furthermore, the liquid refrigerant in the receiver (62) has a higher density than the gas refrigerant in the receiver (62). Therefore, even when the difference between the refrigerant pressure in the receiver (62) and the refrigerant pressure in the internal heat exchanger (57) is small, the mass flow rate of the refrigerant flowing out of the receiver (62) can be ensured, and the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) can be increased sufficiently in a relatively short time.

[0166] <Second Outflow Operation> For example, when the outdoor air temperature is very low, the refrigerant pressure in the receiver (62) during the outflow operation may be lower than the refrigerant pressure in the internal heat exchanger (57). On the other hand, in the first outflow operation, the pressure of the refrigerant sucked into the high-stage compressor (52) is substantially equal to the refrigerant pressure in the internal heat exchanger (57). Therefore, even if the first outflow operation is performed in such a case, the refrigerant cannot be caused to flow out of the receiver (62). Therefore, in such a case, the refrigeration system (10) performs the second outflow operation instead of the first outflow operation.

[0167] The gas outflow operation of the second outflow operation will be described with reference to FIG.

[0168] In the gas outflow operation of the second outflow operation, the controller (90) closes the first motor-operated valve (MV1). The difference between the gas outflow operation of the second outflow operation and the gas outflow operation of the first outflow operation is that the first motor-operated valve (MV1) is closed.

[0169] In the gas outflow operation of the second outflow operation, the first motor-operated valve (MV1) is in a closed state, and the first motor-operated valve (MV1) blocks communication between the high-stage compressor (52) and the internal heat exchanger (57). Therefore, the high-stage compressor (52) draws refrigerant only from the receiver (62) out of the receiver (62) and the internal heat exchanger (57).

[0170] As in the gas outflow operation of the first outflow operation, the gas refrigerant flowing out from the gas outlet of the receiver (62) passes through the gas side connecting pipe (41) and the intermediate connecting pipe (43) and is then sucked into the high-stage compressor (52). Therefore, in the gas outflow operation of the second outflow operation, the mass of the refrigerant stored in the receiver (62) decreases, and as a result, when normal operation is resumed, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0171] The liquid outflow operation of the second outflow operation will be described.

[0172] In the liquid outflow operation of the second outflow operation, the controller (90) closes the first motor-operated valve (MV1). The difference between the liquid outflow operation of the second outflow operation and the liquid outflow operation of the first outflow operation is that the first motor-operated valve (MV1) is closed.

[0173] In the liquid outflow operation of the second outflow operation, the first motor-operated valve (MV1) is in a closed state, and the first motor-operated valve (MV1) blocks communication between the high-stage compressor (52) and the internal heat exchanger (57). Therefore, the high-stage compressor (52) draws refrigerant only from the receiver (62) out of the receiver (62) and the internal heat exchanger (57).

[0174] As in the liquid outflow operation of the first outflow operation, the liquid refrigerant flowing out from the liquid outlet of the receiver (62) passes through the liquid side connecting pipe (42) and the intermediate connecting pipe (43) and is then sucked into the high-stage compressor (52). Therefore, in the liquid outflow operation of the second outflow operation, the mass of the refrigerant stored in the receiver (62) decreases, and as a result, when normal operation is resumed, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0175] --Operation of the Controller in Defrosting Operation-- The operation performed by the controller (90) in the defrosting operation will be described with reference to FIG.

[0176] In the defrosting operation, the controller (90) controls the refrigeration system (10) so that the amount of heat applied to the in-compartment heat exchanger to melt the frost (hereinafter referred to as the "defrosting heat amount") becomes a target heat amount.

[0177] The controller (90) of this embodiment uses the current (hereinafter referred to as the “input current”) supplied to the motor of the high-stage compressor (52) as an index indicating the amount of heat for dehumidification. The controller (90) stores, as a target current, the value of the input current at which the amount of heat for defrosting becomes the target amount of heat. The controller (90) of this embodiment causes the refrigeration system (10) to selectively perform a normal operation, an inflow operation, or an outflow operation so that the input current of the high-stage compressor (52) becomes the target current.

[0178] <Control in Normal Operation> In normal operation, the controller (90) controls the rotation speed of the high-stage compressor (52) and the aperture of the fourth expansion valve (EV4). The controller (90) controls the rotation speed of the high-stage compressor (52) with priority over the control of the aperture of the fourth expansion valve (EV4).

[0179] When normal operation is initiated, the controller (90) sets the opening of the fourth expansion valve (EV4) to an initial opening and controls the rotation speed of the high-stage compressor (52) based on the input current. The controller (90) controls the rotation speed of the high-stage compressor (52) so that the input current becomes a target current. When the input current is lower than the target current, the controller (90) increases the rotation speed of the high-stage compressor (52). When the input current is higher than the target current, the controller (90) decreases the rotation speed of the high-stage compressor (52).

[0180] When the rotation speed of the high-stage compressor (52) reaches a reference speed (e.g., an upper limit speed) but the input current is lower than the target current, the amount of heat for defrosting is insufficient, but the amount of heat for defrosting cannot be increased by controlling the rotation speed of the high-stage compressor (52). Therefore, in this case, the controller (90) controls the opening of the fourth expansion valve (EV4).

[0181] The controller (90) controls the opening degree of the fourth expansion valve (EV4) based on the pressure of the refrigerant discharged from the high stage compressor (52) (specifically, the measurement value of the high stage discharge pressure sensor (78)). The controller (90) reduces the opening degree of the fourth expansion valve (EV4) as much as possible within a range in which the measurement value of the high stage discharge pressure sensor (78) is maintained at or below a reference pressure.

[0182] Specifically, the controller (90) controls the aperture of the fourth expansion valve (EV4) so ​​that the measurement value of the high-stage discharge pressure sensor (78) falls within a reference pressure range. The reference pressure range is a pressure range that includes the reference pressure. The maximum value of the reference pressure range is the reference pressure.

[0183] When the measurement value of the high stage discharge pressure sensor (78) is lower than the lowest value of the reference pressure range, the controller (90) reduces the opening of the fourth expansion valve (EV4). When the measurement value of the high stage discharge pressure sensor (78) is higher than the highest value of the reference pressure range (=reference pressure), the controller (90) increases the opening of the fourth expansion valve (EV4). When the measurement value of the high stage discharge pressure sensor (78) is within the reference pressure range, the controller (90) maintains the opening of the fourth expansion valve (EV4).

[0184] When the opening of the fourth expansion valve (EV4) reaches the reference opening but the input current is higher than the target current, the amount of heat for defrosting is excessive, but the amount of heat for defrosting cannot be reduced by controlling the opening of the fourth expansion valve (EV4). Therefore, in this case, the controller (90) controls the rotation speed of the high-stage compressor (52) again.

[0185] <Switching Between Normal Operation and Inflow Operation> When an inflow start condition is satisfied during normal operation, the controller (90) switches the operation performed by the refrigeration system (10) from the normal operation to the inflow operation.

[0186] The inflow start condition is a condition indicating that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is excessive. In this embodiment, the inflow start condition is a condition that “the pressure of the refrigerant discharged from the high-stage compressor (52) (specifically, the value measured by the high-stage discharge pressure sensor (78)) is equal to or greater than the upper limit pressure.” In the controller (90), the upper limit pressure is set to a value slightly lower than the maximum pressure that the refrigerant circuit (30) can withstand.

[0187] When the refrigeration system (10) starts the inflow operation, a portion of the refrigerant discharged from the high-stage compressor (52) flows into the receiver (62), and the mass of the refrigerant circulating between the high-stage compressor (52) and the in-compartment heat exchanger (57) decreases.

[0188] When an inflow termination condition is satisfied during the inflow operation, the controller (90) switches the operation performed by the refrigeration system (10) from the inflow operation to the normal operation.

[0189] The inflow termination condition is a condition that indicates that the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is appropriate. In this embodiment, the inflow termination condition is a condition that "the pressure of the refrigerant discharged from the high-stage compressor (52) (specifically, the value measured by the high-stage discharge pressure sensor (78)) is lower than the upper limit pressure." When the inflow termination condition is satisfied, the refrigeration system (10) terminates the inflow operation and resumes normal operation.

[0190] The inflow end condition may be a condition that "the duration of the inflow action reaches a predetermined time (for example, 5 seconds)."

[0191] <Switching Between Normal Operation and Outflow Operation> When an outflow start condition is satisfied during normal operation, the controller (90) switches the operation performed by the refrigeration system (10) from the normal operation to the outflow operation.

[0192] The outflow start condition is a condition indicating that the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is insufficient. In this embodiment, the outflow start condition is a condition that “the temperature of the refrigerant discharged from the high-stage compressor (52) (specifically, the value measured by the high-stage discharge temperature sensor (73)) is equal to or higher than an upper limit temperature.”

[0193] When the refrigeration system (10) starts the outflow operation, the refrigerant flowing out from the receiver (62) is sucked into the high-stage compressor (52), and the mass of the refrigerant circulating between the high-stage compressor (52) and the in-compartment heat exchanger (57) increases.

[0194] When an outflow termination condition is satisfied during the outflow operation, the controller (90) switches the operation performed by the refrigeration system (10) from the outflow operation to normal operation. The outflow termination condition is a condition that "the duration of the outflow operation reaches a predetermined time (e.g., one minute)."

[0195] The outflow termination condition may be a condition that “the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (57) is lower than a reference degree of superheat.” In this case, the controller (90) calculates the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (57) using the measurement value of the fourth refrigerant temperature sensor (84) and the measurement value of the low stage suction pressure sensor (75).

[0196] <Control in Outflow Operation> The controller (90) causes the refrigeration system (10) to perform one of the first outflow operation and the second outflow operation as the outflow operation.

[0197] The controller (90) selects one of the first outflow operation and the second outflow operation based on the difference (Pr-Ps) between the pressure of the refrigerant in the receiver (62) (specifically, the measured value Pr of the receiver pressure sensor (79)) and the pressure of the refrigerant sucked into the high-stage compressor (52) (specifically, the measured value Ps of the high-stage suction pressure sensor (77)).

[0198] When the pressure difference (Pr-Ps) is equal to or greater than the reference value, the "pressure of the refrigerant in the receiver (62)" is somewhat higher than the "pressure of the refrigerant drawn into the high-stage compressor (52)." In this case, the high-stage compressor (52) can draw refrigerant from both the receiver (62) and the internal heat exchanger (57). Therefore, in this case, the controller (90) causes the refrigeration system (10) to perform the first outflow operation.

[0199] When the pressure difference (Pr-Ps) is less than the reference value, the "pressure of the refrigerant in the receiver (62)" is close to the "pressure of the refrigerant sucked into the high-stage compressor (52)," or the "pressure of the refrigerant in the receiver (62)" is lower than the "pressure of the refrigerant sucked into the high-stage compressor (52)." In this case, the high-stage compressor (52) cannot suck refrigerant from both the receiver (62) and the internal heat exchanger (57). Therefore, in this case, the controller (90) causes the refrigeration system (10) to perform the second outflow operation, and causes the high-stage compressor (52) to suck refrigerant only from the receiver (62).

[0200] In the first outflow operation, the controller (90) causes the refrigeration system (10) to perform a gas outflow operation. If the outflow start condition is satisfied again during normal operation performed by the refrigeration system (10) after the first outflow operation has ended, it can be determined that the mass of refrigerant flowing out of the receiver (62) in the first outflow operation was small. Therefore, in the second outflow operation, the controller (90) causes the refrigeration system (10) to perform a liquid outflow operation to increase the mass of refrigerant flowing out of the receiver (62).

[0201] Feature (1) of First Embodiment In the refrigeration system (10) of this embodiment, the controller (90) controls the aperture of the fourth expansion valve (EV4), which is a pressure reducing valve, during defrosting operation. During defrosting operation, the refrigerant discharged from the high-stage compressor (52) (point B in FIG. 8 ) is decompressed as it passes through the fourth expansion valve (EV4), and the decompressed refrigerant (point D in FIG. 8 ) is supplied to the internal heat exchanger (57). Therefore, the pressure of the refrigerant flowing out of the internal heat exchanger (57) (point E in FIG. 8 ) is lower than when the refrigerant discharged from the high-stage compressor (52) is supplied to the internal heat exchanger (57) without being decompressed. As a result, the pressure difference between the refrigerant drawn into the high-stage compressor (52) (point A in FIG. 8 ) and the refrigerant discharged from the high-stage compressor (52) (point B in FIG. 8 ) is increased.

[0202] When the pressure difference between the refrigerant sucked into the high-stage compressor (52) and the refrigerant discharged from the high-stage compressor (52) increases, the amount of heat imparted to the refrigerant increases in the process of compressing the refrigerant by the high-stage compressor (52). Therefore, according to this embodiment, the amount of heat available for melting frost on the internal heat exchanger (57) can be increased compared to when the refrigerant discharged from the high-stage compressor (52) is supplied to the internal heat exchanger (57) without being depressurized, and as a result, the time required for defrosting the internal heat exchanger (57) can be shortened.

[0203] Feature (2) of First Embodiment The refrigeration system (10) of this embodiment performs a normal operation, an outflow operation, and an inflow operation during defrosting operation. During the outflow operation, the mass of the refrigerant stored in the receiver (62) decreases, and the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases. During the inflow operation, the mass of the refrigerant stored in the receiver (62) increases, and the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) decreases.

[0204] The refrigeration system (10) of this embodiment performs the inflow operation and the outflow operation, thereby adjusting the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) during the defrosting operation. Therefore, the defrosting operation can be continuously performed while the pressure and temperature of the refrigerant discharged from the high-stage compressor (52) are maintained at appropriate values, and the frost adhering to the internal heat exchanger (57) can be reliably melted by the defrosting operation.

[0205] Feature (3) of First Embodiment The refrigeration system (10) of this embodiment performs a first outflow operation or a second outflow operation as the outflow operation. In the first outflow operation, the high-stage compressor (52) draws in the refrigerant that has flowed out of the receiver (62) and the refrigerant that has flowed out of the internal heat exchanger (57). In the second outflow operation, the high-stage compressor (52) draws in the refrigerant that has flowed out of the receiver (62) but does not draw in the refrigerant that has flowed out of the internal heat exchanger (57).

[0206] Here, for example, when the outside air temperature is very low (for example, about −20° C. to −30° C.), the refrigerant pressure in the receiver (62) may become lower than the refrigerant pressure in the internal heat exchanger (57). In a situation where the refrigerant pressure in the receiver (62) is lower than the refrigerant pressure in the internal heat exchanger (57), the high-stage compressor (52) cannot draw refrigerant from the receiver (62), and therefore the refrigerant cannot be caused to flow out of the receiver (62) by the first outflow operation.

[0207] On the other hand, in the second outflow operation, the refrigerant in the receiver (62) can be sucked into the high-stage compressor (52) regardless of the refrigerant pressure in the internal heat exchanger (57). Therefore, even when the refrigerant pressure in the receiver (62) is lower than the refrigerant pressure in the internal heat exchanger (57), the refrigerant can be made to flow out of the receiver (62), thereby increasing the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0208] Therefore, according to this embodiment, the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) during defrosting operation can be increased by the outflow operation regardless of the refrigerant pressure in the receiver (62).

[0209] Feature (4) of First Embodiment The refrigeration system (10) of this embodiment performs a gas outflow operation or a liquid outflow operation as the outflow operation. In the gas outflow operation, gas refrigerant flowing out of the receiver (62) is sucked into the high-stage compressor (52). In the liquid outflow operation, liquid refrigerant flowing out of the receiver (62) is sucked into the high-stage compressor (52).

[0210] For example, when the outside air temperature is relatively low, the pressure of the refrigerant in the receiver (62) becomes relatively low, and the difference between the refrigerant pressure in the receiver (62) and the refrigerant pressure in the internal heat exchanger (57) during the outflow operation may become small. In this case, when the gas outflow operation is performed, the flow rate of the gas refrigerant flowing out of the receiver (62) becomes small, and it may take a long time to increase the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0211] In such a case, the refrigeration system (10) of the present embodiment performs a liquid outflow operation instead of a gas outflow operation. In the liquid outflow operation, the liquid refrigerant flowing out of the receiver (62) is sucked into the high-stage compressor (52). The density of the liquid refrigerant is significantly higher than the density of the gas refrigerant. Therefore, the refrigeration system (10) of the present embodiment performing the liquid outflow operation can maintain a high mass flow rate of the refrigerant flowing out of the receiver (62) even when the difference between the refrigerant pressure in the receiver (62) and the refrigerant pressure in the internal heat exchanger (57) is relatively small. As a result, the time required to increase the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) can be kept short.

[0212] Here, if a relatively large amount of liquid refrigerant flows into the compression mechanism of the high-stage compressor (52), the compression mechanism may be damaged. On the other hand, in the refrigeration system (10) of this embodiment, the refrigerant drawn into the high-stage compressor (52) passes through the accumulator (52a) and then flows into the compression mechanism of the high-stage compressor (52). Therefore, in the liquid outflow operation, the liquid refrigerant flowing out of the receiver (62) flows into the accumulator (52a), and the refrigerant vaporized in the accumulator (52a) is drawn into the compression mechanism of the high-stage compressor (52). Therefore, in the refrigeration system (10) of this embodiment, even in the liquid outflow operation, the compression mechanism of the high-stage compressor (52) is not damaged by drawing in a relatively large amount of liquid refrigerant.

[0213] Second Embodiment A second embodiment will be described. In this embodiment, the transport container (1) of the first embodiment has a modified configuration of the refrigeration unit (10).

[0214] The refrigeration system (10) of this embodiment differs from the refrigeration system (10) of embodiment 1 in the configuration of the refrigerant circuit (30). Here, the refrigerant circuit (30) of this embodiment differs from the refrigerant circuit (30) of embodiment 1 in the following points.

[0215] As shown in Figure 14, in the refrigerant circuit (30) of this embodiment, a drain pan heater (63) is provided in the defrosting pipe (32). In the defrosting pipe (32), the drain pan heater (63) is arranged downstream of the fourth expansion valve (EV4). The drain pan heater (63) is a pipe attached to the drain pan. The drain pan heater (63) heats the drain pan with the refrigerant flowing therethrough.

[0216] - Defrosting Operation of Refrigeration Unit - In the defrosting operation performed by the refrigeration unit (10) of this embodiment, refrigerant discharged from the high-stage compressor (52) flows into the defrosting pipe (32), similar to the defrosting operation performed by the refrigeration unit (10) of embodiment 1. In the defrosting operation performed by the refrigeration unit (10) of this embodiment, the refrigerant that has flowed into the defrosting pipe (32) dissipates heat to the drain pan while passing through the drain pan heater (63), and then flows into the internal heat exchanger (57).

[0217] During the defrosting operation, frost peeled off from the internal heat exchanger (57) may fall into the drain pan. In the defrosting operation performed by the refrigeration system (10) of this embodiment, the drain pan is heated by the refrigerant flowing through the drain pan heater (63). Therefore, the frost that has fallen into the drain pan from the internal heat exchanger (57) is heated and melted by the drain pan, and is discharged as drain water to the space outside the compartment.

[0218] Third Embodiment A third embodiment will be described. In this embodiment, the transport container (1) of the second embodiment is modified in terms of the configuration of the refrigeration unit (10).

[0219] The refrigeration system (10) of this embodiment differs from the refrigeration system (10) of embodiment 2 in the configuration of the refrigerant circuit (30). Here, the refrigerant circuit (30) of this embodiment differs from the refrigerant circuit (30) of embodiment 2 in the following points.

[0220] As shown in Fig. 15, in the refrigerant circuit (30) of this embodiment, a reheat heat exchanger (58) and a third solenoid valve (SV3) are provided in the defrosting pipe (32), and the reheating pipe (33) and the fifth expansion valve (EV5) are omitted. In the refrigerant circuit (30) of this embodiment, the defrosting pipe (32) also serves as the reheating pipe (33) that sends refrigerant to the reheating heat exchanger (58). In the defrosting pipe (32) of this embodiment, the reheating heat exchanger (58) is arranged upstream of the fourth expansion valve (EV4), a drain pan heater (63) is arranged upstream of the reheating heat exchanger (58), and the third solenoid valve (SV3) is arranged upstream of the drain pan heater (63). The path change mechanism (67) of this embodiment does not include the fourth expansion valve (EV4).

[0221] —Operation of Controller— In the refrigeration system of the present embodiment, the controller (90) controls the aperture of the fourth expansion valve (EV4) in both the defrosting operation and the dehumidifying operation. In the defrosting operation, the controller (90) of the present embodiment controls the aperture of the fourth expansion valve (EV4) in the same manner as the controller (90) of the first and second embodiments. In the dehumidifying operation, the controller (90) of the present embodiment controls the aperture of the fourth expansion valve (EV4) in the same manner as the controller (90) of the first and second embodiments controls the aperture of the fifth expansion valve (EV5). In other words, in the dehumidifying operation, the controller (90) of the present embodiment controls the aperture of the fourth expansion valve (EV4) so ​​that the temperature of the air passing through the reheat heat exchanger (58) (specifically, the measurement value of the second air temperature sensor (87)) becomes a set temperature.

[0222] Fourth Embodiment A fourth embodiment will be described. In the refrigeration system (10) of this embodiment, the controller (90) performs substantially the same operations as the controller (90) of the first embodiment during the defrosting operation of the refrigeration system (10). Here, the operations performed by the controller (90) of this embodiment during the defrosting operation of the refrigeration system (10) will be described with reference to FIG. 16 .

[0223] As described above, when the defrosting start condition is satisfied during the cooling operation, the refrigeration system (10) temporarily suspends the cooling operation and performs the defrosting operation. When the refrigeration system (10) starts the normal operation of the defrosting operation, the controller (90) first performs the process of step ST1.

[0224] <Step ST1> In the process of step ST1, the controller (90) sets the opening of the fourth expansion valve (EV4) to an initial opening. After the process of step ST1 ends, the controller (90) performs the process of step ST2.

[0225] <Step ST2> In the process of step ST2, the controller (90) controls the rotation speed of the high-stage compressor (52) based on the input current. The controller (90) controls the rotation speed of the high-stage compressor (52) so that the input current becomes the target current. The operation of this controller (90) is the same as the operation of the controller (90) of the first embodiment, in which the controller (90) controls the rotation speed of the high-stage compressor (52) based on the input current. After the process of step ST2 is completed, the controller (90) performs the process of step ST3.

[0226] <Step ST3> In the process of step ST3, the controller (90) determines whether the defrost termination condition is met. If the defrost termination condition is met, the controller (90) terminates the defrosting operation of the refrigeration system (10) and restarts the cooling operation of the refrigeration system (10). On the other hand, if the defrost termination condition is not met, the controller (90) performs the process of step ST4.

[0227] <Step ST4> In the process of step ST4, the controller (90) compares the measurement value Pdh of the high-stage discharge pressure sensor (78) with the upper limit pressure. If the condition that "the measurement value Pdh of the high-stage discharge pressure sensor (78) is equal to or greater than the upper limit pressure" is met, the controller (90) performs the process of step ST9. On the other hand, if this condition is not met, the controller (90) performs the process of step ST5.

[0228] <Step ST5> In the process of step ST5, the controller (90) compares the measurement value Tdh of the high-stage discharge temperature sensor with the upper limit temperature. If the condition that "the measurement value Tdh of the high-stage discharge temperature sensor is equal to or greater than the upper limit temperature" is met, the controller (90) performs the process of step ST11. On the other hand, if this condition is not met, the controller (90) performs the process of step ST6.

[0229] <Step ST6> In the process of step ST6, the controller (90) compares the input current of the high-stage compressor (52) with a target current, and compares the rotation speed RSh of the high-stage compressor (52) with a reference speed. If at least one of the conditions that "the input current of the high-stage compressor (52) is equal to or greater than the target current" and "the rotation speed RSh of the high-stage compressor (52) is lower than the reference speed" is satisfied, the controller (90) performs the process of step ST2. On the other hand, if neither of these two conditions is satisfied, the controller (90) performs the process of step ST7.

[0230] <Step ST7> In the process of step ST7, the controller (90) compares the opening degree of the fourth expansion valve (EV4) with a reference opening degree. If the condition that "the opening degree of the fourth expansion valve (EV4) is equal to or less than the reference opening degree" is met, the controller (90) performs the process of step ST11. On the other hand, if this condition is not met, the controller (90) performs the process of step ST8.

[0231] <Step ST8> In the process of step ST8, the controller (90) controls the aperture of the fourth expansion valve (EV4) so ​​that the measurement value Pdh of the high-stage discharge pressure sensor (78) falls within the reference pressure range. The operation of this controller (90) is the same as the operation of the controller (90) in the first embodiment, in which the controller (90) controls the aperture of the fourth expansion valve (EV4) based on the measurement value Pdh of the high-stage discharge pressure sensor (78). After completing the process of step ST8, the controller (90) performs the process of step ST2.

[0232] <Step ST9> The condition in the process of step ST4 that “the measured value Pdh of the high-stage discharge pressure sensor (78) is equal to or greater than the upper limit pressure” is an inflow start condition. As described in the description of the first embodiment, the inflow start condition is a condition indicating that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is excessive.

[0233] Therefore, in the process of step ST9, the controller (90) switches the operation performed by the refrigeration system (10) from the normal operation to the inflow operation. Thereafter, the controller (90) performs the process of step ST10.

[0234] <Step ST10> In the process of step ST10, the controller (90) determines whether the condition that “the measured value Pdh of the high-stage discharge pressure sensor (78) is lower than the upper limit pressure” is satisfied. This condition is the inflow termination condition. As described in the description of the first embodiment, the inflow termination condition is a condition that indicates that the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is appropriate.

[0235] If the inflow end condition is satisfied, the controller (90) performs the process of step ST2. On the other hand, if the inflow end condition is not satisfied, the controller (90) causes the refrigeration system (10) to continue the inflow operation.

[0236] <Step ST11> The condition in the process of step ST5 that “the measured value Tdh of the high-stage discharge temperature sensor is equal to or higher than the upper limit temperature” is the outflow start condition. As described in the description of the first embodiment, the outflow start condition is a condition indicating that the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is insufficient.

[0237] Therefore, in the process of step ST11, the controller (90) switches the operation performed by the refrigeration system (10) from the normal operation to the outflow operation, and then the controller (90) performs the process of step ST12.

[0238] <Step ST12> In the process of step ST12, the controller (90) determines whether the condition that "the duration of the outflow operation reaches a predetermined time" is met. This condition is the outflow termination condition. As described in the description of the first embodiment, the outflow termination condition is a condition for terminating the outflow operation of the refrigeration system (10).

[0239] If the outflow termination condition is satisfied, the controller (90) performs the process of step ST2. On the other hand, if the outflow termination condition is not satisfied, the controller (90) causes the refrigeration system (10) to continue the outflow operation.

[0240] Other Embodiments The refrigeration system (10) of the above embodiment may be modified as follows: The following modifications may be combined or substituted as appropriate, as long as the functionality of the refrigeration system (10) is not impaired.

[0241] First Modification A plurality of fourth expansion valves (EV4-1, EV4-2) may be provided in the refrigerant circuit (30) of the refrigeration system (10) of any of the first to fourth embodiments. Here, the differences between the first modification and the refrigeration system (10) of the first embodiment will be described.

[0242] As shown in Fig. 17 , in the refrigerant circuit (30) of this modified example, two fourth expansion valves (EV4-1, EV4-2) are provided in the defrosting pipe (32). The two fourth expansion valves (EV4-1, EV4-2) are connected in parallel in the defrosting pipe (32). One fourth expansion valve (EV4-1) is a first pressure reducing valve. The other fourth expansion valve (EV4-2) is a second pressure reducing valve. Note that three or more fourth expansion valves may be connected in parallel in the defrosting pipe (32) of this modified example.

[0243] In the refrigeration system (10) of this modified example, the controller (90) sets the fourth expansion valves (EV4-1, EV4-2) to the same opening degree. When changing the opening degrees of the fourth expansion valves (EV4-1, EV4-2), the controller (90) increases or decreases the opening degrees of the fourth expansion valves (EV4-1, EV4-2) by the same amount. However, the controller (90) may be configured to individually control the opening degrees of the fourth expansion valves (EV4-1, EV4-2).

[0244] -Second Modification- The refrigeration system (10) of any of the first to fourth embodiments may be configured to perform only a single-stage compression refrigeration cycle. Here, the differences between the refrigeration system (10) of the third embodiment and the refrigeration system (10) of the first to fourth embodiments will be described.

[0245] 18, the refrigerant circuit (30) of the refrigeration system (10) of this modified example does not include the low-stage compressor (51), the low-stage connecting pipe (44), and the high-stage connecting pipe (45). The refrigerant circuit (30) of this modified example is provided with one compressor (52) that corresponds to the high-stage compressor of the refrigerant circuit (30) of the third embodiment.

[0246] In the main circuit (31) of the refrigerant circuit (30) of this modified example, the other end of the internal heat exchanger (57) is connected to the suction pipe of the compressor (52). The first motor-operated valve (MV1) is provided in the main circuit (31) between the internal heat exchanger (57) and the suction pipe of the compressor (52). The other end of the intermediate connecting pipe (43) is connected to the main circuit (31) between the first motor-operated valve (MV1) and the compressor (52).

[0247] In the cooling operation of the refrigeration system (10) of this modified example, the refrigerant circuit (30) operates in a single-stage compression refrigeration cycle. In the refrigerant circuit (30), the compressor (52) is activated, and refrigerant circulates in the main circuit (31). The refrigerant discharged from the compressor (52) dissipates heat to the outside air in the external heat exchanger (56). The refrigerant flowing out of the external heat exchanger (56) passes through the first flow path (61a) of the internal heat exchanger (61), the first expansion valve (EV1), the receiver (62), the first solenoid valve (SV1), and the second expansion valve (EV2), in that order, and then flows into the internal heat exchanger (57). The refrigerant flowing into the internal heat exchanger (57) absorbs heat from the air flowing through the internal flow path (20) and evaporates. The evaporated refrigerant in the internal heat exchanger (57) is drawn into the compressor (52). The compressor (52) compresses the sucked refrigerant and discharges it.

[0248] -Third Modification- The controller (90) of the refrigeration system (10) of any of the first to fourth embodiments may control the aperture of the fourth expansion valve (EV4) based on the temperature of the refrigerant discharged from the high-stage compressor (52) (specifically, the measurement value of the high-stage discharge temperature sensor (73)) during normal defrosting operation. The controller (90) reduces the aperture of the fourth expansion valve (EV4) as much as possible within a range in which the measurement value of the high-stage discharge temperature sensor (73) is maintained at or below a reference temperature.

[0249] Specifically, the controller (90) controls the opening of the fourth expansion valve (EV4) so ​​that the measurement value of the high-stage discharge temperature sensor (73) falls within a reference temperature range. The reference temperature range is a temperature range that includes the reference temperature. The maximum value of the reference temperature range is the reference temperature.

[0250] When the measurement value of the high stage discharge temperature sensor (73) is lower than the minimum value of the reference temperature range, the controller (90) reduces the opening of the fourth expansion valve (EV4). When the measurement value of the high stage discharge temperature sensor (73) is higher than the maximum value of the reference temperature range (=reference temperature), the controller (90) increases the opening of the fourth expansion valve (EV4). When the measurement value of the high stage discharge temperature sensor (73) is within the reference temperature range, the controller (90) maintains the opening of the fourth expansion valve (EV4).

[0251] In addition, the controller (90) of this modified example may control the aperture of the fourth expansion valve (EV4) based on both the temperature and pressure of the refrigerant discharged from the high-stage compressor (52) during normal operation of the defrosting operation. In this case, the controller (90) reduces the aperture of the fourth expansion valve (EV4) as much as possible within a range in which the measurement value of the high-stage discharge pressure sensor (78) is maintained at or below the reference pressure and the measurement value of the high-stage discharge temperature sensor (73) is maintained at or below the reference temperature.

[0252] Fourth Modification The controller (90) of the refrigeration system (10) of any of the first to fourth embodiments may select either the gas outflow operation or the liquid outflow operation based on the outside air temperature in the first outflow operation of the defrosting operation.

[0253] When the outdoor air temperature is relatively low, the pressure of the refrigerant in the receiver (62) becomes relatively low. If the gas discharge operation is performed under such conditions, the flow rate of the gas refrigerant flowing out of the receiver (62) decreases, and it may take a long time to increase the mass of the refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57). Therefore, the controller (90) of this modified example causes the refrigeration system (10) to perform the gas discharge operation when the outdoor air temperature is equal to or higher than a reference outdoor air temperature, and causes the refrigeration system (10) to perform the liquid discharge operation when the outdoor air temperature is lower than the reference outdoor air temperature.

[0254] Fifth Modification The refrigeration system (10) of the first and second embodiments may be configured without the reheat heat exchanger (58) and the reheat pipe (33). In the refrigeration system (10) of this modification, the third solenoid valve (SV3) and the fifth expansion valve (EV5) provided in the reheat pipe (33) are also omitted. Therefore, the path change mechanism (67) of this modification does not include the third solenoid valve (SV3).

[0255] Sixth Modification The first solenoid valve (SV1) may be omitted from the refrigeration system (10) of the first and second embodiments. When the first solenoid valve (SV1) is held in a closed state in the refrigeration system (10) of the first and second embodiments, the second expansion valve (EV2) is held in a fully closed state in the refrigeration system (10) of this modification. The path change mechanism (67) of this modification does not include the first solenoid valve (SV1).

[0256] -Seventh Modification- The use of the refrigeration system (10) of the first to fourth embodiments is not limited to air conditioning the interior space (5) of the transport container (1). The refrigeration system (10) of the first to fourth embodiments may be used to air condition the interior space of a stationary refrigerator or a freezer warehouse, for example.

[0257] Although the embodiments and modifications have been described above, it will be understood that various modifications in form and detail are possible without departing from the spirit and scope of the claims. Furthermore, elements of the above embodiments, modifications, and other embodiments may be combined or substituted as appropriate. Furthermore, the terms "first," "second," "third," etc. in the specification and claims are used to distinguish between terms to which these terms are attached, and do not limit the number or order of those terms.

[0258] As described above, the present disclosure is useful for refrigeration devices and shipping containers.

[0259] 1 Shipping container 2 Container body 5 Internal space (target space) 10 Refrigeration unit 30 Refrigerant circuit 32 Defrosting piping (bypass passage) 50 Compressor 56 External heat exchanger (heat source side heat exchanger) 57 Internal heat exchanger (use side heat exchanger) 58 Reheat heat exchanger 62 Receiver 65 Expansion valve 67 Path change mechanism 90 Controller EV4 Fourth expansion valve (pressure reducing valve) EV4-1 Fourth expansion valve (first pressure reducing valve) EV4-2 Fourth expansion valve (second pressure reducing valve)

Claims

1. A refrigeration system (10) for air-conditioning a target space (5), comprising a refrigerant circuit (30) having a compressor (50), a heat source side heat exchanger (56), an expansion valve (65), and a utilization side heat exchanger (57), wherein the refrigerant circuit (30) comprises: a bypass passage (32) for sending refrigerant discharged from the compressor (50) to the utilization side heat exchanger (57) bypassing the heat source side heat exchanger (56) and the expansion valve (65); and a pressure reducing valve (EV4) having a variable opening for reducing the pressure of the refrigerant flowing through the bypass passage (32), wherein the refrigeration system (10) performs a refrigeration cycle in which the heat source side heat exchanger (56) functions as a radiator and the utilization side heat exchanger (57) functions as an evaporator, and performs a cooling operation in which air cooled in the utilization side heat exchanger (57) is blown into the target space (5); The refrigeration system performs a defrosting operation to melt frost adhering to the utilization side heat exchanger (57) by supplying the refrigerant discharged from the compressor (50) to the utilization side heat exchanger (57) through the bypass passage (32), and includes a controller (90) that controls an opening degree of the pressure reducing valve (EV4) based on one or both of the pressure and the temperature of the refrigerant discharged from the compressor (50) during the defrosting operation.

2. The refrigeration device according to claim 1, wherein the refrigerant circuit (30) has a receiver (62) arranged between the heat source side heat exchanger (56) and the utilization side heat exchanger (57), and the refrigeration device (10) performs, during the defrosting operation, a normal operation of circulating refrigerant between the compressor (50) and the utilization side heat exchanger (57) while blocking the inflow and outflow of refrigerant into the receiver (62), an outflow operation of causing refrigerant to flow out of the receiver (62) to reduce the amount of refrigerant stored in the receiver (62), and an inflow operation of causing refrigerant to flow into the receiver (62) to increase the amount of refrigerant stored in the receiver (62).

3. The refrigeration system according to claim 2, wherein the outflow operation is started when the temperature of the refrigerant discharged from the compressor (50) becomes higher than a reference temperature during the normal operation.

4. The refrigeration system according to claim 2 or 3, wherein the normal operation is started when the degree of superheat of the refrigerant flowing out of the utilization side heat exchanger (57) becomes lower than a reference degree of superheat during the outflow operation.

5. The refrigeration system according to any one of claims 2 to 4, wherein the outflow operation includes a gas outflow operation for causing gas refrigerant to flow out of the receiver (62), and a liquid outflow operation for causing liquid refrigerant to flow out of the receiver (62).

6. The refrigeration system according to any one of claims 2 to 4, wherein in the outflow operation, the refrigerant flowing out of the receiver (62) is sucked into the compressor (50).

7. The refrigeration system according to claim 6, wherein the outflow operation includes: a first outflow operation in which the compressor (50) draws refrigerant from both the receiver (62) and the utilization side heat exchanger (57); and a second outflow operation in which the compressor (50) draws refrigerant from the receiver (62) but does not draw refrigerant from the utilization side heat exchanger (57).

8. A refrigeration system according to any one of claims 2 to 7, wherein the inflow operation is started when the pressure of the refrigerant discharged from the compressor (50) becomes higher than a reference pressure during the normal operation.

9. A refrigeration system as described in any one of claims 2 to 8, wherein, during the inflow operation, a portion of the refrigerant discharged from the compressor (50) passes through the heat source side heat exchanger (56) and flows into the receiver (62), and the remainder of the refrigerant discharged from the compressor (50) passes through the bypass passage (32) and is supplied to the utilization side heat exchanger (57).

10. A refrigeration system as described in any one of claims 1 to 9, wherein the refrigerant circuit (30) has a path change mechanism (67) that changes the flow path of the refrigerant in the refrigerant circuit (30), and the controller (90) controls the path change mechanism (67) to switch between the cooling operation and the defrosting operation.

11. A refrigeration system as described in any one of claims 2 to 9, wherein the refrigerant circuit (30) has a path change mechanism (67) that changes the flow path of the refrigerant in the refrigerant circuit (30), and the controller (90) controls the path change mechanism (67) to switch between the cooling operation and the defrosting operation, and to switch between the normal operation, the outflow operation, and the inflow operation in the defrosting operation.

12. A refrigeration system according to claim 10 or 11, wherein the path change mechanism (67) has a plurality of control valves controlled by the controller (90).

13. A refrigeration system according to any one of claims 1 to 12, wherein the pressure reducing valve includes a first pressure reducing valve (EV4-1) and a second pressure reducing valve (EV4-2) connected in parallel.

14. A refrigeration system as described in any one of claims 1 to 13, in which the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32), and the air heated in the utilization side heat exchanger (57) is blown into the target space (5) in a heating operation.

15. A refrigeration system as set forth in any one of claims 1 to 14, wherein the refrigerant circuit (30) has a reheat heat exchanger (58) arranged upstream of the pressure reducing valve (EV4) in the bypass passage (32) and exchanging heat between the air that has passed through the utilization side heat exchanger (57) and the refrigerant.

16. The refrigeration system according to any one of claims 1 to 15, wherein the refrigerant circuit (30) is filled with carbon dioxide as a refrigerant.

17. A transport container comprising: a refrigeration unit (10) according to any one of claims 1 to 16; and a container body (2) that forms the target space (5) that is air-conditioned by the refrigeration unit (10).

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