Refrigeration equipment and transport containers

Abstract

In refrigeration systems, the time required for defrosting the heat exchanger is reduced. [Solution] The refrigerant circuit (30) of the refrigeration unit (10) includes a defrosting pipe (32) and a fourth expansion valve (EV4). The refrigeration unit (10) performs a defrosting operation to melt the frost adhering to the internal heat exchanger (57). During the defrosting operation, the refrigerant discharged by the compressor (50) flows through the defrosting pipe (32), is depressurized as it passes through the fourth expansion valve (EV4), and then flows into the internal heat exchanger (57). During the defrosting operation, the controller (90) controls the opening degree of the fourth expansion valve (EV4) based on either or both the pressure and / or temperature of the refrigerant discharged by the compressor (50).

Description

Technical Field

[0001] The present disclosure relates to a refrigeration device and a transport container.

Background Art

[0002] Patent Document 1 discloses a heat pump device that performs 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 the refrigerant flows bypassing the heat radiator and the decompression means. In the defrosting operation for melting the frost adhering to the air heat exchanger, the refrigerant discharged from the compressor is supplied to the air heat exchanger through the high-pressure side defrosting circuit, and the frost adhering to the air heat exchanger is warmed and melted by the refrigerant.

[0003] In the defrosting operation performed by the heat pump device of Patent Document 1, the refrigerant circulates between the compressor and the air heat exchanger through the high-pressure side defrosting circuit. This defrosting operation is an operation for melting the frost adhering to the air heat exchanger by using the heat imparted to the refrigerant in the compressor.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the defrosting operation performed by the heat pump device of Patent Document 1, the refrigerant discharged from the compressor flows into the air heat exchanger without being substantially decompressed. Therefore, the difference between the pressure of the refrigerant sucked by the compressor and the pressure of the refrigerant discharged by the compressor becomes small, and there is a possibility that the amount of heat imparted to the refrigerant by the compressor decreases. If the amount of heat imparted to the refrigerant by the compressor in the defrosting operation is small, the amount of heat available for melting the frost adhering to the air heat exchanger decreases, and the time required for defrosting may become long.

[0006] The purpose of this disclosure is to shorten the time required for defrosting a heat exchanger in a refrigeration system that performs a refrigeration cycle. [Means for solving the problem]

[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) that sends the refrigerant discharged by the compressor (50) to the utilization side heat exchanger (57) by bypassing the heat source side heat exchanger (56) and the expansion valve (65), and a variable-opening pressure reducing valve (EV4) that reduces the pressure of the refrigerant flowing through the bypass passage (32), and the refrigeration system (10) has the heat source side heat exchanger (56) The system performs a refrigeration cycle in which the heat exchanger (57) functions as a heat sink and the heat exchanger on the utilization side functions as an evaporator. This cycle includes a cooling operation in which the air cooled in the heat exchanger (57) is blown out to the target space (5), and a defrosting operation in which the refrigerant discharged by the compressor (50) is supplied to the heat exchanger (57) through the bypass passage (32) to melt frost adhering to the heat exchanger (57). In the defrosting operation, the system includes a controller (90) that controls the opening degree of the pressure reducing valve (EV4) based on either or both of the pressure and temperature of the refrigerant discharged by the compressor (50).

[0008] In the first embodiment, 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 depressurized as it passes through the pressure reducing valve (EV4). Therefore, compared to the case where the refrigerant discharged by the compressor (50) is supplied to the utilization-side heat exchanger (57) without depressurizing it, the difference between the pressure of the refrigerant drawn in by the compressor (50) (suction pressure) and the pressure of the refrigerant discharged by the compressor (50) (discharge pressure) becomes larger. When the difference between suction pressure and discharge pressure becomes larger, the amount of heat imparted to the refrigerant during the process in which the compressor (50) compresses the refrigerant increases. Therefore, according to this embodiment, compared to the case in which the refrigerant discharged by the compressor (50) is supplied to the utilization-side heat exchanger (57) without depressurizing, the amount of heat available to melt the frost adhering to the utilization-side heat exchanger (57) increases, and the time required for defrosting the utilization-side heat exchanger (57) is shortened.

[0009] A second aspect of the present disclosure is the first aspect described above, wherein the refrigerant circuit (30) has a receiver (62) positioned between the heat source side heat exchanger (56) and the utilization side heat exchanger (57), and the refrigeration device (10) in the defrosting operation performs a normal operation in which the refrigerant is circulated between the compressor (50) and the utilization side heat exchanger (57) with the inflow and outflow of refrigerant to the receiver (62) blocked, an outflow operation in which refrigerant is discharged from the receiver (62) to reduce the amount of refrigerant stored in the receiver (62), and an inflow operation in which refrigerant is discharged into the receiver (62) to increase the amount of refrigerant stored in the receiver (62).

[0010] The refrigeration system (10) of the second embodiment performs normal operation, outflow operation, and inflow operation during defrosting. The outflow operation and inflow operation are operations to adjust the amount of refrigerant circulating between the compressor (50) and the utilization-side heat exchanger (57) during defrosting. During 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. During 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 this disclosure is that, in the second aspect described above, if the temperature of the refrigerant discharged from the compressor (50) becomes higher than a reference temperature during the execution of the normal operation, the discharge operation is initiated.

[0012] If the temperature of the refrigerant discharged from the compressor (50) becomes high during normal operation, it can be inferred that the amount of refrigerant circulating between the compressor (50) and the user-side heat exchanger (57) is insufficient. Therefore, in the third embodiment of the refrigeration system (10), if the temperature of the refrigerant discharged from the compressor (50) becomes higher than the reference temperature during normal operation, the normal operation is terminated and the discharge operation is started.

[0013] A fourth aspect of this disclosure is that, in the second or third aspect described above, if the degree of superheat of the refrigerant discharged from the user-side heat exchanger (57) falls below a reference degree of superheat during the discharge operation, the normal operation is initiated.

[0014] If the superheating degree of the refrigerant discharged from the user-side heat exchanger (57) during the discharge operation decreases, it can be inferred that the amount of refrigerant circulating between the compressor (50) and the user-side heat exchanger (57) is appropriate. Therefore, in the fourth embodiment of the refrigeration system (10), if the superheating degree of the refrigerant discharged from the user-side heat exchanger (57) during the discharge operation falls below the reference superheating degree, the discharge operation is terminated and normal operation is started.

[0015] A fifth aspect of this disclosure is, in any one of the second to fourth aspects described above, the discharge operation includes a gas discharge operation that discharges gaseous refrigerant from the receiver (62) and a liquid discharge operation that discharges liquid refrigerant from the receiver (62).

[0016] The refrigeration device (10) of the fifth embodiment performs either a gas discharge operation or a liquid discharge operation as an outflow operation.

[0017] A sixth aspect of this disclosure is that, in any one of the second to fourth aspects described above, the compressor (50) draws in the refrigerant that has flowed out from the receiver (62) during the discharge operation.

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

[0019] A seventh aspect of the present disclosure, in the sixth aspect described above, the discharge operation includes a first discharge operation in which the compressor (50) draws refrigerant from both the receiver (62) and the utilization-side heat exchanger (57), and a second discharge 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 embodiment performs either a first discharge operation or a second discharge operation as a discharge operation. In the first discharge operation, the compressor (50) draws in the refrigerant discharged from the receiver (62) and the refrigerant discharged from the utilization-side heat exchanger (57). In the second discharge operation, the compressor (50) draws in the refrigerant discharged from the receiver (62) but does not draw in the refrigerant discharged from the utilization-side heat exchanger (57).

[0021] An eighth aspect of this disclosure is that, in any one of the second to seventh aspects described above, if the pressure of the refrigerant discharged from the compressor (50) becomes higher than the reference pressure during the execution of the normal operation, the inflow operation is initiated.

[0022] If the pressure of the refrigerant discharged from the compressor (50) becomes high during normal operation, it can be inferred that the amount of refrigerant circulating between the compressor (50) and the user-side heat exchanger (57) is excessive. Therefore, in the refrigeration system (10) of the eighth embodiment, if the pressure of the refrigerant discharged from the compressor (50) becomes higher than the reference pressure during normal operation, the normal operation is terminated and the inflow operation is started.

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

[0024] In the inflow operation performed by the refrigeration device according to the ninth aspect, a part of the refrigerant discharged from the compressor (50) flows into the receiver (62), and the amount of refrigerant stored in the receiver (62) increases. Also, in this inflow operation, the rest of the refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32), and the frost adhering to the utilization side heat exchanger (57) melts.

[0025] In a tenth aspect of the present disclosure, in any one of the first to ninth aspects, the pressure reducing valve includes a first pressure reducing valve (EV4-1) and a second pressure reducing valve (EV4-2) connected in parallel.

[0026] In the bypass passage (32) of the refrigeration device (10) according to the tenth aspect, the first pressure reducing valve (EV4-1) and the second pressure reducing valve (EV4-2) are connected in parallel.

[0027] In an eleventh aspect of the present disclosure, in any one of the first to tenth aspects, a heating operation is performed in which the refrigerant discharged from the compressor (5) 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).

[0028] In the eleventh aspect, the refrigeration device (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 the 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).

[0029] A twelfth aspect of the present disclosure, in any one of the first to eleventh aspects, includes a refrigerant circuit (30) located upstream of the pressure reducing valve (EV4) in the bypass passage (32) and a reheat heat exchanger (58) that exchanges heat between the refrigerant and the air that has passed through the utilization-side heat exchanger (57).

[0030] In the twelfth embodiment, a reheat heat exchanger (58) is provided in the refrigerant circuit (30). The reheat heat exchanger (58) exchanges heat between the refrigerant supplied through the bypass passage (32) and the air that has passed through the utilization-side heat exchanger (57). The refrigerant that has passed through the reheat heat exchanger (58) is sent to the utilization-side heat exchanger (57) after passing through a pressure reducing valve (EV4).

[0031] A thirteenth aspect of this disclosure is that, in any one of the first to twelfth aspects described above, the refrigerant circuit (30) is filled with carbon dioxide as a refrigerant.

[0032] The refrigerant circuit (30) of the 13th embodiment performs a refrigeration cycle by circulating carbon dioxide, which is filled as a refrigerant.

[0033] A fourteenth aspect of this disclosure is a transport container (1) comprising a refrigeration device (10) according to any one of the first to thirteenth aspects described above, and a container body (2) that forms the target space (5) where air conditioning is performed by the refrigeration device (10).

[0034] A transport container (1) in the 14th embodiment comprises a refrigeration unit (10) and a container body (2). The container body (2) forms a target space (5). The refrigeration unit (10) provides air conditioning for the target space (5). [Brief explanation of the drawing]

[0035] [Figure 1] Figure 1 is a schematic perspective view of the refrigeration apparatus of Embodiment 1. [Figure 2] Figure 2 is a schematic cross-sectional view of the transport container according to Embodiment 1. [Figure 3] Figure 3 is a piping diagram showing the configuration of the refrigeration system of Embodiment 1. [Figure 4] Figure 4 is a block diagram showing the configuration of the controller in Embodiment 1. [Figure 5] Figure 5 is a piping diagram corresponding to Figure 3, showing the flow of refrigerant during cooling operation. [Figure 6] Figure 6 is a piping diagram corresponding to Figure 3, showing the flow of refrigerant during dehumidification operation. [Figure 7] Figure 7 is a piping diagram corresponding to Figure 3, showing the refrigerant flow during normal operation of heating and defrosting. [Figure 8] Figure 8 is a Mollier diagram (pressure-enthalpy diagram) showing the changes in the state of the refrigerant in the refrigerant circuit during defrosting operation. [Figure 9] Figure 9 is a piping diagram corresponding to Figure 3, showing the flow of refrigerant during the inflow operation. [Figure 10] Figure 10 is a piping diagram corresponding to Figure 3, showing the refrigerant flow during the gas discharge operation of the first discharge operation. [Figure 11] Figure 11 is a piping diagram corresponding to Figure 3, showing the flow of refrigerant during the liquid discharge operation of the first discharge operation. [Figure 12] Figure 12 is a piping diagram corresponding to Figure 3, showing the refrigerant flow during the gas discharge operation of the second discharge operation. [Figure 13] Figure 13 is a state transition diagram showing the operation of the controller during defrosting. [Figure 14] Figure 14 is a piping diagram showing the configuration of the refrigeration system of Embodiment 2. [Figure 15] Figure 15 is a piping diagram showing the configuration of the refrigeration system of Embodiment 3. [Figure 16] Figure 16 is a flowchart showing the operations performed by the controller during the defrosting operation of the refrigeration system in Embodiment 4. [Figure 17] Figure 17 is a piping diagram showing the configuration of a refrigeration system in a first modified example of another embodiment. [Figure 18] Figure 18 is a piping diagram showing the configuration of a refrigeration system in a second modified example of another embodiment. [Modes for carrying out the invention]

[0036] Embodiment 1 Embodiment 1 will now be described. This embodiment is a transport container (1) equipped with a refrigeration device (10).

[0037] -Shipping container- As shown in Figure 1, the transport container (1) comprises a container body (2) and a refrigeration unit (10). The transport container (1) is a reefer container that allows for temperature control inside the container.

[0038] The transport container (1) of this embodiment is mainly used for maritime transport. This transport container (1) is transported loaded onto ships or the like. However, the use of the transport container (1) is not limited to maritime transport. The transport container (1) may also be used for land transport. In this case, the transport container (1) is transported by automobiles such as trucks or by rail.

[0039] -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 in a horizontal shape. An opening is formed at one end of the container body (2) in the longitudinal direction. The opening of the container body (2) is closed by the refrigeration unit (10). The container body (2) forms an internal space (5) for storing cargo. The internal space (5) is the space to be air-conditioned by the refrigeration unit.

[0040] -Refrigeration equipment- As shown in Figure 2, the refrigeration unit (10) is attached to the opening of the container body (2). The refrigeration unit (10) in this embodiment is a transport refrigeration unit. The refrigeration unit (10) comprises a casing (11), a refrigerant circuit (30), and a controller (80). The refrigeration unit (10) adjusts the temperature of the air (internal air) in the internal space (5).

[0041] <Casing> The casing (11) includes a partition wall (12) and a partition plate (15).

[0042] An internal flow channel (20) is formed inside the partition wall (12). An external chamber (23) is formed outside the partition wall (12). The internal flow channel (20) and the external chamber (23) are separated by the partition wall (12).

[0043] The bulkhead (12) comprises an outer wall (13) and an inner wall (14). The outer wall (13) is located on the outside of the container body (2). The inner wall (14) is located on the inside of the container body (2).

[0044] The outer wall (13) closes the opening of the container body (2). The outer wall (13) is attached to the periphery of the opening of the container body (2). The lower part of the outer wall (13) bulges inward toward the inside of the container body (2). The outer chamber (23) is formed by the lower part of the outer wall (13).

[0045] 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 positioned at a distance from the exterior wall (13). An insulating material (16) is provided between the interior wall (14) and the exterior wall (13).

[0046] The partition plate (15) is positioned inside the container body (2) relative to the interior wall (14). An internal airflow channel (20) is formed between the partition wall (12) and the partition plate (15). An air intake (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 airflow channel (20) extends from the air intake (21) to the air outlet (22).

[0047] <Refrigerant Circuit> The refrigerant circuit (30) is a closed circuit filled with refrigerant. The refrigerant circuit (30) circulates the refrigerant to perform a vapor compression type 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 explained in more detail later.

[0048] Each of the external heat exchanger (56), internal heat exchanger (57), and reheat heat exchanger (58) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and air.

[0049] The external heat exchanger (56) is located at the top 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 outside air. The external heat exchanger (56) is generally rectangular in shape.

[0050] The internal heat exchanger (57) is located in the internal flow path (20). The internal heat exchanger (57) is a user-side heat exchanger that exchanges heat between the refrigerant and the air inside the chamber.

[0051] The reheat heat exchanger (58) is located 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.

[0052] Although not shown in the diagram, a drain pan is provided below the internal heat exchanger (57). The drain pan receives the condensate water generated in the internal heat exchanger (57). The condensate water that falls into the drain pan is discharged outside the chamber.

[0053] <External fan> The refrigeration unit (10) is equipped with an external fan (26). The external fan (26) is a propeller fan. The external fan (26) is located in the external chamber (23). The external fan (26) is also located inside the cylindrical external heat exchanger (56). The external fan (26) sends external air to the external heat exchanger (56).

[0054] <Interior fan> The refrigeration unit (10) is equipped with an internal fan (27). The internal fan (27) is a propeller fan. The internal fan (27) is positioned in the internal airflow path (20). The internal fan (27) is also positioned above the internal heat exchanger (57). The internal fan (27) sends internal air to the internal heat exchanger (57).

[0055] <Sensor> The refrigeration device (10) includes a first air temperature sensor (86), a second air temperature sensor (87), and a humidity sensor (88).

[0056] The first air temperature sensor (86) is positioned upstream of the internal fan (27) in the internal flow path (20). The first air temperature sensor (86) measures the temperature of the air that flows into the internal flow path (20) through the air intake (21).

[0057] The second air temperature sensor (87) and humidity sensor (88) are positioned 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).

[0058] <Electrical component box> As shown in Figure 1, the refrigeration unit (10) has an electrical component box (28). The electrical component box (28) is located at the top of the outer compartment (23). Electrical components such as an inverter board and a control board are housed inside the electrical component box (28).

[0059] - Refrigerant Circuit - As shown in Figure 3, the refrigerant circuit (30) is a closed circuit filled with refrigerant. The refrigerant filled in the refrigerant circuit (30) in this embodiment is carbon dioxide.

[0060] The refrigerant circuit (30) comprises a main circuit (31), defrosting piping (32), and reheating piping (33). The refrigerant circuit (30) also comprises 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).

[0061] <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 order by piping.

[0062] 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 motorized 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 motorized valve (MV1) is positioned downstream of the first check valve (CV1). The first check valve (CV1) allows the flow of refrigerant in the direction of outflow from the low-stage compressor (51) and blocks the flow of refrigerant in the reverse direction.

[0063] The 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 motorized valve (MV2) are provided in the piping connecting the discharge pipe of the high-stage compressor (52) and one end of the external heat exchanger (56). The second motorized valve (MV2) is positioned downstream of the second check valve (CV2). The second check valve (CV2) allows the flow of refrigerant in the direction of outflow from the high-stage compressor (52) and prevents the flow of refrigerant in the reverse direction.

[0064] The other end of the external heat exchanger (56) is connected to the inlet of the receiver (62). The piping connecting the other end of the external heat exchanger (56) and the inlet of the receiver (62) is provided with the first flow path (61a) of the internal heat exchanger (61) and the first expansion valve (EV1). The first expansion valve (EV1) is located downstream of the internal heat exchanger (61).

[0065] The 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 the piping connecting the liquid outlet of the receiver (62) and one end of the internal heat exchanger (57). The second expansion valve (EV2) is located downstream of the first solenoid valve (SV1).

[0066] The other end of the internal heat exchanger (57) is connected to the suction pipe of the low-stage compressor (51).

[0067] In the main circuit (31), the first expansion valve (EV1) is located upstream of the receiver (62), and the second expansion valve (EV2) is located downstream of the receiver (62). The first expansion valve (EV1) and the second expansion valve (EV2) are the expansion valves (65) of the refrigerant circuit (30).

[0068] <Gas side connection pipe> One end of the gas-side connecting pipe (41) is connected to the 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 located downstream of the second solenoid valve (SV2).

[0069] <Liquid-side connecting pipe> One end of the liquid-side connecting pipe (42) is connected 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). A third expansion valve (EV3) is provided in the liquid-side connecting pipe (42).

[0070] <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 between the first electric valve (MV1) and the high-stage compressor (52) in the main circuit (31). A third check valve (CV3) is provided in the intermediate connecting pipe (43). The third check valve (CV3) allows the flow of refrigerant from one end of the intermediate connecting pipe (43) to the other end, and prevents the flow of refrigerant in the reverse direction.

[0071] <Low-stage connecting pipe> The low-stage connecting pipe (44) is a pipe that bypasses the low-stage compressor (51) and allows refrigerant to flow. One end of the low-stage connecting pipe (44) is connected between the suction pipe of the low-stage compressor (51) and the internal heat exchanger (57) in the main circuit (31). The other end of the low-stage connecting pipe (44) is connected between the first check valve (CV1) and the first motorized valve (MV1) in the main circuit (31). A fourth check valve (CV4) is provided in the low-stage connecting pipe (44). The fourth check valve (CV4) allows the flow of refrigerant from one end of the low-stage connecting pipe (44) to the other end, and prevents the flow of refrigerant in the reverse direction.

[0072] <High-stage connecting pipe> The high-stage connecting pipe (45) is a pipe that bypasses the high-stage compressor (52) and allows refrigerant to flow. One end of the high-stage connecting pipe (45) is connected between the first check valve (CV1) and the first motorized valve (MV1) in the main circuit (31). The other end of the high-stage connecting pipe (45) is connected between the second check valve (CV2) and the second motorized valve (MV2) in the main circuit (31). A fifth check valve (CV5) is provided in the high-stage connecting pipe (45). The fifth check valve (CV5) allows the flow of refrigerant from one end of the high-stage connecting pipe (45) to the other end, and prevents the flow of refrigerant in the reverse direction.

[0073] <Defrost piping> One end of the defrosting pipe (32) is connected between the second check valve (CV2) and the second electric valve (MV2) in the main circuit (31). 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 between the second expansion valve (EV2) and the internal heat exchanger (57) in the main circuit (31). The defrosting pipe (32) forms a bypass passage that sends the refrigerant discharged by the high-stage compressor (52) to the internal heat exchanger (57), bypassing the external heat exchanger (56), the first expansion valve (EV1), and the second expansion valve (EV2).

[0074] A fourth expansion valve (EV4) is provided in the defrosting piping (32). The fourth expansion valve (EV4) is a variable-opening pressure reducing valve that reduces the pressure of the refrigerant flowing through the defrosting piping (32).

[0075] <Reheat piping> One end of the reheat piping (33) is connected between the second check valve (CV2) and the second electric valve (MV2) in the main circuit (31). In the main circuit (31), one end of the reheat piping (33) is located downstream of the other end of the high-stage connecting pipe (45). The other end of the reheat piping (33) is connected between the second expansion valve (EV2) and the internal heat exchanger (57) in the main circuit (31).

[0076] The reheat piping (33) is equipped with a third solenoid valve (SV3), a reheat heat exchanger (58), and a fifth expansion valve (EV5). The reheat heat exchanger (58) is located downstream of the third solenoid valve (SV3). The fifth expansion valve (EV5) is located downstream of the reheat heat exchanger (58).

[0077] <Expansion valve> The first expansion valve (EV1), second expansion valve (EV2), third expansion valve (EV3), fourth expansion valve (EV4), and fifth expansion valve (EV5) are all so-called electronic expansion valves. Each expansion valve (EV1 to EV5) comprises a valve body and a stepping motor that drives the valve body. When the valve body is moved by the stepping motor, the opening degree of the expansion valves (EV1 to EV5) changes continuously.

[0078] <Electric valve> The first electric valve (MV1) and the second electric valve (MV2) are both variable-opening valves. Each electric valve (MV1, MV2) comprises a valve body and a stepping motor that drives the valve body. When the valve body is moved by the stepping motor, the opening degree of the electric valves (MV1, MV2) changes continuously.

[0079] <Solenoid valve> The first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) are each on-off valves. Each solenoid valve (SV1 to SV3) comprises a valve body and a solenoid that drives the valve body. When the valve body is moved by the solenoid, the solenoid valves (SV1 to SV3) open and close.

[0080] <Low-stage compressor, high-stage compressor> The low-stage compressor (51) and the high-stage compressor (52) are both fully enclosed scroll compressors. Although not shown in the diagram, each of the low-stage compressor (51) and the high-stage compressor (52) comprises 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 a refrigerant and compresses it.

[0081] 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). The low-stage compressor (51) and the high-stage compressor (52) each compress the refrigerant drawn in from the suction pipe and discharge the compressed refrigerant from the discharge pipe. The low-stage compressor (51) and the high-stage compressor (52) are compressors (50) provided in the refrigerant circuit (30).

[0082] Note that the low-stage compressor (51) and the high-stage compressor (52) are not limited to scroll compressors. The low-stage compressor (51) and the high-stage compressor (52) may be, for example, a rotary compressor or a reciprocating compressor.

[0083] <External heat exchanger, internal heat exchanger> As described above, the external heat exchanger (56), the internal heat exchanger (57), and the reheat heat exchanger (58) are each fin-and-tube heat exchangers that exchange heat between the refrigerant and the air. The external heat exchanger (56) exchanges heat between the refrigerant and the outside air (outside air). The internal heat exchanger (57) and the reheat heat exchanger (58) each exchange heat between the refrigerant and the inside air.

[0084] <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) has a first flow path (61a) and a second flow path (61b). The first flow path (61a) of the internal heat exchanger (61) is located 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 located 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).

[0085] <Receiver> The receiver (62) is a container-shaped component for storing the refrigerant. The receiver (62) also functions as a gas-liquid separator. The receiver (62) separates the gas-liquid two-phase refrigerant flowing in from the inlet into liquid refrigerant and gaseous refrigerant. In the receiver (62), the liquid refrigerant accumulates at the bottom of the receiver (62) and flows out through a liquid outlet formed at the bottom of the receiver (62). In the receiver (62), the gaseous refrigerant accumulates at the top of the receiver (62) and flows out through a gas outlet formed at the top of the receiver (62).

[0086] <Sensors related to low-stage compressors> In the main circuit (31), a low-stage suction temperature sensor (70) and a low-stage suction pressure sensor (75) are provided in the piping connected to the suction pipe of the low-stage compressor (51). The low-stage suction temperature sensor (70) measures the temperature of the refrigerant suctioned by the low-stage compressor (51). The low-stage suction pressure sensor (75) measures the pressure of the refrigerant suctioned by the low-stage compressor (51).

[0087] In the main circuit (31), a low-stage discharge temperature sensor (71) and a low-stage discharge pressure sensor (76) are provided in the 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 by the low-stage compressor (51). The low-stage discharge pressure sensor (76) measures the pressure of the refrigerant discharged by the low-stage compressor (51).

[0088] <Sensors related to high-stage compressors> 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 electric valve (MV1). The high-stage suction temperature sensor (72) measures the temperature of the refrigerant suctioned by the high-stage compressor (52). The high-stage suction pressure sensor (77) measures the pressure of the refrigerant suctioned by the high-stage compressor (52).

[0089] In the main circuit (31), a high-stage discharge temperature sensor (73) and a high-stage discharge pressure sensor (78) are provided in the 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 by the high-stage compressor (52). The high-stage discharge pressure sensor (78) measures the pressure of the refrigerant discharged by the high-stage compressor (52).

[0090] <Other sensors> The refrigerant circuit (30) is equipped with a receiver pressure sensor (79), first to fourth refrigerant temperature sensors (81 to 84), and a heat exchanger temperature sensor (85).

[0091] The receiver pressure sensor (79) is connected between the receiver (62) and the second solenoid valve (SV2) in the gas-side connecting pipe (41). The receiver pressure sensor (79) measures the pressure of the refrigerant stored in the receiver (62).

[0092] The first refrigerant temperature sensor (81) is installed in the piping between the external heat exchanger (56) and the internal heat exchanger (61) in the main circuit (31). 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).

[0093] The second refrigerant temperature sensor (82) is installed in the piping 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 that has flowed out from the liquid outlet of the receiver (62).

[0094] The third refrigerant temperature sensor (83) is installed in the piping between the second expansion valve (EV2) and the internal heat exchanger (57) in the main circuit (31). The third refrigerant temperature sensor (83) is positioned near 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).

[0095] The fourth refrigerant temperature sensor (84) is installed in the piping between the internal heat exchanger (57) and the low-stage compressor (51) in the main circuit (31). The fourth refrigerant temperature sensor (84) is positioned 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).

[0096] The heat exchanger temperature sensor (85) is attached to the internal heat exchanger (57). The heat exchanger temperature sensor (85) measures the temperature of the internal heat exchanger (57).

[0097] -Controller- As shown in Figure 4, the controller (90) comprises 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 an electrical components box (28).

[0098] The controller (90) receives the measured values ​​from sensors installed in the refrigeration unit (10). Based on the measured values ​​from the sensors that are input, the controller (90) controls the equipment installed in the refrigeration unit (10). For example, the controller (90) controls the rotational speed of the low-stage compressor (51), the rotational speed of the high-stage compressor (52), the opening degree of the first to fifth expansion valves (EV1 to EV5), the opening degree of the first to second electric valves (MV1, MV2), the rotational speed of the external fan (26), the rotational speed of the internal fan (27), and so on.

[0099] - Refrigeration System Operation - The operation of the refrigeration unit (10) will be explained. The refrigeration unit (10) performs cooling, dehumidifying, heating, and defrosting operations.

[0100] <Cooling operation> Cooling operation is an operation to cool the air inside the storage chamber. During cooling operation, the refrigeration unit (10) blows the air cooled by the internal heat exchanger (57) into the storage chamber space (5).

[0101] During the cooling operation of the refrigeration unit (10), the refrigerant circuit (30) performs the refrigeration cycle. During the cooling operation, the refrigerant circulates in the main circuit (31) of the refrigerant circuit (30), the external heat exchanger (56) functions as a heat 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). Also during the cooling operation, the refrigerant flows through the gas-side connecting pipe (41) and the intermediate connecting pipe (43).

[0102] During 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 airflow channel (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 airflow channel (20) through the air intake (21). The internal air flowing through the internal airflow channel (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).

[0103] 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 into the chamber space (5) through the air outlet (22) (specifically, the value measured by the second air temperature sensor (87)) reaches the set temperature.

[0104] The cooling operation will be explained with reference to Figure 5.

[0105] During cooling operation, the controller (90) controls the rotational speed of the low-stage compressor (51) and the high-stage compressor (52). The controller (90) also controls the opening degree of the first expansion valve (EV1) and the second expansion valve (EV2), and holds the third expansion valve (EV3), the fourth expansion valve (EV4), and the fifth expansion valve (EV5) in a fully closed state. Furthermore, the controller (90) holds the first motor valve (MV1) and the second motor valve (MV2) in a fully open state, holds the first solenoid valve (SV1) and the second solenoid valve (SV2) in an open state, and holds the third solenoid valve (SV3) in a closed state.

[0106] The high-stage compressor (52) compresses the inhaled refrigerant to a pressure higher than the critical pressure of the refrigerant and discharges it. The refrigerant discharged from the high-stage compressor (52) exchanges heat with the 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 exchanging heat with the refrigerant flowing in 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 depressurized as it passes through the first expansion valve (EV1) and becomes a gas-liquid two-phase state. After that, the refrigerant flows into the receiver (62) and is separated into liquid refrigerant and gaseous refrigerant.

[0107] The liquid refrigerant flowing out from the receiver (62) is depressurized as it passes 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 from the internal heat exchanger (57) is drawn into the low-stage compressor (51). The low-stage compressor (51) compresses the drawn-in refrigerant and discharges it. The refrigerant discharged from the low-stage compressor (51) is drawn into the high-stage compressor (52).

[0108] The gaseous refrigerant flowing out of the receiver (62) flows through the gas-side connecting pipe (41) and into the second flow path (61b) of the internal heat exchanger (61), absorbing 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 drawn into the high-stage compressor (52) together with the refrigerant discharged from the low-stage compressor (51).

[0109] <Dehumidification operation> Dehumidification operation is performed to dehumidify the air inside the storage compartment (5) while maintaining the temperature inside the compartment. During dehumidification operation, the refrigeration unit (10) blows air that has been dehumidified in the internal heat exchanger (57) and then heated in the reheat heat exchanger (58) into the storage compartment (5).

[0110] In the dehumidification operation of the refrigeration unit (10), the controller (90) operates the external fan (26) and the internal fan (27), similar to the cooling operation. 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 intake (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 order, and is then supplied to the internal space (5) through the air outlet (22).

[0111] The dehumidification operation will be explained with reference to Figure 6.

[0112] During dehumidification operation, the controller (90) controls the rotational speed of the low-stage compressor (51) and the high-stage compressor (52). The controller (90) also controls the opening of the first expansion valve (EV1), the second expansion valve (EV2), and the fifth expansion valve (EV5), and holds the third expansion valve (EV3) and the fourth expansion valve (EV4) in a fully closed state. Furthermore, the controller (90) holds the first electric valve (MV1) and the second electric valve (MV2) in a fully open state, and holds the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) in an open state.

[0113] Dehumidification operation differs from cooling operation in that the controller (90) controls the opening degree of the fifth expansion valve (EV5) and holds the third solenoid valve (SV3) in the open position. Furthermore, dehumidification operation differs from 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 piping (33).

[0114] During dehumidification operation of the refrigeration unit (10), the refrigerant circuit (30) performs the refrigeration cycle. In dehumidification operation, similar to cooling operation, the refrigerant circulates in the main circuit (31) of the refrigerant circuit (30), the external heat exchanger (56) functions as a heat 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).

[0115] During dehumidification 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). As a result, moisture in the air condenses in the internal heat exchanger (57) to form condensate. The condensate generated in the internal heat exchanger (57) flows into the drain pan and is discharged outside the unit. Consequently, 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).

[0116] In dehumidification operation, the refrigerant flows through the gas-side connecting pipe (41) and the intermediate connecting pipe (43), similar to cooling operation. The gaseous refrigerant flowing out from the receiver (62) passes through the gas-side connecting pipe (41) and the intermediate connecting pipe (43) in sequence and is drawn into the high-stage compressor (52) together with the refrigerant discharged by the low-stage compressor (51).

[0117] During dehumidification, a portion of the refrigerant discharged from the high-stage compressor (52) is supplied to the reheat heat exchanger (58) through the reheat piping (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 released heat in the reheat heat exchanger (58) is depressurized as it passes through the fifth expansion valve (EV5). The refrigerant that has passed through the fifth expansion valve (EV5) flows into the main circuit (31) and, together with the refrigerant that has passed through the second expansion valve (EV2), flows into the internal heat exchanger (57).

[0118] During dehumidification operation, the controller (90) controls the rotational speed of the low-stage compressor (51) so that the humidity of the air that has passed through the reheat heat exchanger (58) reaches the target humidity. Specifically, the controller (90) controls the rotational speed of the low-stage compressor (51) so that the value measured by the humidity sensor (88) reaches the set humidity. If the value measured by the humidity sensor (88) is higher than the set humidity, the controller (90) increases the rotational speed of the low-stage compressor (51) to lower the evaporation temperature of the refrigerant in the internal heat exchanger (57). If the value measured by the humidity sensor (88) is lower than the set humidity, the controller (90) decreases the rotational speed of the low-stage compressor (51) to raise the evaporation temperature of the refrigerant in the internal heat exchanger (57).

[0119] Furthermore, during dehumidification operation, the controller (90) controls the opening of the fifth expansion valve (EV5) so that the temperature of the air that has passed through the reheat heat exchanger (58) reaches the set temperature. Specifically, the controller (90) controls the opening of the fifth expansion valve (EV5) so that the value measured by the second air temperature sensor (87) reaches the set temperature. If the value measured by the second air temperature sensor (87) is higher than the set temperature, the controller (90) reduces the opening of the fifth expansion valve (EV5) in order to reduce the flow rate of the refrigerant in the reheat heat exchanger (58). If the value measured by the second air temperature sensor (87) is lower than the set temperature, the controller (90) increases the opening of the fifth expansion valve (EV5) in order to increase the flow rate of the refrigerant in the reheat heat exchanger (58).

[0120] During dehumidification, the air flowing through the internal passage (20) experiences a decrease in both temperature and absolute humidity as it passes through the internal heat exchanger (57), and a rise in temperature as it passes through the reheat heat exchanger (58). Therefore, by performing dehumidification, the refrigeration unit (10) can maintain the temperature of the internal space (5) while lowering the humidity of the internal air.

[0121] <Heating operation> The heating operation is an operation to heat the air inside the storage compartment. During the heating operation, the refrigeration unit (10) blows the air heated in the internal heat exchanger (57) into the storage compartment space (5). The heating operation is performed, for example, when the outside temperature is lower than the set temperature of the storage compartment space (5), in order to maintain the temperature inside the storage compartment space (5) at the set temperature.

[0122] During the heating operation of the refrigeration unit (10), the controller (90) keeps the external fan (26) in a stopped state and operates 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 intake (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).

[0123] The heating operation will be explained with reference to Figure 7.

[0124] During heating operation, the controller (90) keeps the low-stage compressor (51) in a stopped state and controls the rotational speed of the high-stage compressor (52). The controller (90) also keeps the first expansion valve (EV1) in an open state, keeps the second expansion valve (EV2), third expansion valve (EV3), and fifth expansion valve (EV5) in a closed state, and controls the opening degree of the fourth expansion valve (EV4). Furthermore, the controller (90) keeps the first electric valve (MV1) in a fully open state, keeps the second electric valve (MV2) in a fully closed state, and keeps the first solenoid valve (SV1), second solenoid valve (SV2), and third solenoid valve (SV3) in a closed state.

[0125] During heating operation, the refrigerant circuit (30) 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 depressurized as it passes through the fourth expansion valve (EV4). After passing through the fourth expansion valve (EV4), the refrigerant flows into the internal heat exchanger (57) and dissipates heat to the air passing through the internal heat exchanger (57). The refrigerant that flows out of the internal heat exchanger (57) is drawn into the high-stage compressor (52) through the low-stage connecting pipe (44). The high-stage compressor (52) compresses the drawn-in refrigerant and discharges it.

[0126] During heating operation, the controller (90) controls the opening of the fourth expansion valve (EV4) so ​​that the temperature of the air that has passed through the internal heat exchanger (57) reaches the set temperature. Specifically, the controller (90) controls the opening of the fourth expansion valve (EV4) so ​​that the value measured by the second air temperature sensor (87) reaches the set temperature. If the value measured by the second air temperature sensor (87) is higher than the set temperature, the controller (90) reduces the opening of the fourth expansion valve (EV4) to decrease the flow rate of the refrigerant in the internal heat exchanger (57). If the value measured by the second air temperature sensor (87) is lower than the set temperature, the controller (90) increases the opening of the fourth expansion valve (EV4) to increase the flow rate of the refrigerant in the internal heat exchanger (57).

[0127] -Defrosting operation of the refrigeration system- Defrosting is performed to melt the frost that has accumulated on the internal heat exchanger (57) during cooling operation.

[0128] During 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 condenses as frost on the internal heat exchanger (57). When frost accumulates on the internal heat exchanger (57), it obstructs the airflow through the internal heat exchanger (57) and hinders heat exchange between the refrigerant and the air.

[0129] Therefore, when the defrosting start condition is met during the cooling operation, the refrigeration system (10) temporarily suspends the cooling operation and performs a defrosting operation. The defrosting start condition is, for example, "the cumulative value of the time spent in a cooling operation where the evaporation temperature of the refrigerant in the internal heat exchanger is lower than 0°C has reached a predetermined time (for example, 2 hours)."

[0130] Furthermore, if the defrosting termination condition is met during the defrosting operation, the refrigeration unit (10) terminates the defrosting operation and resumes the cooling operation. The defrosting 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)."

[0131] During defrosting, the refrigeration unit (10) primarily 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 unit (10) temporarily suspends normal operation and performs inflow operation. Also, 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 unit (10) temporarily suspends normal operation and performs outflow operation.

[0132] In normal operation, inflow operation, and outflow operation, the controller (90) keeps the internal fan (27) in a stopped state. Therefore, no air flows in the internal flow path (20). Also, in normal operation, inflow operation, and outflow operation, the controller (90) keeps the low-stage compressor (51) in a stopped state and operates the high-stage compressor (52).

[0133] <Normal operation> The normal operation of the defrosting process will be explained with reference to Figure 7.

[0134] In normal operation, the controller (90) keeps the external fan (26) and the internal fan (27) in a stopped state. The controller (90) also keeps the low-stage compressor (51) in a stopped state and controls the rotational speed of the high-stage compressor (52). The controller (90) also keeps the first expansion valve (EV1) in an open state, keeps the second expansion valve (EV2), the third expansion valve (EV3), and the fifth expansion valve (EV5) in a closed state, and controls the opening degree of the fourth expansion valve (EV4). The controller (90) also keeps the first electric valve (MV1) in a fully open state, keeps the second electric valve (MV2) in a fully closed state, and keeps the first solenoid valve (SV1), the second solenoid valve (SV2), and the third solenoid valve (SV3) in a closed state.

[0135] In the refrigerant circuit (30) during normal operation, the refrigerant flows in the same way as during heating operation. In the refrigerant circuit (30) during normal operation, the inflow and outflow of refrigerant to the receiver (62) are blocked, and the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57).

[0136] The state of the refrigerant in the refrigerant circuit (30) during normal operation will be explained with reference to the Mollier diagram (pressure-enthalpy diagram) in Figure 8.

[0137] In the refrigerant circuit (30), the refrigerant in state A is drawn into the high-stage compressor (52) and compressed by the high-stage compressor (52) to state B. During the process in which the refrigerant is compressed by the high-stage compressor (52), the pressure of the refrigerant increases, as does the enthalpy of the refrigerant.

[0138] The refrigerant discharged from the high-stage compressor (52) flows through the defrosting pipe (32) to the fourth expansion valve (EV4). During the process from the high-stage compressor (52) to the fourth expansion valve (EV4), the refrigerant releases some heat, causing its state to change from point B to point C. The refrigerant in state C is depressurized as it passes through the fourth expansion valve (EV4) and becomes state D.

[0139] The refrigerant in state D flows into the internal heat exchanger (57) and dissipates heat. In the internal heat exchanger (57), the frost attached to the internal heat exchanger (57) is heated by the refrigerant and melts. Also, in the internal heat exchanger (57), the pressure of the refrigerant decreases due to pressure loss as the refrigerant passes through the internal heat exchanger (57). Therefore, at the outlet of the internal heat exchanger (57), the refrigerant is in state E. The refrigerant that has flowed out of the internal heat exchanger (57) dissipates some heat as it flows towards the high-stage compressor (52) and is in state A.

[0140] <Inflow operation> The inflow operation is an operation to introduce refrigerant into the receiver (62) and increase the mass of refrigerant stored in the receiver (62). The refrigeration system (10) performs the inflow operation to reduce the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0141] The inflow operation during defrosting will be explained with reference to Figure 9.

[0142] During the inflow operation, the controller (90) activates the external fan (26) and opens the second electric valve (MV2). The difference between the inflow operation and the normal operation is that the external fan (26) is activated and the second electric valve (MV2) is opened. Also, during the inflow operation, the controller (90) controls the opening degree of the first expansion valve (EV1) as needed.

[0143] During the inflow operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57), just as in normal operation. During 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 flows into the external heat exchanger (56) dissipates heat into the outside air and then flows into the receiver (62) through the first expansion valve (EV1).

[0144] During the inflow operation, the first solenoid valve (SV1) and the second solenoid valve (SV2) are closed, so no refrigerant flows out of the receiver (62). Therefore, during the inflow operation, the mass of refrigerant stored in the receiver (62) increases, and as a result, the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) decreases.

[0145] Furthermore, if the pressure of the refrigerant discharged by the high-stage compressor (52) during the inflow operation is equal to or greater 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) is lower than the critical pressure of the refrigerant.

[0146] <Outflow operation> The discharge operation is an operation to discharge refrigerant from the receiver (62) and reduce the mass of refrigerant stored in the receiver (62). The refrigeration system (10) performs the discharge operation to increase the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0147] The refrigeration device (10) performs either a first discharge operation or a second discharge operation as the discharge operation. In addition, during the first discharge operation and the second discharge operation, the refrigeration device (10) performs either a gas discharge operation or a liquid discharge operation.

[0148] <First Outflow Operation / Gas Outflow Operation> The gas discharge operation of the first discharge operation will be explained with reference to Figure 10.

[0149] During the gas discharge operation of the first discharge operation, the controller (90) opens the second solenoid valve (SV2). The difference between the gas discharge operation of the first discharge operation and the normal operation is that the second solenoid valve (SV2) is open.

[0150] In the first discharge operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57), similar to normal operation. In the first discharge operation, the gaseous refrigerant in the receiver (62) flows into the gas-side connecting pipe (41). The gaseous refrigerant that flows into the gas-side connecting pipe (41) passes through the intermediate connecting pipe (43) and is drawn into the high-stage compressor (52) together with the refrigerant that has flowed out from the internal heat exchanger (57).

[0151] During the first discharge operation, the second electric valve (MV2) is closed, so no refrigerant flows into the receiver (62). Therefore, during the first discharge operation, the mass of refrigerant stored in the receiver (62) decreases, and as a result, the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0152] <First Outflow Operation / Liquid Outflow Operation> For example, when the outside 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 discharge operation may become small. If a gas discharge operation is performed in this case, the flow rate of gaseous refrigerant discharged from the receiver (62) will be small, 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 cases, the refrigeration system (10) performs a liquid discharge operation instead of a gas discharge operation.

[0153] The liquid discharge operation of the first discharge operation will be explained with reference to Figure 11.

[0154] In the liquid discharge operation of the first discharge 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 discharge operation and the gas discharge operation of the first discharge operation is that the second solenoid valve (SV2) is closed, and the first solenoid valve (SV1) and the third expansion valve (EV3) are open.

[0155] In the liquid discharge operation of the first discharge operation, the refrigerant circulates between the high-stage compressor (52) and the internal heat exchanger (57), similar to the gas discharge operation of the first discharge operation. In the liquid discharge operation of the first discharge operation, the liquid refrigerant in the receiver (62) flows into the liquid-side connecting pipe (42). The liquid refrigerant that flows into the liquid-side connecting pipe (42) passes through the intermediate connecting pipe (43) and is drawn into the high-stage compressor (52) together with the refrigerant discharged from the internal heat exchanger (57).

[0156] During the liquid discharge operation of the first discharge operation, the second electric valve (MV2) is closed, so no refrigerant flows into the receiver (62). Therefore, during the liquid discharge operation of the first discharge operation, the mass of refrigerant stored in the receiver (62) decreases, and as a result, the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0157] Furthermore, the liquid refrigerant in the receiver (62) has a higher density than the gaseous refrigerant in the receiver (62). Therefore, even when the pressure difference between the refrigerant in the receiver (62) and the refrigerant in the internal heat exchanger (57) is small, it is possible to secure a sufficient mass flow rate of refrigerant flowing out of the receiver (62), and the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) can be sufficiently increased in a relatively short time.

[0158] <Second Outflow Operation> For example, in situations where the outside air temperature is very low, the refrigerant pressure in the receiver (62) during the discharge operation may be lower than the refrigerant pressure in the internal heat exchanger (57). On the other hand, in the first discharge operation, the pressure of the refrigerant drawn in by the high-stage compressor (52) is substantially equal to the refrigerant pressure in the internal heat exchanger (57). Therefore, even if the first discharge operation is performed in such a case, it is not possible to discharge the refrigerant from the receiver (62). In such cases, the refrigeration system (10) performs a second discharge operation instead of the first discharge operation.

[0159] The gas discharge operation of the second discharge operation will be explained with reference to Figure 12.

[0160] In the gas discharge operation of the second discharge operation, the controller (90) closes the first motor valve (MV1). The difference between the gas discharge operation of the second discharge operation and the gas discharge operation of the first discharge operation is that the first motor valve (MV1) is closed.

[0161] In the second discharge operation, the first electric valve (MV1) is closed, and the connection between the high-stage compressor (52) and the internal heat exchanger (57) is blocked by the first electric valve (MV1). Therefore, the high-stage compressor (52) draws refrigerant only from the receiver (62) of the internal heat exchanger (57).

[0162] Similar to the gas discharge operation in the first discharge operation, the gaseous refrigerant discharged from the gas outlet of the receiver (62) is drawn into the high-stage compressor (52) after passing through the gas-side connecting pipe (41) and the intermediate connecting pipe (43). Therefore, in the gas discharge operation of the second discharge operation, the mass of refrigerant stored in the receiver (62) decreases, and as a result, the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases when normal operation is resumed.

[0163] The liquid discharge operation of the second discharge operation will now be described.

[0164] During the liquid discharge operation of the second discharge operation, the controller (90) closes the first motorized valve (MV1). The difference between the liquid discharge operation of the second discharge operation and the liquid discharge operation of the first discharge operation is that the first motorized valve (MV1) is closed.

[0165] In the liquid discharge operation of the second discharge operation, the first electric valve (MV1) is in the closed state, and the connection between the high-stage compressor (52) and the internal heat exchanger (57) is blocked by the first electric valve (MV1). Therefore, the high-stage compressor (52) draws refrigerant only from the receiver (62) of the internal heat exchanger (57) and the receiver (62).

[0166] Similar to the liquid discharge operation of the first discharge operation, the liquid refrigerant discharged from the liquid outlet of the receiver (62) is drawn into the high-stage compressor (52) after passing through the liquid-side connecting pipe (42) and the intermediate connecting pipe (43). Therefore, in the liquid discharge operation of the second discharge operation, the mass of refrigerant stored in the receiver (62) decreases, and as a result, the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases when normal operation is resumed.

[0167] -Controller operation during defrosting- The operations performed by the controller (90) during defrosting will be explained with reference to Figure 13.

[0168] During defrosting operation, the controller (90) controls the refrigeration system (10) so that the amount of heat supplied to the internal heat exchanger to melt the frost (hereinafter referred to as "defrosting heat amount") becomes the target heat amount.

[0169] In this embodiment, the controller (90) uses the current supplied to the motor of the high-stage compressor (52) (hereinafter referred to as "input current") as an indicator of the amount of heat required for dehumidification. The controller (90) stores the value of the input current when the amount of heat required for defrosting becomes the target amount as the target current. In this embodiment, the controller (90) causes the refrigeration system (10) to selectively perform normal operation, inflow operation, and outflow operation so that the input current of the high-stage compressor (52) becomes the target current.

[0170] <Control during normal operation> In normal operation, the controller (90) controls the rotational speed of the high-stage compressor (52) and the opening degree of the fourth expansion valve (EV4). The controller (90) prioritizes controlling the rotational speed of the high-stage compressor (52) over controlling the opening degree of the fourth expansion valve (EV4).

[0171] When normal operation begins, the controller (90) sets the opening of the fourth expansion valve (EV4) to its initial opening and controls the rotational speed of the high-stage compressor (52) based on the input current. The controller (90) controls the rotational speed of the high-stage compressor (52) so that the input current becomes the target current. If the input current is lower than the target current, the controller (90) increases the rotational speed of the high-stage compressor (52). If the input current is higher than the target current, the controller (90) decreases the rotational speed of the high-stage compressor (52).

[0172] If the rotational speed of the high-stage compressor (52) has reached the reference speed (for example, the 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 rotational speed of the high-stage compressor (52). In this case, the controller (90) controls the opening degree of the fourth expansion valve (EV4).

[0173] The controller (90) controls the opening degree of the fourth expansion valve (EV4) based on the pressure of the refrigerant discharged by the high-stage compressor (52) (specifically, the value measured by the high-stage discharge pressure sensor (78)). The controller (90) minimizes the opening degree of the fourth expansion valve (EV4) as much as possible within the range in which the value measured by the high-stage discharge pressure sensor (78) is kept below the reference pressure.

[0174] Specifically, the controller (90) controls the opening of the fourth expansion valve (EV4) so ​​that the measurement value of the high-stage discharge pressure sensor (78) falls within the reference pressure range. The reference pressure range is the range of pressures that includes the reference pressure. The highest value within the reference pressure range is the reference pressure.

[0175] If the measurement value from the high-stage discharge pressure sensor (78) is lower than the minimum value of the reference pressure range, the controller (90) reduces the opening of the fourth expansion valve (EV4). If the measurement value from the high-stage discharge pressure sensor (78) is higher than the maximum value of the reference pressure range (= reference pressure), the controller (90) increases the opening of the fourth expansion valve (EV4). If the measurement value from 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).

[0176] If the opening of the fourth expansion valve (EV4) has reached the standard opening but the input current is higher than the target current, the amount of heat required for defrosting is excessive, but the amount of heat required for defrosting cannot be reduced by controlling the opening of the fourth expansion valve (EV4). In this case, the controller (90) controls the rotational speed of the high-stage compressor (52) again.

[0177] <Switching between normal operation and inflow operation> When the inflow start condition is met during normal operation, the controller (90) switches the operation performed by the refrigeration system (10) from normal operation to inflow operation.

[0178] 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 that "the pressure of the refrigerant discharged by the high-stage compressor (52) (specifically, the measurement value of 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.

[0179] When the refrigeration unit (10) starts its inflow operation, a portion of the refrigerant discharged from the high-stage compressor (52) flows into the receiver (62), reducing the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0180] When the inflow termination condition is met during the inflow operation, the controller (90) switches the operation performed by the refrigeration device (10) from inflow operation to normal operation.

[0181] The inflow termination condition is a condition that indicates that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is appropriate. In this embodiment, the inflow termination condition is that "the pressure of the refrigerant discharged by the high-stage compressor (52) (specifically, the measurement value of the high-stage discharge pressure sensor (78)) is lower than the upper limit pressure." When the inflow termination condition is met, the refrigeration system (10) terminates the inflow operation and resumes normal operation.

[0182] Furthermore, the inflow termination condition may also be "the duration of the inflow operation has reached a predetermined time (for example, 5 seconds)."

[0183] <Switching between normal operation and discharge operation> When the conditions for initiating discharge are met during normal operation, the controller (90) switches the operation performed by the refrigeration system (10) from normal operation to discharge operation.

[0184] The discharge initiation condition is a condition indicating that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is insufficient. In this embodiment, the discharge initiation condition is that "the temperature of the refrigerant discharged by the high-stage compressor (52) (specifically, the measurement value of the high-stage discharge temperature sensor (73)) is above the upper limit temperature."

[0185] When the refrigeration unit (10) starts its discharge operation, the refrigerant discharged from the receiver (62) is drawn into the high-stage compressor (52), and the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases.

[0186] When the discharge termination condition is met during the discharge operation, the controller (90) switches the operation performed by the refrigeration device (10) from discharge operation to normal operation. The discharge termination condition is that "the duration of the discharge operation has reached a predetermined time (for example, 1 minute)."

[0187] The discharge termination condition may also be that "the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (57) is lower than the reference degree of superheating." In this case, the controller (90) uses the measurement value from the fourth refrigerant temperature sensor (84) and the measurement value from the low-stage suction pressure sensor (75) to calculate the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (57).

[0188] <Control during outflow operation> The controller (90) causes the refrigeration unit (10) to execute either the first discharge operation or the second discharge operation as the discharge operation.

[0189] The controller (90) selects either a first discharge operation or a second discharge operation based on the difference (Pr-Ps) between the pressure of the refrigerant in the receiver (62) (specifically, the value Pr measured by the receiver pressure sensor (79)) and the pressure of the refrigerant drawn in by the high-stage compressor (52) (specifically, the value Ps measured by the high-stage suction pressure sensor (77)).

[0190] If the pressure difference (Pr-Ps) is above the standard value, the "refrigerant pressure in the receiver (62)" is somewhat higher than the "refrigerant pressure drawn in by the high-stage compressor (52)". In this case, the high-stage compressor (52) can draw in 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 discharge operation.

[0191] If the pressure difference (Pr-Ps) is below the standard value, then the "refrigerant pressure in the receiver (62)" is close to the "refrigerant pressure drawn in by the high-stage compressor (52)", or the "refrigerant pressure in the receiver (62)" is lower than the "refrigerant pressure drawn in by the high-stage compressor (52)". In this case, the high-stage compressor (52) cannot 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 a second discharge operation, causing the high-stage compressor (52) to draw refrigerant only from the receiver (62).

[0192] In the first discharge operation, the controller (90) causes the refrigeration unit (10) to perform a gas discharge operation. If the discharge start condition is met again during the normal operation performed by the refrigeration unit (10) after the completion of the first discharge operation, it can be determined that the mass of refrigerant discharged from the receiver (62) in the first discharge operation was small. Therefore, in the second discharge operation, the controller (90) causes the refrigeration unit (10) to perform a liquid discharge operation in order to increase the mass of refrigerant discharged from the receiver (62).

[0193] -Features of Embodiment 1 (1)- In the refrigeration system (10) of this embodiment, the controller (90) controls the opening degree 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 Figure 8) is depressurized as it passes through the fourth expansion valve (EV4), and the depressurized refrigerant (point D in Figure 8) is supplied to the internal heat exchanger (57). Therefore, compared to the case where the refrigerant discharged from the high-stage compressor (52) is supplied to the internal heat exchanger (57) without depressurization, the pressure of the refrigerant flowing out of the internal heat exchanger (57) (point E in Figure 8) becomes lower, and as a result, the pressure difference between the refrigerant inhaled by the high-stage compressor (52) (point A in Figure 8) and the refrigerant discharged by the high-stage compressor (52) (point B in Figure 8) becomes larger.

[0194] When the pressure difference between the refrigerant inhaled by the high-stage compressor (52) and the refrigerant discharged by the high-stage compressor (52) increases, the amount of heat imparted to the refrigerant during the compression process by the high-stage compressor (52) increases. Therefore, according to this embodiment, compared to the case where the refrigerant discharged by the high-stage compressor (52) is supplied to the internal heat exchanger (57) without depressurizing, the amount of heat available to melt the frost adhering to the internal heat exchanger (57) can be increased, and as a result, the time required for defrosting the internal heat exchanger (57) can be shortened.

[0195] -Features of Embodiment 1 (2)- The refrigeration system (10) of this embodiment performs normal operation, outflow operation, and inflow operation during defrosting. During the outflow operation, the mass of refrigerant stored in the receiver (62) decreases, and the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) increases. During the inflow operation, the mass of refrigerant stored in the receiver (62) increases, and the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) decreases.

[0196] The refrigeration system (10) of this embodiment can adjust the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) during defrosting by performing inflow and outflow operations. As a result, the defrosting operation can be continued while maintaining the pressure and temperature of the refrigerant discharged by the high-stage compressor (52) at appropriate values, and the frost adhering to the internal heat exchanger (57) during the defrosting operation can be reliably melted.

[0197] -Features of Embodiment 1 (3)- The refrigeration system (10) of this embodiment performs either a first discharge operation or a second discharge operation. In the first discharge operation, the high-stage compressor (52) draws in the refrigerant discharged from the receiver (62) and the refrigerant discharged from the internal heat exchanger (57). In the second discharge operation, the high-stage compressor (52) draws in the refrigerant discharged from the receiver (62) but does not draw in the refrigerant discharged from the internal heat exchanger (57).

[0198] Here, for example, if the outside air is very cold (e.g., around -20°C to -30°C), the refrigerant pressure in the receiver (62) may be 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 discharged from the receiver (62) by the first discharge operation.

[0199] On the other hand, in the second discharge operation, the refrigerant in the receiver (62) can be drawn 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), refrigerant can be discharged from the receiver (62), increasing the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57).

[0200] Therefore, according to this embodiment, the mass of 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).

[0201] -Features of Embodiment 1 (4)- The refrigeration system (10) of this embodiment performs either a gas discharge operation or a liquid discharge operation as an outlet operation. In the gas discharge operation, the gaseous refrigerant discharged from the receiver (62) is drawn into the high-stage compressor (52). In the liquid discharge operation, the liquid refrigerant discharged from the receiver (62) is drawn into the high-stage compressor (52).

[0202] For example, in situations where the outside 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 discharge operation may become small. In this case, if the gas discharge operation is performed, the flow rate of gaseous refrigerant discharged from the receiver (62) will be small, 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).

[0203] In such cases, the refrigeration system (10) of this embodiment performs a liquid discharge operation instead of a gas discharge operation. In the liquid discharge operation, the liquid refrigerant discharged from the receiver (62) is drawn into the high-stage compressor (52). The density of the liquid refrigerant is significantly higher than that of the gaseous refrigerant. Therefore, according to the refrigeration system (10) of this embodiment that performs a liquid discharge operation, 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, the mass flow rate of the refrigerant discharged from the receiver (62) can be kept high, and 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.

[0204] In this case, 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) before flowing into the compression mechanism of the high-stage compressor (52). Therefore, in the liquid discharge operation, the liquid refrigerant discharged from the receiver (62) flows into the accumulator (52a), and the refrigerant vaporized inside the accumulator (52a) is drawn into the compression mechanism of the high-stage compressor (52). Consequently, in the refrigeration system (10) of this embodiment, even in the liquid discharge operation, the compression mechanism of the high-stage compressor (52) will not be damaged by drawing in a relatively large amount of liquid refrigerant.

[0205] Embodiment 2 Embodiment 2 will now be described. This embodiment is a modified version of Embodiment 1 in which the configuration of the refrigeration unit (10) is changed in the transport container (1).

[0206] 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, we will explain the differences between the refrigerant circuit (30) of this embodiment and the refrigerant circuit (30) of Embodiment 1.

[0207] As shown in Figure 14, in the refrigerant circuit (30) of this embodiment, a drain pan heater (63) is provided in the defrosting piping (32). In the defrosting piping (32), the drain pan heater (63) is located 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 through it.

[0208] -Defrosting operation of the refrigeration system- In the defrosting operation performed by the refrigeration system (10) of this embodiment, a portion of the refrigerant discharged from the high-stage compressor (52) flows into the defrosting piping (32), similar to the defrosting operation performed by the refrigeration system (10) of Embodiment 1. In the defrosting operation performed by the refrigeration system (10) of this embodiment, the refrigerant that flows into the defrosting piping (32) dissipates heat to the drain pan while passing through the drain pan heater (63), and then flows into the internal heat exchanger (57).

[0209] During defrosting, frost that has detached 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 from the internal heat exchanger (57) into the drain pan is heated by the drain pan, melts, and is discharged to the outside space as drain water.

[0210] Embodiment 3 Embodiment 3 will now be described. This embodiment is a modified version of Embodiment 2 in which the configuration of the refrigeration device (10) is changed in the transport container (1).

[0211] 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, we will explain the differences between the refrigerant circuit (30) of this embodiment and the refrigerant circuit (30) of Embodiment 2.

[0212] As shown in Figure 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 piping (32), and the reheat piping (33) and the fifth expansion valve (EV5) are omitted. In the refrigerant circuit (30) of this embodiment, the defrosting piping (32) also serves as the reheat piping (33) that supplies refrigerant to the reheat heat exchanger (58). In the defrosting piping (32) of this embodiment, the reheat heat exchanger (58) is located upstream of the fourth expansion valve (EV4), the drain pan heater (63) is located upstream of the reheat heat exchanger (58), and the third solenoid valve (SV3) is located upstream of the drain pan heater (63).

[0213] -Controller operation- In the refrigeration system of this embodiment, the controller (90) controls the opening degree of the fourth expansion valve (EV4) in both defrosting and dehumidification operations. In defrosting operations, the controller (90) of this embodiment controls the opening degree of the fourth expansion valve (EV4) in the same way as the controller (90) of embodiments 1 and 2. In dehumidification operations, the controller (90) of this embodiment performs the same opening degree control on the fourth expansion valve (EV4) as the controller (90) of embodiments 1 and 2 performs on the fifth expansion valve (EV5). In other words, in dehumidification operations, the controller (90) of this embodiment controls the opening degree of the fourth expansion valve (EV4) so ​​that the temperature of the air that has passed through the reheat heat exchanger (58) (specifically, the measurement value of the second air temperature sensor (87)) reaches the set temperature.

[0214] Embodiment 4 Embodiment 4 will now be described. In the refrigeration system (10) of this embodiment, the controller (90) performs substantially the same operations as the controller (90) of Embodiment 1 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 Figure 16.

[0215] As described above, when the defrosting start conditions are met during cooling operation, the refrigeration unit (10) temporarily suspends cooling operation and performs defrosting operation. When the refrigeration unit (10) starts normal operation of defrosting operation, the controller (90) first performs the process in step ST1.

[0216] <Step ST1> In step ST1, the controller (90) sets the opening degree of the fourth expansion valve (EV4) to its initial opening degree. Once step ST1 is completed, the controller (90) performs step ST2.

[0217] <Step ST2> In step ST2, the controller (90) controls the rotational speed of the high-stage compressor (52) based on the input current. The controller (90) controls the rotational speed of the high-stage compressor (52) so that the input current becomes the target current. This operation of the controller (90) is the same as the operation of the controller (90) in Embodiment 1, which controls the rotational speed of the high-stage compressor (52) based on the input current. When the processing of step ST2 is completed, the controller (90) performs the processing of step ST3.

[0218] <Step ST3> In step ST3, the controller (90) determines whether the defrosting termination condition is met. If the defrosting 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 defrosting termination condition is not met, the controller (90) performs the process in step ST4.

[0219] <Step ST4> In step ST4, the controller (90) compares the measured value Pdh from the high-stage discharge pressure sensor (78) with the upper limit pressure. If the condition that "the measured value Pdh from the high-stage discharge pressure sensor (78) is equal to or greater than the upper limit pressure" is met, the controller (90) proceeds to step ST9. On the other hand, if this condition is not met, the controller (90) proceeds to step ST5.

[0220] <Step ST5> In step ST5, the controller (90) compares the measured value Tdh from the high-stage discharge temperature sensor with the upper limit temperature. If the condition that "the measured value Tdh from the high-stage discharge temperature sensor is equal to or greater than the upper limit temperature" is met, the controller (90) performs the process in step ST11. On the other hand, if this condition is not met, the controller (90) performs the process in step ST6.

[0221] <Step ST6> In step ST6, the controller (90) compares the input current of the high-stage compressor (52) with the target current and compares the rotational speed RSh of the high-stage compressor (52) with the reference speed. If at least one of the following conditions is met: "the input current of the high-stage compressor (52) is equal to or greater than the target current" and "the rotational speed RSh of the high-stage compressor (52) is lower than the reference speed", the controller (90) performs the process in step ST2. On the other hand, if neither of these two conditions is met, the controller (90) performs the process in step ST7.

[0222] <Step ST7> In 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 less than or equal to the reference opening degree" is met, the controller (90) performs the process in step ST11. On the other hand, if this condition is not met, the controller (90) performs the process in step ST8.

[0223] <Step ST8> In step ST8, the controller (90) controls the opening of the fourth expansion valve (EV4) so ​​that the measured value Pdh from the high-stage discharge pressure sensor (78) falls within the reference pressure range. This operation of the controller (90) is the same as the operation of the controller (90) in Embodiment 1, which controls the opening of the fourth expansion valve (EV4) based on the measured value Pdh from the high-stage discharge pressure sensor (78). After the processing of step ST8 is completed, the controller (90) performs the processing of step ST2.

[0224] <Step ST9> The condition in step ST4, "the measured value Pdh of the high-stage discharge pressure sensor (78) is equal to or greater than the upper limit pressure," is the inflow start condition. As described in the description of Embodiment 1, the inflow start condition is a condition that indicates that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is excessive.

[0225] Therefore, in step ST9, the controller (90) switches the operation performed by the refrigeration unit (10) from normal operation to inflow operation. After that, the controller (90) performs the process of step ST10.

[0226] <Step ST10> In step ST10, the controller (90) determines whether the condition "the measured value Pdh of the high-stage discharge pressure sensor (78) is lower than the upper limit pressure" is met. This condition is the inflow termination condition. As described in the description of Embodiment 1, the inflow termination condition indicates that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is appropriate.

[0227] If the inflow termination condition is met, the controller (90) performs the process in step ST2. On the other hand, if the inflow termination condition is not met, the controller (90) causes the refrigeration device (10) to continue the inflow operation.

[0228] <Step ST11> The condition in step ST5, "the measured value Tdh of the high-stage discharge temperature sensor is above the upper limit temperature," is the discharge initiation condition. As described in the description of Embodiment 1, the discharge initiation condition indicates that the mass of refrigerant circulating between the high-stage compressor (52) and the internal heat exchanger (57) is insufficient.

[0229] Therefore, in step ST11, the controller (90) switches the operation performed by the refrigeration unit (10) from normal operation to discharge operation. After that, the controller (90) performs the process in step ST12.

[0230] <Step ST12> In step ST12, the controller (90) determines whether the condition "the duration of the discharge operation has reached a predetermined time" is met. This condition is the discharge termination condition. As described in the description of Embodiment 1, the discharge termination condition is the condition for terminating the discharge operation of the refrigeration device (10).

[0231] If the discharge termination condition is met, the controller (90) performs the process in step ST2. On the other hand, if the discharge termination condition is not met, the controller (90) causes the refrigeration system (10) to continue the discharge operation.

[0232] Other embodiments The following modifications may be applied to the refrigeration device (10) of the above embodiment. These modifications may be combined or substituted as appropriate, as long as they do not impair the function of the refrigeration device (10).

[0233] -First variation- The refrigerant circuit (30) of the refrigeration device (10) in the above embodiments 1 to 3 may be provided with a plurality of fourth expansion valves (EV4-1, EV4-2). Here, we will explain the differences between this modified example and the refrigeration device (10) of Embodiment 1, when this modified example is applied to the refrigeration device (10) of Embodiment 1.

[0234] As shown in Figure 17, in the refrigerant circuit (30) of this modified example, two fourth expansion valves (EV4-1, EV4-2) are provided in the defrosting piping (32). In the defrosting piping (32), the two fourth expansion valves (EV4-1, EV4-2) are connected in parallel. One of the fourth expansion valves (EV4-1) is the first pressure reducing valve. The other fourth expansion valve (EV4-2) is the second pressure reducing valve. In addition, in the defrosting piping (32) of this modified example, three or more fourth expansion valves may be connected in parallel.

[0235] In the modified refrigeration system (10), the controller (90) sets each of the fourth expansion valves (EV4-1, EV4-2) to the same opening degree. When changing the opening degree of the fourth expansion valves (EV4-1, EV4-2), the controller (90) expands or contracts the opening degree of each of the fourth expansion valves (EV4-1, EV4-2) by the same amount. However, the controller (90) may be configured to control the opening degree of each of the fourth expansion valves (EV4-1, EV4-2) individually.

[0236] -Second variation- The refrigeration apparatus (10) in the above embodiments 1 to 3 may be configured to perform only a single-stage compression refrigeration cycle. Here, we will explain the differences between this modified example and the refrigeration apparatus (10) of embodiment 3, which is applied to the refrigeration apparatus (10) of embodiment 3.

[0237] As shown in Figure 18, in the refrigerant circuit (30) of the refrigeration device (10) of this modified example, the low-stage compressor (51), the low-stage connecting pipe (44), and the high-stage connecting pipe (45) are omitted. The refrigerant circuit (30) of this modified example is provided with one compressor (52) which corresponds to the high-stage compressor of the refrigerant circuit (30) of Embodiment 3.

[0238] In the main circuit (31) of the refrigerant circuit (30) in this modified example, the other end of the internal heat exchanger (57) is connected to the suction pipe of the compressor (52). The first electric valve (MV1) is installed between the internal heat exchanger (57) and the suction pipe of the compressor (52) in the main circuit (31). The other end of the intermediate connecting pipe (43) is connected between the first electric valve (MV1) and the compressor (52) in the main circuit (31).

[0239] In the cooling operation of the refrigeration system (10) of this modified example, the refrigerant circuit (30) performs a single-stage compression refrigeration cycle. In the refrigerant circuit (30), the compressor (52) operates and the refrigerant circulates in the main circuit (31). The refrigerant discharged by 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 in order 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) of the internal heat exchanger (61), and then flows into the internal heat exchanger (57). The refrigerant that flows into the internal heat exchanger (57) absorbs heat from the air flowing through the internal flow path (20) and evaporates. The refrigerant evaporated in the internal heat exchanger (57) is drawn into the compressor (52). The compressor (52) compresses the refrigerant it has drawn in and discharges it.

[0240] -Third variation- In the refrigeration system (10) of embodiments 1 to 3 described above, the controller (90) may control the opening degree of the fourth expansion valve (EV4) based on the temperature of the refrigerant discharged by the high-stage compressor (52) (specifically, the value measured by the high-stage discharge temperature sensor (73)) during normal operation of defrosting. The controller (90) minimizes the opening degree of the fourth expansion valve (EV4) as much as possible within the range in which the value measured by the high-stage discharge temperature sensor (73) is kept below the reference temperature.

[0241] 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 the reference temperature range. The reference temperature range is the range of temperatures that includes the reference temperature. The highest value within the reference temperature range is the reference temperature.

[0242] If the value measured by the high-stage discharge temperature sensor (73) is lower than the lowest value in the reference temperature range, the controller (90) reduces the opening of the fourth expansion valve (EV4). If the value measured by the high-stage discharge temperature sensor (73) is higher than the highest value in the reference temperature range (= reference temperature), the controller (90) increases the opening of the fourth expansion valve (EV4). If the value measured by 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).

[0243] In this modified example, the controller (90) may control the opening degree of the fourth expansion valve (EV4) based on both the temperature and pressure of the refrigerant discharged by the high-stage compressor (52) during normal defrosting operation. In this case, the controller (90) minimizes the opening degree of the fourth expansion valve (EV4) as much as possible within the range in which the measured value of the high-stage discharge pressure sensor (78) is kept below the reference pressure and the measured value of the high-stage discharge temperature sensor (73) is kept below the reference temperature.

[0244] -Fourth variation- In the first discharge operation of the defrosting operation, the controller (90) of the refrigeration system (10) in the above embodiments 1 to 3 may select either a gas discharge operation or a liquid discharge operation based on the outside air temperature.

[0245] When the ambient temperature is relatively low, the pressure of the refrigerant in the receiver (62) becomes relatively low. If a gas discharge operation is performed in such a situation, the flow rate of gaseous refrigerant discharged from the receiver (62) will be small, 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, the controller (90) of this modified example causes the refrigeration system (10) to perform a gas discharge operation when the ambient temperature is above the standard ambient temperature, and to perform a liquid discharge operation when the ambient temperature is below the standard ambient temperature.

[0246] -Fifth variation- The applications of the refrigeration devices (10) in embodiments 1 to 3 described above are not limited to air conditioning the interior space (5) of a transport container (1). The refrigeration devices (10) in embodiments 1 to 3 may, for example, be used to air condition the interior space of a stationary refrigerator or cold storage warehouse.

[0247] While embodiments and modifications have been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims. Furthermore, elements of the embodiments, modifications, and other embodiments described above may be combined or substituted as appropriate. In addition, the designations "first," "second," "third," etc. in the specification and claims are used to distinguish the phrases to which these designations are given, and do not limit the number or order of such phrases. [Industrial applicability]

[0248] As described above, this disclosure is useful for refrigeration equipment and transport containers. [Explanation of Symbols]

[0249] 1. Shipping container 2 Container body 5. Interior space (target space) 10 Refrigeration equipment 30 Refrigerant Circuit 32 Defrosting piping (bypass passage) 50 Compressors 56 External heat exchanger (heat source side heat exchanger) 57 Internal heat exchanger (user side heat exchanger) 58 Reheat heat exchanger 62 Receivers 65 Expansion valve 90 Controllers EV4 4th Expansion Valve (Pressure Reducing Valve) EV4-1 Fourth expansion valve (first pressure reducing valve) EV4-2 4th Expansion Valve (2nd Pressure Reducing Valve)

Claims

1. A refrigeration system (10) for air conditioning a target space (5) is provided, 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), The above refrigerant circuit (30) is, A bypass passage (32) sends the refrigerant discharged by the compressor (50) to the utilization-side heat exchanger (57), bypassing the heat source-side heat exchanger (56) and the expansion valve (65). The system includes a variable-opening pressure reducing valve (EV4) that reduces the pressure of the refrigerant flowing through the bypass passage (32), The above-mentioned refrigeration device (10) is The above heat source side heat exchanger (56) functions as a heat radiator and the above utilization side heat exchanger (57) functions as an evaporator in a refrigeration cycle, and the cooling operation involves blowing the cooled air from the utilization side heat exchanger (57) into the target space (5), The refrigerant discharged by the compressor (50) is supplied to the utilization-side heat exchanger (57) through the bypass passage (32) to perform a defrosting operation to melt the frost adhering to the utilization-side heat exchanger (57). In the defrosting operation described above, the controller (90) controls the opening degree of the pressure reducing valve (EV4) based on either or both of the pressure and temperature of the refrigerant discharged by the compressor (50). Refrigeration equipment.

2. The above-mentioned refrigerant circuit (30) has a receiver (62) positioned between the heat source side heat exchanger (56) and the utilization side heat exchanger (57), The above-mentioned refrigeration device (10) operates during the defrosting operation, With the inflow and outflow of refrigerant to the receiver (62) blocked, the normal operation involves circulating the refrigerant between the compressor (50) and the heat exchanger (57) on the user side. An outflow operation is performed to release refrigerant from the receiver (62) and reduce the amount of refrigerant stored in the receiver (62), The receiver (62) is opened to allow refrigerant to flow in, thereby increasing the amount of refrigerant stored in the receiver (62). The refrigeration apparatus according to claim 1.

3. During the execution of the above-mentioned normal operation, if the temperature of the refrigerant discharged from the compressor (50) becomes higher than the reference temperature, the above-mentioned discharge operation will be initiated. The refrigeration apparatus according to claim 2.

4. During the above-mentioned discharge operation, if the superheat level of the refrigerant discharged from the user-side heat exchanger (57) falls below the reference superheat level, the above-mentioned normal operation will begin. The refrigeration apparatus according to claim 2.

5. The above outflow operation is, A gas discharge operation is performed to discharge the gaseous refrigerant from the receiver (62) mentioned above, This includes a liquid discharge operation that discharges liquid refrigerant from the receiver (62) described above. A refrigeration apparatus according to any one of claims 2 to 4.

6. In the above discharge operation, the compressor (50) draws in the refrigerant that has discharged from the receiver (62). A refrigeration apparatus according to any one of claims 2 to 4.

7. The above outflow operation is, The compressor (50) performs a first outflow operation in which it draws refrigerant from both the receiver (62) and the utilization-side heat exchanger (57), The compressor (50) includes a second outflow operation in which it draws refrigerant from the receiver (62) but does not draw refrigerant from the utilization-side heat exchanger (57). The refrigeration apparatus according to claim 6.

8. During the execution of the above-mentioned normal operation, if the pressure of the refrigerant discharged from the compressor (50) becomes higher than the reference pressure, the above-mentioned inflow operation will be initiated. A refrigeration apparatus according to any one of claims 2 to 4.

9. In the above inflow operation, a portion of the refrigerant discharged from the compressor (50) flows into the receiver (62) through the heat source side heat exchanger (56), and the remaining refrigerant discharged from the compressor (50) is supplied to the utilization side heat exchanger (57) through the bypass passage (32). A refrigeration apparatus according to any one of claims 2 to 4.

10. The above-mentioned pressure reducing valve includes a first pressure reducing valve (EV4-1) and a second pressure reducing valve (EV4-2) connected in parallel. A refrigeration apparatus according to any one of claims 1 to 4.

11. The refrigerant discharged by the compressor (50) is supplied to the utilization-side heat exchanger (57) through the bypass passage (32), and a heating operation is performed in which the heated air from the utilization-side heat exchanger (57) is blown out into the target space (5). A refrigeration apparatus according to any one of claims 1 to 4.

12. The above refrigerant circuit (30) is, The bypass passage (32) has a reheat heat exchanger (58) located upstream of the pressure reducing valve (EV4) that exchanges heat between the refrigerant and the air that has passed through the utilization-side heat exchanger (57). A refrigeration apparatus according to any one of claims 1 to 4.

13. The above refrigerant circuit (30) is filled with carbon dioxide as a refrigerant. A refrigeration apparatus according to any one of claims 1 to 4.

14. A refrigeration apparatus (10) according to any one of claims 1 to 4, The system comprises a container body (2) that forms the target space (5) where air conditioning is performed by the above-mentioned refrigeration device (10), and a container body (2). Shipping container.

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