Refrigeration cycle device and method for controlling the refrigeration cycle device

Abstract

This refrigeration circuit device comprises: a higher circuit having a first compressor, a condenser, a first pressure-reducing device, and a gasket heat exchanger, and in which a high-side refrigerant circulates; and a lower circuit having a second compressor, a gasket heat exchanger, a second pressure-reducing device, and an evaporator, and in which a low-side refrigerant circulates. The gasket heat exchanger exchanges heat between the high-side refrigerant and the low-side refrigerant; the low-side refrigerant is a non-azeotropic refrigerant mixture; and the pressure of the low-side refrigerant remaining in the lower circuit after the second compressor has stopped is maintained at or below a pressure that enables the low-side refrigerant to maintain incombustibility.

Description

[Technical field]

[0001] The present disclosure relates to a refrigeration cycle device having a dual cascade refrigeration cycle and a control method for the refrigeration cycle device. [Background technology]

[0002] Conventionally, a refrigeration cycle device equipped with a dual refrigeration cycle is known that includes a low-stage circuit through which a low-stage refrigerant circulates, a high-stage circuit through which a high-stage refrigerant circulates, and a cascade condenser that exchanges heat between the low-stage refrigerant and the high-stage refrigerant (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 030236 Summary of the Invention [Problem to be solved by the invention]

[0004] In the refrigeration cycle device of Patent Document 1, a non-azeotropic refrigerant mixture is used as the low-stage refrigerant circulating in the low-stage circuit. In this case, when the low-stage circuit is stopped, the refrigerant with a low boiling point among the refrigerants contained in the non-azeotropic refrigerant mixture is gasified and accumulates in the entire low-stage circuit, and the composition of the liquid refrigerant may fluctuate. In particular, when the low-stage circuit has a welded piping, if the gasified refrigerant leaks from the welded part, the composition fluctuation of the liquid refrigerant becomes significant. And, when the refrigerant with a high boiling point among the refrigerants contained in the non-azeotropic refrigerant mixture is flammable, the flammability of the liquid refrigerant increases due to the composition fluctuation of the refrigerant, and the risk of flammability increases when the refrigerant leaks.

[0005] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a refrigeration cycle device that can suppress fluctuations in refrigerant composition after the low-stage circuit is stopped, and a control method for a refrigeration cycle device. [Means for solving the problem]

[0006] The refrigeration cycle device according to the present disclosure includes a high-stage circuit having a first compressor, a condenser, a first pressure reduction device, and a cascade heat exchanger, and in which a high-stage refrigerant circulates; a low-stage circuit having a second compressor, a cascade heat exchanger, a second pressure reduction device, and an evaporator, and in which a low-stage refrigerant circulates; A control device; The cascade heat exchanger exchanges heat between a high-temperature side refrigerant and a low-temperature side refrigerant, and the low-temperature side refrigerant is a non-azeotropic mixture refrigerant consisting of CO2 and a flammable refrigerant. The control device controls the high-stage circuit so that the pressure of the low-stage side refrigerant remaining in the low-stage circuit is equal to or lower than a threshold value after the second compressor is stopped, the threshold value being a pressure at which the low-stage side refrigerant can maintain its non-flammability; The pressure of the low-stage side refrigerant remaining in the low-stage circuit after the second compressor is stopped is maintained at or below a pressure at which the low-stage side refrigerant can maintain its non-flammability.

[0007] A control method for a refrigeration cycle apparatus according to the present disclosure is a control method for a refrigeration cycle apparatus including a high-stage circuit having a first compressor, a condenser, a first pressure reduction device, and a cascade heat exchanger, and through which a high-stage side refrigerant circulates, and a low-stage circuit having a second compressor, a cascade heat exchanger, a second pressure reduction device, and an evaporator, and through which a low-stage side refrigerant circulates, wherein the cascade heat exchanger exchanges heat between the high-stage side refrigerant and the low-stage side refrigerant, and the low-stage side refrigerant is a non-azeotropic mixture refrigerant consisting of CO2 and a flammable refrigerant, The method further includes a step of controlling the high-stage circuit by the control device so that the pressure of the low-stage side refrigerant remaining in the low-stage circuit is equal to or lower than a threshold value after the second compressor is stopped, the threshold value being a pressure at which the low-stage side refrigerant can maintain non-flammability, The pressure of the low-stage side refrigerant remaining in the low-stage circuit after the second compressor is stopped is maintained at or below a pressure at which the low-stage side refrigerant can maintain its non-flammability. Effect of the Invention

[0008] According to the present disclosure, after the second compressor is stopped, the pressure of the low-stage side refrigerant remaining in the low-stage circuit is maintained below a pressure at which the low-stage side refrigerant can maintain its non-flammability, thereby suppressing fluctuations in the composition of the refrigerant after the low-stage circuit is stopped. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram of a refrigeration cycle device according to a first embodiment. [Diagram 2] 4 is a graph showing the relationship between the combustion properties and pressure P of a low-temperature side refrigerant. [Diagram 3]4 is a flowchart showing the operation of the refrigeration cycle device according to the first embodiment. [Figure 4] 6 is a flowchart showing the operation of the refrigeration cycle device according to the second embodiment. [Diagram 5] FIG. 2 is a schematic configuration diagram of a refrigeration cycle device according to a first modified example. [Figure 6] FIG. 11 is a schematic configuration diagram of a refrigeration cycle device according to a second modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment will be described with reference to the drawings. In each drawing, the same reference numerals are used to denote the same or equivalent parts. In addition, the size relationship of each component in the drawings may differ from the actual relationship. Furthermore, the highs and lows of temperature, pressure, etc. in the following description are not determined in relation to absolute values, but are determined relatively in terms of the state or operation of a system or device, etc.

[0011] Embodiment 1 A refrigeration cycle apparatus 100 according to the first embodiment will be described. The refrigeration cycle apparatus 100 includes a dual cascade refrigeration cycle that circulates refrigerants independently, and is used for applications such as freezing, refrigeration, hot water supply, and air conditioning. In the present embodiment, the refrigeration cycle apparatus 100 will be described as an example used as a refrigeration device that cools a freezer or the like.

[0012] Fig. 1 is a schematic diagram of a refrigeration cycle apparatus 100 according to a first embodiment. As shown in Fig. 1, the refrigeration cycle apparatus 100 according to the present embodiment includes a high-stage circuit 1, a low-stage circuit 2, and a control device 3. The high-stage circuit 1 is a high-temperature circuit in which a high-stage refrigerant circulates, and the low-stage circuit 2 is a low-temperature circuit in which a low-stage refrigerant having a boiling point lower than that of the high-stage refrigerant circulates. The high-stage circuit 1 and the low-stage circuit 2 are both equipped with a cascade heat exchanger 14, and the cascade heat exchanger 14 exchanges heat between the high-stage refrigerant circulating in the high-stage circuit 1 and the low-stage refrigerant circulating in the low-stage circuit 2.

[0013] The high-temperature circuit 1 includes a first compressor 11, a condenser 12, a first pressure reducing device 13, and a cascade heat exchanger 14. The first compressor 11, the condenser 12, the first pressure reducing device 13, and the cascade heat exchanger 14 are connected by piping in this order. The high-temperature side refrigerant circulating through the high-temperature circuit 1 is, for example, a single refrigerant or a mixed refrigerant of an HFC system such as R134a, R32, or R410A, or an HFO system such as HFO-1234yf.

[0014] The first compressor 11 is, for example, a capacity-controllable inverter type compressor. The first compressor 11 draws in a high-temperature side refrigerant, compresses it into a high-temperature and high-pressure state, and discharges it to circulate the high-temperature side refrigerant in the high-temperature circuit 1.

[0015] The condenser 12 is, for example, a fin-tube type heat exchanger. The condenser 12 exchanges heat between air and the high-side refrigerant, and condenses and liquefies the high-side refrigerant. The refrigeration cycle device 100 includes a first fan 15 for supplying air to the condenser 12. The first fan 15 is, for example, a propeller fan or a cross-flow fan whose air volume is adjustable. The condenser 12 may be, for example, a plate-type heat exchanger that exchanges heat between water or brine and the high-side refrigerant. In this case, the first fan 15 may be omitted.

[0016] The first pressure reducing device 13 is, for example, an electronic expansion valve whose opening degree can be controlled. The first pressure reducing device 13 is connected to the condenser 12 and reduces the pressure of the high-side refrigerant flowing out from the condenser 12 to expand it. The first pressure reducing device 13 may be a capillary tube or a temperature-sensing expansion valve.

[0017] The cascade heat exchanger 14 is, for example, a plate-type heat exchanger. The cascade heat exchanger 14 includes a high-stage side flow passage 141 connected to the high-stage circuit 1 and a low-stage side flow passage 142 connected to the low-stage circuit 2, and exchanges heat between the high-stage side refrigerant flowing through the high-stage side flow passage 141 and the low-stage side refrigerant flowing through the low-stage side flow passage 142. The high-stage side flow passage 141 of the cascade heat exchanger 14 functions as an evaporator, evaporating and gasifying the high-stage side refrigerant. The low-stage side flow passage 142 of the cascade heat exchanger 14 functions as a condenser, condensing and liquefying the low-stage side refrigerant.

[0018] The low-stage circuit 2 includes a second compressor 21, a cascade heat exchanger 14, a second pressure reducing device 23, and an evaporator 24. The second compressor 21, the cascade heat exchanger 14, the second pressure reducing device 23, and the evaporator 24 are connected by piping in this order. The low-stage side refrigerant circulating in the low-stage circuit 2 is a non-azeotropic mixed refrigerant having a lower boiling point than the high-stage side refrigerant. By using a non-azeotropic mixed refrigerant, a low evaporation temperature that cannot be obtained with a single refrigerant can be obtained. In this embodiment, a non-azeotropic mixed refrigerant containing CO2 and R290 (propane) is used as the low-stage side refrigerant. CO2 is a low-boiling point refrigerant, and R290 is a high-boiling point refrigerant having a higher boiling point than CO2. By using natural refrigerants such as CO2 and R290, the environmental load can be reduced. In addition, the cooling capacity is improved by mixing CO2 with the low-stage side refrigerant, and the COP is improved and the triple point of CO2 is lowered by mixing R290, making it possible to use it at low temperatures.

[0019] The second compressor 21 is, for example, a capacity-controllable inverter type compressor. The second compressor 21 draws in the low-stage side refrigerant, compresses it into a high-temperature and high-pressure state, and discharges it to circulate the low-stage side refrigerant in the low-stage circuit 2.

[0020] The second pressure reducing device 23 is, for example, an electronic expansion valve whose opening degree can be controlled. The second pressure reducing device 23 is connected to the low-side flow passage 142 of the cascade heat exchanger 14, and reduces the pressure of the low-side refrigerant flowing out from the low-side flow passage 142 to expand it. The second pressure reducing device 23 may be a capillary tube or a temperature-sensing expansion valve.

[0021] The evaporator 24 is, for example, a fin tube type heat exchanger. The evaporator 24 exchanges heat between air and the low stage side refrigerant, and evaporates and gasifies the low stage side refrigerant. The refrigeration cycle device 100 includes a second fan 25 for supplying air to the evaporator 24. The second fan 25 is, for example, a propeller fan or a cross-flow fan capable of adjusting the air volume. The evaporator 24 may be, for example, a plate type heat exchanger that exchanges heat between water or brine and the low stage side refrigerant. In this case, the second fan 25 may be omitted.

[0022] The refrigeration cycle apparatus 100 also includes a pressure sensor 26 that detects the pressure P of the low stage side refrigerant that accumulates in the low stage circuit 2 when the low stage circuit 2 is stopped. Since the pressure P of the low stage side refrigerant when the low stage circuit 2 is stopped is approximately uniform within the low stage circuit 2, the pressure sensor 26 is provided at an arbitrary location in the low stage circuit 2. In the example of FIG. 1, the pressure sensor 26 is provided in a pipe that connects the low stage side flow path 142 of the cascade heat exchanger 14 and the second pressure reducing device 23. The pressure P of the low stage side refrigerant detected by the pressure sensor 26 is transmitted to the control device 3.

[0023] Instead of the pressure sensor 26, a sensor for detecting another physical quantity (e.g., condensation temperature) that can be converted into the pressure P of the low stage side refrigerant may be provided, and the control device 3 may convert it into the pressure P. The refrigeration cycle device 100 may further include various sensors (not shown), such as an outside air temperature sensor for detecting the outside air temperature, an indoor temperature sensor for detecting the temperature inside the freezer, and a sensor for detecting the temperature or pressure of the refrigerant at any location in the high stage circuit 1 and the low stage circuit 2.

[0024] The control device 3 controls the overall operation of the refrigeration cycle apparatus 100. The control device 3 is composed of a processing device including a memory for storing data and programs required for control and a CPU for executing the programs, or dedicated hardware such as an ASIC or an FPGA, or both. The control device 3 of this embodiment controls the high-stage circuit 1 based on the pressure P of the low-stage side refrigerant detected by the pressure sensor 26 when the low-stage circuit 2 is stopped. The control device 3 also controls each device of the high-stage circuit 1 and the low-stage circuit 2, as well as the first fan 15 and the second fan 25, based on information received from various sensors and operation details instructed by the user.

[0025] The operation of the refrigeration cycle apparatus 100 of this embodiment will be described based on the flow of refrigerant circulating through each refrigerant circuit. First, the operation of the high-stage circuit 1 will be described. When an instruction to start the operation of the refrigeration cycle apparatus 100 is given, the first compressor 11 and the second compressor 21 are driven. Then, the first compressor 11 of the high-stage circuit 1 sucks in the high-stage side refrigerant, compresses it into a high-temperature, high-pressure state, and discharges it. The high-stage side refrigerant discharged from the first compressor 11 flows into the condenser 12. The condenser 12 exchanges heat between the air supplied from the first fan 15 and the high-stage side refrigerant, and condenses and liquefies the high-stage side refrigerant.

[0026] The high stage side refrigerant condensed and liquefied in the condenser 12 passes through the first pressure reducing device 13. The first pressure reducing device 13 reduces the pressure of the condensed and liquefied high stage side refrigerant. The high stage side refrigerant reduced in pressure by the first pressure reducing device 13 flows into the high stage side flow passage 141 of the cascade heat exchanger 14. The high stage side refrigerant that flows into the high stage side flow passage 141 exchanges heat with the low stage side refrigerant flowing in the low stage side flow passage 142 of the cascade heat exchanger 14, and is evaporated and gasified. The high stage side refrigerant evaporated and gasified in the cascade heat exchanger 14 is sucked into the first compressor 11 again.

[0027] Next, the operation of the low stage circuit 2 will be described. The second compressor 21 of the low stage circuit 2 sucks in low stage side refrigerant, compresses it, and discharges it in a high temperature and high pressure state. The low stage side refrigerant discharged from the second compressor 21 flows into the low stage side flow passage 142 of the cascade heat exchanger 14. The low stage side refrigerant that flows into the low stage side flow passage 142 is heat exchanged with the high stage side refrigerant flowing in the high stage side flow passage 141 of the cascade heat exchanger 14, and is condensed and liquefied.

[0028] The low stage side refrigerant condensed and liquefied in the cascade heat exchanger 14 passes through the second pressure reducing device 23. The second pressure reducing device 23 reduces the pressure of the low stage side refrigerant. The low stage side refrigerant reduced in pressure by the second pressure reducing device 23 flows into the evaporator 24. The evaporator 24 exchanges heat between the air supplied from the second fan 25 and the low stage side refrigerant, and evaporates and gasifies the low stage side refrigerant. At this time, the low stage side refrigerant absorbs heat from the air, thereby cooling the freezing compartment. The low stage side refrigerant evaporated and gasified in the evaporator 24 is sucked back into the second compressor 21.

[0029] When an instruction to stop the refrigeration cycle apparatus 100 is given, the first compressor 11 and the second compressor 21 are stopped, and the circulation of the refrigerant in the high stage circuit 1 and the low stage circuit 2 is stopped. At this time, if a refrigerant with a low boiling point among the low stage side refrigerant residing throughout the low stage circuit 2 is gasified, the composition of the liquid refrigerant in the low stage circuit 2 changes. For example, in the case where a non-azeotropic mixture refrigerant of CO2 and R290 is used as the low stage side refrigerant as in this embodiment, the proportion of flammable R290 in the liquid refrigerant increases due to the gasification of CO2, which has a lower boiling point than R290.

[0030] Moreover, the evaporator 24 of the low stage circuit 2 is disposed inside a room such as a freezer room, and is connected to the second compressor 21 by an extension pipe. Therefore, a welded part connected to the extension pipe is provided on the suction side of the second compressor 21. Then, when the low stage circuit 2 is stopped, if gasified CO2 leaks from the welded part, the proportion of flammable R290 in the liquid refrigerant in the low stage circuit 2 further increases. As a result, the flammability of the low stage side refrigerant in the low stage circuit 2 increases, and the risk of flammability increases when the refrigerant leaks.

[0031] Therefore, the control device 3 of this embodiment continues the operation of the high stage circuit 1 even after the low stage circuit 2 is stopped, and controls the capacity of the high stage circuit 1 to keep the pressure P of the low stage side refrigerant below the pressure value at which the low stage side refrigerant maintains its non-flammability. FIG. 2 is a graph showing the relationship between the flammability of the low stage side refrigerant and the pressure P. The graph in FIG. 2 is a graph for the case in which the low stage side refrigerant is a non-azeotropic refrigerant mixture as in this embodiment, and the refrigerant with the higher boiling point is flammable. As shown in FIG. 2, the higher the pressure P of the low stage side refrigerant, the higher the flammability. Therefore, in order to maintain the low stage side refrigerant non-flammable, the pressure P of the low stage side refrigerant is set to a threshold value P T The threshold P must be less than or equal to T is uniquely determined by the physical properties of the refrigerant constituting the low-temperature side refrigerant. In this embodiment, the threshold value P T is preset and stored in the control device 3. The control device 3 determines whether the pressure P of the low-side refrigerant detected by the pressure sensor 26 is equal to or lower than the threshold value P T The capability of the higher level circuit 1 is controlled as follows.

[0032] 3 is a flowchart showing the operation of the refrigeration cycle apparatus 100 according to the embodiment 1. When an instruction to operate the refrigeration cycle apparatus 100 is given by an instruction from a user or the like, the control device 3 drives the first compressor 11 and the second compressor 21 (S1). As a result, a high-stage side refrigerant is circulated in the high-stage circuit 1, and a low-stage side refrigerant is circulated in the low-stage circuit 2, thereby cooling the freezing compartment.

[0033] Then, the control device 3 judges whether or not to stop the operation of the refrigeration cycle device 100 in response to an instruction from a user or the like (S2). If the operation is not to be stopped (S2: NO), the high-temperature circuit 1 and the low-temperature circuit 2 continue to operate until a stop instruction is received.

[0034] On the other hand, when the operation is stopped (S2: YES), the control device 3 stops the second compressor 21 (S3). This stops the circulation of the low-stage side refrigerant in the low-stage circuit 2. At this time, the operation of the first compressor 11 continues.

[0035] Then, the pressure sensor 26 detects the pressure P of the low-stage side refrigerant (S4). The control device 3 determines whether the pressure P of the low-stage side refrigerant detected by the pressure sensor 26 is greater than or equal to the threshold value P T It is determined whether the pressure P of the low-side refrigerant is equal to or lower than the threshold value P (S5). T If the pressure P of the low-stage refrigerant is equal to or lower than the threshold P (S5: YES), the process proceeds to step S7 while maintaining the capacity of the high-stage circuit 1. T If it is greater than the threshold value P (S5: NO), the control device 3 increases the capacity of the high-stage circuit 1 (S6). Here, the control device 3 may increase the operating frequency of the first compressor 11 by a predetermined constant value, or may increase the low-stage refrigerant pressure P and the threshold value P T Alternatively, the value may be increased by a value according to the difference between the

[0036] By increasing the capacity of the high stage circuit 1, the temperature of the high stage side refrigerant flowing through the high stage side flow passage 141 of the cascade heat exchanger 14 decreases. As a result, the temperature of the low stage side refrigerant that is heat exchanged with the high stage side refrigerant in the cascade heat exchanger 14 decreases, and the pressure P of the low stage side refrigerant decreases. The decrease in the pressure P of the low stage side refrigerant decreases the gas density in the low stage circuit 2, and the mass of the gas refrigerant in the low stage circuit 2 decreases. In other words, the decrease in the pressure P of the low stage side refrigerant reduces the gas amount of the low boiling point refrigerant (CO2) that remains in the entire low stage circuit 2 after the low stage circuit 2 is stopped, and the composition fluctuation of the liquid refrigerant in the low stage circuit 2 can be minimized.

[0037] The control device 3 judges whether or not to start the operation of the refrigeration cycle device 100 in response to an instruction from a user or the like (S7). If the operation is not to be started (S7: NO), the process returns to step S4 and the subsequent processes are repeated. If the operation is to be started (S7: YES), the process proceeds to step S1 and the second compressor 21 is driven to circulate the low-stage side refrigerant through the low-stage circuit 2.

[0038] As described above, in the refrigeration cycle device 100 of this embodiment, even after the low-stage circuit 2 is stopped, the high-stage circuit 1 continues to operate, and the pressure P of the low-stage side refrigerant is increased to a threshold value P at which the low-stage side refrigerant can maintain its non-flammability. TThe high stage circuit 1 is controlled so that the composition of the low stage side refrigerant remaining in the low stage circuit 2 after the low stage circuit 2 is stopped can be suppressed. As a result, even when a non-azeotropic mixed refrigerant containing a flammable refrigerant is used as the low stage side refrigerant, an increase in the risk of flammability at the time of refrigerant leakage can be suppressed. In addition, when a mixed refrigerant containing CO2 is used as the low stage side refrigerant as in this embodiment, the freezing point of CO2 can be lowered, so that cooling below the freezing point (-56°C) is also possible.

[0039] Embodiment 2 A refrigeration cycle apparatus 100 according to the second embodiment will be described. Fig. 4 is a flowchart showing the operation of the refrigeration cycle apparatus 100 according to the second embodiment. This embodiment differs from the first embodiment in the operation of the refrigeration cycle apparatus 100. The configuration of the refrigeration cycle apparatus 100 is the same as that of the first embodiment.

[0040] 4, when an instruction to operate the refrigeration cycle apparatus 100 is given by an instruction from a user, the control device 3 drives the first compressor 11 and the second compressor 21 (S11). As a result, a high-stage side refrigerant is circulated in the high-stage circuit 1, and a low-stage side refrigerant is circulated in the low-stage circuit 2.

[0041] Then, the control device 3 judges whether or not to stop the operation of the refrigeration cycle device 100 in response to an instruction from a user or the like (S12). If the operation is not to be stopped (S12: NO), the high-temperature circuit 1 and the low-temperature circuit 2 continue to operate until a stop instruction is received.

[0042] On the other hand, when the operation is stopped (S12: YES), the control device 3 performs a pump-down operation in the low-stage circuit 2 (S13). Specifically, the control device 3 fully closes the second pressure reducing device 23 and continues the operation of the second compressor 21. Since the second pressure reducing device 23 is closed, the low-stage side refrigerant in the low-stage circuit 2 is recovered on the high-pressure side of the low-stage circuit 2, that is, between the discharge port of the second compressor 21 and the refrigerant inlet of the second pressure reducing device 23. As a result, the low-pressure side of the low-stage circuit 2, that is, between the refrigerant outlet of the second pressure reducing device 23 and the suction port of the second compressor 21, becomes negative pressure (below atmospheric pressure).

[0043] Thereafter, the control device 3 stops the second compressor 21 (S14). This stops the circulation of the low-stage side refrigerant in the low-stage circuit 2. Note that a solenoid valve may be provided between the low-stage side flow path 142 of the cascade heat exchanger 14 and the second pressure reducing device 23, and the solenoid valve may be closed to perform the pump-down operation. Also, a pressure sensor may be provided on the low-pressure side of the low-stage circuit 2, that is, between the refrigerant outlet of the second pressure reducing device 23 and the suction port of the second compressor 21 to detect the low-pressure pressure of the low-stage side refrigerant, and the second compressor 21 may be stopped when the low-pressure pressure of the low-stage side refrigerant becomes equal to or lower than atmospheric pressure. This makes it possible to prevent a breakdown or the like caused by the second compressor 21 continuing to be driven even after the refrigerant runs out on the low-pressure side of the low-stage circuit 2.

[0044] The subsequent steps S15 to S18 are the same as the steps S4 to S7 in the first embodiment, and the pressure P of the low-side refrigerant is equal to or lower than the threshold value P T The high-stage circuit 1 is controlled as follows. In this embodiment, however, the pressure sensor 26 detects the pressure P of the low-stage side refrigerant recovered to the high-pressure side of the low-stage circuit 2. That is, the pressure sensor 26 is provided between the discharge port of the second compressor 21 and the refrigerant inlet of the second pressure reducing device 23.

[0045] In the refrigeration cycle apparatus 100 of this embodiment, the low pressure side of the low stage circuit 2 can be made negative pressure by performing pump-down operation after the low stage circuit 2 is stopped. This makes it possible to prevent leakage of gas refrigerant from welded parts provided on the low pressure side of the low stage circuit 2. As a result, when the refrigeration cycle apparatus 100 is stopped, it is possible to further suppress composition fluctuations of the liquid refrigerant remaining in the low stage circuit 2.

[0046] The above is a description of the embodiment, but the present disclosure is not limited to the above embodiment, and various modifications or combinations are possible within the scope of the present disclosure. For example, the low-side refrigerant is not limited to a non-azeotropic refrigerant mixture of CO2 and R290, and may be other non-azeotropic refrigerant mixtures. However, the effects of the above embodiment can be obtained particularly when the low-side refrigerant is a non-azeotropic refrigerant mixture containing CO2 and a flammable refrigerant.

[0047] In addition, in the above embodiment, the control device 3 is configured to control the entire refrigeration cycle device 100, but a control device 3 may be provided in each of the high-temperature circuit 1 and the low-temperature circuit 2, and the operation of the high-temperature circuit 1 and the low-temperature circuit 2 may be controlled individually.

[0048] In the above embodiment, the capacity of the high-stage circuit 1 is controlled by controlling the operating frequency of the first compressor 11, but the present invention is not limited to this. For example, instead of or in addition to the operating frequency of the first compressor 11, the capacity of the high-stage circuit 1 may be controlled by controlling the opening degree of the first pressure reducing device 13 of the high-stage circuit 1 or the rotation speed of the first fan 15. In this case, the control device 3 controls the capacity of the high-stage circuit 1 when the pressure P of the low-stage side refrigerant is equal to or exceeds the threshold value P T If it is greater, the opening degree of the first pressure reducing device 13 and the rotation speed of the first fan 15 are increased to increase the capacity of the high-temperature circuit 1 .

[0049] The low-temperature circuit 2 of the refrigeration cycle apparatus 100 may include a receiver 22. Fig. 5 is a schematic configuration diagram of a refrigeration cycle apparatus 100A according to Modification 1. As shown in Fig. 5, the low-temperature circuit 2 of the refrigeration cycle apparatus 100A includes a receiver 22 between the cascade heat exchanger 14 and the second pressure reducing device 23. The receiver 22 temporarily stores the low-temperature side refrigerant flowing out from the low-temperature side flow passage 142 of the cascade heat exchanger 14. The receiver 22 stores surplus refrigerant generated due to fluctuations in the cooling load.

[0050] Even when the refrigeration cycle device 100A includes the receiver 22, after the low-stage circuit 2 is stopped, the high-stage circuit 1 is continued to operate, and the pressure P of the low-stage side refrigerant is increased to a threshold value P at which the low-stage side refrigerant can maintain its non-flammability.T In addition, when the refrigeration cycle apparatus 100A includes the receiver 22, the high-side refrigerant pressure is controlled to be equal to or lower than the threshold P at which the low-side refrigerant can maintain its non-flammability even during the operation of the low-side refrigerant circuit 2. T The higher level circuit 1 may be controlled so that:

[0051] In the above embodiment, the high stage circuit 1 is controlled according to the pressure P of the low stage side refrigerant detected by the pressure sensor 26 after the low stage circuit 2 is stopped, but the present invention is not limited to this. For example, the control device 3 may control the high stage circuit 1 according to the temperature of the low stage side refrigerant corresponding to the pressure P of the low stage side refrigerant. Alternatively, the low stage circuit 2 may be provided with a pressure relief device that opens when the pressure or temperature rises to a reference value, and the pressure relief device may keep the pressure P of the low stage side refrigerant remaining in the low stage circuit 2 below a pressure value at which the low stage side refrigerant maintains its non-flammability.

[0052] Fig. 6 is a schematic configuration diagram of a refrigeration cycle apparatus 100B according to Modification 2. As shown in Fig. 6, the refrigeration cycle apparatus 100B includes a pressure relief device 27. The pressure relief device 27 is provided at an arbitrary position in the low-stage circuit 2. When a pump-down operation is performed as in the second embodiment, the pressure relief device 27 is provided on the high-pressure side of the low-stage circuit 2. The pressure relief device 27 is a pressure relief valve or a fusible plug, and operates when the pressure P or temperature of the low-stage side refrigerant exceeds a threshold value P T When the pressure P of the low-side refrigerant is equal to or higher than the threshold value P, the valve or valve is opened, and the gas refrigerant is discharged to the outside, decreasing the pressure P of the low-side refrigerant. T is a pressure value or temperature at which the low stage side refrigerant can maintain its non-flammability, as in the above embodiment. As a result, even after the low stage circuit 2 is stopped, the pressure P of the low stage side refrigerant can be maintained at or below the pressure value at which the low stage side refrigerant can maintain its non-flammability. In the case of this modified example, the high stage circuit 1 may be stopped after the low stage circuit 2 is stopped. Alternatively, the high stage circuit 1 may be continued to be driven even after the low stage circuit 2 is stopped, and pressure control by the pressure relief device 27 and control of the high stage circuit 1 based on the pressure P of the low stage side refrigerant may be combined. [Explanation of symbols]

[0053] 1 high-stage circuit, 2 low-stage circuit, 3 control device, 11 first compressor, 12 condenser, 13 first pressure reducing device, 14 cascade heat exchanger, 15 first fan, 21 second compressor, 22 receiver, 23 second pressure reducing device, 24 evaporator, 25 second fan, 26 pressure sensor, 27 pressure relief device, 100, 100A, 100B refrigeration cycle device, 141 high-stage side flow path, 142 low-stage side flow path.

Claims

1. a high-stage circuit including a first compressor, a condenser, a first pressure reducing device, and a cascade heat exchanger, in which a high-stage refrigerant circulates; a low-stage circuit including a second compressor, the cascade heat exchanger, a second pressure reducing device, and an evaporator, in which a low-stage refrigerant circulates; A control device, The cascade heat exchanger exchanges heat between the high-temperature side refrigerant and the low-temperature side refrigerant, The low-temperature side refrigerant is CO ２ and a flammable refrigerant. the control device controls the high-stage circuit so that a pressure of the low-stage side refrigerant remaining in the low-stage circuit becomes equal to or lower than a threshold value after the second compressor is stopped, The threshold value is a pressure at which the low-temperature side refrigerant can maintain non-flammability, The refrigeration cycle apparatus, wherein a pressure of the low-stage side refrigerant remaining in the low-stage circuit after the second compressor is stopped is maintained at or below a pressure at which the low-stage side refrigerant can maintain its non-flammability.

2. The refrigeration cycle apparatus according to claim 1 , wherein the control device increases the capacity of the high-stage circuit when the pressure of the low-stage side refrigerant is greater than the threshold value.

3. The refrigeration cycle apparatus according to claim 1 , wherein the control device increases an operation frequency of the first compressor when the pressure of the low-temperature side refrigerant is higher than the threshold value.

4. The refrigeration cycle apparatus according to any one of claims 1 to 3, wherein the control device performs a pump-down operation of the low-stage circuit before stopping the second compressor.

5. a high-stage circuit including a first compressor, a condenser, a first pressure reducing device, and a cascade heat exchanger, in which a high-stage refrigerant circulates; A control method for a refrigeration cycle apparatus including a second compressor, the cascade heat exchanger, a second pressure reducing device, and an evaporator, and a low-stage circuit in which a low-stage side refrigerant circulates, comprising: The cascade heat exchanger exchanges heat between the high-temperature side refrigerant and the low-temperature side refrigerant, The low-temperature side refrigerant is CO ２ and a flammable refrigerant. A step of controlling the high-stage circuit by a control device so that a pressure of the low-stage side refrigerant remaining in the low-stage circuit becomes equal to or lower than a threshold value after the second compressor is stopped, The threshold value is a pressure at which the low-temperature side refrigerant can maintain its non-flammability, A control method for a refrigeration cycle device, comprising: maintaining a pressure of the low-stage side refrigerant remaining in the low-stage circuit after the second compressor is stopped at a pressure at which the low-stage side refrigerant can maintain its non-flammability or lower.

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