Refrigeration cycle device and method for controlling the refrigeration cycle device

Abstract

This refrigeration circuit device comprises: a higher circuit provided with 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 provided with a second compressor, a gasket heat exchanger, a receiver, 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 high pressure of the low-side refrigerant that circulates through the lower circuit 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 receiver is provided in the low-stage circuit to store excess refrigerant, and a non-azeotropic refrigerant mixture is used as the refrigerant in the low-stage circuit. In this case, the composition of the refrigerant circulating in the low-stage circuit may fluctuate due to the refrigerant gas having a low boiling point among the refrigerants contained in the non-azeotropic refrigerant mixture being retained in the receiver. If the refrigerant having a high boiling point among the refrigerants contained in the non-azeotropic refrigerant mixture is flammable, the flammability of the refrigerant circulating in the low-stage circuit increases due to the refrigerant composition fluctuation, and the risk of flammability increases in the event of a refrigerant leak.

[0005] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a refrigeration cycle device that can suppress fluctuations in the composition of a refrigerant, and a method for controlling a refrigeration cycle device. [Means for solving the problem]

[0006] The refrigeration cycle device according to the present disclosure includes a high-stage circuit including 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 including a second compressor, a cascade heat exchanger, a receiver, 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 high-pressure pressure of the low-stage side refrigerant circulating in the low-stage circuit is equal to or lower than a threshold value, the threshold value being a pressure at which the low-stage side refrigerant can maintain its non-flammability, The high pressure of the low stage side refrigerant circulating in the low stage circuit 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 including 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 including a second compressor, a cascade heat exchanger, a receiver, 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 includes a step of controlling the high-stage circuit by the control device so that a high-pressure pressure of the low-stage side refrigerant circulating in the low-stage circuit is equal to or lower than a threshold value, the threshold value being a pressure at which the low-stage side refrigerant can maintain non-flammability, The high pressure of the low stage side refrigerant circulating in the low stage circuit 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, the high pressure of the low stage side refrigerant circulating in the low stage circuit is maintained at or below the pressure at which the low stage side refrigerant maintains its non-flammability, thereby suppressing fluctuations in the refrigerant composition. [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 combustibility of a low-temperature side refrigerant and a high-temperature pressure PH. [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. 11 is a schematic configuration diagram of a refrigeration cycle device according to a 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 receiver 22, a second pressure reducing device 23, and an evaporator 24. The second compressor 21, the cascade heat exchanger 14, the receiver 22, 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 by 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, mixing CO2 into the low-temperature refrigerant improves the cooling capacity, and mixing R290 improves the COP and lowers the triple point of CO2, 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 receiver 22 is disposed between the cascade heat exchanger 14 and the second pressure reducing device 23, and 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.

[0021] 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 refrigerant outlet of the receiver 22, and reduces the pressure of the low-side refrigerant flowing out from the receiver 22 to expand it. The second pressure reducing device 23 may be a capillary tube or a temperature-sensing expansion valve.

[0022] 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.

[0023] In addition, the refrigeration cycle device 100 has a high pressure P H The pressure sensor 26 is provided in a pipe connecting the low-stage side flow path 142 of the cascade heat exchanger 14 and the receiver 22. The pressure sensor 26 can be provided on the high-pressure side of the low-stage circuit 2, that is, at any position between the discharge port of the second compressor 21 and the refrigerant inlet of the second pressure reducing device 23. The high-pressure pressure P of the low-stage side refrigerant detected by the pressure sensor 26 H is transmitted to the control device 3.

[0024] In place of the pressure sensor 26, the high pressure P H A sensor is provided to detect other physical quantities (e.g., condensation temperature) that can be converted into the high pressure P H The refrigeration cycle apparatus 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.

[0025] The control device 3 controls the overall operation of the refrigeration cycle device 100. The control device 3 is configured with 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 is configured to detect the high pressure P H The control device 3 controls the high-temperature circuit 1 based on the information received from the various sensors and the operation content instructed by the user. In addition, the control device 3 controls each device of the high-temperature circuit 1 and the low-temperature circuit 2, as well as the first fan 15 and the second fan 25 based on the information received from the various sensors and the operation content instructed by the user.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] The low stage side refrigerant condensed and liquefied in the cascade heat exchanger 14 flows into the receiver 22. The low stage side refrigerant flowing out of the receiver 22 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.

[0030] The receiver 22 of the low stage circuit 2 stores surplus liquid refrigerant generated according to the operating conditions or load conditions of the refrigeration cycle device 100. At this time, among the low stage side refrigerant in the receiver 22, the refrigerant with a low boiling point becomes gas and accumulates in the receiver 22, and the circulation composition of the low stage side refrigerant that flows out of the receiver 22 and circulates in the low stage circuit 2 fluctuates. For example, as in the present embodiment, when a non-azeotropic mixture refrigerant of CO2 and R290 is used as the low stage side refrigerant, CO2, which has a boiling point lower than that of R290, accumulates as gas in the receiver 22. As a result, the proportion of R290, which is a flammable refrigerant, increases in the circulation composition of the low stage side refrigerant. This increases the flammability of the low stage side refrigerant circulating in the low stage circuit 2, and increases the risk of flammability when the refrigerant leaks.

[0031] Therefore, the control device 3 of this embodiment controls the capacity of the high-stage circuit 1 to reduce the high-stage pressure P H The pressure P is set to a value below which the low-side refrigerant maintains its non-flammability. H2 is a graph showing the relationship between the high pressure P of the low-temperature side refrigerant and the high-boiling-point refrigerant, as in the present embodiment. H Therefore, in order to keep the low-side refrigerant non-flammable, the high-pressure P H Threshold 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 detects the high pressure P H is the threshold 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] The pressure sensor 26 detects the high pressure P H The control device 3 detects the high pressure P of the low source side refrigerant detected by the pressure sensor 26 (S2). H is the threshold P T It is determined whether the high pressure P H is the threshold P T If it is equal to or lower than the pressure P (S3: YES), the process proceeds to step S5 while maintaining the capacity of the high-stage circuit 1. H is the threshold P TIf it is greater than the high-stage refrigerant pressure P 1 (S3: NO), the control device 3 increases the capacity of the high-stage circuit 1 (S4). Specifically, the control device 3 increases the operation frequency of the first compressor 11 of the high-stage circuit 1. Here, the control device 3 may increase the operation frequency of the first compressor 11 by a predetermined constant value, or may increase the high-pressure pressure P 1 of the low-stage refrigerant by a predetermined constant value. H and the threshold P T Alternatively, the value may be increased by a value according to the difference between the

[0034] By increasing the capacity of the high-stage circuit 1, the temperature of the high-stage refrigerant flowing through the high-stage flow passage 141 of the cascade heat exchanger 14 is reduced. As a result, the temperature of the low-stage refrigerant that is heat exchanged with the high-stage refrigerant in the cascade heat exchanger 14 is reduced, and the high-pressure pressure P H The high pressure P of the low-side refrigerant decreases. H As a result, the gas density in the receiver 22 decreases, and the mass of the gas refrigerant in the receiver 22 decreases. That is, the high pressure P H As a result, the amount of low boiling point refrigerant gas remaining in the receiver 22 decreases, and the composition fluctuation of the low stage refrigerant that flows out of the receiver 22 and circulates through the low stage circuit 2 can be minimized.

[0035] The control device 3 judges whether or not to stop the operation of the refrigeration cycle apparatus 100 in response to an instruction from a user or the like (S5). If the operation is not to be stopped (S5: NO), the process returns to step S2 and the subsequent processes are repeated. On the other hand, if the operation is to be stopped (S5: YES), the first compressor 11 and the second compressor 21 are stopped (S6). As a result, during the operation of the refrigeration cycle apparatus 100, the high pressure P H is the threshold P T The following is maintained:

[0036] As described above, in the refrigeration cycle device 100 of the present embodiment, the high pressure P H However, the threshold P at which the low-temperature refrigerant can maintain its non-flammability is TThe high stage circuit 1 is controlled so that the composition of the low stage side refrigerant flowing out from the receiver 22 and circulating through the low stage circuit 2 is suppressed. As a result, even when a non-azeotropic mixed refrigerant containing a flammable refrigerant is used as the low stage side refrigerant, the increase in the risk of flammability at the time of refrigerant leakage can be suppressed. Furthermore, 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.

[0037] 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 after a stop command is issued. The configuration of the refrigeration cycle apparatus 100 is the same as that of the first embodiment.

[0038] 4, the operation (steps S1 to S4) during operation of the refrigeration cycle apparatus 100 is the same as that of embodiment 1. The control device 3 judges whether or not to stop the operation of the refrigeration cycle apparatus 100 in response to an instruction from a user or the like (S5). If the operation is not to be stopped (S5: NO), the process returns to step S2 and the subsequent processes are repeated.

[0039] On the other hand, when the operation is stopped (S5: YES), the control device 3 performs pump-down operation in the low-stage circuit 2 (S11). Here, 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 located downstream of the receiver 22 is closed, the low-stage side refrigerant in the low-stage circuit 2 is recovered in the low-stage side flow passage 142 of the cascade heat exchanger 14 and the receiver 22. Thereafter, the control device 3 stops the second compressor 21 (S12). This stops the circulation of the low-stage side refrigerant in the low-stage circuit 2.

[0040] Note that a solenoid valve may be provided between the refrigerant outlet of the receiver 22 and the second pressure reducing device 23, and the pump-down operation may be performed by closing the solenoid valve. Also, a pressure sensor for detecting the low pressure of the low-side refrigerant may be provided on the low-pressure side of the low-stage circuit 2, i.e., between the refrigerant outlet of the second pressure reducing device 23 and the suction port of the second compressor 21, and the second compressor 21 may be stopped when the low pressure of the low-stage side refrigerant becomes equal to or lower than atmospheric pressure. This makes it possible to prevent breakdowns and the like caused by the second compressor 21 continuing to operate even after the refrigerant runs out on the low-pressure side of the low-stage circuit 2.

[0041] The pressure sensor 26 detects the high pressure P H The control device 3 detects the high pressure P H is the threshold P T It is determined whether the high pressure P H is the threshold P T If it is equal to or lower than the pressure P (S14: YES), the process proceeds to step S16 while maintaining the capacity of the high-stage circuit 1. H is the threshold P T If it is greater (S14: NO), the control device 3 increases the capacity of the high-temperature circuit 1 (S15). Specifically, similarly to the first embodiment, the control device 3 increases the operation frequency of the first compressor 11 of the high-temperature circuit 1.

[0042] Here, even if the circulation of the low stage side refrigerant in the low stage circuit 2 stops, the low boiling point refrigerant may gasify in the receiver 22 of the low stage circuit 2, causing a change in the composition of the liquid refrigerant in the receiver 22. For example, in the case of this embodiment, CO2, which is a low boiling point refrigerant, gasifies in the receiver 22, and the proportion of R290, which is a flammable refrigerant, increases in the composition of the liquid refrigerant in the receiver 22. In addition, since a large amount of liquid refrigerant exists in the receiver 22 due to the pump-down operation, the risk of refrigerant leakage from the receiver 22 increases.

[0043] In contrast, in this embodiment, even after the low-stage circuit 2 is stopped, the high-pressure pressure P HHowever, the threshold P at which the low-temperature refrigerant can maintain its non-flammability is T As a result, the temperature of the low-stage refrigerant remaining in the low-stage flow passage 142 of the cascade heat exchanger 14 decreases, and the high-pressure pressure P H Then, the low-temperature side refrigerant whose temperature has been reduced in the cascade heat exchanger 14 flows into the receiver 22 by natural convection so as to maintain the balance of temperature and pressure, so that the gas density in the receiver 22 decreases and the mass of the gas refrigerant in the receiver 22 decreases. As a result, the amount of gas refrigerant of the low-boiling point refrigerant (CO2) in the receiver 22 decreases, and the composition fluctuation of the liquid refrigerant in the receiver 22 can be minimized.

[0044] 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 (S16). If the operation is not to be started (S16: NO), the process returns to step S13 and repeats the subsequent processes. If the operation is to be started (S16: 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.

[0045] As described above, in the refrigeration cycle apparatus 100 of the present embodiment, in addition to the same effects as those of the first embodiment, the composition fluctuation of the liquid refrigerant remaining in the receiver 22 can be suppressed even when the refrigeration cycle apparatus 100 is stopped. As a result, even when a non-azeotropic refrigerant mixture containing a flammable refrigerant is used as the low-side refrigerant, an increase in the risk of flammability when refrigerant leaks from the receiver 22 can be suppressed.

[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 high-pressure pressure P H is the threshold 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] In the above embodiment, the high pressure P of the low-side refrigerant detected by the pressure sensor 26 H In the above embodiment, the high-stage circuit 1 is controlled in response to the high-stage refrigerant pressure P H Alternatively, the control device 3 may control the high-temperature circuit 1 in accordance with the condensation temperature of the low-temperature side refrigerant corresponding to the high-pressure pressure P H may be set to a pressure value at which the low stage side refrigerant maintains its non-flammability or lower. The cooling load of the low stage circuit 2 is calculated based on, for example, the indoor temperature of the freezer or other room to be cooled. In this case, the control device 3 increases the capacity of the high stage circuit 1 when the cooling load of the low stage circuit 2 increases, and decreases the capacity of the high stage circuit 1 when the cooling load of the low stage circuit 2 decreases. As a result, the high pressure pressure P H Even if the pressure of the low-side refrigerant increases, the capacity of the high-side circuit 1 increases, and the high-side refrigerant pressure P H As a result, the high pressure P HIn this modification, the pressure sensor 26 may be omitted, and the high pressure P of the low stage side refrigerant detected by the pressure sensor 26 may be maintained at or below the pressure value at which the low stage side refrigerant maintains its non-flammability. H Control based on the above may also be performed in combination.

[0050] In addition, a pressure relief device is provided in the low-side circuit 2, which opens when the pressure or temperature rises to a reference value, and the high-pressure pressure P H may be set to a pressure value at which the low stage side refrigerant maintains its non-flammability or lower. Fig. 5 is a schematic diagram of a refrigeration cycle apparatus 100A according to a modified example. As shown in Fig. 5, the refrigeration cycle apparatus 100A includes a pressure relief device 27. The pressure relief device 27 is provided in the refrigerant pipe or receiver 22 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 is provided with a pressure relief valve or a fusible plug that is operative to reduce the high pressure P H Or the condensation temperature is threshold P T When the pressure reaches or exceeds this limit, the valve or valve is opened, and the gas refrigerant is discharged to the outside, and the high pressure P H The threshold P T is a pressure value or temperature at which the low-side refrigerant can maintain its non-flammability, as in the above embodiment. H In this modification, the high pressure P of the low stage side refrigerant detected by the pressure sensor 26 can be maintained at or below the pressure value at which the low stage side refrigerant can maintain its non-flammability. H Alternatively, the control of the high-temperature circuit 1 based on the cooling load may be omitted, or these controls may be combined. [Explanation of symbols]

[0051] 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 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 receiver, 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 high-pressure pressure of the low-stage side refrigerant circulating through the low-stage circuit is equal to or lower than a threshold value, The threshold value is a pressure at which the low-temperature side refrigerant can maintain non-flammability, The high pressure of the low stage side refrigerant circulating through the low stage circuit 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 a capacity of the high-stage circuit when the high-stage pressure of the low-stage side refrigerant is higher than the threshold value.

3. 3 . The refrigeration cycle apparatus according to claim 1 , wherein the control device increases an operation frequency of the first compressor when the high pressure of the low-temperature side refrigerant is higher than the threshold value. 4 .

4. The refrigeration cycle device according to any one of claims 1 to 3, wherein the control device continues to drive the first compressor even after the second compressor is stopped, and controls the high-stage circuit so that the high-pressure pressure of the low-stage side refrigerant is equal to or lower than the threshold value.

5. The refrigeration cycle apparatus according to claim 1 , wherein the control device increases the capacity of the high-stage circuit when a cooling load of the low-stage circuit increases.

6. 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 receiver, a second pressure reducing device, and an evaporator, in which a low-stage refrigerant circulates; A control method for a refrigeration cycle apparatus 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 high-pressure pressure of the low-stage side refrigerant circulating in the low-stage circuit is equal to or lower than a threshold value, The threshold value is a pressure at which the low-temperature side refrigerant can maintain non-flammability, A control method for a refrigeration cycle device, comprising: maintaining the high pressure of the low stage side refrigerant circulating through the low stage circuit at or below a pressure at which the low stage side refrigerant can maintain its non-flammability.

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