WO2024154207A1

WO2024154207A1 - Binary refrigeration device

Info

Publication number: WO2024154207A1
Application number: PCT/JP2023/001050
Authority: WO
Inventors: 拓未西山; 智隆石川; 崇憲八代; 裕弥井内; 寛也石原; 英希大野
Original assignee: 三菱電機株式会社
Priority date: 2023-01-16
Filing date: 2023-01-16
Publication date: 2024-07-25

Abstract

A binary refrigeration device (200) comprises: a first refrigeration cycle (1) that includes a first compressor (10), a first heat exchanger (12), a first expansion valve (13), a second heat exchanger (103), a second expansion valve (14), a third heat exchanger (15), and a heater (20), and through which a first refrigerant circulates; a second refrigeration cycle (2) that includes a second compressor (100), a fourth heat exchanger (101), a third expansion valve (102), and a second heat exchanger (103), and through which a second refrigerant circulates; and a control device (600). The control device (600) controls the first refrigeration cycle and the second refrigeration cycle in a cooling mode or a defrosting mode. The defrosting mode is a mode in which the first refrigerant is made to circulate in the opposite direction from that of the cooling mode. The heater (20) is provided to the third heat exchanger (15). When in the defrosting mode, the control device (600) performs control in which the flow path of the first refrigerant in the third heat exchanger (15) is heated by the heater (20) while the first refrigerant is flowing into the third heat exchanger (15).

Description

Dual refrigeration system

This disclosure relates to a dual refrigeration system.

Patent Document 1 discloses that in an air conditioner equipped with a dual refrigeration cycle, when frost is removed from a heat exchanger used as an evaporator during cooling operation of the low-temperature refrigerant circuit, reverse cycle defrosting is performed by sending hot gas from the compressor to the heat exchanger via a four-way valve.

Furthermore, Patent Document 1 discloses that in an air conditioner equipped with a dual cascade refrigeration cycle, when reverse cycle defrosting is performed, the compressor is operated in the high-temperature refrigerant circuit, and heat is exchanged in an intermediate heat exchanger between the low-temperature refrigerant circuit and the high-temperature refrigerant circuit, thereby suppressing the rise in the temperature of the refrigerant in the low-temperature refrigerant circuit.

Patent No. 5595245

However, as in Patent Document 1, when reverse cycle defrosting is performed, the period required to defrost the heat exchanger increases due to factors such as the properties of the refrigerant. One example of the reason for this is that when a refrigerant made of carbon dioxide is used, the saturation temperature of the refrigerant is relatively low, and when the refrigerant is in a two-phase state, the temperature difference between the refrigerant and the frost to be defrosted cannot be sufficiently ensured, resulting in low defrosting capacity.

The present disclosure aims to solve the problems described above, and its purpose is to provide a cascade refrigeration system that can improve defrosting capacity.

The present disclosure relates to a cascade refrigeration system. The cascade refrigeration system includes a first refrigeration cycle including a first compressor, a first heat exchanger, a first expansion valve, a second heat exchanger, a second expansion valve, a third heat exchanger, and a heater, and in which a first refrigerant circulates; a second refrigeration cycle including a second compressor, a fourth heat exchanger, a third expansion valve, and a second heat exchanger, and in which a second refrigerant circulates; and a control device that controls the first refrigeration cycle and the second refrigeration cycle. The second heat exchanger exchanges heat between the first refrigerant and the second refrigerant. The control device controls the first refrigeration cycle and the second refrigeration cycle in a cooling mode or a defrosting mode. The cooling mode is a mode in which the first refrigerant discharged from the first compressor returns to the first compressor via the first heat exchanger, the first expansion valve, the second heat exchanger, the second expansion valve, and the third heat exchanger. The defrost mode is a mode in which the first refrigerant discharged from the first compressor returns to the first compressor via the third heat exchanger, the second expansion valve, the second heat exchanger, the first expansion valve, and the first heat exchanger. The heater is provided in the third heat exchanger. In the defrost mode, the control device controls the flow path of the first refrigerant in the third heat exchanger to be heated by the heater while the first refrigerant is flowing into the third heat exchanger.

In the dual cascade refrigeration system disclosed herein, in defrost mode, while the first refrigerant is flowing into the third heat exchanger, the flow path of the first refrigerant in the third heat exchanger is heated by a heater, thereby improving the defrosting capacity.

1 is a diagram showing the configuration of a dual refrigeration system according to a first embodiment. FIG. FIG. 13 is a diagram showing the arrangement of a heater in the third heat exchanger. FIG. 13 is a diagram showing the state of the third heat exchanger in the defrosting mode. 5 is a flowchart of control executed by the control device when the cascade refrigeration system operates in a defrost mode. FIG. 11 is a diagram showing an example of a change in the detected temperature of a third temperature sensor in a defrosting mode. FIG. 11 is a diagram showing the configuration of a dual refrigeration system according to a second embodiment. FIG. 11 is a diagram showing the configuration of a dual refrigeration system according to a third embodiment. FIG. 13 is a diagram showing the configuration of a dual refrigeration system according to a fourth embodiment. FIG. 13 is a diagram showing the arrangement of a sixth temperature sensor in the cascade refrigeration system of the fifth embodiment. 13 is a flowchart of control executed by a control device when the cascade refrigeration system of the fifth embodiment operates in a defrosting mode. FIG. 13 is a diagram showing an example of a change in power consumption in a defrost mode of the cascade refrigeration device of the fifth embodiment.

Below, the embodiments of the present invention will be described in detail with reference to the drawings. Several embodiments will be described below, but it is planned from the beginning of the application that the configurations described in each embodiment will be appropriately combined. Note that the same or equivalent parts in the drawings will be given the same reference numerals and their description will not be repeated.

Embodiment 1.
(Overall configuration of the cascade refrigeration system 200)
FIG. 1 is a diagram showing the configuration of a cascade refrigeration system 200 according to a first embodiment. The cascade refrigeration system 200 includes a first refrigeration cycle 1, a second refrigeration cycle 2, and a control device 600. The first refrigeration cycle 1 is a low-stage refrigeration cycle. The second refrigeration cycle 2 is a high-stage refrigeration cycle. In the cascade refrigeration system 200, a multi-stage refrigeration cycle is configured in which the first refrigeration cycle 1 and the second refrigeration cycle 2 are connected by a second heat exchanger 103. In the cascade refrigeration system 200, cooling is performed in a low temperature range, for example, at minus several tens of degrees, by using such a multi-stage refrigeration cycle.

The first refrigeration cycle 1 includes a first compressor 10, a four-way valve 11, a first heat exchanger 12, a first expansion valve 13, a second heat exchanger 103, a second expansion valve 14, a third heat exchanger 15, and a heater 20. In the first refrigeration cycle 1, a first refrigerant circulates through the first compressor 10, the four-way valve 11, the first heat exchanger 12, the first expansion valve 13, the second heat exchanger 103, the second expansion valve 14, and the third heat exchanger 15.

The heater 20 is provided in the third heat exchanger 15. The heater 20 performs a heating operation when melting and removing frost that has adhered to the third heat exchanger 15.

A blower fan 12F is provided in correspondence with the first heat exchanger 12. A blower fan 15F is provided in correspondence with the third heat exchanger 15.

The second refrigeration cycle 2 includes a second compressor 100, a fourth heat exchanger 101, a third expansion valve 102, and a second heat exchanger 103. In the second refrigeration cycle 2, a second refrigerant circulates through the second compressor 100, the fourth heat exchanger 101, the third expansion valve 102, and the second heat exchanger 103. A blower fan 101F is provided corresponding to the fourth heat exchanger 101.

The first refrigerant is, for example, a carbon dioxide refrigerant. Carbon dioxide refrigerant has a boiling point lower than -52°C. A refrigerant other than carbon dioxide refrigerant may be used as the first refrigerant. For example, a refrigerant containing carbon dioxide as a main component may be used as the first refrigerant. The second refrigerant is, for example, a propane refrigerant. Note that a refrigerant other than propane refrigerant may be used as the second refrigerant. Furthermore, the first refrigerant and the second refrigerant may be the same type of refrigerant, or different types of refrigerants may be used.

When a refrigerant with a boiling point lower than -52°C is used as the first refrigerant, piping and equipment with a relatively low pressure resistance, such as 4.25 MPaA, may be used in the first refrigeration cycle 1, rather than piping and equipment for high pressure resistance. In this way, when piping and equipment with low pressure resistance are used in the first refrigeration cycle 1, the cost required to manufacture the cascade refrigeration system 200 can be reduced because the thickness of the piping can be made thinner and the flow rate of the first refrigerant can be increased.

The second heat exchanger 103 is a cascade heat exchanger, and exchanges heat between the first refrigerant flowing through the piping between the first expansion valve 13 and the second expansion valve 14 in the first refrigeration cycle 1 and the second refrigerant flowing through the piping between the third expansion valve 102 and the second compressor 100 in the second refrigeration cycle 2. The second heat exchanger 103 may be a double heat exchanger or a plate type heat exchanger.

In the cascade refrigeration system 200, the various devices constituting the first refrigeration cycle 1 and the second refrigeration cycle 2 are housed separately in an outdoor unit 3, which is an outdoor unit, and an indoor unit 4, which is an indoor unit. The outdoor unit 3 is installed, for example, outdoors. The indoor unit 4 is installed, for example, inside a freezing room such as a freezer.

The outdoor unit 3 houses the first compressor 10, four-way valve 11, first heat exchanger 12, first expansion valve 13, second heat exchanger 103, second expansion valve 14, third heat exchanger 15, and heater 20 in the first refrigeration cycle 1, and the second compressor 100, fourth heat exchanger 101, third expansion valve 102, and second heat exchanger 103 in the second refrigeration cycle 2.

The indoor unit 4 houses the second expansion valve 14, the third heat exchanger 15, and the heater 20 in the first refrigeration cycle 1. An extension pipe 6 is provided between the second heat exchanger 103 of the outdoor unit 3 and the second expansion valve 14 of the indoor unit 4. An extension pipe 7 is provided between the four-way valve 11 of the outdoor unit 3 and the third heat exchanger 15.

In the first refrigeration cycle 1, various sensors are provided. A first pressure sensor 31 for detecting the pressure of the first refrigerant discharged from the first compressor 10 and a first temperature sensor 32 for detecting the temperature of the first refrigerant discharged from the first compressor 10 are provided in the piping between the outlet of the first refrigerant in the first compressor 10 and the inlet of the four-way valve 11. A second pressure sensor 33 for detecting the pressure of the first refrigerant sucked into the first compressor 10 and a second temperature sensor 34 for detecting the temperature of the first refrigerant sucked into the first compressor 10 are provided in the piping between the third heat exchanger 15 and the inlet of the first compressor 10. A third temperature sensor 35 for detecting the temperature of the first refrigerant is provided between the first heat exchanger 12 and the second heat exchanger 103. A fourth temperature sensor 36 for detecting the temperature of the first refrigerant flowing out from the third heat exchanger 15 toward the second expansion valve 14 in a defrost mode described later is provided in the piping between the third heat exchanger 15 and the second expansion valve 14.

Various sensors are provided in the second refrigeration cycle 2. A third pressure sensor 41 is provided in the piping between the second refrigerant outlet of the second compressor 100 and the fourth heat exchanger 101 to detect the pressure of the second refrigerant discharged from the second compressor 100. A fourth pressure sensor 42 is provided in the piping between the second heat exchanger 103 and the inlet of the second compressor 100 to detect the pressure of the second refrigerant sucked into the second compressor 100, and a fifth temperature sensor 43 is provided to detect the temperature of the second refrigerant sucked into the second compressor 100.

The indoor unit 4 is provided with a sixth temperature sensor 23 that detects the temperature inside the freezer compartment in which the indoor unit 4 is installed.

The control device 600 is housed in the outdoor unit 3. The control device 600 controls the devices housed in the outdoor unit 3 and the devices housed in the indoor unit 4.

The control device 600 may be housed in the indoor unit 4, or may be provided in both the outdoor unit 3 and the indoor unit 4. When the control device 600 is provided in both the outdoor unit 3 and the indoor unit 4, the control device provided in the outdoor unit 3 and the control device provided in the indoor unit 4 communicate with each other, and these control devices control the devices housed in the outdoor unit 3 and the devices housed in the indoor unit 4.

The control device 600 is composed of a CPU (Central Processing Unit) 601, memory 602 (ROM (Read Only Memory) and RAM (Random Access Memory)), an input/output buffer (not shown), etc. The CPU 601 deploys the programs stored in the ROM into the RAM etc. and executes them. The programs stored in the ROM are programs in which the processing procedures of the control device 600 are written. The control device 600 controls various devices in the cascade refrigeration device 200 in accordance with these programs. The control of such devices is not limited to processing by software, but can also be processed by dedicated hardware (electronic circuits).

The control device 600 receives detection information from various sensors, such as the first pressure sensor 31, the second pressure sensor 33, the third pressure sensor 41, the fourth pressure sensor 42, the first temperature sensor 32, the second temperature sensor 34, the third temperature sensor 35, the fourth temperature sensor 36, the fifth temperature sensor 43, and the sixth temperature sensor 23.

The control device 600 controls, for example, the first compressor 10, the four-way valve 11, the first expansion valve 13, the second expansion valve 14, the heater 20, the second compressor 100, the third expansion valve 102, and the

fans

12F, 15F, and 101F, depending on the detection information of the various sensors mentioned above and other information.

The control device 600 can control the operation of the first refrigeration cycle 1 and the second refrigeration cycle 2 in a cooling mode or a defrosting mode.

The cooling mode is an operating mode in which the room in which the third heat exchanger 15 is provided is cooled by heat exchange using the third heat exchanger 15. In this mode, the first refrigerant discharged from the first compressor 10 passes through the four-way valve 11, the first heat exchanger 12, the first expansion valve 13, the second heat exchanger 103, the second expansion valve 14, and the third heat exchanger 15, and returns to the first compressor 10. In the cooling mode, the first heat exchanger 12 is used as a condenser, and the third heat exchanger 15 is used as an evaporator.

The defrost mode is an operating mode in which frost adhering to the third heat exchanger 15 is removed. In the defrost mode, the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 passes through the third heat exchanger 15, the second expansion valve 14, the second heat exchanger 103, the first expansion valve 13, the four-way valve 11, and the first heat exchanger 12, and returns to the first compressor 10. In the defrost mode, the third heat exchanger 15 is used as a condenser, and the first heat exchanger 12 is used as an evaporator.

In this way, in the defrosting mode, the high-temperature, high-pressure gaseous first refrigerant discharged from the first compressor 10 is circulated in the opposite direction to the cooling mode and supplied to the third heat exchanger 15 to remove the frost that has adhered to the third heat exchanger 15. This defrosting is called reverse hot gas defrosting.

The heater 20 is provided in a position in the third heat exchanger 15 where it can heat the downstream side of the flow path of the first refrigerant in the third heat exchanger 15 when the first refrigerant is flowing into the third heat exchanger 15 in defrost mode.

The four-way valve 11 is configured to switch the direction in which the first refrigerant flows between the cooling mode and the defrost mode. In the cooling mode, the flow path in the four-way valve 11 is the flow path shown by the solid line, and in the defrost mode, the flow path is the flow path shown by the dashed line. In the cooling mode, the flow path of the four-way valve 11 is switched to the first flow path so that the first refrigerant discharged from the first compressor 10 is supplied to the first heat exchanger 12, as shown by the solid line inside the four-way valve 11. In the defrost mode, the flow path of the four-way valve 11 is switched to the second flow path so that the first refrigerant discharged from the first compressor 10 is supplied to the third heat exchanger 15, as shown by the dashed line inside the four-way valve 11.

The heat transfer area of the second heat exchanger 103 is, for example, within a range of 3% to 50% of the combined heat transfer area of the first heat exchanger 12 and the fourth heat exchanger 101. It is preferable that the heat transfer area of the second heat exchanger 103 is within a range of 8% to 30% of the combined heat transfer area of the first heat exchanger 12 and the fourth heat exchanger 101.

In addition, the second refrigeration cycle 2 may have a lower pressure resistance for the piping and equipment than the first refrigeration cycle 1 by selecting a second refrigerant that has a lower pressure at the same temperature (e.g., saturation temperature) than the first refrigerant that is charged in the first refrigeration cycle 1.

(Cooling mode operation status)
Next, the operating state of the cooling mode will be described with reference to FIG.

First, the operating state of the first refrigeration cycle 1 will be described. In the cooling mode, the first refrigerant compressed in the first compressor 10 becomes high-temperature, high-pressure superheated vapor and is discharged from the first compressor 10. The first refrigerant discharged from the first compressor 10 flows into the first heat exchanger 12 via the four-way valve 11. In the first heat exchanger 12, the first refrigerant exchanges heat with the outdoor air and is condensed.

In the cooling mode, the first expansion valve 13 is opened to the maximum degree. The first refrigerant flowing out of the first heat exchanger 12 flows into the second heat exchanger 103. In the second heat exchanger 103, heat is exchanged between the first refrigerant and the second refrigerant. In the second heat exchanger 103, the first refrigerant is further condensed. The first refrigerant flowing out of the second heat exchanger 103 flows into the second expansion valve 14. In the second expansion valve 14, the first refrigerant is expanded.

The first refrigerant that flows out of the second expansion valve 14 flows into the third heat exchanger 15. In the cooling mode, the heater 20 is not operated. In the third heat exchanger 15, the first refrigerant exchanges heat with the air inside the freezer and is evaporated. This causes the air inside the freezer to be cooled by the third heat exchanger 15. The first refrigerant that flows out of the third heat exchanger 15 flows into the suction side of the first compressor 10 via the four-way valve 11.

The control device 600 controls the frequency of the first compressor 10 and the opening of the second expansion valve 14 so that the evaporation temperature of the third heat exchanger 15 and the suction superheat of the first compressor 10 reach preset target values in response to the detection information of the second pressure sensor 33 and the second temperature sensor 34. The control device 600 controls the first expansion valve 13 so that the opening of the first expansion valve 13 reaches its maximum opening.

The control device 600 controls the first compressor 10 to stop operation when the internal temperature becomes equal to or lower than the internal temperature set by the user, according to the information detected by the sixth temperature sensor 23. In a configuration example in which information communication is not possible between the outdoor unit 3 and the indoor unit 4, the control device 600 may control the first compressor 10 to stop operation when the pressure detected by the second pressure sensor 33 becomes equal to or lower than the pressure corresponding to the internal temperature set by the user.

The control device 600 may control the frequency of the second compressor 100 in accordance with the detection information of the first pressure sensor 31 so that the condensing temperature of the first heat exchanger 12 becomes a preset target value. The control device 600 may control the frequency of the second compressor 100 while maintaining the rotation speed of the fan 12F at a maximum speed so that the condensing temperature of the first heat exchanger 12 becomes a preset target value. When controlling the condensing temperature of the first heat exchanger 12 to become a preset target value, the control device 600 may control the rotation speed of the fan 12F so that the temperature detected by the third temperature sensor 35 becomes the outside air temperature + α°C (for example, α = 0°C - 2°C) in order to minimize an increase in the rotation speed of the fan 12F.

Next, the operating state of the second refrigeration cycle 2 will be described. In the cooling mode, the second refrigerant compressed in the second compressor 100 becomes high-temperature, high-pressure superheated vapor and is discharged from the second compressor 100. The second refrigerant discharged from the second compressor 100 flows into the fourth heat exchanger 101. In the fourth heat exchanger 101, the second refrigerant exchanges heat with the outdoor air and is condensed.

The second refrigerant flowing out of the fourth heat exchanger 101 flows into the third expansion valve 102. In the third expansion valve 102, the second refrigerant is expanded. The second refrigerant flowing out of the third expansion valve 102 flows into the second heat exchanger 103. In the second heat exchanger 103, the second refrigerant is evaporated by heat exchange with the first refrigerant. The second refrigerant flowing out of the second heat exchanger 103 flows into the second compressor 100.

The control device 600 controls the opening of the third expansion valve 102 according to the detection information of the fourth pressure sensor 42 and the fifth temperature sensor 43 so that the suction superheat of the second compressor 100 becomes a preset target value.

The control device 600 controls the rotation speed of the fan 101F according to the detection information of the third pressure sensor 41 so that the condensation temperature of the fourth heat exchanger 101 becomes the set target value (saturation temperature).

(Defrost mode operation status)
Next, the operation state of the defrosting mode will be described with reference to FIG.

First, the operating state of the first refrigeration cycle 1 will be described. In the defrost mode, the first refrigerant compressed in the first compressor 10 becomes high-temperature, high-pressure superheated vapor and is discharged from the first compressor 10. The first refrigerant discharged from the first compressor 10 flows into the third heat exchanger 15 via the four-way valve 11. In the third heat exchanger 15, the first refrigerant exchanges heat with the air inside the storage unit and is condensed. In the defrost mode, the heater 20 is operated to heat. The fan 15F of the third heat exchanger 15 is stopped.

In the defrost mode, the first refrigerant, which has become superheated vapor, flows into the third heat exchanger 15 and the heater 20 performs heating operation, so that the frost adhering to the third heat exchanger 15 is melted and removed by the thermal energy of the first refrigerant and the thermal energy of the heater 20.

In the defrost mode, the second expansion valve 14 is opened to the maximum degree. The first refrigerant that flows out of the third heat exchanger 15 flows into the second heat exchanger 103. In the second heat exchanger 103, heat is exchanged between the first refrigerant and the second refrigerant. In the second heat exchanger 103, the first refrigerant is cooled.

The first refrigerant flowing out of the second heat exchanger 103 flows into the first expansion valve 13. In the first expansion valve 13, the first refrigerant is expanded. The first refrigerant flowing out of the first expansion valve 13 flows into the first heat exchanger 12. In the first heat exchanger 12, the first refrigerant exchanges heat with the outdoor air and is evaporated. The first refrigerant flowing out of the first heat exchanger 12 flows into the suction side of the first compressor 10 via the four-way valve 11.

The basic operating state of the second refrigeration cycle 2 in the defrost mode is the same as the operating state in the cooling mode.

(Configuration of heater 20)
Next, the configuration of the heater 20 will be described. Fig. 2 is a diagram showing the arrangement of the heater 20 in the third heat exchanger 15. Fig. 2(A) shows a cross-sectional view of the third heat exchanger 15, which shows a schematic positional relationship between the heater 20 and a tube 61 through which the first refrigerant flows in the third heat exchanger 15. Fig. 2(B) shows a side view of the third heat exchanger 15, which shows a schematic positional relationship between the tube 61 and the heater 20.

Referring to FIG. 2(A), the third heat exchanger 15 is a multi-tube heat exchanger, and the tubes 61 through which the first refrigerant flows are arranged vertically in multiple stages, for example five stages. The first flow path 60 of the third heat exchanger 15 is a flow path through which the first refrigerant flows into the third heat exchanger 15 in the defrost mode. The first flow path 60 branches into multiple tubes 61, for example five tubes.

The tubes 61 of each stage are extended inside the third heat exchanger 15 in the longitudinal direction (horizontal direction) of the third heat exchanger 15 and are folded back multiple times. The second flow path 62 of the third heat exchanger 15 is a flow path through which the first refrigerant flows out of the third heat exchanger 15 in the defrost mode. The tubes 61 of each stage merge into the second flow path 62.

In FIG. 2(A), the solid line connecting two tubes 61 indicates that the tube 61 is bent and folded back in a U-shape at the front side of the cross section, like the shape of the tube 61 at the left end of FIG. 2(B). In FIG. 2(A), the dashed line connecting two tubes 61 indicates that the tube 61 is bent and folded back in a U-shape at the back side of the cross section, like the shape of the tube 61 at the right end of FIG. 2(B). In FIG. 2, each tube 61 is shown to have a shape having multiple paths that go back and forth multiple times in the longitudinal direction of the third heat exchanger 15. Specifically, in FIG. 2, each tube 61 goes back and forth twice in the longitudinal direction of the third heat exchanger 15, forming four rows of paths extending in the longitudinal direction.

The third heat exchanger 15 exchanges heat between the first refrigerant and the indoor air, and the air for heat exchange is supplied from the fan 15F shown in FIG. 1. As shown in FIG. 2(A), the air 15a supplied to the third heat exchanger 15 from the fan 15F shown in FIG. 1 is blown in a direction from the first row to the fourth row of each tube 61.

As shown in Figures 2(A) and 2(B), the heater 20 includes a plurality of heater members 21. Each heater member 21 is a rod-shaped member whose heating element extends in the longitudinal direction of the third heat exchanger 15 so as to follow the extending direction of the third and fourth rows of paths of each tube 61 inside the third heat exchanger 15. In the third heat exchanger 15, a plurality of heater members 21 are provided corresponding to the multiple stages of tubes 61.

In Fig. 2(A) and Fig. 2(B), an example is shown in which heater members 21 are not provided above the first stage tube 61 and below the fifth stage tube 61, but heater members 21 may be provided above the first stage tube 61 and below the fifth stage tube 61.

In FIG. 2(A), each heater member 21 is provided in a position where it can directly heat the third and fourth rows of paths of each tube 61. In this way, each heater member 21 is provided on the downstream side of each tube 61 provided in the third heat exchanger 15 in the direction in which the first refrigerant flows in defrost mode. Since each tube 61 in the third heat exchanger 15 has four rows of paths, the third and fourth rows of paths are downstream of 1/2 of the total length of each tube 61 in the direction in which the first refrigerant flows in defrost mode.

The multiple heater members 21 may have the same heat capacity, or may include heater members with different heat capacities. In the following cases, the multiple heater members 21 may include heater members with different heat capacities. For example, in the third heat exchanger 15, ice roots caused by frost may form in the lower internal portion. In such cases, in order to melt such ice roots in the defrost mode and prevent frost from remaining, it is sufficient to configure at least the heater member 21 in the bottom row of the multiple heater members 21 to have a larger heat capacity than the heater member 21 in the top row.

Furthermore, in the third heat exchanger 15, a phenomenon may occur in which a greater amount of frost adheres to a portion where the airflow speed from the fan 15F is faster than that of the portion where the airflow speed from the fan 15F is slower than that of the portion where the airflow speed from the fan 15F is faster. In such a case, in order to prevent frost from remaining in the portion where the amount of frost adheres more in the defrost mode in the third heat exchanger 15, at least the heater member 21 provided at the position where the airflow speed from the fan 15F is the fastest may be configured to have a greater heat capacity than the heater member 21 provided at the position where the airflow speed from the fan 15F is the slowest.

(State of the third heat exchanger 15 in the defrosting mode)
Next, the state of the third heat exchanger 15 in the defrost mode will be described. Fig. 3 is a diagram showing the state of the third heat exchanger 15 in the defrost mode. In Fig. 3, the temperature distribution in the third heat exchanger 15 and the state of the entire first refrigeration cycle 1 are shown divided into an initial period A after the start of the defrost mode, a middle period B, and a final period C after the start of the defrost mode.

In the initial period A, the middle period B, and the end period C shown in FIG. 3, the temperature distribution in the state of the third heat exchanger 15 and the state of the entire first refrigeration cycle 1 are shown. (A1) in the initial period A, (B1) in the middle period B, and (C1) in the end period C are temperature distribution diagrams showing the relationship between the position D in each tube 61 in the third heat exchanger 15 and the ambient temperature T of each tube 61.

Temperature distribution diagrams (A1), (B1), and (C1) show the distribution of the ambient temperature T between the inlet position D1 of the first refrigerant in the third heat exchanger 15 and the outlet position D2 of the first refrigerant in the third heat exchanger 15 in the defrost mode.

(A2) at the initial stage A, (B2) at the middle stage B, and (C2) at the end stage C are Mollier diagrams showing the state of the first refrigerant. In Mollier diagrams (A2), (B2), and (C2), the vertical axis shows pressure p, and the horizontal axis shows specific enthalpy h. In Mollier diagrams (A2), (B2), and (C2), the saturated liquid line and saturated vapor line are shown by curved lines.

The Mollier diagrams (A2), (B2), and (C2) show the compression process a of the first refrigerant, the condensation process b of the first refrigerant, the expansion process c of the first refrigerant, and the evaporation process d of the first refrigerant in the first refrigeration cycle 1 during defrost mode, and correspond to the devices involved in each process.

In the compression process a, the first refrigerant is compressed by the first compressor 10, and the pressure and specific enthalpy of the first refrigerant increase. In the condensation process b, the first refrigerant is condensed by the third heat exchanger 15, and the specific enthalpy of the first refrigerant decreases while the pressure of the first refrigerant is maintained. Furthermore, in the condensation process b, the temperature of the first refrigerant further decreases in the third heat exchanger 15, so the specific enthalpy of the first refrigerant further decreases. In the expansion process c, the first refrigerant is expanded by the first expansion valve 13, and the pressure of the first refrigerant decreases while the specific enthalpy of the first refrigerant is maintained. In the evaporation process d, the first refrigerant is evaporated by the first heat exchanger 12, and the specific enthalpy of the first refrigerant increases while the pressure of the first refrigerant is maintained.

In the initial stage A of the defrost mode, the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 is supplied from the inlet in the defrost mode to the third heat exchanger 15, but as shown in the temperature distribution diagram (A1), the frost melts only in the area close to the inlet position D1 of the first refrigerant in the third heat exchanger 15, and the ambient temperature T rises from T2 to T1. In such an initial stage A of the defrost mode, the third heat exchanger 15 is in a state where the specific enthalpy h is on the saturated vapor line, as shown in the Mollier diagram (A2).

In the middle period B of the defrost mode, in the third heat exchanger 15, as shown in the temperature distribution diagram (B1), the thermal energy of the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 and the thermal energy generated by the heater 20 melt the frost from the inlet position D1 of the first refrigerant to a position between the inlet position D1 of the first refrigerant and the outlet position D2 of the first refrigerant, causing the ambient temperature T to rise. In such a middle period B of the defrost mode, as shown in the Mollier diagram (B2), the specific enthalpy h of the third heat exchanger 15 becomes high, and the degree of superheat becomes higher than in the early period A of the defrost mode.

At the end of the defrost mode C, as shown in the temperature distribution diagram (C1), the frost melts in the third heat exchanger 15 from the inlet position D1 of the first refrigerant to the outlet position D2 of the first refrigerant due to the thermal energy of the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 and the thermal energy generated by the heater 20, causing the ambient temperature T at the outlet position D2 to rise, for example, from T2 to T3. At the end of the defrost mode C, as shown in the Mollier diagram (C2), the specific enthalpy h of the third heat exchanger 15 becomes even higher, and the degree of superheat becomes higher than in the middle of the defrost mode B.

In the defrost mode, the transition from the initial state A to the middle state B, and the transition from the middle state B to the final state C can be performed by the conventional technology that does not use heating by the heater 20. However, as in the first embodiment, by adding thermal energy by heating the heater 20 during the defrosting of the third heat exchanger 15 by the thermal energy of the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 in the defrost mode, it is possible to shorten the time required to transition from the initial state A to the middle state B, and the time required to transition from the middle state B to the final state C, and the time required to defrost the entire third heat exchanger 15 can be increased. Therefore, in the first embodiment, the defrosting capacity of the cascade refrigeration system 200 can be improved. In particular, in the defrost mode, the time required to transition from the middle state B to the final state C can be significantly shortened.

(Defrost mode control)
Next, the control executed by the control device 600 when the cascade refrigeration system 200 operates in the defrost mode will be described.

FIG. 4 is a flow chart of the control executed by the control device 600 when the cascade refrigeration system 200 operates in the defrost mode. In FIG. 4, the main control items during operation in the defrost mode are written within the boxes for each step.

In step S1, the control device 600 determines whether or not it is currently the start time of defrost mode operation. In the following, operation in defrost mode may be referred to as defrost operation. Whether or not it is currently the start time of defrost operation in step S1 is determined, for example, when the operation duration of cooling operation reaches a preset operation duration time, or when the current time becomes a time preset as the time to execute defrost operation.

If the control device 600 determines in step S1 that the current time is not the start time of the defrost operation, the current operating state is a cooling operation state, and therefore the control device 600 continues operation in the cooling mode in step S15. Hereinafter, the operation in the cooling mode may be referred to as the cooling operation. On the other hand, if the control device 600 determines in step S1 that the current time is the start time of the defrost operation, the control device 600 reduces the frequency of the first compressor 10 to a preset frequency for the defrost operation in step S2. Note that the first compressor 10 may be stopped in step S2. By executing step S2, it is possible to prevent the pressure of the first refrigerant flowing through the first refrigeration cycle 1 from becoming higher than necessary in the defrost mode.

In step S3, the control device 600 switches the four-way valve 11 to the second flow path corresponding to the defrost mode. As a result, in the defrost mode, the high-temperature, high-pressure first refrigerant discharged from the first compressor 10 passes through the four-way valve 11 and is supplied to the third heat exchanger 15. In the defrost mode, the control device 600 controls the opening of the second expansion valve 14 to the maximum opening. In the defrost mode, the control device 600 stops the fan 15F and controls the rotation speed of the first compressor 10 by controlling the frequency of the first compressor 10 so that the discharge temperature of the first compressor 10 detected by the first temperature sensor 32 becomes a preset temperature.

In step S4, the control device 600 determines whether the discharge pressure of the first compressor 10 detected by the first pressure sensor 31 is equal to or lower than the first threshold value. If it is determined in step S4 that the discharge pressure of the first compressor 10 is equal to or lower than the first threshold value, the control device 600 performs heating by the heater 20 in step S7. The first threshold value is set to a pressure value that makes it possible to prevent the pressure of the first refrigerant from excessively increasing in the first refrigeration cycle 1, even if heating is performed by the heater 20. The first threshold value is set to, for example, 3 MPaA.

On the other hand, if it is determined in step S4 that the discharge pressure of the first compressor 10 is higher than the first threshold, the control device 600 starts the second compressor 100 in step S5. Then, in step S6, the second refrigeration cycle 2 is operated under the same operating conditions as those in the cooling mode, and the process proceeds to step S7, where heating is performed by the heater 20. Specifically, in step S6, the control device 600 controls the second compressor 100, the fan 101F, and the third expansion valve 102 with the same operating conditions as those in the cooling mode as target values. In this way, if it is determined in the defrost mode that the discharge pressure of the first compressor 10 is higher than the first threshold, the control device 600 checks the operating state of the second refrigeration cycle 2 and then starts heating by the heater 20.

When the control device 600 determines in step S4 that the discharge pressure of the first compressor 10 is higher than the first threshold value, it executes the operation of the second refrigeration cycle 2 in steps S5 and S6, so that the first refrigerant of the first refrigeration cycle 1 is cooled by the second refrigerant of the second refrigeration cycle 2 in the second heat exchanger 103. This makes it possible to prevent the first refrigerant of the first refrigeration cycle 1 from rising to an excessively high pressure due to heating by the heater 20.

In step S8, the control device 600 determines whether the discharge pressure of the first compressor 10 detected by the first pressure sensor 31 is less than the second threshold value. The second threshold value is a pressure value that is preset as a target value for the discharge pressure of the first compressor 10 in the defrost mode.

If it is determined in step S8 that the discharge pressure of the first compressor 10 is less than the second threshold, the control device 600 controls the first compressor 10 to increase the frequency of the first compressor 10 to increase the rotation speed thereof in step S9, and proceeds to step S11. On the other hand, if it is determined in step S8 that the discharge pressure of the first compressor 10 is equal to or greater than the second threshold, the control device 600 controls the first compressor 10 to decrease the frequency of the first compressor 10 if the discharge pressure of the first compressor 10 exceeds the second threshold in step S10, and proceeds to step S11.

In the defrost mode, steps S8 to S10 control the discharge pressure of the first compressor 10 so that it is maintained at the target value of the discharge pressure in the defrost mode.

In step S11, the control device 600 determines whether the temperature detected by the fourth temperature sensor 36, i.e., the temperature of the first refrigerant flowing from the third heat exchanger 15 toward the second expansion valve 14, is equal to or higher than the third threshold value. The third threshold value is a preset temperature for determining that defrosting of the third heat exchanger 15 in the defrost mode has ended. The third threshold value is set to a temperature higher than the frost temperature, for example, 20°C.

If it is determined in step S11 that the temperature detected by the fourth temperature sensor 36 is less than the third threshold value, the control device 600 determines in step S12 whether the second refrigeration cycle 2 is in operation. In step S12, for example, the control device 600 determines that the second refrigeration cycle 2 is in operation if the second compressor 100 is operating, and determines that the second refrigeration cycle 2 is not in operation if the second compressor 100 is stopped.

If it is determined in step S12 that the second refrigeration cycle 2 is in operation, the control device 600 proceeds to step S7 and repeats the processes from step S7 onwards. On the other hand, if it is determined in step S12 that the second refrigeration cycle 2 is not in operation, the control device 600 proceeds to step S4 and repeats the processes from step S4 onwards.

If it is determined in step S11 that the temperature detected by the fourth temperature sensor 36 is less than the third threshold, the control device 600 proceeds through step S12 and executes step S4 or step S7 to continue supplying high-temperature first refrigerant to the third heat exchanger 15 and heating the third heat exchanger 15 by the heater 20 until the temperature detected by the fourth temperature sensor 36 becomes equal to or greater than the third threshold.

If it is determined in step S11 that the temperature detected by the fourth temperature sensor 36 is equal to or higher than the third threshold value, the control device 600 stops heating by the heater 20 in step S13. The control device 600 switches the operating state to cooling operation in step S14. This switches the flow path of the four-way valve 11 to the first flow path, and the first refrigerant discharged from the first compressor 10 is supplied to the first heat exchanger 12 via the four-way valve 11.

In this way, when the defrosting operation is performed, if the condition that the temperature detected by the fourth temperature sensor 36 is equal to or higher than the third threshold is met, the control device 600 determines that the defrosting has ended, and the heating of the third heat exchanger 15 by the heater 20 is stopped, and the supply of the first refrigerant from the first compressor 10 to the third heat exchanger 15 via the four-way valve 11 is stopped.

(Change in temperature of the fourth temperature sensor 36 in the defrosting mode)
Next, an example of a change in the detected temperature of the fourth temperature sensor 36 in the defrost mode will be described. Fig. 5 is a diagram showing an example of a change in the detected temperature of the fourth temperature sensor 36 in the defrost mode. In Fig. 5, the vertical axis indicates the detected temperature T of the fourth temperature sensor 36, and the horizontal axis indicates the elapsed time t from the start of the defrost operation.

In FIG. 5, the solid line a shows the progress of the detected temperature T of the cascade refrigeration system 200 of the first embodiment, and the dashed line b shows the progress of the detected temperature T of a conventional cascade refrigeration system that is configured not to perform heating by the heater 20 in the defrost mode.

In the configuration of embodiment 1 in which thermal energy is added by heating with heater 20 while defrosting is being performed using thermal energy from the first refrigerant in the defrost mode, the timing t1 at which the detected temperature T starts to rise as shown by solid line a is earlier than the timing t2 at which the detected temperature T starts to rise as shown by dashed line b in the configuration of the conventional technology. As a result, in the configuration of embodiment 1, it is possible to make the timing at which defrosting of the third heat exchanger 15 ends earlier than in the configuration of the conventional technology.

The above-described first embodiment can provide the following effects. As shown in Figs. 1 to 5, in the cascade refrigeration system 200, in the defrost mode, while the first refrigerant is flowing into the third heat exchanger 15, the flow path of the first refrigerant in the third heat exchanger 15 is heated by the heater 20, so that the third heat exchanger 15 can be heated simultaneously by the thermal energy of the first refrigerant and the thermal energy of the heater 20. By performing such heating, the timing at which defrosting of the third heat exchanger 15 ends is earlier than in the past. Therefore, in the cascade refrigeration system 200, the period required for defrosting can be shortened compared to the past. By obtaining such effects, the defrosting capacity of the cascade refrigeration system 200 can be improved.

When carbon dioxide, a high-pressure refrigerant, is used as the first refrigerant as in the first embodiment, the temperature difference between the saturation temperature of the first refrigerant and the frost temperature is small in the defrost mode, so that defrosting by superheating the heater 20 has a greater effect in shortening the time required for defrosting the third heat exchanger 15.

In the cascade refrigeration system 200, the defrosting capacity can be improved even if a refrigerant made of carbon dioxide or a refrigerant made of a mixed gas containing carbon dioxide as a main component is used as the first refrigerant. Therefore, in the cascade refrigeration system 200, a refrigerant that has little performance degradation due to pressure loss and a small GWP (Global Warming Potential), such as a refrigerant made of carbon dioxide or a refrigerant containing carbon dioxide as a main component, can be used as the first refrigerant. As the first refrigerant, for example, R32, R290, R1234yf, and R1234ze(E) can be used as refrigerants that have a small GWP and a high COP (Coefficient of Performance). When such a refrigerant is used as the first refrigerant, the COP of the entire first refrigeration cycle 1 can be improved.

Refrigerants made of carbon dioxide or containing carbon dioxide as the main component are low-toxicity and non-flammable refrigerants, so by using such a refrigerant as the first refrigerant in the cascade refrigeration system 200, the occurrence of fire and toxicity-related accidents can be suppressed even if the first refrigerant leaks into the freezer, for example.

As shown in FIG. 4, when the control device 600 determines that the temperature detected by the fourth temperature sensor 36 is equal to or higher than the third threshold, it stops heating by the heater 20 and switches the operation mode from the defrost mode to the cooling mode, thereby preventing frost from remaining on the third heat exchanger 15.

As shown in FIG. 4, the control device 600 determines whether or not to end the defrost mode depending on the temperature detected by the fourth temperature sensor 36, so that it can end the defrost mode at an appropriate timing corresponding to the change in the degree of frost adhesion, which changes depending on environmental conditions such as the surrounding temperature.

As shown in FIG. 4, in the defrost mode, when the control device 600 determines that the discharge pressure of the first compressor 10 is higher than the first threshold, it checks the operating state of the second refrigeration cycle 2 and then starts heating with the heater 20. This makes it possible to prevent the pressure of the first refrigerant in the first refrigeration cycle 1 from suddenly increasing to an excessive pressure due to heating by the heater 20.

As explained using FIG. 2, when the heat capacity of at least the heater member 21 in the lowest tier is configured to be greater than the heat capacity of the heater member 21 in the highest tier, even if ice is formed due to frost in the lower part inside the third heat exchanger 15, the ice can be melted in defrost mode so that no frost remains.

As explained using FIG. 2, when the heater members 21 are configured so that at least the heater member 21 arranged at the position where the air speed from the fan 15F is the fastest has a larger heat capacity than the heater member 21 arranged at the position where the air speed from the fan 15F is the slowest, it is possible to prevent frost from remaining in the third heat exchanger 15 in the defrost mode in areas where there is a large amount of frost.

In the defrost mode, the inlet side of the third heat exchanger 15 is heated by the thermal energy of the first refrigerant, and the downstream side of the third heat exchanger 15 is heated by the thermal energy of the heater 20 at the same time, so that defrosting of the third heat exchanger 15 can be completed early.

The cascade refrigeration system 200 can reduce the time required for defrosting compared to conventional systems, thereby preventing the temperature inside the freezer from rising during defrost mode.

Specifically, as shown in FIG. 2, the heater 20 that heats the flow path of the first refrigerant in the defrost mode is provided in the flow path of the first refrigerant provided in the third heat exchanger 15 at a position that heats the downstream side of the direction in which the first refrigerant flows in the defrost mode. Therefore, in the third heat exchanger 15, frost that has formed in a position that is difficult to melt by the high-temperature first refrigerant supplied from the first compressor 10 to the third heat exchanger 15 in the defrost mode can be melted from the beginning of the defrost mode.

Embodiment 2.
(Overall configuration of cascade refrigeration system 200A)
As the second embodiment, a first example of a cascade refrigeration system will be described, in which a part of the configuration of the first refrigeration cycle 1 is different from that of the first embodiment. Fig. 6 is a diagram showing the configuration of a cascade refrigeration system 200A of the second embodiment.

The cascade refrigeration system 200A shown in FIG. 6 differs from the cascade refrigeration system 200 shown in FIG. 1 in that a bridge circuit 400 is provided and that the position of the first expansion valve 13 is different. The bridge circuit 400 is a bridge connection of four check valves. The bridge circuit 400 is connected to the first heat exchanger 12, the second heat exchanger 103, the first expansion valve 13, and the second expansion valve 14.

In the cooling mode, the first refrigerant flowing out of the first heat exchanger 12 passes through the bridge circuit 400 and flows into the second heat exchanger 103. The first refrigerant that has been heat exchanged in the second heat exchanger 103 flows into the first expansion valve 13. The first refrigerant flowing out of the first expansion valve 13 passes through the bridge circuit 400 and flows into the second expansion valve 14.

In defrost mode, the first refrigerant flowing out of the second expansion valve 14 flows into the second heat exchanger 103 via the bridge circuit 400. The first refrigerant that has been heat exchanged in the second heat exchanger 103 flows into the first expansion valve 13. The first refrigerant flowing out of the first expansion valve 13 flows into the first heat exchanger 12 via the bridge circuit 400.

The control device 600 controls the first expansion valve 13 and the second expansion valve 14 in the cooling mode and the defrost mode in the same way as the cascade refrigeration system 200 in FIG. 1. The cascade refrigeration system 200A configured in this way can operate in the cooling mode and the defrost mode in the same way as the cascade refrigeration system 200 in FIG. 1 described above.

Embodiment 3.
(Overall configuration of cascade refrigeration system 200B)
As the third embodiment, a second example of a cascade refrigeration system will be described, in which a part of the configuration of the first refrigeration cycle 1 is different from that of the first embodiment. Fig. 7 is a diagram showing the configuration of a cascade refrigeration system 200B of the third embodiment.

The cascade refrigeration system 200B shown in FIG. 7 differs from the cascade refrigeration system 200A shown in FIG. 6 in that a receiver 500 is provided between the second heat exchanger 103 and the first expansion valve 13. The receiver 500 is a tank having an internal space for storing the first refrigerant that flows in from the second heat exchanger 103. The receiver 500 is capable of storing the surplus first refrigerant in the first refrigeration cycle 1. The first refrigerant stored in the receiver 500 flows into the first expansion valve 13.

In cooling mode, the first refrigerant flowing out of the first heat exchanger 12 passes through the bridge circuit 400 and flows into the second heat exchanger 103. The first refrigerant that has been heat exchanged in the second heat exchanger 103 flows into the receiver 500 and is stored. The first refrigerant stored in the receiver 500 flows into the first expansion valve 13. The first refrigerant flowing out of the first expansion valve 13 passes through the bridge circuit 400 and flows into the second expansion valve 14.

In defrost mode, the first refrigerant flowing out of the second expansion valve 14 passes through the bridge circuit 400 and flows into the second heat exchanger 103. The first refrigerant that has been heat exchanged in the second heat exchanger 103 flows into the receiver 500 and is stored. The first refrigerant stored in the receiver 500 flows into the first expansion valve 13. The first refrigerant flowing out of the first expansion valve 13 passes through the bridge circuit 400 and flows into the second expansion valve 14.

The control device 600 controls the first expansion valve 13 and the second expansion valve 14 in the cooling mode and the defrost mode in the same way as the cascade refrigeration system 200 in FIG. 1. In the cascade refrigeration system 200B configured in this way, it is possible to operate in the cooling mode and the defrost mode in the same way as the cascade refrigeration system 200 in FIG. 1 described above.

Embodiment 4.
(Overall configuration of cascade refrigeration system 200C)
As the fourth embodiment, a third example of a cascade refrigeration system will be described, in which a part of the configuration of the first refrigeration cycle 1 is different from that of the first embodiment. Fig. 8 is a diagram showing the configuration of a cascade refrigeration system 200C of the fourth embodiment.

The cascade refrigeration system 200C shown in FIG. 8 differs from the cascade refrigeration system 200B shown in FIG. 7 in that a fifth heat exchanger 502 and a bypass valve 501 are provided between the outlet side of the receiver 500 and the first expansion valve 13, and between the suction side of the first compressor 10 and the first expansion valve 13. The fifth heat exchanger 502 is a cascade heat exchanger. The fifth heat exchanger 502 may be a double heat exchanger or a plate type heat exchanger.

The fifth heat exchanger 502 exchanges heat between the first refrigerant flowing through the piping between the receiver 500 and the first expansion valve 13 and the first refrigerant flowing through the piping branched off from the suction side of the first compressor 10. In the fifth heat exchanger 502, the first refrigerant flowing through the piping between the receiver 500 and the first expansion valve 13 is the high temperature side, and the first refrigerant flowing through the piping branched off from the suction side of the first compressor 10 is the low temperature side. As a result, the first refrigerant flowing out of the receiver 500 is cooled in the fifth heat exchanger 502 and flows into the first expansion valve 13.

The provision of the fifth heat exchanger 502 provides the following effects. When expanding a refrigerant with an expansion valve such as the first expansion valve 13, if the refrigerant is a two-phase flow, the operation of the valve may become unstable and abnormal noise may occur during expansion. However, the first refrigerant flowing into the first expansion valve 13 is cooled by the fifth heat exchanger 502 to improve the degree of liquefaction, so that in the defrost mode, when the first refrigerant is expanded by the first expansion valve 13, the operation of the valve can be stabilized and the generation of abnormal noise during expansion can be suppressed. Furthermore, the first refrigerant flowing into the first expansion valve 13 is cooled by the fifth heat exchanger 502 to improve the degree of liquefaction, so that in the defrost mode, the degree of superheat when evaporating the first refrigerant can be reduced in the first heat exchanger 12, and heat exchange can be performed efficiently.

In addition, if the receiver 500 is not provided, the fifth heat exchanger 502 and the bypass valve 501 may be provided between the second heat exchanger 103 and the first expansion valve 13.

Embodiment 5.
(Configuration regarding placement of sixth temperature sensor 37)
As the fifth embodiment, an example will be described in which, in the defrost mode, the heater 20 is stopped in response to the temperature detected by the sixth temperature sensor 37 provided inside the third heat exchanger 15. Fig. 9 is a diagram showing the arrangement of the sixth temperature sensor 37 in a cascade refrigeration system 200D of the fifth embodiment.

The configuration of the third heat exchanger 15 shown in FIG. 9 differs from that shown in FIG. 2 in that a sixth temperature sensor 37 is provided inside the third heat exchanger 15. The sixth temperature sensor 37 is a sensor that detects the temperature inside the third heat exchanger 15.

The sixth temperature sensor 37 is provided inside the third heat exchanger 15 at a bend (connection between rows) between the second and third rows of the fourth row of tubes 61 from the top. In this way, the sixth temperature sensor 37 is provided on the forward flow side of the third heat exchanger 15 in the direction in which the first refrigerant flows in defrost mode. The sixth temperature sensor 37 may be provided in any of the first to fifth rows of tubes 61 from the top inside the third heat exchanger 15. The sixth temperature sensor 37 may also be provided in multiple rows of tubes 61 inside the third heat exchanger 15.

The sixth temperature sensor 37 may also be provided on the downstream side of the third heat exchanger 15 in the direction in which the first refrigerant flows in the defrost mode. The sixth temperature sensor 37 may be provided at any position in the third heat exchanger 15 at least in the defrost mode, so long as it is capable of detecting the temperature of the first refrigerant and understanding the status of defrosting using the thermal energy of the first refrigerant.

The sixth temperature sensor 37 may also be provided in a portion of the third heat exchanger 15 other than the bent portion connecting the rows of tubes 61.

In this way, by providing a sixth temperature sensor 37 that detects the temperature of the first refrigerant inside the third heat exchanger 15, it is possible to grasp the status of defrosting using the thermal energy of the first refrigerant.

(Defrost mode control)
Next, the control executed by the control device 600 when the cascade refrigeration system 200D operates in the defrost mode will be described.

FIG. 10 is a flow chart of the control executed by the control device 600 when the cascade refrigeration system 200D of embodiment 5 operates in the defrost mode. In FIG. 10, the main control items during operation in the defrost mode are written within the boxes of each step.

Steps S1 to S10 are the same as those in the flowchart shown in FIG. 4. In step S20, the control device 600 determines whether the detection value of the sixth temperature sensor 37 is equal to or greater than the fourth threshold value.

The fourth threshold is preset to a temperature at which it is assumed that, in defrosting operation, the frost adhering to the third heat exchanger 15, and the portion of the tube outside the range of the tube 61 that can be melted by the heater 20, is being melted by the thermal energy of the first refrigerant. Therefore, in step S20, the control device 600 is able to grasp the progress of defrosting by the thermal energy of the first refrigerant in the defrosting mode, and is able to determine whether or not defrosting using the heater 20 is necessary.

If it is determined in step S20 that the detection value of the sixth temperature sensor 37 is less than the fourth threshold, the process returns to step S7. On the other hand, if it is determined in step S20 that the detection value of the sixth temperature sensor 37 is equal to or greater than the fourth threshold, the control device 600 stops heating by the heater 20 in step S21.

The control device 600 performs the same process as step S11 in FIG. 4 in step S22. If it is determined in step S22 that the temperature detected by the fourth temperature sensor 36 is equal to or higher than the third threshold, the control device 600 switches the operating state to cooling operation in step S23, similar to step S14 in FIG. 4.

(Change in temperature of the fourth temperature sensor 36 in the defrosting mode)
Next, an example of change in power consumption in the defrost mode of the cascade refrigeration system 200D will be described. Fig. 11 is a diagram showing an example of change in power consumption in the defrost mode of the cascade refrigeration system 200D of embodiment 5. In Fig. 11, the vertical axis indicates the power consumption W in the defrost mode of the cascade refrigeration system 200D, and the horizontal axis indicates the elapsed time t from the start of the defrost operation.

In FIG. 11, the solid line a indicates the change in power consumption of the cascade refrigeration system 200D of embodiment 5, and the dashed line b indicates the change in power consumption of the

cascade refrigeration systems

200, 200A, 200B, and 200C of embodiments 1 to 4.

In the first to fourth embodiments, in the defrost mode, defrosting by thermal energy of the first refrigerant and defrosting by thermal energy generated by heating the heater 20 are terminated simultaneously, so the power consumption W increases uniformly as shown by the dashed line b. In contrast, in the fifth embodiment, in the defrost mode, defrosting by thermal energy generated by heating the heater 20 is terminated earlier than defrosting by thermal energy of the first refrigerant, so the rate of increase in the power consumption W decreases midway as shown by the solid line a. As a result, in the fifth embodiment, the power consumption in the defrost mode can be suppressed by stopping the heater 20 early in the defrost mode.

As described above, in the fifth embodiment, when it is determined that the detection value of the sixth temperature sensor 37 is equal to or greater than the fourth threshold value, the control device 600 stops heating by the heater 20. Therefore, compared to the first to fourth embodiments, the heater 20 can be stopped earlier in the defrost mode. In this way, by stopping the heater 20 earlier in the defrost mode, the amount of power consumption in the defrost mode can be reduced.

In addition, in the fifth embodiment, the sixth temperature sensor 37 is provided inside the third heat exchanger 15 at the bend (connection between the rows) between the second and third rows of the tubes 61, so that the sixth temperature sensor 37 is prevented from erroneously detecting the actual temperature of the first refrigerant in the tubes 61 due to receiving thermal energy from the heater 20.

<Other Modifications>
In the above-described embodiment, the first refrigerant having a boiling point lower than −52° C. to be filled into the first refrigeration cycle 1 may be carbon dioxide or R23. Assuming that such a first refrigerant leaks into the room, the characteristics required for the first refrigerant are preferably a refrigerant categorized as A1 in ASHRAE34, that is, a refrigerant that is low toxic and non-flammable. Specifically, the first refrigerant is most preferably carbon dioxide, and a mixed refrigerant mainly composed of carbon dioxide may be used.

When using the common fluorocarbon refrigerants R404 or R410A as the first refrigerant, a first or second class refrigerant piping is used depending on the maximum operating pressure, but in the case of carbon dioxide, a high-pressure refrigerant with a boiling point lower than -52°C as in this embodiment, a fourth class refrigerant piping is generally used.

For example, the minimum pressure-resistant elements of air conditioners and freezers, such as multi-air conditioners for buildings that use R410A, have a pressure resistance of 4.25 MPaA or less. The cascade refrigeration system of this embodiment may be configured to use a high-pressure refrigerant in the first refrigeration cycle 1, while the minimum pressure-resistant elements in each element of the first refrigeration cycle 1 have a pressure resistance of 4.25 MPaA or less.

(summary)
Hereinafter, the embodiments will be summarized with reference to the drawings again.

(Section 1) The present disclosure relates to a cascade refrigeration system. As shown in FIG. 1 etc., the cascade refrigeration system 200 includes a first refrigeration cycle 1 including a first compressor 10, a first heat exchanger 12, a first expansion valve 13, a second heat exchanger 103, a second expansion valve 14, a third heat exchanger 15, and a heater 20, in which a first refrigerant circulates; a second refrigeration cycle 2 including a second compressor 100, a fourth heat exchanger 101, a third expansion valve 102, and a second heat exchanger 103, in which a second refrigerant circulates; and a control device 600 that controls the first refrigeration cycle 1 and the second refrigeration cycle 2. The second heat exchanger 103 exchanges heat between the first refrigerant and the second refrigerant. The control device 600 controls the first refrigeration cycle and the second refrigeration cycle in a cooling mode or a defrosting mode. The cooling mode is a mode in which the first refrigerant discharged from the first compressor 10 returns to the first compressor 10 via the first heat exchanger 12, the first expansion valve 13, the second heat exchanger 103, the second expansion valve 14, and the third heat exchanger 15. The defrosting mode is a mode in which the first refrigerant discharged from the first compressor 10 returns to the first compressor 10 via the third heat exchanger 15, the second expansion valve 14, the second heat exchanger 103, the first expansion valve 13, and the first heat exchanger 12. The heater is provided in the third heat exchanger. In the defrosting mode, the control device 600 controls the flow path of the first refrigerant in the third heat exchanger to be heated by the heater while the first refrigerant is flowing into the third heat exchanger 15.

(2) In the cascade refrigeration system described in 1, as shown in FIG. 2 etc., the heater 20 is provided in the flow path of the first refrigerant provided in the third heat exchanger 15 at a position that heats the downstream side of the direction in which the first refrigerant flows in the defrost mode.

(3) In the cascade refrigeration system described in 1 or 2, as shown in FIG. 4, a first pressure sensor 31 is further provided to detect the pressure of the first refrigerant discharged from the first compressor 10, and in the defrost mode, the control device 600 determines whether the pressure of the first refrigerant detected by the first pressure sensor 31 is equal to or lower than a first threshold, and then starts heating by the heater 20 (step S7).

(4) In the cascade refrigeration system described in 3, as shown in FIG. 4, when the control device 600 determines that the pressure of the first refrigerant detected by the first pressure sensor 31 is equal to or lower than the first threshold in the defrost mode, it immediately starts heating by the heater 20 (step S7).

(Section 5) In the cascade refrigeration system described in Section 3, as shown in FIG. 4, when the control device determines that the pressure of the first refrigerant detected by the first pressure sensor 31 is higher than the first threshold in the defrost mode, it starts control to circulate the second refrigerant in the second refrigeration cycle 2 (steps S5, S6), and then starts heating by the heater (step S7).

(Item 6) In the cascade refrigeration system described in any one of Items 1 to 5, as shown in FIG. 1, a fourth temperature sensor 36 is further provided for detecting the temperature of the first refrigerant flowing out from the third heat exchanger 15 to the second expansion valve 14 in the defrost mode. As shown in FIG. 4, in the defrost mode, the control device 600 ends the defrost mode and ends heating by the heater in response to the temperature of the first refrigerant detected by the fourth temperature sensor 36 becoming equal to or higher than the third threshold (steps S13, S14).

(7) In the cascade refrigeration system described in any one of paragraphs 1 to 5, as shown in FIG. 8, a fourth temperature sensor 36 is provided for detecting the temperature of the first refrigerant flowing from the third heat exchanger 15 to the second expansion valve 14 in the defrost mode, and as shown in FIG. 9, a sixth temperature sensor 37 is provided for detecting the temperature of the first refrigerant in the flow path of the first refrigerant inside the third heat exchanger 15 in a range that is not directly heated by the heater 20 in the defrost mode. As shown in FIG. 10, the control device 600 ends heating by the heater 20 in the defrost mode when the temperature of the first refrigerant detected by the sixth temperature sensor 37 reaches the fourth threshold (steps S20, S21), and ends the defrost mode when the temperature of the first refrigerant detected by the fourth temperature sensor 36 reaches the third threshold (steps S22, S23).

(Item 8) In the cascade refrigeration system described in Item 7, as shown in FIG. 9, the sixth temperature sensor 37 is provided inside the third heat exchanger 15, in the flow path of the first refrigerant, upstream of the range heated by the heater 20 in the direction in which the first refrigerant flows in defrost mode.

(Item 9) In the dual cascade refrigeration system described in any one of items 1 to 8, as shown in Figs. 2 and 9, the third heat exchanger 15 has tubes 61, which are the flow path of the first refrigerant, arranged in multiple stages in the vertical direction, and the heater 20 includes multiple heater members 21 arranged in the vertical direction corresponding to the multiple stages of tubes 61. Of the multiple heater members 21, the heater member 21 in the bottom stage has a larger heat capacity than the heater member 21 in the top stage.

(Item 10) In the dual cascade refrigeration system described in any one of items 1 to 9, as shown in FIG. 2 and FIG. 9, the third heat exchanger 15 further includes a fan 15F for blowing air to the third heat exchanger 15, the third heat exchanger 15 includes tubes 61, which are the flow path of the first refrigerant, arranged in multiple stages in the vertical direction, and the heater 20 includes multiple heater members 21 arranged in the vertical direction corresponding to the multiple stages of tubes 61. Of the multiple heater members 21, the heater member 21 arranged at the position where the wind speed of the air blown from the fan 15F is the fastest has a larger heat capacity than the heater member 21 arranged at the position where the wind speed of the air blown from the fan 15F is the slowest.

(11) In the binary refrigeration system described in any one of paragraphs 1 to 10, the boiling point of the first refrigerant is lower than -52°C.

(12) In the binary refrigeration system described in any one of paragraphs 1 to 11, the first refrigerant is carbon dioxide.

(13) In the binary refrigeration system described in any one of paragraphs 1 to 11, the first refrigerant is a mixed gas containing carbon dioxide as the main component.

(14) In the cascade refrigeration system described in any one of paragraphs 1 to 13, the first refrigerant and the equipment constituting the first refrigeration cycle have a minimum pressure resistance of 4.25 MPaA or less.

The embodiments disclosed herein should be considered in all respects as illustrative and not restrictive. The scope of the present disclosure is indicated by the claims, not by the description of the embodiments above, and is intended to include all modifications within the meaning and scope of the claims.

10 first compressor, 12 first heat exchanger, 13 first expansion valve, 103 second heat exchanger, 14 second expansion valve, 15 third heat exchanger, 20 heater, 1 first refrigeration cycle, 100 second compressor, 101 fourth heat exchanger, 102 third expansion valve, 2 second refrigeration cycle, 600 control device, 200, 200A, 200B, 200C, 200D cascade refrigeration system.

Claims

a first refrigeration cycle including a first compressor, a first heat exchanger, a first expansion valve, a second heat exchanger, a second expansion valve, a third heat exchanger, and a heater, in which a first refrigerant circulates;
a second refrigeration cycle including a second compressor, a fourth heat exchanger, a third expansion valve, and the second heat exchanger, in which a second refrigerant circulates;
a control device for controlling the first refrigeration cycle and the second refrigeration cycle,
The second heat exchanger exchanges heat between the first refrigerant and the second refrigerant,
The control device controls the first refrigeration cycle and the second refrigeration cycle in a cooling mode or a defrosting mode,
the cooling mode is a mode in which the first refrigerant discharged from the first compressor returns to the first compressor via the first heat exchanger, the first expansion valve, the second heat exchanger, the second expansion valve, and the third heat exchanger,
the defrosting mode is a mode in which the first refrigerant discharged from the first compressor returns to the first compressor via the third heat exchanger, the second expansion valve, the second heat exchanger, the first expansion valve, and the first heat exchanger,
The heater is provided in the third heat exchanger,
The control device, in the defrost mode, controls the heater to heat a flow path of the first refrigerant in the third heat exchanger while the first refrigerant is flowing into the third heat exchanger.
The cascade refrigeration system according to claim 1, wherein the heater is provided in a position in the flow path of the first refrigerant provided in the third heat exchanger so as to heat the downstream side of the flow direction of the first refrigerant in the defrosting mode.
a pressure sensor for detecting a pressure of the first refrigerant discharged from the first compressor,
3. The cascade refrigeration apparatus according to claim 1, wherein in the defrost mode, the control device starts heating by the heater after determining whether a pressure of the first refrigerant detected by the pressure sensor is equal to or lower than a first threshold value.
The cascade refrigeration system of claim 3, wherein the control device immediately starts heating by the heater when it is determined that the pressure of the first refrigerant detected by the pressure sensor is equal to or lower than the first threshold value in the defrost mode.
The cascade refrigeration system of claim 3, wherein the control device starts control to circulate the second refrigerant in the second refrigeration cycle and then starts heating by the heater when the pressure of the first refrigerant detected by the pressure sensor is determined to be higher than the first threshold value in the defrost mode.
a first temperature sensor that detects a temperature of the first refrigerant flowing out from the third heat exchanger to the second expansion valve in the defrosting mode;
The control device terminates the defrost mode and terminates heating by the heater in response to the temperature of the first refrigerant detected by the first temperature sensor becoming equal to or higher than a second threshold value in the defrost mode. The binary refrigeration device according to any one of claims 1 to 5.
a first temperature sensor that detects a temperature of the first refrigerant flowing out from the third heat exchanger to a side of the second expansion valve in the defrosting mode;
a second temperature sensor configured to detect a temperature of the first refrigerant in a flow path of the first refrigerant in the third heat exchanger, the flow path being not directly heated by the heater in the defrost mode;
The control device, in the defrost mode, terminates heating by the heater in response to the temperature of the first refrigerant detected by the second temperature sensor reaching a third threshold value, and terminates the defrost mode in response to the temperature of the first refrigerant detected by the first temperature sensor reaching a fourth threshold value. The dual refrigeration device according to any one of claims 1 to 5.
The cascade refrigeration system according to claim 7, wherein the second temperature sensor is provided inside the third heat exchanger in a position upstream of the flow path of the first refrigerant in the direction in which the first refrigerant flows in the defrost mode from the range heated by the heater.
The third heat exchanger is provided such that a flow path of the first refrigerant is divided into a plurality of stages in a vertical direction,
the heater includes a plurality of heater members provided in a vertical direction corresponding to the plurality of stages of flow paths,
The cascade refrigeration system according to any one of claims 1 to 8, wherein the heater member at the bottom stage of the plurality of heater members has a larger heat capacity than the heater member at the top stage.
Further comprising a fan for blowing air to the third heat exchanger,
The third heat exchanger is provided such that a flow path of the first refrigerant is divided into a plurality of stages in a vertical direction,
the heater includes a plurality of heater members provided in a vertical direction corresponding to the plurality of stages of flow paths,
The dual refrigeration apparatus according to any one of claims 1 to 9, wherein the plurality of heater members are arranged such that the heater member arranged at a position where the wind speed of the air from the fan is the fastest has a larger heat capacity than the heater member arranged at a position where the wind speed of the air from the fan is the slowest.
The cascade refrigeration system according to any one of claims 1 to 10, wherein the first refrigerant has a boiling point lower than -52°C.
The binary refrigeration system according to any one of claims 1 to 11, wherein the first refrigerant is carbon dioxide.
The binary refrigeration system according to any one of claims 1 to 11, wherein the first refrigerant is a mixed gas containing carbon dioxide as a main component.
The dual refrigeration system according to any one of claims 1 to 13, wherein the equipment constituting the first refrigeration cycle has a minimum pressure resistance of 4.25 MPaA or less.