Cascade refrigeration system

Abstract

The utility model relates to the technical field of cascade refrigeration, and provides a cascade refrigeration system which comprises a first circulation system, a second circulation system and a third circulation system, the first circulation system comprises a first evaporator, an oil-free centrifugal compressor, a cascade heat exchanger and a first liquid reservoir, and the cascade heat exchanger comprises a first condenser; an outlet of the first evaporator, the oil-free centrifugal compressor, the first condenser, the first liquid reservoir and an inlet of the first evaporator are communicated in sequence; a refrigerant of the first circulation system is carbon dioxide. The oil-free centrifugal compressor is adopted, oil-free operation is carried out in the first circulating system, the system is simplified, the influence of lubricating oil on the heat exchange efficiency is eliminated, the reliability and safety of the cascade refrigeration system are improved, and the refrigerant charge amount is reduced.

Description

Technical Field

[0001] The utility model relates to the technical field of cascade refrigeration, in particular to a cascade refrigeration system. Background Art

[0002] In the field of traditional industrial refrigeration, refrigeration systems widely rely on piston compressors and screw compressors as their core power sources. During operation, these compressors must rely on the circulation of refrigeration lubricating oil to ensure the lubrication and sealing of mechanical parts. However, this design mechanism also introduces a complex oil management system, including auxiliary equipment such as oil separators, oil coolers and oil return systems.

[0003] In a cascade refrigeration system, when carbon dioxide (CO2) is used as the low-temperature side refrigerant, the working pressure of the CO2 refrigeration system is much higher than that of the traditional system, and the behavior characteristics of the lubricating oil under high pressure conditions change, making the distribution and recovery of the lubricating oil in the system more complicated and difficult. If the oil return is not smooth, it will not only affect the lubrication condition inside the compressor and increase the risk of wear, but also may affect the refrigeration efficiency due to the accumulation of oil on the evaporator or other low-temperature components, and even cause system failure, endangering the reliability, safety and operating efficiency of the entire refrigeration system. Utility Model Content

[0004] The utility model provides a cascade refrigeration system, which is used to solve the problem in the prior art that the oil return in the compressor is not smooth, thus affecting the reliability, safety and operation efficiency of the refrigeration system.

[0005] The utility model provides a cascade refrigeration system, comprising: a first circulation system, wherein the first circulation system comprises a first evaporator, an oil-free centrifugal compressor, a cascade heat exchanger and a first liquid storage tank, wherein the cascade heat exchanger comprises a first condenser; an outlet of the first evaporator, the oil-free centrifugal compressor, the first condenser, the first liquid storage tank and an inlet of the first evaporator are connected in sequence; and the refrigerant of the first circulation system is carbon dioxide.

[0006] According to a cascade refrigeration system provided by the utility model, the first circulation system also includes a first gas-liquid separator, and the first gas-liquid separator is provided with a first air inlet, a first liquid inlet, a first air outlet and a first liquid outlet; the first air inlet is connected to the outlet of the first evaporator, the first air outlet is connected to the inlet of the oil-free centrifugal compressor, the first liquid outlet is connected to the inlet of the first evaporator, and the first liquid inlet is connected to the outlet of the first liquid storage tank.

[0007] According to a cascade refrigeration system provided by the utility model, the first circulation system also includes a shielded pump, which is arranged between the first liquid outlet and the inlet of the first evaporator and is used to drive the liquid in the first gas-liquid separator to flow to the first evaporator.

[0008] According to a cascade refrigeration system provided by the utility model, the first circulation system further includes a first expansion valve, and the first expansion valve is arranged between the outlet of the first liquid storage tank and the first liquid inlet.

[0009] According to a cascade refrigeration system provided by the utility model, the oil-free centrifugal compressor is a permanent magnet variable frequency compressor.

[0010] According to a cascade refrigeration system provided by the utility model, it also includes a second circulation system, and the cascade heat exchanger also includes a second evaporator, and the second evaporator is connected end to end with the second circulation system.

[0011] According to a cascade refrigeration system provided by the utility model, the second circulation system includes an oil compressor, a second condenser, a second liquid reservoir and an oil separator, the outlet of the second evaporator, the air inlet of the oil compressor, the air outlet of the oil compressor, the inlet of the oil separator, the air outlet of the oil separator, the second condenser, the second liquid reservoir and the inlet of the second evaporator are connected in sequence; the oil outlet of the oil separator is connected to the oil inlet of the oil compressor.

[0012] According to a cascade refrigeration system provided by the utility model, the second circulation system also includes a second gas-liquid separator, and the second gas-liquid separator is provided with a second air inlet, a second liquid inlet, a second air outlet and a second liquid outlet; the second air inlet is connected to the outlet of the second evaporator, the second air outlet is connected to the air inlet of the oil compressor, the second liquid outlet is connected to the inlet of the second evaporator, and the second liquid inlet is connected to the outlet of the second liquid storage tank.

[0013] According to a cascade refrigeration system provided by the utility model, the second liquid outlet of the second gas-liquid separator is higher than the inlet of the second evaporator.

[0014] According to a cascade refrigeration system provided by the utility model, the second circulation system also includes a cooler, which is arranged between the oil outlet of the oil separator and the oil inlet of the oil compressor; and / or, the second circulation system also includes a second expansion valve, which is arranged between the outlet of the second liquid reservoir and the second liquid inlet.

[0015] The cascade refrigeration system provided by the utility model adopts an oil-free centrifugal compressor, and the first circulation system operates without oil, which simplifies the system, eliminates the influence of lubricating oil on heat exchange efficiency, improves the reliability and safety of the cascade refrigeration system, and reduces the refrigerant charge. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 It is a schematic diagram of the cascade refrigeration system provided by the utility model;

[0018] Reference numerals:

[0019] 10. The first circulatory system;

[0020] 11. first evaporator; 12. first gas-liquid separator; 121. first air inlet; 122. first liquid inlet; 123. first air outlet; 124. first liquid outlet; 13. oil-free centrifugal compressor; 14. cascade heat exchanger; 15. first liquid storage tank; 16. shielded pump; 17. first expansion valve;

[0021] 20. Second circulatory system;

[0022] 21. Oil compressor; 22. Second condenser; 23. Second liquid storage tank; 24. Oil separator; 25. Cooler; 26. Second expansion valve; 27. Second gas-liquid separator; 271. Second air inlet; 272. Second liquid inlet; 273. Second air outlet; 274. Second liquid outlet. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the utility model clearer, the technical solution of the utility model will be described clearly and completely in conjunction with the drawings in the utility model. Obviously, the described embodiments are part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0024] In the description of the embodiments of the present utility model, it should be noted that, unless otherwise clearly specified and limited, the terms "connected" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments of the present utility model can be understood according to specific circumstances.

[0025] In the description of the embodiments of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present invention.

[0026] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present utility model, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0027] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0028] The disclosure below provides many different embodiments or examples for realizing different structures of the utility model. In order to simplify the disclosure of the utility model, the components and settings of specific examples are described below. Of course, they are merely examples, and the purpose is not to limit the utility model. In addition, the utility model can repeat reference numbers and / or letters in different examples. This repetition is for the purpose of simplification and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the utility model provides various specific examples of processes and materials, but those of ordinary skill in the art can be aware of the applicability of other processes and / or the use of other materials.

[0029] Combine the following Figure 1 The cascade refrigeration system of the utility model is described.

[0030] The cascade refrigeration system provided by the embodiment of the utility model comprises: a first circulation system 10, the first circulation system 10 comprises a first evaporator 11, an oil-free centrifugal compressor 13, a cascade heat exchanger 14 and a first liquid storage tank 15, the cascade heat exchanger 14 comprises a first condenser; the outlet of the first evaporator 11, the oil-free centrifugal compressor 13, the first condenser, the first liquid storage tank 15 and the inlet of the first evaporator 11 are connected in sequence; the refrigerant of the first circulation system 10 is carbon dioxide. It should be noted that the gas discharged from the exhaust port of the oil-free centrifugal compressor 13 does not contain oil at all.

[0031] The first circulation system 10 is a low-temperature side refrigeration system, and the refrigerant is carbon dioxide. The low-temperature liquid carbon dioxide in the first liquid storage tank 15 is transported to the first evaporator 11 (such as a cold air blower), and the low-temperature liquid carbon dioxide absorbs external heat in the first evaporator 11 and becomes carbon dioxide gas; the carbon dioxide gas enters the oil-free centrifugal compressor 13, and the oil-free centrifugal compressor 13 compresses the carbon dioxide gas into superheated gaseous carbon dioxide under the condensation pressure; the superheated gaseous carbon dioxide under the condensation pressure enters the cascade heat exchanger 14, and is condensed into carbon dioxide liquid under the action of the first condenser in the cascade heat exchanger 14, and the carbon dioxide liquid enters the first liquid storage tank 15 for storage, and becomes low-temperature and low-pressure carbon dioxide liquid after throttling and pressure reduction, and enters the first evaporator 11 to start the next cycle.

[0032] The oil-free centrifugal compressor 13 in the embodiment of the utility model can be a permanent magnet variable frequency compressor to improve the efficiency of the compressor at partial load.

[0033] The cascade refrigeration system provided by the embodiment of the utility model adopts an oil-free centrifugal compressor 13, and the first circulation system 10 operates without oil, which simplifies the system, eliminates the influence of lubricating oil on heat exchange efficiency, improves the reliability and safety of the cascade refrigeration system, and reduces the refrigerant charge.

[0034] The prior art uses piston compressors or screw compressors, both of which require the installation of a compressor lubricating oil system (including a lubricating oil separation system, a cooling system, a conveying system, a distribution system, and an oil circuit control system). The embodiment of the utility model uses an oil-free centrifugal compressor 13. Since the first circulation system 10 side operates without oil, there is no oil return problem, which simplifies the system, and the location of the refrigeration room is not restricted. The refrigeration room can be set on the top roof of the cold storage, does not occupy the ground area, and improves the land utilization rate. In addition, the embodiment of the utility model uses an oil-free centrifugal compressor 13, which has low vibration, low noise, and is easy to install.

[0035] The first circulation system 10 also includes a first gas-liquid separator 12, which is provided with a first air inlet 121, a first liquid inlet 122, a first air outlet 123 and a first liquid outlet 124; the first air inlet 121 is connected to the outlet of the first evaporator 11, the first air outlet 123 is connected to the inlet of the oil-free centrifugal compressor 13, the first liquid outlet 124 is connected to the inlet of the first evaporator 11, and the first liquid inlet 122 is connected to the outlet of the first liquid reservoir 15.

[0036] like Figure 1 As shown, the low-temperature liquid carbon dioxide in the first gas-liquid separator 12 is transported to the first evaporator 11, and the low-temperature liquid carbon dioxide absorbs external heat in the first evaporator 11, and exists in gas-liquid two-phases; the gas-liquid two-phase carbon dioxide enters the first gas-liquid separator 12 through the outlet of the first evaporator 11, and carbon dioxide gas and carbon dioxide liquid are separated; wherein the carbon dioxide liquid enters the first evaporator 11 through the first liquid outlet 124 of the first gas-liquid separator 12 for circulation; the carbon dioxide gas enters the oil-free centrifugal compressor 13 through the first gas outlet 123 of the first gas-liquid separator 12, and the oil-free centrifugal compressor 13 compresses the carbon dioxide gas into superheated gaseous carbon dioxide under the condensation pressure; the superheated gaseous carbon dioxide under the condensation pressure enters the cascade heat exchanger 14, and is condensed into carbon dioxide liquid under the action of the first condenser in the cascade heat exchanger 14, and the carbon dioxide liquid enters the first liquid storage tank 15 for storage, and becomes low-temperature and low-pressure carbon dioxide liquid after throttling and pressure reduction, and enters the first gas-liquid separator 12 to start the next cycle. The first gas-liquid separator 12 in the embodiment of the utility model can be a low-pressure circulation barrel.

[0037] The first circulation system 10 further includes a shielded pump 16 , which is disposed on a pipeline between the first liquid outlet 124 of the first gas-liquid separator 12 and the inlet of the first evaporator 11 , and is used to drive the liquid in the first gas-liquid separator 12 to flow toward the first evaporator 11 .

[0038] The motor and pump of the shielded pump 16 are sealed in a pressure vessel filled with the conveying medium. This structure eliminates the rotary shaft sealing device of the traditional centrifugal pump, so it can be completely leak-free. The shielded pump 16 can convey liquids with pressure and temperature that do not produce crystallization and solidification, and block the delivery of gases.

[0039] Specifically, the inlet of the shielded pump 16 is connected to the first liquid outlet 124 of the first gas-liquid separator 12 through a pipeline, and the outlet of the shielded pump 16 is connected to the inlet of the first evaporator 11 through a pipeline, and the shielded pump 16 can drive the carbon dioxide liquid in the first gas-liquid separator 12 to flow into the first evaporator 11 for heat exchange. At the same time, the shielded pump 16 can prevent the carbon dioxide gas in the first gas-liquid separator 12 from entering the first evaporator 11.

[0040] Furthermore, the first circulation system 10 further includes a first expansion valve 17, which is disposed on a pipeline between the first liquid storage tank 15 and the first gas-liquid separator 12. Figure 1 As shown, the inlet of the first expansion valve 17 is connected to the outlet of the first liquid reservoir 15 through a pipeline, and the outlet of the first expansion valve 17 is connected to the first liquid inlet 122 of the first gas-liquid separator 12. The first expansion valve 17 is used to adjust the flow rate and pressure in the pipeline between the first liquid reservoir 15 and the first gas-liquid separator 12. The carbon dioxide liquid in the first liquid reservoir 15 can be converted into low-temperature and low-pressure carbon dioxide liquid after passing through the first expansion valve 17. The first expansion valve 17 can be a low-temperature expansion valve.

[0041] In the embodiment of the utility model, a first filter element is disposed at the outlet of the first liquid storage container 15 to prevent pollutants from entering the first gas-liquid separator 12 along with the carbon dioxide liquid.

[0042] The first gas outlet 123 and the first liquid outlet 124 of the first gas-liquid separator 12 are both provided with a second filter element to prevent pollutants from entering the oil-free centrifugal compressor 13 with the carbon dioxide gas or entering the first evaporator 11 with the carbon dioxide liquid.

[0043] The cascade refrigeration system provided by the embodiment of the utility model further includes a second circulation system 20, and the cascade heat exchanger 14 further includes a second evaporator, and the second evaporator is connected end to end with the second circulation system 20, that is, the inlet of the second evaporator is connected to the outlet of the second circulation system 20 through a pipeline, and the outlet of the second evaporator is connected to the inlet of the second circulation system 20 through a pipeline. The first circulation system 10 and the second circulation system 20 share a cascade heat exchanger 14.

[0044] The second circulation system includes an oil compressor 21, a second condenser 22, a second liquid reservoir 23 and an oil separator 24. The air inlet of the oil compressor 21, the air outlet of the oil compressor 21, the inlet of the oil separator 24, the air outlet of the oil separator 24, the second condenser 22, the second liquid reservoir 23 and the inlet of the second evaporator are connected in sequence; the oil outlet of the oil separator 24 is connected to the oil inlet of the oil compressor 21.

[0045] It should be noted that the exhaust end of the oil compressor 21 contains a certain amount of lubricating oil particles, which play the role of lubrication, cooling, sealing and cleaning in the system.

[0046] The refrigerant in the second circulation system 20 can be ammonia or halogenated hydrocarbons and mixtures thereof, and the second circulation system is a refrigeration cycle on the high temperature side. The low-temperature and low-pressure liquid refrigerant in the second liquid storage tank 23 enters the second evaporator for heat exchange, and becomes gaseous after heat exchange. The gaseous refrigerant enters the oil compressor 21 for compression and condensation. Due to the effect of the lubricating oil, the gaseous refrigerant after compression and condensation contains oil. The oily refrigerant enters the oil separator 24 for separation, wherein the separated oil enters the oil compressor 21 through the oil outlet of the oil separator 24 and the oil inlet of the oil compressor 21 for recycling; the separated gaseous refrigerant enters the second condenser 22 through the gas outlet of the oil separator 24 and the inlet of the second condenser 22 for condensation, so that the gaseous refrigerant is converted into liquid refrigerant; the liquid refrigerant enters the second liquid storage tank 23, and the liquid refrigerant in the second liquid storage tank 23 becomes a low-temperature and low-pressure refrigerant liquid after throttling and pressure reduction, and enters the second evaporator to start the next cycle.

[0047] The second circulation system also includes a second gas-liquid separator 27, which is provided with a second air inlet 271, a second liquid inlet 272, a second air outlet 273 and a second liquid outlet 274. The second air inlet 271 is connected to the outlet of the second evaporator, the second air outlet 273 is connected to the air inlet of the oil compressor 21, the second liquid outlet 274 is connected to the inlet of the second evaporator, and the second liquid inlet 272 is connected to the outlet of the second liquid storage tank 23.

[0048] like Figure 1 As shown, the low-temperature and low-pressure liquid refrigerant in the second gas-liquid separator 27 enters the second evaporator for evaporation, and becomes gas-liquid two-phase after heat exchange; the gas-liquid two-phase refrigerant enters the second gas-liquid separator 27, and is separated into refrigerant gas and refrigerant liquid; wherein the refrigerant liquid enters the second evaporator through the second liquid outlet 274 of the second gas-liquid separator 27 for circulation; the refrigerant gas enters the oil compressor 21 through the second gas outlet 273 of the second gas-liquid separator 27 and the steam inlet of the oil compressor 21 for compression and condensation. The gaseous refrigerant compressed and condensed by the oil compressor 21 enters the second condenser 22 for condensation, so that the gaseous refrigerant is converted into liquid refrigerant, and the liquid refrigerant enters the second liquid storage 23. The liquid refrigerant in the second liquid storage 23 becomes a low-temperature and low-pressure refrigerant liquid after throttling and pressure reduction, and enters the second gas-liquid separator 27 through the second liquid inlet 272 to start the next cycle.

[0049] The second liquid outlet 274 of the second gas-liquid separator 27 in the embodiment of the utility model is higher than the inlet of the second evaporator, and the liquid refrigerant in the second gas-liquid separator 27 enters the second evaporator under the action of gravity. In an optional embodiment, a driving pump is provided between the second gas-liquid separator 27 and the second evaporator, and the driving pump is used to drive the liquid refrigerant in the second gas-liquid separator 27 to flow into the second evaporator.

[0050] The second circulation system in the embodiment of the utility model also includes a cooler 25, which is arranged between the oil separator 24 and the oil compressor 21, wherein the inlet of the cooler 25 is connected to the oil outlet of the oil separator 24 through a pipeline, and the outlet of the cooler 25 is connected to the second inlet of the oil compressor 21 through a pipeline. The cooler 25 can cool the oil separated by the oil separator 24.

[0051] The second circulation system 20 further includes a second expansion valve 26, which is disposed on a pipeline between the second liquid storage tank 23 and the second gas-liquid separator 27. The inlet of the second expansion valve 26 is communicated with the outlet of the second liquid storage tank 23, and the outlet of the second expansion valve 26 is communicated with the second liquid inlet 272 of the second gas-liquid separator 27. The second expansion valve 26 is used to adjust the flow rate and pressure of the refrigerant entering the second gas-liquid separator 27. The second expansion valve 26 may be a high-pressure expansion valve.

[0052] The oil compressor 21 in the embodiment of the utility model can be a piston compressor or a screw compressor.

[0053] The embodiments of the utility model can be applied to a carbon dioxide pressure-critical cascade refrigeration system.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the utility model, rather than to limit it. Although the utility model has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the utility model.

Claims

1. A cascade refrigeration system, characterized in that: include: a first circulation system, the first circulation system comprising a first evaporator, an oil-free centrifugal compressor, a cascade heat exchanger and a first liquid receiver, the cascade heat exchanger comprising a first condenser; The outlet of the first evaporator, the oil-free centrifugal compressor, the first condenser, the first liquid receiver and the inlet of the first evaporator are connected in sequence; the refrigerant of the first circulation system is carbon dioxide.

2. The cascade refrigeration system according to claim 1, characterized in that: The first circulation system further comprises a first gas-liquid separator, wherein the first gas-liquid separator is provided with a first gas inlet, a first liquid inlet, a first gas outlet and a first liquid outlet; The first air inlet is communicated with the outlet of the first evaporator, the first air outlet is communicated with the inlet of the oil-free centrifugal compressor, the first liquid outlet is communicated with the inlet of the first evaporator, and the first liquid inlet is communicated with the outlet of the first liquid reservoir.

3. The cascade refrigeration system according to claim 2, characterized in that: The first circulation system further includes a shielded pump, which is disposed between the first liquid outlet and the inlet of the first evaporator and is used to drive the liquid in the first gas-liquid separator to flow toward the first evaporator.

4. The cascade refrigeration system according to claim 2, characterized in that: The first circulation system further includes a first expansion valve, which is disposed between the outlet of the first liquid storage tank and the first liquid inlet.

5. The cascade refrigeration system according to claim 1, characterized in that: The oil-free centrifugal compressor is a permanent magnet variable frequency compressor.

6. The cascade refrigeration system according to any one of claims 1 to 5, characterized in that: It also includes a second circulation system, and the cascade heat exchanger also includes a second evaporator, and the second evaporator is connected end to end with the second circulation system.

7. The cascade refrigeration system according to claim 6, characterized in that: The second circulation system includes an oil compressor, a second condenser, a second liquid reservoir and an oil separator. The outlet of the second evaporator, the air inlet of the oil compressor, the air outlet of the oil compressor, the inlet of the oil separator, the air outlet of the oil separator, the second condenser, the second liquid reservoir and the inlet of the second evaporator are connected in sequence; the oil outlet of the oil separator is connected to the oil inlet of the oil compressor.

8. The cascade refrigeration system according to claim 7, characterized in that: The second circulation system further comprises a second gas-liquid separator, wherein the second gas-liquid separator is provided with a second gas inlet, a second liquid inlet, a second gas outlet and a second liquid outlet; The second air inlet is communicated with the outlet of the second evaporator, the second air outlet is communicated with the air inlet of the oil compressor, the second liquid outlet is communicated with the inlet of the second evaporator, and the second liquid inlet is communicated with the outlet of the second liquid storage device.

9. The cascade refrigeration system according to claim 8, characterized in that: The second liquid outlet of the second gas-liquid separator is higher than the inlet of the second evaporator.

10. The cascade refrigeration system according to claim 8, characterized in that: The second circulation system further comprises a cooler, which is arranged between the oil outlet of the oil separator and the oil inlet of the oil compressor; And / or, the second circulation system further includes a second expansion valve, and the second expansion valve is arranged between the outlet of the second liquid storage tank and the second liquid inlet.

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