A cascade unit based on R717 and CO2 as dual natural working fluids

Abstract

The utility model provides a kind of based on R717 and CO2 double natural working medium's cascade unit, including R717 circulation system, CO2 circulation system and integrated chassis, form prefabricated pry dress structure;R717 circulation system contains semi-closed screw compressor, plate shell condenser etc., CO2 circulation system includes screw compressor, supercritical piston compressor etc., and two systems are heat-exchanged by plate shell type R717 / CO2 condensation evaporator;Integrated chassis adopts rectangular frame layout, sets up hoisting point, convenient transportation installation.The utility model uses R717 and CO2 double natural working medium, GWP is extremely low and has no ozone layer destruction, meets green environmental protection requirement;Environment protection is efficient, refrigeration efficiency is improved 15%~20%, installation period is shortened more than 50%, leakage rate is less than or equal to 5ppm;Prefabricated pry dress structure realizes factory integrated assembly, reduces on-site construction amount, shortens project construction period.

Description

Technical Field

[0001] This utility model belongs to the field of refrigeration equipment technology, specifically relating to a cascade unit based on R717 and CO2 as dual natural working fluids. Background Technology

[0002] In the field of refrigeration technology, traditional refrigeration units often use synthetic refrigerants such as Freon. However, Freon refrigerants have a high global warming potential (GWP) and damage the ozone layer, failing to meet environmental protection requirements. With increasing environmental awareness and the introduction of relevant regulations, the search for environmentally friendly refrigerants has become a development trend in the refrigeration industry.

[0003] Natural refrigerants R717 and CO2 have garnered widespread attention due to their excellent environmental performance (low GWP and no ozone layer depletion). R717 offers high refrigeration efficiency but is toxic and corrosive; CO2 exhibits good chemical stability and safety, but its refrigeration efficiency is lower at high temperatures. Combining R717 and CO2 into a cascade chiller can fully leverage the advantages of both, improving both the performance and environmental friendliness of the refrigeration system.

[0004] Currently, there are some shortcomings in the cascade chiller units on the market. For example, some cascade chiller units have unreasonable structural designs, leading to difficulties in installation and maintenance; some units have problems with compressor sealing, which can easily cause refrigerant leakage, affecting the normal operation and safety of the unit; and some units lack an effective oil management system, resulting in poor compressor lubrication and reducing the service life of the unit.

[0005] In view of this, it is particularly important to develop a cascade unit based on R717 and CO2 as dual natural working fluids to overcome the above problems. Utility Model Content

[0006] The purpose of this invention is to propose a cascade cooling unit based on R717 and CO2 as dual natural refrigerants. This unit adopts a prefabricated skid-mounted structure, which is convenient for installation and transportation. Through optimized sealing structure and oil management system, the safety and reliability of the unit are improved. At the same time, by utilizing the cascade cycle of R717 and CO2 as dual natural refrigerants, a highly efficient and environmentally friendly cooling effect is achieved.

[0007] This invention proposes a cascade compressor unit based on R717 and CO2 as dual natural working fluids, comprising an R717 circulation system, a CO2 circulation system, and an integrated base frame. The R717 circulation system and the CO2 circulation system are integrated and mounted on the integrated base frame to form a prefabricated skid-mounted structure. The R717 circulation system includes an R717 compressor, an R717 plate-shell condenser, and an R717 horizontal oil separator. The exhaust port of the R717 compressor is connected to the inlet port of the R717 horizontal oil separator, and the exhaust port of the R717 horizontal oil separator is connected to the inlet port of the R717 plate-shell condenser. The CO2 circulation system includes a CO2 screw compressor. The system comprises a screw compressor, a horizontal CO2 oil separator, a supercritical CO2 reciprocating compressor, and an R717 / CO2 condenser-evaporator. The exhaust ports of the CO2 screw compressor and the CO2 reciprocating compressor are respectively connected to the inlets of the horizontal CO2 oil separator and the supercritical CO2 oil separator. The outlet of the horizontal CO2 oil separator is connected to the CO2 side inlet of the R717 / CO2 condenser-evaporator. The liquid outlet of the R717 plate condenser is connected to the R717 side inlet of the R717 / CO2 condenser-evaporator. The outlet of the R717 side of the R717 / CO2 condenser-evaporator is connected to the inlet of the R717 compressor.

[0008] By adopting the above technical solutions, using R717 and CO2 as dual natural refrigerants, the system has extremely low GWP and no ozone layer depletion, meeting green and environmental protection requirements. The cascade cycle combines the characteristics of the two refrigerants, expanding the refrigeration temperature range. The oil-cooling system optimizes compressor lubrication and operating efficiency, reducing energy consumption. The prefabricated skid-mounted structure enables factory integrated assembly, reducing on-site construction and shortening the project construction cycle. The semi-hermetic compressor design reduces the risk of R717 leakage, and the sophisticated oil separator and oil equalization system ensure stable compressor operation and improve unit reliability.

[0009] Preferably, the R717 circulation system further includes an R717 plate oil cooler, the oil outlet of the R717 horizontal oil separator is connected to the oil inlet of the R717 compressor via an oil pipe, and the R717 plate oil cooler is mounted on the oil pipe.

[0010] By adopting the above technical solution, the R717 plate oil cooler is installed on the oil pipe to cool the lubricating oil, ensuring compressor lubrication, improving operational stability and efficiency, and extending the service life of the equipment.

[0011] Preferably, the CO2 circulation system further includes a CO2 plate-shell oil cooler, a CO2 supercritical oil separator, a CO2 reciprocating compressor oil reservoir, and a CO2 precooler; the oil outlet of the CO2 horizontal oil separator is connected to the oil inlet of the CO2 screw compressor and the CO2 supercritical reciprocating compressor respectively; the gas outlet of the CO2 horizontal oil separator is connected to the inlet of the CO2 precooler; the gas outlet of the CO2 precooler is connected to the CO2 side inlet of the R717 / CO2 condenser-evaporator; the CO2 side outlet of the R717 / CO2 condenser-evaporator is connected to the gas inlet of the CO2 evaporator; the gas outlet of the CO2 evaporator is connected to the gas inlet of the CO2 screw compressor and the CO2 supercritical reciprocating compressor respectively; an oil equalization pipe is provided between the CO2 horizontal oil separator and the CO2 reciprocating compressor oil reservoir; the exhaust port of the CO2 supercritical reciprocating compressor is connected to the CO2 supercritical oil separator.

[0012] By adopting the above technical solutions, the CO2 circulation system has been improved, achieving efficient oil separation, cooling and precooling, ensuring compressor lubrication, and improving refrigeration efficiency and system stability.

[0013] Preferably, the CO2 circulation system further includes a CO2 float valve, which is located at the liquid outlet on the CO2 side of the R717 / CO2 condenser-evaporator.

[0014] By adopting the above technical solution, a float valve is installed at the CO2 side outlet of the R717 / CO2 condenser-evaporator, which can accurately control the flow rate, improve energy efficiency, ensure equipment safety, and enhance system adaptability.

[0015] Preferably, the R717 compressor is configured as a semi-hermetic screw compressor.

[0016] By adopting the above technical solution, this design can effectively prevent R717 leakage and ensure the safe and stable operation of the unit.

[0017] Preferably, the integrated base frame is a rectangular frame structure, the R717 horizontal oil separator and the CO2 horizontal oil separator are arranged side by side in the middle of the integrated base frame, the R717 plate condenser is arranged on one side of the R717 horizontal oil separator, the CO2 precooler and the R717 / CO2 condenser-evaporator are arranged on one side of the CO2 horizontal oil separator, and the R717 compressor, the CO2 screw compressor and the CO2 piston compressor are respectively arranged above the CO2 horizontal oil separator.

[0018] By adopting the above technical solutions, the rectangular frame integrated base frame makes the layout compact and reasonable, and the arrangement of the positions of each component is conducive to pipeline connection, which can improve space utilization and overall unit stability.

[0019] Preferably, the integrated base frame is provided with several lifting points.

[0020] By adopting the above technical solutions, the lifting points on the integrated base frame facilitate the overall lifting and transportation of the unit, improve the convenience of handling, and reduce the manpower and time costs during the installation process.

[0021] Preferably, an oil equalization regulating valve is provided on the oil equalization pipe between the CO2 horizontal oil separator and the CO2 piston compressor oil reservoir. The oil equalization regulating valve is used to adjust the oil quantity balance between the CO2 horizontal oil separator and the CO2 piston compressor oil reservoir.

[0022] By adopting the above technical solution, the oil equalization regulating valve can accurately adjust the oil balance between the CO2 horizontal oil separator and the oil reservoir, ensuring compressor lubrication and improving the stability and reliability of unit operation.

[0023] Preferably, the R717 / CO2 condenser-evaporator has a plate-shell structure, and the R717 flow channels and CO2 flow channels inside the R717 / CO2 condenser-evaporator are arranged alternately.

[0024] By adopting the above technical solutions, the plate-shell structure and staggered flow channel design increase the contact area between R717 and CO2, enhance the heat exchange effect, and improve the refrigeration efficiency of the cascade unit.

[0025] Compared with the prior art, the beneficial results of this utility model are as follows:

[0026] (1) By adopting the dual natural working fluid cascade cycle of R717 and CO2 to replace the traditional Freon refrigerant, the GWP value approaches 0, there is no ozone layer destruction, and the environmental performance is significantly improved.

[0027] (2) The prefabricated skid-mounted integrated design concentrates the dual system components on a rectangular base frame, shortening the on-site construction cycle by more than 50%, and the hoisting point design facilitates transportation; the plate-shell condenser-evaporator and the staggered flow channel enhance the heat exchange efficiency, improving it by 15%-20% compared to the traditional structure.

[0028] (3) The R717 cycle system semi-hermetic compressor effectively reduces refrigerant leakage rate; while reducing the risk of environmental pollution, it improves system energy efficiency and enhances system operation stability. Attached Figure Description

[0029] The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and, together with the description, serve to explain the principles of the present invention. Other embodiments and many anticipated advantages of the embodiments will be readily recognized as they become better understood through reference to the following detailed description. Elements in the drawings are not necessarily to scale. The same reference numerals refer to corresponding similar parts.

[0030] Figure 1 A front view of a cascade unit based on R717 and CO2 dual natural working fluids according to an embodiment of the present invention is shown.

[0031] Figure 2 A top view of a cascade unit based on R717 and CO2 dual natural working fluids according to an embodiment of the present invention is shown;

[0032] Figure 3 A side view of a cascade unit based on two natural working fluids, R717 and CO2, according to an embodiment of the present invention is shown.

[0033] The meaning of each number in the diagram:

[0034] 1. R717 circulating system; 11. R717 semi-hermetic screw compressor; 12. R717 plate condenser; 13. R717 plate oil cooler; 14. R717 horizontal oil separator;

[0035] 2. CO2 circulation system; 21. CO2 screw compressor; 22. CO2 horizontal oil separator; 23. CO2 plate oil cooler; 24. CO2 supercritical reciprocating compressor; 25. CO2 supercritical oil separator; 26. CO2 reciprocating compressor oil receiver; 27. CO2 precooler; 28. R717 / CO2 condenser-evaporator; 29. ​​CO2 float valve;

[0036] 3. Integrated base frame. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the relevant utility model and not intended to limit the scope of the utility model. Furthermore, it should be noted that, for ease of description, only the parts relevant to the utility model are shown in the accompanying drawings.

[0038] It should be noted that, where there is no conflict, the embodiments and features in the embodiments of this utility model can be combined with each other. The present utility model will now be described in detail with reference to the accompanying drawings and embodiments.

[0039] This utility model proposes a cascade unit based on R717 and CO2 as dual natural working fluids, such as... Figure 1 As shown, it mainly consists of an R717 circulation system 1, a CO2 circulation system 2, and an integrated base frame 3. All components of the R717 circulation system 1 and the CO2 circulation system 2 are integrated and installed on the integrated base frame 3, forming a prefabricated skid-mounted structure. This structural design allows the unit to be assembled and commissioned in the factory, and after being transported to the site, it only requires simple connection and fixing before it can be put into use, greatly improving installation efficiency.

[0040] Specifically, refer to Figure 1 and Figure 2 The R717 circulation system 1 includes an R717 compressor, an R717 plate-and-shell condenser 12, an R717 plate-and-shell oil cooler 13, and an R717 horizontal oil separator 14. In this embodiment, the R717 compressor is configured as an R717 semi-hermetic screw compressor 11. The exhaust port of the R717 semi-hermetic screw compressor 11 is connected to the inlet of the R717 horizontal oil separator 14, and the oil outlet of the R717 horizontal oil separator 14 is connected to the oil inlet of the R717 semi-hermetic screw compressor 11 via an oil pipe. The R717 plate-and-shell oil cooler 13 is mounted on the oil pipe, and the exhaust port of the R717 horizontal oil separator 14 is connected to the inlet of the R717 plate-and-shell condenser 12.

[0041] In this embodiment, the working process of the R717 high-temperature cycle system mainly includes: the exhaust gas of the R717 semi-hermetic screw compressor 11 is discharged into the R717 horizontal oil separator 14, the separated refrigerant gas enters the R717 plate condenser 12, the condensed refrigerant liquid enters the evaporation section of the R717 / CO2 condenser-evaporator 28 after throttling, and the evaporated refrigerant gas returns to the R717 semi-hermetic screw compressor 11; the R717 horizontal oil separator 14 is cooled by the R717 plate oil cooler 13 and then supplied to the R717 semi-hermetic screw compressor 11.

[0042] Specifically, the R717 semi-hermetic screw compressor 11 compresses gaseous R717 and discharges high-temperature, high-pressure gas, which then enters the R717 horizontal oil separator 14. In the R717 horizontal oil separator 14, lubricating oil separates from the gaseous R717. The lubricating oil flows back to the oil inlet of the R717 semi-hermetic screw compressor 11 through an oil pipe, ensuring compressor lubrication. The separated gaseous R717 enters the R717 plate condenser 12, where it exchanges heat with cooling water and cools into liquid R717. The liquid R717 flows out from the outlet of the R717 plate condenser 12 and enters the R717 side of the R717 / CO2 condenser-evaporator 28. In the R717 / CO2 condenser-evaporator 28, the liquid R717 absorbs heat from the CO2 side and evaporates into gaseous R717. Then, the gaseous R717 returns to the inlet of the R717 semi-hermetic screw compressor 11, completing one cycle.

[0043] Further, continue to refer to Figure 1 and Figure 2 The CO2 circulation system 2 includes a CO2 screw compressor 21, a CO2 horizontal oil separator 22, a CO2 plate oil cooler 23, a CO2 supercritical piston compressor 24, a CO2 supercritical oil separator 25, a CO2 piston compressor oil reservoir 26, a CO2 precooler 27, and an R717 / CO2 condenser-evaporator 28.

[0044] Combination Figure 2 and Figure 3 The exhaust ports of the CO2 screw compressor 21 and the CO2 piston compressor are connected to the inlets of the CO2 horizontal oil separator 22 and the CO2 supercritical oil separator 24, respectively. The outlet of the CO2 horizontal oil separator is connected to the inlet of the CO2 side of the R717 / CO2 condenser-evaporator.

[0045] The oil outlet of the CO2 horizontal oil separator 22 is connected to the oil inlet of the CO2 screw compressor 21 and the CO2 supercritical reciprocating compressor 24, respectively. The gas outlet of the CO2 horizontal oil separator 22 is connected to the inlet of the CO2 precooler 27. The gas outlet of the CO2 precooler 27 is connected to the gas inlet on the CO2 side of the R717 / CO2 condenser-evaporator 28. The gas outlet on the CO2 side of the R717 / CO2 condenser-evaporator 28 is connected to the gas inlet of the CO2 evaporator. The gas outlet of the CO2 evaporator is connected to the gas inlet of the CO2 screw compressor 21 and the CO2 supercritical reciprocating compressor 24, respectively. An oil equalization pipe is provided between the CO2 horizontal oil separator 22 and the CO2 reciprocating compressor oil reservoir 26. The exhaust port of the CO2 supercritical reciprocating compressor 24 is connected to the CO2 supercritical oil separator 25.

[0046] Reference Figure 1 The CO2 circulation system 2 also includes a CO2 float valve 29, which is located at the liquid outlet on the CO2 side of the R717 / CO2 condenser-evaporator 28. The float valve 29 at the CO2 side liquid outlet of the R717 / CO2 condenser-evaporator 28 allows for precise flow control, improved energy efficiency, enhanced equipment safety, and improved system adaptability.

[0047] In this embodiment, the working process of the CO2 cryogenic cycle system mainly includes:

[0048] CO2 main system: The exhaust gas from the CO2 screw compressor 21 enters the CO2 horizontal oil separator 22, the exhaust gas is discharged to the CO2 precooler 27, then enters the condensation section of the R717 / CO2 condenser-evaporator 28, and then is discharged into the system through the CO2 float valve 29;

[0049] CO2 auxiliary system: The exhaust gas from the CO2 supercritical piston compressor 24 enters the CO2 supercritical oil separator 25, and after being regulated by the gas regulating valve, it is connected to the main system;

[0050] The oil separated by the CO2 horizontal oil separator 22 is cooled and then supplied to the CO2 screw compressor 21; the refrigeration oil separated by the CO2 supercritical oil separator 25 is supplied to the CO2 reciprocating compressor oil reservoir 26, and then to the CO2 supercritical reciprocating compressor 24; the CO2 horizontal oil separator 22 and the CO2 oil reservoir are designed with an oil equalization system.

[0051] Specifically, refer to Figure 1 and Figure 2 The CO2 screw compressor 21 and the CO2 reciprocating compressor compress gaseous CO2 and discharge it as high-temperature, high-pressure gas, which then enters the CO2 horizontal oil separator 22 and the CO2 supercritical oil separator 25, respectively. In the CO2 horizontal oil separator 22 and the CO2 supercritical oil separator 25, the lubricating oil separates from the gaseous CO2, and the lubricating oil flows back to the oil inlet of the CO2 screw compressor 21 and the CO2 supercritical reciprocating compressor 24, respectively. The separated gaseous CO2 enters the CO2 precooler 27 for preliminary cooling. Then, the gaseous CO2 enters the CO2 side of the R717 / CO2 condenser-evaporator 28, where it exchanges heat with R717 and is cooled into liquid CO2. Liquid CO2 enters the CO2 evaporator, where it absorbs heat from the object being cooled and evaporates into gaseous CO2. The gaseous CO2 then returns to the inlet of the CO2 screw compressor 21 and the CO2 supercritical piston compressor 24, completing one cycle.

[0052] In this embodiment, an oil equalization pipe and an oil equalization regulating valve are provided between the CO2 horizontal oil separator 22 and the CO2 reciprocating compressor oil reservoir 26. Because the operating conditions of the CO2 screw compressor 21 and the CO2 supercritical reciprocating compressor 24 may differ, the lubricating oil distribution between them may be uneven. The design of the oil equalization pipe and the oil equalization regulating valve can achieve oil balance between the CO2 horizontal oil separator 22 and the CO2 reciprocating compressor oil reservoir 26, ensuring good lubrication of the compressor.

[0053] In this embodiment, the integrated base frame 3 is a rectangular frame structure. The R717 horizontal oil separator 14 and the CO2 horizontal oil separator 22 are arranged side-by-side in the middle of the integrated base frame 3. The R717 plate condenser 12 is located on one side of the R717 horizontal oil separator 14. The CO2 precooler 27 and the R717 / CO2 condenser-evaporator 28 are located on one side of the CO2 horizontal oil separator 22. The R717 semi-hermetic screw compressor 11, the CO2 screw compressor 21, and the CO2 piston compressor are respectively located above the CO2 horizontal oil separator 22. The integrated base frame 3 has several lifting points.

[0054] The rectangular frame integrated base makes the layout compact and reasonable, and the arrangement of the components facilitates pipeline connection, which can improve space utilization and overall unit stability. The lifting points on the integrated base facilitate the overall lifting and transportation of the unit, improve the convenience of handling, and reduce the labor and time costs during the installation process.

[0055] This utility model's technical solution adopts a factory-prefabricated integrated unit, upgrading intelligent manufacturing and accelerating delivery. Relying on industry-leading factory prefabrication technology, the unit completes precise assembly and rigorous multi-dimensional testing throughout the entire process before leaving the factory. By moving the complex installation process to a standardized factory environment, it completely avoids assembly errors and safety risks caused by human operation and environmental differences in traditional on-site construction. At the same time, the prefabricated design greatly simplifies on-site construction procedures, shortening the on-site construction cycle compared to traditional units. This efficient delivery model significantly saves customers time and management costs, enabling rapid project implementation.

[0056] Modular skid-mounted design and high-efficiency heat exchange ensure compact, efficient, and energy-saving operation. Utilizing a highly integrated skid-mounted design, the R717 side features a high-efficiency plate-and-shell heat exchanger. Thanks to its compact structure and superior heat exchange performance, it significantly reduces refrigerant charge while ensuring efficient system operation, effectively lowering operating costs and potential environmental risks. This design not only improves space utilization and transportation convenience but also creates long-term value for users through its economical and environmentally friendly technological advantages.

[0057] By using CO2 and R717 as the system's refrigerants, the system achieves efficient cooling and heating while significantly reducing greenhouse gas emissions, minimizing the environmental impact of equipment operation at its source. The product empowers enterprises with sustainable development through green technology, helping the industry transform and upgrade towards low-carbon and clean technologies.

[0058] R717 semi-hermetic screw compressor technology: upgraded sealing for enhanced safety and reliability. Targeting the characteristics of R717 refrigerant, the unit innovatively incorporates a semi-hermetic screw compressor. Through a precise sealing structure design and the application of high-strength materials, it fundamentally solves the R717 leakage problem commonly found in traditional open-type screw compressors. This technology significantly reduces the risk of refrigerant loss, not only improving equipment operational safety and stability but also effectively reducing maintenance costs and environmental impact caused by leaks, providing users with a safer and more environmentally friendly refrigeration solution.

[0059] The specific embodiments of this utility model have been described above, but the scope of protection of this utility model is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this utility model should be included within the scope of protection of this utility model. Therefore, the scope of protection of this utility model should be determined by the scope of the claims.

[0060] In the description of this utility model, it should be understood that the terms "upper," "lower," "inner," "outer," etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model. The word "comprising" does not exclude the presence of elements or steps not listed in the claims. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The simple fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used for improvement. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A cascade turbine unit based on R717 and CO2 as dual natural working fluids, characterized in that, It includes an R717 circulation system, a CO2 circulation system, and an integrated chassis, wherein the R717 circulation system and the CO2 circulation system are integrated and installed on the integrated chassis to form a prefabricated skid-mounted structure; The R717 circulation system includes an R717 compressor, an R717 plate condenser, and an R717 horizontal oil separator; the exhaust port of the R717 compressor is connected to the inlet of the R717 horizontal oil separator, and the outlet of the R717 horizontal oil separator is connected to the inlet of the R717 plate condenser. The CO2 circulation system includes a CO2 screw compressor, a CO2 horizontal oil separator, a CO2 supercritical reciprocating compressor, and an R717 / CO2 condenser-evaporator; the exhaust ports of the CO2 screw compressor and the CO2 reciprocating compressor are respectively connected to the inlets of the CO2 horizontal oil separator and the CO2 supercritical oil separator, and the exhaust port of the CO2 horizontal oil separator is connected to the CO2 side inlet of the R717 / CO2 condenser-evaporator; The liquid outlet of the R717 plate condenser is connected to the liquid inlet on the R717 side of the R717 / CO2 condenser-evaporator, and the gas outlet on the R717 side of the R717 / CO2 condenser-evaporator is connected to the gas inlet of the R717 compressor.

2. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The R717 circulation system also includes an R717 plate oil cooler. The oil outlet of the R717 horizontal oil separator is connected to the oil inlet of the R717 compressor via an oil pipe. The R717 plate oil cooler is installed on the oil pipe.

3. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The CO2 circulation system also includes a CO2 plate-shell oil cooler, a CO2 supercritical oil separator, a CO2 piston compressor oil reservoir, and a CO2 precooler; The oil outlet of the CO2 horizontal oil separator is connected to the oil inlet of the CO2 screw compressor and the CO2 supercritical reciprocating compressor, respectively. The gas outlet of the CO2 horizontal oil separator is connected to the inlet of the CO2 precooler. The gas outlet of the CO2 precooler is connected to the gas inlet on the CO2 side of the R717 / CO2 condenser-evaporator. The gas outlet on the CO2 side of the R717 / CO2 condenser-evaporator is connected to the gas inlet of the CO2 evaporator. The gas outlet of the CO2 evaporator is connected to the gas inlet of the CO2 screw compressor and the CO2 supercritical reciprocating compressor, respectively. An oil equalization pipe is provided between the CO2 horizontal oil separator and the oil reservoir of the CO2 reciprocating compressor. The exhaust port of the CO2 supercritical reciprocating compressor is connected to the CO2 supercritical oil separator.

4. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The CO2 circulation system also includes a CO2 float valve, which is located at the liquid outlet on the CO2 side of the R717 / CO2 condenser-evaporator.

5. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The R717 compressor is configured as a semi-hermetic screw compressor.

6. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The integrated base frame is a rectangular frame structure. The R717 horizontal oil separator and the CO2 horizontal oil separator are arranged side by side in the middle of the integrated base frame. The R717 plate condenser is located on one side of the R717 horizontal oil separator. The CO2 precooler and the R717 / CO2 condenser-evaporator are located on one side of the CO2 horizontal oil separator. The R717 compressor, the CO2 screw compressor and the CO2 piston compressor are respectively located above the CO2 horizontal oil separator.

7. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The integrated base frame is equipped with several lifting points.

8. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 3, characterized in that, An oil equalization regulating valve is provided on the oil equalization pipe between the CO2 horizontal oil separator and the CO2 piston compressor oil reservoir. The oil equalization regulating valve is used to adjust the oil quantity balance between the CO2 horizontal oil separator and the CO2 piston compressor oil reservoir.

9. The cascade unit based on R717 and CO2 as dual natural working fluids according to claim 1, characterized in that, The R717 / CO2 condenser-evaporator has a plate-shell structure, and the R717 flow channels and CO2 flow channels inside the R717 / CO2 condenser-evaporator are arranged alternately.

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