Mixed refrigerant two-stage screw type refrigeration compressor unit and refrigeration system

Abstract

The utility model relates to the technical field of natural gas liquefaction, in particular to a mixed refrigerant two-stage screw type refrigeration compressor unit and a refrigeration system. Comprising a low-pressure-stage screw compressor, a high-pressure-stage screw compressor, a low-pressure-stage first-stage oil separator, a high-pressure-stage first-stage oil separator, an intercooler, a low-pressure-stage oil cooler, a high-pressure-stage oil cooler, an anti-explosion low-pressure-stage oil pump and an anti-explosion high-pressure-stage oil pump. And a refrigerant output port of the low-pressure-stage screw compressor is connected with the low-pressure-stage first-stage oil separator through a pipeline. The low-pressure-stage screw compressor and the high-pressure-stage screw compressor are adopted and matched with the corresponding parts such as the oil separator, the cooler and the oil pump, and the efficient and stable refrigeration effect is achieved. Particularly, due to the arrangement of the multi-stage oil separator, the separation effect of refrigerant gas and lubricating oil is improved, stable operation of the refrigerating system is ensured, and the efficiency and performance of the whole refrigerating system are improved.

Description

Technical Field

[0001] The utility model relates to the technical field of natural gas liquefaction, in particular to a mixed refrigerant two-stage screw refrigeration compressor unit and a refrigeration system. Background Art

[0002] The main component of natural gas is methane. Depending on the specific conditions of the underground reservoir, natural gas also includes small amounts of ethane, propane, butane, pentane, etc. as well as water, hydrogen, nitrogen, carbon dioxide and other gases.

[0003] Most natural gas exists in gaseous form. The most common method of transporting natural gas from the wellhead to the gas processing unit and from there to the natural gas users is to use a high-pressure gas pipeline. However, when the natural gas source is far away from the target market and the gas source is relatively scarce, the cost of pipeline transportation is very high. In this case, it is necessary or desirable to liquefy the natural gas into liquefied natural gas for transportation and use.

[0004] Currently, common natural gas liquefaction refrigeration processes include cascade refrigeration cycle, mixed refrigerant refrigeration cycle and expansion refrigeration cycle.

[0005] The mixed refrigerant used in the mixed refrigerant refrigeration cycle is usually composed of methane, ethylene, propane, n-butane, isopentane, nitrogen, etc. mixed in specific components and specific ratios. During the refrigeration cycle, the evaporating gas of the refrigerant needs to be compressed and heated by a refrigeration compressor unit. Traditional single-stage screw refrigeration compressor units are generally used in small and medium-sized refrigeration systems, while two-stage screw refrigeration compressor units are generally used in large refrigeration systems due to their complex structure and higher efficiency; the two-stage screw refrigeration compressor unit includes a low-pressure screw compressor and a high-pressure screw compressor, which can perform staged compression on the refrigerant, thereby improving refrigeration efficiency and reducing power consumption.

[0006] When using a two-stage screw refrigeration compressor unit, a large amount of lubricating oil in the compressor will seep into the mixed refrigerant due to long-term operation. If the oil in the mixed refrigerant cannot be effectively separated, a large amount of lubricating oil will accumulate in the terminal plate-fin cold box, resulting in a decrease in the evaporative heat exchange effect and affecting the production of liquefied natural gas. Utility Model Content

[0007] The utility model aims at the technical problems existing in the prior art and provides a mixed refrigerant two-stage screw refrigeration compressor unit to solve the problem that the lubricating oil in the refrigeration compressor unit is easy to gather in the terminal plate-fin cold box and affect the heat exchange.

[0008] The utility model solves the above technical problems with the following technical solutions: a mixed refrigerant two-stage screw refrigeration compressor unit, comprising a low-pressure screw compressor, a high-pressure screw compressor, a low-pressure first-stage oil separator, a high-pressure first-stage oil separator, an intercooler, a low-pressure oil cooler, a high-pressure oil cooler, an explosion-proof low-pressure oil pump and an explosion-proof high-pressure oil pump;

[0009] The refrigerant outlet of the low-pressure screw compressor is connected to the low-pressure first-stage oil separator through a pipeline; the oil separation outlet of the low-pressure first-stage oil separator is connected to the low-pressure oil cooler through a pipeline, and the oil outlet of the low-pressure oil cooler is connected to the explosion-proof low-pressure oil pump through a pipeline, and the explosion-proof low-pressure oil pump is input into the cooling and lubrication system of the low-pressure screw compressor through a pipeline; the gas separation outlet of the low-pressure first-stage oil separator is connected to the intercooler through a pipeline;

[0010] The refrigerant input port of the high-pressure screw compressor is connected to the gas output port of the intermediate cooler; the refrigerant output port of the high-pressure screw compressor is connected to the high-pressure first-stage oil separator through a pipeline, the oil separation port of the high-pressure first-stage oil separator is connected to the high-pressure oil cooler through a pipeline, the oil output port of the high-pressure oil cooler is connected to the explosion-proof high-pressure oil pump through a pipeline, and the explosion-proof high-pressure oil pump inputs the cooling and lubrication system of the high-pressure screw compressor through a pipeline.

[0011] On the basis of the above technical solution, the present invention can also be improved as follows.

[0012] Furthermore, it also includes an explosion-proof low-pressure main motor and an explosion-proof high-pressure main motor; the explosion-proof low-pressure main motor is connected to the power input shaft of the low-pressure screw compressor through a low-pressure coupling; the explosion-proof high-pressure main motor is connected to the power input shaft of the high-pressure screw compressor through a high-pressure coupling.

[0013] Furthermore, the refrigerant inlet pipeline of the low-pressure screw compressor is also provided with a low-pressure suction stop check valve; the refrigerant inlet pipeline of the high-pressure screw compressor is also provided with a high-pressure suction stop check valve.

[0014] Furthermore, the gas separation port of the high-pressure first-stage oil separator is connected in series with a high-pressure second-stage oil separator and a high-pressure third-stage oil separator in sequence through a pipeline.

[0015] Furthermore, the input and output pipelines of the explosion-proof low-pressure oil pump are respectively equipped with a low-pressure oil coarse filter and a low-pressure oil fine filter; the input and output pipelines of the explosion-proof high-pressure oil pump are respectively equipped with a high-pressure oil coarse filter and a high-pressure oil fine filter.

[0016] Furthermore, it also includes a cooling water tower, the water inlet end of the cooling water tower is connected in parallel with the heat exchange medium outlets of the intermediate cooler, the high-pressure oil cooler and the low-pressure oil cooler through pipelines; the discharge end of the cooling water tower is connected in parallel with the heat exchange medium inlet of the intermediate cooler, the high-pressure oil cooler and the low-pressure oil cooler through pipelines.

[0017] Furthermore, a cooling water circulation pump is also provided on the water inlet pipeline of the cooling water tower.

[0018] Furthermore, the filter elements of the low-pressure first-stage oil separator, the high-pressure first-stage oil separator, the high-pressure second-stage oil separator and the high-pressure third-stage oil separator are all made of polymer materials.

[0019] Furthermore, the shaft seal O-rings of the low-pressure screw compressor and the high-pressure screw compressor are made of perfluoroether material.

[0020] Moreover, the mixed refrigerant two-stage screw refrigeration compressor unit provided by the utility model has at least the following beneficial effects compared with the prior art:

[0021] 1. The two-stage screw compressor with low pressure and high pressure is adopted, and the corresponding oil separator, cooler and oil pump are matched to achieve efficient and stable refrigeration effect. In particular, the multi-stage oil separator is set to improve the separation effect of refrigerant gas and lubricating oil, ensure the stable operation of the refrigeration system, and improve the efficiency and performance of the entire refrigeration system.

[0022] 2. The explosion-proof design and the shaft seal O-ring made of perfluoroether material improve the safety of the system and can maintain stable performance in high temperature, high pressure and corrosive environments. At the same time, the use of cooling water towers and cooling water circulation pumps realizes the recycling of cooling water, improves refrigeration efficiency and reduces energy consumption.

[0023] The utility model also designs a refrigeration system, including the mixed refrigerant two-stage screw refrigeration compressor unit and the plate-fin cold box as described above;

[0024] The gas outlet of the high-pressure three-stage oil separator is input into the refrigerant circulation pipeline of the plate-fin cold box through a pipeline.

[0025] The mixed refrigerant compressed and transported by the mixed refrigerant two-stage screw refrigeration compressor unit is input into the plate-fin cold box and can fully exchange heat with the passing natural gas, thereby ensuring the production of liquefied natural gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the mixed refrigerant two-stage screw refrigeration compressor system of the utility model;

[0027] Figure 2This is a schematic diagram of the cooling water cooling principle of the utility model.

[0028] In the accompanying drawings, the components represented by the reference numerals are listed as follows:

[0029] Low-pressure suction stop check valve 1, low-pressure screw compressor 2, low-pressure coupling 3, explosion-proof low-pressure main motor 4, low-pressure first-stage oil separator 5, intercooler 6, high-pressure suction stop check valve 7, high-pressure screw compressor 8, high-pressure coupling 9, explosion-proof high-pressure main motor 10, high-pressure first-stage oil separator 11, high-pressure exhaust stop check valve 12, high-pressure second-stage oil separator 13, high-pressure third-stage oil separator 14, high-pressure oil cooler 15, high-pressure oil coarse filter 16, explosion-proof high-pressure oil pump 17, high-pressure oil fine filter 18, low-pressure oil cooler 19, low-pressure oil coarse filter 20, explosion-proof low-pressure oil pump 21, low-pressure oil fine filter 22, cooling water tower 23, cooling water circulation pump 24. DETAILED DESCRIPTION

[0030] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0031] It should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integrally formed structure. For ordinary technicians in this field, the specific meanings of such terms in this patent can be understood according to specific circumstances.

[0032] like Figure 1 and Figure 2 As shown, the mixed refrigerant two-stage screw refrigeration compressor unit of the utility model comprises a low-pressure screw compressor 2, a high-pressure screw compressor 8, a low-pressure primary oil separator 5, a high-pressure primary oil separator 11, an intercooler 6, a low-pressure oil cooler 19, a high-pressure oil cooler 15, an explosion-proof low-pressure oil pump 21 and an explosion-proof high-pressure oil pump 17;

[0033] The refrigerant outlet of the low-pressure screw compressor 2 is connected to the low-pressure primary oil separator 5 through a pipeline; the oil separation port of the low-pressure primary oil separator 5 is connected to the low-pressure oil cooler 19 through a pipeline, and the oil outlet of the low-pressure oil cooler 19 is connected to the explosion-proof low-pressure oil pump 21 through a pipeline, and the explosion-proof low-pressure oil pump 21 is input into the cooling and lubrication system of the low-pressure screw compressor 2 through a pipeline; the gas separation port of the low-pressure primary oil separator 5 is connected to the intercooler 6 through a pipeline;

[0034] The refrigerant input port of the high-pressure screw compressor 8 is connected to the gas output port of the intermediate cooler 6; the refrigerant output port of the high-pressure screw compressor 8 is connected to the high-pressure first-stage oil separator 11 through a pipeline, the oil separation port of the high-pressure first-stage oil separator 11 is connected to the high-pressure oil cooler 15 through a pipeline, the oil output port of the high-pressure oil cooler 15 is connected to the explosion-proof high-pressure oil pump 17 through a pipeline, and the explosion-proof high-pressure oil pump 17 is input into the cooling and lubrication system of the high-pressure screw compressor 8 through a pipeline.

[0035] Low pressure stage compression and oil separation:

[0036] The mixed refrigerant first enters the low-pressure screw compressor 2 for preliminary compression;

[0037] The compressed refrigerant and lubricating oil mixture enters the low-pressure first-stage oil separator 5 through the pipeline.

[0038] In the low-pressure first-stage oil separator 5, the refrigerant and the lubricating oil are effectively separated. The separated lubricating oil enters the low-pressure oil cooler 19 through a pipeline for cooling.

[0039] The cooled lubricating oil is pumped back to the cooling and lubricating system of the low-pressure screw compressor 2 by the explosion-proof low-pressure oil pump 21, thus completing the recycling of the lubricating oil.

[0040] At the same time, the separated refrigerant gas enters the intercooler 6 through the gas separation port of the low-pressure stage primary oil separator 5 for cooling.

[0041] High pressure stage compression and oil separation:

[0042] The refrigerant gas cooled by the intercooler 6 enters the high-pressure screw compressor 8 for further compression.

[0043] The compressed refrigerant and lubricating oil mixture enters the high-pressure first-stage oil separator 11 through the pipeline again.

[0044] In the high-pressure first oil separator 11, the refrigerant and the lubricating oil are effectively separated again. The separated lubricating oil enters the high-pressure oil cooler 15 through a pipeline for cooling.

[0045] The cooled lubricating oil is pumped back to the cooling and lubricating system of the high-pressure screw compressor 8 by the explosion-proof high-pressure oil pump 17, thereby completing the recycling of the high-pressure lubricating oil.

[0046] In this embodiment, the unit uses a low-pressure screw compressor 2 and a high-pressure screw compressor 8 for staged compression. This design can reduce the exhaust temperature of the compressor and reduce power consumption, thereby improving the efficiency of the entire refrigeration system. The intercooler 6 is used to cool the refrigerant to ensure that it reaches a suitable temperature before high-pressure compression.

[0047] The low-pressure stage and high-pressure stage are each equipped with a primary oil separator to effectively separate the lubricating oil and refrigerant generated during the compression process. The separated lubricating oil is cooled by the low-pressure stage oil cooler and the high-pressure stage oil cooler respectively, and then re-input into the corresponding compressor cooling and lubrication system through the explosion-proof oil pump, realizing the recycling of the lubricating oil.

[0048] As an embodiment, it also includes an explosion-proof low-pressure main motor 4 and an explosion-proof high-pressure main motor 10; the explosion-proof low-pressure main motor 4 is connected to the power input shaft of the low-pressure screw compressor 2 through a low-pressure coupling 3; the explosion-proof high-pressure main motor 10 is connected to the power input shaft of the high-pressure screw compressor 8 through a high-pressure coupling 9.

[0049] The explosion-proof low-pressure main motor 4 provides power to the low-pressure screw compressor 2 through the low-pressure coupling 3, driving it to perform preliminary compression of the refrigerant gas.

[0050] The explosion-proof high-pressure main motor 10 provides power to the high-pressure screw compressor 8 through the high-pressure coupling 9, so that the compressor can further compress the refrigerant gas.

[0051] In this embodiment, the explosion-proof low-pressure main motor 4 and the explosion-proof high-pressure main motor 10 are flameproof or positive-pressure explosion-proof motors, which provide reliable safety protection for the mixed refrigerant two-stage screw refrigeration compressor unit and ensure stable operation of the unit in a workplace where an explosive environment may exist.

[0052] Electrical components such as the explosion-proof low-voltage main motor 4, the explosion-proof high-voltage main motor 10, the explosion-proof low-voltage oil pump 21 and the explosion-proof high-voltage oil pump 17 are all in accordance with corresponding explosion-proof requirements to ensure safe use.

[0053] As an implementation mode, the refrigerant inlet pipeline of the low-pressure screw compressor 2 is further provided with a low-pressure suction stop check valve 1; the refrigerant inlet pipeline of the high-pressure screw compressor 8 is further provided with a high-pressure suction stop check valve 7. The design of the double check valve can prevent the refrigerant from flowing back.

[0054] As an implementation manner, the gas separation port of the high-pressure first-stage oil separator 11 is connected in series with the high-pressure second-stage oil separator 13 and the high-pressure third-stage oil separator 14 in sequence through pipelines.

[0055] The filter elements of the low-pressure first-stage oil separator 5, the high-pressure first-stage oil separator 11, the high-pressure second-stage oil separator 13 and the high-pressure third-stage oil separator 14 are all polymer material filter elements, which can effectively filter out tiny particles and lubricating oil in the refrigerant gas to ensure the purity of the refrigerant gas.

[0056] First, the refrigerant and lubricating oil mixture compressed by the high-pressure screw compressor 8 enters the high-pressure first-stage oil separator 11. In this oil separator, most of the lubricating oil is separated due to gravity and centrifugal force, and the oil-gas separation content is 3-5ppm; while the refrigerant gas continues to flow to the next stage.

[0057] Next, the separated refrigerant gas enters the high-pressure secondary oil separator 13. Here, through a more sophisticated separation mechanism, such as using a special bellows structure or centrifugal force, the trace amount of lubricating oil remaining in the gas is further removed, and the oil-gas separation content is 1-3ppm. In this way, the purity of the refrigerant gas is further improved.

[0058] Finally, the refrigerant gas enters the high-pressure three-stage oil separator 14. This is the last stage of the oil separation process, and the oil-gas separation content is 0.04-0.06ppm, which is intended to ensure that the refrigerant gas contains almost no lubricating oil components. The three-stage separation can ensure that the final oil-gas separation effect is less than the standard of 0.1ppm, so that the heat exchange effect of the terminal plate-fin cold box is good, and the production of liquefied natural gas is guaranteed.

[0059] As an implementation method, the input and output pipelines of the explosion-proof low-pressure oil pump 21 are respectively equipped with a low-pressure oil coarse filter 20 and a low-pressure oil fine filter 22; the input and output pipelines of the explosion-proof high-pressure oil pump 17 are respectively equipped with a high-pressure oil coarse filter 16 and a high-pressure oil fine filter 18. The coarse oil filter and the fine oil filter can effectively remove large particles of impurities and pollutants in the oil, prevent these impurities from entering the interior of the oil pump, and thus protect the oil pump from damage; the filter can be a metal mesh filter or a wire gap filter.

[0060] The main function of the low-pressure oil filter 20 and the high-pressure oil filter 16 is to filter out large particles of impurities in the lubricating oil, such as metal particles, sand particles, etc., to prevent these impurities from entering the oil pump and causing damage to the operation of the oil pump. Its filtering accuracy is relatively low, but it can effectively protect the oil pump from large particles of impurities.

[0061] The low-pressure oil filter 22 and the high-pressure oil filter 18 are installed at the output end of the oil pump to further filter out the tiny particles and impurities in the lubricating oil. Its filtering accuracy is higher, which can ensure that the lubricating oil entering each component of the refrigeration compressor unit reaches a very high cleanliness, thereby extending the service life of the unit and improving the refrigeration effect.

[0062] As an embodiment, a cooling water tower 23 is further included, wherein the water inlet end of the cooling water tower 23 is sequentially connected in parallel with the heat exchange medium outlets of the intercooler 6, the high-pressure oil cooler 15, and the low-pressure oil cooler 19 through a pipeline; the discharge end of the cooling water tower 23 is sequentially connected in parallel with the heat exchange medium inlet of the intercooler 6, the high-pressure oil cooler 15, and the low-pressure oil cooler 19 through a pipeline. The main function of the cooling water tower is to reduce the temperature of the cooling water by the contact between water and air and the principle of evaporative heat dissipation.

[0063] In addition, a cooling water circulation pump 24 is also provided on the water inlet pipeline of the cooling water tower 23. The cooling water tower and the cooling water circulation pump play a vital role in the mixed refrigerant two-stage screw refrigeration compressor unit. Together with the intermediate cooler, the high-pressure oil cooler and the low-pressure oil cooler, they constitute the cooling cycle of the refrigeration system.

[0064] As an implementation method, the shaft seal O-rings of the low-pressure screw compressor 2 and the high-pressure screw compressor 8 are made of perfluoroether, which has excellent high temperature resistance and can maintain stable elasticity and resilience in high temperature environments without hardening and embrittlement. In the refrigeration compressor unit, the main function of the shaft seal O-ring is to prevent leakage of refrigerant or lubricating oil and ensure the normal operation of the system. Therefore, the shaft seal O-ring needs to have good sealing performance, high temperature resistance and chemical corrosion resistance.

[0065] The utility model also designs a refrigeration system, including the mixed refrigerant two-stage screw refrigeration compressor unit and the plate-fin cold box as described above;

[0066] The gas outlet of the high-pressure three-stage oil separator 14 is input into the refrigerant circulation pipeline of the plate-fin cold box through a pipeline.

[0067] The mixed refrigerant compressed and transported by the mixed refrigerant two-stage screw refrigeration compressor unit is input into the plate-fin cold box and can fully exchange heat with the passing natural gas, thereby ensuring the production of liquefied natural gas.

[0068] It should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. Unless otherwise clearly specified and limited, the terms "installed", "connected" and "connected" 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 a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the utility model can be understood according to the specific circumstances.

[0069] Although the preferred embodiments of the utility model have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the utility model.

[0070] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A mixed refrigerant two-stage screw refrigeration compressor unit, characterized in that: Including low-pressure screw compressor, high-pressure screw compressor, low-pressure primary oil separator, high-pressure primary oil separator, intercooler, low-pressure oil cooler, high-pressure oil cooler, explosion-proof low-pressure oil pump and explosion-proof high-pressure oil pump; The refrigerant outlet of the low-pressure screw compressor is connected to the low-pressure first-stage oil separator through a pipeline; the oil separation outlet of the low-pressure first-stage oil separator is connected to the low-pressure oil cooler through a pipeline, and the oil outlet of the low-pressure oil cooler is connected to the explosion-proof low-pressure oil pump through a pipeline, and the explosion-proof low-pressure oil pump is input into the cooling and lubrication system of the low-pressure screw compressor through a pipeline; the gas separation outlet of the low-pressure first-stage oil separator is connected to the intercooler through a pipeline; The refrigerant input port of the high-pressure screw compressor is connected to the gas output port of the intermediate cooler; the refrigerant output port of the high-pressure screw compressor is connected to the high-pressure first-stage oil separator through a pipeline, the oil separation port of the high-pressure first-stage oil separator is connected to the high-pressure oil cooler through a pipeline, the oil output port of the high-pressure oil cooler is connected to the explosion-proof high-pressure oil pump through a pipeline, and the explosion-proof high-pressure oil pump inputs the cooling and lubrication system of the high-pressure screw compressor through a pipeline.

2. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: It also includes an explosion-proof low-pressure main motor and an explosion-proof high-pressure main motor; the explosion-proof low-pressure main motor is connected to the power input shaft of the low-pressure screw compressor through a low-pressure coupling; the explosion-proof high-pressure main motor is connected to the power input shaft of the high-pressure screw compressor through a high-pressure coupling.

3. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: The refrigerant inlet pipeline of the low-pressure screw compressor is also provided with a low-pressure suction stop check valve; the refrigerant inlet pipeline of the high-pressure screw compressor is also provided with a high-pressure suction stop check valve.

4. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: The gas separation port of the high-pressure first-stage oil separator is connected in series with the high-pressure second-stage oil separator and the high-pressure third-stage oil separator in sequence through a pipeline.

5. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: The input and output pipelines of the explosion-proof low-pressure oil pump are respectively equipped with a low-pressure oil coarse filter and a low-pressure oil fine filter; the input and output pipelines of the explosion-proof high-pressure oil pump are respectively equipped with a high-pressure oil coarse filter and a high-pressure oil fine filter.

6. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: It also includes a cooling water tower, wherein the water inlet end of the cooling water tower is connected in parallel with the heat exchange medium outlets of the intermediate cooler, the high-pressure oil cooler and the low-pressure oil cooler through pipelines; the discharge end of the cooling water tower is connected in parallel with the heat exchange medium inlet of the intermediate cooler, the high-pressure oil cooler and the low-pressure oil cooler through pipelines.

7. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 6, characterized in that: A cooling water circulation pump is also provided on the water inlet pipeline of the cooling water tower.

8. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 4, characterized in that: The filter elements of the low-pressure first-stage oil separator, the high-pressure first-stage oil separator, the high-pressure second-stage oil separator and the high-pressure third-stage oil separator are all polymer material filter elements.

9. The mixed refrigerant two-stage screw refrigeration compressor unit according to claim 1, characterized in that: The shaft seal O-rings of the low-pressure screw compressor and the high-pressure screw compressor are made of perfluoroether material.

10. A refrigeration system, characterized in that: It comprises a mixed refrigerant two-stage screw refrigeration compressor unit and a plate-fin cold box as claimed in any one of claims 1 to 9; The gas outlet of the high-pressure three-stage oil separator is input into the refrigerant circulation pipeline of the plate-fin cold box through a pipeline.

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