Seven-stage heat exchange ultra-low temperature refrigeration device and refrigeration method thereof

Abstract

The present application relates to the field of refrigeration, and particularly relates to a seven-stage heat exchange ultra-low temperature refrigeration device and a refrigeration method thereof, which comprises a seven-stage refrigeration system in communication with a compressor, wherein the seven-stage refrigeration system is arranged in series from front to back at an inlet end and comprises a first-stage heat exchanger, a first-stage phase separator, a first-stage capillary tube, a second-stage heat exchanger, a second-stage phase separator, a second-stage capillary tube, a third-stage heat exchanger, a third-stage phase separator, a third-stage capillary tube, a fourth-stage heat exchanger, a fourth-stage phase separator, a fourth-stage capillary tube, a fifth-stage heat exchanger, a fifth-stage capillary tube, a sixth-stage heat exchanger, a sixth-stage capillary tube, a seventh-stage heat exchanger and a seventh-stage capillary tube. The present application has the advantage that the seven-stage heat exchange ultra-low temperature refrigeration device can greatly reduce the refrigeration temperature compared with the existing ultra-low temperature refrigeration device, and can not only adsorb water vapor and bromine gas, but also adsorb chlorine gas, ammonia gas and carbon dioxide, thereby completely replacing the role of liquid nitrogen.

Description

Technical Field

[0001] This invention relates to the field of refrigeration, specifically to a seven-stage heat exchange ultra-low temperature refrigeration device and its refrigeration method. Background Technology

[0002] Currently, domestic cryogenic refrigeration equipment can only reach a refrigeration temperature of 140K, which has the following limitations: too few types of adsorbed gases. According to... Figure 2 As indicated, at a temperature of 140K, only water vapor and bromine have good adsorption properties. Therefore, liquid nitrogen must be used to adsorb other types of impurities, such as chlorine, ammonia, and carbon dioxide.

[0003] However, commonly used liquid nitrogen has the following limitations: 1. High cost. Transporting and storing liquid nitrogen in factories is very inconvenient. Factories need large liquid nitrogen tanks for storage. Furthermore, liquid nitrogen is relatively expensive. 2. Liquid nitrogen poses certain safety hazards. Liquid nitrogen leaks can cause asphyxiation.

[0004] There is a product on the market called a cold pump, which cools at around 10K. However, it also has the following limitations: 1. The cold pump has a low load capacity and low pumping speed, requiring it to be used in conjunction with liquid nitrogen and cannot be used alone. 2. The cold pump needs to be regenerated periodically during use. Regeneration interrupts the equipment's operation, affecting production. Furthermore, the regeneration frequency increases with the age of the equipment. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an ultra-low temperature refrigeration device that can achieve rapid and continuous cooling without liquid nitrogen, and the device has a low cooling temperature, low cost and long service life.

[0006] To achieve the above objectives, a seven-stage heat exchange cryogenic refrigeration device is designed, including a compressor and a seven-stage refrigeration system connected to the compressor. The seven-stage refrigeration system is arranged in series from the inlet end, consisting of a first-stage heat exchanger, a first-stage phase separator, a first-stage capillary tube, a second-stage heat exchanger, a second-stage phase separator, a second-stage capillary tube, a third-stage heat exchanger, a third-stage phase separator, a third-stage capillary tube, a fourth-stage heat exchanger, a fourth-stage phase separator, a fourth-stage capillary tube, a fifth-stage heat exchanger, a fifth-stage capillary tube, a sixth-stage heat exchanger, a sixth-stage capillary tube, a seventh-stage heat exchanger, and a seventh-stage capillary tube. Adjacent heat exchangers form a loop through piping, and the seventh-stage capillary tube and the seventh-stage heat exchanger form a loop through piping. The first-stage capillary tube... One end of the capillary tube is connected to the first-stage phase separator, and the other end is connected to the loop between the second and third-stage heat exchangers. One end of the second-stage capillary tube is connected to the second-stage phase separator, and the other end is connected to the loop between the third and fourth-stage heat exchangers. One end of the third-stage capillary tube is connected to the third-stage phase separator, and the other end is connected to the loop between the fourth and fifth-stage heat exchangers. One end of the fourth-stage capillary tube is connected to the fourth-stage phase separator, and the other end is connected to the loop between the fifth and sixth-stage heat exchangers. One end of the fifth-stage capillary tube is connected to the fifth-stage heat exchanger, and the other end is connected to the loop between the sixth and seventh-stage heat exchangers. One end of the sixth-stage capillary tube is connected to the sixth-stage heat exchanger, and the other end is connected to the loop between the seventh-stage heat exchanger and the refrigeration capillary tube.

[0007] The fifth-stage capillary adjusts the evaporation ratio by increasing or decreasing the diameter, length, and number of capillary tubes, so that the liquid refrigerant enters the fifth-stage capillary tube and undergoes phase change and cooling before entering the sixth-stage heat exchanger. The fifth-stage capillary tube has a diameter of 0.45mm to 0.9mm, a number of 2 to 4 tubes, and a length of 0.5m to 2.5m.

[0008] The sixth-stage capillary adjusts the evaporation ratio by increasing or decreasing the diameter, length, and number of capillary tubes, so that the liquid refrigerant enters the sixth-stage capillary tube and undergoes phase change and cooling before entering the seventh-stage heat exchanger. The sixth-stage capillary tube has a diameter of 0.45mm to 0.9mm, a number of 2 to 4 tubes, and a length of 0.5m to 3.5m.

[0009] The present invention also has the following preferred technical solutions:

[0010] The compressor is connected to the seven-stage refrigeration system via a water-cooled heat exchanger and a dryer filter.

[0011] A refrigeration method using the above-mentioned seven-stage heat exchange cryogenic refrigeration equipment is also provided, the method being as follows:

[0012] After being compressed by the compressor, the mixed refrigerant becomes a high-temperature, high-pressure refrigerant. After being cooled by cooling water, it becomes a room-temperature, high-pressure refrigerant. The mixed refrigerant undergoes gas-liquid separation after passing through the first-stage heat exchanger and the first-stage phase separator. The liquid portion enters the first-stage capillary tube, where its pressure drops and it transforms into a gas, cooling down before entering the second-stage heat exchanger circuit. There, it exchanges heat with the gas from the first-stage phase separator, causing the mixed refrigerant to cool down and partially liquefy. This remaining portion then enters the second-stage phase separator for further gas-liquid separation. The liquid portion enters the second-stage capillary tube, where its pressure drops and it transforms into a gas, cooling down before entering the second-stage phase separator circuit. In the three-stage heat exchanger loop, the gaseous portion after the second-stage phase separator exchanges heat with the refrigerant in the third-stage heat exchanger, causing the temperature of the mixed refrigerant to drop and partially liquefy. The remaining mixed refrigerant then enters the third-stage phase separator for gas-liquid separation. The liquid portion enters the third-stage capillary tube, where its pressure drops and it transforms into a gas, cooling down before entering the fourth-stage heat exchanger loop. There, it exchanges heat with the gaseous portion after the third-stage phase separator, causing the temperature of the remaining mixed refrigerant to drop and partially liquefy. The remaining mixed refrigerant then enters the fourth-stage phase separator for gas-liquid separation. The liquid portion enters the fourth-stage capillary tube, where its pressure drops and it transforms into a gas, cooling down before entering the fifth-stage heat exchanger loop. The circuit of the first-stage heat exchanger exchanges heat with the gas portion after the fourth-stage phase separator in the fifth-stage heat exchanger. This causes the temperature of the remaining refrigerant to drop and partially liquefy. The remaining mixed refrigerant, after passing through the fifth-stage heat exchanger, cannot undergo gas-liquid separation due to the refrigerant's inherent limitations. Based on its pressure characteristics (currently 90 psi, pressure after capillary tube is 20 psi), the evaporation ratio is adjusted by increasing or decreasing the diameter, length, and number of capillary tubes. This allows the liquid refrigerant portion to enter the fifth-stage capillary tube for phase change and cooling before entering the sixth-stage heat exchanger. The capillary tubes used have a diameter of 0.45 mm to 0.9 mm, a number of 2 to 4, and a length of... The remaining mixed refrigerant, after passing through the sixth-stage heat exchanger (0.5m to 2.5m), experiences a temperature drop and again cannot undergo gas-liquid separation. Based on its pressure characteristics (currently 70psi, then 20psi after passing through the capillary tube), the evaporation ratio is adjusted by increasing or decreasing the diameter, length, and number of capillary tubes. This allows the liquid refrigerant portion to enter the sixth-stage capillary tube and undergo a phase change and temperature reduction before entering the seventh-stage heat exchanger. The capillary tubes used have a diameter of 0.45mm to 0.9mm, a number of 2 to 4, and a length of 0.5m to 3.5m. After passing through the seventh-stage heat exchanger and the seventh-stage capillary tube, the temperature of the remaining refrigerant drops below 100K.

[0013] Compared with the prior art, the advantages of this invention are:

[0014] The seven-stage heat exchange cryogenic refrigeration equipment of this invention significantly reduces the cooling temperature compared to existing cryogenic refrigeration equipment. After the temperature is reduced to 115K–120K, as… Figure 2 As shown, the equipment can not only adsorb water vapor and bromine, but also chlorine, ammonia and carbon dioxide, completely replacing the role of liquid nitrogen. Especially in the semiconductor etching process, it plays the role of adsorbing impurity gases instead of liquid nitrogen. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the seven-stage heat exchange ultra-low temperature refrigeration equipment of the present invention;

[0016] Figure 2 It is a common impurity gas in vacuum semiconductor equipment (the vapor pressure on the y-axis is the absolute pressure).

[0017] In the diagram: 1. Compressor; 2. Water-cooled heat exchanger; 3. Dryer filter; 4. First-stage heat exchanger; 5. First-stage phase separator; 6. First-stage capillary tube; 7. Second-stage heat exchanger; 8. Second-stage phase separator; 9. Second-stage capillary tube; 10. Third-stage heat exchanger; 11. Third-stage phase separator; 12. Third-stage capillary tube; 13. Fourth-stage heat exchanger; 14. Fourth-stage phase separator; 15. Fourth-stage capillary tube; 16. ... 17. Fifth-stage heat exchanger; 18. Sixth-stage heat exchanger; 19. Sixth-stage capillary tube; 20. Seventh-stage heat exchanger; 21. Refrigeration capillary tube; 22. Seventh-stage capillary tube; 23. Refrigeration solenoid valve; 24. Refrigeration manual valve; 25. Oil separator one; 26. Oil separator two; 27. Defrost solenoid valve; 28. Defrost manual valve; 29. ​​Recirculation manual valve; 30. Buffer solenoid valve; 31. Gas receiver; 32. Safety valve. Detailed Implementation

[0018] The invention will be further described below with reference to the accompanying drawings. The structure and principle of the invention are very clear to those skilled in the art. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0019] like Figure 1 As shown, to achieve the above objectives, the seven-stage heat exchange cryogenic refrigeration equipment of the present invention includes a compressor 1, a high-pressure end bellows, a low-pressure end bellows, a water-cooled exchanger 2, an oil separator 1 25, an oil separator 26, a dryer filter 3, a phase separator, a seven-stage heat exchanger, a capillary tube, a refrigeration solenoid valve 23, a defrost solenoid valve 27, a buffer solenoid valve 30, a refrigeration manual valve 24, a defrost manual valve 28, a circuit manual valve 29, a safety valve 32, a gas storage tank 31, and an automatic control circuit system.

[0020] The seven-stage heat exchange ultra-low temperature refrigeration equipment has a seven-stage refrigeration system, a defrosting system, and a buffer system.

[0021] The seven-stage refrigeration system is arranged in series from the inlet end, consisting of compressor 1, high-pressure bellows, water-cooled exchanger 2, dryer filter 3, first-stage heat exchanger 4, second-stage heat exchanger 7, third-stage heat exchanger 10, fourth-stage heat exchanger 13, fifth-stage heat exchanger 16, sixth-stage heat exchanger 18, seventh-stage heat exchanger 20, refrigeration capillary tube 21, refrigeration solenoid valve 23, refrigeration manual valve 24, return manual valve 29, and low-pressure bellows, connected by copper pipes. One end of the return manual valve 29 is connected to the loop between the fourth-stage heat exchanger 13 and the fifth-stage heat exchanger 16.

[0022] The defrosting system is arranged in series from the inlet end, consisting of compressor 1, high-pressure bellows, oil separator one 25, oil separator two 26, defrosting solenoid valve 27, defrosting manual valve 28, and circuit manual valve 29, connected by copper pipes. A circulation loop is formed between oil separator one 25 and oil separator two 26. One end of oil separator one 25 and oil separator two 26 is connected to the loop between compressor 1 and the first-stage heat exchanger 4.

[0023] The buffer system consists of a third-stage phase separator 11, a buffer solenoid valve 30, a gas storage tank 31, a capillary tube, and the loop pipe of the first-stage heat exchanger 4, arranged in series from the inlet end, connected by copper pipes. One end of the gas storage tank 31 is connected to the loop between the first-stage heat exchanger 4 and the second-stage heat exchanger 7, and the other end is connected to the buffer solenoid valve 30. The other end of the buffer solenoid valve 30 is connected to the third-stage phase separator 11. One end of the safety valve 32 is connected to the loop between the compressor 1 and the first-stage heat exchanger 4.

[0024] The automatic circuit control system includes a main board, water temperature board, defrosting board, 24V reset switch, main switch, circuit breaker, contactor, thermal overload protector, transformer, and pressure control switch.

[0025] The seven-stage refrigeration system is arranged in series from the inlet end, consisting of the first-stage heat exchanger 4, the first-stage phase separator 5, the first-stage capillary tube 6, the second-stage heat exchanger 7, the second-stage phase separator 8, the second-stage capillary tube 9, the third-stage heat exchanger 10, the third-stage phase separator 11, the third-stage capillary tube 12, the fourth-stage heat exchanger 13, the fourth-stage phase separator 14, the fourth-stage capillary tube 15, the fifth-stage heat exchanger 16, the fifth-stage capillary tube 17, the sixth-stage heat exchanger 18, the sixth-stage capillary tube 19, the seventh-stage heat exchanger 20, and the seventh-stage capillary tube 22. Adjacent heat exchangers are connected by pipes to form a loop, and the seventh-stage capillary tube 22 and the seventh-stage heat exchanger 20 are connected by pipes to form a loop.

[0026] The first-stage capillary tube 6 is connected at one end to the first-stage phase separator 5 and at the other end to the loop between the second-stage heat exchanger 7 and the third-stage heat exchanger 10. The second-stage capillary tube 9 is connected at one end to the second-stage phase separator 8 and at the other end to the loop between the third-stage heat exchanger 10 and the fourth-stage heat exchanger 13. The third-stage capillary tube 12 is connected at one end to the third-stage phase separator 11 and at the other end to the loop between the fourth-stage heat exchanger 13 and the fifth-stage heat exchanger 16. The fourth-stage capillary tube 15 is connected at one end to the fourth-stage phase separator 14 and at the other end to the loop between the fifth-stage heat exchanger 16 and the sixth-stage heat exchanger 18. The fifth-stage capillary tube 17 is connected at one end to the fifth-stage heat exchanger 16 and at the other end to the loop between the sixth-stage heat exchanger 18 and the seventh-stage heat exchanger 20. The sixth-stage capillary tube 19 is connected at one end to the sixth-stage heat exchanger 18 and at the other end to the loop between the seventh-stage heat exchanger 20 and the refrigeration capillary tube 21.

[0027] The fifth-stage capillary 17 adjusts the evaporation ratio by increasing or decreasing the diameter, length, and number of capillary tubes, so that the liquid refrigerant enters the fifth-stage capillary 17 and undergoes phase change and cooling before entering the sixth-stage heat exchanger 18. The fifth-stage capillary 17 has a diameter of 0.45 mm to 0.9 mm, a number of 2 to 4 tubes, and a length of 0.5 m to 2.5 m.

[0028] The sixth-stage capillary tube 19 adjusts the evaporation ratio by increasing or decreasing its diameter, length, and number. This allows the liquid refrigerant to partially enter the sixth-stage capillary tube 19, undergo phase change and cooling, and then enter the seventh-stage heat exchanger 20. The sixth-stage capillary tube 19 has a diameter of 0.45mm–0.9mm, consists of 2–4 tubes, and has a length of 0.5m–3.5m. The compressor 1 is connected to the seven-stage refrigeration system via a water-cooled heat exchanger 2 and a dryer filter.

[0029] The specific steps of the refrigeration method using the seven-stage heat exchange cryogenic refrigeration equipment of the present invention are as follows:

[0030] After being compressed by compressor 1, the mixed refrigerant becomes a high-temperature, high-pressure refrigerant. After being cooled by cooling water, it becomes a room-temperature, high-pressure refrigerant (around 300K).

[0031] After passing through the first-stage heat exchanger 4 and the first-stage phase separator 5, the mixed refrigerant undergoes gas-liquid separation. The liquid portion enters the first-stage capillary tube 6, where it experiences pressure drop and phase change to gas, cooling down before entering the circuit of the second-stage heat exchanger 7. There, it exchanges heat with the gas that has passed through the first-stage phase separator 5, causing the temperature of the mixed refrigerant to drop to 230K and partially liquefy.

[0032] Then, this part of the mixed refrigerant enters the second-stage phase separator 8, where it is separated into gas and liquid again. After the liquid enters the second-stage capillary tube 9, its pressure drops and it turns into a gas, which cools down and enters the circuit of the third-stage heat exchanger 10. It exchanges heat with the gas part after passing through the second-stage phase separator 8 in the third-stage heat exchanger 10, which makes the temperature of the mixed refrigerant drop to 200K and partially liquefy.

[0033] The remaining mixed refrigerant then enters the third-stage phase separator 11 for gas-liquid separation. The liquid enters the third-stage capillary tube 12, where its pressure drops and it transforms into a gas, cooling down before entering the circuit of the fourth-stage heat exchanger 13. There, it exchanges heat with the gas portion that has passed through the third-stage phase separator 11, causing the temperature of the remaining mixed refrigerant to drop to 160K and partially liquefy.

[0034] The remaining mixed refrigerant enters the fourth-stage phase separator 14 for gas-liquid separation. The liquid portion enters the fourth-stage capillary tube 15, where its pressure drops and it transforms into a gas, cooling down before entering the circuit of the fifth-stage heat exchanger 16. There, it exchanges heat with the gas portion that has passed through the fourth-stage phase separator 14, causing the temperature of the remaining refrigerant to drop to 135K and partially liquefy.

[0035] After the remaining mixed refrigerant passes through the fifth-stage heat exchanger 16, it cannot undergo gas-liquid separation due to the limitations of the refrigerant itself. Therefore, based on its pressure characteristics (current pressure is 90 psi, and pressure is 20 psi after passing through the capillary tube), we adjust its evaporation ratio by increasing or decreasing the diameter, length, and number of capillary tubes, so that the liquid refrigerant part enters the fifth-stage capillary tube 17 and undergoes phase change cooling before entering the sixth-stage heat exchanger 18.

[0036] The capillary tubes used have a diameter of 0.45mm to 0.9mm, a number of 2 to 4, and a length of 0.5m to 2.5m. The remaining mixed refrigerant, after passing through the sixth-stage heat exchanger 18, cools to 120K and again cannot undergo gas-liquid separation. Based on its pressure characteristics (current pressure 70psi, pressure after passing through the capillary tube 20psi), we adjust its evaporation ratio by increasing or decreasing the diameter, length, and number of capillary tubes. This allows the liquid refrigerant portion to enter the sixth-stage capillary tube 19, undergo phase change and cooling, and then enter the seventh-stage heat exchanger 20.

[0037] The capillary tubes used have a diameter of 0.45mm to 0.9mm, a number of 2 to 4, and a length of 0.5m to 3.5m. The remaining refrigerant, after passing through the seventh-stage heat exchanger 20 and the seventh-stage capillary tube 22, has its temperature reduced to 100K.

[0038] The above are merely specific embodiments of this invention, but the scope of protection of this invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in this invention, based on the technical solutions and novel concepts of this invention, should be covered within the scope of protection of this invention.

Claims

1. A seven-stage heat exchange ultra-low temperature refrigeration apparatus comprising a compressor, characterized in that Also included A seven-stage refrigeration system in communication with the compressor, the seven-stage refrigeration system being arranged in series from front to back from the inlet end with a first-stage heat exchanger, a first-stage phase separator, a first-stage capillary tube, a second-stage heat exchanger, a second-stage phase separator, a second-stage capillary tube, a third-stage heat exchanger, a third-stage phase separator, a third-stage capillary tube, a fourth-stage heat exchanger, a fourth-stage phase separator, a fourth-stage capillary tube, a fifth-stage heat exchanger, a fifth-stage capillary tube, a sixth-stage heat exchanger, a sixth-stage capillary tube, a seventh-stage heat exchanger, and a seventh-stage capillary tube, a loop being formed by piping between adjacent two heat exchangers, a loop being formed by piping between the seventh-stage capillary tube and the seventh-stage heat exchanger, one end of the first-stage capillary tube being connected to the first-stage phase separator and the other end being connected to the loop between the second-stage heat exchanger and the third-stage heat exchanger, one end of the second-stage capillary tube being connected to the second-stage phase separator and the other end being connected to the loop between the third-stage heat exchanger and the fourth-stage heat exchanger, one end of the third-stage capillary tube being connected to the third-stage phase separator and the other end being connected to the loop between the fourth-stage heat exchanger and the fifth-stage heat exchanger, one end of the fourth-stage capillary tube being connected to the fourth-stage phase separator and the other end being connected to the loop between the fifth-stage heat exchanger and the sixth-stage heat exchanger, one end of the fifth-stage capillary tube being connected to the fifth-stage heat exchanger and the other end being connected to the loop between the sixth-stage heat exchanger and the seventh-stage heat exchanger, one end of the sixth-stage capillary tube being connected to the sixth-stage heat exchanger and the other end being connected to the loop between the seventh-stage heat exchanger and the refrigeration capillary tube, The fifth-stage capillary tube adjusts the evaporation ratio by increasing or decreasing the diameter, length, and number of the capillary tubes, so that part of the liquid refrigerant enters the sixth-stage heat exchanger after phase change cooling in the fifth-stage capillary tube, the diameter of the fifth-stage capillary tube being 0.45 mm to 0.9 mm, the number being 2 to 4, and the length being 0.5 m to 2.5 m; The sixth-stage capillary tube adjusts the evaporation ratio by increasing or decreasing the diameter, length, and number of the capillary tubes, so that part of the liquid refrigerant enters the seventh-stage heat exchanger after phase change cooling in the sixth-stage capillary tube, the diameter of the sixth-stage capillary tube being 0.45 mm to 0.9 mm, the number being 2 to 4, and the length being 0.5 m to 3.5 m.

2. A seven-stage heat exchange ultra-low temperature refrigeration device according to claim 1, characterized in that The compressor is connected to the seven-stage refrigeration system through a water-cooled exchanger and a drying filter.

3. A method of refrigeration using the seven-stage heat exchange ultra-low temperature refrigeration apparatus according to claim 1 or 2, characterized by The method is specifically as follows: The mixed refrigerant is compressed by the compressor, and becomes high-temperature and high-pressure refrigerant, which is cooled by cooling water to become normal-temperature and high-pressure refrigerant. The mixed refrigerant is separated into gas and liquid after passing through the first-stage heat exchanger and the first-stage phase separator. The liquid enters the first-stage capillary tube, and is reduced in pressure and changed into gas to be cooled and enter the circuit of the second-stage heat exchanger, and exchanges heat with the gas after the first-stage phase separator in the second-stage heat exchanger, so that the temperature of the mixed refrigerant is reduced and part of the mixed refrigerant is liquefied. Then the mixed refrigerant enters the second-stage phase separator to be separated into gas and liquid again. The liquid enters the second-stage capillary tube, and is reduced in pressure and changed into gas to be cooled and enter the circuit of the third-stage heat exchanger, and exchanges heat with part of the gas after the second-stage phase separator in the third-stage heat exchanger, so that the temperature of the mixed refrigerant is reduced and part of the mixed refrigerant is liquefied. Then the remaining mixed refrigerant enters the third-stage phase separator to be separated into gas and liquid. The liquid enters the third-stage capillary tube, and is reduced in pressure and changed into gas to be cooled and enter the circuit of the fourth-stage heat exchanger, and exchanges heat with part of the gas after the third-stage phase separator in the fourth-stage heat exchanger, so that the temperature of the remaining mixed refrigerant is reduced and part of the remaining mixed refrigerant is liquefied. Then the remaining mixed refrigerant enters the fourth-stage phase separator to be separated into gas and liquid. The liquid enters the fourth-stage capillary tube, and is reduced in pressure and changed into gas to be cooled and enter the circuit of the fifth-stage heat exchanger, and exchanges heat with part of the gas after the fourth-stage phase separator in the fifth-stage heat exchanger, so that the temperature of the remaining refrigerant is reduced and part of the remaining refrigerant is liquefied. The remaining mixed refrigerant cannot be separated into gas and liquid after passing through the fifth-stage heat exchanger due to the limitation of the refrigerant itself. According to the pressure characteristics: the current pressure is 90 psi, and the pressure after passing through the capillary tube is 20 psi, the evaporation ratio is adjusted by increasing or decreasing the diameter, length and number of the capillary tube, so that part of the liquid refrigerant enters the fifth-stage capillary tube and is changed into gas to be cooled and enter the sixth-stage heat exchanger. The diameter of the capillary tube is 0.45 mm to 0.9 mm, the number is 2 to 4, and the length is 0.5 m to 2.5 m. The temperature of the remaining mixed refrigerant is reduced after passing through the sixth-stage heat exchanger, and the remaining mixed refrigerant cannot be separated into gas and liquid either. According to the pressure characteristics: the current pressure is 70 psi, and the pressure after passing through the capillary tube is 20 psi, the evaporation ratio is adjusted by increasing or decreasing the diameter, length and number of the capillary tube, so that part of the liquid refrigerant enters the sixth-stage capillary tube and is changed into gas to be cooled and enter the seventh-stage heat exchanger. The diameter of the capillary tube is 0.45 mm to 0.9 mm, the number is 2 to 4, and the length is 0.5 m to 3.5 m. The temperature of the remaining refrigerant is reduced after passing through the seventh-stage heat exchanger and the seventh-stage capillary tube to be below 100 K.

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