A supercritical Brayton cycle bypass impurity removal system and method

Abstract

A supercritical Brayton cycle bypass impurity removal system and method, the system includes a heat source, a supercritical Brayton cycle system, an impurity removal bypass and a CO2 compressor intermediate stage shunt throttling refrigeration system connected in sequence; the impurity removal system serves the supercritical CO2 power generation system, and is intended to remove gas impurities in the system, improve the CO2 purity in the system, and ensure system performance. The system uses the compressor intermediate stage to pump air and cool itself to form a low-temperature cooling medium, and then uses this part of the medium to cool the low-pressure CO2 medium in the bypass to make it liquid. The liquid CO2 in the bypass shunts and removes impurity gases such as O2 and N2 mixed in the system in the separator. The impurity removal bypass does not need to be operated all the time, and only operates when the impurity content of the system is detected to be too high. The system makes the most of the CO2 system's own equipment, does not need to add complex chemical systems and chemical agents, reduces the investment and operating costs of the impurity removal process, and maintains the purity of the supercritical CO2 power generation system medium.

Description

Technical Field

[0001] The present invention relates to the technical field of power generation systems, and in particular to a supercritical Brayton cycle bypass impurity removal system and method. Background Art

[0002] Under the background of energy shortage and environmental crisis, people pay more and more attention to improving energy utilization. Among many thermodynamic cycles, supercritical Brayton cycle is the most advantageous cycle. New supercritical working fluids (carbon dioxide, helium and nitrous oxide, etc.) have inherent advantages such as high energy density, high heat transfer efficiency and simple system. They can greatly improve the heat-to-work conversion efficiency, reduce the size of equipment and have high economy.

[0003] However, this type of cycle also has an obvious technical difficulty. The supercritical carbon dioxide Brayton cycle uses carbon dioxide as the working gas instead of water. The amount of carbon dioxide used in the circulation system is large, and the purity requirement is high. Generally, industrial-grade carbon dioxide is used, which requires a certain initial investment. However, it is difficult for large-scale industrial-grade systems to maintain the high purity of carbon dioxide in the system for a long time without mixing or residual other impurity gases. Therefore, it is necessary to purify the carbon dioxide in the system for a long time to ensure its purity, thereby ensuring system performance. However, since the CO2 in the system is in a supercritical state and has no phase change, it is difficult to separate it from N2 and O2. If chemical reactions are used to remove N2 and O2, it is more complicated. It is not easy to ensure that other chemical elements are not mixed in. In addition, N2 is an inert gas, and it is not easy to separate it by chemical methods.

[0004] In the traditional steam Rankine cycle, there are devices similar to deaerators that can remove oxygen from the system, but the premise is that there is liquid water in the steam Rankine cycle, and the gas can be separated from the liquid water. The supercritical CO2 system itself has no liquid and the working fluid is all supercritical. Summary of the invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a supercritical Brayton cycle bypass impurity removal system and method, which utilizes the system's own working fluid characteristics and maximizes the use of the system's own equipment to remove N2 and O2 impurities, and adopts a method with relatively low technical difficulty and high feasibility to improve the thermal efficiency of the system.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A supercritical Brayton cycle bypass impurity removal system, comprising a turbine 2, the inlet of the turbine 2 is connected to the outlet of the working medium side of the heat source 1, the outlet of the turbine 2 is connected to the inlet of the heat release side of the high-temperature regenerator 3, the outlet of the heat release side of the high-temperature regenerator 3 is connected to the inlet of the heat release side of the low-temperature regenerator 4, the outlet of the heat release side of the low-temperature regenerator 4 is split into two paths, one of which is connected to the inlet of the recompressor 5, and the outlet of the recompressor 5 is connected to the outlet of the heat absorption side of the high-temperature regenerator 3 after merging with the working medium of the outlet of the heat absorption side of the low-temperature regenerator 4. The inlet of the low-temperature regenerator 4 is connected, and another path branched out from the outlet of the heat release side of the low-temperature regenerator 4 is connected to the inlet of the working medium side of the precooler 6. The outlet of the working medium side of the precooler 6 is divided into two paths. The bypass is connected to the inlet of the bypass throttling and pressure reducing device 7. The outlet of the bypass throttling and pressure reducing device 7 is connected to the hot side inlet of the bypass cooler 8. The hot side outlet of the bypass cooler 8 is connected to the upper inlet of the gas-liquid separation tower 9. The lower outlet of the gas-liquid separation tower 9 is connected to the inlet of the booster pump 10. The outlet of the booster pump 10 is connected to the filter The bypass side inlet of the bypass mixer 11 is connected, the working fluid side outlet mainstream of the precooler 6 is connected to the main flow inlet of the filter bypass mixer 11, the outlet of the filter bypass mixer 11 is connected to the main road inlet of the cooling bypass mixer 12, the outlet of the cooling bypass mixer 12 is connected to the inlet of the main compressor 13, the intermediate stage exhaust bypass outlet of the main compressor 13 is connected to the working fluid side inlet of the cooling bypass cooler 14, and the working fluid side outlet of the cooling bypass cooler 14 is connected to the cooling bypass throttling The inlet of the pressure reducer 15 is connected, the outlet of the cooling bypass throttling pressure reducer 15 is connected to the cold side inlet of the bypass cooler 8, the cold side outlet of the bypass cooler 8 is connected to the bypass side inlet of the cooling bypass mixer 12, the final main outlet of the main compressor 13 is connected to the heat absorption side inlet of the low-temperature regenerator 4, the outlet working fluid of the heat absorption side of the low-temperature regenerator 4 is connected to the heat absorption side inlet of the high-temperature regenerator 3 after merging with the outlet working fluid of the compressor, and the heat absorption side outlet of the high-temperature regenerator 3 is connected to the inlet of the heat source 1.

[0008] A liquid outlet is arranged at the lower part of the gas-liquid separation tower 9, and an exhaust port is arranged at the upper part of the gas-liquid separation tower 9.

[0009] The heat source 1 is a boiler, a waste heat exchanger or solar energy.

[0010] A method for removing impurities by bypassing a supercritical Brayton cycle comprises the following steps;

[0011] The impurity removal system is designed to maintain the purity of CO2 in the system. It does not need to be turned on all the time, but only needs to be turned on when the content of impurity gases such as O2 and N2 in the system exceeds the standard.

[0012] When the purity of CO2 in the system is sufficient, the supercritical CO2 in the system is first heated by the heat source 1, and then the supercritical working fluid enters the turbine system 2 to do work. The high-temperature and low-pressure exhaust steam after doing work enters the high-temperature regenerator 3 and the heat release side of the low-temperature regenerator 4 in turn to release heat. The low-temperature and low-pressure working fluid after releasing heat is divided into two paths, one path enters the precooler 6 working fluid side to be cooled, and the other path enters the recompressor 5. The high-pressure working fluid after being pressurized enters the high-temperature regenerator 3 heat absorption side to absorb heat. After being cooled by the precooler 6, all the working fluids pass through the filtering bypass mixer 11 and the cooling bypass mixer 12 in turn, and then enter the main compressor 13. After being compressed, it flows out from the last stage of the main compressor, enters the heat absorption side of the low-temperature regenerator 4 to absorb heat, and then merges with the working fluid compressed by the recompressor 5, and then enters the heat absorption side of the high-temperature regenerator 3. After absorbing heat in the high-temperature regenerator 3, it enters the heat source 1 to be heated, completing the entire working fluid circulation process;

[0013] When the content of impurity gases such as O2 and N2 in the system exceeds the standard, it is necessary to open the impurity removal bypass and the shunt throttling cooling bypass. The difference from normal operation is that after the CO2 working medium flows out of the precooler 6, a part of the working medium is shunted into the impurity removal bypass. In the bypass, the working medium first passes into the bypass throttling reducer 7 for pressure reduction and throttling. The pressure is reduced to below the critical point of CO2, and the working medium is in a subcritical state. Then it enters the bypass cooler 8. The subcritical CO2 working medium is further cooled to the saturation temperature or a liquid state slightly lower than the saturation temperature, while the impurity gases N2 and O2 remain in a gaseous state due to their lower liquefaction temperature. Then the liquid CO2 working medium and the impurity gases enter the gas-liquid separation tower 9. In the gas-liquid separation tower 9, the liquid CO2 flows out from the liquid outlet at the bottom, while the gaseous impurities such as N2 and O2 are discharged from the exhaust port at the top of the separation tower 9. After impurity removal The pure subcritical CO2 liquid working medium then flows into the booster pump 10, and after the pressure is increased to the supercritical state, it enters the filtering bypass mixer 11 to mix with the mainstream from the precooler 6, and then passes through the cooling bypass mixer 12 to enter the main compressor 13. At this time, the intermediate stage exhaust bypass of the main compressor 13 is opened, and a part of the working medium compressed to a certain pressure flows into the shunt throttling cooling bypass from the exhaust outlet, first enters the cooling bypass cooler 14 for preliminary cooling, and then enters the cooling bypass throttling reducer 15. After throttling, it is further reduced to a low temperature lower than the CO2 liquefaction temperature in the previous impurity removal bypass, and then passes into the low temperature side of the bypass cooler 8 to cool the impurity removal bypass working medium, and then the cooling bypass working medium inlet cooling bypass mixer 12 is mixed with the mainstream and enters the main compressor 13. The other operating processes of the system are the same as those during normal operation.

[0014] Beneficial effects of the present invention:

[0015] The present invention extracts a part of the working fluid from the point closest to the critical point in the supercritical CO2 system, first reduces its pressure to a subcritical state close to the critical pressure, then cools it down and liquefies it, and then diverts the gas impurities in the working fluid. The cooling medium used to cool this part of the working fluid is also CO2 in the system. This part of CO2 comes from the middle-stage exhaust of the compressor in the system. This part of the exhaust is first cooled down and pre-cooled, and then throttled to further cool it. This part of CO2 can be cooled down to a temperature lower than the required CO2 liquefaction temperature while maintaining the supercritical state, so the CO2 bypassed by impurities can be cooled down and liquefied.

[0016] The present invention can effectively remove N2 and O2 in the system, maintain the purity of the system and ensure the performance of the system, while making maximum use of the system's own equipment, minimizing energy consumption and not adding any additional chemical agents. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the structure of the system of the present invention.

[0018] Among them, heat source 1, turbine 2, high temperature regenerator 3, low temperature regenerator 4, recompressor 5, precooler 6, bypass throttling pressure reducer 7, bypass cooler 8, gas-liquid separation tower 9, booster pump 10, filter bypass mixer 11, cooling bypass mixer 12, main compressor 13, cooling bypass cooler 14, cooling bypass throttling pressure reducer 15 DETAILED DESCRIPTION

[0019] The present invention is further described in detail below in conjunction with embodiments.

[0020] like Figure 1As shown: a supercritical Brayton cycle bypass impurity removal system, comprising a turbine 2, the inlet of the turbine 2 is connected to the outlet of the working medium side of the heat source 1, the outlet of the turbine 2 is connected to the inlet of the heat release side of the high-temperature regenerator 3, the outlet of the heat release side of the high-temperature regenerator 3 is connected to the inlet of the heat release side of the low-temperature regenerator 4, the outlet of the heat release side of the low-temperature regenerator 4 is split into two paths, one of which is connected to the inlet of the recompressor 5, and the outlet of the recompressor 5 is connected to the outlet of the heat absorption side of the low-temperature regenerator 4 after merging with the high-temperature regenerator 3. The hot side inlet is connected, and the other path branched off from the heat release side outlet of the low-temperature regenerator 4 is connected to the working medium side inlet of the precooler 6. The working medium side outlet of the precooler 6 is divided into two paths. The bypass is connected to the inlet of the bypass throttling pressure reducer 7. The outlet of the bypass throttling pressure reducer 7 is connected to the hot side inlet of the bypass cooler 8. The hot side outlet of the bypass cooler 8 is connected to the upper inlet of the gas-liquid separation tower 9. The lower outlet of the gas-liquid separation tower 9 is connected to the inlet of the booster pump 10. The outlet of the booster pump 10 is connected to The bypass side inlet of the filter bypass mixer 11 is connected, the working fluid side outlet mainstream of the precooler 6 is connected to the main flow inlet of the filter bypass mixer 11, the outlet of the filter bypass mixer 11 is connected to the main road inlet of the cooling bypass mixer 12, the outlet of the cooling bypass mixer 12 is connected to the inlet of the main compressor 13, the intermediate stage exhaust bypass outlet of the main compressor 13 is connected to the working fluid side inlet of the cooling bypass cooler 14, and the working fluid side outlet of the cooling bypass cooler 14 is connected to the cooling bypass section The inlet of the flow reducer 15 is connected, the outlet of the cooling bypass throttling reducer 15 is connected to the cold side inlet of the bypass cooler 8, the cold side outlet of the bypass cooler 8 is connected to the bypass side inlet of the cooling bypass mixer 12, the final main outlet of the main compressor 13 is connected to the heat absorption side inlet of the low-temperature regenerator 4, the outlet working fluid of the heat absorption side of the low-temperature regenerator 4 is connected to the heat absorption side inlet of the high-temperature regenerator 3 after merging with the working fluid at the compressor outlet, and the heat absorption side outlet of the high-temperature regenerator 3 is connected to the inlet of the heat source 1.

[0021] A liquid outlet is arranged at the lower part of the gas-liquid separation tower 9, and an exhaust port is arranged at the upper part of the gas-liquid separation tower 9.

[0022] The heat source 1 is a boiler, a waste heat exchanger or solar energy.

[0023] The supercritical Brayton cycle body includes: heat source 1, turbine 2, high temperature regenerator 3, low temperature regenerator 4, recompressor 5, precooler 6, main compressor 13; the intermediate stage shunt throttling refrigeration system includes: bypass cooler 8, cooling bypass mixer 12, cooling bypass cooler 14, cooling bypass throttling pressure reducer 15; impurity removal bypass includes: bypass throttling pressure reducer 7, bypass cooler 8, gas-liquid separation tower 9, booster pump 10, filtering bypass mixer 11. The refrigerant of the intermediate stage shunt throttling refrigeration system is the CO2 refrigerant in the system, and the compressor is the main compressor 13 of the supercritical Brayton cycle system, and no other refrigerants and compressors are needed. The bypass cooler 8 is a common device for the intermediate stage shunt throttling refrigeration system and the impurity removal bypass.

[0024] The specific implementation process is as follows: when the purity of CO2 in the system is sufficient, the supercritical CO2 in the system is first heated by the heat source 1, and then the supercritical working fluid enters the turbine system 2 to perform work. The high-temperature and low-pressure exhaust steam after the work enters the high-temperature regenerator 3 and the low-temperature regenerator 4 in turn to release heat on the heat release side. After releasing heat, the low-temperature and low-pressure working fluid is divided into two paths, one path enters the precooler 6 working fluid side to be cooled, and the other path enters the re-compressor 5. The pressurized high-pressure working fluid enters the high-temperature regenerator 3 heat absorption side to absorb heat, and then enters the low-temperature regenerator 4 to release heat. The second working medium branched from the outlet of the heat release side directly enters the precooler 6. After being cooled, all the working medium passes through the filter bypass mixer 11 and the cooling bypass mixer 12 in sequence, and then enters the main compressor 13. After being compressed, it flows out from the final stage of the main compressor, enters the heat absorption side of the low-temperature regenerator 4 to absorb heat, and then merges with the first working medium compressed by the recompressor, and then enters the heat absorption side of the high-temperature regenerator 3. After absorbing heat in the high-temperature regenerator 3, it enters the heat source 1 to be heated, completing the entire working medium circulation process;

[0025] When the content of impurity gases such as O2 and N2 in the system exceeds the standard, it is necessary to open the impurity removal bypass and the shunt throttling cooling bypass. The difference from normal operation is that after the CO2 working medium flows out of the precooler 6, a part of the working medium is shunted into the impurity removal bypass. In the bypass, the working medium first passes into the bypass throttling reducer 7 for pressure reduction and throttling. The pressure is reduced to below the critical point of CO2, and the working medium is in a subcritical state. Then it enters the bypass cooler 8. The subcritical CO2 working medium is further cooled to the saturation temperature or a liquid state slightly lower than the saturation temperature, while the impurity gases N2 and O2 remain in a gaseous state due to their lower liquefaction temperature. Then the liquid CO2 working medium and the impurity gases enter the gas-liquid separation tower 9. In the gas-liquid separation tower 9, the liquid CO2 flows out from the liquid outlet at the bottom, while the gaseous impurities such as N2 and O2 are discharged from the exhaust port at the top of the separation tower 9. After impurity removal The pure subcritical CO2 liquid working medium then flows into the booster pump 10, and after the pressure is increased to the supercritical state, it enters the filtering bypass mixer 11 to mix with the mainstream from the precooler 6, and then passes through the cooling bypass mixer 12 to enter the main compressor 13. At this time, the intermediate stage exhaust bypass of the main compressor 13 is opened, and a part of the working medium compressed to a certain pressure flows into the shunt throttling cooling bypass from the exhaust outlet, first enters the cooling bypass cooler 14 for preliminary cooling, and then enters the cooling bypass throttling reducer 15. After throttling, it is further reduced to a low temperature lower than the CO2 liquefaction temperature in the previous impurity removal bypass, and then passes into the low temperature side of the bypass cooler 8 to cool the impurity removal bypass working medium, and then the cooling bypass working medium inlet cooling bypass mixer 12 is mixed with the mainstream and enters the main compressor 13. The other operating processes of the system are the same as those during normal operation.

[0026] but Figure 1 Other layouts of the supercritical Brayton cycle system shown do not affect the application of the present invention, and the content of the present invention is also applicable to other layouts of the supercritical cycle system. Therefore, the supercritical Brayton cycle system in the present invention is a supercritical Brayton cycle system in a broad sense, and is not limited to the layout shown in the figure. For example, other supercritical Brayton cycle systems may use a multi-stage turbine system, or a turbine system with reheat, or may not use a split recompression system, that is, only one main compressor is used without a recompressor in the figure, and the two regenerators in the figure are combined into one regenerator, and so on.

[0027] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A supercritical Brayton cycle bypass impurity removal system, characterized in that: The invention comprises a turbine (2), wherein the inlet of the turbine (2) is connected to the outlet of the working medium side of the heat source (1), the outlet of the turbine (2) is connected to the inlet of the heat release side of the high-temperature heat regenerator (3), the outlet of the heat release side of the high-temperature heat regenerator (3) is connected to the inlet of the heat release side of the low-temperature heat regenerator (4), the outlet of the heat release side of the low-temperature heat regenerator (4) is split into two paths, one of which is connected to the inlet of the recompressor (5), the outlet of the recompressor (5) is connected to the inlet of the heat absorption side of the high-temperature heat regenerator (3) after the working medium is merged with the outlet of the heat absorption side of the low-temperature heat regenerator (4), and the outlet of the low-temperature heat regenerator (4) is connected to the inlet of the heat absorption side of the high-temperature heat regenerator (3), and the outlet of the low-temperature heat regenerator (4) is connected to the inlet of the heat absorption side of the low-temperature heat regenerator (3). Another path branched off from the heat release side outlet of the precooler (4) is connected to the working medium side inlet of the precooler (6), the working medium side outlet of the precooler (6) is divided into two bypass paths connected to the inlet of the bypass throttling pressure reducer (7), the outlet of the bypass throttling pressure reducer (7) is connected to the hot side inlet of the bypass cooler (8), the hot side outlet of the bypass cooler (8) is connected to the upper inlet of the gas-liquid separation tower (9), the lower outlet of the gas-liquid separation tower (9) is connected to the inlet of the booster pump (10), and the outlet of the booster pump (10) is connected to the filter bypass mixer The bypass side inlet of the precooler (6) is connected to the bypass side inlet of the precooler (6), the working fluid side outlet mainstream is connected to the main flow inlet of the filter bypass mixer (11), the outlet of the filter bypass mixer (11) is connected to the main flow inlet of the cooling bypass mixer (12), the outlet of the cooling bypass mixer (12) is connected to the inlet of the main compressor (13), the intermediate stage exhaust bypass outlet of the main compressor is connected to the working fluid side inlet of the cooling bypass cooler (14), the working fluid side outlet of the cooling bypass cooler (14) is connected to the cooling bypass throttling pressure reducer (15 ), the outlet of the cooling bypass throttling depressurizer (15) is connected to the cold side inlet of the bypass cooler (8), the cold side outlet of the bypass cooler (8) is connected to the bypass side inlet of the cooling bypass mixer (12), the final main outlet of the main compressor (13) is connected to the heat absorption side inlet of the low-temperature regenerator (4), the outlet working fluid of the heat absorption side of the low-temperature regenerator (4) is connected to the heat absorption side inlet of the high-temperature regenerator (3) after merging with the outlet working fluid of the compressor, and the heat absorption side outlet of the high-temperature regenerator (3) is connected to the inlet of the heat source (1); The gas-liquid separation tower (9) is provided with a liquid outlet at the lower part, and an exhaust port is provided at the upper part of the gas-liquid separation tower (9); The heat source (1) is a boiler, a waste heat exchanger or solar energy.

2. The impurity removal method of a supercritical Brayton cycle bypass impurity removal system according to claim 1, characterized in that: The steps include: The impurity removal system is designed to maintain the purity of CO2 in the system. It does not need to be turned on all the time, but only needs to be turned on when the content of impurity gases such as O2 and N2 in the system exceeds the standard. When the purity of CO2 in the system is sufficient, the supercritical CO2 in the system is first heated by the heat source (1), and then the supercritical working fluid enters the turbine system (2) to perform work. After the work is performed, the high-temperature low-pressure exhaust steam enters the high-temperature regenerator (3) and the low-temperature regenerator (4) to release heat in turn. After releasing heat, the low-temperature low-pressure working fluid is divided into two paths, one of which enters the precooler (6) to be cooled on the working fluid side, and the other enters the recompressor (5). The pressurized high-pressure working fluid enters the high-temperature regenerator (3) to absorb heat on the heat absorption side and is discharged from the low-temperature regenerator (4) to release heat. The second working fluid diverted from the outlet directly enters the precooler (6). After being cooled, all the working fluid passes through the filtering bypass mixer (11) and the cooling bypass mixer (12) in sequence, and then enters the main compressor (13). After being compressed, it flows out from the final stage of the main compressor, enters the heat absorption side of the low-temperature regenerator (4) to absorb heat, and then merges with the first working fluid compressed by the recompressor, and then enters the heat absorption side of the high-temperature regenerator (3). After absorbing heat in the high-temperature regenerator (3), it enters the heat source (1) to be heated, thereby completing the entire working fluid circulation process; When the content of impurity gases such as O2 and N2 in the system exceeds the standard, it is necessary to start the impurity removal bypass and the shunt throttling cooling bypass. The difference from normal operation is that after the CO2 working medium flows out of the precooler (6), a part of the working medium is shunted into the impurity removal bypass. In the bypass, the working medium first passes into the bypass throttling reducer (7) for throttling and pressure reduction. The pressure is reduced to below the critical point of CO2, and the working medium is in a subcritical state. Then it enters the bypass cooler (8). The subcritical CO2 working medium is further cooled to a liquid state at or slightly lower than the saturation temperature, while the impurity gases N2 and O2 remain in a gaseous state due to their lower liquefaction temperature. Then the liquid CO2 working medium and the impurity gases enter the gas-liquid separation tower (9). In the gas-liquid separation tower (9), the liquid CO2 flows out from the liquid outlet at the bottom, while the gaseous impurities such as N2 and O2 are discharged from the exhaust port at the top of the separation tower (9). The pure subcritical CO2 after impurity removal is The supercritical CO2 liquid working medium then flows into the booster pump (10), where the pressure is increased to a supercritical state and then enters the filter bypass mixer (11) to mix with the mainstream from the precooler (6), and then passes through the cooling bypass mixer (12) to enter the main compressor (13). At this time, the intermediate exhaust bypass of the main compressor (13) is opened, and a part of the working medium compressed to a certain pressure flows into the shunt throttling cooling bypass from the exhaust outlet, first enters the cooling bypass cooler (14) to be initially cooled, and then enters the cooling bypass throttling reducer (15). After throttling, it is further reduced to a low temperature lower than the liquefaction temperature of CO2 in the previous impurity removal bypass, and then enters the low temperature side of the bypass cooler (8) to cool the impurity removal bypass working medium. Subsequently, the cooling bypass working medium enters the cooling bypass mixer (12) to mix with the mainstream and then enters the main compressor (13). The other operating processes of the system are the same as those during normal operation.

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