Two-stage compression liquid adding system and liquid adding method based on R744

Abstract

The present invention belongs to the field of R744 refrigeration technology, and specifically relates to a two-stage compression liquid adding system and liquid adding method based on R744. The system of the present invention includes a liquid storage tank, the inlet end of the liquid storage tank is connected to the pressurized pipe section, the outlet end of the liquid storage tank is connected to the filling pipe section, the outlet of the filling pipe section is connected to the filling equipment, and the liquid adding system also includes a return pipe section with a seventh switch valve V7, the inlet of the return pipe section is connected to the inlet of the sixth switch valve V6, and the outlet of the return pipe section is connected to the liquid storage tank. The present invention pressurizes and liquefies R744 online by coupling two-stage compression with multi-stage cooling, and reduces the high-pressure gas pipeline by artificially dividing the high-pressure gas phase zone and the high-pressure liquid phase zone, which can greatly reduce the investment cost, and the risk of use and maintenance is significantly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of R744 refrigeration, and in particular relates to a two-stage compression liquid adding system and a liquid adding method based on R744. Background Art

[0002] R744 is carbon dioxide, which is colorless, odorless, non-toxic, non-flammable, non-explosive, and has excellent thermal properties. It is an environmentally friendly substance second only to water and air in the industrial field. Under the same application conditions, R744 has the highest operating pressure compared to conventional refrigerants. For example, in an automobile air-conditioning compressor system, if the refrigerant R134a is used, the common operating conditions are -1℃ evaporation and 63℃ condensation, and the corresponding evaporation pressure is 2.82bar and the condensation pressure is 18.04bar; if the refrigerant R410a is used, the common operating conditions are -1℃ evaporation and 63℃ condensation, and the corresponding evaporation pressure is 7.73bar and the condensation pressure is 40.94bar. Similarly, under common operating conditions, the refrigerant R744 evaporates at -1℃ and condenses at 63℃, with an evaporation pressure of 33.9bar and a condensation pressure exceeding 120bar.

[0003] It can be seen that the filling equipment used for R744 has very high pressure resistance requirements for all components and pipelines; especially in traditional filling equipment, multiple tanks are required to pack and store R744, and each tank must withstand extremely high working pressure. The filling pipe section directly connected to each tank is also always under extremely high working pressure. As the position of the filled equipment changes, the filling pipe section needs to travel a longer distance, resulting in a long high-pressure gas pipeline and high investment cost. At the same time, since these filling pipe sections and the components on them will inevitably pass through some spaces with high ambient temperatures in the venue, the R744 in the pipe will be passively heated, causing the pressure to further increase. For example, the R744 filling equipment placed in a factory area, the filling pipe section passes by a running air compressor, and the local cylinder temperature of the air compressor may reach 100°C. At this time, if the filling pipe section is heated, the pressure in the already high-pressure filling pipe section will be further increased, even to about 150bar, resulting in the risk of cracking or even explosion of the filling pipe section, which needs to be solved urgently. Summary of the invention

[0004] One of the purposes of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a two-stage compression liquid adding system based on R744, which pressurizes and liquefies R744 online by coupling two-stage compression with multi-stage cooling, and reduces the high-pressure gas pipeline by artificially dividing the high-pressure gas phase area and the high-pressure liquid phase area, which can greatly reduce the investment cost, and the risks of use and maintenance are significantly reduced. Another purpose of the present invention is to provide a liquid adding method for a two-stage compression liquid adding system based on R744, so as to further ensure the simplicity and efficiency of the operation process.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A two-stage compression liquid adding system based on R744, characterized in that it comprises a liquid storage tank, the inlet end of the liquid storage tank is connected to the pressurized pipe section, the outlet end of the liquid storage tank is connected to the filling pipe section, and the outlet of the filling pipe section is connected to the equipment to be filled, wherein:

[0007] The pressurized pipe section includes a gas storage tank, a first switch valve V1, a cooler, a first compressor, a first air cooler, a buffer tank, a second compressor, a second air cooler and a second switch valve V2 arranged in sequence along the gas flow direction;

[0008] The filling pipe section includes a third switch valve V3, a fourth switch valve V4, a flow pump, a flow regulating valve LV, a flow meter L1 and a sixth switch valve V6 which are arranged in sequence along the liquid flow direction; the filling pipe section also includes a bypass pipeline provided with a fifth switch valve V5, and the fourth switch valve V4 and the flow pump are combined with the bypass pipeline to form a parallel pipeline; a temperature and pressure measuring point for pressure and temperature measurement functions is set on a section of the filling pipe section between the flow meter L1 and the sixth switch valve V6;

[0009] The liquid adding system further comprises a return pipe section with a seventh switch valve V7, the inlet of the return pipe section is connected to the inlet of the sixth switch valve V6, and the outlet of the return pipe section is connected to the liquid storage tank.

[0010] Preferably, the liquid adding system also includes an evacuation pipe section with a vacuum pump, and an eighth switch valve V8 and a ninth switch valve V9 are respectively arranged at both ends of the evacuation pipe section, the end where the eighth switch valve V8 is located is connected to the outlet of the sixth switch valve V6, and the end where the ninth switch valve V9 is located is connected to a filling pipe section between the third switch valve V3 and the fourth switch valve V4.

[0011] Preferably, the liquid adding system also includes a heat exchanger, which is located on a filling pipe section between the parallel pipe and the flow regulating valve LV; the heat exchanger, the cooler, the first air cooler and the second air cooler are all connected to the constant temperature water tank through the cooling water pipe; a water pump is correspondingly arranged on each cooling water pipe, and the cooling water pipe where the cooler, the first air cooler and the second air cooler are located is also correspondingly provided with an eleventh switch valve V11, a twelfth switch valve V12 and a thirteenth switch valve V13, and a water temperature regulator is arranged on the cooling water pipe where the heat exchanger is located.

[0012] Preferably, the liquid adding system further comprises a cooling unit, and the cooling unit is connected to the heat exchange coil of the liquid storage tank via a pressure pump and a tenth switch valve V10.

[0013] Preferably, a liquid level meter, a safety valve and a camera for video monitoring of the liquid in the tank are provided at the liquid storage tank.

[0014] Preferably, the flow pump is a plunger pump.

[0015] Preferably, a one-way valve CV is also provided on the return pipe section.

[0016] Preferably, a liquid adding method using the two-stage compression liquid adding system based on R744 is characterized by comprising the following steps:

[0017] 1) Pressurization process

[0018] The pressurizing pipe section is opened, and the gaseous R744 in the gas storage tank first passes through the first switch valve V1 and the cooler and enters the first compressor for pressurization. The first compressor pressurizes the R744 to the intermediate pressure; then, the R744 enters the first air cooler for cooling, and the cooled R744 enters the buffer tank and then the second compressor. After cooling through the second air cooler at the outlet of the second compressor, the R744 continues to enter the liquid storage tank through the second switch valve V2 to complete the pressurization process;

[0019] 2) Vacuuming process:

[0020] Connect the equipment to be charged to the outlet of the sixth switch valve V6, open the eighth switch valve V8 and the vacuum pump, and evacuate the equipment to be charged through the evacuation pipe section. After reaching the set vacuum requirement, close the eighth switch valve V8 and the vacuum pump, and open the sixth switch valve V6 to complete the vacuuming process; this process is carried out simultaneously or sequentially with the pressurization process;

[0021] 3) Filling process

[0022] a) Open the filling pipe section, and the liquid 744 passes through the third switch valve V3, the fourth switch valve V4, the flow pump, and the heat exchanger in sequence to reach the flow regulating valve. The flow regulating valve LV adjusts the flow of R744, and then the flow value is obtained by the flow meter L1. After the final temperature and pressure values ​​are obtained at the temperature and pressure measuring point, the liquid enters the equipment to be filled through the sixth switch valve V6 to complete the filling process;

[0023] b) When the final temperature and pressure values ​​obtained at the temperature and pressure measuring point do not meet the requirements of the equipment to be charged, the sixth switch valve V6 is closed, the seventh switch valve V7 is opened, and the liquid 744 returns to the liquid storage tank from the return pipe section until the final temperature and pressure values ​​measured at the temperature and pressure measuring point meet the temperature and pressure requirements of the equipment to be charged for R744. At this time, the seventh switch valve V7 is closed, the sixth switch valve V6 is reopened, and then the next step is entered;

[0024] c) When the final pressure value measured at the temperature and pressure measuring point meets the temperature and pressure requirements of the equipment to be charged with R744, the fourth switch valve V4 is closed, the fifth switch valve V5 is opened, and then step a) is performed.

[0025] The beneficial effects of the present invention are:

[0026] 1) The present invention provides a safe and reliable R744 liquid adding system and liquid adding method, which solves the problem of excessive pressure in the liquid storage tank and the entire system when the ambient temperature of R744 is high through coupling of two-stage compression and multi-stage cooling.

[0027] 2) In the pressurized pipe section, the present invention adopts a two-stage compression method, so that each compressor has better operating conditions and a low pressure ratio. The entire system has lower noise, better overall life and stability. The low-pressure stage has relatively broad requirements for the state of suction, and has lower requirements for the oil return equipment, which reduces the cost of some oil circuit equipment. At the same time, the two-stage compression method is combined with the combined structure of the flow pump at the rear section, so that the power of the flow pump can be lower, the extreme working conditions are less, the extreme flow is smaller, and the initial investment cost and maintenance cost can be significantly reduced. At the same time, the present invention pressurizes and liquefies R744 online by coupling two-stage compression with multi-stage cooling, and relies on artificial division of high-pressure gas phase zone and high-pressure liquid phase zone to reduce the high-pressure gas pipeline. The caliber of the high-pressure gas pipeline is usually several times that of the high-pressure liquid pipeline, so reducing the high-pressure gas pipeline can greatly reduce the comprehensive cost of pipelines, construction, and maintenance, and the risk of use and maintenance is significantly reduced.

[0028] 3) In fact, if the pressure and temperature range of the refrigerant R744 that needs to be charged to the equipment to be charged is required to be more than four times, and the equipment use environment is sensitive to noise, and the maintenance frequency of the oil circuit is required to be low, it is recommended to select the above technical solution described in the present invention. According to the above solution of the present invention, the working conditions of each compressor are better, the requirements for the oil return equipment are low, and the investment cost is low; the equipment of the present invention can bring a wider temperature and pressure application range to the equipment to be charged, and the effect is remarkable. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the piping arrangement diagram of the present invention.

[0030] The actual correspondence between the reference numerals and component names of the present invention is as follows:

[0031] a-Equipment to be filled;

[0032] 10-liquid storage tank; 11-heat exchange coil; 12-liquid level gauge; 13-safety valve; 14-camera;

[0033] 20-pressurized pipe section; 21-gas storage tank; 22-cooler; 23-first compressor;

[0034] 24-first air cooler; 25-buffer tank; 26-second compressor; 27-second air cooler;

[0035] 30-charging pipe section; 31-flow pump; 32-heat exchanger; 40-return pipe section;

[0036] 50-evacuation pipe section; 51-vacuum pump; 60-constant temperature water tank; 61-water temperature regulator;

[0037] 70-Cooling unit; 71-Pressure pump DETAILED DESCRIPTION

[0038] For ease of understanding, here we combine Figure 1 The specific structure and working mode of the present invention are further described as follows:

[0039] The specific pipeline layout of the present invention is as follows Figure 1 As shown, in actual operation, the gaseous R744 in the gas storage tank 21 first enters the first compressor 23 through the first switch valve V1 and the pipeline of the cooler 22 for pressurization. The first compressor 23 pressurizes the R744 gas from 10 bar, 20 ° C to 40 bar, 90 ° C to reach the intermediate pressure. Then R744 enters the first air cooler 24 for cooling, and the cooled refrigerant enters the buffer tank 25 and then enters the second compressor 26. After the second air cooler at the outlet of the second compressor 26 cools the refrigerant R744, the refrigerant continues to enter the second switch valve V2 and the liquid storage tank 10. The outlet of the liquid storage tank 10 is connected to the third switch valve V3, and then to the fourth switch valve V4 and the plunger pump as the flow pump 31, and the fifth switch valve V5 is connected in parallel to the plunger pump. The refrigerant continues to enter the heat exchanger 32, and the flow rate of the refrigerant R744 is adjusted by the flow regulating valve LV, and then the specific flow value is measured by the flow meter L1. After passing through the temperature and pressure measuring point, it passes through the sixth switch valve V6 used to switch and control the flow, and then enters the equipment a to be filled to complete the filling process.

[0040] In the above process, the fourth switch valve V4 and the fifth switch valve V5 are used to select bypassing the flow pump 31 or using the flow pump 31; that is, the flow pump 31 is used as a supplement to the two-stage compressor to fine-tune the pressure at the equipment inlet, and is used for working conditions when the equipment inlet pressure fluctuates in a large range.

[0041] The auxiliary water equipment is as follows: a constant temperature water tank 60 is configured to adjust the internal water to the required temperature. The constant temperature water tank 60 is equipped with a heat source electric heating and a cold source cooling unit. At the same time, the pipelines configured separately are: a cooling water pipeline and an eleventh switch valve V11 configured for the cooler 22. A cooling water pipeline and a twelfth switch valve V12 configured for the first air cooler 24. A cooling water pipeline and a thirteenth switch valve V13 configured for the second air cooler 27. A cooling water pipeline and a water temperature regulator 61 configured for the heat exchanger 32. Since the control of R744 in the liquid storage tank 10 is relatively strict, a cooling unit 70 and a corresponding pressure pump 71 and a tenth switch valve V10 are configured for it separately. In addition, the liquid storage tank 10 is the liquid phase working medium storage component with the highest pressure in the system, so it is configured with a liquid level meter 12, a safety valve 13, a camera 14 for video monitoring of the liquid in the tank, and a pressure measurement point. At the same time, the liquid storage tank 10 is also equipped with a heat exchange coil 11, etc.

[0042] Other functional pipeline configurations are as follows:

[0043] Return pipe section 40: A return pipe section 40 is set between the temperature and pressure measuring point and the equipment a to be filled. Since the distance from the liquid storage tank 10 to the equipment a to be filled is very far, and the equipment a to be filled has very high requirements for the state of the refrigerant to be filled, if the temperature and pressure of R744 do not meet the requirements for some reason when it is about to enter the equipment, the return pipe section 40 can play a role at this time. Specifically: R744 returns to the liquid storage tank 10 after passing through the seventh switch valve V7 and the one-way valve CV, and is re-adjusted to a suitable temperature by the liquid storage tank 10 and a suitable pressure by the plunger pump until the state point measured by the temperature and pressure measuring point meets the temperature and pressure requirements of the equipment a to be filled with R744; at this time, the seventh switch valve V7 is closed, and then the aforementioned process pipeline is used normally.

[0044] Evacuate the pipe section 50: The evacuate the pipe section 50 is provided with a vacuum pump 51, a ninth switch valve V9 cooperating with the vacuum pump 51 to evacuate the pump inlet pipeline of the plunger pump to a vacuum, and an eighth switch valve V8 cooperating with the vacuum pump 51 to evacuate the pump outlet pipeline of the plunger pump to a vacuum.

[0045] The evacuation process is placed before the pressurization process. At this time, the device a to be filled is connected, the eighth switch valve V8 and the vacuum pump 51 are opened, and the device a to be filled is evacuated. After the vacuum degree required by the device is reached, the eighth switch valve V8 and the vacuum pump 51 are closed.

[0046] Further functions of each part:

[0047] The gaseous R744 can continue to enter the buffer tank 25 and the second compressor 26 to be compressed only after being cooled by the first air cooler 24. Therefore, the function of the first air cooler 24 is to ensure that the superheat of the gas entering the second compressor 26 is reduced to a reasonable range, that is, to ensure that the exhaust temperature of the second compressor 26 is within a safe and reasonable range.

[0048] The buffer tank 25 is used to make up for the flow difference between the first compressor 23 and the second compressor 26, and can ensure the safe and stable operation of the two compressors; with the buffer tank 25 buffering and coarse adjustment of the circulation volume of the two compressors, the flow of R744 in the entire liquid adding system of the present invention will be more stable and adjustable, with very small fluctuations; the compressor has lower power and lower noise when running, and has better overall life and stability.

[0049] After being compressed by the two-stage compressor, the exhaust temperature of the refrigerant R744 here can reach up to 150°C; directly entering the liquid storage tank 10, which needs to be precisely adjusted in temperature, will cause a large temperature difference in the liquid storage tank 10, resulting in a drastic change in the temperature of the liquid inside the liquid storage tank 10. The second air cooler 27 is used to roughly adjust and control the outlet exhaust temperature of the second compressor 26 to avoid the above situation.

[0050] The heat exchange coil 11 is arranged at the liquid storage tank 10, and the liquid storage tank 10 has sufficient volume; through the analysis of the working conditions of the two-stage compressor and the plunger pump, the volume of the liquid storage tank 10 is generally about 6-8 times the amount of liquid supplied to the device a. The large capacity and the water temperature regulating circuit configured for the heat exchange coil 11 enable the refrigerant R744 in the liquid storage tank 10 to accurately adjust and control the temperature, and the R744 inventory, temperature and pressure are very stable, which can withstand various changes in the system flow, and the working stability performance is further improved.

[0051] Generally speaking, the device a to be filled is far away from the liquid storage tank 10, which may be hundreds to thousands of meters. Therefore, although the pressure and temperature of R744 coming out of the third switch valve V3 are very stable and close to the temperature and pressure state required by the device a to be filled. However, during the aforementioned transmission of hundreds to thousands of meters, the filling pipe section 30 may pass through harmful heat sources, cold sources and pipelines with large resistance, etc., causing the temperature and pressure to gradually increase and deviate from the target value when reaching the device a to be filled. Therefore, the plunger pump of the filling pipe section 30 can be placed near the device a to be filled, and the pressure signal fed back by the temperature and pressure measuring point is received to accurately adjust the pressure state of the refrigerant for a second time. Similarly, the heat exchanger 32 on the filling pipe section 30 is also placed near the device a to be filled, and the temperature signal fed back by the temperature and pressure measuring point is received to accurately adjust the temperature state of the refrigerant for a second time. The short distance can also make the adjustment faster, without parameter hysteresis, so that the state of the refrigerant filled into the device is more accurate. The sixth switch valve V6 is directly arranged at the entrance of the device a to be filled, and the pipeline can be directly cut off when necessary. At the same time, even if encountering harmful heat sources, the heat exchanger 32 can ensure the constant temperature effect of the relatively high-pressure filling pipe section 30, and cooperate with temperature and pressure measurement points to ensure that unexpected pressurization problems are avoided.

[0052] The flow control valve LV at the outlet of the heat exchanger 32 can control the flow parameters of the liquid R744.

[0053] The constant temperature water tank 60 provides cooling water for each cooling and heat exchange equipment. The temperature of the constant temperature water tank 60 is precisely adjusted by controlling the heat source electric heating and the cold source chiller. Then, the temperature state of the refrigerant side is precisely controlled by adjusting the heat exchanger 32, the cooler 22, the first air cooler 24, and the second air cooler 27. More specifically, the water temperature regulator 61 can achieve the temperature adjustment function of the refrigerant that is about to enter the device a to be filled by controlling the heat exchanger 32; since the higher the temperature, the better the fluidity of the refrigerant, it is suitable to heat the refrigerant to the required temperature here, and then enter the device a to be filled with better fluidity.

[0054] When the liquid from the liquid storage tank 10 passes through the aforementioned long pipeline, and the pressure still meets the equipment requirements and there is no need to further adjust the pressure, the plunger pump can be bypassed by closing the fourth switch valve V4 and opening the fifth switch valve V5, that is, the plunger pump can be used to meet the requirements. When using the plunger pump, the fifth switch valve V5 can be closed and the fourth switch valve V4 can be opened.

[0055] In actual operation, the present invention also has the following characteristics:

[0056] 1. The two-stage pressurization structure of the present invention can increase the pressure of the gaseous R744 output from the gas storage tank 21 to 80-100 bar, and coupled with a multi-stage cooling system to control the temperature of R744 within the range of 15-20° C. The system can complete the charging conditions of pressurization and cooling at one time.

[0057] 2. The flow pump 31 used for system infusion is preferably a plunger pump. The plunger pump has the characteristics of accurate liquid delivery and can manually adjust the flow accuracy. When the liquid in the liquid storage tank 10 reaches the required pressure for use, the plunger pump can deliver R744 to the equipment to be filled more accurately and quickly.

[0058] 3. The state of liquid R744 can be monitored through the temperature and pressure measurement points. If the state does not meet the process requirements of R744, the liquid supply is stopped, and the R744 is returned to the liquid storage tank 10 by controlling the return pipe section 40. After readjusting to the state required by the R744 process, it is then transported to the equipment to be filled by the plunger pump. The return pipe section 40 can respond quickly and process the R744 liquid quickly, which is very simple and efficient.

[0059] 4. The present invention can partition the working fluid state of each component, use different safety protection devices respectively, and focus on monitoring the liquid storage tank 10 equipment with a large capacity of liquid phase working fluid, that is, the liquid storage tank 10 equipment is multiple-monitored in terms of pressure, working fluid leakage, video, etc. When the pressure is greater than 10MPa, the safety valve 13 can be opened to safely release the overflow pressure. At the same time, the high-pressure liquid phase pipeline located after the liquid storage tank has a smaller container volume than the high-pressure gas phase pipeline under the same flow rate due to the large density of the liquid phase working fluid. The required system pipeline is thinner and the valve diameter is smaller. The filling pipe section 30 after the third switch valve V3 does not need to bear the high-pressure gas phase load of a large amount of working fluid for a long time and a long distance, and the risk of pipe burst and pipe cracking can be further reduced.

[0060] 5. When the two sets of compressors of the present invention compress the gas, a large amount of heat will be generated during the pressure increase process, and the exhaust temperature is too high and needs to be cooled. Therefore, the device is provided with corresponding cooling components. While ensuring that the R744 pressure remains unchanged, the exhaust temperature is controlled, the working range of the compressor is widened, the safe and efficient operation of the system is maintained, and the system has a very wide working pressure range.

[0061] 6. The liquid storage tank 10 and its heat exchange coil 11 can achieve the dual functions of liquid storage and temperature control. In this system, the component that stores the most R744 liquid phase is the liquid storage tank 10. Accurately controlling and monitoring the temperature and pressure of this component is the key to the safety and stability of the entire system. In addition, the purpose of stepless regulation of the temperature and pressure of the liquid storage tank 10 can also be achieved by cooperating with the pressurized pipe section 20 through the cooling unit 70.

[0062] 7. The present invention can also be used in conjunction with a conventional electrical control system, which can be mainly composed of a controller, an actuator, a communication module, a power module, and a display unit. The controller is responsible for monitoring all electrical equipment in the system, and needs to upload equipment fault information to the general assembly workshop monitoring system to assist maintenance in quickly locating faults and improving maintenance efficiency. Other underlying components need to reserve data interfaces according to the requirements of the general assembly workshop monitoring system, and upload relevant information through the factory network to achieve basic maintenance and automated operation functions.

[0063] Of course, it is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims involved.

[0064] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0065] The techniques, shapes, and structural parts not described in detail in the present invention are all well-known techniques.

Claims

1. A liquid adding method using a two-stage compression liquid adding system based on R744, characterized in that: The two-stage compression liquid adding system based on R744 comprises a liquid storage tank (10), the inlet end of the liquid storage tank (10) is connected to a pressurizing pipe section (20), the outlet end of the liquid storage tank (10) is connected to a filling pipe section (30), and the outlet of the filling pipe section (30) is connected to a device to be filled, wherein: The pressurizing pipe section (20) comprises a gas storage tank (21), a first switch valve V1, a cooler (22), a first compressor (23), a first air cooler (24), a buffer tank (25), a second compressor (26), a second air cooler (27) and a second switch valve V2 which are arranged in sequence along the gas flow direction; The filling pipe section (30) comprises a third switch valve V3, a fourth switch valve V4, a flow pump (31), a flow regulating valve LV, a flow meter L1 and a sixth switch valve V6 which are arranged in sequence along the flow direction of the liquid; the filling pipe section (30) also comprises a side branch pipeline provided with a fifth switch valve V5, and the fourth switch valve V4 and the flow pump (31) are combined with the side branch pipeline to form a parallel pipeline; a temperature and pressure measuring point having a pressure and temperature measurement function is provided on a section of the filling pipe section between the flow meter L1 and the sixth switch valve V6; The liquid adding system further comprises a return pipe section (40) having a seventh switch valve V7, wherein the inlet of the return pipe section (40) is connected to the inlet of the sixth switch valve V6, and the outlet of the return pipe section (40) is connected to the liquid storage tank (10); The liquid adding system further comprises an evacuation pipe section (50) with a vacuum pump (51), an eighth switch valve V8 and a ninth switch valve V9 being arranged at two ends of the evacuation pipe section (50), the end where the eighth switch valve V8 is located being connected to the outlet of the sixth switch valve V6, and the end where the ninth switch valve V9 is located being connected to a section of the filling pipe section between the third switch valve V3 and the fourth switch valve V4; The liquid adding system further comprises a heat exchanger (32), the heat exchanger (32) being located on a section of the filling pipe between the parallel pipe and the flow regulating valve LV; the heat exchanger (32), the cooler (22), the first air cooler (24) and the second air cooler (27) are all connected to a constant temperature water tank (60) via a cooling water pipe; a water pump is correspondingly provided on each cooling water pipe, an eleventh switch valve V11, a twelfth switch valve V12 and a thirteenth switch valve V13 are correspondingly provided on the cooling water pipe where the cooler (22), the first air cooler (24) and the second air cooler (27) are located, and a water temperature regulator (61) is provided on the cooling water pipe where the heat exchanger (32) is located; The following steps are involved: 1) Pressurization process: The pressurizing pipe section (20) is opened, and the gaseous R744 in the gas storage tank (21) first passes through the first switch valve V1 and the cooler (22) and enters the first compressor (23) for pressurization. The first compressor (23) pressurizes the R744 to an intermediate pressure. Thereafter, the R744 enters the first air cooler (24) for cooling. The cooled R744 enters the buffer tank (25) and then enters the second compressor (26). After cooling through the second air cooler (27) at the outlet of the second compressor (26), the R744 continues to enter the liquid storage tank (10) through the second switch valve V2, completing the pressurization process. 2) Vacuuming process: Connect the equipment to be filled to the outlet of the sixth switch valve V6, open the eighth switch valve V8 and the vacuum pump (51), and evacuate the equipment to be filled through the evacuation pipe section (50). After reaching the set vacuum requirement, close the eighth switch valve V8 and the vacuum pump (51), and open the sixth switch valve V6 to complete the vacuuming process; this process is carried out simultaneously with the pressurization process or in sequence; 3) Filling process: a) opening the filling pipe section (30), the liquid 744 sequentially passes through the third switch valve V3, the fourth switch valve V4, the flow pump (31), and the heat exchanger (32) to reach the flow control valve LV, the flow control valve LV adjusts the flow of R744, and then obtains the flow value through the flow meter L1, and then obtains the final temperature value and pressure value through the temperature and pressure measuring point, and then enters the device to be filled through the sixth switch valve V6, completing the filling process; b) When the final temperature and pressure values ​​obtained at the temperature and pressure measuring point do not meet the requirements of the equipment to be charged, the sixth switch valve V6 is closed, the seventh switch valve V7 is opened, and the liquid 744 returns to the liquid storage tank (10) from the return pipe section (40) until the final temperature and pressure values ​​measured at the temperature and pressure measuring point meet the temperature and pressure requirements of the equipment to be charged for R744. At this time, the seventh switch valve V7 is closed, the sixth switch valve V6 is reopened, and then the next step is entered; c) When the final pressure value measured at the temperature and pressure measuring point meets the temperature and pressure requirements of the equipment to be charged with R744, the fourth switch valve V4 is closed, the fifth switch valve V5 is opened, and then step a) is performed.

2. The liquid adding method according to claim 1 using a two-stage compression liquid adding system based on R744, characterized in that: The liquid adding system further comprises a cooling unit (70), wherein the cooling unit (70) is connected to the heat exchange coil (11) of the liquid storage tank (10) via a pressure pump (71) and a tenth switch valve V10.

3. The liquid adding method according to claim 2 using a two-stage compression liquid adding system based on R744, characterized in that: The liquid storage tank (10) is provided with a liquid level meter (12), a safety valve (13) and a camera (14) for video monitoring of the liquid in the tank.

4. The liquid adding method according to claim 1 using a two-stage compression liquid adding system based on R744, characterized in that: The flow pump (31) is a plunger pump.

5. The liquid adding method according to claim 1 using a two-stage compression liquid adding system based on R744, characterized in that: The return pipe section (40) is also provided with a one-way valve CV.

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