Cascade control liquid methane supercooling device and supercooling temperature control method

Abstract

The invention provides a cascade control liquid methane supercooling device and a supercooling temperature control method. The supercooling device comprises a heat exchanger, an inlet of the heat exchanger is connected with a delivery pump through a first pipeline, an outlet of the heat exchanger is connected with a three-way reversing valve through a second pipeline, and the three-way reversing valve is communicated with a liquid methane storage tank through a third pipeline. The conveying pump extracts liquid methane in the liquid methane tank car through the conveying pipe and conveys the liquid methane into the heat exchanger through the first pipeline. The three-way reversing valve is communicated with the liquid methane tank car through a fourth pipeline to form a circulation loop, and the three-way reversing valve is electrically connected with the distributed control module. The distributed control module comprises a main control module and an auxiliary control module. A temperature transmitter electrically connected with the main control module is arranged on the second pipeline and used for detecting the temperature of liquid methane flowing out of the heat exchanger. The first pipeline is provided with a pneumatic adjusting valve electrically connected with the auxiliary control module and used for adjusting the flow of liquid methane flowing into the heat exchanger. The main control module is electrically connected with the auxiliary control module and used for forming a cascade control circuit. And a refrigerant for refrigerating the liquid methane is injected into the heat exchanger through a refrigerant filling valve. According to the supercooling device, the supercooling temperature of the liquid methane is automatically adjusted in a cascade control mode, the control precision of the supercooling temperature of the liquid methane is improved, manual operation in the supercooling process of the liquid methane is reduced, the working efficiency of on-site supercooling operation is improved, and the safety of the supercooling process is improved.

Description

Technical Field

[0001] The invention relates to the technical field of liquid methane supercooling, and in particular to a cascade control liquid methane supercooling device and a supercooling temperature control method. Background Art

[0002] In the ground test of liquid oxygen-methane rocket engines, in order to obtain the characteristics of the engine under supercooled medium, liquid methane needs to be supercooled with liquid nitrogen, that is, liquid nitrogen is used to supercool the temperature of liquid methane from 112K (Kelvin temperature) to 102K. Traditional liquid methane supercooling operation, manually controlling the opening of the liquid methane valve to adjust the liquid methane supercooling temperature, has the disadvantages of cumbersome procedures, low control accuracy, and low personnel safety.

[0003] In addition, in the liquid oxygen and methane supercooling control, the temperature can be monitored through the control module to control the flow of liquid nitrogen and liquid methane in real time, thereby realizing temperature control of the supercooled liquid methane. However, the accuracy of this control method is low, and the temperature of the supercooled liquid methane has a large error with the calibration temperature, and accurate temperature control cannot be achieved, which will affect the subsequent experimental data. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a cascade controlled liquid methane subcooling device and a subcooling temperature control method to solve the problems of complicated process, low temperature control accuracy and low safety of the subcooling process in the prior art.

[0005] The present invention provides a cascade controlled liquid methane subcooling device, the subcooling device comprises: a heat exchanger, wherein the inlet of the heat exchanger is connected to a delivery pump through a first pipeline, the outlet of the heat exchanger is connected to a three-way reversing valve through a second pipeline, and the three-way reversing valve is connected to a liquid methane storage tank through a third pipeline;

[0006] The delivery pump extracts liquid methane from the liquid methane tank truck through the delivery pipe, and delivers the liquid methane to the heat exchanger through the first pipeline;

[0007] The three-way reversing valve is connected to the liquid methane tanker through a fourth pipeline to form a circulation loop, and the three-way reversing valve is electrically connected to a distributed control module;

[0008] The distributed control module includes a main control module and a sub-control module;

[0009] The second pipeline is provided with a temperature transmitter electrically connected to the main control module for detecting the temperature of the liquid methane flowing out of the heat exchanger;

[0010] The first pipeline is provided with a pneumatic regulating valve electrically connected to the auxiliary control module, for regulating the flow of the liquid methane into the heat exchanger;

[0011] The main control module is electrically connected to the sub-control module to form a cascade control circuit;

[0012] The heat exchanger is injected with a refrigerant for refrigerating the liquid methane through a refrigerant filling valve.

[0013] Furthermore, a mass flow meter electrically connected to the auxiliary control module is provided on the third pipeline for detecting the flow rate of the subcooled liquid methane flowing out of the outlet of the heat exchanger.

[0014] In an embodiment of the present invention, a liquid level gauge for detecting the liquid level of the coolant in the heat exchanger is provided on the heat exchanger.

[0015] In an embodiment of the present invention, the distributed control module further comprises a valve switching module for controlling the switching of the three-way reversing valve.

[0016] In an embodiment of the present invention, the first pipeline is connected to the inlet of the inner coil inside the heat exchanger, and the outlet of the inner coil is connected to the second pipeline; the refrigerant injected into the heat exchanger exchanges heat by contacting with the inner coil.

[0017] The present invention also provides a cascade control liquid methane subcooling temperature control method, characterized in that the temperature control method comprises:

[0018] Step S1: connecting the liquid methane tank truck with the heat exchanger and the liquid methane storage tank through pipelines;

[0019] Step S2: allowing liquid methane to circulate between the liquid methane tank truck and the heat exchanger through a three-way reversing valve;

[0020] Step S3: injecting liquid nitrogen into the heat exchanger until the maximum liquid level;

[0021] Step S4: When the temperature transmitter displays that the temperature reaches a preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control;

[0022] Step S5: the main control module calculates the deviation between the actual temperature signal value at the heat exchanger outlet and the set temperature value and performs PID adjustment, and transmits the output signal value to the auxiliary control module as the set value;

[0023] Step S6: the auxiliary control module calculates the deviation between the actual flow signal value detected by the mass flow meter and the set value and performs PID adjustment, thereby controlling the opening of the pneumatic control valve, and looping step S5;

[0024] Step S7: After the mass flow meter detects that the accumulated flow reaches the set value, the three-way reversing valve is controlled to switch so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen inside the heat exchanger is discharged;

[0025] Step S8: Turn off the delivery pump and discharge all the liquid methane in the inner coil of the heat exchanger to the liquid methane tank truck.

[0026] In an embodiment of the present invention, step S2: the step of circulating liquid methane between the liquid methane tank truck and the heat exchanger through the three-way reversing valve specifically includes:

[0027] Step S21: The PID modes of the main controller and the sub-controller are both selected as manual mode, the pneumatic regulating valve is adjusted to a fully open state, and the second pipeline is connected to the fourth pipeline through the three-way reversing valve.

[0028] Step S22: Start the delivery pump. Liquid methane is delivered from the liquid methane tank truck to the heat exchanger by the delivery pump. After flowing through the inner coil in the heat exchanger, it returns to the liquid methane tank truck through the three-way reversing valve and the fourth pipeline.

[0029] In the embodiment of the present invention, step S4: when the temperature transmitter displays that the temperature reaches a preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control, the steps specifically include:

[0030] When the temperature transmitter detects that the temperature at the heat exchanger outlet is about 102K, the distributed control module controls the three-way reversing valve to switch the second pipeline to the third pipeline, and calculates the cumulative volume of liquid methane after supercooling through the mass flow meter;

[0031] The temperature setting value of the main controller is set to 102K, the PID mode of the main controller is set to automatic mode, and the PID mode of the sub-controller is set to cascade mode.

[0032] In an embodiment of the present invention, step S7: after the mass flow meter detects that the accumulated flow reaches a set value, the three-way reversing valve is controlled to be switched so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen inside the heat exchanger is discharged specifically includes:

[0033] Step S71: the mass flow meter transmits the accumulated value of the subcooled methane volume flow rate to the distributed control module in real time;

[0034] Step S72: When the accumulated value of the volume flow reaches the set value, the distributed control module controls the three-way reversing valve to switch the second pipeline to connect the fourth pipeline;

[0035] Step S73: The auxiliary control module controls the pneumatic regulating valve to be fully opened, and at the same time opens the refrigerant discharge valve of the heat exchanger to discharge liquid nitrogen.

[0036] According to the above embodiments, the cascade control liquid methane subcooling device and subcooling temperature control method provided by the present invention have at least the following benefits:

[0037] The supercooling device uses a cascade control method to automatically adjust the liquid methane supercooling temperature, thereby improving the control accuracy of the liquid methane supercooling temperature, reducing manual operations during the liquid methane supercooling process, improving the efficiency of on-site supercooling operations, saving liquid methane supercooling operation time, and improving the safety of the supercooling process.

[0038] In addition, the supercooling device is easy to operate, safe and reliable, has a simple overall structure and is easy to maintain.

[0039] It should be understood that the above general description and the following detailed description are merely exemplary and illustrative and are not intended to limit the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The following drawings are part of the specification of the present invention, and illustrate exemplary embodiments of the present invention. Together with the description of the specification, the accompanying drawings are used to explain the principles of the present invention.

[0041] Figure 1 A pipeline connection diagram of a cascade controlled liquid methane subcooling device provided by the present invention.

[0042] Figure 2 A control structure diagram of a cascade controlled liquid methane subcooling device provided by the present invention.

[0043] Description of reference numerals:

[0044] 1-heat exchanger, 2-first pipeline, 3-second pipeline, 4-third pipeline, 5-fourth pipeline, 6-delivery pump, 7-three-way reversing valve, 8-liquid methane storage tank, 9-liquid methane tank truck, 10-main control module, 11-auxiliary control module, 12-temperature transmitter, 13-pneumatic regulating valve, 14-mass flow meter, 15-liquid level meter. DETAILED DESCRIPTION

[0045] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0046] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0047] The present invention provides a cascade controlled liquid methane subcooling device, such as Figure 1 and 2The structure diagram and control connection diagram of the supercooling device are shown. In a specific implementation, the supercooling device includes: a heat exchanger 1, the inlet of the heat exchanger 1 is connected to a delivery pump 6 through a first pipeline 2, the outlet of the heat exchanger 1 is connected to a three-way reversing valve 7 through a second pipeline 3, and the three-way reversing valve 7 is connected to a liquid methane storage tank 8 through a third pipeline 4.

[0048] The delivery pump 6 extracts liquid methane from the liquid methane tank truck 9 through the delivery pipe, and delivers the liquid methane to the heat exchanger 1 through the first pipeline 2. Specifically, the first pipeline 2 is connected to the inlet of the inner coil inside the heat exchanger 1, and the outlet of the inner coil is connected to the second pipeline 3.

[0049] The three-way reversing valve 7 is connected to the liquid methane tank truck 9 through the fourth pipeline 5 to form a circulation loop, and the three-way reversing valve 7 is electrically connected to the distributed control module, and the three-way reversing valve 7 is controlled by the distributed control module to perform valve switching.

[0050] The distributed control module comprises a main control module 10 and a sub-control module 11. The second pipeline 3 is provided with a temperature transmitter 12 electrically connected to the main control module 10 for detecting the temperature of the liquid methane flowing out of the heat exchanger 1.

[0051] The first pipeline 2 is provided with a pneumatic regulating valve 13 electrically connected to the auxiliary control module 11 for controlling and regulating the flow of the liquid methane into the heat exchanger 1, thereby achieving temperature regulation.

[0052] The third pipeline 4 is provided with a mass flow meter 14 electrically connected to the auxiliary control module 11 for detecting the flow rate of the subcooled liquid methane flowing out of the outlet of the heat exchanger 1 .

[0053] The main control module 10 is electrically connected to the sub-control module 11 to form a cascade control circuit. In this embodiment, the main control module 10 calculates the deviation between the liquid methane temperature value detected at the outlet of the heat exchanger 1 and the set value and performs PID regulation, wherein PID regulation is a classic control system regulation method, which adjusts the output signal of the system through three parts: proportion, integration and differentiation, to achieve fast, accurate and stable control of the system. The PID control method is widely used in various control systems, and its core goal is to achieve the dynamic response, steady-state accuracy and robustness of the system. The main control module outputs a flow set value to the sub-control module 11, and the sub-control module 11 calculates the deviation between the actual flow value detected by the mass flow meter 14 and the set flow value and performs PID regulation, and outputs a control signal to the start regulating valve 13 to control the opening of the pneumatic regulating valve 13, thereby achieving the temperature adjustment of the liquid methane. The entire temperature adjustment process is controlled in series, and the temperature change curve during the adjustment process is more stable, making the temperature control of liquid methane more precise, approaching or reaching the set temperature value, and there will be no sudden changes in temperature adjustment.

[0054] The heat exchanger 1 is injected with a refrigerant for cooling the liquid methane through a refrigerant filling valve, and the refrigerant injected into the heat exchanger 1 exchanges heat with the inner coil by contact. In this embodiment, the preferred refrigerant is liquid nitrogen.

[0055] In a specific embodiment of the present invention, a liquid level meter 15 for detecting the liquid level of the coolant in the heat exchanger 1 is provided on the heat exchanger 1 .

[0056] In a specific embodiment of the present invention, the distributed control module further comprises a valve switching module for controlling the switching of the three-way reversing valve 7, which is used to receive a control signal and perform valve regulation and switching according to different situations.

[0057] The present invention also provides a cascade control liquid methane subcooling temperature control method, the temperature control method comprising:

[0058] Step S1: Connect the liquid methane tank truck with the heat exchanger and the liquid methane storage tank through pipelines.

[0059] Step S2: The liquid methane is circulated between the liquid methane tank truck and the heat exchanger through the three-way reversing valve. Specifically, in order to prevent the liquid methane in the inner coil of the heat exchanger from being frozen by liquid nitrogen, the liquid methane in the pipeline needs to flow "self-circulating" in advance before the liquid nitrogen is injected into the heat exchanger. Start the delivery pump, and the liquid methane is transported from the liquid methane tank truck to the heat exchanger by the delivery pump. After flowing through the inner coil of the heat exchanger, it returns to the liquid methane tank truck through the second pipeline, the three-way reversing valve and the fourth pipeline, realizing the "self-circulating" flow of liquid methane to avoid freezing in the inner coil.

[0060] Step S3: Inject liquid nitrogen into the heat exchanger until the highest liquid level is reached. Specifically, after the liquid methane starts to flow in a "self-circulating" manner, immediately inject liquid nitrogen into the heat exchanger until the liquid level gauge for detecting the liquid nitrogen level in the heat exchanger reports a high signal, and then stop injecting liquid nitrogen. At this time, the entire inner coil of the heat exchanger is immersed in liquid nitrogen, which is convenient for heat exchange and cooling of the liquid methane.

[0061] Step S4: When the temperature transmitter shows that the temperature reaches the preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control. Specifically, when the temperature transmitter detects that the temperature of the liquid methane flowing out of the heat exchanger outlet reaches the preset value, the liquid methane can be stored. At this time, the control signal is transmitted to the distributed control module, and the three-way reversing valve is controlled by the distributed control module to switch the valve, so that the supercooled liquid methane is transported from the second pipeline and the third pipeline to the liquid methane storage tank.

[0062] Step S5: The main control module calculates the deviation between the actual temperature signal value at the heat exchanger outlet and the set temperature value and performs PID adjustment, and transmits the output signal value to the auxiliary control module as the set value.

[0063] Specifically, the temperature transmitter converts the temperature of the liquid methane at the outlet of the heat exchanger into a 4-20mA current signal and transmits it to the main control module in the distributed control module. The temperature of the liquid methane at the outlet is used as the real-time value of the main control module, and the deviation is calculated from the set value of the main control module. PID adjustment is performed based on this temperature deviation, and the flow percentage value of the set temperature is calculated. This value is output to the sub-control module as the flow setting value of the sub-controller.

[0064] Step S6: The auxiliary control module calculates the deviation between the actual flow signal value detected by the mass flow meter and the set value and performs PID adjustment, thereby controlling the opening of the pneumatic control valve and looping step S5.

[0065] Specifically, the mass flow meter converts the real-time mass flow value of the supercooled liquid methane into a 4-20mA current signal and transmits it to the sub-control module in the distributed control module. The mass flow value is used as the actual value of the sub-control module and the deviation is calculated with the set value of the sub-control module, and PID adjustment is performed according to the deviation. Then the sub-control module outputs the output value to the pneumatic control valve. In this embodiment, the opening of the pneumatic control valve is controlled by the output value, and then the flow of liquid methane in the inner coil of the heat exchanger is adjusted, so as to accurately control the temperature of the liquid methane at the outlet of the heat exchanger. In addition, by looping steps S5 and S6, the temperature of the liquid methane is adjusted in real time, so that the temperature change is small and the regulation is more precise.

[0066] Step S7: After the mass flow meter detects that the cumulative flow reaches the set value, the three-way reversing valve is controlled to switch, so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen inside the heat exchanger is discharged. Specifically, after the mass flow meter detects that the cumulative value has been reached, the signal is transmitted to the distributed control module, and the valve control module in the distributed control module controls the three-way reversing valve to switch the valve, so that the liquid methane in the heat exchanger flows into the liquid methane tank truck through the second pipeline and the fourth pipeline, and then is delivered to the heat exchanger through the delivery pump, thereby forming a circulation loop. At the same time, the liquid nitrogen inside the heat exchanger is discharged through the refrigerant discharge valve at the bottom of the heat exchanger.

[0067] Step S8: Turn off the delivery pump and discharge all the liquid methane in the inner coil of the heat exchanger to the liquid methane tank truck. Specifically, after turning off the delivery pump, nitrogen is used to press all the liquid methane remaining in the pipeline into the liquid methane tank truck, and the supercooling operation is completed.

[0068] In a specific embodiment of the present invention, step S2: the step of circulating liquid methane between the liquid methane tank truck and the heat exchanger through the three-way reversing valve specifically includes:

[0069] Step S21: The PID modes of the main controller and the sub-controller are both selected as manual mode, the pneumatic regulating valve is adjusted to a fully open state, and the second pipeline is connected to the fourth pipeline through the three-way reversing valve.

[0070] Step S22: Start the delivery pump. Liquid methane is delivered from the liquid methane tank truck to the heat exchanger by the delivery pump. After flowing through the inner coil in the heat exchanger, it returns to the liquid methane tank truck through the three-way reversing valve and the fourth pipeline.

[0071] In a specific embodiment of the present invention, step S4: when the temperature transmitter displays that the temperature reaches a preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control, the steps specifically include:

[0072] When the temperature transmitter detects that the temperature at the heat exchanger outlet is about 102K, the distributed control module controls the three-way reversing valve to switch the second pipeline to the third pipeline, and calculates the cumulative volume of liquid methane after supercooling through the mass flow meter.

[0073] The temperature setting value of the main controller is set to 102K, the PID mode of the main controller is set to automatic mode, and the PID mode of the sub-controller is set to cascade mode. Specifically, the main controller is set to automatic mode, and the sub-controller is set to cascade mode. The entire temperature control module achieves the purpose of cascade control and achieves precise temperature adjustment.

[0074] In a specific embodiment of the present invention, step S7: after the mass flow meter detects that the accumulated flow reaches the set value, the three-way reversing valve is controlled to be switched so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen inside the heat exchanger is discharged specifically includes:

[0075] Step S71: The mass flow meter transmits the accumulated value of the subcooled methane volume flow rate to the distributed control module in real time.

[0076] Step S72: When the accumulated value of the volume flow reaches the set value, the distributed control module controls the three-way reversing valve to switch the second pipeline to connect the fourth pipeline.

[0077] Step S73: The auxiliary control module controls the pneumatic regulating valve to be fully opened, and at the same time opens the refrigerant discharge valve of the heat exchanger to discharge liquid nitrogen.

[0078] The above description is only an illustrative specific implementation manner of the present invention. Without departing from the concept and principle of the present invention, any equivalent changes and modifications made by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A cascade controlled liquid methane subcooling device, characterized in that: The supercooling device comprises: a heat exchanger (1), wherein the inlet of the heat exchanger (1) is connected to a delivery pump (6) via a first pipeline (2), the outlet of the heat exchanger (1) is connected to a three-way reversing valve (7) via a second pipeline (3), and the three-way reversing valve (7) is connected to a liquid methane storage tank (8) via a third pipeline (4); The delivery pump (6) extracts liquid methane from the liquid methane tanker (9) through a delivery pipe, and delivers the liquid methane to the heat exchanger (1) through the first pipeline (2); The three-way reversing valve (7) is connected to the liquid methane tanker (9) via a fourth pipeline (5) to form a circulation loop, and the three-way reversing valve (7) is electrically connected to a distributed control module; The distributed control module comprises a main control module (10) and a secondary control module (11); The second pipeline (3) is provided with a temperature transmitter (12) electrically connected to the main control module (10) for detecting the temperature of the liquid methane flowing out of the heat exchanger (1); The first pipeline (2) is provided with a pneumatic regulating valve (13) electrically connected to the auxiliary control module (11) for regulating the flow rate of the liquid methane flowing into the heat exchanger (1); The main control module (10) is electrically connected to the auxiliary control module (11) to form a cascade control circuit; The heat exchanger (1) is injected with a refrigerant for refrigerating the liquid methane through a refrigerant filling valve.

2. The cascade control liquid methane subcooling device according to claim 1, characterized in that: The third pipeline (4) is provided with a mass flow meter (14) electrically connected to the auxiliary control module (11) for detecting the flow rate of the subcooled liquid methane flowing out of the outlet of the heat exchanger (1).

3. The cascade control liquid methane subcooling device according to claim 1, characterized in that: The heat exchanger (1) is provided with a liquid level meter (15) for detecting the liquid level height of the coolant in the heat exchanger (1).

4. The cascade control liquid methane subcooling device according to claim 1, characterized in that: The distributed control module also has a valve switching module for controlling the switching of the three-way reversing valve (7).

5. The cascade control liquid methane subcooling device according to claim 1, characterized in that: The first pipeline (2) is connected to the inlet of the inner coil inside the heat exchanger (1), and the outlet of the inner coil is connected to the second pipeline (3); The refrigerant injected into the heat exchanger (1) exchanges heat by contacting with the inner coil.

6. A cascade control liquid methane subcooling temperature control method, characterized in that: The temperature control method includes: Step S1: connecting the liquid methane tank truck with the heat exchanger and the liquid methane storage tank through pipelines; Step S2: allowing liquid methane to circulate between the liquid methane tank truck and the heat exchanger through a three-way reversing valve; Step S3: injecting liquid nitrogen into the heat exchanger until the maximum liquid level; Step S4: When the temperature transmitter displays that the temperature reaches a preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control; Step S5: the main control module calculates the deviation between the actual temperature signal value at the heat exchanger outlet and the set temperature value and performs PID adjustment, and transmits the output signal value to the auxiliary control module as the set value; Step S6: the auxiliary control module calculates the deviation between the actual flow signal value detected by the mass flow meter and the set value and performs PID adjustment, thereby controlling the opening of the pneumatic control valve, and looping step S5; Step S7: After the mass flow meter detects that the accumulated flow reaches the set value, the three-way reversing valve is controlled to switch so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen inside the heat exchanger is discharged; Step S8: Turn off the delivery pump and discharge all the liquid methane in the inner coil of the heat exchanger to the liquid methane tank truck.

7. The cascade control liquid methane subcooling temperature control method according to claim 6, characterized in that: Step S2: The step of circulating the liquid methane between the liquid methane tank truck and the heat exchanger through the three-way reversing valve specifically includes: Step S21: The PID modes of the main controller and the sub-controller are both selected as manual mode, the pneumatic regulating valve is adjusted to a fully open state, and the second pipeline is connected to the fourth pipeline through the three-way reversing valve. Step S22: Start the delivery pump. Liquid methane is delivered from the liquid methane tank truck to the heat exchanger by the delivery pump. After flowing through the inner coil in the heat exchanger, it returns to the liquid methane tank truck through the three-way reversing valve and the fourth pipeline.

8. The cascade control liquid methane subcooling temperature control method according to claim 6, characterized in that: Step S4: When the temperature transmitter displays that the temperature reaches a preset value, the three-way reversing valve is switched so that the supercooled liquid methane flows to the liquid methane storage tank, and the control mode is set to cascade control. Specifically, the steps include: When the temperature transmitter detects that the temperature at the heat exchanger outlet is about 102K, the distributed control module controls the three-way reversing valve to switch the second pipeline to the third pipeline, and calculates the cumulative volume of liquid methane after supercooling through the mass flow meter; The temperature setting value of the main controller is set to 102K, the PID mode of the main controller is set to automatic mode, and the PID mode of the sub-controller is set to cascade mode.

9. The cascade control liquid methane subcooling temperature control method according to claim 6, characterized in that: Step S7: After the mass flow meter detects that the accumulated flow reaches the set value, the three-way reversing valve is controlled to be switched so that the liquid methane circulates between the liquid methane tank truck and the heat exchanger, and the liquid nitrogen in the heat exchanger is discharged. Specifically, the steps include: Step S71: the mass flow meter transmits the accumulated value of the subcooled methane volume flow rate to the distributed control module in real time; Step S72: When the accumulated value of the volume flow reaches the set value, the distributed control module controls the three-way reversing valve to switch the second pipeline to connect the fourth pipeline; Step S73: The auxiliary control module controls the pneumatic regulating valve to be fully opened, and at the same time opens the refrigerant discharge valve of the heat exchanger to discharge liquid nitrogen.

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