A method and system for controlling cryogenic helium circulation at room temperature pressurization

Abstract

This invention discloses a method and system for controlling cryogenic helium circulation for ambient temperature pressurization, comprising: establishing a mapping relationship between cooling time of the load in different temperature ranges and under different circulating helium flow rates; selecting the circulating helium flow rate with the shortest cooling time in each temperature range as the optimal flow rate value; during the system cooling process, acquiring the actual temperature of the load in real time and determining the temperature range to which the actual temperature belongs; and automatically adjusting the flow controller according to the determination result to set the circulating helium flow rate to the optimal flow rate value corresponding to the current temperature range. This invention uses MATLAB to write a calculation program to calculate the optimal helium flow rate corresponding to different temperature ranges of the load. Under this flow rate, the cooling rate is the fastest, and this calculation is written into the PLC module to realize automatic and rapid cooling of the system; it simplifies the operation process, making the system easier to operate, improving the overall cooling rate of the system, and shortening the cooling time.

Description

Technical Field

[0001] This invention relates to a cryogenic helium circulation control method and system, and more particularly to a cryogenic helium circulation control method and system for room temperature pressurization. Background Technology

[0002] Cryogenic cooling systems are widely used in fields such as superconducting magnets and magnetoencephalography (MEG). Common cooling methods include cold helium circulation cooling, direct cooling by the cryostat, and liquid helium immersion cooling. Considering economic cost and vibration requirements, cold helium circulation cooling is the ideal method, balancing cost and low vibration, and providing a stable and uniform cryogenic environment for the magnet. The working principle involves a compressor or cryogenic fan supplying a circulating helium gas with adjustable pressure and flow rate. This gas exchanges heat thoroughly with the cryostat's cold head, carrying the cooling energy to the load, such as the magnet, for cyclic cooling. This heat exchange method can provide high-pressure, temperature-controlled circulating helium gas while reducing the impact of cryostat vibration on the load.

[0003] Cold helium circulation systems can be categorized into cryogenic pressurization systems and ambient temperature pressurization systems based on the power source used. Cryogenic pressurization systems utilize cryogenic fans for power, offering advantages such as integration into the cold chamber, simplified structure, and reduced cold loss due to the pressurization process occurring at low temperatures. However, they are more expensive and require more complex maintenance and disassembly. The heat output of the cryogenic fan depends on the inlet and outlet pressure difference, placing high demands on the flow resistance of the load section. Ambient temperature pressurization systems use a circulating compressor or mechanical pump for power. Cryogenic helium is cooled to ambient temperature via a heat exchanger before entering the compressor. The pressurization process occurs at ambient temperature, and the pressurized helium exchanges heat with the cold head via another heat exchanger before entering the load to cool it. The circulating equipment remains at ambient temperature, facilitating maintenance and reducing costs. Currently, common ambient temperature pressurization helium circulation systems have relatively simple control processes and methods, but suffer from significant pressure fluctuations in the circulating helium, a less intelligent cooling process, and a longer cooling time, hindering system temperature control. Summary of the Invention

[0004] Purpose of the invention: This invention proposes a method and system for controlling cryogenic helium circulation at room temperature pressurization, which can realize low-pressure replenishment and high-pressure recovery of the circulating helium system, as well as automatic and rapid heating and cooling of the system.

[0005] Technical solution: This invention includes the following steps:

[0006] Step 1: Establish the cooling time mapping relationship of the load in different temperature ranges and under different circulating helium flow rates, and select the circulating helium flow rate value with the shortest cooling time in each temperature range as the optimal flow rate value.

[0007] Step 2: During the system cooling process, obtain the actual temperature of the load in real time and determine the temperature range to which the actual temperature belongs;

[0008] Step 3: Based on the judgment result, automatically adjust the flow controller to set the circulating helium flow rate to the optimal flow rate value corresponding to the current temperature range.

[0009] The mapping relationship in step one is established by combining MATLAB simulation calculation with experimental correction.

[0010] The size of the temperature range is adjusted based on the computational load and the flow controller.

[0011] The method also includes: when the system shuts down or the load fails, the high-pressure helium gas in the pipeline is forcibly discharged into the gas storage through the back pressure valve by the compressor.

[0012] The present invention also provides a control system based on the aforementioned method for controlling cryogenic helium circulation for ambient temperature pressurization, comprising a gas storage unit, a compressor, a cold box, and a heat exchanger and a chiller cold head located within the cold box, and further comprising:

[0013] A flow controller is installed on the pipeline between the compressor outlet and the cold box;

[0014] A temperature sensor is installed on the load to collect the load temperature in real time.

[0015] The PLC controller is electrically connected to the temperature sensor and the flow controller respectively. The PLC controller has a pre-stored table of the correspondence between different load temperature ranges and the optimal flow rate of circulating helium. Based on the load temperature fed back by the temperature sensor, the PLC controller automatically retrieves the optimal flow rate value from the corresponding table and controls the flow controller to perform the corresponding flow rate adjustment.

[0016] The outlet of the gas storage unit is connected to the inlet of the compressor via a first pressure reducing valve, and the inlet of the gas storage unit is connected to the outlet of the compressor via a back pressure valve.

[0017] A second pressure reducing valve is also installed on the outlet pipe of the compressor, which is located upstream of the flow controller.

[0018] The heat exchanger is a counter-flow shell-and-tube heat exchanger, with its inner tube inlet connected to the return gas end of the load and its outer tube inlet connected to the output end of the flow controller.

[0019] The load is installed inside the test Dewar, and both the cold box and the test Dewar are independent vacuum-insulated containers.

[0020] In the pre-stored correspondence table in the PLC controller, the optimal flow rate value corresponding to each temperature range is the critical flow rate value with the shortest cooling time; and when the load temperature decreases, the flow controller adjusts the flow rate setting value accordingly.

[0021] Beneficial effects: The present invention has the following advantages:

[0022] (1) The present invention uses MATLAB to write a calculation program to calculate the optimal helium flow rate corresponding to different temperature ranges of the load. Under this flow rate, the cooling rate is the fastest. The result is written into the PLC module to realize the automatic cooling and rapid cooling of the system. The operation process is simplified, making the system easier to operate, improving the overall cooling rate of the system, and shortening the cooling time.

[0023] (2) The present invention uses a pressure reducing valve and a back pressure valve to control the connection between the compressor and the gas storage, so as to achieve pressure stability of the circulation pipeline and high pressure recovery of helium at a low cost, ensuring that the compressor is in a better working pressure range. When the system return gas pressure is low, the gas storage will replenish the system through the pressure reducing valve. When the compressor outlet pressure is higher than the set pressure of the back pressure valve, helium will flow into the gas storage, reducing the waste of helium.

[0024] (3) The present invention uses a helium compressor to provide power for the helium circulation of the system. Compared with the cryogenic fan, it reduces the cost of use and can meet the load of high flow resistance. At the same time, the working environment of the helium compressor is at room temperature, which is convenient for inspection and maintenance. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the present invention;

[0026] Figure 2 This is a schematic diagram of the shell-and-tube heat exchanger of the present invention;

[0027] Figure 3 The cooling time corresponding to different flow rates is calculated for the load temperature to drop from 290K to 285K in this invention. Detailed Implementation

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] Example 1

[0030] The cryogenic helium circulation control method for room temperature pressurization in this embodiment includes the following steps:

[0031] Step 1: Establish a mapping relationship between the cooling time of the load under different temperature ranges and different circulating helium flow rates, and select the circulating helium flow rate with the shortest cooling time in each temperature range as the optimal flow rate. The mapping relationship is established using MATLAB simulation calculations combined with experimental correction. A MATLAB program is used to calculate the cooling time under different flow rates corresponding to different temperature ranges of the load, determining the optimal helium flow rate at which the load cools fastest. The calculation results are then corrected through experiments.

[0032] Step Two: During the system cooling process, the actual temperature of the load is acquired in real time, and the temperature range to which this actual temperature belongs is determined. Once the PLC reads the temperature range of the load temperature, it automatically adjusts the flow controller to the optimal helium flow rate. This achieves automatic and rapid cooling of the system. The size of the temperature range can be adjusted appropriately; in this embodiment, considering both computational complexity and the number of flow controller adjustments, a temperature range of 5K is chosen.

[0033] Step 3: Based on the judgment result, automatically adjust the flow controller to set the circulating helium flow rate to the optimal flow rate value corresponding to the current temperature range.

[0034] Step 4: When the system shuts down or the load fails, the high-pressure helium in the pipeline is forcibly discharged into the gas storage via the back pressure valve through the compressor. The compressor 3 controls its inlet pressure through the first pressure reducing valve 2 to ensure a stable return pressure within its optimal operating pressure range. The outlet pressure of the compressor 3 is controlled by the back pressure valve 4. When the outlet helium pressure exceeds the opening pressure of the back pressure valve 4, the high-pressure helium flows back to the gas storage 1 for high-pressure helium recovery during system reheating or magnet failure.

[0035] Example 2

[0036] like Figure 1As shown, the cryogenic helium circulation control system for ambient temperature pressurization in this embodiment includes: a gas storage tank 1, a compressor 3, a flow controller 6, a heat exchanger 7, a refrigerator cold head 8, a cold box 10, a temperature sensor, and a PLC controller. The flow controller 6 is installed on the pipeline between the outlet of the compressor 3 and the cold box 10. The heat exchanger 7 and the refrigerator cold head 8 are located inside the cold box 10. The temperature sensor is installed on the load 12 to collect the load temperature in real time. The load 12 is also equipped with a heater. The PLC controller is electrically connected to both the temperature sensor and the flow controller 6. The PLC controller has a pre-stored table of correspondence between different load temperature ranges and the optimal flow rate of circulating helium. Based on the load temperature fed back by the temperature sensor, the PLC controller automatically retrieves the optimal flow rate value from the corresponding table and controls the flow controller 6 to perform corresponding flow rate adjustments. In the pre-stored table of correspondence in the PLC controller, the optimal flow rate value corresponding to each temperature range is the critical flow rate value with the shortest cooling time. When the load temperature decreases, the flow controller correspondingly lowers the flow rate setpoint.

[0037] The refrigeration circuit of this cryogenic helium circulation control system consists of a refrigeration unit cold head 8 and its matching compressor, and is independent of the circulation pipeline. The cooling capacity is transferred to the circulation pipeline through the slit heat exchanger 9 on the refrigeration unit cold head 8. The circulation pipeline is powered by the compressor 3, which can pressurize the low-pressure helium to above 20 bar. The compressor is located at room temperature, which facilitates disassembly and maintenance.

[0038] Gas reservoir 1 is a stainless steel buffer tank with a pressure resistance of up to 3.5 MPa. It is used for low-pressure gas replenishment at the compressor inlet and high-pressure gas recovery at the outlet after shutdown. Its volume is calculated based on the system gas pipeline volume and the system circulating operating pressure. The outlet of gas reservoir 1 is connected to the inlet of compressor 3 through the first pressure reducing valve 2. When the inlet pressure of compressor 3 is low, gas reservoir 1 replenishes gas to the system through the first pressure reducing valve 2 to ensure the stability of the compressor 3 inlet pressure, thereby maintaining the pressure stability of the entire circulating pipeline system. The inlet of gas reservoir 1 is connected to the outlet of compressor 3 through the back pressure valve 4. When the outlet pressure of compressor 3 is too high, the circulating pipeline system will discharge helium to the gas reservoir through the back pressure valve 4, realizing high-pressure helium recovery when the system reheats or the magnet fails, thereby reducing helium waste.

[0039] A second pressure reducing valve 5 is also installed on the outlet pipe of compressor 3. This second pressure reducing valve 5 is located upstream of flow controller 6 and is used to precisely regulate the working pressure and flow rate of helium in the circulation pipeline. In the circulation pipeline, the flow rate of helium is an important factor affecting the cooling rate of the system. In this embodiment, a calculation program was written in MATLAB to calculate the optimal cooling flow rate at different load temperatures. The calculation results were then corrected through experiments, and the corrected results were written into the PLC module. This enables the flow controller to automatically adjust to the optimal cooling flow rate according to the load temperature, thereby shortening the overall cooling time of the system.

[0040] like Figure 2 As shown, heat exchanger 7 is a counter-flow shell-and-tube heat exchanger, externally wrapped with several layers of double-layer insulation material. The length and diameter of the shell-and-tube heat exchanger can be calculated based on heat exchange and flow resistance, ensuring sufficient heat exchange while meeting flow resistance requirements. The inner tube 7-1 of heat exchanger 7 is made of copper and is filled with cryogenic helium gas returned from the test Dewar. The outer tube 7-2 is made of stainless steel and is connected to ambient temperature helium gas entering the cold box from the circulating compressor. The two gases flow in opposite directions and exchange heat. Through heat exchange, the temperature of the cryogenic helium gas rises to near ambient temperature and then flows out of the cold box 10 and into the compressor 3; the ambient temperature helium gas decreases in temperature, is pre-cooled, and then enters the slit heat exchanger 9, making full use of the cold energy of the return gas and reducing waste.

[0041] Load 12 is installed inside test Dewar 11. Both the cold chamber 10 and test Dewar 11 are independent, vacuum-insulated containers. The cold chamber 10 and test Dewar 11 are stainless steel cylindrical devices, but other shapes are also acceptable. They must meet the pressure resistance requirement of one atmosphere and employ good welding techniques. Before use, a vacuum pump should be used to evacuate the container to 5 × 10⁻⁶ ppm. -2 Maintain a good vacuum level below Pa.

[0042] The above-described cryogenic helium circulation process is as follows: Helium at rated pressure is replenished to the system pipelines and gas storage, and the refrigeration unit's cold head 8 and compressor 3 are turned on for circulating cooling. Helium is compressed into high-pressure helium by compressor 3, passes through the second pressure reducing valve 5 and flow controller 6, enters the cold box 10, undergoes heat exchange in heat exchanger 7, then enters the slit heat exchanger 9 of the cold head, and subsequently enters the load 12 of the test Dewar 11 to cool the load 12. The return gas then exchanges heat with room-temperature helium in heat exchanger 7, reaching near-room temperature, before entering compressor 3 again, completing the entire circulation process. When the magnet reheats or fails to quench, the high-pressure helium flows back into gas storage 1 through the back pressure valve 4 at the output end of compressor 3, achieving high-pressure helium recovery.

[0043] The system's rapid cooling and automatic temperature control are achieved through PLC-controlled flow controller 6. By inputting the pre-calculated optimal helium flow rate for different temperature ranges of the load into the PLC, the PLC automatically sets the flow rate of flow controller 6 when it reads the load temperature. This enables a rapid and automatic cooling process for the entire cold helium system, reducing the workload of commissioning personnel and shortening the system's cooling time.

[0044] The calculation of cooling time for different flow rates was implemented using a MATLAB program. First, a load magnet was assumed, and its cold mass was calculated. Then, through heat transfer calculations of the system structure and the cooling capacity curves of the chiller at different temperatures, the cooling time of the magnet within a certain temperature range could be calculated. This example calculated the cooling time corresponding to different flow rates when the target load temperature dropped from 290K to 285K. Figure 3 As shown in the figure, in the initial stage of cooling, the cooling time corresponding to high flow rate helium is relatively short, but when the flow rate is greater than 1.4 g / s, the cooling time shows an increasing trend.

Claims

1. A method for controlling cryogenic helium gas circulation for room temperature pressurization, characterized in that, Includes the following steps: Step 1: Establish the cooling time mapping relationship of the load in different temperature ranges and under different circulating helium flow rates, and select the circulating helium flow rate value with the shortest cooling time in each temperature range as the optimal flow rate value. Step 2: During the system cooling process, obtain the actual temperature of the load in real time and determine the temperature range to which the actual temperature belongs; Step 3: Based on the judgment result, automatically adjust the flow controller (6) to set the circulating helium flow rate to the optimal flow rate value corresponding to the current temperature range.

2. The cryogenic helium gas circulation control method for room temperature pressurization according to claim 1, characterized in that, The mapping relationship in step one is established by combining MATLAB simulation calculation with experimental correction.

3. The cryogenic helium circulation control method for room temperature pressurization according to claim 1, characterized in that, The size of the temperature range is adjusted based on the computational load and the flow controller.

4. The cryogenic helium circulation control method for room temperature pressurization according to claim 1, characterized in that, Also includes: When the system stops or the load (12) is overloaded, the high-pressure helium in the pipeline is forced into the gas storage (1) through the back pressure valve (4) via the compressor (3).

5. A control system based on the cryogenic helium circulation control method for ambient temperature pressurization as described in any one of claims 1 to 4, comprising a gas storage chamber (1), a compressor (3), a cold box (10), and a heat exchanger (7) and a refrigerator cold head (8) located within the cold box (10), characterized in that, Also includes: A flow controller (6) is installed on the pipeline between the outlet of the compressor (3) and the cold box (10); A temperature sensor is installed on the load (12) to collect the load temperature in real time; The PLC controller is electrically connected to the temperature sensor and the flow controller (6). The PLC controller has a pre-stored table of the correspondence between different temperature ranges of the load and the optimal flow rate of circulating helium. The PLC controller automatically retrieves the optimal flow rate value from the corresponding table based on the load temperature fed back by the temperature sensor and controls the flow controller (6) to perform the corresponding flow rate adjustment.

6. The cryogenic helium circulation control system for ambient temperature pressurization according to claim 5, characterized in that, The outlet of the gas reservoir (1) is connected to the inlet of the compressor (3) through the first pressure reducing valve (2), and the inlet of the gas reservoir (1) is connected to the outlet of the compressor (3) through the back pressure valve (4).

7. The cryogenic helium circulation control system for ambient temperature pressurization according to claim 5, characterized in that, A second pressure reducing valve (5) is also provided on the outlet pipe of the compressor (3), which is located upstream of the flow controller (6).

8. The cryogenic helium circulation control system for ambient temperature pressurization according to claim 5, characterized in that, The heat exchanger (7) is a counter-flow shell-and-tube heat exchanger, with its inner tube inlet connected to the return gas end of the load (12) and its outer tube inlet connected to the output end of the flow controller (6).

9. The cryogenic helium circulation control system for ambient temperature pressurization according to claim 5, characterized in that, The load (12) is installed inside the test Dewar (11), and both the cold box (10) and the test Dewar (11) are independent vacuum-insulated containers.

10. The cryogenic helium circulation control system for ambient temperature pressurization according to claim 5, characterized in that, In the pre-stored correspondence table in the PLC controller, the optimal flow rate value corresponding to each temperature range is the critical flow rate value with the shortest cooling time; and when the load temperature decreases, the flow controller (6) adjusts the flow rate setting value accordingly.

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