High-voltage power cable manufacturing method based on non-crosslinked polyethylene insulation

Abstract

The invention discloses a manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, and belongs to the technical field of cable manufacturing. The problems of high energy consumption, high carbon emission, long production period and large deformation rate of high-voltage cable production in the prior art are solved. According to the method disclosed by the invention, the non-crosslinked PE material is combined with a low-temperature vulcanization process, so that the decomposition pollution of the crosslinking agent is eliminated; three-stage low-temperature vulcanization is adopted, the vulcanization temperature is reduced by 50% or above compared with that of a traditional process, and HDPE molecular chains are arranged in order in cooperation with segmented heat preservation; moisture interference is avoided through dynamic temperature control nitrogen full-dry type cooling, and the cooling shrinkage rate of the cable core is reduced through matching of the flow velocity and the temperature gradient; mathematical models of the nitrogen flow velocity, the temperature gradient and the wire core diameter are established, real-time matching of the nitrogen flow velocity is achieved, and cooling uniformity is ensured.

Description

Technical Field

[0001] The present invention relates to a manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, belonging to the technical field of cable manufacturing. Background Art

[0002] Under the background of the accelerating global energy structure transformation towards low-carbon, the demand for environmentally friendly cables in the power transmission system is becoming increasingly urgent. However, traditional 110 kV crosslinked polyethylene (XLPE) insulated cables face many environmental and process challenges during production:

[0003] High energy consumption: The high-temperature vulcanization crosslinking process needs to be adopted during production, maintaining a high temperature above 300 °C for a long time, resulting in high energy consumption.

[0004] High carbon emissions: Approximately 1.8 tons of CO2 are generated during the production of each ton of material, with a large carbon emission intensity.

[0005] Long production cycle: The insulation layer has a large thickness (≥15 mm), which exacerbates the degassing difficulty and extends the production cycle to 20 - 30 days.

[0006] Process defects: The conventional water cooling process is likely to cause the deformation rate of a large cross-section wire core (2500 mm 2 ) to be > 0.5%, and at the same time, the oxidation risk of the high-temperature vulcanization tube will affect the dielectric properties of the insulation layer.

[0007] To solve the above problems, developing non-crosslinked polyethylene (PE) insulated cables and reforming the production process has become a breakthrough solution. This solution is expected to significantly reduce energy consumption and carbon emissions, while shortening the production cycle and improving process defects, meeting the urgent demand of the power transmission system for environmentally friendly cables. Summary of the Invention

[0008] Aiming at the problems existing in the prior art, the present application proposes a manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation. The cable produced by this method has good environmental protection and economic benefits on the premise of ensuring product performance.

[0009] To solve the above technical problems, the technical solution adopted by the present invention is a manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, including the following steps:

[0010] 1) Treat the metal conductor material and form a cable conductor;

[0011] 2) After covering an insulation shielding layer on the cable conductor, form a cable core wire;

[0012] 3) Vulcanize the covered cable core wire;

[0013] 4) Post-treat the vulcanized cable core wires to form a cable;

[0014] In step 2), the insulating shielding layer coated on the cable conductor includes an insulating layer, and the material of the insulating layer includes non-crosslinked polyethylene (PE);

[0015] In step 3), a multi-stage vulcanization method is adopted during vulcanization, and the vulcanization temperature is 55 - 70 °C.

[0016] Optimally, in the above-mentioned manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, in step 1), the process of treating the metal conductor material to form a cable conductor includes:

[0017] Drawing and annealing copper or aluminum materials, and then coating an antioxidant on the surface; then stranding the metal wires that have been drawn and surface-coated with an antioxidant to form a cable conductor.

[0018] Optimally, in the above-mentioned manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, in step 2), the insulating shielding layer coated on the cable conductor includes an inner shielding layer, an insulating layer, and an outer shielding layer, which are arranged in sequence from the inside out;

[0019] When coating the insulating shielding layer on the cable conductor, the inner shielding layer, the PE insulating layer, and the outer shielding layer are extruded synchronously; during extrusion, the temperature of the extruder is controlled in three stages, the temperature of the feeding section is 120 - 130 °C, the temperature of the compression section is 140 - 150 °C, and the temperature of the homogenization section is 160 - 170 °C.

[0020] Optimally, in the above-mentioned manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, in step 2), the melt flow index (MFI) of the non-crosslinked polyethylene (PE) of the insulating layer is 0.1 - 0.5 g / 10 min, 190 °C / 5 kg;

[0021] The density of the non-crosslinked polyethylene (PE) of the insulating layer ≥ 0.92 g / cm 3 ;

[0022] The dielectric strength of the non-crosslinked polyethylene (PE) of the insulating layer at 20 °C ≥ 30 kV / mm.

[0023] Optimally, in the above-mentioned manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, the non-crosslinked polyethylene (PE) of the insulating layer includes ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE), and the molecular weight of the non-crosslinked polyethylene (PE) of the insulating layer is 1.5 million, and the melt index is 0.5.

[0024] Optimally, in the above-mentioned manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, the material of the insulating layer includes a modified additive and an anti-tracking agent; the modified additive includes nano boron nitride and a grafted voltage stabilizer;

[0025] The weight ratio of nano boron nitride is 2-3 wt%, and the weight ratio of the grafted voltage stabilizer is 1.5 wt%; The tracking resistance agent includes organosilicon-modified alumina, and the weight ratio of organosilicon-modified alumina is 1 wt%.

[0026] Optimally, in the above method for manufacturing a high-voltage power cable based on non-crosslinked polyethylene insulation, in step 3), the insulation layer is heat-set through a three-stage vulcanization tube, and the temperature of the tube wall decreases gradually along the traveling direction of the cable core; The three-stage vulcanization tube includes a first stage, a second stage, and a third stage arranged in sequence along the traveling direction of the cable core;

[0027] The vulcanization temperature of the first stage is 65-70 °C, and the heat preservation time is ≥30 s; The vulcanization temperature of the second stage is 60-65 °C; The vulcanization temperature of the third stage is 55-60 °C; The total vulcanization time of the insulation layer is 3-5 minutes.

[0028] Optimally, in the above method for manufacturing a high-voltage power cable based on non-crosslinked polyethylene insulation, in step 4), when post-treating the vulcanized cable core, first cool the cable core, and then sequentially weld the metal sheath and extrude the outer sheath layer on the cooled cable core;

[0029] When cooling the cable core, a segmented multi-stage cooling channel is adopted, and the multi-stage cooling channel is axially divided into a preheating zone, a gradient cooling zone, and a constant temperature setting zone according to the cable core;

[0030] Nitrogen at 40-45 °C is introduced into the preheating zone; The gradient cooling zone is divided into 4 sub-zones, and the 4 sub-zones of the gradient cooling zone are arranged in sequence along the axis of the cable core. The temperatures of the nitrogen introduced into the 4 sub-zones of the gradient cooling zone are 55 °C, 45 °C, 35 °C, and 25 °C in sequence; Nitrogen circulation at 25 °C is maintained in the constant temperature setting zone;

[0031] The purity of the nitrogen introduced into the multi-stage cooling channel is ≥99.999%, and the dew point is ≤-70 °C.

[0032] Optimally, in the above method for manufacturing a high-voltage power cable based on non-crosslinked polyethylene insulation, the length ratio of the preheating zone, the gradient cooling zone, and the constant temperature setting zone of the multi-stage cooling channel is 1:3:1;

[0033] The cooling rate of the 4 sub-zones of the gradient cooling zone is ≤2 °C / min;

[0034] Nitrogen circulation at 25 °C is maintained in the constant temperature setting zone for 5-8 minutes.

[0035] Optimized, for the above manufacturing method of high-voltage power cables based on non-crosslinked polyethylene insulation, the nitrogen gas flow rate introduced into the multi-stage cooling channels is dynamically matched. The nitrogen gas flow rate V is adjusted based on the cable cross-sectional area S, where V = 0.05×S + 3, and the fluctuation range of the nitrogen gas flow rate V is ±0.2 m / s;

[0036] The cavity of the multi-stage cooling channel is at a slightly positive pressure, with a pressure of 1.05 - 1.1 atm;

[0037] Based on the core diameter D and the cooling rate ΔT / Δt, calculate the nitrogen gas flow rate and the temperature gradient, satisfying ΔT / Δt ≤ 0.02×D.

[0038] The beneficial effects of this application are as follows:

[0039] In the technical solution of this application, through the combination of non-crosslinked PE material and low-temperature vulcanization process, the decomposition pollution of the crosslinking agent is eliminated, and at the same time, the compactness of the insulating layer is ensured (porosity < 0.01%).

[0040] The technical solution of this application adopts three-stage low-temperature vulcanization, and the vulcanization temperature is reduced by more than 50% compared with the traditional process. With segmented heat preservation, the HDPE molecular chains are arranged orderly, and the breakdown voltage is increased by 15% - 20%.

[0041] In the technical solution of this application, a dynamic temperature-controlled nitrogen gas cooling method is adopted. Nitrogen gas full-dry cooling is used to avoid moisture interference. Through the matching of the flow rate and the temperature gradient (algorithm control), the core cooling shrinkage rate ≤ 0.05%, which is much lower than the industry standard (≤ 0.2%).

[0042] Voltage adaptability design is adopted in this application. Considering the characteristics of the insulation layer thickness (10 - 15 mm) for the 110 kV voltage level, through segmented cooling + low-speed temperature reduction, the temperature difference between the inside and outside of the insulation layer ≤ 3°C, avoiding internal cavities caused by uneven cooling of the thick-wall structure (the cavity rate of the traditional process ≥ 0.1%, and that of this application ≤ 0.03%).

[0043] In this application, dynamic flow rate-temperature coupling control is adopted to establish a mathematical model of nitrogen gas flow rate (V), temperature gradient (ΔT), and core diameter (D), realizing real-time matching of V = f(ΔT, D) to ensure cooling uniformity (the eccentricity of the insulation layer ≤ 5%).

[0044] In this application, micro-impurity monitoring and treatment are added. An on-line gas chromatograph is set in the nitrogen gas circuit to detect the impurity content such as CO and CH4 in real time (the threshold ≤ 1 ppm), and the purification program is automatically started when the standard is exceeded.

[0045] The technical solution of this application also has the following advantages.

[0046] Environmental protection: It can eliminate the peroxide cross-linking process, with VOCs emissions reduced by more than 90%. Cost-effectiveness: The production cycle is shortened by 25%, and the nitrogen recycling rate > 95%; Product performance: The eccentricity of the insulation layer ≤ 8% (national standard ≤ 15%), partial discharge < 5 pC (under 1.5U0). Improvement in insulation performance: Power frequency breakdown field strength ≥ 40 kV / mm (traditional non-crosslinked PE cable ≤ 35 kV / mm); Partial discharge ≤ 3 pC (under 1.5U0, U0 = 64 kV). Deformation control: The radial shrinkage rate of the core after cooling ≤ 0.03%, and the axial bending degree ≤ 0.1 mm / m. Economy: Nitrogen consumption is reduced to 0.5 m 3 / km (traditional process ≥ 1.2 m 3 / km). Brief Description of the Drawings

[0047] Figure 1 It is a schematic diagram of the structure of the trial-produced cable for the technical solution of this application;

[0048] In the figure, 1 is the compacted segmented copper conductor, 2 is the wrapped conductor shield, 3 is the extruded conductor shield, 4 is the non-crosslinked polyethylene insulation, 5 is the insulation shield, 6 is the water-blocking buffer layer, 7 is the corrugated aluminum sheath, 8 is the anticorrosion layer, the flame-retardant PVC outer sheath and the conductive layer. Detailed Embodiments

[0049] The following further elaborates on the technical features of the present invention in combination with specific embodiments.

[0050] The steps of the manufacturing method of the high-voltage power cable based on non-crosslinked polyethylene insulation of this application are as follows:

[0051] Core steps of the manufacturing process:

[0052] 1) Core pretreatment

[0053] The copper / aluminum conductor is coated with an antioxidant on the surface after wire drawing and annealing, and preheated to 40 - 50 °C to enhance the adhesion with the insulation layer.

[0054] 2) Extrusion coating

[0055] Adopt a three-layer co-extrusion process, and the inner shield layer, PE insulation layer, and outer shield layer are extruded synchronously. The temperature of the extruder is controlled in three sections. The temperature of the feeding section is 120 - 130 °C, the temperature of the compression section is 140 - 150 °C, and the temperature of the homogenization section is 160 - 170 °C.

[0056] 3) Vulcanization treatment

[0057] The insulation layer is heat-set through a three-stage vulcanization tube, and the temperature of the tube wall decreases gradually:

[0058] ■ The first stage: 65 - 70 °C (holding time ≥ 30 s);

[0059] ■Second stage: 60 - 65°C;

[0060] ■Third stage: 55 - 60°C;

[0061] The total vulcanization time is controlled within 3 - 5 minutes to avoid thermal aging of the material.

[0062] 4) Post - treatment

[0063] After cooling, the wire core is successively welded to the metal sheath and the outer sheath layer is extruded.

[0064] Among them, other improvements are made in each step, including:

[0065] Replacement of materials

[0066] The insulating layer material is replaced from cross - linked polyethylene XLPE to non - cross - linked polyethylene PE. The material properties include:

[0067] The melt index (MFI) is 0.1 - 0.5 g / 10 min (190°C / 5 kg); the density ≥ 0.92 g / cm 3 ; the dielectric strength ≥ 30 kV / mm (20°C). By combining the non - cross - linked PE material with the low - temperature vulcanization process, the decomposition pollution of the cross - linking agent is eliminated, and at the same time, the compactness of the insulating layer is ensured. For example, the porosity < 0.01%.

[0068] The material system of this application adopts a special high - voltage material system. The base material takes the form of UHMWPE / HDPE blend, with a molecular weight of 1.5 million and a melt index of 0.5. The vulcanization temperature of this application is reduced by more than 50% compared with the traditional process. With segmented heat preservation, the HDPE molecular chains are arranged orderly, and the breakdown voltage is increased by 15% - 20%

[0069] And modified additives are added to this material system, including 2 - 3 wt% of nano - boron nitride to increase the thermal conductivity to 0.45 W / (m·K); 1.5 wt% of grafted voltage stabilizer to inhibit space charge; 1 wt% of silicone - modified alumina as an anti - tracking agent.

[0070] In the above vulcanization and cooling process, the structure of the cooling system is improved. A segmented multi - stage cooling channel is adopted, which is divided into a pre - heating zone (A), a gradient cooling zone (B), and a constant - temperature shaping zone (C) along the axial direction of the cable. The length ratio of each zone is 1:3:1. When the total length of the segmented multi - stage cooling channel is 20 m, the pre - heating zone (A), the gradient cooling zone (B), and the constant - temperature shaping zone (C) are 4 m, 12 m, and 4 m respectively.

[0071] During the vulcanization and cooling process, nitrogen at 40 - 45°C is introduced into the pre - heating zone (A) to slowly reduce the surface temperature of the wire core from 70°C at the vulcanization outlet to 60°C, avoiding stress concentration caused by sudden cooling due to the contraction of molecular chains.

[0072] The gradient cooling zone (B) is divided into 4 sub-zones, and the nitrogen temperatures are 55°C, 45°C, 35°C, and 25°C in sequence. Each sub-zone is equipped with an independent temperature control unit, and the cooling rate ≤ 2°C / min, which is better than the conventional 3°C / min.

[0073] Maintain nitrogen circulation at 25°C in the constant temperature setting zone (C) for 5 - 8 minutes to eliminate residual thermal stress.

[0074] Moreover, during the vulcanization cooling process, the nitrogen parameters are precisely controlled.

[0075] The nitrogen purity and humidity need to be maintained at a nitrogen purity ≥ 99.999% and a dew point ≤ -70°C, which can effectively prevent trace moisture from affecting the insulation performance. Nitrogen full-dry cooling is adopted to avoid moisture interference. By matching the flow rate with the temperature gradient, the cooling shrinkage rate of the wire core ≤ 0.05%, which is much lower than the industry standard of ≤ 0.2%.

[0076] Aiming at the characteristics of the insulation layer thickness of 10 - 15 mm for the 110 kV voltage grade, through segmented cooling + low-speed cooling, the temperature difference between the inside and outside of the insulation layer ≤ 3°C, avoiding internal cavities caused by uneven cooling of the thick-wall structure. Under the traditional process, the cavity rate of the cable wire core ≥ 0.1%, while in this application ≤ 0.03%.

[0077] The nitrogen flow rate is dynamically matched. Based on the cable cross-sectional area S, the nitrogen flow rate V is adjusted: V = 0.05×S + 3 (m / s). For example, when the cross-sectional area is 800 mm 2 ², V = 0.05×800 + 3 = 7 m / s. The flow rate fluctuation range is ±0.2 m / s, which is adjusted in real time by a closed-loop PID control system.

[0078] The pressure in the segmented multi-stage cooling channel needs to be controlled to prevent external air from infiltrating. In this embodiment, a slightly positive pressure of 1.05 - 1.1 atm is maintained in the cooling cavity.

[0079] In this application, through the optimization of the lower sealing device, the deformation of the cable wire core is suppressed, including the optimization of the lower sealing device and the stress compensation algorithm of the cable wire core. Specifically,

[0080] A double-layer elastic sealing ring (silicone + polytetrafluoroethylene) is adopted, with a radial pressure ≤ 0.08 MPa and an axial frictional force ≤ 50 N / m.

[0081] A temperature sensor (accuracy ±0.5°C) is embedded in the inner wall of the sealing ring to monitor the surface temperature of the wire core in real time and feedback it to the nitrogen temperature control system.

[0082] Based on the wire core diameter (D) and the cooling rate (ΔT / Δt), the nitrogen flow rate and the temperature gradient are automatically calculated to satisfy the formula:

[0083] ΔT / Δt ≤ 0.02 × D, where the unit of ΔT / Δt is °C / (min·mm). For example, when the core diameter is 50 mm, the maximum allowable cooling rate is 1 °C / min.

[0084] Moreover, in this embodiment, the energy efficiency of the nitrogen cycle can also be improved by means of heat exchanger integration and gas purification.

[0085] Heat exchanger integration: Recover the waste heat of the cooling exhaust gas, about 50 - 60 °C, for preheating the newly injected nitrogen, and the energy saving rate is ≥ 35%.

[0086] Configure a gas purification module: Configure a molecular sieve adsorption tower to remove trace low-molecular substances volatilized from the insulation layer, such as residual antioxidants, to ensure the purity of the nitrogen cycle.

[0087] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions, or substitutions made by those of ordinary skill in the art within the scope of the essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation, characterized in that: It includes the following steps: 1) Treat the metal conductor material and form a cable conductor; 2) After coating an insulating shielding layer on the cable conductor, form a cable core wire; 3) Cure the coated cable core wire; 4) Post-treat the cured cable core wire and form a cable; In step 2), the insulating shielding layer coated on the cable conductor includes an insulating layer, and the material of the insulating layer includes non-crosslinked polyethylene (PE); In step 3), a multi-stage curing method is adopted during curing, and the curing temperature is 55-70 °C.

2. The manufacturing method of the high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, wherein: In step 1), the process of treating the metal conductor material and forming a cable conductor includes: Drawing and annealing copper or aluminum materials, and then coating an antioxidant on the surface; then stranding the drawn metal wires with an antioxidant-coated surface to form a cable conductor.

3. The manufacturing method of the high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, characterized in that: In step 2), the insulating shielding layer coated on the cable conductor includes an inner shielding layer, an insulating layer, and an outer shielding layer, and the inner shielding layer, insulating layer, and outer shielding layer are arranged in sequence from the inside out; When coating the insulating shielding layer on the cable conductor, the inner shielding layer, PE insulating layer, and outer shielding layer are extruded synchronously; during extrusion, the temperature of the extruder is controlled in three sections. The temperature of the feeding section is 120-130 °C, the temperature of the compression section is 140-150 °C, and the temperature of the homogenization section is 160-170 °C.

4. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, wherein: In step 2), the melt index (MFI) of the non-crosslinked polyethylene (PE) of the insulating layer is 0.1-0.5 g / 10 min, 190 °C / 5 kg; The density of the non-crosslinked polyethylene (PE) of the insulating layer is ≥ 0.92 g / cm 3 ; The dielectric strength of the non-crosslinked polyethylene (PE) of the insulating layer at 20 °C is ≥30 kV / mm.

5. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, wherein: The non-crosslinked polyethylene (PE) of the insulating layer includes ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE), the molecular weight of the non-crosslinked polyethylene (PE) of the insulating layer is 1.5 million, and the melt index is 0.

5.

6. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, wherein: The material of the insulating layer includes a modified additive and an anti-tracking agent; the modified additive includes nano boron nitride and a grafted voltage stabilizer; The weight ratio of nano boron nitride is 2-3 wt%, and the weight ratio of the grafted voltage stabilizer is 1.5 wt%; The anti-tracking agent includes organosilicon-modified alumina, and the weight ratio of organosilicon-modified alumina is 1 wt%.

7. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, wherein: In step 3), the insulating layer is heat-set through a three-stage curing tube, and the temperature of the tube wall decreases gradually along the traveling direction of the cable core wire; the three-stage curing tube includes a first stage, a second stage, and a third stage arranged in sequence along the traveling direction of the cable core wire; The curing temperature of the first stage is 65-70 °C, and the holding time is ≥30 s; the curing temperature of the second stage is 60-65 °C; the curing temperature of the third stage is 55-60 °C; the total curing time of the insulating layer is 3-5 minutes.

8. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 1, characterized in that: In step 4), when post-treating the vulcanized cable core, first cool the cable core, and then sequentially weld a metal sheath and extrude an outer sheath layer on the cooled cable core; When cooling the cable core, a segmented multi-stage cooling channel is adopted. The multi-stage cooling channel is axially divided into a preheating zone, a gradient temperature reduction zone, and a constant temperature shaping zone according to the cable core; Nitrogen at 40-45 °C is introduced into the preheating zone; the gradient temperature reduction zone is divided into 4 sub-zones. The 4 sub-zones of the gradient temperature reduction zone are arranged in sequence along the axis of the cable core. The temperatures of the nitrogen introduced into the 4 sub-zones of the gradient temperature reduction zone are 55 °C, 45 °C, 35 °C, and 25 °C in sequence; nitrogen circulation at 25 °C is maintained in the constant temperature shaping zone; The purity of the nitrogen introduced into the multi-stage cooling channel is ≥99.999%, and the dew point is ≤ -70 °C.

9. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 8, characterized in that: The length ratio of the preheating zone, the gradient temperature reduction zone, and the constant temperature shaping zone of the multi-stage cooling channel is 1:3:1; The temperature reduction rate of the 4 sub-zones of the gradient temperature reduction zone is ≤2 °C / min; Nitrogen circulation at 25 °C is maintained in the constant temperature shaping zone for 5-8 minutes.

10. The manufacturing method of a high-voltage power cable based on non-crosslinked polyethylene insulation according to claim 8, characterized in that: The nitrogen flow rate introduced into the multi-stage cooling channel is dynamically matched. The nitrogen flow rate V is adjusted based on the cable cross-sectional area S, V = 0.05×S + 3, and the fluctuation range of the nitrogen flow rate V is ±0.2 m / s; The inside of the multi-stage cooling channel is at a slightly positive pressure, and the pressure is 1.05-1.1 atm; Based on the core diameter D and the cooling rate ΔT / Δt, calculate the nitrogen flow rate and the temperature gradient to satisfy ΔT / Δt ≤ 0.02×D.

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