Dynamic Simulation and Automatic Control Study of the Precooling Process at 80 K for Large Helium Refrigerators

In the Comprehensive Research Facility for Fusion Technology （CRAFT）, a large-scale helium refrigeration system provides a long-term stable cryogenic environment for superconducting magnet testing. This paper investigates the complex dynamic characteristics of the 80 K precooling process of this system. Based on EcosimPro and its cryogenic library CRYOLIB, a combined approach of dynamic simulation and process analysis was adopted. In particular, a four-stream plate-fin heat exchanger was reconstructed in the model, and a full-process model of the 80 K precooling loop was established.Simulation results showed that the cold box system has very large thermal inertia. The controlled object exhibits a pronounced long time-delay response and strong nonlinearity associated with gas-liquid phase change. At the initial stage of cooldown, the model predicted an excessive temperature drop rate of up to 5 K/min and a hazardous temperature difference exceeding 40 K. These results indicate that, under open-loop operation, the process is exposed to substantial thermal-shock risk and cannot satisfy safe cooldown requirements.To address these issues, an optimized control strategy was developed. It includes a dual-constraint control loop based on both temperature difference and temperature drop rate, as well as pressure and liquid-level control loops for the liquid-nitrogen separator. The dual-constraint design is intended to simultaneously suppress local thermal gradients and limit the overall cooldown slope, while separator pressure and inventory are regulated to maintain stable operating conditions during transients.Further simulation-based validation showed that the optimized strategy keeps both the temperature difference and the temperature drop rate within predefined safety thresholds. Meanwhile, the pressure and liquid level of the liquid-nitrogen separator remain within expected operating ranges. Therefore, the proposed strategy effectively prevents thermal shock in this large-scale, high-inertia cryogenic system and significantly improves the reliability and operational safety of the 80 K precooling process in the CRAFT helium refrigeration system.