Predicting Long-Term Boil-Off Gas in Cryogenic Tanks Using Thermal Network and Thermodynamic Equilibrium Approaches

This study proposes an analytic model to predict boil-off gas (BOG) generation in cryogenic liquid storage tanks. The model simulates evaporation under isobaric conditions, where the liquid phase remains saturated, and the vapor phase is superheated. The simulation is divided into two stages: heat transfer analysis and thermodynamic response. In the first stage, a steady-state thermal network is developed to quantify heat ingress into the tank. This includes external heat convection, insulation conduction, and natural convection within the liquid and vapor phases. Additionally, heat transfer at the liquid-vapor interface is incorporated, calculating heat inputs to both phases. In the second stage, the liquid and vapor are modeled as lumped masses with uniform temperature in each phase. Their temperature variations and evaporation rates are analyzed based on heat inputs from the thermal network. The integration of steady-state heat transfer and dynamic thermodynamic responses allows for time-dependent predictions of evaporation behavior over extended periods. The model was validated using a 1.0 m³ liquid nitrogen storage tank, demonstrating close agreement with the BoilFAST model in predicting BOG generation, heat inputs, and vapor temperatures. This study provides a computationally efficient and reliable method for predicting long-term evaporation in cryogenic systems.