A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part III: Unsteady-state conditions (evacuation period)

This paper analyses the evacuation period of a 300 L super-insulated cryogenic storage tank for liquid nitrogen. Storage tank and radiation shields are the same as in part I of this paper. The present analysis extends application of stationary fluid networks to unsteady-states to determine local, residual gas pressures between shields and the evacuation time of a multilayer super-insulation. Parameter tests comprise magnitude of desorption from radiation shields, spacers and container walls and their influence on length of the evacuation period. Calculation of the integrals over time-dependent desorption rates roughly confirms weight losses of radiation shields obtained after heating and out-gassing the materials, as reported in the literature. After flooding the insulation space with dry N2-gas, the evacuation time can enormously be reduced, from 72 to 4 h, to obtain a residual gas pressure of 0.01 Pa in-between shields of this storage tank. Permeation of nitrogen through container walls is of no importance for residual gas pressures. The simulations finally compare freezing H2O-layers adsorbed on shields, spacers and container walls with flooding of the materials.