Mist formation in a water vapor cold trap and evaluation of its removal rate

Mist formation conditions in a mixture of non-condensable gas (nitrogen or helium) and water vapor, mass transfer rates onto a cooled wall under the mist formation and choking time are determined in a case of a laminar flow in a vertical round tube cooled from the outside at constant temperatures of 77K to 263 K. This is a basic study of a cold trap for a fusion fuel clean-up (fcu) system. If mist is formed in the trap, it makes tritium spikes at the trap outlet, and consequently it causes tritium leakage to the outlet. The limiting condition of mist formation in the present system is quantitatively expressed by the critical supersaturation model (CSM). When the inlet vapor concentration is within the CSM mist formation condition, the outlet concentration is very high immediately after the gas supply and, afterward, it drops very quickly. When within the no-mist formation condition, the outlet concentration is almost constant throughout the frosting time. The trap shows a good removal performance for a long time until choking. This difference in the time variations is successfully explained by the relaxation of the mist formation condition with an increase in the thermal resistance of frost formed on cooled surfaces.