Improvement in pipe chilldown process using low thermal conductive layer

•Experimental investigation of the chilldown process in piping using liquid nitrogen.•Promotion of boiling heat transfer using a thin layer with low thermal conductivity.•Increase in the minimum heat flux point temperature induced by the layer.•Total mass of liquid nitrogen consumed in the process reduced by up to 64%. A method for reducing the time and total mass of cryogenic fluid required for a chilldown process in piping was experimentally investigated in this study. The inner wall of a pipe with an outer diameter of 1/4″ (=6.35mm) was coated with Polytetrafluoroethylene, which has a low thermal conductivity. Liquid nitrogen (LN2) was supplied to the pipe at a constant tank pressure of 120–170kPa. The fluctuations of the two-phase flow, which were composed of LN2 and gas phase nitrogen, were observed. A pipe without an insulating layer and three other pipes with insulating layers of thicknesses 23μm, 63μm, and 91μm, respectively, were used in the experiment. The results indicated that the temperature of the minimum heat flux point (MHF) was higher for the pipe with the insulating layer. This increased temperature caused earlier transition to nucleate boiling. Furthermore, the total mass of LN2 consumed in the chilldown process could be retrenched up to a maximum of 64%. The heat flux decreased after reaching the MHF point; however, heat flux after MHF point is not dominant to overall chilldown time. The effect of the layer to increase the temperature of MHF point is dominant to overall chilldown time, which results in the decrease in the chilldown time and the total mass of LN2 consumed in the chilldown process.