Experimental Investigation of Double Pipe Heat Exchangers for Cryogenic Applications

This study aims to directly recover cold energy from liquid nitrogen evaporation for cooling applications utilizing a double-pipe heat exchanger. The double-pipe heat exchanger has an inner tube inner diameter of 10.7mm, an inner tube outer diameter, an outer tube inner diameter of 20mm, and an outer tube outer diameter of 24mm. Experiments were carried out with saturated liquid nitrogen with an inlet temperature of 93 K and mass flux of 66, 144, and 238 kg/s-m2 flowing at the tube side while simultaneously being heated by a heat transfer fluid (HTF) at the annular side of the heat exchanger. Ethylene glycol-water mixture (35% wt.) with inlet temperatures ranging from 295 to 300 K and flow rates ranging from 0.03 to 0.22 kg/s. The annulus side of the double-pipe heat exchanger was split into three sections to avoid severe ice accumulation due to the inevitable decrease in the HTF temperature as the HTF flows along. The results show that the outer surface heat flux of the inner tube increases substantially with the increase of liquid nitrogen flow rate, which suggests that the flowing nitrogen is in the film boiling region. Furthermore, experimental results show the feasibility of utilizing film boiling for liquid nitrogen cold energy recovery. The vapor blanket formed at the wall surface prevents the wall temperature from overcooling while recovering large amounts of cold energy due to the immense temperature difference between the HTF and nitrogen.