Impact of Excessive Sublimation Cooling on the Internal Temperature of Passive Shippers Cooled by Dry Ice.

The internal temperature of a passive thermal shipper employing dry ice as the coolant often decreases to less than the equilibrium sublimation point (-78.5°C) during shipment. In this paper, we demonstrate that in many cases, this unexpected phenomenon is due to excessive sublimation cooling by the dry ice. We performed a series of experiments and determined the dry ice temperature with the change in dry ice sublimation rates under various external conditions. The results show that the sublimation temperature decreases to less than -80°C and decreases further when the dry ice sublimation rate is artificially increased. The lowest temperature observed in the experiments was -100.6°C, which occurred when the dry ice nuggets were subjected to a forced-air cooling condition, which is an extreme situation that is not expected to happen in actual shipping environments. Several types of shippers were also tested to investigate the impact of shipper material and shipper design. The lowest temperature observed in these shippers was -93.2°C when the shipper was placed on its side instead of upright. Based on the results from these controlled experiments, we conclude that the temperature inside a shipper cooled by dry ice will decrease because of excessive sublimation cooling effect but will not reach less than -100°C in normal shipping environments.