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 drops to below 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 of the dry ice. We performed a series of experiments and determined the dry ice temperature with the change of dry ice sublimation rates under various external conditions. The results show that the sublimation temperature dropped to below -80°C and decreased further when the dry ice sublimation rate was artificially increased. The lowest temperature obtained from the experiments was -100.6°C, 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 materials 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 drop due to excessive sublimation cooling effect but will not reach below -100°C under normal shipping environments.