Assessment of the cold storage performance of a device used in deep frozen cold chain

Dry-shipper is a technical name for a device used to transport deep frozen biomedical samples. The device contains a porous material filled with liquid nitrogen to avoid the hazard of a spillage during transportation. The factors influencing the evaporation rate of liquid nitrogen in the device is of crucial importance to guarantee the safe storage of the frozen samples. We measured the weight and temperature of the dry-shipper during normal operation. The mass of a fully charged dry-shipper decreased linearly with time indicating a fairly constant rate of liquid nitrogen evaporation. The temperature of the sample holder remained at a fairly constant value for about 30 days. After all the liquid nitrogen had evaporated, we would expect an exponential increase of the dry-shipper temperature. However, the increase observed is linear. The reasons for this uncharacteristic behaviour of the device are investigated. The heat leak and the heat capacity of the system is determined from the evaporation rate and the warmup curve, respectively. The experimental data and the accompanying physical model give insights into the main physical parameters contributing to the evaporation of liquid nitrogen in the dry-shipper.