Keeping Medicine from Freezing Without Power: A Water-Ballast Carry Box Sized to Hold 2–8 °C Through a Winter Night

Many common medicines, including insulin and a large share of vaccines, must be kept between 2 °C and 8 °C. Warmth degrades them slowly, but freezing can destroy potency in a single night, and freezing is the risk wherever medicine is carried or stored through winter weather with no powered refrigeration. This paper designs and sizes a passive carry box that protects medicine by thermal ballast rather than by machinery. The box has two parts: a small inner chamber holding the medicine, and a surrounding jacket of ordinary water that is packed warm. A thermal partition between the jacket and the chamber meters the heat flow, so the jacket can start far warmer than 8 °C without cooking the medicine, and then feed heat inward as the night cools. A two-node lumped thermal model, integrated numerically, gives the time the chamber stays inside 2–8 °C. At an ambient temperature of −20 °C with the water starting at 24 °C, protection lasts 3.30 hours with 1 kg of water, 5.21 hours with 2 kg, 8.50 hours with 4 kg, 11.49 hours with 6 kg, 14.34 hours with 8 kg, and 17.12 hours with 10 kg. A 12-hour winter night at −20 °C therefore needs 6.4 kg of water ballast. The same mass of water mixed directly with the medicine, starting at the warmest legal packing temperature of 8 °C, protects for only 6.86 hours at 6 kg, so the partitioned warm jacket is worth roughly two-thirds more time than the obvious single-mass design. The starting temperature is a genuine design window, not a detail: with 8 kg at −20 °C, starts between 22 °C and 26 °C deliver at least 12 hours, while a 27 °C start drives the chamber above 8 °C within 3.93 hours. At −30 °C the design largely fails: even 10 kg holds the band for only 8.74 hours, because the partition that protects the medicine from a warm jacket also starves it when the jacket is losing heat too fast. The sizing rule, the window, and the cold limit are all stated so that a reader can reproduce or refute them from the assumptions alone.