Abstract: A payload loaded into a chamber is cycled to a low temperature of about -320.degree. F. using liquid nitrogen fed to a heat exchanger evaporator that is located at the top of the chamber so that gaseous nitrogen vapor from the evaporator, at substantially the same temperature as the liquid nitrogen, is circulated to a payload in the chamber below, and, at the same time, gas from the chamber is circulated upward to highly thermally conductive fins on the heat exchanger that are cooled by the liquid nitrogen evaporation. Thus, heat from the payload is fed from the gas circulating upward to the heat exchanger to evaporate the liquid nitrogen and so the payload located at the bottom of the chamber is cooled by gas kinetics and is never touched by the liquid nitrogen.