Evaporative heat sinks are used for space vehicles which perform autonomous reentry. They profit of the latent heat of evaporation to reject expendable heat in form of the vapour flow of the evaporation fluid into the environment. The Ammonia Boiler Assembly of HERMES Spaceplane is such an evaporative heat sink. Ammonia from dedicated storage devices is evaporated cooling the water of the closed ATCS water loop.Due to operation at different thermal loads and different ambient pressures, the evaporation process runs at different saturation conditions: The evaporation temperature varies between -33 °C and +5 °C. Potential freezing in the water loop according to the different mission phases with + 6 °C or 8 °C outlet temperatures is avoided using a special routing of the two fluids, water and ammonia, within the evaporator, by application of a special heat exchanger design and the selection of a two stage concept.