This study shows that the main design features of a direct contact humidification-dehumidification (HD) desalination process can be determined based on the method of the constructal design. A direct contact HD unit has three main sections: the humidifier, the dehumidifier and the heat source. To analyze and optimize such unit, two flow models for air circulation are considered: natural and forced circulations. The objective is increasing the water production rate over the volume occupied by the plant. The parameters considered in this work are the air flow rate, tube diameters ratio, temperatures and flow rates of inlet saline and fresh water. The Lagrangian multipliers and genetic algorithm (GA) methods are used to optimize the production rate subject to global constraint (fixed volume). The constructal design developed in this paper explains the influences of the inlet cold and hot water temperatures and air flow rate on the basic design parameters of HD system architecture. It shows that the optimal configuration of HD system is consequently temperature dependent. Finally, the analysis shows that if the humidifier inlet water temperature is high, recycling of the humidifier outlet water results in about 15-25% reduction in the specific thermal energy consumption. (c) 2012 Elsevier B.V. All rights reserved.