In the present work, the design and analysis of a humidification-dehumidification (HD) unit is conducted by applying the concept of constructal theory proposed by Bejan. This study is an innovative design method which includes new useful data on the evaluation of geometric aspect ratio of a humidification-dehumidification desalination process. According to the constructal theory, the acquisition of geometry is the mechanism by which the global flow system achieves global flow performance under global constraints (e.g., availability of space, volume, etc). The circulation of air and water vapor mixture is driven by buoyancy in one large tube with packing bed (the humidifier) and between many parallel smaller tubes (the dehumidifier). Considering the global constraint (fixed flow volume), a procedure is developed to optimize the fresh water production rate by applying the Lagrange multipliers method. In this study, the air circulation rate, the rate of water production, the tube diameters and the number of condenser tubes are significant features of the flow configuration inducted by the constructal design. Furthermore, the effect of changing in the size of the flow system on the flow architecture is obtained. (C) 2010 Elsevier B.V. All rights reserved.