Humidification dehumidification (HDH) is a promising technology for small-scale seawater desalination and has widespread application in drinking and industrial water treatment systems. This paper demonstrates the significance of a novel parameter known as the 'modified heat capacity rate ratio' (HCR) in the thermal design of HDH systems and in simultaneous heat and mass exchange (HME) devices. HCR is of particular importance in developing a fundamental understanding of the concept of thermodynamic balancing. A pilot-scale HDH unit (with a peak production capacity of 700 l/day) has been constructed and detailed experiments have been performed on this unit. Based on these experiments, the recently developed theories behind the design of HDH systems with or without mass extraction and injection are validated. Furthermore, important concepts with regard to design and optimization of HME devices have been studied in the present experiments. (C) 2012 Elsevier Ltd. All rights reserved.