Desiccant cooling systems are spreading as a promising technology to reduce the energy consumption and environmental impact of conventional electric driven vapour compression systems for air conditioning purposes. Desiccant wheels (DWs) are the key component of the desiccant cooling systems which have received substantial attention. Desiccant Wheel if equipped with a purge section will show better performance, however in most cases purge section is not considered or a fixed purge angle is assumed. In this study, analysis of the purge angle effects on energy and dehumidification performances of DW is carried out and a novel optimal purge angle definition is introduced. A mathematical model is developed and validated in order to model the coupled heat and mass transfer processes in a DW. In addition, the effect of process and regeneration air velocities, regeneration air temperature, rotational speed, desiccant layer thickness, channel length (DW length) and channel hydraulic diameter on the purge angle are studied. The results showed that purge angle is a function of outlet air humidity profile, while the process air velocity as an operating parameter and channel length as a design parameter presented the most substantial effect on the profile. Furthermore, implementation of the optimal purge angle, improves the DW coefficient performance (DCOP) and results in desired conditions of outlet process air without the necessity of substantial increase in the DW size. (c) 2017 Elsevier Ltd. All rights reserved.