Air gap width has an important influence on the performance of air gap membrane distillation (AGMD) process. In this study, a novel double-pipe AGMD module (DP-AGMD-M) consisted of PVDF hollow fiber membrane and heat exchange capillary copper tubes was successfully developed. The evenly distribute and easily control of the air gap width in DP-AGMD-M can be implemented. The effect of the air gap width (d(a)), hot feed temperature (T-1), hot feed flow rate (Q), temperature difference (Delta T) between the hot feed outlet temperature and the cold feed inlet temperature, and the effective membrane module length (L) on the performance of DP-AGMD-M were experimental studied. The concept of the equivalent membrane distillation flux J(AGMD);) was firstly introduced in this paper and used to evaluate the comprehensive performance of DP-AGMD-M. The optimal performances, including membrane distillation flux (J), gained output ratio (GOR) and J(AGMD) were obtained. Within the experimental range, the maximum J arrived at 11.4 kg/(m(2) . h), the maximum GOR reached 6.6 and the maximum J(AGMD) was 29.6 kg/(m(2) . h). (C) 2016 Elsevier B.V. All rights reserved.