In this paper, melting of a phase change material (PCM) in a multi-tube heat exchanger (MTHX) is investigated. Water, as the heat transfer fluid (HTF), flows through the inner tube/tubes and the outer one while RT35 as the PCM fills the middle. The aim of this study is to investigate the effect of number of inner tubes as a geometrical parameter during charging process. Also consequences of increasing operational parameters including the HTF mass flow rate and inlet temperature are studied. In order to understand the effects of the proposed configurations, a comparison between double pipe and simple MTHX is carried out. Results show that as the inlet temperature increases melting process accelerates and complete melting time reduces, whereas, similar mass flow rate increase doesn't reduce the melting time to such an extent. By increasing the number of inner tubes from 1 to 4 in the shell side of the MTHX, melt region enlarges and its including vortices strengthens which leads to a dominated convective heat transfer and thus a higher melting rate. This increase in number of tubes leads to 29% reduction in melting time. (c) 2015 Elsevier Ltd. All rights reserved.