In this paper, melting and solidification processes of phase change material embedded in metallic porous foam in a multi-tube heat exchanger are investigated numerically under local non-equilibrium thermal condition. RT 35 is used as phase change material in middle shell of 3D multi-tube heat exchanger. Also, water flows across inner tube/tubes and outer tube as a heat transfer fluid (HTF). The effects of number of inner tubes, their arrangement as well as porosity of metallic foam on thermal characteristics of heat storage unit are studied. Results show that increasing number of inner tubes and adding metallic foam enhance melting and solidification rates significantly. A composite of phase change material/metallic foam with porosity of epsilon = 0.7 engenders shorter melting and solidification time comparing to pure PCM. Arrangement of inner tubes has no effect on melting rate of metal foam/PCM composite. By inserting metallic foam with porosities epsilon = 0.9and epsilon = 0.7, melting time is decreased by 14% and 55%, respectively. Highest melting rate is for case with four inner tubes for all porosities of metallic foam. Regarding solidification process in pure PCM, case with four tubes shows shorter solidification time. Also, highest solidification rate amid composites of phase change material/metallic foam is seen for case with three tubes. Consequently, inserting metallic foam is more efficient in solidification process rather than the melting one. Moreover, increasing number of inner tubes has more influence on phase change rates in metal foam/PCM composites compared to pure PCM.