In this study, a method to track the melting front of phase change materials (PCMs) is developed under simultaneous charging and discharging (SCD) inside horizontal triplex tube heat exchangers. For SCD, the heat transfer mode of internal heating and external cooling is investigated. According to the heat transfer mechanism within the PCMs, two models of pure conduction (PC) and combined conduction and natural convection (CCNC) are developed and solved using ANSYS Fluent (version 17.2). Generally, during charging (i.e. melting), the buoyancy forces induce the liquid PCM to move upwards; therefore, the upper section of the storage is affected. However, this motion is limited during SCD due to the simultaneous cooling from the outer tube. Therefore, a front tracking method is developed assuming that natural convection only contributes melting to the upper section of the storage unit. On the other hand, it is assumed that the melting front for the lower section behaves similar to the PC model. In this study, three PCMs are used in three geometries to develop the correlations using two dimensionless numbers; i.e. liquid fraction of the PC model and the storage radius ratio. Another PCM is then used to verify the method. The correlations can provide results in the range of +/- 15% discrepancy.