In this paper a new modeling of an indirect evaporative cooling system (IEC) with consideration of wall longitudinal heat conduction (LHC) and effect of spray water temperature variation along the exchanger surface in a cross-flow configuration is presented. The resultant coupled equations of heat and mass transfer are discretized using finite difference method (FDM) and solved by an iterative method. Comparing the numerical results of the presented simulation against experimental data revealed excellent agreement with them, this shows a small margin of error in the calculation (around 3%). After validation, this mathematical model is used in a two-stage system of indirect/direct evaporative cooling system, which shows that a two-stage indirect/direct evaporative cooling system in comparing to a one-stage IEC has around 50%higher wet-bulb effectiveness with the same parameters of inlet air and exchanger. Furthermore, the presented model is used for numerical simulation of a counter-flow regenerative evaporative cooler which shows around 60%higher wet-bulb effectiveness in comparing to a one-stage IEC. The numerical results of this study show applicability of the presented model for both sub-and above-wet bulb cooling applications. (C) 2015 Elsevier B.V. All rights reserved.