SYNOPSIS Computation Fluid Dynamics (CFD) is applied to predicting the pressure loss coefficient of tube bundles for closed-wet cooling towers. The prediction involves the single-phase air flow and two-phase flow of gas and water droplets. The accuracy of CFD modelling of the pressure loss coefficient is assessed for single-phase air flow over closely-spaced tube bundles for a range of flow velocities applied in closed-wet cooling towers. The pressure loss coefficient for both single- and two-phase flow over tube bundles with large transverse pitches is then determined by CFD modelling. It is shown that the pressure loss coefficient decreases with transverse pitch but increases with water-to-air mass flow ratio.