The present research study was carried out on the performance of generator air coolers for the hydropower plant of Aswan High Dam at Egypt. The study is carried out under varying operating conditions associated with diurnal variations of the generation from power units and with the place that air-water coolers are located. Air temperatures and cooling water pressures were measured together with recording the generated power so that investigating the performance characteristics of the original and new air coolers. Both coolers were designed with excess surface area enabling the coolers to work well for long periods under dirt conditions. The effectiveness values for both cooler designs were in good agreement with those obtained by the related previous research works at the same flow condition. The maximum and minimum values of the actual fouling factor for both coolers give representative average values for the original cooler twice those representative average values for the new cooler. Also, the malfunction of some original coolers was equivalent to the reduction in the heat transfer area. The hot air temperatures showed good uniformity indicating good flow behaviour of the circulating air so that satisfying the normal design temperature range. (C) 2019 Elsevier Ltd. All rights reserved.