The heated cooling water is fed to the cooling tower and evenly distributed onto the packed packing (heat exchanger) via spray nozzles. It then trickles down through this heat exchanger into the water collection basin, while the air sucked in by a fan is sucked through the cooling tower in the countercurrent.
Due to the optimized mixing of the air with the trickling water, about 2% of the cooling water evaporates and the water is cooled by the evaporation heat as well as by convection - depending on the operating point. The coolest water temperature that can be achieved depends not only on the tangible air temperature, but also on its water vapor content (expressed by the cooling limit temperature or wet bulb temperature).
In the cooler seasons, water temperatures can be reached that are partly below tap water temperature. On summer days with high humidity, the wet bulb temperature in Central Europe can reach values between 18°C and 21°C. This results in water temperatures between 22°C and 25 °C, depending on the size of the cooling tower. The smaller the approximation to the wet bulb temperature, the larger the cooling tower becomes.
For an economically designed cooling tower, the approach (difference between wet bulb temperature and cold water temperature) should therefore not be less than 4°C.