In a cooling tower, hot, humidified water is evenly distributed through a dedicated water distribution system, exchanging heat with the dry air inside the tower. This process involves the flow of high-temperature water molecules toward the low-pressure air, accompanied by direct air-to-water heat transfer and evaporative heat transfer. This effectively dissipates waste heat and cools the water.
Let's take a closer look at the cooling principle in a counterflow circular cooling tower. First, hot water is pressurized by a water pump and delivered through a series of pipes and nozzles to the cooling tower's water distribution system. Here, small holes in the water distribution pipes distribute the hot water evenly to the filling. Dry, low-humidity air then enters the tower through the air inlet grille at the bottom, which is driven by a fan. As the hot water flows through the filling, it forms a water film that exchanges heat with the air. At this stage, the hot, humidified air is extracted from the top of the tower, while the cooling water drips into the lower basin and then returns to the main unit through an outlet pipe.
It is important to note that the air entering the tower is typically dry and low in temperature. In this environment, there is a significant difference in water molecule concentration and kinetic pressure between the water and the air. When the fan is running, the static pressure inside the tower forces the water molecules to continually evaporate into the air, converting them to water vapor. This reduces the average kinetic energy of the remaining water molecules, thus lowering the temperature of the circulating water.
Furthermore, the evaporative cooling process is not affected by whether the air temperature (i.e., the dry bulb temperature) is higher or lower than the water temperature. As long as water molecules continue to evaporate into the air, the water temperature will continue to decrease. However, it is important to realize that the evaporation of water from the air is not final. When the air in contact with the water reaches saturation, further evaporation is impossible, and dynamic equilibrium is achieved. At this point, the number of water molecules evaporating equals the number returning from the air to the water, and the water temperature remains constant. Therefore, the drier the air in contact with the water, the easier evaporation occurs, and thus the easier it is for the water temperature to decrease.
