Recirculation－The fan draws air through the entire louver face of a crossflow tower; therefore, a pressure differential is created along the vertical plane of the structure. Air entering the upper sections is closer to the fan and travels through the tower at a higher velocity than air at lower levels. Bernoulli’s theorem states that when the velocity of a fluid(air) is increased, its pressure is decreased; this effect produces negative pressure at the upper levels of the cooling tower and, under typical atmospheric wind conditions, sets up air recirculation, Fig.5 Hot moist air with a high wet-bulb temperature is pulled back into the cooling tower, which greatly reduces its performance.
The air-intake louvers of a counterflow tower are near the base, a considerable distance from the fan discharge plume, so it is difficult for air to recirculate under normal conditions. Counterflow towers can have recirculation problems if the discharge plume hits a barrier adjacent to the fan outlet, which forces the hot moist air downward. This problem can be avoided by installing an elevated fan cylinder to discharge the air above the barrier.