A cooling tower is a complex system designed to cool the circulating water by evaporation. The water is atomized, then circulated through the tower, where it’s further cooled by a forced airflow caused by the tower fan. The temperature of the water can be varied in several ways. The most common is to vary the speed of the fan, thus affecting the rate of evaporative cooling of the water spray, and so the temperature of the basin (return) water.
A bypass valve can be used to modulate the temperature of the water returned from the cooling tower. This is most often used when the basin water is too cold to return to the loop, when the fan is off, or at a minimum speed, or at some other intermediate condition. For example, consider a cooling tower with a two-speed fan. If the return water is too warm with the fan off, and too cold with the fan on low speed, then the fan is kept on low speed and a portion of the supply water is diverted to temper the basin water.
The ultimate purpose of the cooling tower is to provide water at a steady temperature to the condenser side of the chiller. This enables the chiller to operate at maximum efficiency. The multi-speed fan, or in some instances, a variable-speed fan (which may not require a bypass valve), is an energy-conserving appurtenance to minimize the fan power required to cool the water to setpoint.
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Excerpt from Energy Conservation Projects.