Manipulation of the tower airflow is an invaluable tool to retard ice formation and to reduce or eliminate ice already formed. Reducing the entering airflow velocity brings less cold air into contact with the circulating water. It also alters the path of the falling water, allowing it to impinge upon - and melt - ice previously formed by random droplets that wind gusts or normal splashing may have caused to go outside the warm mainstream of water.
There are three choices available to the designer for controlling airflow through the tower: single-speed fans, two-speed fans, and fans equipped with a variable-frequency drive (VFD). Single-speed fans afford the least opportunity for airflow variation, and towers so equipped require maximum vigilance on the part of the user to determine the proper cyclic operation of the fans that will result in best ice control.
Two-speed fan motors offer greater operating flexibility and should be considered the minimum level of control for cooling towers used in cold climates. Fans may be individually cycled back and forth between full speed and half speed as required to achieve balance between cooling effect and ice control. Keep in mind that the number of cycles may be limited by the maximum allowable motor insulation temperature; an abnormal number of speed changes per hour may cause it to be exceeded. Because cooling tower capacity varies directly with the fan speed, while fan horsepower varies with the cube of the speed, operating all fans at half speed provides the same level of cooling as operation of half the fans at full speed, but this approach uses only a fourth of the energy.
On towers having two or more fans evacuating a common plenum, those fans should be brought to the off position in unison to prevent a down draft of cold moisture-laden air from icing up the mechanical equipment of an inoperative fan. On multicell towers (inline configuration) equipped with a separate plenum for each fan, individual fans may be cycled as necessary to control ice. However, it must be understood that cycling the fan on a particular cell accomplishes nothing with respect to deicing of adjacent cells. Individual cell ice control must be accomplished independently. This is because of the temperature gradients about to be discussed.
VFDs offer the greatest level of flexibility because they permit all fans to operate at the same speed for minimum energy consumption. In addition, all cells will then produce the same cold water temperature, eliminating the temperature gradient between cells.