If you see ice, do not shut down. Increase heat load. Increase water flow. Do not stop the fan unless you intend to scrap the cell.
Watch the fan exhaust. A healthy winter plume is wispy and dissipates quickly. A dangerous plume is thick, heavy, and drifts horizontally without rising. This indicates the water is entering the cold air basin at a temperature too low to melt the ice forming upstream. cooling tower handbook
For nine months of the year, the cooling tower is the unglamorous workhorse of the industrial plant—loud, wet, and largely ignored. But when the mercury dips below 32°F (0°C), this same piece of equipment transforms overnight into the plant’s most vulnerable asset. Winter operation is not about efficiency; it is about survival. If you see ice, do not shut down