Freezing Weather Operation
During periods of low ambient air temperatures, 35°F to 40°F (2°C to
▲ Note4°C) or below, it is advisable to maintain the cold water temperature at or above 70°F (21°C) to retard the formation of ice and to assist in its control. The methods used in Minimizing Tower Energy Use, described on page 13 are those which the operator would use to maintain water temperature at or above 70°F (21°C).
In combinations of low ambient air temperatures and reduced loads, fan speed manipulation may not be sufficient to maintain an acceptable water temperature. Even with fans off, the natural movement of air through the tower may be enough to continue reducing the water temperature. When that happens, it will become necessary to open the bypass valve, if so equipped, and allow total circulation to flow directly into the cold water basin until such time as the basin water inventory rises to a temperature level sufficiently high to preclude frequent changeovers from normal flow to bypass flow. That temperature level might be 85°F to 90°F (29°C to 32°C), but should be determined by operator experimentation.
Deicing
The formation of ice on towers operating in freezing weather cannot be completely prevented, but can be acceptably controlled. Ice will form on the relatively dry parts of the tower that are in contact with the incoming air. Primarily, this includes the columns, structural framing, and fill at the air inlets. Ice forming characteristics on any given tower will vary, depending on velocity and direction of wind, circulating water rate and heat load.
The Class 800 tower is designed with few structural components in the falling water zone, which prevents damage due to falling ice. However, structural damage can still result from excessive buildup of ice attached to the fill.
A regularly programmed inspection (one walk around the tower per shift) during cold weather operation is the best way to determine if ice buildup on the fill is occurring. If ice is observed and determined to be acceptable, maintain full (total) design flow over the tower. Excessive ice formation may be controlled by regulating air and water flow through the tower by one or more of the following procedures:
•Shut the fan down. This reduces the cooling rate to a minimum and increases the quantity of warm water at the air inlet to a maximum. Ex- cept for extreme cold conditions or extended freezing conditions, this procedure will normally control ice formation. If the tower has
11
