Temperature Control and Energy Management
The wet-bulb temperature of the ambient air varies significantly on a daily basis, and considerably from season to season. As the wet-bulb temperature reduces, the tower becomes capable of producing colder and colder water—or it becomes capable of producing a given cold water temperature at reduced airflow through the tower. These characteristics are the “opposing forces” referred to on page 4.
Maximizing Tower Performance
If your operating system is one which benefits from the coldest possible water; that is, if colder water allows you to increase your output—or allows you to operate your system at significantly lower cost, then continuous full speed operation of the fan(s) may be your best mode of operation.
In this mode of operation, concern for the cold water temperature level would be limited to the potential for the tower to form ice during freezing weather. (See Caution note on page 8 and Freezing Weather Operation on pages 11 and 12.) Although the 70°F (21°C) cold water temperature indicated on page 11 is appropriate for cold weather start-up and operation, acceptable temperatures during full operation in spring, summer, and fall may be appreciably lower, perhaps as low as 50°F (10°C) or less. Refer to
your performance curves for expected tower cold water temperatures at varying flow rates, ranges, and wet bulb temperatures.
Minimizing Tower Energy Use
Most systems gain no operating or production benefits from water tem- peratures below a certain level, and that level is not usually below the aforementioned 70°F (21°C). When a reducing ambient wet-bulb permits the tower to reach that target cold water temperature level, further reduc- tions in the wet-bulb temperature permit manipulation of fan speeds or operation to maintain that temperature level.
Single-speed fans can be cycled on and off for cold water temperature control, with the steps of control depending upon the number of fan cells in the tower. Two-speed motors offer twice as many control steps—with the added bonus that half-speed (which produces half of the normal air- flow through the tower) requires less than 20% of the full-speed power requirement.
Excessive cycling of motors causes overheating of the windings and ultimate failure of the motor. The total amount of starting time (period of high inrush current) should not exceed 30 seconds per hour. On fans 20 feet diameter and smaller, this may allow 4 or 5 starts per hour. On larger fans, 1 or 2 starts per hour may be the limit. Determine the
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