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13
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
▲ Note
▲ Caution