12
Freezing Weather Operation
• Under extended extreme cold conditions, it may be necessary to operate
the fan in reverse. This forces warm air out through the air inlets, melting any
accumulated ice. Reversal may be at either full or half speed; however, full
speed is recommended if adequate heat load is available. Reverse operation
of the fan should be avoided, where possible, and should not exceed 15
to 20 minutes.
Reverse operation of fans for prolonged periods during subfreezing
weather can cause severe damage to fans and fan cylinders. Ice can
accumulate inside fan cylinders at fan blade plane of rotation and
fan blade tips will eventually strike this ring of ice, damaging the
fan blades or cylinder. Ice can also accumulate on fan blades and
be thrown off, damaging fan cylinder or blades. Reverse operation
of fans with adjacent fans not operating increases probability of
icing. The low discharge velocity of moist air from fan cylinders
in which fans are not in operation can result in moisture-laden air
being pulled into the adjacent cylinder in which the fan is operating
in reverse, increasing this ice buildup. Therefore, fans each side of
the one operating in reverse must be operated in forward rotation
at full or half speed, or all fans must be operated in reverse. Allow a
minimum of 10 minutes delay between reverse operation and forward
operation during subfreezing weather to permit ice to dissipate from
fan blades and fan cylinders.
• With no heat load on the circulating water, icing cannot be controlled
effectively by air control alone during freezing weather. Towers must not
be operated with reduced water rate and/or no heat load during freezing
weather. If a bypass directly into the cold water basin is used, all water must
be bypassed.
Water flow over the tower must never be modulated
during freezing weather.
See Fan Drive on pages 9 and 10 for fan speed change and reversing
precautions.
Intermittent Operation
When the unit is operated intermittently during winter weather, it is
necessary that the water be drained from the tower piping to insure
protection against freezing and possible rupture.
Caution
Caution
Caution
