SPX Marley NCWD, User Manual

Page: 17 / 32

17
If the minimum allowable cold water temperature is established
at or near minimum heat load, it should be safe for all operating
conditions.
Having established the minimum allowable cold water temperature, maintain-
ing that temperature can be accomplished by fan manipulation, as outlined
in Item 4 under Tower Operation. However, in towers of more than one
cell, where fans are manipulated sequentially, please realize that the water
temperature will be significantly lower in the cell or cells operating at the
highest fan speed than the net cold water temperature produced by the entire
tower would indicate. Wintertime operation of multicell towers at low cold
water temperature levels requires that the operator be especially watchful.
2. As cold air enters the louvers, it causes the water flowing over the fill to be
drawn inward toward the center of the tower. Thus, under fan operation, the
louvers and lower periphery of the tower structure remain partly dry, see-
ing only random splashing from within the tower—plus normal atmospheric
moisture from the entering air. Such lightly wetted areas are most subject
to freezing.
Therefore, if excessive ice forms on the louvers, stop the fan for a few
minutes. With the fan off, the water flow will increase in the vicinity of the
louvers and reduce the ice buildup.
3. Under extended extreme cold conditions, it may be necessary to operate
the fan in reverse. This forces warm air out through the louvers, melting
any accumulated ice—adequate heat load must be available. Reversal of
fan should only be done at half speed or less. Reverse operation of the fan
should be used sparingly and should only be used to control ice, not to
prevent it. Reverse fan operation should not need to exceed 1 or 2 minutes.
Monitoring is required to determine the time required to melt accumulated
ice.
Operating the fan in reverse at half speed for prolonged periods dur-
ing 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. Allow a
minimum of 10 minute delay between reverse operation and forward
operation during subfreezing weather to permit ice to dissipate from
fan blades and fan cylinders. See Fan Drive Caution note on page 11
for fan speed change and reversing precautions.
Caution
Warning
operation
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