24
In more severe climates, the incorporation of a defrost cycle can be used to manage the
formation of ice in the unit. During the defrost cycle, the fans are reversed at
half speed
while the system pump flows water through the unit’s water distribution system. Operating
the unit in reverse will melt any ice that has formed in the unit or on the air intake louvers.
Please note that the fans may need to be cycled off prior to a defrost cycle to allow the
water temperature to rise.
The defrost cycle requires the use of two speed motors with
reverse cycle starters or reversible variable frequency drives.
All motors supplied by
EVAPCO are capable of reverse operation.
The defrost cycle should be incorporated into the normal control scheme of the cooling
tower system. The control system should allow for either a manual or automatic method of
controlling frequency and length of time required to completely defrost the ice from the unit.
The frequency and length of the defrost cycle is dependent on the control methods and
ambient cold weather conditions. Some applications will build ice quicker than others which
may require longer and more frequent defrost periods.
Frequent inspection of the unit will
help “fine tune” the length and frequency of the defrost cycle.
Forced Draft Units
Defrost cycles are NOT recommended for forced draft units, since allowing the leaving water
temperature set point to rise causes the fans to be off for very long periods of time. This is
not recommended for forced draft towers because of the potential for freezing the fan drive
components. Therefore, the defrost cycle is an inappropriate method of ice management for
forced draft units. However, low speed fan operation or variable frequency drives maintain a
positive pressure in the unit which helps prevent ice formation on the fan drive components.
For more information on cold weather operation, please request a copy of EVAPCO’s Engineering
Bulletin 23
Summary of Contents for AT 110-112
Page 39: ...39 ...
