16
Operation and Maintenance Instructions
flow into the fan section. During extremely low ambient
conditions, this moist air may condense and freeze on the
cold components of the drive system. When conditions
change and cooling is needed, any amount of ice that has
formed on the drive system can severely damage the fans
and fan shafts.
Therefore, fans MUST be cycled during
low ambient operation to avoid long periods of idle
fan operation. Excessive cycling can damage the fan
motors. Limit the number of cycles to a maximum of
six per hour.
Two speed or pony motors offer a better method of
control. This additional step of capacity control will reduce
water temperature differentials and the amount of time
that the fans are off. This method of capacity control has
proven effective for applications where load variations are
excessive and cold weather conditions are moderate.
The use of a variable frequency drive provides the most
flexible method of capacity control for forced draft units.
The VFD control system allows the fans to run at nearly
an infinite range of speeds to match the unit capacity to
the system load. During periods of reduced load and low
ambient temperatures, the fans must maintain enough
speed to ensure positive air flow through the unit. This
positive air flow in the unit will prevent moist air from
migrating towards the cold fan drive components reducing
the potential for condensation to form and freeze on
them. The VFD control system should be implemented for
applications that experience fluctuating loads and severe
cold weather conditions.
Ice Management
When operating an evaporative cooling unit in extreme
ambient conditions, the formation of ice is inevitable. The key
to successful operation is to control or manage the amount
of ice that builds up in the unit. If extreme icing occurs, it
can lead to severe operational difficulties as well as potential
damage to the unit. Following these guidelines will minimize
the amount of ice that forms in the unit leading to better
operation during the cold weather season.
Induced Draft Units
When operating an induced draft unit during the cold
weather season, the control sequence must have a method
to manage the formation of ice in the unit. The simplest
method of managing the amount of ice buildup is by
cycling the fan motors off while keeping the pump on.
During these periods of idle fan operation, the warm water
that is absorbing the building load flows thru the coil to
help melt the ice that has formed in the coil, basin or louver
areas.
NOTE: Using this method during periods of high winds
can cause blow through, resulting in splash-out and ice
formation. To help prevent blow through and splash-out,
maintain a minimum 50% fan speed.
In more severe climates, the incorporation of a defrost
cycle can be used to mitigate and/or eliminate any
detrimental effects of ice in the unit. During the defrost
cycle, the fans are reversed at half speed while the
recirculating pump flows water through the unit’s water
distribution system. Operating the unit in reverse will melt
any ice or frost that may have formed in the unit or on the
air intake louvers. 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 cooler or condenser 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 coolers
or condensers 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
download a copy of EVAPCO’s Engineering Bulletin 23 at
evapco.com
Troubleshooting:
see Bulletin 116-E, pages 31-33
