Forced Draft Unit Capacity Control; Ice Management; Induced Draft Units; Forced Draft Units - EVAPCO ESW Series Operation And Maintenance Instruction

Forced Draft Unit Capacity Control

The most common methods of capacity control are cycling the single speed fan motors, using two-speed motors or pony
motors and utilizing variable frequency drives to control the cooler or condenser fans. Although capacity control methods for
forced draft units are similar to those used for induced draft units, there are slight variations.
The simplest method of capacity control for forced draft units is to cycle the fan(s) on and off. However, this method of control
results in larger temperature differentials and periods of time with the fans off. When the fans are cycled off, the water falling
through the unit can draw air 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 thru, resulting in splash-out and ice
formation. To help prevent blow thru 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
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