Forced Draft Unit Capacity Control; Ice Management; Induced Draft Units - EVAPCO ATC-E Operation And Maintenance Instruction Manual

The best method of capacity control during cold weather operation is the use of a variable frequency drive (VFD) . This
allows the closest control of the leaving water temperature by allowing the fan(s) to run at the appropriate speed to
closely match the building load . As the building load decreases, the VFD control system may operate for long periods of
time at fan speeds below 50% . Operating a low leaving water temperature and low air velocity through the unit can cause
ice to form . It is recommended that the minimum speed of the VFD be set at 50% of full speed to minimize the potential
for ice to form in the unit .

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 . Warning: Using
this method will cause blow thru, resulting in splash-out and ice formation . To help prevent blow thru and splash-out,
maintain a minimum 50% fan speed, consult local legislation as described in the section "Capacity Control" .
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 .
Operation and Maintenance Instructions
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