EVAPCO AT 110-112 Operation And Maintenance Instructions Download Page 8

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Basic Cooling Tower Sequence of Operation

System Off / No Load

The system pumps and fans are off. If the basin is full of water a minimum basin water temperature of 4ºC must

be maintained to prevent freezing. This can be accomplished with the use of optional basin heaters. See the

“Cold Weather Operation” section of this bulletin for more details on cold weather operation and maintenance.

System / Condensing Temperature Rises

The system pump turns on. The unit will provide approximately 10% cooling capacity with only the pump running.

NOTE

: If the load is such that simply running the system pump with the unit fan motor idle is sufficient,

motor space heaters (if equipped) should be energized while the motor is idle. Alternatively, the motor can

be energized twiced daily for a minimum of 10 minutes to protect the motor insulation from damage.
If the system temperature continues to rise, the unit fan is cycled on. For a variable speed controller, the

fans are turned on to minimum speed. See the “Fan System – Capacity Control” section of this bulletin for

more details on fan speed control options. If the system temperature continues to rise, then the fan speed

is increased as required, up to full speed.

NOTE

: During sub-freezing weather the minimum recommended speed for variable speed controllers

is 50%. ALL FANS IN OPERATING CELLS OF MULTIPLE CELL UNITS MUST BE CONTROLLED

TOGETHER TO PREVENT ICING ON THE FANS.

System Temperature Stabilizes

Control the leaving water temperature by modulating the fan speeds with variable speed drives or by

cycling fans on and off with single or two-speed drives.

System Temperature Drops

Decrease the fan speed, as required.

System Off / No Load

The system pump turns off. The starter interlock will energize any optional basin heaters in cold weather.
The recirculation pump should not be used as a means of capacity control, and should not be cycled

frequently. Excessive cycling can lead to scale build-up, and reduce wet and dry performance.

Bypass Mode

During winter months when cooling load is minimal, bypass mode may be used as a form of capacity

control. Bypass mode allows the water to “bypass” the tower’s water distribution system and deposits the

inlet water directly into the cold water basin. Alternatively, the incoming water bypass can be piped directly

to the return condenser header pipe. Please note: The location of the bypass valve should be 4,5 m

below the cooling tower cold water basin elevation to assure proper operation and prevent cavitation. This

bypass mode should be continued until the total water inventory reaches an acceptable temperature level

(usually about 27°C), at which time the bypass may be closed to cause total flow over the fill.
EVAPCO does NOT recommend a partial water bypass due to the potential for freezing the heat transfer

media during low ambient operation.

Optional Defrost Cycle

In more severe climates, the incorporation of a defrost cycle may be used to manage the ice formation in

and on the unit. During the defrost cycle, the cooling tower fan(s) are reversed at no more than half speed

while the system pump flows water through the cooling tower’s water distribution system. Operating the

unit in “reverse” will melt any ice that has formed in the unit or on the intake louvers. All multi-speed or

VFD duty motors supplied by EVAPCO for induced draft units, are capable of reverse operation.

Defrost cycles are NOT recommended for forced draft cooling towers. In these units, allowing the leaving

water temperature set point to rise causes the fans to be off for very long periods of time, which increases

the fan drive component potential for freezing. In lieu of a defrost cycle, forced draft units should be

operated at low speed (with a 2-speed motor) or minimum speed (no lower than 25% with a variable

frequency drives) in order to maintain positive pressure inside the unit to help prevent ice formation on the