EVAPCO AT Series Operation & Maintenance Instructions Manual page 22

Cold Weather Operation (Continued)
unit's cold water basin.
Unit Accessories
The appropriate accessories to prevent or minimize ice formation during cold weather operation are relatively simple and inexpensive.
These accessories include cold water basin heaters, the use of a remote sump, electric water level control and vibration cut out switches.
Each of these optional accessories ensure that the cooling tower will function properly during cold weather operation.
Cold Water Basin Heaters
Optional basin heaters can be furnished with the cooling tower to prevent the water from freezing in the basin when the unit is
idle during low ambient conditions. The basin heaters are designed to maintain 40°F basin water temperature at a 0°F (or -20°F
& -40°F) ambient temperature. The heaters are only energized when the condenser water pumps are off and no water is flowing
over the tower. As long as there is a heat load and water is flowing over the tower, the heaters do not need to operate. Other
types of basin heaters to consider would include: hot water coils, steam coils or steam injectors.
Remote Sumps
A remote sump located in an indoor heated space is an excellent way to prevent freezing in the cold water basin during idle or no
load conditions because the basin and associated piping will drain by gravity whenever the circulating pump is idle. EVAPCO can
provide connections in the cold water basin to accommodate for remote sump installations.
Electric Water Level Control
Optional electric water level control packages can be furnished to replace the standard mechanical float and valve assembly. The
makeup water pressure for electronic water level control should be maintained between 5 and 125 PSIG. The electric water level
control eliminates the freezing problems experienced by the mechanical float. In addition, it provides accurate control of the basin
water level and does not require field adjustment even under varying load conditions. Please note: the standpipe assembly, makeup
piping and solenoid valve must be heat traced and insulated to prevent them from freezing.
Vibration Cut Out Switches
During severe cold weather conditions, ice can form on the fans of cooling towers causing excessive vibration. The optional
vibration switch shuts the fan off avoiding potential damage to or failure of the drive system.
Capacity Control Methods for Cold Weather Operation
Induced draft and forced draft cooling towers require separate guidelines for capacity control during cold weather operation.
The sequence of control for a cooling tower operating at low ambient conditions is much the same as a cooling tower operating under
summer conditions provided that the ambient temperature is above freezing. When the ambient temperatures are below freezing,
additional precautions must be taken to avoid the potential for damaging ice formation or equipment damage.
It is very important to maintain close control of the cooling tower during winter operation. EVAPCO recommends that an absolute
MINIMUM leaving water temperature of 42° F must be maintained; obviously, the higher the water temperature from the tower, the
lower the potential for ice formation. This assumes that proper water flow over the tower is maintained.
Induced Draft Unit Capacity Control
The simplest method of capacity control is cycling the fan motor on and off in response to the leaving water temperature of
the tower. However, this method of control results in larger temperature differentials and longer periods of down time. During
extremely low ambient conditions, the moist air may condense and freeze on the fan drive system. Therefore, fans must be cycled
during extremely low ambient conditions to avoid long periods of idle time whether water is flowing over the fill or in bypass.
The number of start/stop cycles must be limited to no more than six per hour.
A better method of control is the use of two speed fan motors. This allows an additional step of capacity control. This additional
step reduces the water temperature differential, and therefore, the amount of time the fans are off. In addition, two speed motors
provide savings in energy costs, since the tower has the potential to operate on low speed during reduced load requirements.
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 percent. 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 percent of full speed to minimize the potential for ice to form in
the unit. Space heaters in the motors are suggested to help keep the high humidity air from condensing in the motors during idle
22