15
Operation and Maintenance Instructions
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 ensures that the cooler or condenser will function
properly during cold weather operation.
Cold Water Basin Heaters
Optional basin heaters can be furnished with the unit 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 (4°C) basin water
temperature at a 0°F (-18°C) ambient temperature. The
heaters are only energized when the recirculating pumps
are off and no water is flowing over the heat exchanger
coil. As long as there is a heat load and water is flowing
over the heat exchanger coil, 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
recirculating pump is idle. EVAPCO units built for remote
sump operation do not include recirculating water pumps.
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
100 psig (35 and 690 kPa). 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, make up 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 coolers or condensers require
separate guidelines for capacity control during cold weather
operation.
The sequence of control for a unit operating at low ambient
conditions is much the same as a cooler or condenser
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.
The most effective way to avoid ice formation in and on a
closed circuit cooler or condenser during the winter is to
run the unit DRY. In dry operation, the recirculation pump is
turned off, the basin drained, and air passes over the coil.
Instead of using evaporative cooling to cool the process fluid
or condense the refrigerant, sensible heat transfer is utilized,
so there is no recirculation water to freeze. If this method will
be used on a forced draft unit, be sure to verify that the motor
and drives have been properly sized to handle the reduction
in static pressure experienced when the spray water is turned
off.
It is very important to maintain close control of the cooler or
condenser during winter operation. EVAPCO recommends
that an absolute MINIMUM leaving water temperature of 42°
F (6°C) must be maintained for cooler applications. The higher
the leaving temperature from the cooler or condenser, the
lower the potential for ice formation.
Induced Draft Unit Capacity Control
The simplest method of capacity control is cycling the
fan motor on and off in response to the leaving fluid
temperature of the cooler or condenser. 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 when water is flowing over the coil. 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 cooler or condenser has the
potential to operate on low speed for the 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%. 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
