19
EVAPORATIVE-COOLED
CONDENSER cont.
Water Treatment System:
• All AAON evaporative-cooled condensers
come equipped with a water treatment system
that should be maintained by a local water
treatment professional trained in the water
treatment of evaporative condensers. This
system consists of a controller, three chemical
pumps and storage tanks, a conductivity sensor, a
motorized ball valve for water bleed, and a water
meter.
• One chemical pump and tank is typically used
for a descaling chemical to prevent scale from
forming in the condenser. The other two pumps
and tanks are typically used for two different
biocides (to kill any microorganisms that could
grow in the condenser). Two biocides are used
to prevent organisms from becoming resistant to
one chemical.
• The mineral content of the water must be
controlled. All make-up water has minerals in it.
As water is evaporated from the condenser, these
minerals remain. As the mineral content of the
water increases, the conductivity of the water
increases. The water treatment controller
monitors this conductivity. As the water
conductivity rises above set point, the controller
will open a motorized ball valve on the discharge
side of the condenser pump and dumps water
into the condenser drain until conductivity is
lowered. While the motorized ball valve is
opened, the controller will not disperse
chemicals.
• The chemicals are dispersed by the water
treatment controller based on the scheduled input
by the water treatment professional. (See the
separate manual for the water treatment controls
for specific programming information.)
• The water meter measures the quantity of
make-up water used by the condenser.
• Any water treatment program must be
compatible with stainless steel, copper,
aluminum, ABS plastic and PVC. Batch feed
processes should never be used as concentrated
chemicals can cause corrosion. Never use
hydrochloric acid (muratic acid) as it will
corrode stainless steel.
Sequence of Operation:
• On a call for cooling, the condenser pump is
activated. A pressure switch in the pump
discharge is bypassed for six seconds by a time
delay relay in order for the pump to establish
recirculating water flow. If flow is not proven
within the six seconds, the pressure switch opens,
breaking the safety circuit, thereby shutting down
the entire system. This pressure switch is set to
close at 3 psi and open at 1 psi.
• A Johnson Controls S350C measures the water
temperature in the pump discharge line. If the
sump water temperature exceeds 105°F, the
cooling system will be shut down thereby
preventing damage to the evaporative condenser.
• If a fault occurs in the evaporative condenser
fan motor VFD, normally closed fault terminals
on the VFD will interrupt the safety circuit,
thereby shutting down the system.
• If the VFD does fault and cannot be reset, there
is a VFD bypass switch mounted near the VFD.
This switch has four positions—line, off, drive,
and test. The “line” position will bypass the
VFD, sending power to the motor. In this
position, the condenser fans will run at full
speed. The “off” position will not allow power
to pass through the switch. This functions as a
disconnect switch. The “drive” position runs
power through the VFD. This is the normal
operation for the switch. The “test” position
routes power to the VFD but not to the motor.
This is useful for running tests on the VFD
without sending power to the motor.
• A Johnson Controls A350P controls the VFD
speed. This device sends a 0-10 VDC signal to
the VFD. This controller is set to maintain a
sump temperature of 70°F. On a rise in sump
temperature, the controller increases the voltage
to the VFD, increasing the speed of the
condenser fans. Conversely, on a drop in sump
temperature, the controller will decrease the
voltage to the VFD, decreasing the speed of the
condenser fans.
• An outside air thermostat does not allow the
condenser to operate when the ambient
temperature is below 35°F.
