AAON LL-540 - Manual (Page 53)

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aluminum, ABS plastic and PVC. Batch
feed processes should never be used as
concentrated chemicals can cause corrosion.
Never use hydrochloric acid (muriatic acid)
as it will corrode stainless steel.
Sequence of Operation for LL Series units
without Diagnostics
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-cooled condenser.
If a fault occurs in the evaporative-cooled
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.
Sequence of Operation for LL Series units
with Diagnostics
These units operate the same way as
described in the previous section, except the
unit controller operates the evaporative-
cooled condenser. These units can also
operate down to 0°F when equipped with the
flooded condenser option as described in the
low ambient section.
On units with diagnostics, each refrigerant
system has suction and discharge pressure
transducers and suction and discharge
temperature sensors. Each compressor has a
current transformer. The condenser pump is
the first stage of condenser control and the
condenser fans are the second stage of
condenser control. The unit controller brings
on the condenser stages based on the
discharge pressures of each system. The
system with the highest pressure will control
the condenser staging. The first stage of
condenser will activate at 180 psig (R-22)
and deactivate at 160 psig. The second stage
of condensing will activate at 195 psig and
deactivate at 185 psig. The unit controller