AAON LZ Series, Установка, эксплуатация и обслуживание

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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-condensed chiller.
If a fault occurs in the evaporative-condensed
chiller 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 LZ Series units
with Diagnostics
These units operate the same way as
described in the previous section, except the
unit controller operates the evaporative-
condensed chiller. These units can also
operate down to 0°F when equipped with the
low ambient head pressure control.
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 unit controller will monitor the sump
temperature and if it exceeds 105°F, it will
reduce the number of compressors that are
running. The unit controller monitors the
condenser pump pressure switch. If this
switch opens, it will not allow the
compressors to operate. The unit controller
also monitors the VFD fault status. If it
receives a VFD fault, it will activate the
alarm contacts on the unit controller. The
VFD will no longer run, but the compressors
will run until the pressures or temperatures
get too high. The controller will reduce the
number of compressors to try to keep as much
of the unit running as possible. The VFD
bypass switch can be set to “line” to operate
the condenser fans at full speed.
Units with diagnostics can operate below
35°F down to 0°F. This is possible because of
the finned de-superheat coil. When the