McQuay AGZ 035A, Installation, Operation And Maintenance Manual

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62 AGZ 035A through 065A IOMM AGZ-3
If an alarm condition exists which prevents normal operation of both refrigerant circuits, the chiller
will be disabled and the display will indicate OFF:Alarm.
Assuming none of the above “OFF” conditions are true, the controller will examine the internal time
clock schedule to determine if the chiller should start. The operating mode will be OFF:TimeClock if
the time schedule indicates an “OFF” time period.
Start - Up
If none of the “OFF” conditions are true, the controller will initiate a start sequence. The chilled
water pump relay output is energized any time the chiller is enabled and the chiller will remain in the
“Waiting For Flow” mode until the field installed flow switch indicates the presence of chilled water
flow. If flow is not proven within 30 seconds, the alarm “Loss of Water Flow” will be activated and
the chiller will continue to wait for proof of chilled water flow. Once flow is established, the alarm
will automatically be cleared and the unit will go into the “Wait For Load” mode.
Once flow is established, the controller will sample the leaving water temperature and compare it
against the leaving water temperature set point, the control band, the start up delta T and the start
timers. If all values indicate a start (stage 1 cooling), the controller will start the lead circuit by
energizing that circuits liquid line solenoid valve (SV). When the circuit pressure exceeds the
LPCutIn pressure, the compressor on that circuit with the lowest starts will be started. If additional
cooling is required (stage 2 cooling), the controller will energize the lag circuit liquid line solenoid
valve (SV). When that circuit’s pressure exceeds the LPCutIn pressure, the compressor with the
lowest starts will be started. If additional cooling is required (stage 3 cooling), the next compressor
with the lowest starts will be started. Stage 4 cooling will start the last compressor.
When the evaporator water temperature is satisfied, the reverse will occur with the compressor with
the most run hours being stopped in each stage down. If the chiller is operating at minimum capacity
(stage 1) and the chilled water temperature falls below the Leaving Evap Water Set Point, control
band and shut down delta T, the controller will stop the last operating compressor. The controller
enters the “Waiting For Load” mode and monitor the leaving water temperature.
Low Ambient Start
If the “LOW AMBIENT” option was selected and the out side air temperature is below 50°F, the low
ambient start logic will replace the normal starting logic. The compressor with the lowest starts on
the lead circuit will start when the liquid line solenoid valve is energized. The LPCutIn will be
ignored and the refrigerant freeze protect set point will be ignored for 180 seconds to allow time for
the refrigerant pressure to build. After the 180 second time and the evaporator pressure is below the
freeze stat set point, the circuit will shut down and the alarm “LoEvapPressure” will be indicated.
The chilled water freeze alarm set point is enforced during low ambient starts and will shut down the
chiller if a water freeze conditions is detected. During the low ambient start period, the evaporator
pressure is monitored and if the evap pressure drops to 2 psi for more than 20 seconds, the chiller will
shut off and a “LoEvap Pressure” alarm will be generated. This protects the compressor from
operating in a vacuum during start-up.
Pumpdown
As the system chilled water requirements diminish, the controller will stage down the compressors.
As the system load continues to drop, the liquid line solenoid valve will be de-energized and the
refrigerant circuit will go through a one time pump down sequence. As the evaporator pressure falls
below the LPCutOut set point, the compressor and condenser fan will stop. The evaporator pump
output relay will remain energized and the unit status will indicate “Waiting For Load”.