McQuay AGR 070AS, Operation Manual

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64 AGR 070A through 100A IOMM AGR-1
Soft Loading
Soft loading limits the number of available stages when the unit is started to prevent excessive power
consumption and possible overshoot of the leaving water temperature set point. Soft loading is in
effect whenever the unit is started from an “off” cycle. On initial start-up, the chiller will turn on the
chilled water pump and sample the loop water temperature for a time equal to the Load Delay set
point (“LoadDelay”). If cooling is required at the end of the time-delay, a start procedure is initiated.
On entering the “Stage mode” the controller starts a countdown timer to indicate how long the unit
has been in the cool stage mode. The number of stages allowed during soft loading is determined by
the Soft Load Maximum Stages (“SoftLdMaxStg”). The duration of the soft load sequence is
determined by the Soft Load Timer. If the Soft Load Timer is set to zero, no soft loading will take
place. When the soft load option is enabled, any time remaining in the Soft Load Timer will be
displayed in the menu Soft Load Set points under item “Time Left”. The following set points may be
adjusted in SoftLoadSpts menu:
•
Soft Load: This is the amount of time soft loading will be in effect after the controller begins
staging. If set to zero, soft loading is canceled.
•
SoftLdMaxStg: Determines the maximum number of cooling stages which may be energized
while soft loading is in effect.
•
LoadDelay: The amount of time allowed for the controller to sample the loop water temperature
before initiating cooling.
Manual Operation
Manual operating modes are available to simplify setup and troubleshooting of the unit. Any of the
following manual modes may be selected from the Control Mode Menu.
Note: These manual settings are intended to aid in troubleshooting and should not be
considered to be normal operating modes. The equipment should not be left unattended
during manual operation as the automatic staging controls are disabled. The chiller will
remain in the manual mode until Automatic operation is selected or a problem alarm occurs
which will change back to the “Automatic mode”.
•
ManualOff: Manual unit off.
•
Auto1Off2: Automatic Circ#1, Circ#2 off.
•
Auto2Off1: Automatic Circ#2, Circ#1 off.
•
ManualStaging: Manual Staging, Circ#1 & 2.
Compressor Staging
The AGR Reciprocating chiller has two refrigerant circuits and each circuit has one reciprocating
compressor. When in the Automatic mode and stage 1 of cooling is required, the compressor with the
lowest number of starts will be the first compressor to start. Stage 2 of cooling will start the
compressor in the other refrigerant circuit. Stages 3 through 6 will cause the compressors to load all
cyclinders.
If a compressor is not available for starting due to a safety, the number of cooling stages will be
decreased. In normal operation the second stage of cooling will have a compressor from each
refrigerant circuit operating. The exception to this is when “LOW AMBIENT” is selected and the
outside air temperature is below the low ambient setting. The staging logic will be modified to start
the compressor with the lowest starts and stage 2 will load the compressor on that refrigerant circuit.
Both compressors on one refrigerant circuit will operate at low ambient to improve head pressure
control. If additional cooling is required, the next refrigerant circuit will be started.