McQuay AGR 090AS User Manual

54 AGR 055A through 100A IOMM AGR
Freezestat Control
On startup the Global UNT controller will check the evaporator pressure after the SLPT(starting low
pressure time) timer times out. If the evaporator pressure is greater than the LPHL (low pressure
high limit) value, the Global UNT controller will continue to stage up as additional cooling is
required. If the evaporator pressure is less than the LPHL, the Global UNT controller will de-
energize BO7 or BO8 (depends on circuit starting) which will stop the compressor. This is a
Freezestat alarm (alarm will indicate on the optional Zone Terminal) and will require the resetting
by depressing the FREEZESTAT RESET on the control panel. The circuit will not go through a
pumpdown.
If the chiller has been operating and the evaporator pressure in either circuit is less than the
LPFS (low pressure Freezestat setpoint) the Global UNT controller starts the FSTP (Freezestat time
period). If the evaporator pressure is below the LPHL after the FSTP times out, BO7 or BO8
(depends on evaporator circuit with the low pressure) will be de-energized stopping the operating
compressor(s) and closing that refrigerant circuit’s liquid line solenoid valve. This is a Freezestat
alarm (alarm will indicate on the optional Zone Terminal) and will require the resetting by
depressing the FREEZESTAT RESET on the control panel. There is a five minute time delay after
the freezestat reset switch has been depressed. The circuit will not go through a pumpdown upon
alarm.
Starting Timers
The Global UNT controller has a starting timer which delays checking the evaporator pressure for
260 seconds. After the timer times out, the evaporator pressure is checked. If the pressure is above
the LPHL (low pressure high limit, (default 57psi)) value, the controller stages up on leaving water
temperature requirements. If the evaporator pressure is below the LPHL, then the controller will
stop that circuit and indicate a freezestat alarm on the (optional) Zone Terminal.
Head Pressure Control
There are two or three condenser fans per refrigerant circuit. These fans are cycled on and off
according to the circuit’s condenser pressure. Fan #1 of a refrigerant circuit will start when the
compressor of that refrigerant circuit has started. This fan will not cycle off until the circuit is
stopped. The second fan on a refrigerant circuit will cycle on and off according to its head pressure
controller (PC12 or PC22). The third fan on a refrigerant circuit is controlled by an ambient
temperature control.
Optional Head Pressure Control (SpeedTrol)
The first condenser fan on each circuit can have the SpeedTrol option. The fan rpm’s is varied to
maintain a minimum condenser head pressure for stable operation at low ambient temperatures.
Wind baffles may be required in addition to the SpeedTrol option for low ambient operation if the
unit is positioned where winds can blow directly into the coils.
Alarms
Circuit Alarm Conditions
The “Circuit Alarm Conditions” are those alarms which only affect one circuit and can stop the
compressors of that circuit. These alarms are:
• Compressor Motor Protection
• High Condenser Pressure
• Freezestat Protection

McQuay AGR 090AS Specifications

General

Brand	McQuay
Model	AGR 090AS
Category	Chiller
Language	English