McQuay AGZ 065A User Manual

IOMM AGZ-3 AGZ 035A through 065A 43
Pumpdown Control
The AGZ units have a one time pump down. The pump down is activated when a circuit’s liquid line
solenoid valve is de-energized. After the circuit’s liquid line solenoid valve is de-energized, the
operating compressor(s) on that refrigerant circuit will operate until the low pressure switch opens
and stops that compressor(s). The compressor will not cycle on if the evaporator pressure exceeds the
low pressure cut-in value. The compressor will start if the Global UNT controller calls for cooling.
The Pumpdown and Stop switch will open its respective circuit’s digital input to the Global UNT
controller. This will open the associate circuit’s output and open the circuit’s solenoid valve. If a
compressor is operating on that circuit, the compressor will operate until the low pressure switch
stops the compressor. If a compressor is not running, the switch does not activate a pumpdown for
that circuit but will not allow the circuit to stage on and operate.
Certain alarm conditions will not permit chiller pumpdown. These alarm conditions are:
•
High Condenser Pressure
•
Bad Evap Pressure Sensor
•
Compressor Motor Protect
•
Freezestat Alarm
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 that 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.

McQuay AGZ 065A Specifications

General

Brand	McQuay
Model	AGZ 065A
Category	Chiller
Language	English