IOM 686 AGZ 030A through 065A 43
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 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 number one 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).
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.
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
The Alarm Light on the control panel will illuminate if either circuit has a high head pressure
occurrence or a compressor motor protection occurrence. Freeze protection would be indicated on the
optional Zone Terminal. If a circuit is off and the panel alarm light is not on, try depressing the
Freezestat reset on the control panel. There is a five minute time delay after depressing the reset
button.
To Next Page
Loading...
Need help?
General
