14
ACZ 010A through 039A
IOMM ACZ1
Sequence of Operation
The following sequence of operation is typical for Models ACZ 010A through ACZ 039A. It can
vary depending upon options.
Start-Up
With the control circuit power on, 115V power is applied through the control circuit fuse F1 to the
compressor crankcase heaters, the compressor motor protections and the primary of the 24V control
circuit transformer. The 24V transformer provides power to the microprocessor controller.
If an optional remote time clock or remote manual switch is field wired to the unit (terminals 25 and
35), it must be closed in order to start the unit. The operation of the unit is then under the control of
the field supplied staging thermostat. The two compressors will start when the normally open staging
contacts close.
Equipment Protection Alarms
The following conditions will shut down the unit and activate the alarm circuit:
•
No water or air flow
•
Low evaporator pressure
•
High condenser pressure
•
Motor protection system
•
Phase voltage protection (Optional)
•
Outside ambient temperature
•
Sensor failures
The following alarms will limit unit operation:
•
Condenser pressure stage down, unloads unit at high discharge pressures
•
Low ambient lockout, shuts off unit at low ambient temperatures
•
Low evaporator pressure hold, holds stage #1 until pressure rises
•
Low evaporator pressure unload, shuts off stage #2
Unit Enable Selection
Enables unit operation from local keypad, digital input, or Building Automation System.
Unit Mode Selection
Selects standard cooling or test operation mode. (Test is for service personnel only.)
Condenser Fan Control
Control of condenser fans is provided by the MicroTech II controller. The control steps condenser
fans based on discharge pressure.
Shutdown
As the Stage #2 external staging thermostat is satisfied, it will stage off the lag compressor unloading
the unit. The Stage #1 will de-energize the liquid line solenoid valve SV1 and the lead compressor
will pump down the unit and shut off on Low Suction Pressure at 40 psig. If the low pressure cutoff
point cannot be reached in 120 seconds, the compressor will time off. The compressor crankcase
heaters will energize when the compressors shut off, keeping the small amount of refrigerant in the
plate heat exchanger from migrating to the compressor. See page 29 for detailed explanation of
compressor staging.