McQuay AGR 070AS, Operation Manual

Page: 65 / 100

IOMM AGR-1 AGR 070A through 100A 65
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 circuit will start when a compressor in the
circuit has started and the condenser pressure exceeds fan on setpoint (230psig R-22). This fan will
not cycle off until the circuit is stopped. The second fan will cycle on and off to maintain minimum
lift pressure. If the condenser pressure increases rapidly, the second fan will be turned on at 290psig
(R-22 setpoint) and remain on until minimum lift pressure regains control. The third fan will operate
based on the minimum lift pressure control.
Pumpdown Control
The AGR units have a limited recycling pump down. Certain alarm conditions will not permit chiller
pumpdown. These alarm conditions are:
•
High Condenser Pressure
•
Bad Evap Pressure Sensor
•
Phase Voltage Monitor (optional)
•
Volts Ratio Protect
•
Mechanical High Pressure Switch
•
Compressor Motor Protect
•
Ground Fault (optional)
Safety Systems
MicroTech continuously performs self diagnostic checks, monitoring all system temperatures,
pressures and safeties, and will automatically shut down a refrigerant circuit or the entire unit should a
fault occur. The cause of the shutdown, time and date of occurrence and conditions at the same time
of alarm will be retained in memory and can be displayed for the operator to review.
The MicroTech controller has several safety systems to safeguard against potential damage of the
chiller equipment. There are alarm and pre-alarm conditions. Alarm conditions cause the
compressors to shut off and/or lockout. Pre-alarm conditions cause the unit to stage down in capacity
in an attempt to avoid an alarm condition. For alarm conditions, the red “Alarm Status” light on the
control panel will be solid. For all pre-alarm conditions, the light will pulse at the rate of 0.5 seconds
on, 4 seconds off. When an alarm of pre-alarm condition exists, the MicroTech display automatically
switches to the alarm menu to alert the user of the condition.
The MicroTech controller alarms can either be of the manual reset type or the auto reset type. Critical
alarm conditions such as High Condenser Pressure, Refrigerant Freeze Protection, and Low
Evaporator Pressure which protect against equipment damage are manual reset, which means they
lockout compressor operation and must be cleared at the MicroTech keypad before operation can
resume. Auto-clearing alarm conditions cause the compressors to shut off on the affected circuit until
the condition is corrected. When the condition is corrected, the chiller will resume normal operation
on that circuit, provided the anti-cycle timers have cleared and sufficient load exists. All pre-alarm
conditions are auto-clearing. After the controller stages down the unit on an alarm condition and the
potential alarm condition has been avoided, the alarm will clear y itself and the unit will resume
normal operation. If the alarm condition is not avoided, the alarm will be logged on the MicroTech
display, causing lockout of compressors. Thus, the alarm must be manually cleared.
When a McQuay Chiller with a MicroTech controller is used with the McQuay RMS (Remote
Monitoring and Sequencing) Panel, the user must note that alarm nomenclature varies. The RMS has
three possible categories of alarms: faults, problems, and warnings. The chiller alarm condition
corresponds with the RMS “fault” condition. Thus, there are system and circuit faults. Chiller pre-
alarm conditions correspond to the RMS “problem” designation. Reciprocating chillers do not have
warning alarms.