McQuay AGS 206A Operation Manual Download

Page: 52 / 66

AGSB-5

Sequence of Operation

The following sequence of operation is typical for McQuay models AGS chillers. The
sequence may vary depending on the software revision or various options that may be
installed on the chiller.

Off Conditions

Power is supplied to each power panel located between condenser sections. Optionally, the
power may be supplied to a single power connection located in a terminal box on the base
of the unit.

With power supplied to the unit, 115 VAC power is applied through the control fuse F1 to
the compressor heaters (HTR1, HTR2, (and HTR3), evaporator heater, and the primary of
the 24V control circuit transformer.

CAUTION

Compressor heaters must be on for at least 12 hours

prior to start-up to avoid compressor damage.

The 24V transformer provides power to the MicroTech II controller and related components.
With 24V power applied, the controller will check the position of the front panel system
switch. If the switch is in the "stop" position, the chiller will remain off, and the display
will indicate the operating mode to be

OFF: System Sw.

The controller will then check the

pumpdown switches. If any of the switches are in the "stop" position, that circuit’s
operating mode will be displayed as

OFF: PumpDwnSw.

If the switches for both circuits

are in the "Stop" position, the unit status will display

OFF: PumpdownSw’s

. If the remote

start/stop switch is open the chiller will be

OFF: RemoteSw

. The chiller may also be

commanded off via communications from a separate communicating panel such a BAS
protocol interface. The display will show

OFF: RemoteComm

if this operating mode is in

effect. If an alarm condition exists which prevents normal operation of both refrigerant
circuits, the chiller will be disabled and the display will indicate

OFF: Alarm

. If the

control mode on the keypad is set to "Manual Unit Off," the chiller will be disabled and the
unit status will display

OFF: ManualMode

. Assuming none of the above stop conditions

are true, the controller will examine the internal time schedule to determine whether the
chiller should be permitted to start.

Alarm

The red alarm light in back of the left arrow key on the controller will be illuminated when
one or more of the cooling circuits has an active alarm condition which results in the circuit
being locked out and manual reset is required. Unless the alarm condition affects all
circuits, the remaining circuits will operate as required. Events will not cause the key to
light.

Start-up

If none of the above "off" conditions are true, the MicroTech II controller will initiate a start
sequence and energize the chilled water pump output relay. The chiller will remain in the

WaitForFlow

mode until the field installed flow switch indicates the presence of chilled

water flow. If flow is not proven within 30 seconds, the alarm output will be turned on, the
keypad display will be

WaitForFlow,

and the chiller will continue to wait for proof of

chilled water flow. Once flow is established, the controller will sample the chilled water
temperature and compare it against the Leaving Chilled Water Setpoint, the Control Band,
and the Start-up Delta-Temperature, which have been programmed into the controller’s
memory.