OM 135
PFS 155C - 315C
3
Unit Layout and Principles of Operation
Control Center
All electrical controls are enclosed in a control center with keylocked, hinged access doors. The
control center is composed of two separate compartments, MicroTech Controller and shelf-mounted
starter.
The MicroTech control panel contains a model 280 microprocessor based controller which provides all
monitoring and control functions required for the safe, efficient operation of the chiller. The operator
can monitor all operating conditions by using the panel’s built in 4-line by 40-character keypad /
display or by using an IBM compatible computer running McQuay Monitor software. In addition to
providing all normal operating controls, the MicroTech controller monitors all safety devices on the
unit and will take corrective action if the chiller is operating outside of it’s normal design conditions. If
a fault condition develops, the controller will shut the system down and activate an alarm output.
Important operating conditions at the time an alarm condition occurs are retained in the controller’s
memory to aid in troubleshooting and fault analysis.
The system is protected by a simple password scheme which only allows access by authorized
personnel. A valid password must be entered into the panel keypad by the operator before any
setpoints may be altered.
Sequence of Operation
The following sequence of operation is typical for McQuay models PFSXXXC screw water chillers.
The sequence may vary depending on the software revision or various options that may be installed
on the chiller.
Off conditions
With power supplied to the unit, 115 VAC power is applied through the circuit breaker (CB) to the
primary of the 24V control circuit transformer and Output Board Relays. The compressor heater (HTR)
is energized through the normally closed contacts on the starter. Note: Before start-up, the
compressor heater must be on for at least 12 hours to insure there is not any liquid refrigerant in the
compressors. The 115V / 24V transformer provides power to the MicroTech controller, two 24V center
tapped transformers 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. (Be certain the individual
compressor switches inside the top right door are in the “on” position. These switches may be used
to lock out a specific compressor.) If the system switch is in the “Auto” position the controller will
then check the remote start/stop switch. If the remote start stop input is open, the chiller will be OFF:
RemoteSw. The chiller may also be commanded off via communications from a separate
communicating panel such as the Chiller System Controller Panel or an Open Protocol interface. The
display will show OFF: RemoteComm if this operating mode is in effect. If an alarm condition exists
which prevents normal operation, 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: Manual Mode. 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. The operating mode will be OFF: TimeClock if the time schedule indicates time remaining in
an “off” time period.
Ice Mode Operation
For operations requiring ice mode feature, logic in MicroTech must be programmed to adjust the
operating parameters for the specific application . Refer to the control section of this manual for
additional information.
