McQuay PFS 155C User Manual

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4 PFS 155C - 315C OM 135

Alarm

The alarm light on the MicroTech control panel will be illuminated when any alarm condition exists.

Unless the alarm condition affects both compressors, the remaining compressor will operate as

required. Refer to the control section of this manual for additional information.

Start-up

If none of the above “off” conditions are true, the MicroTech controller will initiate a start sequence

and energize the chilled water pump output relay. If flow is not proven within 15 seconds, the alarm

output will be turned on and the keypad display will be No Evap Flow. When evap flow is confirmed,

the controller will sample the chilled water temperature and compare it against the Leaving Chilled

Water Setpoint and the Start-up Delta Temperature, which have been programmed into the controller’s

memory. If the leaving chilled water temperature is above the Leaving Chilled Water Setpoint plus the

adjustable Start-up Delta Temperature (which is defined outside of the control band), the controller

will then start the condenser pump. The same time and alarm conditions exist for the condenser flow.

The compressor with the lowest number of starts will be the lead compressor. The controller will open

the electronic expansion valves to regulate the pressure difference between the evaporator and the

condenser by pulling the evaporator pressure low once the compressor starts. The controller will start

the lead compressor and energize the compressor suction injection and motor cooling solenoid valves.

Liquid injection is energized through the auxiliary starter contacts. The lead compressor will increase

capacity by staging up according to the chilled water leaving evap setpoint. The lead compressor will

stage to 100% capacity before starting the lag compressor. If additional cooling capacity is required,

the controller will energize the lag compressor and stage it up according to setpoint. The compressors

capacity control solenoids will automatically be controlled as required to meet the cooling needs of

the system.

The electronic expansion valves are operated by the MicroTech controller through adaptive control

strategies (use of different system temperatures and pressures) to maintain desired refrigerant control

to the evaporator at all conditions.

Condenser Control

The condenser pump will be started in conjunction with the above logic to provide condenser water

flow. The minimum entering condenser water temperature for full load air conditioning duty shall be

65°F with nominal cooling tower water flow of 3 GPM. When condenser water temperatures are lower

and or chiller load is less than 100%, the condenser water flow or entering condenser water

temperature must be controlled to maintain satisfactory operating refrigerant pressures. The

MicroTech controller can control condenser water entering temperature by a condenser water bypass

valve based upon entering condenser water temperature. It can also control condenser water flow by

a flow control valve based upon condenser pressure. When using a flow control valve, a minimum

water flow through the condenser must be maintained. This minimum water flow is based upon

system conditions and can vary with each installation.

Shutdown

As the system chilled water requirements lessen, the compressors will be unloaded. As the system

load continues to drop, the lag compressor (compressor with the most run hours) will be stopped. A

continuing load reduction will start the hot gas option or the chiller can shut down on the shut down

delta setpoint below the control band.

Hot gas control will allow the unit to maintain minimum operating parameters if proper condenser

control is available. This type of control is standard and selectable from the keypad. If hot gas

control is not selected, the unit will shut down on the shut down delta setpoint. This feature can be

used to minimize chiller run time under light load conditions.

The condenser water pump will be stopped upon chiller shutdown and the chilled water pump output

relay will remain energized until the time schedule’s “on” time expires, the remote stop switch is

opened, the system switch is moved to the stop position, or a separate communications panel such as

the Chiller System Controller or an Open Protocol interface disables the chiller.

Questions and Answers:

K
kingemmaAug 4, 2025
What to do if McQuay PFS 155C Chiller compressor will not load or unload?
- Y
  ypayneAug 4, 2025
  If the McQuay Chiller compressor will not load or unload, check the solenoids for proper operation and replace them if necessary.