McQuay WDC

24 Product Manual PM WSC/WDC
Condenser water pump control relay
Water flow through the condenser should be discontinued when the chiller is inoperative. Continuous flow
through a cooling tower, without inclusion of building heat in the water, will overcool condenser water if tower
bypass is not employed and will unnecessarily depress the chiller’s refrigerant pressure. Where energy
conservation is desirable, cessation of condenser water flow when the chiller is not operating provides a
practical, inexpensive method of saving power.
Alarm circuit
Terminals are provided in each unit control panel to supply 24 volt AC power to an external alarm circuit. A 25
VA relay coil may be connected to these terminals. The coil will be deenergized when any of the unit’s or
system’s protective controls function. The alarm is not included.
Operating Sequence
With the control panel "Stop-Auto" switch in the "Auto" position, the unit will start, provided that:
1. The chilled water sensor is calling for cooling.
2. No time delay is restraining operation.
3. A remote start-stop switch is not open, preventing unit operation.
4. No safety switch has been tripped and not reset.
5. Compressor is unloaded and lubricant temperature and pressure are within prescribed limits.
The statement "Waiting to load", and the countdown period in seconds assigned to it, assumes that the water
temperature sensed by the chilled water temperature sensor may not represent the entire chilled water system
temperature if the chilled water pump has been shut off. This delay interval provides time for the chilled water
pump to circulate system water and impart a valid system water temperature to the chilled water sensor.
Temperature control operation
Temperature sensors are negative coefficient thermistors selected for extended accuracy and close control.
During compressor operation from 10% to 100% capacity, chilled water temperature will be held to within ±0.2
degrees F (0.12 degrees C). As building cooling load is decreased, the compressor inlet vanes will close as
required to match building load down to 10% of full capacity. A further decrease in the cooling load will lower
the leaving chilled water temperature. The control system will permit a total of 3 to 10 degrees F (1.6 to 5.5
degrees C) (user adjustable) overcooling of the chilled water, preventing rapid restarting and/or elevation of the
chilled water temperature above the setpoint. When the chilled water temperature is depressed to the shutoff
differential setpoint, the compressor motor is de-energized. The oil pump motor continues to run during the
compressor coast-down period and is timed off automatically.
If there is still some load on the chilled water, its temperature will rise until it reaches the cycle-on temperature
setting. At this point the compressor will initiate its start cycle and commence operation.