Unit installation
48
NORMAL OPERATION AND CYCLING
Once the unit has been started, all operations are fully automatic. After an initial period at minimum capacity on the
lead compressor, the control system will adjust the unit load depending on the chilled liquid temperature and rate of
temperature change. If high heat load is present, the controller will increase the capacity of the lead compressor
and/or start-up the other compressor. If very little heat load is present, the lead compressor will continue at minimum
capacity or may simply stop again to avoid overcooling the liquid. If the latter is the case, one compressor will restart
automatically should the liquid temperature rise again. Once a compressor is running, discharge pressure rises as
refrigerant is pumped into the air cooled condenser coils. This pressure is controlled by stages of fans to ensure
maximum unit efficiency while maintaining sufficient pressure for correct operation of the condensers and expansion
valves. When a compressor is running the controller monitors oil pressure, motor current, and various other system
parameters such as discharge pressure, chilled liquid temperature, etc. Should any problems occur; the control
system will immediately take appropriate action and display the nature of the fault (Refer Microprocessor Controller
Section for details).
COMPRESSOR STAGING AND SEQUENCE OF OPERATION
Sequence of Operation:
The following describes the sequence of operation for a two-screw compressor chiller unit.
Operation is similar for a one or four compressor unit.
For initial start-up, the following conditions must be met:
All power supplied to the unit shall be energized for 12 hours.
Control power switch on for at least 5 minutes.
All safety conditions satisfied.
Press ESC on the microcomputer keypad.
Chilled water pump running and chilled water flow switch contact closed.
Customer interlock contact closed, if any.
STAGE ON SEQUENCE:
Staging ON & OFF sequence shall be accomplished by the Leaving water temperature control selection.
Stage # 1:
If the Leaving water temperature is greater than the Stage 1-ON water temperature set point value, the compressor #1
liquid line solenoid & slider control valves shall be switched ON. Now the compressor is in the minimum or unloaded
capacity. The compressor capacity is varied to achieve the full/part load capacity as per the load demand.
As the discharge pressure of Compressor # 1 rises, the corresponding fans are energize accordingly to the fan stage-
ON set point. If the discharge pressure falls below the fan stage- OFF set point value, the corresponding fans will turn
off.
Stage # 2:
If the Leaving Water Temperature is greater than the Stage 2- ON water temperature set point value, the Compressor
# 2 liquid line solenoid & slider control valves shall be switched ON. Now the compressor is in the minimum or
unloaded capacity. The compressor capacity is varied to achieve the full/ part load capacity as per the load demand.
As the discharge pressure of Compressor # 2 rises, the corresponding fans are energize accordingly to the fan stage-
ON set point. If the discharge pressure falls below the fan stage-OFF set point value, the corresponding fans will turn
off.
STAGE – OFF SEQUENCE
During the staging OFF, the first-in last-out sequence is adopted, if equalization of compressor timing is not selected.
Else the more used is switched off.
As the applied load decreases and when the leaving water temperature falls below the stage 2-OFF water
temperature set point value, stage 2 is turned off.
If the leaving water temperature falls below the stage 1-OFF water temperature set point value, the stage 1 is turned
off.