User manual

Operation

31 Liebert

iCOM

Figure 29 Second cooling source and two-step compressorized cooling

Minimum Chilled Water Temperature—This feature allows the user to select a minimum chilled

water temperature to prevent simultaneous operation of the chilled water (free-cooling) and

compressor(s) on units factory-programmed to operate both sources simultaneously. This feature is

enabled in the Service menu under Setpoints, parameter Minimum CW Temp.

If the fluid temperature sensed is below the Minimum CW Temp Value, (Service Menu, Setpoints),

and the Delta T (Temperature Difference) Between Room and Glycol (see 3.3.1 - Differential

Temperatures / Controls (Comparator Circuit) is satisfied, the control will operate ONLY the

chilled water for cooling. The compressor(s) is locked out. If cooling is not achieved at this time (i.e.,

loss of chilled water flow), the control will override this setting and activate a compressor(s). When

this happens, a free-cooling lockout for one hour event is produced. Free-cooling will not resume until

one hour has elapsed.

If the fluid temperature is above the minimum CW value, the control will operate the chilled water

(free-cooling) and compressor(s) simultaneously if needed. If the Minimum CW Temp is disabled, the

control may operate both sources as load requires.

7571 72 73 7470

Increasing Temperature

Cool 2

Cool 1

+ 100%

Cooling

Valve 100%

Open

Valve

Closed

½ Dead-

band

76 77 78 79

+ 200%

Cooling

2nd

Source

Comp

½ Proportional Band ½ Proportional Band

Band 1: 2

Source Band 2: Compressors

Temp Setpoint: 70°F

Proportional Band: 8°F

Deadband: 2°F