User manual

Operation
21 Liebert
®
iCOM
®
3.1.3 General Compressor Requirements
Low-Pressure Time Delay
When the compressor starts, the low-pressure input is ignored for a selected period of time based on
the setting of the Low Pressure Alarm Delay (Service Menu, Options Setup). This time is usually set
to 3 minutes on air-cooled units, and to 0 or 1 minute on water cooled units. When this time is
expired, a second timer starts to operate if the low-pressure input is active. This second timer is active
during normal compressor operation to avoid compressor trips due to bubbles in the refrigerant or
other influences creating short trips of the low-pressure switch. The low-pressure device input is
ignored if the compressor is not operating. Exception: Pump Down (see Pump Down).
The advanced low pressure routine is automatically used when a unit is equipped with the Liebert
iCOM controller, unloading compressors (4-Step and Digital Scroll), low-pressure transducers and
has variable fan speed (VFD or EC fans). The low-pressure routine monitors the low-pressure
transducers and interacts with the fan and compressor to prevent the units coil from freezing.
Advanced DX Freeze Protection
Applying fan speed to direct expansion systems generates the issue of how to avoid freezing
condensate on the coil when the unit operates below 100% fan speed. Liebert iCOM's advanced freeze
protection now provides the ability to predict freeze conditions and correct this condition
automatically by adjusting the fan speed and compressor capacity.
Compressor Type Support
• Digital scroll (all sizes)
• Four-step (all sizes)
• No standard scroll support
Freeze Protection
• Fan and compressor will override sensors when freezing is detected
• Fan ramps up first, then compressor unloads
NOTE
Low-pressure condition could be read through contacts or through pressure transducers with
threshold setting.
NOTE
Pressure transducers have been installed in all Liebert DS
™
units. However, the Liebert Deluxe
System\3
™
units have all shipped with low-pressure switches, which cannot utilize the
advanced freeze protection routine described above. Units equipped with switches have a
higher minimum fan speed setting due to this limitation. A Liebert Deluxe System3 unit can be
updated to pressure transducers as a field upgrade if lower fan speed settings are desired. To
see the pressure readings, go to Service Menus > Diagnostics/Service mode, Page 1.