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3.3.3 Chilled Water Control
The chilled water control valve is adjusted proportionally as the temperature control varies the
requirement for cooling from 0% to 100%. Units with the optional variable speed drive (VSD) control
the fan speed in a similar manner, except that the minimum fan speed is 60% when the cooling
requirement is less than 60%. Also, the fan is operated at 100% on a call for any heating and/or
humidification.
A three-point actuator is used for chilled water cooling, as well as free cooling, or heating, with either
valves or dampers. These actuators are driven through two digital outputs: Open and Close.
The Three-Point Actuator Running Time (Service Menu, Options Setup) defines the time needed to
move the valve or damper from closed to open. The Actuator Direction (Service Menu, Options Setup)
defines if the valve/damper works in direct or reverse mode (the controller outputs are exchanged;
Open becomes Close and Close becomes Open).
Any time the unit receives the Power On signal, the valve performs a 3P Reset: the Close command is
given continuously for a time of 110% of the 3P actuator run time. This calibrates the valve or damper
with the controller and ensures that it is closed. A 3P Reset is also performed if the fan is switched off
for any reason (timer off, unit off, etc.).
During normal operation the 3P actuator is driven in steps. After start of control (unit on or free-cool-
ing / glycol cooling is enabled) the 3P actuator moves (without stepping) to the requested position,
after that the stepping starts.
Figure 17 Three-point actuator control (example: cooling)
3.4 Temperature Control—Second Cooling Source
Certain cooling units are available with a second source of cooling within the unit. These typically are
compressorized models with an additional chilled water or free-cooling coil.
3.4.1 Differential Temperatures / Controls (Comparator Circuit)
Delta T (Temperature Difference) Between Room and Glycol
The comparator circuit determines if the glycol / chilled water temperature of the second cooling
source is low enough to provide at least partial cooling capacity. The comparator circuit has three set-
tings (DT Between Room / FC Type, [Service Menu, Setpoints]):
•No
•Contact
•Value
The No setting is for standard compressorized and chilled water units that do not have a second cool-
ing source. The Contact setting is used when an external control is being used to determine when the
second cooling source is to be activated. The external control communicates to the Liebert unit via a
contact closure
• Closed = cooling enabled
• Open = cooling disabled.
+ 100%
Cooling
Cooling
½ Proportional Band
0% - 3P Closed
Requested 3P Position
100% - 3P Fully Open
0%
Setpoint
+ Temp