Liebert Precision Cooling System DS - Manual (Page 130)

Maintenance
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To lower the head pressure setting, turn the square adjusting screw clockwise until the high pressure
gauge indicates the desired setting. To raise the head pressure setting, turn the adjusting screw coun-
terclockwise until the desired setting is obtained. Consult the factory if your unit is equipped with
valves from other manufacturers.
Testing Function of Valve
First, turn off the refrigeration system. When the refrigeration system has been off for approximately
10 to 15 minutes, the water flow should stop. If the water continues to flow, the valve is either
improperly adjusted (with head pressure too low) or the pressure-sensing capillary is not connected
properly to the condenser.
Location
The water regulating valves are located in the condenser fluid supply line.
12.11.2Motor Ball Valve—Digital Scroll Compressors
On digital scroll units discharge pressure is controlled by a motorized ball valve. During unloaded
operation, the pressure changes during each digital cycle could result in excessive repositions with a
pressure operated water regulating valve. The control algorithm for the motorized ball valve uses an
intelligent sampling rate and adjustable pressure thresholds to reduce valve repositions. The valve
assembly consists of the brass valve, linkage and actuator.
Control
The valve actuator operates on 24VAC power and is controlled by a 2-10VDC proportional control sig-
nal. The valve full open to full close time is 60 seconds. At 2VDC the valve is closed; at 10VDC the
valve is fully open. There is a 20-second delay to position the motorized ball valve before starting the
compressor.
Control Method
The control utilizes an upper and lower pressure threshold with a 35 PSI (241 kPa) deadband to
reduce valve movement. If the liquid pressure is between the upper and lower threshold the valve
remains at the current position. If the liquid pressure exceeds the upper threshold the valve opens,
and if the pressure falls below the lower threshold the valve closes. There are multiple adjustment bands to
ease discharge pressure back into control range.
Adjustment
Both pressure thresholds can be shifted simultaneously over a 50 PSI (345 kPa) range (the 35 PSI
[241 kPa] differential remains constant). The ball valve setpoint offset parameter in the Service menu
can be adjusted from 0 to 50 PSI (345 kPa) to raise or lower the control band similar to the pressure
adjustment on a water regulating valve. Changing the setpoint offset will adjust the pressure thresh-
olds for both circuits. Units are factory set at a 30 PSI (207 kPa) setpoint offset (30 PSI [207 kPa]
above minimum). This results in a 220 PSIA (1517 kPa) lower threshold and a 255 PSIA (1758 kPa)
upper threshold pressure.
Startup
The setpoint offset is adjusted to the minimum value during startup, then transitions to the set value
once the compressor reaches normal operating pressures. Due to the control dead band it is possible
for each circuit to stabilize at different pressures within the dead band. Additionally changes in fluid
temperature could cause pressure changes that do not result in valve movement within the dead
band. Fan cycling stats should be set to prevent continuous fluid temperature swings greater than
10 o F (5.6 o C) (see 12.11.3 - Drycooler Settings).
Location
The motorized ball valves are located in the condenser fluid return line. Three-way valves are piped
in a mixing arrangement with the common port at the valve outlet.
Manual Control
The valve can be manually set by disconnecting AC power, depressing the manual override button on
the valve actuator, and adjusting the valve position with the handle. You also have the option to control
the MBV’s through the Service menu using manual mode to override the normal control.