Emerson Liebert Challenger With Liebert iCOM Control 3000/ITR - Manual (Page 50)

Component Operation and Maintenance
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7.4.8 Motorized Ball Valve—Digital Scroll Compressor
On water-cooled and glycol-cooled digital scroll units, the discharge pressure is controlled by a motor-
ized 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. 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. Additionally changes in fluid temperature
could cause pressure changes that do not result in valve movement within the dead band on drycool-
ers. Fan cycling stats should be set to prevent continuous fluid temperature swings greater than 10 o F
(5.6 o C) (see 7.4.10 - Drycooler Settings).
Location
The motorized ball valves are located in the condenser fluid return line. The three-way valve is 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 con-
trol the motorized ball valves through the Service menu using manual mode to override the normal
control.
7.4.9 Regulating Valve—Scroll Compressor
The water regulating valve automatically regulates the amount of fluid necessary to remove the heat
from the refrigeration system, permitting more fluid to flow when load conditions are high and less
fluid to flow when load conditions are low. The valve consists of a brass body, balance spring, valve
seat, valve disc holders, capillary tube to discharge pressure, and adjusting screw.
Standard Valve - 150 psig (1034 kPa) system for 3 & 5 ton units (Johnson Controls Valve)
High Pressure Valve - 350 psig (2413 kPa) system for 5 ton units (Johnson Controls Valve)
Adjustment—The valve may be adjusted with a standard refrigeration service valve wrench or
screw driver. Refer to Table 7 for recommended refrigerant pressures.
To lower the head pressure setting, turn the square adjusting screw clockwise until the high pressure
gauge indicates the desired setting.