Emerson 3000/ITR Motorized Ball Valve-Digital Scroll Compressor, Control Method, Startup

Component Operation and Maintenance

7.4.8Motorized 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 10oF (5.6oC) (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.

7.4.9Regulating 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.

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