Emerson Vission 20/20 Operation And Service Manual page 161

Appendix B • Vission 20/20 Application Procedures
Pulldown OPERATION EXAMPLE
Assumptions:
• Compressor is off
• Pulldown is selected
• "Initiate Pulldown at Next Start" is selected
• Current suction pressure = 80 PSIG
• Auto-cycle setpoints are enabled
• Pulldown setpoints are setup per the Setpoint Selection Example
Variables:
• Step Pressure = 5.0 PSIG
• Delay Per Step = 20 hours
• Stop Pressure = 20 PSIG
• Auto-cycle Differential = 4 PSIG
Operator presses Unit Start Auto button and the compressor starts. Two items occur:
• The Pulldown feature is now operational
• When Pulldown feature is active:
• Pumpdown is disabled (Pulldown and Pumpdown operation modes are mutually exclusive)
• Low Suction Pressure Stop Load and Unload setpoints are active (Make sure that these setpoints do not
confl ict with the Pulldown Stop setpoint)
• The Pulldown setpoints are immediately calculated:
Initial Pulldown setpoint = Current Suction Pressure 80 psig minus Step Pressure (5 psig) = 75 psig
Auto-Cycle Start Pressure = Pulldown setpoint (75 psig) plus Auto-Cycle Differential (4 psig) = 79 psig
Auto-cycle Stop Pressure = Pulldown setpoint (75 psig) minus Auto-Cycle Differential (4 psig) = 71 psig
The compressor will maintain the suction pressure at 75 psig for the fi rst 20 hours, and then the next calculation of
Pulldown setpoints will be calculated:
Subsequent Pulldown setpoint = Suction Pressure setpoint (75 psig) minus Step Pressure (5 psig) = 70 psig.
Auto-Cycle Start Pressure = Pulldown Setpoint (70psig) plus Auto-Cycle Differential (4 psig) = 74 psig
Auto-Cycle Stop Pressure = Pulldown Setpoint (70 psig) minus Auto-Cycle Differential (4 psig) = 66 psig
After 20 hours of running at 70 psig, the next set of Pulldown setpoints are calculated. This is repeated until the target
setpoint (Stop Pressure setpoint) is reached. The Pulldown operation is then disabled and the compressor will con-
tinue to operate at this setpoint.
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
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