Emerson 20/20 service manual Pumpdown Control

Section 6 • Compressor Control

NOTE

VFD installation is not covered in this manual. A VFD that is not properly installed and configured has the potential of causing intermittent and dangerous problems. Please consult your VFD manual.

1 Step VFD Control:

•Enables the first step in the VFD control algorithm. This check box is not deselectable by the operator.

Capacity Slide Position:

•Defines the minimum and maximum positions for the capacity slide. While in 1 step control these values should be 0% for minimum and 100% for maximum.

VFD Speed:

•Defines the minimum and maximum speed for the motor speed. While in 1 step control these values should reflect the full range of the VFD.

2 Step VFD Control:

•Enables the second step in the VFD control algorithm. Capacity Slide Position:

•Defines the minimum and maximum position of the capacity slide in the 2 step VFD control.

VFD Speed:

•Defines the minimum and maximum speed for the motor in the 2 step VFD control.

P = Proportional (gain) setpoint:

•Used to adjust the motor speed action in direct pro- portion to the difference between the control set- point and the process variable (SP - PV error). This is a unit-less quantity and is used for coarse adjustment. This setpoint should be set to the lowest value that gives adequate control system response. Increasing the proportional setting increases the control sys- tem’s sensitivity to small process fluctuations and the tendency to hunt.

I = Integral (reset) setpoint:

•Used to adjust the capacity control action, integrating the error over time, to account for a small error that has persisted for a long time. This quantity is used for fine adjustment. This setpoint is used to smooth out process variations. This setpoint should be set high enough to prevent hunting but low enough to pre- vent control system overshoot.

D = Derivative (rate) setpoint:

• Used to adjust the capacity control action, accounting

for how fast the error is changing, positively or nega- tively. A standard PID loop variable, it is not used for our applications.

Pumpdown Control

The Pumpdown Control defines a method of “pumping” down a chiller, which is to draw off refrigerant from the chiller. This feature can be enabled or disabled from this menu which is the part of Compressor Control Screen. If Pumpdown is enabled, this feature will only function when the compressor is running in local Auto Mode and Control Mode Configured is Suction Pressure.

If Pumpdown Feature is enabled, then;

•The Auto-cycle functionality is ignored. Pumpdown mode will cause the compressor to cycle off via the Pumpdown Stop Pressure setpoint. Normally, the Pumpdown Stop Pressure setpoint will be set lower than the Auto-cycle Stop setpoint. Therefore, as the suction pressure is pulled down, the compressor is prevented from shutting down prematurely via the Auto-cycle Stop setpoint by automatically ignoring the Auto-cycle feature.

•The compressor will be placed into “Stop” mode af- ter the suction pressure is equal to, or goes below the Pumpdown Stop Pressure.

Pumpdown:

•This checkbox enables the Pumpdown feature. If this box is unchecked, Pumpdown setpoints are ig- nored and the user is not allowed to edit Pumpdown setpoints.

Stop Pressure:

•This setpoint defines the suction pressure value at which the compressor will cycle off. Normally, this setpoint is set below the Suction Pressure Auto-cycle Stop Pressure setpoint.

Stop Delay:

•This setpoint delays the compressor from stopping when the suction pressure is equal to or less than the Stop Pressure.

Min Slide Position:

•The minimum capacity slide is the setpoint that the compressor is allowed to run at. By forcing the compressor capacity to operate at a value above minimum, we insure that the suction pressure will be pulled down to the Stop Pressure setpoint.

Pumpdown Operation (Run/Stop):

• This button starts/stops the Pumpdown operation.

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