Emerson 20/20 service manual Pulldown Control

Section 6 • Compressor Control

This button is active only when compressor is in local Auto mode and Control Mode Configured is Suction Pressure. This button will display “Run” when Pumpdown operation has not started or stopped, while button will display “Stop” when Pumpdown op- eration is running.

When Pumpdown feature is enabled, Pulldown check- box is automatically grayed out. Similarly when Pulldown feature is enabled, Pumpdown checkbox is automatically grayed out and hence, the user will not be able to oper- ate Pumpdown feature. This is done to keep Pumpdown and Pulldown features mutually exclusive.

Pulldown Control

The Pulldown Control defines a method of slowly pull- ing the suction pressure down from a high value. This is sometimes required on systems that have liquid recircu- lation systems or on new building to prevent structural damage by limiting the rate at which to build is cooled.

This feature can be enabled or disabled from this menu which is the part of Compressor Control Screen. If Pulldown is enabled, this feature will only function when the compressor is running in local Auto, Auto Sequencing mode and the Control mode is Suction Pressure 1.

The Pulldown feature provides a method to slowly pull the suction pressure down to operating conditions. The pulldown method used is to step the suction pressure down over a defined time interval.

Example:

Assume the suction pressure is at 85 psig and the set- point we want to get to is 20 psig. The operator wants to allow 48 hours of pulldown time. Pick a reasonable step pressure of 5 psig for every step. This defines a change of (80 – 20 = 60) psig.

•Note: First step is applied immediately. So first step starts at (85 – 5 = 80) psig

•Number of steps = delta 60 psig change * 1 step/5 psig = 12 steps.

•Delay per Step = 48 hours / 12 steps = 4 hours/step.

•So for the first 4 hours, the compressor runs at 80 psig.

•Next 4 hours @ 75 psig

•Next 4 hours @ 70 psig

•And so forth.

After the 12th step (running at 25 psig), 48 hours will have elapsed, and the new setpoint becomes 20 psig, achieving the 20 psig setpoint after 48 hours. After the pulldown setpoint is equal to or is less than the control setpoint, the pulldown feature will disable itself.

Pulldown:

•This checkbox enables the Pulldown feature. If this box is unchecked, Pulldown setpoints are ignored and the operator is not allowed to edit Pulldown setpoints.

Initiate Pulldown at Next Start:

•This checkbox when enabled, turns on the Pulldown process at the next start cycle, Pulldown operation will work in the following manners:

•Pulldown only works when Control mode is Suction Pressure 1.

•If not started in Suction Pressure 1 then Pulldown process will not run until stopped and restarted in Suction Pressure 1.

•If started in Suction Pressure 1 and changed after start, then Pulldown process will be sus- pended and restart once Control mode is changed back to Suction Pressure 1.

Step Pressure:

•This setpoint defines the step decrements at which the suction pressure value will be controlled at.

Delay Per Step:

•This setpoint defines the time increment at which the compressor will be controlled at each step.

Stop Pressure:

•This setpoint defines the suction pressure value at which Pulldown operation will get completed. When suction pressure value is equal to or goes below this setpoint, Pulldown feature disables itself. Also “Pulldown” and “Initiate Pulldown at Next Start” checkboxes will be automatically deselected as nor- mally this is one time use feature.

Auto Cycle Differential:

•This setpoint defines the offset pressure values for Auto Cycle Start Pressure and Stop Pressure from the Suction Pressure setpoint. Auto Cycle Start Pressure setpoint will be Suction Pressure setpoint increment- ed by this setpoint pressure value, while Auto Cycle Stop Pressure value will be Suction Pressure setpoint decremented by this setpoint pressure value.

When Pulldown feature is enabled, Pumpdown checkbox is automatically grayed out. Similarly when Pumpdown feature is enabled, Pulldown checkbox is automatically

Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 1.8.5153 6 – 5