Pulldown Control - Emerson Vilter Vission 20/20 Operation And Service Manual

Section 6 • Compressor Control
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 opera-
tion. This button is active only when compressor is
in local Auto mode and Control Mode Confi gured 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 defi nes 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 page,
see Figure 6-7. Compressor Control Screen - Pumpdown/
Pulldown Control. 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 defi ned 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 defi nes a change of
(80 – 20 = 60) psig.
• Note: First step is applied immediately. So fi rst step
starts at (85 – 5 = 80) psig
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 1.9
• 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 fi rst 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 defi nes the step decrements at which
the suction pressure value will be controlled at.
Delay Per Step:
• This setpoint defi nes the time increment at which the
compressor will be controlled at each step.
Stop Pressure:
• This setpoint defi nes 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.
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