B - 3
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
Appendix B
•
Vission 20/20 Application Procedures
Vission 20/20 Compressor Control
Setup
SCOPE
Vission 20/20 programs – version 4550.1 and later.
Pulldown
The Pulldown feature provides a method of slowly pull-
ing the suction pressure down from a high value, by
slowly lowering the suction pressure control setpoint
over a time period. This feature is sometimes required
on systems that have liquid recirculation systems. On
these systems, if the suction pressure is pulled down
too fast, the pumps can cavitate causing vibration and
damage to the pumps. By slowly lowering the suction
pressure setpoint the suction pressure can be slowly
lowered preventing liquid recirculation pump cavitation.
Pulldown is also be used for new plant startups. Pulling
the suction pressure (and resultant temperature) of new
buildings down too quickly can cause structural dam-
age, so limiting the suction pressure Pulldown rate will
prevent this, allowing time to de-humidify the rooms as
the temperature in the rooms are pulled down.
Pulldown can only be activated when controlling in
Suction Pressure Control mode (Setpoint #1).
NOTE
In new plant construction Pulldown applications,
water freezing in the concrete will lead to structural
damage. For new plant construction Pulldown
applications, it is highly recommended that the
Auto-Cycle be enabled while running Pulldown.
During Pulldown, when the Suction Pressure
Control setpoint is slowly lowered, the Auto-
Cycle Start and Stop setpoints are also slowly
lowered. The Auto-Cycle Stop setpoint will turn the
compressor off should the suction pressure fall too
fast. For additional safety, the Low Suction Pressure
Alarm and Trip setpoints should also be set so that
the suction pressure will not reach a point that can
cause building damage due to water freeze.
SETUP
The Pulldown section in the Compressor Control Menu
provides;
• Selection to enable / disable the Pulldown process.
• Selection to initiate the Pulldown process at the next
compressor start.
• Step pressure de
fi
nes the “steps” (in psig) in which
the suction pressure setpoint is decremented.
• Delay per Step setting which de
fi
nes how long the
compressor will be controlled at the current suction
pressure setpoint.
• Stop pressure setpoint de
fi
nes the point at which the
Pulldown function will stop operation. Normal com-
pressor control will then resume, with the control
setpoint being set to the Pulldown “Stop Pressure”
setting.
• Auto-cycle Differential setpoint de
fi
nes a differential
above and below the suction pressure control set-
point. These points de
fi
ne the auto-cycle start and
stop pressure setpoints. The auto-cycle Start pres-
sure is the suction pressure se auto-cycle
differential setpoint. The Auto-cycle Stop pressure
is the suction pressure setpoint - auto-cycle differen-
tial setpoints.
SELECTION FOR Pulldown SECTION OF
COMPRESSOR CONTROL MENU
(Reference Figure B-1)
Pulldown
• Enables access to Pulldown control setpoints.
Uncheck the box to disable the Pulldown setpoints.
Initiate Pulldown at Next Start
• Enables the Pulldown feature when the compressor
starts
Step Pressure
• This setpoint de
fi
nes the step increments which the
suction pressure will be controlled at.
Delay Per Step
• De
fi
nes the time increment at which the compressor
will be controlled for each step.
Stop Pressure
• Pressure at which the Pulldown feature is deacti-
vated. After Pulldown has completed, the suction
pressure setpoint will remain at this setting and the
compressor will continue to control at this pres-
sure.
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