B - 3
Appendix B
•
Vission 20/20 Application Procedures
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
Vission 20/20 Compressor Control Setup
SCOPE
Vission 20/20 programs – version 1.6.4550.1 and later.
PULLDOWN
The Pulldown feature provides a method to slowly pull
down the suction pressure from a high value, by slowly
lowering the suction pressure control setpoint over a
time period. This feature is sometimes required on sys-
tems 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 pre-
venting liquid recirculation pump cavitation. Pulldown
is also to be used for new plant startups. Pulling the suc-
tion pressure (and resultant temperature) of new build-
ings down too quickly can cause structural damage, so
limiting the suction pressure Pulldown rate will prevent
this, allowing time to de-humidify the rooms as the tem-
perature in the rooms is 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 the options to be selected for the Pulldown fea-
ture, see
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
Initiate Pulldown at Every Start
• Enables the Pulldown feature on every compressor
start.
Step Pressure
•
Step pressure defines the “steps” (in psig) in which
the suction pressure setpoint is decreased.
Delay Per Step
•
Delay per Step setting which defines how long the
compressor will be controlled at the current suction
pressure setpoint, in hours and minutes. Minimum
range of 1, maximum of 1259.
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 pressure.
Auto-cycle Differential
•
This setpoint defines a differential above and below
the suction pressure control setpoint. These points
define the auto-cycle start and stop pressure set
-
points. The auto-cycle Start pressure is the suction
pressure se auto-cycle differential setpoint.
The Auto-cycle Stop pressure is the suction pressure
setpoint - auto-cycle differential setpoints.
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