6 – 10
Section 6
•
Compressor Control
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
is automatically grayed out and hence, the user will not
be able to operate the Pumpdown feature. This is done
to keep the 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 the building should
be cooled.
This feature can be enabled or disabled from this page,
see Figure 6-11. If Pulldown is enabled, this feature will
only function when the compressor is running in local
Auto, Auto Sequencing mode and when 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.
1.
Note: the first step is applied immediately. So first
step starts at (85 – 5 = 80) psig
2. Number of steps = delta (60 psig change * 1 step/5
psig) = 12 steps.
3. Delay per Step = (48 hours / 12 steps) = 4 hours/
step.
4.
So for the first 4 hours, the compressor runs at 80
psig.
5. Next 4 hours @ 75 psig
6. Next 4 hours @ 70 psig
7. And so forth.
Figure 6-11. Compressor Control Screen - Pumpdown/Pulldown Control
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