10 – 6
Section 10
•
Compressor Sequencing
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
Slow Unload Timer:
• If process temperature drops below the process tem-
perature control setpoint then this timer ‘s value is
used to make periodic sequencing decisions.
Fast Unload Timer:
• If process temperature drops below the fast unload
temp offset setpoint then this timer’s value is used to
make periodic sequencing decisions.
Process Control Setpoints - Pressure
The compressor sequencing screen defines settings that
are used by the master compressor for sequencing de-
pending on the Process Control Mode.
When the Process Control Mode selected is Pressure, se-
quencing uses pressure setpoints, see Figure 10-4.
PRESSURE SETPOINTS
Start Offset:
•
Defines the offset from the process pressure control
setpoint to start slave compressor. If the process
pressure surpasses the start offset setpoint and mas-
ter compressor’s capacity has reached max trigger
setpoint then the sequencing algorithm allows the
starting and loading of slave compressors to cater for
increasing load requirements.
Setpoint:
• The target setpoint is a read-only value here. This
setpoint can be changed by logging on to the
“Compressor Control” Screen.
Fast Load Temp Offset:
•
Defines the offset from the process pressure control
setpoint to monitor compressor load. If process pres-
sure surpasses this setpoint’s value then sequencing
decisions are made according to Fast Load Timer.
Fast Unload Pressure Offset:
•
Defines the offset from the process pressure con
-
trol setpoint to monitor compressor load. If pro-
cess pressure drops below this setpoint’s value then
sequencing decisions are made according to Fast
Unload Timer.
CAPACITY LOAD/UNLOAD TIMERS
One of the following Capacity Load/Unload timers is
used to make sequencing decisions periodically. Process
Pressure setpoints are monitored to identify which one
of the following timers will be used.
Slow Load Timer:
• If process pressure surpasses the process pressure
control setpoint then this timer’s value is used to
make periodic sequencing decisions.
Figure 10-4. Compressor Sequencing Screen - Process Control Setpoints for Pressure
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