Section 3
SUPERVISOR CONTROLLER SEQUENCING
19
As machines are started and loaded, there will be only one machine that is modulating, the
others will be forced to "FULL LOAD". The modulating (trim) machine will load at 2psig (.14
bar) above the load pressure parameter, 102 psig (7.03 bar) in this example, and unload at
2 psig (.14 bar) below unload pressure parameter, 108 psig (7.4 bar) in this example. The
reason for this narrower load/unload band is to keep all machines from loading and unload-
ing at the same time. The trim machine will be identified on the display as "TRIM" operating
state. If the pressure rises above the unload pressure -2 psig (.14 bar)(108 psig (7.4bar))
the trim machine will unload (display will say "UNLOADED") and the next machine in the
sequence will switch from full load to trim. If after Recovery Time the pressure is still above
108 psig (7.4 bar) then the trim machine will unload and so on until all machines are
unloaded. Then if the pressure falls below load pressure + 2 psig (.14 bar) or 102 psig (7.03
bar) the first machine in the sequence will load and be the trim machine. If after the
Recovery Time the pressure is still below 102 psig (7.03 bar) then the trim machine will go
to full load and the next machine in the sequence will load and become the trim machine.
System Setup and Tuning
The following shows the settings for a system of low pressure 100-110 psig (6.9 to 7.6 bar)
machines. The Trim High & Low Setpoints are not settable parameters, they are calculated
from the Unload and Load pressures. The Trim High Setpoint is 2 PSI (.14 bar) below the
Unload pressure and the Trim Low Setpoint is 2 PSI (.14bar) above the Load pressure.
100 psig (6.9 bar) Unload
98 psig (6.8 bar) Trim High Setpoint (calculated parameter)
92 psig (6.3 bar) Trim Low Setpoint (calculated parameter)
90 psig (6.2 bar) Load
85 psig (5.9 bar) Lowest pressure
10 Recover time (seconds)
Note that the unload pressure is set to 100 psi (6.9 bar) and not 110 psi (7.6 bar) as is nor-
mal on a stand alone machine. A stand alone machine set up at 100 psi (6.9 bar) to 110 psi
(7.6 bar) , starts to modulate above 100 psi (6.9 bar) and the horsepower is reduced.
However, in a sequenced system a machine may be forced to full load, which would draw
more than the rated horsepower above 100 psi (6.9 bar). Therefore the modulation band of
machines in a system should be set up to be 10 psi (.7 bar) lower than their rated pressure.
For energy efficiency reasons it is desirable to have all running machines either at full load
or unloaded except for the one that will hereafter be called the trim machine. The trim
machine unloads when the line pressure rises above the Trim High setpoint.
The Trim High and Low setpoints are necessary because the trim machine must unload
before the fully loaded machines and load before the unloaded machines, otherwise all
machine would load and unload at the same time.
The LOWEST pressure parameter specifies the lowest pressure allowed before immediate-
ly starting a machine. A machine may also be started if the line pressure drops below the
Trim Low setpoint for a period of time.
The RECOVER time is used to keep multiple machines from loading, unloading or starting
To Next Page
Loading...
Need help?
General
