Sullair LS20TS Operator's Manual And Parts List page 16

Section 2
DESCRIPTION
UNLOAD MODE - EXCESS OF 360 PSIG (24.8
BAR)
As the required air is reduce, the discharge pres-
sure and line pressure rise above the 360 psig (24.8
bar), the Supervisor activates the unload solenoid
valve which in turn pressurizes the blowdown valve.
This opens the blowdown valve, which allows dis-
charge air to flow back to the compressor inlet.
Since the inlet poppet valve is fully closed, the air
pressure from the sump tank will be relieved
through the blowdown valve, which will reduce the
sump pressure. Some of the outgoing air will flow
directly into the compressor inlet to avoid cavitation
during this time of blowdown. As the sump pres-
sure drops, motor power consumption is also
reduced. The unload pressure regulator is set to
maintain the unload pressure at a designated value,
between 140 and 175 psig (9.6 to 12 bar). At this
time, the compressor is in the unload mode.
When the line pressure drops back to the load point
(cut-in pressure) of the pressure transducer, usual-
ly 350 psig (24.1 bar), the compressor will change
state back to the modulating mode as discussed
10
earlier. This cycle will repeat itself, maintaining the
demand required on the system line supply.
SHUTDOWN MODE
When the compressor is shut down based on a
manual shutdown, a safety shutdown or an auto-
matic shutdown from the Supervisor touch pad, the
unload solenoid valve is de-energized allowing air
to flow through the check valve and into the poppet
valve keeping the inlet poppet valve closed. This
also opens the blowdown valve to allow com-
pressed air to vent back above inlet poppet valve
through the inlet filter. The control lines are proper-
ly sized and routed to allow the compressed air to
relieve to atmosphere rather than below the inlet
poppet valve.
If the compressor Supervisor Controller is in the
automatic mode, the compressor will shut down
after running unloaded for a specific amount of
time. The reverse also holds true, if the system line
pressure requires additional compressed air, the
compressor will start automatically to satisfy this
demand.