Sullair LS20T Operator's Manual And Parts List page 21

Figure 2-4A Control System Diagram- Functional Components
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 sole-
noid valve which in turn pressurizes the blowdown
valve. This opens the blowdown valve, which
allows discharge 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
earlier. This cycle will repeat itself, maintaining the
demand required on the system line supply.
Section 2
DESCRIPTION
15