Section 2
DESCRIPTION
10
Figure 2 ---5 Pneumatic Control System
pressure switch usually set at 115 PSIG (792kPa).
The pressure regulator a nd the solenoid valve re-
main closed during this phase. The inlet valve re-
mains fully open for maximum capacity.
MODULATING MODE -- 115 TO 125 PSIG (792
TO 862kPa)
If less than the rated capacity of compressed air is
being used, the s ervice line pressure will rise above
115 PSIG (792kPa). The pressure regulator valve
gradually opens, applying air pressure through the
control line to the inlet valve piston. This causes the
inlet valve to partially close reducing the amount of
air entering the compressor until it matches t he
amount of air being used. The control system func-
tions continually in this manner, between the limits
of 115 to 125 PSIG (792 to 862kPa), in response to
varying demands from the s ervice line.
The pressure regulator has an orifice which vents a
small amount of air to the atmosphere when the
pressure regulator controls the inlet valve. The ori-
fice also bleeds a ny accumulated moisture from
the control lines.
UNLOAD -- IN EXCESS OF 125 PSIG (862kPa )
LINE PRESSURE
When no air is being used,the service line pressure
rises to the setting (cut---out pressure) of the pres-
sure switch. the pressure switch opens, interrupt-
ing the electrical power to the solenoid valve. At this
time, the solenoid valve allows dry sump tank air
pressure to be applied directly to the inlet valve pis-
ton and keep it closed. Simultaneously, the sole-
noid valve sends a pneumatic signal to the blow -
down valve. The blowdown valve opens to the
sump to the atmosphere reducing the sump pres-
suretoapproximately30to35PSIG(207to
281kPa). The check valve in the air service line
pressure prevents line pressure from returning to
the sump.
When the line pressure drops to the low setting
(cut---in pressure) of the pressure switch (usually
115 PSIG [792kPa]), the pressure switch closes,
r e --- e n e r g i z i n g t h e 3 --- w a y s o l e n o i d v a l v e a n d a l -
lowing the blowdown valve to clos e. The re---ener-
gized solenoid valve again prevents pressure from
reaching the inlet valve. The inlet valve is fully open
and t he compressor delivers full rated capacity.
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