SN55, SN55S, SN55V, SN75, SN75S, SN75V User Manual
2: Description
02250247-571 R04
Subject to EAR, ECCN EAR99 and related export control restrictions.
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and an inlet valve. The functional description of the control
system is described below in four distinct phases of opera-
tion. For explanatory purposes, this description will apply to a
compressor with an operating range of 100 to 110 psig (6.9 to
7.6 bar). A compressor with any other pressure range would
operate in the same manner except stated pressures.
Start mode—0 to 50 psig (0 to 3.5 bar)
When the controller Start button is depressed, the separator/
sump tank pressure will quickly rise from 0 to 50 psig (0 to
3.4 bar). The compressor initially starts unloaded with the
solenoid valve open and the inlet valve closed. It then
switches to full load when full rpm has been achieved. During
this period, both the pressure regulator and the solenoid valve
are closed, the inlet valve is fully open and the compressor
pumps at full rated capacity. The rising compressor air pres-
sure is isolated from the service line in this phase by the mini-
mum pressure valve set at approximately 50 psig (3.4 bar).
Full load mode—50 to 100 psig (3.4 to 6.9 bar)
When the compressed air pressure rises above 50 psig
(3.4 bar), the minimum pressure valve opens allowing com-
pressed air to flow into the service line. From this point on, the
line air pressure is continually monitored by the controller. The
pressure regulator and the solenoid valve remain closed during
this phase. The inlet valve is in the fully open position as long as
the compressor is running at 100 psig (6.9 bar) or below.
Modulating mode—100 to 110 psig (6.9 to 7.6 bar)
[SN55, SN75]
If less than the rated capacity of compressed air is being
used, the service line pressure will rise above 100 psig
(6.9 bar). The pressure regulator valve gradually opens,
directing air pressure to the inlet control valve, reducing air
entering the compressor until it matches the amount of air
being used. The control system functions continually in this
manner between the limits of 100 to 110 psig (6.9 to 7.6 bar)
in response to varying demands from the service line. The
integrated inlet valve has an orifice which vents a small
amount of air to the compressor inlet when the pressure
regulator controls the inlet control valve. The orifice also
bleeds any accumulated moisture from the control lines.
Modulating mode with spiral valve—100 to 110 psig
(6.9 to 7.6 bar) [SN55S, SN75S]
As air demand drops below the rated capacity of the com-
pressor, the line pressure will rise above 100 psig (6.9 bar).
As a result, the differential pressure regulator for the spiral
valve gradually opens, applying air pressure to the spiral
valve actuator. Air pressure at the actuator expands the dia-
phragm. The rack, in turn, engages with the pinion mounted
on the spiral valve shaft assembly. This results in a rotary
motion. As the spiral valve rotates, it starts opening the
bypass ports gradually. Excess air is then being returned
back internally to the suction end of the compressor unit.
Now the compressor is fully compressing only that amount
of air, which is being used. As air demand keeps dropping
further, the spiral valve keeps opening more and more until
all the bypass ports are fully open. At this point, the spiral
valve has moved into the unload (minimum) position.
The spiral valve provides a modulation range from 100 to
50%. During this period, the pressure rises approximately
from 100 to 105 psig (6.9 to 7.2 bar).
Unload mode—in excess of 110 psig (7.6 bar)
When a relatively small amount or no air is being used,
the service line pressure continues to rise. When it
exceeds 110 psig (7.6 bar), or 105 psig (7.2 bar) for spiral
valve machine, the controller control system de-ener-
gizes the solenoid valve allowing separator/sump tank air
pressure to be supplied directly to close the inlet valve.
Simultaneously, the solenoid valve sends a pneumatic
signal to the blowdown valve. The blowdown valve opens
to the atmosphere, located in the compressor separator/
sump tank, reducing the separator/sump tank pressure to
approximately 17 psig (1.2 bar). The check valve in the
air service line prevents line pressure from returning to
the separator/sump tank.
When the line pressure drops to the low setting (cut-in pres-
sure; usually 100 psig (6.9 bar) on low pressure (7 bar)
compressors and 125 psig (8.6 bar) on high pressure
(9 bar) compressors, 150 psig (10.3 bar) on (10 bar) com-
pressors, 175 psig (12.0 bar) on (12 bar) compressors), the
controller energizes the solenoid valve and allows the blow-
down valve to close. The re- energized solenoid valve again
prevents line pressure from reaching the inlet control valve.
Should the pressure begin to rise, the pressure regulator will
resume its normal function as previously described.
Load / no load control
If desired by the customer, the compressor can be set to oper-
ate load/no load without modulating control. This control mode
is often selected when a large amount of compressed air stor-
age (air tank) is available. From the Control Parameters
page on the STS Controller, change from Modulation to
Load/Unload. On a machine rated for 100 psig (7 bar) the
compressor will run in the full load mode up to 100 psig (7 bar).
If less than the rated capacity is required, pressure will rise
above 100 psig and the controller will de- energize the solenoid
valve, causing the compressor to run in the unload mode.
When the system pressure falls to 90 psig (6.3 bar), the con-
troller energizes the solenoid valve, causing the compressor to
return to the full load mode. The compressor will thus operate
to keep the system pressure in the range of 90 – 100 psig (6.3
– 6.9 bar).
