Atlas Copco XAHS 146 Dd S3A APP Instruction Manual page 23

The system comprises:
RV Regulating valve
UA Unloader assembly
SR Speed regulator
The compressor is provided with a continuous
regulating system. This system is provided with a
blow-down valve which is integrated in the unloader
assembly (UA). The valve is closed during operation
by air receiver pressure and opens by air receiver
pressure via the compressor element when the
compressor is stopped.
When the air consumption increases, the air receiver
pressure will decrease and vice versa. This receiver
pressure variation is sensed by the regulating valve
which, by means of control air to the unloader,
matches the air output to the air consumption. The air
receiver pressure is maintained between the pre-
selected working pressure and the corresponding
unloading pressure.
When starting the compressor, the unloader valve
(UV) is kept open by spring force, the engine runs at
maximum speed. The compressor element (CE) takes
in air and pressure builds up in the receiver (AR).
The air output is controlled from maximum output
(100%) to no output (0%) by:
1. Speed control of the engine between maximum
load speed and unloading speed (the output of a
screw compressor is proportional to the rotating
speed).
2. Air inlet throttling.
3. Blow off valve (BOV).
If the air consumption is equal to or exceeds the
maximum air output, the engine speed is held at
maximum load speed and the unloading valve is fully
open.
If the air consumption is less than the maximum air
output, the regulating valve supplies control air to
unloader valve (UV) to reduce the air output and
holds air receiver pressure between the normal
working pressure and the corresponding unloading
pressure of approx. 1.5 bar (22 psi) above the normal
working pressure.
When the air consumption is resumed, the blow off
valve (BOV) closes and the unloader valve (UV)
gradually opens the air intake and the speed regulator
(SR) increases the engine speed.
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The construction of the regulating valve (RV) is such
that any increase (decrease) of the air receiver
pressure above the pre-set valve opening pressure
results in a proportional increase (decrease) of the
control pressure to the unloading valve and the speed
regulator.
Part of the control air is vented to atmosphere, and
any condensate discharged, through the vent holes
(VH).