Oil System (See Fig. 2.4); Continuous Regulating System - Atlas Copco XAS 185 JD7 Instruction Book

A blow-down valve (BDV) is fitted in the unloader assembly
to automatically depressurize the air receiver (AR) when
the compressor is stopped.
2.5

Oil system (see Fig. 2.4)

The system comprises:
AR/OS Air receiver/oil separator
RV/UA Regulation Valve/Unloader Valve
OC Oil cooler
OF Oil filter
The lower part of the air receiver (AR) serves as oil tank.
Air pressure forces the oil from the air receiver/oil separator
(AR/OS) through the oil cooler (OC) and oil filter (OF) to the
compressor element (CE).
The compressor element has an oil gallery in the bottom of
its casing. The oil for rotor lubrication, cooling and sealing
is injected through holes in the gallery.
Lubrication of the bearings is ensured by oil injected into
the bearing housings.
The injected oil, mixed with the compressed air, leaves the
compressor element and re-enters the air receiver, where it
is separated from the air as described in section 2.4. The
oil that collects in the bottom of the oil separator element is
returned to the system through scavenging line (SL), which
is provided with a flow restrictor (FR).
The oil filter by-pass valve opens when the pressure drop
over the filter is above normal because of a clogged filter.
The oil then by-passes the filter without being filtered. For
this reason, the oil filter must be replaced at regular
intervals (see section 4.2).

2.6 Continuous regulating system

(see Fig. 2.4)
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
(BDV) which is integrated in the unloader assembly (UA).
The valve is closed during operation by outlet pressure of
the compressor element and opens by air receiver
pressure 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,
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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. The unloader valve is closed.
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 (21.75 psi) above the normal
working pressure.
When the air consumption is resumed, the unloader valve
gradually opens the air intake and the speed regulator increases
the engine speed.
The construction of the regulating valve 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.
Part of the control air is vented to atmosphere, and any
condensate discharged, through the vent holes (VH).
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