Regulating System - Atlas Copco XAHS 486

Regulating System - Atlas Copco XA(V,H,T)S 436-526 PNB  with Engine 6BTA 5.9C-169

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AIR FLOW

Air drawn through the airfilter (AFce) into the compressor element (CE) is compressed. At the element outlet, compressed air and oil pass into the air receiver/oil separator (AR/OS). The check valve (CV) prevents blow-back of compressed air when the compressor is stopped. In the air receiver/oil separator (AR/OS), most of the oil is removed from the air/oil mixture. The oil collects in the receiver and on the bottom of the separator element.

The air leaves the receiver via a minimum pressure valve (MPV) which prevents the receiver pressure from dropping below the minimum working pressure, even when the air outlet valves are open. This ensures adequate oil injection and prevents oil consumption. The minimum pressure valve (MPV) also functions as a check valve. The system comprises temperature switch (TS).

OIL SYSTEM

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), the oil filters (OF) and the oil stop valve (OSV) to the compressor element (CE). When the compressor is stopped, check valve incorporated in the unloader valve prevents the oil from flowing back from compressor element to air filter. 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 AIR FLOW. The oil that collects in the bottom of the oil separator element is returned to the system through a 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.

CONTINUOUS REGULATING SYSTEM

The compressor is provided with a continuous regulating system and a blowoff valve (BOV) 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 (RV) which, by means of control air to the unloader assembly (UA), matches the air output to the air consumption. The air receiver pressure is maintained between the preselected working pressure and the corresponding unloading pressure.

When starting the compressor, the throttle valve (TV) is kept closed via receiver pressure. The compressor element (CE) takes in air and pressure builds up in the receiver (AR). The throttle valve (TV) 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. If the air consumption is equal to or exceeds the maximum air output, the

engine speed is held at maximum load speed and the throttle valve (TV) is fully open.

If the air consumption is less than the maximum air output, air receiver pressure increases and the regulating valve supplies control air to throttle valve (TV) to reduce the air output and holds air receiver pressure between the normal working pressure and the corresponding unloading pressure. Unloading pressure = normal working pressure 12 bar (174 psi)+1.5 bar (21.75 psi)=13.5 bar (195.8 psi).

When the air consumption is resumed, the blow off valve (BOV) closes and the throttle valve (TV) gradually opens the air intake and the electronic speed regulator increases the engine speed. The construction of the regulating valve (RV) is such that any increase (decrease) of the air receiver pressure above the preset valve opening pressure results in a proportional increase (decrease) of the control pressure to the throttle valve and the electronic speed regulator. Part of the control air is vented to atmosphere, and any condensate discharged, through the vent holes.

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