Thursday, December 25, 2008

Positive Displacement Compressors-Vane 3


The lubricating-cooling system consist of oil filter, minimum pressure valve (MPV), a heat exchanger.

And in some designs (usually the bigger horsepower units), temperature control (regulating) valve is used to maintain a relatively constant temperature regardless of compressor load.

Oil Circulation
The use of Compressor lubrication is very important! and cannot be denial? Especially the grade and type of lubricant use.

Oil circulation is usually provided by the pressure-differential system. Circulation in the pressure-differential system is achieved by using higher-pressure air to generate the oil flow. A preset minimum pressure valve (MPV), usually about 80 psi, allows the quick pressure build up in the oil sump before the MPV opens to discharge compressed air to the air system.

The pressure differential between the fully compressed air, and the pressure within the cell as it passes or rotates. The pressure differences thus created, caused the oil to be injected via orifices into all moving parts, including the rotor bearings located in the end covers of the stator..

In all and every cases, the compressed air pressure is relieved automatically from the discharge side of the compressor during shutdown.

Therefore, with this design and operation, there will be oil presence in the compressed air. It is not desirable to have oil in the compressed air, and it is necessary to remove the oil before it is send to the air system.

Oil Separation
Therefore; as air and oil mixture passes from the compression chamber, the oil, which is heavier, is mechanically separated by direction and velocity changes, or use of baffles, or of both, and allowed to drain back into the oil chamber or oil sump for recirculation.

Relatively clean air leaves the sump and enters the separator, where most of the remaining oil is removed by coalescence called air/oil separator/s.

The coalescence (separating media) are ceramic tubes or combination of mesh & fibre glass (Boros Silicate).

The Scavenging Line
The separated oil (residue oil) drains from the coalescing media to collection well for subsequent return via a combination of orifice and check valve to the intake side of the compressor.

Oil Carryover
The amount of oil carried over is less than 5 to 1 ppm of air measured by weight.
Air losses through the oil return (scavenging) line is controlled by an orifice.
The resulting pressure drop through the separator should be approximately 1 to 3 psig, when new. Should change or replace the separator when the pressure difference is at or more than 15 psig.

Heat Exchanger
Air-cooled or water-cooled heat exchanger are used for the oil. The air-cooled heat exchanger is of radiator type, with a cooling fan to provide air circulation. The water-cooled is shell & tube type, and need an external water circulation and cooling system.

Oil circulation at start-up
On start-up, oil bypass the heat exchanger (cooler) and circulates through the compressor and sump with cooling. This provide rapid oil supply & heating on start-up. When correct oil temperature is reached, the temperature regulator modulates the oil flow to maintain the optimum temperature.

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