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11. What are The Techniques for Separating The Oil and Gas Mixture in An Air Compressor?

In the oil-injected screw air compressor unit, two different oil-gas separation methods are employed based on their respective separation mechanisms.

One method is known as the mechanical method, also referred to as collision or cyclonic separation. It relies on the gravitational force and centrifugal force to separate larger-sized oil droplets from the gas. Experimental tests have shown that this mechanical method effectively separates oil droplets with a diameter greater than 1μm.

The other method is the affinity coalescence method, achieved through a specialized component made of specific materials (commonly called an oil fine separator or separator core). This component facilitates the coalescence of oil droplets with a diameter below 1μm into larger droplets before they are further separated.

Currently, oil-injected screw compressor units typically employ both these distinct oil-gas separation methods simultaneously. The mechanical method serves as the coarse separation, while the affinity coalescence method acts as the fine separation.

In the early days, the mechanical method primarily used the collision approach, where an obstacle was placed in the flow direction of the oil-gas mixture. When the mixture collided with the obstacle, the oil droplets in the mixture would gather on the surface of the obstacle and then fall to the bottom of the separator under the influence of gravity. When employing the collision approach, the speed of the mixture’s collision with the obstacle had a specific range, and its optimal value depended on the density of the gas and lubricating oil. Generally, the optimal collision speed was around 3m/s.

Modern compressor units typically use the cyclonic separation method or a combination of both methods for coarse separation. In this approach, a cyclone channel is set up in the oil-gas separator. As the oil-gas mixture enters the separator, larger oil droplets are flung to the wall by the action of centrifugal force in the cyclone channel and then fall to the bottom of the separator under the influence of gravity.

After the oil-gas mixture is separated using the mechanical method, approximately 99.7% of the oil is already separated, and at this point, the diameter of the remaining oil droplets is mostly below 1μm, making it impractical to further separate them using the mechanical method.

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