The discharge in the treatment of compressed air from aftercoolers and air dryers is a mixture of oil and water. The water is the vapor that was removed from the air. The oil is from the lubrication of the air compressor.

The common elements of all separators are a large container where the condensate rests at atmospheric pressure to allow the oil to rise to the surface where it can be skimmed off. The clean water is drawn from the bottom of the container and flows through an activated carbon filter before being discharged. The oil content remaining in the water is in the 15 to 20 ppm range. Other traps and filters are used by different manufacturers to improve the performance of their systems. The activated carbon filters have to be replaced periodically to stay within the oil content limits of local law for discharge.

The listed specifications are based on a mild climate as found in most of the U.S. Cold climates require smaller units (multiply the air flow by 0.67 to determine the size of unit needed), while in humid, tropical climates, the size of the unit has to be twice the actual air flow.

The size of the separator is based on the air flow capacity of the compressor, adjusted by the type of oil used and the type of compressor. Synthetic oil needs more separation time than mineral oil, thus a larger capacity separator is recommended.

Piston models create more oil mist than screw type compressors, thus they need a larger separator. A separator rated for a screw compressor of 100 scfm w/mineral oil is rated at 75 scfm w/synthetic oil. The same model for a piston compressor rated at 75 scfm w/mineral oil is rated at 60 scfm w/synthetic oil.

A typical compressed air system has a number of places where condensate is produced. The storage vessel, aftercoolers, and refrigerant air dryers are all removing moisture and oil from the air. Separators, therefore, all come with more than one input port. In most systems, the compressor does not run full time. The cfm capacity listed here assumes that the compressor runs 50% of the time. For other loads the cfm capacity should be increased or decreased.