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A coalescing filter is typically used in a compressed air system to remove liquid and oil aerosols. A coalescing filter is most efficient when located downstream of an air dryer. The air dryer removes most of the liquid oil condensed in the compressor discharge line. Measuring the percent of oil removed by a coalescing filter is useful for determining efficiency. This paper covers a laboratory method to reproduce oil aerosols much like the filter will see in an actual application. High duty cycles produce the maximum amount of oil from the compressor. The air dryer acts as pre-filter for the coalescing filter. The coalescing filter element and its associated housing should be tested as a unit since the element's inherent efficiency is effected by the design of the housing.

Oil carryover is present in all pneumatic systems where the compressed air is generated by oil lubricated compressors. With the increased use of electronic and small orifice components on trucks and coaches, the problem of contamination and failure related to oil carryover in the compressed air system has increased. Even though the rate of oil carryover out of the compressor has improved over recent years, problems persist. This paper will describe how the oil is transported out of the compressor, the various states oil is generated including vapor pressure profiles and size distribution of the condensed aerosols. Also reviewed is the oil removal effectiveness of various components downstream of the compressor. A model of oil liquefaction from vapor and aerosol states will be presented. The effect of liquefied oil combined with other contaminates in the pneumatic system and various design criteria for downstream equipment will be discussed.