Oil vapour is condensed by simple cooling only to the extent that the reduced residual oil value is only acceptable for a few fields of application. The condensed oil upon cooling, precipitates on the internal walls of the compressed air line, and is separated to nearly 100% if the usual depth filters are fitted, and will flow into the bottom of the filter. The portion of the condensed oil, forming fine oil droplets in the air stream, always totally reaches the filter material and is retained. When assessing filters, the oil content in aerosol form, the oil mist concentration, before and after the filter, is of primary significance. The most penetrating particle size amounts to 0.15 - 0.45 mm even though an average value of 0.3 mm is stated in literature. This does, however, depend on type, viscosity, temperature and quantity of the oil particles fed to the filter. Such filters which display a good separation behaviour faced with these particle sizes, are equally effective with other sizes: Smaller particles are retained by fibre filters as a consequence of Brownian molecular motion. Larger particles are removed mainly through inertial impaction and direct interception.
