The dependence of the differential pressure drop ∆p and the level of internal oil saturation S on the flow velocity of the air were investigated experimentally for a typical oil mist filter composed of oleophilic glass microfiber layers. Over a wide range of filter face velocities (v = 5–70 cm/s) and liquid loading rates (R = 15–125 mg/(m2 s)), and within the accuracy of the measurements, the ‘‘wet” pressure drop of the filter ∆p - ∆p0 (i.e. the increase in ∆p over the ‘‘dry” pressure drop ∆p0 ) was constant and did not show a systematic dependence on v. When decomposing the wet pressure drop into its components ∆p-jump and channel-∆p, the ∆p-jump was also independent of the oil loading rate. The level of internal liquid saturation S was inversely proportional to v, with an empirical fit function S = 1⁄ (1 + v/v*). The characteristic velocity v0 was found to depend on the oil loading rate, and presumably also depends on the media structure which was not varied here. This filter behavior is consistent with the ‘‘jump-and-channel” model proposed recently by Kampa et al. (2014).
The experiments further showed that the ‘‘steady-state” pressure drop under constant filter operating conditions underwent a gradual increase with time (termed ‘‘∆p-creep”) that depends on operating conditions. This ∆p-creep diminishes gradually and was found to become stronger with increasing loading rate and filter face velocity. At the highest rate of increase (i.e. v = 70 cm/s, R = 125 mg/(m2 s)), an experiment lasting for 1100 h did not suffice to attain an asymptotic level for ∆p. Creep was found to be associated with a gradual increase in saturation and must therefore be classified as an(other) instability phenomenon in oil mist filters.