H.E. Kolb, J. Meyer, G. Kasper

Chemical Engineering Science, 2017, 166, 107-114, https://dx.doi.org/10.1016/j.ces.2017.03.028

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.