Flow velocity dependence of the pressure drop of oil mist filters

- The excess Δp of oil mist filters does not depend on the air flow rate.
- Higher air flow rates are compensated for by a reduced internal saturation.
- Long-term operation under constant conditions does not lead to a steady-state Δp.
- Continuing Δp creep was observed even after 1100 h of non-stop operation.

The dependence of the differential pressure drop Δp 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/(m2s)), and within the accuracy of the measurements, the “wet” pressure drop of the filter Δp-Δp0 (i.e. the increase in Δp over the “dry” pressure drop Δp0) was constant and did not show a systematic dependence on v. When decomposing the wet pressure drop into its components Δp-jump and channel-Δp, the Δp-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 v∗ 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 “Δp-creep”) that depends on operating conditions. This Δp-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/(m2s)), an experiment lasting for 1100 h did not suffice to attain an asymptotic level for Δp. 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.