Transient Phenomena of Oil Mist Filtration
Filtration of oil droplets (“mist”) from gases by fibrous media is a common practice for many industrial applications such as metals cutting, the exploitation of natural gas, crankcase ventilation or oil-lubricated screw compressors. Mist filters are widely accepted to reach a steady state, wherein the aerosol loading rate equals the liquid removal rate and ∆p remains constant.
Schematic of wettable multi-layer filter (LEFT) and evolution of pressure drop ∆p until steady state (RIGHT).
The Film-and-Channel Model by Kampa et al. (2014) phenomenologically describes the loading behavior of mist filters to consist of an internal and an external contribution of ∆p. The former is termed the “channel-∆p” while the latter, typically a steep increase of ∆p, is called “∆p-jump”. The channel-∆p is associated with the formation of distinct oil channels within the media (through which the coalesced oil has to be “pumped”), while the ∆p-jump is due to an oil film through which the air flow has to break through. For wettable (oleophilic) filter media this film occupies the downstream face of the filter, whereas for non-wettable (oleophobic) media it occurs on the front face. The oil channels and the film also make separate and definable contributions to the overall efficiency of mist filters.
Important but widely unknown factors for the performance of mist filters in steady state are the role of media properties and the influence of the air flow. Apart from transient phenomena prior to steady state the phenomenon of a slow ∆p increase (“creep”) after steady state operation poses another unsolved issue in oil mist filtration. Understanding the phenomena of multiphase flow in non-woven filter media and its implications for ∆p and efficiency is key for further development of improved filters.