T.K. Müller, J. Meyer, E. Thébault, G. Kasper

Powder Technology, 2014, 253, 247-255

A light coating of oil is known to increase the dust holding capacity of fibrous filter media. The underlying causes for this effect were investigated by performing filtration experiments with arrays of nylon and stainless steel fibers (diameters of 20, 30, and 44 μm) coated with precisely defined amounts of oil. The single fiber efficiency in the inertial regime (Stokes numbers >0.5) was measured as a function of dust load, using 3.5 μm polystyrene and 2.1 μm silica particles in combination with various types of oils (0W-30, WD-40). Additionally, the influence of an oil film on the growth of particle deposit structures was investigated by optical microscopy.
It was found that dust deposition on oily fibers occurs in two distinct stages: at first particles are immersed in the oil film without any appreciable increase in fiber efficiency. Once the film is saturated with particles, further deposition leads to quasi-normal dendritic growth typical of dust deposition on dry fibers, and a sharp increase in single fiber efficiency. The maximum packing density inside the film which is reached at the transition from particle immersion to regular growth, was approximately 46% by volume, regardless of film thickness. There were no indications of flow-induced particle rearrangement inside the film during the first stage.
Comparative measurements were also made with standard paper media containing varying quantities of oil. The oil caused an increase in dust holding capacity by factors between about 1.5 and 3 compared to dry media, due to a delayed upswing of the filter pressure drop with dust loading. The penetration of dust through the media more than doubled.
We conclude that particle immersion in the oil film is responsible for both the delayed increase in fiber efficiency and fiber drag, and that this delay is roughly proportional to the amount of oil loading.