Fundamental investigation on the influence of local oil distribution on the separation behaviour of coalescence filters in gas purification

Gas-carried, submicron oil droplets (so-called oil mist) usually occur as an undesirable by product, for instance in the crankcase ventilation of combustion engines or in oil-lubricated screw compressors. Solutions are therefore being worked on to reduce these oil emissions for a variety of reasons, such as health protection, long-term maintenance of components, etc. A common and efficient method of separating these droplets from the gas stream is to use coalescence filters made of fibreglass.
For unloaded air filters, the separation of liquid particles follows the same principles as the separation of solid particles. With the coalescence of oil inside the filter, dust and droplet filtration begin to differ fundamentally. In this process, chains of droplets are formed, while during the separation of solid particles dendritic or compact porous structures emerge. Depending on the wettability of the fibres, the oil is present in form of so-called barrel shape or clam shell droplets (see figure). At the interfaces of the filter the formation of an oil film is possible.

Figure: SEM image of solidified droplets on fibres of a wettable filter media

The main emphasis of this work is to examine fundamentally how collected oil droplets in oil mist filters change the filtration efficiency. Through µ-CT imaging (see video) and image analysis, parameters such as wetting angle, spacing, arrangement and diameter of the coalesced droplets on the fibres are to be determined. In addition to the mean oil volume fraction or the global mean saturation, the local oil distribution on different size scales (saturated and weakly saturated areas or local nearly closed oil films) should be considered. Thus, the change of the separation efficiency by separated oil should be investigated considering the different saturations and oil accumulation structures in flow direction.

Video: µ-CT scan of oil structures (brown) on fibres (yellow: glass microfibre; green: PE-fibre) of a wettable filter media (length: 350 µm; width: 350 µm; thickness 160 µm).