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Particles in an air stream that pass a filter can be removed in several different ways. If the particles are larger than the opening in the filter material they are separated mechanically. This usually applies for particles greater than 1 mm. The filter's efficiency in this regard increases with a tighter filter material, consisting of finer fibres. Particles between 0.1 µm and 1 µm can be separated by the air stream going around the filter material's fibres, while the particles through their inertia continue straight on. These then hit the filter material's fibres and adhere to the surface. The efficiency of the filter in this regard increases with an increased flow velocity and a tighter filter material consisting of finer fibres. Very small particles (<0.1 µm) move randomly in the air stream influenced by collisions with air molecules. They "hover" in the air flow changing direction the whole time, which is why they easily collide with the filter material's fibres and adhere there. The efficiency of the filter in this regard increases with a reduction in the stream velocity and a tighter filter material consisting of finer fibres. The separating capacity of a filter is a result of the different sub-capacities as set out above. In reality, each filter is a compromise, as no filter is efficient across the entire particle scale, even the effect of the stream velocity on the separating capacity for different particle sizes is not a decisive factor. For this reason particles between 0.2 µm and 0.4 µm are the most difficult to separate. The separating efficiency for a filter is specified for a specific particle size.

A separation efficiency of 90-95% is frequently stated, which means that 5-10% of all particles in the air go straight through the filter. Furthermore, a filter with a stated 95% separation efficiency for the particle size 10 µm can let through particles that are 30-100 µm in size. Oil and water in aerosol form behave as other particles and can also be separated using a filter. Drops that form on the filter mater-ial's fibres sink to the bottom of the filter due to gravitational forces. The filter can only separate oil in aerosol form. If oil in vapour form is to be separated the filter must contain a suitable adsorption mate-rial, usually active carbon. All filtering results in a pressure drop, that is to say, an energy loss in the compressed air system. Finer filters with a tighter structure cause a greater pressure drop and become blocked more quickly, which demands more frequent filter replacement resulting in higher costs. Accordingly, filters must be dimensioned so that they not only handle the nominal flow, but also have a greater capacity threshold so they can manage a pressure drop due to a degree of blockage.