Filter, which separates contaminations primarily on the inside (bulk) of the filtering material.
Filter elements with several layers are used for this and are made of metal powder (sintered filer or fibrous materials (fleece), such as cellulose, plastic, glass or metal fibres (Figure T 11). This increases the dirt holding capacity, but the flow resistance is also greater than when using surface filters.
The disorganised stacking of fibres –– which are often not of equal thickness –– results in pores of different sizes, meaning that the contamination still has particles of widely different sizes after it passes through the filter. It is therefore misleading if volume (bulk) filters made of non-woven fabrics are marked with a filtration grade (e.g. 10μm).
Figure T 11: Structure of fleece (fibre) materials