Depth filter elements generally consist of a layer of fibres of varying thickness. The structure of depth filter elements consist of individual fibres entwined with each other in a random pattern, resulting in a porous structure.
Figure 4.7.2.1
Labyrinth-like passages, are formed between the fibres and these are neither of uniform nor measurable size. The internal flow channels include diameters which, in part, represent a much coarser mesh than the diameters of the particles to be eliminated from a dispersed system. Separation takes place along the entire path followed by the aerosol through the depth filter element. Filtration with depth filter elements is based essentially on the principle of random deposition of particles under the influence of the various cumulative physical effects. Depth filter elements separate out particles in the fibre bed near to the filter element surface. In this case, the direction of flow is from inside to outside. Such filters are used primarily in order to filter the water and oil condensate phase out of the compressed air. In the course of flow from inside to outside, the wet phase is deposited on the filter element material. The accumulation of small particles, leading to bigger drops, causes these to flow into the condensate collecting chamber due to the direction of the gas flow from inside to outside and under the influence of gravity. Deposition at the surface of the fibre takes place mainly through surface forces. As these are relatively small, care has to be taken that the deposited particles do not again become detached and re-entrained into the gas stream. This requirement is most readily met through a low velocity of flow. In order to achieve a large separation performance, a large surface per unit volume is necessary. The unit for this volume is specified by m2/m3.
Figure 4.7.2.2
Fig. 4.7.2.2 shows how classic depth filters function and why these, when filtering solid particles, can become loaded after a brief service life. The fluid to be purified penetrates the filter structure, the solid constituents to be removed are caught in the deepest layers of the filter. With increasing contamination, the resistance to flow limits the throughput gas, so that the differential pressure in the element increases. When filtering liquid particles, they agglomorate to form larger drops and leave the filter structure. Solids trapped in the filter media cannot be removed and are held permanently. Through the fibrous maze and the large storage capacity associated with this, these filter elements achieve a relatively long service life up to saturation, following which such elements are exchanged.
Figure 4.7.2.3
The materials for such depth filter elements are plastics, metal or glass fibres as well as woven or wound systems. Pleated filter elements offer a large surface, although the effective element depth is reduced to a few millimetres.
