For depth filtration, we offer needle felts and various filter cartridges.
These filtration media do not have a definable mesh opening as sieve textiles used for surface filtration, have. The fibres of these materials are arranged at random and so they form pores with different sizes. Depth filtration is not only a mechanical separation of particles on the surface, but mainly an adsorptive and mechanical filtration inside the medium.
Taking a look at the structure of needle felt, it becomes more clearly: Needle felt consists of single fibres, which are brought in mixed up positions using needle passages during production. Number, thickness and prick-deepness of the needles determine the condensation of the fibres and thereby the achievable grade of separation of the needle felt.
The particles intended to be separated are hold back within the three-dimensional structure of the needle felt. It has to be considered that a part of those particles will pass the needle felt. So in fact, the grade of separation of a needle felt is about 60 – 70%.
In order to work contrary to this effect, for example 25 µm instead of 50 µm needle felt can be used.
But the three-dimensional structure of the needle felt also offers some advantages:
The multitude of pores offers a high solid loading effect together with high flow rates. That is: Although the filter media is already loaded with a high rate of solids, the suspension still finds open pores and drains to pass through. Therefore, a filter cake on the surface of the needle felt is build up only slowly. This again promotes a long lifetime of the filter element.
Depth filtration media often show an asymmetric structure of the pores, i.e. the pores become smaller, the deeper they are situated within the media. This is advantageous with regard to unfiltered liquids containing a wide range of particle-sizes, as the bigger particles can enter the filter medium and therefore do not block the surface immediately. This way, the usual life of filter elements can be lengthened additionally.