Blend of nanofibres & coarse fibres help filter fine dust particles
The removal of particles with fibre filters means that the particles must first come into contact with the filter medium and then adhere to the fibres due to the Van-der-Waals forces and the electrostatic forces which act between the fibres and the particles.
For small particles, these forces are so large that they cannot be easily removed from the fibres. This usually only occurs if washing liquids are used.
The diagram illustrates the removal of particles from fibres and shows the flow around the cross-section of a fibre. Particles of different sizes move towards the fibre with the flow (red streamlines).
Due to their inertia, large, heavy particles (1) cannot follow the streamlines around the fibre. They come into contact with the fibre in the angstrom region and are removed. Small particles (2) follow the streamlines, but due to their size they come into contact with the fibre and are therefore removed (barrier effect).
Very small particles (3) also follow the streamlines. However, this movement is dominated by an undirected random movement: particles less than a micrometre in size are so small, that the impulses of the moving air molecules are sufficient to deflect the particles from their linear path.
This effect is known as Brownian Motion or diffusion. There is a certain probability that the random path of the particles will bring them into contact with the surface of the fibre, and that they will be removed. Particles which are removed by the diffusion movement are distributed around the entire fibre.
Small and very small particles are almost exclusively removed by diffusion. For effective removal it is therefore necessary to deliberately use Brownian Motion.
The probability that a particle which is moving randomly will come into contact with a fibre depends on the available surface area of the fibre and the length of the diffusion path. The smaller the selected diameter of the fibre, the larger the specific surface area for the same volume.
In addition, the distances between the fibres is reduced. Both of these effects mean that with a reduction in the fibre diameter of the filter media, the removal performance for fine dust increases considerably.
Filter media which contain nano- or microfibres are therefore characterised by a very high removal performance in the fine dust range.
As well as the degree of removal, the mechanical strength of the medium and the dust storage capacity are also essential quality criteria.
Therefore nanofibres are combined with coarser fibres. Nanofibres can be integrated into the substrate medium or applied to the supporting layer like a spider's web. In both cases, they increase the degree of removal.
MANN+HUMMEL, development partner and series supplier to the international automobile industry uses nanofibres media for interior and air filters.
These high performance media supplement the range of media and are used for series production and for MANN-FILTER branded products.