Depth filter

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A depth filter ( English depth filter ) is used for deposition of particles from the flowing fluids . The separation effect takes place in the depth of the filter medium. Unlike the surface filter is in the depth filtration the formation of a filter cake expressly not desired.

Transport and separation

Essentially, particle separation using a depth filter can be divided into two processes: transport (particles are transported to the filter medium due to the fluid movement) and deposition (particles are separated out in the filter medium by various separation mechanisms).

The following act as separation mechanisms:

Sieve effects only play a minor role.

The effect of the individual separation mechanisms depends heavily on the particle diameter : Ultrafine particles are mainly separated by diffusion, with particles larger than 0.5 µm in diameter, the barrier effect and inertia dominate. The adhesion of the particles on the filter medium is carried out due to van der Waals forces and electrostatics . The latter is particularly important in electret filters .

In contrast to surface filtration, the particles are separated off solely by the filter medium. By narrowing the flow channels due to the particle separation, the separation effect is improved with a simultaneous increase in the pressure loss.

The deposition of particles on the surface of the filter medium in the form of a filter cake is generally undesirable in deep-bed filtration, as this makes it more difficult for subsequent particles to enter the filter medium and locally increased pressure losses occur. If a filter cake has formed, it may be necessary to replace the filter medium immediately.

Due to the lower absorption capacity of the filter medium, depth filtration is only suitable for low particle concentrations in the untreated fluid. The amount of dust that a filter can store until a specified pressure drop is reached is called the dust storage capacity .

Filter media

tissue

While the fabrics initially consisted of natural fibers and metal chips , nowadays glass fibers and other synthetic fibers are preferred . Since there is always a size distribution of the fibers, it can be assumed that particulate filters made of fiber fleeces are inhomogeneous . The fiber diameter is usually between 1 µm and 50 µm, the mean distance between fibers is between three and nine times the fiber diameter.

There are different types of packaging for depth filters made of fabric, depending on the cleaning objective . Coarse particles are removed with filter mats , while filter cartridges and cassette filters are also suitable for removing fine particles.

Pourings

Bulky bed filters are operated as a fixed bed, moving bed or fluid bed. Loose debris can be cleaned by backwashing.

application

Drinking and service water are freed from particles by means of depth filtration. In nature, depth filtration always occurs when surface water penetrates the natural soil and becomes groundwater .

During gas cleaning, depth filters are used to further clean gases that have already been largely freed from particles. They are used in particular for air conditioning systems. Depending on the application, depth filters for gas cleaning are divided into:

literature

Individual evidence

  1. ↑ a b c d e f g h Matthias Stieß: Mechanical process engineering 2 . Springer-Verlag Berlin, Heidelberg, New York, ISBN 3-540-55852-7 , pp. 138-139.
  2. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin. P. 12
  3. ↑ a b c Albert Hellmann, Kilian Schmidt, Siegfried Ripperger, Christoph Thelen, Carsten Möhlmann: Determination of the separation effect of fibrous filter media against ultrafine particles . In: Hazardous substances - cleanliness. Air . 72, No. 11/12, 2012, ISSN  0949-8036 , pp. 484-488.
  4. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin, p. 68.
  5. ^ A b c d Franz Joseph Dreyhaupt (editor): VDI-Lexikon Umwelttechnik . VDI-Verlag Düsseldorf, 1994, ISBN 3-18-400891-6 , p. 1191.
  6. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin. P. 6.
  7. ↑ DIN EN 14799: 2007-07 Air filters for general air cleaning; Terminology; Trilingual version EN 14799: 2006. Beuth Verlag, Berlin, p. 15.
  8. ↑ Tobias Lücke, René Adam: Investigations into the separation efficiency of fiber filters for the filtration of suspended matter. In: Dust - cleanliness. Air . 54, No. 12, 1994, ISSN  0949-8036 , pp. 443-448.
  9. ^ Franz Joseph Dreyhaupt (editor): VDI-Lexikon Umwelttechnik. VDI-Verlag Düsseldorf, 1994, ISBN 3-18-400891-6 , p. 1045.
  10. ↑ DIN EN 13443 Part 2: 2007-10 Systems for the treatment of drinking water inside buildings - Mechanically acting filters - Part 2: Filter fineness 1 µm to below 80 µm - Requirements for execution, safety and testing; German version EN 13443-2: 2005 . Beuth Verlag, Berlin, p. 5.
  11. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin, p. 19.