Functional principles of depth loading filters

Overview:
Filtering separators such as Depth Loading Filters enable removal of extremely fine particles (down to < 0.1 µm),

Depth Loading Filters are primarily employed for concentrations < 2 mg/m³. They generally consist of random-laid fabric with high degrees of porosity. The fine networks of thin fibres enable removal of solid particles from a stream of air on a sustainable basis. When the stream of air flows through the fibre layer, the solid particles must impact upon a fibre, adhere to it, and remain there.

The dominant phenomena involved in the transport mechanisms encountered here are as follows:

  • Inertia (effective for particles > 1 µm)
  • The blocking effect (especially effective for particles 0.3 … 1 µm in size)
  • Diffusion (Brownian molecular movement, effective for particles < 0.3 µm).

The adherence of the particles to the fibres is based on van der Waals forces and on capillary forces. Please consult the illustrations below for more details.

The principles of operation of such fibre filters are based on the following three phenomena (which are graphically explained in greater detail in the illustrations provided here).

The inertia effect:

Photo: Handbuch der Klimatechnik, volume 1, C. F. Müller publishers

Particles smaller than the pores of the fibre layer penetrate into the filter medium. If the particles approach a fibre, and if they cannot directly follow the streamline because of their mass, they are trapped in the tangle of filter fibres in accordance with the inertia effect.

The blocking effect:

Photo: Handbuch der Klimatechnik, volume 1, C. F. Müller publishers

Smaller particles, which cannot be trapped on the basis of the inertia effect, follow along in the flow of air. If the interval between the streamline and the fibre is less than dT/2 of the particle, with the result that the particle impacts the fibre, the particle will be separated according to the sieve effect.

Diffusion effect:

Photo: Handbuch der Klimatechnik, volume 1, C. F. Müller publishers

In cases dominated by the diffusion effect, the particles of dust follow the air stream almost without inertia. They move, however, randomly – the smaller the particles, the more random the motion – along their path as determined by the flow of air. This “thermal fluctuation movement,” which will change in accordance with temperature, increases the probability that these particles as well will collide with a fibre and be trapped there.


Definition of terms

Dust-holding capacity:
The dust-holding capacity, given in units of g/m², is a measure of the capability of a filter to hold dust. It indicates how many grams of dust per gross impingement surface area can be stored in order to raise the pressure difference (at constant air-flow impingement velocity) from the initial value to a stipulated final value.

Operational life cycle:
The operational life cycle is the time that a filter can remain in operation in a system without replacement, or without cleaning of its filter material. In addition to the amount of dust being filtered, and the dust-holding capacity of the filter, the length of the operational life cycle will depend on parameters characteristic of the entire plant system: e.g., the slope of the blower characteristic curves, the pressure difference of the system or at the air filter, as well as the tolerable deviation of the air flow from its rated value.