Theory of Fabric Filtration

FILTRATION PROCESS

The separation of solids from a liquid by means of porous medium or screen which retains the solids and allows the liquid to pass is called filtration. Very small items (such as bacteria) can also be removed (separated) from fluids by filtration process.
     

          

In general, the pores of the medium are larger than the particles which are to be removed, and the filter works efficiently only after an initial deposit has been trapped in the medium.

•         Cake filtration implies that the initially collected dust layers serve as the primary filtration media for subsequent filtration. The role of the textile fabric is primarily that of an initiating support structure for the buildup of the dust layer.

•         Noncake filtration, as the name implies, does not depend upon a dust cake as the primary filtration media. The textile fabric, most often a felt or other nonwoven fabric, serves not only as an initiating support for dust collection but, also, as a primary part of the filtration media through out the filtration cycle.

The basic equation of filtration through porous media is Darcy's law:

where:

Dp        =         the pressure drop across the media

k          =          a constant,

l           =          media thickness, and

Vf        =         face velocity

•         Pressure drop and face velocity are readily measured, but media thickness is not when that media is a fragile dust cake collected on the surface of a fabric. A more easily measured variable in the laboratory is the dust cake areal density, W - the mass per unit area of the dust cake. 

•         W is related to dust cake thickness as follows

Where:

        l              =         the dust cake thickness

       r_p            =         the density of the material making up the dust cake particles

      r_R             =         the bulk density of the dust cake layer,

e                               =          the dust cake porosity.

•         Porosity is simply a measure of the voids in the layer. It says nothing directly about the fluid flow properties of the layer or the shape and density of the connecting links between voids. Fluid flow behavior through porous media depends on permeability which is contained in the constant k of Darcy’s law.  Substituting for “l” in Darcy’s law gives a form of Darcy's equation commonly used in the fabric filtration.

 •         K2 is a constant of proportionality between

   (DP /Vf) and W where:

•         S, the drag of the porous media, is a measure of aerodynamic resistance analogous to electrical resistance.  Drag rather than pressure drop is the measure of filter aerodynamic resistance because its use preserves the concept of a media layer property, independent of flow velocity.