With surface or sieve filter elements, separation of particles takes place on a surface positioned to the direction of flow. In the ideal case, a surface filter element is endowed with a narrow pore size distribution, moreover, accurately equidistant from each other.
Figure 4.7.1.1
Particles larger than the pores are screened off and retained on the surface. Separation and the rate of penetration is dependent on the pore size. The surface has the effectiveness of a sieve which, due to its apertures, prevents the solid particles dispersed in the aerosol from passing through. As a result, the diameters of the flow channels are smaller than the diameters of the solid particles to be separated, so that the surface filter is penetrable solely by the dispersion medium. It results from these characteristics that the use of a surface filter generally leads to a very high pressure loss which can be kept within tolerable limits only through reducing the flow velocity. Surface filtration has no significance as far as the separation of liquid particles is concerned. Thin walled sintered or cellulose acetate impregnated tubes, pleated in a star shaped pattern, are used as filter element materials. Naturally, these surface filter elements are capable of achieving a filtering effect only if the matter to be separated is present in the form of solid particles or as droplets. Drying by means of such filter elements, i.e. removing the water vapour phase from the compressed air, is not possible. As a rule, the complete filters consist of a two-part housing, in which the filter element and its sealing 'o' ring is fixed by means of a retaining device. Attachment by tie rod is preferred to all other forms of mounting as only in this way can a positive and strong filter element location be guaranteed.
Figure 4.7.1.2
The particles and condensate droplets removed by the filter element drop into the condensate collecting chamber due to gravity. When the maximum filling height is reached, the condensate is discharged by an automatically opening drain. Surface filter elements inside a filter housing are, in principle, arranged for flow from the outside towards the inside. A reverse direction of flow would lead to a build-up of separated particles on the inside of the element, leading to degradation of the effective filtering surface through the accumulation of solid particles.
