5.5.3. Pre-filters


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Fig. 5.24: Filtrations mechanism of surface filters
Pre-filters filter solid impurities to a particle size of approx. 3 µm out of the compressed air but filter out very little oil and moisture. Pre-filters take the load off high performance filters and dryers when the air is very dusty. Finer filters can be dispensed with if the demands on the quality of the compressed air are low.


Operating principle

Pre-filters operate according to the principle of superficial filtration. They have a purely sifting effect. The pore size determines the size of particle that can be filtered out. The impurities remain only on the outer surface of the filter elements. Standard materials for filter elements are:
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Fig. 5.25: BOGE Pre-filter, Series V
    • Sintered bronze.
    • Highly molecular polyethylene.
    • Sintered ceramics.
    • Bronze or brass wire (coarse filtration).
    • Pleated cellulose paper inserts.

Air flows through the filter from the outside towards the inside. An opposite direction of flow would allow the separated particles to build up inside the filter element. The growing collection of solid matter would block the effective area of the filter.


Features
    • Re-usability.
      Because the separated particles are only collected on the surface of the pre-filter element it is possible to clean the element.