Grundlagen der mechanischen Schwebstoff-Filtrierung

Abstract

Mechanical filtration of suspended matter is the designation that is applied to a process, wherein a stream of gas carrying suspended matter (dust, smoke, mist, or aerosol spray) is made to pass through a porous substance (filtration medium) in such a manner as to cause the suspended particles to attach themselves and adhere to the filtration medium, thus becoming separated from the gas which, now freed from the particles, passes on through the filter. The distinguishing characteristics of a suspended matter filter are the degree of separation achieved, and the amount of resistance offered to the passage of the gas stream. Separation of the suspended particles is achieved, depending on the size of the particles, either by diffusion, screening, interception, or inertia, by electrostatic forces, or else by a combination of these effects. The actual degree of separation can only be defined in a meaningful manner when considered in conjunction with the velocity of the gas, the degree of concentration of the suspended matter and its composition, the size of particles involved, and the dust load deposited on the filter. The drop in pressure due to the resistance of the filter depends mainly on the gas velocity, and on the amount of dust that has accumulated on the filtration medium. The suitability of a filter is determined, amongst other things, by such factors as the area of filtration surface, the amount of space required for its accommodation, its reliability and resistance to changes in temperature and to chemical attack.
“Useful life” is the term used to describe the life period of a filter under certain pre-determined conditions (gas velocity, concentration of the dust load and its composition) until the moment when a definite final resistance value is reached. This latter is greatly affected by the gas velocity, the concentration of the dust load and its composition, as well as by the quality of the pre-filters used. By employing single or multi-stage prefilters, the useful life of suspended matter filters may in certain cases be extended several times. The optimum combination of filters bears a very close relationship to the initial cost and the operating costs, the gas velocity as it passes through the individual filtration stages, the available fan-capacity, and also to the prevailing dust load conditions. Where larger plant types are concerned, it should be well worth while to make these relationships the subject of a serious study.