Filter selection, particle size distribution

The term essentially a drag coefficient for the dust cake particles, should be a function of the median particle size and particle size distribution, the particle shape, and the packing density. Experimental data are the only reflable source for predicting cake resistance to flow. Bag filters are often selected for some desired maximum pressure drop (500—1750 Pa = 3.75-13 mm Hg) and the cleaning interval is then set to limit pressure drop to a chosen maximum value. 

In the field of oil production, the combination of narrow particle size distribution glass microspheres w ith a new Sonic Filter Tester has been one of the most significant developments in recent years. The method is unparalleled in terms of accuracy and repeatability, w hich gives the engineer great confidence in the selection of sand screens. 

There are a variety of designs [23] of vacuum filter, the selection of which depend strongly on the particle size distribution and density of the suspended solids in the feed slurry. The main types are drum, disc or horizontal belt filters. In each case the filter medium is usually a porous cloth, the permeability of which is chosen to suit the feed particle size. 

The most important factors influencing the selection of the filter type include particle size distribution in the solids to be separated, the specific gravity of these solids and the solids content of the slurry to be filtered. 

However, where there is a coarser particle size distribution, this more often goes hand-in-hand with a higher concentration of solids in the slurry to be filtered. In this case, one can assume that continuous vacuum filtration will be successful. The particle size distribution curve of the solids to be separated is thus a suitable indicator for the selection of filter type [8]. 

The filter medium is that critical component which determines whether or not a filter will perform adequately. Within the context of solid/Uquid separation the term filter medium can be defined as any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining components (Purchas and Sutherland, 2002). The principal role of a filter medium is to cause a clear separation of particulates (which may be solid particles, liquid droplets, colloidal material, or molecular or ionic species) from the liquid with the minimum consumption of energy. In order to achieve this, careful selection of the medium must take into account many factors criteria by which a medium is assessed include the permeability of the clean medium, its particle retention capability and the permeability of the used medium. Serious loss of permeability may follow plugging or blinding of pores in the filter medium, and can determine the lifetime of the medium if an uneconomic filtration rate results. Permeability and particle retention are dependent on the structure of the medium, but interaction of media structure with the shape and size distribution of the particles challenging the medium is also of crucial importance. 

Particulate filtration removes suspended liquid or solid materials whose size, shape, and mass allow them to remain airborne at the air velocity conditions present. Filters are available in a range of efficiencies, with higher efficiency indicating removal of a greater proportion of particles and of smaller particles. Moving to medium efficiency pleated filters is advisable to improve lAQ and increase protection for equipment. However, the higher the efficiency of the filter, the more it will increase the pressure drop within the air distribution system and reduce total airflow (unless other adjustments are made to compensate). It is important to select an appropriate filter for the specific application and to make sure that the HVAC system will continue to perform as designed. Filters are rated by different standards which measure different aspects of performance. 

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