Dust collection electrostatic effects

Dust collection mechanisms include aU of the entrapment processes described in Section 1C, with the effect of electrostatic forces also being very important in many systems. Filtration of dusts is achieved by depth filtration - for which the filter pad is very effective - and by the combination of surface and cake filtration that the pleated sheet media do so well. The two materials have vied with one another over the years as to which can achieve the highest degree of separation, a history of success that is largely determined by the development of new materials of one kind or the other. 

Over the years this effect was used in electrostatic precipitators to collect airborne dust in smokestacks,where the dust was collected as a solid cake. Winslow in 1949 found a similar but reversible effect with silica particles mixed in oil. The slurry stiffened when the field of around 2kVmm" was applied but became fluid again when the field was removed. In modem formulations, carbonaceous particles of diameter 3.5 pm and of resistivity 1 M lm are mixed to around 40% volume fraction in a silicone fluid of resistivity lOOTIJm. 

Where cost-effectiveness or initial capital cost is not a critical factor, flat panel fine filters can be combined with pocket type filters, to provide second-stage filtration, or electrostatic filters may be used for the retention of even finer particles. If the latter are of the dry type, collection of dust from the electrostatic filters must be by pocket or automatic roll filter. Dry-type electrostatic filters usually combine such dust collectors in an integral unit. Both pocket type filters and electrostatic filters are particularly suitable for variable air volume systems, where air volumes can vary from 20% to 110% of design value. Other types of filters are less effective for variable volume flow. Variable air volume systems are becoming increasingly prominent because of the energy savings that are possible. 

---