Particle size, filter efficiency

Bag filters are composed of woven material that intercepts small particles on the filter surface by impingement and electrostatic attraction. The efficiency of collection increases as the depth of the filter cake increases, making these filters highly efficient for small diameter, even sub-micron, particle size. Filters are periodically shaken or back-flushed to remove particulate accumulation. These filters are usually constructed of woven materials suitable for operation at low temperatures up to about 350°C. 

Panicle size, concentration, and chemical composition arc usually the aerosol properties of most interest. Also imponant in certain applications arc particle charge, crystal stmeture and optical properties. In Industry, particles are collected to recover a desirable product or reduce emissions and occiiputional exposures. The efficiency of filters, scrubbers and other such devices depends primarily on particle size. As shown in Chapter a minimum is often found when the efficiency of particle rentoval is plotted a.s a function of particle size. The efficiency minimum or window" occurs in the particle size range near a few tenths of a micron for reasons that differ depending on the mechani.sms of particle collection. A similar efficiency mintniLini is observed for particle deposition in the lung as a function of particle size. The explanations for the efficiency minima in the lung and certain types of filters are similar. 

Stricter control of particle size distribution improves powder handling (qv) and transport through the system, and appHcation efficiency. It also reduces blinding of the final filter (69). Additives have been developed which improve the triboelectric charging characteristics (70). 

Because of their inherently high efficiency on dusts in all particle-size ranges, fabric filters have been used for collection of fine dusts and fumes for over 100 years. The greatest limitation on filter application has been imposed by the temperature limits of available fabric materials. The upper limit for natural fibers is about 90°C (200°F). The major new developments in filter technology that have been made since 1945 have followed the development of fabrics made from glass and synthetic fibers, which has extended the temperature limits to about 230 to 260°C (450 to 500°F). The capabihties of available fibers to resist high temperatures are still among the most severe limitations on the possible applications of fabric filters. 

TABLE 17-11 Comparison of Air Filters by Percent Removal Efficiency for Various Particle Sizes ... 

The particle size and porosity of the filter media, since operating efficiency is directly related to the available biofilm surface area. 

The concentration of indoor pollutants is a function of removal processes such as dilution, filtration, and destruction. Dilution is a function of the air exchange rate and the ambient air quality. Gases and particulate matter may also be removed from indoor air by deposition on surfaces. Filtration systems are part of many ventilahon systems. As air is circulated by the air-conditioning system it passes through a filter which can remove some of the particulate matter. The removal efficiency depends on particle size. In addition, some reactive gases like NOj and SOj are readily adsorbed on interior surfaces of a building or home. 

Small solid particles, present in dust and grit emissions, have very low settling velocities (Table 4.4) The collection efficiencies of simple cyclones are tlierefore, as shown in Figure 17.3, relatively low. Fabric filters, electrostatic precipitators or wet scrubbers may be required to remove particles <5 pm in size with an acceptable efficiency. Therefore the cost of pollution control inevitably increases when dealing with particle size distributions skewed towards the lower end. 

Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a range of actual operating efficiencies of 95 to 99.9%. Several factors determine fabric filter collection efficiency. These include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning mechanism. In general, collection efficiency increases with increasing filtration velocity and particle size. 

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed. 

In EUROVENT 4/9 the dust spot efficiency has been replaced by measurement of the degree of separation of particles within the 0.2-3 pm range, with a particle size of 0.4 pm used for classification of the filter. 

HEPA and ULPA filters have been developed. In the CEN EN 1822 1998 test method, the filter s efficiency is determined for the most penetrating particle size (MPPS). Depending on the filter s total level of separation and leakage, the filter is classified as HIO, Hll,.. ., H14 and U15, U16, or U17. HEPA filters are commonly used for inlet air in the pharmaceutical, optical, and food industries. 

It is important to be aware of the filter s properties in different environments. Figure 9.2 shows how, in the case of new filters, separation varies with particle size and filter class. The filter class is based on the average efficiency, and a new filter normally has much lower initial efficiency. In the case of electrostatically charged filters, separation may be significantly higher for new filters. The figure should be seen as an indication of minimum separation during actual operation. 

Maximum penetrating particle size The particulate size for which a filter has minimum removal efficiency under test conditions. 

Minimum filter efficiency The value of a filter s efficiency relating to its performance classification under specified operating conditions. See also Maximum penetrating particle size. 

Particles are present in outdoor air and are also generated indoors from a large number of sources including tobacco smoking and other combustion processes. Particle size, generally expressed in microns (10-6 m) is important because it influences the location where particles deposit in the respiratory system (U.S. Environmental Protection Agency 1995), the efficiency of particle removal by air filters, and the rate of particle removal from indoor air by deposition on surfaces. 

Efficiency of any air cleaner or filter is a function of the particle size present in the indoor air and the velocity and volume ot air flowing through the device. 

An air intake for a compressor should have a high capacity to remove abrasive materials, including those of small particle size, and good accumulating ability, that is, to collect large quantities of impurities without any significant decrease in filtering efficiency and air flow. 

Sand filters usually provide efficient SS removal for suspended matter above 30 x particle size, but they may need to be periodically sanitized using chlorine, formaldehyde, or similar substances (especially filters with very fine grain media). It is not uncommon for filters to contain microbial slimes and therefore act as a source of microbiological reinfection. 

Special baghouse filters are designed for high-efficiency filtration with a unique three-layer construction. The dust filtration is effective for a wide range of particle sizes. The layered design... 

Theoretical dependence of filter efficiency of a single collector (proportional to the rate at which particle contacts occur between particles and the filter grain by mass transport) on particle diameter. For particles of small diameters transport by diffusion increases with decreasing size. Contact opportunities of the larger particles with the filter grain are due to interception and sedimentation they increase with increasing size. 

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