Filtration mechanism filters efficiency having

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

It is noticed that for such a filter containing fibres of the same size, it is unavoidable to have the minimum filter efficiency in filtering particles of certain sizes, as shown in Fig. 10.2. For very small particles less than dpi in diameter, the primary filtration mechanism is diffusion. For particles between dpi and dp2, the filter is less efficient, as the particles are too large for a great diffusion effect and too small for a large interception effect. For particles of diameter above dp2, the filter is very efficient again because the interception, along with inertial impaction, effects is predominant in the filtration. 

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. 

AA, MAA and ITA also act as plasticisers and ideally, it should not be necessary to add any additional plasticiser comonomer such as MA but, for textile tow precursor, it is probably necessary to increase the acid comonomer content, or add an additional plasticiser comonomer to facilitate the introduction of crimp, which aids in the packaging of the large textile tow. Freedom from adventitious impurities in the dope by careful selection of raw materials, efficient dope filtration (e.g., the use of membrane filters) and clean room conditions will yield a PAN precursor that will have less faults and hence, produce a carbon fiber with better mechanical properties. 

In some cases of pilot scale filtration, entire units have been enclosed as a secondary containment precaution (see Chapter 8). A recent commercial development is the MBR-Sultzer dynamic filter which is available in three sizes. Dynamic filtration is the same as cross flow filtration with little or no recirculation. The cross flow effect is derived from the spinning of the inner surface filter. This type of filter is more efficient, has a lower pump rate and a much higher linear velocity across the filter surface, than conventional cross flow filtration units. There is also little or no damaging effect on sensitive cells. The medium size has the same capacity as the Westfalia SA-7 separator. Van Hemert and Tiesjema concluded that the dynamic filter is suitable for work requiring strict aseptic and primary containment conditions. The use of a double mechanical seal on the rotating shaft could offer a higher degree of containment if required. 

In the past 10 years, electrospun nanofibrous membranes have been spotlighted as an effective filter media to capture fine particles. In addition to the basic studies of electrospinning process to better understand the membrane construction process, researchers from all over the world focus on the study of the relationships between the structure characteristics of electrospun nanofibrous membranes (fiber diameter, pore size, porosity, surface area, etc.) and filtration performances (filtration efficiency, pressure, air permeability, etc.). In this chapter, recent advances in fabricating nanofibrous filter media via electrospinning process have been reviewed. In particular, filtration performances and relevant mechanical properties are discussed in detail. It is interesting that the electrospun nanofibrous membranes have been able to outperform conventional nonwoven membranes fabricated essentially by using the meltblown or spunbonded process. 

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