Filtration mechanism filter efficiency

Abstract Industrial filtration operations involving textile filter media are found in the production of countless items that we encounter in our everyday lives, and yet more operations are engaged in protecting the environment. The chapter attempts to provide a brief overview of the filtration mechanisms that are involved both in industrial dust collection and solid-liquid separation processes, and the equipment types that are engaged in each. The chapter also describes the types of textile filter media that are employed in the various operations, their component strengths and weaknesses, and some of the finishing treatments that are applied to enhance their efficiency. 

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. 

Wood pulp fibres are frequently less than 1 pm in fibre size and dominate wet-laid nonwoven filter media they are able to provide higher filtrafion efficiency and are environmentally fiiendly. Synthetic fibres (eg, polyolefin, polyester, and polycarbonate (PC) fibres) could work as electret fibres to provide additional electrostatic filtration mechanism, and also could be made binder-fitee, thermobond nonwoven fabrics if they are made Ifom thermoplastic polymers. 

Filtration mechanism of nonwoven fabrics and their filter efficiency... 

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. 

Other models of the filter efficiency for the filtration of solid and liquid parficles in a Newtonian liquid, due to the particle direct interception and additional clogging mechanism, might be usefid in the blood filtration." " ... 

The particle collection efficiency of these air filters can be enhanced by the static charging of the fibres, " and this can be realised by using mixed fibres of various layouts of electret filters " such additional filtration mechanism is more useful for filters used in respiration devices.The ionic species generated by the negative polarity corona (CO, ) seem to be more effective for corona charging of a PP nonwoven... 

In contrast to membrane filtration, in which adsorption is an undesirable phenomenon because of the possible loss of target substance, membrane adsorption is in fact based on this phenomenon and high sorption capacities improve in this case the efficiency of the operation from the economical point of view. However, virtually all membrane materials (surfaces), even the ones designed primarily with membrane filtration in mind, exhibit a more or less pronounced but nonspecific adsorption tendency, due to the general possibility of establishing hydrophobic, dipole-dipole and electrostatic interactions as well as hydrogen bonds with the various dissolved molecules. It was also observed by many investigators that mechanical filter may retain particles/molecules that are smaller than the filter s pores, presumably as a result of an impaction of the particles on the filter surface. However, whatever the cause, if the forces... 

Values of overall filter efficiency are given In Table 1 for four particle diameters, and for enhanced and non-enhanced filtration. The filter thickness was taken to be 1 mm. All other quantities were taken to be the same as In the dendrite growth simulations. It can be seen that In both cases the efficiency Increases with particle size, and that electrical enhancement is more pronounced for smaller particles. These results are in keeping with expectations. That is, as the particle size increases, particle inertia becomes the dominant collection mechanism. 

In a 500 ml. three-necked flask, equipped with a thermometer, mechanical stirrer and efficient reflux condenser, dissolve 16 g. of sodium hydroxide pellets in 95 ml. of hot methyl alcohol. Add 49 g. of guanidine nitrate, stir the mixture at 50-65° for 15 minutes, and then cool to about 20°. Filter oflF the separated sodium nitrate and wash with two 12 ml. portions of methyl alcohol. Return the combined filtrates to the clean reaction flask, add 69 g. of sulphanilamide (Section IX,9) and stir at 50-55° for 15 minutes. Detach the reflux condenser and, with the aid of a still-head ( knee-tube ), arrange the apparatus for distillation from an oil bath with stirring about 100 ml. of methyl alcohol are recovered. Add 12 g. of pure cycZohexanol. Raise the temperature of the oil bath to 180-190° and continue the distillation. Reaction commences with the evolution of ammonia when the uiternal temperature reaches 145°. Maintain the... 

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