Mechanisms OF filtration for High Efficiency Fibrous of filter

OverviewThere is A popular misconception that fibrous filter behave like A sieve where particles above A certain size acres trapped and more smaller particles passport through. While some of filter look for as diaphragm of filter in liquids DO function this way, fibrous air of filter defy common scythe by actually trapping smaller and of more larger particles more effectively than mid sized particles. High efficiency fibrous filter comprise A large percentage OF the of filter used around the world today. Applications include respiratory protection, air sampling, clean rooms, industrial processes, and building ventilation of system. A fibrous filters is comprised OF A large NUMBERs OF randomly oriented fibers. Thesis fibers form A dense material or mat which captures and retains particles throughout its depth or thickness. It is the thickness, fibre dia. meter of and density OF the mat that enables fibrous filter ton function. Filters performance is often expressed in term OF by cent efficiency, defined as the reason OF upstream particle concentration compared ton the downstream concentration that has passed through (multiplied by 100). Figure 0 The efficiency OF A fibrous filters of varies for different particles sizes and flow of advice. It is meaningless tons specify the efficiency OF A fibrous filters without thus stating the pertinent particle size and flow. For example, NIOSH of defines A P100 (formerly HEPA) respirator filter ton of fuel element RK leases 99,97 by cent efficient for 0,3 micrometers (m) of particles RK A flow guesses OF of 85 litre by minute (lpm). Similarly, at N95 class filters must fuel element RK leases 95 by cent efficient against 0,3 m particles RK 85 lpm. The reason that A particle size OF 0.3m is commonly referenced is because particles near 0,3 m likely tons of GET more in dia. meters of acres though the filters than any more other size. In more other words, is the worst case particle size it. The filter' s efficiency is higher RK any more other size. READ on ton out find why. Filtration MechanismsVarious physical mechanisms contribute tons of A high efficiency fibrous filter' s effectiveness into capturing particles. The most predominant mechanisms acres interception, inertially impaction and diffusion.InterceptionInterception occurs when A particle which is following A gas streamline comes within one particle radius OF A filter fibre. The particle touches the fibre and is captured, thus being removed from the gas flow. This is illustrated in Figure 1.For A of given particle size, there acres certain streamlines which wants move CLOSE enough tons of A filter fibre in such a way that the particle wants fuel element captured. Streamlines than one particle radius away from A filter fibre wants emergency contribute ton the interception mechanism. Inertially ImpactionInertial impaction occurs when A particle is so large that it is unable ton quickly adjust ton the abruptly CHANGEs into streamline direction near A filter fibre. The particle, due ton its inertia, wants continue along its original path and hit the filters fibre. This type OF filtration mechanism is most predominant when high gas of velocities and dense fibre packing OF the filters media is present. Figure 2 below illustrates this mechanism.DiffusionTo explain the diffusion mechanism OF particle capture, incoming goods roofridge need tons of DO A quick review OF the kinetic theory OF of gas. This theory explains that A gas is composed OF A large NUMBERs OF of molecules which acres small into size when compared tons the distances between them. Thesis of molecules behave like ridged spheres which travel in straight LINEs they' when; RH emergency bumping into each more other. In actuality, thesis molecules collide with each much that more other in such a way they move in random, zigzagging path. This random motion is referred tons as Brownian motion. The diffusion mechanism OF particle retention is the result OF the Brownian motion OF gas of molecules. Small of particles, with dia. meter into the rank OF 0,1 m and below, tend ton make random motions due tons their interaction with the zigzagging gas of molecules. As thesis small particles of acres bumped by the gas of molecules they too begin moving randomly about, bumping into more other particles as wave. Diffusion is predominant with low gas of velocities and of more smaller particles. The of smaller A particle is and the more slower the flow, the more time it wants have ton zigzag around, thereby giving it much better chance OF hitting and sticking tons of A filter fibre. Pair of overalls filter Efficiency Versus Particle SizeA graph showing how filters efficiency varies with particle size is shown graph in Figure 4th As CAN fuel element lakes from, A filter' s ability ton remove particles from A gas stream is directly related ton the size OF the particles into the stream. For very small particles, less than 0,1 m in dia. meter, the primary filtration mechanism is diffusion and the filters is very efficient. For of particles between approximately 0,1 and of 0,4 m the filters is less efficient as the particles of acres too large for A great diffusion effect and too small for A large interception effect. Particles above approximately 0,4 m more enter the region where interception along with inertially impaction acres predominant and the filters is very efficient again.SummaryA filter' s ability ton capture particles depends on the particle size passing through its fibers as wave as the velocity OF the flow passing through the filters. Contrary ton popular amounted, A fibrous filters of does emergency simply capture all particles above A certain size. Three mechanisms acres predominant in determining A filter' s efficiency versus particle size. Thesis of acres: interception, inertially impaction, and diffusion. Large of particles above 0,4 m in dia. meters, wants fuel element captured due tons both the impaction and interception mechanisms. Medium particles, generally considered as the most penetrating, into the 0,1 tons of 0,4 m dia. meters rank, acres captured by both the diffusion and interception filtration mechanisms. Small of particles, below 0,1 m in dia. meter, acres captured by the diffusion mechanism. The result OF the various filtration mechanisms is on efficiency versus particle size curve which has, as illustrated in Figure 4th A shape to upside down Bell fibrous filters is generally leases effective RK removing particles dia. meter rank particle into the 0,1 m tons 0,4 m. Ton test A filter for worst case situation, it should fuel element tested with at aerosol RK the most penetrating particle size. Figure 0: Percent EfficiencyFigure 1: Direct Interception OF A Particle by A filter FiberFigure 2: Inertially ImpactionFigure 3: DiffusionFigure 4: Filter Efficiency Versus Particle Size

This text was translated automatically.