พงศ์ธวัช

There are five ways particles are trapped in fine nonwoven HEPA type filter media, sieve effect, impaction, interception, Brownian diffusion, and static charge effect.
The first is the most obvious, sieve effect. This stops large particles that are just too big to fit through the open areas of the filter. For the type filter we are interested in, this would include all particles above 5um in size and larger. As you go smaller in particle size, say between 1um to 5um, occasionally some of these particles get through, but the efficiency for removal is still well into the 99.9999+% range. This is still due primarily to sieve effect and the beginning of inertial impaction effect.
Inertial impaction occurs when large particles are unable to quickly adjust to changes in the flow stream around fibers. The particle, due to its inertia, impacts a fiber and is captured. Figure 2 above shoes how this works. This effect is dominant from around the 0.5um region up to around 5um.
The next effect is interception. Interception occurs when a particle following a gas stream comes within one particle radius of a fiber. When this occurs the particle is trapped by the fiber. Particles that are farther than one particle diameter will not be removed by this process. This is one reason for the high fiber volume density of the 200CFM media. The more dense, the higher the probability of particle capture. This effect is dominant from about 0.1um up to about 1um.
Brownian diffusion is perhaps the most mysterious of the filtering effects since it tends to defy common sense. Very fine particles in the air stream will collide with gas molecules and create a random path through the media. The smaller the particle the longer the particle will zigzag around. This random motion increases the probability of the particle contacting a fiber. This effect is dominant for all particles smaller than 0.1um.