ไส้กรองอากาศแบบ HEPA กรองฝุ่นได้โดยวิธี sieve effect, impaction, interception, Brownian diffusion, and static charge effect
อันแรกก็ง่ายๆ คืออนุภาคที่ใหญ่กว่าช่องว่างของไส้กรองจะผ่านไปไม่ได้
สามอันต่อมาทำงานตามรูปนี้
รูปกับข้อมูลมาจาก paper นี้: New technologies in HEPA filter development
คำอธิบายเค้าเขียนไว้ดีแล้ว ก็ขอลอกมาลงเลยละกัน:
There are five ways particles are trapped in fine nonwoven HEPA typefilter media, sieve effect, impaction, interception, Browniandiffusion, and static charge effect.
The first is the most obvious,sieve effect. This stops large particles that are just too big to fitthrough the open areas of the filter. For the type filter we areinterested in, this would include all particles above 5um in size andlarger. As you go smaller in particle size, say between 1um to 5um,occasionally some of these particles get through, but the efficiencyfor removal is still well into the 99.9999+% range. This is still dueprimarily to sieve effect and the beginning of inertial impactioneffect.
Inertial impaction occurs when large particles are unableto quickly adjust to changes in the flow stream around fibers. Theparticle, due to its inertia, impacts a fiber and is captured. Figure 2above shoes how this works. This effect is dominant from around the0.5um region up to around 5um.
The next effect is interception.Interception occurs when a particle following a gas stream comes withinone particle radius of a fiber. When this occurs the particle istrapped by the fiber. Particles that are farther than one particlediameter will not be removed by this process. This is one reason forthe high fiber volume density of the 200CFM media. The more dense, thehigher the probability of particle capture. This effect is dominantfrom about 0.1um up to about 1um.
Brownian diffusion is perhaps themost mysterious of the filtering effects since it tends to defy commonsense. Very fine particles in the air stream will collide with gasmolecules and create a random path through the media. The smaller theparticle the longer the particle will zigzag around. This random motionincreases the probability of the particle contacting a fiber. Thiseffect is dominant for all particles smaller than 0.1um. |
