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Looking at the filtration principle of HEPA filter from the filtration effect
Looking at the filtration principle of HEPA filter from the filtration effectWhen it comes to the fiber filtration principle of high-efficiency filters, it can be summarized as five major filtration effects: interception, inertia, diffusion, gravity, and static electricity. Each filtering effect corresponds to a different filtering principle, which is introduced below:1. Interception (or contact, hook) effect The fibers are intricately arranged within the fiber layer, forming numerous grids. When a particle of a certain size moves along the airflow streamlines just near the fiber surface, if the distance from the streamline (also the center line of the particle) to the fiber surface is equal to or less than the particle radius, the particle will be intercepted and deposited on the fiber surface. Down, this effect is called the interception effect. The sieve effect is an interception effect. 2. Inertia effect Due to the complex arrangement of fibers, when the airflow passes through the fiber layer, its streamlines must undergo severe turns. When the particle mass is large or the speed (which can be regarded as the speed of airflow) is large, when the streamline turns, the particle cannot follow the streamline due to inertia while bypassing the fiber and colliding with the fiber to be deposited. If the particles do not hit the fiber surface head-on due to inertia but just hit within the interception effect range, the interception of the particles depends on the combined effect of these two effects. 3. Diffusion effect The Brownian motion of the particles is generated due to the collision of the thermal motion of gas molecules with the particles, which is more significant for smaller particles. At room temperature, the diffusion distance of 0.1um particles per second reaches 17um, which is several to dozens of times larger than the distance between fibers. This gives the particles a greater chance to move to the fiber surface and deposit. The Brownian motion of particles larger than 0.3um is weakened, which is generally not enough to cause them to leave the streamline and collide with the fiber by Brownian motion. 4. Gravity effect When the particles can pass through the fiber layer, they will be displaced from the streamlines under the action of gravity, that is, they will be deposited on the fibers due to gravity settlement. Since the time it takes for the airflow to pass through the fiber filter, especially the filter paper filter, is much less than 1 second, it is effective for particles with a diameter of less than 0.5um. It has passed through the fiber layer before settling on the fiber, so the gravitational settlement can be completely ignored. 5. Electrostatic effect For various reasons, fibers and particles may become charged, creating an electrostatic effect that attracts particles. In addition to consciously charging fibers or particles, if the fiber surface is charged due to friction during fiber processing, or the surface of the fiber is charged due to particle induction. This kind of charge cannot exist for a long time, and the electric field strength is also very weak. The attraction generated is very small and can be completely ignored. |
