The two stages of the filtration process
The first stage is the stabilization stage. In this stage, the filter's capture efficiency and resistance to particles does not change with time, but is determined by the inherent structure of the filter, the nature of the particles and the characteristics of the airflow. The change in thickness of the filter structure due to particle deposition etc. is small. This stage is important for filters when filtering airflows with very low particle concentrations (such as filtering clean room air).
The second stage is the unstable stage. In this stage, the capture efficiency and resistance do not depend on the properties of the particles, but change with time, mainly due to the deposition of particles, the erosion of gas, the influence of water vapor, etc. Variety. Although this stage is much longer than the previous stage and is of decisive significance for general industrial filters, it is of little significance in air cleaning technology.
Three effects of filtering
1. Interception effect
The fibers are intricately arranged in the fiber layer, forming countless grids. When a particle of a certain size moves along the airflow streamline just near the fiber surface, if the distance from the streamline (also the centerline of the particle) to the fiber surface is equal to or less than the particle radius, the particle will be intercepted on the fiber surface and When deposited, this effect is called interception effect, and the sieve effect belongs to the interception effect.
2. Inertia effect
When the airflow passes through the fiber layer, due to the complex arrangement of the fibers, the airflow streamline has to turn violently many times. When the mass of the particles is large or the speed (which can be seen as the speed of the airflow) is large, when the streamline turns, the particles cannot follow the streamline and bypass the fiber at the same time due to inertia, so they leave the streamline and approach the fiber, and collide with the fiber. deposited
If the particles do not hit the fiber surface head-on but just hit the interception effect range due to inertia, the particles are intercepted by the combined action of these two effects.
3. Diffusion effect
The Brownian motion of the particles is generated due to the collision of the thermal motion of the gas molecules with the particles, and the smaller the particles, the more significant the effect is. At room temperature, the diffusion distance of 0.1um particles per second reaches 17μum, which is several times to dozens of times larger than the distance between fibers, which makes the particles have a greater chance to move to the fiber surface and deposit), while particles larger than 0.3μum The Brownian motion of the particles is weakened, generally not enough to rely on Brownian motion to leave the streamline and collide with the fiber.
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