Dust mask filter dust principle

Each layer of filter material of the dust mask is generally composed of a non-woven fabric, and the filtering function is realized by means of fiber filtration. There are generally five principles of fiber filter dust: electrostatic adsorption, gravity deposition, inertial deposition, diffusion deposition effects and direct capture.

(1) Electrostatic adsorption

The floating particles in the air of the operating environment often carry positive or negative charges. The filters of modern dust masks are mostly electret fibers with polarized charges. When these charged particles in the air pass through the electrets, they are strongly adsorbed on the electret fibers; and particles that are not charged in the floating particles will be polarized in the electric field formed by the electrets and will also be polarized. Adsorption on the fiber.

(2) Gravity sedimentation

It means that some particles of relatively large mass that pass through the mask filter deviate from the streamline direction due to gravity and deposit on the fibers.

(3) Inertial deposition

When particles in the air pass through the curved mesh channel of the filter material, the particles with larger momentum will deviate from the streamline direction due to inertia, collide with the fiber and be trapped, and be trapped due to intermolecular forces. This is inertial deposition. The occurrence of inertial deposition depends on the aerodynamic diameter of the particles. Particles with large aerodynamic diameters are more likely to impinge on the fibers than particles with smaller diameters. For masks, inertial deposition is more pronounced when the density of large particles in the gas stream is higher and the velocity is faster.

(4) Diffusion deposition effect

Particles in the gas stream collide with gas molecules that undergo continuous thermal motion and generate random Brownian motions that cause the fine particles to move randomly. This movement causes random collisions between the particles and the fibers. The dust particles are affected by the gravity of the fiber molecules. It is adsorbed. The diffusion deposition effect is the most easy to capture small-scale particles and low-speed particles.

(5) Direct capture

Direct capture means that the larger dust particles that move with the air flow are trapped by the mechanical filtering of the filter material, or the particles are in contact with the fibers when the flow lines are close to the fibers and are captured by the fibers. Generally large particle size particles are mainly filtered by direct capture, and irregularly shaped particles are more easily trapped than spherical particles. The ratio of the diameter of the particles to the diameter of the filter fibers affects the interception efficiency.

When the filter material filters the polydisperse particles in the air, under the combined effect of several filtration mechanisms, the relatively small particles are deposited on the fibers first due to diffusion, so when the particle size is small to large, the diffusion efficiency gradually weakens. Larger particles are deposited under the influence of interception and inertia.

In this way, the filter efficiency curve related to the particle size has a lowest point, and the total efficiency at the particle size is the smallest, and thus the fine particles are the particles that are least likely to be trapped in the filter material.

By detecting the filtration efficiency of the various diffuser particles in the gas flow through the filtering material of various dust masks, it was found that the filtration efficiency corresponding to the 0.31 μm particle was always the lowest. Therefore, the 0.3-micron particle size is usually used as a particle size indicator to determine the filtration efficiency of the filter media.

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