Air filter paper selection and filtration mechanism

When selecting the air filter material, first determine the filter efficiency of the end air filter according to the cleanroom level requirements of the clean room, and then select the matching efficiency of the upper filter at each level. When selecting the filter, the resistance should also be considered as the initial resistance and end. Resistance, dust holding capacity, etc. High end-resistance, long service life, but large air volume, generally consider the end-to-initial resistance ratio to be 2. For the air-conditioning system, a higher-efficiency filter than designed should be used. Although the one-time investment cost is large, various drawbacks such as the obstruction of the air duct and the deterioration of the fan performance can be avoided. As a result, the life of the filter is prolonged, the number of cleanings is reduced, and the cost is greatly saved.

Air filter paper products

In the filter paper selection should mainly consider several points:

1. The effective area has a large effective area, that is, the filter paper has a large area, the dust holding capacity is large, the resistance is small, and the service life is long. Of course, the cost increases accordingly.

2. The finer the fiber diameter, the better the interception effect and the higher filtration efficiency.

3, the binder content of the filter content of binder is high, the paper's tensile strength is high, the filtration efficiency is high, the hair loss phenomenon is small, the filter material background dust is small, the resistance is good, but the resistance increases accordingly.

Filtration mechanism of fiber filter material

The barrier material is used to filter the particles in the gas or liquid. According to the position where the particles are trapped, they can be divided into two categories: one is surface filtration and the other is deep filtration. The so-called surface filtration means that the particles are trapped on the surface of the material. Microporous membranes and microporous membrane composites, metal meshes, etc. are all surface filters. Deep filtration uses the capillary structure inside the barrier material, fiber network structure, etc. to trap the particles in the gaps of the barrier material. Deep filtration is further divided into high porosity and low porosity. High-porosity barrier materials have less filtration resistance, higher efficiency, and wider application. When the glass fiber filter material is used to block gas or liquid particles, surface filtration and deep filtration are all present, but with the material structure, working conditions (physical and chemical properties of the particles, gas temperature, humidity, gas pressure, etc. ) The difference is that there are some changes, sometimes based on surface filtration, sometimes based on deep filtration.

When the average particle size of the particles in the gas or liquid is small, the particle concentration is low, and the static electricity is not large, the network and capillary structure of the fiber filter material play a major role in deep filtration. When the fiber filter material is used for barrier separation of gas or liquid particles, it mainly has the following effects.

1. Interception (or contact, hooking) The fibers in the fiber layer are arranged in an intricate array and formed into numerous meshes (mesh structures). When a particle of a certain size moves along the direction of fluid flow to the vicinity of the surface of the fiber, if the distance from the center of the particle's motion locus to the surface of the fiber is equal to or less than the radius of the particle, the particle is intercepted and precipitated on the surface of the fiber. It is called interception effect.

2. Inertia effect Due to the complicated arrangement of the fibers, when the fluid passes through the surface of the fiber, the particles in the fluid must undergo severe bending along with the trajectory of the fluid. When the particle mass is large or the speed is large, the particles will not pass around the fiber due to inertia when they turn, and thus approach the fiber and collide with the fiber to precipitate.

3. Diffusion effect The Brown movement of particles due to the collision of particles due to the thermal motion of fluid molecules is more pronounced for smaller particles. At room temperature, 0.1 μm particles spread at a distance of 17 μm per second, which is a few to several tens of times greater than the distance between the fibers. This gives the particles a greater chance of moving to the surface and sedimenting. Particles larger than 0.3 μm Brown's weakened movement is generally not enough to rely on Brown's movement to make it away from the trajectory and collide with the fiber.

4. Gravity effect When the particles pass through the fiber layer, the displacement from the movement trajectory occurs under the action of gravity, which is due to gravity sedimentation and deposition on the fiber. Since the air flow passes through the fibrous filter material for much less than 1 s, particles smaller than 0.5 um have passed through the fibrous layer when it has not settled onto the fiber, so gravitational settlement is completely negligible.

5. Electrostatic effects Due to various reasons, fibers and particles may be charged to generate electrostatic effects that attract particles, but in addition to consciously causing the fibers or particles to be charged with static electricity, if they are charged by friction during fiber processing, or Because the surface of the fiber is charged due to particle induction, this charge can neither exist for a long time, but also has a weak electric field strength, and the resulting attraction is very small and can be completely ignored.

In a fibrous filter material, the trapping of particles may be the result of all the mechanisms, or it may be the result of one or more mechanisms, which are mainly caused by the size of the particles, the shape of the particles, the density of the particles, the fibers The thickness, the cross-sectional shape of the fiber, the porosity of the fiber layer, the velocity of the fluid, the temperature of the fluid, the size of the particles and the static electricity charged (and the temperature of the gas as well as the gas) and other factors. Changes in these factors not only affect the filtration mechanism of the particles, but also directly affect the filtration efficiency of the fiber filter material.

Source: HC Network