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High efficiency filter is mainly used to capture dust and suspended solids below 0.5um, which is often used as the terminal filtration of various filtration systems. According to the different mechanical characteristics of the movement of dust or microorganism with different particle sizes in the fluid, the filtration mechanism involves the following aspects:
1. Sieve filtration
When the dust particles are larger than the membrane pore, the dust particles will be trapped on the surface of the filter membrane, which is called screening effect.
2. Inertial impact
When the air flows through the filter membrane, the larger particles in the air flow are trapped due to the inertial effect, which is called inertial collision.
3. Interception
When the air flows through the membrane hole, the smaller particles will go around with the airflow. If the radius of the particles is larger than the distance from the center of the particles to the edge of the hole wall, the particles will be intercepted and captured because of contacting with the hole wall. This effect is called interception.
4. Diffusion
The smaller particles in the air flow will move Brownian under the impact of gas molecules, and they will be captured by contacting with the pore wall of the filter membrane. This diffusion is called diffusion.
5. Electrostatic action
The potential difference between the charged particles and the filter membrane is formed. Under the action of Coulomb force, the particles and the pore wall of the filter membrane are adsorbed and collided and trapped. This effect is called electrostatic interaction.
Generally speaking, all kinds of air filtration mechanism are not effective at the same time, but one or several kinds of combination work. Moreover, with the change of the pore space, air flow velocity, particle size and other reasons, the effects of various mechanisms on the filtration performance of different membranes are also different.
It was thought that the filtration efficiency of the filter for 0.3 μ m dust particles was the lowest, so the filtration efficiency of 0.3 μ m particles was regarded as a typical value in the evaluation of high efficiency filter. There are many methods to produce and measure 0.3 μ m dust, so there are DOP, oil mist, sodium flame and other methods.
Through research and experiment, it is found that by using the laser particle counter which can measure 0.1 μ m particle size dust, not only the dust concentration can be measured, but also the particle size of each dust can be measured. Therefore, it is convenient to measure the typical value with the lowest efficiency, and the dust particle size corresponding to that typical value is MPPs The lowest efficiency corresponding to MPPs is MPPs efficiency. MPPs principle: the aerosol generator produces aerosol that meets the test requirements, and its count value diameter should be in the range of 0.1 μ m ~ 0.3 μ M. Filter the aerosol in the front of the clean air sampling port with the air filter. Then aerosol detection device (such as optical particle counter) is used to sample the aerosol upstream and downstream of the filter, and the concentration of a certain size range of aerosol is measured. The transmittance or filtration efficiency of the tested filter is calculated by the ratio of aerosol concentration in upstream and downstream. Relevant standards: iest-rp-cc-007.1-1992, en 1822-5:2000, GB / T 6165-2008.
