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    Information Of HEPA That You May Like To Know
    Click:522 Date:2020-05-29 17:25:00 Information Source:

    he main purpose of HEPA high efficiency screens is to remove fine particles from the air, including PM2.5 (fine particles less than 2.5 microns in diameter). HEPA is not a brand or trademark, but an international standard. In this article, we will introduce the function and principle of HEPA filter.

     

    The basis of any HEPA screen is 0.5 to 5 microns of cross fiber. The distance between the fibers is about 5 to 50 microns. So, how can a screen with such a large gap block 0.01-0.3 micron particles?

     

    When it comes to the principle of filter screen, we will think of a funnel: when the diameter of particles is larger than the gap between fibers, particles will be stopped by the filter screen. But this principle is only effective in the lower level of the filter. The principle of HEPA filter screen is more complex because the gap between the fibers of HEPA filter screen is 100 times larger than the trapped particles. Looking at the following photos, you can imagine the proportion of PM2.5 size to the gap between the filter fibers:

    image 

    In fact, HEPA filter screen "sticks" PM2.5 micro dust by fiber adsorption. The diameter of the fiber is about the same as that of the micro dust, and the "mesh" space between the fibers is even larger. The "wide" space between the fibers allows the air flow to pass smoothly. However, due to the layer by layer arrangement, the fine particles will inevitably collide with one of the fibers, which will be adsorbed. At this micro scale, the intermolecular van der Waals force between fiber and dust is enough to overcome the influence of air flow and "stick" the two together.

     

    Different from the primary effect (g) and medium effect (f) filters, particles in HEPA filters will be effectively blocked and will never leave the fiber surface as long as they contact the fiber. This effect is called "interception". In addition, as long as the particles continuously entering the filter screen touch the particles that have been stopped, they will also stick to the existing particles. So over time, a layer of particles will accumulate on the fiber of the filter screen:

    image 

    Different functions of HEPA filter screen: what kind of fine particles are the most difficult to stop?

    1. The minimum particle dust with diameter less than 0.1 μ m will make irregular Brownian motion in the air due to its small weight, and there will be some differences in the direction of air flow, so there are more opportunities to impact the fiber. This kind of filtration is called "diffusion", and this kind of particle is quite easy to block.

     

    2. Particles with a diameter of more than 0.3 μ m weigh a little more, so their blocking principle is different. When the air flow contacts the fiber, the air flow will change direction around the fiber. However, due to the large weight of some particles, they can not adjust the direction with the air flow, so they will contact with the fiber and will be blocked by the fiber. This kind of filtration is called "inertial impact", and this kind of particles are easy to block.

     

    3. Fine particles with a diameter of about 0.1-0.3 microns are the most difficult to block. They are most likely to follow the air flow, around the HEPA fiber, not easy to contact the fiber, and because of the larger weight, they will not shake in the air. The most difficult particles to block are called "the most penetrating particles" (MPPs).

     

    The filtration effect of HEPA filter screen of all grades is calculated according to the filtration effect of MPPs. For example, the filtering effect of H11 filter screen is more than 95%; this means that the filtering effect of the most penetrating particles is not less than 95%, while the filtering effect of particles smaller than 0.1 μ m and particles larger than 0.3 μ m will be higher (97%, or even 99%).

     

    After a long time, the gap between the fibers of HEPA filter screen will become smaller and smaller, and some gaps will disappear completely. In this way, the filter efficiency of the filter screen will not decrease, but the air resistance of the filter screen will increase, so that the air volume of the air purifier or the fresh air system will decrease, and the noise will increase. Therefore, HEPA filter screen must be replaced regularly. The particulate matter on the filter screen which has been used for several months can be seen in the following photos:

    image 

     

    The higher the mesh grade, the finer the fiber, and the smaller the gap between the fibers. As a result, higher grade screens are more likely to be blocked because the gap between the fibers quickly shrinks. In order to delay the service life of the high-efficiency filter screen, there must be a primary efficiency (grade g) filter screen or a medium efficiency (grade F) filter screen in front of the high-efficiency HEPA filter screen, otherwise the high-efficiency filter screen will be blocked by large particles quickly:

    image


    The effect of HEPA screens depends on many elements:

    -Fiber diameter

    -Fiber density

    -Material of fiber

    If the fiber is very thin and dense, the particles are more likely to contact the fiber. And the interception effect of different materials is also different. In addition, the dust capacity of the filter screen is also very important: a good filter screen should effectively block PM2.5 for several months before it needs to be replaced.

    When purchasing fresh air purification system, the following problems should be paid attention to:

    1. PM2.5 filtration efficiency (grade of HEPA filter screen).

    In order to effectively filter PM2.5 in air, manufacturers of fresh air system usually use H11, H12 or H13 filter screen. The H11 filter can filter 95% of the most penetrating particles (0.1-0.3 μ m), while the filtration efficiency of other particles is close to 100%. H12 screen will block more than 99% of the most penetrating particles, but its resistance will be greater. Although H13 filter can filter 99.97% PM2.5, it will increase the wind resistance of the whole system a lot, so it is unnecessary to use this filter in domestic fresh air system and air purifier.

     

    2. Wind resistance of HEPA filter screen

    The higher the filter grade is, the greater the resistance is. Therefore, there is no need to use H13 or higher filter screen in the household fresh air system: such filter screen has too much wind resistance, and the filtering effect is redundant for ordinary families. The purpose of the filter screen is limited to the operating room, the dust-free workshop for the manufacture of electronic original parts, etc.

     

    3. Dust capacity of HEPA filter screen

    The PM2.5 value of the first test air outlet is the most superficial data. What many users won't notice is the size of the screen. The relatively large and thick filter screen generally has a large opening area, so the dust capacity will be larger and the service life will be longer, so it does not need to be replaced frequently.

     

    4. Replacement cost of HEPA filter screen

    Some fresh air system manufacturers will deliberately raise the price of consumables and lower the price of equipment, hoping to make money on the filter screen after selling the equipment. However, the general manufacturer will tell the user the replacement frequency and price of each filter screen, so that the user can judge the replacement cost of one year by himself. If the equipment has a cleanable primary filter screen, the cost of replacing the filter screen will be lower.

     

    After understanding the principle and function of HEPA filter screen, you will be easier to choose the fresh air purification system suitable for you.

     


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