HEPA filters (igh Efficiency Particulate Arrestance) are distinguished by the presence of several degrees of air purification, including most pathogens of allergic reactions greater than 1 μm: pathogens of respiratory diseases, fungal spores, microparticles of animal skin or hair, pollen and waste powder mites products.
Fine HEPA filters are made of a special material whose fibers are intertwined in a special order to trap pollutants from the air. The properties of the material, its thickness and pore size determine the cleaning class and the efficiency of the filter. MORE ABOUT HOW TO CHOOSE THE RIGHT AIR Purifier HERE.Talking about filtration, one usually imagines a fine grille or sieve that lets in particles of the same size and does not leak larger pollutants. The HEPA filter is designed differently and purifies the air from particles smaller than the distance between the filter fibers themselves. The filter material itself is composed of layers such as “harmonic” and its fibers are arranged in a non-linear manner so that the air flow passes repeatedly through these filter fibers. The effectiveness of the filter is also influenced by the fibers themselves: material, diameter and density of application. A HEPA filter can be used in conjunction with a pre-filter (usually carbon-activated) to extend the usage life of the more expensive HEPA filter. In such setup, the first stage in the filtration process is made up of a pre-filter which removes most of the larger dust, hair, PM10 and pollen particles from the air. The second stage high-quality HEPA filter, which filters out the finer particles that escapes from the pre-filter. You can see what a device with such filters looks like from this link.
The purification mechanism of HEPA filters is based on three physics processes
PROCESS – 1 DIFFUSION. The smallest particles with a diameter less than 0.1µm, that is, their size is smaller than the distance between the filter fibers, are constantly in a chaotic motion. Roughly speaking, their mass is so small that, in addition to moving in the general direction of the air stream, their trajectory is constantly changing. As a result, when the total air flow goes around the filter fibers, the smallest particles are thrown out of the flow due to their random motion and are trapped on the filter fibers.
PROCESS – 2 INERTIA. Particles with a heavier diameter of more than 0.3µm fall into the inertial filter fibers. The total air flow around the obstacles that the filter material creates and the “ coarse ” particles fail to quickly change their direction of movement, causing them to become trapped in the filter.
PROCESS – 3 HANGING. Particles that are too large for diffusion and too small for inertia do not get stuck in the filter material passing through it. But because of the structure of the microfibers, the particles do not have to be stuck in them – just touch them. Touching the microfiber, the particle adheres to it, the next particle adheres to it, the next particle to it, etc. thus cleaning the middle particles. In practice, all three processes occur simultaneously and affect all particles, regardless of their size. The separation is theoretically performed, since the efficiency of each process on each type of particle depends on their size.