How does an electrostatic filter work?

Electrostatic air filters are intended to filter particles, which are considered infectious for a patient. These particles can be of different types, like pollen (10-30 mm), which can cause asthma or allergenic responses, street dust (about 10 mm), which can bear bacteria or virus or be allergenic, water droplets, which are containing bacterias or viruses (1-20 mm). Also single bacteria (1-10 mm) and viruses (0,3-3 mm) are possible.

All medical air filters are made of a matt of fibers, usually synthetic or glass fibres. Electrostatic charged filters are made of synthetic fibers. Mechanical filters can be made of both types.

All these particles pass into the filter matt in the various openings between the fibers. The particle will collide with several fibers on its way through the filter matt until it loses its speed (inertia) and is resting on a fiber inside it. The possibility for a filter to catch a particle will depend on the initial speed and size (mass) of the particle, the number of fibers available for catching it, as well as how much speed (inertia) it loses at each collision with a fiber.
If a fiber causes the particle to lose more speed on collision with the fiber, fewer fibers are needed to catch the particle.

Electrostatic filters contain fibers with permanently charged areas on the surface. These charges can be either positive or negative. When a particle moves through a matt of such fibers its surface touch the charged fibers. The particle surface is then polarized by the charges on the fiber in such a way that the particle will get more positive or negative areas. The negative areas of the particle are attracted to the positive areas of the fiber surface and vice versa. Also with particles, which contain permanently charged areas from the start, will have its positive areas attracted to the negative areas of the fiber and vice versa.

The result of this charging and attraction will be that the particle will lose more speed (inertia) at each collosion with an electrostatic fiber than when colliding with a noncharged fiber (as used in so called mechanical filters).

Thus an electrostaically charged filter needs fewer fibers than a noncharged filter to catch a certain particle. The positive effect of fewer fibers in the filter will be a more open structure of the filter with less breathing resistance for the same filtering efficiency and filter volume.


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