What is a HEPA Filter: Operating Principles and Non-Obvious Facts

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These are high-performance filters, whose main purpose is to remove fine particles from the air, including PM2.5 and PM10 (with a diameter of less than 2.5 and 10 microns, respectively). HEPA is not a brand or brand, but a class of filters defined by the international standards EN 1822-1: 2009

Let’s take a look at the HEPA filter, tell you about the principle of its operation and the main effects due to which particles are deposited on the filter.

The basis of any HEPA filter is randomly arranged fibers of different thicknesses, approximately 0.5-5 microns. The distance between the fibers is about 5-50 microns. The diameter of fine particles is within a few microns or even a few fractions of a micron. The question arises: how does a filter with such large pores trap such small particles?

We usually think of the filter as a fishing net or net: if the object being filtered is larger than a cell, it gets stuck. This mechanism is called straining. It works for particles whose diameter exceeds the pore size in the filter. 

How does a HEPA filter “catch” fine dust?

The main difference between HEPA and coarse and fine filters is that the particle does not need to get stuck in the fibers to filter. If a speck of dust just touches the filter material, this is already enough for effective precipitation. This is due to two processes: adhesion and autohesic.

Adhesion is the interaction of dust with the settling surface, in our case with HEPA fibers. Due to the adhesion, the first layer of dust appears on clean fibers.

Autohysis – or stickiness, is the interaction of dust particles with each other. Due to autogenic interaction, the particles continue to layer on top of each other, forming multilayer conglomerates on the fibers. 

The nature of adhesion and autohesion lies in the molecular interaction of particles with each other and with fibers (Van der Waals forces). These forces appear at a distance of one to several hundred particle diameters. For the smallest particles, the attraction to the fiber and dust layer is so great that the particles settle in the HEPA filter virtually forever. The figures confirm this: for particles less than 10 microns, the tensile strength of the dust layer is more than 600 Pa.

So, due to the forces of attraction, the particle sticks almost tightly to the fiber of the HEPA filter, it is only necessary to touch its surface. This explains the retention of particles on the filter, but still no answer to the question:

How do the smallest particles touch the HEPA filter fiber?

As we found out, the sieve effect has nothing to do with it – the smallest particles fly freely through the pores. Other mechanisms operate in HEPA filters.

Any particle is trapped in the air flow, and if there are no forces in the filter that deflect the particle from the air flow line towards the fiber, then there will be no precipitation. As a result, the particle will pass through the filter along with the flow. Therefore, the question “How do the particles touch the fiber?” can be paraphrased as “How do the particles get out of the air stream?” and the answer to it will be different, depending on the size and mass of the particle.

The smallest particles (with a diameter of less than 0.1 microns) have a small mass and are constantly in chaotic Brownian motion. Their trajectory constantly fluctuates relative to the air flow line. During vibrations, the particle exits the flow, touches the fiber, and is deposited. 

The entanglement effect works when the particle has approached the fiber surface by a distance of its radius. Such a touch is sufficient for its deposition. This mechanism works not only for MPPS. It is universal and works for particles of any size. Dust particles can remain in the air stream, make diffusive vibrations relative to the stream line, or fly out of the stream due to inertia – in any case, if the particle touches the fiber, it is deposited.

The effectiveness of this mechanism depends on the particle size. The larger the particle, the more likely it is to touch the fiber. 

In fact, in a HEPA filter, all mechanisms act simultaneously on a particle, so the total efficiency of the HEPA filter is equal to the sum of the contributions of each effect:

general information = screen size + engagement point + coefficient of inertia + diffusion coefficient

If you constantly load HEPA with aerosol with large particles, the filter life is significantly reduced. This is due to the sieve effect: large particles quickly clog the filter and reduce its permeability. To avoid the screen effect, one or more prefilters of a lower class are installed before the HEPA filter: G and / or F. They protect the HEPA from premature clogging. If there are prefilters, then HEPA works strictly “by specialty” – filtration of fine particles. That leaves three effects:

general information = engagement point + coefficient of inertia + diffusion coefficient

What does the effectiveness of a HEPA filter depend on?

The effectiveness of HEPA depends not only on the size of the particles to be filtered, but also on the parameters of the filter itself:

  • Fiber diameter in the HEPA filter
  • Fiber packing density
  • Fiber Material

The thinner the fibers and the denser they are packed, the larger the area of their contact with particles. And the better the fibers “cling”, the more effective the deposition. If the material from which the filter is made has a high specific conductivity, then the fibers can be charged in the air stream. In this case, electrostatic attraction forces (Coulomb forces) arise between the fibers and the particles. They further increase the efficiency of the HEPA filter. We will not discuss this effect in more detail here, but we will talk about electrostatic deposition in another article.

As a result, the area of the fibers increases, and this is associated with a paradoxical fact: over time, the effectiveness of HEPA does not decrease, but increases. On the other hand, when the filter becomes dirty, its permeability decreases, its resistance increases, the pressure drop across the filter increases, and, as a result, the performance of the device in which it is installed decreases. If the filter is completely clogged and the performance of the device has dropped to almost zero, the only way out is to replace the filter. The frequency of replacement depends on the filter capacity. This metric determines how much dust the HEPA can deposit before the pressure drop across it becomes critical.

Now that we have an idea of the HEPA filter, let’s put together the principle of its operation in points:

  1. The filter gets an air stream with dust particles of different sizes, from 10 microns and less
  2. Large particles escape from the air stream due to the inertia effect, small particles-due to the diffusion effect
  3. All particles that come out of the flow and touch the fiber are deposited on the filter
  4. The particles are firmly held on the fiber due to the forces of attraction (Van der Waals)

We will also collect all the non-obvious facts about the HEPA filter in one place:

  • A HEPA filter can trap particles of all sizes
  • Dust stays in the HEPA filter almost forever. Vacuuming HEPA is useless – just change it.
  • Over time, the efficiency of the HEPA filter only increases.

That’s all for now: we talked about the principles of fine dust deposition and retention in HEPA filters. If you have any questions, we will be happy to answer them in the comments.

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