HEPA Filters: Using physical tricks to capture particles

Developed for the military; essential in the industrial sector today

Air filters are classified based on the size of the particles they remove from the air. Coarse dust filters remove particles larger than 10 micrometres (e.g., hair and sand), while particle filters also capture particles measuring between 1 and 10 micrometres (e.g., pollen, spores, cement dust). High-efficiency particulate air filters, which include EPA, ULPA, and HEPA filters, capture even smaller particles. They remove particles with diameters of less than 1 micrometre (e.g., aerosols, bacteria, coronaviruses) from the air.

HEPA (high-efficiency particulate air) filters were developed in the wake of atomic weapons research in the 1940s. The new technology was created to remove even the smallest radioactive particles from the air. Today, HEPA filters are used in operating rooms, cleanrooms, laboratories, and aircraft air conditioning systems. Semi-conductor manufacturers, nuclear technology, and many other industries also rely on them to reduce air contaminants in research and production facilities to a minimum.

Particles adhere to the filter fibres

HEPA filters consist of a multi-layer mat of very fine fibres made of glass or other materials such as cellulose. A single filter fibre has a diameter of only 1 to 10 micrometres. Unlike the coarser prefilters, HEPA filters do not filter in accordance with the sieve principle. The fibre network of a HEPA filter is not homogeneous enough for that: the size of the gaps between individual fibres varies. Instead, they exploit the physical movement properties of different small particles in order to capture them.

• Inertia effect: Due to their inertia, larger particles do not follow the air current’s fine changes in direction as it moves through the fibre mat. They collide with the fibres instead and are attached to them as a result of adhesive forces.
• Blocking effect: Medium-sized particles follow the air current around the fibres, but at some point inevitably touch the fibre mat and stick to it.
• Diffusion effect: Particles smaller than 1 micrometre do not move evenly through the filter. Instead, as a result of diffusion their movement is chaotic: they constantly collide with gas molecules. As a result, the particles move far into the filter until they eventually get caught on a fibre.

By exploiting these physical effects, HEPA filters can also remove particles that are considerably smaller than the gaps between the individual fibres in the filter. Filter mats are usually folded before installation in order to increase the useful filter surface in a frame with a constant size. The diffusion effect in particular is increased by folding, which is beneficial for removing the smallest particles.

Only 5 in 100,000 particles escape an H14 filter

High-efficiency particulate air filters (EPA, HEPA, and ULPA) are classified in accordance with EN 1822. The higher the filter class, the more particularly heavy, filterable particles in the 0.1-0.3 micrometre size range remain in the fibre mat. A class H14 HEPA filter has a removal rate of 99.995%. Only 5 particles in 100,000 enter the filter and come out on the other side.

Alongside disposable filters, there are also filters that can be cleaned and reused. However, cleaning them often requires great effort and never removes all of the particles from the filter. And even reusable HEPA filters are designed for only a limited number of cleaning cycles. As reusable filters are usually more expensive than the disposable variants, they do not necessarily save money.

Orientation on filter classes makes purchase decision easier

Manufacturers have recognised the value of HEPA filters as an effective marketing instrument, and accordingly often highlight the keyword ‘HEPA’ when promoting their devices. The spectrum of HEPA devices ranges from air purifiers to household appliances such as vacuum cleaners. However, purchasers should exercise caution because ‘HEPA-based’ or ‘HEPA-like’ filters do not always deliver the filtering performance they expect. That is why those who require a filter with certified high separation efficiency should look for HEPA filter classes H13 and H14. The H14 classification is necessary when viruses are to be removed.

Due to their fine, dense structure, HEPA filters generate a high level of differential pressure. Not every filtering device has a fan that is strong enough to adequately flow through them. And HEPA filters should always be arranged downstream from prefilters. Prefilters capture the coarse particles that would quickly clog the final filter.