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Mechanical filtration (HEPA)

The concept of air pollution is not new and it has history of itself. Particle pollutants in the air have been of concern at least throughout our calendar years: in A.D. 61, the Roman philosopher Seneca complained about the miasma of chimney smoke that constantly hung over Rome. In 1306, King Edward I of England banned the burning of coal in London due to the heavy pollutants left in the air. Time revolutionize the solution of every other problem. Similarly, air pollution solutions have been revolutionized throughout the centuries.

The first air filter was a protective air respirator that was worn over a person’s mouth and nose. Its purpose was to filter air breathed through the unit, thereby protecting the wearer from inhaling any gases, fumes, vapors or harmful dusts particles. This concept was developed in the 16th century. The concept was developed in late 1800 century when a patent was granted for respiratory masks providing fresh air for uses of Turkish baths. Air pollutant particles are diverse and can range from visible to non-visible particles. Some of them are visible but most of the particles, that can damage human health, are not visible to naked eye.

Hence, it was necessary to develop such techniques that can tackle particles of such size. During the late 1940s, the USA developed the first HEPA filters to aid in the protection against radioactive chemical warfare agents. These “collective protector filters,” were later renamed “HEPA filters,” which was an acronym for “High-Efficiency Particulate Air” filter units. After World War II, the U.S. government declassified the HEPA filter technology, allowing it to be used for commercial and residential use. HEPA filters are composed of a mat of randomly arranged fibers. The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting function are fiber diameter, filter thickness, and face velocity. The air space between HEPA filter fibers is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Unlike membrane filters at this pore size, where particles as wide as the largest opening or distance between fibers cannot pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles.

HEPA filters have very high particle removal efficiency (better than 99.97%) for all particle sizes. But one must remember the cost which is low air flow through the filter. This means very large filtration areas like laminar flow ceilings in hospital’s operational area. Another problem is noise: high pressure drop in the filter requires efficient fan to push air through the filter. This means high energy consumption and loud noise. Neither of which is a nice feature in a household purifier.

Mechanical filters were first used in removing particulate matter from industrial gas streams (process intake and process outlet). Patent databases currently show almost 30,000 patents with words air+filter in the text. There are also several hundred patents on purifiers based on mechanical filtration – the latest ones using a concept like a robot vacuum cleaner making the air purifier to wander around the house.

Mechanical filter is exactly what the name indicates: a filter which captures solid airborne particulates based on their mechanical properties – mainly impaction and diffusion. Before studying these two mechanisms we must remember one more thing: gravity. This force tends to remove heavy particles from the air. Due to this, only filtration of particles smaller than 10 µm (10-6m), is considered. Impaction arises from particle mass: at a curve, larger particles tend to move directly and not follow curved gas streamline. This is used in advantage in filters and in practice it affects particles larger than 1 µm in diameter.

Diffusion arises from bombardment of particles by gas molecules. This makes small particles to follow a zigzag rather than a straight route in the air. Inside a filter, zig-zagging drives small particles to filter fibers, where they get bogged down. Diffusion is generally effective for particles smaller than about 0.1 µm. Particles, which easiest penetrate through a filter; follow both impaction and diffusion poorly. This leads to the fact that one should mainly be concerned of filter’s (or purifier’s) efficiency in removing the particles sandwiched in between diffusion and impaction, i.e. from 0.1 to 1 µm.

In this jungle of strange inventions, one should stay calm and go with the facts. Clean Air Delivery Rate (CADR) is a figure of merit that is the volume over time (like m3/h) of clean air flowing out from a purifier. In other words: CADR is the particle removal efficiency (ε) of the purifier times the volumetric flow of air through the purifier. Air purifier with 90% efficiency and volumetric flow of 600 m3/h has CADR of 540 m3/h. This is much better than a HEPA-based purifier with practically 100% efficiency and 150 m3/h volumetric flow – ending CADR to be 150 m3/h, less that 30% of the CADR of the first purifier. Using CADR, consumers are less likely to be misled by a high efficiency filter that is filtering a small amount of air, or by a high volume of air that is not being filtered very well.

In the next blog we’ll have a look on electrostatic precipitators used for room air purification