Powerful, thorough, quiet: How do air purifiers work?

Air purifying in different stages

As well as droplets and aerosols that spread the coronavirus and other diseases, fine dust, mould spores, pollen and other health hazards can pollute indoor air. Air filter devices equipped with corresponding filters remove these pollutants: They draw in the air with a fan, push it through various fine filters and direct it back into the room. In order for a filter device to meet the requirements for filter performance, air flow rate and noise emissions, its design must be well thought-out and balanced.

Air filters are usually made of cellulose, fine-mesh polyester or glass fibre media. They are divided into filter classes based on their separation efficiency and the size of the smallest particles they filter out of the air. Coarse dust filters serve as pre-filters in air purifiers. They trap larger impurities such as dust or hair that would otherwise quickly clog the finer filters downstream. After this, fine dust and other small particles are separated by fine filters. Particles smaller than 1 µm, such as aerosols and viruses, only get trapped in HEPA filters. This topic is looked at in a separate technical article. Class H 14 HEPA filters have a separation efficiency of at least 99.995%.

With the help of activated charcoal filters, gaseous pollutants can also be removed from the air. Activated charcoal is very porous and therefore has a large inner surface area: For 4 g of activated charcoal, this corresponds roughly to the area of a football field. Through this inner surface area, activated charcoal adsorbs pollutant molecules such as radon or chemical vapours. At the same time, it acts as a catalytic converter and accelerates the decomposition of gases such as ozone.

Sound insulation is expensive but necessary

The fan must be designed for the high device counterpressure generated by the filters being passed through in sequence, especially the fine H 14 filters. Since the counterpressure increases as the filters become more soiled, it should also have some power reserves. In order to sufficiently reduce the viral load in the room, the fan must circulate the room air six times per hour, according to Prof. Kähler from the Bundeswehr University Munich. On the other hand, HEPA filters must not be subjected to too high an air flow rate, as this would impair their performance.

The fan and the air flow inside the air purifier both lead to high noise levels. Sound insulating elements such as noise-absorbing mats, acoustically decoupled fans and silencers at the air inlet and outlet reduce the operating noise, but are expensive and take up a lot of space in the device. Even so, depending on where the air filter device is placed, effective sound insulation is essential. While a loud air purifier can still be tolerated in a foyer, it is very disturbing in a meeting room. Large fan blades reduce noise emissions because they displace more air at low speeds.

With UVC lamps and ionisers, the design is all-important

There are some things to consider if UVC lamps are used in an air purifier along with filters to get rid of mould, bacteria and viruses. The UVC irradiation must be powerful in order to achieve any disinfecting effect at all. Even the strongest UVC radiation is ineffective against UV-resistant bacteria though. The special lamps must also be completely encapsulated. This prevents the powerful light from radiating out of the device and damaging the retina. Furthermore, the UVC rays should not fall on plastic parts, as plasticizers may be emitted.

As well as that, some models are equipped with ionizers. These produce electrically charged molecules that adhere to pollutants such as house dust or pollen, and bind them together. This makes the particles heavier, and they fall to the ground. However, ozone is produced when ions are generated. This is a colourless gas that can cause headaches, coughing and breathing difficulties, and can be dangerous for asthma patients. Ionizers should therefore collect the ozone in the device instead of releasing it into the room air.