How it works
A CR Box consists of 4 Merv 13 (or F7 class) paper filters that are taped together to a box fan to create a literally multi-pass air filtration system. Merv-13 is chosen for this purpose because of the relatively cheaper price tag as compared to HEPA filters.
There are two important factors to consider when designing a proper air filtration system, filtration material and air flow (air exchange):
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Merv-13 filters have decent filtration ability
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Have high air exchange which will expedite the air cleaning process.
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Filtration Process
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The fan sucks air from the surroundings and passes it through the filters
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The filtered air is pushed back into the room by the fan
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The process is repeated multiple times, allowing the device to clean the air in the room.
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Will it work on SARS-CoV2?
Sure, SARS-CoV2 virus is approximately 0.1μm in size. Merv 13 filters can trap particles as small as 0.3μm. However, SARS-CoV2 do not exist alone, but usually bonded to a larger particle. Research shows that they normally bonded to aerosol particles ranging from 1μm to 5μm, well within the capability of the Merv 13 filters.
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Link: https://www.ashrae.org/news/ashraejournal/debunking-myths-about-merv-air-filtration
Have we tested our CR Boxes?
Yes, we have tested them using a very crude testing method. We found a small room and activated an aerosol source (in our case a nebulizer) and we measure the air particles using a simple laser aerosol particle counter.
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Particle measurement done prior to the activation of the nebulizer.
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Particles measured again after nebulization.
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Control arm: measure particle number continuously over 30 minutes
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Repeat with CR Box activated over the 30 minute period
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For comparisons, we also measure various commercially made HEPA filtration devices that we can find. (Note the commercial HEPA filters come in various sizes).
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The results are quite encouraging, as we found out that the CR Box either perform almost as good or even better than the more expensive HEPA air filters. We think this is due to the higher air flow from the fan as compared to the HEPA filters. We found out that the CR box was able to remove 88.7% of 0.3μm particles and 95.8% of the 2.5μm particles over the 30 minute period.
How big of a room can the CR Box accommodate?
Based on the estimated CADR that was calculated by indoor air quality experts and taking into account the smaller fan that we use, we assume that one CR Box would be enough for a 50-60sqm classroom. Of course the smaller the room, the more effective the device is.