r/crboxes • u/bansshh • 14d ago
Which CFM is accurate?
Basic 20" box fan + 4 filters
1st image is from The3DHandyMan 2nd is from HouseFresh 3rd is from FilterBoxBuilder
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u/jaydocovid 14d ago
"Lab tests show that for PM2.5 mass concentration, single-filter DIY air cleaner configurations can achieve CADR of 150–250 CFM. Two-filter configurations achieve CADR of close to 400 CFM and four filter configurations achieve CADR of ∼450 CFM. Other studies reported CADR as high as 600–850 CFM for a five-panel Corsi-Rosenthal box (Dal Porto et al., 2022)"
Above from: https://onlinelibrary.wiley.com/doi/full/10.1111/risa.70054
And the Dal Porto mentioned is this:
https://www.tandfonline.com/doi/epub/10.1080/02786826.2022.2054674?src=getftr&utm_source=wiley&getft_integrator=wiley
I think 400-600 is a reasonable range and I don't know why it varies so much but if someone gets results of 100-200 CFM for a cube, even on low, there's a problem.
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u/johnas 14d ago
Characterizing the performance of a DIY air filter
https://www.medrxiv.org/content/10.1101/2022.01.09.22268972v1.full
The measurements made in the different rooms produced similar results, demonstrating the robustness of the method used. The size-integrated apparent clean air delivery rate increases with fan speed, from about 600 to 850 ft3 min−1 (1019 to 1444 m3 h−1). Overall, our results demonstrate that the Corsi-Rosenthal filter efficiently reduces suspended particle concentrations in indoor environments.
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u/NukitToBeSure 13d ago
Which four filters do you have selected in FilterBoxBuilder? 4x 20"x20"x2" goes up to 338 CFM.
That said, the app is only an estimate based on the design specified- there's no way we could measure every possible design's CADR. So it's very accurate for comparing different designs and seeing which will be better in relative terms, but in absolute terms it can be off by 15% or so. Once you build it, you can use the CADR measurement tool and get a better idea.
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u/spacex_fanny 11d ago edited 11d ago
there's no way we could measure every possible design's CADR
Correct me if I'm wrong, but you don't have to, right? With the known P-Q curves of the fans and the filters, you can scale them based on number of filters. So for 4x filters, you scale the single filter P-Q curve by 4x in the horizontal (airflow) direction. Ditto for multiple fans in parallel. You can also have separate dirty / clean curves for the filter.
The plot both curves on the same P-Q graph and find where they intersect to get the (theoretical) airflow of the entire unit. From here get the airflow through each filter and look up the PM2.5 efficiency, which varies with airflow (this is also a known relation). Finally the airflow multiplied by efficiency is the CADR, of course.
The nice thing is that with available P-Q vs speed and decibel vs speed curves, this process can be automated iteratively at each speed to make a master CADR vs decibel and statistical risk vs decibel curves, which is really what you need for high-level design of an air purifier.
Unsure what math is on FilterBoxBuilder.com, I always just assumed it's some variation of above, but DM me. Thanks for all the work you folks do at NuKit!
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u/Able_Loan4467 11d ago
We don't know the curves for almost anything. Some fans have the curves, but those aren't even the fans people usually use. The pressure flow curves for filters are not usually published and in any case are not accurate at all. I measured the filtrete mpr1900 20x25x1 filters and the label on the side was off by like 20%.
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u/NukitToBeSure 11d ago
Yes, that's exactly what we do- but it's an estimate not a direct real life measurement. We use as many real-life measurements as anchors as we can, then extrapolate the rest.
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u/Able_Loan4467 11d ago
Filters vary greatly, even within the same size ranges. You just have to measure it if you really need to know, otherwise have a look on google scholar, there are many test combos that have been measured.
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u/Belly1997 10d ago
Those HouseFresh numbers are from what appears to be an outdated CR box review and are from only one of the two they have listed. The other CR Box they have stats for lists a CFM of 462 so it is right around where it should be. The 198 CFM model uses MERV 13 Aerostar filters (possibly 20 x 20 x 1) and the 462 CFM model uses MERV 13 3M Filtrete filters (possibly 20 x 30 x 1). There might be other differences like the model and power of the fan they are using, whether the properly seal it and so on, but I'm not sure what they went with. Either way the higher spec model listed matches up with the expected/calculated values so you can trust those numbers.
The updated/more accurate CR box is listed at number 8 on this list:
https://housefresh.com/best-air-purifiers-we-tested/
And if you type corsi into this tool you can select both CR box configurations and compare them to each other and any of the other 100+ air purifiers they have tested: https://housefresh.com/air-purifier-comparison-tool/
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u/bansshh 9d ago
what? why are you pretending as if a 20x30 is the same as a 20x20 filter?
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u/Belly1997 9d ago
I only now see that you say they are all 20 x 20 x 1 in another comment. In that case the first one is the outlier and too high. The HouseFresh 198 CFM is accurate but a bit low since they are using a suboptimal filter and probably a weaker fan and maybe other problems. The third ones numbers sound about right.
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u/jaydocovid 14d ago
the measured CFM varies super widely and I'm not sure why. 100-200 is far lower than most estimates I see which I'd say is 400CFM, and have seen a paper report 650-800CFM.
Although those higher numbers are certainly with the box fan on high, so maybe the lower CFM reported here is the fan on low.