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Boxfan Ambient Air Filter Hack Revisited #4: Two Fans Are Better Than One

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Blog entry by clagwell posted 06-27-2020 06:34 PM 602 reads 0 times favorited 4 comments Add to Favorites Watch
« Part 3: New Fan and More New Filters Part 4 of Boxfan Ambient Air Filter Hack Revisited series Part 5: CFM and Efficiency - Part A, the Fan »

Well, that’s satisfying. Two fans nearly doubles the CFM with the MERV 12/MERV 2 combination. The point of using the MERV 2 prefilter is to make the more expensive MERV 12 last longer before needing replacement so I didn’t bother with testing the MERV 12 alone for this round.

Of course what’s shown needs a bit of work to make it useful but I think the concept is valid. Fans are cheap. The pressure adds nicely at lower flow rates. Note that there’s not nearly as much gain with the lower resistance MERV 8 so it looks like 400 to 600 CFM would be a good target for stacking filters and fans in whatever combination you like.

To my surprise, it looks like this hack is actually comparable in performance to a low end ambient air filter. The MERV 12 is probably even a better filter than what they have.

The next post will deal with the effect of CFM and filter efficiency on overall performance and will get a little math intense, so be forewarned!

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?



4 comments so far

View Redoak49's profile

Redoak49

4754 posts in 2767 days


#1 posted 06-27-2020 07:53 PM

Could you explain how you measured and calculated the cfm. Is this the average across the surface? Did you use the test rig from the previous post?

View clagwell's profile

clagwell

250 posts in 570 days


#2 posted 06-27-2020 08:29 PM

I used the test rig as shown in part 2. I did a 20 point traverse. Four by five across the inlet. At each point I used the averaging function of the hot wire anemometer. I then averaged those twenty readings. Filters were mounted on the intake side. Measurements were made on the intake side.
The HWA generated a .csv file with the data that I imported into a spreadsheet and averaged the traverse points there.

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?

View Redoak49's profile

Redoak49

4754 posts in 2767 days


#3 posted 06-27-2020 08:45 PM

When I did my tests, I found some strange flows in the corners and near the edges with air going the opposite direction. Also, the air flow near the center drops. I think these fans create strange airflow that are difficult to measure. That is why I just presented the figure I did in one of your posts.

View clagwell's profile

clagwell

250 posts in 570 days


#4 posted 06-27-2020 08:52 PM

Yes, I found the same thing. It’s typical behavior for an axial fan. That’s why I used a duct and moved the measurement plane away from the fan and to the intake side of the filters. The flow is much more uniform there.

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?

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