Boxfan Ambient Air Filter Hack Revisited #2: New Tests and New Filters

  • Advertise with us
Blog entry by clagwell posted 06-27-2020 01:38 PM 835 reads 0 times favorited 2 comments Add to Favorites Watch
« Part 1: Seven years ago... Part 2 of Boxfan Ambient Air Filter Hack Revisited series Part 3: New Fan and More New Filters »

Last year I acquired a Hot Wire Anemometer. I intended to do some dust collector testing with it but that didn’t work out. It seems a HWA is really a tool for HVAC work where the pressures and velocities are a lot lower. That should be perfect for testing a box fan filter.

In order to get the measurement points away from the complex flow patterns near the fan and have a well defined area for the traverse I fabricated a test duct with the vertical traverse calibration points. (Ok, so it’s just a cardboard box with some packing tape and marked with a Sharpie. It still works.)

The HWA is shock mounted to an orthogonal motion traverse jig (well, it’s actually a cheap microphone boom stand with the shock mount mic clip that happened to be attached when I grabbed it but it does the job).

I tried both directions for the fan and got the most consistent results on the suction side. I used twenty traverse points. The HWA includes an averaging function so I averaged at least ten readings for each point. That’s a minimum of 200 readings for the total traverse.

The old filter was packed full of drywall dust so I bought a new one, a DuPont MERV 12.

With the new test rig I measured 980 CFM for the fan by itself. Adding the MERV 12 filter reduced that to 250 CFM. I still have the prefilter I bought originally so I added that and got 190 CFM. The prefilter might be about a MERV 8, I’m not sure. Retail MERV markings were not common seven years ago.

I tried cleaning the old filter, which I think was probably a MERV 13, but it tested 300 CFM. Subjectively it was much more than I remember from when it was new. More than likely it was damaged by the high velocity of the shopvac. A backlight shows plenty of damage to the filter media.

So it may have been MERV 13 when new but it certainly isn’t now.

The next thing to revisit is the effect of the shroud. I could remove it and retest but with the price of the fans it makes sense to just buy another one. That’s next.

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

2 comments so far

View Redoak49's profile


5393 posts in 3234 days

#1 posted 06-27-2020 04:06 PM

The results will be interesting but one of the issues is that you have turbulent flow and difficult to avoid.

View clagwell's profile


387 posts in 1037 days

#2 posted 06-27-2020 04:30 PM

The Reynolds number at 250 CFM in that duct is about 17,000, so yeah, turbulence is impossible to avoid. Turbulence by itself is not really the issue. I think it’s the rotation that causes the biggest errors, especially with a hot wire anemometer which is not particularly sensitive to direction. Proper testing would use an air straightener but that requires a booster fan to make up the loss. That’s way beyond the scope of this little project!

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

Have your say...

You must be signed in to post the comments.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics