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Boxfan Ambient Air Filter Hack Revisited #1: Seven years ago...

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Blog entry by clagwell posted 06-27-2020 12:00 PM 363 reads 0 times favorited 6 comments Add to Favorites Watch
no previous part Part 1 of Boxfan Ambient Air Filter Hack Revisited series Part 2: New Tests and New Filters »

A few years ago I decided to try the trick of slapping a furnace filter on the back of a box fan to make an ambient air cleaner. I wasn’t at all happy with the performance. It seemed like the filter almost completely choked off the airflow.

The filter clamps can hold up to three inches of filters but I could barely get any air through a single filter let alone two or three.

Air flow rate is at least as important in an ambient air cleaner as filter efficiency so I really wanted to know how much air was actually being filtered. I bought a cheap ($18) vane type anemometer:

Turns out that it’s difficult to get good readings close to a fan and, in addition, the anemometer vane had a lot of friction and couldn’t read low velocities at all; it simply stopped spinning in slow air. It quit working altogether after a few years and I was happy to get rid of it.

My measurements were rough, but I got around 800 to 1000 CFM for the fan alone. Adding the one high quality (or, at least, expensive) filter dropped the flow to about 80 CFM. The addition of a prefilter made the flow unreadable with that anemometer.

One thing I noticed while moving the anemometer around the front of the fan was a reversal of the flow direction in the corners when the filter was on the back. So the resistance of the filter was causing air circulation around the blade tips front to back. That reminded me that ducted fans are used for high thrust/low velocity applications, like hovercraft.

So I added a shroud to the boxfan.

When I checked the airflow I got about 120 CFM through the filter. A 50% increase is good but not spectacular. In my 6000 cubic foot shop that’s only 1.2 room exchanges per hour, hardly good ventilation.

That was seven years ago. I’ve used that air cleaner a few times but not as a shop air cleaner. I now have much better tools for measuring airflow so I thought it would be interesting to test this thing again. That’s for the next post.

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



6 comments so far

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Redoak49

4754 posts in 2766 days


#1 posted 06-27-2020 12:41 PM

I measured a similar box fan and filter and had a huge variation of cfm over the surface. I will try to find the map of cfm that I made.

View clagwell's profile

clagwell

250 posts in 570 days


#2 posted 06-27-2020 01:51 PM


I measured a similar box fan and filter and had a huge variation of cfm over the surface. I will try to find the map of cfm that I made.

- Redoak49


Yes, that’s a real issue. Any numbers obtained are very rough and even questionable when used only for comparison purposes. In the next blog post I show my new setup. It yields much more believable results. Here’s a map of the velocities from one traverse using the new setup:

-- 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 2766 days


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

Her is some data that I got from a 20” Lasko Fan on the Medium setting. I took the readings directly on the face of the fan at nine places across and up and down for a total of 81 readings with a fan anemometer. The filter when used was placed on the incoming air side. There were areas where the air was flowing backward.

First with no filter…

Second with a pleated filter

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clagwell

250 posts in 570 days


#4 posted 06-27-2020 05:02 PM

Oh, nice data map! Thank you for posting those. Can I steal them? ;)

That really illustrates the issues I had doing that (although I didn’t have the patience for 81 data points!). That reverse flow in the corners is what prompted the addition of the shroud as well as adding the testing duct. The shroud reduces that recirculation and the duct isolates those effects from the measurement plane.

Thanks again, those really clarify the difficulties with flow measurements.

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

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Redoak49

4754 posts in 2766 days


#5 posted 06-27-2020 10:00 PM

Given the air flows in FPM that I measured and averaging over the area, I go1147 CFM with no filter and 624 cfm with the filter.

I really struggled with the data presentation and tried all kinds of graphs and ended up with the simple color coded data square.

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clagwell

250 posts in 570 days


#6 posted 06-27-2020 10:19 PM

1147 CFM isn’t surprising, especially if you’re measuring on the downstream side of the fan with the filter on the intake side. I had higher readings on that side along with the wild variations that you saw. I decided back then to make all of my measurements on the intake side of the filter. There was a lot less variation across the filter and it’s really the flow into the filter that counts, not what comes out of the fan.
That’s a good flow rate for a filter. What kind of filter was it?

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

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