Boxfan Ambient Air Filter Hack Revisited

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Blog series by clagwell updated 06-28-2020 05:00 PM 6 parts 4517 reads 18 comments total

Part 1: Seven years ago...

06-27-2020 12:00 PM by clagwell | 6 comments »

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 wan...

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Part 2: New Tests and New Filters

06-27-2020 01:38 PM by clagwell | 2 comments »

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. (...

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Part 3: New Fan and More New Filters

06-27-2020 04:50 PM by clagwell | 0 comments »

New fan: The first thing to determine is how much effect the fan shroud has. Testing the same set of filters as the previous post yields this: With no filter there’s no real difference with the shroud. It increases flow with the MERV 12 by about 14% and 12% with the addition of the MERV 8. That’s not a huge improvement but adding the shroud wasn’t very difficult, at least with the old fan. Unfortunately, the new fan has a different grill that protrudes into the met...

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Part 4: Two Fans Are Better Than One

06-27-2020 06:34 PM by clagwell | 4 comments »

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 n...

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Part 5: CFM and Efficiency - Part A, the Fan

06-28-2020 12:08 PM by clagwell | 6 comments »

Cleaning the air in your shop, whether with an exhaust fan or ambient air cleaner, is a form of dilution ventilation. That is, clean air is introduced into the room and mixes with the dusty air. The concentration of dust decreases gradually. It doesn’t all leave at the same time. Think of a barrel of dirty water. The best way to change that to clean water would be to empty the barrel and refill it with clean water. But if you can’t do that the next choice is to run clean ...

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Part 6: CFM and Efficiency - Part B, the Filter

06-28-2020 05:00 PM by clagwell | 0 comments »

In the last post we looked at the effect of CFM on the length of time it takes to reduce the dust concentration in a room. That applies to either venting outside and bringing in clean air or passing the air through a perfect filter. Lacking a perfect filter we have to ask what effect the filter capture efficiency has on the air clearing rate? Let’s start with the example from the previous post and assume 1,000 particles per cubic foot to start. At 500 CFM we’re sending 500,0...

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