Should purifiers have the filter on the front or back?

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When I made my first DIY purifier, I put the filter on the front:

 

 

It turned out that I wasn’t the only one interested in breathing clean air in China, so I started a small social enterprise called Smart Air to ship these low-cost purifiers to fellow-air breathers who don’t have $1,000 to blow on an IQ Air. In the last four years, we’ve shipped over 30,000 of these simple machines to people all over China, and we’ve done hundreds and hundreds of tests comparing different designs.

Should the filter go in front or back?

But wait. After I published instructions for how to make it and test data, a ton of people have told me I’m wrong–I should have put the filter behind the fan! Duh! Here’s a more-respectful-than-usual suggestion on my live test video:

 

 

I’m fascinated by how many people have this intuition. But when I push further, most people can’t come up with a good reason. For some people, I think it’s purely an intuition about having air come from a pure source. But here are the two most coherent reasons I’ve heard:

Claim 1: It “protects” the fan.

This is probably the best reason I’ve heard. Yes, putting the filter behind the fan will prevent dust from hitting the motor and collecting on it. I think there’s a small, non-zero benefit of that, although I haven’t seen anyone actually test it.

Gut check: people run “unprotected” fans (we just call them “fans”) all the time, and it doesn’t seem to cause much harm.

Claim 2: It improves performance.

This is an odd idea to me. I’ve asked several physics PhDs, and they’ve said it shouldn’t make a difference. The amount of air coming in is equal to the amount of air coming out.

But who knows? Maybe there’s something about the air flow process that I don’t understand. I don’t like when people point to a theory and assume that proves the matter, without actually testing it. So I went out and tested it!

It’s hard to test with that first fan I used (above) because putting the filter on the back would require me to tape around the back screen. That would alter the fan, so we can’t compare apples to apples.

Fortunately, I tested lots of different fans.

And eventually I settled on this tube-shaped fan I call the Cannon.
It’s perfect for this test because I can easily put the HEPA filter on the front or back. So that’s what I did! I ran 20 tests of 0.5 micron particulate in my apartment in Beijing. I also had a third-party lab test the two versions in a CADR test.

The results? Putting the HEPA on the intake side actually did slightly worse! I was surprised. I thought they’d perform the same.

Now this next part is hard for me to understand, but the Smart Air team is fortunate to have an aeronautical engineer. Here’s an important detail about the Cannon fan I didn’t mention:

 

The fan isn’t in the middle. Instead, it’s about 2/3rds of the way down the tube. And according to our engineer, it helps to have more distance between the fan and the filter so that a column of pressure can build up.

Again, I still haven’t grasped the physics, so I invite anyone with more expertise to add their explanation here. But two independent datasets show that putting the HEPA on the front leads to higher reductions in particulate.

If the fan were in the exact middle, we hypothesize that performance would be equal with the HEPA on the front or the back.

Conclusion: Putting the filter on the front or back of the fan should provide the same reductions in particulate, as long as the distance from the fan is the same.

Input-Output Testing

I think this is a fascinating demonstration of how I bumbled into a good design. I didn’t know much about air flow dynamics, but I did lots of input-output tests before I chose this setup. In other words, I was agnostic about what was going on inside the fan, but I tested different fans, measured the output I cared about (reduction in particulate), and that led me to a good design—even though I couldn’t explain why at the time.

Speculation: Does turbulent air flow explain why “filter first” is worst?

Now we’re not 100% sure on this, but based on research on guide vanes, putting a filter before the fan will make the air going into the fan turbulent. Making the fan operate in turbulent air flow (as opposed to laminar flow) has two consequences:

A. One is that the efficiency of the fan is reduced.

B. The other is that the fan will be noisier.

However, we don’t know whether this effect would be large enough to be meaningful.

Putting the filter after the fan means turbulent air (from the fan) is going into the filter. We’re unsure what effect that has on the efficiency of the filter.

Thanks to Paddy Robertson for his expertise in helping me understand this question.

Thomas Talhelm

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

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