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Boxfan Ambient Air Filter Hack Revisited #5: CFM and Efficiency - Part A, the Fan

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Blog entry by clagwell posted 06-28-2020 12:08 PM 580 reads 0 times favorited 6 comments Add to Favorites Watch
« Part 4: Two Fans Are Better Than One Part 5 of Boxfan Ambient Air Filter Hack Revisited series Part 6: CFM and Efficiency - Part B, the Filter »

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 water into the barrel and let the dirty water overflow out. The water gradually gets clean but it takes a lot more than one barrel of water to do it and it never really gets completely clean. That’s what happens when you ventilate your shop.

So how quickly does the air clean up? Turns out that the change in dust concentration is yet another example of exponential decay with the decay constant equal to the room air exchange rate. For example, say your shop has a volume of 5000 cubic feet and you fan does 500 CFM. We multiply 500 CFM by 60 Minutes/Hour to get 30,000 cubic feet per hour. Divide that by 5000 cubic feet and we have 6 room changes per hour. So in 1/6 hour or 10 minutes the dust concentration is reduced by 63%. It goes down another 63% in the next 10 minutes, and again in the next 10 minutes. If you wait long enough there’s not enough dust left to bother with. That 1/6 hour or ten minutes is also known as the time constant.

63% is kind of an oddball value. A little math shows that in 2.3 time constants the dust level is reduced by a factor of 10. That seems more useful to me. Using the previous example, we reduce the dust by a factor of 10 every 23 minutes, or a factor of 100 in 46 minutes and a factor of 1000 in 69 minutes.

Time required in minutes to reduce dust by a factor of 10 = 2.3 x (Room Volume in cubic feet) divided by (Fan flow in cubic feet per minute)

or:

T10 = 2.3 x Vol/CFM

Next we’ll look at the effect of filter efficiency.

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



6 comments so far

View Dark_Lightning's profile

Dark_Lightning

4103 posts in 3887 days


#1 posted 06-28-2020 03:20 PM

You can mitigate the amount of dust that circulates by placing that fan assembly next to the source. I use a shop vac with a Dust Deputy on the big power tools, but there is still some that gets away. I’d have to figure out a way to get that fan in a useful location. My “shop” is pretty tight on space. Interesting project!

-- Steven.......Random Orbital Nailer

View Redoak49's profile

Redoak49

4754 posts in 2767 days


#2 posted 06-28-2020 11:33 PM

Your math is interesting but does it depend on perfect mixing which is not likely the case.

It might be simple to set up an experiment using a cheap air quality monitor like below to see the actual rate. However, the results will be highly dependent on location of filter and sensor. This entire discussion is interesting to me and a helpful discussion.

View clagwell's profile

clagwell

250 posts in 570 days


#3 posted 06-29-2020 08:20 AM

Yes, good mixing is assumed for the math to work. I think that’s appropriate for ambient air cleaning. Placement of shop air cleaners is usually chosen to maximize circulation and eliminate dead spots.

The next section points out the main problem with trying to correlate with actual measurements if you’re filtering, namely, the variation of filter efficiency with particle size. Then there’s also the variation in dust concentration with particle size. Somewhere I have a magazine review of shop air cleaners that plots particle counts over time for the various cleaners. I’ll se if I can find it.

That’s a nice particle counter. Everyone who’s interested in dust control should have one. That version may be very low priced but it actually contains a fairly high-end sensor. The packaging is kind of a hack job but what’s inside is actually pretty good. It’s a Plantower PM5003 that’s used in a lot of much more expensive units.

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

View clagwell's profile

clagwell

250 posts in 570 days


#4 posted 06-29-2020 09:58 AM

It might be simple to set up an experiment using a cheap air quality monitor like below to see the actual rate.

- Redoak49

That’s available from Amazon now. No long wait for shipping from China.

However, if you’re willing to wait it looks like there’s a new version that has data export capability.

-- 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


#5 posted 06-29-2020 10:48 AM

I would love to have the data capability but do not understand the data transmission or software needed.

View clagwell's profile

clagwell

250 posts in 570 days


#6 posted 06-29-2020 11:14 AM



I would love to have the data capability but do not understand the data transmission or software needed.

- Redoak49


If you’re asking about the unit in the Aliexpress link I just saw it today. I don’t know anymore than what I got from a quick look. Looks like they provide software and the data is via usb.

If you’re asking about the sensor itself the data transmission is just 9600 baud serial at 3.3V. You have to write your own software.

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

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