giant Cypress

Dust thou breathe, and unto dust thou lungs shalt return, redux

It’s Safety Week 2011. Last year I wrote about considerations when thinking about dust, and probably due to my day job, and the past weekend when my whole family seemed to be dealing with allergies, dust is on my mind again this year.

There is a lot of information regarding dust collection out on the internet, including one of the most comprehensive resources on the subject, Bill Pentz’s website. There are many excellent options out there, from large cyclone systems, to clever designs that can put a cyclone in even the smallest workshops, to Phil Thien’s cyclone separator lid, which gives you many of the advantages of a cyclone in a manner that can be made in most any woodworking shop. But one of the critical tools for managing dust in a shop is often overlooked and often misunderstood — the air cleaner.

The overall strategy behind dust collection is to get the dust at the source. Of course, this is the best strategy. But it’s nearly impossible to collect all the dust at the source, even with the best dust shrouds and dust collection systems. Some of that dust is going to escape. So you need to have a plan B in your shop, which is where the air cleaner comes in.

Since dust will be circulating in the air whether or not the air cleaner is present, it only makes sense to incorporate a secondary plan to trap the dust that escapes. An air cleaner will be circulating air, but it will be trapping dust particles as it runs. The fine dust particles are so fine that they stay suspended for quite a long time, so that any additional “airborne” time that air circulation would contribute will be relatively minor.

I think that the problem with air cleaners is that often they are very much undersized for the room that they are installed in. A common recommendation is to have an air cleaner that circulates the room volume 6 times an hour, or once every 10 minutes. If you assume only 75% efficiency, which is much lower than typical manufacturer’s specs of 90% or more, after doing a back of the envelope calculation (okay, I used a spreadsheet) it will take about 2 hours to clear 99.99% of the dust. If the air cleaner works as specified, you get to the 99.99% level in 15 minutes less, which is slightly better.

If you increase the CFM capacity of your air cleaner so that you circulate the room volume once every 2-1/2 minutes, however, you’ll be exchanging the air just over 25 times an hour. In this case, at 75% efficiency, you’ll clear 99.99% of the dust in just about half an hour. Again, if the air cleaners perform at 90% efficiency as claimed, the results are slightly better, but it’s clear that boosting the total CFM of the air cleaners has a much more significant impact on how quickly the dust gets cleared out.

Taking that dust escaping into the air is a given even with a powerful dust collector or cyclone system, I think that air cleaners are a very cost effective way of dealing with this issue. If your air cleaner doesn’t have enough CFM, it’s much cheaper and easier to just add more air cleaners to your workshop space than it is to upgrade your cyclone. Two of the three-speed 1100 CFM air cleaners that nearly every manufacturer sells is enough to achieve 25 room air exchanges per hour in a 20’ x 20’ shop. My shop is 11’ x 20’, and the JDS Air-Tech 750-ER that I have gets me to 25 air exchanges per hour at its very conservatively rated 750 CFM. Of course, trapping the dust at the source still remains the best option, and I don’t want to talk anyone out of getting a good dust collector or cyclone system. But using air cleaners is an important part of dust management in any workshop.

(Photo from JDS Company.)