Hello all,
*Background, Question below, feel free to skip*
Early this morning I came across and followed references to Bill Pentz’s site, and have been on there an this forum all day (so much for getting any work done today). I see there is controversy as to if his conclusions are valid. But I’ve drank of the Kool-aid so to speak, and am looking to design my dust strategy to comply with his guidelines. I wish to err on the side of my health. OK, that said, space being a huge issue and summer heat being what it is here in Austin, TX, I cannot take the open air woodworking approach. My shop is in the attached garage (it was actually in a bedroom with table saw in garage, but now I’m redoubling my efforts to get dust production out of the house!). I’m now playing around with the idea of eventually mounting one of the clearvue units in my attic above the garage and ducting up and back to it (I’ve seem some pics of another guy that did this on the clearvue gallery), but until I can afford to do that, and as a supplement…
*Question*
Is there an air purifier/air cleaner (as in ambient filter, not dust collector type thing) that will filter down to .5 microns?
I have a Delta 50-875 air purifier, but am not sure what to do about filtration of airborne dust. Since the filters on this unit are not proven by anyone but the manufacturer’s claims, and that only down to 1 micron. I’m afraid I’ve got a useless device, that I perhaps shouldn’t even bother mounting to the ceiling? (Is it worse to filter down to 1 micron, but keep redistributing the smaller stuff, or not run an air cleaner at all and help let the stuff settle out?)
Are there any good devices out there, or better (certified) filters for the Delta?
Replies
I have been reading, Controlling Dust in the Workshop/Rick Peters and he's pretty much against air cleaners. His opinion is that if you need an air cleaner, your dust collection system needs radical improvement. He also states that an air cleaner keeps all air/dust circulating till it finally gets pulled through the air cleaner. A Good point to ponder, I'am starting to re-think my plans about dust collection and air cleaning.
The new Jet canister dust collector filters go down to 2 microns, While the Jet air cleaners inner bag filter goes down to 1 micron.
Work Safe, Count to 10 when your done for the day !!
Bruce S.
What Rick Peters says makes no logical sense. It's like saying that if your basement floods, not to bother getting a little transfer pump to drain your basement because it will circulate the water, causing more damage to your belongings, until your basement is dry.On the Sawmill Creek forum, you'll be able to find test results that Bill Pentz did that shows that there will be dust in the air that is not captured by even very well designed and powerful cyclone/DC systems. It appears that dust escaping the point of capture is a given.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. Going back to the flooded basement analogy, a bigger pump is better. 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 increase the CFM capacity of your air cleaner so that you circulate the room volume once every 2 1/2 minutes, you'll be exchanging the air just over 25 times an hour. Even assuming only 75% efficiency, which is much lower than manufacturer's specs, you'll clear out 99.9999% of the dust in just under 25 minutes. For comparison, at the "recommended" once every 10 minutes rate, it will take 3 hours and 20 minutes to clear dust down to the same level.Again, taking that dust escaping into the air is a given even with a powerful DC/cyclone system, I think that air cleaners are a very cost effective way of dealing with this. If your air cleaner doesn't have enough CFM, it's cheaper and easier to just add more air cleaners to your workshop space than it is to upgrade your DC/cyclone.Of course, trapping the dust at the source is the best option, and I don't want to talk anyone out of getting a good DC/cyclone system. Also, the most efficient way of cleaning the air in a room is to open windows and doors and to use an exhaust fan. But to say that an air cleaner will not help at all with dust control just seems silly.
well said!I don't agree with your numbers regarding how much will be picked up after 25 minutes. Some of the small stuff will always blow through the filter. I would take a slightly different perspective regarding the air filter. Since the OP already has an air filter, one of the questions was whether or not it is worth it to run it. I think it's definitely worth it as the air filter will take out SOME of the ultra-fine particles that are floating around in the shop. It won't get ALL of the particles, but some is better than none in this case.The sub micron stuff is going to float around anyway, so why not run it through a filter than can pick up some of it? You won't get it all, but the overall volume will be down. And that's a good thing. :-)Glen
Hi Glen,Here's how I did the math:My air filter is specified by the manufacturer to operates at 99% efficiency for 5 micron particles, and 91% efficiency for 1 micron particles. I am assuming that this is optimistic, and decided to work with 75% efficiency, meaning that as air passes through, the air cleaner takes out 75% of the particles, leaving 25% in the air.My air filter has 750 CFM. My workshop is 20' x 11' x 8', which is 1760 cubic feet, divided by 750 CFM means that it could circulate all the air in the room in 2.35 minutes, or 25.5 times per hour.After the first 2.35 minutes, 25% of the dust remains.After another 2.35 minutes, 25% x 25% = 6.25% of the dust remains after 4.7 minutes total.After another 2.35 minutes, 25% x 25% x 25% = 1.56% of the dust remains after 7 minutes total.Do this a few more times, and you'll get down to 0.0001% of the dust remaining after 10 cycles, or 23.5 minutes. That's how I came up with 99.9999% of the dust removed.I actually made myself an Excel spreadsheet so I could play with these numbers. In the real world, the biggest impact on these numbers is the speed of the turnover of air in the room, which is directly related to CFM. Dust collection efficiency certainly has some impact, but if your efficiency is above 60% you're in the decreasing rate of return phase. Aiming for a CFM that is 1/2 your room volume seems to hit the sweet spot.Wilbur
Wilbur,
I've been out of college for too many years to recall exactly how you should figure out a problem like the one you ran about dust removal, but I do know that your method won't give an accurate answer and that the math and the physics behind it is rather more complicated.
The basic problem is that the cleaned air come out of the filters is mixing in with the rest of the air in the room, so that some of the air going through the filters on the next round is already clean in a sense and you won't be removing any additional dust from it but it will be using up some of the air removing capacity of the unit. What it boils down to is that as you remove more and more dust from the air the remaining dust gets harder and harder to capture because it is getting more and more spread out in the cleaned air.
John White, Shop Manager, Fine Woodworking Magazine
Hi John,To my understanding, the efficiency rating of filters refers to the percentage of dust particles it will trap on one pass. This is treated as a linear function, and, to my knowledge, is not dependent on the number of dust particles: if the efficiency is 95%, then if 1000 particles go in, 50 will come out; if 100 particles go in, 5 come out; and if 10 particles go in, 1 comes out. It may be that filters don't act in an entirely linear manner, but I have not been able to find any real data to back this up.On the ASHRAE website there are formulas that are used to calculate "mass balance", or the number of particles in the air based on a number of factors. These equations are more complicated than my analysis, since they are designed to cover situations where new particles are being generated while the filtration unit is working, whereas my analysis just looks at how long it takes an air cleaner to filter out dust in a room. If you assume a situation where you're done woodworking (or in my case, turning kiln dried wood into powder), and run an air cleaner, the main factors that come into play are: the filter efficiency, which corresponds to the manufacturer's specs; the "recirculation factor", which takes into account the incomplete mixing of air that you mentioned; and the "volumetric flow", which is roughly equivalent to the CFM of the air cleaner. These are treated as linear variables in the information provided by ASHRAE.Here's the link to the ASHRAE Standard 62.1-2004. The formulas are in Appendix D:http://www.realread.com/prst/pageview/browse.cgi?book=1931862672It is true that incomplete mixing and circulation will impact the efficiency of the air filter, but I'm accounting for this by the fact that I'm not using an efficiency of 91%, which is what the manufacturer states, but I'm knocking that factor down to 75% to cover the recirculation and incomplete mixing issues. This is a bit of an assumption, but efficiency will not have a major detrimental effect until you get below 50%.Here's a real world experience with this issue. Back in Chicago, one of the clubs where I used to go see rock bands installed a filter for cigarette smoke. This situation is even tougher to manage than our wood dust issue because people were constantly lighting up and adding new smoke to the air, the number of cigarette smoke particles generated in a rock club is more than the amount of wood dust generated in a typical hobbyist shop, and smoke particles are even smaller than wood dust particles. Once the smoke filter was installed, I was able to see a show in that club and come out without the usual stench of second hand smoke.Anyway, I understand that the two weaknesses of my model are [1] that my estimate of 75% efficiency is too high, but I have a hard time believing that the recirculation and incomplete mixing issue would cause the total efficiency of the system to drop below 50%, and [2] if the filters do not act in a linear fashion, which would go against how manufacturers spec the filters and how ASHRAE believes how filters behave.Finally, if you are concerned about dead space in your workshop that your air cleaner can't get to, the easy thing to do is to hang a second air cleaner in that spot. This is far easier and likely less expensive than trying to upgrade your dust collector.By the way, I just bought your book, "Care and Repair of Shop Machines", a few weeks ago, and I think it's just great.Wilbur
Edited 5/1/2007 2:38 pm ET by wilburpan
Thanks for the compliment on my book.
I'm sure that your calculations give a good rough approximation of how efficient a filter system is, at least for the first few passes, but as the amount of particles diminish, the efficiency of capturing the remaining dust drops off quickly.
Your numbers showing 99%+ efficiency after just a few minutes aren't realistic, probably something like 90% to 95% is the best that can be achieved even after an hour of running if I recall my engineering courses correctly. If I can, I'll find the proper way of determining a systems effectiveness and post it here.
John W.
The basic problem is that the cleaned air come out of the filters is mixing in with the rest of the air in the room, so that some of the air going through the filters on the next round is already clean in a sense and you won't be removing any additional dust from it but it will be using up some of the air removing capacity of the unit.
This is correct, and is why, in the example above, more than 25% of the dust remains after 2.35 minutes. The density of dust trails off asymtotically and at an unfortunately slow rate. It's a straightforward calculus problem, that I'm too lazy to work out right now. ;)
Barry,You don't have to do the math -- I did it for you. ;)This is an asymptotic process, but not a slow one.Here are some graphs based on the model showing how dust is cleared based on the efficiency of the clearing system. The numbers are based on my shop, where the air cleaner can circulate the room volume in 2.35 minutes.The first graph shows the drop in the percent of dust particles as the efficiency of the system changes. What was surprising to me is how quickly the initial falloff is in the percentage of dust particles. Even at an efficiency of only 50%, the drop off is very quick, and it's not really until you get to efficiency levels of 40% or less that you really see a slowdown in the clearance of the dust.The second graph is plotted on a log scale, so that you can see the dropoff more easily. I placed the X axis so that it intersects at 0.0001%, or 99.999% dust clearance. The point at which the line crosses the X axis tells you how long it will take to get dust clearance down to that level. As you can see, at 40% efficiency you get to 99.999% clearance in just over an hour. 65% efficiency puts you at about 30 minutes to get to the same level.John White has raised the issue of whether or not efficiency remains constant as you clear dust out. If this is true, the first part of the graph still will be accurate. Over the first 20 minutes, even a system that is only 40% efficient will still clear almost 99% of the dust.My point still is, IF you have enough CFM in your air cleaner for the size of your room, air cleaners would have to be incredibly inefficient to really just be "cleaning the clean air". There is the question as to whether dust located far from the air cleaner will eventually be processed by the air cleaner. This is why I think that having enough CFM for your room is important. My shop is a 20' x 10' room, and my air cleaner is hung towards one end of the shop. I have found that if I turn my air cleaner on and stand at the opposite side of the room, I can feel a slight air current generated by the air cleaner. Then again, I've got 750 CFM in only 1760 cubic feet.Everything that I've found on this issue points to only three factors that impact this: the amount of air processed by the filter (CFM), the efficiency of the filter (manufacturer's specs), and how quickly the air in the room is recirculated, which is the hardest to control. Luckily, there are very easy ways to address the last issue: install a second (or third or fourth) air cleaner in your "dead zone", or set up a fan to blow air in the direction of your air cleaner intake.
You may have done the math, but you did it incorrectly. ;) You neglected to account for John White's point that the output of the cleaner mixes immediately with the still-dirty air in the shop, so a portion of the air entering the cleaner is air that's already been cleaned.
OK I did the math and for the example you gave:
After the first 2.35 minutes, 47.2% of the dust remains (not 25%).
After another 2.35 minutes, 22.3% of the dust remains (not 6.25%).
After another 2.35 minutes, 10.5% of the dust remains (not 1.56%).
Finally, after 23.5 minutes, you're down to 0.055%, not 0.0001%
The dust decreases, but not as fast as you claimed.
The derivation is as follows:
A room with volume V has an initial quantity of dust r0 at time t=0, when the cleaner is switched on. The amount of dust in the room any time thereafter as a function of time is r(t). The efficiency of the cleaner filter is e (0.75 in the example), and an amount of air equal to the shop volume V moves through the cleaner in time T (2.35 minutes in the example).
Here's the calculus part:
The instantaneous amount -dr by which the dust decreases during instant dt is:
dr = - r(t) * e * (dt/T)
This comes from the fact that the quantity of dust entering the cleaner during this instant is equal to the fraction of shop air entering it (dt/T) times the amount of dust in the shop at that time (r(t)), and the cleaner removes only a fraction e) of it. Solving for r(t):
dr/dt = -(e/T)*r(t), or finally r(t) = r0 * exp(-e*(t/T))
That's what you need to plug into those Excel spreadsheets. ;)
Hi Barry,Thanks for the calculus refresher! It's been a while.I did plug your formula back into my spreadsheet. It was quite interesting what happened. The behavior of the dust collection model changed most at higher efficiencies, whereas the curves for the low efficiency filters changed very little. With the addition of your equations, my initial calculations are off by just under a factor of 2. Instead of needing a CFM to room volume ratio of 0.42 to get 99.9999% clearance in 23.5 minutes, you need a CFM to room volume ratio of 0.77. Of course, all you need to get there is to buy another air cleaner.However, if you examine the curves in the corrected model, the fact that the impact of lowering the efficiency of the system is minimal until you get to a filter efficiency of 45% or so is still true.Your derivation assumes perfect mixing of the clean and dusty air in the room, which isn't going to completely happen because the clean outflow air needs to take some time before it starts to mix with the dirty air at the intake. So my original model could be seen as a best case scenario, and your corrected model as a worst case scenario, with actual performance being somewhere in between.Of course, its better to assume worst case scenarios when thinking about these things. When I incorporate your correction into the model, it seems that my room will be 99.99% dust free in just about a half hour with the parameters we've been working with (750 CFM, 1760 cubic feet, 75% efficiency). And since we are now accounting for mixing, I feel a little bit better about assuming the efficiency per the manufacturer's specs, which is 91%. In which case, the dust level in my room gets down to 99.999% in that 30 minute time span. In any case, getting the dust down to at least less than 0.01% in half an hour still seems pretty good to me.I'm happy to stand corrected, but I think that the issue of air cleaners being useful additions to dust management still stands. I still think that air cleaners are underappreciated, and that the usual advice of 6-10 air exchanges per hour is inadequate. Certainly the issue of "cleaning the clean air" doesn't stop air cleaners from being effective, again assuming that you have enough CFM for our room.I've attached the revised graphs and my spreadsheet for your amusement. Enjoy! Please let me know if you have any other ideas for tweaking this model.Wilbur
Would not the decreased efficiency based on "Re-cleaning already cleaned air" be eliminated by simply exhausting the air cleaner and using a fresh air intake rather than exhausting the cleaner back into the shop?
You've just "reinvented" shop ventilation, and it is more efficient than recirculating the air and you can also eliminate the filters since some body else down wind will be now breathing the dust, a simple fan will do it.
John W.
Your derivation assumes perfect mixing of the clean and dusty air in the room, which isn't going to completely happen because the clean outflow air needs to take some time before it starts to mix with the dirty air at the intake. So my original model could be seen as a best case scenario, and your corrected model as a worst case scenario, with actual performance being somewhere in between.
I'd agree with that.
Thank you for posting the math, I really didn't want to go back to my engineering texts to dig that stuff out, it was bad enough the first time.
John W.
Wilbur,
I think the math is a bit optimistic, (the collector can not capture dust that does not reach it) however I see what you're saying. The more air complete air changes, the more dust will be collected.
I think everybody is in agreement that the use of a filter is a good thing...
Buster
I would have to say I agree with Bruce, if you need an air purifier while you're in the shop then you should probably look closer at improving dust collection at the source. My understanding is that air purifiers will keep small particles suspended longer.
That said, I see no harm in running the purifier after you're done working for the day. Most have a timer that will turn the filter off after an hour or two. It will filter out some of the fine particles after you've left for the day. Your Delta is probably more than fine for this task.
I would save your money and use it to upgrade your dust collection at the source. If your shop is not 100% Pentz certified yet, I wouldn't stop wood working. My view is that shop dust collection is a system. Build it as you go, and you'll end up with a much better system than if you tried to shove it all in at once.
Buster
"Is there an air purifier/air cleaner (as in ambient filter, not dust collector type thing) that will filter down to .5 microns?"
By the way, to answer the original question, it doesn't look like anyone makes a drop in HEPA filter for air cleaner units, which is what you would need for filtering 0.5 micron particles. However, Wynn Environmental (http://www.wynnenv.com/) says that they make a 12" x 12" x 24" box style HEPA filter that could be mounted on the outflow side of the air cleaner. It's not a drop-in unit, but the frame is made of wood, and so theoretically you could fit it to the outflow side. That's the best I could find.
There are stand alone HEPA filtration units, but they tend to be small 200-400 CFM units for a few hundred dollars, or commercial units that provide 1000-1500 CFM, but cost about $1,500.
Thanks, I'll check it out. :)-Mettius
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