Airflow at the exhaust with the carbon filter attached and the tent zipped up ranges from 60 - 108 CFM and averages 83. Airflow at the 6" x 12" inlet is roughly the same, as we’d expect. Comparison with the previous measurements with the tent open (post #99) shows that closing the tent cut the airflow by 12 CFM.
We know from earlier measurements (posts #51 and #55) that my 4" inline fan can’t extract enough air to control the temperature at high light settings, even with the carbon filter removed.
I found a very good deal on a new Vivosun Aerozesh G6 fan, which is rated 388 CFM. That’s double my G4’s rating, so I expect I’ll be able to get adequate airflow at speeds below 100%, which should be quieter, too.
My previous measurements show that my 4" 205-CFM filter cuts the G4’s airflow by 50%. That finding implies that a filter’s rated CFM must be at least double the fan’s rating to achieve the fan’s rated CFM. I say “at least” because the fan’s max CFM should be a “diminishing returns” function of the filter’s rated CFM, e.g.:
AC Infinity’s 8" filters provide the lowest cost per CFM and the greatest CFM per inch of filter-length. Their 20" model comes very close to meeting my criterion and is an excellent value. Their 23" XL filter exceeds my criterion and is a close second WRT value.
Have I missed a manufacturer? (Spider Farmer and Mars Hydro don’t offer filters with rated CFMs > 500.)
Just keep in mind that different types of fans can respond differently as restrictions increase, so that might not always pan out. You’d want to know from the filter company what pressure the fan needs to make in order to hit the filters specced cfm. Then check the performance map for the fan from the fan manufacturer if the fan can make at least that much pressure specced from the filter company, & how much air it’s moving then. There are at least a couple other filter companies out there that I know of , Can Filters, & there’s another one that I can’t think of that makes what seems to be a well-designed filter with an internal conical baffle to even out the airflow along the length of the filter, & I think both of those companies spec the cfm & pressure numbers for their filters. You should (I think maybe) be able to convert your airspeed readings to pressure to see what pressure your fan makes at max, & compare that to the filters pressure requirement for its specced cfm.
You’re right – thanks for catching my overgeneralization. For Vivosun’s G series 6" and 8" fans, though, I expect it’s accurate. I’ll see if I can get performance maps from them.
An AC customer service rep has been trying to find out for me how they determine their filters’ CFM ratings. No luck yet. I expect there’s an industry standard test method, but I haven’t identified it yet.
I notice that Canfilters says their CFM ratings denote the airflow that provides 0.1 second of contact time with the carbon. They appear to be a US manufacturer, so I may learn more about standard testing procedures by calling them.
Yeah the G-series looks like a hybrid design, so good for the application. I guess I just mentioned it as a general reminder for anyone reading. You could always buy the fan first, do your own bench test with the blocking of the intake at 25% increments to make your own flow map now that you have the gauge, & then worry about trying to find a filter with specs that fit it. Probably you could buy or do a DIY filter around 18" to 2’ long, & it will probably be close in terms of getting close to max flow of your fan while still being effective at odor control, if the filter’s designed well enough.
Aren’t all the fans that are being marketed for our grow tents/rooms mixed-flow (hybrid) designs?
Yes, and thank you for doing so.
I toyed briefly with the idea of designing a DIY filter, but realized that many competent engineers have explored this problem already and everyone’s settled on the design we know. I’m not apt to do better.
Aqua denotes the filter(s) that offer the lowest cost/CFM in each size category. Yellow denotes close seconds.
Cost/CFM tends to increase with the thickness of the carbon layer, so the “best value” filters presumably don’t last as long or filter as well.
Finally, just for fun, I fit a simple predictive equation to the data. A straight line fit works fairly well (R2 = 0.97), but those familiar with regression can see from the residuals that there’s a 2nd-order effect that isn’t accounted for.
@PhotoFinisH Is this the unusual filter design you were trying to think of? It’s an Active Air Inline. Inlet air passes through the carbon layer twice before exiting.
Not what I was thinking of, but that’s interesting & I’ll take a closer look at it. The one I was thinking of has a conical inner wall tapering in from the ‘capped’ end down to the ‘open’ end, allowing for more carbon to be installed down near the fan, which should spread out the flow down the whole filter more evenly. I forget who makes it.
These days it’s probably easier to just buy a good one that will perform they way you need it to, but I think the inner wall on at least the couple cheap commercial filters that I have can be improved for better flow & better anti-odor effectiveness.
Factory inner wall:
Should probably flow a little better with the thinner materials, & better odor effectiveness since more carbon is open to the airflow, & also in my case because the filter is built long enough to where the fabric ‘chokes’ are needed to spread the flow evenly across the whole length of the filter, it’s also spreading the flow across a bigger overall carbon area, & it’s also big enough to not be much of a restriction. Most of the drag that isn’t intentionally induced from the chokes is down to minimal losses through the carbon & minimal wall materials, & the air itself. But with the correct type & size of fan, those are minimal. But even if I were to build a diy version to the same length as the couple shorty commercial filters that I do have, I think the diy would flow more with its inner wall ‘design’. I’d have to check the commercial one, but pretty sure it would have the same 1" thick carbon ‘wall’ as a diy one typically ends up with.
These days mostly yes, but there are also the in-line centrifugal fans, & then the axial ‘booster’ fans out there, & they’re usually all on the menu plus probably some other types at most of the grow supply places.
Please post more photos. That looks more like a complete DIY filter to me than a replacement for the inner wall.
I asked my wife and a friend of hers recently for nylon stockings headed for the trash. They both laughed and said I’d have to buy new ones because they haven’t worn them in years!
It is my diy filter, & I’m not sure it would be worth it to try & remove & replace an inner wall of a pre-built filter unless you wanted to compare the flow rates. These days you can probably find something off the shelf that flows well enough & is effective enough. But in looking at my off the shelf filter, the inner wall looked more restrictive than my diy filter, just food for thought. The trade-off with the nylons is durability, but metal foil tape patches small holes well enough. I think I also did two nylon layers over the inner & outer screens. I should be able to find the diy I followed, that might be more helpful if you wanted to see how to build one, since mine is still installed & mostly wrapped in towels for the chokes.
Here’s the link: Carbon Filter DIY
Not 100% sure if that’s an offending link, so if so, go to overgrow com & search for the thread titled " Building a large carbon filter - anyone have the instructions from the old OG? " , & follow the link in there for copies of the original thread. Plus there are some other ideas further down in the first link.
