DIY: Pro grade 400cfm Air filter

After trying a few of the DIY filters on here, and not being please with the airflow reductions they caused, I decided to design and build a filter for a 400cfm exhaust fan (or Mechanibus's Stanley blower @ 310cfm) that would allow optimal air flow. I took pictures so that anyone who wants to do the same will know how.

Components:
(1)\t1 6”x24” Metal round duct: $3.98
(2)\t1 8”x24” Metal round duct: $5.20
(3)\t1 6” crimped duct cap: $3.96
(4)\t1 8” crimped duct cap: $4.93
(5)\t1 6” to 8” reducer: $7.68
(6)\t1 36”x84” roll fiberglass bug screen: $5.49
(7)\t2 6.25”x36” Aluminum gutter guards: $3.88
(8)\t16 #8x1/2” self tapping screws: $2.14
(9)\t28 #8x1/4” pan head screws: $1.96
(10) 2.5lb medium-coarse grit, air grade activated carbon: $13.60
(11) 25”x 6” rag (use old hand towel or something)

Total cost: $52.82 + tax. Annual cost to run: $13.60 – $27.20



Minimum Tools:
(1)\tDrill
(2)\tHacksaw
(3)\tBit driver
(4)\tRazor knife
(5)\t2” Aluminum tape
(6)\tMetal shears (or strong scissors)
(7)\tMeasuring tape
(8)\tMarker
(9)\t180grt Sand paper


Recommended advanced tools:
(1)\t10” Bandsaw
(2)\tAir cutoff tool or Dremel with cutoff disc
(3)\tDe-burring knife
(4)\t10” bench mounted belt sander

Time to complete with advanced tools: 68 minutes.
Time to complete without advanced tools: I’d say 2 or more hours depending on your skill with a hacksaw.



A word about safety!
\tEvery time someone who has been smoking pot gets hurt and goes to the emergency room, they add that to the “statistics” against marijuana use. Do us all a favor, leather gloves and safety glasses guys. Cut sheet metal is very sharp, tends to get squirrelly when cutting with power tools, and loves to throw little pieces at your eyes; be safe. Having had to have a piece of metal that flew around my glasses drilled out of my eye; my policy on eye and hand protection is not “fuck it, that’s why God gave you two”.



Part one: Shell fabrication

Take and assemble your 6” round duct (If you plan to cut to length with tin snips, do not lock the sections yet. Use tape to hold the halves together.)



Install the 6” duct into the inside of the reducer crimp first so that the crimp only extends about halfway into the 6” collar.




Install 3 self taping screws to hold the reducer in place

Flip the assembly over and mark a ring around the duct at the level of the reducer (this will be handy later)



Now assemble your 8” duct the same way you did the 6”.

http://img843.imageshack.us/img843/7686/44205997.jpg

Install the 8” duct into the reducer crimp first until it bottoms out against the inside crimp (make sure it is all the way down. Sometimes the rivets like to catch on lips)



Install a shop towel around the 6” duct and force down between the two ducts to center the 6” duct in the 8” duct (I know this pic is hard to make out).



Leave the towel in place for now, and install 4 self taping screws evenly around the 8” duct securing it to the collar.



Mark the location of the collar lip on the 8” duct all the way around. Then measure 12” up the duct from the reducer and mark that in a few spots around the duct.



Use a template to connect the marks into a line around the duct (I used a piece of paper lined up and taped in place.




Now cut through the assembled ducts (make sure the towel is still in place to hold the inner duct square). I used a 10” bandsaw and made the cut in a few seconds. If you don’t have a bandsaw, you can make this cut with a hacksaw but it will take some time.

A word about proper hacksaw usage: Do yourself a favor and use a new blade. Thin sheetmetal tends to catch between the saws teeth. A new blade will reduce this. Also, use slow deliberate strokes. I know it seems counter-intuitive, but fast back strokes will not make it cut any faster. It will just wear out the blade, slow down the cut speed, and give you a crappy jagged cut (it will also wear you out faster).




(If you made this cut with tin snips, permanently lock the duct halves together now.)
Now remove your shop towel from the discarded top section of pipe and reinstall in the filter housing. Install the 6” duct cap (do not use screws to hold it in).



Now install the 8” duct cap and secure two ways. First use 4 self tapping screws to hold it on the 8” duct. Then use 4 self tapping screws to attach it to the 6” duct cap (this essentially makes the 2 duct caps into one cap/spacer. Important later!).



Now put alignment marks on all the pieces you can so it can be reassembled later (don’t forget to put one from the 6” duct to the reducer on the inside of both).



Congratulations. You have now completed this section. See next post for more.

 

Part two: Turning solid duct into a filter. The inner chamber.

So you made yourself one handsome filter housing; now take it all apart  (hope you put in those alignment marks).

Take the 6” duct and mark a ring 2” up from the cut end. (I reinstalled the cap to make the duct more rigid while I work with it. Optional but helpful)



Now take one of your gutter guards and cut it into 3 12” lengths.



This next bit is a little tricky to describe. Starting at the duct seam: measure over 1/4” and place one gutter guard on the duct so the side of the guard lines up with the mark. Center it vertically between the two previously marked rings and put 4 marks on the duct; In each of the second hole in from the sides and the second hole in from the top and bottom. Now remove the guard. Using the self tapping screws from earlier put one screw in each mark. Remove the screws and put the guard back on the duct using the screws. It should look like this:



Install the other 2 guard sections the same way so that they sit evenly around the duct. Approximately 3/8” between guards and a max of 2” between screws.



Now mark a dotted line on both sides of each guard in the inner grid space next to the screws. As shown on the bottom guard in this pic (disregard the line in the top guard, I miss marked it).



Now remove the guards and you should see 3 marked “boxes” that measure about 4”x10” and three 2inch “columns”. Cut out the boxes with your air cutoff tool (or hacksaw for the less fortunate) If using the hacksaw method, drill ½” holes in the 4 corners of the “boxes”. Remove the blade from the saw and pass the blade through the holes before reattaching it to the saw. That will allow you to cut the sides.




Now get your bug screen and cut a single piece measuring 18.75”x13”.



Starting over one of the columns, wrap the screen around the duct so that the 13” side is vertical and 18.75” side wraps around. Secure the ends with tape to hold it in place. Reinstall the guards using 4 pan head screws each (not the self tapping screws) you should end up with something like this: (I left the screen wide so you could see how it is installed)



First put tape over the columns and screws vertically. Now wrap the top and bottom of the guards with tape (be sure to press it down really well so you get a nice seal around/in between the guards).



If it looks like that then you are done with this section. See next post for more.

 

Part three: Turning solid duct into a filter. The outer wall.


We are going to being doing the same sort of thing to the outer wall that we did with the inner wall. The main difference is that the outer wall will have four smaller air passages instead of the three we did last time.


Take your other gutter guard and cut it into 4 9” lengths.



Mark a ring 2” below the ring that represents the reducer’s location. Then mark a ring 2” up from the duct cap location. Reinstall the duct cap if you have not already.

Starting at the duct seam: measure over 1/4” and place one gutter guard on the duct so the side of the guard lines up with the mark. Center it vertically between the two previously marked rings and put 4 marks on the duct; in each of the second hole in from the sides and the second hole in from the top and bottom. Now remove the guard. Using the self tapping screws from earlier put one screw in each mark. Remove the screws and put the guard back on the duct using the screws. Install the other 3 guard sections the same way so that they sit evenly around the duct. Approximately 3/8” between guards and a max of 2” between screws.
It should look like this:



Now mark a dotted line on both sides of each guard in the inner grid space next to the screws. Now remove the guards and you should see 4 marked “boxes” that measure about 4”x7.5” and four 2inch “columns”.



Cut out the boxes with your air cutoff tool (or hacksaw for the less fortunate)



Now get your bug screen and cut a single piece measuring 25”x10”. Starting over one of the columns, wrap the screen around the duct so that the 10” side is vertical and 25” side wraps around. Secure the ends with tape to hold it in place. Reinstall the guards using 4 pan head screws each (not the self tapping screws) you should end up with something like this:



First put tape over the columns and screws vertically. Now wrap the top and bottom of the guards with tape (be sure to press it down really well so you get a nice seal around/in between the guards). Do not overlap the reducer position ring as this will make it very difficult to do final assembly in the next post.

 

Part four: All that work actually pays off. Final assembly


Align the four screw holes in both duct caps and screw the inner and outer wall sections back together with self tapping screws. If you can’t seem to get the holes to line up, temporarily reinstall the section into the reducer using those alignment marks we made back in section 1.



Fill the filter with carbon up to about ½” above the inner wall’s vent opening. I hope you remembered to rinse your carbon like you should (I didn’t and ended up with a mess).



Now take that rag we were using for alignment earlier and make a nice tight top seal over your carbon. The reducer screws will hold it in place after installation.



Reinstall your reducer using the alignment marks and self tapping screws.



You are done. Marvel at your fabrication skills, and install it in a place of prominence that others may bask in your glory.

Whenever the filter stops being effective (est. 6-12 months depending on environment) simply remove it from your exhaust setup, remove the reducer and rag, dump the carbon and replace it with new. Reinstall rag and reducer and viola, brand spanking new filter for another run.

 

Thanks for the comments guys.

housefullOweed, did you actually +rep me? Cause it doesn't show up in my CP. Just wondering if the system is faulty.

 

Sweet. If you guys build one, make sure to post up a pic (especially if you make any changes).

 

Thanks man.

I've been thinking of making a proportionally scaled down version for a <100cfm fan using a 3" to 4" reducer and 6" long from base to reducer bottom. You would only need (3) 2.5"x6" air channels on the 3" inner chamber, and (4) 3.5"x4.5" channels on the 4" outer wall. I estimate a need for < 9oz of carbon; which is basically one of those wall mart containers. I don't even think you would need the gutter guards, if you can get the screen on tight enough. Maybe use some aluminum screen instead of fiberglass (for extra rigidity). )

 

This.

Or you can get something like this and crush/grind it to a smaller size.
5 lbs Activated Carbon LARGE AIR PHASE pellets coarse - eBay (item 300387518196 end time Nov-26-10 01:00:10 PST)

Generally speaking, the finer the pellet size the longer it will absorb odors. I have in the pasty used coarse pellets; when they stop working after a few months, you can break them up and use them some more.