How to build a "mondo ghetto" laminar flow hood.
Posted 30 December 2005 - 08:34 PM
- Salem likes this
Posted 31 December 2005 - 09:52 AM
Problem: My FOAF needed a sterile micro-environment to work in and had NO intention of dropping $600 (or more) on a flow hood. My friend feels that $600 is a little bit too much to spend on $250 worth of parts. Being without real carpentry skills (or tools), constructing a wooden hood was out of the question. Also, he wanted Plexiglas between his often sneezing nose (allergies) and his projects. Sneezed mucous particles travel fast enough to easily slice through any laminar flow stream.
So, with that in mind, this is what he came up with, which I gladly share with you. Ladies and Gentlemen, without further adieu, I proudly introduce the mondo ghetto laminar flow hood!
Since my friend started working in the mondo ghetto, contamination has become a thing of the past. 4 agar plates given a 180 second exposure to the laminar stream generated 1 contaminated plate after two weeks in the incubator. 4 plates at 30/60 second exposure consistently show no contamination at all. These .3 micron HEPA filters are the SHIT!
Wanna make one? You’ll need to acquire the following:
1 blower (mine is a 420 CFM squirrel cage type)
1 speed control for blower (DON’T use a light dimmer)
1 8x8 .3micron HEPA filter with wooden frame (you can go bigger, if you feel the need)
3 HEPA rated furnace filters that fit onto your blower tub
Roll of duct tape
1 tube goop (or other strong silicone glue)
1 caulking gun
3 tubes silicone caulk
1 tube construction adhesive
8 angle brackets
22 short wood screws (shorter than the HEPA frame is thick)
12 long (1.5-2 inch) screws with nuts
24 washers for long screws
Two big plastic tubs (must be large enough to fit blower and mount HEPA)
Two PVC joining sections, at least an inch larger around than your arms
A huge plastic tub to act as the glove box/flow hood area
A piece of Plexiglas large enough to cover the huge tub
1 florescent strip light
We begin by making the lower HEPA tub. Take one of your identical tubs and trace the shape of the 8x8 HEPA on it. Cut this square out carefully (you’ll note that my friend split his tub and had to glue/screw it back together; don’t try to build this drunk). Mount the 8 angle brackets (two per side) onto the HEPA with your short wood screws and goop (drill pilot holes). Make CERTAIN the screws aren’t long enough to penetrate into the body of the HEPA! Once everything is dry, slide the HEPA into the hole and mark where the angle brackets need mounting holes drilled. Using the long screws/nuts, secure the HEPA to your tub. Using the caulking gun, put a thick bead around the edges to seal it in.
Now, put the lid on this tub and then set the blower onto the lid. Using a ruler or a paper template, cut a hole in the lid for the blower but don’t mount it yet. Cut almost the whole bottom out of the top tub and secure it to the blower mounting lid. Try to leave an inch or so of lip on the bottom for the glue to stick to. My friend used silicone caulk, construction adhesive and a screw on each corner. The yellow/beige stuff in the corner is the construction adhesive.
Once the top tub/lid is completely dry (48 hours), mount the blower on it (screws/nuts are the best way to go). Put a thin bead of silicone around the mouth of the blower to seal it tightly to the lid. Now attach the motor speed control to the blower and cut a hole in the tub edge for the cord. Use silicone caulk to seal the hole.
Now, glue your three furnace HEPAs together with silicone and let dry for 24 hours. Use duct tape to seal the edges and make one thick HEPA. Cover the bottom cardboard edges with duct tape (gluing straight to the cardboard frame won’t hold very well). Use goop to glue the big filter over the opening in the blower tub. Once the goop is dry, put a nice thick silicone bead on the edge to seal the filter/tub completely. Below is a side view of the three filters glued/taped together and then glued to the top tub. If you squint, you might be also see the silicone bead.
This is what the pre-filter HEPA looks like after a few months of use (look how yellow it is…can you tell my friend smokes a lot?). If it wasn’t for the pre-filter, all that crap would currently be clogging his HEPA and fouling his blower motor. I wish he had a HEPA for my lungs...
Once everything is dry, set the top tub on the bottom and it should look like this:
Now, we need to create the glove box/flow hood portion. Take your huge plastic tub and determine a comfortable distance apart for the two arm holes. This will vary depending on how broad your shoulders are. Cut two large round holes and fit the PVC joiners into them. Use goop to seal the joiner’s edges.
Please note: it is VERY important that these joiners be a good bit bigger around than your arms. For the laminar flow to work, air must be able to escape the box freely. The air pressure escapes around your arms.
Now cut a square hole on the other side of your tub to slide the HEPA through. Note: this does not have to be an air tight seal. Ideally, the three tubs can be pulled apart for storage. If you glue the HEPA to the flow hood tub, moving it will be a HUGE pain in the butt because you'll need two people. Once this thing is set up, it is FAR from stealthy.
Assemble your hood and crank it up, with the fan speed control on low. Place a lit candle about 4 inches in front of the HEPA and adjust the fan speed until the flame bends to almost horizontal. If the air stream blows the candle out, reduce the motor speed immediately. Remember, too much air pressure can wreck your HEPA.
Once everything is built, let the mondo ghetto run for 24 hours to clear everything out of the HEPA. After 24 hours, do the candle test to verify flow rate. Now, test your mondo ghetto by exposing a series of agar plate to the laminar air stream for progressively longer periods of time. My friend's mondo ghetto will maintain zero contamination up to 120 seconds of exposure. My friend starts to see contams regularly above 180 seconds with (on average) 1 in 4 plates showing undesired growth.
It is not known what flaw in the design causes this. Any input you could offer is greatly appreciated. I suspect that if my friend was using a full 12x12 HEPA, this wouldn't happen. That, however, is pure speculation.
Make note of the setting on your speed control, in case you bump it. My friend purchased his with a covered plastic box, because the full speed setting on his fan would very quickly roast his little HEPA.
"Candle test" your HEPA after about 40 hours of use to check the flow rate. Squirrel cage blowers will loose a little pressure and the furnace HEPAs will deliver less air as they clog with crap.
When you are ready to work, put the Plexiglas on the glove box and the light on the Plexiglas. Here is the finished mondo ghetto laminar flow hood (same pic from the beginning):
· Change your three furnace HEPAs when the top one gets brown and cruddy looking. Leave the furnace HEPA covered with a plastic trash bag when not in use.· Save the box your 8x8 HEPA is shipped in. You can use this box to cover the HEPA when not in use. If you toss the box or it isn’t a good fit, a shower cap or a plastic bag (secured with rubber bands) can also be used. Keeping the HEPAs covered when not in use will make them last longer.
· My friend always lets the mondo ghetto run for 2 hours before using it. 2 is probably overkill, but I would suggest letting it run for at least 1 hour before use.
· A 420cfm blower is TOTAL overkill for an 8x8 HEPA. A 420cfm could probably power a 12x24 filter. My friend just happened to have one lying around and a speed controller was far cheaper than a new fan. If you are buying a blower new, I recommend an axial model over the squirrel cage style. They cost a little more, but they are a better value in the long run (less power consumed and better maintenance of pressure over time).
· You can buy a used blower, but I would not recommend it unless you can check the output beforehand.
· The fan motor control is not critical, but it’s important. If your fan’s output is too high, you will blow holes in the HEPA. You won’t see these holes, though, just the contaminants that leak through and wreck your projects. Not enough air pressure is far better than too much.
· The HEPA furnace pre-filters aren’t critical, but they are important. Pre-filtering the air will make your HEPA last much longer. It will also make the blower last marginally longer, because it won’t get clogged with crap.
· If your blower isn’t generating enough pressure, you can squeeze a little more out by gluing the blower tub to the HEPA tub and the HEPA tub to the glove box tub. I didn’t do this, because I wanted something I could tear down and stash a little easier. Also, a fully glued together mondo ghetto can only be safely moved by two people.
· Tyvek sleeves with elastic at the wrist/elbow are PERFECT for this environment. I don’t even wash up before throwing on gloves and sleeves, though I do wear a filter mask (again, I sneeze a lot).
· If you want stealthy end results, be far smarter than me and don’t use transparent tubs. Homer say D'OH!
· If you are shorter, use a narrower tub for the flow hood section. You want to be able to reach in and hold your work directly in front of the HEPA, almost touching it. That’s hard to do if your arms won’t reach all the way across the bottom of the tub.
· Don’t carry the blower tub around by the furnace HEPAs (use the tub handles). Cardboard framed furnace HEPAs tear up pretty easily.
· Once the mondo ghetto is complete, let it run for 24 hours to clean everything out.
· PLEASE test your mondo ghetto on plates before exposing a project!
Any and all feedback is greatly appreciated and desired. I'm always anxious to improve on ANYTHING.
- Salem likes this
Posted 03 January 2006 - 09:54 AM
BE AWARE, the 8x8 is usually a special order and they probably won't have them in stock. You will probably have to wait 2-3 weeks for them to fabricate one. 12x12 and larger are stocked items.
You can shop around other places, just make certain it the unit is a %99.99 effective at .3 microns. You can also spend about 3 times as much on a "high capacity" model that lets you blow an assinine amount of air through it (and not worry about putting holes in the filter at high pressure).
The HEPA pre-filters can be picked up in a 3 pack from your local Wally World for $20-$30.