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Author: Subject: fume hood design
bob800
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[*] posted on 28-7-2010 at 17:27
fume hood design


I'm thinking of building a homemade fume hood. I know there's other threads on this topic, but I'm very new to fume hood design and need some basic information. Would <a href="http://www.instructables.com/id/Building-a-small-fume-hood-for-stinky-projects/">this</a> be acceptable for an amateur lab? Is the range hood ok for chemistry or do I need to buy a separate blower?

Also, what are baffles and what are they used for? Is plexiglass a suitable material for the sash? And finally what size dimensions\blower ratings are suitable for a small hood?

Any advice on building fume hoods would be greatly appreciated,
thanks
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cnidocyte
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[*] posted on 29-7-2010 at 03:48


No that fume hood isn't intended for chemistry. To make a fume hood use gypsum plasterboard for the walls since its relatively fireproof. Plexiglass can be used but it has poor solvent resistance so you may need to replace it if it gets splashed with solvent. Polycarbonate is a bit better but still isn't very resistant to non polar solvents. Ideally you could use a pane of normal glass on the inside and plexiglass/polycarbonate on the outside. The ordinary glass is highly resistant to chemicals and the plexiglass/polycarbonate is shatter resistant. If you don't want to spend too much money on it right now though plexiglass by itself is fine.

I've been wondering the same thing about baffles. What the hell are they? I looked up on wikipedia and a load of other sites but could only find vague references as to what they might be.
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[*] posted on 29-7-2010 at 06:19


I believe baffles help reduce air turbulance and so it the air is drawn out of the fume cupboard quicker without any being blown back out towards the user.

That fume cupboard would be ok however the wooden walls might need replacing every so often due to spilled chemicals. If the wood is painted with an epoxy paint it should be fine.
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[*] posted on 29-7-2010 at 09:09


Flammable plastics, heat, and solvents -> splattered flaming plastic stuck to your flesh. A very bad idea. A thin piece of polycarbonate glued to window glass or between two pieces of glass is a poor person's bulletproof glass.

Range hood motors are in the air flow for cooling & to save money. Solvent vapors or corrosives will destroy the motor. The motor should be outside.

There's a cement board which is very heat resistant and quite cheap.

Discarded PC cases are a source of epoxy painted steel. It's a pain to cut up or unfold, but it is strong and usually free. "Pop" rivets (blind rivets that look like nails applied from one side) are good to stick the pieces together.

You need to change the air in the hood quickly. You need to move air into the hood faster than toxic gases fall out of the hood door. So if the door opening is 2' x 3' = 6 sq feet, moving the air 1'/sec into the hood = 360 cfm (cubic feet per minute).

The hood needs to be bigger than the biggest apparatus you plan to use in it plus enough work space to prepare any reagents plus space for a light. The smallest useful hood is 12" wide by 18" high IMnsHO.

Perhaps the fume hood discussions should be gathered together & made sticky under "reagents and apparatus" or part of "writing the textbook"?

An architect who designs chem labs wrote on a photography web site saying that the aerodynamics are critical, especially for heavier than air vapors like mercury. Gases & vapors go where they want to which is often not where you think they will.

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bob800
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[*] posted on 29-7-2010 at 10:19


Thanks for all the help. Maybe <a href="http://developer.nicejewel.com/fumehood1.php">this</a> fume hood setup is better for a lab?

So would a blower like <a href="http://www.grainger.com/Grainger/DAYTON-PSC-Blower-1TDT2?Pid=search">this one</a> work? It's pretty expensive though, is it possible to get one less than $100 with 300 or more cfm? Will the motor last if I put it at the end of the vent? Also, what does the "@ 0.000-In. SP" mean in "CFM @ 0.000-In. : 549"?

thanks
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[*] posted on 30-7-2010 at 11:34


The jewelry maker's hood would probably be OK for HOLDING COLD INORGANIC fumes. It would be hazardous for anything heated above 50C or so and could fog or melt the door from any organic solvent other than ethanol or isopropanol.

You couldn't work in it because it doesn't have a real sash and there's no way to be sure that fumes wouldn't swirl out when the door is open.
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[*] posted on 30-7-2010 at 14:05


"@ 0.000-In. SP", in "CFM @ 0.000-In. : 549" means zero inches of static pressure. Generally you are blowing through a fan of some kind (although the math may work for compressors), and their resistance to flow, by design or dust, is measured in said inches, which I believe are vertical inches of water displaced.

My (poorly researched) rule of thumb is that 1" SP = 1/2 the cfm.
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bob800
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[*] posted on 30-7-2010 at 17:15


Thanks. Do you think this fan : <a href="http://www.horticulturesource.com/product_info.php?products_id=6386">http://www.horticulturesource.com/product_info.php?products_id=6386</ a> would work? It's 471 cfm and it looks like the motor is separated from the air flow.
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Lambda-Eyde
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[*] posted on 30-7-2010 at 17:27


What you're after is a squirrel cage fan/blower.


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bob800
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[*] posted on 30-7-2010 at 17:34


I thought the motor is supposed to be (ideally) separated from the fan to prevent it from being destroyed from corrosive vapors. Is that the case for these squirrel blowers? Or would they last long enough even though the motor is in the air flow?
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entropy51
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[*] posted on 30-7-2010 at 17:36


Based on about 30 years of using a hood not too different from the link you posted, I think that hood would be good for many kinds of experiments. Just follow S.C. Wack's advice:
Quote: Originally posted by S.C. Wack  
As I've pointed out before, the original fume cupboards were powered by flame. Then ordinary fans. This was suitable for a long time.

If you're having explosions, you need to either stop pouring stuff on the floor of the box instead of in glassware, use coolant with your condenser and receiver when distilling ether or acetaldehyde, or get a bigger fan.
The trick to using a hood is not to let the vapors loose in it. Good technique, scrubbing traps, very cold water in the condenser, no ignition sources make the use of a simple hood quite safe for halogens and many organic vapors. Avoiding runaway reactions, which seem to be inexplicably common around here, is important. No need to run a 10 mole reaction unless you're manufacturing something to sell.

My hood is not too different from that one, but my hood is larger, with double plastic doors opening from each side. I also have an L-shaped baffle or "airfoil" under the doors so that a nice laminar flow of air enters and sweeps across the floor of the hood on its way to the exhaust port. I've worked with halogens, phosgene, AlCl3, Grignards, H2S and lots more without trouble.

I've yet to need to put on a gas mask, which I believe is an admission of lousy technique.

Before using a DIY hood you should test the draft with some sort of smoke generator to be sure that the air flow is adequate. Some dry ice in a beaker of water makes a pretty good smoke generator.

[Edited on 31-7-2010 by entropy51]
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bob800
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[*] posted on 30-7-2010 at 17:45


Quote: Originally posted by entropy51  

My hood is not too different from that one, but my hood is larger, with double plastic doors opening from each side. I also have an L-shaped baffle or "airfoil" under the doors so that a nice laminar flow of air enters and sweeps across the floor of the hood on its way to the exhaust port. I've worked with halogens, phosgene, AlCl3, Grignards, H2S and lots more without trouble.


What type of blower are you using?
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entropy51
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[*] posted on 30-7-2010 at 18:15


Quote: Originally posted by bob800  

What type of blower are you using?
It's a Dayton squirrel cage blower that looks a lot like the link you posted. Its over 20 years old and I can't read the model number, but it looks like it says 1300 CFM. It could be bigger and I will replace it with a larger one when it quits, but so far it has kept me out of the emergency room. I can make Br2 in 10 mL batches in it without catching so much as a whiff. I work on the smallest scale possible to avoid unpleasant surprises.

I wouldn't evaporate ether in an open container in a hood like this, but I have distilled ether in it many times with an ice cold condenser and receiver.
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bob800
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[*] posted on 30-7-2010 at 18:21


So something like this would work: <a href="http://www.kalyx.com/store/proddetail.cfm/ItemID/897933/CategoryID/12000/SubCatID/2745/file.htm">http://www.kalyx.com/store/proddetail.cfm/It emID/897933/CategoryID/12000/SubCatID/2745/file.htm</a>? I don't understand why these squirrel cage blowers are so much more expensive per cfm then something like this: <a href="http://www.horticulturesource.com/product_info.php?products_id=6386">http://www.horticulturesource.com/product_info.php?products_id=6386</ a>
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[*] posted on 30-7-2010 at 18:28


Blowers like the one pictured a few posts above are perfectly suitable for a fume hood. The motor is out of the air flow. The ones bob posted immediately above will work for a while, depending on how much acid & solvent fumes they have to endure. If you're careful, they'll work for years.

Blowers differ a lot in price and construction depending on whether they have to push against resistance, such as air filters (you were going to put some charcoal or something to absorb the nasties before your neighbors breathe them?). "Back pressure" usually rated in inches of water is the key rating.

If the air flow is adequate, then the concentration of flammable material in air is below the amount needed to explode or sustain combustion. That's why the flame-powered hoods worked.

And as S.C.Wack & entropy51 have pointed out, a hood is not an excuse for sloppy setup. It's a life preserver (or eye/lung/hand preserver). Some preparations require moving fuming beakers around. That's fine, and that's what a fume hood is for - just put the fuming mess into something which captures and disposes of the fumes ASAP. That's how I handle aqua regia and other lung-unfriendly things.


[Edited on 31-7-2010 by densest]
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entropy51
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[*] posted on 30-7-2010 at 18:29


Quote: Originally posted by bob800  
So something like this would work: <a href="http://www.kalyx.com/store/proddetail.cfm/ItemID/897933/CategoryID/12000/SubCatID/2745/file.htm">http://www.kalyx.com/store/proddetail.cfm/It emID/897933/CategoryID/12000/SubCatID/2745/file.htm</a>? I don't understand why these squirrel cage blowers are so much more expensive per cfm then something like this: <a href="http://www.horticulturesource.com/product_info.php?products_id=6386">http://www.horticulturesource.com/product_info.php?products_id=6386</ a>
$100 is not expensive for a blower, but it looks like the motor is in the gas stream, which is a no no. Nobody said chemistry was an inexpensive hobby. A good blower will last a long time if you scrub your waste gas stream before it goes into the blower.
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[*] posted on 30-7-2010 at 18:36


@bob800 - the blower you referenced will not push against an air filter. If you release corrosive or solvent vapors, it will degrade and eventually fail. Murphy's law guarantees it will be at the most disastrous time.

I use a similar fan to vent my glass working area. For that, it's fine - there are a small quantity of nitrogen oxides which would be very bad for my lungs but are not sufficient to corrode the fan motor. I would not use it to remove acid fumes.
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bob800
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[*] posted on 30-7-2010 at 18:37


Thanks so much for all the help. So if my hood is 2' wide by 1.5' high by 14 inches back then would this blower work?: <a href="http://www.kalyx.com/store/proddetail.cfm/ItemID/897933/CategoryID/12000/SubCatID/2745/file.htm">http://www.kalyx.com/store/proddetail.cfm/It emID/897933/CategoryID/12000/SubCatID/2745/file.htm</a> EDIT: Oops, that's the one you said was in the gas flow. How about the 400 cfm version of this one: <a href="http://www.planetnatural.com/site/search_engine.html?mv_session_id=j4Fisob8&search_clear=1&criteria=active+air+blowers&x=0&y=0"& gt;http://www.planetnatural.com/site/search_engine.html?mv_session_id=j4Fisob8&search_clear=1&criteria=active+air+blowers&x=0&y=0</ a>

[Edited on 31-7-2010 by bob800]

[Edited on 31-7-2010 by bob800]
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[*] posted on 30-7-2010 at 18:46


I wouldn't use it - the motor is inside the air stream. I've had to replace such a fan twice in the hood over my stove.

Something like 1963k37 from http://www.mcmaster.com is good. It's about $400, which means you could probably find one surplus for $100 or less.

Don't skimp on your safety!!!!!!!!!!!! I'm still regretting a very "minor" injury to one finger. It took a year to recover from a day of too much nitrogen oxides.

If you want to do chemistry on a small budget, you have to scrounge good equipment from unlikely places. There are a lot of surplus equipment dealers around the planet. Your job is to know better than they do what something is worth or to know how to use something that they have better than anyone else local.

[Edited on 31-7-2010 by densest]
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bob800
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[*] posted on 30-7-2010 at 18:47


Sorry, I posted the wrong blower. See my edited post.
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[*] posted on 30-7-2010 at 18:50


That one looks good. Do the math for your hood - I use an arbitrary value of 1 foot per second of air into the hood through the door.
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bob800
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[*] posted on 30-7-2010 at 18:53


What's the formula for calculating that?
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[*] posted on 30-7-2010 at 19:05


I gave it above - cfm of fan / ( 60 * area of opening in square feet)
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bob800
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[*] posted on 30-7-2010 at 19:08


Thanks - sorry, I didn't realize it was the same thing
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bob800
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[*] posted on 30-7-2010 at 19:12


OK- So if my opening is 1.5 feet high, 2 feet long and 465 cfm then it would be 2.66 seconds? Is that correct?

[Edited on 31-7-2010 by bob800]
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