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Author: Subject: Fume Hood Exhaust
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[*] posted on 15-10-2018 at 15:40
Fume Hood Exhaust


I have a Labconco Protector Workstation which is ductless. I have the option to vent from the blower and am looking to get this outside. The current filter is a formaldehyde carbon filter and while I do occasionally deal with it, the major chemical being used is concentrated sulfuric acid. While some vapors will get absorbed, I'm looking to push the air outside, just in case, breathing sulfuric acid vapors isn't the best.

At any rate, the blower fan is out the top and will probably use Type I PVC. How long is too long is a run? I have a portable AC unit venting outside but the exhaust vent is only 15" off the ground so there would be (3) 90 degree turns. I think that is too much. But being in the garage, should I just knock another hole above the fume hood and vent it out there? Any ideas would be welcomed. Thinking of getting a decent shed at some point in the future.
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[*] posted on 15-10-2018 at 16:20


try using the search engine.

https://www.sciencemadness.org/whisper/viewthread.php?tid=67...




The single most important condition for a successful synthesis is good mixing - Nicodem
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macckone
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[*] posted on 15-10-2018 at 20:26


You will also need to replace the formaldehyde filter with an acid filter, otherwise acid vapor will destroy the system.
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Deathunter88
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[*] posted on 16-10-2018 at 01:25


You don't need a fume hood for concentrated sulfuric acid at all, since it is not volatile at normal temperatures and pressures. The only time I can think of where you would is if you were distilling sulfuric acid.
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macckone
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[*] posted on 16-10-2018 at 20:05


Deathhunter88:
Not volatile and doesn't have fumes are two very very different things.
Mercury is not volatile but the vapor is quite toxic even at low levels.
Sulfuric acid vapor is also bad to breath even at very low concentrations.
On a humid day you can see the fumes coming off sulfuric acid and mixing with the water vapor.
And fumes can eat metal very quickly.
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Deathunter88
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[*] posted on 17-10-2018 at 08:04


Quote: Originally posted by macckone  
Deathhunter88:
Not volatile and doesn't have fumes are two very very different things.
Mercury is not volatile but the vapor is quite toxic even at low levels.
Sulfuric acid vapor is also bad to breath even at very low concentrations.
On a humid day you can see the fumes coming off sulfuric acid and mixing with the water vapor.
And fumes can eat metal very quickly.


The reason that Mercury vapors are so toxic at low concentrations is because it accumulates within the body. Sulfuric acid doesn't do that. Using the vapor pressure for sulfuric acid of 0.0005mmHg, I calculated that the vapor concentration in a sealed tank would be 2.5mg/m^3. Looking at this paper: https://www.ncbi.nlm.nih.gov/books/NBK208279/ on the acceptable exposure limits, you can see that such a concentration has an exposure limit of 30 minutes. Given the fact that you will not have your nose stuck up next to the acid, it remains my firm assertion that the amateur working with concentrated sulfuric acid does not need a fume hood.
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macckone
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[*] posted on 17-10-2018 at 20:04


That is at 25C.
If you move that up to 50C the vapor pressure increases 10 times.
And at 100C it is more like .2mmHG or about 50 times.
That means your concentration is also higher and your allowed time is much lower.
It also means you get a good bit of fuming.

With sulfuric acid the damage can be cumulative as opposed to the chemical itself.
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[*] posted on 18-10-2018 at 13:45


Specifying a fume cupboard for just 1 chemical seems rather shortsighted.
What if you want to use hydrochloric acid next time?
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[*] posted on 18-10-2018 at 21:21


unionized:
general acid resistance is a good goal of a fume hood.
it is relatively easy to get resistance to a wide range of chemicals simply by using stainless steel for the fan and an epoxy finish for the other parts. But you need to either coat the fan with epoxy which can easily cause it to be unbalanced or put an acid absorbing filter in front of the fan.
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[*] posted on 19-10-2018 at 00:40


If you're venting outside, and don't do really crazy stuff in your hood, just get a cheap fan that is easily replaced when it stops working after some time. That is how I did it and I only replaced my fan once over the years. I used to use HCl on a regular basis.
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[*] posted on 19-10-2018 at 20:29


Tsjerk: cheap and replaceable is definitely a way to go.
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