syntelman - 26-8-2007 at 00:17
Does anybody know in general how the exhaust/duct of fume hoods is installed? Is a explosion proof fan connected between the fume hood and an existing
ventilation system or do you need a separate air duct for fume hoods as solvent fumes etc may escape?
I have no idea how standard air ducts are constructed and all this may sound a bit confusing as English is not my native language and the terms may be
a bit mixed up. 
Thanks!
Mr. Wizard - 26-8-2007 at 07:27
If you want a 'rated' (=expensive and recognized safe by the industry) hood you will have to have an explosion proof motor. This motor will have to be
sealed in order to keep out flammable fumes, solvents and acid fumes. If you just want to pull out some nasty smells and a little toxic vapor you
might get by with a cooking exhaust fan. These fans , mounted on a sheet metal hood, are not designed to be corrosion or explosion resistant, but they
are cheap, and may even be free. After the installation of an 'over the stove' microwave oven these items are usually brought back by the workmen and
tossed in the trash.
Under NO circumstances are the vents from a hood ever connected to an existing air conditioning or heating ventilation system. I realize you may not
have meant to do that, but your meaning wasn't clear. Use a separate duct. I know of an instance where a plastic parts maker stunk up a whole
building doing this. It's also very common for dental labs to do this while they 'burn out' their lost wax molds. Their hood designs didn't pull
enough air to overcome the HVAC system's return.
You have to use the correct materials for a fume hood. If you are going to expose the hood and the ducts to flammable concentrations of solvents then
design for it with fireproof ductwork and explosion rated motors. If you are going to make an occasional sample of H2S then you might be OK with a
used kitchen hood and some drywall sideboards. You have to design your hood with what you are going to put through it. If you want it to handle
everything it will be expensive. Give us an idea of what you are doing and we can give more specific advice.
syntelman - 26-8-2007 at 07:47
Thanks for your very informative answer!
And I may have been unclear, but as stated I have no knowledge of how air ducts/vents are constructed in general. However I suspected that to be the
case. 
I am only in general interested in the technical parts of actually installing a "real"/'rated' fume hood correctly (of course rules and regulations
vary), i.e setting up a laboratory for small scale organic and inorganic synthesis. I have received quotes for fume hoods and yes they are expensive,
but occasionally I see used ones sold pretty cheap so the remaining question is how to solve the exhaust.
But as I understand it it's all a matter of getting a explosion proof motor and ductwork and hook it up to a vent that leads outside? Do you have any
tips for makers of such motors and what they possibly could cost?
12AX7 - 26-8-2007 at 09:12
Ducting is sheetmetal, usually with such a bead rolled on the sides that it can be snapped together. This is easily seen at the hardware store.
Else, it can be done with plain sheetmetal bonded with solder, rivets, sheetmetal screws, etc. Sections are usually constructed as modular pipe which
merely slides together, with a rippled end that slides tightly inside the full-size section.
Exhaust fans come with duct fittings, for obvious reasons, so interface is not a problem.
Tim
Mr. Wizard - 26-8-2007 at 13:00
I have no idea what prices are for these things are in Europe. I can only guess they cost a LOT more than here in the US. Since you will be in one
place for a while I suggest you figure out how much air you will have to move, and then get a heating and air conditioning handbook and figure out how
big the ducts have to be to carry the flow you will need. Sometimes it's better to let a professional do the job, as the tools and skills are a
little beyond many amateurs. You know yourself best.
I suggest using a search engine to get real numbers and prices.
If you are doing this in a commercial building or where other people live you have a moral obligation to follow safety rules regarding the
fireproofing and venting of such appliances. You may be required to put in automatic fire protection devices as they do in restaurants.
leu - 26-8-2007 at 19:30
If you had used the search engine you would have found this thread which has some useful information in it:
https://sciencemadness.org/talk/viewthread.php?tid=8949&...
syntelman - 26-8-2007 at 22:41
Thanks! However I'm more looking for what rules and regulations apply to installation of 'rated' fume hoods (or more specifically where to lead the
exhaust etc). I am not really interested in constructing a DIY model. And as Mr. Wizard says you have moral obligations to follow when working in a
commercial building (which most likely will be the casy if any).
I have tried contacting local enviromental departments but they really don't have a clear answer on how this stuff works. I shall try to contact a
local maker of fume hoods (who charge almost 3000USD for a complete hood which doesn't include any fan! ) instead, they must know. Thanks again for all the help!
SpectraMist - 1-9-2007 at 12:01
I found this thread looking for similar information for a fume hood I will be using with aerosol spray paint.
I was considering the cheaper alternatives but know there is black particulate matter as well as flammable fumes.
I didn't want to get a grain elevator explosion going in my art studio so I have a question about sealed fans.
Are they readily available by themselves or must they be purchased along with the hood unit?
What if the sealed fan goes out on the hood units? Could I just order a replacement fan and can you suggest suppliers?