Hexavalent
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Fumehood Efficiency
Having recently acquired a recirculating fume hood (there is nowhere in the garage that I can install ducting and a roof stack) and performing a quick
smoke test, it has come to my attention that, although the face velocity is good, the CFM rating doesn't seem to be too high, i.e. it doesn't move a
huge amount of air in an x amount of time. Are there any known ways in which I can help this? I keep it clear and uncluttered at all times. Help?
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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BromicAcid
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Keep the doors closed as much as possible, some hoods even have alarms to let you know if the doors are open for too long.
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entropy51
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Hexavalent, there was an earlier thread about these ductless hoods that you might want to consult, although I don't think it answers your specific
question. Here is a post I made in that thread, which included a link to an evaluation of this type of hoods, which is unfortunately dead it appears.
Quote: Originally posted by entropy51  | The main recommendations that I have seen for ductless fume hoods with absorption filters is that they are intended only for removal of nuisance odors
and not for removal of toxic gases and vapors. I certainly wouldn't depend on one for NH3, H2S, CO etc. (or any flammable vapor) unless there was a
duct to outside air following the filter.
Recirculating the exhaust of a fume hood violates most of the safety codes for fume hood performance. |
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Hexavalent
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Thanks a lot for your input, I will re-consider my options regarding the fumehood positioning. Would it be acceptable if I ran this; http://www.ebay.co.uk/itm/100mm-x-10M-ALUMINIUM-FLEXIBLE-DUC...
across the garage floor or roof to the nearest window?
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Hexavalent
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Or even this;
http://www.ebay.co.uk/itm/6-Hydroponics-Flexible-Ducting-150...
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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entropy51
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I don't know if the blower in your hood has enough
capacity to overcome the pressure drop in an external duct or not. In principle you could certainly use a flexible duct like that, but you would
probably want to keep the length of the duct as short as possible. The exhaust duct on my home made hood is about 4 feet long from the blower outlet
to the window and that works well.
It is possible that you would obtain a higher air flow with an external duct if you connected it in such a way as to bypass the presumbably high flow
resistance of the absorber filter.
One caveat about the aluminum duct is that acid fumes such as HCl will eventually corrode it so you have to keep a close eye on it to prevent leaks.
It is cheap enough however to consider it disposable and just replace it frequently.
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Hexavalent
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Again, thanks for your seasoned input. Unfortunately, the duct would have to be 10m long to get to the nearest window or door and I can't go through
the walls or through the roof. So, perhaps I should just consider using it for nuisance odours for now and do those experiments that can generate
toxic gasses/vapors outside with a fan blowing to the side (possibly things like bromine vapour, chloramine, hydrazine etc.). Maybe when I'm older and
find my own home I can rip the filters out and just blast it through a window (a lab space will be a priority when house-hunting!).
As for corrosion, I was always under the impression that acid fumes would not have long enough to hang around and corrode ducts . . .am I right?
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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watson.fawkes
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The rule of thumb is that 1 unit
length of flexible duct has air resistance equivalent to 4-5 times the length of smooth duct. The choice of duct material could easily make or break
your particular circumstances. | Quote: Originally posted by Hexavalent | | As for corrosion, I was always under the impression that acid fumes would not have long enough to hang around and corrode ducts . . .am I right?
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No. It's the steady-state concentration of fumes that matter, not how fast they are moving. If you have a
single release that starts then ends, moving air will clear it out quickly. On the other hand, if you've got a continuous point source of acid fume
emission, then you have continuous contact of somewhat-diluted acid fumes with your duct.
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Hexavalent
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OK, again thanks for your input. Would I be better just using it for nuisance odours and not toxic vapors/gasses then, instead handling them outside
with a fan by my side?
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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entropy51
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| Quote: Originally posted by Hexavalent | | OK, again thanks for your input. Would I be better just using it for nuisance odours and not toxic vapors/gasses then, instead handling them outside
with a fan by my side? |
I think so, providing that you realize that if you are doing chemistry you will
likely generate more than nuisance odors.
These hoods are adequate for measuring out a little aqueous ammonia or HCl for making a dilute solution, but not for working with these gases on a
significant scale. I sure would not try distilling Br2 in one of these boys. I would also be hesitant to use anything more flammable than a little
methanol or ethanol in one of these hoods. If a Grignard took off in one of them I would expect an ether explosion.
Other members with more experience with ductless hoods may have better advice, but in most labs the Occupational Health and Safety people strictly
limit the use of ductless hoods since they do not meet the required standards for "hoods".
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Hexavalent
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What about distilling 250ml of xylene in it?
The fan is actually isolated from the motor, I believe.
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entropy51
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Hexavalent, I am by no means an expert on these
hoods, but I would be concerned about a spill overwhelming the absorbing capacity of the filter, perhaps just momentarily. If this happened flammable
vapor could build up in the cabinet, be ignited by your heat source, or spill out into the room where the vapor might find an ignition source.
I think another problem with these hoods is that there is no warning that the absorber has become saturated and is no longer effective.
My concerns may be misplaced, but I just don't think these hoods are intended for serious chemistry work. I think them more appropriate for working
with small amounts of stinky stuff in a biology lab.
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Hexavalent
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there is no warning that the absorber has become saturated and is no longer effective.[/rquote]
On my hood there is a filter saturation alarm that seems to beep whenever, as you say, it is temporarily flooded.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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