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evercurious
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Questions about Fume Hoods and Filtering Exhaust
Hello everyone - this is my first post here and I have a couple big questions (bottom one is probably the most important, if you don't want to read
everything!):
I know there are a lot of posts on fume hoods here. I honestly have searched for 2-3+ hours on a comprehensive overview of the best way to build a
fume hood system, but haven't been able to find it. The best system I could find that incorporated the concepts of having an essentially explosion
proof and corrosion proof system was provided by magpie, but it was over a series of posts. Also, it didn't outline everything that was done.
I like the blower (Grainger 7c651) that was used in that example, but it requires a lot of piping. Is there any type of belt-driven induction type
blower that could be used so that a pipe would shoot straight up (vertically) from a fume hood and immediately (horizontally) outside a basement
window (I know 90 degree bends aren't good, so it could take place over a longer distance)?
***Also, and the primary reason for my post, is there is virtually NOTHING to be found on filtering the exhaust! I don't want to be exhausting things
like HCl gas outside and having my neighbours kids get sick from it. How would one go about filtering exhaust? Would it involve putting activated
carbon in the ductwork prior to the blower (or after the blower?)? Should the carbon be packed tightly together, or should it be layered by resting on
top of filter paper (or something porous that won't corrode)? Is activated carbon even a good media for absorbing HCl gas (I've read differing
accounts on this from various 'reports')? I'm really flabbergasted that I haven't been able to find anything on this, surely I'm not the only one
that's concerned about this!
Thanks in advance!
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watson.fawkes
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Quote: Originally posted by evercurious  | Is there any type of belt-driven induction type blower that could be used so that a pipe would shoot straight up (vertically) from a fume hood and
immediately (horizontally) outside a basement window (I know 90 degree bends aren't good, so it could take place over a longer distance)?
***Also, and the primary reason for my post, is there is virtually NOTHING to be found on filtering the exhaust! I don't want to be exhausting things
like HCl gas outside and having my neighbours kids get sick from it. How would one go about filtering exhaust? |
Exhausting contaminants at ground level is not a good idea. Every filtration system will eventually fail. If you're concerned about
the hazard, you'll need, at the least, monitors to detect the failure, one per contaminant class. It's much cheaper to buy more pipe and exhaust at
the roof level. See the recent thread on exhaust gas velocity for more on this topic.
Wide sweep bends have much lower static pressure drop than narrow turns, it's true, but they're acceptable if you've got an otherwise intractable flue
geometry problem. In other words, if you have to, you can always buy a bigger blower and pay a bit more to operate it. This can be cheaper than a lot
of carpentry.
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evercurious
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Couple of questions:
Firstly, I couldn't find the topic about "exhaust gas velocity" that you mentioned.
Second, if I really did want to filter, could you give me some suggestions? I suspect that if I used a lot of activated carbon, in the proper setup,
and replaced it often enough (like after every experiment), then it would be alright - what do you think of this?
Third, if I was venting things like hydrogen chloride gas at the rooftop level, would any of it settle to ground level and harm anyone - or would it
just be taken up into the air and not pose a threat to surrounding homes?
Thanks!
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hissingnoise
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Some growers use ozone generators to remove - ooops, wrong topic???
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watson.fawkes
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| Quote: Originally posted by evercurious | Firstly, I couldn't find the topic about "exhaust gas velocity" that you mentioned. [...] Third, if I was venting things like hydrogen chloride gas at
the rooftop level, would any of it settle to ground level and harm anyone - or would it just be taken up into the air and not pose a threat to
surrounding homes?
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See this thread.
| Quote: | | Second, if I really did want to filter, could you give me some suggestions? I suspect that if I used a lot of activated carbon, in the proper setup,
and replaced it often enough (like after every experiment), then it would be alright - what do you think of this? |
There is no general advice about this. The nature of the contaminants, what's effective against them, how long the mitigation
materials last, how you determine when their lifetime is up, etc.—these are all relevant to what's needed. This is one of the very good
reasons not to consider this. Do you really want to research your exhaust system for design and operational adequacy every time you use it? If you
must, though, I'd suggest stepping out and reading about industrial counterflow stack scrubbers, as well, since you may need to neutralize the
contaminants, as you may not be able to just filter them out. Any chemical engineering handbook has adequate information about this topic. Perusal of
such a reference should convince you, unless you have some specific need, that a duct run to the roof is preferable. You can put this out the outside
of an existing structure, I might point out.
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evercurious
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Thanks Watson for your time. The 'this thread' link didn't work for me though :/
Just wanted to clarify, if I ran my exhaust through the roof, it would be safe? I really don't want to end up hurting anyone.
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entropy51
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I think you might want to "filter" the fumes before they enter your exhaust system. You mention HCl and that will eventually corrode
many ducts and blowers if a lot of it goes into the exhaust. The effluent from reaction flasks can be absorbed in caustic, or even H2O in the case of
HCl. This principle can also be used to "scrub" a lot of other effluvia before it goes it up the stack. When done this way, only the fugitive
emissions that escape the scrubber will enter the exhaust, and they will be diluted if the air flow is adequate.
My fume hood exhausts at ground level, not ideal, but by scrubbing the fumes before they enter the exhaust I have avoided problems. No dead grass,
pets, or children. The shrub that hides the exhaust outlet has not shown any adverse efffects on its health either. I have used some fairly bad
stuff in the hood, but the scale is obviously important and I generally work on the smallest scale possible.
I would add the caveat that H2S and thiols might cause problems, but that is true even with a vertical exhaust located on the roof.
With respect to hood design, my experience is that it need not be elaborate. I think adequate air flow is the most important variable here, based on
empirical evidence. If the flow is insufficient all the fancy airfoils and baffles in the world will not help. As S.C. Wack has pointed out, fume
hoods are not rocket science. I know all about the ASHRAE and ACGIH standards, and they are generally overkill for the amateur chemist. The perfect
is the enemy of the good enough.
I have worked in several manufacturing and research labs and none of them used filters, charcoal or otherwise, on the exhaust. The only
filtered exhausts I have seen are HEPA filters on the exhaust of biological safety cabinets in which virulent microorganisms were handled, and that is
quite different from the amateur chemist's situation.
P.S. A member recently reported that his lab was "visited" after a neighbor observed him installing a fume hood duct on the roof and called police.
For what it's worth. A good cover story, like a kitchen or bathroom fan exhaust, might be a good idea.
[Edited on 26-1-2010 by entropy51]
[Edited on 26-1-2010 by entropy51]
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evercurious
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Thanks for your help! I was thinking of pumping the HCl gas through IPA - I remember someone in my class said that it's very stable and has a higher
affinity for HCl than water (thoughts?)
Just out of curiosity, if I were doing a distillation setup, on the inlet for the vacuum, could I just run a hose from that to a bucket full of water?
I've been thinking about this quite a bit but I'm not sure if that would increase the pressure in the system. Here is what I mean (number 9 is where I
would attach the hose):
http://en.wikipedia.org/wiki/File:Simple_distillation_appara...
Thanks!
PS - any ideas for a belt drive induction type blower (one that doesn't have the motor inline)?
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gsd
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@evercurious
1) If you have to scrub HCl vapours try using dilute alkali solutions. Using IPA for scrubbing HCl may be lot of fun but then what about subsequent
scrubbing of escaping vapours of IPA? How about installing a flare to do that?
2) You can always put the #9 nozzle to the bucket. The pressure effet will not be much- only corresponding to the depth of discharge tube below water
level; but just give some thought to the suck-back.
3) In several threads devoted to the fume hood this idea of belt driven motor is adequately discussed.
My humble suggestion to you is just read through an excellent book called "Practical Organic Chemistry" by Vogel before attempting your distillation.
gsd
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entropy51
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| Quote: | | any ideas for a belt drive induction type blower (one that doesn't have the motor inline)? |
It need not be
belt driven; my motor is mounted on the blower shaft, but out of the gas flow. The motor can even be in the gas stream: google "bilge blower".
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evercurious
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Thanks everyone! Couple more questions:
1) Should I just dilute the HCl gas through a bucket (made of PVC - or what other types of plastics are ok with HCl?) full of water? I'm assuming the
water should be very cold.
2) What specific brand of motor do you have entropy? I have read about the bilge blowers, but I was under the impression that they could deteriorate
in the presence of acids like HCl, and then expose components that could lead to an explosion.
3) I'm still not sure how I could put activated carbon inside the exhaust piping. I know it should be before the blower/fan. However, I'm not sure how
to put it in there - like, most carbon filters have metal on the outside holding it, that would be destroyed by HCl gas. How would I go about building
some sort of canister system to put in the piping? I was thinking of carbon between sheets of filter paper - but I need advice!
[Edited on 26-1-2010 by evercurious]
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entropy51
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1. If you dunk the hose in water the HCl may dissolve so fast that the water will suck back into the flask. Connect the hose to an inverted funnel
whose wide end just touches the water surface. Glass is probably better than plastic, but unless the acidic water becomes very concentrated plastic
will be OK. If the water gets warm (as it does when scrubbing NH3) just add some ice cubes.
2. My blower is from a manufacturer that doesn't exist any more as far as I know (BMDS). (If you don't let corrosive vapors get into the blower they
last a long time.) The idea is to keep corrosives out of the gas stream by scrubbing. The only fumes that should enter the blower are those that
escape when you pour from one container to another and when you take a setup apart.
3. My advice? Forget the charcoal. It just cuts down the air flow.
You're making this much harder than it really is.
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Magpie
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I was also going to suggest the inverted funnel absorber for highly water soluble gases such as HCl and HBr. This is common practice in beginner
organic labs and works well, at least on the small scale used there. You haven't told us what scale you plan to operate on so I will presume it is
small scale. On that basis I would also forget charcoal absorbers and filters. Just wait until the neighbors aren't playing next to your exhaust, or
if you are really worried wait for a good wind. Here's a sketch of the inverted funnel. Note that a small gap is kept between the water surface and
the lip of the funnel to prevent suckback. I recommend you invest in a lab maual or two, eg, "Organic Laboratory Techniques" by Pavia et al.
Edit: By all means use a scrubber (bubbler) to absorb noxious gases if the gas is not soluble enough for use of the inverted funnel. You will,
however have to be expecialy careful to prevent suckback if using a bubbler (submerged outlet). Inserting a safety bottle in the effluent gas train
will provide protection here.
[Edited on 26-1-2010 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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| Quote: Originally posted by evercurious | The 'this thread' link didn't work for me though :/
Just wanted to clarify, if I ran my exhaust through the roof, it would be safe? I really don't want to end up hurting anyone. |
Try this one instead. If you run a duct up to the roof, and you have adequate exhaust gas velocity, you'll be fine. You don't want a
barely adequate fan, letting heavier-than-air fumes fall back down to the ground before diffusing.
If all you're ever going to emit is HCl, highly soluble as it is, the suggestion to use a scrubber is quite adequate.
If you still want to filter, build a breech-loading system into the duct work and make yourself a charcoal cartridge for it: a metal tube (perhaps a
piece of duct), wire cloth on each end for mechanical stability and to hold the charcoal in, and a volume of charcoal. Make the whole thing out of
metal so that you can bake it as-is when you want to regenerate it. Start with a couple inches thick and see if that works. You will need to upsize
your fan to take into account the static pressure loss through a filtration bed.
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Magpie
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I just happen to be setting up for making an alkyl bromide where HBr will be evolved, so here's a photo of the inverted funnel gas absorber:
The single most important condition for a successful synthesis is good mixing - Nicodem
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evercurious
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Thanks for all your help everyone! I really appreciate it. Just a few more questions:
1.@ magpie/entropy : the inverted funnel idea - if I plan on performing a reaction in which a fair bit of HCl will come into play, would I be better
served to simply stick a hose into a bucket of water (while keeping a trap in place to allow for suckback)?
2. Regarding the HCl in the water, what kind of bucket should I store it in - I know from dealing with Muriatic Acid in the past, that the bucket
needs to be really well sealed. Other than PVC, what kind of plastics would be alright? (and to get rid of it, do I just dilute it with NaOH?)
3.@ watson.fawkes : I really like your idea for the carbon filter being a breech loader. My only concern is the wire cloth - wouldn't HCl gas eat
right through it - or are there certain kinds of metals or kinds of wire traps that could withstand it?
4. Do I need to 'ground' the piping system that I am going to have - does the suction of the fan create static electricity inside the ducting that
could cause a spark/explosion when dealing with VOCs?
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Magpie
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| Quote: Originally posted by evercurious |
1.@ magpie/entropy : the inverted funnel idea - if I plan on performing a reaction in which a fair bit of HCl will come into play, would I be better
served to simply stick a hose into a bucket of water (while keeping a trap in place to allow for suckback)?
2. Regarding the HCl in the water, what kind of bucket should I store it in - I know from dealing with Muriatic Acid in the past, that the bucket
needs to be really well sealed. Other than PVC, what kind of plastics would be alright? (and to get rid of it, do I just dilute it with NaOH?)
4. Do I need to 'ground' the piping system that I am going to have - does the suction of the fan create static electricity inside the ducting that
could cause a spark/explosion when dealing with VOCs? |
1. What's a fair bit? You haven't told us what you are trying to do or the scale of your anticipated HCl generation. If you want specifics you have
to give specifics.
2. Any plastic bucket will do - I guess they make most out of PVC. Why save the weak muriatic acid you are making? Just send it down the drain with
lots of flush water, of course.
4. I have electrically isolated PVC ducting myself and have wondered about this. I tried to get it to spark to a ground but never found anything.
The single most important condition for a successful synthesis is good mixing - Nicodem
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entropy51
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You can just stick the hose in a jug of water, but be sure to use a trap! And the trap should be large enough to hold ALL of the water and then some.
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watson.fawkes
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| Quote: Originally posted by Magpie | | 4. I have electrically isolated PVC ducting myself and have wondered about this. I tried to get it to spark to a ground but never found anything.
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Static electricity in ducts come from pushing particulate exhaust through it. There are known cases of
ungrounded PVC ducting in wood shops exploding. It's rare, but it happens. As I understand it, the particles generate static electricity by rubbing on
the duct walls, just like any other triboelectric effect. The solution is to run a ground-bonded conductor along the interior of the duct. The
conductor limits the ΔV that can develop, and the ground bonding references that to earth.
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watson.fawkes
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| Quote: Originally posted by evercurious | | 3.@ watson.fawkes : I really like your idea for the carbon filter being a breech loader. My only concern is the wire cloth - wouldn't HCl gas eat
right through it - or are there certain kinds of metals or kinds of wire traps that could withstand it? |
You'll want to get familiar with materials compatibility charts, generally. Stainless steel mesh should be fine for HCl, either dry
or weak/cool mist. I don't know off the top of my head what material to use for hot or concentrated acid, but I'd be a bit surprised if you were doing
that in your basement. Alternately, you could make your cartridge out of plastic, but you'll have to take out and put back in the charcoal each time
you regenerate. You'll incur a big, black, dusty mess and greater charcoal losses each cycle.
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evercurious
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@ Watson.fawkes - could I just wrap a grounding wire around the PVC ducting and attach it to a grounded source like an electrical box? I don't really
want to run something on the inside of the vent, in order to avoid other issues.
@ Magpie - 1. I plan on doing some aqua regia concoctions to isolate gold plating found on old computer parts. This is just one of the examples in
which I will be using HCl. What's the absorptivity of HCl in water? Like, how much water would I need per gram of HCl?
2. What type of material did you use for your ductwork? PVC?
@ Everyone - any suggestions for a brand/model type of blower? I saw yours Magpie, but I'm not sure if I want to install that much ductwork. Is there
anything that I can used that's relatively easy to install without accessing it at funny angles, explosive proof, and not an 'inline' motor?
THANKS!
[Edited on 27-1-2010 by evercurious]
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watson.fawkes
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| Quote: Originally posted by evercurious | | @ Watson.fawkes - could I just wrap a grounding wire around the PVC ducting and attach it to a grounded source like an electrical box? I don't really
want to run something on the inside of the vent, in order to avoid other issues. |
If you're not running a lot
of particulates through a PVC duct, you don't need to ground at all. But if you do ground, yes, you have to ground the inside, because that's where
the charge builds up. If you ground the outside, the charge won't magically migrate through the PVC, which is an insulator. And also yes, it's a pain,
which is why most serious woodworking duct is made of metal, just to avoid the entire issue. In the present case, that metal would have to be
stainless or better, which is pricey. So ground the inside, or ground not at all, because grounding the outside is a waste of time and materials.
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Magpie
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| Quote: Originally posted by evercurious |
@ Magpie - 1. I plan on doing some aqua regia concoctions to isolate gold plating found on old computer parts. This is just one of the examples in
which I will be using HCl. What's the absorptivity of HCl in water? Like, how much water would I need per gram of HCl?
2. What type of material did you use for your ductwork? PVC?
@ Everyone - any suggestions for a brand/model type of blower? I saw yours Magpie, but I'm not sure if I want to install that much ductwork. Is there
anything that I can used that's relatively easy to install without accessing it at funny angles, explosive proof, and not an 'inline' motor?
[Edited on 27-1-2010 by evercurious] |
1. My handbook (recommend you get one) says that HCl is soluble in cold water at 82.3g/100mL. But you will likely be disposing of this byproduct
long before that.
2. My ducting is 8" IPS 63 psi irrigation PVC with schedule 40 fittings. The pipe itself is cheap enough but the fittings are expensive.
Standard squirrel cage blowers, belt driven, open one face, have the same configuration as mine. They inherently make a 90 deg turn. But, as far as
I know they are designed to be mounted such that the fan shaft is horizontal. Ie, the shaft bearings are for radial loads only, not axial (thrust)
loads. All I can suggest is that you search the Grainger and McMaster-Carr catalogs.
[Edited on 27-1-2010 by Magpie]
[Edited on 27-1-2010 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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evercurious
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@ Magpie
1. Your Grainger blower, can it handle HCl fumes without being damaged?
2. The exhaust area for that blower is a rectangular shape - where did you find rectangular shaped PVC piping? I've only ever seen circular ones
(forgive me if this seems like a silly question - maybe it can be purchased from pluming supply stores?)
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Magpie
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| Quote: Originally posted by evercurious | @ Magpie
1. Your Grainger blower, can it handle HCl fumes without being damaged?
2. The exhaust area for that blower is a rectangular shape - where did you find rectangular shaped PVC piping? I've only ever seen circular ones
(forgive me if this seems like a silly question - maybe it can be purchased from pluming supply stores?) |
1. The blower internals are epoxy coated. Whatever HCl fumes I generate are minimal and are diluted with 450 cubic feet per minute of air. I
haven't inspected the internals of my blower in 5 years, but it seems to be working fine. I only occaisionally use HCl - my hobby is chemistry, not
metal dissolving.
2. All those blowers have rectangular outlets. I had a metal fabricator make a transition piece out of stainless steel. Yes, it was expensive.
The single most important condition for a successful synthesis is good mixing - Nicodem
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