Sciencemadness Discussion Board - Questions about Fume Hoods and Filtering Exhaust

Questions about Fume Hoods and Filtering Exhaust

evercurious - 24-1-2010 at 19:08

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!

watson.fawkes - 25-1-2010 at 08:05

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.

evercurious - 25-1-2010 at 09:29

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!

hissingnoise - 25-1-2010 at 11:05

Some growers use ozone generators to remove - ooops, wrong topic???

watson.fawkes - 25-1-2010 at 13:05

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?

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.

evercurious - 25-1-2010 at 14:40

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.

entropy51 - 25-1-2010 at 15:39

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]

evercurious - 25-1-2010 at 20:56

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)?

gsd - 25-1-2010 at 21:42

@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

entropy51 - 26-1-2010 at 05:30

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".

evercurious - 26-1-2010 at 10:34

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]

entropy51 - 26-1-2010 at 11:11

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.

Magpie - 26-1-2010 at 11:35

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.

inverted funnel.bmp - 706kB

[Edited on 26-1-2010 by Magpie]

watson.fawkes - 26-1-2010 at 14:23

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.

Magpie - 26-1-2010 at 16:01

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:

inverted funnel.jpg - 67kB

evercurious - 26-1-2010 at 16:07

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?

Magpie - 26-1-2010 at 16:17

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.

entropy51 - 26-1-2010 at 16:26

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.

watson.fawkes - 26-1-2010 at 17:53

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.

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.

watson.fawkes - 26-1-2010 at 18:04

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.

evercurious - 26-1-2010 at 18:53

@ 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]

watson.fawkes - 26-1-2010 at 19:37

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.

Magpie - 27-1-2010 at 09:10

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]

evercurious - 30-1-2010 at 07:35

@ 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?)

Magpie - 30-1-2010 at 09:40

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.

evercurious - 4-2-2010 at 14:08

Quote: Originally posted by Magpie

Quote: Originally posted by evercurious

Regarding your second point - I'm having a difficult time figuring out how to construct a simple fumehood without spending a fortune on custom parts. Any suggestions as to a type of blower or fan that would work adequately for what I want - without having to get a custom made part? Is it possible to find square shaped PVC piping that I could use instead - or is a transition piece really needed?

Magpie - 4-2-2010 at 18:50

Quote: Originally posted by evercurious

... I'm having a difficult time figuring out how to construct a simple fumehood without spending a fortune on custom parts.

Any suggestions as to a type of blower or fan that would work adequately for what I want - without having to get a custom made part?

Is it possible to find square shaped PVC piping that I could use instead - or is a transition piece really needed?

> What's so simple about a fumehood? If they were simple vendors would not be able to sell them for upwards of $10,000.

>You can build it right, or you can throw something together. I'm the wrong guy to ask for the latter.

>I'm not aware of rectangular PVC ducting.

watson.fawkes - 4-2-2010 at 19:31

Quote: Originally posted by evercurious

Regarding your second point - I'm having a difficult time figuring out how to construct a simple fumehood without spending a fortune on custom parts [...] Is it possible to find square shaped PVC piping that I could use instead - or is a transition piece really needed?

It's either time or money, so you're voting for time. I hope your workshop is moderately well equipped, because you're voting to make the requisite parts yourself. If you don't want to use stainless for materials costs, you could use PVC sheet; contact a supplier to plastics fabricators in a yellow pages.

I've seen centrifugal blowers with round output ports; they're all made from cast plastic. I seem to recall they're more money that the ones from metal for equivalent size, and I've never seen them particularly large. Perhaps you'll have better luck than me in locating one.

evercurious - 4-2-2010 at 20:21

So everyone on here who has built themselves a fume hood has had a custom-made transition piece built for their blower? I guess I'm just wondering what the alternatives are. What did you use watson?

watson.fawkes - 4-2-2010 at 20:41

Quote: Originally posted by evercurious

So everyone on here who has built themselves a fume hood has had a custom-made transition piece built for their blower? I guess I'm just wondering what the alternatives are. What did you use watson?

I do my own fabrication. My shop, not in a residential space, is moderately well equipped.

You can buy transitions off the shelf in galvanized sheet metal (zinc coated mild steel). For light duty, they'll work OK. Corrosives will eat them. If you're dealing with corrosives, you need adequate materials. I don't know how much HCl you're planning on removing, whether it's wet or dry, or what temperature it is, so I really don't know what level of protection against corrosion would be adequate for your situation. It's possible that a powder-coated or epoxy-coated standard transition would do fine for you. What I can say, is that if you need heroic measures, it's possibly something not to do in your basement.

un0me2 - 6-2-2010 at 14:14

From my understanding H2S & Thiols can be oxidised to be a little less noxious using hypochlorite solution, amines can be absorbed into acid, acids into base and ammonia into water/dilute acid. The problems that still occur are mainly to do with solvents, most of which will be absorbed into activated charcoal. I've heard/read that various other strategies can be/are being used, but this would neutralize most of the more noxious fumes from home chemistry. Of course, brushless motors in anything that is going to be exposed to flammable vapors is a good idea.

PS In the past I've generally utilized the fact that I resided in a low-density/rural area, and only had to neutralize the MOST noxious/dangerous fumes, the rest could be allowed to run free. Now I am in a suburban setting and I too will have to look at neutralizing/deodorizing the outflow from my experiments.

[Edited on 6-2-2010 by un0me2]

evercurious - 8-2-2010 at 09:56

What about the materials I should use to line the hood with - I have read about people using wood then just covering it with a layer of epoxy resin. Would that be adequate, or would seeing if I can get some polypropylene and coating that with epoxy be a better idea? The wood would definitely be easier...