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I found this build-plan recycling on the internet (I believe it was originally published in a book written by the infamous clandestine chemist Hobart Huson) which seemed interesting to me. I do not have the money to pay for a professional fume hood, nor do I have the space to fit such thing. I have to switch the function of my work workplace sometimes because of lack of space (it is also used for woodworking, another hobby of me), so a simple demountable fume hood would be ideal. As said, I found this one with a simple build description, but I was wondering about its effectiveness and safety.

Demountable fume hood

As shown on the picture, the fume hood consists of a demountable frame, a thick plastic sheet covering the frame that is attached to a tube, and a simple exhaust system connected to an other tube that is placed outside the building, so that the fumes can diffuse in the atmosphere. The following four points concern me:

  • Is the thick plastic sheet enough the prevent most fumes from diffusing in my workplace?
  • Is this plastic sheet a potential danger in case of fire? Is there some fire resistant (at least until I'm able to pull my fire blanket) plastic sheet that should (must) be used?
  • Should I be worried about damaging the environment with these exhaust fumes as there is no filtration in this system? Or is reading the MSDS (i.e. the environment section) for the chemicals being used enough?
  • There is said nothing about the necessity of an explosion proof fan in the build instructions. To what extend should I worry about this when sucking off flammables?

Take note of the fact that we are talking about amateur chemistry here. There are some nasty fumes that need to be removed, like chlorine for example, but no dense fogs of hydrogen cyanide or phosgene.

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Since the chemical will not be in contact with the plastic for very long, very little will diffuse through the plastic. If you are concerned about fire, you could wrap the outside (not the inside) with aluminum foil; this should keep the fire contained. Have a container of sand handy in case the fire is extremely hot and the aluminum starts to burn. As long as you have a high air flow and the volatiles are coming from a small area, there should be no problem. I can't comment about the environmental effect unless you provide more detail. –  LDC3 May 24 at 17:41
    
You might want to consider building in a simple filtering system to prevent damage to the pump - as that is most likely the most expensive part of the whole construction. (Unfortunately I cannot provide any idea on how to achieve something like that.) –  Martin May 25 at 9:59
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3 Answers 3

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If you do a little reading about fume cupboards, and in particular, benchtop extraction hoods, you will discover that they are constructed from materials not too dissimilar to your design; consider the difference being amateur use versus professional use. To address your four main concerns:

Is the thick plastic sheet enough the prevent most fumes from diffusing in my workplace?

Provided the plastic is thick enough. So, certainly thicker than cling wrap, and I'd be happy using the types of material common to decent portable greenhouses. It needs to be robust enough to withstand the extraction forces from your exhaust fan. If your exhaust is working correctly, any chemicals will not be in contact with the plastic long enough to be a concern. Be sure you run a post-purge extraction once you have finished your day's chemistry to remove all traces of chemical fumes.

Is this plastic sheet a potential danger in case of fire? Is there some fire resistant (at least until I'm able to pull my fire blanket) plastic sheet that should (must) be used?

Most fume hoods are made from combustible material. I have, in my 20+ year career as a research chemist, seen many fume hoods destroyed by fire. This includes hoods made from wood, various forms of plastic, even those with metal frames and plastic components. If you have a fire, yes the plastic sheet will burn, and depending on the material, it may burn quickly or slowly - you will have to judge that. This, then, will increase the volume of your fire, and the amount of combustible material. So you will need to manage that in your workshop layout. So, yes, this is a hazard, but one that you should be able to plan for, and adequately implement a risk mitigation strategy. But the real concerns are not whether the plastic sheet would be a danger if there is fire, but (a)will the plastic sheet increase the risk of fire, and (b)what effect will your exhaust fan have in the event of fire. If you are doing chemistry that involves refluxing, as your picture suggests, you need to make sure that the size of the hood is big enough that you are not heating your plastic, and that the air flow is sufficient to extract any hot air from the roof of your hood. You also need to consider that fires in fume hoods are generally more active than fires on open bench areas because they are actively fanned and have a fresh air source driven into the fire zone because of your extraction fan. A fire blanket in your workshop is not a sufficient measure against the type of fire you might have doing chemistry, Buy yourself at least one dry chemical extinguisher, which are sold in all major retailers that sell fire blankets. If you are using chlorine, then you should NOT use the ABC type dry extinguishers that use ammonium based material, as they will react violently with chlorine. You should use a BC type, which is based on sodium bicarbonate.

Should I be worried about damaging the environment with these exhaust fumes as there is no filtration in this system? Or is reading the MSDS (i.e. the environment section) for the chemicals being used enough?

Yes, you should be worried. Reading the MSDS is not enough. You need to read it, understand it, and then take appropriate measures to mitigate any hazardous risk to the local environment. If you are extracting chlorine gas, where is it going? Apart from being pulled through your pump system, which will be corroded pretty badly after only a few uses, what is directly outside at your pump exhaust? If you position your exhaust just outside a window of your building, what local wind channelling exists to push that exhaust straight into the next window of your, or your neighbour's house? You could consider installing a carbon filter like the types used on water systems, as these are not overly expensive, but for many commercial systems, no scrubbing or filtration occurs, but the key issue is where is your exhaust going? So, understand your design, consider your risks, and plan your exhaust appropriately.

There is said nothing about the necessity of an explosion proof fan in the build instructions. To what extend should I worry about this when sucking off flammables?

I'm pretty sure that all building codes (certainly all the ones I have ever encountered) will specify that any motor used in an area containing flammable vapours (and dust, if you need to consider that for your woodworking) should be of the type approved for such an environment. What dust extraction do you use for your woodworking? If you get an arc from the pump motor, you will very likely get ignition. Again, this is something I have seen in research laboratories for all sorts of non-approved electrical equipment, including pumps, air heaters, fridges. So, if you are exhausting flammable vapours, this is a real risk.

The final consideration you haven't really mentioned is the exhaust pump itself. You need to make sure that it is of suitable size to provide correct laminar flow into the front of the hood, around all your equipment, into the exhaust outlet, and pushed back through the exhaust port after the pump. You should test this using a smoke pellet, or something similar. the larger the opening of your hood, the larger the exhaust fan needs to be, which is why you should always lower the sash hood on fume cupboards when doing chemistry.

Good luck.

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"If you get an arc from the pump motor, you will very likely get ignition." What exactly do you mean by that? For woodworking I use an adjustable fan that can suck up to 360 cph (cubic meter per hour), which should be more than enough I guess. Here is a picture: i.imgur.com/A92uVf8.png (it's a PrimaKlima PK-125-EUP, used for growing manily I believe). Thanks for your thorough answer! –  Jori May 25 at 7:10
    
Arcing is something that occurs in electrical equipment where there is a faulty ground connection. It is not all that uncommon, and you may have seen it in hair dryers or battery drills etc. It causes a small spark. Equipment, such as fridges and pumps being used where there is a high level of flammable vapours are specifically designed for such an environment, and are very expensive; you will have to be the judge as to whether your conditions are producing significant flammable vapours. For instance, I would not use a kitchen exhaust hood to extract flammable vapours from refluxing acetone. –  long May 26 at 4:50
    
With refluxing? Isn't the point of refluxing that as less as possible vapor escapes the system? Also 360 cph seems quite much to me. I don't think I can afford an explosive proof fan. –  Jori May 26 at 6:22
    
Yes, absolutely. But with all things, you need to plan for worst case scenarios. I'm not suggesting you should buy a spark proof fan, just be aware of the potential hazard, assess your risk, and plan your procedures accordingly. Nobody can give you the 'you'll have no problems' thumbs up, based on the info provided, we can only make you aware of potential risks. You still need to make your own assessment based on what procedures and chemicals you will be using. –  long May 26 at 6:30
    
of course and I completely agree with you, but it is not possible for me to buy such thing anytime soon and I do not believe that many other hobby chemist could afford it either. I was just curious why you took the aceton refluxing as example and not something like alcohol solvent evaporation. I could hardly imagine that with a proper reflux condenser any significant vapor that could set off an explosion can escape. –  Jori May 26 at 6:43
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Since the chemicals will not be in contact with the plastic for very long, very little will diffuse through the plastic.

If you are concerned about fire, you could wrap the outside (not the inside) with aluminum foil; this should keep the fire contained. Have a container of sand handy in case the fire is extremely hot and the aluminum starts to burn. In fact, sand would be handy for most fires since it will decrease the amount of oxygen getting to the flames.

As long as you have a high air flow and the volatiles are coming from a small area, there should be no problem. Added: The concentration of the volatiles will not be high enough for a spark to ignite.

I can't really comment about the environmental effect unless you provide more detail. If the MSDS doesn't mention a hazardous exposure from the fumes, then you probably don't need a fume hood. You can get some activated charcoal (from a pet store) and pass the fumes through the charcoal, but I don't know how effective that would be.

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What do you consider to be high air flow? –  Jori May 26 at 6:44
    
It looks like they are using a leaf blower; that would be more than needed. –  LDC3 May 26 at 12:56
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Is the thick plastic sheet enough the prevent most fumes from diffusing in my workplace?

Yes. Actually, you should care more about the integrity of the tube

Is this plastic sheet a potential danger in case of fire?

Depends. But care more about inflammable and unmeltable working surface (metal or ceramic). Some old institutions work just fine with fume hoods using wood frames.

Is there some fire resistant (at least until I'm able to pull my fire blanket) plastic sheet that should (must) be used?

PVC is self-extinguishing plastic that is extremely hard to lit up. However, when burned, it produce toxic vapours, and is a spark risk.

Should I be worried about damaging the environment with these exhaust fumes as there is no filtration in this system?

Care about your car. It affects environment more than most other fumes you can produce.

There is said nothing about the necessity of an explosion proof fan in the build instructions. To what extend should I worry about this when sucking off flammables?

The engine must not touch the air sucked of, as there is a spark in common electric engines.The fan itself must not touch anything to not produce sparks. Everything should be oiled.

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