Sciencemadness Discussion Board

Factors that decide if a fume hood is good or bad?

Draeger - 27-3-2020 at 14:40

So, recently I've found a collection of seemingly good looking fume hoods for not so high prices, compared to what you'd usually see.

Basically, I just want to know which factors I should look for to determine if a fume hood is worth buying or not since I know nothing about fume hoods.

CouchHatter - 27-3-2020 at 17:11

Look up the different classes of fume hoods, if they are labeled. Some recirculate air, which is fine for a biological lab but no good for mine. I found a shiny, cheap Class II Type A2 that was useless to me because it recirculated something like 60% of the air.

Make sure the sash is easy to raise and lower. A hood doesn't have to be airtight to be good.

Sulaiman - 27-3-2020 at 17:54

I do not have a fume cupboard and I have little experience, but one thing that I know to be important is

could your exhaust fumes get to your neighbours ?

If the answer is yes then you should consider scrubbing any noxious/smelly/toxic gasses before they go out towards your neighbours.

My father was from Ahlen, I've been there a few times so I know that Westfalia is quite rural,
but if you live in a town or suburb you must consider your neighbours.
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some other considerations;
. the fan will suck in external air via the fume cupboard,
in winter this means a continuous flow of cold air into your workspace,
so you will work in the cold, or significantly spend money on heating.

. if you plan on doing distillations etc. inside the fume cupboard then it needs to be tall, and preferably wide also.
Just a heating mantle, rbf, column and head with thermometer can be quite tall if you use NS24 or larger glassware.

. check that there is some method of handling a large (e.g. a litre or more) spill of liquid

. if your fume hood uses some type of filtering system,
ensure that you can get new filters that fit, at reasonable cost.

. if the fan is not powerful enough, wind direction may force air to flow in reverse - out of the cupboard towards yourself.
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these are just my thoughts - not based experience,
others may have more specific recommendations.

Herr Haber - 28-3-2020 at 07:12

I too have neighbours and of course thought exactly like Sulaiman since I live in a building.
The sucking cold air in definitely affects what you are doing if it involves heat and the moving air worsens it. Time to keep the al foil around.


I would like to build myself a half open enclosure for my needs. The idea being that I could add or remove parts such as a shield. Also, I cant build a closed space that would fit my reaction aparatus AND the 60 cm condenser :)

Then I thought about how much air gets mixed with the fumes before going outside.
Usually... lots.

Let's say I'm generating Bromine, Chlorine, NOx, NH3 etc. for whatever reason. First, if possible I'll scrub as much as I can. Maybe getting a useful reagent in the process btw. Of course, if those are not by-products hopefully the better part will be used up in the reaction.
Then, I'll have it sucked away by a 300+ cubic meter / hour exhaust fan.
I'm actually at that point also worried about solvents. The adjustable inline fan I have in mind is for home installations. Motors, brushes... things to think about !

That also means diluting a lot the very little I plan to let out.

I am looking for a quality air ducting, that could survive a few corrosives and be easily replacable. The ones that look like an acordeon are usually cheap and I could position it where needed possibly attaching one of those carbon filters that can be found in hydroponics shops.

So far I only had one incident that drew a neighbour attention. More than anything she was kidding when she asked me if I wasnt afraid of dyeing orange the outer wall. Way too much concentrated acid on silverware at once that day :)

Other that that I regularly ask the neighbours around if my chemistry activities are bothering them and they all seem surprised when I ask.
Now they waive back when they see me all gloved up, with my lab coat, gas mask on putting a big three liters reactor on the edge of my window !
"Nice blue, that is copper isnt it?" was one of the last comments

There are a lot of discussions on the forum about building a fume hood. My supplier would be more than happy to design me one for thousands of Euros.

The inspiration from ScienceMadness and what's available in the hardware store will be more than enough for my usual need I hope.

Draeger - 28-3-2020 at 10:04

I have no idea how moving gasses work, so like, lemme just ask that.

Firstly, would it be ok to dump out gasses horizontally? I'm asking since I saw most people let it fly out their roof, but that's not possible for me.

Secondly, would it be okay to throw out the gasses without scrubbing with no neighbors, except those in the same house? I live on the top floor of a house, and there's other people in flats, all on floors below me, so could gasses I vent out reach them and be toxic to them? I know it is quite unspecific, but maybe there's some general rule or something that prevents it.

[Edited on 28-3-2020 by Draeger]

RedDwarf - 29-3-2020 at 05:44

I remember when I was at school (>40 years ago) and we had labs in separate buildings on different levels. We were doing something that involved generating something obnoxious safely in the fume cupboard. It was a hot day so the first years in the general science lab had their windows open... there were a lot of very sick looking 11 year olds that afternoon by the time they worked out what was happening. Short message - if you live in close proximity with others the only way to use a fume hood is to make sure that the outlet is scrubbed.

Draeger - 30-3-2020 at 14:45

I'm also wondering; how strong does the fan need to be? I know that it depends on the length the gasses need to travel, but what if the exit was right behind the hood and only needed to travel like a meter or so?

Idk if this is unrealistic to ask or whatever, I really don't know much about how this is supposed to really work, even after reading the fume hood thread.

CouchHatter - 30-3-2020 at 20:10

Hope this helps. It's a bunch of resources I complied when I built my hood.

Duct distance is not as much a factor as turns and bends, IIRC. The blower is usually far away from the user because it's LOUD. Some members have built hoods with smaller blowers close to the hood face. There is some variability in hood designs, but if you're buying a non-custom hood then the face openings will probably be all about the same size, AND a big factor in choosing your non-inline blower motor.

Abromination - 31-3-2020 at 09:24

Quote: Originally posted by CouchHatter  
Hope this helps. It's a bunch of resources I complied when I built my hood.

Duct distance is not as much a factor as turns and bends, IIRC. The blower is usually far away from the user because it's LOUD. Some members have built hoods with smaller blowers close to the hood face. There is some variability in hood designs, but if you're buying a non-custom hood then the face openings will probably be all about the same size, AND a big factor in choosing your non-inline blower motor.

Not sure if this is in there or not, but this doc was super helpful when I built mine, I believe you sent it to me some time last year.
Great resource.
https://drive.google.com/open?id=1KzthNZQWE5O8RFWazeEt7a3hnp...