Sciencemadness Discussion Board

idea for slight improvement of fume hood

jimwig - 20-6-2009 at 09:22

okay you are toying (haha) with something that can kill you or make you wish you were dead.

let us say that you are using a good fume hood but the flow rate is insufficient to stop the vapor from spreading out even a little bit..... you need the super sucker!!!!

take a 2 - 4 inch diameter flex tubing and stick up the chimney..... then put the other end just as close to the Rx as possible... by doing this you have brought the suction down right to the problem... the venturi effect in the chimney will create what will hopefully be a better localized removal of the danger..........

have i tried this ?--- well sort of..... i used this principle in gold dredging and it worked (underwater) so be advised.....

check it out with some harmless smoke etc. in your hood --- ha smoke in the hood get it hahaha!

watson.fawkes - 20-6-2009 at 11:40

Quote: Originally posted by jimwig  

take a 2 - 4 inch diameter flex tubing and stick up the chimney..... then put the other end just as close to the Rx as possible.
The issue with a fume hood is not to get most of the noxious fumes out as fast as possible but to get all of the fumes out at whatever rate that actually takes. The idea that you can replace general ventilation with point source removal is flawed. You can, however, supplement your general ventilation.

Now as to your specific suggestion, just sticking a tube up in the flue make not create much net flow at all. It depends on what the flow resistance in the tube is. What would certainly work is a supplemental fan in your tube, at sufficient flow rate to take out the bulk of your fumes. The fan in the hood will work better, because there's lower concentration in the general air. Alternately, you could partition the plenum before the exhaust fan to provide a second duct for point source removal.

Even more than in the general case, supplemental exhaust really needs a fan with a motor out of the flow path. Such a fan is getting the concentrated bad stuff, whatever that stuff is.

And after all that, there are still trick to get the air flow for point source removal to work well. The best is a shroud, naturally. Barring that, a flow plate (just a plate with a hole in it) at the intake orifice does a lot to increase efficiency.

jimwig - 20-6-2009 at 16:17

pushing a flow into a finite space (finite established flow rate) {with extra local motor fan} will perhaps help but not much..... the space is already involved in exhausting the (supposedlly) maximum. so pushilng more into that existing flow just doesn't seem to accomplish the goal... which i assume is as i said a very localized suction.

the amount of gas (?) is still limited by the suction established by the roof unit...... you arre not really adding anything.

same with my idea. but the total flow is not compromised by any unsupported flow/suctiion such as would be the case with a small additional motor. only if the motor adctually added to the total vacuum would this work significantly. it would seem.

using a venturi effect it seems would because the exhaust chimney) space is still doing the exact work as before... only now the direct suction is focused on a much smaller area.... the fume hood still evacuates the work area. only now part of the suction is right at the target as opposed to the entire area.

this has worked in the past but not with a fume hood.

my experience is limited with regard to that particular application

But (and i may be wrong, dammit) BUT i would ceertainly like to try it.

and btw thanks for your insight

edit --- i think what i am getting at is that the unit (fume hood) as existant is at unity and to add anything without increasing the whole would be pushing for over unity. does that sound right?

[Edited on 21-6-2009 by jimwig]

jimwig - 20-6-2009 at 16:31

farther and this is what i thihk you were getting at; IS the suction at unity (the whole at maximum and consistant throughout) would not furnish any increase at the end of the super sucker. that is not what i meant. it would focus (?) - i lost it. i forgot my point... sorry......unity suction vs unity exhaust and ........ fuck.

i guess you are correct and my onlly thought would be that focus -- but still that would not change anything in terms of

what i need to do is actually do the experiment.... because i think the unity suction being equal is not equal... vortexes and currents being affected by the fune hood in actuality would not be equal throughout the space...... and by directing a localized and more connected flow suction might overcome the imperfection and bypass the vapors et al to the exhaust.

i am at a loss here to express my idea. is any of this making sense.....??

The_Davster - 20-6-2009 at 16:54

A length of flexible hose with beefy computer fans mounted on both ends, and a small box on one end makes a great localized exhaust system. I would not trust natural wind currents to protect my health.

jimwig, clean up your posts, they are hard to understand.

watson.fawkes - 21-6-2009 at 07:37

Quote: Originally posted by jimwig  
the amount of gas (?) is still limited by the suction established by the roof unit...... you arre not really adding anything.
This is just plain wrong. Please learn what a "fan curve" is. (There's a recent, extensive thread on this board, for example.) Briefly, a fan exhausts more if it works against less pressure differential. Supplementary fans lower the pressure difference.

More generally, you seem to be lacking the basic physical concepts needed to reason about this topic. My recommendation is to read a basic HVAC technician's manual. These are geared for beginners and will get you started. You can move on from there.

User - 21-6-2009 at 14:33

A simple trick my chemistry teacher told me was to put things that have a low boiling point ( low weight molecules ) higher in the hood.
For example a lift or chimney could be used.

I have no idea if this is still relevant, the guy worked in labs over 50 years so some of the things that used to matter could be outdated.It could well be that the hoods from that time where by far not so good as they are now.

[Edited on 21-6-2009 by User]