Fan for a modified funnel

artsyuno · #1 2010-11-10, 11:33pm

I'm in the middle of a studio build and I am going to put in a ventilation system that is kind of an oversized funnel. My countertop will be against the wall and the vent opening will sit at the wall or fairly close to it and vent straight out through the wall with the fan on the outside of the wall (so maybe 1 turn on the outside of the wall so that the fan aims downward). The dimensions of the opening will be ~ 16"x16". It'll look a lot like the one in the photos below, but will have all rigid 8" or 10" ducting (depending on fan cfm) and won't go down and through the floor. Any thoughts on what the appropriate fan strength might be?

JoeDeM · #2 2010-11-11, 8:06am

I would get a vortex or a Fantech fan the vortex 8" is a 747 cfm, but it really depends how far you are going to run the duct. The fan in your picture is a terrible fan, and should only be used as a booster fan with another fan. Vortex fan can usually be found in local hydroponics stores.

laserglass · #3 2010-11-11, 8:54am

one of these would work. there is a 6 inch diameter round vent on the side of the fan you cannot see. get some duct hardware and use adhesive and screws to attach it to the fan input. then use duct tape to seal and to connect to your hood. i use this fan for ventilation on my major burner without any problem. and it cheap and easy to find.

Dale M. · #4 2010-11-11, 9:19am

So far there has not been any real studies on "funnel system" and how effective they are other than several comments "they work fine" which is not very definitive....

With the funnel system the actual air flow drops off radically the further you move away from funnel opening....

My best guess would be to use at least 500 to 750 cfm for fan and have funnel at a position that is slightly above the torch combustion area so flame and combustion by products (which tends to rise) are more or less aimed directly into it.....

Having funnel at bench top level is going to reduce efficiency of capture of combustion byproducts...

Note in this picture the hot gas rising from torch... This is what you have to capture....

This IS the concept on how to capture it....

If you have funnel on same plane as torch (at bench top) it will take a very large volume of air movement (more CFM) to capture combustion by products...

Also funnel will have to be a lot closer to tip of flame for better capture, a lot closer than if you used a canopy hood for fume cabinet (box)...

In my opinion people who have the funnel low down on bench didn't exactly get it right..... IT may work, but the cost (in CFM -Fan size ) is a lot higher than if funnel is in line with rising gas plume..

Dale

artsyuno · #5 2010-11-11, 9:20am

I'm not using the fan in the photo. The fan will be a real inline fan, not a booster. The venting will go straight through the wall and then down, so I guess a foot or two, one turn and another foot or two at most. My contractor will have the installation done and seems to have a good handle on how to do it. We're just trying to figure out the needed cfm. If I use the usual formula, it comes out at 222 cfm, but that seems low and I know this set up may vary from an overhead hood, but I prefer pulling the fumes away from me rather than past my face.

artsyuno · #6 2010-11-11, 9:29am

Thanks, Dale. The plan I've been working with is for the funnel to be slightly raised and tilted so that the flame goes into it and it comes forward over the top a bit (vs. Andrea Guarino style where it sits below the flame). I'll give my contractor this info and the photo you just linked, though. Maybe we need to go in a little higher and more angled than I'd anticipated.

Dale M. · #7 2010-11-11, 9:37am

Quote:

Originally Posted by artsyuno

I'm not using the fan in the photo. The fan will be a real inline fan, not a booster. The venting will go straight through the wall and then down, so I guess a foot or two, one turn and another foot or two at most. My contractor will have the installation done and seems to have a good handle on how to do it. We're just trying to figure out the needed cfm. If I use the usual formula, it comes out at 222 cfm, but that seems low and I know this set up may vary from an overhead hood, but I prefer pulling the fumes away from me rather than past my face.

The usual formula does not apply here because the capture area become infinity larger the more you move away from funnel opening....

Where the formula does work is with hood or fume cabinet opening is a constant size and in constant relation ship with worker...

The is a graphic example of air movement loss the further from funnel/duct opening one moves and as you move inches away air flow drops off dramatically....

Its on this site it details suction loss as you move further away from funnel/duct opening.......

http://www.engineeringtoolbox.com/ex...et-d_1008.html

Dale

artsyuno · #8 2010-11-11, 9:49am

That's very helpful. Thanks.

Lorraine Chandler · #9 2010-11-17, 10:10pm

You might also want to check out the decibel ratings as some people have white noise or ringing in the ears if the fan is really loud.