Questions regarding my ventilation set up...

[Firelilly]

I'm going to run this by y'all, because I've sort of had one notion all along on what the final set up for my ventilation would be like and my husband, as it turns out, has had another. :P We've both decided that it would be a good idea to hear what others with more experience have to say on the subject.

What we have is a 600 CFM squirrel cage fan that will be mounted under the workbench surface. There will be about a three foot length of round 6'' duct coming from the fan to the height of the workbench and then a round to rectangle vent that will be above the workbench surface.

The 6'' x 10'' rectangle vent will be directly in front of my torch, about 2.5 feet from the flame. In my mind I thought that would be all that was needed, considering how powerful the fan is and what a short jog of duct work there will be.

But DH is considering fabricating, from aluminum sheet, a larger hood that will project from the 6''x10'' vent and sit on the workbench itself. I know I have seen pics where folks have hoods like this set up on their work surface, but I'm wondering if it's really necessary in this instance?

I know DH want's to make things as safe as possible which, of course, is the right headed way to be. But I'm hoping the aluminum hood would be overkill. I'd like to keep the work surface more open and not have a big hood sitting on the workbench if I don't have to.

Any input would be welcome.

Thanks very much!

Lil

[Mr. Smiley]

I would think that if you go bigger on the duct than 6" flex, it may be better. Since you don't have far to go anyway, regular smooth hard duct would be best. I know it's not easiest. Then you "might" have enough flow and be able to avoid the hood. The hood is a good thing, because if something doesn't get sucked right into the opening, it is going to go up... it's hot. Then the hood kind of traps it and directs it into the exhaust. There are some guys here than are much better at ventilation than I am. One thing you can try is the incense test. Put one where your torch is and see if the smoke all gets sucked out. ;)

[Firelilly]

The input flange on the fan is 6''. So I'm not sure there would be any benefit to trying to fit a collar on that would help transition to larger duct work. It would still bottleneck back to 6'' so it wouldn't really make a dif' would it? I don't know...maybe there's something I'm not understanding here.

Yepper, I'll for sure try the incense trick. If it seems to do a good job of drawing air without a hood, maybe we could put together a hood that I could just use when or if I do something that's more particularly dangerous to work with...like if I fume beads or something. And not use it for everyday torching soft glass?

Thanks so much for the input, Mister. O:) I appreciate it!

Lil

[Dale M.]

First off I'm not real excited about a downdraft system... Yes it works.... And I still think one needs to consider a hood over the work space. With a open bench, air is drawn from all over and anything siting on bench will obstruct or divert air flow.... With a fume hood or barley box arrangement, it guarantees a fresh air flow past your face without any obstruction or diversion from other surrounding objects.

Also with hood you may be able to use less then a 600cfm fan and there for cut down the noise and "draft" the large volume of air movement may cause...

You also need to figure face velocity in this equation.... With a open system it hard to tell if you have sufficient capture, with fume hood or just a simple hood you can figure face velocity and pretty much guarantee capturing contaminated air and pulling in plenty of fresh air... Face velocity equation is roughly 100cfm air for each square foot of hood opening. Hood opening is not a slot cut in bench top you are trying to draw air through

Also with down draft system it will be trying to suck your flame down the vent and probably cause a lot of turbulence in flame...

Also going to 8 inch duct will reduce static pressure and allow for better air flow.

Dale

[Firelilly]

D'oh! Ok, well not what I was hoping to hear...but thank you very much for the information. It's much appreciated.

Lil

[Firelilly]

Dale, I got to thinking that when you were talking about a downdraft system that maybe you thought I meant the vent was going to end flush with the tabletop?

I'm talking about this piece, sticking above the table's surface, directly in front of the torch.

18096--------- table surface--------18097

Is this what you thought I meant? I'm thinking the fan will be pulling the flame straight forward. Which I suppose could still wreak havoc if the fan is too powerful? We went with a fairly high powered fan because everything I was reading seemed to be saying people weren't going high enough with the CFM's. Now you have me wondering if the fan is overkill. :P

I asked DH early on if maybe we could put the fan on a rheostat. We're wondering if that's going to shorten the life of the fan's motor or not. The makers won't indorse such a thing for obvious reasons. But I don't think that necessarily means the rheostat is a terrible idea. We're figuring to go with it.

15961 Ahhhh!

O:)
Lil

[Dale M.]

Aaah..... A mini hood. Should work. It makes more sense that flush with table top......

Putting fan on rheostat will only be successful if fan is designed for variable speed operation.... Yes slowing it down with a rheostat if it is not designed to be, will cause it to overheat and fail sooner ...

Dale

[Firelilly]

Yeah, I didn't think we were quite on the same page there for a minute. O:)

Thanks so much, Dale! I appreciate you taking the time.

Lil