Fume hood venturi.

[goldenchild]

SOOOOO, all I need is a jet engine,,hmmm That I could probably do....

Goldenchild. Are you sure it will work? I took this design from something 4metals drew up in another thread and if I recall he has built many fume hoods.... Not doubting you brother, just getting tired of reinventing this thing.

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=4776&hilit=fume+hood

At the least your current configuration isn't working I am currently working on mine and may take some pictures later in the day. I'm in the testing phases right now. I can say that with the configuration above I am getting results. There are leaks and nothing is sealed but there is still suction so I'm sure it will work when all is in place. Like I mentioned you should do this piecemeal making sure one piece works before moving onto the next and making nothing permanent until the end. Hence the leaks. My set up is much smaller only having one downpipe but the principle is the same. I think your blower is more than enough to accommodate all of your downpipes.

Edit Keep in mind that with 4metals' design the fumes are being drawn in and exhausted directly through the blowers.

 

[eeTHr]

goldenchild---

I made a fume hood out of a plastic tub somewhat like the one in your picture. It seems to work pretty good for awhile, but it turned out that the heat made the plastic brittle.

One day, when I wasn't using it, my cat jumped on top of it, and the whole top shattered and fell in.

Have you ever seen a cat run really fast? :mrgreen:


Maybe yours is a different kind. :?:

 

[goldenchild]

Was yours made of #2 (HDPE), #4 (LDPE) or #5 (PP) plastic?

 

[eeTHr]

I don't know. I got it at Lowe's. It was just one of those rubbery kind of translucent tubs, with a top that clamped-on at the handles. It's for storing stuff, I guess. It was somewhat flexible, and seemed to hold up to acids well.

 

[goldenchild]

Ok so here is the configuration I decided on. I took this picture right before sealing everything. Obviously the blower hasn't been set up and any suggestions on what to use to hold up an odd shaped object like this would be appreciated :roll: I also have not joined the two 90 degree elbows yet. I'm thinking of joining them together with a union or something similar. This way if I have to move anything for maintenance and what not I can detach the hood from the exhaust pipe. Anyone see any downside to that? After that I guess the next step will be to film the fume hood in action 8)

 

[Barren Realms 007]

You can get a couple of flanges with a gasket between them that will last longer and give you less trouble than a union and will possibly cost you less.

 

[eeTHr]

Every time you add a 90, it adds a restriction factor to your flow.
You can use a 45, and twist it 45, to reach the wall. Run that directly through the wall, and put the second 45 outside, and twist it to go up the outside wall (I assume that's where the existing goes, to above your roof).

This would reduce the total bends from 4 x 90 = 360, to 2 x 45 = 90 degrees. This would reduce back pressure from your exhaust line, and increase overall suction.

 

[Barren Realms 007]

Every time you add a 90, it adds a restriction factor to your flow.
You can use a 45, and twist it 45, to reach the wall. Run that directly through the wall, and put the second 45 outside, and twist it to go up the outside wall (I assume that's where the existing goes, to above your roof).

This would reduce the total bends from 4 x 90 = 360, to 2 x 45 = 90 degrees. This would reduce back pressure from your exhaust line, and increase overall suction.

Under normal circumstnces this would be true. But with him reducing down to 2" at his processing container and then expanding back up to 4" after he leaves this it will not make that much of a difference because his restriction will be at the 2".

 

[eeTHr]

I think the smaller PVC at the "T" would have a higher flow rate within it, and also put some load on the motor, thus reducing total air volume. When the air exits the "T" restriction, it will spread out and slow down, and there will be less flow volume, but only proportional to the power limitation of the blower motor causing it to slow down.

In other words, if the motor didn't bog down at all, there would merely be a trade of medium air speed for high speed at the smaller diameter "T." Then a trade of high speed for medium speed after the "T." But since the restriction will cause the blower motor to slow some amount, yes, there will be reduced air flow overall.

Kind of like when a electrical transformer raises the voltage, but lowers the current, with the power remaining the same, except for losses due to less than 100% efficiency of all transformers.

But I also think that adding futther resistance to the exhaust, of a type which has no trade-off but is pure flow reduction, would be non-optimum.

The difference between 360 degrees and 90 seems significant. I know there are formulas for this when running water pipes, and similarly for designing air conditioning systems.

A good way to determine the amount of suction gained, by eliminating 270 degrees of turn, would be to measure both configurations with a vacuum guage at the hood pipe.

 

[Barren Realms 007]

All of those are good point's eeTHr. If I were close I would take my air velocity meter and see what the readings are under the different circumstances, but I'm not. Let's see how he does when he cranks that baby up. 8)

All the point's we both have laid out might be mute because it is such a short run they will not come into play.

One thing I would recomend is him putting a drain port on the pipe to drain any condensation he gets downstream of the hood.

 

[eeTHr]

It might be good to tip the horizontal piping a little down grade on the exhaust end, so the condensation doesn't flow back down into the hood at the wrong time.

And, as you said, put a 1/2" PVC ball valve at the lowest point. This would require drilling and PVC primer and glue, but that's pretty easy.

The short distance of the exhaust run, won't reduce the negative effects of the 360 degrees worth of turns. There is even a drag factor per foot of straight pipe, which must be calculated per building codes for water pipes. I'm not familiar with the codes for air ducts, but there is no reason not to believe that there is some significant drag factor in the straight sections there, also.

 

[callicom]

Page 71 of Ammen's book shows a fume hood venturi setup.

Jim

this hood looks like the blower is pushing in a smaller pipe thats inserted inside the bigger pipe causing a siphonning effect, is that right :?:

 

[glondor]

I tried this with my set up, it did not work for me. 3 inch inside 4 inch passing the T. No draw at the hood.

 

[Barren Realms 007]

I tried this with my set up, it did not work for me. 3 inch inside 4 inch passing the T. No draw at the hood.

I wonder if it would work with a 2" pipe inserted.


I have never done it but my dad use to tell me a story of a room where the air was being sucked out of a room rather than being blown into a room. When investigated the problem he found that someone had installed the Wye backwards in the duct run.

I still think you are trying to move too much air thru the pipe from that blower.

Ask your friend that gave you the blower if he has an old bathroom vent fan (they put out 100-150 CFM) and set it up to blow into the pipe. If he dosen't I can send you one I have the gut's to probably 50 of them in my shop and I'll be glad to send one to you.

Try a hand held hair dryer taped in there.

 

[eeTHr]

Someone suggested an electric leaf blower, I think on the other thread about hoods. They do put out some good CFMs. They are loud, though, so I'm thinking of sound proofing around it, with an opening for the air intake.

It was also mentioned somewhere that it should be 100 CFM per square foot of opening in the front, in order to have sufficient suction when working with the door open.

 

[eeTHr]

I just took a quick look at Lowe's for leaf blowers, to check approximate prices and CFM. Here's a 12 Amp Black & Decker for $50 plus tax.

It says 245 CFM (at 200 MPH). That would pull an opening of 20" w x 16" H, for a dual hot plate setup, or 18" x 18" for a single.

But that's not counting efficiency loss due to the venturi effect. Does anyone know the maximum effeciency for the venturi, and the best configuration for achieving that?

Edit: They also have one made by Task Force, for the same price, rated at 380 CFM.

 

[goldenchild]

P.S. A 100-150 cfm fan/ hair dryer/ leaf blower won't be enough for glondor's huge hood. Not even close.

 

[4metals]

When you open the door were you able to smell the ongoing reaction?

 

[goldenchild]

When you open the door were you able to smell the ongoing reaction?

No. In fact, the video only shows when the reaction was very tame. I got a little over excited and added too much nitric almost causing a boil over. A bunch of NOx gas got produced but was still all good. Even when open. This design really sucks :lol:

 

[eeTHr]

goldenchile---

Thanks for making a video!

But now I'm wondering why the sides of the hood tub didn't pull inward when you closed the door, since I didn't see any air inlet holes in the hood.

Does the reduced air flow, in turn, reduce the venturi effect? Maybe the venturi system is self-compensating in that respect? If that is so, it would be another interesting benefit of the venturi system.

When I had my plastic tub hood, I set it on it's side, and used the lid as the front door, with side hinges. I only had a small suction fan at first, but when the door was about an inch from being closed, the suction would close it the rest of the way. It wasn't enough suction to use it with the door open, but I had to drill some air vent holes to get a flow of air through the hood and thus out the exhaust duct.

When I changed to a shop vac, it had more suction, and I had to drill more holes, or the sides would start pulling in.

It seems like you would need at least a few air inlet holes, in order for the fumes to flow out of the hood, though. (But I didn't see any buildup of fumes in the video.)

Glad to see it working, whatever is going on with it!


P.S. I looked back to see if you posted the rated CFM for that blower, but couldn't find it. What is the blower's CFM?