Fume hood venturi.

[glondor]

Hey all I need the assistance of the fume hood experts. Every attempt to vent with the venturi effect has failed. I have increased the size of the blower 3 times thinking I was not moving enough air. ( thanks to Craig the furnace guy in Hamilton). In each case I get a slight positive pressure at the hood inlets instead of a vacuum. The pipes increase in size from 3 inch to 4 inch to 6 inch as they leave the building. Air flow is strong. I get no draw in the hood, just a slight positive pressure. Can any one give me some help? I will answer any questions regarding this.

 

[Barren Realms 007]

Your should have your blower attached at the straight part of the "Y" and you will have less air turbulance.

The fan and blower you are using will move 2,000 CFM or more, way more air than you need.

 

[glondor]

I will try to rework it. Thanks BR. My hood is large. Dual hood, Two intakes in left side and one intake in right. Need to move a lot of air.

 

[Barren Realms 007]

Your main restriction will be in the size of your vent pipe.

 

[goldenchild]

I don't know if I'm missing something here but... your blower is on the wrong side. In the third picture it should be blowing from right to left and then up through the ceiling. Also it looks like your piping is all the same width. At the hole above the fume hood there should be constriction to create the venturi affect.

 

[glondor]

I had it that way in the beginning Golden child. It did not draw either. Small blower top left, now blocked and reworked to top right.

 

[goldenchild]

Well. I'm pretty sure it definately won't work with the blower on the exhaust side. The small blower may possibley not be enough? There are possible leaks in the setup? Did you test just putting the blower on one section of the pipe with the constrictive piece and seeing if it drew? You gotta do this piecemeal.

Edit I didn't even see the second pic. It looks like your piping is too wide at the down pipes. To make it easier I bet you used Ts that matched the pipe widths. 3 inch? It would probably work better if you had reducers at each side of the piping with a 1-1.25 quarter Ts in between to create the venturi affect. Also you will probably need some kind of fixture to close the hood or have one hell of a blower!

 

[glondor]

Thanks for your help Golden Child. I gave up on the small blower blowing across the intakes in favour of the larger one drawing from them because I thought the small one was too small. I was hoping the way I have it now would work. I will re arrange the Y pipe at the blower to blow straight through as per Barren realms suggestion and also go to a larger pipe just after the Y and see if that works.

Any other suggestions welcomed. I need the damned thing to work.

 

[goldenchild]

Thanks for your help Golden Child. I gave up on the small blower blowing across the intakes in favour of the larger one drawing from them because I thought the small one was too small. I was hoping the way I have it now would work. I will re arrange the Y pipe at the blower to blow straight through as per Barren realms suggestion and also go to a larger pipe just after the Y and see if that works.

Any other suggestions welcomed. I need the damned thing to work.

Just keep it simple. Give me a sec to draw something up.

 

[jimdoc]

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

Jim

 

[goldenchild]

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

Jim

Dang lol.

 

[rusty]

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

Jim

Dang lol.

Print version

 

[samuel-a]

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

Jim

Yup, that's prcatically what i built here

 

[glondor]

I did base my first attempt on a super drawing by 4metals in a "build your own fume hood thread" Here>> http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=4776&hilit=hood+vent Nice drawing with multiple intakes. I also referenced your photo Samuel A. I will work on it tomorrow. Maybe later tonight. I will try to rearrange the pipes.

Thanks for all your help. I will keep you posted.

 

[glondor]

Ok I did a straight through run of the 4 inch, using a sanitary t at the junction with the 3 inch pipe in the hood. No joy. Blows out all 3 vents really really good. I guess I have to rearrange the pipes again to the other side of the hood and try blowing across the interior vents.

 

[goldenchild]

Again. The blower is on the wrong side. The only suction you MAY achieve is right at the joint. The fan has to go on the other side.

 

[Barren Realms 007]

Take the Tee out and put the Wye back in. Also try taking some cardboard and tape and close up one side of your blower.

 

[goldenchild]

Take the Tee out and put the Wye back in. Also try taking some cardboard and tape and close up one side of your blower.

I don't get it. Unless he puts a jet engine on the exhaust side there won't be enough suction at the down pipes.

 

[glondor]

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.

https://goldrefiningforum.com/threads/how-to-build-an-acid-resistant-hood-scrubber.4776/

 

[eeTHr]

I'm planning to build a venturi system also, so I've been following these posts whenever I see them.

The first suggestion someone posted on this was to have the "downpipe" (from the hood going up to the exhaust), smaller than the exhaust duct, to product more air pressure and prevent the flow from starting off going into the hood, thus producing some suction out of the hood, which would continue in that direction once started. This sounds logical.

Next, came the suggestion of the exhaust duct narrowing down at the "T" to the hood, accompanied by a drawing. The drawing had the same size small pipe from the hood going into the "T." This narrow section of the exhaust ducting would increase the speed of the air through the "T," and seems like it would produce more suction effect.

I had a hard time getting my mind around the "blower on the wrong side" part, which someone mentioned, but now I see that since the blown air passes over each "T," it puts them all in series, with the same air speed over each one. Whereas having all the hood pipes joining, before getting to the venturi "T," places the hood pipes all in parallel, and will divide the air speed by the number of hood pipes.

As a knee-jerk reaction, I like the design pictured in the book best. Because it just looks like it would draw more. But looks don't necessarily count, I guess.

One interesting thing I know---my brother made a suction pump with a water hose, just coming out of a faucet, and pumped the air out of his car air conditioner with it. It was a venturi set-up, and simply used a "T" connector. If I remember correctly, the air line was, in fact, smaller than the water line, at the "T" connector.

I was planning on seeing if I could get a "Y" PVC connector, with the angled branch for the hood ducting, smaller than the straight branch. And if that didn't pan out, I was thinking of using a 90 degree fitting, and drilling a hold at the elbow, so a smaller pipe from the blower would insert and go a few inches up the exhaust end for a suction design similar to the book picture.

I hope this discussion continues, because it would be helpful to many, if less expensive blowers can be used because the fumes wouldn't go through them.

I'm not ready to start my project yet, or I would post some vacuum readings from the different configurations.

Thanks to you guys for posting what you have so far!