# Fume Hood Exhaust



## tyck (Oct 13, 2018)

Hello Everyone,

Need a bit of advice please, and appreciate any feedback. Build fume hood from scratch following many tutorials/videos and it worked okay before the acid fumes nuked my blower a month or so ago, see pic




When I say it worked okay for 3 months or so, I was able to refine and melt about 2 separate gold bar oz.

So I ended up purchasing a couple more blowers from a party here on the forum that work fine, and after reading and doing more research, I wanted to avoid my first mistake and decided to go with a new exhaust design, see next image:





Now I got this 2nd design idea from here on the forum somewhere from this image, and tried to implement it




the think is, there is a significant amount of forced air blowing outside of my garage exit vent the way it should be, but, there is also a small amount of air blowing inward of my fume hood as well. So I'm not getting total suction as I would expect I would, as I did when I had my first setup where the blower was straight up and down with the inline configuration.

What am I doing wrong, and what should I do to make things right and working again?

Thanks in advance.


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## FrugalRefiner (Oct 13, 2018)

The [stt]venture[/stt] venturi should be as close to the exhaust exit as possible. The 90 ° elbow after the [stt]venture[/stt] venturi will restrict the air flow and cause the back pressure.

Dave

Edit: It looks like auto-correct changed venturi to venture and I didn't catch it. Sorry for any confusion.


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## Smack (Oct 13, 2018)

Looks like a Vortex blower in the second pic.


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## tyck (Oct 13, 2018)

Okay, so by the venture, you mean the V-trap fitting should be moved up towards the top instead of the bottom where I have it? And I should replace the 90 el with a 45 el?

Also, is the Vortex blower i'm using not the correct blower for this configuration?

Thanks for the feedback guys!


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## nickvc (Oct 14, 2018)

I’m thinking that your fan should be connected to the pipe going out of the wall in a straight line so the vacuum is produced by the by the air pressure leaving the building ideally you want th e air from your your pump going in a straight line and the exhaust from your hood connected at a 45 degree angle I think.


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## g_axelsson (Oct 14, 2018)

Move the venturi to as close to the exhaust as possible and try to avoid any bends in the tubes between the venturi and the exhaust.

In your current setup the long tube and the bend must handle the air from the blower and the fume hood. The drag and the volume of air creates a back pressure and if that is higher than the suction created in the venturi then air starts to go the other way into the fume hood to balance out everything.

If you have the venturi close to the exit there will be less back pressure and the venturi will create a suction in the hood. The longer the piping and the more bends there are between the venturi and the hood the higher will the resistance to the flow be but it will never reverse the flow, only getting lower.

You also need to see that there are air getting into the room so you don't create a low pressure in the room, that will also impact on the efficiency of your hood.

Göran


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## tyck (Oct 14, 2018)

Thanks Göran, 

I think I understand a bit better now, unfortunately I can't go straight up through the ceiling due to the Spanish tile roofing I would damage beyond any repair I could do.

With that said, I could replace the 90 elbow with a 45 degree elbow fitting, and move the venturi up next to the 45 elbow.

Would you also recommend that I shorten the length of the pipe from the blower to the venturi, perhaps maybe a foot or two away instead of the length it currently is?

Thanks again for all the valuable feedback!


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## speed (Oct 16, 2018)

What does the 'nozzle' look like on the inside? This is the important bit. It must be matched well to the pressure/flow potential if your blower and the size of the actual duct. 

I agree tho the 90deg bend could well be a problem.


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## g_axelsson (Oct 16, 2018)

The closer you can get to the exhaust the better. If that doesn't work you might need to redesign the venturi part. Moving the blower closer to the venturi should also increase the efficiency of the blower. All pipes adds drag to the system.

There are a ton of HVAC programs out there for anyone wanting to do calculations on drag, pressure and ducting. I don't know if there is any program that can also calculate the effect from a venturi in the ducting. I'm no expert on HVAC programs, I just googled it once a couple of years ago.

To get the best efficiency out of the venturi you could try to run it without the piping to the hood and exhaust, then you could experiment with changing geometries of the venturi.

And as in any system transporting gas or fluids, you want a smooth inside without sharp corners, that way the flow could stay laminar and not turbulent. 

Here are some links I collected a while ago.
http://goldrefiningwiki.com/mediawiki/index.php/Venturi

I have a problem with the website currently, a disk crashed recently so there are some issues with graphics.

Göran


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## kurtak (Oct 17, 2018)

I don't have a lot of time these days to post - but I log on to read everyday & have been following this thread

one of the "first" things you NEED to do in your current design - is you "need" to plumb the "intake" end of your blower so it is "pulling" air from "out side" your lab & preferably from a wall opposite that of the hood vent

Right now the intake of you blower is pulling air from inside the lab - which means the intake of your blower is also trying to pull air from the face of your hood because the hood is in the same room you are pulling air from - in other word the intake of the blower is fighting with the air trying to go in the face of the hood

So - you need to plumb a pipe from outside the lab to your blower intake - this pipe needs to be larger in diameter & then reduced "at the point" of hooking up to the blower intake



> Move the venturi to as close to the exhaust as possible and try to avoid any bends in the tubes between the venturi and the exhaust.



Goran's advice here is spot on 

There are a number of other things you can do to improve the air flow of your current design but I need to get ready for work right now --- that said - I will "try" to post more tomorrow &/or next couple days & will "try" to post some drawings

Kurt


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## kurtak (Oct 18, 2018)

In addition to what I posted yesterday another thing you can do to improve your air flow is to use a "long sweep" 90 degree elbow instead of the "short close" 90 degree (& VERY abrupt) elbows you are currently using 

In fact - the more you can "stretch out your point A to point B 90 degree turn the better (point A being the point of your hood in take & point B being your outside exhaust)

Therefore you can use 2 - 45 degree elbows (with a length of pipe between elbows) or even better 4 - 22&1/2 degree elbows

Kurt


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## Platdigger (Oct 18, 2018)

I like Nicks idea of having a straight pipe from the blower.
Seems like that 45 so close to the venturi could be at least part of the problem.


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## anachronism (Oct 19, 2018)

You also need to use a back plate or baffle. This spreads the point of suction so it catches the heavier gases at the bottom and prevents them spilling out of the front into your workspace.


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