Fume Hood Fan Speed Doubt

[didit]

Hello All. Quick into, I am an absolute newbie and haven't even attempted my first refine yet. Long story short I had a very serious health issue (abated hopefully) and it made me realize I need to empty the bucket a little so to speak. I've always wanted to do precious metal refining and micro scrapping and have spent the last 9 months studying up and planning. I renovated my 12' x16' shop to be a bit more comfortable place to recover and refine. I've been reading Hoke but admittedly am only half way though it (extremely informative but not exactly a page turner if you know what I mean). I've also been visiting the forum and researching, reading, and watching lots of youtube videos on various topics related to the process.

I have completed my shop table with melamine top and am in the process of building the hood (also melamine). Before I went any further and started using flex seal on the crevices or drilling holes in the wall I wanted to run my design plan by you folks and see if you think my fan will be capable of handling the size of the hood. The hood just seems a lot bigger than I imagined.

The face of the hood is 39 3/4 inches by 34 1/2 inches. So my calculations are thus

Facex100/144=CFM

39.75 x 34.25 x 100 / 144 = 945.44 CFM

The fan I have is a12 inch fan rated at 1060 CFM.

I think it should work but since I am using a Venturi design I am not how much it reduces the equations.

Any thoughts, comments, suggestions, and advice would be appreciated.

P.S. I hope the .pdf file of my concept plan downloads but if not I can explain the design as well. The pieces with fan on the table are for the reduction of 2 inch and 4 inch pvc pipe.

 

[ChopperGreg]

Why pull air from a long piece of PVC pipe?

 

[didit]

Why pull air from a long piece of PVC pipe?

The plan is to pull intake air from outside the shop so as not to create a pressure change within the shop. I went through a lot of trouble to put in air condition as well and don't want too much of that. It really hot were I live.

 

[didit]

The plan is to pull intake air from outside the shop so as not to create a pressure change within the shop. I went through a lot of trouble to put in air condition as well and don't want too much of that. It really hot were I live.

Oh and sorry. I just realized I didn't finish the concept drawing all the way. The pvc pipe will extend to the outside of the shop both ways. Just imagine the pipe going through the walls.

 

[didit]

Why pull air from a long piece of PVC pipe?

Here is a more accurate depiction. Sorry about the confusion on that.

 

[ChopperGreg]

I’d suggest either making the intake pipe as short as possible, make it as large a diameter as you can, or both.

Air drag on the inside will limit how much air that fan can move. The same applies to your Venturi injection pipe, your junction and your outlet pipe. If you can start with 8in or bigger PVC, I recommend it, because those will be the biggest factors in how much air your inline fan will actually be able to move, regardless of its rated capacity.

Other than that, place the fan as close to the junction, as possible, to minimize the length of reduced diameter pipe.

Don’t forget that you will have to figure the size of the vent, in your hood door, to maximize the flow through your hood.

 

[didit]

I’d suggest either making the intake pipe as short as possible, make it as large a diameter as you can, or both.

Air drag on the inside will limit how much air that fan can move. The same applies to your Venturi injection pipe, your junction and your outlet pipe. If you can start with 8in or bigger PVC, I recommend it, because those will be the biggest factors in how much air your inline fan will actually be able to move, regardless of its rated capacity.

Other than that, place the fan as close to the junction, as possible, to minimize the length of reduced diameter pipe.

Don’t forget that you will have to figure the size of the vent, in your hood door, to maximize the flow through your hood.

I understand what you are saying. Right now the intake pipe is 6 inch but I might be able to increase it to an 8 inch. I'll see how big I can make it but unfortunately I'm constrained by a side building adjacent that wall and I don't want to go through the roof. I can make some measurements and see if I can increase the intake.

I could probably increase the outflow as well but that 4 inch long sweep wye was not easy to come by so I'd have to look what's available in a larger size that's affordable.

As far as the door it will be a vertical sliding sash. Will be most likely open to about 2 inches when in use and because of the ceiling will not open larger than 24 inches.

 

[Yggdrasil]

I understand what you are saying. Right now the intake pipe is 6 inch but I might be able to increase it to an 8 inch. I'll see how big I can make it but unfortunately I'm constrained by a side building adjacent that wall and I don't want to go through the roof. I can make some measurements and see if I can increase the intake.

I could probably increase the outflow as well but that 4 inch long sweep wye was not easy to come by so I'd have to look what's available in a larger size that's affordable.

As far as the door it will be a vertical sliding sash. Will be most likely open to about 2 inches when in use and because of the ceiling will not open larger than 24 inches.

And make sure the exhaust and intake are separated so you don't drag the exhaust in again.

 

[4metals]

First your calculations are correct for the required CFM. In the configuration of your venturi, I have had great success using a long tapered piece to compress the air and I always had the discharge just past the opening. In Ammens excellent book on refining he has a sketch which better describes what I just said.
You will never get the total exhaust that the fan puts out to suck through your hood. But if you have a front piece of plexiglass that slides up and down, you can keep it half way up and still have room to reach your forearms in and work. That will assure the maximum velocity for the longest time. If you need to reach in further or higher up (you will) the glass can be raised higher for just when you are needing the height.

You will also benefit from a small fume scrubber for the more concentrated NOx.

 

[4metals]

I blew up a section of Ammens sketch to accentuate the slight restriction he has in the discharge tube which look like 2 bumps on the pipe. I have seen broken venturi tubes, much larger, with similar restrictions. I am assuming the placement of this restriction causes the airflow to suck more air out of the hood as the airpath expands causing the suction.

 

[4metals]

I am assuming that @didit has extended the section of smaller ID pipe to a point past the sweep from the hood below. The drawing is very nice but does not indicate what is going on inside the pipe.

 

[didit]

I am assuming that @didit has extended the section of smaller ID pipe to a point past the sweep from the hood below. The drawing is very nice but does not indicate what is going on inside the pipe.

The illustration you posted from Ammens book is probably the best I've seen on the venturi. I know the smaller pipe needs to be just past the up-take pipe opening but am not just sure just how far that should be. My plan is to build a temporary rolling cart to place the fan on so I can move the 2 inch pipe back and forth inside the 4 inch pipe until I find the spot that creates the maximum suction from the hood. Once I know that spot I can make it more permanent.

I also might increase the intake as ChopperGregg suggested but for now will continue using the 2 and 4 inch pipe since I already have it. I can always upsize that later.

If I add a scrubber later it will probably be in the form of pvc canisters placed on the outside of the building adjacent the hood wall like I read in another post.

First though I just want to get a functioning hood in place with the material I have. Baby steps.

 

[4metals]

When you find the sweet spot for the tip of the smaller pipe in relation to the hood penetration, let us know what distance that is. The more input we get from DIY folks the closer we can come to perfection.

 

[4metals]

When you finish your build please post a photo of the setup in this thread.

 

[didit]

I am assuming that @didit has extended the section of smaller ID pipe to a point past the sweep from the hood below. The drawing is very nice but does not indicate what is going on inside the pipe.

That is probably one of the better illustrations I have seen for the venturi. Since I don't know exactly where the 2 inch pipe needs to be other than just past the up hole from the hood I plan on making a temporary rolling cart to put the fan on so I can adjust the 2 inch pipe back and forth inside the 4 inch pipe. That way I can find how far to push the 2 inch inside until I receive the maximum suction. That's the plan anyway.

I think ChopperGreg's point about a larger pipe is a good idea but think I'll continue with the pipe set of 2 inch into 4 inch I have now and see what it does sinch I already have it. I can always upsize in the future if I think it necessary.

The original plan had a 6 inch fan but after reading up I realized it wasn't going to be powerful enough. That's why the piping are the sizes I chose. I upgraded to a 12 inch fan and just have to adjust for the reduction to the pipe from it.

If I do a scrubber I will most likely go with a design I saw on another post whereas pvc canisters were mounted on the outside of the building on the adjacent wall. I'll look further into later. First, I just want to get a functioning exhaust in place. Baby steps.

 

[didit]

When you find the sweet spot for the tip of the smaller pipe in relation to the hood penetration, let us know what distance that is. The more input we get from DIY folks the closer we can come to perfection.

That I can do. I'll maybe draw up a diagram with measurements in a document that can be shared easily. It would be nice to know as I myself have yet to find the answer and believe me when I say I have been looking.

 

[4metals]

It would be nice to know as I myself have yet to find the answer and believe me when I say I have been looking.

I have never seen a drawing from which I could deduce that from a diameter of X compressed to a diameter of Y you can expect to draw Z CFM (as a percentage of total CFM put into the system. I am sure the metric exists but the concept is quite simple and surely companies that manufacture venturi systems know them. But in a capitalist world the profits diminish when too many folks can make their own, hence the secrecy.

 

[snoman701]

It's going to be a matter of pressures, angle of venturi, material venturi is made out of, etc. As of twenty years ago, a lot of this was still very much a black magic in which it took time testing it on a flow bench. There's a lot of stuff that seems counterintuitive as well, as rough surfaces perform better as it creates micro turbulence that speeds up the flow at the edges. It's really weird. I just know that I was working on a patent in 2003 that we had a pretty bright mathematics doctorate candidate working on, trying to do the computer simulation, and it wasn't as straightforward as one would expect.

 

[4metals]

There's a lot of stuff that seems counterintuitive as well, as rough surfaces perform better as it creates micro turbulence that speeds up the flow at the edges.

I like that suggestion, as any slight resistance where the air is expanding from the compression will force the system to make up the air from somewhere else, in this case inside the hood.

 

[didit]

It's going to be a matter of pressures, angle of venturi, material venturi is made out of, etc. As of twenty years ago, a lot of this was still very much a black magic in which it took time testing it on a flow bench. There's a lot of stuff that seems counterintuitive as well, as rough surfaces perform better as it creates micro turbulence that speeds up the flow at the edges. It's really weird. I just know that I was working on a patent in 2003 that we had a pretty bright mathematics doctorate candidate working on, trying to do the computer simulation, and it wasn't as straightforward as one would expect.

I came across this article about air flow because I was curious what happens to the air speed of a 6 inch 440 cfm fan when reduced to 2 inches. I was going to use the 6 inch fan at first in my hood but realized it was too small. I'm not changing the piping so the intake will be a 6 inch pipe reducing upward to a new 12 inch fan so according to this article the velocity will decrease going into the fan. However, I think it's easier to 'suck' air rather than 'push' it so I don't think it'll be an issue.

Regardless the article has a lot a really nice diagrams that explain the volume and velocity part when ducting is reduced.