Fume Hood Fan Speed Doubt

[didit]

I've still have a lot of work to do on my dyi fume hood but thought I'd share an update of the contraption I put together to try and find the best air flow spot. With this temporary cart I can move the fan back and forth so the 2 inch pipe can be at different spots inside the 4 inch pipe.

I taped sheets of paper to the inside of the hood hoping I would be able to see if it would move but so far it doesn't move much unless the door is completely closed creating a vacuum (which I'm sure we don't want a complete vacuum). Although, when the door is completely closed I noticed the front bottom of the plexi glass gets pulled in about a 1/4 inch so at least I know something is happening.

My friend lent me a manometer to measure the air flow which I will use as soon as I can figure it out. He didn't leave instructions so when I have free time I'll get back to it and find the 'sweet spot'.

When the sweet spot is found, I'll cut the pipe and move the fan closer to the venturi, make a shelf for the fan to set on, and then put in the 6 inch piping to the outside for intake air.

When all is completely said and done I'll post it in the 'homemade fume hood thread' that 4 metals referenced.

 

[didit]

other pic of cart

 

[4metals]

One question, why did you reduce the intake side of the fan? I believe that may reduce the volume of air the fan can force through the restriction forming the venturi.

Once you figure out how to quantify your CFM through the hood, try removing the funnel on the intake side to see if you can increase the CFM pulled by the venturi.

 

[Yggdrasil]

other pic of cart

I suspect the design of the Y bend might favor full size flow through.
Try to remove the 2" restriction and run it directly, if this do not work make a small restriction just before the inlet from below.

 

[4metals]

I suspect the design of the Y bend might favor full size flow through.
Try to remove the 2" restriction and run it directly, if this do not work make a small restriction just before the inlet from below.

That is a possibility, meaning the taper to 4", just before the further restriction to 2" should be continued as 4". Sounds worthy of a try. Since the system is configured to test a few configurations it should not be too difficult.

However if the air can somehow blow into the hood rather than drawing from it, there is need for extending the pipe past the hood intake.

 

[didit]

One question, why did you reduce the intake side of the fan? I believe that may reduce the volume of air the fan can force through the restriction forming the venturi.

Once you figure out how to quantify your CFM through the hood, try removing the funnel on the intake side to see if you can increase the CFM pulled by the venturi.

The fan is a 12 inch inline fine so it is made to connect to 12 inch ducting. I would connect 12 inch ducting to it but am constrained by the building construction and do not have a way to duct it to the outside with 12 inch piping (I didn't think about using hose though). I can only drill a 6 to 8 inch opening. I tried it out prior to putting that funnel on and it didn't seem to affect the out flow of the 2 inch opening on the opposite side much. But I do think you are correct that if does reduce airflow. I'm just not sure if it is significant enough though.

 

[didit]

I suspect the design of the Y bend might favor full size flow through.
Try to remove the 2" restriction and run it directly, if this do not work make a small restriction just before the inlet from below.

Hmmm. I hadn't thought of that because I thought the entire basis to move air from the hood chamber was to use a venturi design which calls for a compression of the air to a small diameter and then to a larger. I could try it though doing it directly though. That's kind of the point at this phase. Find what works best.

 

[didit]

I suspect the design of the Y bend might favor full size flow through.
Try to remove the 2" restriction and run it directly, if this do not work make a small restriction just before the inlet from below.

This is a rough sketch of what I think you are suggesting? Does this sketch look right?

 

[Yggdrasil]

This is a rough sketch of what I think you are suggesting? Does this sketch look right?

Yes, but it needs to be tested and you may need to make a restriction just before the branching so it have a pre compression.
And the expansion happening inside the Y part and after.

 

[didit]

Yes, but it needs to be tested and you may need to make a restriction just before the branching so it have a pre compression.
And the expansion happening inside the Y part and after.

I like it. Thanks for the advice. I'll try and draw up another sketch because I think I know what you mean by 'pre-compression' inside the Y want to see if I got it right. I'll get you a drawing soon and then I'll hopefully get to testing by the weekend.

 

[4metals]

You do not want to compress the air leaving the hood as that will reduce flow. The taper needs to be inside the pipe. When I get to my laptop this evening I will post a drawing of the taper inside a duct from Ammens book.

 

[didit]

You do not want to compress the air leaving the hood as that will reduce flow. The taper needs to be inside the pipe. When I get to my laptop this evening I will post a drawing of the taper inside a duct from Ammens book.

I think you already have posted the one from Ammens in this thread. That's what I based my design on. My 2 inch pipe is the taper and goes somewhere just past the opening in the Y. I was thinking if I understand Yggdrasil's response correctly that the taper would be in the Y if using 4 inch direct flow above if that makes sense.

On a side note, I have been looking at 12 inch dryer vent hose and think I'll go ahead and take off the intake funnel and just make it 12 inch. With the hose I should have more flexibility as to were it can be cut into the wall.

 

[Yggdrasil]

You do not want to compress the air leaving the hood as that will reduce flow. The taper needs to be inside the pipe. When I get to my laptop this evening I will post a drawing of the taper inside a duct from Ammens book.

What I meant is the compression expansion happening inside the Venturi.
There is a compression and speed increase induced by the restricting diameter and just when it comes out there is a pressure drop and speed reduction.
This pressure drop is what drives the Venturi principle.

One can even drive it by a high pressure liquid stream (High pressure washer or similar) centered in the exit of the restriction.
Messy, but in principle it can be done.

 

[Yggdrasil]

I think you already have posted the one from Ammens in this thread. That's what I based my design on. My 2 inch pipe is the taper and goes somewhere just past the opening in the Y. I was thinking if I understand Yggdrasil's response correctly that the taper would be in the Y if using 4 inch direct flow above if that makes sense.

On a side note, I have been looking at 12 inch dryer vent hose and think I'll go ahead and take off the intake funnel and just make it 12 inch. With the hose I should have more flexibility as to were it can be cut into the wall.

Like This:
There are two venturi systems here, one driving the fume hood and one driving the Vacuum system.


Edit to add:

This is a principle sketch, not a design.

 

[didit]

Like This:
There are two venturi systems here, one driving the fume hood and one driving the Vacuum system.
View attachment 65171

Edit to add:

This is a principle sketch, not a design.

I understand that one. The current design I have is replicating what the fan part of that diagram does just with a 2 inch pipe going into a the 4 inch which seems to be working somewhat.

I'm trying to understand what Yggdrasil is saying if I just basically eliminate the 2 inch pipe and run straight 4 inch. I think he means one of these options that I put in this drawing. Do one of these options seem like it might work better in your opinion? Or should I stay on the course I'm on similar to the diagram you just posted?

 

[Yggdrasil]

I understand that one. The current design I have is replicating what the fan part of that diagram does just with a 2 inch pipe going into a the 4 inch which seems to be working somewhat.

I'm trying to understand what Yggdrasil is saying if I just basically eliminate the 2 inch pipe and run straight 4 inch. I think he means one of these options that I put in this drawing. Do one of these options seem like it might work better in your opinion? Or should I stay on the course I'm on similar to the diagram you just posted?

This will not work.
You need to drop the lower one and cut the upper one in half and then move it to the other side of the Y

Like this:

 

[Yggdrasil]

I understand that one. The current design I have is replicating what the fan part of that diagram does just with a 2 inch pipe going into a the 4 inch which seems to be working somewhat.

I'm trying to understand what Yggdrasil is saying if I just basically eliminate the 2 inch pipe and run straight 4 inch. I think he means one of these options that I put in this drawing. Do one of these options seem like it might work better in your opinion? Or should I stay on the course I'm on similar to the diagram you just posted?

What creates the suction is the pressure drop when the air comes out of the restriction (traffic cone),
alongside with the speed of the air that will drag air along with it.

 

[didit]

What creates the suction is the pressure drop when the air comes out of the restriction (traffic cone),
alongside with the speed of the air that will drag air along with it.

Makes sense. It looks like I should just keep going with the original plan. It's pulling air from the hood. Just a matter of finding the best spot. Once the fan is moved closer to the venture it should be even better.

 

[4metals]

This is the image from Ammens book. Note the end of the pipe is just past the opening of the hood. If it ended before the hood opening there is a strong possibility it would blow into the hood.

Your venturi jpeg is making things more confusing and it isn't helping. This ain't rocket science. Your jpeg implies the tapered restriction is past the opening but it seems to fill the entire pipe. You need to leave a space to allow the air to decompress and draw additional air with it. And that additional air comes from inside your hood.

 

[didit]

This is the image from Ammens book. Note the end of the pipe is just past the opening of the hood. If it ended before the hood opening there is a strong possibility it would blow into the hood.
View attachment 65178
Your venturi jpeg is making things more confusing and it isn't helping. This ain't rocket science. Your jpeg implies the tapered restriction is past the opening but it seems to fill the entire pipe. You need to leave a space to allow the air to decompress and draw additional air with it. And that additional air comes from inside your hood.I