Fume Hood Fan Speed Doubt

[didit]

No I get it. The current setup I have is exactly what is in the image you just posted from Ammens. I was thinking about trying a different approach by using Yggdrasil suggestion but I misunderstood his comment. Not a language thing, I was just totally overthinking it. I do thank the both of you for replying to post though. It's really helpful.

Right now the hood does have air flow. I'm going to use the manometer to find the best spot (smoke test too). Then I'll take off the intake funnel and see how much it increases the flow rate. I'll you guys posted.

 

[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.

You were right. I removed the intake funnel and the suction increased in the hood. I still can't figure out this manometer so the paper guide is showing that there is internal upward air flow. The suction should only get better with finding the best pipe spot and moving the fan closer. Now I just have to figure out how to intake it from the outside in 12 inch diameter.

 

[didit]

I made a TEMPORARY baffle so I could see how it would increase suction. It makes a world of difference. Now I will make a more permanent one out of some type of plastic panel. Still testing the best spot for airflow into the venturi. Definitely making progress though.

 

[didit]

FOUND IT! Using a manometer similar to what air conditioner service people use to measure pressure differentials, I was able to find the distance at which the 2 inch pipe (restriction) inside the 4 inch pipe creates the most suction. The black arrow in the second picture indicates where the 2 inch pipe ends.

Hope this post helps other DYI builders.

Cheers.

 

[Naughtius Maximus]

Cigars or incense are a good way to test.

 

[didit]

Cigars or incense are a good way to test.

Yep. Way ahead of you. I a few more tweaks and that's the next step. It doesn't pull as well as I would like when open to 18 inches.

 

[4metals]

It doesn't pull as well as I would like when open to 18 inches.

I think the majority of the compression of the airflow is in the large funnel that tapers down to 2". I would also try to taper down to a 3" pipe inserted into the sweep tee as it carries over twice as much air as a 2" pipe. That should increase your airflow.

 

[didit]

I think the majority of the compression of the airflow is in the large funnel that tapers down to 2". I would also try to taper down to a 3" pipe inserted into the sweep tee as it carries over twice as much air as a 2" pipe. That should increase your airflow.

I can try that. I'm really curious what is going to happen when I hook up the intake. Right now I just leave the door to the lab open so it's all outside air going into the fan. I noticed if I have the door to the lab closed the hood hardly sucks at all because of the pressure change. So I'm curious what happens when I'm able to get intake air from the outside of the lab but have the lab door closed creating an inside pressure change. Fun times.

 

[4metals]

Over the years of using this type of hood I have learned that a refinery with other exhaust sources like a hood over a melt furnace or incinerator, compete for "in the building" air and hamper the quantity of air a venturi can draw out of your hood. That is why I only use them in a room that is relatively sealed off from other blowers generating negative pressure and that room needs make up air. Make up air for a small refiner is just an open window. When the government gets involved make up air means costly systems.