# Homemade fume scrubbing eductor, venturi vacuum pump



## Martijn (Oct 25, 2014)

On my way to making my first fume hood, I read some designs of scrubbers here on the forum, and decided that the eductor vacuum setup would be my best option. 
I want to use the vacuum to let the fumes from a nitric digestion bubble through a cooled flask to get most Nitric oxides out and maybe regain some. Any remaining fumes will be sucked trough the eductor, mixing them with the water in a reservoir.
The pump feeding the eductor will also supply some nozzles spraying down with the main airflow into the same container.

So, after some attempts to make an eductor, I made one that works great! 
It's made of a simple Plastic T-piece of 10mm inner diameter with an inserted nozzle and on the discharge i made a smooth restriction, like a real venturi mixing chamber. Here's a drawing of what i made:


The two inserted parts are made from a plastic rod. 
The nozzle needed something to swirl the water around inside it to make the nozzle actually spray instead of shooting a narrow beam, so I put a plastic cap of a ball pen inside, and drilled three holes in it, under an angle and to the side. That part is not on the drawing.
It gave a nice spray cone, which mixes the air and water in the mixing chamber. 

Next I needed to know what size supply pump it needs. 
So to test the required pressure and the created vacuum with this venturi, I hooked it up to my tap water, which is at 3,1Bar. I put a valve and a big manometer in, to control and measure pressure, and hooked the vacuum up to a 25mm see-through hose and a bubble flask to get an idea of flow created at different pressure settings. The hose I put in a bucket of water to let it suck a column of water up to measure the vacuum.

While testing, I figured out that the best way to position the venturi is with inlet and outlet horizontally, just underneath the surface of the reservoir it spills back in, which I used just to catch the water for this test, normally it would be part of the water loop in the scrubber.
For every cm it's under water you lose a cm of water column (1mBar) of vacuum. 

The vacuum it makes goes up with 3 mBar every 100mBar of inlet pressure, already starting at 0,1 bar! the maximum (actually minimum) I got out of it is -94mBar. Here's a graph of the results:



the pressure needed to make a 30 cm bubble flask, is just over 1 Bar, so a simple pump will do. I will need 20 cm to bubble the fumes through water, and a flow rate of about 10 bubbles per second would be enough. That much will cost 2 mBar of vacuum, but there's enough left. 
If I wanted to draw the fumes directly out of the reaction flask, it would take as little as 0.2 Bar of supply pressure to create sufficient suction. Any overpressure from the reaction will not build up because it can easily escape.

On to building the fume hood and the main scrubber with spray nozzles. Can't wait to start safely in a good fume hood.  

I hope this info will be helpful to someone. It only cost me a half day of work to make, and the parts I had laying around. The cheapest one I could find was over 90 Euro, so well worth the trouble imo.


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## METLMASHER (Mar 20, 2015)

Well done, Martijn!

That seems a bit beyond my skill. ( That's never really stopped me though. )

An update photo(s) would be a great addition for your tutorial, I know your scrubber will be excellent in use.


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