how to build an acid resistant hood & scrubber

[jeneje]

I have read this thread and have a question. I have built a 5' hood and two 12" x12" stacks. In the first stack I have done what you suggested cut up 2-liter bottles for the packing. I have a lot of birds and wildlife around my house, which brings me to my question. Can I use the same packing in the second tower to remove the rest of the contaminates. And will it be safe for my wild neighbors, I don't want to endanger them. I will be using this system to refine e-scrap. I have several tons of pins and boards along with jewelry scrap. I will be using electrolysis with acid and glycerin. I will also be using AR and a nitric acid bath to remove the foils from the pins. Will this system be safe to me and my friends?

Thanks
Ken

 

[4metals]

This design scrubber definitely works but the design has a finite capacity. Every design does. This scrubber can handle a moderate quantity of NO_x, as in the quantity outgassing off a 100 oz karat digestion. You have to slowly work up to your efficient scrubbing capacity and once you learn what it is, don't exceed it without upgrading. You also have to determine when to change out the scrubbing liquid to lower the TDS and restore efficiency. It's a learning curve.

Then both you and the birds will be happy.

 

[jeneje]

Thanks 4metals, but I have another question, how tall should the scrubber be at 12" square using a 3500 cfm blower. I made my 10" square 4'6" tall I don't think it will work. I noticed that after the fact yours in the diagram was 12" square. Thanks.

 

[4metals]

Jeneje

An option is to use the majority of the exhaust for the hood and throttle down the draw that sucks through the scrubber if you have 4 cubic feet of packed scrubbing area, with a full flow through the scrubber you will turn about 14.5 times the air per second more than needed. You want the air to be in the packing being scrubbed for 8 seconds, the more the better. With a 4 cubic foot packed bed you should be looking for 120 cfm, that's pretty small, double the height and aim for a 250 - 300 cfm flow. You could also reverse the pulleys on the blower (unless it is direct drive) to lessen the cfm. I would break the exhaust into 2 streams, one to scrub with a slow flow to assure scrubbing, and the balance to a hood for the fugitive emissions. (un-scrubbed)

 

[golddie]

you should have 250 cfm

Can anyone say how many cfm is this
http://www.grainger.com/Grainger/wwg/search.shtml?searchQuery=2c863&op=search&Ntt=2c863&N=0&sst=subset
http://www.grainger.com/Grainger/wwg/search.shtml?searchQuery=3k771&op=search&Ntt=3k771&N=0&sst=subset

 

[4metals]

That blower with no resistance is 900 cfm with 1/4" it's 788 cfm and with 1/2" it's 655 cfm

Generally a scrubber adds 1/4 to 1/2 an inch of resistance, then you have to add in the fittings resistance.

 

[golddie]

Hi 4metals
I appreciate your help.
My scrubber is working very well.
I am not able to understand the meaning of your sentence.
resistance
no resistance
quarter inch
half inch
What would these be?
Thanks

 

[4metals]

Goldie,

Air flowing through a straight pipe would have little resistance offered from the pipe, if you were to add an elbow, the air has to bounce around the turn, colliding with the side of the pipe during the turn and it causes a measurable resistance. If you have lots of turns, there is more resistance. Actually even the straight pipe has some resistance so long pipe runs also have resistance.

Resistance is measures in inches of water column. A blower drawing its flow through a pipe could lift the water up a pipe by 1, 2 or more inches, depending on the design of the blower. Let's say your blower could pull a water column of 1 inch without restrictions. Now add your scrubber, duct, fittings and packing and the same blower will only pull say 1/2 inch of water. Then the resistance would be 1/2 inch and the pump which unimpeded pulled 1 inch now pulls 1/2 inch.

When you buy blowers somewhere on the specs they give different flows at different resistances. Then you simply select a blower to deliver what you want given the resistance of your system and your flow requirements.

 

[Emmjae]

I have only scanned this post but from what little I have read I believe your problem is not enough fresh air intake. I also agree that you need to close off part or all of the front of your hood.

I built a crude fume hood 4 months ago 60"L x 30"D x 39"H with a front opening of 46" x 30". I am using 2 - 120 cfm bathroom exhaust fans made of all plastic except for the motor and each vented thru the roof with 4" ducting. I also keep 1 of these fans running 24 hrs. a day.

I put a piece of 1/4" plexiglass on the front with crude hinges and barrel bolts to keep it held down and used 1/4" x 3/4" closed cell foam weather stripping to seal the edges. I kept the bottom 10" of my hood open.
I am also in a small 9' x 9'-6" room but I do have a window.

Now for my point . If I close off my window and open my hood front and rely on drawing all my fresh air from my house I have the same problems you do. When I close my hood front the fume control gets better. However, if I just crack open my window, I can actually feel the air drawing in and will sometimes even howl if it's not open enough. I then get no noticeable fumes at all in my room until I do something outside the hood. But I still don't get any fumes in my house.

I can only assume my system must be keeping control of most of my fumes in my little room because I have had several pieces of raw steel beside my hood for a month or so and I have no signs of any oxidation or on any of my tools.

I am working on a small scrubber system that I can incorporate into my hood and will eventually build a bigger and better hood system once I'm happy with my scrubber results. In the mean time I keep things small and use lots of watch glasses.

 

[rbramsey]

Hi qst42know
I thought it would be a better idea to have 2 pipes that was 3" in the sides of the hood way I the system might work better than to have 6" in center.

FYI, two 3" pipes are not = one 6" pipe. It takes 4 - 3" pipes to equal the area of 1 - 6" pipe. Two 3" pipes would be equivalent to 4.5" pipe. This is not accounting for any efficiency loss or drag.

Area of a circle = pi * R * R
area of 6" pipe = 3.14159 * 3 * 3 = 28.2743 sq in.
area of 3" pipe = 3.14158 * 1.5 * 1.5 = 7.0686 sq in

# of 3" pipes = 6" pipe
7.0686x = 28.2743
x= 28.2743 / 7.0686 = 3.9999

I thought this would be important with you using this for a fume hood.
Richard

 

[Drewbie]

When thinking about managing fumes, you have to think "pressure differentials".

If you have your fume hood in an enclosed room, your fume extraction isn't going to work very well, because your fan is working its guts out lowering the pressure in the room and once it gets to a certain pressure differential, it's going to run out of puff and there will be no further air extraction.

You HAVE to have a source of fresh air somewhere else in the room or things will not go as planned.

If you pressurize the room from another location with a big-arse fan, and the fume hood air exit is the only way the air can get out of the room, then you won't even NEED a fan inside the fume hood

If you have a fume hood that's not quite enough capacity, it may be a much easier and cheaper solution to pressurize the room slightly, rather than having to re-build the fan hood with bigger fans and pipes.

 

[Emmjae]

I do my siphoning outside but directly in front of my hood. I keep my main beaker at the hood opening and my waste container on the floor. You will get some smell obviously but it's at a minimum if you keep it in the path of the fresh air drawn into the fume hood. Buckets are done on the floor in front of the fume hood also.
My light is just a basic 4' twin tube 32-watt fluorescent shop light. I mounted it as close to my hood opening as possible to keep it out of the direct fumes. My final light setup will be a Vapor Proof fluorescent although so far I show no signs of corrosion on my original setup.

Mike

 

[4metals]

An easy way to add a light is cut a round hole in the top of the hood and silicone in one of these type pie dishes


and use an aluminum reflector above it.


The light is outside of the hood so it does not corrode. The pyrex pie dish is glass so acid fumes do not effect it.

 

[Refiner232121]

4metals' way is good but a bit of work.
If you do it like Mike's the only problem is that the screws will rust.
For that problem you can always paint the screws with oil paint.

 

[4metals]

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