# So should I have 2 fume hoods or 1



## PreciousMexpert (Oct 22, 2010)

I would like to build a fume hood similar to this.
I am wondering whether to have 2 blowers or 1.
The reason whether to have the second blower is that what happens when you remove the tube from the reaction vessel some gas will escape into the air and that might be bad for me and the environment.
So should I have 2 blowers or 1.
Also where do I place the second blower if I decide to have a second one
Thanks


http://goldrefiningforum.com/phpBB3/memberlist.php?mode=viewprofile&u=9899


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## 4metals (Oct 22, 2010)

I always like to have 2 blowers, one scrubbed and one not. In a conventional hood the exhaust requirement of 100 cfm per square foot of opening means you need a huge blower for the hood you show in the picture. A 4 x 8 opening needs 3200 cfm for good exhaust. 

The scrubbed exhaust is only sucking out of the ends of all of the tubes so even if you give yourself a factor of 10 the area of a 2" diameter drop is 31.4 sq inches, you can have 3 drops per 100 CFM of blower. In an 8 foot hood, having 6 drops should be adequate. So a blower giving you 200 cfm after the resistance of the scrubber would do fine. 

For the hood itself, you can place sliding doors so the hood can be closed off partially. If you arrange your doors so you always have it half closed you can get by with 1600 cfm for the main blower. 

Once your chemicals are added and the reactions are cooking, sealed reactors can allow you to shut off the main blower and just scrub the reaction. Big benefit in putting up reactions at night because you can shut down the main blower. Where I've done most of my refining, that means not coming in to ice in the refinery in the winter.


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## PreciousMexpert (Oct 22, 2010)

Hi 4metals!
Thanks for your help


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## PreciousMexpert (Oct 22, 2010)

I notice that he does not have a condenser and he probably closes the passage of the holes partially.
Is that good


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## 4metals (Oct 22, 2010)

If you look at the picture closely you will see that every flex tube has a shut off valve at the top to regulate flow. I'm sure the exhaust goes to a scrubber. A condenser doesn't eliminate the need for scrubbing it only lowers your nitric consumption by condensing the fume back into the reactor. Usually a condensed aqua regia reaction will go to completion with aqua regia at a 5:1 ratio.


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## PreciousMexpert (Oct 22, 2010)

Hi 4metals
When you say 
condensed aqua regia reaction will go to completion with aqua regia at a 5:1 ratio.
what would be the approximate ratio for something like what you see in the picture
Exactly what do you mean by 5:1 ratio
Thanks for your help


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## 4metals (Oct 22, 2010)

In a condensed reactor the condensed fume, mostly nitric, falls back into the reactor essentially replenishing the nitric. For stone removal I like it because you can really heat up the reaction and dissolve up those last bits of chloride encrusted metal so cleanup is easier. For digesting cornflake shotted gold I don't see the same benefit. Karat gold scrap which has been melted and has a silver assay below say 9% does not have to be heated above 130 degrees and the savings in nitric are minimal. 

The ratio is the amount of hydrochloric compared to the amount of nitric 5:1 condensed. Without a condenser anywhere between 3.5:1 to 4:1 works. It is always best to measure out your acids, add all of the HCl and 1/3 of the nitric and slowly add the excess nitric until the reaction stops. This way you can avoid adding too much nitric and dealing with all of the aggravation getting rid of it before you drop the gold.


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## PreciousMexpert (Oct 22, 2010)

Hi 4metals
Thanks for your help
I tried to find a plastic container to use a hood and couldn't 
how about using Plexiglas
http://en.wikipedia.org/wiki/Poly%28methyl_methacrylate%29
I said I should make a smaller version of what you see in the picture maybe 
4 feet by 3 feet
I thought about making the walls with Plexiglas and attaching them together with plastic nuts and bolts and gluing everything with acid resistant glue
Thanks again


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## 4metals (Oct 22, 2010)

Build one out of plywood and get it coated inside and out with the coating they use to spray bed liners in pickup trucks. Check with your surfing buddies, all the surfers where I came from use the spray on bed liners to keep the salt water from rotting out their pickup truck bed. 

Quite a while ago I posted on how to build a hood then a scrubber was added into the mix.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=4776


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## PreciousMexpert (Oct 22, 2010)

Hi 4metals
Thanks for your help
I appreciate it very much


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## PreciousMexpert (Oct 22, 2010)

Hi 4metals


> Place a stop so the door always stays 3 or 4 inches up from the bottom,


Is this to create a draft.
how about blowing compressed air into that opening
Here is something about that idea
http://www.ehs.ufl.edu/lab/fumehood.htm#design



> Do not position fans or air conditioners so as to direct airflow across the face of the hood. This can interfere with airflow and containment of hazardous chemicals.


This is written right underneath the picture


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## qst42know (Oct 22, 2010)

Compressed air could work but it would take a very large compressor. I have a 5hp compressor that won't run a pneumatic Gast vacuum generator to capacity. A blower will give the velocity and volume required more efficiently.


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## goldnugget77 (Oct 23, 2010)

qst42know that is very helpful thanks
I know a situation where refining is done and right above where this is taking place there are heating pipes
and other plumbing pipes in the ceiling which is about 5 feet away from the fume hood.
The person just started doing this so we don't know what the effect will be.
My question is that will any harm be done to these pipes
and how can the pipes be protected
Thanks


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## qst42know (Oct 23, 2010)

I don't have any experience running or designing a refinery but I would expect a good coat of paint would go a long way in protecting nearby metals.


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## goldsilverpro (Oct 23, 2010)

In my experience, most paints don't stand up too well on hoods, even epoxy. One place I worked had acid proof tile/mortar on the floor and inside the fume hoods. Worked great, easy to keep clean, but hell on glassware. I have seen wooden fume hoods coated with several coats of boat resin (applied on fiberglass cloth). One place sent their wooden hoods out to have a coating of PVC "chopped" on them. These "chop" guns can apply many types of pure plastics dry (but molten), without solvents, at about any thickness you want - not terribly expensive. On some of the wooden silver cells I made, I had a "chopped" PVC coating applied to them - lasted forever if you didn't bang them around.


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## goldnugget77 (Oct 23, 2010)

Hi Folks
Thanks for your reply
I am thinking if there could be major damage to these pipes.
They are hot all the time 
i said maybe I could make a compartment but I dont know if that will work


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## qst42know (Oct 23, 2010)

Paint isn't a permanent solution but it's better than exposed. Kind of like your car. I get that it's no protection for direct exposure. The better the prep work the longer it will last and at least it is some level of protection for casual exposure.


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## goldnugget77 (Oct 23, 2010)

qst42know
What do you think of the idea of making a plastic cover and sealing it all over
that way there is no exposure


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## qst42know (Oct 23, 2010)

I guess I don't understand what you are trying to cover?

Something within the room you are working in, or something in the actual fume hood?


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## goldnugget77 (Oct 23, 2010)

what i am trying to cover are steel pipes
they are about 4 inches in diameter
and they are worm or mildly hot
they are in the same room where the fume hood is and are directly exposed to the atmosphere.
there are also smaller pipes 2 inch pipes that are also steel 
all this is about 3 to 4 feet away from the fume hood


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## Barren Realms 007 (Oct 23, 2010)

goldnugget77 said:


> what i am trying to cover are steel pipes
> they are about 4 inches in diameter
> and they are worm or mildly hot
> they are in the same room where the fume hood is and are directly exposed to the atmosphere.
> ...



If your fume hood keeps the contaminents out of the room you should not have to worry about the pipes.


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## qst42know (Oct 23, 2010)

If it were me anything in the hood is at risk and you need to move the hood or the pipes. Everything in the room (except for things like sprinkler heads) needs a good coat of quality paint.


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## goldnugget77 (Oct 23, 2010)

Hi people
Thanks
I have a better idea of what to do now


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## Harold_V (Oct 24, 2010)

Barren Realms 007 said:


> goldnugget77 said:
> 
> 
> > what i am trying to cover are steel pipes
> ...


The notion of keeping the atmosphere acid fume free is interesting, but far from reality. I ran my fume hood almost constantly, but over a period of about ten years, anything metallic that was not protected was corroded, albeit not very badly. That included some items made of stainless. I also recall that the anodized aluminum window frames were slightly corroded. 

If you desire to keep things looking good in a lab, be prepared to re-paint occasionally, and to apply protective covering to all metal surfaces. 

Harold


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## Lou (Oct 24, 2010)

My advice: paint now, not later! Now the metal is bare and not oxidized. The paint will stick and won't spall and come off like it would if you were trying to hastily remedy a situation.


Epoxy paint is the best paint to use. I'll check what brand it is. 

Also, I work on epoxy countertops. The only thing that gives them trouble is concentrated KOH and HF. They're practically bulletproof for aqua regia. 

http://labfabricators.com/


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## Frankk12 (Oct 25, 2010)

I am wondering if it would be OK to use oil paint instead of PVC paint or the other paint 4metals was talking about.
Thanks


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## 4metals (Oct 25, 2010)

It's a question of degree, painting the new unoxidized pipes with either an epoxy paint or a pvc coating will give you a high degree of protection. Left over builders paint you have in the basement will still do the job but to a much lesser degree. If you're setting up a hood and plan on doing some serious work, put the time and effort into protecting the surrounding area up front and you will have less maintenance and upkeep to do later.


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