# Fan/Blower recommendation for standard hood?



## MGH (Oct 28, 2013)

Hi all,

I’m looking for a recommendation on a fan/blower to use for this fume hood.

I just bought this hood on a GovDeals auction. I had to drive 7 hours each way to pick it up, but with a winning auction price of $41 I couldn’t pass it up.

This is a six foot hood with an operating sash height of 18 inches and effective width of 5.5 feet. In searching so far I understand that I need a fan that will pull at least 825 cfm. Is there a certain model or style of fan that is recommended? I am still trying to put this together on a small budget, so I’m wondering if a squirrel cage fan or restaurant equipment fan would adequate (if I can find one cheap). But of course I don’t want to go to the trouble of installing anything that I’ll have to replace in six months if it’s just going to get eaten up by acid fumes.

I do plan on setting up a scrubber system as well. I’m hoping to get the hood hardware set in place first.

For those eagle-eyed readers out there, yes I do have the panels that snap onto the brackets at the back of the hood  .

Thanks for any advice and direction,
Matt H.


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## niteliteone (Oct 28, 2013)

You can try USPlastics they carry some nice blowers that are chemical resistant 
here is the link;
http://www.usplastic.com/catalog/search.aspx?search=exhaust%20fan&page=1


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## Smack (Oct 29, 2013)

Those are baffles that go in the back of the hood and should be fastened with nylon rivets from the factory. Are you going to set it up on epoxy resin counter top? Will have to be set up on a 30" standard counter top so plan your base cabinets accordingly. I'm guessing a 6"x20" exhaust hole in the top? And does it have a flange or a transition in place? You can spend 2k on a exhaust blower for that hood easily, not easy to find a used one that will work for how your going to need it set up. The type of exhaust blower will determine to some degree what kind of ducting you'll have to use. Piecing ducting together with odds and ends is a nightmare. Noxx got a nice chemical proof blower from somewhere and he put the link up but like I said, they ain't cheap. 
PVC Ducting- http://www.harvelduct.com
Fume Hood Guide- http://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=12641
Best price on counter tops I could find- http://keurindustries.com Just over $600.00 on a 6' epoxy resin top with marine edge.


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## MGH (Oct 30, 2013)

Thanks guys for the great links and advice on the overall setup. I was a little worried about the price of an appropriate blower, and that $2000 price tag confirms it. No regrets though. I still got a good deal on a hood that I can set up... eventually.

To answer a couple of questions:


Smack said:


> Are you going to set it up on epoxy resin counter top?


 Maybe. I was planning on starting off with building a table out of 2x4s with a plywood top (same as I've done with aquarium stands before) then laying a 1/4 to 1/2 inch thick sheet of polymer on top for the work surface. I'd need to do a little research on acid resistance and hardness. An epoxy resin surface would be great, but if I can go a little cheaper I may try on this point.



Smack said:


> And does it have a flange or a transition in place?


 Yes, it has a transition already in place. I haven't taken the exact measurements yet, but I think it's more like 4 x 20 inches.

I was hoping to get this all installed before winter really set in. But I've done enough refining so far to know that one of my top five rules is "Don't try to rush things". At least now I have a much better idea of what to look for in searching for used materials. Thanks again for the help. Any other advice is equally welcome.

-Matt H.


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## niteliteone (Oct 30, 2013)

MGH said:


> Thanks guys for the great links and advice on the overall setup. I was a little worried about the price of an appropriate blower, and that $2000 price tag confirms it. No regrets though. I still got a good deal on a hood that I can set up... eventually.
> 
> To answer a couple of questions:
> 
> ...


Matt,
To get up and running cheaper, you can start out with a metal squirrel cage blower by cleaning it real good and painting the insides with an epoxy paint. This should last a year or two depending on how much you use it, giving time to get the proper blower assembly. I usually pick up used blowers for $25 from a commercial roofer I know, so their cheap used. 
I would but the plastic ducting from the start though, since the cost is not much different than the cheaper metal stuff.


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## jonn (Oct 30, 2013)

I may have a blower you'd be interested in. It's resin, came out of a lab connected to a fume hood. Pm me if interested and I will send pictures. Jonn


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## MGH (Oct 30, 2013)

niteliteone said:


> To get up and running cheaper, you can start out with a metal squirrel cage blower by cleaning it real good and painting the insides with an epoxy paint.


 I thought about this too - glad to know it wasn't such a crazy idea. I'll definitely keep this in mind.



jonn said:


> I may have a blower you'd be interested in. It's resin, came out of a lab connected to a fume hood. Pm me if interested and I will send pictures. Jonn


PM sent


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## goldsilverpro (Oct 30, 2013)

I bought a steel squirrel cage blower from a scrap metal yard for a song and it lasted for 2 or 3 years until it fell apart.


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## Smack (Oct 30, 2013)

MGH said:


> Smack said:
> 
> 
> > And does it have a flange or a transition in place?
> ...



4x20" is the exhaust size, a transition would be going (transitioning) from the 4x20" to something like a 10" round ducting. Here is a picture of my exhaust blower, it has an aluminum impeller that I had epoxy coated by a Powder Coating Company and then had to have the impeller rebalanced. It moves just under 900 CFM through 8" round ducting before any scrubber or ducting.


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## 4metals (Oct 30, 2013)

A lot depends on where you are working. If this is for a business and you are in a building where other people work, gassing everybody out of the building isn't an option. If you're in a backyard shed you can leave for a few hours when a blower fails its a different story. All of the systems I put in are in places where you have to do everything possible to keep the blowers running, proper materials of construction, preventative maintenance and paying constant attention to the "vibes" the equipment puts out. Motors usually put out a distinctive sound when they work, and start wining when a bearing is drying out or the balance is out of whack. 
I am sure the cheap and dirty blowers that GSP used sang a different tune as they aged and were slowly consumed by the fumes. If you are going to go with a metal blower and a spray can of epoxy, pay attention to the blower sounds.


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## niteliteone (Oct 31, 2013)

When you hear that first "Tat-Tat-Tat" it is time to shut it down and replace it while it still works somewhat. Don't wait for that "KA-CHUNG" cause then it stops working that instant. :shock:


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## MGH (Oct 31, 2013)

This is coming together a little better now – at least in my head. Thank you, Smack for the correction. Now I think I better understand what you meant by mentioning that the type of blower will determine what kind of ducting will be needed. So I’ll need to transition from my rectangular exhaust to [probably] a round duct which will connect to the blower – and of course the sizing of that duct needs to match the in feed for the blower. Then of course I’ll need a little more ducting to get from the blower to outside.

The hood and blower will be installed in a detached garage behind the house. So I will be able to monitor suspicious blower noises and simply vacate the work area as a last resort if something fails.

Thanks again everyone for the tips, advice, and warnings.


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## rickbb (Oct 31, 2013)

You can make those cheap squirrel cage fans last longer by taking them apart and painting all interior surfaces.

If you start with a new one then you won't have a rust layer to worry with before painting it.

If you go that route, don't close up those little holes you see inside the housing under the fan cage. You may think your sealing it up good to prevent fume leakage, but those are vent holes to draw air across the motor to keep it cool. If those are blocked your motor will over heat and stop. (Don't ask how I know this.  )


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## Palladium (Oct 31, 2013)

Everybody's right. I guess its according to what you are doing like 4metals said. I personally use the squirrel cage metal fan and i get about 2 years. It's cheap and it's effective. Now when it comes to a question of personal safety and the business workplace you better buy the best you can. If it fails and someone even claims they can't smell right anymore because of them smelling something when the system shut down, even if it was only 2 mins, then a jack in the box lawyer is going to have a field day with negligence on your part. Welcome to liberalism!


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## gold4mike (Nov 1, 2013)

I used an inexpensive (mostly) plastic blower from Harbor Freight for my hood. It's lasted almost two years so far and is still going strong. I used six inch plastic pipe through the roof of my shed for the exhaust and used clear plexiglass to make a transition box that friction fits to the blower. I put the box together with RTV silicone.

I bought a second identical blower so I could lift off the transition box and replace the blower in less than five minutes. I fully expected to have replaced the blower by now but it's doing better than expected. They offer a 1-year full replacement warranty for $10 and it's tied to the item number of the blower rather than the serial number so you can get it replaced free if it fails within a year.

Here's a link:

http://www.harborfreight.com/catalogsearch/result?q=blower


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## MGH (Nov 1, 2013)

Thanks everyone. You guys have been a great help.

One more question – more of an HVAC topic. In many of the metal squirrel cage fans I’ve come across (including what I see in the Harbor Freight link above), they appear to be set up to pull in ambient air without any direct ducting, and then discharge through some attached duct. To pull air from the hood I would obviously need to attach a duct to the infeed of the blower. Could someone point out how this is typically accomplished? Or do I just need to find a blower that has a flange ready to receive some ducting?

Thanks again,


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## philddreamer (Nov 1, 2013)

Something like this:
http://www.ebay.com/itm/HO-400CFM-Ventilation-6-Inch-Inline-Cooling-Duct-Fan-Exhaust-Air-Blower-Vent-/370927597513?pt=US_Hydroponics&hash=item565d01f7c9

Phil


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## gold4mike (Nov 1, 2013)

I simply stuck a fitting that looks like a toilet flange on the intake side of the blower using more RTV silicone and friction fit a short piece of plastic pipe into it so it can be quickly removed in case I need to swap the blower.


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## rickbb (Nov 1, 2013)

Yes, a plastic toilet bowl floor flange will fit almost perfectly on the inlet side of a standard squirell cage fan. It's what i have on mine, a little RTV sicilone and your good to go.


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## niteliteone (Nov 2, 2013)

MGH said:


> Thanks everyone. You guys have been a great help.
> 
> One more question – more of an HVAC topic. In many of the metal squirrel cage fans I’ve come across (including what I see in the Harbor Freight link above), they appear to be set up to pull in ambient air without any direct ducting, and then discharge through some attached duct. To pull air from the hood I would obviously need to attach a duct to the infeed of the blower. Could someone point out how this is typically accomplished? Or do I just need to find a blower that has a flange ready to receive some ducting?
> 
> Thanks again,


Real easy to make a flange and connect it with a few screws and RTV silicon.
These pictures are of the one I will be installing on my shop next month. The current flange is 16" but I will be cutting it down to 12" to fit the exhaust stack I have.


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## Smack (Nov 4, 2013)

Couldn't find this link in my bookmarks the other day, just another company for ducting supplies: http://www.harrisonplastic.com/


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## MGH (Nov 4, 2013)

Thanks again to everyone for all the advice and links. They are incredibly helpful. I think I've found cheap a local blower on Craigslist, but can't meet to check it out until Wednesday. Once I get a blower I'll try to get it all set up sooner rather than later.


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## MGH (Nov 17, 2013)

Just a progress update.

I built a table for the hood out of 2x4’s, using ¾” plywood for the structure of the work surface, then covered that with a 3/16” sheet of polypropylene. It’s a calculated risk (or risky optimistic hope) using polypropylene rather than PVC, but a 4’x 8’ sheet was $64 rather than $171 for PVC.

I was also able to pick up a used furnace fan on Craigslist for $20. It blows very well – should have a decent amount of life left in it as long as I can give its insides a good coat of paint. Disassembling and painting the fan is my next step.

I’m still not exactly sure what I’ll do about ducting. I haven’t forgotten the links you guys provided. Hopefully I’ll have another update in a couple of weeks.

-Matt


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## MGH (Nov 26, 2013)

Another update, with a request for more input at the end…

I disassembled, painted, and then reassembled the blower. I also had a “well duh!” moment to use one of the 7 or 8 old rheostats I saved from the dumpster at work one day so I can vary the speed of the fan if needed.

I have the hood-to-blower ducting fairly complete. I decided to use 4 inch diameter landscape drainage pipe (high density polyethylene). The reason was twofold: it was fairly inexpensive, but mostly it was because of physical space restrictions. I didn’t have very many options on where to put this hood. The table top height is a little low at about 31 inches, and even so the top of the hood nearly butts up against the roof joists in the garage (though it’s open space above the joists). One of the joists runs right over the top of the 6x23 inch fume outlet of the hood so I can’t attach any standard transition. I cut a piece of the same 3/16 inch polypropylene I used for the work surface inside the hood to make a plate that overlaps the fume outlet by ½ inch on all sides. Then I cut circles to accept female couplers for the 4 inch drainage tubes. I attached the plate to the fume outlet using RTV silicone. The couplers are a dry fit. I employed the same basic design to mate the “ducting” to the inlet of the blower, using male couplers on that end.

I’m still kind of stuck on the blower-to-exit ducting. I can’t find any satisfactory materials at the local home improvement store. I thought about using 6 inch PVC pipe to go straight up and out, but I think that I need to stick with at least an 8 inch diameter, and probably 10 inch. The 6 inch diameter is the largest size I can get locally in a straight walled pipe. I’ve browsed the links for plastics suppliers, and they have what I would like to use, but a single piece of 10” PVC is about $248 (10 foot minimum). A flange fitting to butt the duct to the blower would be $108. A rain cap would be $500+. Ouch.

Again, there’s a time to bite the bullet and just do it right, but is this really what others have spent for exhaust outlets, or have you guys found cheaper options?


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## Palladium (Nov 26, 2013)

Than thing looks like a solid rocket booster for the space shuttle. Good job!
Only two things i would question. The poly pipe may not stand up to nirtic. I've had stuff like that basically melt and turn to goo.
The second thing is even though the fan might work with a rheostat i think i read somewhere it will cause that type of fan to over heat and burn out. Maybe something about the way they are wound. I'm not sure about that someone with more electrical experience will have to weigh in. Looking good !!!


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## Smack (Nov 26, 2013)

I agree Palladium, that electric motor is most likely not made to run at a decreased voltage and will not last. You should write down the info off the motor and ask someone at a local electric repair shop.


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## Palladium (Nov 26, 2013)

Reading it again i misread it the first time to think it was polyester instead of polyethylene for some reason. :roll: 
You should be ok.


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## Palladium (Nov 26, 2013)

I built a box around mine and placed it outside where it can direct vent. All ducting was built from plywood as well as the box outside that houses the fan. You could build a vertical shaft straight up with the fan sitting outside on top to draw and exhaust the air.


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## Smack (Nov 26, 2013)

The ol carpet drying blower, those will move lots of air and are all over the net for sale.


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## Palladium (Nov 26, 2013)

Yep! $20 is what i gave for that one and it lasted over a year. When it came time to replace it i used some old scrap wood and built another enclosure, removed the old fan (box and all) and threw it all away. The new fan and box slides right over the old air shaft and replacement is easy. Total cost of 2nd fan $35. The fan i use moves about 3200 cubic feet a min. That means the air in my shop is replaced about 3 times a minute.


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## Harold_V (Nov 27, 2013)

MGH said:


> I also had a “well duh!” moment to use one of the 7 or 8 old rheostats I saved from the dumpster at work one day so I can vary the speed of the fan if needed.


That may not be as wise as you think. Not all motors will respond to being operated from a dimmer. I recommend you do some research before assuming yours will, as you may risk a fire if it won't.

Harold


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## MGH (Nov 27, 2013)

Palladium, Smack, and Harold,

Thanks for the warning about running the motor with a rheostat. I will certainly hold off on that until I’m sure. Fortunately, I have some flexibility in using the rheostat. I’ve wired a dedicated 20 amp circuit for the blower which powers a standard 120V outlet which is controlled by a switch I installed on the face of the fume hood. I can plug the rheostat into this outlet and then plug the blower into the rheostat, or just plug the blower directly into the outlet without the rheostat. For the time being, I’ll just assume that I’ll run without the rheostat at all.

Palladium, your fan-on-the-outside setup looks like a good solution. I don’t think I can go that route in my scenario, but it gives me some ideas. By the way, I have that exact same black and gray blanket in my garage too. 

I’m still thinking about how to get the exhaust out and through the roof. It’ll be a relatively short, straight up run of just three or four feet since the blower will already sit close to the roof anyway (sorry, I don’t think I’m going to call so much attention to myself with a stack 7 feet above the roof line). Since it’s short, and now weighing the costs, I’m back to thinking about using metal ducting and painting the insides. I don’t anticipate being in this house for more than another two or three years (growing family), so I’ll have to disassemble whatever I do anyway. Not committed yet. I’m going to wait it out at least a couple days over Thanksgiving and see what might gel up in my mind.


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## Palladium (Nov 27, 2013)

That fan you have will move well over 3,000 cubic feet of air a min. It draws some much air i can stand over an open vessel of nitric (25 gal) while it is at full temp and putting off noxx and never even smell the first hint of vapors. You shouldn't need that much draw for your fume hood. Instead of varying the fan speed you could control air flow by means of blocking one or two of the 4 circular intakes you have mounted in the top of the fume hood. You don't need a riser sitting real high above the structure because that fan will blow 30 ft straight up above the roof line.


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## niteliteone (Nov 27, 2013)

That "Rheostat" would work great on a "Shaded Pole" motor (think ceiling fan type motor), but most low voltage (100v - 240v) single phase blowers will use a PSC (Permanent Split Capacitor) motor which will burn out within a few minutes under load with the low voltage from the rheostat.


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## Smack (Nov 28, 2013)

Whether you have a factory made hood or make one from scratch, you should look into an alarm like the one I'm using on my hood. You can see in the picture that it will sound the alarm when the sash is open about 80%.


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## MGH (Dec 11, 2013)

Update:

My hood is now operational. I will be the first to admit that I’ve made several compromises that would not even be options for a hood in a more permanent installation or in a place of business. This project reflects a current priority for thriftiness and the recognition that I’ll likely be moving the setup in one to three years from now. My primary concern was personal safety while doing refining work, and it does a great job. I’ve started a batch of gold filled, and while dissolving base metals of 400g there were of course plenty of brown fumes, but I couldn’t catch even a hint of their odor (neither while in front of the hood, nor while outside downwind of the exhaust).

I finished hooking up my ducting from the hood to the blower and ultimately went the cheapest and most locally accessible route for materials for exhausting from the blower to outside. I used cylindrical 8 inch diameter steel ducting – the kind that comes preformed, and that you snap together along a seam to form the tube just prior to installation. I spray painted the interior of all ducting pieces with epoxy paint prior to assembly. I placed a flat piece of sheet metal across the exit of the blower, and cut a circle out of that and used a starter thimble to accept the 8 inch ducting. Then I attached a single 5 foot section of ducting to go from the blower straight up and out the roof (which exits very near the roof ridge and extends about 4 feet above it). I wanted to have a concentric cylinder “rain cap”, but I couldn’t make it happen on the cheap, so I simply mounted an adjustable elbow at the end of the exhaust at about 45 degrees, and then added another small section of straight pipe after that which I cut at a steep angle to prevent any rain from falling in.

After hooking up all the exhaust ducting, I found that my hood would not suck. This wasn’t unexpected. There was so much restriction in the four 4 inch drainage pipes going from the hood to the blower, that the air simply took the path of least resistance and entered the blower on the motor side which was still wide open (picture attached). To make more resistance on the motor side, I spread some chicken wire across the motor and blower housing for structural support, and then layered over that some foil-lined bubble wrap insulation that I had laying around (sorry, no picture of that). This does not make an air tight seal on the motor side – and this is by design so that the motor gets a little supply of fresh air – but it does restrict the flow enough to force air through the other side which draws air through the hood.

I completely dropped the notion of using a rheostat to dial down the voltage and blower speed. It’s fairly quite at full speed anyway. From outside the garage you might not even notice it unless you’re listening for it, and even then it’s just a dull hum. I’ll just run it at full voltage or turn it off.

So like I said, it’s not perfect, but it works. We’ll see how long it lasts. I’d say I’m a relatively low volume refiner (not more than an ounce or two of gold per month) so I’m hoping it’ll last at least up until the time we move to a new house. Then hopefully I can invest more in proper exhaust ducting and maybe even a plastic blower.

Thanks again to everyone for the tips and warnings. I hope to make this a more robust installation sometime in the future, at which point I’ll definitely be incorporating more of the advice already given in this thread. For now, I just wish the weather would warm up a little. I finished the hood and exhaust construction while we had a little warmer weather, but right after that we dove into an arctic blast. I’m torn between wanting to do some refining and just admitting that it’s not worth it in this kind of cold!

-Matt H.


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## Smack (Dec 11, 2013)

Good job Matt, but one thing you might want to do over is your wiring for the blower, never a good idea to have exposed wire connections, they need to be in a junction box. In most cases if there is a fire and they find wiring exposed like that, they will not cover the damages. I would seriously think about fixing that one thing. Have fun with it and be safe.


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## MGH (Dec 11, 2013)

Smack said:


> In most cases if there is a fire and they find wiring exposed like that, they will not cover the damages.


Dang. I did all the other wiring right when I branched of a circuit for the hood lights and outlets and added a circuit for the blower itself. That's all in conduit, connections inside boxes. I just couldn't figure out how to get all those connections that are on the blower to be inside a box - especially with the capacitor. I assume that whole assembly is usually inside the furnace box when it's used for its intended purpose and that's how it's all isolated. Not sure what to do with the blower motor wires in this application. Could I just mount a box right there and make the connections inside it even though the connections on the capacitor would still be in open air? Would that make any difference to an investigator? I'm open to suggestions. Thanks.


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## Smack (Dec 12, 2013)

Ya, they've gotten real picky over that type of thing. If you can take the capacitor off the blower housing you could put it in a box with the wires. You could get a bigger box too like a 6x6 steel junction box. I ran mostly rigid conduit for mine and had to put in a 6x6 and a 8x8 box to have plenty of room for all the wires, connections and room for future expansion.


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## butcher (Dec 12, 2013)

One side of the fan is open (motor end side), the fan will pull air from side of least resistance easier, than trying to pull air from the hood through all of that corrugated pipe, You will most likely need to enclose this open side more, the motor itself is sitting in the path of corrosive fumes, the motors shaft and bushings may not hold up well to these fumes, the motor can corrode with out much problem, but corrosion of the motors bushings can overheat the motor, the motor end rear of shaft needs to pull air through motor to cool windings (this should be left open to air flow), wiring will need enclosed, you have restricted out put of the fan with the smaller round duct, probably not much of a problem if fan is run at low speed, since you also restricted flow on the fume hood side inlet.

I just do not see this as the best option for a fume hood fan, it may work for a while, but personally I would be looking for another fan.


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## Harold_V (Dec 13, 2013)

Unless the air moved by a squirrel cage fan is extremely well filtered, there will be buildup on the wheel that eventually creates problems with air restriction as well as balance. They are not the best choice for the purpose, although they do work. 

Harold


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## jeneje (Dec 13, 2013)

I see you vented your fumes outside over your roof. Acids and roofing materials do not mix well with each other. In the not so near future I would consider building a scrubber to run the fumes through before exiting them outside above my roof.
Ken

edited; To explain this a little better, shingles are made from asphalt, asphalt contains up to 6% sulfur. The sulfur will mix with the acids causing decomposition of the asphalt, in return causing failure to the roof. 

The reason I mention this, is, you commented on not being there over two or three years. The cost to replace a roof is very expensive, running between $165.00 to as much as $450.00 per 100 square feet of roof area. It is by far cheaper to construct a scrubber, verse replacing a new roof, depending on the size of your home.


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## tedpgh (Jan 1, 2014)

Good job getting a setup together and making good use of your hood.

Be advised though the narrow diameter and ribbing of the tubing you use is likely restricting your airflow (A LOT).

Your setup may suffice, but if you find it's not pulling well enough then you'll want to use something smooth and wider.

-Ted


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## MGH (Jan 2, 2014)

Hi again all. Thank you to everyone for their interest and taking the time to comment. I don’t want to be one of those people who gets super defensive with any amount of criticism – I hope this won’t read that way – but I do want to clarify a couple things in case there has been any misunderstanding.



butcher said:


> One side of the fan is open (motor end side), the fan will pull air from side of least resistance easier, than trying to pull air from the hood through all of that corrugated pipe, You will most likely need to enclose this open side more, the motor itself is sitting in the path of corrosive fumes, the motors shaft and bushings may not hold up well to these fumes...





tedpgh said:


> Be advised though the narrow diameter and ribbing of the tubing you use is likely restricting your airflow (A LOT).


You guys are absolutely right. I had already encountered the restriction due to the corrugated drain pipe, and placed some additional restriction on the motor side (just didn’t post a picture in my earlier post). There is still fresh air flow that gets in through this restriction on the motor side and passes across the motor, while providing enough restriction to force air through the hood. I’m not sure how much of the motor or bushing are in the path of the corrosive fumes. This fan has a kind of plate down the middle of the wheel portion which would seem to provide some separation between incoming air from the hood side and the motor. I used the drain pipe rather than smooth pipe both because of cost, and the difficulty of working around the wood framing of the roof. It was, of course, a compromise.



jeneje said:


> I see you vented your fumes outside over your roof. Acids and roofing materials do not mix well with each other. In the not so near future I would consider building a scrubber to run the fumes through before exiting them outside above my roof.


Of course a scrubber will be a priority. And of course I cannot assume that there is no contact between the fumes and shingles, but the exhaust exits several feet above the roof line, is pointed at an upward angle, and the flow is probably somewhere around 2000 – 3000 cfm – hopefully carrying the fumes up and away from the roof. The point about shingle damage is well taken, but this is a calculated risk as I think there must be minimal fume-to-shingle contact, and because the roof on this detached garage is already about 20 years old anyway, and I wouldn’t be surprised if I have to replace it before selling the house regardless of any refining activities.



Harold_V said:


> Unless the air moved by a squirrel cage fan is extremely well filtered, there will be buildup on the wheel that eventually creates problems with air restriction as well as balance. They are not the best choice for the purpose, although they do work.


I have no rebuttal here. This point is also well taken. The blower and wheel assembly inside are reasonably accessible for inspection and cleaning. I will be sure to keep an eye on particulate buildup (and corrosion). 

So my future needs are still: low flow alarm, scrubber, and better ducting and blower all-around. And again, not wanting to sound defensive or dismissive, but I think I accomplished my intermediate goal of setting up a functional hood which provides personal safety and a convenient/secure place to do some refining, all with minimal investment (the total spend was $577 for hood, blower, ducting, paint, polypropylene sheet, conduit, wiring, etc.). My first refining in the setup was about 800 grams of gold filled jewelry, base metals digested in two 400 gram batches. There were of course plenty of NOx fumes, but at no time could I detect any hint of their odour, neither while standing in front of the hood, or outside, downwind of the exhaust (I’ll post a picture in the gallery section). I know this doesn’t mean that the fumes simply ceased to exist, but I think the hood is serving its purpose for small refining activities.

Thanks as always to the GRF and to the individuals who shared their experience and knowledge in this thread in particular. This place is great!


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## Harold_V (Jan 2, 2014)

MGH said:


> Harold_V said:
> 
> 
> > Unless the air moved by a squirrel cage fan is extremely well filtered, there will be buildup on the wheel that eventually creates problems with air restriction as well as balance. They are not the best choice for the purpose, although they do work.
> ...


What's important for you to take away from my comment is the fact that any deposits you see will have value. Do not discard anything that is removed from the venting system of a fume hood, as, in spite of your most careful attempts, you're going to lose traces. That's why I had a filter system in my hood.

Understand that I refined commercially, and that my hood ran pretty much non-stop. It got cleaned out about every two years, with a yield of about eight ounces of gold from the recovered material. The yield seemed to be quite reliable, as the hood was cleaned out several times while I was in operation. 



> So my future needs are still: low flow alarm, scrubber, and better ducting and blower all-around. And again, not wanting to sound defensive or dismissive, but I think I accomplished my intermediate goal of setting up a functional hood which provides personal safety and a convenient/secure place to do some refining,


Unless you happened to be born with a genius level IQ, and already know all there is to know about refining, there is much you stand to learn by paying attention to comments offered by those of us who have been there, done that. It took three generations of hood design before I had addressed the pertinent issues. Part of the reason for that is that it is not always obvious to the novice what problems must be addressed. Filtering a hood is one of them, and it's more than obvious to me that precious few understand the importance. 

Harold


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## rickbb (Jan 7, 2014)

As Harold pointed out, it's a continuous process to tweak it and get it right. Don't stop now just because your getting some good results.

I've used commercial fume hoods in several jobs for 20+ years. When I started my hobby in recovery and refining of e-scrap and built my own hood, I had to re-work my hood 3 times to get it where it needed to be. And I still need to get a better scrubber working even for my small volumes.


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## Plasticman (Mar 28, 2014)

If you ever want a real professional exhaust system, please give us a chance to supply this for you.


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