Fan/Blower recommendation for standard hood?

[Smack]

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

 

[MGH]

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.

 

[MGH]

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

 

[MGH]

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?

 

[Palladium]

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 !!!

 

[Smack]

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.

 

[Palladium]

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.

 

[Palladium]

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.

 

[Smack]

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

 

[Palladium]

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.

 

[Harold_V]

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

 

[MGH]

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.

 

[Palladium]

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.

 

[niteliteone]

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.

 

[Smack]

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%.

 

[MGH]

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.

 

[Smack]

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.

 

[MGH]

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.

 

[Smack]

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.

 

[butcher]

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.