fume hood tryal

[callicom]

this is what i`ve done, i made a 10 inch by 10 inch box that stops just short of the outside, around it i built a 14 inch by 14 inch box and cut two slots in it. i was expecting the exiting air to create a kind of siphoning at the two slots but instead it blow in a little. here`s some pics

 

[callicom]

is it posible that the box from the blower just needs to be extended to lets say just a few inches from the outside. i hope you guys could make out the pics because they should be turned 90 degrees clockwise except for the second pic
1- the blower on top of small box thats inside the larger box
2- the two slots cut in the larger box that are to vacuum
3- the hole before i cut out the vinyl
4- the exit of the smaller box that i think should be extended further
5- a view of the whole set up
again i`m sory about the sideways pictures, coes anuone have any input, am i just wasting my time or does the idea make some sense and has a posibility of working by extending the fan outlet to near the outside :?: thanks in advance

 

[goldenchild]

The last picture. I would have stopped the wood column right where the top of the hood meets with the wood. Maybe even in the top most box. Then have a pvc pipe come out of the box and go into the top of the fumehood. I think I know why you created the column. Because of the shape of the opening on the blower. There are adapters for that where you could get rid of the column completely and only use pvc. Also the column has to have some kind of sealant if you are to achieve good suction http://www.florahydroponics.com/product.aspx?id=1538 That site has alot of goodies. It's also where I got my blower.



Now that I think of it even more. To make it super simple... If the blower isn't too heavy and can be supported by the top of the fumehood just cut out a hole there and put the blower directly on the hood. Then put a pvc pipe on the blowers exhaust, a hole in the wall and out goes your exhaust. If you're not planning on a scrubber of course.

 

[callicom]

i didn`t think i could have the fumes go through the blower :!: from what i could understand if the fumes were to go through the blower it would ruin :!: :?:

 

[butcher]

My mind wanders or wonders, Could a feller take an old room fan from the second hand store, the kind with a plastic fan blade, and small motor, dismantle it,

Build fume hood with the small fan motor mounted in front of the hood adding a extension stainless steel rod (between motor and fan blade) and bearings for rod mounted to hood, this rod can go through hood (and the back board inside hood that allows air in hood escape around the back board, this board is stand off spaced a few inches from back wall of hood, and the board is smaller than the inside of the back wall, where air goes around this board the gaps at top, bottom and sides allow air to flow around this back board, as fumes of many acids are heavier than air and flow around this bottom of this back board) the plastic fan blade mounted on back, outside the back wall of the hood (a round hole in back wall) .
(Pipe fumes further from his shop if needed)

The motor and shaft could be mounted to one side of front door (Glass) if your front glass lifted making it hard to mount fan above. Another option extension for the fan could even be flexible shaft. Or like an old electric weed whacker (plastic fan blade mounted on end), allowing you to mount its motor on side of hood and extension shaft making a bend through the hood, dog gonnit wish I could draw a picture of what I am trying to say, cause I sure do not know if I can write it where someone would get the picture.

Other thoughts, look for squirrel cage fans with plastic fans and motor and bearings mounted outside fumes path (if cage was metal you could epoxy or fiberglass coat it ), some types of Bathroom exhaust fans may also work

 

[callicom]

I kind of see what you say, the final would have the motor outside the ventilation shaft and the blade inside, that would probably work, but i`m trying to salvage what i`ve already made,
what i`m trying to do is build a type of vertical ventury hood, what i didn`t do is the cone shape that comes from the blower that might be the reason it doesn`t work. From looking at the picture in hokes book i better go to a horisontal set up. back to the drawing board and hopefully this time it will draw the air instead of blowing it :!: i`ll post other pics

 

[butcher]

Here is a thought, expirimenting would be needed:

say you refined in a room fan blows fresh air into room, this air has to escape the room, the only means for it to escape is through your fume hood, pushing fumes outside with the fresh air. openings in fume hood would need to be sized to handle the cfm (cubic foot of air per minute) of your fan, and if fan was capable of a few more cfm all the better.

only draw back would be opening the door as it would need to be closed for the fume hood to work.

 

[Geo]

that's used in paint shops. its called "positive air pressure environment". one way to combat loss of positive pressure would be to add a small space just inside the door with another door( maybe a storm door) so you can open the outer door and enter then shut that door and open the next door to enter the room. the space needs to be only a few feet wide, just enough to walk in and shut the door. works the same with exiting the room.

 

[old thompson]

I am pretty sure that "Positve air" refers to air comming in. I'm pretty certain you want "negative air" all the way around. If air is comming in the fumes might stay around. Air going out is what you want, it is much more efficient than pumping air in. Nature abhors a vacum.

 

[butcher]

You cannot pump air into a room without air leaving the room.
you cannot take air out of the room without air coming into the room.

 

[old thompson]

You cannot pump air into a room without air leaving the room.
you cannot take air out of the room without air coming into the room.

Not trying to start a pissing match, but sure you can. Air is not like a liquid that cannot be compressed. You can compress air and as far as I know it is more efficient to pump it out than it is to pump it in.

But, I don't Know everything. Please correct me if I am wrong..

 

[Geo]

positive air pressure environment means the the air pressure inside is above the atmospheric pressure outside. by pushing air into the room your compressing the air. now if the only outlet for the pressure is a vent in the back of your fume hood you would have positive airflow through the fume hood to the neutral air pressure outside. as the air travels through the hood on its way to the neutral pressure it will carry any fumes with it as long as the pressure on the inside of the room is kept at a positive pressure. theres no other way around it. not trying to argue but the distinction of positive or negative air pressure depends on which side of the fan your standing on.

 

[old thompson]

I see what you are saying Geo, but the way I am also seeing it is that I can pump up a tire to 3 atmospheres and if there is a hole in that tire most of the stink stays inside the tire.

I think that I would rather be in the vacum. The fresh air must go into a vacum, it does not need to get out of pressure except for that tiny hole in the tire.

I hope my thoughts on this are clear. Atmospheric air pressure will fill a vacum. but if you compress air in a container it will not nesescarily come out as fast as you fill it.

 

[butcher]

Blow into a coke bottle, or suck on one, there comes a point where no more air can be blown in or sucked out.

If your room had no where for air to escape. Your fan would not put any more air in. and your fan would just be spinning its wheels circulating air inside itself.

Your tire expands, you are using a high pressure pump to pump and pressurize the air (an air fan cannot do that to your room).

the fan can supply air to the room depending on its CFM (size), it cannot pressurize your room (to any extent) the exhaust duct size determines how much Cfm can flow out of the room, so if your fan CFM generated was slightly greater than the size of the exhaust hole or ductwork, all the air you pumped into the room would go out the room, this would change the complete air in the room several times a minute (calculations of these can be made). No air would escape fume hood to your room all fumes would be pushed outdoors.

 

[old thompson]

What happens if there is a hole in the bottom of the Coke bottle?

Can't I force air into the Coke bottle faster than it can get out?

Same Coke bottle, if I draw air out through the hole (either one) the atmoshere forces it in.

A fume hood is like a Coke bottle with holes on each end. That is, unless you have a closed system that you would use in your bedroom, and nobody does that as far as I Know.

 

[butcher]

I have no more comments.

 

[Harold_V]

I have one comment, for what it's worth. Keep in mind, I used a fume hood for not only chemical use, but for incineration. The results I sought, and achieved, were flawless.

I would NOT rely on a pressurized room for exhaust. I would rely on a vacuum for exhaust. The vacuum should be created by the hood itself, so there's always a NEGATIVE pressure in the room in which it is contained. That way none of the fumes should escape the fume hood. That may or may not be the case if the pressure inside the fume hood is no different from the pressure inside the room in which it is housed and used.

Harold

 

[callicom]

as far as positive pressure in the room it not what i`m looking for and anyway there`s too many escapes in that barn, then again it would be possible to build a smaller more air tight room inside with the mentioned strategy (positive air).
The atempt was to create the same effect as the ventury by the blower pushing the air down the 10 inch by ten inch shaft that exausted outside this smaller shat being covered by the larger 16x16 shaft with just the two holes for the fume hood. its basically the same principle as the other men are trying with the pvc pipes.
heres what i`ll modify today
1- i`m going to end the ten by ten shaft closer to the outside and coneshape the last foot.
2- if that don`t work i`m going to take that whole set up and make shelves out of it :lol:
3-i`m going to buy pvc pipes and keep reading that other ventury post 8)
thanks for all the input

 

[callicom]

well as you guys could see on the two pictures i``ve taken off the outer vent shaft and the inner shaft is exposed. Now i`ve added that little part at the bottom to direct the air directly out and i funneled it also to constrain the air flow, i also brought it to about 3 inches from the outside.
Well the result after lighting a cigarette next to the hole in the wall is that it was drawing the smoke outside, so it seems to be working, how performant it is is left to find out. i`m going to rebuild the outer venting shaft and i`ll post the results :|

 

[Geo]

sorry to sidetrack your thread. i was not recommending anything, i was speaking hypothetically about the possibilty. i would not recommend that anyone do what i was talking about because there are much easier ways of doing it.