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Author: Subject: homebuilt fume hood
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[*] posted on 17-9-2008 at 03:09


Wow that's a huge difference, but that is the same kind of fans as that you wanted to order.
So same goes for you, order them not at conrad but at the website you just gave me.
I will go for the 200B, but I can get it via e-bay and there it is 80 euro's so well yeah it is still brand new and including shipping costs concerning conrad this is really cheap, concerning the website you just gave it is the same.
About the msn thing I will pm you my e-mail adress where you can add me on.
EDIT: it is already in my profile the mail adress.

[Edited on 17-9-2008 by DNA]
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[*] posted on 17-9-2008 at 08:01


Quote:
Originally posted by DNA
I'll also get something else then this flexible tube for a dryer.
That's a good thing.

The rule of thumb, as I recall, is that flexible tube has internal resistance of about three times that of ordinary galvanized steel. In practice that means it's a "last meter" option, but not good at all for long runs. For a fixed fume hood, there's generally no need for that flexibility.

As for using bends, if all you are trying to do is to shift the run of a pipe, two 90° bends are much worse than two 45°ones, and two 45° bends are worse than two 22-1/2° ones.
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[*] posted on 17-9-2008 at 14:53


From DNA:

Quote:

And what is the difference between having a circulair or rectangular opening magpie?


Assuming you mean the outlet of a fan, rectangular is pretty common for squirrel cage fans. Not much you can do about it. I had to build a special plenum to adapt my fan rectangular outlet to the louvred and screened outlet in the wall of my garage. I had a sheet metal shop build it out of stainless steel. Cost me big $. :( I can draw a sketch of this plenum if you would like.

For any ducting a rectangular cross-section is going to have more friction loss (pressure requirement) than a circular duct with the same cross-sectional area. However, as you've probably noticed, retangular ducting is normal for home heating and air-conditioning. I suppose it takes up less space. There are always economic tradeoffs involved.

If I didn't answer your question, let me know.

[Edited on 17-9-2008 by Magpie]

[Edited on 17-9-2008 by Magpie]
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[*] posted on 17-9-2008 at 15:06


From Jor:
Quote:

I think I'm getting the AXC 150B, the one with 530m3, and AXC 160A with 440m3. The PDF says the product is suitable for long ducting, and can overcome wuite some presuure. Not sure what is the best one for me. Wich one would you guys choose (Magpie ?)


I want to see the fan curves first. :P
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[*] posted on 17-9-2008 at 16:03


http://www2.produktinfo.conrad.com/datenblaetter/550000-5749...


;)
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[*] posted on 17-9-2008 at 17:39


Quote:
Originally posted by Jor
http://www2.produktinfo.conrad.com/datenblaetter/550000-5749...
I'll reinforce what others have said. Inline fans are not appropriate for fume hoods. Period. Even if they're cheaper. Even if they're quieter. Even if they're easier to install. Even if they're smaller. To review the risks:
  • Corrosion and then Electrical Fire. The real hazard of a corroded fan is an electrical fire. Acid fumes will corrode an inline fan. The motor will eventually open if you are lucky and short into a fire if you are not.
  • Explosion. Any fan that can generate a spark in the gas flow is an explosion risk when combustible fumes are present. Some motors inherently spark, and every corroded motor will eventually spark.
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[*] posted on 17-9-2008 at 17:54


The brochure for Jor's candidates say the the fan is in-line but that the motor is "external".

Edit: I don't understand how the motor can not be in the flow path for these fans. They don't really look like they have a squirrel cage, but instead are just axial fans - like a kitchen or bathroom fan.

If this is true I can't recommend them either.

[Edited on 17-9-2008 by Magpie]
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[*] posted on 17-9-2008 at 22:44


:(:(:(
It's really impossible to find one with an external fan...
I thought external rotor motor means that it is outside.

However, if I cannot find a good fan, I will just buy this one. I have seen many people having hoods made themselves with common fans, and they work good as well, for long times. Take garage chemist's hood, it has an ordinary kitchen exhaust fan, and he is passing the most nasty fumes through it (works with things like acid chlorides and stuff).
I will look further, but if I cant find a better one, I will go for the motor from conrad.

[Edited on 17-9-2008 by Jor]
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[*] posted on 18-9-2008 at 01:28


No no people, the fan that jor and me have been showing you actually have the motor outside.
It is explosion proof and has also got several certificates of safety, waterproof and what not.
So although it is an inline fan it is suitable I think, because the motor is placed outside the duct.
It is a sort of axial fan combined with the advantages of a squirrel cage fan as they say on the specification sheet.
this is the same fan as that I posted the specifications for where magpie calculated the pressure drop for which was quite a nice pressure drop.
I'll have a look for the exact certificates and things that it has.
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[*] posted on 18-9-2008 at 01:31


Here I have some pictures for you guys so you can see the motor being outside and more exact pictures of that particular fan ;)

http://img175.imageshack.us/img175/4114/afb024ic0.jpg

http://img66.imageshack.us/img66/1393/afb025ev4.jpg

http://img98.imageshack.us/img98/764/afb026aat7.jpg

EDIT: pictures are way too big to post them on the forum, so I included the links.
[Edited on 18-9-2008 by DNA]

[Edited on 18-9-2008 by DNA]
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[*] posted on 18-9-2008 at 05:44


Quote:
Originally posted by DNA
Here I have some pictures for you guys so you can see the motor being outside [...]
http://img98.imageshack.us/img98/764/afb026aat7.jpg
That little white box is an electrical junction box, not a motor. A fan with an inline impeller and an external motor must be connected by some sort of belt or chain, which would require two openings, which wouldn't seal. The reason that other motor configurations can work is that the couplings is a shaft and the penetration has a bearing. The bearing seal is the weak link at that point. Viton bearing seals are available, if that really becomes a problem.
Quote:
It is explosion proof and has also got several certificates of safety, waterproof and what not.
It's possible for a motor to be rated explosion-proof and still not be suitable for acid vapor service. Those are two different hazards. "Waterproof" is a property that can change in the face of corrosive vapors; assume it will change unless specifically designed for it. Since it's cheaper to use another configuration, assume that such a fan is a specialty product and even more expensive than you think it ought to be.

I also looked up "conrad.com" (the site of the fan information), which I was unfamiliar with because it's a European concern and I don't live there. The upshot is that it's not an industrial supplier and (assuming it was going to be your source) it's unlikely you'll find anything suitable there. It's possible, but don't count on it. One of the threshold tests of being a serious hobbyist is whether you can be a business at least for the purpose of gaining access to wholesale suppliers. (Hint: business cards work wonders.)
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[*] posted on 18-9-2008 at 08:05


DNA, how did you order? The version I want and the version you have are both not in their online catalogue...
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[*] posted on 18-9-2008 at 08:28


I thought it might be good to repost the picture of my fan. As you can see it is a centrifugal type with "squirrel cage" and externally mounted motor. This style I can recommend.

As watson.fawkes indicated trying to buy equipment like this can present obstacles. The local wholesale distributor for this fan would not sell to me. So I asked an HVAC contractor to buy it for me from the distributor. Since he was a nice guy he did so without any price markup.

blower_7C651.jpg - 9kB
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[*] posted on 18-9-2008 at 13:48


My fan I didn't order via conrad but I bought it via ebay.
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[*] posted on 16-10-2008 at 08:22


We now are working here on the ductwork of the ventilation-system, and we run into a problem. A local supplier tells us that it absolutely unadvisable to take a 200mm duct out of the roof (we have an old roof). He advises 150mm, standard size. Now from the hood I will first go up 3 meter with PVC pipe, and then motor will be attached. Next we have to put an adapter to convert 200mm to 150mm between the motor and the roof (distance motor/roof is about 1 meter.. I was wondering, how does this affect the volume of air that can be moved. technical details of my motor says it can move 760m3/h. Will it decrease by 50% (surface area of a 150mm circle is about 50% of the surfae area of a 200mm diameter) , or less? And is it advisalble to put the adapter (200/150) right under the roof, or immediately behind the motor?
Sash is now done! :P
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[*] posted on 16-10-2008 at 08:54


Please post more pics when you can Jor, it is a deligth to see a buildind hood..



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[*] posted on 16-10-2008 at 10:13


Quote:
Originally posted by Jor
We now are working here on the ductwork of the ventilation-system, and we run into a problem. A local supplier tells us that it absolutely unadvisable to take a 200mm duct out of the roof (we have an old roof). He advises 150mm, standard size.
Having done some work on old houses, this answer makes zero sense to me. Frankly, my first suspicion is that your supplier has much lower margins on the larger vent (which may mean he has to go to the effort of special-ordering it) and is trying to sell you something for his benefit. Regardless, worry about flashing. Worry a lot. Bad flashing can ruin a structure over time. I once lived in a house (rented, thankfully) where some failed flashing caused a stud cavity to fill halfway up with water when it rained. Really. I "fixed" the immediate problem by getting up in the attic and drilling a drainage hole in the (closed) soffit board. The only concern I can imagine with an old roof is to get all the layers of flashing necessary (one for each roof layer, in an ideal world) in place correctly, but I can't imagine how that would differ by diameter.
Quote:
Now from the hood I will first go up 3 meter with PVC pipe, and then motor will be attached. Next we have to put an adapter to convert 200mm to 150mm between the motor and the roof (distance motor/roof is about 1 meter.. I was wondering, how does this affect the volume of air that can be moved.
The effect of a constriction in the pipe is to increase the static resistance of your venting system. This will move the operating point of your fan to a point with less flow. If you fan is of marginal capacity (I don't know one way or another), your hood will have more transient releases. That's bad, and worth fixing up front. It's also the reason why slightly over-specifying the blower and putting in a damper is frequently a worthy cost for prudence. (Variable speed control also works, but not always applicable.)
Quote:
And is it advisalble to put the adapter (200/150) right under the roof, or immediately behind the motor?
If you have to use a smaller exit vent, make the extra static resistance as low as possible by keeping the constriction as short as possible. This means putting the adapter right under the roof.

As long as I'm addressing static resistance, you get static resistance on the intake air side as well. To wit, you can't blow much air out of a sealed room. Have you planned a source of make-up air? This supplies the same volume of air that you're going to exhaust out your vent. If you have hard winters where you live, figuring out how you want to balance your personal comfort vs. the heating cost of exhaust.
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[*] posted on 16-10-2008 at 10:30


Good points by watson.fawkes.

I agree that I would not give up easily on keeping that duct 200mm all the way. No matter how short the 150mm piece is it will act like a restricting orifice. I'll take a crack at seeing what it might cost you in airflow, ie, I'll do some calculations.

I don't see why you can't do this but then again I don't know the construction of your roof. I installed the vent pipe for my gas hot water heater. It was a piece of cake. Then again I only had to penetrate a 1/2" (12mm) piece of plywood and the asphalt shingles. The vent pipe was only 3" (75mm) diameter resulting in virtually no structural weakening of the roof. I installed a galvanized steel flashing. I have had no problems in over 5 years.

Is your installer concerned about structural weakening of the roof? If so can it be reinforced locally at the penetration point at a reasonable cost?

Or is it the flashing that he is concerned with. Perhaps he doesn't have or can't get standard flashing in a 200mm size. In that case you might have to have one custom made by a sheet metal shop.



[Edited on 16-10-2008 by Magpie]

[Edited on 16-10-2008 by Magpie]
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[*] posted on 16-10-2008 at 11:39


Let's say it like this, it is much harder for us to install a 200mm exit, says the supplier. After all, money is limited, and time. I'm getiing help of someone (I could never do it alone).
A reason it si much more difficult, is that a 150mm would fit by removing one 'pan' (don't know the english word, it's a ceramic material on house roofs), and when taking 200mm we have to remove two, where also quite some open area will be leftover, and it's hard to close that again (my Englsh is real bad here, try to udnerstand :P). Another small thing, the neigbours must be wondering, when a huge 200ml pipe comes out a garage. I have nice neighbours dont get me wrong, we even go with them to football games sometimes (Feyenoord!!), but I don't know their responce to a chemistry lab.

Hmm, yes the flashing is really important... Or I would get water in the blower and hood ... Or a bird ;)
But wich one is recommended? I can imagine flashing gives quite some resistance, as it blocks the exit.

I need about 450m3 in for my hood. Maximum capacity of my blower is said to be 760m3. it has 4 speeds, 25%, 50%, 75% and 100%. If put at full power, even with the resistance, 450m3 should be achievable right? Even a little les (350-400m3) is also enough in my opinion (I don't do highly toxic work, I work in small amounts, and I can almost fully close the sash, with no bypass, So that really effectively sucks away all fumes, when having alot.

Ok, i will put the adapter right under the roof then.

When I'm having the hood on, I will open a door, just a little. I know it will be quite cold in winters, but it is anyway, even without hood.

Also I use the hood only when neccesary. Because I have no external motor, especially HCl (very corrosive to metals, more than SO2, NOx, etc) will be used outside a hood. i really don't care about breathign a little HCl, as long as it;s not a cloud (I had this experience when opeing a 5L jug of lab-grade 37% HCl :o).

I have this hood especially for nasties, like chlorine, bromine, NO2 organic solvent vapours (like benzene, DCM and chloroform), and carcinogens. As my rotor blades of the blower are plastic coated, this is good for vapours like HCl, HBr, acetic acid, NO2, SO2, etc. It's NOT good for Br2 and Cl2. Would it help to buy a can of Teflon spray and spray a thin layer on the blades?

Yes Magpie I have realised it will add resistance, no matter hwo short the 150mm piece. But what i wonder is, how much. I have some extra power (760m3), more than I need, as noted before.

the flashign is indeed also a problem with 200mm. In local stores I have not seen it at 200mm size...
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[*] posted on 16-10-2008 at 12:41


OK, I think I understand your problem now. You have ceramic or stone tiles for roofing. This is much more difficult to work with than my plywood and asphalt shingles.

When I looked back at my spread sheet for a previous calculation I did for you I saw that the diameter was 150mm. So what is the outlet diameter of your fan? If it is 150mm you might as well make the whole duct system out of 150mm duct, ie, all the way from the hood to the stack outlet.

How high above the roof will your vent extend? I have a 75mm vent pipe and a 100mm PVC vent pipe for my gas hotwater heater and my gas furnace, respectively. They only extend about 0.5m so no exterior support is needed against the force of strong winds. But for chemical vapors you might like it a little longer. You will have to balance this need against any requirement for structural support and appearance to the neighbors. You want to blend right in and not be a topic of neighborhood speculation. I think I would make it look as normal as possible. Those ceramic tiles can take the chemical abuse anyway! ;)

Flashing is the seal between the roof and the vent pipe. It keeps water out of your attic, not your vent pipe. If you need to keep water out of your vent pipe, which you probably do, then there are devices for this such as conical "witch hats" or T's. I'd use whatever is customary in your area.

Edit:
My previous calculation for 150mm ID smooth duct, 4m long, 2ea 90deg ells, 1 pipe entrance, 1 pipe exit, gave 9.3mm H2O pressure drop for 450m3/hr. If this fits within your fan curve you will be OK. I don't know which fan curve you have.

[Edited on 16-10-2008 by Magpie]
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[*] posted on 16-10-2008 at 13:31


Yea my diameter was indeed 150mm at that time, but after all i decided to buy for 30 euros more a blower wich has 4 speeds, with a max of 760m3, so i could try all speeds and decide wich one works best. this way I can also put it on lowest speed when working with things like ammonia, wich just smells but is tot really toxic. This conserves energy. having only the possiblity to turn it at full blast, I didnt like that.
This blower had 200mm and I never even realised this would be a problem.
Why would I put the exit of the exhaust to high in the air? To get it as far away from me as possible? It's close to mine and the neighbours garden anyway. Even with this hood, I can only work with small amounts and smelly substances only at night. For example when I make bromine I do this at 0:00-1:00 at night. this will still be the case.
When I use large amounts, I always use a washbottle.
We really don't want to work for a long time on the roof. This will draw the attention. Although people are not as paranoia here as in the US, it might a bit weird, an exhaust pipe on the roof of a garage.

What do you think about the idea to spray TEFLON on my blower blades wich are palstic coated? Good idea to help them survive chlorine and bromine, or will the layer of teflon be to thin?

By the way, although my motor is not external, it's not right in the path or airflow. It is sealed within some kind of case, protected by thich metal and plastic. The wires for electricity (just the big one you plug in not the small colored ones) are inside the path though, just as some kind of battery looking thing where it is connected. But it's right in the middle, and most air will not go through the middle.

There are no 2 90 degrees angles in the duct anymore, just a single 45 degrees angle. There will probably be the 200-150mm adaption in the last half meter. So what do you think, will I get 400m3or more ? I think I can,I can't imagine a single 200-150mm adaption to take about half of the power of my blower.

WOW, I can talk English decently when talking about normal things, but when trying to explain technical details and such , it gets quite hard! I hope you can udnerstand me :P
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[*] posted on 16-10-2008 at 14:14


Don't worry about your English; you are doing great. I understand you perfectly!

Post your fan curve if at all possible. If it has been posted before let me know which one it is.

Yes, high stacks are just to get the fumes away from structures and people. Industrial specifications in the US are 10 ft (3.5m). If you keep your noxious odor generation to late at night I wouldn't think it would ever be a problem. Or, as evil_lurker says, when he gets a good southwest wind he is "good to go." I use my fan during the day. I'm just careful about not doing it when the neighbors are in their backyard.

I'm thinking that spraying the blades with Teflon might be overkill. Likely it wouldn't hurt anything except possibly cause the fan to be imbalanced, but not likely.

[Edited on 16-10-2008 by Magpie]
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[*] posted on 16-10-2008 at 14:41


Also where you place your hands (where the glass slides down, have a vent blowing air straight up over your arms, so the risk of contamination from outside agents is minimalized.

*Not required, but its a favourite feature of mine.

Can't you buy an old one that is being discarded by a local college or university and repair it? I know in Canada, when a department has to keep to budget, they often get new equipment they don't need.

I found this

http://www.labequip.com/itemcatalog/stkno/26322/Kewaunee-Sci...

It does not come with a fan, base cabinet or work surface but it offers a shell and should cut back on your construction time.

Here is a smaller, but more complete hood (only $200)

http://cgi.ebay.com/BAKER-COMPANY-SG-600-FUME-HOOD-6-16822_W...

[Edited on 16-10-2008 by sbovisjb1]

[Edited on 16-10-2008 by sbovisjb1]




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[*] posted on 16-10-2008 at 15:02


Quote:
Originally posted by Jor
A reason it si much more difficult, is that a 150mm would fit by removing one 'pan' (don't know the english word, it's a ceramic material on house roofs), and when taking 200mm we have to remove two, where also quite some open area will be leftover, and it's hard to close that again
The ordinary English usage is to call each piece of ceramic a "tile". (You already knew that word, I'm sure, but it's used for overlapping roofing as well as for flat, butt-jointed, and grouted pieces.)

OK. This does make sense (I've visited Netherlands; I know exactly the style you're referring to.) A smallish constriction isn't going to be that bad; you just have to pay for it with extra fan (which it seems you have). On the other hand, if you haven't already bought your vent, see if your supplier can obtain a rectangular vent, say, 150mm by 250mm, or whatever is standard. You can orient that along the tile width. You'd also need a different shaped adapter, but you already need an adapter of some kind regardless.
Quote:
Another small thing, the neigbours must be wondering, when a huge 200ml pipe comes out a garage. I have nice neighbours dont get me wrong, we even go with them to football games sometimes (Feyenoord!!), but I don't know their responce to a chemistry lab.
If it looks like a chimney, with a standard chimney cap, it shouldn't attract much notice. Another hint: if it looks old, it won't be as noticeable. Cheap spray paint is your friend here. It doesn't need to last, since when the paint goes your vent will <i>be</i> old.
Quote:
Hmm, yes the flashing is really important... Or I would get water in the blower and hood ... Or a bird ;)
But wich one is recommended? I can imagine flashing gives quite some resistance, as it blocks the exit.
I believe you're thinking of a chimney cap, which cover the top of the vent. Just make sure that the exit area beneath the chimney cap is of adequate size (a little bigger than the pipe cross-section) and you'll be fine.

Flashing is different. It's a membrane, either rigid (metal) or flexible (EPDM, for example), that takes on all the special shapes required to fit obstructions to the basic repeating pattern of your roofing material. The traditional flashing for tile is sheet copper. It's easy to shape in the field and it solders easily. Roofing is a "shifted sandwich" of tiles or shingle. Flashing for a vent slips into this sandwich and acts like just another piece of roofing. This general article and this specific construction detail should get you started.
Quote:
When I'm having the hood on, I will open a door, just a little. I know it will be quite cold in winters, but it is anyway, even without hood.
You might also consider a second fan and an outside (source) vent and damper, if the layout of everything permits. You'd then just pipe some incoming air to near the top of your fume hood.
Quote:
Also I use the hood only when neccesary. Because I have no external motor [...]
I do understand your limitation. But I would recommend putting a bit of forethought into how you might replace the fan, if necessary. This involves access to the fan site itself, the ability to cut into the duct, and where the electricity is already routed. The idea is to make fan replacement less than a full construction project.
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[*] posted on 16-10-2008 at 22:10


Not entirely off topic.. and nifty :)

I built a small Tesla Rotor (Google Tesla Turbine) to play with out of CDs, and it worked much better than I expected for airflow. An advantage of this design is that the air exits axially (or enters, depending on direction) to the rotor, so you could make a simple unit out of your preferred non corrodible material and link it to a motor externally with an air bearing and a rare earth magnet. This is similar to how it is used for corrosive and heavy solids bearing liquid transport by oil companies.

If you made the rotor assembly out of cheap enough materials, you could just dispose of it every few weeks and replace it, thus obviating problems with wear.

Lots of work, but elegant. Its on The List.
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