Sciencemadness Discussion Board

motor for fumehood

Jor - 28-4-2008 at 11:51

Im still in doubt what source of suction I should use for my fume hood. UTFSE yielded little information about this subject.

What do you guys advice? A kitchen exhaust as the source, or a 'ventilator' (don't know English word) wich can be conncented to a pipe? Like this:

Or something else?

Second: what should be the capacity of the suction? Say I'm making a hood of approx 0,8m W, 1m H, 0,6 D, would 500-600 m3/h be sufficient? What would be minimum, and what would be maximum (turbulence).

Any pictures of the suction source (damn , can't find the proper English word, I'm Dutch) you guys using would be welcome, as well as capacity.

Pulverulescent - 28-4-2008 at 13:02

I think the word you're looking for is extractor fan (former MF enthusiast?) as for the motor, if your fumes are corrosive the motor should be sealed for protection.

That's just off the top of my head.

I (sniff!) don't have one myself!


Maja - 28-4-2008 at 13:43

I'm interested in sealing motor. What is the proper or effective one ?

ScienceGeek - 28-4-2008 at 13:58

I have used my kitchen fan for over 2 years now, with gasses passing through like Chlorine and Nitrogen Dioxide. I inspect the fan now and then, and there doesn't seem to be any damage! Also, the motor is more than powerful enough to "suck up" most gases!

chemrox - 28-4-2008 at 14:19

I kept a storeroom clear of ether, alchol and acetone fumes for years with a box fan mounted in a window that it fit well. There were acid bottles in there as well including nitiric and HCl. No problem although the steel blades had some rust after two years of running. I think you should focus on the volume you need to remove and the rate at which you wish to change the volume. I've seen small, homemade hoods that used computer fans. The proximity of the fan to the fume source is important. If the fan can be kept at distance of a few feet or more you can probably get by with a conventional motor. Vendors of building supplies have various sizing criteria for fans you can look up. I'm using a twleve inch exhaust fan with a variable speed control to change the air in a former kitchen that had an inadequate exhaust fan. Rahter than cut a whole in the exterior wall we decided to sacrifice a window. It almost seems like overkill until I start pouring an acid chloride...anyway try some conventional sizing parameters based on volume and rate of change..

Jor - 28-4-2008 at 14:59

Sciencegeek, have been passing flammable vapors through kitchen exhaust? What your exhaust's capacity?

Chemrox. I have no idea at what rate I want to refresh the volume. I will work with small amounts of corrosives, like HCl, HNO3, NO2, halogens, etc. Also flammables. No extreme flammables though, like ether, CS2 and acetone. Acetone and ether are not bad for the health anyway. Besides, I will do some simple synthesises. No work though with volatile extreme toxins or carcinogens. Possibly some acid chlorides. What refresh rate do you recommend? I can get many extractor fans, with different capacity. The higher the capacity, the higher the cost. I think my hood will be a total of 0,5m3.

How many meters from ground level should exit of exhaust pipe be? I live in a city. Considering I do not work with things like H2S.

Any pictures of how you guys did all this would be very welcome.

bio2 - 28-4-2008 at 15:04

There is a very good thread here by Magpie on this subject. There are others as well.

All these questions have been long ago answered.

.............. I have no idea at what rate I want to refresh the volume...........

Try typing "fume hood design" into a search engine.

Don't be so damn lazy!

Magpie - 28-4-2008 at 15:05

You can see my fume hood and parameters at the 17th post of "Tour My Lab." Your design target should be around 30cm/sec face velocity for your hood, IIRC.

[Edited on 28-4-2008 by Magpie]

Jor - 28-4-2008 at 15:45


I have read Magpie's topic. I based a good part of my plans on his photo's, however he did not mention the capacity needed!
I have done multiple search-actions. It will yield same internet pages all the time. I did not think of fume hood design yet. Will try.
Please don't call me lazy. I hate that. I know pretty much chemistry, but I am VERY bad at do-it-yourself and contructing!

I find it hard to find a motor removed from the blower. Every blower I encountered until now was not 'removed' from the motor. Should it be removed because of the corrosive fumes or flammables? If it's only flammables, I think it's not of real concern. Vapors should be diluted before they enter the blower. Ofcourse, as mentioned before I will not work with things like ether or carbon disulphide. However corrosive fumes ruin the motor even at low concentrations.

I am very sorry about my stupidness bio2, and also to others , but can you guys see if the motor is on the outside on the following product:

Please ignore English mistakes. I'm very bad at this language!! :-P

bio2 - 28-4-2008 at 16:00

No offense intended Jor, sorry.

If you convert face velocity to the volume for your area then you have the answer you seek.

Magpie - 28-4-2008 at 18:05

Jor, I took another look at that first picture you posted. I was mistaken. I see the rectangular outlet on the bottom now. So that would not be the type you insert in a duct. Also the fumes may not be in contact with the motor.

I do not think you are lazy or stupid, quite the contrary! I will help you where I can.

"Face velocity" is a term used to indicate the velocity of the air, taken as an average, that enters the opening or "face" of a fume hood. You say your opening (or face) is 0.8m W x 1m H. The face then has area, A = 0.8m^2. If Q is flow and FV is face velocity, then FV = Q/A . IIRC your target FV should be ~0.3m/s. Then Q=FV*A= (0.3m/s)(0.8m^2)=0.24m^3/s = 864 m^3/hr.

Now this is for a target FV of 30cm/s. I have seen standards for up to 50 cm/s. I'm happy with 30cm/s and think that 50 cm/s would be a little drafty, especially if trying to use a bunsen burner or keep a vessel hot when the room air is cold. As far as what a minimum would be, well anything is better than nothing. Your 500-600m3/hr would be far better than nothing.

Actually, the blower I have is a Grainger model 7C651, that moves 475ft3/min at 0.750 inches H2O static pressure. See picture below.

This is equivalent to 807m3/hr at 19mm H2O static pressure.

I have no idea where the motor is on that blower in the second picture. What is the normal purpose for that blower -moving leaves?

[Edited on 28-4-2008 by Magpie]

[Edited on 28-4-2008 by Magpie]

[Edited on 28-4-2008 by Magpie]

[Edited on 29-4-2008 by Magpie]

blower 7C651.jpg - 9kB

Jor - 29-4-2008 at 01:47

Sorry bio2, saw that as an offence. I now realise it was't. Well it was very late and I was tired. Maybe that's the reason.

I have now realised that the blower in the first pic is unsuitable. It's only 180m3/h.

You just did calculation, and ended with 864m3/h. However you took H=1 and W=0,8. I always thought one had to take the H and W of the gap when the sash is fully open, wich is approx at 40cm-50cm up I think. Or am I wrong here? If I'm right, that means I would only need approx 430m3/h !

And could you answer the question in my previous post , if you like? About the motor and corrosive gasses, etc.

Magpie - 29-4-2008 at 08:17

Jor, from your first post:

Second: what should be the capacity of the suction? Say I'm making a hood of approx 0,8m W, 1m H, 0,6 D, would 500-600 m3/h be sufficient? What would be minimum, and what would be maximum (turbulence).

My calculations were based on the numbers you supplied. What else could I have used? But you are correct, the sash open area is what is needed for the calculation.

Here a word of caution: Your face velocity (FV) will vary with the size of the opening, that is, if you close the sash somewhat the FV will increase. This may be too drafty. That is why I used a constant opening design. My FV is constant no matter where I place the sash. When I lower the sash to close off the lower opening the upper opening exactly compensates by growing larger.

Visualize a large window with a sash covering only half the opening. No matter where you place the sash the opening area is the same.


If I'm right, that means I would only need approx 430m3/h !

Yes, that is correct for the shorter height.

[Edited on 29-4-2008 by Magpie]

On the question of motor exposure to fumes I think its best not to have the motor exposed to fumes. Corrosion would likely be a problem (HCl, etc) and explosion is also a risk. But since the fumes are highly diluted explosion risk may be very low. If you just spill a liter of ether, well then, all bets are off.

Explosion proof motors are available for fume hoods. I don't have one. But I have my motor located well outside the fume path, connected to my blower using a belt.

BTW, I make no claim to being an expert in this field.

[Edited on 29-4-2008 by Magpie]

Jor - 29-4-2008 at 09:03

Yes I understand what you mean. Your FV is constant, because you have a bypass.
Thank you very much for the help Magpie. I will ask again if anything jumps into mind.

The project should start in , say, 1 month.
When I'm done (if I ever succeed), I will provide you with a picture.

Magpie - 29-4-2008 at 10:44

Sounds good Jor. Just be sure your duct size is large enough. Mine is 8" (20cm) ID.

Up thread I posted a picture of my blower. I removed the motor and mounted it right behind the blower for a more solid mounting. It has worked great now for over 2 years with no maintenance. :D

[Edited on 29-4-2008 by Magpie]

Magpie - 29-4-2008 at 13:58

Jor says:

Your FV is constant, because you have a bypass.

Yes! I thought I should follow up on this important point. So shown below is a picture of my fume hood. You can see the sash in the middle whose position is adjustable up or down. The open area at the top is a bypass, if you will.

FYI, here's my dimensions:

Window opening without sash: 143.5cm H; 114cm W
Sash: 74.9cm H

This is just a standard 4' x 5' window from Home Depot except it only has one sash instead of two.

hood sash.jpg - 57kB

Jor - 29-4-2008 at 15:26

Im so jealous :P
Very beautiful fume hood.
I see you used baffles for airflow. Did you just remove pieces of tthe wall in the back?

I once saw your shed in a picture. Seems that you're pumping out your fumes only a few meter away from you. How are you sure they are not pulled in again, considering you are generating a low pressure in shed by removing air?

Magpie - 29-4-2008 at 16:05

Yes, I have two baffles. They are mounted on channel pieces so they are about 5cm from the back wall, and are removeable. You can see more detail in "Tour My Lab." With these baffles airflow is directed to three levels.

My total duct length is about 5 meters. This includes 1 long-radius 90 degree bend and 2 each 22.5 degree bends.

My lab is in my garage. The outlet of my duct is a rectangular louvered vent that was pre-existing in the peak of my garage attic. I sealed up my garage pretty good with a new insulated double-door, and use of caulking, etc. Yes, if air goes out air must come in. ;) So in the opposite peak of my garage attic I installed a second louvered vent. This pulls air out of the attic of my house. So it is entering the house attic through all the soffet vents (under the eaves), and the peak vent at the opposite end of the house some 13 meters away. That way I minimize any eating of my own fumes! Here's a picture of the vent I installed:

BTW, Jor what time is it in the Netherlands now? Shouldn't you be in bed? Or are you addicted to ScienceMadness? :P

[Edited on 29-4-2008 by Magpie]

inlet vent.jpg - 87kB

Jor - 30-4-2008 at 01:12

Very nice. I guess this all costed you loads of money ? ;-)

Yea it was pretty late. 1:26 AM. I am currently having holiday and I had a very nice convo on MSN :-P
Besides I never go to bed early.

Magpie - 30-4-2008 at 10:25


Very nice. I guess this all costed you loads of money ? ;-)

By poor struggling student standards - yes. By retiree lifelong dream hobby standards - not so bad.

The fumehood itself I had fabricated out of 1/16" (14 ga) aluminum to my drawings. That was a major expense for materials and labor. The rest of the labor I did myself.

[Edited on 1-5-2008 by Magpie]

RxnJackson - 21-5-2008 at 12:08

Sorry this is so late.........

Blowers for Diesel and Gas Marine engines work very well as sparkless blower systems. My setup contains a 4" In line hose blower that comes as a "Bilge Exhasuter" for boats. Marine sources commonly have problems with fumes and gases collecting in the bilge and becoming a hazard so sparkless motors are a must. I forget the exact displacement but it's around 167cu/ft/min or so. It also is very reasonable price, about $40 for the motor, and you need a DC power supply ($40-$100) depending upon what you buy. Just remember you need some wattage the little radioshack wall adapters for DC are no good, a solid powersupply will run more than the blower itself.

If this topic is still up for discussion and your intersted in some pics of the setup let me know and Ill post them.

DJF90 - 21-5-2008 at 13:08

167 cuft/min is about 120 cubic meters per hour... I'm sure there were previous comments suggesting 500-800 cubic meters per hour as a goot target. Although your bilge blower isn't shifting as much air as would be preferred, it is definately much better than nothing at all. Pictures would be good though :)

Jor - 22-5-2008 at 05:41

Can you show us your pictures.

I'm not gonna need 500 cubics, I will make a very small fumehood. 300-400 wil be enough.

Phosphor-ing - 22-5-2008 at 06:10

In another thread sometime last year bio2 told me that the bilge blower would probably not hold up to continous use. It is designed to only run a minute or 2 at a time.

Insane idea

kalacrow - 23-5-2008 at 15:41

This won't solve your problem.. but I just had to post this :)

I have made a couple of simple Tesla Pumps that pump air, using CDROMs as the vanes. I suddenly realized "Hey.. I bet if you ganged up a LOT of those to one, good sized motor, and used regular garden hose for the air exchange, you could make a simple setup from nothing but spare components plus a motor. And for a motor you could just use one from a dryer."


I know..its a LOT of work, but if you don't have cash.. and you DO have time, it's possible to make something like this. It would probably be pretty quiet as well, and certainly explosion proof. Here is a fellow who is working in Tesla Turbines, which are simply reverse Tesla blowers (believe it or not)

I bet you could get scrap lexan and plexi, glue a whole crew of these up, and have a lot of fun!

bio2 - 24-5-2008 at 10:38

A bilge fan or pump will have the DC (duty cycle) on the motor nameplate.

These are typically available.

Standard Blower
Intermittently rated motor 20 minutes on - 20 minutes off. This duty is sufficient for most standard blower applications. 105 Cubic feet - 3 Cubic metres per minute air flow.

Heavy Duty Blower
Continuously rated motor maximum 1 hour with 50% duty cycle = 1 hour on then 1/2 hour off for cool down. FLANGE MOUNT Suits 3" 75mm blower hose. 150 cubic feet - 4.23 cubic metres per minute capacity.

Extra Heavy Duty Flexmount
Continuously rated extra heavy duty motor with min. 5,000 hour motor life. Suitable for commercial or high use engine room air intake or extraction. Ideal for continuous operation applications where engine rooms with generator sets require the constant venting of warm air.