Mateo_swe
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Registered: 24-8-2019
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Fan for DIY fume/extraction hood
I have built a fume extraction hood and are just finishing the last painting on it.
For fan i have used a smaller explosion proof fan that was made for boat engine room use.
But i am worrying a bit that this fan will not have enough airflow because its a quite small fan.
I have thought about this and i could add another fan of the same model.
But having dual fans can cause problems if they arent exactly preforming the same.
I have some other fans i could use and they look similar to the attached pictures.
The fan with the metal casing and the one with black plastic casing are very similar and have a brushless motor mounted at the axle midpoint.
The fan that are of the other "long" type has a brushless motor mounted on the side but air could reach the motor even if its not in the airstream.
Its not sealed from the airstream.
Do you think i dare use one of these fans if the one i now have mounted turns out to be too small?
They arent explosion proof rated but have brushless motors that shouldn't produce sparks.
Maybe i could use the long type fan with the motor mounted on the side and then try isolate/seal the motor so any airstream with vapours cant reach
the motor.
What do you think?
Sad that explosion proof fans are so expensive in these sizes.
  
[Edited on 2021-8-20 by Mateo_swe]
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macckone
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Are you building an fume/extraction hood or flow hood, they are different things.
The first removes vapors via pulling the air.
The second provides a stream of particle free air via pushing the air.
Flow hoods are usually used for biological experiments where a full clean room is not feasible.
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Mateo_swe
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Its an fume/extraction hood that has a fan on top sucking the air + fumes out of the hood and to the outside.
I see if i can change the wording.
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macckone
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You need about 100 fpm of air velocity with whatever back pressure your outlet generates.
for an 18" x 4ft sash opening that would be 600 cfm at whatever back pressure.
assume 2 in of h2o for back pressure just as a guess assuming there aren't a lot of turns and the run is pretty straight.
So you need a fan that will pull 600 cfm and 2 in of h2o.
box fans can handle the volume but not the pressure.
if the fan opening is too small it won't work well but it should not be too different from the ducting.
air flow engineering is fairly complicated beyond rough guesses as we don't know what your setup is.
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Fyndium
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I use duct fans that are used in cannabis growing facilities. They provide high pressure differential with high flow rates and are readily connected
with normal air ducting equipment. Unless you generate huge amounts of flammable gases, there is no practical risk of ignition because the ratio of
air to fuel is way high to cross LEL. Primarily I control toxic and malodorous fumes with them, though.
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Mateo_swe
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Hmm, i have to measure the with and height of the front when fully open.
I cant remember if the with is 100cm or 120cm, i check.
My diameter of the air pipes are pretty thin, just 100mm or maybe 120mm.
Its quite easy to change to wider pipes, like 200mm as the fumehood is close to the outlet going to the outside.
Im pretty sure the current fan isnt near 600 cfm.
I think the fan no3 (the one to the right) in the pictures can deliver the needed air-pull.
Maybe i make changes to use that one instead and also get some wider duct pipes to the outside.
I will use diethyl ether in the fumehood (not often, but i will do on rare occasions) and i dont want have any accidents, so i will isolate the
brushless motor from the airstream.
Should be pretty easy as its mounted on the side.
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