# Home made air scrubber CFM question



## kyscroller (Mar 20, 2013)

I've happened on a large power roof vent assembly with a fan motor combo rated ar 2,000 cfm. (it was free)Reading most of the prior posts on home made scrubbers they are rated at a 1,000 or less. Would it hurt to push this much air for a shop thats only about 300 sq ft.? My wife says something has to be done. She uses the other side of our garage for a stained glass studio (the other 300 sq ft.). Also does the size of the box matter for this much air movement. I was going to try and keep it to 14"x14". We'd appreciate any help and suggestions. Thanks.


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## crank49 (Apr 7, 2010)

Are you talking about a shop *air filter*?

I have to ask because to an environmental engineer a scrubber is a device that uses liquid to clean the air stream.

For an air filter you want the velocity of air passing through the filter media to be a low as possible. 200 to 300 fpm (feet per minute) is a good target.

You calculate this by dividing the CFM of the fan by the filter area in square feet. So a 12" x 12" filter = 1 sq.ft. and if you pulled 2000 cfm through a 1 sq.ft. filter you would have 2000 FPM. About ten times more than you want. For a unit to handle 2000 CFM you need at least three filters of 20" x 25".

2000 CFM is a lot of air movement. Any thing more than 25 FPM of air moving by your face is going to be uncomfortable. You figure that number by the flow traveling through the filter, assuming it is passing through the shop from end to end in a circular motion divided by the cross section area of the shop. I'm going to guess your shop is 24×25 and has a 10 ft ceiling. Half of the width, 12 ft, X the height, 10 ft, is 120 sq. ft.
Then 2000 cfm / (12×10) = 16.6 FPM so this would not be too uncomfortable.

Another number to look at is air changes per hour. 600sq.ft x 10ft height = 6000 cubic feet of air. 2000 CFM is going to filter all the air in that shop once every 3 minutes. Or 20 times an hour. You should have a very clean shop.


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## Loren (May 30, 2008)

One thing you could do is exhaust it to the outside and 
when you turn it on it will just suck all the dusty air out 
of the shop pretty quick.

In terms of recirculating, it would probably blow holes in
some filters.

I'll bet it's pretty loud. I have a dust collector in that CFM
class and it is loud.


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## mummykicks (Jun 19, 2013)

Look and see if you can buy different pulley's for it (I assume it's a belt between the motor and blower). 'Gear' it down to a more reasonable flow rate.


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## kyscroller (Mar 20, 2013)

My shop is 14' x 23' +/- with an 8' ceiling. (it's a lower level garage). So if I build a box to handle 3 - 20"x25" filters inboard and 1 out would that be fine? My wife says the cleaner the better so more air moving thru won't hurt anything. I don't want to vent outside because the heat will go out with the dust.


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## crank49 (Apr 7, 2010)

Here is one I helped a fellow LJ with recently.
http://lumberjocks.com/projects/90829


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## bigblockyeti (Sep 9, 2013)

Sounds like you'd be pushing it with only three filters and without good support behind them, that kind of CFM usually has decent pressure too, it would collapse and inhale at least one of the filters. You'd have a mess on your hands that would be difficult to describe (personal experience). I now have a JET unit and it has three filters with ALOT of pleats to make up substantial surface area. The three stage filtration seems to be the industry standard as the outer filter needs to be blown out every couple weeks, the second stage, every 3-4 months and the final stage, yearly at most. You would be asking quite a bit of just one filter stage to handle all of the filtration down to what would be considered a safe level. Not to discourage you, it can be done, but with the power of the fan you have, you'd be spending a lot on filters that would handle that level of air flow.


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## MrRon (Jul 9, 2009)

You will probably need a filter area of around 6-8 SF located on the suction side of the fan and some ductwork to project the "clean" air away from the suction end of the fan. If they are too close together, the exhausted air will take a shortcut back to the suction end. Keep the velocity down so you don't have papers blowing around.


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## crank49 (Apr 7, 2010)

It's all in the calculations. Three 20×25 filters equal 10.4 sq.ft. 
Then 2000 CFM divided across 10.4 sq.ft. is equal to 181 FPM flow. 
That will not collapse any thing.
A home HVAC using a 20×25 filter typically will be pulling about 800 CFM through one of those filters. that calculates out to about 230 FPM, more than the three filter 2000 CFM system I recommended.

BUT, lets back up here. I'm not talking about stacking three filters on top of each other.
I'm saying three filters side by side. You need the surface area to slow the flow down.
Now, if you want to double up, or triple the thickness that's good too, but you need the 10 sq.ft. of area.


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## kyscroller (Mar 20, 2013)

Looks like I need to rethink this and come up with a way to put a speed control on the motor because it's a direct drive. Thanks everyone for the comments.


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## Finn (May 26, 2010)

Many direct drive fan motors are multi-speed. Open up the motor and see if there is a way to select a lower speed.


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## crank49 (Apr 7, 2010)

What kind of fan is it?
Is it a centrifugal, with an impeller in a scroll housing?
Or a squirrel cage?

If the fan has an inlet and an outlet all you have to do to reduce the flow is choke it down.
The only thing you have to watch out for is if the motor is in the air stream make sure it gets enough flow for cooling.

Choking a fan down DOES NOT make the motor work harder. Just the opposite.
The amps will be reduced with the air flow restricted.


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## kyscroller (Mar 20, 2013)

The fan setup is like you find in a box fan. This was one of those roof fans with a bowl shape dome covering it. From tip to tip of the blades is only about 12". How small could it be choked down and still be effective? 
Are we choking the inflow of air or the out flow to hold more air back?


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## crank49 (Apr 7, 2010)

Then that is a propeller type fan, if I understand your description.
That won't develop much pressure.
But, on the other hand you may not need much. I think for that fan I would build a tower about 6 ft tall and about 16" square. Put four 16×24 filters around it. Mount the fan on top.
This will pull dusty air from the lower level of the shop and discharge filtered air across the ceiling. 
Should help reduce the turbulence.
You could use real good filters and maybe over wrap the filters with the spun fiber type filter as a pre filter stage.
Don't worry about choking it down. But, it it's too windy in your shop, insert a blast gate across the tower, below the fan inlet. That can be your throttle.
Put the whole thing on casters and you can roll it out of your way.


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