DC as a air filter - Novel or Bad idea?

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Forum topic by SnowFrog posted 10-11-2011 07:14 PM 808 views 0 times favorited 9 replies Add to Favorites Watch
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102 posts in 1969 days

10-11-2011 07:14 PM

Topic tags/keywords: question

Hi there;

I have reached the point that I am seriously contenplating upgrading my shopvac based DC to a proper DC. In the 1200CFM range. I am also planning on building a proper Air filter.

So in preparation I have been reading articles, reviews, blogs and following just about any links related to Dc and Air Filltration. One thing I expected to see but never came across is a design where a DC, say with a 1 micron canister would be used as an Air filteration when it is not being used on a tool.

What I am imagining is a design similar to a downdraft table but mounted on a ceiling or a wall.
The filtering capacity pretty well match what is most popular (1 micron).
At 1200 CFM it is about 3-4 time the capacity of many small shop air filter.
Some DC come with a 99 minute timer so the duty cycle of the DC is not an issue.

So you could blast gate between the tool and the air filtration when not using the tool

So except for noise, would this concept actually work?

My shop is less than 1200 Cubic Feet – so 1200 CFM will turn my shop inside-out every minute.

-- One can dream, about a passion not yet fully fulfilled!

9 replies so far

View Bertha's profile


12989 posts in 2116 days

#1 posted 10-11-2011 07:40 PM

I’m not an engineer by any stretch, but I know a few here, lol. It seems like overkill to use a overgrown blower fan and a filter for air handling. I have a 2hp DC and it howls like a banshee in my small shop. I also have an “ambient” JET filter that I can listen to Bach over. My DC rips papers out of my hands; I forget to turn my air filter off. It just strikes me as a very inefficient use of a machine, in general. I’m not an engineer either, so I could be totally wrong. I’m interested in the answers to this question. Good luck!

-- My dad and I built a 65 chev pick up.I killed trannys in that thing for some reason-Hog

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12989 posts in 2116 days

#2 posted 10-11-2011 07:42 PM

I just re-read my post. I should mention that I’m not an engineer, lol. :)

-- My dad and I built a 65 chev pick up.I killed trannys in that thing for some reason-Hog

View hairy's profile


2377 posts in 2955 days

#3 posted 10-11-2011 07:44 PM

You would do better by attaching a furnace filter to a fan.

-- stay thirsty my friends...

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1730 posts in 2491 days

#4 posted 10-11-2011 08:11 PM

Not really, Snowfrog.

A cfm flow rating is the product of the velocity (ft/min) times the cross sectional area of the inlet (sq ft) If your impeller housing inlet is 6” (~0.2 sq ft) in diameter, the velocity is ~6000 ft/min (~60 mph) – at the inlet.

If you had no ducting, and moved away from the inlet, the cross sectional area suddenly jumps depending on the dimensions of the space. By the time you get across the room, the velocity is effectively zero and dust stays where it is.

-- Adversity doesn't build reveals it.

View dbray45's profile


3147 posts in 2199 days

#5 posted 10-11-2011 08:33 PM

Ask your HVAC guy/gal if they have a small old air handler that they have replaced and are scrapping. With this you can easily add the filters to make a thorough air cleaner. It will still cost you $75 – 100 in filters and to wire and possibly duct it for a few more $ but it will be very efficient and a lot quieter (1/3 – 1/2hp motor). It will be dusty and dirty to clean – including the squirrel cage but it will be worth it.

And a lot of them have 2 or 3 speed motors

-- David in Damascus, MD

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5688 posts in 2731 days

#6 posted 10-11-2011 09:33 PM

you could made a box with a 20×20 filter hooked up to your dust collector but there are better ways

I use a jet air cleaner in addition to 3 air cleaner units i made from squirrell cage A/C motors. The work really good and I purchased permanent anc washable electrostatic filters to use in all of them. There is also another good feature in that they are on wheels and I cn move them where i need them.
I have a downdraft table hooked up to my dust collection system as well. With all the grinding and sanding i do every bit is needed to clean the air.

View SnowFrog's profile


102 posts in 1969 days

#7 posted 10-12-2011 07:30 PM

Ok – I think I get the idea that it is not advisable.

SawKerf – A question, just for my understanding. How does what you are saying differ than a regular air filtration unit? Say the unit is 400CFM. The air velocity as I move across the room will still drop to zero.

So if this hypotetical “downdraft” inlet to the DC is the same size as say a 16×16 filter, the fact that the DC pull 1200cfm is still 3 times the cubic Feet being pulled through to the filter, No?

-- One can dream, about a passion not yet fully fulfilled!

View crank49's profile


3979 posts in 2394 days

#8 posted 10-12-2011 07:58 PM

A little note of interest, related to flow.

A 400 CFM flow would be quite small, but in filters they are looking for air changes per hour. So look at the cubic feet of volume of the whole shop, say a typical garage, at 24’ x 28’ x 9’ hi = ~6050 cubic feet. You want to filter that volume about 6 times per hour. 6050 X 6 = 36,300 cubic feet per hour. Now divide the cubic feet per hour by 60 to get CFM required. 36,300 CFH / 60 = 605 CFM.

So, yes you could filter the air in a shop with a DC, but while running a 2HP motor and listening to a 90db roar. A 600 CFM filter might have a 1/6 HP motor and only hum about 50-60db. That 2HP motor is costing about $0.15 to $0.20 per hour to run for the electricity.

And, you really need to filter the air down to .3 microns or better, not 1 micron.
The fine dust is far more harmfull than the coarse stuff.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

View Sawkerf's profile


1730 posts in 2491 days

#9 posted 10-12-2011 09:22 PM

Snow frog -

You’re right about the airflow dropping off as you move away from the filtration unit. That’s why you usually see multiple units spaced around a shop. Each one handles a part of the space.

-- Adversity doesn't build reveals it.

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