Air Filtration Direction

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Forum topic by Tommy Evans posted 11-03-2013 04:03 PM 1673 views 0 times favorited 11 replies Add to Favorites Watch
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Tommy Evans

146 posts in 2172 days

11-03-2013 04:03 PM

Topic tags/keywords: question sanding

I recently bought this mop sander for $75. After using it for 5 minutes I KNEW I needed an air filter.
I bought a $19.97 box fan and mounted it on 1/4 hardboard. 12×20 side and top filters with a 20×20 rear filter. Those are “better” merv, don’t remember the exact number and I know they could be better, but this was a test.

On the front I put a cheap filter just to keep out the bigger stuff from the better filters. I’ve been pleasently surprised with it’s operation and have had to vacuum the front filter 4 or 5 times.

But then I started looking at the past threads here about filtration and notice that ALL the diy units have the filters on the suction side of the fan! Mine are on the “outgoing air” side, except for the cheap pre-filter.

I’ve never seen a store bought unit, so don’t know how they are made.
I plan on mounting this contraption on a shelf on the wall up by the ceiling next to the HF dust collector (it still has the 5 micron bag) But will move it to behind the mop sander when that is used. It is light enough for that.

Is there some rule that the filters should be before the fan because of some kind of hvac or suction or vacuum or air flow physics? Should the outgoing air flow be unimpeded?
You know what I mean?

peace, T

11 replies so far

View AandCstyle's profile


3052 posts in 2255 days

#1 posted 11-04-2013 12:56 AM

Tommy, I am not any kind of an expert about fans, but when I made an exhaust fan I put the filter in front of the fan. My rationale was that I wanted to keep gunk off the blades of the fan because the added weight might cause the motor to burn out sooner. I don’t know if that is a significant concern for a $20 fan, but that is why I did it the way I did. FWIW

-- Art

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Tommy Evans

146 posts in 2172 days

#2 posted 11-04-2013 01:28 AM

Thank you, Art. I’ve read of concerns of not having TEFC motors for the fan.

Possibly one area of my concern to be answered is – Can a fan blade push air through the resistance of the filters equally as it can pull air through the same resistance? or vice versa.

Google is not helping, so far. ;)

peace, T

View rockindavan's profile


299 posts in 2633 days

#3 posted 11-04-2013 02:51 AM

Theoretically yes, the fan should be able to spin faster if there is less restriction if fan is pre filter. On the other hand there is going to be more back pressure if the filters are post fan, so it should end up working out to about the same cfm either way. I would say keep the gunk off the fan and put it after the filters. On production air filters, the squirrel cage fan is after filters.

View Grandpa's profile


3259 posts in 2673 days

#4 posted 11-04-2013 03:04 AM

squirrel cage fans are the best. That is why they are used on furnaces and swamp coolers. I have had attic fans or whole house fans shut down because of resistance in the attic exhaust. I am always concerned about fires with fans as you are using. I would never leave it alone and running. Stay with it and you will probably be okay. the squirrel cage style can “slip” in the air and not overheat.

View Tommy Evans's profile

Tommy Evans

146 posts in 2172 days

#5 posted 11-04-2013 03:48 AM

Thank you rockindavan and Grandpa.
I’ve just read many web pages… from medical marijauna grow rooms to electronic clean fetishists. I’ll give this one link I had found….

I would have problems with not prefiltering before a merv 13 filter.

BUT finally answer came from Bill Pentz… “Updates—This system needed some changes. I flipped the blower upside down so instead of blowing into the filter, it sucks through the filter and blows out a strong stream of air. This let me put a swivel duct on top of the fan to direct that fast moving air stream. When aimed properly this airflow stirred all the air in my home. ”

I had noticed that my setup had very slow discharge air, almost un-noticeable. I guess we want to move that air around in the shop to get it moving into the filters.

So, at this point, I think I will go with at least 2 prefilters on the intake side and then try the final high merv filter on discharge. If it kills the air flow too much I’ll put it on the intake side.

Somewhere in my readings someone had mentioned having the intake filters arranged in a cube, (like my discharge filters) so there is more surface area to allow less resistance. I’ll have to think about that.

Thank you for letting me think out loud to y’all, it helps a lot.
peace, T

View crank49's profile


4030 posts in 2968 days

#6 posted 11-04-2013 02:05 PM

I have a fan I modified to be a filter and I have one I bought that was made at the factory that way.. Put all the filters before the fan.

Before the fan will help the blades and motor stay cleaner. There is no advantage at all to putting a filter after the fan.
The fan will know no difference whether it is pulling or pushing.

Be careful how many filter layers you put in the airstream, however. These box fans have very limited pressure capacity. Too many layers of filtration will result in almost no CFM of air flow. No flow, no benefit.

View Tommy Evans's profile

Tommy Evans

146 posts in 2172 days

#7 posted 11-04-2013 02:19 PM

“These box fans have very limited pressure capacity. Too many layers of filtration will result in almost no CFM of air flow. No flow, no benefit.”


Yes, that is the crux of the matter – no flow, no benefit. Keeping the box fan light will help in being able to place it at the point of dust creation, mainly behind the mop sander and “store it” (running or not; as needed) on the wall shelf.
thank you, Michael.

peace, T

View firefighterontheside's profile (online now)


18176 posts in 1854 days

#8 posted 11-04-2013 02:36 PM

I think that with the filters on the exhaust side you will lose some of your cfm instead of it all going thru the filters. The square face of the box fan will have air back up thru the corners when there is resistance from the filter. Imagine if you put a solid piece of plywood over the exhaust side. All of the airflow would come back out the intake side. Does that make sense.

-- Bill M. "People change, walnut doesn't" by Gene.

View bigblockyeti's profile


5120 posts in 1718 days

#9 posted 11-04-2013 02:45 PM

Given your limited flow, that could be a problem with trying to get the dust out of the entire shop, but if you’re keeping the fan very close to the sander you can probably make it work. Since you’re using a $20 box fan, I’ll assume you’re trying to do this on a budget as most more effective solutions will almost certainly end up costing more one way or another. 1st: Keep the filters on the intake side, it will better protect the fan from getting dust in the motor and switch, which could cause the fan to fail or worse yet pose a fire hazard. While most fans, axial or centrifugal can blow with greater pressure than they can suck, this won’t be a big difference with the performance of most inexpensive box fans. 2nd: Most of the commercial units will have a prefilter, medium filter requiring period cleaning and fine filter requiring even less maintenance. It would be helpful to model your air cleaner in this fashion; Start with a cheap furnace filter as a prefilter, then a better furnace filter behind that, and finally a good filter rated to remove much of the finer dust. These types are typically reusable, though more expensive initially. 3rd: Since you’re using this exclusively for capturing the dust from one machine, think about making a simple or collapsible dust hood for the sanding area leading into the cleaner. 4th: Just for safe measure, wear a dust mask to keep from inhaling any of the fine dust not contained by your homemade filter.

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Tommy Evans

146 posts in 2172 days

#10 posted 11-04-2013 03:36 PM

Thanks Bill & yeti.
I understand one error of my ways- I was first disappointed that the air filtration did not pull in more saw dust. That’s the wrong way to think of it. The saw dust sitting on the floor, table, etc is not the danger, it’s the little wee bits one can’t see that pose more of a risk and the reason for this type of unit. Yes, a dust mask is kept right at that machine!

Now a question about my HF dust collector, but that will be a seperate thread.


View SuperCubber's profile


1026 posts in 2282 days

#11 posted 11-20-2013 08:48 PM

+1 to firefighterontheside

-- Joe | Spartanburg, SC | "To give anything less than your best is to sacrafice the gift." - Steve Prefontaine

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