Using filters on box fans

[Phil Thien]

I had someone write recently and ask "how well do those filters strapped to box fans actually work?"

I thought it deserved some more research, and I've been trying different filters, etc., to figure out how well a box fan w/ filter can function as an air cleaner.

Because I didn't intend this to be permanent, I wanted a quick way to hold the filters to the fan.

I was able to weave a piece of clothesline through the grill in such a way that it retains the filter in three corners. The fourth corner can be tied in a bow, or you can use on of the cord stops I found on an old bag that was being discarded.

It actually works very nicely, and took all of two or three minutes.

[Jack Wilson50]

Paul:

Any feedback on which type of filter works best?

Thanks for the research.

Jack

[Larry Prem]

Most air cleaners are two stage. The other filter is 5 micron, the inner 1 micron.
With just one filter you are either sacrificing efficiency, or the filter will clog faster.

A filter on a boxfan is not on par with an air cleaner. It's certainly better than nothing, but I would not spend any time optimizing it.

[Van Huskey]

Here is some info (though somewhat hard to interpret) about using box fan filters.

http://www.americanallergysupply.com/box-fan-air-cleaner.htm

[Phil Thien]

Most air cleaners are two stage. The other filter is 5 micron, the inner 1 micron.
With just one filter you are either sacrificing efficiency, or the filter will clog faster.

A filter on a boxfan is not on par with an air cleaner. It's certainly better than nothing, but I would not spend any time optimizing it.

Larry, the question wasn't which one works better.

The question was, "how well can a box fan work?"

[Sid Matheny]

"how well do those filters strapped to box fans actually work?"



From the looks of the filter on my fan they work well. Not like an air cleaner of corse but they do help. Anything that filters dust out of the shop air is a help. Several work better than one but just one in the right place can make a big difference.

Sid

[Van Huskey]

This does pose the question how well a box fan based filter would work if equipped with a proper pre-filter system and a pocket filter for the fines. They are about 2000 CFM (with no static pressure). I have considered building one or two into the walls between my shop and my "mechanical" room. There ain't any magic to air filtration just movin' air through filters.

[Phil Thien]

This does pose the question how well a box fan based filter would work if equipped with a proper pre-filter system and a pocket filter for the fines. They are about 2000 CFM (with no static pressure). I have considered building one or two into the walls between my shop and my "mechanical" room. There ain't any magic to air filtration just movin' air through filters.

An axial fan won't likely pull much air through two filters.

I noticed a huge hit to CFM just placing a filter in front of the box fan.

The 20" Lasko fan I'm using claims to move 2500-CFM free-air, on high speed. I'm guessing it moves about 500 to 750-CFM early-on with a "higher-end" Filtrete filter (on that same high speed). That is pure speculation, mind you. Measuring it would be a challenge.

But a couple of important points: The filter I'm using is 20" square. About 40% larger than your typical Jet air cleaner (12 x 24 or 288 sq. inches). So it may actually be doing a pretty decent job.

Only time will tell how the fan will handle a filter as it gets loaded. That is what I'm working on now, I'm pre-loading filters.

If I was a betting man, I'd say two box fans with quality filters may five an air cleaner a run for the money. But my mind may change once that filter starts loading.

[ian maybury]

The issue Van/Phil could well be in the last point 'just movin air through filters'. Not saying it's definitely an issue as i've no experience and have never investigated them, but the so far as i can remember axial flow fans typically only generate a few inched if that of static pressure. Which unless the filter was very low pressure drop over its life might possibly lead to flow problems.

ian

[Van Huskey]

My thought would be to use a pair of fans with a common plenum and one set of LARGE filters from Filters America for tons of surface area. The idea being I am doing it for fun and if it doesn't meet expectations all I have to do is take the box fans out and build in a proper fan.

[Ole Anderson]

For a while I used a 1/12 hp 24" box fan with a cheap Merv 4 pre-filter in front of a filtrete filter. It worked, but the air flow is a small fraction of what the fan put out without the filters and a small fraction of what an ambient air cleaner with a 1/3 hp centrifugal fan can generate. And having to run it on HIGH all of the time just to have any airflow at all was an annoyance regarding noise. I have since moved on to a DustRight ambient air cleaner and a full dust collection system powered by a 2 hp Dust Gorilla.

[Alan Schaffter]

After looking at the dust in my shop during and after completing recent project, I have decided, that my dust collection system with 3 hp motor/14" impeller, 3D Pentz cyclone, efficient ducting, dual parallel filters/direct vent, fancy auto gates, central shop vac for routers/ROS, ambient air cleaner, etc., etc. only go so far, maybe not far enough. I still generate dust in various ways that is not collected. The solutions- a good mask or live with it. I'm living with it.

[Mike Heidrick]

I use this method for spraying out of rattle cans. I use el cheapo the cheapest filters I can find. Simple tape on. Works awesome.

[michael veach]

Don't leave the fan running unattended. The motor can become very hot due to the reduced air flow.

[Greg Peterson]

I still generate dust in various ways that is not collected. The solutions- a good mask or live with it. I'm living with it.

IME, ample DC capacity is only half the battle. The real struggle, in my opinion, is getting the DC as close as possible to the cutter creating the dust. I believe the euro machines do a good job of addressing this issue. What passes for DC collection on most of the machines I have seen, it appears most manufacturers give DC little thought.

I got rid of my DeWalt RAS. I did not get anywhere near what I put into it, but I simply could not tolerate the mess it made. And I tried numerous DC techniques and shop made hoods.

[Alan Lightstone]

I built a home-made knock-down spray booth with a 20" Lasko Fan and an air filter. While it did work, air flow was dramatically decreased. I think more like 300cfm, not 2500.

As far as how it works for dust, I actually looked at my Dylos air quality meter and could see the particle count going down with the fan on. But it was a fraction of how effective my Jet 1000 air filter worked.

I tend to keep it going in the background. Doesn't hurt, probably helps a little.

That being said, I'm now working on replacing it with a 3500 cfm 20" impeller 1/3HP explosion proof fan. I'm sure it handles the SP of the filters far better. I'm planning on three 20" filters in parallel. Hopefully in a few weeks, I'll see how it does.

[Larry Edgerton]

http://www.sawmillcreek.org/images/attach/jpg.gif

I have posted up this picture before but it is pertinent to this discussion. As far as I am concerned the factory built air cleaners are a joke. They just do not move enough air, and they do not have enough filter area. The reality of a woodshop is that a lot of dust is created and so you need more area to catch the dust.

I did a lot of reading on air filtration when I was off road racing. At $40K a pop protecting a motor is important, and the conditions are as bad as it gets. The one key to filtration efficiency is to slow the air down going through the filter. Ideally you would have zero pressure difference from one side of the filter to the other. This of course can not happen but it is the goal. A filter with too much of a pressure difference is going to do one of two things. It is going either let too many contaminants through or it is going to restrict air flow, or both in some cases. On a motor that results in excess wear or loss of performance. A filter system for a shop has the same constraints even though it has a different goal.

What I did with this filter design is transfer this same principal to cleaning the air in my shop. In an effort to slow down the air through the filter and place as little load as possible on the fan I used eight filters. To pull the air from all directions I arranged them in an octagon shape. The results were satisfactory. I painted a work van directly under this fan, and there was no paint on the floor. The filters turned the color of the van almost immediately. As a bonus the filters lasted quite a long time because of the surface area, but eventually you could hear the fan pulling more amps and it was time to change them. I made it come on with my main light switch so it was always on. Two of these would clear the air in a 140' x 56' building in very short order.

I will have a similar arrangement in my new shop but with ductwork to help the filtered air reach the corners for a more complete flow pattern. I knew my old shop was temporary so I never bothered with the ductwork.

I have a commercial portable dust filter that is nothing more than a heavy duty box fan with a roll of filter paper that pulls up from the bottom, and it works somewhat for spot sanding but does not move enough air.

Another thing is that I have a theory that the fines want to ride the heated air up, so pulling the air to a point above you is a natural part of the air flow, the work done providing the heat.

Phil. Heres a thought. Take 5 filters the same size that you have and duct tape them into a box shape, then tape that to you fan and see how that works. Your filters will have less of a pressure difference and will be better able to catch the fines that you are really after. You will increase the amount of air in the shop that actually gets filtered and put less load on the fan.

Larry

[Michael W. Clark]

I think there was a Wood Magazine or Woodsmith comparison of air cleaners and they put a box fan in for fun. I think it got the Top Value award. Airflow may drop off as it gets dirty more quickly. Looking forward to your testing.

Mike

[Alan Schaffter]

http://www.sawmillcreek.org/images/attach/jpg.gif

" . . . but eventually you could hear the fan pulling more amps and it was time to change them.

Larry

I think this gets to the heart of the problem. There are only so many air passages in filter media and depending on size and the size of the dust particles, either the dust will pass through or it won't. If a dust particle is stopped then that passage is now blocked- permanently or until the filter is cleaned and maybe even after that. So the question becomes, how quickly will the filter clog and how often should it be changed (or cleaned). Pressure drop gives the easiest indication of the filter status, but how does that relate to filter area? Filters are such a lose - lose proposition.

[Howard Acheson]

>>>> you could hear the fan pulling more amps and it was time to change them.

I know it's counter-intuitive, but a fan motor will draw less amps as the airflow becomes more restricted. Stopping the air flow completely will result is the least amperage draw. It takes amps to move air. If the air is not moving, little amperage is used.

The other point I would make is that it is far better to remove the dust at the point of generalion than it is to use a whole room air cleaner. Placement of the fan and direction of air flow versus operator location is a key element.

[Larry Edgerton]

I would agree that collecting at the source is the best option, but real world conditions do not allow 100% collection at the source. So what are you going to do about the rest of the dust? Me, I have an air filter to augment my normal dust collection. I have a commercial shop that specializes in one off items, and setting up the perfect dust collection for every setup is a sure way for me to go out of business.

I will re-phrase my amp comment.

I can tell when the filters are getting plugged by the way the motor sounds, and that was actually my point.

Larry

[Greg Peterson]

The other point I would make is that it is far better to remove the dust at the point of generalion than it is to use a whole room air cleaner. Placement of the fan and direction of air flow versus operator location is a key element.

+1. Capturing the fines is critical and the most difficult task. Once the fines get into the wild, capturing them requires an entirely different system. I use a box fan on the floor in conjunction with my air scrubber. The intended effect is put particles into, and keep them in suspension, so that the air scrubber can collect them. My Dylos unit confirms the effectiveness of this method in my small, L shaped shop. YMMV.

[mreza Salav]

I built an air-cleaner using a used squirrel cage fan (from and old furnace) that I got for $20 plus a 1/4HP motor.

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It has two layers of filters on two sides: the outer (that is not shown in the pictures and is attached with rubber bands)
is cheaper that I change more frequently and the inner is better quality (more expensive).
It works extremely well. I have it running most of the time until about an hour or so after I leave the shop. I have checked the air quality
and with it running (even when I'm working with machines or doing sanding) the air quality in the shop is better than in the house!
Of course you'd see a spike in the number of particles doing certain operations but they drop pretty quickly when this is running.

[Phil Thien]

The other point I would make is that it is far better to remove the dust at the point of generalion than it is to use a whole room air cleaner. Placement of the fan and direction of air flow versus operator location is a key element.

Agreed that capturing them at the source is best. But for many (most?) operations, it just isn't possible to catch 100% at the source.

[Ole Anderson]

What I did with this filter design is transfer this same principal to cleaning the air in my shop. In an effort to slow down the air through the filter and place as little load as possible on the fan I used eight filters. To pull the air from all directions I arranged them in an octagon shape. The results were satisfactory. I painted a work van directly under this fan, and there was no paint on the floor. The filters turned the color of the van almost immediately. As a bonus the filters lasted quite a long time because of the surface area, but eventually you could hear the fan pulling more amps and it was time to change them. I made it come on with my main light switch so it was always on. Two of these would clear the air in a 140' x 56' building in very short order.

Phil. Heres a thought. Take 5 filters the same size that you have and duct tape them into a box shape, then tape that to you fan and see how that works. Your filters will have less of a pressure difference and will be better able to catch the fines that you are really after. You will increase the amount of air in the shop that actually gets filtered and put less load on the fan.

Larry

I believe Larry is on to something here. Propeller fans generate a lot of cfm at a very low pressure differential, so increasing the filter area as he did, will allow the fan to work near it's maximum cfm, which is a lot more air than a typical ambient air filter will remove. So if you have the room, this looks like a good solution. Start with a coarse, low loss filter followed up by a higher number MERV filter, or a Filtrete to get quality filtration, that is doubling up the filters like they do in an ambient air cleaner, a coarse furnace style pre-filter followed by a one micron bag.

[Phil Thien]

Phil. Heres a thought. Take 5 filters the same size that you have and duct tape them into a box shape, then tape that to you fan and see how that works. Your filters will have less of a pressure difference and will be better able to catch the fines that you are really after. You will increase the amount of air in the shop that actually gets filtered and put less load on the fan.

Larry

Larry, good thought. Right now I want to see just what a single filter on a single fan can accomplish. But connecting five filters to each other would be an interesting experiment.

What kind of fan do you have in that unit? Is it a blower (like from a furnace), or a conventional propeller fan?

[Andrew Joiner]

http://www.sawmillcreek.org/images/attach/jpg.gif


Larry

Finally a look into Larry's shop. I'd love to see more.

[Andrew Joiner]

I have a commercial shop that specializes in one off items, and setting up the perfect dust collection for every setup is a sure way for me to go out of business.



I totally agree. Even Bill Pentz says with the best dust collection and filters he opens the windows and wears a respirator. You just can't get all the super fine dust.

[Alan Schaffter]

Move that dust making machine up to an open garage door, then roll this up just inside and blow it all outside: :D

http://www.jmfx.net/files/25hp_wind_machine3.jpg?0

[Michael W. Clark]

I think anytime you are dealing with fine dusts, the lower the Air/Cloth ratio (CFM/ft2), the better. Of course, this comes at a price of real estate and filters required. I know users report good results from the overhead purchased filter units, but the air they claim to move and the small filter areas, does not seem like a sustainable solution. Personally, I like mreza's approach but I would want to maximize the filter area as much as practical. You try to get all the dust at the hood or tool, but for some operations, its just not practical.

Phil, I have a box fan with filters and I use it in a small shop (14x22). I have it mounted to the ceiling about mid-way of the shop and it can clear the air in a few minutes. I don't know the PM count reduction, but I can "see" a difference when breaking down sheet goods with a circ saw.

The more filter area, the less resistance to flow, and the fan can move more air. This also helps keep the fan motor cool by allowing more air to flow over it. The fan motor will draw more amps at a higher flow, but it will sound like its working harder at a lower flow.

Larry, you got me thinking. If you take your arrangement and duct the outlet to the floor, this would be great for shops with higher ceilings during the heating season. Basically, pulling air in and recovering the heat in the rafters, cleaning the air, and sending the warm air back to the floor where it is most appreciated.

Dust definitely rises with hot air. I've done dust collection system audits and balances in foundries and aluminum recyclers. You can see the dust difference from the ground to the rafters. It forms a cloud up there at times. And guess what, all the ductwork is in the rafters, sitting there high above the floor at about 130F+ ambient. Great times...

Mike

[Chris Parks]

What passes for DC collection on most of the machines I have seen, it appears most manufacturers give DC little thought.

There speaks a man with experience.

[Howard Acheson]

The following is something I wrote a number of years ago for a woodworking club newsletter. Some of the info may be of interest to those participating in this thread.

Here are some general rules based on Nagyszalanczy's "Workshop Dust Control" and some of my own experiences and thoughts.

The two most important criteria for an air cleaner are the CFM and the filters. You want a CFM factor that will clean the size of your shop and a filter that removes the particle size that you are concerned about.

To determine the size or required air flow, use this formula: Volume of your shop (Length x width x Height) times Number of air changes per hour (typically 6 - 8) divided by 60. This will give you an answer in Cubic Feet per Minute which is how air cleaners are measured. MOST AIR CLEANER MANUFACTURERS RATE THE CFM OF THE FAN ONLY, but there are losses due to the filters. If you are building your own or if the air cleaner you are purchasing rates only the fan, figure you will lose about 25 - 40% for filtering losses.

As important as the air cleaner size is how and where you mount it. Try to mount at about 8-10 feet above the floor (no lower than 6'or 2/3 of the floor to ceiling distance if less than 8' ceiling). Mount along the longest wall so the intake is approximately 1/3 the distance from the shorter wall. Mount no further than 4-6 inches from the wall.

The exhaust is the largest determiner of the circulation pattern. You are trying to encourage circulation parallel to the floor/ceiling so ceiling mounting is not recommended. Use a smoke stick (or a cigar) to observe and maximize circulation. Use a secondary fan to direct air to the intake if necessary. Also, consider that a standard 24" floor fan moves a lot of air and, in some shops, just positioning it in a doorway with a window or other door open can accomplish as much or more than an air cleaner. It's all in the circulation patterns.

The exhaust is the clean air so that is where you want to position yourself. Do not place the air cleaner over the a dust producer. That will guarantee that the operator will be in direct line between the dust producer and the air cleaner. The operator wants to be in the clean air stream. If the dust has to pass your nose to get to the air cleaner, you get no benefits. If you have an odd shaped shop, two smaller units may be better than one large one.

DO NOT RELY ON A AIR CLEANER TO ACT AS A DUST COLLECTOR. The purpose of and air cleaner is to keep airborne dust in suspension and reduce airborne dust as quickly as possible AFTER THE DUST PRODUCER HAS BEEN TURNED OFF.

Finally, if you are looking for health benefits, you will not find any air cleaner manufacturer that makes health claims because there are few health benefits. CATCHING DUST AT IT'S SOURCE IS THE BEST LONG TERM GOAL. Rick Peters', author of "Controlling Dust in the Workshop", makes the point that spending your money getting the dust at its source is a better investment than trying to capture it after it is already airborne. If the dust is in the air, it's going to be in your nose and lungs too. Robert Witter of Oneida Air Systems has noted that "overhead cleaners can only lower ambient dust levels AFTER THE SOURCE OF EMISSIONS IS SHUT DOWN, and they take several hours to do this. This is why they are not used in industry." The absolute best answer, if health is the primary concern, is to use a NIOSH approved respirator. The dust cleaner will help keep your shop cleaner but have minimal or no health benefits. OSHA takes this position too. They measure the number of particles per a volume and most air cleaners will not satisfy their specs.

[Prashun Patel]

I love my boxfan/filter. I believe the best way to optimize is to use multiple fans/filters and to put them as close to the work as possible, and to change the filters often.

[Mac McQuinn]

Clever idea on the hold-down. With a metal box fan, I used a set of (4) thin metal "L" brackets with magnetic tape on one end and put two on the top, two on the bottom for a quick filter change. While the 1" thick Alergen Ultimate filter seems to work well for me, I do wonder if one the thicker furnace filters would give you longer life with less (CFM) drop at the other side of filter. Lets us know your findings.
Mac

[Larry Edgerton]

What kind of fan do you have in that unit? Is it a blower (like from a furnace), or a conventional propeller fan?

Phil, I am using a squirrel cage fan. From my understanding they will pull air better when there are restrictions. Plus I get them at a salvage yard for nothing.

I use regular furnace filters, not the cheap ones but not the most expensive. I spray them with a light coat of K&N filter oil before installation, and this has not cause a problem with finishes. It is a very tacky oil made for the purpose. I always got it free when we raced, but those days are over.

My new one will be a system like Mreza mentions above, but as the furnace filters catch most of the dust I will have a 2 to 1 ratio, or I may just use a sock at the end of the exhaust duct. Not sure.

Larry