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Important Note: The device I describe in this article should never be used in any environment where potentially explosive fumes can build up. This includes areas with flammable dust, fuels, and any substances containing VOCs (Volatile Organic Compounds). This is because the simple box fan upon which this design is based does NOT use an explosion-proof motor.

I put this together for my outbuilding where I have my model railroad, because I was forever having problems with dust, and the dust was starting to get so bad it was driving me crazy. The dust we get here is relentless, especially since it's driven by the winds. I've found that since this unit has been in service that my allergies just don't bother me much out there, so I found a side benefit!

This unit does pretty much the same thing as those really expensive models, and the cost is minimal. It's worked so well that I thought I'd share it here. It is currently cleaning a 12 x 16 space, and doing a good job at it. The highest-cost item in this whole build will be the air filter, unless you don't have a spare box fan sitting around. Best of all is that a replacement filter doesn't cost a hundred bucks, plus you can easily see when the air filter needs replacing. (My particular design uses one of those washable 'lifetime' filters, but more about that in a minute.)

One note on the air filter itself: you get out of it what you put into it. A version for household locations would include two filters, just like HEPA filters do: On the outside you use a regular, cheap, $3 filter to pick up the big stuff like dust bunnies and pet hair, and behind it you use a $15 good anti-allergen filter. The anti-allergen filters in the hardware store sometimes are hard to tell from the inexpensive ones, so here's a photo to help you recognize the type.



I liberated a spare fan from the house for this, and I first tried putting the filter on the exhaust side of the fan, because the controls are on the intake side. I added a lamp timer, because I didn't want it running all the time. Here's a photo of my first try at it, and I had it working for a couple months like this:



But on the advice of marlboro 180 and tiretrx, I relocated the filter to the intake side. What a difference! Having the filter on the exhaust side doesn't work well because of the way air flows through a fan. The blades are trying to beat the air through the filter instead of pulling it through.

Having the filter on the intake side really is the way it should be built. But for this kind of fan, this meant that every time I wanted to turn the fan on and off I'd have to take everything apart. Here's how it looks now:



So a lamp timer becomes necessary, even if it's only used as a power switch. The lamp timer gives us the advantage of saving juice and also lets us tailor the amount of run-time to the amount of filtering necessary for the room.

Okay, let's build it.

PARTS:

Box fan (square), with an overall outside dimension of about 20 inches; typically you'll find them running about 22 inches square.

$3 furnace filter from the hardware store (size: 20x20x1)

Anti-Allergy Air Filter from the hardware store (size: 20x20x1)

Two bungee cords, about 48 inches long

Lamp timer (optional, but will be necessary if your fan's controls are on the intake side)

4 short lengths of 1" ID aluminum C-channel (optional)

ASSEMBLE:

Stack the air filters on the intake side of the fan, making sure to pay attention to the airflow arrows on the side; the expensive filter goes on the inside, next to the fan, the cheap filter goes on the outside. Wrap the bungee cords around fan and filters, then hook the ends on each other to hold the air filters in place.

Here's a shot of it running. It doesn't make any more noise than the original fan did.



The unit takes a few days to really start cleaning up the air, if it's on a timer and running only periodically. I've included a photo of my lamp timer so you can see that I have the fan set to run periodically throughout the day. Here's a shot of how I have the lamp timer set up:



NOTES:

My version of the CAC (Cheapskate Air Cleaner) looks a little different because I used one of those 'Allergy-Free from Aller-Pure' air filters. I'd bought it on the recommendation of Paul Harvey many years ago, but eventually found it to be incompatible with my heat pump system; so it was just sitting around.

The permanent type air filter is a good fit for this design because the metal chassis isn't distorted by the bungee cords, but it's expensive to buy one of these kinds of filters outright. As an alternative, you can use some lengths of 1" ID aluminum C-channel (which I specify above) to slip over the outside edges of the filters at the bungee cords to keep them from being crushed.

You will have to change the cheap outer filter periodically. But because it keeps most of the trash out of the expensive filter, you won't have to change the expensive one as often.

You can expect about 1/3 the airflow from the finished unit. Not quite enough to cool you on a hot day, but enough to circulate the air in a good-sized room and clean it.

Warning: The fan used in this device does not contain an explosion-proof motor. The build up of dust and volatile organic compounds (VOC's) has, in the past, lead to many documented fires related to the fan much like the one in this article. Therefore, keep your filters clean and do NOT use this device where such conditions may exist.

Originally posted on MyTractorForum.com July 2011.
 
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