Shop air filter

[GregH]

Recently, while visiting a friend's basement workshop, he showed me his ceiling-hung air filter, and he remarked on how effective it is. So.... I've started looking at one for my shop. It strikes me that they are rather expensive ($3-400) for what appears to be a box, blower/motor, and a couple furnace filters. Seems to me, I could build one for a lot less. Any thoughts???? (Dumb question!)

[Bruce Hooke]

I built one myself and I am not sure I save a lot of money, especially when you factor in my time, but I was able build just what I wanted. For example, for not really that much money I was able to get a really good pleated filter from Graingers that should be doing a good job of filtering out even the very small dust particles. I also built in a timer so I can leave the shop and leave the fan running for 20 minutes (or whatever) to clear the air.

It does help a lot if you can manage to get access to Graingers because they are the best supplier I know of for things like squirrel-cage fans, pleated filters, etc. However, Graingers will only sell wholesale so you need to convince them that you are in business. This is not that hard...I think I basically needed to show them a sales tax permit and a business card, or some such, all of which are easy to obtain, but are maybe not something you'd want to get into if you are not trying to start a business. It also helps if you have a business name, which means filing a "Doing Business As" form with your town.

The construction is pretty simple. I built a box, put a pleated filter at one end with a good furnace filter as a pre-filter. At the other end of the box is a squirrel cage fan with a timer and a high and low speed switch. That last is something I almost never use. If it is worth turning on it is worth running at high speed.

[Mrleft8]

If you have a dead computer or 2 you can take the cooling fans from there, otherwise a little dumpster diving should yeild a larger one fairly quickly. Some furnace filters and a plywood box and a switch (if the fan didn't come with one), and Bob's yer uncle. The one I have was a gift from my mother, but I think I'll build another for the front of the shop...

[Pete Dorr]

save time - use the dead computer as the box and duct tape the filter in place of the side access panel

Maybe I'll try that myself :)

[kingplanker]

http://lumberjocks.com/woodspar/blog/1553

this blog has a decent how-to sequence - could be built simpler
http://farm2.static.flickr.com/1370/855140813_005c0c4372.jpg

[Bruce Hooke]

A few things to note:

1. My understanding is that a furnace filter is not going to filter out the smallest dust particles, which are exactly the most dangerous ones to your health. To do that I believe you need to use a pleated "bag" filter. A furnace filter could actually give you a false sense of safety since it will filter out the big stuff that is most visible to the naked eye but leave the small stuff that is what actually harms your lungs.

2. If you use a pleated bag filter then a computer fan or a box fan is not going to have anywhere near enough power to really get the job done. My understanding is that both are designed to work in an environment where there is very little pressure differential between the input and output sides. A dust system on the other hand has a fairly high pressure differential. I find that with a high-quality pleated filter even a good squirrel cage fan is none too powerful. Ideally, when looking at squirrel cage fans you should try to get the specs that show how the fan does at different pressure differentials, since that is what really tells you how it will perform in use.

3. If you want to get serious about designing the system to do the best job for your shop, get a copy of Woodshop Dust Control, which explains how to size the system and where to install it in your shop for maximum effect.

[Brian Palmer]

Some box fans, like the one in the picture, have a temperature sensor for the motor and they automatically shut off if the motor overheats, as may happen if the fan accidentally falls over and is laying flat on the floor. The same thing happens if you put a filter across the front. I found this out by trying to do exactly that. It would cut out when I needed it most.

Consider getting a fan with a bit more "oomph" to use behind a filter, like a fan designed to work with a furnace or A/C unit, and that has enough power to keep pulling air across the filter even after you have some dust on it. Otherwise you'll just be changing a lot of filters, and it won't really move much air through the filter once it has some dust on it.

Brian

[Wayne Jeffers]

Yeah, for a cheap solution, if not the highest filtration levels.

Box fans are (usually) 20" X 20".

Furnace filters come in 20" X 20" size.

Coincidence?

Wayne

[Bruce Hooke]

It is maybe also worth noting that pleated filters reportedly filter best once they have some dust built up on them. However, for that to be applicable you have to have a fan that can still pull air through the filter once there is some dust build-up.

[Wayne Jeffers]

. . . 1. My understanding is that a furnace filter is not going to filter out the smallest dust particles, which are exactly the most dangerous ones to your health. . .

I'm sure this is true. The few times I've used the box fan/furnace filter combo, I've also used a respirator mask.

When I really want to clear the dust from the shop, I have a huge exhaust fan that will change the air in the shop in a couple of minutes. ;)

Wayne