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Filter advice for shop-built air scrubber? - ®smpr_fi_mac® - 11-10-2019

The furnace blower I have is a 1/2hp unit, 3.6A, 220V. It has four speed settings; I'll wire it to run on the high speed.

The design I'm going for will place the blower in the center of a plywood box, with dual filters on each side of it. I have on hand two AC filters, 20x20. I'd like to run these as disposable, with a much finer filter behind each one to catch the fine dust.

What can you guys recommend? I want a lot of surface area unless you recommend smaller. I have a 12' ceiling so a large enclosure isn't a problem.

Is this where a HEPA filter would come into play? I think I remember .5 micron needed to really do a good job? I expect filters in this filtration range will be costly, but I'm okay with that.

I think I should be looking for a MERV rating of 12 or so, maybe?

Also, what depth of filter should I get? 1"? As much as 4" deep?

It'll look something like this one: https://youtu.be/kXImSeA56lE


RE: Filter advice for shop-built air scrubber? - fredhargis - 11-11-2019

Brad, I built one one much like you plan. I didn't put any filter on the discharge (the air was already cleaned) except for a really cheap fiberglas mesh filter the hide the innards. But on the intake side I used 3 and chose Filtrete filter for #2, and #3. The #3 was a Filtrete Ultra Allergen, the #2 was a Filtrete red, and the frfist filter was whatever the cheapest one I could find that had filter media (none of that mesh stuff). I had to change the first one maybe 2 times a year, but #2 and #3 lasted much longer.


RE: Filter advice for shop-built air scrubber? - Cooler - 11-11-2019

I attached a 20" x 20" to a box fan using a Filtrete filter for my old darkroom which was in my dusty basement.  I used the forced air to enter the darkroom and had vents for the air to exit. 

To extend the life of the fairly expensive Filtrete filters I used a fiberglass filter to pre-filter the air.  The fiberglass filters are very cheap but do not filter fine enough.  I don't recall which micron filter I got but the finer the micron the less air flow.

You need a MERV rating of 8 to remove dust.  Anything  above that probably restricts airflow to remove odors, etc.

MERV rating chart:  https://www.allfilters.com/airfilter/mervchart

3M MERV ratings:   http://www.iaqsource.com/article.php/comparing-3m-filtrete-filters/?id=19

MERV 8 rated filters from Amazon:  https://www.amazon.com/20-Merv-Furnace-Filter-Pack/dp/B019I020AK


RE: Filter advice for shop-built air scrubber? - ®smpr_fi_mac® - 11-11-2019

Cooler,

Looking at that first link, I found another link on the page:

https://www.allfilters.com/airfilter/mervefficiency

It adds particle size to MERV rating. It appears that 8 is only good for 3 microns and up.

Is that fine enough?

I do plan on building a second scrubber for the opposite side of my shop next year so I'm okay with a bit less air flow in exchange for better filtration.

What's micron size I should be aiming for?


RE: Filter advice for shop-built air scrubber? - Wild Turkey - 11-11-2019

Back when those room-sized air filters were popular a magazine called "New Shelter" (IIRC) {which has since shut down} ran a test on room sized filters to see which one worked best.

They used an oscillating fan as their "control"

It worked better than all the other filters. Laugh Confused

More research explained why:  Air filters work when a particle impacts a filter medium at a speed high enough that it sticks to that media.

Bottom line was that too fine a filter slows things down too much to catch particles.

More air flowing faster through coarser filters caught more particles than fine filters with slow moving air.

That's why the oscillating fan worked so well -- it moved a lot of air that made the dust particles stick to the walls.

That's why a fan moving lots of air through several furnace filters will catch more particles than a very fine filter that doesn't move a lot of air.

Yes, I've looked for the copy of that article that I kept, but two moves have hidden lots of stuff. Rolleyes


RE: Filter advice for shop-built air scrubber? - ®smpr_fi_mac® - 11-11-2019

That's neat! My plan is a two-stage filter, coarse and fine.

What's too fine, though?


RE: Filter advice for shop-built air scrubber? - ®smpr_fi_mac® - 11-11-2019

Heck, I'm still in the design phase. I'll put three filters in line if it'll work better.


RE: Filter advice for shop-built air scrubber? - MichaelMouse - 11-12-2019

(11-11-2019, 08:53 PM)®smpr_fi_mac® Wrote: Heck, I'm still in the design phase.  I'll put three filters in line if it'll work better.
Micron malarkey is for your face, volume for dust collection.  Use a mask when making dust, and collect at the source.  Standard furnace types will do fine for the airborne.  Principle is not straining, but stopping, like the mask.  When the particle hits something, unless you have extremely high pressure, it'll stop. 

If you're determined to have the finest "filtration," consider electrostatic.  https://www.galco.com/techdoc/hnwl/f100f2044_u_cp.pdf That way it doesn't just hit something, it's dragged to it....


RE: Filter advice for shop-built air scrubber? - ®smpr_fi_mac® - 11-12-2019

(11-12-2019, 07:57 AM)MichaelMouse Wrote: Micron malarkey is for your face, volume for dust collection.  Use a mask when making dust, and collect at the source.  Standard furnace types will do fine for the airborne.  Principle is not straining, but stopping, like the mask.  When the particle hits something, unless you have extremely high pressure, it'll stop. 

If you're determined to have the finest "filtration," consider electrostatic.  https://www.galco.com/techdoc/hnwl/f100f2044_u_cp.pdf That way it doesn't just hit something, it's dragged to it....


I'm really not looking for finest or best. I guess I'm looking for better than the *none* that I have now and will be happy with pretty-dang-good. I didn't realize how confusing it would be for me when I started looking into what to do/use.

I do appreciate all the advice given. It's helping.


RE: Filter advice for shop-built air scrubber? - Cooler - 11-12-2019

(11-11-2019, 08:51 PM)®smpr_fi_mac® Wrote: That's neat!  My plan is a two-stage filter, coarse and fine.

What's too fine, though?

MERV 8  lists all the particulates

MERV 11  adds auto emissions and smoke

MERV 12 adds tobacco  smoke and bacteria


I don't have auto emissions or smoke in my shop and I don't smoke.  I don't think I am  worried about bacteria in the shop either.  I am mostly concerned with particulates like saw dust or grinding dust.  So MERV 8 would be my choice.

Apparently the higher the MERV number the  finer it filters.  The higher the MERV number the lower the airflow.  

I added the fiberglass filter to filter out lint from my dryer

This is a more complete chart:  http://www.ontimeairfilters.com/air-filter-merv-rating

I noted a fiberglass filter that had a MERV 4 rating, and that would not be fine enough for sawdust.