Hi Folks,
Seems like air cleaners are popular these days so I thought I'd throw my design out there for grins.
I built my shop back around 1990 (actually I had a guy pour the slab and I did the rest..a one year wood-working project). Before that everything happened in our attached garage, and dust inside the house was always a problem. While the shop was under construction, our downstairs heat-pump died, including the air handler. Usually air handlers sort of rust from the inside out from humidity condensation..but there's nothing wrong with the fan/motor part. I grabbed it out of the frame and it ran great...220V. and 1000 CFM. I think it's a 3/4 hp. motor so at 220, it's drawing about 2 Amps.
There were no commercial ceiling mount air cleaners then, except for big industrial things...so this is what I came up with. I talked with a local filter supplier and learned the basics about pre-filtering, standard filter sizes and built a box...basically a 22"X22"X72" case out of plywood...(which was soon named "the coffin" by LOML). I wanted to move a lot of air(the shop is about 24 ft.X36 ft.with a 9 ft. ceiling) so there are four 20"X20" intake openings. The metal frame and screen are standard sizes as are the polyester batts. Everything was sealed with marine goop on the inside and the foam rubber in the openings holds the frames tight.
The squirrel-cage motor fan was placed in the center compartment with the output facing into the shop. On either side of that compartment, we have a 4"X20"X20" pleated box filter. I had an HVAC outfit make a transition piece to match the fan opening to standard metal duct and ran the duct out to the middle of the room with a bi-directional vent cover on the end.
The polyester batts (called paint pockets in the filter trade) screen out the "rocks" and the pleated box filters do the rest. These are MERV11 filters that have 95% arrestance of 3 micron particles..the next level up(MERV12) screen Legionella spores ...don't think we need to go that far.
I learned ( more than ever wanted to know) about the filter stuff from the tech expert (Rick) at Global Filter on Westgate road, Raleigh..the trade-offs are arrestance, load-up time, and airflow.
The result is a filter that moves 1000 CFM through the system, so in my shop's roughly 7700 cu.ft., we change the air about 8 times an hour. The flow is strong enough that I feel a breeze blowing by at the work-bench, so as long as I'm sanding "up-wind", I don't even get my glasses dusty.
You can see the Dwyer manometer below the box in one of the pics, to measure the pressure drop across the whole system. The fan is 2 speed, so at the full 1000 CFM, I see about 0.09 inches with all new filters in place...not even 0.10", and according to Rick, I don't need to replace the HEPA type filter until there's about 2.0" drop. The latest ones have been in there about three months and the drop is now all the way up to 0.15". Almost half the drop occurs from the pre-filters, but they are washable...I just vacuum them when they look dirty.
The MERV11's cost $7.75 at Global, and the paint pockets were $1.83 each.
I think they'll last a very long time!
If anyone wants to try it I think I still have the sketches around of the design, and you can pick up old air-handler fans from most HVAC service guys.There used to be a place down on Hodges St. called Motors and Controls. Don't know if they're still there, but they had lots of them in the back that they would give you free, just to get rid of them.
Don
Truths: There is no such thing as a 25 hour day, so why do I keep trying to cram so much into every day so it seems that way! 
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