A number of people have been asking me about the air scrubbers that I build for my shop, and how would they go about building their own. I’m making this available to everyone, as I believe that every wood shop should have one (or more), and anything that can make for a safer and healthier work environment is a good thing!
Any scrubber that you build can be easily modified to accommodate either a timer mechanism, a remote switch, or both. (Cheaply too!)
There are two ways to build an air scrubber – the expensive way, purchasing all new components (e.g. blower) and materials, and the inexpensive way scrounging and recycling components, and using any scrap plywood that you may have. I am always a fan of the second method, for obvious reasons, but also because I like the challenge and satisfaction of revitalizing something that may have been destined for the scrap heap, and reinventing it into something completely brand new.
The first step is to determine your shop’s total cubic feet of air space. Use the formula (Length X Width X Height), divide this number by the number of minutes it takes to exchange one full air volume. The rule of thumb is that a scrubber should be able to exchange the total volume of air a minimum of 6 times each hour. The more exchanges that can occur the better.
Gathering the materials: The heart of the air scrubber is the blower unit. (Without it, all you have is a nice looking box with some filters stuck in one end!) The blower’s CFM (cubic feet per minute) capacity should match the number that was determined from the above formula. The best scrubber blower is a continuous duty “squirrel cage” unit. (The Grainger catalog lists these as Shaded Pole blowers and Furnace Blowers.) This is where scrounging and recycling start to pay off. The blowers that I currently have operating in my shop were salvaged from large home furnaces, and from whole-house humidification units. These units are capable of 1000 CFM’s and 600 CFM’s respectfully. They cost $0.00. (Yes, if you can believe it, at some point in the last 30 years, builders here in the North East thought that sending heavily humidified air throughout the entire house from a large water/blower/duct unit in the basement was a good thing!) Talk to your local furnace installer regarding a used blower unit. When new furnaces are installed the old units are just scrapped, usually with the blowers still fully functional for your needs.
The body of the scrubber can be made from a (approx.) 4’x4′ (x 1/2″) plywood, some 1″x2″ pine, a random assortment of nuts, bolts, washers, and screws. Additionally, you will want some foam gasket, a switch of your choosing, and you’ll most likely need to fabricate a power cord (16/3 SJ) and plug.(grounded triple prong).
Before you start building you’ll need filters. (The box unit’s dimensions are partly determined by the size filters that you choose.) There are pocket filters and there are pleated filters – I use the pleated filters due to their low profile and the fact that many of today’s filters can match the performance of good pocket filters. Either filter will work, it’s your choice. To extend the life of either filter, and to increase the scrubber’s performance, an electrostatic furnace/AC pre-filter must be used! For filters, I use 3M’s Filtrete 1250’s, and The Web (brand name) Electrostatic Replacement Pad for furnaces and AC filters. 3M quotes the 1250 to be 90% efficient with particles 3-10 microns in size. The Web filter, if washed regularly, is quoted as being able to last 5 years. To test the filter’s ability, I stacked up three filters into my 1000 CFM scrubber – two 3M 1250’s and one pre-filter. The unit was then run all day for 2 solid weeks under normal shop conditions (cabinet saws, planers, jointers, hand & machine sanding, etc… a cyclone dust collection system is also used for all machine work.) At the end of two weeks, I removed the filters. The pre-filter was completely covered and the first 1250 was as well to a slightly lesser extent. The third filter, to the naked eye, was a clean as it was straight out of the package!
Now it’s time to build!! You will have to determine the cutting dimensions of the plywood based upon the height and width of the blower, and the filters that you’ve chosen. The height of the box should give an inch+ of clearance over the top of the blower, and enough space at the bottom to accommodate the metal mounting flanges of the exhaust port(s). Because the blower’s air inlet port is only on one side, the blower will be mounted slightly off center in the box by a few inches to give air clearance. The length of the box should be 3″-5″ longer than the longest side of the filter – measuring from the filter to where the blower starts inside the box. (ex. 20″ filter: 25″ from filter to blower). If you choose pocket filters, be sure that the box is long enough so that the pockets aren’t touching the blower unit.
Once you have cut the plywood to the dimensions that you determined, locate where the blower’s flanges will be on the end board, and cut an exhaust port with a jig saw. Cut to the inside dimension of the exhaust port opening, not the outside edge of the flange – if you do, there will be a big hole with nothing to bolt the flange on to!!
Before screwing the side panels together, measure the thickness of the filter stack, and then nail or screw some cleats onto the end opposite the blower, whatever that measurement is, from the end. (This is what the filters will rest against.) You will also want to wire and mount your switch before all four sides are attached. This is where a spring wound timer could be installed. To keep the filters in place, I’ve stretched some copper wire (salvaged by my shop’s electrician across the end of the box – Any thin string will do.
The wiring of the switch and motor is very simple, but is somewhat different depending on what kind of switch you choose (timer, toggle, pull chain, etc…) I would recommend looking up a diagram in any basic home maintenance book for wiring a switch. (This can be found in the big orange book from Home Depot.) (If you have any questions, e-mail me, and I’ll e-mail you back with a diagram of how I wired some of the units that I’ve built.)
Wireless 115 V. remote switches are available at Radio Shack for $25. They also have ones with three channels.
Screw the box together, maybe throw some caulk (or phenoseal) around any seams to prevent air leaks, stick some foam gasket on the cleat so the filter has a good seal – and your off to the races!!
Air scrubber units can be hung from brackets or chains on the ceiling or joists, mounted on casters to roll around the shop, or, if it’s small enough can be placed on a bench top for smaller projects.
If you have any questions, or need a diagram of some of the things I’ve mentioned, e-mail me and I’ll supply you with what’s needed.
Dan Kornfeld, Owner/President – Odyssey Wood Design, Inc.