Eric, Testing results in the video above indicate that you need at least a 4" filter if you are going to attach a single filter to the fan; otherwise, you just do not get enough airflow for it to do much good except maybe right next to where you are sanding for example. Stacking the filters does not really work unless maybe you stack something like a MERV 8 in front of a MERV 11 but it mostly just acts to restrict the air flow and does not increase how much of the finest dust it can collect. I think it would just serve to prolong the life of the MERV 11 filter. You are better off using a 4" filter with a higher MERV rating because it increases the surface area so you do not choke off the airflow. Reducing the outflow by adding a round cowling on the front so that it doesn't suck in unfiltered air from the front should also increase the amount of air that it pulls through the filter without reducing the net air being moved out the front. Another thing mentioned is that you need at least a MERV 11 filter for it to capture the most dangerous particles but moving up to a 13 or 14 is sort of the sweet spot for cost and effectiveness. MERV 15 is even more effective but the price jumps significantly for those. One 4" MERV 15 filter can cost $60-100 compared to a little over $100 I paid for 5 MERV 13 Filters.
BTW, one thing that was shown in a couple articles I read indicates that in a 4 or 5 filter setup, even using 1 inch filters, because the multi filter setup doesn't restrict the air flow by much, it will cycle the air through the filters more often so even MERV 11 filters will eventually remove much of the below 5 micron dust from the air. If you can filter the equivalent of the entire volume of air every 5 minutes for example, even a filter that removes lower percentages of particles below say 5 microns will eventually get most of them by just sending the the air through the filter so many times. So with a MERV 11 filters for example, they should remove between 65 and 79% of the 1 to 3 micron particles in each pass. If you can process the entire volume of your shop air 12 times in an hour for example, you can theoretically capture up to 99% (65% ^ 12) of those particles in an hour.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
