re: Shop Air Filter

If you haven't already, get yourself a cheap anemometer and see if it is sucking in air in from around the edges in the front.  I was amazed at how much air a basic box fan pulls in from the front side of the fan.  As I moved the anemometer from the center to the edge, the anemometer actually reverses direction and the velocity is pretty high.  That probably isn't an issue for moving air around the shop but it is if you are using it to pull or push air through a filter -- you are wasting that much air flow.  I would bet that as the filters clog, it will pull even more air from the front side to make up for it.  You could try, temporarily at least, attaching a piece of cardboard with a hole cutout so that only the flat part of the grill is open and see if that actually increases how much air goes through the filters.  I used my anemometer to measure (estimate anyway) the total amount of air going through the filters as well.    The fan I used for my DIY filter has a cowling that prevents it from sucking in air from the front side of the fan (verified with the anemometer).  I also push air into my filter box so I know that any air going into the box has to exit through the filters so that I can measure the intake airflow to know if the flow is reduced due to clogged filters.   When I stand under my filter, I can feel a slight draft coming down from the filter so I know that it is still cleaning the air, even a year and half later. 

Also, I highly recommend going to thicker filters.  Even 2" filters will help but my four  4" filters have been going for a year and half now with no noticeable reduction in airflow.  If I had 1" filters, I suspect that I would not only have significantly lower air flow and poorer air cleaning but I think I would have probably had to swap them out 3 or 4 times during the last year and a half.