woodshopnerdery wrote
As far as I understand it, the issue is that all filters clog eventually. When they become very clogged the air flow drops significantly. The less air moving through the system the less dust it collects. Pleated filters add more surface area and therefore have a higher initial airflow than felt bags of equivalent micron rating. However, the smaller the particle the filter traps, the less air flow it allows, even when new. So even a new pleated HEPA filter will have less flow than a 1 micron pleated filter of the same dimension. So to get the same air flow through the system with a more efficient filter, you need a more powerful motor.
Why would you have "identical" surface area for a HEPA filter compared to a non-HEPA 1 micron pleated filter? Yes, HEPA filters can capture much smaller particle sizes than traditional pleated filters, but to compensate for the restrictions of a HEPA filter they typically have much more surface area in comparison to a standard filter. I would argue that a properly selected HEPA filter should be far less restrictive than typical standard filters it replaces because they often have vastly superior filter area in the same form factor. I would honestly expect your DC airflow to improve by switching to a HEPA filter from a non-HEPA filter.
woodshopnerdery wrote
Since HEPA filters trap more dust they become clogged faster and the air flow drops faster. So pleated HEPA filters have to be cleaned more often to maintain air flow. It is not easy to fully clean pleated filters. The common solution is to add a cyclone separator to extend the time between filter cleanings. Note cyclones separate large particles, but also separate some of the particles that would clog the filter. However, cyclones add some air friction to the system and air friction means less air velocity. So to maintain the same air flow after adding a cyclone, you need yet again an even more powerful motor.
So, the assumption here is that HEPA filters have identical filter area again when this is often not true. IF a HEPA filter has significantly more surface area, i.e. more pleats than a standard filter then it won't clog faster than a non-HEPA filter necessarily. Cyclones prolong filter life of HEPA and non-HEPA filters (both have to be cleaned), but they do restrict airflow (10-20% in my experience). Whether the drop in airflow is noticeable largely depends on your DC system and ductwork. There are different specs to consider for a DC system, but the main one in a ducted system is trying to maintain at least 4000fpm on horizontal ducting. This is because when the air stream drops below 4000fpm the size dust particles often found in woodworking can drop out of the airstream and settle on the bottom of the those horizontal runs. Taking ductwork apart to clean it out is a real PIA so you want to minimize any dust settling out inside the ductwork. IF you have no ducting, i.e. using short hoses only, you can actually tolerate some drop in airflow from a cyclone and for the most part not compromise dust collection at the tool because you are not limited by that 4000fpm general rule. It is also possible to mitigate airflow restrictions due to a cyclone by putting more than one in parallel. It costs more to go parallel, but that can be mitigated with low cost Chinese cyclones.
woodshopnerdery wrote
So in my opinion, I cannot simply add a canister to my little system. I would need a major upgrade including a cyclone and a more powerful motor. Jet, Laguna and Oneida have 1.5 HP cyclone systems with HEPA filters. If I make a change to my system, it would be to get or make something like that.
I am not trying to talk you into a cyclone or a HEPA filter here, but just give you some perspective based on my own experience. I
chose to run a cyclone and did enough airflow measurements to convince myself that I am better with it than without it in my system. I also
chose to run HEPA filters in my system because my health is important to me. However, these choices largely didn't affect my dust collection ability because my shop is too small for ducting so I am using short, high quality hoses (low SP drop). I considered ducting my shop, but I would have had to go through the attic with it and it would have required at least $2k between cyclone, DC and ductwork. I just can't justify that expense for myself right now, so what I have works, albeit on a much smaller scale. I can't comment on your specific DC system or your shop's needs, but I can certainly understand the cost factor involved. IF you are doing ductwork and have a lower powered DC it can certainly necessitate needing a larger DC if you add HEPA and cyclone to it.
