OK ran numbers on this for quite a while.
2x 20x25x1 followed by a 20x25x4 pleated paper is still not going to work. The 20x25x4 is already incorrectly undersized and will exceed the max static pressure (choking the unit) at only about 25%-33% of that filter’s ASHRAE dust capacity. You have to change it way, way too soon because, due to being undersized, it has to pull that high 576 fpm load through it. Adding another stage without addressing that will make its issues worse, and it won’t be possible to configure it to be highly effective at actually addressing the PM2.5 problem. Depending on what filter you buy, it may be at its limit or past it the moment it’s turned on.
Key problem is that filter cabinet is undersized. It needs 2x 24x24x4 in a “A” configuration, any way you slice it. The first stage absolutely needs to be tackified polyester media, not pleated paper. 20 sq feet for 5 ton will be the minimum to hit the loading capacity profile. So 10 ft of 25” wide media.
Filters have a max dust loading capacity in grams of dust per sq in of area. Tackified polyester media is radically more capacity- 0.23g/sq in, pleated paper is 0.07-0.10g/sq in. Also, tackified polyester media does not show as much static pressure rise until nearing its max load.
But, that ultimate capacity is based on where it builds up 1” of static pressure, where the until will choke, when using standard face flow velocities. If you attempted a 2-stage with standard sizing rules, that would require 2” of static which is impossible. It could be under 1” of static while new, but it will be high enough that it will lose significant cooling capacity and efficiency, and fail with only moderate filter load. One incident of someone doing a lot of hand sanding without dust collection or a problem with the dust collection system can throw the equivalent of weeks of “normal” loading onto it in a short time, so it may not matter how often you waste money changing filters earlier.
The key is to design it with enough filter area on both stages that the total static drop is still within the unit’s static pressure limit when those filters are in their fully loaded states. This is also where the cost-effectiveness comes into play, as well as reliability. To do that, BOTH stages MUST be oversized by 80%-100%. That halves the pressure drop of each stage all any loading level, so the combined resistance is similar to one stage at standard sizing, and the filters can be used up to their load limits
And we really need to address PM2.5. PM2.5 is the actual health risk here, and you don’t get really meaningful capture unless the secondary is a MERV13. A MERV13 is out of the question if it’s only the 4x20x25 cabinet that’s up there now. That will have too much static drop with or without having another stage in front of it.
Now I’d looked at the options before, and rockwool duct board towards the wall looked mechanically problematic. Something has to hold that up, and those units are suspended and do swing a little bit. Rigid ductwork seems to have a high risk of tearing up if it reaches to the wall, or the wall is used to anchor it. And how to access the stage 2 filters, that’s a problem. Flex ducts would be of course not have that issue, but… well, it’d be super weird to use flex ducts like this.
All that goes away if both stages can sit on the unit. It actually can, and be changeable with a pole from the ground and maybe a lift pulley.
Back to the first part- I spent quite awhile going over these numbers. Putting 2x 20x25x1 in front of the 1x 20x25x4 is worse on cost, reliability, and overall HVAC efficiency
Putting this all together, yes I see how to “win” this. I’m drawing up the CAD and BOM now. This will keep the HVAC in the high efficiency range under the whole range of filter loading, get high single-pass clearance of PM2.5, could be changed without clearing the footprint for a large ladder, maximize cost-effectiveness, and should be able to go over 6 months between filter changes (hard to account for, but it’s able to reach 100% filter capacity utilization now (3x-4x better than we have now), increases the filter capacity by 2.3x, moves 60%-70% of the total dust mass to the prefilter, so, roughly, 23x the capacity of the current config before reaching max static drop. Yep, a 1yr change schedule under our existing shop conditions is indeed possible. $208 consumables per unit at that point. That’s substantially less operating cost than we have now. MERV13 filters that are much more effective at capturing PM2.5 cost about 3x more for the same area, but that’s after factoring that in.
But perhaps more important is that is after switching to MERV13, now has about twice the capture efficiency for PM2.5. PM2.5 is the long-term health risk (it takes months for the lungs to clear particles in this range) so that’s a big deal.
I need to know the exact XY dimensions of the end of the units. Also, what’s the total air volume in the shop? Length x width x height?
