OMG THERE'S A BOTTOM TO THE TRAY?!?!
No wonder these stupid things toasty their drives. Did you manufacturers learn NOTHING from AIC's mistakes?
In a 4U enclosure you have 7 vertical inches and 6 rows of drives. A typical 3.5" drive is just over 1" high (ST4000DM000 is 1.03" for ex). So you have 6.18" used. The chassis itself is supposed to be just shy of 7" (usually about 6.95"). So you have just 3/4ths of an inch to hold the top plate, bottom plate, and any interior structure.
In an imaginary world where the chassis was made of zero thickness Unobtanium (2d plane!), you are still left with a cross-sectional area of only .77" x 17" through which the air can flow, or about 13 square inches. Which'd be great if it was a free and clear rectangular duct, but it is actually a mostly-obstructed duct (since 7" x 17" = 119 sq - 91% blockage). So even in our imaginary world of optimal chassis, ... bad.
But in the real world it is worse. Even in the current Supermicros, I'm skeptical if anyone claims more than 1mm clearance above and below a drive (individually enclosed in a grid). But visual estimation does suggest something close to that, so conservatively .75mm times 6 rows times two (above and below) is 9mm (guesstimate). Sticking in a bottom to the trays would effectively eliminate about half of that, I'd guess?
So I'm kind of horrified. I hate the bottomless trays but they're sensible from a strictly engineering PoV. I keep reading about the difficulties the Norco guys have. I continue to come to a better understanding of why that is.
And metal slats to block airflow in blank slots? Use a plastic filler tray. Sufficiently solves the problem a tray bottom does, AND blocks the air without chance of some ops person who is unfamiliar with the chassis not realizing there's a magic slat to open on the tray to enable airflow.
I'm sorry cj 'cuz I know you like your Norco but boy ... sure seems like it wasn't designed by people who had looked at the competition to figure out what worked and failed. And I say that comfortably because AIC made a lot of stupid choices which should have been easily-learned-from. And we sold AIC back in the day. heh