If they had kept the iMac just a little thicker they could have incorporated a huge heat sink with a bigger, slow-spinning fan. That would have made it killer – quiet, cool and powerful.
Where the heat sink is the new iMacs aren't that much thinner. It is mainly the edges that go thinner.
If they weren't so thermodynamically hostile they could do better with the same edge thickness thickness constraints.
A bigger slow-spinning fan was/is possible at the current thickness. An 27"
iMac 2013 teardown pictures
but the 5K model is essentially the same (
5K Teardown )
They could get probably 0.5-1 inch increased diameter fan in there. It would probably be more custom order and cost more, but there is space. There is nothing around the fan on the top and right edge ( as look straight on). Might need to tweak the illuminated Apple logo up 0.25 inch, but that is doable too. (and/or just get rid of the backlight. Need a radio exit. don't need to shine light out the back. )
The problem they have is the Johnny Ive "super clean" OCD restrictions. The outlet vent is only in the back behind the pedestal arm. Most likely this is so you can't see the vents from most angles where arm doesn't hide them. ( "Oh look it is magical it has no vents" ) Similar with the inlet vents ( arrayed on the bottom so no casual viewer will see). Again symmetric OCD drives the bottom edges about as thin as the top two so restricting inlet flow too.
The speaker/air baffles soak up tons of space too. If had pulled air in from the back (instead of lifting it up over and back down again ) that complexity isn't necessary.
The Mac Pro is somewhat built around the "one single fan" sitting on top. The round , cylinder shape flows from just extending the enclosure down from that. Here it appears somewhat stuck in almost "last" in the design. Previous, iMacs had more than one. It is a huge step forward to put the heat sink right next to the the outlet vent, but this is still needs some work. At minimum they should start with the hottest GPU/CPU combo and set the fan specs there. What it looks like they did was set to the mid point of the 27" line up and hoped to cover the rest. There is just wasted space in the current design. (pretty sure could get two fans in there. Add one above/right of the HDD. Needs an outlet. ).
If they weren't trying to drag hot air down that would help too. Mac Pro lets hot rise up and out almost for free ( not just size/diameter as to why the fan is spinning so slow most of the time). The fan doesn't have to fight what hot air naturally wants to do. It also doesn't help to "pre heat" (mix with the heat from the other components with no direct air flow ) the air before it goes to the heat pump radiator.
A huge metal heatsink coupled to uninspired air flow design isn't going to help as much.
