I’d like to add something. Nothing I’ve ever read really delves into the actual cooling mechanism within the computer. The air flows over the components which are cooled by convection. There is a convective heat transfer coefficient between the components and the air.
So with no filter the components get coated by dust etc and that coefficient decreases hence the cooling decreases. Also the fans blades get coated and move the air less efficiently (slower). So the overall cooling efficiency is further diminished.
With an external filter the air is restricted some but it is clean air, the components do not get coated and the cooling efficiency decreases somewhat because there might be a tad less air flow but the cooling coefficient between the components and the air stays high and probably unchanged and the fan efficiency also stays high and so, therefore, does the overall cooling efficiency
As stated in my last post I’ve seen a year or more (with filthers) where the internal dust is negligible. So I conclude that an external filther does more good than harm or computing without a filther is potentially more harmful than with.
Next the internal fans especially in the power supply have struts and/or protective grills that are anything but aerodynamically clean. They interfere with the flow and create noise. I’ve removed those grills and struts (where possible) with no worse problem than the noise is decreased. I’ve also put rheostats on the fans to slow them down to a lower noise level. Ahhhhhhh.
Neither external filthers, removal of fan struts nor slowing the fans down has ever created a heating problem. I did monitor the heating.
I now have an iMac all-in-one and have only an external filther. I don’t dare open that puppy.