Time for real data! Here's a nice line of modern server fans: https://www.delta-fan.com/newspost/ingress-protection-ip55-ip56-durable-reliable-and-weatherproo

Time for real data! Here's a nice line of modern server fans: https://www.delta-fan.com/newspost/ingress-protection-ip55-ip56-durable-reliable-and-weatherproof-7.html https://www.delta-fan.com/newspost/ingress-protection-ip55-i... 40mm, 12.6W gets 21.56cfm (at zero pressure) or 3.416 inH2O (at zero flow). That's 1.7 cfm/W. 140mm, 46.8W gets 282.31cfm or 2.033 inH2O. That's 6.03 cfm/W, over 3x better. Now, I'm being a bit unfair: I ignored pressure. I don't see fan curves, but a vaguely credible figure of merit if we're just trying to estimate efficiency is to multiple the flow and the pressure and divide by power. The little fan gets 0.68 and the big fan gets 1.44. That's still quite a lot better. (It does not mean that fan exceeds 100% efficiency. It's not delivering 282.31cfm at 2.033 in H2O.) But, interestingly, these fans produce almost the same velocity. If the moderately lower available pressure from the large fan isn't a problem (which it shouldn't be -- we're talking about a regime where there is too much TDP per rack, so it's beneficial to spread things out, thus reducing resistance to airflow), these fans really can substitute for each other in the sense that covering the same area with big fans produces about the same airflow as covering the area with small fans. Oh, and the big fan is louder, but not louder than 10 small fans! So I stand by my claim: if you are cooling the contents of your rack by filling it with metal tubes and blowing air through the tubes using fans, use big fans, which you can achieve by using taller units. And 2U isn't enough for that 140mm fan.