Some good points here, and worth looking into. My only reluctance to an evaporative cooler would be that we'd need to have some sort of ongoing water source onsite. I'm not sure if that's an option at some of these sites we're looking at. Literal 40-160 acre flat parcels in the desert. I suppose drilling a well at some point could be a "phase two" option during a later build out.
If the intention is to somewhat cool the room, does the cooled-air system need to match the exact CFM of the miner fans? I more or less thought of it as a way to bring the ambient temperature of the room down a few degrees. I'm sure my lack of an engineering degree is shining right about now... but I more or less thought of this as "injecting" some cooler air into the situation, not necessarily providing ALL of the air that's being exchanged in the room? Or is that dumb?
Yeah, I saw articles on systems like you are describing. It's trading off efficiency for simplicity. It's just a lot more efficient to transfer the heat from a liquid to the surroundings than from air, and liquid holds way more heat per unit volume so you don't have to move it as fast. Keep in mind that you'll need to push around 200cfm of air per miner through your pipe, so you'll need very large pipes and an air system that can handle very big static pressures. So for the scale you're talking about, I think the liquid one with a heat exchanger is probably going to end up simpler in the end. But in my opinion, neither will be as inexpensive or as simple as an evaporative cooling system.
But I'd love to see the results of a test. Just set up a single air loop with a single miner, and bury 50 or 100 feet of 6 or 8" pvc.