Rear-door heat exchanger

A retrofit path rather than a design: leave the servers air-cooled and hang a water coil in the back door, so the air leaving the rack is already at room temperature and the room never gets hot. It requires no change to the node, which is exactly the point, and it is how an existing air-cooled hall buys another generation of density without being rebuilt. The compromise is that it still moves heat with air first, so it stops well short of what direct-to-chip achieves.