Power in is directly heat out (that mechanical motion of fans twirling just isn't giving more than a rounding error). You can only get so much cooling density.

Power in is directly heat out (that mechanical motion of fans twirling just isn't giving more than a rounding error). You can only get so much cooling density. And loading up a rack can use a signifigant portion of cooling. Its pretty easy now-a-days to get 18kW into a rack, which will require 5 tons of cooling. Ie. enough cooling to handle a 3,000 ft^2 house.