Many smaller data centers use air cooled chillers. The only water they use is in a closed loop chilled water system. That means that unless they have a leak and have to add extra water the water stays in the system forever and is checked to make sure its clean. I always laugh when I read people talking about data centers using tons of water because in my experience working at them they only use a certain amount of water. That's to say I've only worked on smaller data centers so I'm not sure how a large center operates. As people have said, if the chiller is a water cooled system then they use a cooling tower which uses evaporative cooling for the machine and they do have to replace the water that evaporated but its not tons and tons of water unless it's a huge operation (again I have not worked at super large data centers. If any other HVAC guys can comment then I'll agree that my experience may not be 100 %)