First off free cooling isn't free it would better described as reduced cooling. My data centers were located in Minnesota so we were able to take advantage of the outside temperatures for a good part of the year.

Two of our sites utilized CRAC units with compressors to cool raised floor space. We set our wet cooling towers to make 48 degree condenser water when the outside temperature reached 40 degrees or colder. Each of our CRAC units had a temperature sensor monitoring the condenser water so we set it to turn off the compressors in each CRAC unit at 48 degrees. This would shut off the compressors and divert the colder condenser to a free cooling coil in the unit. This way all we had were the fans running in each unit. The compressors draw the majority of the power load in each CRAC unit so we were able to take advantage of our wonderful Minnesota weather for at least 5 month per year, saving a fair amount of money typically consumed by cooling.

One issue we had to deal with was icing on our cooling towers when the weather got down to or below zero. We found by reversing the direction of the fans on our cooling towers we were able to keep them from freezing up. We had to monitor them during the coldest part of winter, and we had multiple cooling towers with redundancy factored in so we would only reverse one tower at a time. Typically after an hour they would pretty much clear the ice in the reversed mode. We were able to do this with our facility management system with just a few key strokes. By doing this we were able to cut cooling costs considerably and greatly extend the life of our compressors in the CRAC units.

Ken Koty,