The winter of 2013/14 has been a rough one in the northern U.S. The Great Lakes are 90% frozen, major northern cities are at 50+ days below 0°F (and we’re not done yet!). Even the south had a taste of it, with a couple of ‘Polar Vortexes’ moving to the southern U.S. states.
Data Center cooling systems must be designed to perform no matter what happens with the outside air conditions. Weather changes and the cooling systems must adjust and react accordingly. Knowing that there are no perfect designs, this can become a retro-commissioning opportunity for data center cooling systems during extreme weather events.
This happened at T5@Atlanta during the January Polar Vortex. As you may recall, a large swath of arctic air descended on the U.S., driving temperatures in Atlanta down to 3°F (-16°C). Given that T5@Atlanta is outfitted with a pair of Plate and Frame Heat Exchangers, this of course should make for a great day of Free Cooling. However, our concern was the Cooling Towers freezing, which as it turns out, wasn’t the problem we actually experienced.
In the Control Room, the Critical Facilities Manager was at the helm observing the Alerton Building Automation System status in real time. The automation system was managing the Condenser Water loop and the Cooling Tower so that no freezing in the Tower was occurring. However, we began noticing a Chiller being commanded “ON”, then shut down. A few minutes later, it happened again.
T5 purposely designed the Critical Mechanical Systems to take full advantage of Free Cooling using Waterside Economization. This means when the outdoor air conditions are both cool enough and dry enough, the mechanical data center cooling system can pre-cool the returning water to the Chiller, reducing the work (and thus the energy) the Chiller must do to remove the heat load. As temperatures drop further, the Chiller can shut down completely. Those of us in our industry know this is an important component of an energy efficient, low PUE data center.
That chilly morning in January was certainly was one of those days. So why then was a Chiller being commanded to “Start”? As it turns out, the Building Automation System was programmed for what was thought to be an Extreme Design Day for Atlanta. This means the BAS was programmed to treat any temperature less than 5°F as being bad, and when that occurs, the built-in safety is to start a chiller. Given that both Outdoor Air Temperature sensors were reading less than 5°F, the BAS saw them both as being bad and hence, the Chiller “Start” command was generated.
This extreme weather event highlighted an unexpected vulnerability, one that provided a retro-commissioning opportunity that couldn’t be simulated otherwise.