When operating your data center, often there are backup measures to keep things running during a power outage. Generators may be the first thing that comes to mind, as they bring power back during an outage, but along with that the cooling systems also need operate. As the heat is rejected through the mechanical systems, the last item that rejects the heat to the atmosphere is often the cooling towers. To operate continuously, they need to maintain enough water through a loss of make-up water, as 1% or more of the water is evaporated or lost to drift conditions. In addition to this, the blow down cycle may keep operating during a power outage, which may double that water loss.
Ok, so perhaps 2% losses don’t seem so bad… but added up over a day or more of operation the water levels in the towers will run low. If they run too low they don’t work… and therefore no more cooling for your data center.
Would the typical make-up water flow be sufficient? It just may be – but sometimes when a location loses power for an extended period, the utility companies may not be able to provide an adequate flow. Sometimes this happens right away, sometimes the pressure and/or flow begins to taper down.
Or perhaps there are other issues with the water supply that were unforeseen. It could even happen without a power outage, such as upstream construction accidently taking down your only supply pipe from the utility. What’s the plan for that?
Many cooling towers have an integral sump that collects the cooler water at the bottom of the tower. This sump depth has a minimum to feed the condenser water pumps, but not necessarily too deep, as this adds material cost and weight. Other towers might be field-built, with a separate sump beneath the tower, and yet more might be a combination of a factory tower with a field built sump.
All cooling tower sumps provide enough water for the chiller and cooling tower to operate for some time without make-up water, but the volume of the sump will dictate how long. Increasing the water that can be stored in the sump not only increases the amount of operating time from hours to days, it also helps to create a more uniform water temperature entering the chillers. Another means would be to provide a separate water source, such as a tank, on the site.
How big would a tank or how deep should the sumps be? A good plan would be to match what the generators can do, so let’s go back to the generators and see what they are doing. For example, if they are supporting 50% of the IT power load (and the other power needs to support that load) for 2 days, then the cooling plant should also be able to support 50% of the IT cooling load (and the other cooling needs to support that load) for 2 days. By stepping backward through the load needed and what would be expected of the cooling towers, a total water volume can be estimated during a peak period.