There have been some significant changes to the data center cooling industry in the last couple years. One of the most prominent changes has been the TC 9.9 Committee’s Thermal Guidelines for Data Processing Environments that was released in 2011.
These new guidelines expanded the operating envelope within the data center. Now upon first glance it may not look like these guidelines have changed significantly but they have actually expanded the allowed server entering air temperatures such that we can now safely use innovative technologies to achieve greater efficiency in the data center. This has led to some interesting new technologies being introduced to the white space such as indirect evaporative cooling and chiller assisted cooling systems.
Before we look at these new technologies and how they bring greater efficiency when used with containment, let’s look further at the TC 9.9 guidelines.
One of the details I’d like to point out is that the actual limit on entering air temperature to the server is now raised to 80.6°F. This is significantly higher than the limit in the previous versions of the thermal guidelines. This increase in supply air temperature allows us to raise the return air temperature to the cooling equipment significantly. To get an idea of the impact we can expect to see as a result of these changes, we can look at a traditional return air temperature of 75°F dry bulb and make a straight forward comparison. If we increase the return air temperature by 20°F to 95°F, we see significant increases in individual unit efficiency and capacity and still have our supply air temperatures well within the ASHRAE guidelines.
Psychrometric Chart From ASHRAE
By increasing the return air temperature to 95°F we see about a 66% increase in capacity for a single piece of equipment. This increase is very significant when evaluating the required capital investment in the cooling infrastructure for a mission critical space.
If we look at the same scenario but operate the equipment in a different manner (where we’re actually just reducing the CFM to get back to our original capacity we had at 75°F return air), we can expect to save about 49% in total fan power due to the increased capacity of the cooling coil which is driven by the greater temperature difference between entering air and entering chilled water.
Another way to think about it, if we return the air flow rate to the original value, we can expect a much higher return water temperature on a chilled water unit and up to 22% increase in the chiller efficiency, this is due solely to the higher return water temperature at the chiller.
The last benefit that these guidelines provide us is that by utilizing the higher return air temperatures to the CRAC/CRAH, we’re able to use air or waterside economizers for many more hours during the year than we would be able to at the lower return temperatures. This would decrease the sites power consumption while providing and improved annualized PUE.
Tune in for our next blog entry where we’ll examine how we get these higher return air temperatures by utilizing strategies to eliminate air mixing.