Managing the Expanding Temperature Envelope
Fred Stack is vice president of marketing for Liebert Precision Cooling at Emerson Network Power. Stack is responsible for new product development roadmaps that reflect evolving market demands and incorporate new technology.FRED STACK
The recent expansion of data center temperature limits has raised various questions and concerns throughout the industry. Some data center and IT managers have voiced reservations about elevating the overall data center temperature and reducing their outage ride through time while others seek all possible efficiency gains.
As a refresher, the purpose of the ASHRAE-recommended envelope is to give guidance on maintaining high reliability while efficiently operating data centers. The newest ASHRAE recommended environmental range for class one and two data centers is 18 to 27° C (64.4 to 80.6° F) (dry bulb temperature). The allowable envelope which broadens the range to 32° C (90° F) is where IT equipment manufacturers test product in order to verify that the unit will function with no damage for periods of times generally assumed to be counted in hours or a few days for emergency operation.
These higher temperature ranges are being promoted to emphasize to the industry that these are server input temperatures, not cooling unit return temperatures. The temperatures in the cold aisles can be significantly increased from the averages across the industry for a significant increase in the cooling capacity and efficiency of the cooling equipment.
The higher temperature ranges are also being used to increase the number of hours an air or water economizer can be used in a data center. Water economizers have minimal operational concerns, just some maintenance procedures, while air economizers require data center managers recognize the potential impact of energy to humidify dry air along with the potential impact of particulate and gaseous contamination.
One additional caution has been explained in the ASHRAE publications. That is the move from 25° C to 27° C (77° F to 80.6° F) can have a negative impact on the IT equipment’s power dissipation. Most IT manufacturers start to increase air moving device speed around 23° C (73° F) to improve component cooling and offset the increased ambient air temperature. Increases in the server intake temperature from 23° to 27°C will double the server fan energy consumption. A typical server fan consumes 10 percent of the total server energy at low temperatures.
This small change in input temperature to 27°C (80.6°F) will increase the total server energy by 10 percent. A move to 29°C (84.2°F) increases the total server energy by 20 percent. These increases in server energy, while an improvement to your PUE metric, actually negate all the energy savings from the effort. For this reason it is usually recommended to only move above 24°C (75.2°F) when an economizer provides access to “free cooling.”
Ultimately, raising temperatures in data centers should be carried out in a systematic manner to best determine where hot spots may occur. Care should be taken to ensure that data centers are operating efficiently without risking infrastructure shutdown because of high temperatures or contamination.
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