It’s a well-known fact that data center cooling systems are some of server farms' biggest energy consumers. One of the most effective ways to reduce cooling energy costs has been to maximize the use of naturally cool outside air, a method called airside economization, or free cooling.
Climate imposes obvious time and place limits on the use of free cooling. You cannot use it just anywhere, and you cannot use it all the time.
But a group within ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) has for many years worked on expanding those limits. Until recently, the efforts of ASHRAE’s TC 9.9 (the technical committee focused on data centers) have been to show that if operators accept temperature levels on the data center floor that are just a little bit higher than customary, they will not only get energy savings from using less mechanical-cooling capacity but also be able to use free cooling in more places and for longer periods of time – all while keeping temperatures within normal warranty by IT equipment manufacturers.
After having raised the inlet-air temperature in its recommendations three times (once in 2004, once in 2008, and again in 2011), the committee is now working on expanding the relative-humidity part of the envelope. The “relative” part is very important here, because humidity and temperature are inseparable.
TC 9.9 develops its recommendations together with IT manufacturers. There are three types of envelopes in its data center cooling guidelines:
- Recommended: Environmental conditions that ensure high reliability while operating in the most energy efficient manner.
- Allowable: A wider envelope than the recommended range. Manufacturers test their equipment within this envelope to make sure it functions under the conditions it describes.
- Prolonged Exposure: These are operating conditions well outside the recommended range, encompassing extremes of the allowable range. While short-term “excursions” into this envelope may be acceptable, anything more than a short excursion can reduce equipment reliability and longevity.
There is a trend among operators overall to dial data center cooling systems to increase operating temperature and humidity while staying within the ASHRAE envelopes. It lets them save on both operating and capital costs while keeping expensive IT gear under warranty.
Impact of Lower Relative Humidity Minimal
TC 9.9 recently completed a study together with the University of Missouri on the impact of low humidity on electrostatic discharge (ESD) on the data center floor. The study showed that with ESD-rated floors, the difference in the effect of discharge on IT equipment failure at 8 percent relative humidity versus 25 percent relative humidity is minimal, Don Beaty, the committee’s co-founder, said.
Based on the study’s results, ASHRAE is planning to expand its relative-humidity recommendations for data centers, which will ultimately further expand the amount of free-cooling opportunities around the world.
The committee is planning to publish results of the study later this year and then work on updating its official recommendations. “The point of expanding the relative-humidity envelope is to enable more free-cooling hours per year in cold climates without the need for artificial humidification,” Beaty said.
In Data Center Thermodynamics Everything is Linked
The relationship between temperature and humidity is quite simple: cooler air is dryer. The danger of dry air for electronics lies in static electricity, which accumulates more the less humid the room is and causes ESD.
TC 9.9’s envelope is in the form of a psychrometric chart, since it’s more complex than simply a temperature range and a humidity range. There are different classes of equipment, and recommendations are different for each classes. The way the committee expanded temperature ranges last time, in fact, was by adding more classes of equipment that is rated to perform in hotter conditions.
The humidity part itself is really multiple parameters: relative humidity, minimum dew point, and maximum dew point.
ESD Shoes Not Terribly Important
The study compared the presence of ESD in less humid conditions in data centers with ESD floors (ESD stands for electrostatic discharge). These are floors made of material that conducts and dissipates static electricity.
The researchers tested the combination of ESD floors with ESD shoes (also frequently used in data centers) and ESD floors without ESD shoes, and found that presence or absence of the shoes also did not have a significant impact on hardware failure rates. This should come as a relief to many. “People aren’t that disciplined about using ESD shoes,” Beaty said.