Temperature is the hottest topic (pardon the pun) when it comes to maintaining the proper environmental conditions in a data center—particularly in the context of energy consumption and cost—but humidity is also important. But with ASHRAE’s recently expanded allowable and recommended ranges for temperature and humidity, is water vapor still a concern?
Humidity: What You Can’t See Can Hurt Your Data Center
Liquid water is generally a bad thing in your data center, but in the air, it’s something you need in the right proportions. Too much humidity can lead to condensation, which can in turn cause corrosion or—in sufficient amounts—electrical shorts. But too little humidity promotes buildup of electrostatic charge, and discharges of static electricity can damage or destroy sensitive electronics.
Part of the solution is data center measurement and monitoring. Installing humidity sensors (along with temperature sensors) provides information that enables maintaining proper environmental conditions. That’s clear enough. What’s not so clear is what exactly you should measure. Traditionally, relative humidity (RH) has been the metric of choice, with 45% to 55% RH being the espoused ideal range. But inherent difficulties with RH mean it is being used less frequently as a metric. Furthermore, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) recently expanded its recommended and allowable temperature ranges—as well as its humidity ranges. Given, then, that the traditional view of maintaining 45% to 55% RH in the data center seems to be in flux, what’s the best approach?
Measuring Humidity: Absolute or Relative?
The difficulty with attempting to maintain a relative humidity in the data center is that RH is dependent on temperature: it is a measure of the percentage of water vapor content that air can hold at a given temperature. But because warmer air holds more water, an RH of, say, 50% at 65°F would be significantly lower than 50% at 80°F. The difficulty in the context of data centers is that these facilities deal with both warm air and cool air, which are ideally kept as separate as possible. Cool air flows into a server inlet, is heated and then is ejected as warm exhaust. The water content of this air hasn’t changed during this process (barring, of course, condensation), but the relative humidity of the exhaust is lower than that of the cool air at the server inlet.
An alternative to relative humidity is absolute humidity, which can be expressed as, for instance, the mass of water per mass of dry air. A more familiar measure of absolute humidity is the dew point: the temperature at which water in the air begins condensing (or the temperature at which the RH is 100% for a given air mass). The advantage of measuring and monitoring dew point temperatures instead of relative humidity is that the dew point at the server inlet is the same as that at the server exhaust outlet. A sensor can thus be placed at the server inlet (or the outlet) without the need to worry about getting a humidity measurement at the outlet as well.
Naturally, then, measuring absolute humidity enables companies to have less of a “moving target” with regard to maintaining a specific humidity in their data centers. Furthermore, the recently updated ASHRAE temperature and humidity ranges show a clear recognition of absolute humidity as being important—not just relative humidity.
Expanded ASHRAE Guidelines
A recent whitepaper from The Green Grid (“Updated Air-Side Free Cooling Maps: The Impact of ASHRAE 2011 Allowable Ranges”) discusses the new ASHRAE recommended and allowable ranges in the context of free cooling. Over the recommended temperature range (18°C to 27°C, or about 64°F to 81°F), a portion (temperatures below about 23°C or about 73°F) has a corresponding maximum humidity of 50% (RH). The other portion has a maximum absolute humidity of about 0.011 (measured in grams of water per grams of dry air). The previous (2004) ASHRAE recommended range maintained the traditional RH values of 40% to 55%.
Interestingly, however, the new ASHRAE guidelines still maintain the same overall humidity range—although the recommended humidity varies to some extent with temperature, the absolute humidity should never fall below about 0.006 (same units as above), nor should it ever exceed 0.011. For certain allowable ASHRAE ranges (which should only be used when the data center’s IT equipment can withstand them), the absolute and relative humidity can go beyond the recommended range. Not only do these expanded guidelines give data center operators more leeway with their cooling infrastructure, they also enable more use of free cooling (air-side or water-side economization)—in many areas of the world, throughout the entire year.
Should You Worry About Humidity?
The wider ASHRAE guidelines mean that facilities do not need to sweat humidity as much as they did when 40% to 55% RH was the rule. Furthermore, a growing recognition of absolute humidity (such as dew point) as a better metric means less measurement variation from one side the server (the inlet) to the other (the exhaust outlet). Of course, humidity is still a concern: too much or too little can still cause problems for your IT equipment.
Maintaining a certain humidity range when using mechanical cooling methods often requires addition of (or removal of) water from the air, but this generally involves a fairly closed system. When free cooling is used extensively, the natural variations in temperature and humidity of outside air can complicate the situation, simply by making humidifiers work harder, for instance. Just “opening the windows” of your facility sounds like a great cooling option (it’s certainly cheap), but doing so on particularly rainy or dry days can wreak havoc on your equipment, unless you take steps to regulate water content in the air.
So, should you still sweat humidity in the data center? In some sense, yes: too much or too little water vapor in the air is problematic—nothing about that situation has changed. But as the expanded ASHRAE recommended and allowable operating ranges indicate, many companies and manufacturers are recognizing that the old, tight limits on temperature and humidity are not as necessary as once thought. Thus, although maintaining proper humidity is still critical, it’s not as difficult as it once was (thought to be).
Perhaps the more important industry trend to note is the shift from relative humidity toward absolute humidity as the superior metric. Absolute humidity is just that—absolute, in the sense that it is a measurement of the actual water content of air. Relative humidity measures the percent capacity of air at a given temperature, which can be problematic, because data centers deal with cool air and warm air simultaneously. The current challenge for most facilities is selecting the right temperature and humidity range—whether the recommended range or an allowable range—to maximize the potential for free cooling while still adequately protecting equipment from heat and condensation. So—stay cool and dry, but keep an eye on your energy usage while you’re at it.
Photo courtesy of Sam Bald
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