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In data centers, we have maintained relative humidity levels of 45% to 50% for decades, based on the assumption that high humidity is necessary to avoid static discharge problems. That has now changed.
The danger to IT equipment from static generation and discharge in the data center environment is insignificant at 15% relative humidity (RH), and can be dropped to 8% RH with only minor precautions, according to a research study, released in 2014, commissioned by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) technical committee TC 9.9 (Mission Critical Facilities, Data Centers, Technology Spaces and Electronic Equipment). That data center humidity revelation can help you save energy and improve your power usage effectiveness ratio, called PUE.
If you run higher equipment inlet temperatures in your data center, in accordance with the 2008 and 2011 ASHRAE Thermal Guidelines, maintaining high relative humidity requires adding a considerable amount of moisture; warm air holds more water vapor than cool air. Wasting water and energy to humidify the data center unnecessarily is inconsistent with green practices today.
Reduced data center humidity levels
In 2012, ASHRAE TC 9.9 commissioned an independent research project to determine what humidity level is actually necessary to avoid static damage to IT equipment in the data center. The research was done by faculty and graduate students who specialize in electrostatic discharge (ESD) phenomenon at Missouri University of Science and Technology. They performed tests on all types of data center floor surfaces, with people wearing a variety of shoes and walking on the floors in both predefined and random patterns. The full report was published in October 2014, and is 167 pages long.
The study data shows that, if industry-standard data center facility design and operating practices are followed, static occurrences have a very low probability of harming IT hardware down to the lowest level tested: 8% RH. Therefore, data centers in any climate can save energy by not vaporizing large amounts of water for humidification. Those data centers in cool climates, and particularly those that use free cooling, should no longer need to humidify at all for much of the year.
Precautions for the new humidity range
Data center operators should chiefly consider the combination of floor surface and footwear related to ESD. Most data centers with raised access floors use a grounded high-pressure laminate, which is considered static dissipative, for the floor surface. That is generally accepted to mean between 104 to 109 Ohms resistance. Sealed concrete floors or static dissipative floor coverings tend to perform similarly. Shoes that have some level of conductivity provide sufficient dissipation; avoid footwear with polymer-based sole material, as on running or deck shoes.
The low humidity conclusions apply to encased hardware. Regardless of the humidity level, always use a wrist strap when working inside an equipment case or changing components. If you're operating at the new low humidity levels, the wrist strap is mandatory when you remove equipment covers.
RH or dew point?
Why does the new research discuss RH levels, when the ASHRAE Thermal Guidelines recommend controlling moisture via absolute humidity or dew point (DP)? Static discharge phenomena are more easily related to RH than to DP under the test conditions, but the two are not difficult to reconcile. This research will result in a fourth edition of the ASHRAE TC 9.9 Thermal Guidelines book, in which RH limits will be noted, but the recommendations for data center humidity control will still be via DP sensing.
RH is relative to temperature, so the higher the air temperature, the more moisture the air can actually hold at the same RH level. This is why rain occurs when heat and humidity have been high, and the temperature falls as the sun sets. Cooler air holds less moisture, so the water vapor condenses into water droplets. In a data center, we never want the room temperature to reach the dew point temperature. However, since data center temperature varies over a wide range -- particularly in a modern facility with aisle containment -- the location in the room at which RH is actually measured greatly influences how much moisture the cooling system then adds or removes from the air. That directly affects how much energy and water the data center uses. Absolute humidity or DP, on the other hand, is the same throughout the room independent of temperature, so it is a more consistent metric for controlling moisture in a space that has widely differing air temperatures.
Implementing the ASHRAE guidelines
All of the ASHRAE TC 9.9 environmental parameters are based on IT equipment inlet temperature and humidity conditions from the cold aisle. The ASHRAE Thermal Guidelines now recommend a DP range from minus 10 degrees Celsius to 15 degrees Celsius (14 degrees Fahrenheit to 59 degrees Fahrenheit), noncondensing. If a data center facility operates near the high end of the ASHRAE-recommended thermal envelope of 27 degrees Celsius (80.6 degrees Fahrenheit), and humidification levels are in accordance with the new ASHRAE limits for DP temperature, inlet air will never fall below 8% RH. Virtually all computer room air conditioners can be controlled via discharge air temperature and dew point: Simply measure both temperature and DP in the supply air to the cold aisle, and operate air conditioners and humidifiers accordingly.
Conservative guidelines for data center humidity
Saving energy is important, but of even higher priority in most data centers is protecting IT equipment and maintaining reliable operation. Such a large change in recommended practices may seem hard to believe. Even if we're confident that the ASHRAE operating limits are safe, most IT people get nervous about operating close to the extremes. No data center has a completely homogeneous environment, so there will always be some temperature variations within an aisle or from top to bottom of racks. If you're unwilling to drop inlet humidity all the way to 8% RH, start by designing and controlling data center temperature and moisture levels somewhat below the ASHRAE limits -- it will still save significant electrical energy and water.
The following suggested parameters and practices take advantage of the newest ASHRAE guidelines for data center humidity, without pushing the limits. Specific designs and operations can vary:
- Data center floors should be static dissipative and properly grounded, as described earlier in the article.
- People in data centers should wear shoes that are reasonably conductive, which means a resistance of no more than 108 Ohms. Clean-room shoes are not required, but avoid polymer soles. Some data center operators choose to require ESD shoes.
- Set cooling to deliver inlet temperatures to the IT hardware near the upper limit of the ASHRAE thermal envelope. Designing for a more conservative 24 degrees Celsius or 75 degrees Fahrenheit should ensure that no cabinet sees excessively warm inlet air.
- Measure humidity in the cold aisle and set for a conservative DP temperature of minus 4.44 degrees Celsius (24 degrees Fahrenheit), which is equivalent to 15% RH at the inlet air temperature suggested above. As air passes through equipment and picks up heat, its temperature increases, creating a temperature differential (TD) between the cold and hot aisles. A TD of 11 degrees Celsius (20 degrees Fahrenheit) between the cold and hot aisles is fairly common, resulting in a 35 degrees Celsius (95 degrees Fahrenheit) hot aisle. But since RH reduces as temperature rises, if you run at 8% inlet RH, the hot aisle could drop to 4% RH. ASHRAE predicts no problem with this lower hot aisle humidity, but no testing was done below 8% RH. Running higher cold aisle inlet humidity will keep the hot aisle from going below the 8% tested limit, if that's a concern.
- Provide and mandate the use of grounding wrist straps whenever work must be done inside a piece of IT hardware.
The upper humidity limit
If you happen to be in a climate region where high humidity is the norm, this new research won't help much. Be aware of the upper ASHRAE Thermal Guidelines limit of 60% RH, which avoids a completely different type of problem: Airborne particulates and gaseous contaminants can combine with water vapor, resulting in corrosive damage to components within the IT equipment at high humidity.
About the author:
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC, with more than 35 years of experience. An expert in data center power and cooling, he helped pioneer building cable design, is a corresponding member of ASHRAE TC9.9 and a voting member of ASHRAE SPC 90.4 which is developing the new energy efficiency standard for data centers. McFarlane also teaches at Marist College's Institute for Data Center Professionals.
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