The data center humidity conundrum

There are several measures for data center humidity, including relative humidity and dew point temperature, and knowing which ones to use can extend hardware life.

Humidity -- too much of it makes us miserable in the summer, too little dries our skin in winter, and it has shocking effects when we walk across a carpet. Our computer hardware doesn’t like these extremes any more than we do.

So what’s the right amount of data center humidity? How do we measure it? There was a time when everyone knew, or at least thought they knew, the answer -- 45% to 55%. But times have changed, and we are forced to think about not only humidity level, but also measurement and control.

Obviously, we don’t want water in neither the liquid or solid state in our data center. But without proper control, both states are possible, including ice, which can form on air conditioner coils and disrupt the cooling if settings are incorrect. We also know that with too little humidity, we can create static electricity that can discharge and ruin sensitive micro-circuits in computers. So it’s clear that we need some moisture in the air, but it must be just the right amount. Years ago, when data centers were kept pretty uniform in temperature and electricity was cheap, achieving perfect humidity wasn’t difficult. Today, it’s a different story, with the wide variations in temperature and high energy costs.

What’s wrong with ‘relative humidity’
could hold. As the name implies, RH has to be “relative” to something, and that “something” is air temperature.

Fifty percent RH means the air contains half the moisture it is capable of holding at that particular temperature, but therein lays the problem. Warm air can hold more moisture than cool air, which is why the air around us is drier in winter and can feel clammy and oppressive in summer. Humidity has usually been measured at the return air intakes of air conditioners, where the air is supposed to be warmer, and therefore holds more moisture. The result: 95 degrees Fahrenheit air at 50% RH is pretty moist, and can only be cooled down to 73 degrees Fahrenheit before it reaches 100% RH, becomes saturated and condenses into water.

If we are able to achieve a 95 degree Fahrenheit return air temperature, which is the approximate goal for modern, high-density data center designs, and we set the humidity at 50% RH for the return air stream, we would want to maintain the supply air temperature to our equipment at above 75 degrees Fahrenheit to avoid condensation.

Conversely, if your supply air temperature is as low as 55 degrees Fahrenheit, which is still too common in many data centers, we’d want to keep the return air RH setting nearer to 40% to avoid condensation. It starts to become clear that because return air temperatures vary so widely in most data centers, RH depends not only on where you measure it, but also on the actual temperatures that you are operating. In short, specifying a single RH number, or even a range, doesn’t really mean much anymore.

So what do I measure?
Humidity should now be measured as “dew point temperature,” according to ASHRAE TC 9.9’s Thermal Guidelines for Data Processing Environments, second edition. Another revision is expected to be published soon, and it may further expand the temperature and humidity envelope for certain equipment classifications.

To understand dew point temperature, first think about the dew you see on the lawn or your car windows on a spring morning after the temperature has dropped overnight. Dew point (DP) is simply the temperature at which a certain amount of vapor will condense into water. This is “absolute humidity,” so the measured number will be the same everywhere in the data center, regardless of actual temperature.

Most computer room air conditioners sold in the last ten years can be set to measure humidity this way, so it’s not all that special to do. And if we know the DP temperature, we also know that we don’t want the temperature anywhere in our data center to reach that number, because we don’t want moisture condensing into water on our equipment. In fact, most of the special units available today for high-density cooling are designed to maintain their temperatures around 4 degrees Fahrenheit above DP so that condensation doesn’t occur, and you don’t need to have condensate drain pipes from the units to get rid of it.

What numbers do I use?
Again, ASHRAE TC9.9 has set standards for DP. Data center humidity should be maintained between 41.9 degrees Fahrenheit DP (5.5 degrees Celsius) and 59 degrees Fahrenheit DP (15 degrees Celsius). And for reasons we won’t go into here, it should also never exceed 60% RH, regardless of temperature. Now, we have a measurement that is meaningful in every situation that makes technical sense and is also easy to utilize. Another nice aspect is that we don’t have to run humidifiers in every air conditioner to accomplish this number. We can put humidifiers in only a few units, or can even run a separate humidification system in the data center. Humidity will be measured and controlled the same way everywhere, and we’ll save a little money on both the air conditioners and the electricity bill.

What happens if humidity is too high?
If you’re down south in the summer, have a lot of air leakage into your building when outdoor humidity is high, or are using air side economizers (will be discussed in a future article), you might bring more moisture into your data center than you want, resulting in possible condensation on or inside equipment in the cold aisles. DP sensors will detect this and will tell the Computer Room Air Conditioners (CRACs) to cool the air inside them to below the DP. This causes the moisture to condense on the cooling coils and harmlessly drip off into the condensate drains.

Remember, most high-density cooling systems, such as in-row units and rear-door coolers, automatically maintain their temperatures above the DP, so they won’t help reduce the room humidity at all. Base cooling using conventional CRACs is still needed with most of these special systems. But it’s very important that the air actually discharged from the CRACs is not below the DP until the humidity inside the whole room is reduced to a proper and stable level. Otherwise, moisture may condense on computing hardware.

Historically, regulating air temperature for DP has been accomplished by using “reheat” on CRACs. Reheat brings the discharged air temperature back up to normal to remove the moisture after it’s been over-cooled. But reheat is one of the biggest energy wasters ever invented. It’s expensive to cool down air, but to over-cool it just to heat it back up again is something to be avoided whenever possible. If you can’t avoid bringing in large quantities of outside air into a humid environment, the best thing to do is to de-humidify it before it enters the data center, which can be done with desiccant dehumidifiers and/or pre-cooling. Using reheat in the CRAC units is highly frowned upon in modern data center design.

It’s more realistic, and more energy efficient, to measure and control humidity in today’s data centers using DP temperature rather than RH. The wide range of air temperatures encountered in today’s data centers make RH numbers relatively meaningless, even within the same facility. DP is an “absolute humidity” measurement that can be used even in many older installations. It is stable throughout the facility and can ensure that temperatures do not cause condensation anywhere in the space.

Robert McFarlane
is a principal in charge of data center design for the international consulting firm Shen Milsom Wilke. McFarlane has spent more than 30 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professionals program, is a data center power and cooling expert, is widely published and speaks at many industry seminars.

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