Our atmosphere (the air around us) is made up of gasses consisting of nitrogen (78%), oxygen (21%), carbon dioxide (0.3%) and water in the form of vapor (humidity). The amount of water in the atmosphere is measured as a Relative Humidity (RH)
Data Center air should contain the proper amount of water vapor to maximize the availability of computing equipment. Air containing too much or too little water vapor can cause failures. At the outer extremities of RH, we can see condensation forming on equipment, or at the other end, static electricity buildup and discharge. Neither of these phenomena are useful in maximizing equipment availability.
Condensation happens at the dew point (always expressed as a temperature). It’s the temperature at which water vapor leaves the air and appears on objects as liquid water. For example, when a cold drink is left outside on a warm summer day droplets of liquid water form on the can or glass. This is because the cold drink cools the surrounding air to a temperature lower than the air’s dew point. We must avoid reaching dew point in a data center.
Low humidity levels (sub 20% RH) and movement of humans and equipment though a data center or even airflow itself can cause a charging effect that can lead to static electrical buildup and potentially electrical discharge (10,000+ volt sparks). There is some scientific arguments about exactly how this works but everyone agrees that low RH aids static buildup and moderate RH prevents or mitigates the discharge.
There are two main approaches that can be taken to maintaining an appropriate level of Relative Humidity and to prevent condensation or static discharge. The first approach involves turning the Data Center into a sealed box with only the minimum flow of fresh air from outside. Ken Brill of the Uptime Institute offers the following advice:
Eliminate dehumidification and humidification. Computer rooms should be machine rooms with no people. As a result, outside air coming into the computer room should be severely restricted. If this is done, there should be no need for de-humidification or humidification.
As a simple diagnostic, have someone do an inventory of the dehumidification or humidification indicator lights on every cooling unit and record the physical location of each unit relative to each other. This survey will take less than 30 minutes in a typical computer room and is likely to indicate humidification on one cooling unit with the immediately adjacent unit simultaneously de-humidifying.
Up to 30% of the units can be in this state, which wastes enormous amounts of energy for no benefit and significant water leakage risk. (Since this water is inside pipes, the magnitude of the flows are invisible without special measurement. If this water was allowed to accumulate in the computer room over the course of a year, it would fill the computer room to the height of about eight feet). Proper engineering plus calibration of cooling unit sensors can totally eliminate this problem, which costs energy and produces no benefit other than for the contractor who provides cooling unit service.
Ken goes on to say that these initiatives will significantly improve both cooling reliability and stability, and they save energy. If you have a 20th Century sealed box data center this is extremely good advice. Note that humidifiers are quite simple devices that create water vapor by introducing steam into the air flow, they use significant amounts of energy.
This approach removes any possibility of using fresh air cooling in the data center as a part of the design specifies the virtual elimination of outside air. Dry side economizers will still provide a useful adjunct to refrigeration plant but using fresh air is not possible.
Note that we can reach the dew point in two circumstances: If Air has a RH of 100% (rain forest conditions) we will see condensation or alternatively very high relative humidity mixed with cold surfaces such as those caused by refrigeration. If we can eliminate cold surfaces and avoid siting our data center in a rain forest perhaps we can avoid this problem?
What causes low RH and it’s associated static discharge problem? The very process of cooling data center air removes large amounts of water vapor resulting in low relative humidity levels. This occurs when warm data center air is drawn through the cooling coil in a computer room air conditioner or air handler. Most cooling coils are maintained at a constant temperature of 43-48°F. This is usually below the dew point of the air in the IT environment so just as in the example of water droplets forming on the cold drink can, water droplets can form on the cold cooling coil. Very large volumes of air (over 160 cubic feet per minute for each kW of computing equipment) flow through the cooling coil at high velocity. In instances where the air stays in contact with the cooling coil long enough to be cooled below its dew point, liquid water called condensate forms on the cooling coil.
Both high and low RH problems can be eliminated if we can remove refrigeration from the data center.
So the alternative to making the data center a closed box? Open the windows, use fresh air cooling, buy only equipment where the vendor will support higher operating temperatures during the summer months. Use air side economizers.