We all know that in the world we live in, water exists in 3 forms – liquid, gas and solid. Water vapor refers to the gas form of water and it is produced from evaporation or boiling of liquid water. Liquid water starts to boil when its vapor pressure reaches its surrounding pressure. 
Below is a graph plotting the relationship of water vapor pressure versus temperature. Note that at the normal boiling point of 100°C, the vapor pressure equals the standard atmospheric pressure of 760 Torr or 101.325 kPa (approximately 7.5 Torr per kPa), and that is where evaporative cooling takes place (water turning into gas form takes a part of the heat with it). Anyway, enough of boring details…
So, what is the significance of water vapor in the context of data centers? Well, we first need to understand two terms – relative humidity and dew point.
- Relative Humidity (RH): The term humidity refers to the amount of water vapor in the air. Usually, when we refer to humidity, we would normally refer to relative humidity. Relative humidity (RH) represents the actual amount of water vapor in the air (i.e. the partial pressure of water vapor in the air) relative to the maximum amount of water vapor the air can hold at a given temperature (i.e. the saturated vapor pressure of water at a given temperature). This ratio of pressures is expressed as a percentage. The saturated vapor pressure of water (in Torr) at a given temperature is illustrated by the graph above. So, if RH is 0%, it means there is no presence of water vapor in the air. If RH is 100%, it means that the air is holding all the water vapor it could possibly can at the given temperature.
- Dew point: The term dewpoint is another important definition that needs to be understood. Dew point is basically a temperature threshold. When the surrounding air is cooled below this temperature, the water vapor will leave the air and appears as liquid water, i.e. condensation occurs. This occurs as the air will not be able to hold at much water vapor in its new lower temperature and the excess water vapor leaves the air as liquid.
Therefore, it is important to understand the relationship of the air temperature and the ability to hold more water vapor. Typically, the higher the air temperature, the more water vapor it will be able to hold. And the reverse is true too.
ASHRAE recommends that RH be maintained within the range of 45% to 55% at the air intake point of IT equipment in data centers.
Someone once asked me about the effects of humidity towards cooling. Actually, the reverse would be more appropriate as the cooling strategy and inlet temperature influences humidity. And what are the consequences of excessive levels of RH or too low RH in the data center?
When the RH levels are too low, there is a higher risk of static electrical discharge. This is contributed by the low humidity cool air circulating through the data center, moving across ungrounded insulated surface, building up static electricity. Research shows that as RH increases, the insulating layer of water molecules that forms on surfaces reduces the occurrence of static electrical discharges.
Most equipment manufacturers nowadays do specify that their equipment will operate normally within a 20% to 80% RH (in terms of a non-condensing humidity). This is as long as the inlet temperature remains above the dew point temperature. Data center environments with high RH and where the operators regularly tweak their CRAC’s setpoints to lower down temperature, would have a higher risk of condensation occurring.
Now that we understood what RH is, what dew point is, what is ASHRAE’s recommended range, what are the effects of excessive RH or too low RH, lets understand how we can control RH.
Firstly, when we design and build a data center facility, creating an effective vapor barrier and air tight construction is very important. Vapor barrier allows us to control leakage or introduction or diffusion of water vapor through the building walls, ceiling and ground. Water vapor propagates through walls when the pressure is different on each side of the wall, hence it is common advise to either build an internal service corridor between the data center and the external facade of the building, or to use a semi-permeable material in the construction of the wall. For the ground, it is common to use vapor barrier sealant like Trocellen Polyethlene foam (topped with anti-static or aluminium foil lamination above).
Secondly, when we look at the cooling strategy, we have to consider a few things. Use of rack-based or row-based cooling systems, such as Emerson’s XDH, XDV or XDO units or APC’s in-row units, means that these cooling systems are purely providing cooling function only. They do not provide humidification or dehumidification function. In such a case, where the data center is completely cooled through use of these cooling systems, there will be a higher need to emphasize on the effectiveness of the vapor barrier and control of fresh air introduction. Perhaps a hybrid solution combining precision cooling (CRAC) for ambient to support humidification/dehumidification and spot / closely-coupled cooling with rack-based or row-based systems, and containment, would solve this handicap. The need for humidification/dehumidification function is because no matter how effective the vapor barrier is, other means of vapor introduction do exist, e.g. in the form of human traffic. Designing and placement of a humidity control system will require some form of estimating / calculation of worst case latent load and desired redundancy.
Thirdly, another form of water vapor introduction is when fresh air is introduced into the room. Typically, this is not a code, but introducing fresh air do help to ensure a good indoor air quality (not stale) and establish a slight positive pressure in the room for cleanliness concerns. Fresh air is also introduced when air side economizer is adopted. However, means to control the quantity of fresh air, or to monitor the humidity levels of the fresh air, should be in place to minimize or limit unnecessary latent load introduction.
Controlling water vapor in data centers to maintain an optimum operating condition as per ASHRAE’s recommendation is both an art and science. It is only through this control that the data center can reach a stable equilibrium of reliability and availability.
Tags: Air Management, Data Center, humidity, protection, RH, strategy, Temperature