ASHREA’s latest guidelines for data centres (2008/2011) set an “allowable” upper level for humidity at 80% RH with a maximum dew point of 17°C. The recommended levels are 60% RH and 15°C dew point. As previously explained, “humidity” is the amount of water vapour (moisture) in the air and “relative humidity” (RH) is the amount of moisture in the air as a percentage of the maximum amount that the air could hold at the same temperature and pressure. For data centres the pressure is atmospheric pressure and the amount of moisture the air can hold is for all practical purposes directly related to its temperature. Excess humidity is a problem in summer because the warm outside air can hold high levels of moisture which, when the temperature is lowered in the data centre, result in excessive levels of relative humidity. For example, air at 27°C can hold a maximum of 0.0228kg of water vapour per kg of dry air: at this condition it is said to be “saturated” and it has an RH of 100%. If it contains only 0.0114kg of water vapour, i.e. half of what it could hold, it has an RH of 50%. This is a fairly typical RH in the UK when the temperature is at 27°C. When this air cooled to a cold aisle temperature of 22 °C the RH will be 68% and the dew-point will be 16°C so we exceed our ASHRAE recommended limits but are within the allowable guidelines. However if the air had instead been at 27°C and 60% RH, then when cooled to 22°C it would be at 82% RH and 18.7°C dew point so would exceed both the recommended and allowable guidelines. Even if the air is not cooled, as for example with a direct fresh air cooling system, the dew point is over 17°C so it still does not meet ASHREA requirements. Such conditions do occur in the UK and in fact there are over 500 hours a year when de-humidification is required, even with latest more relaxed guidelines.
The above analysis is based on there being no moisture added to the outside air. In highly populated spaces, e.g. offices, the moisture added by the occupants (from breathing and perspiration) can be significant and needs to be taken into account. In a data centre however the occupancy level is never going to be high enough to be an issue and so in most cases there will be no moisture added within the space and the only de-humidification load is due to outside air. Unless you have a direct fresh air cooling system then the amount of outside air coming into the data centre will be small, never-the-less some moisture will need to be removed or the RH will gradually creep up until it exceeds the allowable limits.
If you do have a direct fresh air cooling system, unless it is of the evaporative type, it will not be adding any moisture to the air but the volumes of air involved mean the de-humidification load can outweigh the benefit of the lower dry-bulb temperature. There is a point where it is more economic to shut-down the fresh air system even though the outside air is cooler than the hot aisle. Theoretically the optimum point to shut down the fresh air free cooling and go to re-circulation is when the enthalpy (i.e. the total energy from both temperature and moisture content) of the outside air exceeds that of the hot aisle. In practice however, because of the 17°C maximum allowable dew-point, it is best to shut-down the fresh air any time its dew-point exceeds this. If you have direct fresh air and it is of the evaporative (adiabatic) cooling type then the situation is very different indeed. The evaporation upon which these systems rely is adding significantly to the humidity of the air coming into the data centre. Taking our previous example of outside air at 27°C and 50% RH, if this is cooled to 22°C using evaporative cooling the resultant dew-point will be 18.2°C, exceeding the allowable guidelines.
Some argue that the number of hours a year that these conditions exist means it not something to be concerned about but, as previously stated, with any system there are over 500 hours a year when de-humidification is required and the use of evaporative direct free cooling increases this to over 600 hours a year. Of course it is possible to allow the cold aisle temperature to rise and thus reduce its RH but consideration does have to be given to the hot aisle which is likely to be at least 10°C warmer than the cold aisle. In any case there will still be 300 plus hours when, irrespective of the cold aisle temperature, the dew-point will exceed 17°C. Others argue that the humidity guidelines can be ignored but in my view these upper levels really are the maximum that should be considered. As anyone offering data centre design services will know, the “allowable” levels are an upper limit and the ASHRAE 2011 guidelines actually state: “the typical mission-critical data center must give the utmost consideration of the trade-offs before they opt to operate with relative humidity that exceeds 60%”. The reasons given are that above this there is a dramatic increase the incidence of corrosion of copper components.
You also ignore the high dew-point limit at your peril. If a piece of equipment is brought into a data centre and its temperature is below the dew-point then condensation will occur (a phenomenon familiar to anyone who wears glasses if they have been outside in the cold for any length of time and then walk indoors into a humid space). It is quite feasible that equipment could be stored in a warehouse where even in warm weather the temperature might be 18°C and so it can be seen that the maximum 17°C dew-point is a sensible precaution. Direct fresh air free cooling, with or without evaporative cooling, is a good way of improving PUE and reducing energy bills but it will add to the humidification and de-humidification loads. Indirect free cooling systems don’t add to the loads but nor do they eliminate them entirely and these systems, as well as data centres with no free cooling, will still need a means of adding moisture in cold weather and removing it in warm humid weather. Energy efficient methods of humidifying are readily available as described in part 2 but as yet de-humidification is still an energy consuming process. “Dessicant de-humidifiers”, which dry the air by absorbing moisture using a drying agent such as silica-gel, are available but while they use minimal energy in drying the air, once the silica-gel is saturated this then has to be dried. To do this on a continuous basis requires heat which requires energy. Also this is an extra piece of equipment to be purchased and maintained whereas the usual method of de-humidification uses the air conditioning units which you will most likely have anyway (even with a free cooling system we would generally recommend a mechanical cooling system for back up). In my view a means of de-humidifying the air is essential for any data centre and this is best done using mechanical cooling. The best way to minimise the running costs is to minimise the amount of fresh air coming in when that fresh air is above the desired moisture content. If you have any questions relating to data centre humidification or any other aspect of mechanical design and operation please feel free to email firstname.lastname@example.org
Writer bio: My name is Frank Wilman and I was one of the founders of Future-tech back in 1982. I worked on my first “data centre” in 1973 and have design and installed almost every cooling solution available over the years. My passion is mechanical cooling systems as they give us an opportunity to be truly innovative and continually push the boundaries of resilience and efficiency