Dehumidification in data centers when using CW units at high-temperature levels
Most users and planners are now aware that temperature levels when cooling IT equipment in data centers have changed dramatically in recent years.
The main reason for the adjustment of air temperatures is ASHRAE recommendation TC 9.9 2011, which recommends air inlet temperatures to IT equipment in a range from 18 °C up to a maximum of 27 °C. Adding an average temperature difference as air flows through the IT equipment of 10-15 K, this produces return air flow temperatures back to the A/C unit in the range from 28 °C to 42 °C (see blog article "Delta T"). The actually most important "side-effect" of this recommendation, however, is utilization of so-called "free cooling" – that is, cooling of IT equipment as far as possible without the energy-intensive use of compression cooling (see blog article "Free cooling").
Based on their good scalability and their comparatively simple hydraulics, large data centers mostly use chilled water-cooled precision A/C units (so-called CW units), which require centralized chilled water production (see also blog article "Standby management"). To improve the efficiency of the chiller, and to utilize free cooling at comparatively high outside temperatures, chilled water systems are also being run at ever higher water temperatures. A positive side-effect of high water temperatures in conjunction with high air temperatures is that the purely sensible cooling targeted in data centers (in order to avoid cost-intensive humidification) is assured.
In summary: higher air temperatures + higher water temperatures = avoidance of dehumidification in normal cooling operation and improved utilization of free cooling.
To return briefly to the ASHRAE recommendations: The allowed range of relative humidity for the IT equipment is very generously spanned between 20 % and 80 %.
All these factors together mean, in principle and in theory, that nowadays there is no need for dehumidification or humidification in normal cooling of a data center. Sadly, this is another area in which theory and practice differ. There are requirements regarding ESD (electrostatic discharge) from IT equipment; the data center staff is in the room; the room is not 100% air-tight; and humidity is introduced from the outside; doors are opened and closed, etc. The possibly resultant and required humidification is comparatively easy to realize (humidifier in the A/C unit or in the room). The possibly required dehumidification is difficult however.
In the past (when the equipment was operated at lower air and water temperatures), the dehumidification with CW-units was done in the following way:
The chilled water control valve is fully opened in dehumidification mode, increasing the water volume flow through the cooling coil. This increases the total cooling capacity of the unit, and the unit's water outlet temperature falls. The temperature difference between the air and water side increases, and the resultant drop below the dew point causes the required dehumidification. In some cases the speed of the EC fans (if installed) is also reduced in order to boost the effect.
When operating CW units at high air and water temperatures, the problem then arises that the drop below dew point necessary for dehumidification can no longer be achieved, because the general temperature level is simply too high.
So what can be done to provide dehumidification?
The technically most practical way is to use one or more so-called "dual-fluid" units in a GCW design. A dual-fluid unit is a combination of a direct expansion (DX) and a CW unit (see GCW refrigeration system). In the GCW design the unit's refrigerant circuit is closed. The heat is dissipated by way of a water-cooled plate condenser, which in this case is simply connected to the existing chilled water system. In normal cooling operation, the unit's CW circuit is additionally used; for dehumidification, however, the switch is made to DX mode. Dehumidification and the drop below dew point are very much easier to achieve in DX mode because the evaporation temperature is normally lower than the water temperature level, or can be more easily brought to the required dehumidification level by way of controls in the refrigerant circuit (expansion valve). The numbers and/or cooling capacities of these units then depend on the expected dehumidification capacity and the size of the data center.
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