DATA CENTRES
Evaporative cooling strategies
Dave Marshall-George, UK sales manager at Condair, looks at the different ways humidifiers can be used in data centres to provide low cost evaporative cooling.
A
diabatic evaporation is a process whereby water changes from a liquid to the vapour phase. The heat required to evaporate the water is taken from the air
stream. The result being that the air stream is cooled, whilst simultaneously increasing the humidity as well. Adiabatic evaporation can be considered air humidification, as the moisture content of the air stream increases in the process. This process, often referred to as evaporative cooling, can be designed specifically to provide low energy cooling of the air stream.
The use of evaporative cooling in data centre
environmental control strategies is now relatively common place. The nature of evaporative cooling perfectly suits the requirements of a data centre’s climatic operating window. Unlike a regular office temperature of around 23°C, a data centre’s upper temperature condition can be as high as 28°C. When the outdoor climate conditions are suitable, data centre cooling systems may use the outdoor air rather than mechanical cooling in their data halls. During the warmer months, when the outdoor air conditions are too warm to successfully lower the indoor temperature to the desired set point, evaporative cooling can be used to provide additional low energy cooling capacity. A litre of cold water, when evaporated into the air, provides 0.68kW of adiabatic cooling. A single cold water evaporative cooler can supply around 1,000 litres of moisture while operating on less than 1kW of electricity, so they offer great potential for low energy, high capacity cooling. Depending on the condition of the air stream,
32 November 2020
evaporative cooling can offer a temperature reduction of up to 12°C. More and more air handling units are being developed with an evaporative cooling element to take advantage of this low cost, low energy form of temperature control.
Two main strategies have emerged for using cold water humidifiers to provide evaporative cooling in AHUs for data centres; direct evaporative cooling and indirect evaporative cooling.
Direct evaporative cooling uses the humidifier to spray or evaporate water into the filtered, incoming air stream. This strategy is used to increase the cooling effect of free air cooling systems, whereby a high volume of outdoor air is supplied to the data halls and an equivalent amount of warm air exhausted. Direct evaporative cooling is ideal for use in dry climates, where the dry outdoor air offers the greatest potential for absorbing moisture from the humidifier, and therefore providing cooling to the incoming fresh air. A consideration for employing this type of cooling strategy is the level of criticality placed on the introduction of outside air to the indoor environment. For critical data centre operations, a cooling system that is dependent on the introduction of outdoor air may not be acceptable, given the potential risk from external pollutants, such as a nearby fire. An alternative form of evaporative cooling that avoids this risk is indirect evaporative cooling. This strategy uses a heat exchanger, such as a crossflow or thermal wheel, to transfer the thermal energy from an external airstream to a circulating internal airstream.
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