Condair managing director, Ian Eitzen, explains how humidifiers can be used in data centres to provide low cost evaporative cooling.
Adiabatic 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 is that air stream is cooled, whilst simultaneously increasing humidity. 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 commonplace. 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 setpoint, 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, providing low energy, high capacity cooling.
Depending on the condition of the air stream, 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.
The outdoor air is drawn into the AHU, run through the heat recovery (HR) device before being exhausted. Warm air is extracted from the data halls, run through the HR and its temperature reduced by the cooler outdoor airstream, without mixing directly with it. This cooled air is then recirculated back into the data halls to reduce the internal temperature.
The effectiveness of this strategy obviously depends on the temperature of the outdoor air being below the data hall’s target condition. To maximise the potential cooling from this system, and expand its use into times when the outdoor temperature is too high, a cold water humidifier can be applied to the external airstream prior to the HR system. This reduces its temperature and increases the cooling capacity of the system.
As global humidification and evaporative cooling specialists, Condair frequently become involves with a third type of evaporative cooling strategy, indirect exhaust air evaporative cooling.
This method sees a cold water humidifier cooling the warm air that is extracted from the indoor atmosphere. This cooled exhaust air is then fed through an HR unit before being ejected outside.
A fresh air supply brings air in from outside and runs it through the HR unit, where it is cooled by a few degrees by the cooler exhaust airstream. This incoming fresh air can then be introduced to the data halls or cooled further mechanically, if required, before being supplied to the room.
This strategy can reduce the load on mechanical chillers, thus lowering a building’s overall cooling costs. It is also interesting, as it can be useful irrespective of the outdoor climate.
Both the direct and indirect cooling strategies previously outlined rely somewhat on certain outdoor conditions being available, with regards humidity and temperature.
An exhaust air indirect evaporative cooling strategy uses the air being returned from the indoor environment, which is mostly consistent regardless of the ambient outdoor climate. We often see this type of cooling strategy used in hot and humid regions across Asia, as well as parts of Europe, particularly Germany, where the drive to reduce a building’s energy consumption is paramount.
Wherever the data centre or whatever AHU cooling strategy is being employed, there is nearly always an evaporative cooling method that can enhance performance or reduce energy consumption. The data centre industry’s insatiable need for low cost, high capacity cooling has driven effective collaboration between data centre designers, AHU design teams and humidifier manufacturers, such as Condair, to produce more and more innovative solutions. Long may it continue!