Thursday, 15 February 2018

Humidity Control + Energy Saving: is there a solution?

ASHRAE has been working for many years on guidelines that allow a wider tolerance for temperature and humidity. Consequently, the need to humidify has decreased, making the value of humidification equipment less significant in the overall HVAC system of the DC.

However, what if maintaining the humidity also reduced the cooling demand?

One of the most effective solutions involves the use of adiabatic humidifiers: adding moisture to an air stream, absorbing heat in the air, increasing humidity and decreasing the temperature for very little energy consumption (c.1kW electrical power for 70kW cooling)

This evaporative cooling is increasingly used in new generation data centres in which the design conditions are close to the limits suggested by ASHRAE: this is made possible by careful design of air flows and good separation between the air entering the racks and the exhaust air (layout with “hot aisles and cold aisles”).

The higher operating temperature and humidity allow the use of outside air for ‘free cooling’ (e.g. when below 25°C), and when the outside air is hotter and drier, evaporative cooling can be adopted, increasing humidity up to 60% and higher while bringing the temperature down to acceptable values, simply through the evaporation of water.

There are several different adiabatic humidification technologies available, from “wetted media” to washers and spray systems: the principle underlying all of these devices is to maximise the contact surface area between air and water, so as to ensure effective evaporation and perfect absorption in the humidified air stream. The choice of the system depends on numerous factors, ranging from available space to required efficiency and the need for modulation.

In general, the solution needs to be evaluated in terms of TCO (Total Cost of Ownership) throughout the system’s working life, also taking into consideration its resilience in terms of continuous operation as well as water consumption, which in many areas may be a critical factor: indeed, many data centres, together with the classic PUE (Power Usage Effectiveness) for energy consumption also monitor WUE as regards water consumption.

Recently, atomisation systems have become quite popular; these use a system of nozzles and high pressure pumps to create minute droplets of water, thus ensuring optimum absorption. These systems can be controlled by inverters to modulate atomised water production and respond to different load conditions. Other benefits of these systems include very low air pressure drop, no recirculation (and consequently a high level of hygiene, something that unfortunately is often neglected) and the possibility to use one pumping unit with two separate distribution systems; one for summer (evaporative cooling) and one for humidification in winter, meaning significant flexibility - even with vertical air flows.

The effectiveness of such systems depends significantly on local temperature-humidity conditions and in much of Europe both free cooling and evaporative cooling can be exploited for most of the year, to the extent where some data centres are designed to use mechanical cooling as an emergency backup system only.

Guest blog written by Enrico Boscaro, Group Marketing Manager and William Littlewood, Business Development manager, Carel

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