Airside economisers offer real advantages since adiabatic cooling can be used to boost the free cooling hours and reduce mechanical cooling loads and the CO2 footprint, says Iain McNeil
Improvements in IT manufacturing and technology have allowed consultants and designers to expand the environmental envelope in data centres without compromising reliability. Consultants taking advantage of new guidelines like ASHRAE TC 9.1 are turning to fresh air free cooling economisers.

An airside economiser allows large volumes of fresh air to be drawn into a data centre whenever the outdoor conditions are favourable. The outside air is distributed into the server rooms, and then warmed by the servers then extracted and expelled outside.

This shouldn't be confused with water-based free cooling economisers which use heat exchangers to cool a fluid-based cooling system which cools the internal air. Airside economisers provide an attractive design since they eliminate efficiency losses using heat exchangers in water-based economisers.

Airside economisers also have a further advantage - adiabatic cooling can be used to boost the free cooling hours and reduce any mechanical cooling load and CO2 footprint.Adiabatic cooling essentially involves the evaporation of water to produce a corresponding reduction in temperature. Energy to change water from its liquid state to a vapour state is taken directly from the air thus cooling it. Every litre of water evaporated provides 680 Watts of cooling from as little as 3 Watts power. On hot dry days the adiabatic system cools and humidifies the intake air therefore extending the free cooling hours of the airside economiser. Control limits are set to avoid the humidity level increasing beyond the recommended environmental envelope. This type of unit is referred to as direct evaporative cooling (DEC).

Additional savings can be achieved by adding a secondary adiabatic cooler and heat exchanger. Located in the exhaust section, the adiabatic cooler significantly reduces the return temperature on to the heat exchanger which in turn pre-cools the fresh air intake. Since the adiabatic cooler is located within the exhaust section there is no restriction on the humidity level. This type of unit is referred to as indirect evaporative cooling (IEC).

The psychometric chart (right) illustrates the concept of airside economiser with assisted adiabatic cooling using ASHRAE recommended envelope and 23 deg C/60 per cent rH (relative humidity) set point.

The area in blue represents free cooling from the airside economiser. The green area represents the additional benefit using DEC and the yellow area
represents the benefit from IEC. The conditions requiring purely mechanical cooling are reduced to the red area.

A high pressure spray adiabatic system - such as Carel's humiFog - is one of the most efficient technologies, using as little as 2.5W for 650W of cooling power. Typical energy savings using DEC are from 80 to 95 per cent and 30 to 93 per cent for IEC versus traditional mechanical cooling using a chiller.

We must consider the water consumption of the adiabatic unit as well. The carbon dioxide footprint to produce 1kW of cooling using adiabatic is 0.75g CO2 for the water and 2.5g CO2 for the electric, making a total of 3.25g CO2. Compared to a chiller with coefficient of performance (CoP) of 3.0, the carbon footprint will be around 150g CO2. This represents a 97 per cent reduction in carbon footprint using adiabatic cooling.

The above assumes 500g CO2/ kWh for electric and 0.5g CO2/l for water. These figures will, of course, vary depending on specific conditions such as design, location, equipment, CoP etc.

The adiabatic cooling system has one further advantage. During the winter, when the specific humidity is very low, the adiabatic system provides humidification control to meet minimum rH level set by ASHRAE etc, without the requirement for a steam humidifier which uses 750W per litre of water compared to 5W per litre typical for a high pressure spray system.

Not long ago, data centres had a Power Usage Effectiveness (PUE) of around 2.0, meaning that only half of their total power is used in IT with the same again used on services. Changes in equipment such as electrically commutated (ec) fans, electronic expansion valves and low power lighting etc has helped reduce PUE levels to around 1.4-1.6.

The use of fresh air cooling with adiabatic has resulted in data centres claiming PUEs of 1.1-1.2, meaning only 10 to 20 per cent additional power is used for services.

Not all applications will be able to take advantage of airside economisers with assisted adiabatic cooling. However, adiabatic-assisted cooling using fresh air economisers can provide reliable and stable environmental conditions for the lowest energy output, capital and operating costs. These systems offer the lowest environmental impact yet still provide effective operation with low maintenance.

// The author is sales manager - Humidification & Evaporative Cooling Systems at Carel UK //