Networks Europe Nov-Dec 2017 | Page 34

34 COOLING Direct free cooling With direct free cooling, filtered outside air is fed directly into the data centre if the CRAC system determines that it is at the right temperature. This means that the compressors do not have to operate as often, saving electrical energy. However, in urban areas air quality might not be good enough and is often full of particles, which can be hostile to modern IT equipment. Therefore, direct free cooling should only be used where filtering can control the quality of outdoor air. Also, if the outdoor air is too cold, a certain proportion of warm air from the data centre must be mixed in with it to supply controlled, tempered air to the IT equipment. Figure 4 - Indirect free cooling with CRAC and dry cooler Figure 5 - Indirect free cooling with CRAH and chiller Humidification also has to be controlled and methods to humidify or dehumidify air can also be expensive and complex, with large ductwork systems often required. In addition, large openings in the building fabric are needed to bring in the outdoor air, plus an equally large ductwork system and fabric opening for the exhaust air. These openings and ductwork bring a security risk to the data centre that has to be considered and in the event of an external situation such as a fire, the system must have the ability to operate independently of the air inlet. Indirect free cooling With indirect free cooling, no outside air enters the data centre and CRAC systems can be configured to suit requirements. However, indirect free cooling is theoretically less efficient than direct free cooling, as at least one heat transfer must always take place between the air in the data centre and the outdoor air. Single stage indirect free cooling is based on an air/water heat exchanger placed on the hot water return of the chiller. Outside air is blown across the heat exchange and helps removes the heat before it gets to the chiller. Therefore, with a return water temperature of 20°C, if the outside temperature is 19°C or less it starts reducing the activity of the chiller and reduces running costs. Two stage indirect free cooling systems have a considerably smaller footprint than their single stage counterparts, but are less efficient as they function with two heat transfers. In the first heat exchanger, the heat from the air in the data centre is transferred to a liquid, normally glycol. This is then pumped to the outside in relatively small pipes, where heat from the data centre is rejected to the outdoor air via a second heat exchanger. Indirect dynamic free cooling To further improve the efficiency of indirect free cooling, dynamic technology controls the mode in accordance with the current heat load in the data centre, and so increases the time in free cooling mode. Moreover, it has another operating mode – extended free cooling – that further lengthens the operational time and drastically reduces operating costs by cutting energy intensive compressor cooling to a minimum. Liquid asset Air contains a certain amount of water and the total amount depends on the air temperature and barometric pressure. As water absorbs into the air, an adiabatic process takes place – this means the temperature of the air decreases, while the energy content of the air remains unchanged. By using adiabatic assistance in direct free cooling systems the air that enters the data centre is cooled before entering. Indirect free cooling and adiabatic assistance is more conducive however, as the adiabatically assisted outdoor air never enters the data centre. In single stage indirect free cooling systems, adiabatic assistance reduces the temperature of the outdoor air entering the air/air heat exchanger and reduces the amount of mechanical cooling required to satisfy the supply air temperature to the IT equipment. In two-stage indirect free cooling systems, the warm outdoor air can be cooled by adiabatic assistance before entering the dry cooler to either increase the hours of usable free cooling, or ext end the mixed mode at the upper limit to reduce the use of mechanical cooling. Evaluating needs and solutions As the density of installed equipment in the data centre has risen, so too has the amount of heat generated. While being able to fit more kit into a smaller space is generally considered a good thing, the need to control temperature has led to the growing use of free cooling. It should be remembered that no free cooling system is perfect though, and all have limitations. It's therefore important to evaluate what kind of free cooling is best suited to the application on a case-by-case basis. n www.networkseuropemagazine.com