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Issue 11, Aug/Sept





It is expensive if we do not adapt. There is plenty of information on free cooling systems; for example, data centers in Iceland, data centers on sea barges, or using the cooling source from a lake. The problem is that in many cases the free cooling has been applied to legacy designs, with chilled water supplied at 7°C to 10°C, using hot aisle/cold aisle, etc.

Data centers in 75% of the world’s cities have the potential to achieve low PUEs (1.2- 1.3). For costs similar to those of legacy data centers, this can be achieved following these points: • Supply air to IT equipment at or over 25°C; • Physically separate cold and hot air streams using containment systems; and

• Use free cooling combining adiabatic cooling with indirect air-side free cooling. Direct air and water side can also be used.

Adiabatic cooling is achieved when water is evaporated due to differences of vapour pressure (water vapour/air humidity). Typically for an average relative humidity, this type of cooling can achieve water temperatures 5°C to 10°C below the outdoor air temperature (dry bulb). This type of cooling can improve the viability of free cooling systems.

Due to high density problems and lack of air management, manufacturers have developed novel water-cooled rack solutions. The ratio of fl uid density multiplied by specifi c heat of water is more than 4,000 times higher than air. This means that under similar conditions, to transport the same cooling energy an air duct of 2m x 1m can be replaced by a chilled water pipe of diameter 25mm. However, the energy to transport this cooling via fan or pump will be similar.

For free cooling (FC) systems, there will always be a difference of temperature between the air supplied to the IT equipment and the outdoor conditions (dry bulb temperature or wet bulb temperature for adiabatic cooling systems). This is called ‘approach’. Typical approach values are: • FC direct air side – 0.5°C (fan gain only); • FC indirect air side – 4°C; • FC water side, good design – 7°C; and • FC water side, legacy design – 12°C.

This indicates we will have fewer hours of free cooling if we use water-side rather than air- side free cooling solutions. The above options should be analysed when considering free cooling.

Good air management will reduce air bypass and recirculation, which in turn will reduce the air temperatures supplied to the IT equipment. IT equipment fans start to ramp up when air enters above 27°C. Assuming the IT equipment fans were not running at higher speeds, there is no energy saved on the cooling systems by simply lowering the air temperatures. Energy will be saved once the speed of the fans (CRAC, CRAH, AHUs) is reduced and the chilled water (and air supply) temperature set points are increased. As from here, there will be savings of fan energy and cooling system compressor energy, and the number of free cooling hours will also increase. Air management is an enabler to save energy.


The essence of this myth is that the focus of certifi cation is external to the organisation and not for those who operate the data center. There are other concerns. The data center industry has adopted PUE or its inverse – DCiE (data center infrastructure effi ciency) – as the best way to measure its effi ciency. However, there are many building certifi cation schemes (BREEAM, LEED, HQE) that were created for commercial offi ce buildings, with much lower power densities and higher occupants than data centers. These schemes work on a points system that is not necessarily appropriately weighted for data centers and has led to many perverse incentives.

Unless the data center is small compared with the building, a separate energy assessment for the data center should be carried out. 

22ºC 56 27ºC 27ºC

Ing Dr Robert Tozer MSc MBA PhD CEng MCIBSE MASHRAE, Mission Critical Facilities Consultant, London RobertTozer@ Drawings: by Camilo Diaz

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