18 juni 2015

Blueprint for the perfect data centre: Cooling system

written by Marco Alink

A few years ago, I was involved in the construction of a brand new data centre. This is quite a luxury for a data centre manager as I was able to furnish the building entirely in accordance with the latest technologies. At the same time it was also a challenge, however, as the blueprint for a perfect data centre just does not exist. I would like to share with you my decisions and considerations. In this part, I will be talking about the cooling system.

On paper, it may seem that the choice of a particular cooling system depends mainly on which cooling method you opt for. However, the room in which the servers are installed is just as important. Server equipment usually works best at a temperature of around 20 to 22 degrees. A small room heats up more quickly, forcing the cooling system to work harder. In addition, there needs to be a clearance of sixty centimetres between the server rack and the ceiling. A lower ceiling is not really suitable as the cold air cannot be discharged adequately, causing the fans on the servers to work extraordinarily hard. It is also important to cool from below and it is therefore advisable to raise servers from the ground. After all, warm air rises.

Outdoor air cooling

In my view, the best place to locate a data centre is in an open room with a high ceiling. This will allow the heat to rise quite considerably. If you want to cool your data centre using outdoor air, it is even theoretically possible to place server equipment outside under a shelter. Intel has even experimented with this type of set-up in the past on a site with favourable weather conditions (little rain and a relatively stable outside temperature). However, this presents other challenges, such as the risk of fire and theft. Furthermore, the air outside isn't always clean. Small insects can fly around and pollen can be carried by air.

Adiabatic cooling

The classic method for cooling a data centre is still compressor cooling, in which a coolant – usually water – is used to remove warm air and add cold air. The water is channelled outside to allow the air to cool it down and if this is not sufficiently adequate, extra cooling is provided by compressor coolers. The water is then used to transport cold energy which is then allowed to expand in the data centre room to be cooled. Although this is a good system, it requires expensive compressors and pumps that all use energy and bring with them the risk of malfunctioning.

Ample consideration is therefore being given to alternative cooling techniques that do not rely on as much technology. After all, this translates into lower energy bills and a better price for the customer. Oil cooling, for example, is becoming increasingly popular. The data centre equipment is submerged in a bath of mineral oil. Mineral oil has a greater heat absorption rate than air, which means that heat can more easily be dispelled. Another type of cooling system that has been around for a while longer is adiabatic cooling, also known as evaporative cooling. Air is cooled down by sprinkling outdoor coolers with water, causing evaporation to occur.

Plate heat exchanger

The main disadvantage of adiabatic cooling is that it uses a lot of water, which also entails certain costs. A system that is currently attracting more interest is indirect adiabatic cooling. The temperature of the air in the data centre is lowered by means of outside air and a plate heat exchanger. This means that the data centre air is cooled indirectly by outside air when the temperature outdoors is low. When the temperature outside rises, the plate heat exchanger is moistened. The water on the plate in the heat exchanger then evaporates, drawing away the heat from the data centre air. It is a great system as the outside air is not brought into the data centre. However, this type of cooling is still in its infancy. Although we did consider using this system for our new data centre, we ultimately decided against it. However, this method remains interesting enough to keep a close eye on developments.

Intelligent free cooling

In the end, we decided to use yet another cooling method that is known as 'intelligent free cooling'. In this system, we use sensors to monitor the outside air temperature so that we can make use of it whenever possible. When the outside temperature isn't ideal, we use the classic compressor cooling method, using water as a coolant. An important point to mention is that we use the heat that is removed to heat our offices adjacent to the data centre. This is an excellent way to further reduce our operating costs and provide a complete multi-storey building with affordable heating.

The advantage of this system is that we can use the equipment in our data centre without the need for modifications. We don't have the disadvantage that you do with adiabatic cooling of having to add water to the system. Intelligent free cooling is a combination of tried-and-tested technologies, which gives us the reassurance of using a reliable system. Furthermore, it is a highly sustainable cooling system as it uses outside air and also reuses residual heat to heat up our offices.

Did we make the right choice? We think so. The point is that there are a large number of different types of cooling systems that all depend on different things. This makes them difficult to compare and so you will have to place various scenarios side by side. A low PUE (Power Usage Effectiveness) is not the only important consideration. You will also need to make calculations based on partial loads. These are aspects that a commercial data centre must certainly take into consideration.

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