Evaporative cooling is a promising, cost-effective part of many data centre cooling strategies. Could it be right for yours?
Before you answer, here’s a quick test. Blow over the top of your hand. Now lick it and blow over it again. Which felt cooler? Even though you added a warm liquid (your saliva) to your skin, it felt cooler when you blew over it. Why is this? It’s an effect known as “evaporative cooling”. When liquid evaporates, it takes in heat from around it, causing a cooling effect.
In hot climates, people have used evaporative cooling in their homes for generations. They hang a wet sheet in a well-ventilated room, and as the water evaporates from the sheet, it cools the ambient temperature of the room.
How evaporative cooling systems work
Evaporative cooling can be very important in maintaining an optimum temperature in data centres as well. A simple setup can create a very low-cost data centre cooling strategy, using a simple system of low-cost fans that draw air through filters, which are kept wet through a falling stream of water. The incoming air causes evaporation of the water, and the resulting cooler air is then blown into the data centre.
The benefits of using these computer room evaporative cooling systems (CRECS) over standard computer room air conditioning (CRAC) units is that they don’t use any costly refrigerants.
Evaporative cooling uses easily-maintained low-pressure fans and water pumps instead of higher-pressure pumps. Because they are based around an open system of water flows, another advantage is that the CRECS is that they do not suffer leaks within themselves.
As shown in the figure above, the CRECS pump water from a reservoir at the bottom of the unit, which soaks filters on the sides. An air fan pulls in warm air from the surrounding environment.
As the air passes through the filters, it is stripped of any particulates and is cooled through the evaporation of the water. The cooled air is then ducted through to the data centre.
When operating data centres with temperatures of up to 26° C, CRECS help data centre managers save on cooling costs. Capital, energy and maintenance costs will all be lower. So what’s not to like?
When not to use evaporative cooling systems
Evaporative cooling is a good solution for many environments, but there are situations where it may not be the best choice.
In some cases, post-evaporative treatment is required to dehumidify the air so that it is suitable for use within the data centre; overly humid air (for data centres, generally above 70% relative humidity) can condense on the surfaces of data centre equipment causing problems.
Over-drying the air, however, to below 30% relative humidity can also cause problems such as increased static electricity and growth of metallic dendrites on circuit boards and metal cases.
These can lead to short circuits in the long term. Therefore, ensuring that the air’s humidity is kept within best practice parameters is crucial while using evaporative cooling techniques. Some systems use heat transfer systems so that the humid air from the CRECS unit is not used directly in the data centre itself. Instead, data centre designers sometimes place a contained air system with high-surface area heat exchangers between the CRECS and the data centre air.
In areas where the standard environmental air is already quite cold, evaporative cooling is also not a good choice. In this case, simply using cold air passed directly through particulate filters may be sufficient, providing “free air cooling” and limiting energy costs to running the fans.
For areas where the environmental humidity is high, evaporative cooling can be problematic as well. Evaporative cooling works best where the air is dry -- compare how domestic washing dries on a hot, dry day compared to on a hot, humid day. Where humidity is high, it is just too hard for any more moisture to be absorbed by the air, which slows down evaporation.
Will evaporative cooling work for UK data centres?
So, for data centres in places such as the Middle East and other hot, dry areas, evaporative cooling can make economic sense. But can data centre managers in the UK use evaporative cooling techniques? I personally know of a 500 kW data centre in Manchester -- one of the wettest parts of the UK with relatively low temperatures and often high humidity -- where evaporative cooling is used 100% of the time.
However, an organisation may be wary of using 100% evaporative cooling in its data centre, either because of local atmospheric conditions at certain times of the year or merely concern about the immaturity of the approach. In this case, designing a hybrid system that mixes evaporative cooling with other data centre cooling techniques may provide substantial energy savings.
For example, a bypass system makes free air cooling a good choice when the temperature drops below a certain level, in which case the wet filters can be bypassed and just particulate filters used. The water pump can be turned off, saving energy.
If the ambient air temperature is too cold, then you can mix warm exit air from the data centre with the incoming air to raise the temperature. If the humidity is too high, mixing in some of the exit air from the data centre may be enough to bring it back within the target relative humidity levels. But, when the ambient air temperature and humidity levels are optimal, it’s best to use evaporative cooling.
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Those approaches should deal with the weather across the UK in the majority of cases. However, if “the majority of cases” is not good enough for your organisation, then you can install a small CRAC unit to work alongside the CRECS as required. But only as required.
Hosepipe bans, droughts and evaporative cooling
The final issue for the UK has to do with the prolonged drought that is now hitting the south and east of the country. An open CREC system will use up water as it is lost through evaporation, and an organisation’s data centre team could find itself being questioned by both its local water authority and its own management if it uses too much water -- particularly where water is being metered.
Therefore, to minimise water loss, I recommend using evaporated water that has been recovered by a condensing tower. Here, a portion of the water is pumped through a heat exchanger system where the exit air from the data centre is vented. By lowering the temperature of this air, it carries less moisture, which condenses and trickles back to the reservoir for re-use.
Overall, evaporative cooling systems are now at a point where they create good savings on capital, maintenance, operating and energy costs and provide a greener alternative than CRAC units. As a complete data centre cooling system, or even as part of one, CRECS can be a core part of a new and emerging approach to data centre cooling.
Clive Longbottom is a service director at UK analyst Quocirca Ltd. and a contributor to SearchVirtualDataCentre.co.uk.