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June 11, 2012

Liquid cooling: decisions, types, approaches.

Nicole Hemsoth

Despite world economic turmoil of the recent severe European crisis, the demand for high performance computing services stays on the rise. Companies and institutions progressively see computational power as a source of competitive advantage and in many cases as the only optimal solution for many scientific and business challenges, from high energy physics to big data. This trend has brought an unprecedented rise in demand for high computational power. This is posing some sound energy and thermal management challenges.

The energy problem in data centres is two sided. On the one hand, data centres have a problem of energy consumption, which enlarges bills also in countries where the cost of energy is relatively low. On the other hand, there is a problem of peak power demand, so, in other words, a problem of availability.  Megawatt installations are not so uncommon anymore, meaning that a request of power similar to the one that has traditionally belonged to the heavy industry sector is becoming almost the norm, in some occasions requiring special arrangement for power systems and electrical lines.

Thermal management is exacerbated by another trend: density. In many cases, rack powers of 30 kW are well beyond what legacy air cooling can handle. In the modern HPC, the high powers in play often leave few options but resorting to some form of water cooling.

Liquid cooling has many advantages, which derive from the much higher heat capacity per unit volume of water compared to air (we are talking about a factor of 3500 times higher). Liquid cooling implies higher densities, energy savings and the possibility to reuse the thermal energy that the water extracts from the IT equipment. Some additional advantages can be found in terms of lower noise levels, less vibrations and close control of electronics temperatures.

The best approach in deciding what type of cooling to implement is to consider alternatives in relation to technical and business needs, the type of air and liquid cooling system available within budget and a series of variables that play an important role in the decision: the desired density versus space availability, new construction versus existing construction, the proximity to natural sources of cold water like rivers and lakes, the local climate, the cost of energy and the thermal energy recovery possibilities.

For instance, high performance high density requirements may leave little choice than liquid cooling to efficiently manage the extraction of the heat from the supercomputers. While, if the data centre has an economizer and the climate is best suited to air-side economizers (mild temperatures and moderate humidity) than an air cooled DC may have more sense. 

Deciding the cooling system may also take in consideration the type of water cooling to be installed. There are solutions that simply create an extension of the existing liquid-cooling loop closer to the IT equipment like in the case of liquid cooled racks (liquid cooled door, closed-liquid rack). In other solutions, in-row units are embedded in rows of data center cabinets, providing localized air distribution and management. Alternatively, overhead cooling suspends from the ceiling complements a hot aisle/cold aisle arrangement. As hot air rises from the hot aisle, the overhead cooler captures it, conditions it, and releases it back to the cold aisle

More effective cooling can be reached when the liquid is brought in the near proximity of the electronic components like in the case of submerged cooling, spray cooling or direct (embedded) cooling.

In the first case, the electronic components are immerged in oil and water which is kept in circulation through small pumps. In the second, the water is vaporized and tiny drops of water fall on the electronics evaporating immediately and taking away a lot of heat. In the latter, water is taken through metal plates or micro pipes to direct contact with processors, memory and other components.

Another distinction is normally made between hot and cold liquid cooling. The definition of hot liquid cooling can be vary. In Eurotech we think that hot liquid cooling means the technology capable of using a liquid (e.g. water) with a temperature above the server room temperature.  We also accept that, pushing the bar up in terms of max coolant temperature, hot liquid cooling may take place when the water is hot enough to allow thermal energy reuse.

In any kind of liquid cooling, one aspect that needs careful attention is the risk of leaking. This is an issue because the electronic components are upgraded on a routine basis resulting in many systems with the need to disconnect and reconnect the liquid carrying lines. Also, there is the need to consider whether cooling with water brings on all of its potential. For instance, resorting to chillers to cool the water will allow density, but limit the energy savings that are maximized with hot water cooling technologies, thanks to air conditioning avoidance. However, it is no news that new powerful processors with TDP of 150W may require coolant temperatures lower that the ones guaranteed by free cooling in warm climates. An additional downside of increasing water temperature may be the higher operating temperature of electronic components.  This risk needs to be balanced by the advantages coming from levelling temperatures on the mother board and avoiding hot spots at data center level.

Eurotech approach

Eurotech has developed liquid cooling systems for more than 7 years and it was the first in the market to offer a hot liquid cooling with high serviceability. Eurotech Aurora supercomputers have been liquid cooled since product one and day one, allowing for precious competences and know how to be waived within the fabric of the organization. This experience helped the development of our idea of liquid cooling.

Eurotech liquid cooling is:

Hot. That means using hot water of 50+ °C, balancing customer needs, density targets, data center temperature and site temperature/humidity profiles. Eurotech delivers to customers the liquid cooling solution that allows utilizing the water at the maximum temperature possible across the year. 

Direct.  The cooling takes place inside the rack, where aluminum cold plates are put in direct contact with the components, allowing to maximize the heat transfer and heat extraction efficacy. The good side effect is to level out temperatures on board avoiding hot spots.

Green. Eurotech aims to utilize free coolers (liquid to air heat exchangers) in any climate zone. Solutions are designed to avoid air conditioning, while maintaining the highest density possible, and to exploit, if required and wherever it is possible, thermal energy recovery.

Comprehensive. The “cold plates” cool processors, memory, FPGAs, power supply, switches and any other heat generating component, including GPUs or other accelerators. This means that there is not a single heat source in the rack that is not cooled, preventing hot spots at DC level.

Serviceable. Eurotech Aurora HPC boards are hot swappable despite being water cooled thanks to connectors that seal instantaneously when a node card is extracted for maintenance or management purposes. The node cards are blades that a single person can easily manage.

Safe. Eurotech understand that it is imperative to keep water away from electronics. For this reason we have spent several years to develop a system that doesn’t leak and to mature those competencies that guide our customers into the correct and trouble free maintenance of the liquid cooling infrastructure.

Indeed, one of the Eurotech focus is on correct liquid cooling operations and maintenance, which is fundamental to preserve the system safety and integrity and keep performances at top levels.

“The maintenance of liquid cooling systems is not a daunting task” says Paul Arts, Eurotech technical director “but it requires following guidelines many of them are conveniently collected by Ashrae. At Eurotech, we assist our customers in approaching hot water cooling, designing the systems and training the customers in operations and maintenance. If have to spare my 2 cents, areas I would focus my attention are water quality, anti-corrosion precautions, flow rate and dew point temperatures”

Eurotech has experienced that correct operations maximize the life not only of the cooling system but also of the electronic components, rounding up the advantages of using hot water cooling. Eurotech believes in liquid cooling as an approachable and concrete solution for facing energy and thermal issues, especially in those contexts that are climatically unfavourable.

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