Developing a new approach to cooling computer rooms

Server cabinets with integral water-based cooling can accommodate equipment consuming up to 22 kW, compared for around 3.5 kW for conventional server cabinet layouts in air-conditioned rooms.
As the power of computing equipment grows, so too does its power density. The consequence is that the capabilities of computer-room air conditioning are reaching their limits.Over the last two decades or so, cooling computer rooms has grown into a significant business for air-conditioning companies. We are quite used to seeing rows and rows of cabinets pushing out cooled air at enormous air-change rates per hour compared with comfort cooling for people. In addition, such computer suites are designed and built to enable large amounts of cool air to circulate unimpeded. Air is usually supplied under the floor and emerges into the main space through grilles in the floor. The return air path is at high level. Upper limit However, this approach to cooling computer suites, which continue to grow in size with cabinets housing ever-more computing power has an upper limit. According to Simon Terry, director and general manager of Knürr Ltd, conventional computer-room air conditioning (CRAC) has an upper limit of 1 to 1.5 kW/m2. With the power draw of a cabinet being 3 to 4.5 kW, one cabinet requires an overall floor area of around 3 m2. Such quantitative considerations, explains Simon Terry, dictate the layout of computer rooms and data suites — with rows of cabinets drawing in cool air through a perforated front face (the ‘cool’ aisle) and exhausting warm air from the back into the ‘hot’ aisle. However, as we all know, micro-processor and server technology advance at a rapid pace. One major effect is that power densities increase. Simon Terry says that the next generation of Intel processor will have a power consumption of 180 W. He elaborates that some blade servers today have a power density in a 7 U module, and that in a year to 18 months the power density of blade servers will double to 8 kW in a 7 U module. [A ‘U’ is a standard unit of vertical height in computer parlance, and is 1.75 in.] Such power densities imply a total power consumption of 24 kW for a cabinet of typical height. Knürr specialises in server cabinets. The company is a global operation based in Bavaria and in Germany an with a large plant at St Ives in Cambridgeshire. Simon Terry tells us that from its strong position in the UK market — its installed base in London, for example, is over 15 000 cabinets — that cooling computer suites is becoming a critical problem. ‘Conventional computer-room air conditioning is reaching its limits. Leaving more space between server cabinets is the obvious response, but that is expensive in terms of floor space,’ he explains, ‘so a radical new approach is needed.’ Cooled cabinets Knürr’s approach is based on its expertise in making and installing server cabinets. Instead of installing cabinets in a cooled room, the cooling becomes an integral part of the cabinet itself. That is achieved by fitting a heat exchanger into the bottom of the cabinet and supplying it with chilled water from a central plant. Air is moved around the cabinet by a number of fans so that a stable temperature is maintained throughout its height. As realised by Knürr, the concept is called CoolTherm, and a single 19 in cabinet can manage up to 22 kW of heat load, compared with around 3.5 kW for conventional server cabinet layouts. Cabinets are completely sealed, so no air is exhausted into the room. Simon Terry says, ‘The immediate benefit is a massive 80% reduction in present, typical conventional cabinet numbers within large data centres. Resultant floor-space savings of 80% will significantly reduce the increasingly high take-up of large areas of expensive computer-room floor as server power levels rocket. ‘Cabinet installation costs can also be halved, and enormous year-on-year operational savings made in associated reductions in CRAC install and running costs.’ Independent testing He also says that independent detailed testing has shown reductions in total cost of ownership of some 40%. The heat-transfer arithmetic is simple. To handle a heat load load of 1 kW requires air to be circulated at 325 m3/h with a ?T of 10 K. If chilled water is used ot remove the heat, only 90 l/h needs to be circulated. To maintain a temperature of around 25°C in a Cooltherm cabinet containing 22 kW of equipment, the chilled water might have flow/return temperatures of 12/18°C at a flow of 0.4 to 0.9 l/s. The concept has received its first large-scale application in Germany. The project is associated with CERN’s high-energy physics facilities and comprises a thousand 10 kW racks with a total cooling requirement of 1 MW. The application of water-cooled server cabinets is not limited to new projects. The approach lends itself to refurbishment and upgrading existing installations. A useful feature is that a raised floor is not required. Knürr is at 31 Burrel Road, St Ives, Cambs.
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