Cooling technologies have evolved a great deal in the 21 years since my company was founded to colocate telecoms equipment, and have moved far beyond the original CRAC units of 1993. Modern data centres have needed to continuously redesign and regenerate their cooling systems to keep up with an ever fluctuating industry.
Location, Location, Location
Data centres located in technology hotspots such as central London have specific cooling constraints due to the high cost of space. Using available space for colocated customer equipment and business expansion, rather than plant rooms, is essential. The proximity of residential areas in central London means noise pollution becomes an issue, so large fans starting and stopping at night is unacceptable.
While cheaper locations are available, customers continue to demand colocation and data centre facilities in technology hotspots. Central London is arguably one of best-connected areas in Europe and a data centre in the heart of Tech City will always be attractive.
One popular data centre cooling system is to combine Direct Expansion (DX) and cold aisle containment, with all air handling being above the raised floor. Cold air is traversed across the top of the racks and falls into the cold aisle. It passes through the colocated customer equipment, then rises into DX evaporators, before it is chilled and recycled. However, as the technology world continues to evolve, are there more appropriate systems available?
Avoiding The Cooling Pitfalls
Much modern data centre cooling uses water as the heat carrier, but water needs a lot of space for plant and pipework. Adding storage tanks at roof level risks stresses the building was not designed to take, while water leaks on the upper floors are very risky in multi-story data centres. However, adding water storage at ground level uses valuable space, which could otherwise be used for colocated customer racks.
It is sensible to limit water to a single vertical area, containing all the toilets and kitchen facilities, removed from the data halls and other equipment areas. A water leak in a data centre may then flood the toilets, but would have no affect on customers. Leakage control systems are available, using an air vacuum to suck leaking water back into the pipes, but are largely unproven, so operations managers often do not want to take the risk.
Free air cooling and evaporative cooling, where air from outside the building passes over colocated customer equipment, are often considered by data centres. However, city pollutants give rise to long-term corrosion problems, particularly with modern lead-free soldered joints, while many colocation customers are uncomfortable with the idea of natural air, regardless of filtering levels.
Away from a city, the same precautions must be taken to prevent pollution in the event of a vehicle fire in the vicinity of the data centre. Any data centre cooling system using external air sources should be indirect, with a heat exchanger between the data centre and the external air source, to minimise the risk of pollution damage to colocated equipment.
Both free air and evaporative cooling use large volumes of air moving slowly which requires large ducts to channel the air. These take up valuable space within the data centre, are difficult to route around, restrict air circulation and are difficult to modify. In an industry where technology evolves year on year, the expectation is that everything will be changed at some point. To transport heat, a volatile liquid in a 5mm pipe at 30 bar pressure is much more attractive than low pressure air in a 600mm square duct.
After carefully considering all the cooling system options, my company’s data centre in London continues to operate overhead all-DX systems with cold aisle containment. These offer good efficiency (with a COP of 3.5, the Partial PUE is 1.28), the smallest plant footprint, low roof loading, zero leak risk, off-the-shelf equipment, the best ease of modification, the best growth staging, the least visual obtrusion and the least noise pollution. Cooling is a vital cog in the data centre machine and it is essential data centre architects and operations managers find the solution which is most beneficial, not just for their business, but for their colocation customers.