I have been responsible for engineering operations at large data centres for more than 20 years...

...and my long experience supports some of Nick Vaney’s recommendations (“Powering down for data protection”, BSj 08/08), such as raising dataroom temperatures, controlling air flows in the room, inverter fans and raising chilled water temperatures, giving free cooling opportunities that make a considerable difference to energy efficiency.

By definition, every piece of plant in a data centre will normally run part-loaded, so part- load efficiency and operation is the most important metric.

I have developed a network of connections to other operational data centre engineers around the world, and we have swapped and collated information on design and operations, particularly on the subject of energy efficiency. The main conclusion we have reached is that the energy efficiency of a data centre is determined largely by operational practice and simple good housekeeping, with design being a secondary factor.

Nick gives an annual CoP figure of 12 for a water-cooled chiller and cooling tower system at a data centre. Our shared operational experience suggests, however, that packaged air- cooled systems with free cooling actually perform better.

I have no idea why this may be, but suspect that the answer may be due to the part-load operation, ease of control and problematic operation of systems with evaporative towers.

John Cooknell MCIBSE