Chilled water cooling system pipes for Archer at the EPCC. The Cray XC30 cabinets are cooled by water supplied at 18 degrees, which returns at 29 degrees. The plant operates on a closed loop with chillers that operate on refrigeration principles to cool the return water. Advantage is taken of the high return water temperature so that the bulk of the heat from the return water is diverted through external cooling towers when the outside temperature is suitable. This "free cooling" enables the return water to be substantially cooled without the permanent need to run the large electrical chillers, so saving significant amounts of energy. (Image: MW Brown).