IBM 7014-T00 Manual page 174

Figure 116. Example of liquid cooling systems and loops within a data center
Water-related problems
Proper treatment of the water is necessary to avoid the following common water-related problems:
corrosion, microbiological growth, scale formation, and fouling. Any of these problems can significantly
reduce the cooling efficiencies and increase the risk of system downtime.
• Corrosion - corrosion can take on many forms. The common forms of corrosion that are relevant to the
cooling loop include the following forms:
– Uniform corrosion, also referred to as general corrosion, is the spatially uniform removal of metal
from the surface. It is the typical expected mode of corrosion.
– Pitting corrosion is a localized attack of a metal surface that in the case of copper tubes can lead to
water leaks with a typical mean time to failure of around 2 years.
– Galvanic corrosion arises when two metals that are wide apart in the galvanic series are in electrical
contact and immersed in the same water environment. The potential difference that arises between
the two metals in contact, forces electrons to flow from the less noble to the more noble metal. On
the less noble metal surface, corrosion occurs, giving off electrons that are consumed on the more
noble metal surface by a reduction reaction that can take many chemical forms. Examples are the
reduction of metal ions or the consumption of oxygen and water to form hydroxyl ions. Even when not
in electrical contact, aluminum can be galvanically attacked by copper because of dissolved copper
ions in low concentrations that deposit on the aluminum surface forming the galvanic corrosion
couple.
• Microbiological growth - microbiological growth in water cooling systems can lead to deposition,
fouling, and corrosion within the cooling loop. Prevention of microbiological growth involves making
sure that the cooling loop hardware is assembled from components that are free of biological organisms
and treatment with biocides to control the bacteria population. To avoid biological growth, the water
cooling loops must be shipped and stored dry. Every effort must be made to blow out the water and dry
the water cooling loop as much as possible before shipping and storage.
• Scale formation - scaling is the deposition of dense, adherent material on the cooling loop surfaces.
Scaling occurs when the solubilities of salts in water are exceeded because of high concentrations or
because of increased temperature.
• Fouling - fouling of cooling loops is the deposition of non-scale-forming substances such as corrosion
products and organics. Fungi, such as Fusarium sp, are known to grow, foul, and plug filters and fine
finned heat sinks. They generally grow at the water line in cooling tower basins or sumps.
160 Power Systems: Racks and rack features