Page 36 - Climate Control News magazine March 2022

Product Focus
Corrosion protection for data centre components
CORROSION IS A LONG-STANDING PROBLEM FOR HVAC PROFESSIONALS THAT CAN LEAD TO SERIOUS BREAKDOWNS, ESPECIALLY IN THE DATA CENTRE WHERE 24/7 AVAILABILITY IS ESSENTIAL. DEPUTY HEAD OF PRODUCT MANAGEMENT AT STULZ, TOBIAS WOLF, EXPLAINS HOW TO PROTECT YOUR ASSETS AND AVOID COSTLY FAILURES.
FREE COOLING HAS be- come firmly established as a highly effective way to re- duce the energy consumed by chillers through the use of outside air or water. Free cooling sys- tems offer big savings but air to water heat exchangers must be maintained to a high standard.
Air cooled heat exchangers, such as
ers, dry coolers, or those used in free cooling sys- tems, are often exposed to elements such as pol- lution and salty air, which can accelerate corrosion.
The right choice of corrosion protection can help to significantly extend the operational life of a cooling system. Understanding the various corrosion protection measures for air cooled ex- changers, as well as the corrosive processes that take place in a chilled water circuit, serve as the basis for a proactive maintenance strategy. If a heat exchanger leaks due to corrosive processes, it can lead to a failure of the installation, there- fore corrosion has a significant, as well as nega- tive, impact on efficiency and availability.
NATURAL OXIDE
Corrosion is defined as the reaction of a metal with its environment. Damage to the material structure is usually irreversible and requires ex- pensive repairs or even replacement.
Oxygen corrosion is caused by a metal reacting with oxygen to form oxides. In the case of copper and aluminium the resulting natural oxide layer protects the interior of the metal, so oxidation can even be considered beneficial. Conversely, compounds such as nitrogen and sulfur oxides, as well as ammonia, chlorides and carbon mon- oxide, react with copper and aluminium to form acids that can cause microscopic indentations. Known as pitting, this can cause severe corrosion damage within just a few months.
As soon as more than one metal is used in components of a cooling system, especially air exposed ones, galvanic corrosion can occur. In
conjunction with an electrolyte such as salt water, ions of a less noble metal begin to flow towards a more noble metal. Copper and aluminium com- ponents are particularly affected by this, as are metal combinations in en- vironments with high saltwater con- tamination. In particular, galvanic corrosion can cause significant dam-
age to aluminium microchannel air to water heat exchangers connected via copper tubes. Fracture and blockage of the fine microchannels then leads to partial pressure losses or even re- frigerant leakage.
PROTECTION MEASURES
There are now various options for protecting air to water heat exchangers against corrosion. Thicker aluminium fins protect against prema- ture breakage, however, during plant operation the extra fin width blocks airflow, thereby reduc- ing efficiency. A better option is the use of Cu/Cu coil fins, as they eliminate the effects of galvanic corrosion because both the tubes and fins are made entirely of copper.
In addition to structural measures, modern coat- ing techniques can also help. Spray coating provides protection against all types of corrosion and is rela- tively inexpensive compared to other types of coat- ing. However, a well-known problem concerning spray coatings is that they can leave gaps in the top coat and while they are recommended for conven- tional installation conditions, they are less suitable for locations with very high ambient pollution from exhaust gases, acid rain or salty air.
Under test conditions spray coating with alu- minium pigmented polyurethane offers the most effective protection. Athough it has a lower salt spray value, cathodic dip coating is a more reliable technique.
LIQUID ASSET
Most water circuits use a mix of materials that have different chemical properties. Monitoring water parameters in a chilled water circuit is
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condens-
therefore required, in addition to a site specific assessment of individual corrosion risks. Pay- ing attention to the right water quality right from the initial filling stage not only improves system service life, but also ensures higher plant efficiency.
In data centres there are two main water cooled system designs – precision air conditioners with an integrated direct evaporation (DX) refrigera- tion circuit, or chilled water (CW) indoor units connected to a central chiller via air to water heat exchangers. Both variants require an oxygen tight water circuit free of foreign particles and sedi- menting substances. In the closed circuits found in chilled water systems, corrosion damage can occur within a couple of years. This is a problem not only because of possible pipe damage and leaks, but also because flaked off corrosion parti- cles can clog pumps, filters or capillaries.
Oxygen corrosion is caused by dissolved oxy- gen in water. In closed systems it decomposes over time, but a residual oxygen content can gen- erally be assumed. If oxygen, water and a metal react with each other, oxidation or corrosion of the metal occurs. In the case of acid corrosion, metals are attacked by acid due to a pH value that is too low (<5pH) and this begins a decom- position process. The reaction can take place
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