• Tobias Wolf
    Tobias Wolf
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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 become firmly established as a highly effective way to reduce the energy consumed by chillers through the use of outside air or water. Free cooling systems offer big savings but air to water heat exchangers must be maintained to a high standard.

Air cooled heat exchangers, such as condensers, dry coolers, or those used in free cooling systems, are often exposed to elements such as pollution 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 exchangers, 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, therefore corrosion has a significant, as well as negative, 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 expensive 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 monoxide, 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 components are particularly affected by this, as are metal combinations in environments with high saltwater contamination. In particular, galvanic corrosion can cause significant damage 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 refrigerant leakage.

Protection measures

There are now various options for protecting air to water heat exchangers against corrosion. Thicker aluminium fins protect against premature breakage, however, during plant operation the extra fin width blocks airflow, thereby reducing 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 coating techniques can also help. Spray coating provides protection against all types of corrosion and is relatively inexpensive compared to other types of coating. However, a well-known problem concerning spray coatings is that they can leave gaps in the top coat and while they are recommended for conventional 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 aluminium 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 therefore required, in addition to a site specific assessment of individual corrosion risks. Paying 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) refrigeration 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 sediment 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 particles can clog pumps, filters or capillaries.

Oxygen corrosion is caused by dissolved oxygen in water. In closed systems it decomposes over time, but a residual oxygen content can generally 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 decomposition process. The reaction can take place without oxygen and proceeds more aggressively in a more acidic environment.

For electrochemical, or galvanic, corrosion to occur, two materials must be in contact. The less noble metal gives up electrons and is oxidised, with steady decomposition of the material. Meanwhile, with bacterial corrosion, bacteria extract electrons from the metal, whereupon the material oxidises. The waste product of the bacteria is sulfur oxide, which causes a strong odour in the surrounding medium.

Oxygen content also has a massive impact on corrosion in a chilled water circuit. If there is high oxygen content in the water, an enhanced risk of corrosion can also be assumed, but in closed systems this is rapidly degraded. To prevent oxygen corrosion it is possible to chemically bind oxygen, yet other factors also contribute to the corrosiveness of circulating water including pH value, water hardness and conductivity.

A too high or too low pH value should be avoided because different materials have different pH ranges in which they can form a protective oxide layer. If this range in an installed material is undercut or exceeded, the risk of corrosion increases significantly – leading to acid corrosion.

After a system has been filled, a large proportion of dissolved carbonic acid is still contained in the circulating water. This proportion gradually escapes as carbon dioxide and  the pH value increases. An exact adjustment of a pH value is almost impossible. If there is a mix of materials in the pipe network, the range of values in which the pH value does not have a corrosive effect on any components is reduced.

Water hardness is defined by the content of alkaline earth metal ions. On the one hand, there is carbonate hardness, which is also called temporary hardness, and on the other there is permanent hardness. Non-permanent hydrogen carbonates, which are dissolved in water when a circuit is filled, precipitate into carbon dioxide and carbonates locate at the warm sections of a circuit and can cause significant efficiency losses.

The conductivity of water is determined by the amount of anions and cations dissolved in the water including minerals such as magnesium, calcium and hydrogen carbonate, as well as salts and dissolved metal ions. The more particles present, the higher the conductivity and thus the susceptibility to electrolytic corrosion. In chilled water systems lime always precipitates at the warmest point with the lowest flow velocity, and in most cases this is the air to water heat exchanger. As such, there is a risk that it will become clogged, which can also lead to stress cracks.

When checking water conditions, special attention should be paid to salts. Dissolved salts in particular are critical for the chilled water circuit, as they are very reactive and cause precipitation and corrosion. Chloride is the salt of hydrochloric acid and is the most stable parameter in the loop system. Primarily, it serves as a chemical catalyst that also accelerates corrosion processes.

In addition to mandatory leak and function tests, a comprehensive cooling system maintenance strategy should include preventive corrosion protection for air cooled or air exposed heat exchangers.

About the Author

Tobias Wolf is the deputy head of product management at STULZ GmbH.  He has a degree in Mechanical Engineering and a Double Masters in Energy Technologies (ENTECH).  Tobias joined STULZ in 2017 with a focus on chillers and precision air conditioners for data centers, telecommunications and industry. With 14 years experience in refrigeration, air conditioning and environmental technology, as well as energy efficiency and building services engineering, he is  is a knowledgeable expert in this field. 

 

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