Corrosion is a long-standing issue for HVAC professionals that can lead to serious outages, especially in the data center where 24/7 availability is essential. Deputy Head of Product Management at STULZ, Tobias Wolf, explains how to protect your assets and avoid costly breakdowns.
Free cooling has firmly established itself as a very effective way to reduce the energy consumed by chillers through the use of outside air or water. Free-cooling systems offer great savings, but air-to-water heat exchangers must be kept high.
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 salt air, which can accelerate corrosion.
The right choice of corrosion protection can significantly extend the life of a cooling system. Understanding the different corrosion protection measures of air-cooled heat exchangers, as well as the corrosive processes that take place in a chilled water circuit, forms the basis for a proactive maintenance strategy. If a heat exchanger leaks due to corrosive processes, this can lead to plant failure. Therefore, corrosion has a significant, as well as negative, impact on efficiency and availability.
Corrosion is defined as the reaction of a metal with its environment. Damage to the physical structure is usually irreversible and requires expensive repairs or even replacement.
Oxygen corrosion is caused by the reaction of a metal with oxygen to form oxides. In the case of copper and aluminum, the resulting natural oxide layer protects the inside of the metal, so that 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 aluminum to form acids that can cause microscopic indentations. . Known as pitting, this can cause serious corrosion damage in just a few months.
As soon as more than one metal is used in the components of a cooling system, especially those exposed to air, galvanic corrosion can occur. In conjunction with an electrolyte such as salt water, ions of a less noble metal begin to flow to a more noble metal. Copper and aluminum components are particularly affected, as are combinations of metals in environments heavily contaminated with salt water. In particular, galvanic corrosion can cause significant damage to aluminum microchannel air-water heat exchangers connected via copper tubes. Rupture and clogging of the fine microchannels then lead to partial pressure drops or even refrigerant leaks.
There are now different options for protecting air-water heat exchangers against corrosion. Thicker aluminum fins protect against premature breakage, however, during plant operation, the extra width of the fins blocks airflow, reducing efficiency. A better option is to use Cu/Cu coil fins, as they eliminate the effects of galvanic corrosion as the tubes and fins are all 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 with spray coatings is that they can leave gaps in the topcoat and although they are recommended for conventional installation conditions, they are less suitable for locations with high gas pollution. exhaust, acid rain or salty air. .
Under test conditions, aluminum pigmented polyurethane spray coating provides the most effective protection. Although it has a lower salt spray value, cathodic dip coating is a more reliable technique.
Most water circuits use a mixture of materials that have different chemical properties. Monitoring of water parameters in a chilled water circuit is therefore necessary, in addition to a site-specific assessment of individual corrosion risks. Paying attention to good water quality from the initial filling phase not only improves the life of the system, but also ensures greater efficiency of the installation.
In data centers, there are two main designs of water-cooled systems: precision air conditioners with an integrated direct evaporative (DX) refrigeration circuit or chilled water (CW) indoor units connected to a central chiller through air-water heat exchangers. Both variants require an oxygen-tight water circuit free of foreign particles and sedimentary substances. In closed circuits of chilled water systems, corrosion damage can occur within a few years. This is a problem not only because of the possible damage and leaks on the pipes, but also because the loose corrosion particles can clog the 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 usually 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 too low pH value (
For electrochemical or galvanic corrosion to occur, two materials must be in contact. The less noble metal gives up electrons and oxidizes, with constant decomposition of the material. Meanwhile, with bacterial corrosion, the bacteria extract electrons from the metal, after which the material oxidizes. The waste product of bacteria is sulfur oxide, which causes a strong odor in the surrounding environment.
The oxygen content also has a considerable impact on corrosion in a chilled water circuit. If the oxygen content of the water is high, an increased risk of corrosion can also be assumed, but in closed systems this deteriorates rapidly. To prevent oxygen corrosion, it is possible to chemically bind oxygen, but other factors also contribute to the corrosivity of circulating water, including pH value, water hardness and conductivity.
Too high or too low a 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 underestimated or exceeded, the risk of corrosion increases dramatically, resulting in acid corrosion.
After filling a system, a large proportion of dissolved carbonic acid is still contained in the circulating water. This proportion gradually escapes as carbon dioxide and pH value increase. An exact adjustment of a pH value is almost impossible. If there is a mixture of materials in the pipeline network, the range of values in which the pH value does not have a corrosive effect on the components is reduced.
Water hardness is defined by the content of alkaline earth metal ions. On the one hand there is carbonate hardness, also called temporary hardness, and on the other hand there is permanent hardness. Non-permanent hydrogen carbonates, which are dissolved in water when a circuit is filled, precipitate out as carbon dioxide and the carbonates localize in the hot sections of a circuit and can cause significant efficiency losses.
Water conductivity is determined by the amount of dissolved anions and cations in the water, including minerals such as magnesium, calcium, and hydrogen carbonate, as well as dissolved salts and metal ions. The more particles present, the higher the conductivity and therefore the susceptibility to electrolytic corrosion. In chilled water systems, lime always precipitates at the hottest point with the lowest flow velocity, and in most cases this is the air-water heat exchanger. As such, there is a risk of it becoming 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 highly reactive and cause precipitation and corrosion. Chloride is the salt of hydrochloric acid and is the most stable parameter in the loop system. It mainly serves as a chemical catalyst which also accelerates corrosion processes.
In addition to mandatory leak and function testing, a comprehensive cooling system maintenance strategy should include preventative corrosion protection for air-cooled or air-exposed heat exchangers.
About the Author
Tobias Wolf is deputy director of product management at STULZ GmbH. He holds a degree in mechanical engineering and a double master’s degree in energy technologies (ENTECH). Tobias joined STULZ in 2017 focusing on precision chillers and air conditioners for data centers, telecommunications and industry. With 14 years of experience in refrigeration, air conditioning and environmental technology, as well as energy efficiency and building services engineering, he is a knowledgeable expert in this field.