The hardness of the water is determined by its content of alkaline earth metal ions. There is carbonate hardness – also referred to as temporary hardness – on the one hand. This is chiefly caused by cations of magnesium and calcium. On the other hand, there is also a element of permanent hardness, which cannot precipitate as an insoluble solid. In warm parts of the circuit, the non-permanent hydrogen carbonates, which are dissolved in the water when a circuit is filled, precipitate as carbon dioxide and carbonates. The result of this combination is commonly also referred to as “limescale”. This substance adheres to the inside of heat exchangers and pipes, leading to huge losses of efficiency in the entire system.
The conductivity of the water is determined by the quantity of dissolved anions and cations it contains. These include hardness builders such as magnesium, calcium and hydrogen carbonate, but also minerals and dissolved metal ions. The more particles the water contains, the higher its conductivity and therefore its susceptibility to electrolytic corrosion. Moreover, this increases the likelihood of sediments in the water. The best known exponent of this is limescale, which always forms at the warmest point with the slowest flow in a chilled water system, which in most cases is the heat exchanger. There is now an acute risk of heat exchanger clogging, which has a hugely negative impact on efficiency. This is because of the smaller contact surface and the fact that limescale has a far lower thermal conductivity coefficient than copper. Partial limescale deposits can also cause stress cracks in the heat exchanger due to the varying heat expansion. High conductivity does not necessarily constitute a higher risk, however. Corrosion inhibitors increase conductivity, for example, but in this case it is not problematic.
When checking the water conditions, particular attention should be paid to minerals. Dissolved minerals are especially critical for the refrigerant circuit, because they are highly reactive and provoke both precipitation and corrosion. Chloride is the salt produced by hydrochloric acid and is the most stable parameter in the circulatory system. It is primarily used as a chemical catalyst, but it also intensifies corrosive processes. During this acceleration, chloride is not used up, but is retained. In addition, chloride attacks the passive layers on metals, exposing more surface to attack from normal oxygen corrosion. In certain conditions, chloride can cause local damage that results in pitting corrosion.
Sulfate is the salt produced by sulfuric acid and acts as a nutritional basis for any bacteria in the circulatory system. These are then sulfate-reducing bacteria. The combination of calcium and sulfate to form calcium sulfate can also be an indicator of sulfate reduction. Generally, these very hard deposits are responsible for poor temperature transitions and inefficient operation.
Bacterial processes cause nitrate to transform into ammonia and to react with copper pipes to form a copper ammonia complex, which is capable of oxidizing iron without a supply of oxygen while disintegrating back into ammonia and copper. Through the process of complexing, ammonia therefore continually oxidizes the iron constituents of a circuit. Tap water has a relatively low nitrate value, and in technical use can be regarded as critical from approx. 5 mg/l. However, the nitrate level is becoming increasingly relevant in the evaluation of corrosion risks, due to the rising concentration of nitrate in the groundwater, particularly in agricultural areas.
Risk due to added glycol
Depending on their installation location, water circuits are often filled with a water-glycol mixture to prevent the pipework from freezing. However, in certain circumstances glycol can intensify the corrosive properties of the water. With a neutral pH value and a low-oxygen environment, corrosivity scarcely differs from pure water. But if oxygen is dissolved in the water, glycol can break down. This produces acids that acidify the water-glycol mixture, lowering the pH value. Acidification of the mixture can lead to pH values of less than 4. Consequently, as well as the usual corrosion inhibitors, antifreeze concentrates also contain acid neutralizers, which can mitigate the acidification to a certain extent. There are only a limited number of these acidic buffers in the mixture, however, which is why both the pH value and the glycol content of the circuit need to be checked. Furthermore, if the glycol concentration falls below the minimum value specified by the manufacturer, increased bacterial growth can be expected. This results in organic deposits and bacterial corrosion. Common minimum levels are 20 percent by volume for ethylene glycol and 25 percent by volume for propylene glycol. Here, it may be a good idea to contact the glycol manufacturer or to read the manufacturer’s technical data sheets.
Conclusion
Basically, the oxygen content should be kept to a minimum and the system kept free from leakage. Chlorides and sulfates must be reduced in line with the Rule 3.003 from the BTGA, and the use of water with a low mineral content is also recommended. In addition, it is important to keep the pH value within the middle range, to suit the resistance of the materials used, and to check this regularly. Softeners and residual hardness stabilizers are also recommended. The use of demineralized water is disadvantageous as it is acidifying and highly reactive. If pure water is used, the lowest possible conductivity should be aimed for, as well as a nitrate content of less than 5 mg/l. The use of corrosion inhibitors or glycol mixtures in the right concentration is also advisable, as is constant monitoring. Despite the fact that Rule 3.003 from the BTGA is not a standard, but a technical rule, it is extremely pertinent in this field. If plant operators ensure that the system is low in minerals right from the start, in accordance with this Rule, and take care of the water conditions, they can increase the efficiency and service life of their chilled water systems.