What is limescale?
One of the most common causes of limescale deposition is connected to calcium carbonate (CaCo3), which is naturally present in chalk, limestone and marble. Acidic water running over the rocks and penetrating them dissolves limestone into calcium ions (Ca2+) and bicarbonate ions (HCO3-), which harden the water. Limescale formation occurs when calcium (Ca2+) and carbonate ions (CO32-) react in water to form an insoluble particulate matter.3
Ca(HCO3)2 (aq) ---> <--- CaCO3 (s) + H2O + CO2
Figure 1: Among other things, the water hardness results from insoluble CaCO3
Calcium carbonate exists in three crystalline phases. The two most common are aragonite, orthorhombic crystals and calcite, hexagonal crystals (Figure 2). Due to the crystal shape, fewer stick/collect on walls of pipes, heat exchangers, sinks, coffee machines, etc.
Limescale is formed on walls of pipes, flow heaters, heat exchangers, condensers, vaporisers etc. when hard water is heated (or cooled). The calcium and bicarbonate ions precipitate due to the change in solubility and form the hard limescale deposit in the surfaces of heat exchangers and clog pipes and pipe manifolds.1 This depositing in pipes, heat exchangers, etc. is also called “fouling”.1 Fouling in pipes leads to increased drop in pressure and often to complete blockage. In heat exchangers, the heat exchange is reduced. Scaling/fouling is a largescale problem in many industries, households and farms which use water and heat.
It can also lead to unstable operation and unscheduled off-times and loss of earnings. Examples from the industry are for example processes, crude oil, power / gas / water supplies, shipping, mining, air compression and purging.1 If scaling/fouling can be reduced or prevented in these industries, the costs for energy, servicing and replacement of devices can be considerably reduced.1,3
Figure 2: Aragonite
Figure 3: Calcite
1. “Theory of electronic anti-fouling technology to control precipitation fouling in heat exchangers”. Cho, Y. I.; Fan, C.; Choi, B-G. Int. Comm. Heat Mass Transfer, Bd. 24, Nr. 6, S. 757-770, 1997.
2. ”Physical water treatment using RF electronic fields for the mitigation of CaCo3 fouling in cooling water”. TIjing, L. D.; Kim, H. Y.; Lee, D. H.; Kim, C. S.; Cho, Y, I. International Journal of Heat and Mass Transfer, Bd. 53, S. 1426-1437, 2010.
3. “Influence of electronic antifouling on agglomeration of calcium carbonate”. Nasser, A. W. N.; Ruwaie, A. A. H.; Hounslow, M. J.; Salman, A.D. Powder Technology, Bd. 106, Ausgaben 1-2, S. 201-207, 2010