Scaling can be defined as the formation of relatively thick layers of calcium carbonate and/or corrosion products on heat transfer surfaces.
Natural waters dissolve CO2 from the atmosphere to form weak carbonic acid this dissociates into hydrogen, hydrogen carbonate and carbonate ions. Calcium ions in the water, dissolved from minerals, react with bicarbonate ions to form an equilibrium with calcium carbonate and carbon dioxide. As the temperature of water increases, carbon dioxide is driven off and the equilibrium is pushed to the right. Therefore calcium carbonate precipitates in the hottest part of the system.
It is known that the amount of scale produced is related to the hardness of the water but what exactly do we mean by hardness? Total hardness is the sum of all the calcium and magnesium salts in the water, which includes carbonate and bicarbonate but also other salts such as sulphates, chlorides and nitrates.
Since scale builds up primarily on heat transfer surfaces, it results in poor heat transfer with a subsequent loss of efficiency. The problem is most acute in systems where hard water is constantly being replaced, such as in kettles or combi-boilers. However, even in closed recirculating heating systems, scale deposited from the initial fill water and make-up water to replace evaporative losses from the header tank, results in losses of efficiency of 2-6%. In real terms, this means increased fuel bills and higher CO2 emissions.
Scale deposition is usually non-uniform, leading to localised hot spots. This gives rise to nucleate boiling resulting in boiler noise, especially on start-up in the morning. In extreme cases, limescale deposition has resulted in failure of cast iron heat exchangers due to thermal shock.Reduced flow due to scaling can also occur on the hot water side of combi-boilers but it is most often noticed with showerheads, which may become totally blocked. On the secondary waterside, as pumps generate their own heat, scale also has a tendency to form within the pump housing. This can also restrict flow and reduce the life of the pump.
Natural waters contain varying amounts of calcium, magnesium, bicarbonate and carbonate ions, which react to form limescale on heating. This causes problems in domestic hot water and heating systems, such as reduced efficiency, reduced flow and blockages of components and boiler noise.Various methods to reduce or eliminate these problems are the use of water softeners, polyphosphate dosing devices, filtration systems and scale inhibitors.
Phillip Munn, Ph.D., C.Eng., MIM, MICorr, MWMSoc