Calcium carbonate crystalline scale deposition

Although the presence of hardness is reported as calcium carbonate, in reality, for most water supplies the most common major contributors to total dissolved solids are calcium and magnesium bicarbonates. These dissolved solids most readily produce crystalline scales and thus predominantly contribute to boiler system deposits unless removed by some form of pre-boiler, external treatment process. 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Each of the dissolved minerals in any given source of makeup water has a potential for causing difficulties to a greater or lesser extent. For most water supplies, the commonest and highest levels of dissolved solids are the temporary hardness salts, calcium and magnesium bicarbonates. These are also the dissolved solids that most readily produce crystalline scales and thus contribute to cooling system deposits. Bicarbonates and carbonates provide the total alkalinity (M alkalinity) in most cooling waters, and normally there is no free hydroxide alkalinity. 

Fig. 4.2 Calcium carbonate deposits can occur as layer-upon-layer of scale inside a pipe or on a metal surface, reducing both the flow of water and heat-transfer rates. The deposition is often a combination of both amorphous scale and crystalline scale (as depicted in the sketch)...

Layer or layers of minerals (especially calcium carbonate) deposited, by the throwing down, or precipitation, onto a heat-transfer surface, reducing its U value. Scales are often hard and dense and difficult to remove. The scale can be crystalline in nature (a solid body having a characteristic internal structure, with symmetrically arranged plane surfaces and definite angles), or amorphous (lacking any characteristic crystalline shape). 

---