There are some metal cations in the water, which are combined with some anions. When the water is heated, it is easy to form scales due to evaporation and concentration, which adheres to the heated surface and affects the heat conduction. The total concentration of these metal ions in the water is called water hardness.
For example, the most common metal ions in natural water are calcium ions (Ca2 +) and magnesium ions (Mg2 +), which interact with water anions such as carbonate ions (CO32-), bicarbonate ions (HCO3-), and sulfate ions (SO42 -), Chloride ions (CL-), and nitrate ions (NO3-) are combined together to form calcium, magnesium carbonate, bicarbonate, sulfate, chloride, and nitrate hardness. Metal ions such as iron and manganese in water will also form hardness, but because they have very little content in natural water, they can be ignored. Therefore, the total concentration of Ca2 + and Mg2 + is usually regarded as the hardness of water.
The hardness of the water has a great influence on the boiler water. Therefore, the water should be softened or desalted according to the requirements of the boiler for the water quality of various parameters.