Hard water that dissolves salts, which are hardness components such as calcium, magnesium, silica, etc. When used as boiler water, it is concentrated at the heat-transfer surface of the boiler, and when used as condenser cooling water, it becomes saturated and precipitates out and adheres hard.
Scale is a poor conductor of heat with a thermal conductivity of 0.1-3W/(m-K) {0.1-3kcal/mh℃}, which significantly reduces the heat exchange function.