Limescale

Scale is a solid deposition on the walls, for example, of boilers ( hence the name), pots or pipes, the longer the time were filled with water in use. Scale usually consists of the alkaline earth metal salts of calcium and magnesium carbonate.

Formation

In surface water that flows through calcareous soil ( carbon dioxide ) by bound carbonic acid dissolve calcium and magnesium compounds. The sum of calcium and magnesium salts was formerly known as total hardness. By evaporation or heating by hard water from carbon dioxide escapes, the dissolved calcium bicarbonate is converted to insoluble calcium carbonate ( lime = ) and it comes to precipitation, which deposit on the walls of the vessel in existing crystal nuclei. By increasing the temperature, the formation of scale by the following reaction can be written:

Geology and chemistry of the scale see limestone.

Removal and prevention

Is removed scale usually by boiling with dilute acids, such as citric acid or lactic acid. Furthermore, in particular applies vinegar as a home remedy for disposal. When descaling with vinegar but it must be ensured that this may be the existing rubber seals (eg in a coffee machine ) attacks and makes porous. Especially in machinery and piping systems, in which over years or decades scale has deposited scale buildup can usually only be removed mechanically by tapping or splitting. The use of citric acid should be taken to ensure the water is not too much heat, as this may precipitate sparingly soluble calcium citrate (see also Article citric acid).

For the prevention of scale formation, the water can be softened before use.

An entity known as the inner water treatment measure is based on the addition of phosphate in the boiler water. An excess causes the formation of calcium or magnesium phosphate, which settle as sludge on the bottom of the boiler and can be removed by draining from the boiler. A similar procedure is applied to the inner feed water treatment for steam locomotives.

In Italy it is common to place a small chunk of marble in water boiler, which should bind the lime itself. The effectiveness of this method is controversial.

Scale deposits in boilers

In steam boilers, which are operated with improperly treated boiler water, boiler stone falls especially on heating surfaces. Gipshaltiger scale has a thermal conductivity λ from 0.5 to 2.3 W / mK and silicate richer scale from 0.08 to 0.18 W / mK. Compared to steel ( λ = 50 W / mK ) of the scale has a very low thermal conductivity and thus forms an insulating layer on the water chamber. This leads to an increase in temperature to the walls of the heating surfaces, with the result that the material to overheat surfaces expands stronger, and this may lead to cracks in particular to welds.

In water -tube boilers and quick steam generators, the water pipes can grow as much with scale deposits that the flow and thus cooling the wall exposes. Very thick scale surface leads to a reduction of the yield strength of the material due to the high temperature wall. The affected components may vary according to the direction of the compressive stress dent or dent in the tank and this can lead to failure of the boiler ( boiler explosion ).

By the insulating effect of the scale, the heat transfer is reduced in the boiler. The outlet temperature of the flue gas increases considerably, and thus the combustion efficiency decreases.

In the 19th and 20th century steam boilers were mostly operated without water treatment. The scale therefore had to be cut off from the boiler knockers mechanically with pointed hammers. The specific thermal load of the heating surfaces of the rolls and then used fire-tube boiler was relatively small and the boiler had not critical with respect to the deposition of scale areas, so that this operation was possible. Modern steam boiler designs with high Feuerungswirkungsgraden ( flame-tube smoke -tube boilers ) can be damaged by scale deposits after a short period of operation. Therefore, a treatment of the feed water with softener is mandatory and prescribed in the regulations (eg TRD 611 ).