Knowledge of scale that needs to be known in solar water heaters and hot water projects

Views:6     Author:Vickie     Publish Time: 2021-05-27      Origin:Site Inquire

Knowledge of scale that needs to be known in solar water heaters and hot water projects

1. How does scale form?

Generally speaking. Water containing minerals such as calcium (Ca) and magnesium (Mg) salts is called "hard water". (The hardness of water (also called salinity) refers to the amount of calcium and magnesium salts dissolved in the water. The higher the content, the higher the hardness, and vice versa. 1 liter of water containing 10mmgCaO (or equivalent to 10mmgCaO) is called 1 Degree. Soft water is water with a hardness less than 8) River water, lake water, well water and spring water are all hard water. Tap water is hard water that is obtained by sedimentation, removal of sediment, and disinfection and sterilization of river, lake, or well water. It is also hard water. The fresh rain and snow contains no minerals in the water, which is "soft water". After the water is boiled, part of the water evaporates, and calcium sulfate (CaSO4, gypsum is calcium sulfate containing crystal water) that is not easy to dissolve is precipitated. The originally dissolved calcium bicarbonate (Ca(HCo3)2) and magnesium bicarbonate (Mg(HCO3)2) decompose in boiling water and release carbon dioxide (CO2), which becomes insoluble calcium carbonate (CaCO3) and hydrogen. Magnesium oxide (Mg(OH)2) also precipitates. This is the origin of scale.

Scale can be roughly divided into hard scale and soft scale according to its formation cause and different forming state. When the water contains impurities such as carbonate colloid, bacteria and organic matter, the carbonate is similar to the sand in the cement mortar, while the colloid, bacteria and organic matter are equivalent to the cement in the cement mortar. Such viscous substances and carbonates work together to form hard scales that adhere to the container (or pipe surface) under high-temperature boiling conditions. Once the viscous substances such as colloids, bacteria and organic matter are removed (such as by ultrafiltration), even if the concentration of calcium, magnesium ions and carbonate ions in the water is high, only white, loose and easy-to-remove carbonate soft will be produced. Scale without producing hard scale.

2. Why do hot water projects need water treatment?

Scale is inseparable from our lives. Its chemical properties are very stable, which not only affects the heat exchange rate and consumes energy (one millimeter thick scale can consume 10% more energy), but too much scale will block the water pipe and reduce the flow of water, which may cause safety hazards. ; Scale can even endanger people's health.

Hot water projects must undergo water treatment, just like anti-corrosion projects in the metal equipment industry. Proper water treatment for hot water projects can increase thermal efficiency by about 20%-40%; equipment service life can be increased by 1-2 times; equipment maintenance rate can be reduced by 20%-30%.

In Western Europe and North America, water treatment for hot water projects is a necessary link. Design institutes, equipment suppliers, and users pay enough attention to water treatment for hot water projects. Water treatment equipment accounts for 5% of the entire project. -8%. The energy saved as a result is 20%-30% compared with that of underdeveloped countries; the use cost of equipment is reduced by 50%. The energy consumption of scale is well-known. Take France as an example. The annual scale is more than 7,000 tons, which costs more than one billion euros. The degree of energy consumption can be imagined.

3. the harm of scale to solar water heaters

①The solar vacuum tube is the collector of the solar water heater. It directly absorbs sunlight and uses the principle of small proportion of hot water and large proportion of cold water to exchange the water heated by sunlight and cold water in the water tank to heat the water tank. In the water.

The heat collection efficiency is one of the important indicators to measure the performance of the vacuum tube of solar water heaters. The heat collection efficiency of solar water heaters is reflected in the vacuum tube technology, which is mainly determined by the light transmittance, unidirectional reflectivity and vacuum insulation rate of the vacuum tube. The light transmittance determines the heat collection efficiency of the vacuum tube, that is to say: the higher the light transmittance, the higher the heat collection efficiency. This problem can be seen from the energy balance equation of the solar collector: Q=QI-QL and the collector efficiency formula: η=Q∕A*G: the higher the light transmittance, the solar radiation energy absorbed by the collector The higher the QI, the higher the useful energy Q absorbed by the collector, and the higher the collector efficiency η; and vice versa. But the light transmittance is the data measured in the laboratory, and in actual application, when the water is continuously used for about half a month, a visible scale film is formed on the inner wall of the vacuum tube, and the light transmittance can only reach 85% of the new vacuum tube. After about half a year of use, a 1-2mm thick scale can be formed on the inner wall of the vacuum tube, and its light transmittance is only below 50% of that of the new vacuum tube. At this time, the heat collection efficiency is also reduced to below 50% compared with the new vacuum tube;

② In order to avoid the scale problem and the paralysis of the entire system caused by water leakage due to the rupture of the vacuum tube, some high-end solar water heaters, large-scale solar water heaters, and balcony wall-mounted solar water heaters mostly use solar heat pipes, and it is considered that there is no need for water treatment, but the heat pipe is located Most of the heat generated is concentrated in the copper probe. Because of the high temperature and small area of the probe, it is easier to form a scale package on the probe. Because the heat of the heat pipe is exchanged multiple times by the heat medium and the heat sensor, it is more sensitive to scale, and scale exchanges heat. The efficiency impact is greater.

③ The thermal conductivity of scale is extremely low, less than one percent of that of metal. It is attached to the inner wall of the solar collector tube, so that the heat absorbed by the heat absorption tube cannot be transferred to the water in time, causing the temperature to be too high, and it is easy to burst the tube and the kettle will burn This is the reason for the omission. Similarly, your solar water temperature is getting lower year after year, and finally the heat collector tube will be scrapped. Careful users will find that the solar water temperature at home is not as high as last year, which means more scale formation. It is difficult to clean up the scale in the traditional way. We have also cleaned up the scale in the thermos. Is it hard to clean up? Similarly, the diameter of the vacuum tube is small, which makes it more difficult to clean up thoroughly.

④ Scale corrodes the inner tank, causing water leakage, destroying the heat preservation effect, and reducing the service life of the solar water heater. In the past, the solar energy made with white iron at home leaked within a few years, which is the cause of scale corrosion. Current solar water heater tanks are generally made of 304 stainless steel. During production welding, although there is argon protection, it is affected by high temperature, which changes the metal crystal image near the weld, that is, transforms from austenite to martensite. In the process of continuous heating, the water containing chloride ions accelerates the corrosion and perforation of the metal around the weld.

⑤The scale affects the normal use of the water temperature meter and the electric heating system, resulting in damage to the equipment.

⑥ Because the inner surface of the vacuum tube is very smooth, the scale shell will fall off under the impact of the water flow, causing the water circulation in the vacuum tube to be unsmooth and even blocked.

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