Cause Analysis of Damage of Solar Vacuum Tube
1. Hot and cold shock: In daily life, people who have used glass should know that when cold water is poured into the glass just filled with boiling water, or boiling water is poured into the glass just filled with boiling water, the glass will break. The same is true of vacuum tube solar collectors. For some reasons, such as long-term exposure to sunlight, water in strong sunlight at noon will lead to cracking of the vacuum tube mouth.
2. Pressure is too high: due to improper selection of pumps, too high lift, or the use of tap pipe network pressure, resulting in excessive pressure on glass pipe, leading to tube burst. In addition, the one-way valve of the intake pipe is not well sealed, especially when the water tank is lower than the collector, resulting in negative pressure phenomenon of the system, especially when the water tank is lower than the collector. It's easy to burst the pipe, too.
3. Stress: Vacuum tube vacuum degree is not enough, thermal insulation performance is not qualified. In winter, when the night temperature is low, the water in the vacuum tube freezes due to the temperature drop, which causes the vacuum tube to burst. In addition, due to the incorrect installation position of the exhaust valve of the system, the exhaust gas is blocked and the gas resistance increases. After water injection into the system, the maximum stress of the nozzle will approach the applicable stress, which may lead to pipeline explosion. The installation angle of the system is incorrect, and the solar collector can not be installed on the same plane, which will cause the stress between the vacuum tube and the header. When the stress limit is reached, the tube is easy to explode.
4. Air-drying: due to pipe connection caused by pipe blockage, or valve misoperation, water can not flow into the vacuum tube, air-drying caused the explosion of the tube.
5. Scale: The temperature in the vacuum tube is easy to reach 80 or 90 ℃, and it is easy to produce scale, which is attached to the surface of the header and vacuum tube. The thermal conductivity of the scale is small and the scaling layer is thicker, which prevents the heat exchange between the heat absorber of the vacuum tube and the heat conducting fluid in the tube, and forms a certain temperature difference between the inside and outside of the scale. When the solar radiation is good, the outer temperature of the scale is higher. A large temperature difference with the inner layer results in scale cracks, and the cold liquid in the tube enters the inner wall of the glass along the cracks, which easily causes the explosion of the tube.
6. Quality defect of vacuum tube itself: Borosilicate glass is usually used in vacuum tube collector, but borosilicate glass with expansion coefficient greater than 3.410-6 is not easy to process. Due to the change of glass composition and mechanism, the properties of borosilicate glass have changed dramatically, which leads to tube burst. Therefore, borosilicate glass with expansion coefficient of 3.3 is generally chosen.