Great or small shading factor
The smaller the shading factor, the better the performance of blocking sunlight and heat from radiating indoors. As the country’s requirements for building energy conservation are getting higher and higher, in terms of architectural glass requirements, the most important are the shading (shielding) coefficient Sc (or Se) and the heat transfer coefficient K value (or U value). The shading coefficient of ordinary glass is about 0.9, so surface processing of glass, such as heat-reflecting glass, low-emissivity glass, etc., is required to reduce the shading coefficient.
Glass Concept description of shading coefficient
1. The shading coefficient not only includes sunlight (300nm-2500nm spectrum) entering the room through the glass, but also includes the energy of secondary heat transfer of the glass. . The glass body will absorb a part of the energy of sunlight, and its own temperature will rise. At this time, the glass will conduct a second heat transfer to the room by means of radiation and convection. For example, if the direct sunlight transmittance of a piece of brown glass is 50 percent, and the total sunlight transmittance is 63 percent, the extra 13 percent is the secondary radiation absorbed by the glass.
3. We call the value of the solar transmittance of the standard 3mm white glass as the heading ratio. It is 0.889 in my country and 0.87 in the world. It seems that the difference is not very big, but There should be a consensus when it comes to foreign construction projects. For example, calculate the shading coefficient Sc = solar heat gain coefficient (SHGC) / heading ratio. When SHGC is 0.818, the domestic calculation is 0.92, while the foreign calculation is 0.94.
3. The shading coefficient detects the full spectrum energy of solar radiation, including ultraviolet light, visible light and near-infrared light in the 300nm~2500nm band, which can generate heat after entering the room. The smaller the shading factor, the less sunlight enters the room and the less heat it can generate. A low shading factor does not directly mean that the visible light transmittance is also low, because while keeping the visible light transmittance constant, reducing the near-infrared transmittance can also reduce the shading factor.
4. The shading coefficient is a proportional value to 3nm transparent glass, not equal to the total sunlight transmittance of the sample glass. For example, when the shading coefficient of the glass is 0.5, it cannot be considered that this piece of glass can allow 50 percent of the solar radiation heat to enter the room. It should be understood that the solar heat that the glass can transmit is 50 percent of the standard 3nm white glass. When the shading coefficient of the glass is 1, it means that the total direction ratio of sunlight of this sample is equal to that of standard 3nm white glass. When the shading coefficient is 0, it means that the sample can neither directly transmit sunlight nor absorb and transmit sunlight energy for the second time.
5. The heat controlled by the shading coefficient is not the same heat as the heat controlled by the heat transfer coefficient. The latter refers to the heat transfer caused by the temperature difference, and the former is mainly aimed at solar radiation.
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