With the promotion of building energy conservation to four-step energy conservation, the energy-saving performance of windows and doors is more and more demanding. In order to improve the energy-saving performance of doors and windows, the selection of window glass is very important.
Most of the types of hollow glass used at present are double-layer hollow glass, triple-layer hollow glass, double-layer Low-E hollow glass and other products. With the improvement of the requirements for energy-saving performance of doors and windows, the configuration of hollow glass for doors and windows is also developed towards double-glazing Low-E hollow glass(offline, double-silver), triple-layer Low-E hollow glass, triple-layer biplate Low-E or the adoption of technologies such as warm edge and inflation. The energy-saving performance of glass will be significantly improved.
More and more attention has been paid to the selection of Low-E glass, and its product series, specifications and varieties have become more and more detailed. Better understanding of the performance of Low-E glass is a top priority for door and window designers.
Window and door profiles properties of Low-E glass
Low-E glass (also known as low-radiation coated glass) is the abbreviation of Low Emissivity Glass, which is a film product composed of multiple layers of metal or other compounds on the surface of glass. The product has high transmittance to visible light, high reflectivity to infrared (especially mid-to-far infrared), and good thermal insulation performance. It can control sunlight, save energy, regulate heat and improve environment. The surface emissivity of ordinary glass is about 0. 84, and the surface emissivity of on-line Low-E glass is generally below 0. 25. The low-radiation film with a thickness of 80-90 nm has a high reflectivity to the far infrared radiation, and can reflect back more than 80% of the far infrared radiation, so Low-E glass has a good effect of blocking the thermal radiation transmission.
Most of the radiation of the sun can be transmitted indoors through hollow glass, bringing light and warmth to our life. Indoor items will be rediate due to their own warm temperature and a certain amount of heat through the hollow glass and transfer to the outdoor. The hollow glass made of low-radiation coated glass can transmit most of the visible light and solar energy into the room, and effectively prevent the indoor re-radiation (long wave) from passing through the glass to the outdoor, so as to reduce the U value of the door and window products.
In winter, the design of low-radiation hollow glass in northern cities of China mainly considers shielding ultraviolet light, high transmission of visible light (improving the utilization of natural light), high transmission of near-infrared light (obtaining solar thermal energy), low transmission of long wave (between 2. 5---50μm)(preventing indoor thermal energy loss).
In summer, the design of low-radiation hollow glass in cities in southern China mainly considers shielding ultraviolet light, high transmission of visible light (improving utilization of natural light), low transmission of near-infrared light (shielding solar thermal energy) and low transmission of long wave (between 2.5---50μm)(preventing thermal energy radiation of outdoor articles (roads, buildings, etc.) and also preventing loss of indoor cold air).
E-value of Low-E glass, effect on distinguishing between on-line Low-E glass and off-line Low-E glass
Since Low-E glass is a series of film products composed of coating one or several layers of silver metal film or other compounds on the glass surface, the film layer has extremely low surface emissivity and radiates back far infrared thermal energy with wavelength of 2. 5μm-40μm. The lower the emissivity e value of Low-E glass, the lower the U value of glass, and the better the thermal insulation effect.
According to the processing technology of Low-E glass, it is divided into on-line Low-E glass and off-line Low-E glass. The main differences between the two types are that the e-value of Low-E glass is generally about 0.30-0.16 for on-line Low-E glass, about 0.15-0.08 for off-line single-silver Low-E glass, about 0.05-0.07 for double-silver Low-E glass and about 0.02 for triple-silver Low-E glass. The emissivity of ordinary transparent glass is 0.84, meaning that 16% of the wavelength is from 2.5μm to 40μm of the far infrared thermal energy is radiated back. For example, No. 253 in the national glass database is off-line low-radiation coated glass, with the emissivity of 0.08, which means that 92% of the wavelength is 2.5μm-40μm far infrared thermal energy radiation back, making the energy saving effect of hollow glass more obvious.
Off-line Low-E glass
The film system is generally composed of several to over ten layers. The coating material is metallic silver, of which only the silver film plays the role of low radiation, and the other film layers are all protective and transitional film layers. Using vacuum magnetron sputtering process, the glass surface is coated with single-layer and double-layer silver functional film.
In pursuit of higher energy-saving requirements in different regions, low-radiation coated glass with double silver and triple silver appeared successively. The membrane system structure is more complex than that of ordinary Low-E membrane layer.
Low-E glass highlights the shielding effect of glass on solar thermal radiation. It filters sunlight into cold light source to the maximum extent, solves the contradiction that high visible light transmittance and low solar light transmittance can not be considered, and achieves the effect of high visible light transmittance and low solar thermal transmittance.
On-line Low-E Glass
The on-line low-radiation coated glass has large hardness and is not easy to scratch, so it is called hard coating. Its production is carried out at a certain temperature, called chemical vapor deposition. The coating material is tin oxide. The thickness of the film is 20 times that of the off-line film, but it is also quite thin.
The spectrum of on-line Low-E glass is characterized by the conductive film of tin oxide, while that of off-line Low-E glass is characterized by the composite film of silver and tin oxide. Both of them have good transmission to visible light, while the latter has much higher reflection to near-infrared light than the former, and has less absorption and higher reflection to far-infrared radiation than the former. Therefore, the off-line Low-E glass has a lower shading coefficient and a lower heat transfer coefficient than the on-line Low-E glass.
In the application, the glass type shall be selected according to the design requirements for the performance of doors and windows. Meanwhile, the U value of Low-E glass, the shading coefficient SC and the e value of Low-E glass shall also be considered. The glass type that meets both the U value requirements for the glass for doors and windows and the design requirements for the shading coefficient SC shall be selected in the glass database. The glass U value and shading coefficient of different Low-E glass have great fluctuation. The correct understanding of the e value of Low-E glass will be of great reference value for the selection of glass varieties.