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Is it better to use online Low-E or offline Low-E for the door, window, and curtain wall glass?

With the continuous improvement of mankind's awareness of sustainable development and building energy conservation, since the 1980s, low-emissivity coated glass has been rapidly promoted and popularized in developed countries such as Europe and the United States due to its excellent energy-saving performance. Due to the increase in sales and the maturity of the manufacturing level, the price of the product has also been reduced, and the scope of application has rapidly expanded from the initial high-end public buildings to ordinary civil buildings.

Glossary: Low-E coated glass is abbreviated as low-E glass or LOW-E glass, which is named because of its extremely low surface emissivity. The surface emissivity of ordinary glass is about 0.84, and the surface emissivity of Low-E glass is below 0.25. This low-radiation film with a thickness of less than one-hundredth of the hair has a high reflectivity to far-infrared heat radiation and can reflect more than 80% of far-infrared radiation, so Low-E glass has good resistance to heat and heat radiation.

The Brief Introduction about online Low-E Glass and offline Low-E Glass

1. Production Process

In-line Low-E glass is produced by spraying a chemical solution with tin salt as the main component on the surface of the hot glass during the float glass production process to form a single layer of tin oxide (SnO2) compound film with a certain low-radiation function.

Offline Low-E glass is made by using a vacuum magnetron sputtering method to uniformly plate metallic silver (Ag) and other metals and metal compounds with extremely low emissivity on the surface of the glass on a special production line. 

2. Variety and Appearance

There is a single variety of online Low-E glass, which is limited by the large-scale production of float glass. Currently, it is only 6 mm thick, colorless and transparent.

There are many varieties of offline Low-E glass. According to different climate characteristics, high, medium, and low transmittance products can be produced. The colors are silver-gray, light gray, light blue, colorless and transparent, etc., and colored glass can also be used to make green And many other colors. The thickness can be made from 3 to 12 mm.

3. Performance Parameters

The spectrum of online Low-E glass shows the characteristics of tin oxide conductive film, while the spectrum of offline Low-E glass shows the characteristics of silver and tin oxide composite film. Both have a good transmission for visible light, while the latter for near-infrared light. It has much higher reflection than the former, and the latter has less absorption and higher reflection for far-infrared radiation than the former. Therefore, compared with the online Low-E glass, the offline Low-E glass has a low shading coefficient and a low heat transfer coefficient.

4. Energy-Saving

Heat transmitted through glass in warmer season:

Q Warmer Season= U (T outside-T inside) + 630 Sc (w/㎡);

Heat transmitted through glass in colder season:

Q Colder Season= U (T outside-T inside) (w/㎡);

For the above-mentioned online Low-E (model SG 500) insulating glass components, the heat transferred into the room in summer and the heat transferred out of the outside in winter are respectively:

Q Warmer Season=2.40×(35-20)+630×0.72=489.6 w/㎡;

Q Colder Season=2.17×(-5-20)=-54.3 w/㎡ (a negative number means heat is transferred from indoor to outdoor);

For the above-mentioned offline Low-E glass (model CEB11) insulating glass components, the heat transferred into the room in summer and the heat transferred out of the outside in winter are respectively:

Q Warmer Season=2.21×(35-20)+630×0.44=310.4 w/㎡;

Q Colder Season=2.01×(-5-20)=-50.3 w/㎡ (a negative number means heat is transferred from indoor to outdoor)

The calculation conditions are 35°C outdoors in warmer season, -5°C outdoors in colder season and 20°C indoors.

Conclusion: Compared with offline Low-E insulating glass, online Low-E insulating glass receives 179.2 kilowatts of heat per thousand square meters of windows in warmer season, which is 58% more; in winter, it loses 4 kilowatts of heat per thousand square meters of windows, which is more 8%.

5. Technical Content and Cost

The production technology of online Low-E glass belongs to chemical coating, the equipment and process are relatively simple, and the technical content and production cost of the product are low; the production technology of offline Low-E glass belongs to vacuum magnetron sputtering coating, and the equipment and process include Very important technical content, the product is a high-tech crystallization, the production cost is very high.

6. Performance of Tempered Products

The online Low-E glass will endure a high temperature close to the softening point of the glass during the tempering process. At this time, the color of the film may have some changes, resulting in chromatic aberration. Besides, the tempering of online Low-E glass is tempered with a film. The existence of the film makes the heating on both sides of the glass asymmetrical, and the tempering process is difficult to control, which may cause a series of problems such as large tempering deformation. The offline Low-E glass is first tempered and then coated, so there is no problem. To avoid the above-mentioned tempering problem of online Low-E glass, some people use untempered online Low-E glass as the inner sheet of the insulating glass and put the tempered transparent glass on the outside. This usage will not only have a certain impact on the Low-E glass effect in the south but also may cause the stress spots of the outer tempered white glass to be magnified and aggravated by the inner Low-E glass. In contrast, placing tempered offline Low-E glass on the outdoor side does not have this problem.

7. Product Adaptability

In foreign countries, online Low-E glass is mainly used in civil buildings. The reason is that residential buildings are relatively low, and online Low-E glass that is cheap and does not need to be tempered can be used; while offline Low-E glass with excellent functions and more expensive is mainly made of tempered insulating glass, which is used in high-end buildings such as commercial buildings.

8. Product Stability

In reality, some people worry that offline Low-E glass products have film oxidation problems and are not stable enough. Research and experimental results show that the Low-E film in a hollow (laminated) glass protected by dry gas (or isolated from the outside world) will not oxidize at all. If Low-E glass is not used as hollow glass, its energy-saving effect will be greatly weakened or even completely lost. Therefore, Low-E glass (whether in or offline) is generally hollow glass. It is worth pointing out that since it is insulating glass, under the premise that "offline Low-E glass hollow (laminated) glass is a long-term stable product", the advantage of online Low-E glass film surface stability is only for insulating glass (laminated) The processing unit is advantageous. As the end-user of coated glass products, what we accept is stable hollow (laminated) glass products, which should focus on the function of the glass rather than the difficulty of the processing process. Therefore, for end-users of Low-E glass, the so-called "film surface stability" advantage of online Low-E glass has no practical significance. More than 20 years of foreign experience tells us that offline Low-E glass hollow (laminated) glass is a long-term stable product.

The monolithic offline Low-E glass film surface is relatively soft (called "soft film" abroad), and will slowly oxidize when it is attacked by moisture and certain oxidants. Therefore, offline Low-E glass requires very high processing conditions for insulating glass or laminated glass, and cannot be transported over long distances. Therefore, it can only be processed by Low-E glass manufacturers. At present, several domestic first-line glass brands have perfect offline Low-E glass film surface stability, long-distance transportation, low price, and low requirements for processing conditions, so they are affected by most of the domestic glass processing that cannot produce offline Low-E glass. Recommendations of manufacturers (including some installation units).

The Characteristics Analysis Offline Low-E Glass and Online Low-E Glass

1. Varieties and Characteristics

There are various types of online coated glass, including Low-E glass and San-E (sunlight control Low-E) glass, with uniform film layer and natural color. According to different climate characteristics, it can be used as a single piece or mixed with a glass of different colors. Online Low-E glass has good deep processing performance. It can be tempered, bent, laminated, etc. easily like ordinary glass to meet the needs and diversified choices of designers and owners for architectural appearance, glass function, and style. It can meet the more urgent delivery time and patch requirements of the construction industry.
Offline Low-E glass cannot be used as a single piece due to the constraints of the process and film structure, cannot be tempered and laminated in the later stage, and cannot produce hot-bent Low-E glass. Because offline Low-E glass cannot exist independently in the air layer for a long time, it is difficult to meet the requirements of the construction industry's patch construction period.

2. Film Stability

Online Low-E glass is "hard coating", and the shelf life of the film is 30 years. Offline Low-E glass is a "soft coating", the film has poor firmness and can hardly be stored in the open.

On-line Low-E glass is coated with advanced CVD (Chemical Vapor Deposition) technology that represents the development trend of the world's coating process during the molding process of the glass in the tin bath of the float glass production line. At this time, the glass is at a high temperature of nearly 700°C. The fresh state has strong reflection activity, and the film layer is bonded to the glass through a chemical bond, which is very strong. Besides, the film layer is all composed of a semiconductor oxide, which has good chemical and thermal stability and higher hardness than the glass body, so it is called "hard coating".
Online Low-E glass has permanent energy-saving characteristics, its film has permanent energy-saving characteristics, and its shelf life is 30 years.

The offline Low-E glass production uses a vacuum magnetron sputtering process. A single-layer or double-layer functional film of pure silver is plated on the surface of the glass, and a multi-layer dielectric film is required on both sides of the silver. Generally, the film system is composed of several to ten film layers, among which only the silver film layer plays the role of low radiation, and the other film layers are all protective and excessive film layers. Since the offline Low-E coating temperature is normal, the film and glass are bonded by physical bonds, so the film has poor firmness and can even be wiped off by hand. Moreover, silver is easily oxidized, and the film is a "soft coating".

Offline Low-E glass must be processed into the hollow glass in a short time, and the edge coating must be removed when forming the hollow glass. Due to the weak respiration of hollow glass, when water vapor, sulfide, and oxide enter the cavity, the insulation performance of offline Low-E glass will gradually be lost, and the appearance will become dirty and discolored. Therefore, the shelf life of the best products from domestic offline manufacturers is only 10 years.

3. Energy Saving Effect and Durability

The online Low-E glass uses a tin oxide semiconductor film with an emissivity of 0.10 to 0.20. Since the tin dioxide semiconductor layer has very good chemical stability, it can keep the low emissivity permanently unchanged, and the K value of the heat transmission coefficient will not change. Due to the good stability of the film, the online Low-E glass can guarantee that the film performance will never decay whether it is used in a single piece or combined into insulating glass.

Offline Low-E glass uses metallic silver as the radiation functional film for far-infrared heat radiation, and its initial emissivity can reach 0.1 to 0.15, which is very similar to the off-line heat transfer coefficient K value at the beginning. However, due to the chemical reaction between silver and trace sulfur and oxygen in the air, the emissivity of the film will increase significantly once the film is slightly oxidized, which will increase the heat transfer coefficient K value of the glass and change the thermal insulation performance. Poor, serious, and even lead to discoloration and peeling of the glass film layer.

4. Product Cost Performance

At present, the price of online products in the world is generally higher than that of offline products by about 30%. Due to its excellent product performance and permanent energy-saving characteristics, it has been widely used in high-end curtain wall construction in developed countries in Europe and America. The successful launch of online Low-E glass has created a market that cannot be covered by offline products in the home appliance, luxury bus, and other industries.

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