Due to the different structures, the heat transfer mechanism of vacuum glass and insulating glass is also different. Vacuum glass, heat transfer in the central part of the vacuum glass consists of radiative heat transfer and heat transfer of the support, which neglects residual gas heat transfer. Insulating glass consists of gas heat transfer (including conduction and convection) and radiant heat transfer.
It can be seen that to reduce the heat transfer caused by the temperature difference, the vacuum glass and the insulating glass should reduce the radiation heat transfer. The effective method is to use a glass coated with a low-emission film (LOW-E glass), taking into account other optics. Under the conditions of performance requirements, the lower the emissivity (also called emissivity), the better. The difference between the two is that vacuum glass should also reduce the heat transfer of the lattice support as much as possible. At present, Xinliji Company adopts an open circular (or C-type) support with a diameter of 0.5mm according to its own patent, and the lattice spacing 25mm, its thermal conductivity is about 0.5Wm-2k-1. Insulating glass should minimize the heat transfer of the gas. In order to reduce the heat transfer of the gas and take into account factors such as sound insulation and thickness, the thickness of the air layer of the insulating glass is generally 9-24 mm, which is mostly 12 mm. To reduce gas heat transfer, large molecular weight gases (such as inert gases: argon, helium, neon) can be used instead of air, but even so, gas heat transfer still dominates. Compared
Table 1 lists the three low-E glasses currently available for vacuum glass production in the domestic market. These three low-E glasses are plated with “online†Low-E film or “offline†and “hard†with protective layer. Low-E film, both can withstand high temperatures of 500 °C.
Table 2 shows the calculation results of the heat transfer coefficients of the four vacuum glasses made of the above three Low-E glasses.
Generally, a uniform material is characterized by its thermal conductivity (thermal conductivity) λ. It is defined as: 1m thick object, under steady state conditions, when the temperature difference between the two sides is 1K, the heat transferred through the area of ​​1m2 per unit time. The legal unit in China is Wm-1K-1.
Vacuum glass is not a uniform continuous material and is a thin structure. In order to facilitate comparison with other insulation materials, the concept of "apparent thermal conductivity" or "converted thermal conductivity" is often cited. The meaning can be imagined as the value of the thermal conductivity of a plurality of sheets of vacuum glass when laminated to a thickness of 1 m.
In fact, the apparent thermal conductivity can be easily calculated according to the following formula.
Where C is the thermal conductivity of vacuum glass in Wm-2K-1
Where d is the thickness of the vacuum glass in m
From this, it can be calculated that the apparent thermal conductivity of the four vacuum glasses in Table 2 is listed in the last column of the same table.
Table 3 lists the thermal conductivity of several common building materials
The comparison shows that the vacuum glass is extremely thin, so the apparent thermal conductivity is much lower than that of the general thermal insulation material, and it is also ten times smaller than the thermal conductivity of the insulation material specified by the GB4272-92 standard in China, which is 0.12Wm-1K-1, indicating that it is Insulation with excellent performance.
If the red brick wall in Table 3 is taken as an example, it is not difficult to calculate the four types of vacuum glass of No. 1 to 4 in Table 2, which are equivalent to the thickness of 0.60m, 0.66m, 0.86m and 1.10m red brick. performance.
Since the gas heat transfer of the insulating glass exceeds the heat transfer of the support of the vacuum glass, the heat transfer coefficient of the vacuum glass is always lower than that of the insulating glass under the same conditions of the original glass type. Table 4 Nos. 1 to 4 give the parameters of the ordinary hollow glass made of the same four sets of glass in Table 2, and the numbers 5-8 are the parameters of the same four sets of glass made of argon-filled insulating glass.
*12A means 12mm air 4L means 4mm Low-E glass
12Ar means 12mm argon 4 means 4mm white glass
*In-table parameters are calculated using window4.1 software
advantage
From the comparison of the results of Table 2 and Table 4, the vacuum glass has a much lower K value than the insulating glass, and also has the following advantages:
1. Because of the high thermal resistance, anti-condensation and frosting performance is better.
2. Since the interval is a vacuum, it has the following advantages:
Good sound insulation performance, especially low-frequency sound insulation performance is better than insulating glass with the same thickness of glass
There is no internal fog condensation problem in the presence of insulating glass
There is no problem of gas thermal conductivity change when the insulating glass is placed horizontally
There is no problem of bursting of hollow glass transported to low altitude areas of the plateau
3. Since the two sheets of glass form a rigid joint, the wind pressure resistance is higher than that of the glass of the same thickness. For example, vacuum glass made of 4mm glass has a wind pressure resistance higher than 8mm thick glass, which is more than one and a half times that of two 4mm glass hollow glass.
4. Because it is a full glass material seal, and a getter is added inside, the Low-E film used is a “hard filmâ€, which is not an off-line “soft film†that is easy to oxidize and change color. As long as the manufacturing process and equipment are advanced, vacuum glass The service life is much longer than that of insulating glass sealed with organic materials.
5. The thickness is more than double that of the insulating glass, which not only saves the window frame material, but also can be combined into a sandwich vacuum, "vacuum + hollow", "self-cleaning vacuum" and other properties as a piece of glass combined with other glass deep processing technology. "Combined vacuum glass". This compatibility with other deep processing technologies not only promotes the development of other technologies, but also compensates for the inadequacies of vacuum glass. For example, tempered vacuum glass cannot be manufactured at present, but a combination technique can be utilized to solve the safety problem.
Therefore, the characteristics of vacuum glass give it a comprehensive performance advantage.
Sound insulation mechanism
The vacuum glass and the insulating glass are completely different in structure and fabrication. The insulating glass simply bonds two pieces of glass together with an air layer in between, and the vacuum glass is sandwiched between the two pieces of glass to support the film at a high temperature. In the vacuum environment, the two pieces of glass are completely fused, and the two pieces of glass are vacuumed in the middle. The sound is not transmitted under vacuum. Of course, since the support of the vacuum glass becomes a sound bridge, the vacuum cannot reach 100% vacuum. But these supports only account for one-thousandth of the glass. These are just tiny acoustic bridges that can be ignored.
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