Unnecessary composition of external vacuum degree of vacuum insulation panel and improvement measures
Vacuum insulation panel (VIP) is a kind of ultra-insulation fresh-keeping material. It is thin and thin, normally around 17mm. Its thermal conductivity is extremely low, which can reach around 0.004W / (m & middot; K), while the normal fresh-keeping material thermal conductivity It is about 0.03W / (m & middot; K). The scale of use of VIP is relatively broad, and it has been used in many fields such as refrigerators, cold storages, refrigerated containers and medical preservation containers. The performance of the vacuum insulation panel and the length of its service life are largely at the level of the external vacuum of the VIP. Therefore, Bai Wen did a comprehensive study of the ingredients that do not need VIP external vacuum degree: gas penetration and external core material degassing, which provides basis for VIP production. 1.There is no need to change the vacuum level in the plate.
The core layer of VIP is mostly powder or fiber. In the thermal conduction, the heat conduction of gas plays a great role. If the core layer is evacuated, the thermal conductivity of VIP core material can be significantly increased. The performance and extension of the vacuum insulation panel are more unprofitable. Figure 1 shows the relationship between the thermal conductivity of typical core vacuum insulation panels and the gas pressure in the panels.
1. Glass fiber 2. Polyurethane foam 3. Polystyrene foam 4. Sedimentation method SiO2 5. Nanoporous silica material
Figure 1 The relationship between the thermal conductivity of a typical core material vacuum insulation board and the gas pressure in the board
It can be seen from Figure 1 that when the gas pressure drops from atmospheric pressure to 103Pa, the thermal conductivity remains constant, that is, the thermal conductivity in the pressure scale is not dry with the gas pressure in the insulation layer; when the pressure further increases, the pressure is 1 ~ 103Pa Between them, the thermal conductivity also tends to increase with the increase of pressure; when the pressure is lower than 1Pa, the thermal conductivity tends to be constant, and then the percentage of gas thermal conductivity is small. 2, no need to have the vacuum degree of the vacuum insulation composition
2.1, gas penetration
2.1.1. Forms of gas penetration
① When the VIP core material is vacuum-sealed, a large amount of air will enter, and the gas of the department includes nitrogen, oxygen, carbon dioxide, and water vapor;
② Regarding the waste core material, a large amount of air will infiltrate through the edge bond;
③ In the operation, a large number of gases will also enter the VIP through the nominal mulch;
④Because the outside of the VIP is a vacuum condition, the gas and water vapor dissolved in the external core material will produce a vacuum outgassing scene during the operation.
2.1.2, there is no need for gas penetration
The core insulation material is porous and easily attracts moisture. In addition, the core layer data will also release the entire moisture and gas due to the formation of old materials. The thermal conductivity of water is 25 times that of the atmosphere. Research has shown that: when the moisture content is greater than 5% to 10%, the tidal dragon pans out the pore space of the empty medium after the moisture absorption of the adiabatic data, resulting in a sharp increase in its thermal conductivity. Thermal insulation coefficient of 0.03W / m & middot; K, after absorbing 1% moisture, the thermal conductivity increases by 25%, and the current discount increases. At that time, the adiabatic data had stomata communicating with the stomata, and the moisture would permeate the remaining parts under the action of the capillaries, which was even more harmful. With the extension of time, more and more gases penetrate into the vacuum insulation board, and the vacuum pressure in the board will rise back to a certain level, which will increase the vacuum degree of the VIP, and even increase the thermal conductivity. There is no need for adiabatic motivation. Figures 2 and 3 show the changes in pressure as water vapor and oxygen continue to penetrate.
Figure 2 Changes in oxygen permeation rate and pressure in the vacuum plate with time
1. Open-cell foamed plastic 2. Nano-porous core material 3. SiO2 deposition method
Figure 3 Changes in pressure and water vapor penetration in three vacuum plates
Because the theoretical open cell ratio of the foam core material is about 95%, there are still a large number of closed cells. Under vacuum conditions, the gas in the closed-cell foam and the volatile components such as extremely large foaming agents and amine catalysts that dissolve in the foam wall will escape turbulently, resulting in a vacuum outgassing scene; The internal preheating (approximately 50 ° C) increases the nominal heat of the core material, slowing down the escape of the above-mentioned gas, and the released gas causes an increase in the vacuum degree, which improves the thermal insulation performance. The relationship between core material outgassing and pressure is shown in Figure 4. In the past 20 years, with the extension of time, the gas turbulently escapes and is gradually added, so that the external pressure of the core material rises, so the thermal conductivity increases, and the VIP Improved sound insulation.
Figure 420 The relationship between the amount of gas released and the change in external pressure in 3
The degree of vacuum is an insignificant trade-off between the performance of the vacuum insulation panel and the length of service life. In order to ensure a high degree of vacuum in the panel and popularize the sound insulation performance of the VIP, we can start by reducing gas penetration and core material outgassing.
3.1. Reduce gas penetration
3.1.1. Selection of nominal mulch
In the later period, non-metallic films were adopted.Although this film has a strong resistance to gas penetration, non-metals increase the preheating of the board, and the heat will continue to permeate along the edges containing non-metals, which will cause heat to break through. Significant, compared with the nominal barrier that was widely used at the time, the caloric expenditure has increased by 49%. Therefore, in order to make the barrier not only provide strong resistance to gas penetration, but also reduce heat transmission at the maximum level, most of the time, non-metal and plastic composite films were adopted, and the most widely used aluminum-plated film, which can effectively Control the gas and water vapor penetration rate to maintain it at 1 g / m2 or less per day. Table 1 shows the comparison between some new nominal barrier data and traditional nominal barrier data in terms of resistance to gas penetration under typical application conditions.