# Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing
## 3 Results and Discussion
### Effect of Vacuum Pressure Diminution on the Thermal Performance of TVG  
A vacuum insulation is a cavity, not an empty space between two glass panes, as there is no concept of the absolute emptiness in high-vacuum physics, but a reduced mass of atmospheric air. The density of air in a space determines the level of vacuum pressure [35]. This provides thermal insulation, because with a lower density of air, the mean free path between air molecules is enlarged to above 1000 m, which eventually lowers the heat-transfer path between the air molecules in a space. In TVG (the current study), the space between glass panes is sealed with a hermetic edge seal, i.e., Cerasolzer CS-186, and evacuated to high vacuum pressure 0.001 Pa, with the intention of lowering conductive and convective heat transfer to negligible values. However, the radiation heat transfer is curtailed using low-emittance SnO\({}_{2}\) coatings. The influence of vacuum pressure decrement due to the vacuum edge seal is usually caused by either degradation due to adsorption of gaseous molecules, mechanical handling, and/or inconsistent making of the edge seal or the edge seal not being congruent. The FEM results, as shown in Figure 6, predict that when increasing the cavity pressure from 0.001 Pa to 100 kPa, the \(U_{C}\) and \(U_{I}\) values increased from 0.28 Wm\({}^{-2}\) K\({}^{-1}\) and 0.94 Wm\({}^{-2}\) K\({}^{-1}\) to 2.4 Wm\({}^{-2}\) K\({}^{-1}\) and 2.58 Wm\({}^{-2}\) K\({}^{-1}\), respectively. However, due to the ageing process, the TVG may lose the vacuum pressure and this would affect its thermal performance, mainly because of the water vapours evolved from the glass surfaces during the fabrication which had not desorbed from the glass surfaces and remained in the vacuum space. Thus, the use of nonevaporable getters sealed under specific temperature cycles is necessary. This is because water molecules that were outgassed during high-temperature thermal ageing and readsorbed onto the glass surfaces at room temperatures can ruin the stability of the internal vacuum pressure, unless nonevaporable getters are used. Figure 5: The isothermal temperature distribution (in \({}^{\circ}\)C) on the cold-side glass pane surface of the TVG, showing the temperature disparities from the 10-mm wide edge-sealed area towards the centre-of-pane area. Figure 7 shows the surface temperature contours for the hot and cold sides at the cavity pressures of 0.001 Pa, 1 Pa, and 50 kPa, spreading from the vacuum edge seal to the centre-of-pane of one quarter of the TVG along Line B and Line A (as illustrated in Figure 7a). It shows that the temperature gradients between the hot and cold sides increase by decreasing the cavity vacuum pressure from 50 kPa to 0.001 Pa. A reduction in cavity vacuum pressure affects the surface heat transfer quite significantly. However, the temperature gradients on the cold side are larger than on the warm side, due to the direction of the heat flow on the hot side dissipating heat through the edge seal and influencing towards the centre-of-pane area, consequently developing edge effects that spread up to distances of 38 mm and 70 mm from the edge to the centre-of-pane on the hot and cold side surfaces along Line A (which is nearer to the edge corners). Nevertheless, at 50 kPa, the edge effects to the centre-of-pane are smaller along line A (about 26 mm and 30 mm on the hot and cold sides, respectively) because of the diminution of the vacuum pressure affecting temperature gradients. Line B is approximately 130 mm away from the horizontal axis of the TVG quarter; it shows that the edge effects reduce when moving away from the edge corners. When comparing the surface temperature contours along Line A and Line B on the hot and cold sides, the temperature gradients on Line B was increased compared to Line A at the cavity vacuum pressures of 0.001 Pa and 1.0 Pa. By reducing the width of the edge seal, further reduction to the edge effects were achievable.