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Is the thickness of the gas layer of Low-E insulating glass better?

2020-08-28

Is the thickness of the gas layer of Low-E insulating glass better?

Gas layer knowledge
Is the thickness of the gas layer of Low-E insulating glass better?
Taking the commonly used single cavity Low-E insulating glass as an example, the relationship between K value and the number of air layers can be obtained by measuring the Low-E insulating glass with different air layer thickness (see the figure below). The K value of Chinese standard is basically consistent with the U value curve of American Standard in winter. When the air layer is 6 mm, the K value is the highest, when it is 9 mm, the K value decreases by about 0.4 [WW (M2 • K)], and reaches the lowest value when it drops to 12 mm,Therefore, 12mm should be the best thickness of air layer.The results show that the thicker the gas layer is, the better the gas layer is, but there is an optimal thickness of 12mm.

Fig. K value curve of Low-E insulating glass with air layer thickness
In fact, there are two ways to transfer heat in the gas layer

The first is at the molecular level.Through the collision and diffusion between gas molecules, the heat is transferred to the other end. The thicker the gas layer is, the more energy the molecules lose in the process of transfer.On the macro level, the thicker the gas layer is, the greater the thermal resistance is, and the less heat is transferred. The heat transferred through this way is inversely proportional to the thickness of the gas layer.

The second way is the directional flow of gas.The gas in contact with the hot glass surface (near the indoor surface in winter) is heated, and the gas in contact with the cold glass surface (outside the room in winter) has a downward directional flow trend. As a result, the gas circulation heat transfer is formed.When the gas layer is thin, due to the restriction of the gas internal viscous force, only a small range of circulation can be formed, so the heat transferred is very little; while when the gas layer thickness increases, the restriction of the viscous force is weakened, the circulation range is increased, and the heat transfer is increased.The heat transferred in this way is proportional to the thickness of the gas layer.

In short, when the gas layer is thin, the heat transfer of the first path is dominant, and the K value will decrease with the increase of the gas layer thickness; when the gas layer thickness reaches a certain degree, the heat transfer of the second way will occupy the dominant position and the influence will be greater, and the K value will increase.

In fact, if the temperature difference between the glass on both sides of the gas layer is very small, that is, the hot glass is not hot enough and the cold glass is not cold enough, then the directional flow trend of the gas near the glass surface is very weak, and the heat transferred by the gas circulation can only occupy a dominant position when the gas layer is thicker,Therefore, the optimal thickness of gas layer will extend to the thickness with the decrease of temperature difference between two pieces of glass.In winter, the temperature difference between indoor and outdoor in Chinese standard and American standard is nearly 40 ℃, and the optimal thickness of gas layer is 12mm; under European standard condition, the temperature difference between indoor and outdoor is 15 ℃ and the optimal thickness of gas layer is 16mm; the minimum temperature difference of American Standard summer condition is only 8 ℃, so the optimal thickness of gas layer is 25mm (see the figure above).

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