Summary:

In this paper, the rationality of the whole window configuration is preliminarily analyzed through the analysis of the cumulative proportion of heat flow and the temperature difference of the frame glass. It is recommended that the U-value difference between the frame and the glass is not more than 1.5.

  Key words:

Reasonable, box U value, cumulative ratio

It has almost become a consensus to choose broken bridge aluminum and nylon heat insulation strips for energy-saving windows. However, there are still many misunderstandings about how to propose the whole window configuration for specific energy-saving requirements. For example, it is often seen that ordinary aluminum is equipped with Low-E insulating glass, and 14.8 heat insulation strips are used in the configuration of the whole window 1.8, and so on. This involves the rationality of matching, that is, how to match the heat transfer coefficient (ie U value) of the frame and the glass.

1. The importance of energy saving and comfort

In TED's "How Behavioral Science Can Save Energy," the speaker showed that a can of coal has the energy to keep a lightbulb on for a year. However, due to the relationship between conduction and heat dissipation in the process of power generation and transmission, only 10% of the energy is actually converted into light energy for use, and the remaining 90% is wasted. Then, to achieve the same effect (the light bulb is on for a year), a whole carload of coal is needed, and that one can of coal is a drop in the bucket. On the other hand, every kWh of electricity we save actually saves 10 kWh of coal for the global village!

Therefore, our government's goal is to achieve 65% energy saving nationwide by 2020. Table 1 below is the progress so far. It can be seen that half of the provinces and cities are already implementing the 65% standard, while Beijing, Tianjin, Hebei and Shandong have already started to implement the 75% energy saving standard.

However, the actual implementation is not optimistic. According to the sales volume of thermal insulation strips, the 14.8 thermal insulation strips in Northeast and North China still account for the majority. It feels like 14.8, which can take all the world. A considerable part of this is because the configuration of the whole window is not clear, and there are still many misunderstandings. For example, the 14.8 heat insulation strip is used in the configuration of the whole window 1.8, and some even ordinary aluminum and Low-E can be found everywhere. And various problems caused by these unreasonable, such as Low-E glass breakage, condensation, cold radiation, etc., have greatly reduced the comfort experience of occupants.

condensation phenomenon

Second, the method of comparison:

According to the international standard ISO 15099-2003 section 8.3.2 on indoor side convection heat transfer regulations, the calculation of heat transfer is in accordance with formula 130, and according to thermal engineering regulations JGJT 151, China's indoor side boundary is a natural convection state, then comprehensively consider Formula 131-139 in ISO15099, we can get:

The convective heat transfer on the indoor side is approximately proportional to the temperature difference between the indoor side surface temperature and the air

Starting from the contact point between the frame and the wall, the heat loss ratio of each segment is accumulated, which is called the cumulative heat flow ratio. The cumulative ratio of the heat flow at the top of the glass is 1, which is 100%.

Specific to this article, as follows:

1. Select THERM/Window software and open the CR model, so that the accurate surface temperature can be obtained.

2. The U value of the frame is determined by the width of the thermal insulation strip, so the inward opening windows are selected, which are ordinary aluminum, 14.8, and 24 thermal insulation strips, respectively. The corresponding U values of the frame are 5.5, 3.6, and 2.9 W/m2∙K, respectively. Match different glass to obtain glass U value of 1.6, 2.0, 2.7. (Table 3) The glass-to-frame ratio was kept at 70%.

3. The obtained inner surface temperature is shown in Figure 3. The abscissa is the cumulative distance from each point on the inner surface to the sightline (the side length of the inner surface of each grid). Sightline refers to the lower edge of the visible part of the glass. The negative value of the abscissa represents the inner surface of the box, and the positive value represents the inner surface of the frame. The inner surface of the glass. Therefore, in the cumulative ratio diagram obtained, the value corresponding to the abscissa 0 is the ratio represented by the box, because the left side of the y-axis is the box, and the right side is the glass.

4. Because of the existence of glass spacers, the edge part of the glass (transparent part within the range of 63.5mm to the sightline) is larger than the central part, so it is necessary to count the cumulative proportion of heat flow in this section.

U value difference between frame and glass

3. Comparison results:

According to the above steps, the cumulative ratio obtained is shown in Table 4. The yellow part represents the ratio of the frame + glass edge part. The data sorted by the difference in U value of the frame and glass are shown in Table 5 and Figure 6.

As can be seen from the above figure:

For ordinary aluminum windows, the proportion of flow through the frame is more than 55%. That is, more than half of the heat flows away from the window frame. The more heat is lost, the lower the surface temperature inside the frame (Figure 7), and the increased heat exchange with the glass edge portion pulls down the glass edge temperature.

For the node using the 14.8 insulation strip, the heat passing through the frame increased from 39% to 49% as the glass U-value decreased from 2.7 to 2.0 to 1.6.

From the cumulative ratio trend chart in Figure 6, the box and box + glass edge ratios have an inflection point at 2.0. For a common window type with a glass-to-frame ratio of 70%, it is unreasonable if the heat loss from the window frame exceeds 45%, so the difference between the u value of the frame and the glass should be less than 1.5.

Figure 7 shows the temperature distribution on the indoor side of three types of window frames with 1.6mm insulating glass. It can be clearly seen that when the difference between the frame and the glass is small, the temperature difference is also small.

The temperature difference between the frame and the glass surface is large, which is easy to cause:

The temperature difference of the glass plate increases: for example, the surface temperature of the ordinary aluminum window frame is low, which increases the radiative heat exchange with the edge of the Low-E glass, which increases the temperature difference between the center and the edge of the glass, thereby increasing the thermal stress of the glass. The result is Easy to burst. If the glass is cut irregularly and the resulting edge crack is large, the probability of thermal cracking is higher.

Poor comfort: There is radiative heat exchange between the window and the occupant, and the glass and window frame feel significantly different. Therefore, the passive house specifically stipulates that the temperature difference between the window frame and glass and the indoor temperature should not be greater than 3 degrees.

Condensation: The temperature of ordinary aluminum window frames is low, which is more prone to condensation or mildew.

4. Summary

From the above analysis, it can be understood that the greater the difference between the U value of the frame and the glass, the higher the proportion of heat flowing through the window frame, and the greater the surface temperature difference between the frame and the glass, which is detrimental to the thermal comfort of the user. For example, when ordinary aluminum is matched with Low-E, the window frame loses more than 55% of the heat of the entire window, which increases the temperature difference between the center and the edge of the glass, which can easily lead to thermal cracking. If 45% is used as the critical point, it is reasonable that the difference between the U value of the frame and the glass is less than 1.5.