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In the previous article Discuss the relationship between the K value of insulating glass and the gas interlayer①, we focused on the determination of the thermal resistance (R’) of the gas interlayer on insulating glass processing, and the next, we will continue to introduce the calculation of glass K value, the influence of glass type on K value...

4. Calculation of glass K value, the influence of glass type on K value

4.1 Calculation of glass K value

As we all know, glass occupies the largest area of aluminum alloy doors and windows, and the heat transfer of glass is the decisive factor for heat transfer throughout the doors and windows. Therefore, if you want to achieve satisfactory results in the heat transfer of aluminum alloy doors and windows, you must control the type of glass. . 

Here are several examples of the calculation of the K value of glass to compare and explain the K value of glass:

Calculation of the K value of insulating glass: (take 6+9+6 double white insulating glass as an example)   

Find the total thermal resistance of insulating glass R0  

R0=Ri +∑R +Re R0= Ri +R1+R’+R2 +Re  

R1——Thermal resistance of inner glass (㎡·K/W)  

R2——Thermal resistance of external glass (㎡·K/W)  

R’ ——Thermal resistance of gas interlayer (㎡·K/W)   

Known: Ri=0.125 ㎡·K/W 

Summer: Re=0.053 ㎡·K/W   

Winter: Re=0.043 ㎡·K/W  

R’ Lookup Table 1 to get:   

Summer: R’=0.117 ㎡·K/W   

Winter: R’=0.135 ㎡·K/W R1=R2=δ/λ  

δ—the material thickness (m)λ-Thermal conductivity (W/m.K) Lookup 

table 1: The thermal conductivity of glass is 0.76 (W/m.K) 

R1=R2=0.006/0.76=0.00789㎡·K/W

Total thermal resistance of glass 

Summer: R0 =0.125+0.00789+0.117+0.00789+0.053=0.311㎡·K/W

Winter: R0 =0.125+0.00789+0.135+0.00789+0.043=0.319㎡·K/W

6+9+6 double white hollow glass K value: K=1/R0

Summer: K=1/0.311=3.2 W/㎡·K

Winter: K=1/0.319=3.1 W/㎡·K

The same calculation method, the K value list of different types of insulating glass is as follows:

NumberGlass typeSummer K value (W/㎡.K)Winter K value 
(W/㎡.K)
16+6+6 double white hollow glass3.53 3.45
26+9+6 double white hollow glass3.23.1
36+12+6 Double white hollow glass3.13.06
46+6+6 double white hollow glass (filled with argon)3.23.1
56+9+6 double white hollow glass (filled with argon)2.962.9
66+12+6 double white hollow glass (filled with argon)2.92.8

4.2 The influence of glass type on K value

4.2.1 In addition to air filling, the insulating glass interlayer can also be filled with inert gas in a stable state. (such as argon, krypton, etc.) will improve the insulation performance of insulating glass to varying degrees. This is because of the inert gas The thermal conductivity is lower than that of air, (the thermal conductivity of air is 0.024W/mK, the thermal conductivity of argon is 0.016 W/mK, and the thermal conductivity of krypton is 0.0087W/mK). At the same time, the density, specific heat, and dynamic viscosity of the inert gas also play a certain role in the insulation effect. At present, the most commonly used inert gas is argon. This is because the content of argon in the air is relatively abundant, the extraction is easier, and the cost is lower.

4.2.2 The heat transfer coefficient of insulating glass decreases with the increase of the insulating glass interlayer, and the better the thermal insulation effect, but the change of the gas interlayer is not proportional to the change of the gas thermal resistance. This problem can be reflected by the gas thermal resistance (R’) and interlayer thickness (H) curve:

The reflected by the gas thermal resistance (R’) and interlayer thickness (H) curve

The reflected by the gas thermal resistance (R’) and interlayer thickness (H) curve

4.2.3 When the gas interlayer is below 1.6cm, the thermal resistance of the air in summer is from 0.086~0.134㎡·K/W, the difference is 0.048 ㎡·K/W, and the difference is from 0.103~0.154 ㎡·K/W in winter. It is 0.051 ㎡·K/W. When the gas interlayer is above 1.6cm, the thermal resistance of the air in summer is 0.134~0.146㎡·K/W, the difference is 0.012 ㎡·K/W, and the winter is from 0.152~0.181 ㎡·K/W, the difference is 0.027㎡ ·K/W. In comparison, it is found that when the gas interlayer is more than 1.6cm, the change in thermal resistance is minimal. On the contrary, the larger the interlayer, the more air will participate in convection, which will reduce the thermal resistance, and the same is true for the argon-filled interlayer. Therefore, the larger the gas interlayer, the better the thermal insulation effect, and the ideal thickness is 1.6cm or less.  

4.3 Calculation of K value of special insulating glass:  

With the development of the construction industry and the continuous improvement of building energy-saving standards, ordinary insulating glass can no longer meet the requirements of building energy-saving, and some new insulating glass products are needed to replace it. The following examples illustrate the thermal insulation of several products:  

4.3.1 Three-layer insulating glass and K value calculation (take 5+6+5+6+5 white glass insulating glass as an example)

Find the total thermal resistance of insulating glass R0

R0=Ri +∑R +Re R0= Ri +3R1+2R’ +Re

R1—Glass thermal resistance (㎡.K/W)

R’—Thermal resistance of gas interlayer (㎡·K/W)

Known: Ri=0.125 ㎡·K/W

Summer: Re=0.053 ㎡·K/W

Winter: Re=0.043 ㎡·K/W          

R’ Lookup Table 2 to get:

Summer: R’=0.093 ㎡·K/W

Winter: R’=0.11 ㎡·K/W          

R1=δ/λδ—the material thickness (m)λ-Thermal conductivity (W/m·K) 

Lookup table 1: The thermal conductivity of glass is 0.76 (W/m·K)     

R1=R2=0.005/0.76=0.0066㎡·K/W

Total thermal resistance of glass Summer: 

R0 =0.125+0.0066X3+0.093X2+0.053=0.384㎡·K/W

Winter: R0 =0.125+0.0066X3+0.11X2+0.043=0.408㎡·K/W             

Hollow glass K value: K=1/R0           

Summer: K=1/0.384=2.6 W/㎡·K           

Winter: K=1/0.409=2.4 W/㎡·K

The same calculation method, the K value list of different types of three-layer insulating glass is as follows:

NumberGlass typeSummer K value
(W/㎡·K)
Winter K value
(W/㎡·K)
15+6+5+6+5 white glass hollow glass2.62.4
25+9+5+9+5 white glass hollow glass2.32.2
35+12+5+12+5 white glass hollow glass2.252.11
45+6+5+6+5 White glass hollow glass (filled with argon)2.272.14
55+9+5+9+5 white glass hollow glass (charged argon)2.071.96

4.3.2 By calculating the K value of insulating glass, the following conclusions can be drawn:

A. Compared with the K value of ordinary insulating glass, the K value of three-layer insulating glass is greatly reduced. This is due to the multiple gas interlayers of insulating glass, which greatly improves the insulation performance of insulating glass...  

B. Compared with ordinary insulating glass, three-layer insulating glass has increased weight and thickness, which greatly changes the shape and size of door and window frame materials. At the same time, very high requirements are placed on the accessories (corners, hinges, etc.) of doors and windows, which causes an increase in cost. Therefore, three-layer insulating glass is rarely used at this stage.  

C. Insulating glass composed of stained glass and coated glassStained glass is also called heat-absorbing glass. It uses the coloring of the body to increase the absorption of sunlight heat and reduce the penetration of sunlight heat. Due to outdoor convection, part of the absorbed sunlight heat will be taken away, thereby reducing the sun. The degree of radiant heat entering the room, but its control of far-infrared rays is the same as that of ordinary white glass, and the emissivity is 0.837. Therefore, under the same conditions, the K value of the composition of the hollow glass is the same as that of the ordinary white glass.

D. Coated glass is also called heat-reflective glass. It has good transparency in the wavelength range of 0.4μm~0.7μm and has high reflectivity for ultraviolet light with a wavelength of less than 0.4μm and infrared light with a wavelength of 0.7~2.5μm. And 97% of the solar radiation heat is concentrated in the wavelength range of 0.3μm~2.5μm, so it has a good reflection effect on the solar radiation heat but has no obvious reflection effect on the far-infrared heat radiation, so, under the same conditions, In this case, the K value of the hollow glass is similar to that of ordinary white glass.   

4.3.3 Low-E glass and hollow glass compositiona. 

A. Low-E glass is also called low-radiation coated glass. It has a 60% transmittance to solar radiation heat in the wavelength range of 0.3μm~2.5μm and has a high reflection effect on far-infrared rays of 4.5μm~25μm. It reflects back 80% of the far-infrared radiant heat transferred from the room, and can effectively enter the solar radiant heat into the room, so it has the function of a one-way valve.  

B. Low-E glass is divided into two types: one is Low-E glass produced by an online high-temperature pyrolysis deposition method, and the other is Low-E glass produced by offline vacuum sputtering method. The advantage of Low-E glass produced by the online high-temperature pyrolysis deposition method is that it can be hot-bent and toughened, and it does not need to be used in a hollow state. Disadvantages: The thermal performance is poorer than that of Low-E glass produced by offline vacuum sputtering. The emissivity of most online Low-E glass is between 0.35 and 0.5. Unless the thickness of the coating is increased, the thickening of the coating will cause The transparency of the glass to be very poor. The biggest advantage of Low-E glass produced by the offline vacuum sputtering method is: it has good far-infrared reflection ability, K value is far better than online Low-E glass, and its emissivity is between 0.08 and 0.15. The disadvantage is: the silver oxide film is relatively fragile and must be made into the hollow glass before it can be used. It cannot be transported long distances before it is made into a hollow.

C. The K value of hollow glass composed of Low-E glass changes with the change of its emissivity. This is because the change of emissivity will directly affect the change of the thermal resistance of the gas interlayer. It can be compared through the following list. 

Found its changes: (Table 3) The thermal resistance of gas interlayer of Low-E insulating glass with different emissivity: 

(R’)EmissivityWinter state
Interlayer thickness δ (cm)
0.512345 or more
0.080.2430.4170.5170.4950.4790.468
0.150.2360.3850.4660.4480.4350.430
0.350.1640.2700.3200.3080.3010.299
0.50.1500.2380.2760.2680.2620.258
(R’)EmissivitySummer state
Interlayer thickness δ (cm)
0.51234~56 or more
0.080.2270.3770.6150.5890.560.55
0.150.2150.3380.5070.4900.4700.463
0.350.1390.2170.3010.2940.2850.282
0.50.1250.1850.2440.2390.2330.230

D. The thermal resistance value of the gas interlayer decreases with the increase of Low-E glass emissivity, making the K value of Low-E insulating glass greatly increase.e. It can be seen from Table 3 that the thermal resistance value of the Low-E insulating glass interlayer when the thickness of the interlayer is less than or equal to 1.6cm, the thermal resistance value Roof the layer increases with the increase of the thickness of the interlayer, when the gas interlayer When the thickness is greater than or equal to 1.6cm, the thermal resistance value Roof the interlayer decreases slightly with the increase of the thickness of the interlayer.   

Low-E insulating glass with different emissivity K value comparison:

EmissivityGlass typeSummer K value
 (W/㎡·K)
Winter K value
 (W/㎡·K)
0.086+12+6 outer Low-E inner white hollow glass1.71.7
0.156+12+6 outer Low-E inner white hollow glass1.81.8
0.356+12+6 outer Low-E inner white hollow glass2.22.3
0.56+12+6 outer Low-E inner white hollow glass2.32.6

5. Summary

Through the calculation of the heat transfer coefficient of glass, this article aims to understand the heat transfer process of glass and deepen the understanding of the concept of heat transfer of doors and windows. Through the calculation of the heat transfer coefficient, you can understand the development direction of energy saving of doors and windows and know which links are important for energy saving of doors and windows. Links, what aspects of glass and profiles can be improved, and what is the effect after improvement. At the same time, you can simply calculate the K value of the glass according to the calculation example to provide a reference for the design.

The testing equipment for insulating glass argon gas filling K value 1

The testing equipment for insulating glass argon gas filling K value 1

By the way, If you are interested in automatic inflating equipment and manual inflating equipment in the production and processing of insulating glass, please click on the vertical insulating glass air-floating production line and the manual insulating glass gas filling inflator machine to learn more...

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