Discuss the relationship between the K value of insulating glass and the gas interlayer①

1. Foreword

At present, my country's building energy consumption (including construction energy consumption and use energy consumption) is about 1/4 of the country's total energy consumption. In terms of building energy consumption, it is mainly reflected in winter heat preservation and summer cooling, and building exterior protection. Among the four major components of the structure (door and window curtain wall, wall, roof, and ground), the heat transfer coefficient of the door and window curtain wall is the largest, which is the weakest link in building heat preservation and heat insulation. According to statistics, the energy consumption of door and window curtain walls accounts for the total amount of enclosure structures. 1/2 of energy consumption. Under the conditions of heating or air conditioning in winter, the energy consumption of single-glazed windows accounts for 30% to 50% of the total heating load. In summer, the heat of the single-glazed windows caused by the sun's radiation causes the indoor air-conditioning to be cold. The loss of energy is about 20-30% of the air-conditioning load. It can be seen that enhancing the thermal insulation performance of doors, windows, and curtain walls is an important part of reducing building energy consumption and improving the indoor living environment.

Three factors cause energy loss in doors and windows: energy loss caused by heat transfer and air convection in the door and window frames and the glass itself; energy loss caused by indoor and outdoor air circulation caused by gaps between doors and windows; energy loss caused by radiant heat passing through glass and frame materials . Glass accounts for about 75% of the heat transfer area of doors and windows. Therefore, to control the K value of the entire doors and windows, glass is the key.

2. Calculation of K value

In the conduction and convection heat transfer process of doors and windows, the heat transfer coefficient K value is an important indicator to measure the amount of heat transfer. The so-called heat transfer coefficient is under the condition of stable heat transfer, the air temperature difference between the two sides of the maintenance structure is 1K and passes within 1h. The heat is transferred by an area of 1㎡, in units (W/㎡.K). Different from the concept of thermal conductivity, the so-called thermal conductivity λ is the temperature difference between the two sides of a 1m thick object of 1℃ under the condition of stable heat transfer, and the heat transferred through an area of 1㎡ within 1h, in units (W/m.K). The concepts of the two are different, do not confuse them.

K=1/R0 (2-1)

K—— Heat transfer coefficient (W/㎡·K)

R0 ——Total thermal resistance (㎡·K/W)

R0=Ri +∑R +Re (2-2)

Ri——Inner surface heat transfer resistance (㎡·K/W)

R——Thermal resistance of material layer (㎡·K/W)

Re——External surface heat transfer resistance (㎡·K/W)

R=δ/λ (2-3)

Δ——the material thickness (m)

Λ—— Thermal conductivity (W/m·K)

Ri=1/αi

Αi ——Inner surface heat transfer coefficient (W/㎡·K) For glass, αi = 8 W/㎡.K

Ri=1/8=0.125 ㎡·K/W

Re=1/αe

Αe——External surface heat transfer coefficient (W/㎡·K)

αe=19 W/㎡·K in summer

αe=23 W/㎡·K in winter

Summer: Re=1/19=0.053 ㎡.K/W

Winter: Re=1/23=0.043 ㎡·K/W

3. Determination of the thermal resistance (R’) of the gas interlayer

3.1 Mathematical model

The thermal resistance of the gas interlayer depends on the thickness of the two interfaces of the interlayer and the intensity of radiative heat transfer between them. Convective heat transfer also occupies a certain proportion of the total heat transfer of the interlayer. The strength of convective heat transfer is related to the thickness of the interlayer, the set direction and shape of the interlayer, and the sealing performance.

R’=1/hs

Hs- the thermal conductivity of the gas interlayer

Hs=hg+hr

Hg- the thermal conductivity of the gas in the internal layer

Hr-radiative thermal conductivity of gas interlayer

Hr=4σ(1/ε1+1/ε2-1)-1·Tm3

Ε1ε2 The corrected emissivity of the two surfaces of the gap layer at the average absolute temperature Tm

Σ Stephen-Bolsman constant

Hg=Nuλ/s

Nu=A(Gr·Pr)n

constant

Gr-Glaschev criterion

Pr-Plant criterion

N-power exponent

Gr=(9.81s3ΔTρ2)/Tmμ

Pr=μ·c/λ

ΔT-The temperature difference between the glass surface before and after the gas interlayer, K

Ρ-gas density

Μ-dynamic viscosity of the gas

C-specific heat of gas

The heat transfer of the gas interlayer is a very complicated process. According to the above formula, the calculation of the thermal resistance of the gas interlayer less than 16mm is more accurate. As the gas interlayer increases, the convective heat transfer will increase. It is more complicated, and the thermal resistance can be calculated directly in the following table.

3.2 Thermal resistance of air interlayer

^{Table 1 Thermal resistance of air interlayer (R’)}

Calculation of equivalent thermal conductivity of gas interlayer:

Λ’=δ/R’ (2-4)

R’—Thermal resistance of gas interlayer (㎡.K/W), check according to (Table 2)

Δ—the thickness of the gas interlayer (m)

Λ’—Equivalent thermal conductivity of gas interlayer (W/m.K)

3.3 Determination of the thermal resistance (R’) when the insulating glass interlayer is filled with argon

^{Table 2 Thermal resistance (R’) when the insulating glass interlayer is filled with argon gas}

In the last article Discuss the relationship between the K value of insulating glass and the gas interlayer② , Jinan LIJIANG Glass will continue to introduce the calculation of glass K value, the influence of glass type on K value, and will summarize the K value of insulating glass gas filling.