Calculation and Detection Methods of Several Insulating Glass Heat Transfer Coefficients.

1. The preface

Insulating glass is a glass structure with good thermal insulation, heat insulation, and sound insulation properties, and has been widely used in civil buildings. In general buildings, the area of doors and windows accounts for 20% to 30% of the building envelope, while the energy consumption of doors and windows accounts for 40% to 50% of the heat loss of the building envelope, and the energy consumption of glass curtain walls is even greater. This accounts for a considerable proportion of building energy consumption. Glass is the thinnest and most heat-conducting material in the building envelope, accounting for about 90% of the heat consumption of the entire exterior doors, windows, and curtain walls. As an important building energy-saving material, insulating glass is playing an increasingly important role in promoting building energy conservation. But in terms of energy saving in the whole building, glass is the weakest link in the energy saving of building doors, windows, and curtain walls. The heat transfer coefficient of glass is one of the important indicators of the thermal performance of glass, and it is also an important limiting indicator for glass in building energy-saving design standards. Insulating glass is an important part of doors, windows, and curtain walls. The heat transfer coefficient of insulating glass directly affects the heat transfer coefficient of doors, windows, and curtain walls, and even the energy-saving effect of the entire building. Only by testing and calculating the heat transfer coefficient of insulating glass can we truly understand and grasp the energy-saving characteristics of glass, and promote the sustainable development and wide application of energy-saving glass. Therefore, LIJIANG Glass will focus on the introduction of several calculation and test methods for the heat transfer coefficient of insulating glass, providing references for those engaged in the production, design, construction, and testing of architectural glass.

 2.The test and calculation method

2.1 The test method

According to relevant industry standards "Determination of visible light transmittance, direct sunlight transmittance, total solar energy transmittance, ultraviolet transmittance and related window glass parameters of architectural glass", the emissivity of the glass surface is tested by Fourier transform infrared spectrometer, and then passed " Calculation and determination of the steady-state U value (heat transfer coefficient) of insulating glass" or the relevant formulas in "Thermal Engineering Calculation Regulations for Architectural Windows and Glass Curtain Walls" to calculate the steady-state U value (heat transfer coefficient) of the glass

Taking the industry standard "Calculation and Measurement of Insulating Glass Steady State U Value (Heat Transfer Coefficient)" as an example, the main calculation formula of U value (heat transfer coefficient) is as follows:

Average heating power applied to metering area

In the case of a glass indoor surface heat transfer coefficient;

• h_e -- the external surface heat transfer coefficient of glass;
• h_t -- the thermal conductivity coefficient of multilayer glazing system content;
• h_i -- the Indoor surface heat transfer coefficient of glass;
• h_S --the thermal conductivity coefficient of the gas spacer layer;
• N -- the number of gas interval layers;
• M -- the number of material layers;
• d_m -- the thickness of each material layer;
• r_m -- the thermal resistance coefficient of each material;
• h_g -- the gas heat transfer coefficient;
• δ -- the Stefan-Boltzmann constant;
• T_m -- the average absolute temperature of the gas compartment;
• δ₁、δ₂ -- the corrected emissivity of the two glass surfaces in the spacer layer at the average temperature.

The heat transfer coefficient of the outdoor surface of the glass is a function of the wind speed near the glass. When comparing the U value of the insulating glass, the heat transfer coefficient of the outdoor surface can be selected as 23W/m'K). This calculation process does not consider the impact of the U-value improvement due to the emissivity-corrected coating surface on the outside of the glass

From the above formula, it can be seen that when calculating the heat transfer coefficient inside the multilayer glass system and the heat transfer coefficient of the indoor surface of the glass, the emissivity of the glass surface needs to be considered. For ordinary glass (clean, uncoated glass) surfaces, the corrected emissivity is generally taken as 0.837. For the coated glass surface, the standard emissivity is measured by an infrared spectrometer, and the corrected emissivity is calculated according to Table A2 in Appendix A of "Calculation and Measurement of Insulating Glass Steady State U Value (Heat Transfer Coefficient)".

According to the test method of "Determination of visible light transmittance, direct sunlight transmittance, total solar energy transmittance, ultraviolet transmittance and related window glass parameters of architectural glass" and the characteristics of testing equipment on the market, the sample size is generally 100mm in length and 100mm in width. Large-sized samples need to be set up in a test darkroom. For insulating glass, it needs to be cut into a single piece of glass before performing a single-piece test. If the sample is low-emissivity coated insulating glass, it should be tested in time after cutting to avoid the film surface being oxidized and affecting the test results. Before the test, generally wipe the glass surface with absolute ethanol, and the surface of the sample should be kept clean during the test.

2.2 Main features

From the relevant calculation formulas of this method, it can be seen that the glass heat transfer coefficient is related to the glass thickness, cavity type, cavity thickness, and glass surface emissivity, and has nothing to do with the transmittance and front/rear reflectance of the glass in the visible and solar bands. The key issue of this method is how to accurately measure the emissivity of the glass surface. The calculation method of glass heat transfer coefficient in the "Calculation Regulations for Thermal Engineering of Building Doors, Windows, and Glass Curtain Walls" is relatively cumbersome, and generally requires supporting special calculation software (such as Therm (5~7) series, Window (5~7) series etc.), while the calculation method of "Calculation and Measurement of Insulating Glass Steady State U Value (Heat Transfer Coefficient)" is more concise and clear, and the test time is relatively short.

3. The protective hot plate method

3.1 The test method

Use a protective hot plate device to measure the constant heat flow through the metering unit, and calculate the thermal resistance of the insulating glass based on the area of the metering unit and the temperature difference between the hot and cold surfaces of the insulating glass, plus the heat transfer resistance of the inner and outer surfaces, the total thermal resistance is obtained and finally converted to the heat transfer coefficient. The test methods for the protective hot plate method in "Calculation and Determination of the Steady State U Value (Heat Transfer Coefficient) of Insulating Glass" and "Determination of the Steady State Thermal Resistance and Related Properties of Thermal Insulation Materials by the Protective Hot Plate Method" are the same. Taking the standard "Calculation and Measurement of Insulating Glass Steady U Value (Heat Transfer Coefficient)" as an example, the main calculation formula for U value (heat transfer coefficient) i1`s as follows:

In this formula

• R -- the thermal resistance of insulating glass
• h_e -- the outdoor surface heat transfer coefficient of the glass;
• h_i -- the thermal conductivity inside the multi-layer glass system;
• A-- the measuring area
• T₁、T₂ -- the average temperature of the hot and cold surfaces of the sample;
• the average heating power applied to metering area

The measuring device is a protective hot plate double-sample device following the "Determination of Steady-state Thermal Resistance and Related Properties of Thermal Insulation Materials". Flat insulating glass. The difference in thickness of the two samples measured at the edge should not exceed 2%, and the two surfaces should be parallel. The inward or outward deflection of the central area of the outer surface of the sample should not be greater than 0.5mm at 283K. When this method is tested, the sample is placed vertically in the test device, and full contact between the sample and the adjacent panel is ensured. This method does not consider the influence of the coating surface orientation on the U value. If other heat transfer coefficients of the inner and outer surfaces of the glass are used to meet specific conditions, it shall be noted in the test records and reports.

3.2 Main features

The standard "Calculation and Determination of Insulating Glass Steady State U Value (Heat Transfer Coefficient)" is only for insulating glass, and the sample size is generally 800 mm*800 mm. The "Steady-state Thermal Resistance of Thermal Insulation Materials and Related Properties Determination of Protective Hot Plate Method" standard applies to a wider range of tests such as homogeneous thermal insulation materials and is one of the main standards for thermal resistance testing of building thermal insulation materials. , a sample size is a square with a side length of 300mm or 500mm. When testing the heat transfer coefficient of insulating glass using the "Steady-state Thermal Resistance and Related Properties of Thermal Insulation Materials Determination of the Protective Hot Plate Method", a flat-plate thermal conductivity meter is generally used to test the thermal resistance of the glass, and then the heat transfer coefficient is calculated. The commonly used sample size is 300mm* 300mm. In general, the guarded hot plate method is easy to install and the test operation is relatively easy, but the guarded hot plate method to test the heat transfer coefficient of insulating glass is rarely used in actual engineering.

^{Figure 1 The protective hot plate method}

4. The heat flow meter method

4.1 The test method

The heat flow meter measuring device calibrated by the calibration sample with known thermal resistance measures the heat flow of the tested sample, calculates the thermal resistance of the tested sample, and adds the heat transfer resistance of the inner and outer surfaces of the sample to obtain the total thermal resistance, and finally converts is the heat transfer coefficient. The test methods of the heat flow meter method in "Calculation and Measurement of the Steady State U Value (Heat Transfer Coefficient) of Insulating Glass" and "The Heat Flow Meter Method for the Measurement of the Steady State Thermal Resistance and Related Properties of Thermal Insulation Materials" are the same. Taking the insulating glass industry standard as an example, the main calculation formula of the U value (heat transfer coefficient) is as follows:

In this formula

• R-- the thermal resistance of insulating glass
• h_e -- the outdoor surface heat transfer coefficient
• h_i -- the indoor surface heat transfer coefficient
• q -- the heat flux density
• C₁、C₂ -- the constant
• T_m -- the calorimeter measures the average temperature of the section
• V-- the potential difference

The sample is a square flat insulating glass with a side length of 800mm, and the surface of the sample should be flat and parallel. The internal and external deflection of the central area of the outer surface of the sample should not be greater than 0.5mm at 283K. When this method is tested, the thermal resistance of the calibration sample is measured by the guarded hot plate method. Calibration specimens shall be homogeneous, non-hygroscopic material with flat parallel surfaces and similar thermal properties to the specimen being tested.

4.2 Main features

The industry standard "Calculation and Measurement of Insulating Glass Steady State U Value (Heat Transfer Coefficient)" is only for insulating glass, and the sample size is generally 800 mm*800 mm. The industry standard "Determination of Steady-state Thermal Resistance and Related Properties of Thermal Insulation Materials by Protective Hot Plate Method" applies to a wider range of tests such as homogeneous or heterogeneous thermal insulation materials, and is also the main standard for thermal resistance testing of construction engineering materials at present. One, especially for measuring the thermal resistance of inhomogeneous materials. Since the heat flow meter method is a relative measurement method, the thermal resistance of the tested sample is estimated by the ratio of the calibration sample with known thermal resistance, and the test error is relatively large. In general, the heat flow meter method is easy to install and operate, but the heat flow meter method is rarely used in actual engineering to test the heat transfer coefficient of insulating glass.

^{Figure 2 The heat flow meter method}

5. The calibration hot box method

5.1 The test method

According to the "Testing Method for Thermal Insulation Performance of Building Exterior Doors and Windows", the detection of glass heat transfer coefficient is based on the principle of steady-state heat transfer. One side of the glass specimen is a hot box, simulating the indoor temperature conditions of heating buildings in winter; the other side is a cold box, simulating the outdoor temperature and airflow velocity in winter. Under the conditions of sealing the gap of the specimen and maintaining stable air temperature, air velocity, and heat radiation on both sides of the specimen,Measure the calorific value per unit time of the heating device in the hot box, subtract the heat loss through the hot box wall, the test piece frame, the filling plate, the test piece, and the edge of the filling plate, divide by the product of the test piece area and the air temperature difference on both sides, The heat transfer coefficient K value of the specimen can be obtained. 

In the formula, 

• Q -- the heating power of heating device
• M₁ -- the heat flow coefficient of hot box wall determined by calibration test
• △θ₁ -- the difference between the area-weighted average temperature of the inner and outer surface of the hot box wall
• M₂ -- the Specimen frame heat flow coefficient determined by calibration test
• △θ₂ -- the difference between the weighted average temperature of the surface area of the hot side and the cold side of the test piece frame
• S -- the area of the filled plate
• Λ -- the thermal conductivity of the filler plate
• △θ₃ -- the average temperature difference between the hot and cold measuring surfaces of the filled plate
• Φ_edge -- Edge line heat transfer between specimen and filler plate
• A -- the area of the test piece calculated according to the size of the outer edge of the test piece
• T₁ -- the thermometric air temperature
• T₂ -- the cold air temperature

The test piece should be a glass plate of 800mm*1250mm, and the size of the hole for installing the test piece should be less than 820mm*1270mm. When the size of the opening is greater than 820mm*1270mm, the remaining part shall be filled with filler plates with known thermal conductivity. The test piece should be intact, free from cracks, missing corners, obvious deformation, and no damage to the peripheral seal. When installing the test piece, the gap between the test piece and the filling board is generally filled with polystyrene foam plastic strips. If the gap is small and difficult to fill, it can be filled with polyurethane foam, and the seam is sealed on both sides with transparent tape.

5.2 Main features

This method is suitable for testing the heat transfer coefficient of vertically installed glass. The size of the test piece is relatively large, generally up to 1.25~3.0m, and the test cycle is relatively long, generally more than 12 hours. Only used when testing large-size glass specimens. In addition, due to hot summers and cold winters and cold regions, etc. having high requirements on the insulation performance of doors and windows, it is common to use the door and window insulation equipment based on the industry standard requirements of the "Testing Method for Thermal Insulation Performance of Exterior Windows and Doors of Buildings" to test the heat transfer coefficient of glass; In warm-winter regions, the infrared spectrometer is generally used to test the glass emissivity and then calculate the glass heat transfer coefficient. It is relatively rare to use the industry standard "Testing Method for Thermal Insulation Performance of Building Exterior Windows and Doors" to test the glass heat transfer coefficient. At the same time, the calibration of the hot box method requires regular calibration of the heat flow coefficient of the wall of the hot box and the heat flow coefficient of the specimen frame. In particular, attention should be paid to the calibration conditions to minimize the test error caused by the calibrated heat flow coefficient.

^{Figure 3 The calibration hot box method}

6. On-site non-destructive testing method

6.1 Test method

According to the "Technical Conditions and Calculation Methods for On-Site Measurement of Optical and Thermal Parameters of Energy-saving Glass for Buildings", the non-destructive testing method is used to directly measure the thickness, spectral transmittance, spectral reflectance, the corrected emissivity of the film surface, and spacing of each layer of glass and spacers on site. After the basic parameters such as the volume concentration of inert gas in the layer are calculated, the total solar transmittance and heat transfer coefficient are calculated according to the industry standard "Calculation Regulations for Thermal Engineering of Building Doors, Windows, and Glass Curtain Walls". The sample should be flat glass, and the size of the sample should generally be larger than 200mm * 200mm. When measuring, avoid direct sunlight in the measurement area. The surface of the measurement area of the tested sample should be clean and free of obvious scratches.

6.2 Main features

Since the insulating glass can be directly tested without cutting the insulating glass, it provides a non-destructive testing method for the quality control of the engineering site glass and the site inspection and identification of the existing glass. This method is suitable for on-site testing of optical and thermal parameters of energy-saving glass for buildings that have been installed or are to be installed in engineering.

^{Figure 4 On-site non-destructive testing method}

7. The epilogue

Since the actual heat transfer process of insulating glass is often a complex combination of the three basic heat transfer methods of heat conduction, heat convection, and heat radiation, and the performance of the test equipment is different, there are certain differences in the results of the glass heat transfer coefficient measured by different test methods. In the test and calculation methods, the calculation method of the industry standard "Calculation Regulations for Thermal Engineering of Architectural Doors, Windows and Glass Curtain Walls" is more complicated and cumbersome, and generally requires supporting special calculation software for calculation. And Determination" The industry standard calculation method is more concise and clear, and the test time is relatively short. In the protective hot plate method and the heat flow meter method, the test piece is easy to install and the test operation is relatively easy; compared with the protective hot plate method, the test accuracy of the heat flow meter method is relatively low, and the heat flow meter method is used to test the heat transfer coefficient of glass in actual engineering. application is less. In the calibration hot box method, the installation of the specimen is more complicated and the test period is relatively long; compared with the guarded hot plate method, the calibration hot box method has slightly lower test accuracy and is generally only used in the testing of large-sized glass specimens. In the on-site non-destructive testing method, the photothermal parameters of the glass can be directly tested without cutting the insulating glass, which is suitable for on-site quality control of energy-saving glass for construction in engineering and on-site inspection and identification of existing glass.

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