Thermal emissivity of materials in the framework of heating and construction
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Principle of thermal emissivity
The emissivity of a material (ε) is its capacity to absorb and reemit energy by radiation. This physical value indicates how much a surface emits thermal infrared radiation compared to an ideal body called “black body”. The “black body” is the most efficient in terms of heat emission by radiation, ε=1=maximum.
Emissivity of materials for emitting radiation
Difference in emissivity between 2 materials (sleek-reflective white IR / rough-absorbent black IR)
Thermal emissivity in the context of radiation heating
Contribution of emissivity in the radiant power
The emissivity of a surface varies according to several parameters (which makes it difficult to measure):
- the nature of the material.
- surface condition. The rougher (as opposed to sleek) the surface of the emitting material, the better the emissivity.
- the colour. A black pan will have a better emissivity than a silver pan.
- the temperature of the material. The higher the temperature, the better the emissivity.
A few indicative values of emissivity of materials used to transmit heat in the context of heating:
- ε (anodised aluminium) =0.77
- ε (polished aluminium) =0.05 (the surface being therefore a significant non-negligible part)
- ε (glass) =0.93
- ε (smooth metal) =0.25
- ε (metal coated with specific emissive powder to maximise the emissivity) =0.96 – sources www.degxel.co.uk
In clear terms, when we talk about the diffusion efficiency of radiation by a heater, the knowledge of the material is not sufficient to draw a conclusion. Other factors described above are essential.
Thermal emissivity in the context of construction
Thermo-detector of emissivity of the measured material (wall)
- ε (stone)=0.93
- ε (frosted glass)=0.93
- ε (frosted tiles)=0.92
- ε (non-smooth wood)=0.90
- ε (red brick)=0.90
- ε (plaster)=0.90
The surface emissivity is taken into account when measuring the surface temperature of a wall using an infrared thermo-detector. The gun captures the surface temperature of the material without contact. We can thus deduce thermal bridges and the risk of mould.
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